User's Guide
Reference Manual
Release Notes
PDF
Top

Diameter
User's Guide
Version 1.6

Expand All
Contract All

Chapters

- Introduction
  - Top of chapter
- Usage
  - Top of chapter
- Examples
  - Top of chapter
- Standards Compliance
  - Top of chapter
  - RFC 6733
  - RFC 3539

# 4 Standards Compliance

Known points of questionable or non-compliance.

### 4.1 RFC 6733

- There is no support for DTLS over SCTP.

- There is no explicit support for peer discovery (section 5.2). It can possibly be implemented on top of diameter as is but this is probably something that diameter should do.

- The peer state machine's election process (section 5.6.4) isn't implemented as specified since it assumes knowledge of a peer's Origin-Host before sending it a CER. (The identity becoming known upon reception of CEA.) The possibility of configuring the peer's Origin-Host could be added, along with handling of the case that it sends something else, but for many applications this will just be unnecessary configuration of a value that it has no control over.

#### Commentary

A more detailed commentary on RFC 6733 follows. Its purpose is to (hopefully) clarify not only what is supported but how, given that semantics and features discussed in the RFC are not solely the responsibility of the diameter application: in many cases much depends on the configuration a user passes to diameter, the implementation of diameter_app(3) callback modules in particular.

Comments apply to all text following the preceding comment. Be sure to distinguish between capitalized Diameter, the protocol defined by the RFC, and lowercase diameter, the Erlang application to which the commentary applies.

Warning

The commentary is not yet complete. Comments currently stop at chapter 4.

    Fajardo, et al.              Standards Track                    [Page 6]

    RFC 6733                 Diameter Base Protocol             October 2012

    1.  Introduction

       Authentication, Authorization, and Accounting (AAA) protocols such as
       TACACS [RFC1492] and RADIUS [RFC2865] were initially deployed to
       provide dial-up PPP [RFC1661] and terminal server access.  Over time,
       AAA support was needed on many new access technologies, the scale and
       complexity of AAA networks grew, and AAA was also used on new
       applications (such as voice over IP).  This led to new demands on AAA
       protocols.

Note that diameter implements the Diameter protocol as defined in RFC 6733. It also supported the previous version of the protocol, as defined in RFC 3588, when there are differences. (Which will be noted below.) It does not support RADIUS.

       Network access requirements for AAA protocols are summarized in
       Aboba, et al. [RFC2989].  These include:

       Failover

          [RFC2865] does not define failover mechanisms and, as a result,
          failover behavior differs between implementations.  In order to
          provide well-defined failover behavior, Diameter supports
          application-layer acknowledgements and defines failover algorithms
          and the associated state machine.

No comment.

       Transmission-level security

          RADIUS [RFC2865] defines an application-layer authentication and
          integrity scheme that is required only for use with response
          packets.  While [RFC2869] defines an additional authentication and
          integrity mechanism, use is only required during Extensible
          Authentication Protocol (EAP) [RFC3748] sessions.  While attribute
          hiding is supported, [RFC2865] does not provide support for per-
          packet confidentiality.  In accounting, [RFC2866] assumes that
          replay protection is provided by the backend billing server rather
          than within the protocol itself.

          While [RFC3162] defines the use of IPsec with RADIUS, support for
          IPsec is not required.  In order to provide universal support for
          transmission-level security, and enable both intra- and inter-
          domain AAA deployments, Diameter provides support for TLS/TCP and
          DTLS/SCTP.  Security is discussed in Section 13.

Whether or not IPsec is used is transparent to diameter.

The transport protocol used on a given peer connection is also transparent to diameter in that transport to diameter is simply a module that implements the transport protocol documented in diameter_transport(3). A diameter user configures this module as the diameter:transport_opt() transport_module.

While a user can implement their own transport modules, diameter includes implementations for TCP and SCTP: diameter_tcp(3) based on gen_tcp(3) and diameter_sctp(3) based on gen_sctp(3). The former supports TLS but the latter does not currently support DTLS.

       Reliable transport

          RADIUS runs over UDP, and does not define retransmission behavior;
          as a result, reliability varies between implementations.  As
          described in [RFC2975], this is a major issue in accounting, where
          packet loss may translate directly into revenue loss.  In order to

    Fajardo, et al.              Standards Track                    [Page 7]

    RFC 6733                 Diameter Base Protocol             October 2012

          provide well-defined transport behavior, Diameter runs over
          reliable transport mechanisms (TCP, Stream Control Transmission
          Protocol (SCTP)) as defined in [RFC3539].

       Agent support

          RADIUS does not provide for explicit support for agents, including
          proxies, redirects, and relays.  Since the expected behavior is
          not defined, it varies between implementations.  Diameter defines
          agent behavior explicitly; this is described in Section 2.8.

No comment.

       Server-initiated messages

          While server-initiated messages are defined in RADIUS [RFC5176],
          support is optional.  This makes it difficult to implement
          features such as unsolicited disconnect or re-authentication/
          re-authorization on demand across a heterogeneous deployment.  To
          address this issue, support for server-initiated messages is
          mandatory in Diameter.

A diameter user can both send and receive messages.

       Transition support

          While Diameter does not share a common protocol data unit (PDU)
          with RADIUS, considerable effort has been expended in enabling
          backward compatibility with RADIUS so that the two protocols may
          be deployed in the same network.  Initially, it is expected that
          Diameter will be deployed within new network devices, as well as
          within gateways enabling communication between legacy RADIUS
          devices and Diameter agents.  This capability enables Diameter
          support to be added to legacy networks, by addition of a gateway
          or server speaking both RADIUS and Diameter.

RADIUS Attributes can be redefined as Diameter AVP's using diameter's diameter_dict(4) interface but diameter provides no such definitions.

       In addition to addressing the above requirements, Diameter also
       provides support for the following:

       Capability negotiation

          RADIUS does not support error messages, capability negotiation, or
          a mandatory/non-mandatory flag for attributes.  Since RADIUS
          clients and servers are not aware of each other's capabilities,
          they may not be able to successfully negotiate a mutually
          acceptable service or, in some cases, even be aware of what
          service has been implemented.  Diameter includes support for error
          handling (Section 7), capability negotiation (Section 5.3), and
          mandatory/non-mandatory Attribute-Value Pairs (AVPs)
          (Section 4.1).

    Fajardo, et al.              Standards Track                    [Page 8]

    RFC 6733                 Diameter Base Protocol             October 2012

       Peer discovery and configuration

          RADIUS implementations typically require that the name or address
          of servers or clients be manually configured, along with the
          corresponding shared secrets.  This results in a large
          administrative burden and creates the temptation to reuse the
          RADIUS shared secret, which can result in major security
          vulnerabilities if the Request Authenticator is not globally and
          temporally unique as required in [RFC2865].  Through DNS, Diameter
          enables dynamic discovery of peers (see Section 5.2).  Derivation
          of dynamic session keys is enabled via transmission-level
          security.

       Over time, the capabilities of Network Access Server (NAS) devices
       have increased substantially.  As a result, while Diameter is a
       considerably more sophisticated protocol than RADIUS, it remains
       feasible to implement it within embedded devices.

No comment.

    1.1.  Diameter Protocol

       The Diameter base protocol provides the following facilities:

       o  Ability to exchange messages and deliver AVPs

There are two interfaces directly involved in message exchange when using diameter: the function diameter:call/4 for sending outgoing requests, and the application callback interface, documented in diameter_app(3) for receiving incoming request and answers.

       o  Capabilities negotiation

Capabilities negotiation is the responsibility of diameter: a user configures a diameter service and/or transport with capabilities to provide AVP values for CER and CEA messages but it is diameter itself that sends these messages. A user receives notification of a successful capabilities exchange by way of peer_up/3 callbacks.

       o  Error notification

A user can subscribe to events, using diameter:subscribe/1, in order to receive notification of various failures. Errors in Diameter messaging are communicated via the application callbacks handle_request/3, handle_answer/4 and handle_error/4.

       o  Extensibility, required in [RFC2989], through addition of new
          applications, commands, and AVPs

Support for applications, commands and AVP's is extensible using diameter's dictionary interface, as documented in diameter_dict(4). Dictionaries are compiled to Erlang encode/decode modules using diameterc(1) or diameter_make(3).

       o  Basic services necessary for applications, such as the handling of
          user sessions or accounting

Compiled dictionaries are provided for the RFC 3588 and RFC 6733 Diameter applications: common, base accounting and relay. Dictionaries for a number of standardized applications are provided in uncompiled form below the examples subdirectory of the diameter application directory.

       All data delivered by the protocol is in the form of AVPs.  Some of
       these AVP values are used by the Diameter protocol itself, while
       others deliver data associated with particular applications that
       employ Diameter.  AVPs may be arbitrarily added to Diameter messages,
       the only restriction being that the Command Code Format (CCF)
       specification (Section 3.2) be satisfied.  AVPs are used by the base
       Diameter protocol to support the following required features:

       o  Transporting of user authentication information, for the purposes
          of enabling the Diameter server to authenticate the user

       o  Transporting of service-specific authorization information,
          between client and servers, allowing the peers to decide whether a
          user's access request should be granted

    Fajardo, et al.              Standards Track                    [Page 9]

    RFC 6733                 Diameter Base Protocol             October 2012

       o  Exchanging resource usage information, which may be used for
          accounting purposes, capacity planning, etc.

       o  Routing, relaying, proxying, and redirecting of Diameter messages
          through a server hierarchy

       The Diameter base protocol satisfies the minimum requirements for a
       AAA protocol, as specified by [RFC2989].  The base protocol may be
       used by itself for accounting purposes only, or it may be used with a
       Diameter application, such as Mobile IPv4 [RFC4004], or network
       access [RFC4005].  It is also possible for the base protocol to be
       extended for use in new applications, via the addition of new
       commands or AVPs.  The initial focus of Diameter was network access
       and accounting applications.  A truly generic AAA protocol used by
       many applications might provide functionality not provided by
       Diameter.  Therefore, it is imperative that the designers of new
       applications understand their requirements before using Diameter.
       See Section 1.3.4 for more information on Diameter applications.

       Any node can initiate a request.  In that sense, Diameter is a peer-
       to-peer protocol.  In this document, a Diameter client is a device at
       the edge of the network that performs access control, such as a
       Network Access Server (NAS) or a Foreign Agent (FA).  A Diameter
       client generates Diameter messages to request authentication,
       authorization, and accounting services for the user.  A Diameter
       agent is a node that does not provide local user authentication or
       authorization services; agents include proxies, redirects, and relay
       agents.  A Diameter server performs authentication and/or
       authorization of the user.  A Diameter node may act as an agent for
       certain requests while acting as a server for others.

       The Diameter protocol also supports server-initiated messages, such
       as a request to abort service to a particular user.

No comment.

    1.1.1.  Description of the Document Set

       The Diameter specification consists of an updated version of the base
       protocol specification (this document) and the Transport Profile
       [RFC3539].  This document obsoletes both RFC 3588 and RFC 5719.  A
       summary of the base protocol updates included in this document can be
       found in Section 1.1.3.

       This document defines the base protocol specification for AAA, which
       includes support for accounting.  There are also a myriad of
       applications documents describing applications that use this base
       specification for Authentication, Authorization, and Accounting.
       These application documents specify how to use the Diameter protocol
       within the context of their application.

    Fajardo, et al.              Standards Track                   [Page 10]

    RFC 6733                 Diameter Base Protocol             October 2012

       The Transport Profile document [RFC3539] discusses transport layer
       issues that arise with AAA protocols and recommendations on how to
       overcome these issues.  This document also defines the Diameter
       failover algorithm and state machine.

       "Clarifications on the Routing of Diameter Request Based on the
       Username and the Realm" [RFC5729] defines specific behavior on how to
       route requests based on the content of the User-Name AVP (Attribute
       Value Pair).

    1.1.2.  Conventions Used in This Document

       The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
       "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
       document are to be interpreted as described in [RFC2119].

No comment.

    1.1.3.  Changes from RFC 3588

       This document obsoletes RFC 3588 but is fully backward compatible
       with that document.  The changes introduced in this document focus on
       fixing issues that have surfaced during the implementation of
       Diameter (RFC 3588).  An overview of some the major changes are given
       below.

RFC 6733 is not fully backwards compatible with RFC 3588. (For example, in what values of Result-Code values are permissible with the E-bit.) The implications of incompatibilities for diameter are noted where appropriate.

       o  Deprecated the use of the Inband-Security AVP for negotiating
          Transport Layer Security (TLS) [RFC5246].  It has been generally
          considered that bootstrapping of TLS via Inband-Security AVP
          creates certain security risks because it does not completely
          protect the information carried in the CER/CEA (Capabilities-
          Exchange-Request/Capabilities-Exchange-Answer).  This version of
          Diameter adopts the common approach of defining a well-known
          secured port that peers should use when communicating via TLS/TCP
          and DTLS/SCTP.  This new approach augments the existing in-band
          security negotiation, but it does not completely replace it.  The
          old method is kept for backward compatibility reasons.

diameter_tcp(3) supports both methods of negotiating TLS: bootstrapping via Inband-Security and directly following connection establishment.

       o  Deprecated the exchange of CER/CEA messages in the open state.
          This feature was implied in the peer state machine table of RFC
          3588, but it was not clearly defined anywhere else in that
          document.  As work on this document progressed, it became clear
          that the multiplicity of meaning and use of Application-Id AVPs in
          the CER/CEA messages (and the messages themselves) is seen as an
          abuse of the Diameter extensibility rules and thus required
          simplification.  Capabilities exchange in the open state has been
          re-introduced in a separate specification [RFC6737], which clearly
          defines new commands for this feature.

Capabilities exchange in the open state is not supported: an incoming CER in the open state will cause diameter to ask the relevant transport process to terminate, which implies the loss of the peer connection in the case of diameter_tcp(3) and diameter_sctp(3).

Capabilities update, as defined by RFC 6737, is not yet supported. Support will require diameter to handle CUR/CUA in the same way that it handles CER/CEA.

    Fajardo, et al.              Standards Track                   [Page 11]

    RFC 6733                 Diameter Base Protocol             October 2012

       o  Simplified security requirements.  The use of a secured transport
          for exchanging Diameter messages remains mandatory.  However, TLS/
          TCP and DTLS/SCTP have become the primary methods of securing
          Diameter with IPsec as a secondary alternative.  See Section 13
          for details.  The support for the End-to-End security framework
          (E2E-Sequence AVP and 'P'-bit in the AVP header) has also been
          deprecated.

The End-to-End security framework is not supported since it's use is largely unspecified: diameter will set the P-bit in outgoing AVP's as directed by the relevant dictionary and/or prepare_request/3 or handle_request/3 callbacks, but whether or not the P-bit is set on incoming AVP's has no consequence.

As noted above, DTLS is not currently supported and whether or not IPsec is used is transparent to diameter.

       o  Changed Diameter extensibility.  This includes fixes to the
          Diameter extensibility description (Section 1.3 and others) to
          better aid Diameter application designers; in addition, the new
          specification relaxes the policy with respect to the allocation of
          Command Codes for vendor-specific uses.

       o  Clarified Application Id usage.  Clarify the proper use of
          Application Id information, which can be found in multiple places
          within a Diameter message.  This includes correlating Application
          Ids found in the message headers and AVPs.  These changes also
          clearly specify the proper Application Id value to use for
          specific base protocol messages (ASR/ASA, STR/STA) as well as
          clarify the content and use of Vendor-Specific-Application-Id.

       o  Clarified routing fixes.  This document more clearly specifies
          what information (AVPs and Application Ids) can be used for making
          general routing decisions.  A rule for the prioritization of
          redirect routing criteria when multiple route entries are found
          via redirects has also been added (see Section 6.13).

       o  Simplified Diameter peer discovery.  The Diameter discovery
          process now supports only widely used discovery schemes; the rest
          have been deprecated (see Section 5.2 for details).

Peer discover is not currently supported: peers to which a node should connect must be configured. Connection requests are accepted from arbitrary peers but a diameter:transport_opt() capabilities_cb can be used to reject a peer based on an incoming CER or CEA.

       There are many other miscellaneous fixes that have been introduced in
       this document that may not be considered significant, but they have
       value nonetheless.  Examples are removal of obsolete types, fixes to
       the state machine, clarification of the election process, message
       validation, fixes to Failed-AVP and Result-Code AVP values, etc.  All
       of the errata filed against RFC 3588 prior to the publication of this
       document have been addressed.  A comprehensive list of changes is not
       shown here for practical reasons.

    1.2.  Terminology

       AAA

          Authentication, Authorization, and Accounting.

    Fajardo, et al.              Standards Track                   [Page 12]

    RFC 6733                 Diameter Base Protocol             October 2012

       ABNF

          Augmented Backus-Naur Form [RFC5234].  A metalanguage with its own
          formal syntax and rules.  It is based on the Backus-Naur Form and
          is used to define message exchanges in a bi-directional
          communications protocol.

       Accounting

          The act of collecting information on resource usage for the
          purpose of capacity planning, auditing, billing, or cost
          allocation.

       Accounting Record

          An accounting record represents a summary of the resource
          consumption of a user over the entire session.  Accounting servers
          creating the accounting record may do so by processing interim
          accounting events or accounting events from several devices
          serving the same user.

       Authentication

          The act of verifying the identity of an entity (subject).

       Authorization

          The act of determining whether a requesting entity (subject) will
          be allowed access to a resource (object).

       Attribute-Value Pair (AVP)

          The Diameter protocol consists of a header followed by one or more
          Attribute-Value-Pairs (AVPs).  An AVP includes a header and is
          used to encapsulate protocol-specific data (e.g., routing
          information) as well as authentication, authorization, or
          accounting information.

No comment.

       Command Code Format (CCF)

          A modified form of ABNF used to define Diameter commands (see
          Section 3.2).

The @messages section of the diameter_dict(4) format has the CCF as content.

       Diameter Agent

          A Diameter Agent is a Diameter node that provides relay, proxy,
          redirect, or translation services.

    Fajardo, et al.              Standards Track                   [Page 13]

    RFC 6733                 Diameter Base Protocol             October 2012

       Diameter Client

          A Diameter client is a Diameter node that supports Diameter client
          applications as well as the base protocol.  Diameter clients are
          often implemented in devices situated at the edge of a network and
          provide access control services for that network.  Typical
          examples of Diameter clients include the Network Access Server
          (NAS) and the Mobile IP Foreign Agent (FA).

       Diameter Node

          A Diameter node is a host process that implements the Diameter
          protocol and acts as either a client, an agent, or a server.

       Diameter Peer

          Two Diameter nodes sharing a direct TCP or SCTP transport
          connection are called Diameter peers.

       Diameter Server

          A Diameter server is a Diameter node that handles authentication,
          authorization, and accounting requests for a particular realm.  By
          its very nature, a Diameter server must support Diameter server
          applications in addition to the base protocol.

A Diameter Node is implemented by configuring a service using diameter:start_service/2 and one or more transports using diameter:add_transport/2. The service typically represents a Diameter Node but since capabilities can be configured on individual transports it's more accurate to say that the node is a collection of transports advertising the same Origin-Host.

The role of a node (agent, client or server) is not something that's configured explicitly. Transports are either connecting or listening, depending on whether diameter should establish a peer connection and send CER or accept connections and receive CER, but the role a node implements depends largely on dictionary configuration and diameter_app(3) callback implementation.

       Downstream

          Downstream is used to identify the direction of a particular
          Diameter message from the home server towards the Diameter client.

       Home Realm

          A Home Realm is the administrative domain with which the user
          maintains an account relationship.

       Home Server

          A Diameter server that serves the Home Realm.

       Interim Accounting

          An interim accounting message provides a snapshot of usage during
          a user's session.  Typically, it is implemented in order to
          provide for partial accounting of a user's session in case a
          device reboot or other network problem prevents the delivery of a
          session summary message or session record.

    Fajardo, et al.              Standards Track                   [Page 14]

    RFC 6733                 Diameter Base Protocol             October 2012

       Local Realm

          A local realm is the administrative domain providing services to a
          user.  An administrative domain may act as a local realm for
          certain users while being a home realm for others.

       Multi-session

          A multi-session represents a logical linking of several sessions.
          Multi-sessions are tracked by using the Acct-Multi-Session-Id.  An
          example of a multi-session would be a Multi-link PPP bundle.  Each
          leg of the bundle would be a session while the entire bundle would
          be a multi-session.

       Network Access Identifier

          The Network Access Identifier, or NAI [RFC4282], is used in the
          Diameter protocol to extract a user's identity and realm.  The
          identity is used to identify the user during authentication and/or
          authorization while the realm is used for message routing
          purposes.

       Proxy Agent or Proxy

          In addition to forwarding requests and responses, proxies make
          policy decisions relating to resource usage and provisioning.
          Typically, this is accomplished by tracking the state of NAS
          devices.  While proxies usually do not respond to client requests
          prior to receiving a response from the server, they may originate
          Reject messages in cases where policies are violated.  As a
          result, proxies need to understand the semantics of the messages
          passing through them, and they may not support all Diameter
          applications.

       Realm

          The string in the NAI that immediately follows the '@' character.
          NAI realm names are required to be unique and are piggybacked on
          the administration of the DNS namespace.  Diameter makes use of
          the realm, also loosely referred to as domain, to determine
          whether messages can be satisfied locally or whether they must be
          routed or redirected.  In RADIUS, realm names are not necessarily
          piggybacked on the DNS namespace but may be independent of it.

    Fajardo, et al.              Standards Track                   [Page 15]

    RFC 6733                 Diameter Base Protocol             October 2012

       Real-Time Accounting

          Real-time accounting involves the processing of information on
          resource usage within a defined time window.  Typically, time
          constraints are imposed in order to limit financial risk.  The
          Diameter Credit-Control Application [RFC4006] is an example of an
          application that defines real-time accounting functionality.

       Relay Agent or Relay

          Relays forward requests and responses based on routing-related
          AVPs and routing table entries.  Since relays do not make policy
          decisions, they do not examine or alter non-routing AVPs.  As a
          result, relays never originate messages, do not need to understand
          the semantics of messages or non-routing AVPs, and are capable of
          handling any Diameter application or message type.  Since relays
          make decisions based on information in routing AVPs and realm
          forwarding tables, they do not keep state on NAS resource usage or
          sessions in progress.

       Redirect Agent

          Rather than forwarding requests and responses between clients and
          servers, redirect agents refer clients to servers and allow them
          to communicate directly.  Since redirect agents do not sit in the
          forwarding path, they do not alter any AVPs transiting between
          client and server.  Redirect agents do not originate messages and
          are capable of handling any message type, although they may be
          configured only to redirect messages of certain types, while
          acting as relay or proxy agents for other types.  As with relay
          agents, redirect agents do not keep state with respect to sessions
          or NAS resources.

No comment.

       Session

          A session is a related progression of events devoted to a
          particular activity.  Diameter application documents provide
          guidelines as to when a session begins and ends.  All Diameter
          packets with the same Session-Id are considered to be part of the
          same session.

Sessions are not something that diameter is aware of. The function diameter:session_id/1 can be used to construct appropriate values for Session-Id AVP's but logic connecting events in the same session is the responsibility of the diameter user.

       Stateful Agent

          A stateful agent is one that maintains session state information,
          by keeping track of all authorized active sessions.  Each
          authorized session is bound to a particular service, and its state
          is considered active either until it is notified otherwise or
          until expiration.

    Fajardo, et al.              Standards Track                   [Page 16]

    RFC 6733                 Diameter Base Protocol             October 2012

       Sub-session

          A sub-session represents a distinct service (e.g., QoS or data
          characteristics) provided to a given session.  These services may
          happen concurrently (e.g., simultaneous voice and data transfer
          during the same session) or serially.  These changes in sessions
          are tracked with the Accounting-Sub-Session-Id.

       Transaction State

          The Diameter protocol requires that agents maintain transaction
          state, which is used for failover purposes.  Transaction state
          implies that upon forwarding a request, the Hop-by-Hop Identifier
          is saved; the field is replaced with a locally unique identifier,
          which is restored to its original value when the corresponding
          answer is received.  The request's state is released upon receipt
          of the answer.  A stateless agent is one that only maintains
          transaction state.

       Translation Agent

          A translation agent (TLA in Figure 4) is a stateful Diameter node
          that performs protocol translation between Diameter and another
          AAA protocol, such as RADIUS.

       Upstream

          Upstream is used to identify the direction of a particular
          Diameter message from the Diameter client towards the home server.

       User

          The entity or device requesting or using some resource, in support
          of which a Diameter client has generated a request.

No comment.

    1.3.  Approach to Extensibility

       The Diameter protocol is designed to be extensible, using several
       mechanisms, including:

       o  Defining new AVP values

       o  Creating new AVPs

       o  Creating new commands

       o  Creating new applications

    Fajardo, et al.              Standards Track                   [Page 17]

    RFC 6733                 Diameter Base Protocol             October 2012

       From the point of view of extensibility, Diameter authentication,
       authorization, and accounting applications are treated in the same
       way.

Extensibility in diameter is by way of the dictionary interface documented in diameter_dict(4): a diameter user creates applications, commands and AVP's by implementing a new dictionary, compiling the dictionary to a codec module using diameterc(1) or diameter_make(3), and configuring the resulting dictionary module on a service. The dictionary modules provided with diameter are all implemented in this manner.

       Note: Protocol designers should try to reuse existing functionality,
       namely AVP values, AVPs, commands, and Diameter applications.  Reuse
       simplifies standardization and implementation.  To avoid potential
       interoperability issues, it is important to ensure that the semantics
       of the reused features are well understood.  Given that Diameter can
       also carry RADIUS attributes as Diameter AVPs, such reuse
       considerations also apply to existing RADIUS attributes that may be
       useful in a Diameter application.

Reuse in dictionary files is achieved by way of the @inherits section. AVP's are inherited, commands are not.

    1.3.1.  Defining New AVP Values

       In order to allocate a new AVP value for AVPs defined in the Diameter
       base protocol, the IETF needs to approve a new RFC that describes the
       AVP value.  IANA considerations for these AVP values are discussed in
       Section 11.3.

       The allocation of AVP values for other AVPs is guided by the IANA
       considerations of the document that defines those AVPs.  Typically,
       allocation of new values for an AVP defined in an RFC would require
       IETF Review [RFC5226], whereas values for vendor-specific AVPs can be
       allocated by the vendor.

    1.3.2.  Creating New AVPs

       A new AVP being defined MUST use one of the data types listed in
       Sections 4.2 or 4.3.  If an appropriate derived data type is already
       defined, it SHOULD be used instead of a base data type to encourage
       reusability and good design practice.

       In the event that a logical grouping of AVPs is necessary, and
       multiple "groups" are possible in a given command, it is recommended
       that a Grouped AVP be used (see Section 4.4).

       The creation of new AVPs can happen in various ways.  The recommended
       approach is to define a new general-purpose AVP in a Standards Track
       RFC approved by the IETF.  However, as described in Section 11.1.1,
       there are other mechanisms.

Creating new AVP's is an issue for the dictionary designer, not diameter.

    1.3.3.  Creating New Commands

       A new Command Code MUST be allocated when required AVPs (those
       indicated as {AVP} in the CCF definition) are added to, deleted from,
       or redefined in (for example, by changing a required AVP into an
       optional one) an existing command.

    Fajardo, et al.              Standards Track                   [Page 18]

    RFC 6733                 Diameter Base Protocol             October 2012

       Furthermore, if the transport characteristics of a command are
       changed (for example, with respect to the number of round trips
       required), a new Command Code MUST be registered.

       A change to the CCF of a command, such as described above, MUST
       result in the definition of a new Command Code.  This subsequently
       leads to the need to define a new Diameter application for any
       application that will use that new command.

       The IANA considerations for Command Codes are discussed in
       Section 3.1.

Creating new commands is an issue for the dictionary designer, not diameter.

    1.3.4.  Creating New Diameter Applications

       Every Diameter application specification MUST have an IANA-assigned
       Application Id (see Section 2.4).  The managed Application ID space
       is flat, and there is no relationship between different Diameter
       applications with respect to their Application Ids.  As such, there
       is no versioning support provided by these Application Ids
       themselves; every Diameter application is a standalone application.
       If the application has a relationship with other Diameter
       applications, such a relationship is not known to Diameter.

Creating new applications is an issue for the dictionary designer, not diameter.

An application's Application Id is specified in the @id section of a dictionary file.

       Before describing the rules for creating new Diameter applications,
       it is important to discuss the semantics of the AVP occurrences as
       stated in the CCF and the M-bit flag (Section 4.1) for an AVP.  There
       is no relationship imposed between the two; they are set
       independently.

       o  The CCF indicates what AVPs are placed into a Diameter command by
          the sender of that command.  Often, since there are multiple modes
          of protocol interactions, many of the AVPs are indicated as
          optional.

       o  The M-bit allows the sender to indicate to the receiver whether or
          not understanding the semantics of an AVP and its content is
          mandatory.  If the M-bit is set by the sender and the receiver
          does not understand the AVP or the values carried within that AVP,
          then a failure is generated (see Section 7).

The M-bit is set on outgoing AVP's as directed by the relevant dictionary. For incoming AVP's, an M-bit set on an AVP that isn't explicitly included in the definition of the command in question is interpreted as a 5001 error, DIAMETER_AVP_UNSUPPORTED, the consequences of which depend on the value of the diameter:application_opt() answer_errors or request_errors.

       It is the decision of the protocol designer when to develop a new
       Diameter application rather than extending Diameter in other ways.
       However, a new Diameter application MUST be created when one or more
       of the following criteria are met:

    Fajardo, et al.              Standards Track                   [Page 19]

    RFC 6733                 Diameter Base Protocol             October 2012

       M-bit Setting

          An AVP with the M-bit in the MUST column of the AVP flag table is
          added to an existing Command/Application.  An AVP with the M-bit
          in the MAY column of the AVP flag table is added to an existing
          Command/Application.

          Note: The M-bit setting for a given AVP is relevant to an
          Application and each command within that application that includes
          the AVP.  That is, if an AVP appears in two commands for
          application Foo and the M-bit settings are different in each
          command, then there should be two AVP flag tables describing when
          to set the M-bit.

       Commands

          A new command is used within the existing application because
          either an additional command is added, an existing command has
          been modified so that a new Command Code had to be registered, or
          a command has been deleted.

       AVP Flag bits

          If an existing application changes the meaning/semantics of its
          AVP Flags or adds new flag bits, then a new Diameter application
          MUST be created.

       If the CCF definition of a command allows it, an implementation may
       add arbitrary optional AVPs with the M-bit cleared (including vendor-
       specific AVPs) to that command without needing to define a new
       application.  Please refer to Section 11.1.1 for details.

No comment.

    2.  Protocol Overview

       The base Diameter protocol concerns itself with establishing
       connections to peers, capabilities negotiation, how messages are sent
       and routed through peers, and how the connections are eventually torn
       down.  The base protocol also defines certain rules that apply to all
       message exchanges between Diameter nodes.

       Communication between Diameter peers begins with one peer sending a
       message to another Diameter peer.  The set of AVPs included in the
       message is determined by a particular Diameter application.  One AVP
       that is included to reference a user's session is the Session-Id.

       The initial request for authentication and/or authorization of a user
       would include the Session-Id AVP.  The Session-Id is then used in all
       subsequent messages to identify the user's session (see Section 8 for

    Fajardo, et al.              Standards Track                   [Page 20]

    RFC 6733                 Diameter Base Protocol             October 2012

       more information).  The communicating party may accept the request or
       reject it by returning an answer message with the Result-Code AVP set
       to indicate that an error occurred.  The specific behavior of the
       Diameter server or client receiving a request depends on the Diameter
       application employed.

       Session state (associated with a Session-Id) MUST be freed upon
       receipt of the Session-Termination-Request, Session-Termination-
       Answer, expiration of authorized service time in the Session-Timeout
       AVP, and according to rules established in a particular Diameter
       application.

Like Session-Id, session state is maintained by the diameter user: diameter has no session state of its own and does not interpret STR/STA in any way.

       The base Diameter protocol may be used by itself for accounting
       applications.  For authentication and authorization, it is always
       extended for a particular application.

       Diameter clients MUST support the base protocol, which includes
       accounting.  In addition, they MUST fully support each Diameter
       application that is needed to implement the client's service, e.g.,
       Network Access Server Requirements (NASREQ) [RFC2881] and/or Mobile
       IPv4.  A Diameter client MUST be referred to as "Diameter X Client"
       where X is the application that it supports and not a "Diameter
       Client".

       Diameter servers MUST support the base protocol, which includes
       accounting.  In addition, they MUST fully support each Diameter
       application that is needed to implement the intended service, e.g.,
       NASREQ and/or Mobile IPv4.  A Diameter server MUST be referred to as
       "Diameter X Server" where X is the application that it supports, and
       not a "Diameter Server".

       Diameter relays and redirect agents are transparent to the Diameter
       applications, but they MUST support the Diameter base protocol, which
       includes accounting, and all Diameter applications.

       Diameter proxies MUST support the base protocol, which includes
       accounting.  In addition, they MUST fully support each Diameter
       application that is needed to implement proxied services, e.g.,
       NASREQ and/or Mobile IPv4.  A Diameter proxy MUST be referred to as
       "Diameter X Proxy" where X is the application which it supports, and
       not a "Diameter Proxy".

No comment.

    Fajardo, et al.              Standards Track                   [Page 21]

    RFC 6733                 Diameter Base Protocol             October 2012

    2.1.  Transport

       The Diameter Transport profile is defined in [RFC3539].

       The base Diameter protocol is run on port 3868 for both TCP [RFC0793]
       and SCTP [RFC4960].  For TLS [RFC5246] and Datagram Transport Layer
       Security (DTLS) [RFC6347], a Diameter node that initiates a
       connection prior to any message exchanges MUST run on port 5658.  It
       is assumed that TLS is run on top of TCP when it is used, and DTLS is
       run on top of SCTP when it is used.

Which port a transport connects to or listens on is a matter of configuration. Both diameter_tcp(3) and diameter_sctp(3) will default to 3868 if no other value is specified.

       If the Diameter peer does not support receiving TLS/TCP and DTLS/SCTP
       connections on port 5658 (i.e., the peer complies only with RFC
       3588), then the initiator MAY revert to using TCP or SCTP on port
       3868.  Note that this scheme is kept only for the purpose of backward
       compatibility and that there are inherent security vulnerabilities
       when the initial CER/CEA messages are sent unprotected (see
       Section 5.6).

