Diameter Base Protocol
draft-ietf-dime-rfc3588bis-23.txt

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 in both local and roaming situations. This document specifies the message format, transport, error reporting, accounting and security services used by all Diameter applications. The Diameter base protocol as defined in this document must be supported by all Diameter implementations.

Status of this Memo

This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79.

Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

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This Internet-Draft will expire on February 6, 2011.

Copyright Notice

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Table of Contents

1.  Introduction
    1.1.  Diameter Protocol
        1.1.1.  Description of the Document Set
        1.1.2.  Conventions Used in This Document
        1.1.3.  Changes from RFC3588
    1.2.  Terminology
    1.3.  Approach to Extensibility
        1.3.1.  Defining New AVP Values
        1.3.2.  Creating New AVPs
        1.3.3.  Creating New Commands
        1.3.4.  Creating New Diameter Applications
2.  Protocol Overview
    2.1.  Transport
        2.1.1.  SCTP Guidelines
    2.2.  Securing Diameter Messages
    2.3.  Diameter Application Compliance
    2.4.  Application Identifiers
    2.5.  Connections vs. Sessions
    2.6.  Peer Table
    2.7.  Routing Table
    2.8.  Role of Diameter Agents
        2.8.1.  Relay Agents
        2.8.2.  Proxy Agents
        2.8.3.  Redirect Agents
        2.8.4.  Translation Agents
    2.9.  Diameter Path Authorization
3.  Diameter Header
    3.1.  Command Codes
    3.2.  Command Code ABNF specification
    3.3.  Diameter Command Naming Conventions
4.  Diameter AVPs
    4.1.  AVP Header
        4.1.1.  Optional Header Elements
    4.2.  Basic AVP Data Formats
    4.3.  Derived AVP Data Formats
        4.3.1.  Common Derived AVPs
    4.4.  Grouped AVP Values
        4.4.1.  Example AVP with a Grouped Data type
    4.5.  Diameter Base Protocol AVPs
5.  Diameter Peers
    5.1.  Peer Connections
    5.2.  Diameter Peer Discovery
    5.3.  Capabilities Exchange
        5.3.1.  Capabilities-Exchange-Request
        5.3.2.  Capabilities-Exchange-Answer
        5.3.3.  Vendor-Id AVP
        5.3.4.  Firmware-Revision AVP
        5.3.5.  Host-IP-Address AVP
        5.3.6.  Supported-Vendor-Id AVP
        5.3.7.  Product-Name AVP
    5.4.  Disconnecting Peer connections
        5.4.1.  Disconnect-Peer-Request
        5.4.2.  Disconnect-Peer-Answer
        5.4.3.  Disconnect-Cause AVP
    5.5.  Transport Failure Detection
        5.5.1.  Device-Watchdog-Request
        5.5.2.  Device-Watchdog-Answer
        5.5.3.  Transport Failure Algorithm
        5.5.4.  Failover and Failback Procedures
    5.6.  Peer State Machine
        5.6.1.  Incoming connections
        5.6.2.  Events
        5.6.3.  Actions
        5.6.4.  The Election Process
6.  Diameter message processing
    6.1.  Diameter Request Routing Overview
        6.1.1.  Originating a Request
        6.1.2.  Sending a Request
        6.1.3.  Receiving Requests
        6.1.4.  Processing Local Requests
        6.1.5.  Request Forwarding
        6.1.6.  Request Routing
        6.1.7.  Predictive Loop Avoidance
        6.1.8.  Redirecting Requests
        6.1.9.  Relaying and Proxying Requests
    6.2.  Diameter Answer Processing
        6.2.1.  Processing received Answers
        6.2.2.  Relaying and Proxying Answers
    6.3.  Origin-Host AVP
    6.4.  Origin-Realm AVP
    6.5.  Destination-Host AVP
    6.6.  Destination-Realm AVP
    6.7.  Routing AVPs
        6.7.1.  Route-Record AVP
        6.7.2.  Proxy-Info AVP
        6.7.3.  Proxy-Host AVP
        6.7.4.  Proxy-State AVP
    6.8.  Auth-Application-Id AVP
    6.9.  Acct-Application-Id AVP
    6.10.  Inband-Security-Id AVP
    6.11.  Vendor-Specific-Application-Id AVP
    6.12.  Redirect-Host AVP
    6.13.  Redirect-Host-Usage AVP
    6.14.  Redirect-Max-Cache-Time AVP
7.  Error Handling
    7.1.  Result-Code AVP
        7.1.1.  Informational
        7.1.2.  Success
        7.1.3.  Protocol Errors
        7.1.4.  Transient Failures
        7.1.5.  Permanent Failures
    7.2.  Error Bit
    7.3.  Error-Message AVP
    7.4.  Error-Reporting-Host AVP
    7.5.  Failed-AVP AVP
    7.6.  Experimental-Result AVP
    7.7.  Experimental-Result-Code AVP
8.  Diameter User Sessions
    8.1.  Authorization Session State Machine
    8.2.  Accounting Session State Machine
    8.3.  Server-Initiated Re-Auth
        8.3.1.  Re-Auth-Request
        8.3.2.  Re-Auth-Answer
    8.4.  Session Termination
        8.4.1.  Session-Termination-Request
        8.4.2.  Session-Termination-Answer
    8.5.  Aborting a Session
        8.5.1.  Abort-Session-Request
        8.5.2.  Abort-Session-Answer
    8.6.  Inferring Session Termination from Origin-State-Id
    8.7.  Auth-Request-Type AVP
    8.8.  Session-Id AVP
    8.9.  Authorization-Lifetime AVP
    8.10.  Auth-Grace-Period AVP
    8.11.  Auth-Session-State AVP
    8.12.  Re-Auth-Request-Type AVP
    8.13.  Session-Timeout AVP
    8.14.  User-Name AVP
    8.15.  Termination-Cause AVP
    8.16.  Origin-State-Id AVP
    8.17.  Session-Binding AVP
    8.18.  Session-Server-Failover AVP
    8.19.  Multi-Round-Time-Out AVP
    8.20.  Class AVP
    8.21.  Event-Timestamp AVP
9.  Accounting
    9.1.  Server Directed Model
    9.2.  Protocol Messages
    9.3.  Accounting Application Extension and Requirements
    9.4.  Fault Resilience
    9.5.  Accounting Records
    9.6.  Correlation of Accounting Records
    9.7.  Accounting Command-Codes
        9.7.1.  Accounting-Request
        9.7.2.  Accounting-Answer
    9.8.  Accounting AVPs
        9.8.1.  Accounting-Record-Type AVP
        9.8.2.  Acct-Interim-Interval AVP
        9.8.3.  Accounting-Record-Number AVP
        9.8.4.  Acct-Session-Id AVP
        9.8.5.  Acct-Multi-Session-Id AVP
        9.8.6.  Accounting-Sub-Session-Id AVP
        9.8.7.  Accounting-Realtime-Required AVP
10.  AVP Occurrence Table
    10.1.  Base Protocol Command AVP Table
    10.2.  Accounting AVP Table
11.  IANA Considerations
    11.1.  AVP Header
        11.1.1.  AVP Codes
        11.1.2.  AVP Flags
    11.2.  Diameter Header
        11.2.1.  Command Codes
        11.2.2.  Command Flags
    11.3.  Application Identifiers
    11.4.  AVP Values
        11.4.1.  Result-Code AVP Values
        11.4.2.  Experimental-Result-Code AVP
        11.4.3.  Accounting-Record-Type AVP Values
        11.4.4.  Termination-Cause AVP Values
        11.4.5.  Redirect-Host-Usage AVP Values
        11.4.6.  Session-Server-Failover AVP Values
        11.4.7.  Session-Binding AVP Values
        11.4.8.  Disconnect-Cause AVP Values
        11.4.9.  Auth-Request-Type AVP Values
        11.4.10.  Auth-Session-State AVP Values
        11.4.11.  Re-Auth-Request-Type AVP Values
        11.4.12.  Accounting-Realtime-Required AVP Values
        11.4.13.  Inband-Security-Id AVP (code 299)
    11.5.  Diameter TCP, SCTP and TLS/TCP Port Numbers
    11.6.  S-NAPTR Parameters
12.  Diameter protocol related configurable parameters
13.  Security Considerations
    13.1.  TLS Usage
    13.2.  Peer-to-Peer Considerations
14.  References
    14.1.  Normative References
    14.2.  Informational References
Appendix A.  Acknowledgements
    A.1.  RFC3588bis
    A.2.  RFC3588
Appendix B.  S-NAPTR Example
Appendix C.  Duplicate Detection
Appendix D.  Internationalized Domain Names
§  Authors' Addresses

