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rfc:rfc3588

Network Working Group P. Calhoun Request for Comments: 3588 Airespace, Inc. Category: Standards Track J. Loughney

                                                                 Nokia
                                                            E. Guttman
                                                Sun Microsystems, Inc.
                                                               G. Zorn
                                                   Cisco Systems, Inc.
                                                              J. Arkko
                                                              Ericsson
                                                        September 2003
                       Diameter Base Protocol

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 The Diameter base protocol is intended to provide an Authentication,
 Authorization and Accounting (AAA) framework for applications such as
 network access or IP mobility.  Diameter is also intended to work in
 both local Authentication, Authorization & Accounting and roaming
 situations.  This document specifies the message format, transport,
 error reporting, accounting and security services to be used by all
 Diameter applications.  The Diameter base application needs to be
 supported by all Diameter implementations.

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 BCP 14, RFC 2119
 [KEYWORD].

Calhoun, et al. Standards Track [Page 1] RFC 3588 Diameter Based Protocol September 2003

Table of Contents

 1.  Introduction.................................................   6
     1.1.   Diameter Protocol.....................................   9
            1.1.1.   Description of the Document Set..............  10
     1.2.   Approach to Extensibility.............................  11
            1.2.1.   Defining New AVP Values......................  11
            1.2.2.   Creating New AVPs............................  11
            1.2.3.   Creating New Authentication Applications.....  11
            1.2.4.   Creating New Accounting Applications.........  12
            1.2.5.   Application Authentication Procedures........  14
     1.3.   Terminology...........................................  14
 2.  Protocol Overview............................................  18
     2.1.   Transport.............................................  20
            2.1.1.   SCTP Guidelines..............................  21
     2.2.   Securing Diameter Messages............................  21
     2.3.   Diameter Application Compliance.......................  21
     2.4.   Application Identifiers...............................  22
     2.5.   Connections vs. Sessions..............................  22
     2.6.   Peer Table............................................  23
     2.7.   Realm-Based Routing Table.............................  24
     2.8.   Role of Diameter Agents...............................  25
            2.8.1.   Relay Agents.................................  26
            2.8.2.   Proxy Agents.................................  27
            2.8.3.   Redirect Agents..............................  28
            2.8.4.   Translation Agents...........................  29
     2.9.   End-to-End Security Framework.........................  30
     2.10.  Diameter Path Authorization...........................  30
 3.  Diameter Header..............................................  32
     3.1.   Command Codes.........................................  35
     3.2.   Command Code ABNF specification.......................  36
     3.3.   Diameter Command Naming Conventions...................  38
 4.  Diameter AVPs................................................  38
     4.1.   AVP Header............................................  39
            4.1.1.   Optional Header Elements.....................  41
     4.2.   Basic AVP Data Formats................................  41
     4.3.   Derived AVP Data Formats..............................  42
     4.4.   Grouped AVP Values....................................  49
            4.4.1.   Example AVP with a Grouped Data Type.........  50
     4.5.   Diameter Base Protocol AVPs...........................  53
 5.  Diameter Peers...............................................  56
     5.1.   Peer Connections......................................  56
     5.2.   Diameter Peer Discovery...............................  56
     5.3.   Capabilities Exchange.................................  59
            5.3.1.   Capabilities-Exchange-Request................  60
            5.3.2.   Capabilities-Exchange-Answer.................  60
            5.3.3.   Vendor-Id AVP................................  61
            5.3.4.   Firmware-Revision AVP........................  61

Calhoun, et al. Standards Track [Page 2] RFC 3588 Diameter Based Protocol September 2003

            5.3.5.   Host-IP-Address AVP..........................  62
            5.3.6.   Supported-Vendor-Id AVP......................  62
            5.3.7.   Product-Name AVP.............................  62
     5.4.   Disconnecting Peer Connections........................  62
            5.4.1.   Disconnect-Peer-Request......................  63
            5.4.2.   Disconnect-Peer-Answer.......................  63
            5.4.3.   Disconnect-Cause AVP.........................  63
     5.5.   Transport Failure Detection...........................  64
            5.5.1.   Device-Watchdog-Request......................  64
            5.5.2.   Device-Watchdog-Answer.......................  64
            5.5.3.   Transport Failure Algorithm..................  65
            5.5.4.   Failover and Failback Procedures.............  65
     5.6.   Peer State Machine....................................  66
            5.6.1.   Incoming connections.........................  68
            5.6.2.   Events.......................................  69
            5.6.3.   Actions......................................  70
            5.6.4.   The Election Process.........................  71
 6.  Diameter Message Processing..................................  71
     6.1.   Diameter Request Routing Overview.....................  71
            6.1.1.   Originating a Request........................  73
            6.1.2.   Sending a Request............................  73
            6.1.3.   Receiving Requests...........................  73
            6.1.4.   Processing Local Requests....................  73
            6.1.5.   Request Forwarding...........................  74
            6.1.6.   Request Routing..............................  74
            6.1.7.   Redirecting Requests.........................  74
            6.1.8.   Relaying and Proxying Requests...............  75
     6.2.   Diameter Answer Processing............................  76
            6.2.1.   Processing Received Answers..................  77
            6.2.2.   Relaying and Proxying Answers................  77
     6.3.   Origin-Host AVP.......................................  77
     6.4.   Origin-Realm AVP......................................  78
     6.5.   Destination-Host AVP..................................  78
     6.6.   Destination-Realm AVP.................................  78
     6.7.   Routing AVPs..........................................  78
            6.7.1.   Route-Record AVP.............................  79
            6.7.2.   Proxy-Info AVP...............................  79
            6.7.3.   Proxy-Host AVP...............................  79
            6.7.4.   Proxy-State AVP..............................  79
     6.8.   Auth-Application-Id AVP...............................  79
     6.9.   Acct-Application-Id AVP...............................  79
     6.10.  Inband-Security-Id AVP................................  79
     6.11.  Vendor-Specific-Application-Id AVP....................  80
     6.12.  Redirect-Host AVP.....................................  80
     6.13.  Redirect-Host-Usage AVP...............................  80
     6.14.  Redirect-Max-Cache-Time AVP...........................  81
     6.15.  E2E-Sequence AVP......................................  82

Calhoun, et al. Standards Track [Page 3] RFC 3588 Diameter Based Protocol September 2003

 7.  Error Handling...............................................  82
     7.1.   Result-Code AVP.......................................  84
            7.1.1.   Informational................................  84
            7.1.2.   Success......................................  84
            7.1.3.   Protocol Errors..............................  85
            7.1.4.   Transient Failures...........................  86
            7.1.5.   Permanent Failures...........................  86
     7.2.   Error Bit.............................................  88
     7.3.   Error-Message AVP.....................................  89
     7.4.   Error-Reporting-Host AVP..............................  89
     7.5.   Failed-AVP AVP........................................  89
     7.6.   Experimental-Result AVP...............................  90
     7.7.   Experimental-Result-Code AVP..........................  90
 8.  Diameter User Sessions.......................................  90
     8.1.   Authorization Session State Machine...................  92
     8.2.   Accounting Session State Machine......................  96
     8.3.   Server-Initiated Re-Auth.............................. 101
            8.3.1.   Re-Auth-Request.............................. 102
            8.3.2.   Re-Auth-Answer............................... 102
     8.4.   Session Termination................................... 103
            8.4.1.   Session-Termination-Request.................. 104
            8.4.2.   Session-Termination-Answer................... 105
     8.5.   Aborting a Session.................................... 105
            8.5.1.   Abort-Session-Request........................ 106
            8.5.2.   Abort-Session-Answer......................... 106
     8.6.   Inferring Session Termination from Origin-State-Id.... 107
     8.7.   Auth-Request-Type AVP................................. 108
     8.8.   Session-Id AVP........................................ 108
     8.9.   Authorization-Lifetime AVP............................ 109
     8.10.  Auth-Grace-Period AVP................................. 110
     8.11.  Auth-Session-State AVP................................ 110
     8.12.  Re-Auth-Request-Type AVP.............................. 110
     8.13.  Session-Timeout AVP................................... 111
     8.14.  User-Name AVP......................................... 111
     8.15.  Termination-Cause AVP................................. 111
     8.16.  Origin-State-Id AVP................................... 112
     8.17.  Session-Binding AVP................................... 113
     8.18.  Session-Server-Failover AVP........................... 113
     8.19.  Multi-Round-Time-Out AVP.............................. 114
     8.20.  Class AVP............................................. 114
     8.21.  Event-Timestamp AVP................................... 115
 9.  Accounting................................................... 115
     9.1.   Server Directed Model................................. 115
     9.2.   Protocol Messages..................................... 116
     9.3.   Application Document Requirements..................... 116
     9.4.   Fault Resilience...................................... 116
     9.5.   Accounting Records.................................... 117
     9.6.   Correlation of Accounting Records..................... 118

Calhoun, et al. Standards Track [Page 4] RFC 3588 Diameter Based Protocol September 2003

     9.7.   Accounting Command-Codes.............................. 119
            9.7.1.   Accounting-Request........................... 119
            9.7.2.   Accounting-Answer............................ 120
     9.8.   Accounting AVPs....................................... 121
            9.8.1.   Accounting-Record-Type AVP................... 121
            9.8.2.   Acct-Interim-Interval AVP.................... 122
            9.8.3.   Accounting-Record-Number AVP................. 123
            9.8.4.   Acct-Session-Id AVP.......................... 123
            9.8.5.   Acct-Multi-Session-Id AVP.................... 123
            9.8.6.   Accounting-Sub-Session-Id AVP................ 123
            9.8.7.   Accounting-Realtime-Required AVP............. 123
 10. AVP Occurrence Table......................................... 124
     10.1.  Base Protocol Command AVP Table....................... 124
     10.2.  Accounting AVP Table.................................. 126
 11. IANA Considerations.......................................... 127
     11.1.  AVP Header............................................ 127
            11.1.1.  AVP Code..................................... 127
            11.1.2.  AVP Flags.................................... 128
     11.2.  Diameter Header....................................... 128
            11.2.1.  Command Codes................................ 128
            11.2.2.  Command Flags................................ 129
     11.3.  Application Identifiers............................... 129
     11.4.  AVP Values............................................ 129
            11.4.1.  Result-Code AVP Values....................... 129
            11.4.2.  Accounting-Record-Type AVP Values............ 130
            11.4.3.  Termination-Cause AVP Values................. 130
            11.4.4.  Redirect-Host-Usage AVP Values............... 130
            11.4.5.  Session-Server-Failover AVP Values........... 130
            11.4.6.  Session-Binding AVP Values................... 130
            11.4.7.  Disconnect-Cause AVP Values.................. 130
            11.4.8.  Auth-Request-Type AVP Values................. 130
            11.4.9.  Auth-Session-State AVP Values................ 130
            11.4.10. Re-Auth-Request-Type AVP Values.............. 131
            11.4.11. Accounting-Realtime-Required AVP Values...... 131
     11.5.  Diameter TCP/SCTP Port Numbers........................ 131
     11.6.  NAPTR Service Fields.................................. 131
 12. Diameter Protocol Related Configurable Parameters............ 131
 13. Security Considerations...................................... 132
     13.1.  IPsec Usage........................................... 133
     13.2.  TLS Usage............................................. 134
     13.3.  Peer-to-Peer Considerations........................... 134
 14. References................................................... 136
     14.1.  Normative References.................................. 136
     14.2.  Informative References................................ 138
 15. Acknowledgements............................................. 140
 Appendix A.  Diameter Service Template........................... 141
 Appendix B.  NAPTR Example....................................... 142
 Appendix C.  Duplicate Detection................................. 143

Calhoun, et al. Standards Track [Page 5] RFC 3588 Diameter Based Protocol September 2003

 Appendix D.  Intellectual Property Statement..................... 145
 Authors' Addresses............................................... 146
 Full Copyright Statement......................................... 147

1. Introduction

 Authentication, Authorization and Accounting (AAA) protocols such as
 TACACS [TACACS] and RADIUS [RADIUS] were initially deployed to
 provide dial-up PPP [PPP] and terminal server access.  Over time,
 with the growth of the Internet and the introduction of new access
 technologies, including wireless, DSL, Mobile IP and Ethernet,
 routers and network access servers (NAS) have increased in complexity
 and density, putting new demands on AAA protocols.
 Network access requirements for AAA protocols are summarized in
 [AAAREQ].  These include:
 Failover
    [RADIUS] 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.  This is described in
    Section 5.5 and [AAATRANS].
 Transmission-level security
    [RADIUS] defines an application-layer authentication and integrity
    scheme that is required only for use with Response packets.  While
    [RADEXT] defines an additional authentication and integrity
    mechanism, use is only required during Extensible Authentication
    Protocol (EAP) sessions.  While attribute-hiding is supported,
    [RADIUS] does not provide support for per-packet confidentiality.
    In accounting, [RADACCT] 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.  Since within [IKE] authentication occurs
    only within Phase 1 prior to the establishment of IPsec SAs in
    Phase 2, it is typically not possible to define separate trust or
    authorization schemes for each application.  This limits the
    usefulness of IPsec in inter-domain AAA applications (such as
    roaming) where it may be desirable to define a distinct
    certificate hierarchy for use in a AAA deployment.  In order to
    provide universal support for transmission-level security, and
    enable both intra- and inter-domain AAA deployments, IPsec support
    is mandatory in Diameter, and TLS support is optional.  Security
    is discussed in Section 13.

Calhoun, et al. Standards Track [Page 6] RFC 3588 Diameter Based Protocol September 2003

 Reliable transport
    RADIUS runs over UDP, and does not define retransmission behavior;
    as a result, reliability varies between implementations.  As
    described in [ACCMGMT], this is a major issue in accounting, where
    packet loss may translate directly into revenue loss.  In order to
    provide well defined transport behavior, Diameter runs over
    reliable transport mechanisms (TCP, SCTP) as defined in
    [AAATRANS].
 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.
 Server-initiated messages
    While RADIUS server-initiated messages are defined in [DYNAUTH],
    support is optional.  This makes it difficult to implement
    features such as unsolicited disconnect or
    reauthentication/reauthorization on demand across a heterogeneous
    deployment.  Support for server-initiated messages is mandatory in
    Diameter, and is described in Section 8.
 Auditability
    RADIUS does not define data-object security mechanisms, and as a
    result, untrusted proxies may modify attributes or even packet
    headers without being detected.  Combined with lack of support for
    capabilities negotiation, this makes it very difficult to
    determine what occurred in the event of a dispute.  While
    implementation of data object security is not mandatory within
    Diameter, these capabilities are supported, and are described in
    [AAACMS].
 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, described in
    [NASREQ], enables Diameter support to be added to legacy networks,
    by addition of a gateway or server speaking both RADIUS and
    Diameter.

Calhoun, et al. Standards Track [Page 7] RFC 3588 Diameter Based Protocol September 2003

 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).
 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 [RADIUS].  Through DNS, Diameter
    enables dynamic discovery of peers.  Derivation of dynamic session
    keys is enabled via transmission-level security.
 Roaming support
    The ROAMOPS WG provided a survey of roaming implementations
    [ROAMREV], detailed roaming requirements [ROAMCRIT], defined the
    Network Access Identifier (NAI) [NAI], and documented existing
    implementations (and imitations) of RADIUS-based roaming
    [PROXYCHAIN].  In order to improve scalability, [PROXYCHAIN]
    introduced the concept of proxy chaining via an intermediate
    server, facilitating roaming between providers.  However, since
    RADIUS does not provide explicit support for proxies, and lacks
    auditability and transmission-level security features, RADIUS-
    based roaming is vulnerable to attack from external parties as
    well as susceptible to fraud perpetrated by the roaming partners
    themselves.  As a result, it is not suitable for wide-scale
    deployment on the Internet [PROXYCHAIN].  By providing explicit
    support for inter-domain roaming and message routing (Sections 2.7
    and 6), auditability [AAACMS], and transmission-layer security
    (Section 13) features, Diameter addresses these limitations and
    provides for secure and scalable roaming.
 In the decade since AAA protocols were first introduced, 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

Calhoun, et al. Standards Track [Page 8] RFC 3588 Diameter Based Protocol September 2003

 within embedded devices, given improvements in processor speeds and
 the widespread availability of embedded IPsec and TLS
 implementations.