       Diameter clients MUST support either TCP or SCTP; agents and servers
       SHOULD support both.

       A Diameter node MAY initiate connections from a source port other
       than the one that it declares it accepts incoming connections on, and
       it MUST always be prepared to receive connections on port 3868 for
       TCP or SCTP and port 5658 for TLS/TCP and DTLS/SCTP connections.
       When DNS-based peer discovery (Section 5.2) is used, the port numbers
       received from SRV records take precedence over the default ports
       (3868 and 5658).

       A given Diameter instance of the peer state machine MUST NOT use more
       than one transport connection to communicate with a given peer,
       unless multiple instances exist on the peer, in which, case a
       separate connection per process is allowed.

The diameter:service_opt() restrict_connection controls to what extent a diameter service allows multiple connections to the same peer. (As identified by the value of Origin-Host received from it during capabilities exchange.)

       When no transport connection exists with a peer, an attempt to
       connect SHOULD be made periodically.  This behavior is handled via
       the Tc timer (see Section 12 for details), whose recommended value is
       30 seconds.  There are certain exceptions to this rule, such as when
       a peer has terminated the transport connection stating that it does
       not wish to communicate.

The frequency of reconnection attempts is configured with the diameter:transport_opt() connect_timer and watchdog_timer.

       When connecting to a peer and either zero or more transports are
       specified, TLS SHOULD be tried first, followed by DTLS, then by TCP,
       and finally by SCTP.  See Section 5.2 for more information on peer
       discovery.

The order in which different transports are attempted depends on the order of diameter:transport_opt() transport_module and transport_config tuples in transport configuration.

    Fajardo, et al.              Standards Track                   [Page 22]

    RFC 6733                 Diameter Base Protocol             October 2012

       Diameter implementations SHOULD be able to interpret ICMP protocol
       port unreachable messages as explicit indications that the server is
       not reachable, subject to security policy on trusting such messages.
       Further guidance regarding the treatment of ICMP errors can be found
       in [RFC5927] and [RFC5461].  Diameter implementations SHOULD also be
       able to interpret a reset from the transport and timed-out connection
       attempts.  If Diameter receives data from the lower layer that cannot
       be parsed or identified as a Diameter error made by the peer, the
       stream is compromised and cannot be recovered.  The transport
       connection MUST be closed using a RESET call (send a TCP RST bit) or
       an SCTP ABORT message (graceful closure is compromised).

ICMP messages and other transport-level errors aren't directly visible to diameter but transport implementations like diameter_tcp(3) and diameter_sctp(3) propagate these as terminating transport processes.

    2.1.1.  SCTP Guidelines

       Diameter messages SHOULD be mapped into SCTP streams in a way that
       avoids head-of-the-line (HOL) blocking.  Among different ways of
       performing the mapping that fulfill this requirement it is
       RECOMMENDED that a Diameter node send every Diameter message (request
       or response) over stream zero with the unordered flag set.  However,
       Diameter nodes MAY select and implement other design alternatives for
       avoiding HOL blocking such as using multiple streams with the
       unordered flag cleared (as originally instructed in RFC 3588).  On
       the receiving side, a Diameter entity MUST be ready to receive
       Diameter messages over any stream, and it is free to return responses
       over a different stream.  This way, both sides manage the available
       streams in the sending direction, independently of the streams chosen
       by the other side to send a particular Diameter message.  These
       messages can be out-of-order and belong to different Diameter
       sessions.

diameter_sctp(3) allows the sender to specify a stream number explicitly. The stream on which an incoming message is received it passed to handle_request/3 and handle_answer/4 callbacks as transport_data in a #diameter_packet{}.

Ordered or unordered delivery can be configured per transport.

       Out-of-order delivery has special concerns during a connection
       establishment and termination.  When a connection is established, the
       responder side sends a CEA message and moves to R-Open state as
       specified in Section 5.6.  If an application message is sent shortly
       after the CEA and delivered out-of-order, the initiator side, still
       in Wait-I-CEA state, will discard the application message and close
       the connection.  In order to avoid this race condition, the receiver
       side SHOULD NOT use out-of-order delivery methods until the first
       message has been received from the initiator, proving that it has
       moved to I-Open state.  To trigger such a message, the receiver side
       could send a DWR immediately after sending a CEA.  Upon reception of
       the corresponding DWA, the receiver side should start using out-of-
       order delivery methods to counter the HOL blocking.

diameter_sctp(3) does not currently allow the user to switch between ordered and unordered delivery, or to specify the manner of sending per message: one or the other must be configured, the defaults being ordered.

       Another race condition may occur when DPR and DPA messages are used.
       Both DPR and DPA are small in size; thus, they may be delivered to
       the peer faster than application messages when an out-of-order
       delivery mechanism is used.  Therefore, it is possible that a DPR/DPA

    Fajardo, et al.              Standards Track                   [Page 23]

    RFC 6733                 Diameter Base Protocol             October 2012

       exchange completes while application messages are still in transit,
       resulting in a loss of these messages.  An implementation could
       mitigate this race condition, for example, using timers, and wait for
       a short period of time for pending application level messages to
       arrive before proceeding to disconnect the transport connection.
       Eventually, lost messages are handled by the retransmission mechanism
       described in Section 5.5.4.

       A Diameter agent SHOULD use dedicated payload protocol identifiers
       (PPIDs) for clear text and encrypted SCTP DATA chunks instead of only
       using the unspecified payload protocol identifier (value 0).  For
       this purpose, two PPID values are allocated: the PPID value 46 is for
       Diameter messages in clear text SCTP DATA chunks, and the PPID value
       47 is for Diameter messages in protected DTLS/SCTP DATA chunks.

No comment.

    2.2.  Securing Diameter Messages

       Connections between Diameter peers SHOULD be protected by TLS/TCP and
       DTLS/SCTP.  All Diameter base protocol implementations MUST support
       the use of TLS/TCP and DTLS/SCTP.  If desired, alternative security
       mechanisms that are independent of Diameter, such as IPsec [RFC4301],
       can be deployed to secure connections between peers.  The Diameter
       protocol MUST NOT be used without one of TLS, DTLS, or IPsec.

As noted above, DTLS is not currently supported and IPsec usage is transparent to diameter. Security is not enforced by diameter.

    2.3.  Diameter Application Compliance

       Application Ids are advertised during the capabilities exchange phase
       (see Section 5.3).  Advertising support of an application implies
       that the sender supports the functionality specified in the
       respective Diameter application specification.

       Implementations MAY add arbitrary optional AVPs with the M-bit
       cleared (including vendor-specific AVPs) to a command defined in an
       application, but only if the command's CCF syntax specification
       allows for it.  Please refer to Section 11.1.1 for details.

No comment.

    2.4.  Application Identifiers

       Each Diameter application MUST have an IANA-assigned Application ID.
       The base protocol does not require an Application Id since its
       support is mandatory.  During the capabilities exchange, Diameter
       nodes inform their peers of locally supported applications.
       Furthermore, all Diameter messages contain an Application Id, which
       is used in the message forwarding process.

    Fajardo, et al.              Standards Track                   [Page 24]

    RFC 6733                 Diameter Base Protocol             October 2012

       The following Application Id values are defined:

             Diameter common message       0
             Diameter base accounting      3
             Relay                         0xffffffff

These applications are implemented in the dictionary modules diameter_gen_base_rfc6733, diameter_gen_acct_rfc6733 and diameter_relay respectively. There are also RFC 3588 versions or the common and accounting dictionaries: diameter_gen_base_rfc3588 and diameter_base_accounting. (The inconsistent naming is historical.) Dictionary modules are configured using the diameter:application_opt() dictionary.

       Relay and redirect agents MUST advertise the Relay Application ID,
       while all other Diameter nodes MUST advertise locally supported
       applications.  The receiver of a Capabilities Exchange message
       advertising relay service MUST assume that the sender supports all
       current and future applications.

       Diameter relay and proxy agents are responsible for finding an
       upstream server that supports the application of a particular
       message.  If none can be found, an error message is returned with the
       Result-Code AVP set to DIAMETER_UNABLE_TO_DELIVER.

No comment.

    2.5.  Connections vs. Sessions

       This section attempts to provide the reader with an understanding of
       the difference between "connection" and "session", which are terms
       used extensively throughout this document.

       A connection refers to a transport-level connection between two peers
       that is used to send and receive Diameter messages.  A session is a
       logical concept at the application layer that exists between the
       Diameter client and the Diameter server; it is identified via the
       Session-Id AVP.

                 +--------+          +-------+          +--------+
                 | Client |          | Relay |          | Server |
                 +--------+          +-------+          +--------+
                          <---------->       <---------->
                       peer connection A   peer connection B

                          <----------------------------->
                                  User session x

                    Figure 1: Diameter Connections and Sessions

       In the example provided in Figure 1, peer connection A is established
       between the client and the relay.  Peer connection B is established
       between the relay and the server.  User session X spans from the
       client via the relay to the server.  Each "user" of a service causes
       an auth request to be sent, with a unique session identifier.  Once
       accepted by the server, both the client and the server are aware of
       the session.

    Fajardo, et al.              Standards Track                   [Page 25]

    RFC 6733                 Diameter Base Protocol             October 2012

       It is important to note that there is no relationship between a
       connection and a session, and that Diameter messages for multiple
       sessions are all multiplexed through a single connection.  Also, note
       that Diameter messages pertaining to the session, both application-
       specific and those that are defined in this document such as ASR/ASA,
       RAR/RAA, and STR/STA, MUST carry the Application Id of the
       application.  Diameter messages pertaining to peer connection
       establishment and maintenance such as CER/CEA, DWR/DWA, and DPR/DPA
       MUST carry an Application Id of zero (0).

As noted above, diameter is not involved in session management. This is the responsibility of the diameter user.

    2.6.  Peer Table

       The Diameter peer table is used in message forwarding and is
       referenced by the routing table.  A peer table entry contains the
       following fields:

       Host Identity

          Following the conventions described for the DiameterIdentity-
          derived AVP data format in Section 4.3.1, this field contains the
          contents of the Origin-Host (Section 6.3) AVP found in the CER or
          CEA message.

       StatusT

          This is the state of the peer entry, and it MUST match one of the
          values listed in Section 5.6.

       Static or Dynamic

          Specifies whether a peer entry was statically configured or
          dynamically discovered.

       Expiration Time

          Specifies the time at which dynamically discovered peer table
          entries are to be either refreshed or expired.  If public key
          certificates are used for Diameter security (e.g., with TLS), this
          value MUST NOT be greater than the expiry times in the relevant
          certificates.

       TLS/TCP and DTLS/SCTP Enabled

          Specifies whether TLS/TCP and DTLS/SCTP is to be used when
          communicating with the peer.

       Additional security information, when needed (e.g., keys,
       certificates).

The Peer Table is not directly accessible to the diameter user. Information about connected peers can be retrieved using diameter:service_info/2.

    Fajardo, et al.              Standards Track                   [Page 26]

    RFC 6733                 Diameter Base Protocol             October 2012

    2.7.  Routing Table

       All Realm-Based routing lookups are performed against what is
       commonly known as the routing table (see Section 12).  Each routing
       table entry contains the following fields:

       Realm Name

          This is the field that MUST be used as a primary key in the
          routing table lookups.  Note that some implementations perform
          their lookups based on longest-match-from-the-right on the realm
          rather than requiring an exact match.

       Application Identifier

          An application is identified by an Application Id.  A route entry
          can have a different destination based on the Application Id in
          the message header.  This field MUST be used as a secondary key
          field in routing table lookups.

       Local Action

          The Local Action field is used to identify how a message should be
          treated.  The following actions are supported:

          1.  LOCAL - Diameter messages that can be satisfied locally and do
              not need to be routed to another Diameter entity.

          2.  RELAY - All Diameter messages that fall within this category
              MUST be routed to a next-hop Diameter entity that is indicated
              by the identifier described below.  Routing is done without
              modifying any non-routing AVPs.  See Section 6.1.9 for
              relaying guidelines.

          3.  PROXY - All Diameter messages that fall within this category
              MUST be routed to a next Diameter entity that is indicated by
              the identifier described below.  The local server MAY apply
              its local policies to the message by including new AVPs to the
              message prior to routing.  See Section 6.1.9 for proxying
              guidelines.

          4.  REDIRECT - Diameter messages that fall within this category
              MUST have the identity of the home Diameter server(s)
              appended, and returned to the sender of the message.  See
              Section 6.1.8 for redirection guidelines.

    Fajardo, et al.              Standards Track                   [Page 27]

    RFC 6733                 Diameter Base Protocol             October 2012

       Server Identifier

          The identity of one or more servers to which the message is to be
          routed.  This identity MUST also be present in the Host Identity
          field of the peer table (Section 2.6).  When the Local Action is
          set to RELAY or PROXY, this field contains the identity of the
          server(s) to which the message MUST be routed.  When the Local
          Action field is set to REDIRECT, this field contains the identity
          of one or more servers to which the message MUST be redirected.

       Static or Dynamic

          Specifies whether a route entry was statically configured or
          dynamically discovered.

       Expiration Time

          Specifies the time at which a dynamically discovered route table
          entry expires.  If public key certificates are used for Diameter
          security (e.g., with TLS), this value MUST NOT be greater than the
          expiry time in the relevant certificates.

       It is important to note that Diameter agents MUST support at least
       one of the LOCAL, RELAY, PROXY, or REDIRECT modes of operation.
       Agents do not need to support all modes of operation in order to
       conform with the protocol specification, but they MUST follow the
       protocol compliance guidelines in Section 2.  Relay agents and
       proxies MUST NOT reorder AVPs.

       The routing table MAY include a default entry that MUST be used for
       any requests not matching any of the other entries.  The routing
       table MAY consist of only such an entry.

       When a request is routed, the target server MUST have advertised the
       Application Id (see Section 2.4) for the given message or have
       advertised itself as a relay or proxy agent.  Otherwise, an error is
       returned with the Result-Code AVP set to DIAMETER_UNABLE_TO_DELIVER.

Routing does not need specific support in diameter: a user can maintain their own routing table if desired and implement any desired routing in diameter_app(3) callbacks. However, it may be convenient to add more specific routing support to diameter in the future.

    2.8.  Role of Diameter Agents

       In addition to clients and servers, the Diameter protocol introduces
       relay, proxy, redirect, and translation agents, each of which is
       defined in Section 1.2.  Diameter agents are useful for several
       reasons:

An noted above, the role a node plays is largely a question of configuration and diameter_app(3) callback implementation.

       o  They can distribute administration of systems to a configurable
          grouping, including the maintenance of security associations.

    Fajardo, et al.              Standards Track                   [Page 28]

    RFC 6733                 Diameter Base Protocol             October 2012

       o  They can be used for concentration of requests from a number of
          co-located or distributed NAS equipment sets to a set of like user
          groups.

       o  They can do value-added processing to the requests or responses.

       o  They can be used for load balancing.

       o  A complex network will have multiple authentication sources, they
          can sort requests and forward towards the correct target.

       The Diameter protocol requires that agents maintain transaction
       state, which is used for failover purposes.  Transaction state
       implies that upon forwarding a request, its Hop-by-Hop Identifier is
       saved; the field is replaced with a locally unique identifier, which
       is restored to its original value when the corresponding answer is
       received.  The request's state is released upon receipt of the
       answer.  A stateless agent is one that only maintains transaction
       state.

       The Proxy-Info AVP allows stateless agents to add local state to a
       Diameter request, with the guarantee that the same state will be
       present in the answer.  However, the protocol's failover procedures
       require that agents maintain a copy of pending requests.

       A stateful agent is one that maintains session state information by
       keeping track of all authorized active sessions.  Each authorized
       session is bound to a particular service, and its state is considered
       active until either the agent is notified otherwise or the session
       expires.  Each authorized session has an expiration, which is
       communicated by Diameter servers via the Session-Timeout AVP.

       Maintaining session state may be useful in certain applications, such
       as:

       o  Protocol translation (e.g., RADIUS <-> Diameter)

       o  Limiting resources authorized to a particular user

       o  Per-user or per-transaction auditing

       A Diameter agent MAY act in a stateful manner for some requests and
       be stateless for others.  A Diameter implementation MAY act as one
       type of agent for some requests and as another type of agent for
       others.

No comment.

    Fajardo, et al.              Standards Track                   [Page 29]

    RFC 6733                 Diameter Base Protocol             October 2012

    2.8.1.  Relay Agents

       Relay agents are Diameter agents that accept requests and route
       messages to other Diameter nodes based on information found in the
       messages (e.g., the value of the Destination-Realm AVP Section 6.6).
       This routing decision is performed using a list of supported realms
       and known peers.  This is known as the routing table, as is defined
       further in Section 2.7.

       Relays may, for example, be used to aggregate requests from multiple
       Network Access Servers (NASes) within a common geographical area
       (Point of Presence, POP).  The use of relays is advantageous since it
       eliminates the need for NASes to be configured with the necessary
       security information they would otherwise require to communicate with
       Diameter servers in other realms.  Likewise, this reduces the
       configuration load on Diameter servers that would otherwise be
       necessary when NASes are added, changed, or deleted.

       Relays modify Diameter messages by inserting and removing routing
       information, but they do not modify any other portion of a message.
       Relays SHOULD NOT maintain session state but MUST maintain
       transaction state.

           +------+    --------->     +------+     --------->    +------+
           |      |    1. Request     |      |     2. Request    |      |
           | NAS  |                   | DRL  |                   | HMS  |
           |      |    4. Answer      |      |     3. Answer     |      |
           +------+    <---------     +------+     <---------    +------+
        example.net                example.net                example.com

                      Figure 2: Relaying of Diameter messages

       The example provided in Figure 2 depicts a request issued from a NAS,
       which is an access device, for the user bob@example.com.  Prior to
       issuing the request, the NAS performs a Diameter route lookup, using
       "example.com" as the key, and determines that the message is to be
       relayed to a DRL, which is a Diameter relay.  The DRL performs the
       same route lookup as the NAS, and relays the message to the HMS,
       which is example.com's home server.  The HMS identifies that the
       request can be locally supported (via the realm), processes the
       authentication and/or authorization request, and replies with an
       answer, which is routed back to the NAS using saved transaction
       state.

       Since relays do not perform any application-level processing, they
       provide relaying services for all Diameter applications; therefore,
       they MUST advertise the Relay Application Id.

Requests are relayed by returning a relay tuple from a handle_request/3 callback.

    Fajardo, et al.              Standards Track                   [Page 30]

    RFC 6733                 Diameter Base Protocol             October 2012

    2.8.2.  Proxy Agents

       Similar to relays, proxy agents route Diameter messages using the
       Diameter routing table.  However, they differ since they modify
       messages to implement policy enforcement.  This requires that proxies
       maintain the state of their downstream peers (e.g., access devices)
       to enforce resource usage, provide admission control, and provide
       provisioning.

       Proxies may, for example, be used in call control centers or access
       ISPs that provide outsourced connections; they can monitor the number
       and type of ports in use and make allocation and admission decisions
       according to their configuration.

       Since enforcing policies requires an understanding of the service
       being provided, proxies MUST only advertise the Diameter applications
       they support.

No comment.

    2.8.3.  Redirect Agents

       Redirect agents are useful in scenarios where the Diameter routing
       configuration needs to be centralized.  An example is a redirect
       agent that provides services to all members of a consortium, but does
       not wish to be burdened with relaying all messages between realms.
       This scenario is advantageous since it does not require that the
       consortium provide routing updates to its members when changes are
       made to a member's infrastructure.

       Since redirect agents do not relay messages, and only return an
       answer with the information necessary for Diameter agents to
       communicate directly, they do not modify messages.  Since redirect
       agents do not receive answer messages, they cannot maintain session
       state.

       The example provided in Figure 3 depicts a request issued from the
       access device, NAS, for the user bob@example.com.  The message is
       forwarded by the NAS to its relay, DRL, which does not have a routing
       entry in its Diameter routing table for example.com.  The DRL has a
       default route configured to DRD, which is a redirect agent that
       returns a redirect notification to DRL, as well as the HMS' contact
       information.  Upon receipt of the redirect notification, the DRL
       establishes a transport connection with the HMS, if one doesn't
       already exist, and forwards the request to it.

    Fajardo, et al.              Standards Track                   [Page 31]

    RFC 6733                 Diameter Base Protocol             October 2012

                                      +------+
                                      |      |
                                      | DRD  |
                                      |      |
                                      +------+
                                       ^    |
                           2. Request  |    | 3. Redirection
                                       |    |    Notification
                                       |    v
           +------+    --------->     +------+     --------->    +------+
           |      |    1. Request     |      |     4. Request    |      |
           | NAS  |                   | DRL  |                   | HMS  |
           |      |    6. Answer      |      |     5. Answer     |      |
           +------+    <---------     +------+     <---------    +------+
          example.net                example.net               example.com

                     Figure 3: Redirecting a Diameter Message

       Since redirect agents do not perform any application-level
       processing, they provide relaying services for all Diameter
       applications; therefore, they MUST advertise the Relay Application
       ID.

No comment.

    2.8.4.  Translation Agents

       A translation agent is a device that provides translation between two
       protocols (e.g., RADIUS<->Diameter, TACACS+<->Diameter).  Translation
       agents are likely to be used as aggregation servers to communicate
       with a Diameter infrastructure, while allowing for the embedded
       systems to be migrated at a slower pace.

       Given that the Diameter protocol introduces the concept of long-lived
       authorized sessions, translation agents MUST be session stateful and
       MUST maintain transaction state.

       Translation of messages can only occur if the agent recognizes the
       application of a particular request; therefore, translation agents
       MUST only advertise their locally supported applications.

           +------+    --------->     +------+     --------->    +------+
           |      |  RADIUS Request   |      |  Diameter Request |      |
           | NAS  |                   | TLA  |                   | HMS  |
           |      |  RADIUS Answer    |      |  Diameter Answer  |      |
           +------+    <---------     +------+     <---------    +------+
          example.net                example.net               example.com

                    Figure 4: Translation of RADIUS to Diameter

No comment.

    Fajardo, et al.              Standards Track                   [Page 32]

    RFC 6733                 Diameter Base Protocol             October 2012

    2.9.  Diameter Path Authorization

       As noted in Section 2.2, Diameter provides transmission-level
       security for each connection using TLS/TCP and DTLS/SCTP.  Therefore,
       each connection can be authenticated and can be replay and integrity
       protected.

       In addition to authenticating each connection, the entire session
       MUST also be authorized.  Before initiating a connection, a Diameter
       peer MUST check that its peers are authorized to act in their roles.
       For example, a Diameter peer may be authentic, but that does not mean
       that it is authorized to act as a Diameter server advertising a set
       of Diameter applications.

       Prior to bringing up a connection, authorization checks are performed
       at each connection along the path.  Diameter capabilities negotiation
       (CER/CEA) also MUST be carried out, in order to determine what
       Diameter applications are supported by each peer.  Diameter sessions
       MUST be routed only through authorized nodes that have advertised
       support for the Diameter application required by the session.

       As noted in Section 6.1.9, a relay or proxy agent MUST append a
       Route-Record AVP to all requests forwarded.  The AVP contains the
       identity of the peer from which the request was received.

       The home Diameter server, prior to authorizing a session, MUST check
       the Route-Record AVPs to make sure that the route traversed by the
       request is acceptable.  For example, administrators within the home
       realm may not wish to honor requests that have been routed through an
       untrusted realm.  By authorizing a request, the home Diameter server
       is implicitly indicating its willingness to engage in the business
       transaction as specified by any contractual relationship between the
       server and the previous hop.  A DIAMETER_AUTHORIZATION_REJECTED error
       message (see Section 7.1.5) is sent if the route traversed by the
       request is unacceptable.

       A home realm may also wish to check that each accounting request
       message corresponds to a Diameter response authorizing the session.
       Accounting requests without corresponding authorization responses
       SHOULD be subjected to further scrutiny, as should accounting
       requests indicating a difference between the requested and provided
       service.

       Forwarding of an authorization response is considered evidence of a
       willingness to take on financial risk relative to the session.  A
       local realm may wish to limit this exposure, for example, by
       establishing credit limits for intermediate realms and refusing to
       accept responses that would violate those limits.  By issuing an

    Fajardo, et al.              Standards Track                   [Page 33]

    RFC 6733                 Diameter Base Protocol             October 2012

       accounting request corresponding to the authorization response, the
       local realm implicitly indicates its agreement to provide the service
       indicated in the authorization response.  If the service cannot be
       provided by the local realm, then a DIAMETER_UNABLE_TO_COMPLY error
       message MUST be sent within the accounting request; a Diameter client
       receiving an authorization response for a service that it cannot
       perform MUST NOT substitute an alternate service and then send
       accounting requests for the alternate service instead.

No comment.

    3.  Diameter Header

       A summary of the Diameter header format is shown below.  The fields
       are transmitted in network byte order.

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |    Version    |                 Message Length                |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          | Command Flags |                  Command Code                 |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                         Application-ID                        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                      Hop-by-Hop Identifier                    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                      End-to-End Identifier                    |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |  AVPs ...
          +-+-+-+-+-+-+-+-+-+-+-+-+-

The Diameter Header is represented by the diameter_header record defined in diameter.hrl. The diameter_packet record contains a header field whose value will be a decoded #diameter_header{} for incoming messages passed to handle_request/3 and handle_answer/4 callbacks. In the case of outgoing messages, diameter and the relevant dictionary populate the Diameter Header appropriately, although prepare_request/3 and handle_request/3 callbacks can modify header values. (Which can be useful in test.)

       Version

          This Version field MUST be set to 1 to indicate Diameter Version
          1.

        Message Length

          The Message Length field is three octets and indicates the length
          of the Diameter message including the header fields and the padded
          AVPs.  Thus, the Message Length field is always a multiple of 4.

       Command Flags

          The Command Flags field is eight bits.  The following bits are
          assigned:

    Fajardo, et al.              Standards Track                   [Page 34]

    RFC 6733                 Diameter Base Protocol             October 2012

              0 1 2 3 4 5 6 7
             +-+-+-+-+-+-+-+-+
             |R P E T r r r r|
             +-+-+-+-+-+-+-+-+

          R(equest)

             If set, the message is a request.  If cleared, the message is
             an answer.

          P(roxiable)

             If set, the message MAY be proxied, relayed, or redirected.  If
             cleared, the message MUST be locally processed.

          E(rror)

             If set, the message contains a protocol error, and the message
             will not conform to the CCF described for this command.
             Messages with the 'E' bit set are commonly referred to as error
             messages.  This bit MUST NOT be set in request messages (see
             Section 7.2).

          T(Potentially retransmitted message)

             This flag is set after a link failover procedure, to aid the
             removal of duplicate requests.  It is set when resending
             requests not yet acknowledged, as an indication of a possible
             duplicate due to a link failure.  This bit MUST be cleared when
             sending a request for the first time; otherwise, the sender
             MUST set this flag.  Diameter agents only need to be concerned
             about the number of requests they send based on a single
             received request; retransmissions by other entities need not be
             tracked.  Diameter agents that receive a request with the T
             flag set, MUST keep the T flag set in the forwarded request.
             This flag MUST NOT be set if an error answer message (e.g., a
             protocol error) has been received for the earlier message.  It
             can be set only in cases where no answer has been received from
             the server for a request, and the request has been sent again.
             This flag MUST NOT be set in answer messages.

          r(eserved)

             These flag bits are reserved for future use; they MUST be set
             to zero and ignored by the receiver.

Reserved bits are set to 0 in outgoing messages.

    Fajardo, et al.              Standards Track                   [Page 35]

    RFC 6733                 Diameter Base Protocol             October 2012

       Command Code

          The Command Code field is three octets and is used in order to
          communicate the command associated with the message.  The 24-bit
          address space is managed by IANA (see Section 3.1).  Command Code
          values 16,777,214 and 16,777,215 (hexadecimal values FFFFFE-
          FFFFFF) are reserved for experimental use (see Section 11.2).

       Application-ID

          Application-ID is four octets and is used to identify for which
          application the message is applicable.  The application can be an
          authentication application, an accounting application, or a
          vendor-specific application.

          The value of the Application-ID field in the header MUST be the
          same as any relevant Application-Id AVPs contained in the message.

       Hop-by-Hop Identifier

          The Hop-by-Hop Identifier is an unsigned 32-bit integer field (in
          network byte order) that aids in matching requests and replies.
          The sender MUST ensure that the Hop-by-Hop Identifier in a request
          is unique on a given connection at any given time, and it MAY
          attempt to ensure that the number is unique across reboots.  The
          sender of an answer message MUST ensure that the Hop-by-Hop
          Identifier field contains the same value that was found in the
          corresponding request.  The Hop-by-Hop Identifier is normally a
          monotonically increasing number, whose start value was randomly
          generated.  An answer message that is received with an unknown
          Hop-by-Hop Identifier MUST be discarded.

       End-to-End Identifier

          The End-to-End Identifier is an unsigned 32-bit integer field (in
          network byte order) that is used to detect duplicate messages.
          Upon reboot, implementations MAY set the high order 12 bits to
          contain the low order 12 bits of current time, and the low order
          20 bits to a random value.  Senders of request messages MUST
          insert a unique identifier on each message.  The identifier MUST
          remain locally unique for a period of at least 4 minutes, even
          across reboots.  The originator of an answer message MUST ensure
          that the End-to-End Identifier field contains the same value that
          was found in the corresponding request.  The End-to-End Identifier
          MUST NOT be modified by Diameter agents of any kind.  The
          combination of the Origin-Host AVP (Section 6.3) and this field is
          used to detect duplicates.  Duplicate requests SHOULD cause the
          same answer to be transmitted (modulo the Hop-by-Hop Identifier

    Fajardo, et al.              Standards Track                   [Page 36]

    RFC 6733                 Diameter Base Protocol             October 2012

          field and any routing AVPs that may be present), and they MUST NOT
          affect any state that was set when the original request was
          processed.  Duplicate answer messages that are to be locally
          consumed (see Section 6.2) SHOULD be silently discarded.

       AVPs

          AVPs are a method of encapsulating information relevant to the
          Diameter message.  See Section 4 for more information on AVPs.

No comment.

    3.1.  Command Codes

       Each command Request/Answer pair is assigned a Command Code, and the
       sub-type (i.e., request or answer) is identified via the 'R' bit in
       the Command Flags field of the Diameter header.

       Every Diameter message MUST contain a Command Code in its header's
       Command Code field, which is used to determine the action that is to
       be taken for a particular message.  The following Command Codes are
       defined in the Diameter base protocol:

                                                       Section
        Command Name             Abbrev.    Code       Reference
          --------------------------------------------------------
          Abort-Session-Request     ASR       274           8.5.1
          Abort-Session-Answer      ASA       274           8.5.2
          Accounting-Request        ACR       271           9.7.1
          Accounting-Answer         ACA       271           9.7.2
          Capabilities-Exchange-    CER       257           5.3.1
             Request
          Capabilities-Exchange-    CEA       257           5.3.2
             Answer
          Device-Watchdog-Request   DWR       280           5.5.1
          Device-Watchdog-Answer    DWA       280           5.5.2
          Disconnect-Peer-Request   DPR       282           5.4.1
          Disconnect-Peer-Answer    DPA       282           5.4.2
          Re-Auth-Request           RAR       258           8.3.1
          Re-Auth-Answer            RAA       258           8.3.2
          Session-Termination-      STR       275           8.4.1
             Request
          Session-Termination-      STA       275           8.4.2
             Answer

These messages are all defined in diameter's implementation of the common dictionary in modules diameter_gen_base_rfc6733 and diameter_gen_base_rfc3588. Corresponding record definitions are found in diameter_gen_base_rfc6733.hrl and diameter_gen_base_rfc3588.hrl.

    Fajardo, et al.              Standards Track                   [Page 37]

    RFC 6733                 Diameter Base Protocol             October 2012

    3.2.  Command Code Format Specification

       Every Command Code defined MUST include a corresponding Command Code
       Format (CCF) specification, which is used to define the AVPs that
       MUST or MAY be present when sending the message.  The following ABNF
       specifies the CCF used in the definition:

The CCF is what is specified in the @messages section of the diameter_dict(4) format, except as noted below.

       command-def      = "<" command-name ">" "::=" diameter-message

Angle brackets are currently not allowed here. This was a change between RFC 3588 and RFC 6733: the former disallowed them in the grammar but included them in its own command definitions.

       command-name     = diameter-name

       diameter-name    = ALPHA *(ALPHA / DIGIT / "-")

       diameter-message = header   *fixed  *required *optional

       header           = "<Diameter-Header:" command-id
                             [r-bit] [p-bit] [e-bit] [application-id]">"

       application-id   = 1*DIGIT

       command-id       = 1*DIGIT
                          ; The Command Code assigned to the command.

       r-bit            = ", REQ"
                          ; If present, the 'R' bit in the Command
                          ; Flags is set, indicating that the message
                          ; is a request as opposed to an answer.

       p-bit            = ", PXY"
                          ; If present, the 'P' bit in the Command
                          ; Flags is set, indicating that the message
                          ; is proxiable.

       e-bit            = ", ERR"
                          ; If present, the 'E' bit in the Command
                          ; Flags is set, indicating that the answer
                          ; message contains a Result-Code AVP in
                          ; the "protocol error" class.

       fixed            = [qual] "<" avp-spec ">"
                          ; Defines the fixed position of an AVP.

       required         = [qual] "{" avp-spec "}"
                          ; The AVP MUST be present and can appear
                          ; anywhere in the message.