1.  Introduction

Authentication, Authorization and Accounting (AAA) protocols such as TACACS [RFC1492] (Finseth, C., “An Access Control Protocol, Sometimes Called TACACS,” July 1993.) and RADIUS [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) were initially deployed to provide dial-up PPP [RFC1661] (Simpson, W., “The Point-to-Point Protocol (PPP),” July 1994.) and terminal server access. Over time, AAA support was needed on many new access technologies, the scale and complexity of AAA networks grew, and AAA was also used on new applications (such as voice over IP). This lead to new demands on AAA protocols.

Network access requirements for AAA protocols are summarized in [RFC2989] (Aboba, B., Calhoun, P., Glass, S., Hiller, T., McCann, P., Shiino, H., Zorn, G., Dommety, G., C.Perkins, B.Patil, D.Mitton, S.Manning, M.Beadles, P.Walsh, X.Chen, S.Sivalingham, A.Hameed, M.Munson, S.Jacobs, B.Lim, B.Hirschman, R.Hsu, Y.Xu, E.Campell, S.Baba, and E.Jaques, “Criteria for Evaluating AAA Protocols for Network Access,” November 2000.). These include:

Failover

[RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) 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 [RFC3539] (Aboba, B. and J. Wood, “Authentication, Authorization and Accounting (AAA) Transport Profile,” June 2003.).

Transmission-level security

[RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) defines an application-layer authentication and integrity scheme that is required only for use with Response packets. While [RFC2869] (Rigney, C., Willats, W., and P. Calhoun, “RADIUS Extensions,” June 2000.) defines an additional authentication and integrity mechanism, use is only required during Extensible Authentication Protocol (EAP) sessions. While attribute-hiding is supported, [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) does not provide support for per-packet confidentiality. In accounting, [RFC2866] (Rigney, C., “RADIUS Accounting,” June 2000.) assumes that replay protection is provided by the backend billing server, rather than within the protocol itself.

While [RFC3162] (Aboba, B., Zorn, G., and D. Mitton, “RADIUS and IPv6,” August 2001.) defines the use of IPsec with RADIUS, support for IPsec is not required. In order to provide universal support for transmission-level security, and enable both intra- and inter-domain AAA deployments, Diameter provides support for TLS. 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 [RFC2975] (Aboba, B., Arkko, J., and D. Harrington, “Introduction to Accounting Management,” October 2000.), 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 [RFC3539] (Aboba, B. and J. Wood, “Authentication, Authorization and Accounting (AAA) Transport Profile,” June 2003.).