1.1. Diameter Protocol

 The Diameter base protocol provides the following facilities:
  1. Delivery of AVPs (attribute value pairs)
  2. Capabilities negotiation
  3. Error notification
  4. Extensibility, through addition of new commands and AVPs (required

in [AAAREQ]).

  1. Basic services necessary for applications, such as handling of

user sessions or accounting

 All data delivered by the protocol is in the form of an AVP.  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 added arbitrarily to Diameter messages,
 so long as the required AVPs are included and AVPs that are
 explicitly excluded are not included.  AVPs are used by the base
 Diameter protocol to support the following required features:
  1. Transporting of user authentication information, for the purposes

of enabling the Diameter server to authenticate the user.

  1. Transporting of service specific authorization information,

between client and servers, allowing the peers to decide whether a

    user's access request should be granted.
  1. Exchanging resource usage information, which MAY be used for

accounting purposes, capacity planning, etc.

  1. Relaying, proxying and redirecting of Diameter messages through a

server hierarchy.

 The Diameter base protocol provides the minimum requirements needed
 for a AAA protocol, as required by [AAAREQ].  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 [DIAMMIP], or
 network access [NASREQ].  It is also possible for the base protocol
 to be extended for use in new applications, via the addition of new
 commands or AVPs.  At this time the focus of Diameter is 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.

Calhoun, et al. Standards Track [Page 9] RFC 3588 Diameter Based Protocol September 2003

 See Section 2.4 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 authenticate and/or authorize messages
 locally; 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.

1.1.1. Description of the Document Set

 Currently, the Diameter specification consists of a base
 specification (this document), Transport Profile [AAATRANS] and
 applications: Mobile IPv4 [DIAMMIP], and NASREQ [NASREQ].
 The Transport Profile document [AAATRANS] 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.
 The Mobile IPv4 [DIAMMIP] application defines a Diameter application
 that allows a Diameter server to perform AAA functions for Mobile
 IPv4 services to a mobile node.
 The NASREQ [NASREQ] application defines a Diameter Application that
 allows a Diameter server to be used in a PPP/SLIP Dial-Up and
 Terminal Server Access environment.  Consideration was given for
 servers that need to perform protocol conversion between Diameter and
 RADIUS.
 In summary, this document defines the base protocol specification for
 AAA, which includes support for accounting.  The Mobile IPv4 and the
 NASREQ  documents describe applications that use this base
 specification for Authentication, Authorization and Accounting.

Calhoun, et al. Standards Track [Page 10] RFC 3588 Diameter Based Protocol September 2003

1.2. Approach to Extensibility

 The Diameter protocol is designed to be extensible, using several
 mechanisms, including:
  1. Defining new AVP values
  2. Creating new AVPs
  3. Creating new authentication/authorization applications
  4. Creating new accounting applications
  5. Application authentication procedures
 Reuse of existing AVP values, AVPs and Diameter applications are
 strongly recommended.  Reuse simplifies standardization and
 implementation and avoids potential interoperability issues.  It is
 expected that command codes are reused; new command codes can only be
 created by IETF Consensus (see Section 11.2.1).

1.2.1. Defining New AVP Values

 New applications should attempt to reuse AVPs defined in existing
 applications when possible, as opposed to creating new AVPs.  For
 AVPs of type Enumerated, an application may require a new value to
 communicate some service-specific information.
 In order to allocate a new AVP value, a request MUST be sent to IANA
 [IANA], along with an explanation of the new AVP value.  IANA
 considerations for Diameter are discussed in Section 11.

1.2.2. Creating New AVPs

 When no existing AVP can be used, a new AVP should be created.  The
 new AVP being defined MUST use one of the data types listed in
 Section 4.2.
 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).
 In order to create a new AVP, a request MUST be sent to IANA, with a
 specification for the AVP.  The request MUST include the commands
 that would make use of the AVP.

1.2.3. Creating New Authentication Applications

 Every Diameter application specification MUST have an IANA assigned
 Application Identifier (see Section 2.4) or a vendor specific
 Application Identifier.

Calhoun, et al. Standards Track [Page 11] RFC 3588 Diameter Based Protocol September 2003

 Should a new Diameter usage scenario find itself unable to fit within
 an existing application without requiring major changes to the
 specification, it may be desirable to create a new Diameter
 application.  Major changes to an application include:
  1. Adding new AVPs to the command, which have the "M" bit set.
  1. Requiring a command that has a different number of round trips to

satisfy a request (e.g., application foo has a command that

    requires one round trip, but new application bar has a command
    that requires two round trips to complete).
  1. Adding support for an authentication method requiring definition

of new AVPs for use with the application. Since a new EAP

    authentication method can be supported within Diameter without
    requiring new AVPs, addition of EAP methods does not require the
    creation of a new authentication application.
 Creation of a new application should be viewed as a last resort.  An
 implementation MAY add arbitrary non-mandatory AVPs to any command
 defined in an application, including vendor-specific AVPs without
 needing to define a new application.  Please refer to Section 11.1.1
 for details.
 In order to justify allocation of a new application identifier,
 Diameter applications MUST define one Command Code, or add new
 mandatory AVPs to the ABNF.
 The expected AVPs MUST be defined in an ABNF [ABNF] grammar (see
 Section 3.2).  If the Diameter application has accounting
 requirements, it MUST also specify the AVPs that are to be present in
 the Diameter Accounting messages (see Section 9.3).  However, just
 because a new authentication application id is required, does not
 imply that a new accounting application id is required.
 When possible, a new Diameter application SHOULD reuse existing
 Diameter AVPs, in order to avoid defining multiple AVPs that carry
 similar information.

1.2.4. Creating New Accounting Applications

 There are services that only require Diameter accounting.  Such
 services need to define the AVPs carried in the Accounting-Request
 (ACR)/ Accounting-Answer (ACA) messages, but do not need to define
 new command codes.  An implementation MAY add arbitrary non-mandatory
 AVPs (AVPs with the "M" bit not set) to any command defined in an

Calhoun, et al. Standards Track [Page 12] RFC 3588 Diameter Based Protocol September 2003

 application, including vendor-specific AVPs, without needing to
 define a new accounting application.  Please refer to Section 11.1.1
 for details.
 Application Identifiers are still required for Diameter capability
 exchange.  Every Diameter accounting application specification MUST
 have an IANA assigned Application Identifier (see Section 2.4) or a
 vendor specific Application Identifier.
 Every Diameter implementation MUST support accounting.  Basic
 accounting support is sufficient to handle any application that uses
 the ACR/ACA commands defined in this document, as long as no new
 mandatory AVPs are added.  A mandatory AVP is defined as one which
 has the "M" bit set when sent within an accounting command,
 regardless of whether it is required or optional within the ABNF for
 the accounting application.
 The creation of a new accounting application should be viewed as a
 last resort and MUST NOT be used unless a new command or additional
 mechanisms (e.g., application defined state machine) is defined
 within the application, or new mandatory AVPs are added to the ABNF.
 Within an accounting command, setting the "M" bit implies that a
 backend server (e.g., billing server) or the accounting server itself
 MUST understand the AVP in order to compute a correct bill.  If the
 AVP is not relevant to the billing process, when the AVP is included
 within an accounting command, it MUST NOT have the "M" bit set, even
 if the "M" bit is set when the same AVP is used within other Diameter
 commands (i.e., authentication/authorization commands).
 A DIAMETER base accounting implementation MUST be configurable to
 advertise supported accounting applications in order to prevent the
 accounting server from accepting accounting requests for unbillable
 services.  The combination of the home domain and the accounting
 application Id can be used in order to route the request to the
 appropriate accounting server.
 When possible, a new Diameter accounting application SHOULD attempt
 to reuse existing AVPs, in order to avoid defining multiple AVPs that
 carry similar information.
 If the base accounting is used without any mandatory AVPs, new
 commands or additional mechanisms (e.g., application defined state
 machine), then the base protocol defined standard accounting
 application Id (Section 2.4) MUST be used in ACR/ACA commands.

Calhoun, et al. Standards Track [Page 13] RFC 3588 Diameter Based Protocol September 2003

1.2.5. Application Authentication Procedures

 When possible, applications SHOULD be designed such that new
 authentication methods MAY be added without requiring changes to the
 application.  This MAY require that new AVP values be assigned to
 represent the new authentication transform, or any other scheme that
 produces similar results.  When possible, authentication frameworks,
 such as Extensible Authentication Protocol [EAP], SHOULD be used.

1.3. Terminology

 AAA
    Authentication, Authorization and Accounting.
 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).
 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.
 Broker
    A broker is a business term commonly used in AAA infrastructures.
    A broker is either a relay, proxy or redirect agent, and MAY be
    operated by roaming consortiums.  Depending on the business model,
    a broker may either choose to  deploy relay agents or proxy
    agents.

Calhoun, et al. Standards Track [Page 14] RFC 3588 Diameter Based Protocol September 2003

 Diameter Agent
    A Diameter Agent is a Diameter node that provides either relay,
    proxy, redirect or translation services.
 Diameter Client
    A Diameter Client is a device at the edge of the network that
    performs access control.  An example of a Diameter client is a
    Network Access Server (NAS) or a Foreign Agent (FA).
 Diameter Node
    A Diameter node is a host process that implements the Diameter
    protocol, and acts either as a Client, Agent or Server.
 Diameter Peer
    A Diameter Peer is a Diameter Node to which a given Diameter Node
    has a direct transport connection.
 Diameter Security Exchange
    A Diameter Security Exchange is a process through which two
    Diameter nodes establish end-to-end security.
 Diameter Server
    A Diameter Server is one that handles authentication,
    authorization and accounting requests for a particular realm.  By
    its very nature, a Diameter Server MUST support Diameter
    applications in addition to the base protocol.
 Downstream
    Downstream is used to identify the direction of a particular
    Diameter message from the home server towards the access device.
 End-to-End Security
    TLS and IPsec provide hop-by-hop security, or security across a
    transport connection.  When relays or proxy are involved, this
    hop-by-hop security does not protect the entire Diameter user
    session.  End-to-end security is security between two Diameter
    nodes, possibly communicating through Diameter Agents.  This
    security protects the entire Diameter communications path from the
    originating Diameter node to the terminating Diameter node.
 Home Realm
    A Home Realm is the administrative domain with which the user
    maintains an account relationship.
 Home Server
    See Diameter Server.

Calhoun, et al. Standards Track [Page 15] RFC 3588 Diameter Based Protocol September 2003

 Interim accounting
    An interim accounting message provides a snapshot of usage during
    a user's session.  It is typically implemented in order to provide
    for partial accounting of a user's session in the case of a device
    reboot or other network problem prevents the reception of a
    session summary message or session record.
 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 [NAI], 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.
    This is typically accomplished by tracking the state of NAS
    devices.  While proxies typically 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 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.

Calhoun, et al. Standards Track [Page 16] RFC 3588 Diameter Based Protocol September 2003

 Real-time Accounting
    Real-time accounting involves the processing of information on
    resource usage within a defined time window.  Time constraints are
    typically imposed in order to limit financial risk.
 Relay Agent or Relay
    Relays forward requests and responses based on routing-related
    AVPs and realm 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 proxy
    agents, redirect agents do not keep state with respect to sessions
    or NAS resources.
 Roaming Relationships
    Roaming relationships include relationships between companies and
    ISPs, relationships among peer ISPs within a roaming consortium,
    and relationships between an ISP and a roaming consortium.
 Security Association
    A security association is an association between two endpoints in
    a Diameter session which allows the endpoints to communicate with
    integrity and confidentially, even in the presence of relays
    and/or proxies.
 Session
    A session is a related progression of events devoted to a
    particular activity.  Each application SHOULD provide guidelines
    as to when a session begins and ends.  All Diameter packets with
    the same Session-Identifier are considered to be part of the same
    session.

Calhoun, et al. Standards Track [Page 17] RFC 3588 Diameter Based Protocol September 2003

 Session state
    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 by
    expiration.
 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 is a stateful Diameter node that performs
    protocol translation between Diameter and another AAA protocol,
    such as RADIUS.
 Transport Connection
    A transport connection is a TCP or SCTP connection existing
    directly between two Diameter peers, otherwise known as a Peer-
    to-Peer Connection.
 Upstream
    Upstream is used to identify the direction of a particular
    Diameter message from the access device towards the home server.
 User
    The entity requesting or using some resource, in support of which
    a Diameter client has generated a request.

2. Protocol Overview

 The base Diameter protocol may be used by itself for accounting
 applications, but for use in authentication and authorization it is
 always extended for a particular application.  Two Diameter
 applications are defined by companion documents:  NASREQ [NASREQ],

Calhoun, et al. Standards Track [Page 18] RFC 3588 Diameter Based Protocol September 2003

 Mobile IPv4 [DIAMMIP].  These applications are introduced in this
 document but specified elsewhere.  Additional Diameter applications
 MAY be defined in the future (see Section 11.3).
 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.,
 NASREQ and/or Mobile IPv4.  A Diameter Client that does not support
 both NASREQ and Mobile IPv4, MUST be referred to as "Diameter X
 Client" where X is the application which 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 that does not support
 both NASREQ and Mobile IPv4, MUST be referred to as "Diameter X
 Server" where X is the application which it supports, and not a
 "Diameter Server".
 Diameter Relays and redirect agents are, by definition, protocol
 transparent, and MUST transparently 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 which does not support
 also both NASREQ and Mobile IPv4, MUST be referred to as "Diameter X
 Proxy" where X is the application which it supports, and not a
 "Diameter Proxy".
 The base Diameter protocol concerns itself with capabilities
 negotiation, how messages are sent and how peers may eventually be
 abandoned.  The base protocol also defines certain rules that apply
 to all exchanges of messages 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.  The Session-Id is then used in all
 subsequent messages to identify the user's session (see Section 8 for
 more information).  The communicating party may accept the request,
 or reject it by returning an answer message with the Result-Code AVP

Calhoun, et al. Standards Track [Page 19] RFC 3588 Diameter Based Protocol September 2003

 set to indicate 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.

2.1. Transport

 Transport profile is defined in [AAATRANS].
 The base Diameter protocol is run on port 3868 of both TCP [TCP] and
 SCTP [SCTP] transport protocols.
 Diameter clients MUST support either TCP or SCTP, while agents and
 servers MUST support both.  Future versions of this specification MAY
 mandate that clients support SCTP.
 A Diameter node MAY initiate connections from a source port other
 than the one that it declares it accepts incoming connections on, and
 MUST be prepared to receive connections on port 3868.  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.
 When no transport connection exists with a peer, an attempt to
 connect SHOULD be periodically made.  This behavior is handled via
 the Tc timer, 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.
 When connecting to a peer and either zero or more transports are
 specified, SCTP SHOULD be tried first, followed by TCP.  See Section
 5.2 for more information on peer discovery.
 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.
 Diameter implementations SHOULD also be able to interpret a reset
 from the transport and timed-out connection attempts.

Calhoun, et al. Standards Track [Page 20] RFC 3588 Diameter Based Protocol September 2003

 If Diameter receives data up from TCP 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).

2.1.1. SCTP Guidelines

 The following are guidelines for Diameter implementations that
 support SCTP:
 1. For interoperability: All Diameter nodes MUST be prepared to
    receive Diameter messages on any SCTP stream in the association.
 2. To prevent blocking: All Diameter nodes SHOULD utilize all SCTP
    streams available to the association to prevent head-of-the-line
    blocking.

2.2. Securing Diameter Messages

 Diameter clients, such as Network Access Servers (NASes) and Mobility
 Agents MUST support IP Security [SECARCH], and MAY support TLS [TLS].
 Diameter servers MUST support TLS and IPsec.  The Diameter protocol
 MUST NOT be used without any security mechanism (TLS or IPsec).
 It is suggested that IPsec can be used primarily at the edges and in
 intra-domain traffic, such as using pre-shared keys between a NAS a
 local AAA proxy.  This also eases the requirements on the NAS to
 support certificates.  It is also suggested that inter-domain traffic
 would primarily use TLS.  See Sections 13.1 and 13.2 for more details
 on IPsec and TLS usage.

2.3. Diameter Application Compliance

 Application Identifiers are advertised during the capabilities
 exchange phase (see Section 5.3).  For a given application,
 advertising support of an application implies that the sender
 supports all command codes, and the AVPs specified in the associated
 ABNFs, described in the specification.
 An implementation MAY add arbitrary non-mandatory AVPs to any command
 defined in an application, including vendor-specific AVPs.  Please
 refer to Section 11.1.1 for details.