    Fajardo, et al.              Standards Track                   [Page 38]

    RFC 6733                 Diameter Base Protocol             October 2012

       optional         = [qual] "[" avp-name "]"
                          ; The avp-name in the 'optional' rule cannot
                          ; evaluate to any AVP Name that is included
                          ; in a fixed or required rule.  The AVP can
                          ; appear anywhere in the message.
                          ;
                          ; NOTE:  "[" and "]" have a slightly different
                          ; meaning than in ABNF.  These braces
                          ; cannot be used to express optional fixed rules
                          ; (such as an optional ICV at the end).  To do
                          ; this, the convention is '0*1fixed'.

       qual             = [min] "*" [max]
                          ; See ABNF conventions, RFC 5234, Section 4.
                          ; The absence of any qualifier depends on
                          ; whether it precedes a fixed, required, or
                          ; optional rule.  If a fixed or required rule has
                          ; no qualifier, then exactly one such AVP MUST
                          ; be present.  If an optional rule has no
                          ; qualifier, then 0 or 1 such AVP may be
                          ; present.  If an optional rule has a qualifier,
                          ; then the value of min MUST be 0 if present.

       min              = 1*DIGIT
                          ; The minimum number of times the element may
                          ; be present.  If absent, the default value is 0
                          ; for fixed and optional rules and 1 for
                          ; required rules.  The value MUST be at least 1
                          ; for required rules.

       max              = 1*DIGIT
                          ; The maximum number of times the element may
                          ; be present.  If absent, the default value is
                          ; infinity.  A value of 0 implies the AVP MUST
                          ; NOT be present.

       avp-spec         = diameter-name
                          ; The avp-spec has to be an AVP Name, defined
                          ; in the base or extended Diameter
                          ; specifications.

       avp-name         = avp-spec / "AVP"
                          ; The string "AVP" stands for *any* arbitrary AVP
                          ; Name, not otherwise listed in that Command Code
                          ; definition.  The inclusion of this string
                          ; is recommended for all CCFs to allow for
                          ; extensibility.

    Fajardo, et al.              Standards Track                   [Page 39]

    RFC 6733                 Diameter Base Protocol             October 2012

       The following is a definition of a fictitious Command Code:

       Example-Request ::= < Diameter Header: 9999999, REQ, PXY >
                           { User-Name }
                        1* { Origin-Host }
                         * [ AVP ]

No comment.

    3.3.  Diameter Command Naming Conventions

       Diameter command names typically includes one or more English words
       followed by the verb "Request" or "Answer".  Each English word is
       delimited by a hyphen.  A three-letter acronym for both the request
       and answer is also normally provided.

       An example is a message set used to terminate a session.  The command
       name is Session-Terminate-Request and Session-Terminate-Answer, while
       the acronyms are STR and STA, respectively.

       Both the request and the answer for a given command share the same
       Command Code.  The request is identified by the R(equest) bit in the
       Diameter header set to one (1), to ask that a particular action be
       performed, such as authorizing a user or terminating a session.  Once
       the receiver has completed the request, it issues the corresponding
       answer, which includes a result code that communicates one of the
       following:

       o  The request was successful

       o  The request failed

       o  An additional request has to be sent to provide information the
          peer requires prior to returning a successful or failed answer.

       o  The receiver could not process the request, but provides
          information about a Diameter peer that is able to satisfy the
          request, known as redirect.

       Additional information, encoded within AVPs, may also be included in
       answer messages.

The diameter_dict(4) format places no requirement on the naming of commands.

    4.  Diameter AVPs

       Diameter AVPs carry specific authentication, accounting,
       authorization, and routing information as well as configuration
       details for the request and reply.

    Fajardo, et al.              Standards Track                   [Page 40]

    RFC 6733                 Diameter Base Protocol             October 2012

       Each AVP of type OctetString MUST be padded to align on a 32-bit
       boundary, while other AVP types align naturally.  A number of zero-
       valued bytes are added to the end of the AVP Data field until a word
       boundary is reached.  The length of the padding is not reflected in
       the AVP Length field.

    4.1.  AVP Header

       The fields in the AVP header MUST be sent in network byte order.  The
       format of the header is:

           0                   1                   2                   3
           0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                           AVP Code                            |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |V M P r r r r r|                  AVP Length                   |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |                        Vendor-ID (opt)                        |
          +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
          |    Data ...
          +-+-+-+-+-+-+-+-+

       AVP Code

          The AVP Code, combined with the Vendor-Id field, identifies the
          attribute uniquely.  AVP numbers 1 through 255 are reserved for
          reuse of RADIUS attributes, without setting the Vendor-Id field.
          AVP numbers 256 and above are used for Diameter, which are
          allocated by IANA (see Section 11.1.1).

       AVP Flags

          The AVP Flags field informs the receiver how each attribute must
          be handled.  New Diameter applications SHOULD NOT define
          additional AVP Flag bits.  However, note that new Diameter
          applications MAY define additional bits within the AVP header, and
          an unrecognized bit SHOULD be considered an error.  The sender of
          the AVP MUST set 'R' (reserved) bits to 0 and the receiver SHOULD
          ignore all 'R' (reserved) bits.  The 'P' bit has been reserved for
          future usage of end-to-end security.  At the time of writing,
          there are no end-to-end security mechanisms specified; therefore,
          the 'P' bit SHOULD be set to 0.

          The 'M' bit, known as the Mandatory bit, indicates whether the
          receiver of the AVP MUST parse and understand the semantics of the
          AVP including its content.  The receiving entity MUST return an
          appropriate error message if it receives an AVP that has the M-bit

    Fajardo, et al.              Standards Track                   [Page 41]

    RFC 6733                 Diameter Base Protocol             October 2012

          set but does not understand it.  An exception applies when the AVP
          is embedded within a Grouped AVP.  See Section 4.4 for details.
          Diameter relay and redirect agents MUST NOT reject messages with
          unrecognized AVPs.

          The 'M' bit MUST be set according to the rules defined in the
          application specification that introduces or reuses this AVP.
          Within a given application, the M-bit setting for an AVP is
          defined either for all command types or for each command type.

          AVPs with the 'M' bit cleared are informational only; a receiver
          that receives a message with such an AVP that is not supported, or
          whose value is not supported, MAY simply ignore the AVP.

          The 'V' bit, known as the Vendor-Specific bit, indicates whether
          the optional Vendor-ID field is present in the AVP header.  When
          set, the AVP Code belongs to the specific vendor code address
          space.

       AVP Length

          The AVP Length field is three octets, and indicates the number of
          octets in this AVP including the AVP Code field, AVP Length field,
          AVP Flags field, Vendor-ID field (if present), and the AVP Data
          field.  If a message is received with an invalid attribute length,
          the message MUST be rejected.

    4.1.1.  Optional Header Elements

       The AVP header contains one optional field.  This field is only
       present if the respective bit-flag is enabled.

       Vendor-ID

          The Vendor-ID field is present if the 'V' bit is set in the AVP
          Flags field.  The optional four-octet Vendor-ID field contains the
          IANA-assigned "SMI Network Management Private Enterprise Codes"
          [ENTERPRISE] value, encoded in network byte order.  Any vendors or
          standardization organizations that are also treated like vendors
          in the IANA-managed "SMI Network Management Private Enterprise
          Codes" space wishing to implement a vendor-specific Diameter AVP
          MUST use their own Vendor-ID along with their privately managed
          AVP address space, guaranteeing that they will not collide with
          any other vendor's vendor-specific AVP(s) or with future IETF
          AVPs.

    Fajardo, et al.              Standards Track                   [Page 42]

    RFC 6733                 Diameter Base Protocol             October 2012

          A Vendor-ID value of zero (0) corresponds to the IETF-adopted AVP
          values, as managed by IANA.  Since the absence of the Vendor-ID
          field implies that the AVP in question is not vendor specific,
          implementations MUST NOT use the value of zero (0) for the
          Vendor-ID field.

    4.2.  Basic AVP Data Formats

       The Data field is zero or more octets and contains information
       specific to the Attribute.  The format and length of the Data field
       is determined by the AVP Code and AVP Length fields.  The format of
       the Data field MUST be one of the following base data types or a data
       type derived from the base data types.  In the event that a new Basic
       AVP Data Format is needed, a new version of this RFC MUST be created.

       OctetString

          The data contains arbitrary data of variable length.  Unless
          otherwise noted, the AVP Length field MUST be set to at least 8
          (12 if the 'V' bit is enabled).  AVP values of this type that are
          not a multiple of 4 octets in length are followed by the necessary
          padding so that the next AVP (if any) will start on a 32-bit
          boundary.

       Integer32

          32-bit signed value, in network byte order.  The AVP Length field
          MUST be set to 12 (16 if the 'V' bit is enabled).

       Integer64

          64-bit signed value, in network byte order.  The AVP Length field
          MUST be set to 16 (20 if the 'V' bit is enabled).

       Unsigned32

          32-bit unsigned value, in network byte order.  The AVP Length
          field MUST be set to 12 (16 if the 'V' bit is enabled).

       Unsigned64

          64-bit unsigned value, in network byte order.  The AVP Length
          field MUST be set to 16 (20 if the 'V' bit is enabled).

    Fajardo, et al.              Standards Track                   [Page 43]

    RFC 6733                 Diameter Base Protocol             October 2012

       Float32

          This represents floating point values of single precision as
          described by [FLOATPOINT].  The 32-bit value is transmitted in
          network byte order.  The AVP Length field MUST be set to 12 (16 if
          the 'V' bit is enabled).

       Float64

          This represents floating point values of double precision as
          described by [FLOATPOINT].  The 64-bit value is transmitted in
          network byte order.  The AVP Length field MUST be set to 16 (20 if
          the 'V' bit is enabled).

       Grouped

          The Data field is specified as a sequence of AVPs.  These AVPs are
          concatenated -- including their headers and padding -- in the
          order in which they are specified and the result encapsulated in
          the Data field.  The AVP Length field is set to 8 (12 if the 'V'
          bit is enabled) plus the total length of all included AVPs,
          including their headers and padding.  Thus, the AVP Length field
          of an AVP of type Grouped is always a multiple of 4.

    4.3.  Derived AVP Data Formats

       In addition to using the Basic AVP Data Formats, applications may
       define data formats derived from the Basic AVP Data Formats.  An
       application that defines new Derived AVP Data Formats MUST include
       them in a section titled "Derived AVP Data Formats", using the same
       format as the definitions below.  Each new definition MUST be either
       defined or listed with a reference to the RFC that defines the
       format.

    4.3.1.  Common Derived AVP Data Formats

       The following are commonly used Derived AVP Data Formats.

       Address

          The Address format is derived from the OctetString Basic AVP
          Format.  It is a discriminated union representing, for example, a
          32-bit (IPv4) [RFC0791] or 128-bit (IPv6) [RFC4291] address, most
          significant octet first.  The first two octets of the Address AVP
          represent the AddressType, which contains an Address Family,
          defined in [IANAADFAM].  The AddressType is used to discriminate
          the content and format of the remaining octets.

    Fajardo, et al.              Standards Track                   [Page 44]

    RFC 6733                 Diameter Base Protocol             October 2012

       Time

          The Time format is derived from the OctetString Basic AVP Format.
          The string MUST contain four octets, in the same format as the
          first four bytes are in the NTP timestamp format.  The NTP
          timestamp format is defined in Section 3 of [RFC5905].

          This represents the number of seconds since 0h on 1 January 1900
          with respect to the Coordinated Universal Time (UTC).

          On 6h 28m 16s UTC, 7 February 2036, the time value will overflow.
          Simple Network Time Protocol (SNTP) [RFC5905] describes a
          procedure to extend the time to 2104.  This procedure MUST be
          supported by all Diameter nodes.

       UTF8String

          The UTF8String format is derived from the OctetString Basic AVP
          Format.  This is a human-readable string represented using the
          ISO/IEC IS 10646-1 character set, encoded as an OctetString using
          the UTF-8 transformation format [RFC3629].

          Since additional code points are added by amendments to the 10646
          standard from time to time, implementations MUST be prepared to
          encounter any code point from 0x00000001 to 0x7fffffff.  Byte
          sequences that do not correspond to the valid encoding of a code
          point into UTF-8 charset or are outside this range are prohibited.

          The use of control codes SHOULD be avoided.  When it is necessary
          to represent a new line, the control code sequence CR LF SHOULD be
          used.

          The use of leading or trailing white space SHOULD be avoided.

          For code points not directly supported by user interface hardware
          or software, an alternative means of entry and display, such as
          hexadecimal, MAY be provided.

          For information encoded in 7-bit US-ASCII, the UTF-8 charset is
          identical to the US-ASCII charset.

          UTF-8 may require multiple bytes to represent a single character /
          code point; thus, the length of a UTF8String in octets may be
          different from the number of characters encoded.

          Note that the AVP Length field of an UTF8String is measured in
          octets not characters.

    Fajardo, et al.              Standards Track                   [Page 45]

    RFC 6733                 Diameter Base Protocol             October 2012

       DiameterIdentity

          The DiameterIdentity format is derived from the OctetString Basic
          AVP Format.

                            DiameterIdentity  = FQDN/Realm

       The DiameterIdentity value is used to uniquely identify either:

          *  A Diameter node for purposes of duplicate connection and
             routing loop detection.

          *  A Realm to determine whether messages can be satisfied locally
             or whether they must be routed or redirected.

          When a DiameterIdentity value is used to identify a Diameter node,
          the contents of the string MUST be the Fully Qualified Domain Name
          (FQDN) of the Diameter node.  If multiple Diameter nodes run on
          the same host, each Diameter node MUST be assigned a unique
          DiameterIdentity.  If a Diameter node can be identified by several
          FQDNs, a single FQDN should be picked at startup and used as the
          only DiameterIdentity for that node, whatever the connection on
          which it is sent.  In this document, note that DiameterIdentity is
          in ASCII form in order to be compatible with existing DNS
          infrastructure.  See Appendix D for interactions between the
          Diameter protocol and Internationalized Domain Names (IDNs).

       DiameterURI

          The DiameterURI MUST follow the Uniform Resource Identifiers (RFC
          3986) syntax [RFC3986] rules specified below:

          "aaa://" FQDN [ port ] [ transport ] [ protocol ]

                          ; No transport security

          "aaas://" FQDN [ port ] [ transport ] [ protocol ]

                          ; Transport security used

          FQDN               = < Fully Qualified Domain Name >

    Fajardo, et al.              Standards Track                   [Page 46]

    RFC 6733                 Diameter Base Protocol             October 2012

          port               = ":" 1*DIGIT

                          ; One of the ports used to listen for
                          ; incoming connections.
                          ; If absent, the default Diameter port
                          ; (3868) is assumed if no transport
                          ; security is used and port 5658 when
                          ; transport security (TLS/TCP and DTLS/SCTP)
                          ; is used.

          transport          = ";transport=" transport-protocol

                          ; One of the transports used to listen
                          ; for incoming connections.  If absent,
                          ; the default protocol is assumed to be TCP.
                          ; UDP MUST NOT be used when the aaa-protocol
                          ; field is set to diameter.

          transport-protocol = ( "tcp" / "sctp" / "udp" )

          protocol           = ";protocol=" aaa-protocol

                          ; If absent, the default AAA protocol
                          ; is Diameter.

          aaa-protocol       = ( "diameter" / "radius" / "tacacs+" )

          The following are examples of valid Diameter host identities:

          aaa://host.example.com;transport=tcp
          aaa://host.example.com:6666;transport=tcp
          aaa://host.example.com;protocol=diameter
          aaa://host.example.com:6666;protocol=diameter
          aaa://host.example.com:6666;transport=tcp;protocol=diameter
          aaa://host.example.com:1813;transport=udp;protocol=radius

       Enumerated

          The Enumerated format is derived from the Integer32 Basic AVP
          Format.  The definition contains a list of valid values and their
          interpretation and is described in the Diameter application
          introducing the AVP.

    Fajardo, et al.              Standards Track                   [Page 47]

    RFC 6733                 Diameter Base Protocol             October 2012

       IPFilterRule

          The IPFilterRule format is derived from the OctetString Basic AVP
          Format and uses the ASCII charset.  The rule syntax is a modified
          subset of ipfw(8) from FreeBSD.  Packets may be filtered based on
          the following information that is associated with it:

                Direction                          (in or out)
                Source and destination IP address  (possibly masked)
                Protocol
                Source and destination port        (lists or ranges)
                TCP flags
                IP fragment flag
                IP options
                ICMP types

       Rules for the appropriate direction are evaluated in order, with the
       first matched rule terminating the evaluation.  Each packet is
       evaluated once.  If no rule matches, the packet is dropped if the
       last rule evaluated was a permit, and passed if the last rule was a
       deny.

       IPFilterRule filters MUST follow the format:

             action dir proto from src to dst [options]

             action       permit - Allow packets that match the rule.
                          deny   - Drop packets that match the rule.

             dir          "in" is from the terminal, "out" is to the
                          terminal.

             proto        An IP protocol specified by number.  The "ip"
                          keyword means any protocol will match.

             src and dst  <address/mask> [ports]

                          The <address/mask> may be specified as:
                          ipno       An IPv4 or IPv6 number in dotted-
                                     quad or canonical IPv6 form.  Only
                                     this exact IP number will match the
                                     rule.

    Fajardo, et al.              Standards Track                   [Page 48]

    RFC 6733                 Diameter Base Protocol             October 2012

                          ipno/bits  An IP number as above with a mask
                                     width of the form 192.0.2.10/24.  In
                                     this case, all IP numbers from
                                     192.0.2.0 to 192.0.2.255 will match.
                                     The bit width MUST be valid for the
                                     IP version, and the IP number MUST
                                     NOT have bits set beyond the mask.
                                     For a match to occur, the same IP
                                     version must be present in the
                                     packet that was used in describing
                                     the IP address.  To test for a
                                     particular IP version, the bits part
                                     can be set to zero.  The keyword
                                     "any" is 0.0.0.0/0 or the IPv6
                                     equivalent.  The keyword "assigned"
                                     is the address or set of addresses
                                     assigned to the terminal.  For IPv4,
                                     a typical first rule is often "deny
                                     in ip! assigned".

                          The sense of the match can be inverted by
                          preceding an address with the not modifier (!),
                          causing all other addresses to be matched
                          instead.  This does not affect the selection of
                          port numbers.

                          With the TCP, UDP, and SCTP protocols, optional
                          ports may be specified as:

                             {port/port-port}[,ports[,...]]

                           The '-' notation specifies a range of ports
                          (including boundaries).

                          Fragmented packets that have a non-zero offset
                          (i.e., not the first fragment) will never match
                          a rule that has one or more port
                          specifications.  See the frag option for
                          details on matching fragmented packets.

             options:
                frag    Match if the packet is a fragment and this is not
                        the first fragment of the datagram.  frag may not
                        be used in conjunction with either tcpflags or
                        TCP/UDP port specifications.

    Fajardo, et al.              Standards Track                   [Page 49]

    RFC 6733                 Diameter Base Protocol             October 2012

                ipoptions spec
                        Match if the IP header contains the comma-separated
                        list of options specified in spec.  The
                        supported IP options are:

                        ssrr (strict source route), lsrr (loose source
                        route), rr (record packet route), and ts
                        (timestamp).  The absence of a particular option
                        may be denoted with a '!'.

                tcpoptions spec
                        Match if the TCP header contains the comma-separated
                        list of options specified in spec.  The
                        supported TCP options are:

                        mss (maximum segment size), window (tcp window
                        advertisement), sack (selective ack), ts (rfc1323
                        timestamp), and cc (rfc1644 t/tcp connection
                        count).  The absence of a particular option may
                        be denoted with a '!'.

                established
                        TCP packets only.  Match packets that have the RST
                        or ACK bits set.

                setup   TCP packets only.  Match packets that have the SYN
                        bit set but no ACK bit.

                tcpflags spec
                        TCP packets only.  Match if the TCP header
                        contains the comma-separated list of flags
                        specified in spec.  The supported TCP flags are:

                        fin, syn, rst, psh, ack, and urg.  The absence of a
                        particular flag may be denoted with a '!'.  A rule
                        that contains a tcpflags specification can never
                        match a fragmented packet that has a non-zero
                        offset.  See the frag option for details on
                        matching fragmented packets.

                icmptypes types
                        ICMP packets only.  Match if the ICMP type is in
                        the list types.  The list may be specified as any
                        combination of ranges or individual types
                        separated by commas.  Both the numeric values and
                        the symbolic values listed below can be used.  The
                        supported ICMP types are:

    Fajardo, et al.              Standards Track                   [Page 50]

    RFC 6733                 Diameter Base Protocol             October 2012

                        echo reply (0), destination unreachable (3),
                        source quench (4), redirect (5), echo request
                        (8), router advertisement (9), router
                        solicitation (10), time-to-live exceeded (11), IP
                        header bad (12), timestamp request (13),
                        timestamp reply (14), information request (15),
                        information reply (16), address mask request (17),
                        and address mask reply (18).

       There is one kind of packet that the access device MUST always
       discard, that is an IP fragment with a fragment offset of one.  This
       is a valid packet, but it only has one use, to try to circumvent
       firewalls.

       An access device that is unable to interpret or apply a deny rule
       MUST terminate the session.  An access device that is unable to
       interpret or apply a permit rule MAY apply a more restrictive rule.
       An access device MAY apply deny rules of its own before the supplied
       rules, for example to protect the access device owner's
       infrastructure.

    4.4.   Grouped AVP Values

       The Diameter protocol allows AVP values of type 'Grouped'.  This
       implies that the Data field is actually a sequence of AVPs.  It is
       possible to include an AVP with a Grouped type within a Grouped type,
       that is, to nest them.  AVPs within an AVP of type Grouped have the
       same padding requirements as non-Grouped AVPs, as defined in
       Section 4.4.

       The AVP Code numbering space of all AVPs included in a Grouped AVP is
       the same as for non-Grouped AVPs.  Receivers of a Grouped AVP that
       does not have the 'M' (mandatory) bit set and one or more of the
       encapsulated AVPs within the group has the 'M' (mandatory) bit set
       MAY simply be ignored if the Grouped AVP itself is unrecognized.  The
       rule applies even if the encapsulated AVP with its 'M' (mandatory)
       bit set is further encapsulated within other sub-groups, i.e., other
       Grouped AVPs embedded within the Grouped AVP.

       Every Grouped AVP definition MUST include a corresponding grammar,
       using ABNF [RFC5234] (with modifications), as defined below.

             grouped-avp-def  = "<" name ">" "::=" avp

             name-fmt         = ALPHA *(ALPHA / DIGIT / "-")

    Fajardo, et al.              Standards Track                   [Page 51]

    RFC 6733                 Diameter Base Protocol             October 2012

             name             = name-fmt
                                ; The name has to be the name of an AVP,
                                ; defined in the base or extended Diameter
                                ; specifications.

             avp              = header *fixed *required *optional

             header           = "<" "AVP-Header:" avpcode [vendor] ">"

             avpcode          = 1*DIGIT
                                ; The AVP Code assigned to the Grouped AVP.

             vendor           = 1*DIGIT
                                ; The Vendor-ID assigned to the Grouped AVP.
                                ; If absent, the default value of zero is
                                ; used.

    4.4.1.  Example AVP with a Grouped Data Type

       The Example-AVP (AVP Code 999999) is of type Grouped and is used to
       clarify how Grouped AVP values work.  The Grouped Data field has the
       following CCF grammar:

             Example-AVP  ::= < AVP Header: 999999 >
                              { Origin-Host }
                            1*{ Session-Id }
                             *[ AVP ]

          An Example-AVP with Grouped Data follows.

          The Origin-Host AVP (Section 6.3) is required.  In this case:

             Origin-Host = "example.com".

          One or more Session-Ids must follow.  Here there are two:

             Session-Id =
               "grump.example.com:33041;23432;893;0AF3B81"

             Session-Id =
               "grump.example.com:33054;23561;2358;0AF3B82"

    Fajardo, et al.              Standards Track                   [Page 52]

    RFC 6733                 Diameter Base Protocol             October 2012

          optional AVPs included are

             Recovery-Policy = <binary>
                2163bc1d0ad82371f6bc09484133c3f09ad74a0dd5346d54195a7cf0b35
                2cabc881839a4fdcfbc1769e2677a4c1fb499284c5f70b48f58503a45c5
                c2d6943f82d5930f2b7c1da640f476f0e9c9572a50db8ea6e51e1c2c7bd
                f8bb43dc995144b8dbe297ac739493946803e1cee3e15d9b765008a1b2a
                cf4ac777c80041d72c01e691cf751dbf86e85f509f3988e5875dc905119
                26841f00f0e29a6d1ddc1a842289d440268681e052b30fb638045f7779c
                1d873c784f054f688f5001559ecff64865ef975f3e60d2fd7966b8c7f92

             Futuristic-Acct-Record = <binary>
                fe19da5802acd98b07a5b86cb4d5d03f0314ab9ef1ad0b67111ff3b90a0
                57fe29620bf3585fd2dd9fcc38ce62f6cc208c6163c008f4258d1bc88b8
                17694a74ccad3ec69269461b14b2e7a4c111fb239e33714da207983f58c
                41d018d56fe938f3cbf089aac12a912a2f0d1923a9390e5f789cb2e5067
                d3427475e49968f841

       The data for the optional AVPs is represented in hexadecimal form
       since the format of these AVPs is not known at the time of definition
       of the Example-AVP group nor (likely) at the time when the example
       instance of this AVP is interpreted -- except by Diameter
       implementations that support the same set of AVPs.  The encoding
       example illustrates how padding is used and how length fields are
       calculated.  Also, note that AVPs may be present in the Grouped AVP
       value that the receiver cannot interpret (here, the Recover-Policy
       and Futuristic-Acct-Record AVPs).  The length of the Example-AVP is
       the sum of all the length of the member AVPs, including their
       padding, plus the Example-AVP header size.

    Fajardo, et al.              Standards Track                   [Page 53]

    RFC 6733                 Diameter Base Protocol             October 2012

       This AVP would be encoded as follows:

             0       1       2       3       4       5       6       7
          +-------+-------+-------+-------+-------+-------+-------+-------+
       0  |     Example AVP Header (AVP Code = 999999), Length = 496      |
          +-------+-------+-------+-------+-------+-------+-------+-------+
       8  |     Origin-Host AVP Header (AVP Code = 264), Length = 19      |
          +-------+-------+-------+-------+-------+-------+-------+-------+
       16 |  'e'  |  'x'  |  'a'  |  'm'  |  'p'  |  'l'  |  'e'  |  '.'  |
          +-------+-------+-------+-------+-------+-------+-------+-------+
       24 |  'c'  |  'o'  |  'm'  |Padding|     Session-Id AVP Header     |
          +-------+-------+-------+-------+-------+-------+-------+-------+
       32 | (AVP Code = 263), Length = 49 |  'g'  |  'r'  |  'u'  |  'm'  |
          +-------+-------+-------+-------+-------+-------+-------+-------+
                                        . . .
          +-------+-------+-------+-------+-------+-------+-------+-------+
       72 |  'F'  |  '3'  |  'B'  |  '8'  |  '1'  |Padding|Padding|Padding|
          +-------+-------+-------+-------+-------+-------+-------+-------+
       80 |     Session-Id AVP Header (AVP Code = 263), Length = 50       |
          +-------+-------+-------+-------+-------+-------+-------+-------+
       88 |  'g'  |  'r'  |  'u'  |  'm'  |  'p'  |  '.'  |  'e'  |  'x'  |
          +-------+-------+-------+-------+-------+-------+-------+-------+
                                       . . .
          +-------+-------+-------+-------+-------+-------+-------+-------+
       120|  '5'  |  '8'  |  ';'  |  '0'  |  'A'  |  'F'  |  '3'  |  'B'  |
          +-------+-------+-------+-------+-------+-------+-------+-------+
       128|  '8'  |  '2'  |Padding|Padding|  Recovery-Policy Header (AVP  |
          +-------+-------+-------+-------+-------+-------+-------+-------+
       136|  Code = 8341), Length = 223   | 0x21  | 0x63  | 0xbc  | 0x1d  |
          +-------+-------+-------+-------+-------+-------+-------+-------+
       144|  0x0a | 0xd8  | 0x23  | 0x71  | 0xf6  | 0xbc  | 0x09  | 0x48  |
          +-------+-------+-------+-------+-------+-------+-------+-------+
                                        . . .
          +-------+-------+-------+-------+-------+-------+-------+-------+
       352|  0x8c | 0x7f  | 0x92  |Padding| Futuristic-Acct-Record Header |
          +-------+-------+-------+-------+-------+-------+-------+-------+
       328|(AVP Code = 15930),Length = 137| 0xfe  | 0x19  | 0xda  | 0x58  |
          +-------+-------+-------+-------+-------+-------+-------+-------+
       336|  0x02 | 0xac  | 0xd9  | 0x8b  | 0x07  | 0xa5  | 0xb8  | 0xc6  |
          +-------+-------+-------+-------+-------+-------+-------+-------+
                                        . . .
          +-------+-------+-------+-------+-------+-------+-------+-------+
       488|  0xe4 | 0x99  | 0x68  | 0xf8  | 0x41  |Padding|Padding|Padding|
          +-------+-------+-------+-------+-------+-------+-------+-------+

    Fajardo, et al.              Standards Track                   [Page 54]

    RFC 6733                 Diameter Base Protocol             October 2012

    4.5.  Diameter Base Protocol AVPs

       The following table describes the Diameter AVPs defined in the base
       protocol, their AVP Code values, types, and possible flag values.

       Due to space constraints, the short form DiamIdent is used to
       represent DiameterIdentity.

    Fajardo, et al.              Standards Track                   [Page 55]

    RFC 6733                 Diameter Base Protocol             October 2012

                                                +----------+
                                                | AVP Flag |
                                                |  rules   |
                                                |----+-----|
                       AVP  Section             |    |MUST |
       Attribute Name  Code Defined  Data Type  |MUST| NOT |
       -----------------------------------------|----+-----|
       Acct-             85  9.8.2   Unsigned32 | M  |  V  |
         Interim-Interval                       |    |     |
       Accounting-      483  9.8.7   Enumerated | M  |  V  |
         Realtime-Required                      |    |     |
       Acct-            50   9.8.5   UTF8String | M  |  V  |
         Multi-Session-Id                       |    |     |
       Accounting-      485  9.8.3   Unsigned32 | M  |  V  |
         Record-Number                          |    |     |
       Accounting-      480  9.8.1   Enumerated | M  |  V  |
         Record-Type                            |    |     |
       Acct-             44  9.8.4   OctetString| M  |  V  |
        Session-Id                              |    |     |
       Accounting-      287  9.8.6   Unsigned64 | M  |  V  |
         Sub-Session-Id                         |    |     |
       Acct-            259  6.9     Unsigned32 | M  |  V  |
         Application-Id                         |    |     |
       Auth-            258  6.8     Unsigned32 | M  |  V  |
         Application-Id                         |    |     |
       Auth-Request-    274  8.7     Enumerated | M  |  V  |
          Type                                  |    |     |
       Authorization-   291  8.9     Unsigned32 | M  |  V  |
         Lifetime                               |    |     |
       Auth-Grace-      276  8.10    Unsigned32 | M  |  V  |
         Period                                 |    |     |
       Auth-Session-    277  8.11    Enumerated | M  |  V  |
         State                                  |    |     |
       Re-Auth-Request- 285  8.12    Enumerated | M  |  V  |
         Type                                   |    |     |
       Class             25  8.20    OctetString| M  |  V  |
       Destination-Host 293  6.5     DiamIdent  | M  |  V  |
       Destination-     283  6.6     DiamIdent  | M  |  V  |
         Realm                                  |    |     |
       Disconnect-Cause 273  5.4.3   Enumerated | M  |  V  |
       Error-Message    281  7.3     UTF8String |    | V,M |
       Error-Reporting- 294  7.4     DiamIdent  |    | V,M |
         Host                                   |    |     |
       Event-Timestamp   55  8.21    Time       | M  |  V  |
       Experimental-    297  7.6     Grouped    | M  |  V  |
          Result                                |    |     |
       -----------------------------------------|----+-----|

    Fajardo, et al.              Standards Track                   [Page 56]

    RFC 6733                 Diameter Base Protocol             October 2012

                                                +----------+
                                                | AVP Flag |
                                                |  rules   |
                                                |----+-----|
                       AVP  Section             |    |MUST |
       Attribute Name  Code Defined  Data Type  |MUST| NOT |
       -----------------------------------------|----+-----|
       Experimental-    298  7.7     Unsigned32 | M  |  V  |
          Result-Code                           |    |     |
       Failed-AVP       279  7.5     Grouped    | M  |  V  |
       Firmware-        267  5.3.4   Unsigned32 |    | V,M |
         Revision                               |    |     |
       Host-IP-Address  257  5.3.5   Address    | M  |  V  |
       Inband-Security                          | M  |  V  |
          -Id           299  6.10    Unsigned32 |    |     |
       Multi-Round-     272  8.19    Unsigned32 | M  |  V  |
         Time-Out                               |    |     |
       Origin-Host      264  6.3     DiamIdent  | M  |  V  |
       Origin-Realm     296  6.4     DiamIdent  | M  |  V  |
       Origin-State-Id  278  8.16    Unsigned32 | M  |  V  |
       Product-Name     269  5.3.7   UTF8String |    | V,M |
       Proxy-Host       280  6.7.3   DiamIdent  | M  |  V  |
       Proxy-Info       284  6.7.2   Grouped    | M  |  V  |
       Proxy-State       33  6.7.4   OctetString| M  |  V  |
       Redirect-Host    292  6.12    DiamURI    | M  |  V  |
       Redirect-Host-   261  6.13    Enumerated | M  |  V  |
          Usage                                 |    |     |
       Redirect-Max-    262  6.14    Unsigned32 | M  |  V  |
          Cache-Time                            |    |     |
       Result-Code      268  7.1     Unsigned32 | M  |  V  |
       Route-Record     282  6.7.1   DiamIdent  | M  |  V  |
       Session-Id       263  8.8     UTF8String | M  |  V  |
       Session-Timeout   27  8.13    Unsigned32 | M  |  V  |
       Session-Binding  270  8.17    Unsigned32 | M  |  V  |
       Session-Server-  271  8.18    Enumerated | M  |  V  |
         Failover                               |    |     |
       Supported-       265  5.3.6   Unsigned32 | M  |  V  |
         Vendor-Id                              |    |     |
       Termination-     295  8.15    Enumerated | M  |  V  |
          Cause                                 |    |     |
       User-Name          1  8.14    UTF8String | M  |  V  |
       Vendor-Id        266  5.3.3   Unsigned32 | M  |  V  |
       Vendor-Specific- 260  6.11    Grouped    | M  |  V  |
          Application-Id                        |    |     |
       -----------------------------------------|----+-----|

    Fajardo, et al.              Standards Track                   [Page 57]

    RFC 6733                 Diameter Base Protocol             October 2012

    5.  Diameter Peers

       This section describes how Diameter nodes establish connections and
       communicate with peers.

    5.1.  Peer Connections

       Connections between diameter peers are established using their valid
       DiameterIdentity.  A Diameter node initiating a connection to a peer
       MUST know the peer's DiameterIdentity.  Methods for discovering a
       Diameter peer can be found in Section 5.2.