Agent support

[RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) 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 [RFC5176] (Chiba, M., Dommety, G., Eklund, M., Mitton, D., and B. Aboba, “Dynamic Authorization Extensions to Remote Authentication Dial In User Service (RADIUS),” January 2008.), support is optional. This makes it difficult to implement features such as unsolicited disconnect or reauthentication/reauthorization on demand across a heterogeneous deployment. To tackle this issue, support for server-initiated messages is mandatory in Diameter.

Transition support

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

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 [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.). Through DNS, Diameter enables dynamic discovery of peers (see Section 5.2 (Diameter Peer Discovery)). Derivation of dynamic session keys is enabled via transmission-level security.

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

1.1.  Diameter Protocol

The Diameter base protocol provides the following facilities:

• Ability to exchange messages and deliver AVPs
• Capabilities negotiation
• Error notification
• Extensibility, through addition of new applications, commands and AVPs (required in [RFC2989] (Aboba, B., Calhoun, P., Glass, S., Hiller, T., McCann, P., Shiino, H., Zorn, G., Dommety, G., C.Perkins, B.Patil, D.Mitton, S.Manning, M.Beadles, P.Walsh, X.Chen, S.Sivalingham, A.Hameed, M.Munson, S.Jacobs, B.Lim, B.Hirschman, R.Hsu, Y.Xu, E.Campell, S.Baba, and E.Jaques, “Criteria for Evaluating AAA Protocols for Network Access,” November 2000.)).
• Basic services necessary for applications, such as handling of user sessions or accounting

All data delivered by the protocol is in the form of AVPs. Some of these AVP values are used by the Diameter protocol itself, while others deliver data associated with particular applications that employ Diameter. AVPs may be arbitrarily added to Diameter messages, the only restriction being that the Augmented Backus-Naur Form (ABNF, [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.)) Command Code syntax specification (Section 3.2) is satisfied. 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.
• Routing, relaying, proxying and redirecting of Diameter messages through a server hierarchy.

The Diameter base protocol satisfies the minimum requirements for an AAA protocol, as specified by [RFC2989] (Aboba, B., Calhoun, P., Glass, S., Hiller, T., McCann, P., Shiino, H., Zorn, G., Dommety, G., C.Perkins, B.Patil, D.Mitton, S.Manning, M.Beadles, P.Walsh, X.Chen, S.Sivalingham, A.Hameed, M.Munson, S.Jacobs, B.Lim, B.Hirschman, R.Hsu, Y.Xu, E.Campell, S.Baba, and E.Jaques, “Criteria for Evaluating AAA Protocols for Network Access,” November 2000.). The base protocol may be used by itself for accounting purposes only, or it may be used with a Diameter application, such as Mobile IPv4 [RFC4004] (Calhoun, P., Johansson, T., Perkins, C., Hiller, T., and P. McCann, “Diameter Mobile IPv4 Application,” August 2005.), or network access [RFC4005] (Calhoun, P., Zorn, G., Spence, D., and D. Mitton, “Diameter Network Access Server Application,” August 2005.). It is also possible for the base protocol to be extended for use in new applications, via the addition of new commands or AVPs. The initial focus of Diameter was network access and accounting applications. A truly generic AAA protocol used by many applications might provide functionality not provided by Diameter. Therefore, it is imperative that the designers of new applications understand their requirements before using Diameter. See Section 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 provide local user authentication or authorization services; agents include proxies, redirects and relay agents. A Diameter server performs authentication and/or authorization of the user. A Diameter node may act as an agent for certain requests while acting as a server for others.

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

1.1.1.  Description of the Document Set

The Diameter specification consists of an updated version of the base protocol specification (this document) and the Transport Profile [RFC3539] (Aboba, B. and J. Wood, “Authentication, Authorization and Accounting (AAA) Transport Profile,” June 2003.). This document obsoletes RFC 3588. A summary of the base protocol updates included in this document can be found in Section 1.1.3 (Changes from RFC3588).

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

The Transport Profile document [RFC3539] (Aboba, B. and J. Wood, “Authentication, Authorization and Accounting (AAA) Transport Profile,” June 2003.) 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.

1.1.2.  Conventions Used in This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

1.1.3.  Changes from RFC3588

This document obsoletes RFC 3588 but is fully backward compatible with that document. The changes introduced in this document focus on fixing issues that have surfaced during implementation of [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.). An overview of some the major changes are given below.

• Deprecated the use of Inband-Security AVP for negotiating transport layer security. It has been generally considered that bootstrapping of TLS via Inband-Security AVP creates certain security risk because it does not completely protect the information carried in the CER/CEA. This version of Diameter adopted a common approach of defining a well-known secured port that peers should use when communicating via TLS. This new approach augments the existing Inband-Security negotiation but does not completely replace it. The old method is kept for backwards compatibility reasons.
• Deprecated the exchange of CER/CEA messages in the open state. This feature was implied in the peer state machine table of [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) but it was not clearly defined anywhere else in that document. As work on this document progressed, it became clear that the multiplicity of meaning and use of Application Id AVPs in the CER/CEA messages (and the messages themselves) is seen as an abuse of the Diameter extensibility rules and thus required simplification. It is assumed that the capabilities exchange in the open state will be re-introduced in a separate specification which clearly defines new commands for this feature.
• Simplified Security Requirements. The use of a secured transport for exchanging Diameter messages remains mandatory. However, TLS has become the primary method of securing Diameter and IPsec is a secondary alternative. See Section 13 (Security Considerations) for details. The support for the End-to-End security framework (E2ESequence AVP and 'P'-bit in the AVP header) has also been deprecated.
• Diameter Extensibility Changes. This includes fixes to the Diameter extensibility description (Section 1.3 (Approach to Extensibility) and others) to better aid Diameter application designers; in addition, the new specification relaxes the policy with respect to the allocation of command codes for vendor-specific uses (see Section 11.2.1 (Command Codes) for details).
• Application Id Usage. Clarify the proper use of Application Id information which can be found in multiple places within a Diameter message. This includes correlating Application Ids found in the message headers and AVPs. These changes also clearly specify the proper Application Id value to use for specific base protocol messages (ASR/ASA, STR/STA) as well as clarifying the content and use of Vendor-Specific-Application-Id.
• Routing Fixes. This document more clearly specifies what information (AVPs and Application Id) can be used for making general routing decisions. A rule for the prioritization of redirect routing criteria when multiple route entries are found via redirects has also been added (See Section 6.13 for details).
• Simplification of Diameter Peer Discovery. The Diameter discovery process now supports only widely used discovery schemes; the rest have been deprecated (see Section 5.2 (Diameter Peer Discovery) for details).