Calhoun, et al. Standards Track [Page 21] RFC 3588 Diameter Based Protocol September 2003

2.4. Application Identifiers

 Each Diameter application MUST have an IANA assigned Application
 Identifier (see Section 11.3).  The base protocol does not require an
 Application Identifier 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 Identifier, which is used in the message forwarding
 process.
 The following Application Identifier values are defined:
    Diameter Common Messages      0
    NASREQ                        1 [NASREQ]
    Mobile-IP                     2 [DIAMMIP]
    Diameter Base Accounting      3
    Relay                         0xffffffff
 Relay and redirect agents MUST advertise the Relay Application
 Identifier, 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.

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 is a transport level connection between two peers, used
 to send and receive Diameter messages.  A session is a logical
 concept at the application layer, and is shared between an access
 device and a server, and is identified via the Session-Id AVP

Calhoun, et al. Standards Track [Page 22] RFC 3588 Diameter Based Protocol September 2003

        +--------+          +-------+          +--------+
        | 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 its local 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.  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.

2.6. Peer Table

 The Diameter Peer Table is used in message forwarding, and referenced
 by the Realm 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 4.4. 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 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.

Calhoun, et al. Standards Track [Page 23] RFC 3588 Diameter Based Protocol September 2003

 TLS Enabled
    Specifies whether TLS is to be used when communicating with the
    peer.
 Additional security information, when needed (e.g., keys,
 certificates)

2.7. Realm-Based Routing Table

 All Realm-Based routing lookups are performed against what is
 commonly known as the Realm Routing Table (see Section 12).  A Realm
 Routing Table Entry contains the following fields:
 Realm Name
    This is the field that is typically 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 a vendor id and an application id.
    For all IETF standards track Diameter applications, the vendor id
    is zero.  A route entry can have a different destination based on
    the application identification AVP of the message.  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 resolve to a route entry with
       the Local Action set to Local can be satisfied locally, and do
       not need to be routed to another server.
    2. RELAY - All Diameter messages that fall within this category
       MUST be routed to a next hop server, without modifying any
       non-routing AVPs.  See Section 6.1.8 for relaying guidelines
    3. PROXY - All Diameter messages that fall within this category
       MUST be routed to a next hop server.  The local server MAY
       apply its local policies to the message by including new AVPs
       to the message prior to routing.  See Section 6.1.8 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 6.1.7
       for redirect guidelines.

Calhoun, et al. Standards Track [Page 24] RFC 3588 Diameter Based Protocol September 2003

 Server Identifier
    One or more servers the message is to be routed to.  These servers
    MUST also be present in the Peer table. When the Local Action is
    set to RELAY or PROXY, this field contains the identity of the
    server(s) the message must be routed to.  When the Local Action
    field is set to REDIRECT, this field contains the identity of one
    or more servers the message should be redirected to.
 Static or Dynamic
    Specifies whether a route entry was statically configured, or
    dynamically discovered.
 Expiration time
    Specifies the time which a dynamically discovered route table
    entry expires.
 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 MUST follow the protocol
 compliance guidelines in Section 2.  Relay agents MUST NOT reorder
 AVPs, 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 Identifier (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.

2.8. Role of Diameter Agents

 In addition to client and servers, the Diameter protocol introduces
 relay, proxy, redirect, and translation agents, each of which is
 defined in Section 1.3.  These Diameter agents are useful for several
 reasons:
  1. They can distribute administration of systems to a configurable

grouping, including the maintenance of security associations.

  1. They can be used for concentration of requests from an number of

co-located or distributed NAS equipment sets to a set of like user

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

Calhoun, et al. Standards Track [Page 25] RFC 3588 Diameter Based Protocol September 2003

  1. They can be used for load balancing.
  1. 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 either until it is notified otherwise, or by expiration.  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:
  1. Protocol translation (e.g., RADIUS ↔ Diameter)
  1. Limiting resources authorized to a particular user
  1. Per user or 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.

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., Destination-Realm).  This routing decision is
 performed using a list of supported realms, and known peers.  This is
 known as the Realm Routing Table, as is defined further in Section
 2.7.

Calhoun, et al. Standards Track [Page 26] RFC 3588 Diameter Based Protocol September 2003

 Relays MAY be used to aggregate requests from multiple Network Access
 Servers (NASes) within a common geographical area (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 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 NAS,
 which is an access device, for the user bob@example.com.  Prior to
 issuing the request, NAS performs a Diameter route lookup, using
 "example.com" as the key, and determines that the message is to be
 relayed to DRL, which is a Diameter Relay.  DRL performs the same
 route lookup as NAS, and relays the message to HMS, which is
 example.com's Home Diameter Server.  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 NAS using saved transaction state.
 Since Relays do not perform any application level processing, they
 provide relaying services for all Diameter applications, and
 therefore MUST advertise the Relay Application Identifier.

2.8.2. Proxy Agents

 Similarly 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
 provisioning.

Calhoun, et al. Standards Track [Page 27] RFC 3588 Diameter Based Protocol September 2003

 It is important to note that although proxies MAY provide a value-add
 function for NASes, they do not allow access devices to use end-to-
 end security, since modifying messages breaks authentication.
 Proxies MAY be used in call control centers or access ISPs that
 provide outsourced connections, they can monitor the number and types
 of ports in use, and make allocation and admission decisions
 according to their configuration.
 Proxies that wish to limit resources MUST maintain session state.
 All proxies MUST maintain transaction state.
 Since enforcing policies requires an understanding of the service
 being provided, Proxies MUST only advertise the Diameter applications
 they support.

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.  Further, since redirect agents never relay requests, they are
 not required to maintain transaction 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.  DRL has a
 default route configured to DRD, which is a redirect agent that
 returns a redirect notification to DRL, as well as HMS' contact
 information.  Upon receipt of the redirect notification, DRL
 establishes a transport connection with HMS, if one doesn't already
 exist, and forwards the request to it.

Calhoun, et al. Standards Track [Page 28] RFC 3588 Diameter Based Protocol September 2003

                             +------+
                             |      |
                             | 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, and therefore MUST advertise the Relay Application
 Identifier.

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, and 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

Calhoun, et al. Standards Track [Page 29] RFC 3588 Diameter Based Protocol September 2003

2.9. End-to-End Security Framework

 End-to-end security services include confidentiality and message
 origin authentication.  These services are provided by supporting AVP
 integrity and confidentiality between two peers, communicating
 through agents.
 End-to-end security is provided via the End-to-End security
 extension, described in [AAACMS].  The circumstances requiring the
 use of end-to-end security are determined by policy on each of the
 peers. Security policies, which are not the subject of
 standardization, may be applied by next hop Diameter peer or by
 destination realm.  For example, where TLS or IPsec transmission-
 level security is sufficient, there may be no need for end-to-end
 security.
 End-to-end security policies include:
  1. Never use end-to-end security.
  1. Use end-to-end security on messages containing sensitive AVPs.

Which AVPs are sensitive is determined by service provider policy.

    AVPs containing keys and passwords should be considered sensitive.
    Accounting AVPs may be considered sensitive.  Any AVP for which
    the P bit may be set or which may be encrypted may be considered
    sensitive.
  1. Always use end-to-end security.
 It is strongly RECOMMENDED that all Diameter implementations support
 end-to-end security.

2.10. Diameter Path Authorization

 As noted in Section 2.2, Diameter requires transmission level
 security to be used on each connection (TLS or IPsec).  Therefore,
 each connection is authenticated, replay and integrity protected and
 confidential on a per-packet basis.
 In addition to authenticating each connection, each connection as
 well as 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.

Calhoun, et al. Standards Track [Page 30] RFC 3588 Diameter Based Protocol September 2003

 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 6.1.8, a relay or proxy agent MUST append a
 Route-Record AVP to all requests forwarded.  The AVP contains the
 identity of the peer the request was received from.
 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 the 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.
 Similarly, the local Diameter agent, on receiving a Diameter response
 authorizing a session, MUST check the Route-Record AVPs to make sure
 that the route traversed by the response is acceptable.  At each
 step, 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 which would violate those limits.  By issuing an
 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.

Calhoun, et al. Standards Track [Page 31] RFC 3588 Diameter Based Protocol September 2003

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 ...
 +-+-+-+-+-+-+-+-+-+-+-+-+-
 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.
 Command Flags
    The Command Flags field is eight bits.  The following bits are
    assigned:
     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 ABNF
                  described for this command.  Messages with the 'E'

Calhoun, et al. Standards Track [Page 32] RFC 3588 Diameter Based Protocol September 2003

                  bit set are commonly referred to as error
                  messages.  This bit MUST NOT be set in request
                  messages.  See Section 7.2.
    T(Potentially re-transmitted 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 is sent again.  This flag
                  MUST NOT be set in answer messages.
    r(eserved)  - these flag bits are reserved for future use, and
                  MUST be set to zero, and ignored by the receiver.
 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 11.2.1).
    Command-Code values 16,777,214 and 16,777,215 (hexadecimal values
    FFFFFE -FFFFFF) are reserved for experimental use (See Section
    11.3).
 Application-ID
    Application-ID is four octets and is used to identify to which
    application the message is applicable for.  The application can be
    an authentication application, an accounting application or a
    vendor specific application.  See Section 11.3 for the possible
    values that the application-id may use.
    The application-id in the header MUST be the same as what is
    contained in any relevant AVPs contained in the message.

Calhoun, et al. Standards Track [Page 33] RFC 3588 Diameter Based Protocol September 2003

 Hop-by-Hop Identifier
    The Hop-by-Hop Identifier is an unsigned 32-bit integer field (in
    network byte order) and 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 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) and 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 (see 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
    field and any routing AVPs that may be present), and 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.

Calhoun, et al. Standards Track [Page 34] RFC 3588 Diameter Based Protocol September 2003

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:
 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

Calhoun, et al. Standards Track [Page 35] RFC 3588 Diameter Based Protocol September 2003

3.2. Command Code ABNF specification

 Every Command Code defined MUST include a corresponding ABNF
 specification, which is used to define the AVPs that MUST or MAY be
 present.  The following format is used in the definition:
 command-def      = command-name "::=" diameter-message
 command-name     = diameter-name
 diameter-name    = ALPHA *(ALPHA / DIGIT / "-")
 diameter-message = header  [ *fixed] [ *required] [ *optional]
                    [ *fixed]
 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.

Calhoun, et al. Standards Track [Page 36] RFC 3588 Diameter Based Protocol September 2003

 optional         = [qual] "[" avp-name "]"
                    ; The avp-name in the 'optional' rule cannot
                    ; evaluate to any AVP Name which is included
                    ; in a fixed or required rule.  The AVP can
                    ; appear anywhere in the message.
 qual             = [min] "*" [max]
                    ; See ABNF conventions, RFC 2234 Section 6.6.
                    ; The absence of any qualifiers 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.
                    ;
                    ; NOTE:  "[" and "]" have a different meaning
                    ; than in ABNF (see the optional rule, above).
                    ; 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'.
 min              = 1*DIGIT
                    ; The minimum number of times the element may
                    ; be present.  The default value is zero.
 max              = 1*DIGIT
                    ; The maximum number of times the element may
                    ; be present.  The default value is infinity.  A
                    ; value of zero 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, which does not conflict with the
                    ; required or fixed position AVPs defined in
                    ; the command code definition.

Calhoun, et al. Standards Track [Page 37] RFC 3588 Diameter Based Protocol September 2003

 The following is a definition of a fictitious command code:
 Example-Request ::= < "Diameter-Header: 9999999, REQ, PXY >
                     { User-Name }
                   * { Origin-Host }
                   * [ AVP

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:
  1. The request was successful
  1. The request failed
  1. An additional request must be sent to provide information the peer

requires prior to returning a successful or failed answer.

  1. 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.

4. Diameter AVPs

 Diameter AVPs carry specific authentication, accounting,
 authorization, routing and security information as well as
 configuration details for the request and reply.
 Some AVPs MAY be listed more than once.  The effect of such an AVP is
 specific, and is specified in each case by the AVP description.

Calhoun, et al. Standards Track [Page 38] RFC 3588 Diameter Based Protocol September 2003

 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 till 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
    backward compatibility with RADIUS, without setting the Vendor-Id
    field.  AVP numbers 256 and above are used for Diameter, which are
    allocated by IANA (see Section 11.1).
 AVP Flags
    The AVP Flags field informs the receiver how each attribute must
    be handled.  The 'r' (reserved) bits are unused and SHOULD be set
    to 0.  Note that subsequent Diameter applications MAY define
    additional bits within the AVP Header, and an unrecognized bit
    SHOULD be considered an error.  The 'P' bit indicates the need for
    encryption for end-to-end security.
    The 'M' Bit, known as the Mandatory bit, indicates whether support
    of the AVP is required.  If an AVP with the 'M' bit set is
    received by a Diameter client, server, proxy, or translation agent
    and either the AVP or its value is unrecognized, the message MUST
    be rejected.  Diameter Relay and redirect agents MUST NOT reject
    messages with unrecognized AVPs.

Calhoun, et al. Standards Track [Page 39] RFC 3588 Diameter Based Protocol September 2003

    The 'M' bit MUST be set according to the rules defined for the AVP
    containing it.  In order to preserve interoperability, a Diameter
    implementation MUST be able to exclude from a Diameter message any
    Mandatory AVP which is neither defined in the base Diameter
    protocol nor in any of the Diameter Application specifications
    governing the message in which it appears.  It MAY do this in one
    of the following ways:
    1) If a message is rejected because it contains a Mandatory AVP
       which is neither defined in the base Diameter standard nor in
       any of the Diameter Application specifications governing the
       message in which it appears, the implementation may resend the
       message without the AVP, possibly inserting additional standard
       AVPs instead.
    2) A configuration option may be provided on a system wide, per
       peer, or per realm basis that would allow/prevent particular
       Mandatory AVPs to be sent.  Thus an administrator could change
       the configuration to avoid interoperability problems.
    Diameter implementations are required to support all Mandatory
    AVPs which are allowed by the message's formal syntax and defined
    either in the base Diameter standard or in one of the Diameter
    Application specifications governing the message.
    AVPs with the 'M' bit cleared are informational only and 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.
    Unless otherwise noted, AVPs will have the following default AVP
    Flags field settings:
       The 'M' bit MUST be set.  The 'V' bit MUST NOT be set.
 AVP Length
    The AVP Length field is three octets, and indicates the number of
    octets in this AVP including the AVP Code, AVP Length, AVP Flags,
    Vendor-ID field (if present) and the AVP data.  If a message is
    received with an invalid attribute length, the message SHOULD be
    rejected.

Calhoun, et al. Standards Track [Page 40] RFC 3588 Diameter Based Protocol September 2003

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"
    [ASSIGNNO] value, encoded in network byte order.  Any vendor
    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), nor with future IETF
    applications.
    A vendor ID value of zero (0) corresponds to the IETF adopted AVP
    values, as managed by the IANA.  Since the absence of the vendor
    ID field implies that the AVP in question is not vendor specific,
    implementations MUST NOT use the zero (0) vendor ID.

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 four-octets in length is 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).

Calhoun, et al. Standards Track [Page 41] RFC 3588 Diameter Based Protocol September 2003

 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).
 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.  Each of these
    AVPs follows - in the order in which they are specified -
    including their headers and padding.  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 AVP Derived Data Formats MUST include
 them in a section entitled "AVP Derived 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.
 The below AVP Derived Data Formats are commonly used by applications.
 Address
    The Address format is derived from the OctetString AVP Base
    Format.  It is a discriminated union, representing, for example a
    32-bit (IPv4) [IPV4] or 128-bit (IPv6) [IPV6] address, most
    significant octet first.  The first two octets of the Address

Calhoun, et al. Standards Track [Page 42] RFC 3588 Diameter Based Protocol September 2003

    AVP represents the AddressType, which contains an Address Family
    defined in [IANAADFAM].  The AddressType is used to discriminate
    the content and format of the remaining octets.
 Time
    The Time format is derived from the OctetString AVP Base 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 chapter 3 of [SNTP].
    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.
    SNTP [SNTP] 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 AVP Base
    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 [UFT8] transformation format described in RFC 2279.
    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 an 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.

