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[Text version]

This RFC is obsoleted by: RFC 6733

This RFC is updated by: RFC 5719 , RFC 5729 , RFC 6408

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

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
              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
   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
       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
   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.
   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.
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 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:
   -  Delivery of AVPs (attribute value pairs)
   -  Capabilities negotiation
   -  Error notification
   -  Extensibility, through addition of new commands and AVPs (required
      in [AAAREQ]).
   -  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:

   -  Transporting of user authentication information, for the purposes
      of enabling the Diameter server to authenticate the user.
   -  Transporting of service specific authorization information,
      between client and servers, allowing the peers to decide whether a
      user's access request should be granted.
   -  Exchanging resource usage information, which MAY be used for
      accounting purposes, capacity planning, etc.
   -  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. 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.

1.2. Approach to Extensibility

The Diameter protocol is designed to be extensible, using several mechanisms, including:
      -  Defining new AVP values
      -  Creating new AVPs
      -  Creating new authentication/authorization applications
      -  Creating new accounting applications
      -  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.
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:

- Adding new AVPs to the command, which have the "M" bit set.

   -  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).
   -  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
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.

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.
   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.
   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.
   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.
   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],
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 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. 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.

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

          +--------+          +-------+          +--------+
          | 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.
   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.
   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:
   -  They can distribute administration of systems to a configurable
      grouping, including the maintenance of security associations.
   -  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.

- They can do value-added processing to the requests or responses. - They can be used for load balancing.

   -  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:

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

   -  Limiting resources authorized to a particular user
   -  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.
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.
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.

                               +------+
                               |      |
                               | 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

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:

- Never use end-to-end security.

   -  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.

- 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. 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.

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'
                    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.
   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.

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

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.
   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.
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:

   -  The request was successful
   -  The request failed
   -  An additional request must be sent to provide information the peer
      requires prior to returning a successful or failed answer.
   -  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. 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.
      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.

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).
   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
      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.
   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.
      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.
      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.
                      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.
            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.

   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.
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".
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).
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|
       +-------+-------+-------+-------+

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.

                                            +---------------------+
                                            |    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                                |    |     |    |     |    |
   -----------------------------------------|----+-----+----+-----|----|
                                            +---------------------+
                                            |    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                        |    |     |    |     |    |
   -----------------------------------------|----+-----+----+-----|----|

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
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
          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 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 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.
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.
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 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:
   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.
   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.
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. 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
                    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.

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).

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

- Authentication requests that span multiple round trips

   -  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.
   -  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.

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:
   -  the Command-Code is set to the appropriate value
   -  the 'R' bit is set

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

   -  the Origin-Host and Origin-Realm AVPs MUST be set to the
      appropriate values, used to identify the source of the message
   -  the Destination-Host and Destination-Realm AVPs MUST be set to the
      appropriate values as described in Section 6.1.
   -  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:

- the Hop-by-Hop Identifier should be set to a locally unique value

- 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:

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

   -  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
   -  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.
                  +------------------+
                  |     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. 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:
   -  The same Hop-by-Hop identifier in the request is used in the
      answer.

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

   -  The Destination-Host and Destination-Realm AVPs MUST NOT be
      present in the answer message.
   -  The Result-Code AVP is added with its value indicating success or
      failure.
   -  If the Session-Id is present in the request, it MUST be included
      in the answer.
   -  Any Proxy-Info AVPs in the request MUST be added to the answer
      message, in the same order they were present in the request.

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

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

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.
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.
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.

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 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:

   -  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.
   -  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.
   -  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 (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:

      -  1xxx (Informational)
      -  2xxx (Success)
      -  3xxx (Protocol Errors)
      -  4xxx (Transient Failures)
      -  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.
   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.
   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.
   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.
   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:
   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. 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.
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. 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'. 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
   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
   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
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 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
   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
   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
                    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
   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.

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.

   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 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 ]

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 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.

   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.

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): <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. 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:
   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:
   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. 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
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.

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. 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 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 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 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.

   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.

   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.
   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.

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.
   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.
                       +-----------------------------------------------+
                       |                  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  |
   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 |
   ------------------------------+-----+-----+

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.
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].

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].

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].

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:
   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.

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.

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 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. 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.
   [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.
   [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.
   [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.
   [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.

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.
      #
      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
   -------------------------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

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:
   -  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.
   -  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.
   -  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.

- 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. 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.

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.

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

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