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

1.2.  Terminology

AAA

Authentication, Authorization and Accounting.

ABNF

Augmented Backus-Naur Form [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.). A metalanguage with its own formal syntax and rules. It is based on the Backus-Naur Form and is used to define message exchanges in a bi-directional communications protocol.

Accounting

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

Accounting Record

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

Authentication

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

Authorization

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

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.

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

Diameter Node

A Diameter Node is a host process that implements the Diameter protocol, and acts either as a Client, Agent or Server.

Diameter Peer

If a Diameter Node shares a direct transport connection with another Diameter Node, it is a Diameter Peer to that Diameter Node.

Diameter Server

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

Downstream

Downstream is used to identify the direction of a particular Diameter message from the Home Server towards the Diameter Client.

Home Realm

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

Home Server

A Diameter Server which serves the Home Realm.

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 a device reboot or other network problem prevents the delivery 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 [RFC4282] (Aboba, B., Beadles, M., Arkko, J., and P. Eronen, “The Network Access Identifier,” December 2005.), 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. The Diameter Credit Control Application [RFC4006] (Hakala, H., Mattila, L., Koskinen, J-P., Stura, M., and J. Loughney, “Diameter Credit-Control Application,” August 2005.) is an example of an application that defines real-time accounting functionality.

Relay Agent or Relay

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

Redirect Agent

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

Session

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

Stateful Agent

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

User

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

1.3.  Approach to Extensibility

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

• Defining new AVP values
• Creating new AVPs
• Creating new commands
• Creating new applications

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

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

1.3.1.  Defining New AVP Values

In order to allocate a new AVP value for AVPs defined in the Diameter Base protocol, the IETF needs to approve a new RFC that describes the AVP value. IANA considerations for these AVP values are discussed in Section 11.4.

The allocation of AVP values for other AVPs is guided by the IANA considerations of the document that defines those AVPs. Typically, allocation of new values for an AVP defined in an IETF RFC should require IETF Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.), whereas values for vendor-specific AVPs can be allocated by the vendor.

1.3.2.  Creating New AVPs

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

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

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

1.3.3.  Creating New Commands

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

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

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

The IANA considerations for commands are discussed in Section 11.2.1.

1.3.4.  Creating New Diameter Applications

Every Diameter application specification MUST have an IANA assigned Application Id (see Section 2.4 and Section 11.3). The managed Application Id space is flat and there is no relationship between different Diameter applications with respect to their Application Ids. As such, there is no versioning support provided by these application Ids itself; every Diameter application is a standalone application. If the application has a relationship with other Diameter applications, such a relationship is not known to Diameter.

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

• The ABNF indicates what AVPs are placed into a Diameter Command by the sender of that Command. Often, since there are multiple modes of protocol interactions many of the AVPs are indicated as optional.
• The M-bit allows the sender to indicate to the receiver whether or not understanding the semantics of an AVP and its content is mandatory. If the M-bit is set by the sender and the receiver does not understand the AVP or the values carried within that AVP then a failure is generated (see Section 7).

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

M-bit Setting

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

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

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

Commands

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

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

2.  Protocol Overview

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

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

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

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

Diameter Clients MUST support the base protocol, which includes accounting. In addition, they MUST fully support each Diameter application that is needed to implement the client's service, e.g., NASREQ and/or Mobile IPv4. A Diameter Client 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 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 transparent to the Diameter applications but they MUST support the Diameter base protocol, which includes accounting, and all Diameter applications.

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

2.1.  Transport

The Diameter Transport profile is defined in [RFC3539] (Aboba, B. and J. Wood, “Authentication, Authorization and Accounting (AAA) Transport Profile,” June 2003.).

The base Diameter protocol is run on port 3868 for both TCP [RFC793] (Postel, J., “Transmission Control Protocol,” January 1981.) and SCTP [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.). For TLS [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.), a Diameter node that initiate a TLS connection prior to any message exchanges MUST run on port [TBD]. It is assumed that TLS is run on top of TCP when it is used. The remainder of this document uses the term TLS to abbreviate the use of TLS over TCP.

If the Diameter peer does not support receiving TLS connections on port [TBD], i.e. the peer complies only with [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.), then the initiator MAY revert to using TCP or SCTP and on port 3868. Note that this scheme is kept for the purpose of backwards compatibility only and that there are inherent security vulnerabilities when the initial CER/CEA messages are sent un-protected (see Section 5.6 (Peer State Machine)).

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

A Diameter node MAY initiate connections from a source port other than the one that it declares it accepts incoming connections on, and MUST be prepared to receive connections on port 3868 for TCP or SCTP and port [TBD] for TLS connections. 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 (see Section 12 (Diameter protocol related configurable parameters) for details), whose recommended value is 30 seconds. There are certain exceptions to this rule, such as when a peer has terminated the transport connection stating that it does not wish to communicate.