Calhoun, et al. Standards Track [Page 43] RFC 3588 Diameter Based Protocol September 2003

 DiameterIdentity
    The DiameterIdentity format is derived from the OctetString AVP
    Base Format.
       DiameterIdentity  = FQDN
    DiameterIdentity value is used to uniquely identify a Diameter
    node for purposes of duplicate connection and routing loop
    detection.
    The contents of the string MUST be the 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 it is sent on.
 DiameterURI
    The DiameterURI MUST follow the Uniform Resource Identifiers (URI)
    syntax [URI] rules specified below:
    "aaa://" FQDN [ port ] [ transport ] [ protocol ]
                    ; No transport security
    "aaas://" FQDN [ port ] [ transport ] [ protocol ]
                    ; Transport security used
    FQDN               = Fully Qualified Host Name
    port               = ":" 1*DIGIT
                    ; One of the ports used to listen for
                    ; incoming connections.
                    ; If absent,
                    ; the default Diameter port (3868) is
                    ; assumed.
    transport          = ";transport=" transport-protocol
                    ; One of the transports used to listen
                    ; for incoming connections.  If absent,
                    ; the default SCTP [SCTP] protocol is
                    ; assumed.  UDP MUST NOT be used when
                    ; the aaa-protocol field is set to
                    ; diameter.

Calhoun, et al. Standards Track [Page 44] RFC 3588 Diameter Based Protocol September 2003

    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
    Enumerated is derived from the Integer32 AVP Base Format.  The
    definition contains a list of valid values and their
    interpretation and is described in the Diameter application
    introducing the AVP.
 IPFilterRule
    The IPFilterRule format is derived from the OctetString AVP Base
    Format.  It uses the ASCII charset.  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.

Calhoun, et al. Standards Track [Page 45] RFC 3588 Diameter Based Protocol September 2003

    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.
                    ipno/bits  An IP number as above with a mask
                               width of the form 1.2.3.4/24.  In
                               this case, all IP numbers from
                               1.2.3.0 to 1.2.3.255 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.

Calhoun, et al. Standards Track [Page 46] RFC 3588 Diameter Based Protocol September 2003

                    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.
          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.

Calhoun, et al. Standards Track [Page 47] RFC 3588 Diameter Based Protocol September 2003

          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:
                  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.
 The rule syntax is a modified subset of ipfw(8) from FreeBSD, and the
 ipfw.c code may provide a useful base for implementations.

Calhoun, et al. Standards Track [Page 48] RFC 3588 Diameter Based Protocol September 2003

 QoSFilterRule
    The QosFilterRule format is derived from the OctetString AVP Base
    Format.  It uses the ASCII charset.  Packets may be marked or
    metered 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)
       DSCP values                        (no mask or range)
    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 treated as best
    effort.  An access device that is unable to interpret or apply a
    QoS rule SHOULD NOT terminate the session.
 QoSFilterRule filters MUST follow the format:
 action dir proto from src to dst [options]
              tag    - Mark packet with a specific DSCP
                       [DIFFSERV].  The DSCP option MUST be
                       included.
              meter  - Meter traffic.  The metering options
                       MUST be included.
 dir          The format is as described under IPFilterRule.
              proto        The format is as described under
              IPFilterRule.
              src and dst  The format is as described under
              IPFilterRule.

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.

Calhoun, et al. Standards Track [Page 49] RFC 3588 Diameter Based Protocol September 2003

 The AVP Code numbering space of all AVPs included in a Grouped AVP is
 the same as for non-grouped AVPs.  Further, if any of the AVPs
 encapsulated within a Grouped AVP has the 'M' (mandatory) bit set,
 the Grouped AVP itself MUST also include the 'M' bit set.
 Every Grouped AVP defined MUST include a corresponding grammar, using
 ABNF [ABNF] (with modifications), as defined below.
    grouped-avp-def  = name "::=" avp
    name-fmt         = ALPHA *(ALPHA / DIGIT / "-")
    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]
                       [ *fixed]
    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 ABNF grammar:
    Example-AVP  ::= < AVP Header: 999999 >
                     { Origin-Host }
                   1*{ Session-Id }
                    *[ AVP ]
 An Example-AVP with Grouped Data follows.
 The Origin-Host AVP is required (Section 6.3).  In this case:
    Origin-Host = "example.com".

Calhoun, et al. Standards Track [Page 50] RFC 3588 Diameter Based Protocol September 2003

 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"
 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 hex since the format
 of these AVPs is neither 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 which
 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 which the receiver
 cannot interpret (here, the Recover-Policy and Futuristic-Acct-Record
 AVPs).

Calhoun, et al. Standards Track [Page 51] RFC 3588 Diameter Based Protocol September 2003

 This AVP would be encoded as follows:
         0       1       2       3       4       5       6       7
     +-------+-------+-------+-------+-------+-------+-------+-------+
   0 |     Example AVP Header (AVP Code = 999999), Length = 468      |
     +-------+-------+-------+-------+-------+-------+-------+-------+
   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 = 50 |  'g'  |  'r'  |  'u'  |  'm'  |
     +-------+-------+-------+-------+-------+-------+-------+-------+
                                   . . .
     +-------+-------+-------+-------+-------+-------+-------+-------+
  64 |  'A'  |  'F'  |  '3'  |  'B'  |  '8'  |  '1'  |Padding|Padding|
     +-------+-------+-------+-------+-------+-------+-------+-------+
  72 |     Session-Id AVP Header (AVP Code = 263), Length = 51       |
     +-------+-------+-------+-------+-------+-------+-------+-------+
  80 |  'g'  |  'r'  |  'u'  |  'm'  |  'p'  |  '.'  |  'e'  |  'x'  |
     +-------+-------+-------+-------+-------+-------+-------+-------+
                                   . . .
     +-------+-------+-------+-------+-------+-------+-------+-------+
 104 |  '0'  |  'A'  |  'F'  |  '3'  |  'B'  |  '8'  |  '2'  |Padding|
     +-------+-------+-------+-------+-------+-------+-------+-------+
 112 |   Recovery-Policy Header (AVP Code = 8341), Length = 223      |
     +-------+-------+-------+-------+-------+-------+-------+-------+
 120 |  0x21 | 0x63  | 0xbc  | 0x1d  | 0x0a  | 0xd8  | 0x23  | 0x71  |
     +-------+-------+-------+-------+-------+-------+-------+-------+
                                   . . .
     +-------+-------+-------+-------+-------+-------+-------+-------+
 320 |  0x2f | 0xd7  | 0x96  | 0x6b  | 0x8c  | 0x7f  | 0x92  |Padding|
     +-------+-------+-------+-------+-------+-------+-------+-------+
 328 | Futuristic-Acct-Record Header (AVP Code = 15930), Length = 137|
     +-------+-------+-------+-------+-------+-------+-------+-------+
 336 |  0xfe | 0x19  | 0xda  | 0x58  | 0x02  | 0xac  | 0xd9  | 0x8b  |
     +-------+-------+-------+-------+-------+-------+-------+-------+
                                   . . .
     +-------+-------+-------+-------+-------+-------+-------+-------+
 464 |  0x41 |Padding|Padding|Padding|
     +-------+-------+-------+-------+

Calhoun, et al. Standards Track [Page 52] RFC 3588 Diameter Based Protocol September 2003

4.5. Diameter Base Protocol AVPs

 The following table describes the Diameter AVPs defined in the base
 protocol, their AVP Code values, types, possible flag values and
 whether the AVP MAY be encrypted.  For the originator of a Diameter
 message, "Encr" (Encryption) means that if a message containing that
 AVP is to be sent via a  Diameter agent (proxy, redirect or relay)
 then the message MUST NOT be sent unless there is end-to-end security
 between the originator and the recipient and integrity /
 confidentiality protection is offered for this AVP OR the originator
 has locally trusted configuration that indicates that end-to-end
 security is not needed.  Similarly, for the originator of a Diameter
 message, a "P" in the "MAY" column means that if a message containing
 that AVP is to be sent via a  Diameter agent (proxy, redirect or
 relay) then the message MUST NOT be sent unless there is end-to-end
 security between the originator and the recipient or the originator
 has locally trusted configuration that indicates that end-to-end
 security is not needed.
 Due to space constraints, the short form DiamIdent is used to
 represent DiameterIdentity.

Calhoun, et al. Standards Track [Page 53] RFC 3588 Diameter Based Protocol September 2003

                                          +---------------------+
                                          |    AVP Flag rules   |
                                          |----+-----+----+-----|----+
                 AVP  Section             |    |     |SHLD| MUST|    |
 Attribute Name  Code Defined  Data Type  |MUST| MAY | NOT|  NOT|Encr|
 -----------------------------------------|----+-----+----+-----|----|
 Acct-             85  9.8.2   Unsigned32 | M  |  P  |    |  V  | Y  |
   Interim-Interval                       |    |     |    |     |    |
 Accounting-      483  9.8.7   Enumerated | M  |  P  |    |  V  | Y  |
   Realtime-Required                      |    |     |    |     |    |
 Acct-            50   9.8.5   UTF8String | M  |  P  |    |  V  | Y  |
   Multi-Session-Id                       |    |     |    |     |    |
 Accounting-      485  9.8.3   Unsigned32 | M  |  P  |    |  V  | Y  |
   Record-Number                          |    |     |    |     |    |
 Accounting-      480  9.8.1   Enumerated | M  |  P  |    |  V  | Y  |
   Record-Type                            |    |     |    |     |    |
 Accounting-       44  9.8.4   OctetString| M  |  P  |    |  V  | Y  |
  Session-Id                              |    |     |    |     |    |
 Accounting-      287  9.8.6   Unsigned64 | M  |  P  |    |  V  | Y  |
   Sub-Session-Id                         |    |     |    |     |    |
 Acct-            259  6.9     Unsigned32 | M  |  P  |    |  V  | N  |
   Application-Id                         |    |     |    |     |    |
 Auth-            258  6.8     Unsigned32 | M  |  P  |    |  V  | N  |
   Application-Id                         |    |     |    |     |    |
 Auth-Request-    274  8.7     Enumerated | M  |  P  |    |  V  | N  |
    Type                                  |    |     |    |     |    |
 Authorization-   291  8.9     Unsigned32 | M  |  P  |    |  V  | N  |
   Lifetime                               |    |     |    |     |    |
 Auth-Grace-      276  8.10    Unsigned32 | M  |  P  |    |  V  | N  |
   Period                                 |    |     |    |     |    |
 Auth-Session-    277  8.11    Enumerated | M  |  P  |    |  V  | N  |
   State                                  |    |     |    |     |    |
 Re-Auth-Request- 285  8.12    Enumerated | M  |  P  |    |  V  | N  |
   Type                                   |    |     |    |     |    |
 Class             25  8.20    OctetString| M  |  P  |    |  V  | Y  |
 Destination-Host 293  6.5     DiamIdent  | M  |  P  |    |  V  | N  |
 Destination-     283  6.6     DiamIdent  | M  |  P  |    |  V  | N  |
   Realm                                  |    |     |    |     |    |
 Disconnect-Cause 273  5.4.3   Enumerated | M  |  P  |    |  V  | N  |
 E2E-Sequence AVP 300  6.15    Grouped    | M  |  P  |    |  V  | Y  |
 Error-Message    281  7.3     UTF8String |    |  P  |    | V,M | N  |
 Error-Reporting- 294  7.4     DiamIdent  |    |  P  |    | V,M | N  |
   Host                                   |    |     |    |     |    |
 Event-Timestamp   55  8.21    Time       | M  |  P  |    |  V  | N  |
 Experimental-    297  7.6     Grouped    | M  |  P  |    |  V  | N  |
    Result                                |    |     |    |     |    |
 -----------------------------------------|----+-----+----+-----|----|

Calhoun, et al. Standards Track [Page 54] RFC 3588 Diameter Based Protocol September 2003

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

Calhoun, et al. Standards Track [Page 55] RFC 3588 Diameter Based Protocol September 2003

5. Diameter Peers

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

5.1. Peer Connections

 Although a Diameter node may have many possible peers that it is able
 to communicate with, 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 timeframe, 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 shutdown.  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.

5.2. Diameter Peer Discovery

 Allowing for dynamic Diameter agent discovery will make it possible
 for simpler and more robust deployment of Diameter services.  In
 order to promote interoperable implementations of Diameter peer
 discovery, the following mechanisms are described.  These are based

Calhoun, et al. Standards Track [Page 56] RFC 3588 Diameter Based Protocol September 2003

 on existing IETF standards.  The first option (manual configuration)
 MUST be supported by all DIAMETER nodes, while the latter two options
 (SRVLOC and DNS) MAY be supported.
 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 static (manually)
    configured Diameter agent locations.  These will be used if they
    exist and respond.
 2. The Diameter implementation uses SLPv2 [SLP] to discover Diameter
    services.  The Diameter service template [TEMPLATE] is included in
    Appendix A.
    It is recommended that SLPv2 security be deployed (this requires
    distributing keys to SLPv2 agents).  This is discussed further in
    Appendix A.  SLPv2 security SHOULD be used (requiring distribution
    of keys to SLPv2 agents) in order to ensure that discovered peers
    are authorized for their roles.  SLPv2 is discussed further in
    Appendix A.
 3. The Diameter implementation performs a NAPTR query for a server in
    a particular realm.  The Diameter implementation has to know in
    advance which realm to look for a Diameter agent in.  This could
    be deduced, for example, from the 'realm' in a NAI that a Diameter
    implementation needed to perform a Diameter operation on.
    3.1 The services relevant for the task of transport protocol
        selection are those with NAPTR service fields with values
        "AAA+D2x", where x is a letter that corresponds to a transport
        protocol supported by the domain.  This specification defines
        D2T for TCP and D2S for SCTP.  We also establish an IANA
        registry for NAPTR service name to transport protocol
        mappings.
        These NAPTR records provide a mapping from a domain, to the
        SRV record for contacting a server with the specific transport
        protocol in the NAPTR services field.  The resource record
        will contain an empty regular expression and a replacement
        value, which is the SRV record for that particular transport
        protocol.  If the server supports multiple transport
        protocols, there will be multiple NAPTR records, each with a
        different service value.  As per RFC 2915 [NAPTR], the client

Calhoun, et al. Standards Track [Page 57] RFC 3588 Diameter Based Protocol September 2003

        discards any records whose services fields are not applicable.
        For the purposes of this specification, several rules are
        defined.
    3.2 A client MUST discard any service fields that identify a
        resolution service whose value is not "D2X", for values of X
        that indicate transport protocols supported by the client.
        The NAPTR processing as described in RFC 2915 will result in
        discovery of the most preferred transport protocol of the
        server that is supported by the client, as well as an SRV
        record for the server.
        The domain suffixes in the NAPTR replacement field SHOULD
        match the domain of the original query.
 4. If no NAPTR records are found, the requester queries for those
    address records for the destination address,
    '_diameter._sctp'.realm or '_diameter._tcp'.realm.  Address
    records include A RR's, AAAA RR's or other similar records, chosen
    according to the requestor's network protocol capabilities.  If
    the DNS server returns no address records, the requestor gives up.
    If the server is using a site certificate, the domain name in the
    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 or 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
    that 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, or validation of DNS RRs via DNSSEC
    is not sufficient to conclude this.  For example, a web server may
    have obtained a valid TLS 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 configuration of a
    Diameter Server CA.  Alternatively this can be achieved by
    definition of OIDs within TLS or IKE certificates so as to signify
    Diameter Server authorization.
 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 TTL.  If a peer is discovered

Calhoun, et al. Standards Track [Page 58] RFC 3588 Diameter Based Protocol September 2003

 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,
 supported Diameter applications, security mechanisms, etc.)
 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 applications in order to
 ensure that unrecognized commands and/or AVPs are not unnecessarily
 sent to a peer.
 A receiver of a Capabilities-Exchange-Req (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.
 Similarly, a receiver of a Capabilities-Exchange-Req (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.
 Since the CER/CEA messages cannot be proxied, it is still possible
 that an upstream agent receives 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

Calhoun, et al. Standards Track [Page 59] RFC 3588 Diameter Based Protocol September 2003

 message (see Section 7.) with the Result-Code AVP set to
 DIAMETER_UNABLE_TO_DELIVER to inform the downstream 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 Identifier).

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 [SCTP], which allows 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 ]

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.