       Although a Diameter node may have many possible peers with which it
       is able to communicate, it may not be economical to have an
       established connection to all of them.  At a minimum, a Diameter node
       SHOULD have an established connection with two peers per realm, known
       as the primary and secondary peers.  Of course, a node MAY have
       additional connections, if it is deemed necessary.  Typically, all
       messages for a realm are sent to the primary peer but, in the event
       that failover procedures are invoked, any pending requests are sent
       to the secondary peer.  However, implementations are free to load
       balance requests between a set of peers.

       Note that a given peer MAY act as a primary for a given realm while
       acting as a secondary for another realm.

       When a peer is deemed suspect, which could occur for various reasons,
       including not receiving a DWA within an allotted time frame, no new
       requests should be forwarded to the peer, but failover procedures are
       invoked.  When an active peer is moved to this mode, additional
       connections SHOULD be established to ensure that the necessary number
       of active connections exists.

       There are two ways that a peer is removed from the suspect peer list:

       1.  The peer is no longer reachable, causing the transport connection
           to be shut down.  The peer is moved to the closed state.

       2.  Three watchdog messages are exchanged with accepted round-trip
           times, and the connection to the peer is considered stabilized.

       In the event the peer being removed is either the primary or
       secondary, an alternate peer SHOULD replace the deleted peer and
       assume the role of either primary or secondary.

    Fajardo, et al.              Standards Track                   [Page 58]

    RFC 6733                 Diameter Base Protocol             October 2012

    5.2.  Diameter Peer Discovery

       Allowing for dynamic Diameter agent discovery makes possible simpler
       and more robust deployment of Diameter services.  In order to promote
       interoperable implementations of Diameter peer discovery, the
       following mechanisms (manual configuration and DNS) are described.
       These are based on existing IETF standards.  Both mechanisms MUST be
       supported by all Diameter implementations; either MAY be used.

       There are two cases where Diameter peer discovery may be performed.
       The first is when a Diameter client needs to discover a first-hop
       Diameter agent.  The second case is when a Diameter agent needs to
       discover another agent for further handling of a Diameter operation.
       In both cases, the following 'search order' is recommended:

       1.  The Diameter implementation consults its list of statically
           (manually) configured Diameter agent locations.  These will be
           used if they exist and respond.

       2.  The Diameter implementation performs a NAPTR query for a server
           in a particular realm.  The Diameter implementation has to know,
           in advance, in which realm to look for a Diameter agent.  This
           could be deduced, for example, from the 'realm' in an NAI on
           which a Diameter implementation needed to perform a Diameter
           operation.

           The NAPTR usage in Diameter follows the S-NAPTR DDDS application
           [RFC3958] in which the SERVICE field includes tags for the
           desired application and supported application protocol.  The
           application service tag for a Diameter application is 'aaa' and
           the supported application protocol tags are 'diameter.tcp',
           'diameter.sctp', 'diameter.dtls', or 'diameter.tls.tcp'
           [RFC6408].

           The client can follow the resolution process defined by the
           S-NAPTR DDDS [RFC3958] application to find a matching SRV, A, or
           AAAA record of a suitable peer.  The domain suffixes in the NAPTR
           replacement field SHOULD match the domain of the original query.
           An example can be found in Appendix B.

       3.  If no NAPTR records are found, the requester directly queries for
           one of the following SRV records: for Diameter over TCP, use
           "_diameter._tcp.realm"; for Diameter over TLS, use
           "_diameters._tcp.realm"; for Diameter over SCTP, use
           "_diameter._sctp.realm"; for Diameter over DTLS, use
           "_diameters._sctp.realm".  If SRV records are found, then the
           requester can perform address record query (A RR's and/or AAAA

    Fajardo, et al.              Standards Track                   [Page 59]

    RFC 6733                 Diameter Base Protocol             October 2012

           RR's) for the target hostname specified in the SRV records
           following the rules given in [RFC2782].  If no SRV records are
           found, the requester gives up.

       If the server is using a site certificate, the domain name in the
       NAPTR query and the domain name in the replacement field MUST both be
       valid based on the site certificate handed out by the server in the
       TLS/TCP and DTLS/SCTP or Internet Key Exchange Protocol (IKE)
       exchange.  Similarly, the domain name in the SRV query and the domain
       name in the target in the SRV record MUST both be valid based on the
       same site certificate.  Otherwise, an attacker could modify the DNS
       records to contain replacement values in a different domain, and the
       client could not validate whether this was the desired behavior or
       the result of an attack.

       Also, the Diameter peer MUST check to make sure that the discovered
       peers are authorized to act in its role.  Authentication via IKE or
       TLS/TCP and DTLS/SCTP, or validation of DNS RRs via DNSSEC is not
       sufficient to conclude this.  For example, a web server may have
       obtained a valid TLS/TCP and DTLS/SCTP certificate, and secured RRs
       may be included in the DNS, but this does not imply that it is
       authorized to act as a Diameter server.

       Authorization can be achieved, for example, by the configuration of a
       Diameter server Certification Authority (CA).  The server CA issues a
       certificate to the Diameter server, which includes an Object
       Identifier (OID) to indicate the subject is a Diameter server in the
       Extended Key Usage extension [RFC5280].  This certificate is then
       used during TLS/TCP, DTLS/SCTP, or IKE security negotiation.
       However, note that, at the time of writing, no Diameter server
       Certification Authorities exist.

       A dynamically discovered peer causes an entry in the peer table (see
       Section 2.6) to be created.  Note that entries created via DNS MUST
       expire (or be refreshed) within the DNS Time to Live (TTL).  If a
       peer is discovered outside of the local realm, a routing table entry
       (see Section 2.7) for the peer's realm is created.  The routing table
       entry's expiration MUST match the peer's expiration value.

    5.3.  Capabilities Exchange

       When two Diameter peers establish a transport connection, they MUST
       exchange the Capabilities Exchange messages, as specified in the peer
       state machine (see Section 5.6).  This message allows the discovery
       of a peer's identity and its capabilities (protocol version number,
       the identifiers of supported Diameter applications, security
       mechanisms, etc.).

    Fajardo, et al.              Standards Track                   [Page 60]

    RFC 6733                 Diameter Base Protocol             October 2012

       The receiver only issues commands to its peers that have advertised
       support for the Diameter application that defines the command.  A
       Diameter node MUST cache the supported Application Ids in order to
       ensure that unrecognized commands and/or AVPs are not unnecessarily
       sent to a peer.

       A receiver of a Capabilities-Exchange-Request (CER) message that does
       not have any applications in common with the sender MUST return a
       Capabilities-Exchange-Answer (CEA) with the Result-Code AVP set to
       DIAMETER_NO_COMMON_APPLICATION and SHOULD disconnect the transport
       layer connection.  Note that receiving a CER or CEA from a peer
       advertising itself as a relay (see Section 2.4) MUST be interpreted
       as having common applications with the peer.

       The receiver of the Capabilities-Exchange-Request (CER) MUST
       determine common applications by computing the intersection of its
       own set of supported Application Ids against all of the
       Application-Id AVPs (Auth-Application-Id, Acct-Application-Id, and
       Vendor-Specific-Application-Id) present in the CER.  The value of the
       Vendor-Id AVP in the Vendor-Specific-Application-Id MUST NOT be used
       during computation.  The sender of the Capabilities-Exchange-Answer
       (CEA) SHOULD include all of its supported applications as a hint to
       the receiver regarding all of its application capabilities.

       Diameter implementations SHOULD first attempt to establish a TLS/TCP
       and DTLS/SCTP connection prior to the CER/CEA exchange.  This
       protects the capabilities information of both peers.  To support
       older Diameter implementations that do not fully conform to this
       document, the transport security MAY still be negotiated via an
       Inband-Security AVP.  In this case, the receiver of a Capabilities-
       Exchange-Request (CER) message that does not have any security
       mechanisms in common with the sender MUST return a Capabilities-
       Exchange-Answer (CEA) with the Result-Code AVP set to
       DIAMETER_NO_COMMON_SECURITY and SHOULD disconnect the transport layer
       connection.

       CERs received from unknown peers MAY be silently discarded, or a CEA
       MAY be issued with the Result-Code AVP set to DIAMETER_UNKNOWN_PEER.
       In both cases, the transport connection is closed.  If the local
       policy permits receiving CERs from unknown hosts, a successful CEA
       MAY be returned.  If a CER from an unknown peer is answered with a
       successful CEA, the lifetime of the peer entry is equal to the
       lifetime of the transport connection.  In case of a transport
       failure, all the pending transactions destined to the unknown peer
       can be discarded.

       The CER and CEA messages MUST NOT be proxied, redirected, or relayed.

    Fajardo, et al.              Standards Track                   [Page 61]

    RFC 6733                 Diameter Base Protocol             October 2012

       Since the CER/CEA messages cannot be proxied, it is still possible
       that an upstream agent will receive a message for which it has no
       available peers to handle the application that corresponds to the
       Command Code.  In such instances, the 'E' bit is set in the answer
       message (Section 7) with the Result-Code AVP set to
       DIAMETER_UNABLE_TO_DELIVER to inform the downstream agent to take
       action (e.g., re-routing request to an alternate peer).

       With the exception of the Capabilities-Exchange-Request message, a
       message of type Request that includes the Auth-Application-Id or
       Acct-Application-Id AVPs, or a message with an application-specific
       Command Code MAY only be forwarded to a host that has explicitly
       advertised support for the application (or has advertised the Relay
       Application Id).

    5.3.1.  Capabilities-Exchange-Request

       The Capabilities-Exchange-Request (CER), indicated by the Command
       Code set to 257 and the Command Flags' 'R' bit set, is sent to
       exchange local capabilities.  Upon detection of a transport failure,
       this message MUST NOT be sent to an alternate peer.

       When Diameter is run over SCTP [RFC4960] or DTLS/SCTP [RFC6083],
       which allow for connections to span multiple interfaces and multiple
       IP addresses, the Capabilities-Exchange-Request message MUST contain
       one Host-IP-Address AVP for each potential IP address that MAY be
       locally used when transmitting Diameter messages.

          Message Format

             <CER> ::= < Diameter Header: 257, REQ >
                       { Origin-Host }
                       { Origin-Realm }
                    1* { Host-IP-Address }
                       { Vendor-Id }
                       { Product-Name }
                       [ Origin-State-Id ]
                     * [ Supported-Vendor-Id ]
                     * [ Auth-Application-Id ]
                     * [ Inband-Security-Id ]
                     * [ Acct-Application-Id ]
                     * [ Vendor-Specific-Application-Id ]
                       [ Firmware-Revision ]
                     * [ AVP ]

    Fajardo, et al.              Standards Track                   [Page 62]

    RFC 6733                 Diameter Base Protocol             October 2012

    5.3.2.  Capabilities-Exchange-Answer

       The Capabilities-Exchange-Answer (CEA), indicated by the Command Code
       set to 257 and the Command Flags' 'R' bit cleared, is sent in
       response to a CER message.

       When Diameter is run over SCTP [RFC4960] or DTLS/SCTP [RFC6083],
       which allow connections to span multiple interfaces, hence, multiple
       IP addresses, the Capabilities-Exchange-Answer message MUST contain
       one Host-IP-Address AVP for each potential IP address that MAY be
       locally used when transmitting Diameter messages.

       Message Format

             <CEA> ::= < Diameter Header: 257 >
                       { Result-Code }
                       { Origin-Host }
                       { Origin-Realm }
                    1* { Host-IP-Address }
                       { Vendor-Id }
                       { Product-Name }
                       [ Origin-State-Id ]
                       [ Error-Message ]
                       [ Failed-AVP ]
                     * [ Supported-Vendor-Id ]
                     * [ Auth-Application-Id ]
                     * [ Inband-Security-Id ]
                     * [ Acct-Application-Id ]
                     * [ Vendor-Specific-Application-Id ]
                       [ Firmware-Revision ]
                     * [ AVP ]

    5.3.3.  Vendor-Id AVP

       The Vendor-Id AVP (AVP Code 266) is of type Unsigned32 and contains
       the IANA "SMI Network Management Private Enterprise Codes"
       [ENTERPRISE] value assigned to the Diameter Software vendor.  It is
       envisioned that the combination of the Vendor-Id, Product-Name
       (Section 5.3.7), and Firmware-Revision (Section 5.3.4) AVPs may
       provide useful debugging information.

       A Vendor-Id value of zero in the CER or CEA message is reserved and
       indicates that this field is ignored.

    Fajardo, et al.              Standards Track                   [Page 63]

    RFC 6733                 Diameter Base Protocol             October 2012

    5.3.4.  Firmware-Revision AVP

       The Firmware-Revision AVP (AVP Code 267) is of type Unsigned32 and is
       used to inform a Diameter peer of the firmware revision of the
       issuing device.

       For devices that do not have a firmware revision (general-purpose
       computers running Diameter software modules, for instance), the
       revision of the Diameter software module may be reported instead.

    5.3.5.  Host-IP-Address AVP

       The Host-IP-Address AVP (AVP Code 257) is of type Address and is used
       to inform a Diameter peer of the sender's IP address.  All source
       addresses that a Diameter node expects to use with SCTP [RFC4960] or
       DTLS/SCTP [RFC6083] MUST be advertised in the CER and CEA messages by
       including a Host-IP-Address AVP for each address.

    5.3.6.  Supported-Vendor-Id AVP

       The Supported-Vendor-Id AVP (AVP Code 265) is of type Unsigned32 and
       contains the IANA "SMI Network Management Private Enterprise Codes"
       [ENTERPRISE] value assigned to a vendor other than the device vendor
       but including the application vendor.  This is used in the CER and
       CEA messages in order to inform the peer that the sender supports (a
       subset of) the Vendor-Specific AVPs defined by the vendor identified
       in this AVP.  The value of this AVP MUST NOT be set to zero.
       Multiple instances of this AVP containing the same value SHOULD NOT
       be sent.

    5.3.7.  Product-Name AVP

       The Product-Name AVP (AVP Code 269) is of type UTF8String and
       contains the vendor-assigned name for the product.  The Product-Name
       AVP SHOULD remain constant across firmware revisions for the same
       product.

    5.4.  Disconnecting Peer Connections

       When a Diameter node disconnects one of its transport connections,
       its peer cannot know the reason for the disconnect and will most
       likely assume that a connectivity problem occurred or that the peer
       has rebooted.  In these cases, the peer may periodically attempt to
       reconnect, as stated in Section 2.1.  In the event that the
       disconnect was a result of either a shortage of internal resources or
       simply that the node in question has no intentions of forwarding any
       Diameter messages to the peer in the foreseeable future, a periodic

    Fajardo, et al.              Standards Track                   [Page 64]

    RFC 6733                 Diameter Base Protocol             October 2012

       connection request would not be welcomed.  The Disconnection-Reason
       AVP contains the reason the Diameter node issued the Disconnect-Peer-
       Request message.

       The Disconnect-Peer-Request message is used by a Diameter node to
       inform its peer of its intent to disconnect the transport layer and
       that the peer shouldn't reconnect unless it has a valid reason to do
       so (e.g., message to be forwarded).  Upon receipt of the message, the
       Disconnect-Peer-Answer message is returned, which SHOULD contain an
       error if messages have recently been forwarded, and are likely in
       flight, which would otherwise cause a race condition.

       The receiver of the Disconnect-Peer-Answer message initiates the
       transport disconnect.  The sender of the Disconnect-Peer-Answer
       message should be able to detect the transport closure and clean up
       the connection.

    5.4.1.  Disconnect-Peer-Request

       The Disconnect-Peer-Request (DPR), indicated by the Command Code set
       to 282 and the Command Flags' 'R' bit set, is sent to a peer to
       inform it of its intentions to shut down the transport connection.
       Upon detection of a transport failure, this message MUST NOT be sent
       to an alternate peer.

          Message Format

             <DPR>  ::= < Diameter Header: 282, REQ >
                        { Origin-Host }
                        { Origin-Realm }
                        { Disconnect-Cause }
                      * [ AVP ]

    5.4.2.   Disconnect-Peer-Answer

       The Disconnect-Peer-Answer (DPA), indicated by the Command Code set
       to 282 and the Command Flags' 'R' bit cleared, is sent as a response
       to the Disconnect-Peer-Request message.  Upon receipt of this
       message, the transport connection is shut down.

    Fajardo, et al.              Standards Track                   [Page 65]

    RFC 6733                 Diameter Base Protocol             October 2012

          Message Format

             <DPA>  ::= < Diameter Header: 282 >
                        { Result-Code }
                        { Origin-Host }
                        { Origin-Realm }
                        [ Error-Message ]
                        [ Failed-AVP ]
                      * [ AVP ]

    5.4.3.   Disconnect-Cause AVP

       The Disconnect-Cause AVP (AVP Code 273) is of type Enumerated.  A
       Diameter node MUST include this AVP in the Disconnect-Peer-Request
       message to inform the peer of the reason for its intention to shut
       down the transport connection.  The following values are supported:

          REBOOTING                         0
             A scheduled reboot is imminent.  A receiver of a DPR with
             above result code MAY attempt reconnection.

          BUSY                              1
             The peer's internal resources are constrained, and it has
             determined that the transport connection needs to be closed.
             A receiver of a DPR with above result code SHOULD NOT attempt
             reconnection.

          DO_NOT_WANT_TO_TALK_TO_YOU        2
             The peer has determined that it does not see a need for the
             transport connection to exist, since it does not expect any
             messages to be exchanged in the near future.  A receiver of a
             DPR with above result code SHOULD NOT attempt reconnection.

    5.5.  Transport Failure Detection

       Given the nature of the Diameter protocol, it is recommended that
       transport failures be detected as soon as possible.  Detecting such
       failures will minimize the occurrence of messages sent to unavailable
       agents, resulting in unnecessary delays, and will provide better
       failover performance.  The Device-Watchdog-Request and Device-
       Watchdog-Answer messages, defined in this section, are used to pro-
       actively detect transport failures.

    Fajardo, et al.              Standards Track                   [Page 66]

    RFC 6733                 Diameter Base Protocol             October 2012

    5.5.1.  Device-Watchdog-Request

       The Device-Watchdog-Request (DWR), indicated by the Command Code set
       to 280 and the Command Flags' 'R' bit set, is sent to a peer when no
       traffic has been exchanged between two peers (see Section 5.5.3).
       Upon detection of a transport failure, this message MUST NOT be sent
       to an alternate peer.

          Message Format

             <DWR>  ::= < Diameter Header: 280, REQ >
                        { Origin-Host }
                        { Origin-Realm }
                        [ Origin-State-Id ]
                      * [ AVP ]

    5.5.2.  Device-Watchdog-Answer

       The Device-Watchdog-Answer (DWA), indicated by the Command Code set
       to 280 and the Command Flags' 'R' bit cleared, is sent as a response
       to the Device-Watchdog-Request message.

          Message Format

             <DWA>  ::= < Diameter Header: 280 >
                        { Result-Code }
                        { Origin-Host }
                        { Origin-Realm }
                        [ Error-Message ]
                        [ Failed-AVP ]
                        [ Origin-State-Id ]
                      * [ AVP ]

    5.5.3.   Transport Failure Algorithm

       The transport failure algorithm is defined in [RFC3539].  All
       Diameter implementations MUST support the algorithm defined in that
       specification in order to be compliant to the Diameter base protocol.

    5.5.4.  Failover and Failback Procedures

       In the event that a transport failure is detected with a peer, it is
       necessary for all pending request messages to be forwarded to an
       alternate agent, if possible.  This is commonly referred to as
       "failover".

    Fajardo, et al.              Standards Track                   [Page 67]

    RFC 6733                 Diameter Base Protocol             October 2012

       In order for a Diameter node to perform failover procedures, it is
       necessary for the node to maintain a pending message queue for a
       given peer.  When an answer message is received, the corresponding
       request is removed from the queue.  The Hop-by-Hop Identifier field
       is used to match the answer with the queued request.

       When a transport failure is detected, if possible, all messages in
       the queue are sent to an alternate agent with the T flag set.  On
       booting a Diameter client or agent, the T flag is also set on any
       remaining records in non-volatile storage that are still waiting to
       be transmitted.  An example of a case where it is not possible to
       forward the message to an alternate server is when the message has a
       fixed destination, and the unavailable peer is the message's final
       destination (see Destination-Host AVP).  Such an error requires that
       the agent return an answer message with the 'E' bit set and the
       Result-Code AVP set to DIAMETER_UNABLE_TO_DELIVER.

       It is important to note that multiple identical requests or answers
       MAY be received as a result of a failover.  The End-to-End Identifier
       field in the Diameter header along with the Origin-Host AVP MUST be
       used to identify duplicate messages.

       As described in Section 2.1, a connection request should be
       periodically attempted with the failed peer in order to re-establish
       the transport connection.  Once a connection has been successfully
       established, messages can once again be forwarded to the peer.  This
       is commonly referred to as "failback".

    5.6.  Peer State Machine

       This section contains a finite state machine that MUST be observed by
       all Diameter implementations.  Each Diameter node MUST follow the
       state machine described below when communicating with each peer.
       Multiple actions are separated by commas, and may continue on
       succeeding lines, as space requires.  Similarly, state and next state
       may also span multiple lines, as space requires.

       This state machine is closely coupled with the state machine
       described in [RFC3539], which is used to open, close, failover,
       probe, and reopen transport connections.  In particular, note that
       [RFC3539] requires the use of watchdog messages to probe connections.
       For Diameter, DWR and DWA messages are to be used.

       The I- prefix is used to represent the initiator (connecting)
       connection, while the R- prefix is used to represent the responder
       (listening) connection.  The lack of a prefix indicates that the
       event or action is the same regardless of the connection on which the
       event occurred.

    Fajardo, et al.              Standards Track                   [Page 68]

    RFC 6733                 Diameter Base Protocol             October 2012

       The stable states that a state machine may be in are Closed, I-Open,
       and R-Open; all other states are intermediate.  Note that I-Open and
       R-Open are equivalent except for whether the initiator or responder
       transport connection is used for communication.

       A CER message is always sent on the initiating connection immediately
       after the connection request is successfully completed.  In the case
       of an election, one of the two connections will shut down.  The
       responder connection will survive if the Origin-Host of the local
       Diameter entity is higher than that of the peer; the initiator
       connection will survive if the peer's Origin-Host is higher.  All
       subsequent messages are sent on the surviving connection.  Note that
       the results of an election on one peer are guaranteed to be the
       inverse of the results on the other.

       For TLS/TCP and DTLS/SCTP usage, a TLS/TCP and DTLS/SCTP handshake
       SHOULD begin when both ends are in the closed state prior to any
       Diameter message exchanges.  The TLS/TCP and DTLS/SCTP connection
       SHOULD be established before sending any CER or CEA message to secure
       and protect the capabilities information of both peers.  The TLS/TCP
       and DTLS/SCTP connection SHOULD be disconnected when the state
       machine moves to the closed state.  When connecting to responders
       that do not conform to this document (i.e., older Diameter
       implementations that are not prepared to received TLS/TCP and DTLS/
       SCTP connections in the closed state), the initial TLS/TCP and DTLS/
       SCTP connection attempt will fail.  The initiator MAY then attempt to
       connect via TCP or SCTP and initiate the TLS/TCP and DTLS/SCTP
       handshake when both ends are in the open state.  If the handshake is
       successful, all further messages will be sent via TLS/TCP and DTLS/
       SCTP.  If the handshake fails, both ends move to the closed state.

       The state machine constrains only the behavior of a Diameter
       implementation as seen by Diameter peers through events on the wire.

       Any implementation that produces equivalent results is considered
       compliant.

    Fajardo, et al.              Standards Track                   [Page 69]

    RFC 6733                 Diameter Base Protocol             October 2012

          state            event              action         next state
          -----------------------------------------------------------------
          Closed           Start            I-Snd-Conn-Req   Wait-Conn-Ack
                           R-Conn-CER       R-Accept,        R-Open
                                            Process-CER,
                                            R-Snd-CEA

          Wait-Conn-Ack    I-Rcv-Conn-Ack   I-Snd-CER        Wait-I-CEA
                           I-Rcv-Conn-Nack  Cleanup          Closed
                           R-Conn-CER       R-Accept,        Wait-Conn-Ack/
                                            Process-CER      Elect
                           Timeout          Error            Closed

          Wait-I-CEA       I-Rcv-CEA        Process-CEA      I-Open
                           R-Conn-CER       R-Accept,        Wait-Returns
                                            Process-CER,
                                            Elect
                           I-Peer-Disc      I-Disc           Closed
                           I-Rcv-Non-CEA    Error            Closed
                           Timeout          Error            Closed

          Wait-Conn-Ack/   I-Rcv-Conn-Ack   I-Snd-CER,Elect  Wait-Returns
          Elect            I-Rcv-Conn-Nack  R-Snd-CEA        R-Open
                           R-Peer-Disc      R-Disc           Wait-Conn-Ack
                           R-Conn-CER       R-Reject         Wait-Conn-Ack/
                                                             Elect
                           Timeout          Error            Closed

          Wait-Returns     Win-Election     I-Disc,R-Snd-CEA R-Open
                           I-Peer-Disc      I-Disc,          R-Open
                                            R-Snd-CEA
                           I-Rcv-CEA        R-Disc           I-Open
                           R-Peer-Disc      R-Disc           Wait-I-CEA
                           R-Conn-CER       R-Reject         Wait-Returns
                           Timeout          Error            Closed

          R-Open           Send-Message     R-Snd-Message    R-Open
                           R-Rcv-Message    Process          R-Open
                           R-Rcv-DWR        Process-DWR,     R-Open
                                            R-Snd-DWA
                           R-Rcv-DWA        Process-DWA      R-Open
                           R-Conn-CER       R-Reject         R-Open
                           Stop             R-Snd-DPR        Closing
                           R-Rcv-DPR        R-Snd-DPA        Closing
                           R-Peer-Disc      R-Disc           Closed

    Fajardo, et al.              Standards Track                   [Page 70]

    RFC 6733                 Diameter Base Protocol             October 2012

          I-Open           Send-Message     I-Snd-Message    I-Open
                           I-Rcv-Message    Process          I-Open
                           I-Rcv-DWR        Process-DWR,     I-Open
                                            I-Snd-DWA
                           I-Rcv-DWA        Process-DWA      I-Open
                           R-Conn-CER       R-Reject         I-Open
                           Stop             I-Snd-DPR        Closing
                           I-Rcv-DPR        I-Snd-DPA        Closing
                           I-Peer-Disc      I-Disc           Closed

          Closing          I-Rcv-DPA        I-Disc           Closed
                           R-Rcv-DPA        R-Disc           Closed
                           Timeout          Error            Closed
                           I-Peer-Disc      I-Disc           Closed
                           R-Peer-Disc      R-Disc           Closed

    5.6.1.  Incoming Connections

       When a connection request is received from a Diameter peer, it is
       not, in the general case, possible to know the identity of that peer
       until a CER is received from it.  This is because host and port
       determine the identity of a Diameter peer; the source port of an
       incoming connection is arbitrary.  Upon receipt of a CER, the
       identity of the connecting peer can be uniquely determined from the
       Origin-Host.

       For this reason, a Diameter peer must employ logic separate from the
       state machine to receive connection requests, accept them, and await
       the CER.  Once the CER arrives on a new connection, the Origin-Host
       that identifies the peer is used to locate the state machine
       associated with that peer, and the new connection and CER are passed
       to the state machine as an R-Conn-CER event.

       The logic that handles incoming connections SHOULD close and discard
       the connection if any message other than a CER arrives or if an
       implementation-defined timeout occurs prior to receipt of CER.

       Because handling of incoming connections up to and including receipt
       of a CER requires logic, separate from that of any individual state
       machine associated with a particular peer, it is described separately
       in this section rather than in the state machine above.

    5.6.2.  Events

       Transitions and actions in the automaton are caused by events.  In
       this section, we will ignore the I- and R- prefixes, since the actual
       event would be identical, but it would occur on one of two possible
       connections.

    Fajardo, et al.              Standards Track                   [Page 71]

    RFC 6733                 Diameter Base Protocol             October 2012

       Start          The Diameter application has signaled that a
                      connection should be initiated with the peer.

       R-Conn-CER     An acknowledgement is received stating that the
                      transport connection has been established, and the
                      associated CER has arrived.

       Rcv-Conn-Ack   A positive acknowledgement is received confirming that
                      the transport connection is established.

       Rcv-Conn-Nack  A negative acknowledgement was received stating that
                      the transport connection was not established.

       Timeout        An application-defined timer has expired while waiting
                      for some event.

       Rcv-CER        A CER message from the peer was received.

       Rcv-CEA        A CEA message from the peer was received.

       Rcv-Non-CEA    A message, other than a CEA, from the peer was
                      received.

       Peer-Disc      A disconnection indication from the peer was received.

       Rcv-DPR        A DPR message from the peer was received.

       Rcv-DPA        A DPA message from the peer was received.

       Win-Election   An election was held, and the local node was the
                      winner.

       Send-Message   A message is to be sent.

       Rcv-Message    A message other than CER, CEA, DPR, DPA, DWR, or DWA
                      was received.

       Stop           The Diameter application has signaled that a
                      connection should be terminated (e.g., on system
                      shutdown).

    5.6.3.  Actions

       Actions in the automaton are caused by events and typically indicate
       the transmission of packets and/or an action to be taken on the
       connection.  In this section, we will ignore the I- and R- prefixes,
       since the actual action would be identical, but it would occur on one
       of two possible connections.

    Fajardo, et al.              Standards Track                   [Page 72]

    RFC 6733                 Diameter Base Protocol             October 2012

       Snd-Conn-Req   A transport connection is initiated with the peer.

       Accept         The incoming connection associated with the R-Conn-CER
                      is accepted as the responder connection.

       Reject         The incoming connection associated with the R-Conn-CER
                      is disconnected.

       Process-CER    The CER associated with the R-Conn-CER is processed.

       Snd-CER        A CER message is sent to the peer.

       Snd-CEA        A CEA message is sent to the peer.

       Cleanup        If necessary, the connection is shut down, and any
                      local resources are freed.

       Error          The transport layer connection is disconnected,
                      either politely or abortively, in response to
                      an error condition.  Local resources are freed.

       Process-CEA    A received CEA is processed.

       Snd-DPR        A DPR message is sent to the peer.

       Snd-DPA        A DPA message is sent to the peer.

       Disc           The transport layer connection is disconnected,
                      and local resources are freed.

       Elect          An election occurs (see Section 5.6.4 for more
                      information).

       Snd-Message    A message is sent.

       Snd-DWR        A DWR message is sent.

       Snd-DWA        A DWA message is sent.

       Process-DWR    The DWR message is serviced.

       Process-DWA    The DWA message is serviced.

       Process        A message is serviced.

    Fajardo, et al.              Standards Track                   [Page 73]

    RFC 6733                 Diameter Base Protocol             October 2012

    5.6.4.  The Election Process

       The election is performed on the responder.  The responder compares
       the Origin-Host received in the CER with its own Origin-Host as two
       streams of octets.  If the local Origin-Host lexicographically
       succeeds the received Origin-Host, a Win-Election event is issued
       locally.  Diameter identities are in ASCII form; therefore, the
       lexical comparison is consistent with DNS case insensitivity, where
       octets that fall in the ASCII range 'a' through 'z' MUST compare
       equally to their uppercase counterparts between 'A' and 'Z'.  See
       Appendix D for interactions between the Diameter protocol and
       Internationalized Domain Name (IDNs).

       The winner of the election MUST close the connection it initiated.
       Historically, maintaining the responder side of a connection was more
       efficient than maintaining the initiator side.  However, current
       practices makes this distinction irrelevant.

    6.  Diameter Message Processing

       This section describes how Diameter requests and answers are created
       and processed.

    6.1.  Diameter Request Routing Overview

       A request is sent towards its final destination using one of the
       following three combinations of the Destination-Realm and
       Destination-Host AVPs:

       o  A request that is not able to be proxied (such as a CER) MUST NOT
          contain either Destination-Realm or Destination-Host AVPs.

       o  A request that needs to be sent to a home server serving a
          specific realm, but not to a specific server (such as the first
          request of a series of round trips), MUST contain a Destination-
          Realm AVP but MUST NOT contain a Destination-Host AVP.  For
          Diameter clients, the value of the Destination-Realm AVP MAY be
          extracted from the User-Name AVP, or other methods.

       o  Otherwise, a request that needs to be sent to a specific home
          server among those serving a given realm MUST contain both the
          Destination-Realm and Destination-Host AVPs.

       The Destination-Host AVP is used as described above when the
       destination of the request is fixed, which includes:

       o  Authentication requests that span multiple round trips.

    Fajardo, et al.              Standards Track                   [Page 74]

    RFC 6733                 Diameter Base Protocol             October 2012

       o  A Diameter message that uses a security mechanism that makes use
          of a pre-established session key shared between the source and the
          final destination of the message.

       o  Server-initiated messages that MUST be received by a specific
          Diameter client (e.g., access device), such as the Abort-Session-
          Request message, which is used to request that a particular user's
          session be terminated.

       Note that an agent can only forward a request to a host described in
       the Destination-Host AVP if the host in question is included in its
       peer table (see Section 2.6).  Otherwise, the request is routed based
       on the Destination-Realm only (see Section 6.1.6).

       When a message is received, the message is processed in the following
       order:

       o  If the message is destined for the local host, the procedures
          listed in Section 6.1.4 are followed.

       o  If the message is intended for a Diameter peer with whom the local
          host is able to directly communicate, the procedures listed in
          Section 6.1.5 are followed.  This is known as "Request
          Forwarding".

       o  The procedure listed in Section 6.1.6 is followed, which is known
          as "Request Routing".

       o  If none of the above are successful, an answer is returned with
          the Result-Code set to DIAMETER_UNABLE_TO_DELIVER, with the 'E'
          bit set.

       For routing of Diameter messages to work within an administrative
       domain, all Diameter nodes within the realm MUST be peers.

       The overview contained in this section (6.1) is intended to provide
       general guidelines to Diameter developers.  Implementations are free
       to use different methods than the ones described here as long as they
       conform to the requirements specified in Sections 6.1.1 through
       6.1.9.  See Section 7 for more details on error handling.

    6.1.1.  Originating a Request

       When creating a request, in addition to any other procedures
       described in the application definition for that specific request,
       the following procedures MUST be followed:

    Fajardo, et al.              Standards Track                   [Page 75]

    RFC 6733                 Diameter Base Protocol             October 2012

       o  the Command Code is set to the appropriate value;

       o  the 'R' bit is set;

       o  the End-to-End Identifier is set to a locally unique value;

       o  the Origin-Host and Origin-Realm AVPs MUST be set to the
          appropriate values, used to identify the source of the message;
          and

       o  the Destination-Host and Destination-Realm AVPs MUST be set to the
          appropriate values, as described in Section 6.1.