When connecting to a peer and either zero or more transports are specified, TLS SHOULD be tried first, followed by TCP, then by SCTP. 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. Further guidance regarding the treatment of ICMP errors can be found in [RFC5927] (Gont, F., “ICMP Attacks against TCP,” July 2010.) and [RFC5461] (Gont, F., “TCP's Reaction to Soft Errors,” February 2009.). Diameter implementations SHOULD also be able to interpret a reset from the transport and timed-out connection attempts. If Diameter receives data from the lower layer that cannot be parsed or identified as a Diameter error made by the peer, the stream is compromised and cannot be recovered. The transport connection MUST be closed using a RESET call (send a TCP RST bit) or an SCTP ABORT message (graceful closure is compromised).

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. These messages can be out-of-order and belong to different Diameter sessions.
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

Connections between Diameter peers SHOULD be protected by TLS. All Diameter base protocol implementations MUST support the use of TLS. If desired, alternative security mechanisms that are independent of Diameter, such as IPsec [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.), can be deployed to secure connections between peers. The Diameter protocol MUST NOT be used without any security mechanism.

2.3.  Diameter Application Compliance

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

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

2.4.  Application Identifiers

Each Diameter application MUST have an IANA assigned Application Id (see Section 11.3). The base protocol does not require an Application Id since its support is mandatory. During the capabilities exchange, Diameter nodes inform their peers of locally supported applications. Furthermore, all Diameter messages contain an Application Id, which is used in the message forwarding process.

The following Application Id values are defined:

      Diameter Common Messages      0
      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 refers to a transport level connection between two peers that is used to send and receive Diameter messages. A session is a logical concept at the application layer existing between the Diameter client and the Diameter server; it is identified via the Session-Id AVP.

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

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

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

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

2.6.  Peer Table

The Diameter Peer Table is used in message forwarding, and referenced by the Routing Table. A Peer Table entry contains the following fields:

Host identity

Following the conventions described for the DiameterIdentity derived AVP data format in Section 4.3. 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.  Routing Table

All Realm-Based routing lookups are performed against what is commonly known as the Routing Table (see Section 12). A Routing Table Entry contains the following fields:

Realm Name

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

Application Identifier

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

Local Action

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

1. LOCAL - Diameter messages that can be satisfied locally, and do not need to be routed to another Diameter entity.
2. RELAY - All Diameter messages that fall within this category MUST be routed to a next hop Diameter entity that is indicated by the identifier described below. Routing is done without modifying any non-routing AVPs. See Section 6.1.9 for relaying guidelines
3. PROXY - All Diameter messages that fall within this category MUST be routed to a next Diameter entity that is indicated by the identifier described below. The local server MAY apply its local policies to the message by including new AVPs to the message prior to routing. See Section 6.1.9 for proxying guidelines.
4. REDIRECT - Diameter messages that fall within this category MUST have the identity of the home Diameter server(s) appended, and returned to the sender of the message. See Section 6.1.8 for redirect guidelines.

Server Identifier

One or more servers to which the message is to be routed. 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 MUST be redirected to.

Static or Dynamic

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

Expiration time

Specifies the time at which a dynamically discovered route table entry expires.

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 and proxies MUST NOT reorder AVPs.

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

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

2.8.  Role of Diameter Agents

In addition to clients and servers, the Diameter protocol introduces relay, proxy, redirect, and translation agents, each of which is defined in Section 1.3. 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 the agent is notified otherwise, or the session expires. Each authorized session has an expiration, which is communicated by Diameter servers via the Session-Timeout AVP.

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

• 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 Routing Table, as is defined further in Section 2.7.

Relays may, for example, be used to aggregate requests from multiple Network Access Servers (NASes) within a common geographical area (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

The example provided in Figure 2 (Relaying of Diameter messages) 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 Id.

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.

Proxies may, for example, 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.

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.

The example provided in Figure 3 (Redirecting a Diameter Message) 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

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

2.9.  Diameter Path Authorization

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

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 6.1.9, 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.

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 and the padded AVPs. Thus the message length field is always a multiple of 4.

Command Flags

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

       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 value of the application-id field in the header MUST be the same as any relevant application-id AVPs contained in the message.

Hop-by-Hop Identifier

The Hop-by-Hop Identifier is an unsigned 32-bit integer field (in network byte order) 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 when sending the message. 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]

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.
                   ;
                   ; NOTE:  "[" and "]" have a slightly different
                   ; meaning than in ABNF (RFC 5234]). These braces
                   ; cannot be used to express optional fixed rules
                   ; (such as an optional ICV at the end). To do this,
                   ; the convention is '0*1fixed'.

qual             = [min] "*" [max]
                   ; See ABNF conventions, RFC 5234 Section 4.
                   ; The absence of any 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. If an optional rule has a qualifier,
                   ; then the value of min MUST be 0 if present.

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

max              = 1*DIGIT
                   ; The maximum number of times the element may
                   ; be present. If absent, the default value is
                   ; infinity. A value of 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, not otherwise listed in that command code
                   ; definition. Addition this AVP is recommended for
                   ; all command ABNFs to allow for extensibility.



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 has to 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 and routing information as well as configuration details for the request and reply.

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 has been reserved for future usage of end-to-end security. At the time of writing there are no end-to-end security mechanisms specified therefore the 'P' bit SHOULD be set to 0.

The 'M' Bit, known as the Mandatory bit, indicates whether the receiver of the AVP MUST parse and understand the semantic of the AVP including its content. The receiving entity MUST return an appropriate error message if it receives an AVP that has the M-bit set but does not understand it. An exception applies when the AVP is embedded within a Grouped AVP. See Section 4.4 for details. Diameter Relay and redirect agents MUST NOT reject messages with unrecognized AVPs.

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

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.

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 MUST be rejected.