Calhoun, et al. Standards Track [Page 60] RFC 3588 Diameter Based Protocol September 2003

 When Diameter is run over SCTP [SCTP], which allows 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" [ASSIGNNO]
 value assigned to the vendor of the Diameter application.  In
 combination with the Supported-Vendor-Id AVP (Section 5.3.6), this
 MAY be used in order to know which vendor specific attributes may be
 sent to the peer.  It is also envisioned that the combination of the
 Vendor-Id, Product-Name (Section 5.3.7) and the Firmware-Revision
 (Section 5.3.4) AVPs MAY provide very useful debugging information.
 A Vendor-Id value of zero in the CER or CEA messages is reserved and
 indicates that this field is ignored.

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.

Calhoun, et al. Standards Track [Page 61] RFC 3588 Diameter Based Protocol September 2003

 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 [SCTP] MUST
 be advertised in the CER and CEA messages by including a Host-IP-
 Address AVP for each address.  This AVP MUST ONLY be used in the CER
 and CEA messages.

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"
 [ASSIGNNO] value assigned to a vendor other than the device 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.

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 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

Calhoun, et al. Standards Track [Page 62] RFC 3588 Diameter Based Protocol September 2003

 Disconnect-Peer-Answer 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 initiates the transport
 disconnect.

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 its intentions to shutdown 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 }

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 shutdown.
 Message Format
    <DPA>  ::= < Diameter Header: 282 >
               { Result-Code }
               { Origin-Host }
               { Origin-Realm }
               [ Error-Message ]
             * [ Failed-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
 shutdown the transport connection.  The following values are
 supported:

Calhoun, et al. Standards Track [Page 63] RFC 3588 Diameter Based Protocol September 2003

 REBOOTING                         0
    A scheduled reboot is imminent.
 BUSY                              1
    The peer's internal resources are constrained, and it has
    determined that the transport connection needs to be closed.
 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.

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.

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 ]

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.

Calhoun, et al. Standards Track [Page 64] RFC 3588 Diameter Based Protocol September 2003

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

5.5.3. Transport Failure Algorithm

 The transport failure algorithm is defined in [AAATRANS].  All
 Diameter implementations MUST support the algorithm defined in the
 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.
 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 records
 still remaining to be transmitted in non-volatile storage.  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.

Calhoun, et al. Standards Track [Page 65] RFC 3588 Diameter Based Protocol September 2003

 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 [AAATRANS], which is used to open, close, failover,
 probe, and reopen transport connections.  Note in particular that
 [AAATRANS] requires the use of watchdog messages to probe
 connections.  For Diameter, DWR and DWA messages are to be used.
 I- is used to represent the initiator (connecting) connection, while
 the R- 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.
 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 usage, a TLS handshake will begin when both ends are in the
 open state.  If the TLS handshake is successful, all further messages
 will be sent via TLS.  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.

Calhoun, et al. Standards Track [Page 66] RFC 3588 Diameter Based Protocol September 2003

 Any implementation that produces equivalent results is considered
 compliant.
 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,       Closed
                                         R-Disc

Calhoun, et al. Standards Track [Page 67] RFC 3588 Diameter Based Protocol September 2003

                  R-Peer-Disc      R-Disc           Closed
                  R-Rcv-CER        R-Snd-CEA        R-Open
                  R-Rcv-CEA        Process-CEA      R-Open
 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,       Closed
                                   I-Disc
                  I-Peer-Disc      I-Disc           Closed
                  I-Rcv-CER        I-Snd-CEA        I-Open
                  I-Rcv-CEA        Process-CEA      I-Open
 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; and the source port of an
 incoming connection is arbitrary.  Upon receipt of CER, the identity
 of the connecting peer can be uniquely determined from Origin-Host.
 For this reason, a Diameter peer must employ logic separate from the
 state machine to receive connection requests, accept them, and await
 CER.  Once 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 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 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.

Calhoun, et al. Standards Track [Page 68] RFC 3588 Diameter Based Protocol September 2003

5.6.2. Events

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

Calhoun, et al. Standards Track [Page 69] RFC 3588 Diameter Based Protocol September 2003

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-prefix,
 since the actual action would be identical, but would occur on one of
 two possible connections.
 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 shutdown, 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.

Calhoun, et al. Standards Track [Page 70] RFC 3588 Diameter Based Protocol September 2003

 Process-DWA    The DWA message is serviced.
 Process        A message is serviced.

5.6.4. The Election Process

 The election is performed on the responder.  The responder compares
 the Origin-Host received in the CER sent by its peer with its own
 Origin-Host.  If the local Diameter entity's Origin-Host is higher
 than the peer's, a Win-Election event is issued locally.
 The comparison proceeds by considering the shorter OctetString to be
 padded with zeros so that it length is the same as the length of the
 longer, then performing an octet-by-octet unsigned comparison with
 the first octet being most significant.  Any remaining octets are
 assumed to have value 0x80.

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 a combination
 of the Destination-Realm and Destination-Host AVPs, in one of these
 three combinations:
  1. a request that is not able to be proxied (such as CER) MUST NOT

contain either Destination-Realm or Destination-Host AVPs.

  1. 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.
  1. 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:
  1. Authentication requests that span multiple round trips
  1. 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.

Calhoun, et al. Standards Track [Page 71] RFC 3588 Diameter Based Protocol September 2003

  1. 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 forward a request to a host described in the
 Destination-Host AVP only if the host in question is included in its
 peer table (see Section 2.7).  Otherwise, the request is routed based
 on the Destination-Realm only (see Sections 6.1.6).
 The Destination-Realm AVP MUST be present if the message is
 proxiable.  Request messages that may be forwarded by Diameter agents
 (proxies, redirects or relays) MUST also contain an Acct-
 Application-Id AVP, an Auth-Application-Id AVP or a Vendor-Specific-
 Application-Id AVP.  A message that MUST NOT be forwarded by Diameter
 agents (proxies, redirects or relays) MUST not include the
 Destination-Realm in its ABNF.  The value of the Destination-Realm
 AVP MAY be extracted from the User-Name AVP, or other application-
 specific methods.
 When a message is received, the message is processed in the following
 order:
 1. If the message is destined for the local host, the procedures
    listed in Section 6.1.4 are followed.
 2. 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.
 3. The procedures listed in Section 6.1.6 are followed, which is
    known as Request Routing.
 4. If none of the above is 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.
 Note the processing rules contained in this section are intended to
 be used as general guidelines to Diameter developers.  Certain
 implementations MAY use different methods than the ones described
 here, and still comply with the protocol specification.  See Section
 7 for more detail on error handling.

Calhoun, et al. Standards Track [Page 72] RFC 3588 Diameter Based Protocol September 2003

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:
  1. the Command-Code is set to the appropriate value
  1. the 'R' bit is set
  1. the End-to-End Identifier is set to a locally unique value
  1. the Origin-Host and Origin-Realm AVPs MUST be set to the

appropriate values, used to identify the source of the message

  1. the Destination-Host and Destination-Realm AVPs MUST be set to the

appropriate values as described in Section 6.1.

  1. an Acct-Application-Id AVP, an Auth-Application-Id or a Vendor-

Specific-Application-Id AVP must be included if the request is

    proxiable.

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 MUST
 be followed:
  1. the Hop-by-Hop Identifier should be set to a locally unique value
  1. 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 occur:
  1. The Destination-Host AVP contains the local host's identity,

Calhoun, et al. Standards Track [Page 73] RFC 3588 Diameter Based Protocol September 2003

  1. 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
  1. Both the Destination-Host and the Destination-Realm are not

present.

 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 that the local node is
 able to directly communicate with.
 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 applications.
 A Diameter message that may be forwarded by Diameter agents (proxies,
 redirects or relays) MUST include the target realm in the
 Destination-Realm AVP and one of the application identification AVPs
 Auth-Application-Id, Acct-Application-Id or Vendor-Specific-
 Application-Id.  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 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 Realm Routing Table (see Section
 2.7).

6.1.7. 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 separate
 Redirect-Host AVP.

Calhoun, et al. Standards Track [Page 74] RFC 3588 Diameter Based Protocol September 2003

                +------------------+
                |     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 the answer message with the 'E' bit set, and the
 Result-Code AVP set to DIAMETER_REDIRECT_INDICATION uses the hop-by-
 hop field in the Diameter header to identify the request in the
 pending message queue (see Section 5.3) that is to be redirected.  If
 no transport connection exists with the new agent, 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.

6.1.8. 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 the request was
 received from.
 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.  Proxy-Info AVP has certain
 security implications and SHOULD contain an embedded HMAC with a
 node-local key.  Alternatively, it MAY simply use local storage to
 store state information.
 The message is then forwarded to the next hop, as identified in the
 Realm Routing Table.

Calhoun, et al. Standards Track [Page 75] RFC 3588 Diameter Based Protocol September 2003

 Figure 6 provides an example of message routing using the procedures
 listed in these sections.
      (Origin-Host=nas.mno.net)    (Origin-Host=nas.mno.net)
      (Origin-Realm=mno.net)       (Origin-Realm=mno.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

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:
  1. The same Hop-by-Hop identifier in the request is used in the

answer.

  1. The local host's identity is encoded in the Origin-Host AVP.
  1. The Destination-Host and Destination-Realm AVPs MUST NOT be

present in the answer message.

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

failure.

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

in the answer.

  1. Any Proxy-Info AVPs in the request MUST be added to the answer

message, in the same order they were present in the request.

  1. The 'P' bit is set to the same value as the one in the request.
  1. The same End-to-End identifier in the request is used in the

answer.

Calhoun, et al. Standards Track [Page 76] RFC 3588 Diameter Based Protocol September 2003

 Note that the error messages (see Section 7.3) 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 which 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 and
 it MUST issue an STR on behalf of the access device.
 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
 MUST be present in all Diameter messages.  This AVP identifies the
 endpoint that originated the Diameter message.  Relay agents MUST NOT
 modify this AVP.
 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.

Calhoun, et al. Standards Track [Page 77] RFC 3588 Diameter Based Protocol September 2003

 This AVP SHOULD be placed as close to the Diameter header as
 possible. 6.10

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 the message is to be routed to.  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.
 Request messages whose ABNF does not list the Destination-Realm AVP
 as a mandatory AVP are inherently non-routable messages.
 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, and therefore MUST NOT be protected by end-to-end security.

Calhoun, et al. Standards Track [Page 78] RFC 3588 Diameter Based Protocol September 2003

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.  The Grouped
 Data field has the following ABNF 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, and
 contains state local information, and MUST be treated as opaque data.

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).  The
 Auth-Application-Id MUST also be present in all Authentication and/or
 Authorization messages that are defined in a separate Diameter
 specification and have an Application ID assigned.

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).  The Acct-Application-Id MUST also be
 present in all Accounting messages.  Exactly one of the Auth-
 Application-Id and Acct-Application-Id AVPs MAY be present.

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.

Calhoun, et al. Standards Track [Page 79] RFC 3588 Diameter Based Protocol September 2003

 Currently, the following values are supported, but there is ample
 room to add new security Ids.
 NO_INBAND_SECURITY                0
    This peer does not support TLS.  This is the default value, if the
    AVP is omitted.
 TLS                               1
    This node supports TLS security, as defined by [TLS].

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 of the Auth-Application-Id and
 Acct-Application-Id AVPs MAY be present.
 This AVP MUST also be present as the first AVP in all experimental
 commands defined in the vendor-specific application.
 This AVP SHOULD be placed as close to the Diameter header as
 possible.
 AVP Format
 <Vendor-Specific-Application-Id> ::= < AVP Header: 260 >
                                   1* [ Vendor-Id ]
                                   0*1{ Auth-Application-Id }
                                   0*1{ Acct-Application-Id }

6.12. Redirect-Host AVP

 One or more of 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 pertaining to this session.

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.

Calhoun, et al. Standards Track [Page 80] RFC 3588 Diameter Based Protocol September 2003

 When present, this AVP dictates how the routing entry resulting from
 the Redirect-Host is to be used.  The following values are supported:
 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 MAY be sent to the host specified in the
    Redirect-Host AVP.
 ALL_REALM                         2
    All messages destined for the realm requested MAY 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
    MAY be sent to the host specified in the Redirect-Host AVP.
 ALL_APPLICATION                   4
    All messages for the application requested MAY 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 MAY be sent to the host specified in the Redirect-
    Host AVP.
 ALL_USER                          6
    All messages for the user requested MAY be sent to the host
    specified in the Redirect-Host AVP.

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, the
 Result-Code AVP is set to DIAMETER_REDIRECT_INDICATION and the
 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, will be
 cached.  Note that once a host created due to a redirect indication
 is no longer reachable, any associated peer and routing table entries
 MUST be deleted.

Calhoun, et al. Standards Track [Page 81] RFC 3588 Diameter Based Protocol September 2003

6.15. E2E-Sequence AVP

 The E2E-Sequence AVP (AVP Code 300) provides anti-replay protection
 for end to end messages and is of type grouped.  It contains a random
 value (an OctetString with a nonce) and counter (an Integer).  For
 each end-to-end peer with which a node communicates (or remembers
 communicating) a different nonce value MUST be used and the counter
 is initiated at zero and increases by one each time this AVP is
 emitted to that peer.  This AVP MUST be included in all messages
 which use end-to-end protection (e.g., CMS signing or encryption).

7. Error Handling

 There are two different types of errors in Diameter; protocol and
 application errors.  A protocol error is one that occurs at the base
 protocol level, and MAY require per hop attention (e.g., 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.
                        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

Calhoun, et al. Standards Track [Page 82] RFC 3588 Diameter Based Protocol September 2003

 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, and
 therefore the message would be forwarded back to the originator of
 the request.
 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:
  1. 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.
  1. 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.
  1. A command is received with an AVP that is omitted, yet is

mandatory according to the command's ABNF. 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

Calhoun, et al. Standards Track [Page 83] RFC 3588 Diameter Based Protocol September 2003

 (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
 whether an error occurred.  All Diameter answer messages defined in
 IETF applications 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 11.4).  Diameter provides the
 following classes of errors, all identified by the thousands digit in
 the decimal notation:
  1. 1xxx (Informational)
  2. 2xxx (Success)
  3. 3xxx (Protocol Errors)
  4. 4xxx (Transient Failures)
  5. 5xxx (Permanent Failure)
 A non-recognized 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.

Calhoun, et al. Standards Track [Page 84] RFC 3588 Diameter Based Protocol September 2003

 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 and only these errors MUST
 only be used in answer messages whose 'E' bit is set.
 DIAMETER_COMMAND_UNSUPPORTED       3001
    The Request contained a Command-Code that the receiver did not
    recognize or support.  This MUST be used when a Diameter node
    receives an experimental command that it does not understand.
 DIAMETER_UNABLE_TO_DELIVER         3002
    This error is given when Diameter can not 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 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.

Calhoun, et al. Standards Track [Page 85] RFC 3588 Diameter Based Protocol September 2003

 DIAMETER_INVALID_HDR_BITS          3008
    A request was received whose bits in the Diameter header were
    either set to an invalid combination, or to a value that is
    inconsistent with the command code's definition.
 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.
 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.
 DIAMETER_AVP_UNSUPPORTED           5001
    The peer received a message that contained an AVP that is not
    recognized or supported and was marked with the Mandatory bit.  A
    Diameter message with this error MUST contain one or more Failed-
    AVP AVP containing the AVPs that caused the failure.
 DIAMETER_UNKNOWN_SESSION_ID        5002
    The request contained an unknown Session-Id.

Calhoun, et al. Standards Track [Page 86] RFC 3588 Diameter Based Protocol September 2003

 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 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 is not willing to provide service to
    the user.  One or more Failed-AVP AVPs MUST be present, containing
    the AVPs that contradicted each other.
 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 when a CER message is received, and there
    are no common applications supported between the peers.
 DIAMETER_UNSUPPORTED_VERSION       5011
    This error is returned when a request was received, whose version
    number is unsupported.

Calhoun, et al. Standards Track [Page 87] RFC 3588 Diameter Based Protocol September 2003

 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 an unrecognized bit in the Diameter
    header is set to one (1).
 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.
 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) 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 ABNF specification for the command,
 and will instead conform to the following ABNF:

Calhoun, et al. Standards Track [Page 88] RFC 3588 Diameter Based Protocol September 2003

 Message Format
 <answer-message> ::= < Diameter Header: code, ERR [PXY] >
                   0*1< Session-Id >
                      { Origin-Host }
                      { Origin-Realm }
                      { Result-Code }
                      [ Origin-State-Id ]
                      [ Error-Reporting-Host ]
                      [ Proxy-Info ]
                    * [ AVP ]
 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 real-time, and
 SHOULD NOT be expected to 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 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.
 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 which is restricted to 0, 1, or 0-1
 occurrences.