    6.1.2.  Sending a Request

       When sending a request, originated either locally or as the result of
       a forwarding or routing operation, the following procedures SHOULD be
       followed:

       o  The Hop-by-Hop Identifier SHOULD be set to a locally unique value.

       o  The message SHOULD be saved in the list of pending requests.

       Other actions to perform on the message based on the particular role
       the agent is playing are described in the following sections.

    6.1.3.  Receiving Requests

       A relay or proxy agent MUST check for forwarding loops when receiving
       requests.  A loop is detected if the server finds its own identity in
       a Route-Record AVP.  When such an event occurs, the agent MUST answer
       with the Result-Code AVP set to DIAMETER_LOOP_DETECTED.

    6.1.4.  Processing Local Requests

       A request is known to be for local consumption when one of the
       following conditions occurs:

       o  The Destination-Host AVP contains the local host's identity;

       o  The Destination-Host AVP is not present, the Destination-Realm AVP
          contains a realm the server is configured to process locally, and
          the Diameter application is locally supported; or

       o  Both the Destination-Host and the Destination-Realm are not
          present.

    Fajardo, et al.              Standards Track                   [Page 76]

    RFC 6733                 Diameter Base Protocol             October 2012

       When a request is locally processed, the rules in Section 6.2 should
       be used to generate the corresponding answer.

    6.1.5.  Request Forwarding

       Request forwarding is done using the Diameter peer table.  The
       Diameter peer table contains all of the peers with which the local
       node is able to directly communicate.

       When a request is received, and the host encoded in the Destination-
       Host AVP is one that is present in the peer table, the message SHOULD
       be forwarded to the peer.

    6.1.6.  Request Routing

       Diameter request message routing is done via realms and Application
       Ids. A Diameter message that may be forwarded by Diameter agents
       (proxies, redirect agents, or relay agents) MUST include the target
       realm in the Destination-Realm AVP.  Request routing SHOULD rely on
       the Destination-Realm AVP and the Application Id present in the
       request message header to aid in the routing decision.  The realm MAY
       be retrieved from the User-Name AVP, which is in the form of a
       Network Access Identifier (NAI).  The realm portion of the NAI is
       inserted in the Destination-Realm AVP.

       Diameter agents MAY have a list of locally supported realms and
       applications, and they MAY have a list of externally supported realms
       and applications.  When a request is received that includes a realm
       and/or application that is not locally supported, the message is
       routed to the peer configured in the routing table (see Section 2.7).

       Realm names and Application Ids are the minimum supported routing
       criteria, additional information may be needed to support redirect
       semantics.

    6.1.7.  Predictive Loop Avoidance

       Before forwarding or routing a request, Diameter agents, in addition
       to performing the processing described in Section 6.1.3, SHOULD check
       for the presence of a candidate route's peer identity in any of the
       Route-Record AVPs.  In the event of the agent detecting the presence
       of a candidate route's peer identity in a Route-Record AVP, the agent
       MUST ignore such a route for the Diameter request message and attempt
       alternate routes if any exist.  In case all the candidate routes are
       eliminated by the above criteria, the agent SHOULD return a
       DIAMETER_UNABLE_TO_DELIVER message.

    Fajardo, et al.              Standards Track                   [Page 77]

    RFC 6733                 Diameter Base Protocol             October 2012

    6.1.8.  Redirecting Requests

       When a redirect agent receives a request whose routing entry is set
       to REDIRECT, it MUST reply with an answer message with the 'E' bit
       set, while maintaining the Hop-by-Hop Identifier in the header, and
       include the Result-Code AVP to DIAMETER_REDIRECT_INDICATION.  Each of
       the servers associated with the routing entry are added in a separate
       Redirect-Host AVP.

                         +------------------+
                         |     Diameter     |
                         |  Redirect Agent  |
                         +------------------+
                          ^    |    2. command + 'E' bit
           1. Request     |    |    Result-Code =
          joe@example.com |    |    DIAMETER_REDIRECT_INDICATION +
                          |    |    Redirect-Host AVP(s)
                          |    v
                      +-------------+  3. Request  +-------------+
                      | example.com |------------->| example.net |
                      |    Relay    |              |   Diameter  |
                      |    Agent    |<-------------|    Server   |
                      +-------------+  4. Answer   +-------------+

                         Figure 5: Diameter Redirect Agent

       The receiver of an answer message with the 'E' bit set and the
       Result-Code AVP set to DIAMETER_REDIRECT_INDICATION uses the Hop-by-
       Hop Identifier in the Diameter header to identify the request in the
       pending message queue (see Section 5.5.4) that is to be redirected.
       If no transport connection exists with the new peer, one is created,
       and the request is sent directly to it.

       Multiple Redirect-Host AVPs are allowed.  The receiver of the answer
       message with the 'E' bit set selects exactly one of these hosts as
       the destination of the redirected message.

       When the Redirect-Host-Usage AVP included in the answer message has a
       non-zero value, a route entry for the redirect indications is created
       and cached by the receiver.  The redirect usage for such a route
       entry is set by the value of Redirect-Host-Usage AVP and the lifetime
       of the cached route entry is set by Redirect-Max-Cache-Time AVP
       value.

       It is possible that multiple redirect indications can create multiple
       cached route entries differing only in their redirect usage and the
       peer to forward messages to.  As an example, two(2) route entries
       that are created by two(2) redirect indications results in two(2)

    Fajardo, et al.              Standards Track                   [Page 78]

    RFC 6733                 Diameter Base Protocol             October 2012

       cached routes for the same realm and Application Id.  However, one
       has a redirect usage of ALL_SESSION, where matching requests will be
       forwarded to one peer; the other has a redirect usage of ALL_REALM,
       where request are forwarded to another peer.  Therefore, an incoming
       request that matches the realm and Application Id of both routes will
       need additional resolution.  In such a case, a routing precedence
       rule MUST be used against the redirect usage value to resolve the
       contention.  The precedence rule can be found in Section 6.13.

    6.1.9.  Relaying and Proxying Requests

       A relay or proxy agent MUST append a Route-Record AVP to all requests
       forwarded.  The AVP contains the identity of the peer from which the
       request was received.

       The Hop-by-Hop Identifier in the request is saved and replaced with a
       locally unique value.  The source of the request is also saved, which
       includes the IP address, port, and protocol.

       A relay or proxy agent MAY include the Proxy-Info AVP in requests if
       it requires access to any local state information when the
       corresponding response is received.  The Proxy-Info AVP has security
       implications as state information is distributed to other entities.
       As such, it is RECOMMENDED that the content of the Proxy-Info AVP be
       protected with cryptographic mechanisms, for example, by using a
       keyed message digest such as HMAC-SHA1 [RFC2104].  Such a mechanism,
       however, requires the management of keys, although only locally at
       the Diameter server.  Still, a full description of the management of
       the keys used to protect the Proxy-Info AVP is beyond the scope of
       this document.  Below is a list of common recommendations:

       o  The keys should be generated securely following the randomness
          recommendations in [RFC4086].

       o  The keys and cryptographic protection algorithms should be at
          least 128 bits in strength.

       o  The keys should not be used for any other purpose than generating
          and verifying instances of the Proxy-Info AVP.

       o  The keys should be changed regularly.

       o  The keys should be changed if the AVP format or cryptographic
          protection algorithms change.

       The message is then forwarded to the next hop, as identified in the
       routing table.

    Fajardo, et al.              Standards Track                   [Page 79]

    RFC 6733                 Diameter Base Protocol             October 2012

       Figure 6 provides an example of message routing using the procedures
       listed in these sections.

           (Origin-Host=nas.example.net)    (Origin-Host=nas.example.net)
           (Origin-Realm=example.net)       (Origin-Realm=example.net)
           (Destination-Realm=example.com)  (Destination-Realm=example.com)
                                            (Route-Record=nas.example.net)
          +------+      ------>      +------+      ------>      +------+
          |      |     (Request)     |      |      (Request)    |      |
          | NAS  +-------------------+ DRL  +-------------------+ HMS  |
          |      |                   |      |                   |      |
          +------+     <------       +------+     <------       +------+
         example.net    (Answer)   example.net     (Answer)   example.com
              (Origin-Host=hms.example.com)   (Origin-Host=hms.example.com)
              (Origin-Realm=example.com)      (Origin-Realm=example.com)

                      Figure 6: Routing of Diameter messages

       Relay and proxy agents are not required to perform full inspection of
       incoming messages.  At a minimum, validation of the message header
       and relevant routing AVPs has to be done when relaying messages.
       Proxy agents may optionally perform more in-depth message validation
       for applications in which it is interested.

    6.2.  Diameter Answer Processing

       When a request is locally processed, the following procedures MUST be
       applied to create the associated answer, in addition to any
       additional procedures that MAY be discussed in the Diameter
       application defining the command:

       o  The same Hop-by-Hop Identifier in the request is used in the
          answer.

       o  The local host's identity is encoded in the Origin-Host AVP.

       o  The Destination-Host and Destination-Realm AVPs MUST NOT be
          present in the answer message.

       o  The Result-Code AVP is added with its value indicating success or
          failure.

       o  If the Session-Id is present in the request, it MUST be included
          in the answer.

       o  Any Proxy-Info AVPs in the request MUST be added to the answer
          message, in the same order they were present in the request.

    Fajardo, et al.              Standards Track                   [Page 80]

    RFC 6733                 Diameter Base Protocol             October 2012

       o  The 'P' bit is set to the same value as the one in the request.

       o  The same End-to-End identifier in the request is used in the
          answer.

       Note that the error messages (see Section 7) are also subjected to
       the above processing rules.

    6.2.1.   Processing Received Answers

       A Diameter client or proxy MUST match the Hop-by-Hop Identifier in an
       answer received against the list of pending requests.  The
       corresponding message should be removed from the list of pending
       requests.  It SHOULD ignore answers received that do not match a
       known Hop-by-Hop Identifier.

    6.2.2.  Relaying and Proxying Answers

       If the answer is for a request that was proxied or relayed, the agent
       MUST restore the original value of the Diameter header's Hop-by-Hop
       Identifier field.

       If the last Proxy-Info AVP in the message is targeted to the local
       Diameter server, the AVP MUST be removed before the answer is
       forwarded.

       If a relay or proxy agent receives an answer with a Result-Code AVP
       indicating a failure, it MUST NOT modify the contents of the AVP.
       Any additional local errors detected SHOULD be logged but not
       reflected in the Result-Code AVP.  If the agent receives an answer
       message with a Result-Code AVP indicating success, and it wishes to
       modify the AVP to indicate an error, it MUST modify the Result-Code
       AVP to contain the appropriate error in the message destined towards
       the access device as well as include the Error-Reporting-Host AVP; it
       MUST also issue an STR on behalf of the access device towards the
       Diameter server.

       The agent MUST then send the answer to the host that it received the
       original request from.

    6.3.  Origin-Host AVP

       The Origin-Host AVP (AVP Code 264) is of type DiameterIdentity, and
       it MUST be present in all Diameter messages.  This AVP identifies the
       endpoint that originated the Diameter message.  Relay agents MUST NOT
       modify this AVP.

    Fajardo, et al.              Standards Track                   [Page 81]

    RFC 6733                 Diameter Base Protocol             October 2012

       The value of the Origin-Host AVP is guaranteed to be unique within a
       single host.

       Note that the Origin-Host AVP may resolve to more than one address as
       the Diameter peer may support more than one address.

       This AVP SHOULD be placed as close to the Diameter header as
       possible.

    6.4.  Origin-Realm AVP

       The Origin-Realm AVP (AVP Code 296) is of type DiameterIdentity.
       This AVP contains the Realm of the originator of any Diameter message
       and MUST be present in all messages.

       This AVP SHOULD be placed as close to the Diameter header as
       possible.

    6.5.  Destination-Host AVP

       The Destination-Host AVP (AVP Code 293) is of type DiameterIdentity.
       This AVP MUST be present in all unsolicited agent initiated messages,
       MAY be present in request messages, and MUST NOT be present in answer
       messages.

       The absence of the Destination-Host AVP will cause a message to be
       sent to any Diameter server supporting the application within the
       realm specified in Destination-Realm AVP.

       This AVP SHOULD be placed as close to the Diameter header as
       possible.

    6.6.  Destination-Realm AVP

       The Destination-Realm AVP (AVP Code 283) is of type DiameterIdentity
       and contains the realm to which the message is to be routed.  The
       Destination-Realm AVP MUST NOT be present in answer messages.
       Diameter clients insert the realm portion of the User-Name AVP.
       Diameter servers initiating a request message use the value of the
       Origin-Realm AVP from a previous message received from the intended
       target host (unless it is known a priori).  When present, the
       Destination-Realm AVP is used to perform message routing decisions.

       The CCF for a request message that includes the Destination-Realm AVP
       SHOULD list the Destination-Realm AVP as a required AVP (an AVP
       indicated as {AVP}); otherwise, the message is inherently a non-
       routable message.

    Fajardo, et al.              Standards Track                   [Page 82]

    RFC 6733                 Diameter Base Protocol             October 2012

       This AVP SHOULD be placed as close to the Diameter header as
       possible.

    6.7.  Routing AVPs

       The AVPs defined in this section are Diameter AVPs used for routing
       purposes.  These AVPs change as Diameter messages are processed by
       agents.

    6.7.1.  Route-Record AVP

       The Route-Record AVP (AVP Code 282) is of type DiameterIdentity.  The
       identity added in this AVP MUST be the same as the one received in
       the Origin-Host of the Capabilities Exchange message.

    6.7.2.  Proxy-Info AVP

       The Proxy-Info AVP (AVP Code 284) is of type Grouped.  This AVP
       contains the identity and local state information of the Diameter
       node that creates and adds it to a message.  The Grouped Data field
       has the following CCF grammar:

             Proxy-Info ::= < AVP Header: 284 >
                            { Proxy-Host }
                            { Proxy-State }
                          * [ AVP ]

    6.7.3.  Proxy-Host AVP

       The Proxy-Host AVP (AVP Code 280) is of type DiameterIdentity.  This
       AVP contains the identity of the host that added the Proxy-Info AVP.

    6.7.4.  Proxy-State AVP

       The Proxy-State AVP (AVP Code 33) is of type OctetString.  It
       contains state information that would otherwise be stored at the
       Diameter entity that created it.  As such, this AVP MUST be treated
       as opaque data by other Diameter entities.

    6.8.  Auth-Application-Id AVP

       The Auth-Application-Id AVP (AVP Code 258) is of type Unsigned32 and
       is used in order to advertise support of the Authentication and
       Authorization portion of an application (see Section 2.4).  If
       present in a message other than CER and CEA, the value of the Auth-
       Application-Id AVP MUST match the Application Id present in the
       Diameter message header.

    Fajardo, et al.              Standards Track                   [Page 83]

    RFC 6733                 Diameter Base Protocol             October 2012

    6.9.  Acct-Application-Id AVP

       The Acct-Application-Id AVP (AVP Code 259) is of type Unsigned32 and
       is used in order to advertise support of the accounting portion of an
       application (see Section 2.4).  If present in a message other than
       CER and CEA, the value of the Acct-Application-Id AVP MUST match the
       Application Id present in the Diameter message header.

    6.10.  Inband-Security-Id AVP

       The Inband-Security-Id AVP (AVP Code 299) is of type Unsigned32 and
       is used in order to advertise support of the security portion of the
       application.  The use of this AVP in CER and CEA messages is NOT
       RECOMMENDED.  Instead, discovery of a Diameter entity's security
       capabilities can be done either through static configuration or via
       Diameter Peer Discovery as described in Section 5.2.

       The following values are supported:

       NO_INBAND_SECURITY 0

          This peer does not support TLS/TCP and DTLS/SCTP.  This is the
          default value, if the AVP is omitted.

       TLS 1

          This node supports TLS/TCP [RFC5246] and DTLS/SCTP [RFC6083]
          security.

    6.11.  Vendor-Specific-Application-Id AVP

       The Vendor-Specific-Application-Id AVP (AVP Code 260) is of type
       Grouped and is used to advertise support of a vendor-specific
       Diameter application.  Exactly one instance of either Auth-
       Application-Id or Acct-Application-Id AVP MUST be present.  The
       Application Id carried by either Auth-Application-Id or Acct-
       Application-Id AVP MUST comply with vendor-specific Application Id
       assignment described in Section 11.3.  It MUST also match the
       Application Id present in the Diameter header except when used in a
       CER or CEA message.

       The Vendor-Id AVP is an informational AVP pertaining to the vendor
       who may have authorship of the vendor-specific Diameter application.
       It MUST NOT be used as a means of defining a completely separate
       vendor-specific Application Id space.

    Fajardo, et al.              Standards Track                   [Page 84]

    RFC 6733                 Diameter Base Protocol             October 2012

       The Vendor-Specific-Application-Id AVP SHOULD be placed as close to
       the Diameter header as possible.

          AVP Format

          <Vendor-Specific-Application-Id> ::= < AVP Header: 260 >
                                               { Vendor-Id }
                                               [ Auth-Application-Id ]
                                               [ Acct-Application-Id ]

       A Vendor-Specific-Application-Id AVP MUST contain exactly one of
       either Auth-Application-Id or Acct-Application-Id.  If a Vendor-
       Specific-Application-Id is received without one of these two AVPs,
       then the recipient SHOULD issue an answer with a Result-Code set to
       DIAMETER_MISSING_AVP.  The answer SHOULD also include a Failed-AVP,
       which MUST contain an example of an Auth-Application-Id AVP and an
       Acct-Application-Id AVP.

       If a Vendor-Specific-Application-Id is received that contains both
       Auth-Application-Id and Acct-Application-Id, then the recipient MUST
       issue an answer with Result-Code set to
       DIAMETER_AVP_OCCURS_TOO_MANY_TIMES.  The answer MUST also include a
       Failed-AVP, which MUST contain the received Auth-Application-Id AVP
       and Acct-Application-Id AVP.

    6.12.  Redirect-Host AVP

       The Redirect-Host AVP (AVP Code 292) is of type DiameterURI.  One or
       more instances of this AVP MUST be present if the answer message's
       'E' bit is set and the Result-Code AVP is set to
       DIAMETER_REDIRECT_INDICATION.

       Upon receiving the above, the receiving Diameter node SHOULD forward
       the request directly to one of the hosts identified in these AVPs.
       The server contained in the selected Redirect-Host AVP SHOULD be used
       for all messages matching the criteria set by the Redirect-Host-Usage
       AVP.

    6.13.  Redirect-Host-Usage AVP

       The Redirect-Host-Usage AVP (AVP Code 261) is of type Enumerated.
       This AVP MAY be present in answer messages whose 'E' bit is set and
       the Result-Code AVP is set to DIAMETER_REDIRECT_INDICATION.

       When present, this AVP provides hints about how the routing entry
       resulting from the Redirect-Host is to be used.  The following values
       are supported:

    Fajardo, et al.              Standards Track                   [Page 85]

    RFC 6733                 Diameter Base Protocol             October 2012

       DONT_CACHE 0

          The host specified in the Redirect-Host AVP SHOULD NOT be cached.
          This is the default value.

       ALL_SESSION 1

          All messages within the same session, as defined by the same value
          of the Session-ID AVP SHOULD be sent to the host specified in the
          Redirect-Host AVP.

       ALL_REALM 2

          All messages destined for the realm requested SHOULD be sent to
          the host specified in the Redirect-Host AVP.

       REALM_AND_APPLICATION 3

          All messages for the application requested to the realm specified
          SHOULD be sent to the host specified in the Redirect-Host AVP.

       ALL_APPLICATION 4

          All messages for the application requested SHOULD be sent to the
          host specified in the Redirect-Host AVP.

       ALL_HOST 5

          All messages that would be sent to the host that generated the
          Redirect-Host SHOULD be sent to the host specified in the
          Redirect-Host AVP.

       ALL_USER 6

          All messages for the user requested SHOULD be sent to the host
          specified in the Redirect-Host AVP.

       When multiple cached routes are created by redirect indications and
       they differ only in redirect usage and peers to forward requests to
       (see Section 6.1.8), a precedence rule MUST be applied to the
       redirect usage values of the cached routes during normal routing to
       resolve contentions that may occur.  The precedence rule is the order
       that dictate which redirect usage should be considered before any
       other as they appear.  The order is as follows:

    Fajardo, et al.              Standards Track                   [Page 86]

    RFC 6733                 Diameter Base Protocol             October 2012

       1.  ALL_SESSION

       2.  ALL_USER

       3.  REALM_AND_APPLICATION

       4.  ALL_REALM

       5.  ALL_APPLICATION

       6.  ALL_HOST

    6.14.  Redirect-Max-Cache-Time AVP

       The Redirect-Max-Cache-Time AVP (AVP Code 262) is of type Unsigned32.
       This AVP MUST be present in answer messages whose 'E' bit is set,
       whose Result-Code AVP is set to DIAMETER_REDIRECT_INDICATION, and
       whose Redirect-Host-Usage AVP set to a non-zero value.

       This AVP contains the maximum number of seconds the peer and route
       table entries, created as a result of the Redirect-Host, SHOULD be
       cached.  Note that once a host is no longer reachable, any associated
       cache, peer, and routing table entries MUST be deleted.

    7.  Error Handling

       There are two different types of errors in Diameter; protocol errors
       and application errors.  A protocol error is one that occurs at the
       base protocol level and MAY require per-hop attention (e.g., a
       message routing error).  Application errors, on the other hand,
       generally occur due to a problem with a function specified in a
       Diameter application (e.g., user authentication, missing AVP).

       Result-Code AVP values that are used to report protocol errors MUST
       only be present in answer messages whose 'E' bit is set.  When a
       request message is received that causes a protocol error, an answer
       message is returned with the 'E' bit set, and the Result-Code AVP is
       set to the appropriate protocol error value.  As the answer is sent
       back towards the originator of the request, each proxy or relay agent
       MAY take action on the message.

    Fajardo, et al.              Standards Track                   [Page 87]

    RFC 6733                 Diameter Base Protocol             October 2012

                              1. Request        +---------+ Link Broken
                    +-------------------------->|Diameter |----///----+
                    |     +---------------------|         |           v
             +------+--+  | 2. answer + 'E' set | Relay 2 |     +--------+
             |Diameter |<-+ (Unable to Forward) +---------+     |Diameter|
             |         |                                        |  Home  |
             | Relay 1 |--+                     +---------+     | Server |
             +---------+  |   3. Request        |Diameter |     +--------+
                          +-------------------->|         |           ^
                                                | Relay 3 |-----------+
                                                +---------+

            Figure 7: Example of Protocol Error Causing Answer Message

       Figure 7 provides an example of a message forwarded upstream by a
       Diameter relay.  When the message is received by Relay 2, and it
       detects that it cannot forward the request to the home server, an
       answer message is returned with the 'E' bit set and the Result-Code
       AVP set to DIAMETER_UNABLE_TO_DELIVER.  Given that this error falls
       within the protocol error category, Relay 1 would take special
       action, and given the error, attempt to route the message through its
       alternate Relay 3.

                +---------+ 1. Request  +---------+ 2. Request  +---------+
                | Access  |------------>|Diameter |------------>|Diameter |
                |         |             |         |             |  Home   |
                | Device  |<------------|  Relay  |<------------| Server  |
                +---------+  4. Answer  +---------+  3. Answer  +---------+
                           (Missing AVP)           (Missing AVP)

               Figure 8: Example of Application Error Answer Message

       Figure 8 provides an example of a Diameter message that caused an
       application error.  When application errors occur, the Diameter
       entity reporting the error clears the 'R' bit in the Command Flags
       and adds the Result-Code AVP with the proper value.  Application
       errors do not require any proxy or relay agent involvement;
       therefore, the message would be forwarded back to the originator of
       the request.

       In the case where the answer message itself contains errors, any
       related session SHOULD be terminated by sending an STR or ASR
       message.  The Termination-Cause AVP in the STR MAY be filled with the
       appropriate value to indicate the cause of the error.  An application
       MAY also send an application-specific request instead of an STR or
       ASR message to signal the error in the case where no state is
       maintained or to allow for some form of error recovery with the
       corresponding Diameter entity.

    Fajardo, et al.              Standards Track                   [Page 88]

    RFC 6733                 Diameter Base Protocol             October 2012

       There are certain Result-Code AVP application errors that require
       additional AVPs to be present in the answer.  In these cases, the
       Diameter node that sets the Result-Code AVP to indicate the error
       MUST add the AVPs.  Examples are as follows:

       o  A request with an unrecognized AVP is received with the 'M' bit
          (Mandatory bit) set causes an answer to be sent with the Result-
          Code AVP set to DIAMETER_AVP_UNSUPPORTED and the Failed-AVP AVP
          containing the offending AVP.

       o  A request with an AVP that is received with an unrecognized value
          causes an answer to be returned with the Result-Code AVP set to
          DIAMETER_INVALID_AVP_VALUE, with the Failed-AVP AVP containing the
          AVP causing the error.

       o  A received command that is missing AVPs that are defined as
          required in the commands CCF; examples are AVPs indicated as
          {AVP}.  The receiver issues an answer with the Result-Code set to
          DIAMETER_MISSING_AVP and creates an AVP with the AVP Code and
          other fields set as expected in the missing AVP.  The created AVP
          is then added to the Failed-AVP AVP.

       The Result-Code AVP describes the error that the Diameter node
       encountered in its processing.  In case there are multiple errors,
       the Diameter node MUST report only the first error it encountered
       (detected possibly in some implementation-dependent order).  The
       specific errors that can be described by this AVP are described in
       the following section.

    7.1.  Result-Code AVP

       The Result-Code AVP (AVP Code 268) is of type Unsigned32 and
       indicates whether a particular request was completed successfully or
       an error occurred.  All Diameter answer messages in IETF-defined
       Diameter application specifications MUST include one Result-Code AVP.
       A non-successful Result-Code AVP (one containing a non-2xxx value
       other than DIAMETER_REDIRECT_INDICATION) MUST include the Error-
       Reporting-Host AVP if the host setting the Result-Code AVP is
       different from the identity encoded in the Origin-Host AVP.

       The Result-Code data field contains an IANA-managed 32-bit address
       space representing errors (see Section 11.3.2).  Diameter provides
       the following classes of errors, all identified by the thousands
       digit in the decimal notation:

    Fajardo, et al.              Standards Track                   [Page 89]

    RFC 6733                 Diameter Base Protocol             October 2012

       o  1xxx (Informational)

       o  2xxx (Success)

       o  3xxx (Protocol Errors)

       o  4xxx (Transient Failures)

       o  5xxx (Permanent Failure)

       An unrecognized class (one whose first digit is not defined in this
       section) MUST be handled as a permanent failure.

    7.1.1.  Informational

       Errors that fall within this category are used to inform the
       requester that a request could not be satisfied, and additional
       action is required on its part before access is granted.

       DIAMETER_MULTI_ROUND_AUTH 1001

          This informational error is returned by a Diameter server to
          inform the access device that the authentication mechanism being
          used requires multiple round trips, and a subsequent request needs
          to be issued in order for access to be granted.

    7.1.2.  Success

       Errors that fall within the Success category are used to inform a
       peer that a request has been successfully completed.

       DIAMETER_SUCCESS 2001

          The request was successfully completed.

       DIAMETER_LIMITED_SUCCESS 2002

          When returned, the request was successfully completed, but
          additional processing is required by the application in order to
          provide service to the user.

    7.1.3.  Protocol Errors

       Errors that fall within the Protocol Error category SHOULD be treated
       on a per-hop basis, and Diameter proxies MAY attempt to correct the
       error, if it is possible.  Note that these errors MUST only be used
       in answer messages whose 'E' bit is set.

    Fajardo, et al.              Standards Track                   [Page 90]

    RFC 6733                 Diameter Base Protocol             October 2012

       DIAMETER_COMMAND_UNSUPPORTED 3001

          This error code is used when a Diameter entity receives a message
          with a Command Code that it does not support.

       DIAMETER_UNABLE_TO_DELIVER 3002

          This error is given when Diameter cannot deliver the message to
          the destination, either because no host within the realm
          supporting the required application was available to process the
          request or because the Destination-Host AVP was given without the
          associated Destination-Realm AVP.

       DIAMETER_REALM_NOT_SERVED 3003

          The intended realm of the request is not recognized.

       DIAMETER_TOO_BUSY 3004

          When returned, a Diameter node SHOULD attempt to send the message
          to an alternate peer.  This error MUST only be used when a
          specific server is requested, and it cannot provide the requested
          service.

       DIAMETER_LOOP_DETECTED 3005

          An agent detected a loop while trying to get the message to the
          intended recipient.  The message MAY be sent to an alternate peer,
          if one is available, but the peer reporting the error has
          identified a configuration problem.

       DIAMETER_REDIRECT_INDICATION 3006

          A redirect agent has determined that the request could not be
          satisfied locally, and the initiator of the request SHOULD direct
          the request directly to the server, whose contact information has
          been added to the response.  When set, the Redirect-Host AVP MUST
          be present.

       DIAMETER_APPLICATION_UNSUPPORTED 3007

          A request was sent for an application that is not supported.

       DIAMETER_INVALID_HDR_BITS 3008

          A request was received whose bits in the Diameter header were set
          either to an invalid combination or to a value that is
          inconsistent with the Command Code's definition.

    Fajardo, et al.              Standards Track                   [Page 91]

    RFC 6733                 Diameter Base Protocol             October 2012

       DIAMETER_INVALID_AVP_BITS 3009

          A request was received that included an AVP whose flag bits are
          set to an unrecognized value or that is inconsistent with the
          AVP's definition.

       DIAMETER_UNKNOWN_PEER 3010

          A CER was received from an unknown peer.

    7.1.4.  Transient Failures

       Errors that fall within the transient failures category are used to
       inform a peer that the request could not be satisfied at the time it
       was received but MAY be able to satisfy the request in the future.
       Note that these errors MUST be used in answer messages whose 'E' bit
       is not set.

       DIAMETER_AUTHENTICATION_REJECTED 4001

          The authentication process for the user failed, most likely due to
          an invalid password used by the user.  Further attempts MUST only
          be tried after prompting the user for a new password.

       DIAMETER_OUT_OF_SPACE 4002

          A Diameter node received the accounting request but was unable to
          commit it to stable storage due to a temporary lack of space.

       ELECTION_LOST 4003

          The peer has determined that it has lost the election process and
          has therefore disconnected the transport connection.

    7.1.5.  Permanent Failures

       Errors that fall within the permanent failures category are used to
       inform the peer that the request failed and should not be attempted
       again.  Note that these errors SHOULD be used in answer messages
       whose 'E' bit is not set.  In error conditions where it is not
       possible or efficient to compose application-specific answer grammar,
       answer messages with the 'E' bit set and which comply to the grammar
       described in Section 7.2 MAY also be used for permanent errors.

    Fajardo, et al.              Standards Track                   [Page 92]

    RFC 6733                 Diameter Base Protocol             October 2012

       DIAMETER_AVP_UNSUPPORTED 5001

          The peer received a message that contained an AVP that is not
          recognized or supported and was marked with the 'M' (Mandatory)
          bit.  A Diameter message with this error MUST contain one or more
          Failed-AVP AVPs containing the AVPs that caused the failure.

       DIAMETER_UNKNOWN_SESSION_ID 5002

          The request contained an unknown Session-Id.

       DIAMETER_AUTHORIZATION_REJECTED 5003

          A request was received for which the user could not be authorized.
          This error could occur if the service requested is not permitted
          to the user.

       DIAMETER_INVALID_AVP_VALUE 5004

          The request contained an AVP with an invalid value in its data
          portion.  A Diameter message indicating this error MUST include
          the offending AVPs within a Failed-AVP AVP.

       DIAMETER_MISSING_AVP 5005

          The request did not contain an AVP that is required by the Command
          Code definition.  If this value is sent in the Result-Code AVP, a
          Failed-AVP AVP SHOULD be included in the message.  The Failed-AVP
          AVP MUST contain an example of the missing AVP complete with the
          Vendor-Id if applicable.  The value field of the missing AVP
          should be of correct minimum length and contain zeroes.

       DIAMETER_RESOURCES_EXCEEDED 5006

          A request was received that cannot be authorized because the user
          has already expended allowed resources.  An example of this error
          condition is when a user that is restricted to one dial-up PPP
          port attempts to establish a second PPP connection.

       DIAMETER_CONTRADICTING_AVPS 5007

          The Home Diameter server has detected AVPs in the request that
          contradicted each other, and it is not willing to provide service
          to the user.  The Failed-AVP AVP MUST be present, which contain
          the AVPs that contradicted each other.

    Fajardo, et al.              Standards Track                   [Page 93]

    RFC 6733                 Diameter Base Protocol             October 2012

       DIAMETER_AVP_NOT_ALLOWED 5008

          A message was received with an AVP that MUST NOT be present.  The
          Failed-AVP AVP MUST be included and contain a copy of the
          offending AVP.

       DIAMETER_AVP_OCCURS_TOO_MANY_TIMES 5009

          A message was received that included an AVP that appeared more
          often than permitted in the message definition.  The Failed-AVP
          AVP MUST be included and contain a copy of the first instance of
          the offending AVP that exceeded the maximum number of occurrences.

       DIAMETER_NO_COMMON_APPLICATION 5010

          This error is returned by a Diameter node that receives a CER
          whereby no applications are common between the CER sending peer
          and the CER receiving peer.

       DIAMETER_UNSUPPORTED_VERSION 5011

          This error is returned when a request was received, whose version
          number is unsupported.

       DIAMETER_UNABLE_TO_COMPLY 5012

          This error is returned when a request is rejected for unspecified
          reasons.

       DIAMETER_INVALID_BIT_IN_HEADER 5013

          This error is returned when a reserved bit in the Diameter header
          is set to one (1) or the bits in the Diameter header are set
          incorrectly.