4.1.1.  Optional Header Elements

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

Vendor-ID

The Vendor-ID field is present if the 'V' bit is set in the AVP Flags field. The optional four-octet Vendor-ID field contains the IANA assigned "SMI Network Management Private Enterprise Codes" [RFC3232] (Reynolds, J., “Assigned Numbers: RFC 1700 is Replaced by an On-line Database,” January 2002.) value, encoded in network byte order. Any vendor or standardization organization that are also treated like vendors in the IANA managed "SMI Network Management Private Enterprise Codes" space wishing to implement a vendor-specific Diameter AVP MUST use their own Vendor-ID along with their privately managed AVP address space, guaranteeing that they will not collide with any other vendor's vendor-specific AVP(s), nor with future IETF AVPs.

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] (Institute of Electrical and Electronics Engineers, “IEEE Standard for Binary Floating-Point Arithmetic, ANSI/IEEE Standard 754-1985,” August 1985.). 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] (Institute of Electrical and Electronics Engineers, “IEEE Standard for Binary Floating-Point Arithmetic, ANSI/IEEE Standard 754-1985,” August 1985.). 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 Derived AVP Data Formats MUST include them in a section entitled "Derived AVP Data Formats", using the same format as the definitions below. Each new definition MUST be either defined or listed with a reference to the RFC that defines the format.

4.3.1.  Common Derived AVPs

The following are commonly used Derived AVP Data Formats.

Address

The Address format is derived from the OctetString AVP Base Format. It is a discriminated union, representing, for example a 32-bit (IPv4) [RFC791] (Postel, J., “Internet Protocol,” September 1981.) or 128-bit (IPv6) [RFC4291] (Hinden, R. and S. Deering, “IP Version 6 Addressing Architecture,” February 2006.) address, most significant octet first. The first two octets of the Address AVP represents the AddressType, which contains an Address Family defined in [IANAADFAM] (IANA,, “Address Family Numbers,” .). 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 [RFC5905] (Mills, D., Martin, J., Burbank, J., and W. Kasch, “Network Time Protocol Version 4: Protocol and Algorithms Specification,” June 2010.).

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 [RFC5905] (Mills, D., Martin, J., Burbank, J., and W. Kasch, “Network Time Protocol Version 4: Protocol and Algorithms Specification,” June 2010.) 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 [RFC3629] (Yergeau, F., “UTF-8, a transformation format of ISO 10646,” November 2003.) transformation format described in RFC 3629.

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/Realm

DiameterIdentity value is used to uniquely identify either:

• A Diameter node for purposes of duplicate connection and routing loop detection.
• A Realm to determine whether messages can be satisfied locally, or whether they must be routed or redirected.

When a DiameterIdentity is used to identify a Diameter node 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. Note that in this document, DiameterIdentity is in ASCII form in order to be compatible with existing DNS infrastructure. See Appendix D (Internationalized Domain Names) for interactions between the Diameter protocol and Internationalized Domain Name (IDNs).

DiameterURI

The DiameterURI MUST follow the Uniform Resource Identifiers (URI) syntax [RFC3986] (Berners-Lee, T., Fielding, R., and L. Masinter, “Uniform Resource Identifier (URI): Generic Syntax,” January 2005.) 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 if no transport
                      ; security is used and port (TBD) when
                      ; transport security (TLS) is used.

      transport          = ";transport=" transport-protocol

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

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

    protocol           = ";protocol=" aaa-protocol

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

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

    The following are examples of valid Diameter host identities:

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

Enumerated

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

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

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

IPFilterRule filters MUST follow the format:

      action dir proto from src to dst [options]

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

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

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

      src and dst  <address/mask> [ports]

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

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

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

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

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

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

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

          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.

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

Every Grouped AVP defined MUST include a corresponding grammar, using ABNF [RFC5234] (Crocker, D. and P. Overell, “Augmented BNF for Syntax Specifications: ABNF,” January 2008.) (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]

      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). The length of the Example-AVP is the sum of all the length of the member AVPs including their padding plus the Example-AVP header size.

This AVP would be encoded as follows:

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

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.

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

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

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

5.  Diameter Peers

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

5.1.  Peer Connections

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

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 option (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 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. This could be deduced, for example, from the 'realm' in a NAI that a Diameter implementation needed to perform a Diameter operation on.
   
   The NAPTR usage in Diameter follows the S-NAPTR DDDS application [RFC3958] (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.) in which the SERVICE field includes tags for the desired application and supported application protocol. The application service tag for a Diameter application is 'aaa' and the supported application protocol tags are 'diameter.tcp', 'diameter.sctp' or 'diameter.tls'.
   
   The client can follow the resolution process defined by the S-NAPTR DDDS [RFC3958] (Daigle, L. and A. Newton, “Domain-Based Application Service Location Using SRV RRs and the Dynamic Delegation Discovery Service (DDDS),” January 2005.) application to find a matching SRV, A or AAAA record of a suitable peer. The domain suffixes in the NAPTR replacement field SHOULD match the domain of the original query. An example can be found in Appendix B (S-NAPTR Example).
3. If no NAPTR records are found, the requester directly queries for SRV records '_diameter._sctp'.realm, '_diameter._tcp'.realm and '_diameter._tls'.realm depending on the requesters network protocol capabilities. If SRV records are found then the requester can perform address record query (A RR's and/or AAAA RR's) for the target hostname specified in the SRV records. If no SRV records are found, the requester gives up.

If the server is using a site certificate, the domain name in the NAPTR query and the domain name in the replacement field MUST both be valid based on the site certificate handed out by the server in the TLS 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.

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

Diameter implementations SHOULD first attempt to establish a TLS connection prior to the CER/CEA exchange. This protects the capabilities information of both peers. To support older Diameter implementations that do not fully conform to this document, the transport security MAY still be negotiated via Inband-Security AVP. In this case, the 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 Id).