Calhoun, et al. Standards Track [Page 89] RFC 3588 Diameter Based Protocol September 2003

 A Diameter message MAY 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.
 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.  Its Data field has the
 following ABNF grammar:
 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 which
 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).

8. Diameter User Sessions

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

Calhoun, et al. Standards Track [Page 90] RFC 3588 Diameter Based Protocol September 2003

 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 use 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 the home realm is willing to be fiscally
 responsible for.  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.
 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 is used to terminate user
 sessions.  These are used to allow servers that maintain state
 information to free resources.

Calhoun, et al. Standards Track [Page 91] RFC 3588 Diameter Based Protocol September 2003

 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.

8.1. Authorization Session State Machine

 This section contains a set of finite state machines, representing
 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 as an error
 condition, and an answer, if applicable, MUST be returned to the
 originator of the message.
 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'.

Calhoun, et al. Standards Track [Page 92] RFC 3588 Diameter Based Protocol September 2003

 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
 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
 Pending   Failed Service-specific        Cleanup    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      Session-Timeout Expires on     Send STR   Discon
           Access Device

Calhoun, et al. Standards Track [Page 93] RFC 3588 Diameter Based Protocol September 2003

 Open      ASR Received,                  Send ASA   Discon
           client will comply with        with
           request to end the session     Result-Code
                                          = SUCCESS,
                                          Send STR.
 Open      ASR Received,                  Send ASA   Open
           client will not comply with    with
           request to end the session     Result-Code
                                          != SUCCESS
 Open      Authorization-Lifetime +       Send STR   Discon
           Auth-Grace-Period expires on
           access device
 Discon    ASR Received                   Send ASA   Discon
 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             serv.
                                          specific answer
 Idle      Service-specific authorization Send       Idle
           request received, and          failed serv.
           user is not authorized         specific answer
 Open      Service-specific authorization Send       Open
           request received, and user     successful
           is authorized                  serv. specific
                                                answer
 Open      Service-specific authorization Send       Idle
           request received, and user     failed serv.
           is not authorized              specific
                                          answer,
                                          Cleanup
 Open      Home server wants to           Send ASR   Discon
           terminate the service

Calhoun, et al. Standards Track [Page 94] RFC 3588 Diameter Based Protocol September 2003

 Open      Authorization-Lifetime (and    Cleanup    Idle
           Auth-Grace-Period) expires
           on home server.
 Open      Session-Timeout expires on     Cleanup    Idle
           home server
 Discon    Failure to send ASR            Wait,      Discon
                                          resend ASR
 Discon    ASR successfully sent and      Cleanup    Idle
           ASA Received with Result-Code
 Not       ASA Received                   None       No Change.
 Discon
 Any       STR Received                   Send STA,  Idle
                                          Cleanup.
 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        Cleanup    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

Calhoun, et al. Standards Track [Page 95] RFC 3588 Diameter Based Protocol September 2003

 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 serv. Idle
           request received, and          specific
           successfully processed         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 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.
 Both base Diameter AVPs as well as application specific AVPs MAY 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 limits 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, and hence
 accounting connectivity problems are required to cause the serviced
 user to be disconnected.  Otherwise, records produced by the client

Calhoun, et al. Standards Track [Page 96] RFC 3588 Diameter Based Protocol September 2003

 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, which the 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 have an effect also
 to 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

Calhoun, et al. Standards Track [Page 97] RFC 3588 Diameter Based Protocol September 2003

 PendingS  Successful accounting                     Open
           start answer received
 PendingS  Failure to send and buffer     Store      Open
           space available and realtime   Start
           not equal to DELIVER_AND_GRANT Record
 PendingS  Failure to send and no buffer             Open
           space available and realtime
           equal to GRANT_AND_LOSE
 PendingS  Failure to send and no buffer  Disconnect Idle
           space available and realtime   user/dev
           not equal to
           GRANT_AND_LOSE
 PendingS  Failed accounting start answer            Open
           received and realtime equal
           to GRANT_AND_LOSE
 PendingS  Failed accounting start answer Disconnect Idle
           received and realtime 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 record  interim
           can be overwritten) and        record
           realtime not equal to
           DELIVER_AND_GRANT
 PendingI  Failure to send and no buffer             Open
           space available and realtime
           equal to GRANT_AND_LOSE

Calhoun, et al. Standards Track [Page 98] RFC 3588 Diameter Based Protocol September 2003

 PendingI  Failure to send and no buffer  Disconnect Idle
           space available and realtime   user/dev
           not equal to GRANT_AND_LOSE
 PendingI  Failed accounting interim                 Open
           answer received and realtime
           equal to GRANT_AND_LOSE
 PendingI  Failed accounting interim      Disconnect Idle
           answer received and realtime   user/dev
           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
 PendingL  Failure to send and no buffer             Idle
           space available
 PendingL  Failed accounting stop answer             Idle
           received

Calhoun, et al. Standards Track [Page 99] RFC 3588 Diameter Based Protocol September 2003

                  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
                       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

Calhoun, et al. Standards Track [Page 100] RFC 3588 Diameter Based Protocol September 2003

 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

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 pre-paid 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 a RAR message with 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 service-initiated re-auth is
 supported, since some applications do not allow access devices to
 prompt the user for re-auth.

Calhoun, et al. Standards Track [Page 101] RFC 3588 Diameter Based Protocol September 2003

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 indicates the disposition of
 the request.
 A successful RAA message MUST be followed by an application-specific
 authentication and/or authorization message.

Calhoun, et al. Standards Track [Page 102] RFC 3588 Diameter Based Protocol September 2003

 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-Host-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
 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

Calhoun, et al. Standards Track [Page 103] RFC 3588 Diameter Based Protocol September 2003

 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 expires 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 the access
 device 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 ]

Calhoun, et al. Standards Track [Page 104] RFC 3588 Diameter Based Protocol September 2003

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 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
 credit or other reasons that were not anticipated when the session
 was first authorized.  On the other hand, an operator may maintain a
 management server for the purpose of issuing ASRs to administratively
 remove users from the network.
 An access device that receives an ASR with Session-ID equal to a
 currently active session MAY stop the session.  Whether the access

Calhoun, et al. Standards Track [Page 105] RFC 3588 Diameter Based Protocol September 2003

 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
 respond with an Abort-Session-Answer, including a Result-Code AVP to
 indicate what action it took.
 Note that if the access device does stop the session upon receipt of
 an ASR, it issues an STR to the authorizing server (which may or may
 not be the agent issuing the ASR) just as it would if the session
 were terminated for any other reason.

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 server 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, Result-Code is set to DIAMETER_SUCCESS.  If the session
 is not currently active, Result-Code is set to
 DIAMETER_UNKNOWN_SESSION_ID.  If the access device does not stop the
 session for any other reason, Result-Code is set to
 DIAMETER_UNABLE_TO_COMPLY.

Calhoun, et al. Standards Track [Page 106] RFC 3588 Diameter Based Protocol September 2003

 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

 Origin-State-Id is used to allow rapid detection of terminated
 sessions for which no STR would have been issued, due to
 unanticipated shutdown of an access device.
 By including Origin-State-Id in CER/CEA messages, an access device
 allows a next-hop server to determine immediately upon connection
 whether the device has lost its sessions since the last connection.
 By including Origin-State-Id in request messages, an access device
 also allows a server with which it communicates via proxy to make
 such a determination.  However, a server that is not directly
 connected with the access device will not discover that the access
 device has been restarted unless and until it receives a new request
 from the access device.  Thus, use of this mechanism across proxies
 is opportunistic rather than reliable, but useful nonetheless.
 When a Diameter server receives an Origin-State-Id that is greater
 than the Origin-State-Id previously received from the same issuer, it
 may assume that the issuer has lost state since the previous message
 and that all sessions that were active under the lower Origin-State-
 Id have been terminated.  The Diameter server MAY clean up all
 session state associated with such lost sessions, and MAY also issues
 STRs for all such lost sessions that were authorized on upstream
 servers, to allow session state to be cleaned up globally.

Calhoun, et al. Standards Track [Page 107] RFC 3588 Diameter Based Protocol September 2003

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 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 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 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 4.4).  The remainder of the
 Session-Id is delimited by a ";" character, and MAY be any sequence
 that the client can guarantee to be eternally unique; however, the
 following format is recommended, (square brackets [] indicate an
 optional element):

Calhoun, et al. Standards Track [Page 108] RFC 3588 Diameter Based Protocol September 2003

 <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, 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 may include a modem's
 device Id, a layer 2 address, timestamp, etc.
 Example, in which there is no optional value:
    accesspoint7.acme.com;1876543210;523
 Example, in which there is an optional value:
    accesspoint7.acme.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 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.  Great 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.  This is typically used in cases where multiple
 authentication methods are used, and a successful auth response with
 this AVP set to zero is used to signal that the next authentication
 method is to be immediately initiated.  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.
 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.

Calhoun, et al. Standards Track [Page 109] RFC 3588 Diameter Based Protocol September 2003

 An Authorization-Lifetime AVP MAY be present in re-authorization
 messages, and 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.  However, the server MAY
 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.
 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:

Calhoun, et al. Standards Track [Page 110] RFC 3588 Diameter Based Protocol September 2003

 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) [RADIUS] 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 8.9).
 A Session-Timeout AVP MAY be present in a re-authorization answer
 message, and 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) [RADIUS] is of type UTF8String, which
 contains the User-Name, in a format consistent with the NAI
 specification [NAI].

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 following values are defined:

Calhoun, et al. Standards Track [Page 111] RFC 3588 Diameter Based Protocol September 2003

 DIAMETER_LOGOUT                   1
    The user initiated a disconnect
 DIAMETER_SERVICE_NOT_PROVIDED     2
    This value is used when the user disconnected prior to the receipt
    of the authorization answer message.
 DIAMETER_BAD_ANSWER               3
    This value indicates that the authorization answer received by the
    access device was not processed successfully.
 DIAMETER_ADMINISTRATIVE           4
    The user was not granted access, or was disconnected, due to
    administrative reasons, such as the receipt of a Abort-Session-
    Request message.
 DIAMETER_LINK_BROKEN              5
    The communication to the user was abruptly disconnected.
 DIAMETER_AUTH_EXPIRED             6
    The user's access was terminated since its authorized session time
    has expired.
 DIAMETER_USER_MOVED               7
    The user is receiving services from another access device.
 DIAMETER_SESSION_TIMEOUT          8
    The user's session has timed out, and service has been terminated.

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.

Calhoun, et al. Standards Track [Page 112] RFC 3588 Diameter Based Protocol September 2003

 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 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 messages for this session MUST be sent
 to the same authorization server.  This AVP MAY also specify that a
 Session-Termination-Request message for this session MUST be sent to
 the same authorizing server.
 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

Calhoun, et al. Standards Track [Page 113] RFC 3588 Diameter Based Protocol September 2003

 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.
 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 to 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.

Calhoun, et al. Standards Track [Page 114] RFC 3588 Diameter Based Protocol September 2003

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.

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 [ACCMGMT] have been built in to 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 [ACCMGMT], 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.

Calhoun, et al. Standards Track [Page 115] RFC 3588 Diameter Based Protocol September 2003

 The Diameter accounting server MAY override the interim interval or
 the realtime 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.

9.2. Protocol Messages

 A Diameter node that receives a successful authentication and/or
 authorization messages from the Home AAA server MUST collect
 accounting information for the session.  The Accounting-Request
 message is used to transmit the accounting information to the Home
 AAA 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.  A rejected Accounting-Request message MAY cause
 the user's session to be terminated, depending on the value of the
 Accounting-Realtime-Required AVP received earlier for the session in
 question.
 Each Diameter Accounting protocol message MAY be compressed, in order
 to reduce network bandwidth usage.  If IPsec and IKE are used to
 secure the Diameter session, then IP compression [IPComp] MAY be used
 and IKE [IKE] MAY be used to negotiate the compression parameters.
 If TLS is used to secure the Diameter session, then TLS compression
 [TLS] MAY be used.

9.3. Application document requirements

 Each Diameter application (e.g., NASREQ, MobileIP), MUST define their
 Service-Specific AVPs that MUST be present in the Accounting-Request
 message in a section entitled "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 this section.

9.4. Fault Resilience

 Diameter Base protocol mechanisms are used to overcome small message
 loss and network faults of 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 same record to several servers and duplication of messages

Calhoun, et al. Standards Track [Page 116] RFC 3588 Diameter Based Protocol September 2003

 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 starting sending the records in the non-volatile
 memory to the accounting server with 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
 how many accounting records may at most 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 oldest, undelivered or 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.  If strong authentication across agents is required, end-to-
 end security may be used for authentication purposes.
 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
 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

Calhoun, et al. Standards Track [Page 117] RFC 3588 Diameter Based Protocol September 2003

 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_RECORD 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

 The Diameter protocol's Session-Id AVP, which is globally unique (see
 Section 8.8), is used during the authorization phase to identify a
 particular session.  Services that do not require any authorization
 still use the Session-Id AVP to identify sessions.  Accounting
 messages MAY use a different Session-Id from that sent in
 authorization messages.  Specific applications MAY require different
 a Session-ID for accounting messages.
 However, there are certain applications that require multiple
 accounting sub-sessions.  Such applications would send messages with
 a constant Session-Id AVP, but a different Accounting-Sub-Session-Id
 AVP.  In these cases, correlation is performed using the Session-Id.
 It is important to note that receiving a STOP_RECORD 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.
 Furthermore, there are certain applications where a user receives
 service from different access devices (e.g., Mobile IPv4), each with
 their own unique Session-Id.  In such cases, the Acct-Multi-Session-
 Id AVP is used for correlation.  During authorization, a server that

Calhoun, et al. Standards Track [Page 118] RFC 3588 Diameter Based Protocol September 2003

 determines that a request is for an existing session SHOULD include
 the Acct-Multi-Session-Id AVP, which the access device MUST include
 in all subsequent accounting messages.
 The Acct-Multi-Session-Id AVP MAY include the value of the original
 Session-Id.  It's contents are implementation specific, but MUST be
 globally unique across other Acct-Multi-Session-Id, 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 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.
 One of Acct-Application-Id and Vendor-Specific-Application-Id AVPs
 MUST be present.  If the Vendor-Specific-Application-Id grouped AVP
 is present, it must have an Acct-Application-Id inside.
 The AVP listed below SHOULD include service specific accounting AVPs,
 as described in Section 9.3.

Calhoun, et al. Standards Track [Page 119] RFC 3588 Diameter Based Protocol September 2003

 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 ]
              [ 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 and includes the usage AVPs only
 if CMS is in use when sending this command.  Note that the inclusion
 of the usage AVPs when CMS is not being used leads to unnecessarily
 large answer messages, and can not be used as a server's proof of the
 receipt of these AVPs in an end-to-end fashion.  If the Accounting-
 Request was protected by end-to-end security, then the corresponding
 ACA message MUST be protected by end-to-end security.
 Only the target Diameter Server, known as the home Diameter Server,
 SHOULD respond with the Accounting-Answer command.
 One of Acct-Application-Id and Vendor-Specific-Application-Id AVPs
 MUST be present.  If the Vendor-Specific-Application-Id grouped AVP
 is present, it must have an Acct-Application-Id inside.
 The AVP listed below SHOULD include service specific accounting AVPs,
 as described in Section 9.3.

Calhoun, et al. Standards Track [Page 120] RFC 3588 Diameter Based Protocol September 2003

 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-Reporting-Host ]
              [ 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 is the only record of the service.
 START_RECORD                    2
    An 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.

Calhoun, et al. Standards Track [Page 121] RFC 3588 Diameter Based Protocol September 2003

 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

 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 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.

Calhoun, et al. Standards Track [Page 122] RFC 3588 Diameter Based Protocol September 2003

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 together 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 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.

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.

Calhoun, et al. Standards Track [Page 123] RFC 3588 Diameter Based Protocol September 2003

 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 can not be delivered or
    stored.

10. AVP Occurrence Table

 The following tables presents the AVPs defined in this document, and
 specifies in which Diameter messages they MAY, or MAY NOT be present.
 Note that AVPs that can only be present within a Grouped AVP are not
 represented in this table.
 The table uses 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.
 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.