       DIAMETER_INVALID_AVP_LENGTH 5014

          The request contained an AVP with an invalid length.  A Diameter
          message indicating this error MUST include the offending AVPs
          within a Failed-AVP AVP.  In cases where the erroneous AVP length
          value exceeds the message length or is less than the minimum AVP
          header length, it is sufficient to include the offending AVP
          header and a zero filled payload of the minimum required length
          for the payloads data type.  If the AVP is a Grouped AVP, the
          Grouped AVP header with an empty payload would be sufficient to
          indicate the offending AVP.  In the case where the offending AVP
          header cannot be fully decoded when the AVP length is less than

    Fajardo, et al.              Standards Track                   [Page 94]

    RFC 6733                 Diameter Base Protocol             October 2012

          the minimum AVP header length, it is sufficient to include an
          offending AVP header that is formulated by padding the incomplete
          AVP header with zero up to the minimum AVP header length.

       DIAMETER_INVALID_MESSAGE_LENGTH 5015

          This error is returned when a request is received with an invalid
          message length.

       DIAMETER_INVALID_AVP_BIT_COMBO 5016

          The request contained an AVP with which is not allowed to have the
          given value in the AVP Flags field.  A Diameter message indicating
          this error MUST include the offending AVPs within a Failed-AVP
          AVP.

       DIAMETER_NO_COMMON_SECURITY 5017

          This error is returned when a CER message is received, and there
          are no common security mechanisms supported between the peers.  A
          Capabilities-Exchange-Answer (CEA) message MUST be returned with
          the Result-Code AVP set to DIAMETER_NO_COMMON_SECURITY.

    7.2.  Error Bit

       The 'E' (Error Bit) in the Diameter header is set when the request
       caused a protocol-related error (see Section 7.1.3).  A message with
       the 'E' bit MUST NOT be sent as a response to an answer message.
       Note that a message with the 'E' bit set is still subjected to the
       processing rules defined in Section 6.2.  When set, the answer
       message will not conform to the CCF specification for the command;
       instead, it and will conform to the following CCF:

          Message Format

          <answer-message> ::= < Diameter Header: code, ERR [, PXY] >
                            0*1< Session-Id >
                               { Origin-Host }
                               { Origin-Realm }
                               { Result-Code }
                               [ Origin-State-Id ]
                               [ Error-Message ]
                               [ Error-Reporting-Host ]
                               [ Failed-AVP ]
                               [ Experimental-Result ]
                             * [ Proxy-Info ]
                             * [ AVP ]

    Fajardo, et al.              Standards Track                   [Page 95]

    RFC 6733                 Diameter Base Protocol             October 2012

       Note that the code used in the header is the same than the one found
       in the request message, but with the 'R' bit cleared and the 'E' bit
       set.  The 'P' bit in the header is set to the same value as the one
       found in the request message.

    7.3.  Error-Message AVP

       The Error-Message AVP (AVP Code 281) is of type UTF8String.  It MAY
       accompany a Result-Code AVP as a human-readable error message.  The
       Error-Message AVP is not intended to be useful in an environment
       where error messages are processed automatically.  It SHOULD NOT be
       expected that the content of this AVP be parsed by network entities.

    7.4.  Error-Reporting-Host AVP

       The Error-Reporting-Host AVP (AVP Code 294) is of type
       DiameterIdentity.  This AVP contains the identity of the Diameter
       host that sent the Result-Code AVP to a value other than 2001
       (Success), only if the host setting the Result-Code is different from
       the one encoded in the Origin-Host AVP.  This AVP is intended to be
       used for troubleshooting purposes, and it MUST be set when the
       Result-Code AVP indicates a failure.

    7.5.  Failed-AVP AVP

       The Failed-AVP AVP (AVP Code 279) is of type Grouped and provides
       debugging information in cases where a request is rejected or not
       fully processed due to erroneous information in a specific AVP.  The
       value of the Result-Code AVP will provide information on the reason
       for the Failed-AVP AVP.  A Diameter answer message SHOULD contain an
       instance of the Failed-AVP AVP that corresponds to the error
       indicated by the Result-Code AVP.  For practical purposes, this
       Failed-AVP would typically refer to the first AVP processing error
       that a Diameter node encounters.

       The possible reasons for this AVP are the presence of an improperly
       constructed AVP, an unsupported or unrecognized AVP, an invalid AVP
       value, the omission of a required AVP, the presence of an explicitly
       excluded AVP (see tables in Section 10) or the presence of two or
       more occurrences of an AVP that is restricted to 0, 1, or 0-1
       occurrences.

       A Diameter message SHOULD contain one Failed-AVP AVP, containing the
       entire AVP that could not be processed successfully.  If the failure
       reason is omission of a required AVP, an AVP with the missing AVP
       code, the missing Vendor-Id, and a zero-filled payload of the minimum
       required length for the omitted AVP will be added.  If the failure
       reason is an invalid AVP length where the reported length is less

    Fajardo, et al.              Standards Track                   [Page 96]

    RFC 6733                 Diameter Base Protocol             October 2012

       than the minimum AVP header length or greater than the reported
       message length, a copy of the offending AVP header and a zero-filled
       payload of the minimum required length SHOULD be added.

       In the case where the offending AVP is embedded within a Grouped AVP,
       the Failed-AVP MAY contain the grouped AVP, which in turn contains
       the single offending AVP.  The same method MAY be employed if the
       grouped AVP itself is embedded in yet another grouped AVP and so on.
       In this case, the Failed-AVP MAY contain the grouped AVP hierarchy up
       to the single offending AVP.  This enables the recipient to detect
       the location of the offending AVP when embedded in a group.

       AVP Format

             <Failed-AVP> ::= < AVP Header: 279 >
                           1* {AVP}

    7.6.  Experimental-Result AVP

       The Experimental-Result AVP (AVP Code 297) is of type Grouped, and
       indicates whether a particular vendor-specific request was completed
       successfully or whether an error occurred.  This AVP has the
       following structure:

       AVP Format

             Experimental-Result ::= < AVP Header: 297 >
                                     { Vendor-Id }
                                     { Experimental-Result-Code }

       The Vendor-Id AVP (see Section 5.3.3) in this grouped AVP identifies
       the vendor responsible for the assignment of the result code that
       follows.  All Diameter answer messages defined in vendor-specific
       applications MUST include either one Result-Code AVP or one
       Experimental-Result AVP.

    7.7.  Experimental-Result-Code AVP

       The Experimental-Result-Code AVP (AVP Code 298) is of type Unsigned32
       and contains a vendor-assigned value representing the result of
       processing the request.

       It is recommended that vendor-specific result codes follow the same
       conventions given for the Result-Code AVP regarding the different
       types of result codes and the handling of errors (for non-2xxx
       values).

    Fajardo, et al.              Standards Track                   [Page 97]

    RFC 6733                 Diameter Base Protocol             October 2012

    8.  Diameter User Sessions

       In general, Diameter can provide two different types of services to
       applications.  The first involves authentication and authorization,
       and it can optionally make use of accounting.  The second only makes
       use of accounting.

       When a service makes use of the authentication and/or authorization
       portion of an application, and a user requests access to the network,
       the Diameter client issues an auth request to its local server.  The
       auth request is defined in a service-specific Diameter application
       (e.g., NASREQ).  The request contains a Session-Id AVP, which is used
       in subsequent messages (e.g., subsequent authorization, accounting,
       etc.) relating to the user's session.  The Session-Id AVP is a means
       for the client and servers to correlate a Diameter message with a
       user session.

       When a Diameter server authorizes a user to implement network
       resources for a finite amount of time, and it is willing to extend
       the authorization via a future request, it MUST add the
       Authorization- Lifetime AVP to the answer message.  The
       Authorization-Lifetime AVP defines the maximum number of seconds a
       user MAY make use of the resources before another authorization
       request is expected by the server.  The Auth-Grace-Period AVP
       contains the number of seconds following the expiration of the
       Authorization-Lifetime, after which the server will release all state
       information related to the user's session.  Note that if payment for
       services is expected by the serving realm from the user's home realm,
       the Authorization-Lifetime AVP, combined with the Auth-Grace-Period
       AVP, implies the maximum length of the session for which the home
       realm is willing to be fiscally responsible.  Services provided past
       the expiration of the Authorization-Lifetime and Auth-Grace-Period
       AVPs are the responsibility of the access device.  Of course, the
       actual cost of services rendered is clearly outside the scope of the
       protocol.

       An access device that does not expect to send a re-authorization or a
       session termination request to the server MAY include the Auth-
       Session-State AVP with the value set to NO_STATE_MAINTAINED as a hint
       to the server.  If the server accepts the hint, it agrees that since
       no session termination message will be received once service to the
       user is terminated, it cannot maintain state for the session.  If the
       answer message from the server contains a different value in the
       Auth-Session-State AVP (or the default value if the AVP is absent),
       the access device MUST follow the server's directives.  Note that the
       value NO_STATE_MAINTAINED MUST NOT be set in subsequent re-
       authorization requests and answers.

    Fajardo, et al.              Standards Track                   [Page 98]

    RFC 6733                 Diameter Base Protocol             October 2012

       The base protocol does not include any authorization request
       messages, since these are largely application-specific and are
       defined in a Diameter application document.  However, the base
       protocol does define a set of messages that are used to terminate
       user sessions.  These are used to allow servers that maintain state
       information to free resources.

       When a service only makes use of the accounting portion of the
       Diameter protocol, even in combination with an application, the
       Session-Id is still used to identify user sessions.  However, the
       session termination messages are not used, since a session is
       signaled as being terminated by issuing an accounting stop message.

       Diameter may also be used for services that cannot be easily
       categorized as authentication, authorization, or accounting (e.g.,
       certain Third Generation Partnership Project Internet Multimedia
       System (3GPP IMS) interfaces).  In such cases, the finite state
       machine defined in subsequent sections may not be applicable.
       Therefore, the application itself MAY need to define its own finite
       state machine.  However, such application-specific state machines
       SHOULD follow the general state machine framework outlined in this
       document such as the use of Session-Id AVPs and the use of STR/STA,
       ASR/ASA messages for stateful sessions.

    8.1.  Authorization Session State Machine

       This section contains a set of finite state machines, which represent
       the life cycle of Diameter sessions and which MUST be observed by all
       Diameter implementations that make use of the authentication and/or
       authorization portion of a Diameter application.  The term "Service-
       Specific" below refers to a message defined in a Diameter application
       (e.g., Mobile IPv4, NASREQ).

       There are four different authorization session state machines
       supported in the Diameter base protocol.  The first two describe a
       session in which the server is maintaining session state, indicated
       by the value of the Auth-Session-State AVP (or its absence).  One
       describes the session from a client perspective, the other from a
       server perspective.  The second two state machines are used when the
       server does not maintain session state.  Here again, one describes
       the session from a client perspective, the other from a server
       perspective.

       When a session is moved to the Idle state, any resources that were
       allocated for the particular session must be released.  Any event not
       listed in the state machines MUST be considered an error condition,
       and an answer, if applicable, MUST be returned to the originator of
       the message.

    Fajardo, et al.              Standards Track                   [Page 99]

    RFC 6733                 Diameter Base Protocol             October 2012

       In the case that an application does not support re-auth, the state
       transitions related to server-initiated re-auth, when both client and
       server sessions maintain state (e.g., Send RAR, Pending, Receive
       RAA), MAY be ignored.

       In the state table, the event "Failure to send X" means that the
       Diameter agent is unable to send command X to the desired
       destination.  This could be due to the peer being down or due to the
       peer sending back a transient failure or temporary protocol error
       notification DIAMETER_TOO_BUSY or DIAMETER_LOOP_DETECTED in the
       Result-Code AVP of the corresponding Answer command.  The event 'X
       successfully sent' is the complement of 'Failure to send X'.

       The following state machine is observed by a client when state is
       maintained on the server:

                                  CLIENT, STATEFUL
          State     Event                          Action       New State
          ---------------------------------------------------------------
          Idle      Client or device requests      Send         Pending
                    access                         service-
                                                   specific
                                                   auth req

          Idle      ASR Received                   Send ASA     Idle
                    for unknown session            with
                                                   Result-Code =
                                                   UNKNOWN_
                                                   SESSION_ID

          Idle      RAR Received                   Send RAA     Idle
                    for unknown session            with
                                                   Result-Code =
                                                   UNKNOWN_
                                                   SESSION_ID

          Pending   Successful service-specific    Grant        Open
                    authorization answer           Access
                    received with default
                    Auth-Session-State value

          Pending   Successful service-specific    Sent STR     Discon
                    authorization answer received,
                    but service not provided

          Pending   Error processing successful    Sent STR     Discon
                    service-specific authorization
                    answer

    Fajardo, et al.              Standards Track                  [Page 100]

    RFC 6733                 Diameter Base Protocol             October 2012

          Pending   Failed service-specific        Clean up     Idle
                    authorization answer received

          Open      User or client device          Send         Open
                    requests access to service     service-
                                                   specific
                                                   auth req

          Open      Successful service-specific    Provide      Open
                    authorization answer received  service

          Open      Failed service-specific        Discon.      Idle
                    authorization answer           user/device
                    received.

          Open      RAR received and client will   Send RAA     Open
                    perform subsequent re-auth     with
                                                   Result-Code =
                                                   SUCCESS

          Open      RAR received and client will   Send RAA     Idle
                    not perform subsequent         with
                    re-auth                        Result-Code !=
                                                   SUCCESS,
                                                   Discon.
                                                   user/device

          Open      Session-Timeout expires on     Send STR     Discon
                    access device

          Open      ASR received,                  Send ASA     Discon
                    client will comply             with
                    with request to end the        Result-Code =
                    session                        = SUCCESS,
                                                   Send STR.

          Open      ASR Received,                  Send ASA     Open
                    client will not comply         with
                    with request to end the        Result-Code !=
                    session                        != SUCCESS

          Open      Authorization-Lifetime +       Send STR     Discon
                    Auth-Grace-Period expires on
                    access device

          Discon    ASR received                   Send ASA     Discon

    Fajardo, et al.              Standards Track                  [Page 101]

    RFC 6733                 Diameter Base Protocol             October 2012

          Discon    STA received                   Discon.      Idle
                                                   user/device

       The following state machine is observed by a server when it is
       maintaining state for the session:

                                 SERVER, STATEFUL
          State     Event                          Action       New State
          ---------------------------------------------------------------
          Idle      Service-specific authorization Send         Open
                    request received, and          successful
                    user is authorized             service-
                                                   specific
                                                   answer

          Idle      Service-specific authorization Send         Idle
                    request received, and          failed
                    user is not authorized         service-
                                                   specific
                                                   answer

          Open      Service-specific authorization Send         Open
                    request received, and user     successful
                    is authorized                  service-
                                                   specific
                                                   answer

          Open      Service-specific authorization Send         Idle
                    request received, and user     failed
                    is not authorized              service-
                                                   specific
                                                   answer,
                                                   Clean up

          Open      Home server wants to confirm   Send RAR     Pending
                    authentication and/or
                    authorization of the user

          Pending   Received RAA with a failed     Clean up     Idle
                    Result-Code

          Pending   Received RAA with Result-Code  Update       Open
                    = SUCCESS                      session

          Open      Home server wants to           Send ASR     Discon
                    terminate the service

    Fajardo, et al.              Standards Track                  [Page 102]

    RFC 6733                 Diameter Base Protocol             October 2012

          Open      Authorization-Lifetime (and    Clean up     Idle
                    Auth-Grace-Period) expires
                    on home server

          Open      Session-Timeout expires on     Clean up     Idle
                    home server

          Discon    Failure to send ASR            Wait,        Discon
                                                   resend ASR

          Discon    ASR successfully sent and      Clean up     Idle
                    ASA Received with Result-Code

          Not       ASA Received                   None         No Change
          Discon

          Any       STR Received                   Send STA,    Idle
                                                   Clean up

       The following state machine is observed by a client when state is not
       maintained on the server:

                                  CLIENT, STATELESS
          State     Event                          Action       New State
          ---------------------------------------------------------------
          Idle      Client or device requests      Send         Pending
                    access                         service-
                                                   specific
                                                   auth req

          Pending   Successful service-specific    Grant        Open
                    authorization answer           access
                    received with Auth-Session-
                    State set to
                    NO_STATE_MAINTAINED

          Pending   Failed service-specific        Clean up     Idle
                    authorization answer
                    received

          Open      Session-Timeout expires on     Discon.      Idle
                    access device                  user/device

          Open      Service to user is terminated  Discon.      Idle
                                                   user/device

    Fajardo, et al.              Standards Track                  [Page 103]

    RFC 6733                 Diameter Base Protocol             October 2012

       The following state machine is observed by a server when it is not
       maintaining state for the session:

                                  SERVER, STATELESS
          State     Event                          Action       New State
          ---------------------------------------------------------------
          Idle      Service-specific authorization Send         Idle
                    request received, and          service-
                    successfully processed         specific
                                                   answer

    8.2.  Accounting Session State Machine

       The following state machines MUST be supported for applications that
       have an accounting portion or that require only accounting services.
       The first state machine is to be observed by clients.

       See Section 9.7 for Accounting Command Codes and Section 9.8 for
       Accounting AVPs.

       The server side in the accounting state machine depends in some cases
       on the particular application.  The Diameter base protocol defines a
       default state machine that MUST be followed by all applications that
       have not specified other state machines.  This is the second state
       machine in this section described below.

       The default server side state machine requires the reception of
       accounting records in any order and at any time, and it does not
       place any standards requirement on the processing of these records.
       Implementations of Diameter may perform checking, ordering,
       correlation, fraud detection, and other tasks based on these records.
       AVPs may need to be inspected as a part of these tasks.  The tasks
       can happen either immediately after record reception or in a post-
       processing phase.  However, as these tasks are typically application
       or even policy dependent, they are not standardized by the Diameter
       specifications.  Applications MAY define requirements on when to
       accept accounting records based on the used value of Accounting-
       Realtime-Required AVP, credit-limit checks, and so on.

       However, the Diameter base protocol defines one optional server side
       state machine that MAY be followed by applications that require
       keeping track of the session state at the accounting server.  Note
       that such tracking is incompatible with the ability to sustain long
       duration connectivity problems.  Therefore, the use of this state
       machine is recommended only in applications where the value of the
       Accounting-Realtime-Required AVP is DELIVER_AND_GRANT; hence,
       accounting connectivity problems are required to cause the serviced
       user to be disconnected.  Otherwise, records produced by the client

    Fajardo, et al.              Standards Track                  [Page 104]

    RFC 6733                 Diameter Base Protocol             October 2012

       may be lost by the server, which no longer accepts them after the
       connectivity is re-established.  This state machine is the third
       state machine in this section.  The state machine is supervised by a
       supervision session timer Ts, whose value should be reasonably higher
       than the Acct_Interim_Interval value.  Ts MAY be set to two times the
       value of the Acct_Interim_Interval so as to avoid the accounting
       session in the Diameter server to change to Idle state in case of
       short transient network failure.

       Any event not listed in the state machines MUST be considered as an
       error condition, and a corresponding answer, if applicable, MUST be
       returned to the originator of the message.

       In the state table, the event "Failure to send" means that the
       Diameter client is unable to communicate with the desired
       destination.  This could be due to the peer being down, or due to the
       peer sending back a transient failure or temporary protocol error
       notification DIAMETER_OUT_OF_SPACE, DIAMETER_TOO_BUSY, or
       DIAMETER_LOOP_DETECTED in the Result-Code AVP of the Accounting
       Answer command.

       The event "Failed answer" means that the Diameter client received a
       non-transient failure notification in the Accounting Answer command.

       Note that the action "Disconnect user/dev" MUST also have an effect
       on the authorization session state table, e.g., cause the STR message
       to be sent, if the given application has both authentication/
       authorization and accounting portions.

       The states PendingS, PendingI, PendingL, PendingE, and PendingB stand
       for pending states to wait for an answer to an accounting request
       related to a Start, Interim, Stop, Event, or buffered record,
       respectively.

                                CLIENT, ACCOUNTING
          State     Event                          Action       New State
          ---------------------------------------------------------------
          Idle      Client or device requests      Send         PendingS
                    access                         accounting
                                                   start req.

          Idle      Client or device requests      Send         PendingE
                    a one-time service             accounting
                                                   event req

          Idle      Records in storage             Send         PendingB
                                                   record

    Fajardo, et al.              Standards Track                  [Page 105]

    RFC 6733                 Diameter Base Protocol             October 2012

          PendingS  Successful accounting                       Open
                    start answer received

          PendingS  Failure to send and buffer     Store        Open
                    space available and real time  Start
                    not equal to DELIVER_AND_GRANT Record

          PendingS  Failure to send and no buffer               Open
                    space available and real time
                    equal to GRANT_AND_LOSE

          PendingS  Failure to send and no         Disconnect   Idle
                    buffer space available and     user/dev
                    real time not equal to
                    GRANT_AND_LOSE

          PendingS  Failed accounting start answer              Open
                    received and real time equal
                    to GRANT_AND_LOSE

          PendingS  Failed accounting start answer Disconnect   Idle
                    received and real time not     user/dev
                    equal to GRANT_AND_LOSE

          PendingS  User service terminated        Store        PendingS
                                                   stop
                                                   record

          Open      Interim interval elapses       Send         PendingI
                                                   accounting
                                                   interim
                                                   record

          Open      User service terminated        Send         PendingL
                                                   accounting
                                                   stop req.

          PendingI  Successful accounting interim               Open
                    answer received

          PendingI  Failure to send and (buffer    Store        Open
                    space available or old         interim
                    record can be overwritten)     record
                    and real time not equal to
                    DELIVER_AND_GRANT

    Fajardo, et al.              Standards Track                  [Page 106]

    RFC 6733                 Diameter Base Protocol             October 2012

          PendingI  Failure to send and no buffer               Open
                    space available and real time
                    equal to GRANT_AND_LOSE

          PendingI  Failure to send and no         Disconnect   Idle
                    buffer space available and     user/dev
                    real time not equal to
                    GRANT_AND_LOSE

          PendingI  Failed accounting interim                   Open
                    answer received and real time
                    equal to GRANT_AND_LOSE

          PendingI  Failed accounting interim      Disconnect   Idle
                    answer received and            user/dev
                    real time not equal to
                    GRANT_AND_LOSE

          PendingI  User service terminated        Store        PendingI
                                                   stop
                                                   record
          PendingE  Successful accounting                       Idle
                    event answer received

          PendingE  Failure to send and buffer     Store        Idle
                    space available                event
                                                   record

          PendingE  Failure to send and no buffer               Idle
                    space available

          PendingE  Failed accounting event answer              Idle
                    received

          PendingB  Successful accounting answer   Delete       Idle
                    received                       record

          PendingB  Failure to send                             Idle

          PendingB  Failed accounting answer       Delete       Idle
                    received                       record

          PendingL  Successful accounting                       Idle
                    stop answer received

          PendingL  Failure to send and buffer     Store        Idle
                    space available                stop
                                                   record

    Fajardo, et al.              Standards Track                  [Page 107]

    RFC 6733                 Diameter Base Protocol             October 2012

          PendingL  Failure to send and no buffer               Idle
                    space available

          PendingL  Failed accounting stop answer               Idle
                    received

                           SERVER, STATELESS ACCOUNTING
          State     Event                          Action       New State
          ---------------------------------------------------------------

          Idle      Accounting start request       Send         Idle
                    received and successfully      accounting
                    processed.                     start
                                                   answer

          Idle      Accounting event request       Send         Idle
                    received and successfully      accounting
                    processed.                     event
                                                   answer

          Idle      Interim record received        Send         Idle
                    and successfully processed.    accounting
                                                   interim
                                                   answer

          Idle      Accounting stop request        Send         Idle
                    received and successfully      accounting
                    processed                      stop answer

          Idle      Accounting request received;   Send         Idle
                    no space left to store         accounting
                    records                        answer;
                                                   Result-Code =
                                                   OUT_OF_
                                                   SPACE

    Fajardo, et al.              Standards Track                  [Page 108]

    RFC 6733                 Diameter Base Protocol             October 2012

                                SERVER, STATEFUL ACCOUNTING
          State     Event                          Action       New State
          ---------------------------------------------------------------

          Idle      Accounting start request       Send         Open
                    received and successfully      accounting
                    processed.                     start
                                                   answer;
                                                   Start Ts

          Idle      Accounting event request       Send         Idle
                    received and successfully      accounting
                    processed.                     event
                                                   answer
          Idle      Accounting request received;   Send         Idle
                    no space left to store         accounting
                    records                        answer;
                                                   Result-Code =
                                                   OUT_OF_
                                                   SPACE

          Open      Interim record received        Send         Open
                    and successfully processed.    accounting
                                                   interim
                                                   answer;
                                                   Restart Ts

          Open      Accounting stop request        Send         Idle
                    received and successfully      accounting
                    processed                      stop answer;
                                                   Stop Ts

          Open      Accounting request received;   Send         Idle
                    no space left to store         accounting
                    records                        answer;
                                                   Result-Code =
                                                   OUT_OF_
                                                   SPACE;
                                                   Stop Ts

          Open      Session supervision timer Ts   Stop Ts      Idle
                    expired

    Fajardo, et al.              Standards Track                  [Page 109]

    RFC 6733                 Diameter Base Protocol             October 2012

    8.3.  Server-Initiated Re-Auth

       A Diameter server may initiate a re-authentication and/or re-
       authorization service for a particular session by issuing a Re-Auth-
       Request (RAR).

       For example, for prepaid services, the Diameter server that
       originally authorized a session may need some confirmation that the
       user is still using the services.

       An access device that receives an RAR message with the Session-Id
       equal to a currently active session MUST initiate a re-auth towards
       the user, if the service supports this particular feature.  Each
       Diameter application MUST state whether server-initiated re-auth is
       supported, since some applications do not allow access devices to
       prompt the user for re-auth.

    8.3.1.  Re-Auth-Request

       The Re-Auth-Request (RAR), indicated by the Command Code set to 258
       and the message flags' 'R' bit set, may be sent by any server to the
       access device that is providing session service, to request that the
       user be re-authenticated and/or re-authorized.

        Message Format

             <RAR>  ::= < Diameter Header: 258, REQ, PXY >
                        < Session-Id >
                        { Origin-Host }
                        { Origin-Realm }
                        { Destination-Realm }
                        { Destination-Host }
                        { Auth-Application-Id }
                        { Re-Auth-Request-Type }
                        [ User-Name ]
                        [ Origin-State-Id ]
                      * [ Proxy-Info ]
                      * [ Route-Record ]
                      * [ AVP ]

    8.3.2.  Re-Auth-Answer

       The Re-Auth-Answer (RAA), indicated by the Command Code set to 258
       and the message flags' 'R' bit clear, is sent in response to the RAR.
       The Result-Code AVP MUST be present, and it indicates the disposition
       of the request.

    Fajardo, et al.              Standards Track                  [Page 110]

    RFC 6733                 Diameter Base Protocol             October 2012

       A successful RAA message MUST be followed by an application-specific
       authentication and/or authorization message.

        Message Format

             <RAA>  ::= < Diameter Header: 258, PXY >
                        < Session-Id >
                        { Result-Code }
                        { Origin-Host }
                        { Origin-Realm }
                        [ User-Name ]
                        [ Origin-State-Id ]
                        [ Error-Message ]
                        [ Error-Reporting-Host ]
                        [ Failed-AVP ]
                      * [ Redirect-Host ]
                        [ Redirect-Host-Usage ]
                        [ Redirect-Max-Cache-Time ]
                      * [ Proxy-Info ]
                      * [ AVP ]

    8.4.  Session Termination

       It is necessary for a Diameter server that authorized a session, for
       which it is maintaining state, to be notified when that session is no
       longer active, both for tracking purposes as well as to allow
       stateful agents to release any resources that they may have provided
       for the user's session.  For sessions whose state is not being
       maintained, this section is not used.

       When a user session that required Diameter authorization terminates,
       the access device that provided the service MUST issue a Session-
       Termination-Request (STR) message to the Diameter server that
       authorized the service, to notify it that the session is no longer
       active.  An STR MUST be issued when a user session terminates for any
       reason, including user logoff, expiration of Session-Timeout,
       administrative action, termination upon receipt of an Abort-Session-
       Request (see below), orderly shutdown of the access device, etc.

       The access device also MUST issue an STR for a session that was
       authorized but never actually started.  This could occur, for
       example, due to a sudden resource shortage in the access device, or
       because the access device is unwilling to provide the type of service
       requested in the authorization, or because the access device does not
       support a mandatory AVP returned in the authorization, etc.

       It is also possible that a session that was authorized is never
       actually started due to action of a proxy.  For example, a proxy may

    Fajardo, et al.              Standards Track                  [Page 111]

    RFC 6733                 Diameter Base Protocol             October 2012

       modify an authorization answer, converting the result from success to
       failure, prior to forwarding the message to the access device.  If
       the answer did not contain an Auth-Session-State AVP with the value
       NO_STATE_MAINTAINED, a proxy that causes an authorized session not to
       be started MUST issue an STR to the Diameter server that authorized
       the session, since the access device has no way of knowing that the
       session had been authorized.

       A Diameter server that receives an STR message MUST clean up
       resources (e.g., session state) associated with the Session-Id
       specified in the STR and return a Session-Termination-Answer.

       A Diameter server also MUST clean up resources when the Session-
       Timeout expires, or when the Authorization-Lifetime and the Auth-
       Grace-Period AVPs expire without receipt of a re-authorization
       request, regardless of whether an STR for that session is received.
       The access device is not expected to provide service beyond the
       expiration of these timers; thus, expiration of either of these
       timers implies that the access device may have unexpectedly shut
       down.

    8.4.1.  Session-Termination-Request

       The Session-Termination-Request (STR), indicated by the Command Code
       set to 275 and the Command Flags' 'R' bit set, is sent by a Diameter
       client or by a Diameter proxy to inform the Diameter server that an
       authenticated and/or authorized session is being terminated.

        Message Format

            <STR>  ::= < Diameter Header: 275, REQ, PXY >
                       < Session-Id >
                       { Origin-Host }
                       { Origin-Realm }
                       { Destination-Realm }
                       { Auth-Application-Id }
                       { Termination-Cause }
                       [ User-Name ]
                       [ Destination-Host ]
                     * [ Class ]
                       [ Origin-State-Id ]
                     * [ Proxy-Info ]
                     * [ Route-Record ]
                     * [ AVP ]

    Fajardo, et al.              Standards Track                  [Page 112]

    RFC 6733                 Diameter Base Protocol             October 2012

    8.4.2.  Session-Termination-Answer

       The Session-Termination-Answer (STA), indicated by the Command Code
       set to 275 and the message flags' 'R' bit clear, is sent by the
       Diameter server to acknowledge the notification that the session has
       been terminated.  The Result-Code AVP MUST be present, and it MAY
       contain an indication that an error occurred while servicing the STR.

       Upon sending or receipt of the STA, the Diameter server MUST release
       all resources for the session indicated by the Session-Id AVP.  Any
       intermediate server in the Proxy-Chain MAY also release any
       resources, if necessary.

        Message Format

             <STA> ::= < Diameter Header: 275, PXY >
                        < Session-Id >
                        { Result-Code }
                        { Origin-Host }
                        { Origin-Realm }
                        [ User-Name ]
                      * [ Class ]
                        [ Error-Message ]
                        [ Error-Reporting-Host ]
                        [ Failed-AVP ]
                        [ Origin-State-Id ]
                      * [ Redirect-Host ]
                        [ Redirect-Host-Usage ]
                        [ Redirect-Max-Cache-Time ]
                      * [ Proxy-Info ]
                      * [ AVP ]

    8.5.  Aborting a Session

       A Diameter server may request that the access device stop providing
       service for a particular session by issuing an Abort-Session-Request
       (ASR).

       For example, the Diameter server that originally authorized the
       session may be required to cause that session to be stopped for lack
       of credit or other reasons that were not anticipated when the session
       was first authorized.

       An access device that receives an ASR with Session-ID equal to a
       currently active session MAY stop the session.  Whether the access
       device stops the session or not is implementation and/or
       configuration dependent.  For example, an access device may honor
       ASRs from certain agents only.  In any case, the access device MUST

    Fajardo, et al.              Standards Track                  [Page 113]

    RFC 6733                 Diameter Base Protocol             October 2012

       respond with an Abort-Session-Answer, including a Result-Code AVP to
       indicate what action it took.

    8.5.1.  Abort-Session-Request

       The Abort-Session-Request (ASR), indicated by the Command Code set to
       274 and the message flags' 'R' bit set, may be sent by any Diameter
       server or any Diameter proxy to the access device that is providing
       session service, to request that the session identified by the
       Session-Id be stopped.

        Message Format

             <ASR>  ::= < Diameter Header: 274, REQ, PXY >
                        < Session-Id >
                        { Origin-Host }
                        { Origin-Realm }
                        { Destination-Realm }
                        { Destination-Host }
                        { Auth-Application-Id }
                        [ User-Name ]
                        [ Origin-State-Id ]
                      * [ Proxy-Info ]
                      * [ Route-Record ]
                      * [ AVP ]

    8.5.2.  Abort-Session-Answer

       The Abort-Session-Answer (ASA), indicated by the Command Code set to
       274 and the message flags' 'R' bit clear, is sent in response to the
       ASR.  The Result-Code AVP MUST be present and indicates the
       disposition of the request.

       If the session identified by Session-Id in the ASR was successfully
       terminated, the Result-Code is set to DIAMETER_SUCCESS.  If the
       session is not currently active, the Result-Code is set to
       DIAMETER_UNKNOWN_SESSION_ID.  If the access device does not stop the
       session for any other reason, the Result-Code is set to
       DIAMETER_UNABLE_TO_COMPLY.

    Fajardo, et al.              Standards Track                  [Page 114]

    RFC 6733                 Diameter Base Protocol             October 2012

        Message Format

             <ASA>  ::= < Diameter Header: 274, PXY >
                        < Session-Id >
                        { Result-Code }
                        { Origin-Host }
                        { Origin-Realm }
                        [ User-Name ]
                        [ Origin-State-Id ]
                        [ Error-Message ]
                        [ Error-Reporting-Host ]
                        [ Failed-AVP ]
                      * [ Redirect-Host ]
                        [ Redirect-Host-Usage ]
                        [ Redirect-Max-Cache-Time ]
                      * [ Proxy-Info ]
                      * [ AVP ]

    8.6.  Inferring Session Termination from Origin-State-Id

       The Origin-State-Id is used to allow detection of terminated sessions
       for which no STR would have been issued, due to unanticipated
       shutdown of an access device.

       A Diameter client or access device increments the value of the
       Origin-State-Id every time it is started or powered up.  The new
       Origin-State-Id is then sent in the CER/CEA message immediately upon
       connection to the server.  The Diameter server receiving the new
       Origin-State-Id can determine whether the sending Diameter client had
       abruptly shut down by comparing the old value of the Origin-State-Id
       it has kept for that specific client is less than the new value and
       whether it has un-terminated sessions originating from that client.