5.3.1.  Capabilities-Exchange-Request

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

When Diameter is run over SCTP [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.), 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 [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.), 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" [RFC3232] (Reynolds, J., “Assigned Numbers: RFC 1700 is Replaced by an On-line Database,” January 2002.) value assigned to the vendor of the Diameter device. It is 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 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 [RFC4960] (Stewart, R., “Stream Control Transmission Protocol,” September 2007.) MUST be advertised in the CER and CEA messages by including a Host-IP-Address AVP for each address.

5.3.6.  Supported-Vendor-Id AVP

The Supported-Vendor-Id AVP (AVP Code 265) is of type Unsigned32 and contains the IANA "SMI Network Management Private Enterprise Codes" [RFC3232] (Reynolds, J., “Assigned Numbers: RFC 1700 is Replaced by an On-line Database,” January 2002.) value assigned to a vendor other than the device vendor but including the application vendor. This is used in the CER and CEA messages in order to inform the peer that the sender supports (a subset of) the vendor-specific AVPs defined by the vendor identified in this AVP. The value of this AVP MUST NOT be set to zero. Multiple instances of this AVP containing the same value SHOULD NOT be sent.

5.3.7.  Product-Name AVP

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

5.4.  Disconnecting Peer connections

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

5.4.1.  Disconnect-Peer-Request

The Disconnect-Peer-Request (DPR), indicated by the Command-Code set to 282 and the Command Flags' 'R' bit set, is sent to a peer to inform 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 }
               * [ AVP ]

5.4.2.  Disconnect-Peer-Answer

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

   Message Format

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

5.4.3.  Disconnect-Cause AVP

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

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

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

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

5.5.  Transport Failure Detection

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

5.5.1.  Device-Watchdog-Request

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

   Message Format

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

5.5.2.  Device-Watchdog-Answer

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

   Message Format

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

5.5.3.  Transport Failure Algorithm

The transport failure algorithm is defined in [RFC3539] (Aboba, B. and J. Wood, “Authentication, Authorization and Accounting (AAA) Transport Profile,” June 2003.). 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 [RFC3539] (Aboba, B. and J. Wood, “Authentication, Authorization and Accounting (AAA) Transport Profile,” June 2003.), which is used to open, close, failover, probe, and reopen transport connections. Note in particular that [RFC3539] (Aboba, B. and J. Wood, “Authentication, Authorization and Accounting (AAA) Transport Profile,” June 2003.) 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, TLS handshake SHOULD begin when both ends are in the closed state prior to any Diameter message exchanges. The TLS connection SHOULD be established before sending any CER or CEA message to secure and protect the capabilities information of both peers. The TLS connection SHOULD be disconnected when the state machine moves to the closed state. When connecting to responders that do not conform to this document (i.e. older Diameter implementations that are not prepared to received TLS connections in the closed state), the initial TLS connection attempt will fail. The initiator MAY then attempt to connect via TCP or SCTP and initiate the TLS handshake when both ends are in the open state. If the handshake is successful, all further messages will be sent via TLS. If the handshake fails, both ends moves 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

   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

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

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

6.  Diameter message processing

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

6.1.  Diameter Request Routing Overview

A request is sent towards its final destination using 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. For Diameter clients, the value of the Destination-Realm AVP MAY be extracted from the User-Name AVP, or other methods.
• 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).

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

• If the message is destined for the local host, the procedures listed in Section 6.1.4 are followed.
• 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.
• The procedures listed in Section 6.1.6 are followed, which is known as Request Routing.
• 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.

6.1.2.  Sending a Request

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

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

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

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

6.1.7.  Predictive Loop Avoidance

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

6.1.8.  Redirecting Requests

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

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.

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

It is possible that multiple redirect indications can create multiple cached route entries differing only in their redirect usage and the peer to forward messages to. As an example, two(2) route entries that are created by two(2) redirect indications results in two(2) cached routes for the same realm and Application Id. However, one has a redirect usage of ALL_SESSION where matching request will be forwarded to one peer and the other has a redirect usage of ALL_REALM where request are forwarded to another peer. Therefore, an incoming request that matches the realm and Application Id of both routes will need additional resolution. In such a case, a routing precedence rule MUST be used against the redirect usage value to resolve the contention. The precedence rule can be found in Section 6.13.

6.1.9.  Relaying and Proxying Requests

A relay or proxy agent MUST append a Route-Record AVP to all requests forwarded. The AVP contains the identity of the peer 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. The Proxy-Info AVP has security implications as state information is distribute to other entities. As such, it is RECOMMMENDED to protect the content of the Proxy-Info AVP with cryptographic mechanisms, for example by using a keyed message digest. Such a mechanism, however, requires the management of keys, although only locally at the Diameter server. Still, a full description of the management of the keys used to protect the Proxy-Info AVP is beyond the scope of this document. Below is a list of commonly recommended:

• The keys should be generated securely following the randomness recommendations in [RFC4086] (Eastlake, D., Schiller, J., and S. Crocker, “Randomness Requirements for Security,” June 2005.).
• The keys and cryptographic protection algorithms should be at least 128 bits in strength.
• The keys should not be used for any other purpose than generating and verifying tickets.
• The keys should be changed regularly.
• The keys should be changed if the ticket format or cryptographic protection algorithms change.

The message is then forwarded to the next hop, as identified in the Routing Table.

Figure 6 (Routing of Diameter messages) provides an example of message routing using the procedures listed in these sections.

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

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

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 towards the Diameter server.

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

6.3.  Origin-Host AVP

The Origin-Host AVP (AVP Code 264) is of type DiameterIdentity, and 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.

6.4.  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.