Calhoun, et al. Standards Track [Page 124] RFC 3588 Diameter Based Protocol September 2003

                     +-----------------------------------------------+
                     |                  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+ |0  |0+ |0  |0+ |0  |0+ |0  |0+ |0  |0+ |
 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  |
 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  |0  |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  |

Calhoun, et al. Standards Track [Page 125] RFC 3588 Diameter Based Protocol September 2003

 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.
                               +-----------+
                               |  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 |
 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             | 0-1 | 0-1 |
 User-Name                     | 0-1 | 0-1 |
 Vendor-Specific-Application-Id| 0-1 | 0-1 |
 ------------------------------+-----+-----+

Calhoun, et al. Standards Track [Page 126] RFC 3588 Diameter Based Protocol September 2003

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 BCP 26 [IANA].  The following
 policies are used here with the meanings defined in BCP 26: "Private
 Use", "First Come First Served", "Expert Review", "Specification
 Required", "IETF Consensus", "Standards Action".
 This section explains the criteria to be used by the IANA for
 assignment of numbers within namespaces defined within this document.
 Diameter is not intended as a general purpose protocol, and
 allocations SHOULD NOT be made for purposes unrelated to
 authentication, authorization or accounting.
 For registration requests where a Designated Expert should be
 consulted, the responsible IESG area director should appoint the
 Designated Expert.  For Designated Expert with Specification
 Required, the request is posted to the AAA WG mailing list (or, if it
 has been disbanded, a successor designated by the Area Director) for
 comment and review, and MUST include a pointer to a public
 specification. Before a period of 30 days has passed, the Designated
 Expert will either approve or deny the registration request and
 publish a notice of the decision to the AAA WG mailing list or its
 successor.  A denial notice must be justified by an explanation and,
 in the cases  where it is possible, concrete suggestions on how the
 request can be modified so as to become acceptable.

11.1. AVP Header

 As defined in Section 4, the AVP header contains three fields that
 requires IANA namespace management; the AVP Code, Vendor-ID and Flags
 field.

11.1.1. AVP Codes

 The AVP Code namespace is used to identify attributes.  There are
 multiple namespaces.  Vendors can have their own AVP Codes namespace
 which 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 IANA
 controlled AVP Codes namespace.  The AVP Codes and sometimes also
 possible values in an AVP are controlled and maintained by IANA.

Calhoun, et al. Standards Track [Page 127] RFC 3588 Diameter Based Protocol September 2003

 AVP Code 0 is not used. AVP Codes 1-255 are managed separately as
 RADIUS Attribute Types [RADTYPE].  This document defines the AVP
 Codes 257-274, 276-285, 287, 291-300, 480, 483 and 485-486.  See
 Section 4.5 for the assignment of the namespace in this
 specification.
 AVPs may be allocated following Designated Expert with Specification
 Required [IANA].  Release of blocks of AVPs (more than 3 at a time
 for a given purpose) should require IETF Consensus.
 Note that Diameter defines a mechanism for Vendor-Specific AVPs,
 where the Vendor-Id field in the AVP header is set to a non-zero
 value.  Vendor-Specific AVPs codes are for Private Use and should be
 encouraged instead of allocation of global attribute types, for
 functions specific only to one vendor's implementation of Diameter,
 where no interoperability is deemed useful.  Where a Vendor-Specific
 AVP is implemented by more than one vendor, allocation of global AVPs
 should be encouraged instead.

11.1.2. AVP Flags

 There are 8 bits in the AVP Flags field of the AVP header, defined in
 Section 4.  This document assigns bit 0 ('V'endor Specific), bit 1
 ('M'andatory) and bit 2 ('P'rotected).  The remaining bits should
 only be assigned via a Standards Action [IANA].

11.2. Diameter Header

 As defined in Section 3, the Diameter header contains two fields that
 require IANA namespace management; Command Code and Command Flags.

11.2.1. Command Codes

 The Command Code namespace is used to identify Diameter commands.
 The values 0-255 are reserved for RADIUS backward compatibility, and
 are defined as "RADIUS Packet Type Codes" in [RADTYPE].  Values 256-
 16,777,213 are for permanent, standard commands, allocated by IETF
 Consensus [IANA].  This document defines the Command Codes 257, 258,
 271, 274-275, 280 and 282.  See Section 3.1 for the assignment of the
 namespace in this specification.
 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,
 as outlined in [IANA-EXP].

Calhoun, et al. Standards Track [Page 128] RFC 3588 Diameter Based Protocol September 2003

11.2.2. Command Flags

 There are eight bits in the Command Flags field of the Diameter
 header.  This document assigns bit 0 ('R'equest), bit 1 ('P'roxy),
 bit 2 ('E'rror) and bit 3 ('T').  Bits 4 through 7 MUST only be
 assigned via a Standards Action [IANA].

11.3. Application Identifiers

 As defined in Section 2.4, the Application Identifier is used to
 identify a specific Diameter Application.  There are standards-track
 application ids and vendor specific application ids.
 IANA [IANA] has assigned the range 0x00000001 to 0x00ffffff for
 standards-track applications; and 0x01000000 - 0xfffffffe for vendor
 specific applications, on a first-come, first-served basis.  The
 following values are allocated.
    Diameter Common Messages            0
    NASREQ                              1 [NASREQ]
    Mobile-IP                           2 [DIAMMIP]
    Diameter Base Accounting            3
    Relay                               0xffffffff
 Assignment of standards-track application IDs are by Designated
 Expert with Specification Required [IANA].
 Both Application-Id and Acct-Application-Id AVPs use the same
 Application Identifier space.
 Vendor-Specific Application Identifiers, are for Private Use.
 Vendor-Specific Application Identifiers are assigned on a First Come,
 First Served basis by IANA.

11.4. AVP Values

 Certain AVPs in Diameter define a list of values with various
 meanings.  For attributes other than those specified in this section,
 adding additional values to the list can be done on a First Come,
 First Served basis by IANA.

11.4.1. Result-Code AVP Values

 As defined in Section 7.1, the Result-Code AVP (AVP Code 268) defines
 the values 1001, 2001-2002, 3001-3010, 4001-4002 and 5001-5017.
 All remaining values are available for assignment via IETF Consensus
 [IANA].

Calhoun, et al. Standards Track [Page 129] RFC 3588 Diameter Based Protocol September 2003

11.4.2. Accounting-Record-Type AVP Values

 As defined in Section 9.8.1, the Accounting-Record-Type AVP (AVP Code
 480) defines the values 1-4.  All remaining values are available for
 assignment via IETF Consensus [IANA].

11.4.3. Termination-Cause AVP Values

 As defined in Section 8.15, the Termination-Cause AVP (AVP Code 295)
 defines the values 1-8.  All remaining values are available for
 assignment via IETF Consensus [IANA].

11.4.4. Redirect-Host-Usage AVP Values

 As defined in Section 6.13, the Redirect-Host-Usage AVP (AVP Code
 261) defines the values 0-5.  All remaining values are available for
 assignment via IETF Consensus [IANA].

11.4.5. Session-Server-Failover AVP Values

 As defined in Section 8.18, the Session-Server-Failover AVP (AVP Code
 271) defines the values 0-3.  All remaining values are available for
 assignment via IETF Consensus [IANA].

11.4.6. Session-Binding AVP Values

 As defined in Section 8.17, the Session-Binding AVP (AVP Code 270)
 defines the bits 1-4.  All remaining bits are available for
 assignment via IETF Consensus [IANA].

11.4.7. Disconnect-Cause AVP Values

 As defined in Section 5.4.3, the Disconnect-Cause AVP (AVP Code 273)
 defines the values 0-2.  All remaining values are available for
 assignment via IETF Consensus [IANA].

11.4.8. Auth-Request-Type AVP Values

 As defined in Section 8.7, the Auth-Request-Type AVP (AVP Code 274)
 defines the values 1-3.  All remaining values are available for
 assignment via IETF Consensus [IANA].

11.4.9. Auth-Session-State AVP Values

 As defined in Section 8.11, the Auth-Session-State AVP (AVP Code 277)
 defines the values 0-1.  All remaining values are available for
 assignment via IETF Consensus [IANA].

Calhoun, et al. Standards Track [Page 130] RFC 3588 Diameter Based Protocol September 2003

11.4.10. Re-Auth-Request-Type AVP Values

 As defined in Section 8.12, the Re-Auth-Request-Type AVP (AVP Code
 285) defines the values 0-1.  All remaining values are available for
 assignment via IETF Consensus [IANA].

11.4.11. Accounting-Realtime-Required AVP Values

 As defined in Section 9.8.7, the Accounting-Realtime-Required AVP
 (AVP Code 483) defines the values 1-3.  All remaining values are
 available for assignment via IETF Consensus [IANA].

11.4.12. Inband-Security-Id AVP (code 299)

 As defined in Section 6.10, the Inband-Security-Id AVP (AVP Code 299)
 defines the values 0-1.  All remaining values are available for
 assignment via IETF Consensus [IANA].

11.5. Diameter TCP/SCTP Port Numbers

 The IANA has assigned TCP and SCTP port number 3868 to Diameter.

11.6. NAPTR Service Fields

 The registration in the RFC MUST include the following information:
 Service Field: The service field being registered.  An example for a
 new fictitious transport protocol called NCTP might be "AAA+D2N".
 Protocol: The specific transport protocol associated with that
 service field.  This MUST include the name and acronym for the
 protocol, along with reference to a document that describes the
 transport protocol.  For example - "New Connectionless Transport
 Protocol (NCTP), RFC 5766".
 Name and Contact Information: The name, address, email address and
 telephone number for the person performing the registration.
 The following values have been placed into the registry:
    Services Field               Protocol
    AAA+D2T                       TCP
    AAA+D2S                       SCTP

12. Diameter protocol related configurable parameters

 This section contains the configurable parameters that are found
 throughout this document:

Calhoun, et al. Standards Track [Page 131] RFC 3588 Diameter Based Protocol September 2003

 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.
 Realm 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 to.  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.

13. Security Considerations

 The Diameter base protocol assumes that messages are secured by using
 either IPSec or TLS.  This security mechanism is acceptable in
 environments where there is no untrusted third party agent.  In other
 situations, end-to-end security is needed.
 Diameter clients, such as Network Access Servers (NASes) and Mobility
 Agents MUST support IP Security [SECARCH] and MAY support TLS [TLS].
 Diameter servers MUST support TLS and IPsec.  Diameter
 implementations MUST use transmission-level security of some kind
 (IPsec or TLS) on each connection.
 If a Diameter connection is not protected by IPsec, then the CER/CEA
 exchange MUST include an Inband-Security-ID AVP with a value of TLS.
 For TLS usage, a TLS handshake will begin when both ends are in the
 open state, after completion of the CER/CEA exchange.  If the TLS
 handshake is successful, all further messages will be sent via TLS.
 If the handshake fails, both ends move to the closed state.
 It is suggested that IPsec be used primarily at the edges for intra-
 domain exchanges.  For NAS devices without certificate support, pre-
 shared keys can be used between the NAS and a local AAA proxy.
 For protection of inter-domain exchanges, TLS is recommended.  See
 Sections 13.1 and 13.2 for more details on IPsec and TLS usage.

Calhoun, et al. Standards Track [Page 132] RFC 3588 Diameter Based Protocol September 2003

13.1. IPsec Usage

 All Diameter implementations MUST support IPsec ESP [IPsec] in
 transport mode with non-null encryption and authentication algorithms
 to provide per-packet authentication, integrity protection and
 confidentiality, and MUST support the replay protection mechanisms of
 IPsec.
 Diameter implementations MUST support IKE for peer authentication,
 negotiation of security associations, and key management, using the
 IPsec DOI [IPSECDOI].  Diameter implementations MUST support peer
 authentication using a pre-shared key, and MAY support certificate-
 based peer authentication using digital signatures.  Peer
 authentication using the public key encryption methods outlined in
 IKE's Sections 5.2 and 5.3 [IKE] SHOULD NOT be used.
 Conformant implementations MUST support both IKE Main Mode and
 Aggressive Mode.  When pre-shared keys are used for authentication,
 IKE Aggressive Mode SHOULD be used, and IKE Main Mode SHOULD NOT be
 used.  When digital signatures are used for authentication, either
 IKE Main Mode or IKE Aggressive Mode MAY be used.
 When digital signatures are used to achieve authentication, an IKE
 negotiator SHOULD use IKE Certificate Request Payload(s) to specify
 the certificate authority (or authorities) that are trusted in
 accordance with its local policy.  IKE negotiators SHOULD use
 pertinent certificate revocation checks before accepting a PKI
 certificate for use in IKE's authentication procedures.
 The Phase 2 Quick Mode exchanges used to negotiate protection for
 Diameter connections MUST explicitly carry the Identity Payload
 fields (IDci and IDcr).  The DOI provides for several types of
 identification data.  However, when used in conformant
 implementations, each ID Payload MUST carry a single IP address and a
 single non-zero port number, and MUST NOT use the IP Subnet or IP
 Address Range formats.  This allows the Phase 2 security association
 to correspond to specific TCP and SCTP connections.
 Since IPsec acceleration hardware may only be able to handle a
 limited number of active IKE Phase 2 SAs, Phase 2 delete messages may
 be sent for idle SAs, as a means of keeping the number of active
 Phase 2 SAs to a minimum.  The receipt of an IKE Phase 2 delete
 message SHOULD NOT be interpreted as a reason for tearing down a
 Diameter connection.  Rather, it is preferable to leave the
 connection up, and if additional traffic is sent on it, to bring up
 another IKE Phase 2 SA to protect it.  This avoids the potential for
 continually bringing connections up and down.

Calhoun, et al. Standards Track [Page 133] RFC 3588 Diameter Based Protocol September 2003

13.2. TLS Usage

 A Diameter node that initiates a connection to another Diameter node
 acts as a TLS client according to [TLS], and a Diameter node that
 accepts a connection acts as a TLS server.  Diameter nodes
 implementing TLS for security MUST mutually authenticate as part of
 TLS session establishment.  In order to ensure mutual authentication,
 the Diameter node acting as TLS server must request a certificate
 from the Diameter node acting as TLS client, and the Diameter node
 acting as TLS client MUST be prepared to supply a certificate on
 request.
 Diameter nodes MUST be able to negotiate the following TLS 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 cipher
 suite:
    TLS_RSA_WITH_AES_128_CBC_SHA
 Diameter nodes MAY negotiate other TLS cipher suites.

13.3. 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.  When certificate authentication Diameter
 peers may not be known beforehand, and therefore peer discovery may
 be required.
 Note that IPsec is considerably less flexible than TLS when it comes
 to configuring root CAs.  Since use of Port identifiers is prohibited
 within IKE Phase 1, within IPsec it is not possible to uniquely
 configure trusted root CAs for each application individually; the
 same policy must be used for all applications.  This implies, for
 example, that a root CA trusted for use with Diameter must also be

Calhoun, et al. Standards Track [Page 134] RFC 3588 Diameter Based Protocol September 2003

 trusted to protect SNMP.  These restrictions can be awkward at best.
 Since TLS supports application-level granularity in certificate
 policy, TLS SHOULD be used to protect Diameter connections between
 administrative domains.  IPsec is most appropriate for intra-domain
 usage when pre-shared keys are used as a security mechanism.
 When pre-shared key authentication is used with IPsec to protect
 Diameter, unique pre-shared keys are configured with Diameter peers,
 who are identified by their IP address (Main Mode), or possibly their
 FQDN (Aggressive Mode).  As a result, it is necessary for the set of
 Diameter peers to be known beforehand.  Therefore, peer discovery is
 typically not necessary.
 The following is intended to provide some guidance on the issue.
 It is recommended that a Diameter peer implement the same security
 mechanism (IPsec or TLS) across all its peer-to-peer connections.
 Inconsistent use of security mechanisms can result in redundant
 security mechanisms being used (e.g., TLS over IPsec) or worse,
 potential security vulnerabilities.  When IPsec is used with
 Diameter, a typical security policy for outbound traffic is "Initiate
 IPsec, from me to any, destination port Diameter"; for inbound
 traffic, the policy would be "Require IPsec, from any to me,
 destination port Diameter".
 This policy causes IPsec to be used whenever a Diameter peer
 initiates a connection to another Diameter peer, and to be required
 whenever an inbound Diameter connection occurs.  This policy is
 attractive, since it does not require policy to be set for each peer
 or dynamically modified each time a new Diameter connection is
 created; an IPsec SA is automatically created based on a simple
 static policy.  Since IPsec extensions are typically not available to
 the sockets API on most platforms, and IPsec policy functionality is
 implementation dependent, use of a simple static policy is the often
 the simplest route to IPsec-enabling a Diameter implementation.
 One implication of the recommended policy is that if a node is using
 both TLS and IPsec, there is not a convenient way in which to use
 either TLS or IPsec, but not both, without reserving an additional
 port for TLS usage.  Since Diameter uses the same port for TLS and
 non-TLS usage, where the recommended IPsec policy is put in place, a
 TLS-protected connection will match the IPsec policy, and both IPsec
 and TLS will be used to protect the Diameter connection.  To avoid
 this, it would be necessary to plumb peer-specific policies either
 statically or dynamically.