       An access device can also include the Origin-State-Id in request
       messages other than the CER if there are relays or proxies in between
       the access device and the server.  In this case, however, the server
       cannot discover that the access device has been restarted unless and
       until it receives a new request from it.  Therefore, this mechanism
       is more opportunistic across proxies and relays.

       The Diameter server may assume that all sessions that were active
       prior to detection of a client restart have been terminated.  The
       Diameter server MAY clean up all session state associated with such
       lost sessions, and it MAY also issue STRs for all such lost sessions
       that were authorized on upstream servers, to allow session state to
       be cleaned up globally.

    Fajardo, et al.              Standards Track                  [Page 115]

    RFC 6733                 Diameter Base Protocol             October 2012

    8.7.  Auth-Request-Type AVP

       The Auth-Request-Type AVP (AVP Code 274) is of type Enumerated and is
       included in application-specific auth requests to inform the peers
       whether a user is to be authenticated only, authorized only, or both.
       Note any value other than both MAY cause RADIUS interoperability
       issues.  The following values are defined:

       AUTHENTICATE_ONLY 1

          The request being sent is for authentication only, and it MUST
          contain the relevant application-specific authentication AVPs that
          are needed by the Diameter server to authenticate the user.

       AUTHORIZE_ONLY 2

          The request being sent is for authorization only, and it MUST
          contain the application-specific authorization AVPs that are
          necessary to identify the service being requested/offered.

       AUTHORIZE_AUTHENTICATE 3

          The request contains a request for both authentication and
          authorization.  The request MUST include both the relevant
          application-specific authentication information and authorization
          information necessary to identify the service being requested/
          offered.

    8.8.  Session-Id AVP

       The Session-Id AVP (AVP Code 263) is of type UTF8String and is used
       to identify a specific session (see Section 8).  All messages
       pertaining to a specific session MUST include only one Session-Id
       AVP, and the same value MUST be used throughout the life of a
       session.  When present, the Session-Id SHOULD appear immediately
       following the Diameter header (see Section 3).

       The Session-Id MUST be globally and eternally unique, as it is meant
       to uniquely identify a user session without reference to any other
       information, and it may be needed to correlate historical
       authentication information with accounting information.  The
       Session-Id includes a mandatory portion and an implementation-defined
       portion; a recommended format for the implementation-defined portion
       is outlined below.

       The Session-Id MUST begin with the sender's identity encoded in the
       DiameterIdentity type (see Section 4.3.1).  The remainder of the
       Session-Id is delimited by a ";" character, and it MAY be any

    Fajardo, et al.              Standards Track                  [Page 116]

    RFC 6733                 Diameter Base Protocol             October 2012

       sequence that the client can guarantee to be eternally unique;
       however, the following format is recommended, (square brackets []
       indicate an optional element):

          <DiameterIdentity>;<high 32 bits>;<low 32 bits>[;<optional value>]

       <high 32 bits> and <low 32 bits> are decimal representations of the
       high and low 32 bits of a monotonically increasing 64-bit value.  The
       64-bit value is rendered in two part to simplify formatting by 32-bit
       processors.  At startup, the high 32 bits of the 64-bit value MAY be
       initialized to the time in NTP format [RFC5905], and the low 32 bits
       MAY be initialized to zero.  This will for practical purposes
       eliminate the possibility of overlapping Session-Ids after a reboot,
       assuming the reboot process takes longer than a second.
       Alternatively, an implementation MAY keep track of the increasing
       value in non-volatile memory.

       <optional value> is implementation specific, but it may include a
       modem's device Id, a Layer 2 address, timestamp, etc.

       Example, in which there is no optional value:

          accesspoint7.example.com;1876543210;523

       Example, in which there is an optional value:

         accesspoint7.example.com;1876543210;523;mobile@200.1.1.88

       The Session-Id is created by the Diameter application initiating the
       session, which, in most cases, is done by the client.  Note that a
       Session-Id MAY be used for both the authentication, authorization,
       and accounting commands of a given application.

    8.9.  Authorization-Lifetime AVP

       The Authorization-Lifetime AVP (AVP Code 291) is of type Unsigned32
       and contains the maximum number of seconds of service to be provided
       to the user before the user is to be re-authenticated and/or re-
       authorized.  Care should be taken when the Authorization-Lifetime
       value is determined, since a low, non-zero value could create
       significant Diameter traffic, which could congest both the network
       and the agents.

       A value of zero (0) means that immediate re-auth is necessary by the
       access device.  The absence of this AVP, or a value of all ones
       (meaning all bits in the 32-bit field are set to one) means no re-
       auth is expected.

    Fajardo, et al.              Standards Track                  [Page 117]

    RFC 6733                 Diameter Base Protocol             October 2012

       If both this AVP and the Session-Timeout AVP are present in a
       message, the value of the latter MUST NOT be smaller than the
       Authorization-Lifetime AVP.

       An Authorization-Lifetime AVP MAY be present in re-authorization
       messages, and it contains the number of seconds the user is
       authorized to receive service from the time the re-auth answer
       message is received by the access device.

       This AVP MAY be provided by the client as a hint of the maximum
       lifetime that it is willing to accept.  The server MUST return a
       value that is equal to, or smaller than, the one provided by the
       client.

    8.10.  Auth-Grace-Period AVP

       The Auth-Grace-Period AVP (AVP Code 276) is of type Unsigned32 and
       contains the number of seconds the Diameter server will wait
       following the expiration of the Authorization-Lifetime AVP before
       cleaning up resources for the session.

    8.11.  Auth-Session-State AVP

       The Auth-Session-State AVP (AVP Code 277) is of type Enumerated and
       specifies whether state is maintained for a particular session.  The
       client MAY include this AVP in requests as a hint to the server, but
       the value in the server's answer message is binding.  The following
       values are supported:

       STATE_MAINTAINED 0

          This value is used to specify that session state is being
          maintained, and the access device MUST issue a session termination
          message when service to the user is terminated.  This is the
          default value.

       NO_STATE_MAINTAINED 1

          This value is used to specify that no session termination messages
          will be sent by the access device upon expiration of the
          Authorization-Lifetime.

    8.12.  Re-Auth-Request-Type AVP

       The Re-Auth-Request-Type AVP (AVP Code 285) is of type Enumerated and
       is included in application-specific auth answers to inform the client
       of the action expected upon expiration of the Authorization-Lifetime.

    Fajardo, et al.              Standards Track                  [Page 118]

    RFC 6733                 Diameter Base Protocol             October 2012

       If the answer message contains an Authorization-Lifetime AVP with a
       positive value, the Re-Auth-Request-Type AVP MUST be present in an
       answer message.  The following values are defined:

       AUTHORIZE_ONLY 0

          An authorization only re-auth is expected upon expiration of the
          Authorization-Lifetime.  This is the default value if the AVP is
          not present in answer messages that include the Authorization-
          Lifetime.

       AUTHORIZE_AUTHENTICATE 1

          An authentication and authorization re-auth is expected upon
          expiration of the Authorization-Lifetime.

    8.13.  Session-Timeout AVP

       The Session-Timeout AVP (AVP Code 27) [RFC2865] is of type Unsigned32
       and contains the maximum number of seconds of service to be provided
       to the user before termination of the session.  When both the
       Session-Timeout and the Authorization-Lifetime AVPs are present in an
       answer message, the former MUST be equal to or greater than the value
       of the latter.

       A session that terminates on an access device due to the expiration
       of the Session-Timeout MUST cause an STR to be issued, unless both
       the access device and the home server had previously agreed that no
       session termination messages would be sent (see Section 8).

       A Session-Timeout AVP MAY be present in a re-authorization answer
       message, and it contains the remaining number of seconds from the
       beginning of the re-auth.

       A value of zero, or the absence of this AVP, means that this session
       has an unlimited number of seconds before termination.

       This AVP MAY be provided by the client as a hint of the maximum
       timeout that it is willing to accept.  However, the server MAY return
       a value that is equal to, or smaller than, the one provided by the
       client.

    8.14.  User-Name AVP

       The User-Name AVP (AVP Code 1) [RFC2865] is of type UTF8String, which
       contains the User-Name, in a format consistent with the NAI
       specification [RFC4282].

    Fajardo, et al.              Standards Track                  [Page 119]

    RFC 6733                 Diameter Base Protocol             October 2012

    8.15.  Termination-Cause AVP

       The Termination-Cause AVP (AVP Code 295) is of type Enumerated, and
       is used to indicate the reason why a session was terminated on the
       access device.  The currently assigned values for this AVP can be
       found in the IANA registry for Termination-Cause AVP Values
       [IANATCV].

    8.16.  Origin-State-Id AVP

       The Origin-State-Id AVP (AVP Code 278), of type Unsigned32, is a
       monotonically increasing value that is advanced whenever a Diameter
       entity restarts with loss of previous state, for example, upon
       reboot.  Origin-State-Id MAY be included in any Diameter message,
       including CER.

       A Diameter entity issuing this AVP MUST create a higher value for
       this AVP each time its state is reset.  A Diameter entity MAY set
       Origin-State-Id to the time of startup, or it MAY use an incrementing
       counter retained in non-volatile memory across restarts.

       The Origin-State-Id, if present, MUST reflect the state of the entity
       indicated by Origin-Host.  If a proxy modifies Origin-Host, it MUST
       either remove Origin-State-Id or modify it appropriately as well.
       Typically, Origin-State-Id is used by an access device that always
       starts up with no active sessions; that is, any session active prior
       to restart will have been lost.  By including Origin-State-Id in a
       message, it allows other Diameter entities to infer that sessions
       associated with a lower Origin-State-Id are no longer active.  If an
       access device does not intend for such inferences to be made, it MUST
       either not include Origin-State-Id in any message or set its value to
       0.

    8.17.  Session-Binding AVP

       The Session-Binding AVP (AVP Code 270) is of type Unsigned32, and it
       MAY be present in application-specific authorization answer messages.
       If present, this AVP MAY inform the Diameter client that all future
       application-specific re-auth and Session-Termination-Request messages
       for this session MUST be sent to the same authorization server.

    Fajardo, et al.              Standards Track                  [Page 120]

    RFC 6733                 Diameter Base Protocol             October 2012

       This field is a bit mask, and the following bits have been defined:

       RE_AUTH 1

          When set, future re-auth messages for this session MUST NOT
          include the Destination-Host AVP.  When cleared, the default
          value, the Destination-Host AVP MUST be present in all re-auth
          messages for this session.

       STR 2

          When set, the STR message for this session MUST NOT include the
          Destination-Host AVP.  When cleared, the default value, the
          Destination-Host AVP MUST be present in the STR message for this
          session.

       ACCOUNTING 4

          When set, all accounting messages for this session MUST NOT
          include the Destination-Host AVP.  When cleared, the default
          value, the Destination-Host AVP, if known, MUST be present in all
          accounting messages for this session.

    8.18.  Session-Server-Failover AVP

       The Session-Server-Failover AVP (AVP Code 271) is of type Enumerated
       and MAY be present in application-specific authorization answer
       messages that either do not include the Session-Binding AVP or
       include the Session-Binding AVP with any of the bits set to a zero
       value.  If present, this AVP MAY inform the Diameter client that if a
       re-auth or STR message fails due to a delivery problem, the Diameter
       client SHOULD issue a subsequent message without the Destination-Host
       AVP.  When absent, the default value is REFUSE_SERVICE.

       The following values are supported:

       REFUSE_SERVICE 0

          If either the re-auth or the STR message delivery fails, terminate
          service with the user and do not attempt any subsequent attempts.

    Fajardo, et al.              Standards Track                  [Page 121]

    RFC 6733                 Diameter Base Protocol             October 2012

       TRY_AGAIN 1

          If either the re-auth or the STR message delivery fails, resend
          the failed message without the Destination-Host AVP present.

       ALLOW_SERVICE 2

          If re-auth message delivery fails, assume that re-authorization
          succeeded.  If STR message delivery fails, terminate the session.

       TRY_AGAIN_ALLOW_SERVICE 3

          If either the re-auth or the STR message delivery fails, resend
          the failed message without the Destination-Host AVP present.  If
          the second delivery fails for re-auth, assume re-authorization
          succeeded.  If the second delivery fails for STR, terminate the
          session.

    8.19.  Multi-Round-Time-Out AVP

       The Multi-Round-Time-Out AVP (AVP Code 272) is of type Unsigned32 and
       SHOULD be present in application-specific authorization answer
       messages whose Result-Code AVP is set to DIAMETER_MULTI_ROUND_AUTH.
       This AVP contains the maximum number of seconds that the access
       device MUST provide the user in responding to an authentication
       request.

    8.20.  Class AVP

       The Class AVP (AVP Code 25) is of type OctetString and is used by
       Diameter servers to return state information to the access device.
       When one or more Class AVPs are present in application-specific
       authorization answer messages, they MUST be present in subsequent re-
       authorization, session termination and accounting messages.  Class
       AVPs found in a re-authorization answer message override the ones
       found in any previous authorization answer message.  Diameter server
       implementations SHOULD NOT return Class AVPs that require more than
       4096 bytes of storage on the Diameter client.  A Diameter client that
       receives Class AVPs whose size exceeds local available storage MUST
       terminate the session.

    8.21.  Event-Timestamp AVP

       The Event-Timestamp (AVP Code 55) is of type Time and MAY be included
       in an Accounting-Request and Accounting-Answer messages to record the
       time that the reported event occurred, in seconds since January 1,
       1900 00:00 UTC.

    Fajardo, et al.              Standards Track                  [Page 122]

    RFC 6733                 Diameter Base Protocol             October 2012

    9.  Accounting

       This accounting protocol is based on a server directed model with
       capabilities for real-time delivery of accounting information.
       Several fault resilience methods [RFC2975] have been built into the
       protocol in order minimize loss of accounting data in various fault
       situations and under different assumptions about the capabilities of
       the used devices.

    9.1.  Server Directed Model

       The server directed model means that the device generating the
       accounting data gets information from either the authorization server
       (if contacted) or the accounting server regarding the way accounting
       data shall be forwarded.  This information includes accounting record
       timeliness requirements.

       As discussed in [RFC2975], real-time transfer of accounting records
       is a requirement, such as the need to perform credit-limit checks and
       fraud detection.  Note that batch accounting is not a requirement,
       and is therefore not supported by Diameter.  Should batched
       accounting be required in the future, a new Diameter application will
       need to be created, or it could be handled using another protocol.
       Note, however, that even if at the Diameter layer, accounting
       requests are processed one by one; transport protocols used under
       Diameter typically batch several requests in the same packet under
       heavy traffic conditions.  This may be sufficient for many
       applications.

       The authorization server (chain) directs the selection of proper
       transfer strategy, based on its knowledge of the user and
       relationships of roaming partnerships.  The server (or agents) uses
       the Acct-Interim-Interval and Accounting-Realtime-Required AVPs to
       control the operation of the Diameter peer operating as a client.
       The Acct-Interim-Interval AVP, when present, instructs the Diameter
       node acting as a client to produce accounting records continuously
       even during a session.  Accounting-Realtime-Required AVP is used to
       control the behavior of the client when the transfer of accounting
       records from the Diameter client is delayed or unsuccessful.

       The Diameter accounting server MAY override the interim interval or
       the real-time requirements by including the Acct-Interim-Interval or
       Accounting-Realtime-Required AVP in the Accounting-Answer message.
       When one of these AVPs is present, the latest value received SHOULD
       be used in further accounting activities for the same session.

    Fajardo, et al.              Standards Track                  [Page 123]

    RFC 6733                 Diameter Base Protocol             October 2012

    9.2.  Protocol Messages

       A Diameter node that receives a successful authentication and/or
       authorization message from the Diameter server SHOULD collect
       accounting information for the session.  The Accounting-Request
       message is used to transmit the accounting information to the
       Diameter server, which MUST reply with the Accounting-Answer message
       to confirm reception.  The Accounting-Answer message includes the
       Result-Code AVP, which MAY indicate that an error was present in the
       accounting message.  The value of the Accounting-Realtime-Required
       AVP received earlier for the session in question may indicate that
       the user's session has to be terminated when a rejected Accounting-
       Request message was received.

    9.3.  Accounting Application Extension and Requirements

       Each Diameter application (e.g., NASREQ, Mobile IP) SHOULD define its
       service-specific AVPs that MUST be present in the Accounting-Request
       message in a section titled "Accounting AVPs".  The application MUST
       assume that the AVPs described in this document will be present in
       all Accounting messages, so only their respective service-specific
       AVPs need to be defined in that section.

       Applications have the option of using one or both of the following
       accounting application extension models:

       Split Accounting Service

          The accounting message will carry the Application Id of the
          Diameter base accounting application (see Section 2.4).
          Accounting messages may be routed to Diameter nodes other than the
          corresponding Diameter application.  These nodes might be
          centralized accounting servers that provide accounting service for
          multiple different Diameter applications.  These nodes MUST
          advertise the Diameter base accounting Application Id during
          capabilities exchange.

       Coupled Accounting Service

          The accounting message will carry the Application Id of the
          application that is using it.  The application itself will process
          the received accounting records or forward them to an accounting
          server.  There is no accounting application advertisement required
          during capabilities exchange, and the accounting messages will be
          routed the same way as any of the other application messages.

       In cases where an application does not define its own accounting
       service, it is preferred that the split accounting model be used.

    Fajardo, et al.              Standards Track                  [Page 124]

    RFC 6733                 Diameter Base Protocol             October 2012

    9.4.  Fault Resilience

       Diameter base protocol mechanisms are used to overcome small message
       loss and network faults of a temporary nature.

       Diameter peers acting as clients MUST implement the use of failover
       to guard against server failures and certain network failures.
       Diameter peers acting as agents or related off-line processing
       systems MUST detect duplicate accounting records caused by the
       sending of the same record to several servers and duplication of
       messages in transit.  This detection MUST be based on the inspection
       of the Session-Id and Accounting-Record-Number AVP pairs.  Appendix C
       discusses duplicate detection needs and implementation issues.

       Diameter clients MAY have non-volatile memory for the safe storage of
       accounting records over reboots or extended network failures, network
       partitions, and server failures.  If such memory is available, the
       client SHOULD store new accounting records there as soon as the
       records are created and until a positive acknowledgement of their
       reception from the Diameter server has been received.  Upon a reboot,
       the client MUST start sending the records in the non-volatile memory
       to the accounting server with the appropriate modifications in
       termination cause, session length, and other relevant information in
       the records.

       A further application of this protocol may include AVPs to control
       the maximum number of accounting records that may be stored in the
       Diameter client without committing them to the non-volatile memory or
       transferring them to the Diameter server.

       The client SHOULD NOT remove the accounting data from any of its
       memory areas before the correct Accounting-Answer has been received.
       The client MAY remove the oldest, undelivered, or as yet
       unacknowledged accounting data if it runs out of resources such as
       memory.  It is an implementation-dependent matter for the client to
       accept new sessions under this condition.

    9.5.  Accounting Records

       In all accounting records, the Session-Id AVP MUST be present; the
       User-Name AVP MUST be present if it is available to the Diameter
       client.

       Different types of accounting records are sent depending on the
       actual type of accounted service and the authorization server's
       directions for interim accounting.  If the accounted service is a

    Fajardo, et al.              Standards Track                  [Page 125]

    RFC 6733                 Diameter Base Protocol             October 2012

       one-time event, meaning that the start and stop of the event are
       simultaneous, then the Accounting-Record-Type AVP MUST be present and
       set to the value EVENT_RECORD.

       If the accounted service is of a measurable length, then the AVP MUST
       use the values START_RECORD, STOP_RECORD, and possibly,
       INTERIM_RECORD.  If the authorization server has not directed interim
       accounting to be enabled for the session, two accounting records MUST
       be generated for each service of type session.  When the initial
       Accounting-Request for a given session is sent, the Accounting-
       Record-Type AVP MUST be set to the value START_RECORD.  When the last
       Accounting-Request is sent, the value MUST be STOP_RECORD.

       If the authorization server has directed interim accounting to be
       enabled, the Diameter client MUST produce additional records between
       the START_RECORD and STOP_RECORD, marked INTERIM_RECORD.  The
       production of these records is directed by Acct-Interim-Interval as
       well as any re-authentication or re-authorization of the session.
       The Diameter client MUST overwrite any previous interim accounting
       records that are locally stored for delivery, if a new record is
       being generated for the same session.  This ensures that only one
       pending interim record can exist on an access device for any given
       session.

       A particular value of Accounting-Sub-Session-Id MUST appear only in
       one sequence of accounting records from a Diameter client, except for
       the purposes of retransmission.  The one sequence that is sent MUST
       be either one record with Accounting-Record-Type AVP set to the value
       EVENT_RECORD or several records starting with one having the value
       START_RECORD, followed by zero or more INTERIM_RECORDs and a single
       STOP_RECORD.  A particular Diameter application specification MUST
       define the type of sequences that MUST be used.

    9.6.  Correlation of Accounting Records

       If an application uses accounting messages, it can correlate
       accounting records with a specific application session by using the
       Session-Id of the particular application session in the accounting
       messages.  Accounting messages MAY also use a different Session-Id
       from that of the application sessions, in which case, other session-
       related information is needed to perform correlation.

       In cases where an application requires multiple accounting sub-
       sessions, an Accounting-Sub-Session-Id AVP is used to differentiate
       each sub-session.  The Session-Id would remain constant for all sub-
       sessions and is used to correlate all the sub-sessions to a
       particular application session.  Note that receiving a STOP_RECORD

    Fajardo, et al.              Standards Track                  [Page 126]

    RFC 6733                 Diameter Base Protocol             October 2012

       with no Accounting-Sub-Session-Id AVP when sub-sessions were
       originally used in the START_RECORD messages implies that all sub-
       sessions are terminated.

       There are also cases where an application needs to correlate multiple
       application sessions into a single accounting record; the accounting
       record may span multiple different Diameter applications and sessions
       used by the same user at a given time.  In such cases, the Acct-
       Multi-Session-Id AVP is used.  The Acct-Multi-Session-Id AVP SHOULD
       be signaled by the server to the access device (typically, during
       authorization) when it determines that a request belongs to an
       existing session.  The access device MUST then include the Acct-
       Multi-Session-Id AVP in all subsequent accounting messages.

       The Acct-Multi-Session-Id AVP MAY include the value of the original
       Session-Id.  Its contents are implementation specific, but the MUST
       be globally unique across other Acct-Multi-Session-Ids and MUST NOT
       change during the life of a session.

       A Diameter application document MUST define the exact concept of a
       session that is being accounted, and it MAY define the concept of a
       multi-session.  For instance, the NASREQ DIAMETER application treats
       a single PPP connection to a Network Access Server as one session and
       a set of Multilink PPP sessions as one multi-session.

    9.7.  Accounting Command Codes

       This section defines Command Code values that MUST be supported by
       all Diameter implementations that provide accounting services.

    9.7.1.  Accounting-Request

       The Accounting-Request (ACR) command, indicated by the Command Code
       field set to 271 and the Command Flags' 'R' bit set, is sent by a
       Diameter node, acting as a client, in order to exchange accounting
       information with a peer.

       In addition to the AVPs listed below, Accounting-Request messages
       SHOULD include service-specific accounting AVPs.

    Fajardo, et al.              Standards Track                  [Page 127]

    RFC 6733                 Diameter Base Protocol             October 2012

          Message Format

             <ACR> ::= < Diameter Header: 271, REQ, PXY >
                       < Session-Id >
                       { Origin-Host }
                       { Origin-Realm }
                       { Destination-Realm }
                       { Accounting-Record-Type }
                       { Accounting-Record-Number }
                       [ Acct-Application-Id ]
                       [ Vendor-Specific-Application-Id ]
                       [ User-Name ]
                       [ Destination-Host ]
                       [ Accounting-Sub-Session-Id ]
                       [ Acct-Session-Id ]
                       [ Acct-Multi-Session-Id ]
                       [ Acct-Interim-Interval ]
                       [ Accounting-Realtime-Required ]
                       [ Origin-State-Id ]
                       [ Event-Timestamp ]
                     * [ Proxy-Info ]
                     * [ Route-Record ]
                     * [ AVP ]

    9.7.2.  Accounting-Answer

       The Accounting-Answer (ACA) command, indicated by the Command Code
       field set to 271 and the Command Flags' 'R' bit cleared, is used to
       acknowledge an Accounting-Request command.  The Accounting-Answer
       command contains the same Session-Id as the corresponding request.

       Only the target Diameter server, known as the home Diameter server,
       SHOULD respond with the Accounting-Answer command.

       In addition to the AVPs listed below, Accounting-Answer messages
       SHOULD include service-specific accounting AVPs.

    Fajardo, et al.              Standards Track                  [Page 128]

    RFC 6733                 Diameter Base Protocol             October 2012

          Message Format

             <ACA> ::= < Diameter Header: 271, PXY >
                       < Session-Id >
                       { Result-Code }
                       { Origin-Host }
                       { Origin-Realm }
                       { Accounting-Record-Type }
                       { Accounting-Record-Number }
                       [ Acct-Application-Id ]
                       [ Vendor-Specific-Application-Id ]
                       [ User-Name ]
                       [ Accounting-Sub-Session-Id ]
                       [ Acct-Session-Id ]
                       [ Acct-Multi-Session-Id ]
                       [ Error-Message ]
                       [ Error-Reporting-Host ]
                       [ Failed-AVP ]
                       [ Acct-Interim-Interval ]
                       [ Accounting-Realtime-Required ]
                       [ Origin-State-Id ]
                       [ Event-Timestamp ]
                     * [ Proxy-Info ]
                     * [ AVP ]

    9.8.  Accounting AVPs

       This section contains AVPs that describe accounting usage information
       related to a specific session.

    9.8.1.  Accounting-Record-Type AVP

       The Accounting-Record-Type AVP (AVP Code 480) is of type Enumerated
       and contains the type of accounting record being sent.  The following
       values are currently defined for the Accounting-Record-Type AVP:

       EVENT_RECORD 1

          An Accounting Event Record is used to indicate that a one-time
          event has occurred (meaning that the start and end of the event
          are simultaneous).  This record contains all information relevant
          to the service, and it is the only record of the service.

    Fajardo, et al.              Standards Track                  [Page 129]

    RFC 6733                 Diameter Base Protocol             October 2012

       START_RECORD 2

          Accounting Start, Interim, and Stop Records are used to indicate
          that a service of a measurable length has been given.  An
          Accounting Start Record is used to initiate an accounting session
          and contains accounting information that is relevant to the
          initiation of the session.

       INTERIM_RECORD 3

          An Interim Accounting Record contains cumulative accounting
          information for an existing accounting session.  Interim
          Accounting Records SHOULD be sent every time a re-authentication
          or re-authorization occurs.  Further, additional interim record
          triggers MAY be defined by application-specific Diameter
          applications.  The selection of whether to use INTERIM_RECORD
          records is done by the Acct-Interim-Interval AVP.

       STOP_RECORD 4

          An Accounting Stop Record is sent to terminate an accounting
          session and contains cumulative accounting information relevant to
          the existing session.

    9.8.2.  Acct-Interim-Interval AVP

       The Acct-Interim-Interval AVP (AVP Code 85) is of type Unsigned32 and
       is sent from the Diameter home authorization server to the Diameter
       client.  The client uses information in this AVP to decide how and
       when to produce accounting records.  With different values in this
       AVP, service sessions can result in one, two, or two+N accounting
       records, based on the needs of the home organization.  The following
       accounting record production behavior is directed by the inclusion of
       this AVP:

       1.  The omission of the Acct-Interim-Interval AVP or its inclusion
           with Value field set to 0 means that EVENT_RECORD, START_RECORD,
           and STOP_RECORD are produced, as appropriate for the service.

       2.  The inclusion of the AVP with Value field set to a non-zero value
           means that INTERIM_RECORD records MUST be produced between the
           START_RECORD and STOP_RECORD records.  The Value field of this
           AVP is the nominal interval between these records in seconds.
           The Diameter node that originates the accounting information,
           known as the client, MUST produce the first INTERIM_RECORD record
           roughly at the time when this nominal interval has elapsed from

    Fajardo, et al.              Standards Track                  [Page 130]

    RFC 6733                 Diameter Base Protocol             October 2012

           the START_RECORD, the next one again as the interval has elapsed
           once more, and so on until the session ends and a STOP_RECORD
           record is produced.

           The client MUST ensure that the interim record production times
           are randomized so that large accounting message storms are not
           created either among records or around a common service start
           time.

    9.8.3.   Accounting-Record-Number AVP

       The Accounting-Record-Number AVP (AVP Code 485) is of type Unsigned32
       and identifies this record within one session.  As Session-Id AVPs
       are globally unique, the combination of Session-Id and Accounting-
       Record-Number AVPs is also globally unique and can be used in
       matching accounting records with confirmations.  An easy way to
       produce unique numbers is to set the value to 0 for records of type
       EVENT_RECORD and START_RECORD and set the value to 1 for the first
       INTERIM_RECORD, 2 for the second, and so on until the value for
       STOP_RECORD is one more than for the last INTERIM_RECORD.

    9.8.4.  Acct-Session-Id AVP

       The Acct-Session-Id AVP (AVP Code 44) is of type OctetString is only
       used when RADIUS/Diameter translation occurs.  This AVP contains the
       contents of the RADIUS Acct-Session-Id attribute.

    9.8.5.  Acct-Multi-Session-Id AVP

       The Acct-Multi-Session-Id AVP (AVP Code 50) is of type UTF8String,
       following the format specified in Section 8.8.  The Acct-Multi-
       Session-Id AVP is used to link multiple related accounting sessions,
       where each session would have a unique Session-Id but the same Acct-
       Multi-Session-Id AVP.  This AVP MAY be returned by the Diameter
       server in an authorization answer, and it MUST be used in all
       accounting messages for the given session.

    9.8.6.  Accounting-Sub-Session-Id AVP

       The Accounting-Sub-Session-Id AVP (AVP Code 287) is of type
       Unsigned64 and contains the accounting sub-session identifier.  The
       combination of the Session-Id and this AVP MUST be unique per sub-
       session, and the value of this AVP MUST be monotonically increased by
       one for all new sub-sessions.  The absence of this AVP implies no
       sub-sessions are in use, with the exception of an Accounting-Request
       whose Accounting-Record-Type is set to STOP_RECORD.  A STOP_RECORD
       message with no Accounting-Sub-Session-Id AVP present will signal the
       termination of all sub-sessions for a given Session-Id.

    Fajardo, et al.              Standards Track                  [Page 131]

    RFC 6733                 Diameter Base Protocol             October 2012

    9.8.7.   Accounting-Realtime-Required AVP

       The Accounting-Realtime-Required AVP (AVP Code 483) is of type
       Enumerated and is sent from the Diameter home authorization server to
       the Diameter client or in the Accounting-Answer from the accounting
       server.  The client uses information in this AVP to decide what to do
       if the sending of accounting records to the accounting server has
       been temporarily prevented due to, for instance, a network problem.

       DELIVER_AND_GRANT 1

          The AVP with Value field set to DELIVER_AND_GRANT means that the
          service MUST only be granted as long as there is a connection to
          an accounting server.  Note that the set of alternative accounting
          servers are treated as one server in this sense.  Having to move
          the accounting record stream to a backup server is not a reason to
          discontinue the service to the user.

       GRANT_AND_STORE 2

          The AVP with Value field set to GRANT_AND_STORE means that service
          SHOULD be granted if there is a connection, or as long as records
          can still be stored as described in Section 9.4.

          This is the default behavior if the AVP isn't included in the
          reply from the authorization server.

       GRANT_AND_LOSE 3

          The AVP with Value field set to GRANT_AND_LOSE means that service
          SHOULD be granted even if the records cannot be delivered or
          stored.

    10.  AVP Occurrence Tables

       The following tables present the AVPs defined in this document and
       specify in which Diameter messages they MAY or MAY NOT be present.
       AVPs that occur only inside a Grouped AVP are not shown in these
       tables.

       The tables use the following symbols:

       0     The AVP MUST NOT be present in the message.

       0+    Zero or more instances of the AVP MAY be present in the
             message.

    Fajardo, et al.              Standards Track                  [Page 132]

    RFC 6733                 Diameter Base Protocol             October 2012

       0-1   Zero or one instance of the AVP MAY be present in the message.
             It is considered an error if there are more than one instance
             of the AVP.

       1     One instance of the AVP MUST be present in the message.

       1+    At least one instance of the AVP MUST be present in the
             message.

    10.1.  Base Protocol Command AVP Table

       The table in this section is limited to the non-Accounting Command
       Codes defined in this specification.

                           +-----------------------------------------------+
                           |                  Command Code                 |
                           +---+---+---+---+---+---+---+---+---+---+---+---+
       Attribute Name      |CER|CEA|DPR|DPA|DWR|DWA|RAR|RAA|ASR|ASA|STR|STA|
       --------------------+---+---+---+---+---+---+---+---+---+---+---+---+
       Acct-Interim-       |0  |0  |0  |0  |0  |0  |0-1|0  |0  |0  |0  |0  |
         Interval          |   |   |   |   |   |   |   |   |   |   |   |   |
       Accounting-Realtime-|0  |0  |0  |0  |0  |0  |0-1|0  |0  |0  |0  |0  |
         Required          |   |   |   |   |   |   |   |   |   |   |   |   |
       Acct-Application-Id |0+ |0+ |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Auth-Application-Id |0+ |0+ |0  |0  |0  |0  |1  |0  |1  |0  |1  |0  |
       Auth-Grace-Period   |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Auth-Request-Type   |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Auth-Session-State  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Authorization-      |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
         Lifetime          |   |   |   |   |   |   |   |   |   |   |   |   |
       Class               |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0+ |0+ |
       Destination-Host    |0  |0  |0  |0  |0  |0  |1  |0  |1  |0  |0-1|0  |
       Destination-Realm   |0  |0  |0  |0  |0  |0  |1  |0  |1  |0  |1  |0  |
       Disconnect-Cause    |0  |0  |1  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Error-Message       |0  |0-1|0  |0-1|0  |0-1|0  |0-1|0  |0-1|0  |0-1|
       Error-Reporting-Host|0  |0  |0  |0  |0  |0  |0  |0-1|0  |0-1|0  |0-1|
       Failed-AVP          |0  |0-1|0  |0-1|0  |0-1|0  |0-1|0  |0-1|0  |0-1|
       Firmware-Revision   |0-1|0-1|0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Host-IP-Address     |1+ |1+ |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Inband-Security-Id  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Multi-Round-Time-Out|0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |

    Fajardo, et al.              Standards Track                  [Page 133]

    RFC 6733                 Diameter Base Protocol             October 2012

       Origin-Host         |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |
       Origin-Realm        |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |1  |
       Origin-State-Id     |0-1|0-1|0  |0  |0-1|0-1|0-1|0-1|0-1|0-1|0-1|0-1|
       Product-Name        |1  |1  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Proxy-Info          |0  |0  |0  |0  |0  |0  |0+ |0+ |0+ |0+ |0+ |0+ |
       Redirect-Host       |0  |0  |0  |0  |0  |0  |0  |0+ |0  |0+ |0  |0+ |
       Redirect-Host-Usage |0  |0  |0  |0  |0  |0  |0  |0-1|0  |0-1|0  |0-1|
       Redirect-Max-Cache- |0  |0  |0  |0  |0  |0  |0  |0-1|0  |0-1|0  |0-1|
         Time              |   |   |   |   |   |   |   |   |   |   |   |   |
       Result-Code         |0  |1  |0  |1  |0  |1  |0  |1  |0  |1  |0  |1  |
       Re-Auth-Request-Type|0  |0  |0  |0  |0  |0  |1  |0  |0  |0  |0  |0  |
       Route-Record        |0  |0  |0  |0  |0  |0  |0+ |0  |0+ |0  |0+ |0  |
       Session-Binding     |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Session-Id          |0  |0  |0  |0  |0  |0  |1  |1  |1  |1  |1  |1  |
       Session-Server-     |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
         Failover          |   |   |   |   |   |   |   |   |   |   |   |   |
       Session-Timeout     |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Supported-Vendor-Id |0+ |0+ |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Termination-Cause   |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |1  |0  |
       User-Name           |0  |0  |0  |0  |0  |0  |0-1|0-1|0-1|0-1|0-1|0-1|
       Vendor-Id           |1  |1  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
       Vendor-Specific-    |0+ |0+ |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
         Application-Id    |   |   |   |   |   |   |   |   |   |   |   |   |
       --------------------+---+---+---+---+---+---+---+---+---+---+---+---+

    10.2.  Accounting AVP Table

       The table in this section is used to represent which AVPs defined in
       this document are to be present in the Accounting messages.  These
       AVP occurrence requirements are guidelines, which may be expanded,
       and/or overridden by application-specific requirements in the
       Diameter applications documents.

    Fajardo, et al.              Standards Track                  [Page 134]

    RFC 6733                 Diameter Base Protocol             October 2012

                                        +-----------+
                                        |  Command  |
                                        |    Code   |
                                        +-----+-----+
          Attribute Name                | ACR | ACA |
          ------------------------------+-----+-----+
          Acct-Interim-Interval         | 0-1 | 0-1 |
          Acct-Multi-Session-Id         | 0-1 | 0-1 |
          Accounting-Record-Number      | 1   | 1   |
          Accounting-Record-Type        | 1   | 1   |
          Acct-Session-Id               | 0-1 | 0-1 |
          Accounting-Sub-Session-Id     | 0-1 | 0-1 |
          Accounting-Realtime-Required  | 0-1 | 0-1 |
          Acct-Application-Id           | 0-1 | 0-1 |
          Auth-Application-Id           | 0   | 0   |
          Class                         | 0+  | 0+  |
          Destination-Host              | 0-1 | 0   |
          Destination-Realm             | 1   | 0   |
          Error-Reporting-Host          | 0   | 0+  |
          Event-Timestamp               | 0-1 | 0-1 |
          Failed-AVP                    | 0   | 0-1 |
          Origin-Host                   | 1   | 1   |
          Origin-Realm                  | 1   | 1   |
          Proxy-Info                    | 0+  | 0+  |
          Route-Record                  | 0+  | 0   |
          Result-Code                   | 0   | 1   |
          Session-Id                    | 1   | 1   |
          Termination-Cause             | 0   | 0   |
          User-Name                     | 0-1 | 0-1 |
          Vendor-Specific-Application-Id| 0-1 | 0-1 |
          ------------------------------+-----+-----+

    11.  IANA Considerations

       This section provides guidance to the Internet Assigned Numbers
       Authority (IANA) regarding registration of values related to the
       Diameter protocol, in accordance with [RFC5226].  Existing IANA
       registries and assignments put in place by RFC 3588 remain the same
       unless explicitly updated or deprecated in this section.

    11.1.  AVP Header

       As defined in Section 4, the AVP header contains three fields that
       require IANA namespace management: the AVP Code, Vendor-ID, and Flags
       fields.

    Fajardo, et al.              Standards Track                  [Page 135]

    RFC 6733                 Diameter Base Protocol             October 2012

    11.1.1.  AVP Codes

       There are multiple namespaces.  Vendors can have their own AVP Codes
       namespace that will be identified by their Vendor-ID (also known as
       Enterprise-Number), and they control the assignments of their vendor-
       specific AVP Codes within their own namespace.  The absence of a
       Vendor-ID or a Vendor-ID value of zero (0) identifies the IETF AVP
       Codes namespace, which is under IANA control.  The AVP Codes and
       sometimes possible values in an AVP are controlled and maintained by
       IANA.  AVP Code 0 is not used.  AVP Codes 1-255 are managed
       separately as RADIUS Attribute Types.  Where a Vendor-Specific AVP is
       implemented by more than one vendor, allocation of global AVPs should
       be encouraged instead.

       AVPs may be allocated following Expert Review (by a Designated
       Expert) with Specification Required [RFC5226].  A block allocation
       (release of more than three AVPs at a time for a given purpose)
       requires IETF Review [RFC5226].

    11.1.2.  AVP Flags

       Section 4.1 describes the existing AVP Flags.  The remaining bits can
       only be assigned via a Standards Action [RFC5226].

    11.2.  Diameter Header

    11.2.1.  Command Codes

       For the Diameter header, the Command Code namespace allocation has
       changed.  The new allocation rules are as follows:

          The Command Code values 256 - 8,388,607 (0x100 to 0x7fffff) are
          for permanent, standard commands, allocated by IETF Review
          [RFC5226].

          The values 8,388,608 - 16,777,213 (0x800000 - 0xfffffd) are
          reserved for vendor-specific Command Codes, to be allocated on a
          First Come, First Served basis by IANA [RFC5226].  The request to
          IANA for a Vendor-Specific Command Code SHOULD include a reference
          to a publicly available specification that documents the command
          in sufficient detail to aid in interoperability between
          independent implementations.  If the specification cannot be made
          publicly available, the request for a vendor-specific Command Code
          MUST include the contact information of persons and/or entities
          responsible for authoring and maintaining the command.

    Fajardo, et al.              Standards Track                  [Page 136]

    RFC 6733                 Diameter Base Protocol             October 2012

          The values 16,777,214 and 16,777,215 (hexadecimal values 0xfffffe
          - 0xffffff) are reserved for experimental commands.  As these
          codes are only for experimental and testing purposes, no guarantee
          is made for interoperability between Diameter peers using
          experimental commands.

    11.2.2.  Command Flags

       Section 3 describes the existing Command Flags field.  The remaining
       bits can only be assigned via a Standards Action [RFC5226].

    11.3.  AVP Values

       For AVP values, the Experimental-Result-Code AVP value allocation has
       been added; see Section 11.3.1.  The old AVP value allocation rule,
       IETF Consensus, has been updated to IETF Review as per [RFC5226], and
       affected AVPs are listed as reminders.

    11.3.1.  Experimental-Result-Code AVP

       Values for this AVP are purely local to the indicated vendor, and no
       IANA registry is maintained for them.

    11.3.2.  Result-Code AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    11.3.3.  Accounting-Record-Type AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    11.3.4.  Termination-Cause AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    11.3.5.  Redirect-Host-Usage AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    11.3.6.  Session-Server-Failover AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    11.3.7.  Session-Binding AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    Fajardo, et al.              Standards Track                  [Page 137]

    RFC 6733                 Diameter Base Protocol             October 2012

    11.3.8.  Disconnect-Cause AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    11.3.9.  Auth-Request-Type AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    11.3.10.  Auth-Session-State AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    11.3.11.  Re-Auth-Request-Type AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    11.3.12.  Accounting-Realtime-Required AVP Values

       New values are available for assignment via IETF Review [RFC5226].

    11.3.13.  Inband-Security-Id AVP (code 299)

       The use of this AVP has been deprecated.

    11.4.  _diameters Service Name and Port Number Registration

       IANA has registered the "_diameters" service name and assigned port
       numbers for TLS/TCP and DTLS/SCTP according to the guidelines given
       in [RFC6335].

          Service Name:         _diameters

          Transport Protocols:  TCP, SCTP

          Assignee:             IESG <iesg@ietf.org>

          Contact:              IETF Chair <chair@ietf.org>

          Description:          Diameter over TLS/TCP and DTLS/SCTP

          Reference:            RFC 6733

          Port  Number:         5868, from the User Range

    Fajardo, et al.              Standards Track                  [Page 138]

    RFC 6733                 Diameter Base Protocol             October 2012

    11.5.  SCTP Payload Protocol Identifiers

       Two SCTP payload protocol identifiers have been registered in the
       SCTP Payload Protocol Identifiers registry:

        Value | SCTP Payload Protocol Identifier
       -------|-----------------------------------
         46   | Diameter in a SCTP DATA chunk
         47   | Diameter in a DTLS/SCTP DATA chunk

    11.6.  S-NAPTR Parameters

       The following tag has been registered in the S-NAPTR Application
       Protocol Tags registry:

       Tag                | Protocol
       -------------------|---------
       diameter.dtls.sctp | DTLS/SCTP

    12.  Diameter Protocol-Related Configurable Parameters

       This section contains the configurable parameters that are found
       throughout this document:

       Diameter Peer

          A Diameter entity MAY communicate with peers that are statically
          configured.  A statically configured Diameter peer would require
          that either the IP address or the fully qualified domain name
          (FQDN) be supplied, which would then be used to resolve through
          DNS.

       Routing Table

          A Diameter proxy server routes messages based on the realm portion
          of a Network Access Identifier (NAI).  The server MUST have a
          table of Realm Names, and the address of the peer to which the
          message must be forwarded.  The routing table MAY also include a
          "default route", which is typically used for all messages that
          cannot be locally processed.

       Tc timer

          The Tc timer controls the frequency that transport connection
          attempts are done to a peer with whom no active transport
          connection exists.  The recommended value is 30 seconds.

    Fajardo, et al.              Standards Track                  [Page 139]

    RFC 6733                 Diameter Base Protocol             October 2012

    13.  Security Considerations

       The Diameter base protocol messages SHOULD be secured by using TLS
       [RFC5246] or DTLS/SCTP [RFC6083].  Additional security mechanisms
       such as IPsec [RFC4301] MAY also be deployed to secure connections
       between peers.  However, all Diameter base protocol implementations
       MUST support the use of TLS/TCP and DTLS/SCTP, and the Diameter
       protocol MUST NOT be used without one of TLS, DTLS, or IPsec.

       If a Diameter connection is to be protected via TLS/TCP and DTLS/SCTP
       or IPsec, then TLS/TCP and DTLS/SCTP or IPsec/IKE SHOULD begin prior
       to any Diameter message exchange.  All security parameters for TLS/
       TCP and DTLS/SCTP or IPsec are configured independent of the Diameter
       protocol.  All Diameter messages will be sent through the TLS/TCP and
       DTLS/SCTP or IPsec connection after a successful setup.

       For TLS/TCP and DTLS/SCTP connections to be established in the open
       state, the CER/CEA exchange MUST include an Inband-Security-ID AVP
       with a value of TLS/TCP and DTLS/SCTP.  The TLS/TCP and DTLS/SCTP
       handshake will begin when both ends successfully reach the open
       state, after completion of the CER/CEA exchange.  If the TLS/TCP and
       DTLS/SCTP handshake is successful, all further messages will be sent
       via TLS/TCP and DTLS/SCTP.  If the handshake fails, both ends MUST
       move to the closed state.  See Section 13.1 for more details.

    13.1.  TLS/TCP and DTLS/SCTP Usage

       Diameter nodes using TLS/TCP and DTLS/SCTP for security MUST mutually
       authenticate as part of TLS/TCP and DTLS/SCTP session establishment.
       In order to ensure mutual authentication, the Diameter node acting as
       the TLS/TCP and DTLS/SCTP server MUST request a certificate from the
       Diameter node acting as TLS/TCP and DTLS/SCTP client, and the
       Diameter node acting as the TLS/TCP and DTLS/SCTP client MUST be
       prepared to supply a certificate on request.

       Diameter nodes MUST be able to negotiate the following TLS/TCP and
       DTLS/SCTP cipher suites:

             TLS_RSA_WITH_RC4_128_MD5
             TLS_RSA_WITH_RC4_128_SHA
             TLS_RSA_WITH_3DES_EDE_CBC_SHA

       Diameter nodes SHOULD be able to negotiate the following TLS/TCP and
       DTLS/SCTP cipher suite:

             TLS_RSA_WITH_AES_128_CBC_SHA

    Fajardo, et al.              Standards Track                  [Page 140]

    RFC 6733                 Diameter Base Protocol             October 2012

       Note that it is quite possible that support for the
       TLS_RSA_WITH_AES_128_CBC_SHA cipher suite will be REQUIRED at some
       future date.  Diameter nodes MAY negotiate other TLS/TCP and DTLS/
       SCTP cipher suites.

       If public key certificates are used for Diameter security (for
       example, with TLS), the value of the expiration times in the routing
       and peer tables MUST NOT be greater than the expiry time in the
       relevant certificates.

    13.2.  Peer-to-Peer Considerations

       As with any peer-to-peer protocol, proper configuration of the trust
       model within a Diameter peer is essential to security.  When
       certificates are used, it is necessary to configure the root
       certificate authorities trusted by the Diameter peer.  These root CAs
       are likely to be unique to Diameter usage and distinct from the root
       CAs that might be trusted for other purposes such as Web browsing.
       In general, it is expected that those root CAs will be configured so
       as to reflect the business relationships between the organization
       hosting the Diameter peer and other organizations.  As a result, a
       Diameter peer will typically not be configured to allow connectivity
       with any arbitrary peer.  With certificate authentication, Diameter
       peers may not be known beforehand and therefore peer discovery may be
       required.

    13.3.  AVP Considerations

       Diameter AVPs often contain security-sensitive data; for example,
       user passwords and location data, network addresses and cryptographic
       keys.  The following AVPs defined in this document are considered to
       be security-sensitive:

       o  Acct-Interim-Interval

       o  Accounting-Realtime-Required

       o  Acct-Multi-Session-Id

       o  Accounting-Record-Number

       o  Accounting-Record-Type

       o  Accounting-Session-Id

       o  Accounting-Sub-Session-Id

       o  Class

    Fajardo, et al.              Standards Track                  [Page 141]

    RFC 6733                 Diameter Base Protocol             October 2012

       o  Session-Id

       o  Session-Binding

       o  Session-Server-Failover

       o  User-Name

       Diameter messages containing these or any other AVPs considered to be
       security-sensitive MUST only be sent protected via mutually
       authenticated TLS or IPsec.  In addition, those messages MUST NOT be
       sent via intermediate nodes unless there is end-to-end security
       between the originator and recipient or the originator has locally
       trusted configuration that indicates that end-to-end security is not
       needed.  For example, end-to-end security may not be required in the
       case where an intermediary node is known to be operated as part of
       the same administrative domain as the endpoints so that an ability to
       successfully compromise the intermediary would imply a high
       probability of being able to compromise the endpoints as well.  Note
       that no end-to-end security mechanism is specified in this document.

    14.  References

    14.1.  Normative References

       [FLOATPOINT]
                  Institute of Electrical and Electronics Engineers, "IEEE
                  Standard for Binary Floating-Point Arithmetic, ANSI/IEEE
                  Standard 754-1985", August 1985.

       [IANAADFAM]
                  IANA, "Address Family Numbers",
                  <http://www.iana.org/assignments/address-family-numbers>.

       [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
                  September 1981.

       [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7,
                  RFC 793, September 1981.

       [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
                  Requirement Levels", BCP 14, RFC 2119, March 1997.

       [RFC3492]  Costello, A., "Punycode: A Bootstring encoding of Unicode
                  for Internationalized Domain Names in Applications
                  (IDNA)", RFC 3492, March 2003.

    Fajardo, et al.              Standards Track                  [Page 142]

    RFC 6733                 Diameter Base Protocol             October 2012

       [RFC3539]  Aboba, B. and J. Wood, "Authentication, Authorization and
                  Accounting (AAA) Transport Profile", RFC 3539, June 2003.

       [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
                  10646", STD 63, RFC 3629, November 2003.

       [RFC3958]  Daigle, L. and A. Newton, "Domain-Based Application
                  Service Location Using SRV RRs and the Dynamic Delegation
                  Discovery Service (DDDS)", RFC 3958, January 2005.

       [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
                  Resource Identifier (URI): Generic Syntax", STD 66,
                  RFC 3986, January 2005.

       [RFC4004]  Calhoun, P., Johansson, T., Perkins, C., Hiller, T., and
                  P. McCann, "Diameter Mobile IPv4 Application", RFC 4004,
                  August 2005.

       [RFC4005]  Calhoun, P., Zorn, G., Spence, D., and D. Mitton,
                  "Diameter Network Access Server Application", RFC 4005,
                  August 2005.

       [RFC4006]  Hakala, H., Mattila, L., Koskinen, J-P., Stura, M., and J.
                  Loughney, "Diameter Credit-Control Application", RFC 4006,
                  August 2005.

       [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
                  Requirements for Security", BCP 106, RFC 4086, June 2005.

       [RFC4282]  Aboba, B., Beadles, M., Arkko, J., and P. Eronen, "The
                  Network Access Identifier", RFC 4282, December 2005.

       [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
                  Architecture", RFC 4291, February 2006.

       [RFC4960]  Stewart, R., "Stream Control Transmission Protocol",
                  RFC 4960, September 2007.

       [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
                  IANA Considerations Section in RFCs", BCP 26, RFC 5226,
                  May 2008.

       [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
                  Specifications: ABNF", STD 68, RFC 5234, January 2008.

       [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
                  (TLS) Protocol Version 1.2", RFC 5246, August 2008.

    Fajardo, et al.              Standards Track                  [Page 143]

    RFC 6733                 Diameter Base Protocol             October 2012

       [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
                  Housley, R., and W. Polk, "Internet X.509 Public Key
                  Infrastructure Certificate and Certificate Revocation List
                  (CRL) Profile", RFC 5280, May 2008.

       [RFC5729]  Korhonen, J., Jones, M., Morand, L., and T. Tsou,
                  "Clarifications on the Routing of Diameter Requests Based
                  on the Username and the Realm", RFC 5729, December 2009.

       [RFC5890]  Klensin, J., "Internationalized Domain Names for
                  Applications (IDNA): Definitions and Document Framework",
                  RFC 5890, August 2010.

       [RFC5891]  Klensin, J., "Internationalized Domain Names in
                  Applications (IDNA): Protocol", RFC 5891, August 2010.

       [RFC6083]  Tuexen, M., Seggelmann, R., and E. Rescorla, "Datagram
                  Transport Layer Security (DTLS) for Stream Control
                  Transmission Protocol (SCTP)", RFC 6083, January 2011.

       [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
                  Security Version 1.2", RFC 6347, January 2012.

       [RFC6408]  Jones, M., Korhonen, J., and L. Morand, "Diameter
                  Straightforward-Naming Authority Pointer (S-NAPTR) Usage",
                  RFC 6408, November 2011.

    14.2.  Informative References

       [ENTERPRISE]  IANA, "SMI Network Management Private Enterprise
                     Codes",
                     <http://www.iana.org/assignments/enterprise-numbers>.

       [IANATCV]     IANA, "Termination-Cause AVP Values (code 295)",
                     <http://www.iana.org/assignments/aaa-parameters/
                     aaa-parameters.xml#aaa-parameters-16>.

       [RFC1492]     Finseth, C., "An Access Control Protocol, Sometimes
                     Called TACACS", RFC 1492, July 1993.

       [RFC1661]     Simpson, W., "The Point-to-Point Protocol (PPP)",
                     STD 51, RFC 1661, July 1994.

       [RFC2104]     Krawczyk, H., Bellare, M., and R. Canetti, "HMAC:
                     Keyed-Hashing for Message Authentication", RFC 2104,
                     February 1997.

    Fajardo, et al.              Standards Track                  [Page 144]

    RFC 6733                 Diameter Base Protocol             October 2012

       [RFC2782]     Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR
                     for specifying the location of services (DNS SRV)",
                     RFC 2782, February 2000.

       [RFC2865]     Rigney, C., Willens, S., Rubens, A., and W. Simpson,
                     "Remote Authentication Dial In User Service (RADIUS)",
                     RFC 2865, June 2000.

       [RFC2866]     Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

       [RFC2869]     Rigney, C., Willats, W., and P. Calhoun, "RADIUS
                     Extensions", RFC 2869, June 2000.

       [RFC2881]     Mitton, D. and M. Beadles, "Network Access Server
                     Requirements Next Generation (NASREQNG) NAS Model",
                     RFC 2881, July 2000.

       [RFC2975]     Aboba, B., Arkko, J., and D. Harrington, "Introduction
                     to Accounting Management", RFC 2975, October 2000.

       [RFC2989]     Aboba, B., Calhoun, P., Glass, S., Hiller, T., McCann,
                     P., Shiino, H., Walsh, P., Zorn, G., Dommety, G.,
                     Perkins, C., Patil, B., Mitton, D., Manning, S.,
                     Beadles, M., Chen, X., Sivalingham, S., Hameed, A.,
                     Munson, M., Jacobs, S., Lim, B., Hirschman, B., Hsu,
                     R., Koo, H., Lipford, M., Campbell, E., Xu, Y., Baba,
                     S., and E. Jaques, "Criteria for Evaluating AAA
                     Protocols for Network Access", RFC 2989, November 2000.

       [RFC3162]     Aboba, B., Zorn, G., and D. Mitton, "RADIUS and IPv6",
                     RFC 3162, August 2001.

       [RFC3748]     Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and
                     H. Levkowetz, "Extensible Authentication Protocol
                     (EAP)", RFC 3748, June 2004.

       [RFC4301]     Kent, S. and K. Seo, "Security Architecture for the
                     Internet Protocol", RFC 4301, December 2005.

       [RFC4690]     Klensin, J., Faltstrom, P., Karp, C., and IAB, "Review
                     and Recommendations for Internationalized Domain Names
                     (IDNs)", RFC 4690, September 2006.

       [RFC5176]     Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B.
                     Aboba, "Dynamic Authorization Extensions to Remote
                     Authentication Dial In User Service (RADIUS)",
                     RFC 5176, January 2008.

    Fajardo, et al.              Standards Track                  [Page 145]

    RFC 6733                 Diameter Base Protocol             October 2012

       [RFC5461]     Gont, F., "TCP's Reaction to Soft Errors", RFC 5461,
                     February 2009.

       [RFC5905]     Mills, D., Martin, J., Burbank, J., and W. Kasch,
                     "Network Time Protocol Version 4: Protocol and
                     Algorithms Specification", RFC 5905, June 2010.

       [RFC5927]     Gont, F., "ICMP Attacks against TCP", RFC 5927,
                     July 2010.

       [RFC6335]     Cotton, M., Eggert, L., Touch, J., Westerlund, M., and
                     S. Cheshire, "Internet Assigned Numbers Authority
                     (IANA) Procedures for the Management of the Service
                     Name and Transport Protocol Port Number Registry",
                     BCP 165, RFC 6335, August 2011.

       [RFC6737]     Kang, J. and G. Zorn, "The Diameter Capabilities Update
                     Application", RFC 6737, October 2012.

    Fajardo, et al.              Standards Track                  [Page 146]

    RFC 6733                 Diameter Base Protocol             October 2012

    Appendix A.  Acknowledgements

    A.1.  This Document

       The authors would like to thank the following people that have
       provided proposals and contributions to this document:

       To Vishnu Ram and Satendra Gera for their contributions on
       capabilities updates, predictive loop avoidance, as well as many
       other technical proposals.  To Tolga Asveren for his insights and
       contributions on almost all of the proposed solutions incorporated
       into this document.  To Timothy Smith for helping on the capabilities
       Update and other topics.  To Tony Zhang for providing fixes to
       loopholes on composing Failed-AVPs as well as many other issues and
       topics.  To Jan Nordqvist for clearly stating the usage of
       Application Ids.  To Anders Kristensen for providing needed technical
       opinions.  To David Frascone for providing invaluable review of the
       document.  To Mark Jones for providing clarifying text on vendor
       command codes and other vendor-specific indicators.  To Victor
       Pascual and Sebastien Decugis for new text and recommendations on
       SCTP/DTLS.  To Jouni Korhonen for taking over the editing task and
       resolving last bits from versions 27 through 29.

       Special thanks to the Diameter extensibility design team, which
       helped resolve the tricky question of mandatory AVPs and ABNF
       semantics.  The members of this team are as follows:

       Avi Lior, Jari Arkko, Glen Zorn, Lionel Morand, Mark Jones, Tolga
       Asveren, Jouni Korhonen, and Glenn McGregor.

       Special thanks also to people who have provided invaluable comments
       and inputs especially in resolving controversial issues:

       Glen Zorn, Yoshihiro Ohba, Marco Stura, Stephen Farrel, Pete Resnick,
       Peter Saint-Andre, Robert Sparks, Krishna Prasad, Sean Turner, Barry
       Leiba, and Pasi Eronen.

       Finally, we would like to thank the original authors of this
       document:

       Pat Calhoun, John Loughney, Jari Arkko, Erik Guttman, and Glen Zorn.

       Their invaluable knowledge and experience has given us a robust and
       flexible AAA protocol that many people have seen great value in
       adopting.  We greatly appreciate their support and stewardship for
       the continued improvements of Diameter as a protocol.  We would also
       like to extend our gratitude to folks aside from the authors who have

    Fajardo, et al.              Standards Track                  [Page 147]

    RFC 6733                 Diameter Base Protocol             October 2012

       assisted and contributed to the original version of this document.
       Their efforts significantly contributed to the success of Diameter.

    A.2.  RFC 3588

       The authors would like to thank Nenad Trifunovic, Tony Johansson and
       Pankaj Patel for their participation in the pre-IETF Document Reading
       Party.  Allison Mankin, Jonathan Wood, and Bernard Aboba provided
       invaluable assistance in working out transport issues and this was
       also the case with Steven Bellovin in the security area.

       Paul Funk and David Mitton were instrumental in getting the Peer
       State Machine correct, and our deep thanks go to them for their time.

       Text in this document was also provided by Paul Funk, Mark Eklund,
       Mark Jones, and Dave Spence.  Jacques Caron provided many great
       comments as a result of a thorough review of the spec.

       The authors would also like to acknowledge the following people for
       their contribution in the development of the Diameter protocol:

       Allan C. Rubens, Haseeb Akhtar, William Bulley, Stephen Farrell,
       David Frascone, Daniel C. Fox, Lol Grant, Ignacio Goyret, Nancy
       Greene, Peter Heitman, Fredrik Johansson, Mark Jones, Martin Julien,
       Bob Kopacz, Paul Krumviede, Fergal Ladley, Ryan Moats, Victor Muslin,
       Kenneth Peirce, John Schnizlein, Sumit Vakil, John R. Vollbrecht, and
       Jeff Weisberg.

       Finally, Pat Calhoun would like to thank Sun Microsystems since most
       of the effort put into this document was done while he was in their
       employ.

    Appendix B.  S-NAPTR Example

       As an example, consider a client that wishes to resolve aaa:
       ex1.example.com.  The client performs a NAPTR query for that domain,
       and the following NAPTR records are returned:

        ;;        order pref flags service   regexp replacement
        IN NAPTR  50    50   "s"   "aaa:diameter.tls.tcp" ""
                     _diameter._tls.ex1.example.com
        IN NAPTR  100   50   "s"   "aaa:diameter.tcp"     ""
                     _aaa._tcp.ex1.example.com
        IN NAPTR  150   50   "s"   "aaa:diameter.sctp"    ""
                     _diameter._sctp.ex1.example.com

       This indicates that the server supports TLS, TCP, and SCTP in that
       order.  If the client supports TLS, TLS will be used, targeted to a

    Fajardo, et al.              Standards Track                  [Page 148]

    RFC 6733                 Diameter Base Protocol             October 2012

       host determined by an SRV lookup of _diameter._tls.ex1.example.com.
       That lookup would return:

        ;;       Priority  Weight  Port    Target
        IN SRV   0         1       5060    server1.ex1.example.com
        IN SRV   0         2       5060    server2.ex1.example.com

       As an alternative example, a client that wishes to resolve aaa:
       ex2.example.com.  The client performs a NAPTR query for that domain,
       and the following NAPTR records are returned:

        ;;        order pref flags service   regexp replacement
        IN NAPTR  150   50   "a"   "aaa:diameter.tls.tcp"  ""
                     server1.ex2.example.com
        IN NAPTR  150   50   "a"   "aaa:diameter.tls.tcp"  ""
                     server2.ex2.example.com

       This indicates that the server supports TCP available at the returned
       host names.

    Appendix C.  Duplicate Detection

       As described in Section 9.4, accounting record duplicate detection is
       based on session identifiers.  Duplicates can appear for various
       reasons:

       o  Failover to an alternate server.  Where close to real-time
          performance is required, failover thresholds need to be kept low.
          This may lead to an increased likelihood of duplicates.  Failover
          can occur at the client or within Diameter agents.

       o  Failure of a client or agent after sending a record from non-
          volatile memory, but prior to receipt of an application-layer ACK
          and deletion of the record to be sent.  This will result in
          retransmission of the record soon after the client or agent has
          rebooted.

       o  Duplicates received from RADIUS gateways.  Since the
          retransmission behavior of RADIUS is not defined within [RFC2865],
          the likelihood of duplication will vary according to the
          implementation.

       o  Implementation problems and misconfiguration.

       The T flag is used as an indication of an application-layer
       retransmission event, e.g., due to failover to an alternate server.
       It is defined only for request messages sent by Diameter clients or
       agents.  For instance, after a reboot, a client may not know whether

    Fajardo, et al.              Standards Track                  [Page 149]

    RFC 6733                 Diameter Base Protocol             October 2012

       it has already tried to send the accounting records in its non-
       volatile memory before the reboot occurred.  Diameter servers MAY use
       the T flag as an aid when processing requests and detecting duplicate
       messages.  However, servers that do this MUST ensure that duplicates
       are found even when the first transmitted request arrives at the
       server after the retransmitted request.  It can be used only in cases
       where no answer has been received from the server for a request and
       the request is sent again, (e.g., due to a failover to an alternate
       peer, due to a recovered primary peer or due to a client re-sending a
       stored record from non-volatile memory such as after reboot of a
       client or agent).

       In some cases, the Diameter accounting server can delay the duplicate
       detection and accounting record processing until a post-processing
       phase takes place.  At that time records are likely to be sorted
       according to the included User-Name and duplicate elimination is easy
       in this case.  In other situations, it may be necessary to perform
       real-time duplicate detection, such as when credit limits are imposed
       or real-time fraud detection is desired.

       In general, only generation of duplicates due to failover or re-
       sending of records in non-volatile storage can be reliably detected
       by Diameter clients or agents.  In such cases, the Diameter client or
       agents can mark the message as a possible duplicate by setting the T
       flag.  Since the Diameter server is responsible for duplicate
       detection, it can choose whether or not to make use of the T flag, in
       order to optimize duplicate detection.  Since the T flag does not
       affect interoperability, and it may not be needed by some servers,
       generation of the T flag is REQUIRED for Diameter clients and agents,
       but it MAY be implemented by Diameter servers.

       As an example, it can be usually be assumed that duplicates appear
       within a time window of longest recorded network partition or device
       fault, perhaps a day.  So only records within this time window need
       to be looked at in the backward direction.  Secondly, hashing
       techniques or other schemes, such as the use of the T flag in the
       received messages, may be used to eliminate the need to do a full
       search even in this set except for rare cases.

       The following is an example of how the T flag may be used by the
       server to detect duplicate requests.

          A Diameter server MAY check the T flag of the received message to
          determine if the record is a possible duplicate.  If the T flag is
          set in the request message, the server searches for a duplicate
          within a configurable duplication time window backward and
          forward.  This limits database searching to those records where
          the T flag is set.  In a well-run network, network partitions and

    Fajardo, et al.              Standards Track                  [Page 150]

    RFC 6733                 Diameter Base Protocol             October 2012

          device faults will presumably be rare events, so this approach
          represents a substantial optimization of the duplicate detection
          process.  During failover, it is possible for the original record
          to be received after the T-flag-marked record, due to differences
          in network delays experienced along the path by the original and
          duplicate transmissions.  The likelihood of this occurring
          increases as the failover interval is decreased.  In order to be
          able to detect duplicates that are out of order, the Diameter
          server should use backward and forward time windows when
          performing duplicate checking for the T-flag-marked request.  For
          example, in order to allow time for the original record to exit
          the network and be recorded by the accounting server, the Diameter
          server can delay processing records with the T flag set until a
          time period TIME_WAIT + RECORD_PROCESSING_TIME has elapsed after
          the closing of the original transport connection.  After this time
          period, it may check the T-flag-marked records against the
          database with relative assurance that the original records, if
          sent, have been received and recorded.

    Appendix D.  Internationalized Domain Names

       To be compatible with the existing DNS infrastructure and simplify
       host and domain name comparison, Diameter identities (FQDNs) are
       represented in ASCII form.  This allows the Diameter protocol to fall
       in-line with the DNS strategy of being transparent from the effects
       of Internationalized Domain Names (IDNs) by following the
       recommendations in [RFC4690] and [RFC5890].  Applications that
       provide support for IDNs outside of the Diameter protocol but
       interacting with it SHOULD use the representation and conversion
       framework described in [RFC5890], [RFC5891], and [RFC3492].

### 4.2 RFC 3539

RFC 3539 is more difficult to comply to since it discusses problems as much as it requires functionality but all the MUST's are covered, the watchdog state machine being the primary one. Of the optional functionality, load balancing is left to the diameter user (since it's the one deciding who to send to) and there is no Congestion Manager.