Calhoun, et al. Standards Track [Page 135] RFC 3588 Diameter Based Protocol September 2003

 If IPsec is used to secure Diameter peer-to-peer connections, IPsec
 policy SHOULD be set so as to require IPsec protection for inbound
 connections, and to initiate IPsec protection for outbound
 connections.  This can be accomplished via use of inbound and
 outbound filter policy.

14. References

14.1. Normative References

 [AAATRANS]     Aboba, B. and J. Wood, "Authentication, Authorization
                and Accounting (AAA) Transport Profile", RFC 3539,
                June 2003.
 [ABNF]         Crocker, D. and P. Overell, "Augmented BNF for Syntax
                Specifications: ABNF", RFC 2234, November 1997.
 [ASSIGNNO]     Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced
                by an On-line Database", RFC 3232, January 2002.
 [DIFFSERV]     Nichols, K., Blake, S., Baker, F. and D. Black,
                "Definition of the Differentiated Services Field (DS
                Field) in the IPv4 and IPv6 Headers", RFC 2474,
                December 1998.
 [DIFFSERVAF]   Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
                "Assured Forwarding PHB Group", RFC 2597, June 1999.
 [DIFFSERVEF]   Davie, B., Charny, A., Bennet, J., Benson, K., Le
                Boudec, J., Courtney, W., Davari, S., Firoiu, V. and
                D. Stiliadis, "An Expedited Forwarding PHB", RFC 3246,
                March 2002.
 [DNSSRV]       Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR
                for specifying the location of services (DNS SRV)",
                RFC 2782, February 2000.
 [EAP]          Blunk, L. and J. Vollbrecht, "PPP Extensible
                Authentication Protocol (EAP)", RFC 2284, March 1998.
 [FLOATPOINT]   Institute of Electrical and Electronics Engineers,
                "IEEE Standard for Binary Floating-Point Arithmetic",
                ANSI/IEEE Standard 754-1985, August 1985.
 [IANA]         Narten, T. and H. Alvestrand, "Guidelines for Writing
                an IANA Considerations Section in RFCs", BCP 26, RFC
                2434, October 1998.

Calhoun, et al. Standards Track [Page 136] RFC 3588 Diameter Based Protocol September 2003

 [IANAADFAM]    IANA; "Address Family Numbers",
                http://www.iana.org/assignments/address-family-numbers
 [IANAWEB]      IANA, "Number assignment", http://www.iana.org
 [IKE]          Harkins, D. and D. Carrel, "The Internet Key Exchange
                (IKE)", RFC 2409, November 1998.
 [IPComp]       Shacham, A., Monsour, R., Pereira, R. and M. Thomas,
                "IP Payload Compression Protocol (IPComp)", RFC 3173,
                September 2001.
 [IPSECDOI]     Piper, D., "The Internet IP Security Domain of
                Interpretation for ISAKMP", RFC 2407, November 1998.
 [IPV4]         Postel, J., "Internet Protocol", STD 5, RFC 791,
                September 1981.
 [IPV6]         Hinden, R. and S. Deering, "IP Version 6 Addressing
                Architecture", RFC 2373, July 1998.
 [KEYWORDS]     Bradner, S., "Key words for use in RFCs to Indicate
                Requirement Levels", BCP 14, RFC 2119, March 1997.
 [NAI]          Aboba, B. and M. Beadles, "The Network Access
                Identifier", RFC 2486, January 1999.
 [NAPTR]        Mealling, M. and R. Daniel, "The naming authority
                pointer (NAPTR) DNS resource record," RFC 2915,
                September 2000.
 [RADTYPE]      IANA, "RADIUS Types",
                http://www.iana.org/assignments/radius-types
 [SCTP]         Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
                Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
                Zhang, L. and V. Paxson, "Stream Control Transmission
                Protocol", RFC 2960, October 2000.
 [SLP]          Veizades, J., Guttman, E., Perkins, C. and M. Day,
                "Service Location Protocol, Version 2", RFC 2165, June
                1999.
 [SNTP]         Mills, D., "Simple Network Time Protocol (SNTP)
                Version 4 for IPv4, IPv6 and OSI", RFC 2030, October
                1996.

Calhoun, et al. Standards Track [Page 137] RFC 3588 Diameter Based Protocol September 2003

 [TCP]          Postel, J. "Transmission Control Protocol", STD 7, RFC
                793, January 1981.
 [TEMPLATE]     Guttman, E., Perkins, C. and J. Kempf, "Service
                Templates and Service: Schemes", RFC 2609, June 1999.
 [TLS]          Dierks, T. and C. Allen, "The TLS Protocol Version
                1.0", RFC 2246, January 1999.
 [TLSSCTP]      Jungmaier, A., Rescorla, E. and M. Tuexen, "Transport
                Layer Security over Stream Control Transmission
                Protocol", RFC 3436, December 2002.
 [URI]          Berners-Lee, T., Fielding, R. and L. Masinter,
                "Uniform Resource Identifiers (URI): Generic Syntax",
                RFC 2396, August 1998.
 [UTF8]         Yergeau, F., "UTF-8, a transformation format of ISO
                10646", RFC 2279, January 1998.

14.2. Informative References

 [AAACMS]       P. Calhoun, W. Bulley, S. Farrell, "Diameter CMS
                Security Application", Work in Progress.
 [AAAREQ]       Aboba, B., Calhoun, P., Glass, S., Hiller, T., McCann,
                P., Shiino, H., Zorn, G., Dommety, G., Perkins, C.,
                Patil, B., Mitton, D., Manning, S., Beadles, M.,
                Walsh, P., Chen, X., Sivalingham, S., Hameed, A.,
                Munson, M., Jacobs, S., Lim, B., Hirschman, B., Hsu,
                R., Xu, Y., Campbell, E., Baba, S. and E. Jaques,
                "Criteria for Evaluating AAA Protocols for Network
                Access", RFC 2989, November 2000.
 [ACCMGMT]      Aboba, B., Arkko, J. and D. Harrington. "Introduction
                to Accounting Management", RFC 2975, October 2000.
 [CDMA2000]     Hiller, T., Walsh, P., Chen, X., Munson, M., Dommety,
                G., Sivalingham, S., Lim, B., McCann, P., Shiino, H.,
                Hirschman, B., Manning, S., Hsu, R., Koo, H., Lipford,
                M., Calhoun, P., Lo, C., Jaques, E., Campbell, E., Xu,
                Y., Baba, S., Ayaki, T., Seki, T. and A.  Hameed,
                "CDMA2000 Wireless Data Requirements for AAA", RFC
                3141, June 2001.
 [DIAMMIP]      P. Calhoun, C. Perkins, "Diameter Mobile IP
                Application", Work in Progress.

Calhoun, et al. Standards Track [Page 138] RFC 3588 Diameter Based Protocol September 2003

 [DYNAUTH]      Chiba, M., Dommety, G., Eklund, M., Mitton, D. and B.
                Aboba, "Dynamic Authorization Extensions to Remote
                Authentication Dial In User Service (RADIUS)", RFC
                3576, July 2003.
 [IANA-EXP]     T. Narten, "Assigning Experimental and Testing Numbers
                Considered Useful", Work in Progress.
 [MIPV4]        Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
                August 2002.
 [MIPREQ]       Glass, S., Hiller, T., Jacobs, S. and C. Perkins,
                "Mobile IP Authentication, Authorization, and
                Accounting Requirements", RFC 2977, October 2000.
 [NASNG]        Mitton, D. and M. Beadles, "Network Access Server
                Requirements Next Generation (NASREQNG) NAS Model",
                RFC 2881, July 2000.
 [NASREQ]       P. Calhoun, W. Bulley, A. Rubens, J. Haag, "Diameter
                NASREQ Application", Work in Progress.
 [NASCRIT]      Beadles, M. and D. Mitton, "Criteria for Evaluating
                Network Access Server Protocols", RFC 3169, September
                2001.
 [PPP]          Simpson, W., "The Point-to-Point Protocol (PPP)", STD
                51, RFC 1661, July 1994.
 [PROXYCHAIN]   Aboba, B. and J. Vollbrecht, "Proxy Chaining and
                Policy Implementation in Roaming", RFC 2607, June
                1999.
 [RADACCT]      Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
 [RADEXT]       Rigney, C., Willats, W. and P. Calhoun, "RADIUS
                Extensions", RFC 2869, June 2000.
 [RADIUS]       Rigney, C., Willens, S., Rubens, A. and W. Simpson,
                "Remote Authentication Dial In User Service (RADIUS)",
                RFC 2865, June 2000.
 [ROAMREV]      Aboba, B., Lu, J., Alsop, J., Ding, J. and W. Wang,
                "Review of Roaming Implementations", RFC 2194,
                September 1997.
 [ROAMCRIT]     Aboba, B. and G. Zorn, "Criteria for Evaluating
                Roaming Protocols", RFC 2477, January 1999.

Calhoun, et al. Standards Track [Page 139] RFC 3588 Diameter Based Protocol September 2003

 [SECARCH]      Kent, S. and R. Atkinson, "Security Architecture for
                the Internet Protocol", RFC 2401, November 1998.
 [TACACS]       Finseth, C., "An Access Control Protocol, Sometimes
                Called TACACS", RFC 1492, July 1993.

15. Acknowledgements

 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 similarly
 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.

Calhoun, et al. Standards Track [Page 140] RFC 3588 Diameter Based Protocol September 2003

Appendix A. Diameter Service Template

 The following service template describes the attributes used by
 Diameter servers to advertise themselves.  This simplifies the
 process of selecting an appropriate server to communicate with.  A
 Diameter client can request specific Diameter servers based on
 characteristics of the Diameter service desired (for example, an AAA
 server to use for accounting.)
 Name of submitter:  "Erik Guttman" <Erik.Guttman@sun.com> Language of
 service template:  en
 Security Considerations:
    Diameter clients and servers use various cryptographic mechanisms
    to protect communication integrity, confidentiality as well as
    perform end-point authentication.  It would thus be difficult if
    not impossible for an attacker to advertise itself using SLPv2 and
    pose as a legitimate Diameter peer without proper preconfigured
    secrets or cryptographic keys.  Still, as Diameter services are
    vital for network operation it is important to use SLPv2
    authentication to prevent an attacker from modifying or
    eliminating service advertisements for legitimate Diameter
    servers.
 Template text:
 -------------------------template begins here-----------------------
 template-type=service:diameter
 template-version=0.0
 template-description=
   The Diameter protocol is defined by RFC 3588.
 template-url-syntax=
   url-path= ; The Diameter URL format is described in Section 2.9.
             ; Example: 'aaa://aaa.example.com:1812;transport=tcp
    supported-auth-applications= string L M
    # This attribute lists the Diameter applications supported by the
    # AAA implementation.  The applications currently defined are:
    #  Application Name     Defined by
    #  ----------------     -----------------------------------
    #  NASREQ               Diameter Network Access Server Application
    #  MobileIP             Diameter Mobile IP Application
    #
    # Notes:
    #   . Diameter implementations support one or more applications.
    #   . Additional applications may be defined in the future.
    #     An updated service template will be created at that time.

Calhoun, et al. Standards Track [Page 141] RFC 3588 Diameter Based Protocol September 2003

    #
    NASREQ,MobileIP
    supported-acct-applications= string L M
    # This attribute lists the Diameter applications supported by the
    # AAA implementation.  The applications currently defined are:
    #  Application Name     Defined by
    #  ----------------     -----------------------------------
    #  NASREQ               Diameter Network Access Server Application
    #  MobileIP             Diameter Mobile IP Application
    #
    # Notes:
    #   . Diameter implementations support one or more applications.
    #   . Additional applications may be defined in the future.
    #     An updated service template will be created at that time.
    #
    NASREQ,MobileIP
    supported-transports= string L M
    SCTP
    # This attribute lists the supported transports that the Diameter
    # implementation accepts.  Note that a compliant Diameter
    # implementation MUST support SCTP, though it MAY support other
    # transports, too.
    SCTP,TCP
  1. ————————template ends here———————–

Appendix B. NAPTR Example

 As an example, consider a client that wishes to resolve aaa:ex.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+D2S"           ""
    _diameter._sctp.example.com IN NAPTR 100  50  "s"  "AAA+D2T"
    ""  _aaa._tcp.example.com
 This indicates that the server supports SCTP, and TCP, in that order.
 If the client supports over SCTP, SCTP will be used, targeted to a
 host determined by an SRV lookup of _diameter._sctp.ex.com. That
 lookup would return:
 ;;          Priority Weight Port   Target
    IN SRV  0        1      5060   server1.example.com IN SRV  0
    2      5060   server2.example.com

Calhoun, et al. Standards Track [Page 142] RFC 3588 Diameter Based Protocol September 2003

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:
  1. Failover to an alternate server. Where close to real-time

performance is required, failover thresholds need to be kept low

    and this may lead to an increased likelihood of duplicates.
    Failover can occur at the client or within Diameter agents.
  1. Failure of a client or agent after sending of a record from non-

volatile memory, but prior to receipt of an application layer ACK

    and deletion of the record. record to be sent.  This will result
    in retransmission of the record soon after the client or agent has
    rebooted.
  1. 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.
  1. 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
 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.

Calhoun, et al. Standards Track [Page 143] RFC 3588 Diameter Based Protocol September 2003

 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 possible duplicate by setting the T
 flag.  Since the Diameter server is responsible for duplicate
 detection, it can choose to make use of the T flag or not, in order
 to optimize duplicate detection.  Since the T flag does not affect
 interoperability, and may not be needed by some servers, generation
 of the T flag is REQUIRED for Diameter clients and agents, but 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
    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 out of order duplicates, 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 has
    expired, then 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.

Calhoun, et al. Standards Track [Page 144] RFC 3588 Diameter Based Protocol September 2003

Appendix D. Intellectual Property Statement

 The IETF takes no position regarding the validity or scope of any
 intellectual property or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights.  Information on the
 IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11.  Copies of
 claims of rights made available for publication and any assurances of
 licenses to be made available, or the result of an attempt made to
 obtain a general license or permission for the use of such
 proprietary rights by implementers or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard.  Please address the information to the IETF Executive
 Director.

Calhoun, et al. Standards Track [Page 145] RFC 3588 Diameter Based Protocol September 2003

Authors' Addresses

 Pat R. Calhoun
 Airespace, Inc.
 110 Nortech Parkway
 San Jose, California, 95134
 USA
 Phone:  +1 408-635-2023
 Fax:  +1 408-635-2020
 EMail:  pcalhoun@airespace.com
 John Loughney
 Nokia Research Center
 Itamerenkatu 11-13
 00180 Helsinki
 Finland
 Phone:  +358 50 483 6242
 EMail:  john.Loughney@nokia.com
 Jari Arkko
 Ericsson
 02420 Jorvas
 Finland
 Phone: +358 40 5079256
 EMail: Jari.Arkko@ericsson.com
 Erik Guttman
 Sun Microsystems, Inc.
 Eichhoelzelstr. 7
 74915 Waibstadt
 Germany
 Phone:  +49 7263 911 701
 EMail:  erik.guttman@sun.com
 Glen Zorn
 Cisco Systems, Inc.
 500 108th Avenue N.E., Suite 500
 Bellevue, WA 98004
 USA
 Phone:  +1 425 438 8218

Calhoun, et al. Standards Track [Page 146] RFC 3588 Diameter Based Protocol September 2003

Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
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 or assist in its implementation may be prepared, copied, published
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 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
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 The limited permissions granted above are perpetual and will not be
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 This document and the information contained herein is provided on an
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 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Calhoun, et al. Standards Track [Page 147]

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