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

6.7.1.  Route-Record AVP

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

6.7.2.  Proxy-Info AVP

The Proxy-Info AVP (AVP Code 284) is of type Grouped. This AVP contains the identity and local state information of Diameter node that creates and adds it to a message. 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. It contains state information that would otherwise be stored at the Diameter entity that created it. As such, this AVP MUST be treated as opaque data by entities other Diameter entities.

6.8.  Auth-Application-Id AVP

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

6.9.  Acct-Application-Id AVP

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

6.10.  Inband-Security-Id AVP

The Inband-Security-Id AVP (AVP Code 299) is of type Unsigned32 and is used in order to advertise support of the security portion of the application. The use of this AVP in CER and CEA messages is no longer recommended. Instead, discovery of a Diameter entities security capabilities can be done either through static configuration or via Diameter Peer Discovery described in Section 5.2.

The following values are supported:

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 [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.).

6.11.  Vendor-Specific-Application-Id AVP

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

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

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

   AVP Format

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

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

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

6.12.  Redirect-Host AVP

The Redirect-Host AVP (AVP Code 292) is of type DiameterURI. One or more 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 matching the criteria set by the Redirect-Host-Usage AVP.

6.13.  Redirect-Host-Usage AVP

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

When present, this AVP provides a hints about 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 SHOULD be sent to the host specified in the Redirect-Host AVP.

ALL_REALM 2

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

REALM_AND_APPLICATION 3

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

ALL_APPLICATION 4

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

ALL_HOST 5

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

ALL_USER 6

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

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

1. ALL_SESSION
2. ALL_USER
3. REALM_AND_APPLICATION
4. ALL_REALM
5. ALL_APPLICATION
6. ALL_HOST

6.14.  Redirect-Max-Cache-Time AVP

The Redirect-Max-Cache-Time AVP (AVP Code 262) is of type Unsigned32. This AVP MUST be present in answer messages whose 'E' bit is set, 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, SHOULD be cached. Note that once a host is no longer reachable, any associated cache, peer and routing table entries MUST be deleted.

7.  Error Handling

There are two different types of errors in Diameter; protocol 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) 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) 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.

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

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:

• A request with an unrecognized AVP is received with the 'M' bit (Mandatory bit) set, causes an answer to be sent with the Result-Code AVP set to DIAMETER_AVP_UNSUPPORTED, and the Failed-AVP AVP containing the offending AVP.
• A request with an AVP that is received with an unrecognized value causes an answer to be returned with the Result-Code AVP set to DIAMETER_INVALID_AVP_VALUE, with the Failed-AVP AVP containing the AVP causing the error.
• A command is received that is missing AVP(s) that are defined as required in the commands ABNF; examples are AVPs indicated as {AVP}. The receiver issues an answer with the Result-Code set to DIAMETER_MISSING_AVP, and creates an AVP with the AVP Code and other fields set as expected in the missing AVP. The created AVP is then added to the Failed- AVP AVP.

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

7.1.  Result-Code AVP

The Result-Code AVP (AVP Code 268) is of type Unsigned32 and indicates whether a particular request was completed successfully or whether an error occurred. All Diameter answer messages in IETF defined Diameter application specification 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 errors MUST only be used in answer messages whose 'E' bit is set. This document omits some error codes defined in [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.). To provide backward compatibility with [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) implementations these error code values are not re-used and hence the error codes values enumerated below are non-sequential.

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_BIT_IN_HEADER 3011

This error is returned when a reserved bit in the Diameter header is set to one (1) or the bits in the Diameter header defined in Section 3 are set incorrectly.

DIAMETER_INVALID_MESSAGE_LENGTH 3012

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

7.1.4.  Transient Failures

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

DIAMETER_AUTHENTICATION_REJECTED 4001

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

DIAMETER_OUT_OF_SPACE 4002

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

ELECTION_LOST 4003

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

7.1.5.  Permanent Failures

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

To provide backward compatibility with existing implementations that follow [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.), some of the error values that have previously been used in this category by [RFC3588] (Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and J. Arkko, “Diameter Base Protocol,” September 2003.) will not be re-used. Therefore the error values enumerated here maybe non-sequential.

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. The Failed-AVP AVPs MUST be present which contains 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 by a Diameter node that receives a CER whereby no applications are common between the CER sending peer and the CER receiving peer.

DIAMETER_UNSUPPORTED_VERSION 5011

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

DIAMETER_UNABLE_TO_COMPLY 5012

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

DIAMETER_INVALID_AVP_LENGTH 5014

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

DIAMETER_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.

DIAMETER_UNKNOWN_PEER 5018

A CER was received from an unknown peer.

DIAMETER_COMMAND_UNSUPPORTED 5019

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

DIAMETER_INVALID_HDR_BITS 5020

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 5021

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.

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-Message ]
                        [ Error-Reporting-Host ]
                        [ Failed-AVP ]
                        [ Experimental-Result ]
                      * [ 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 an environment where error messages are processed automatically. It SHOULD NOT be expected that the content of this AVP is 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. A Diameter message SHOULD contain only one Failed-AVP that corresponds to the error indicated by the Result-Code AVP. For practical purposes, this Failed-AVP would typically refer to the first AVP processing error that a Diameter node encounters.

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

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

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

AVP Format

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

7.6.  Experimental-Result AVP

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

AVP Format

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

The Vendor-Id AVP (see Section 5.3.3) in this grouped AVP identifies the vendor responsible for the assignment of the result code 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

In general, 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 are used to terminate user sessions. These are used to allow servers that maintain state information to free resources.

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

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

8.1.  Authorization Session State Machine

This section contains a set of finite state machines, 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 case that an application does not support re-auth, the state transitions related to server-initiated re-auth when both client and server sessions maintains state (e.g., Send RAR, Pending, Receive RAA) MAY be ignored.

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

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

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

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

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

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

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

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

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

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

   Open      Session-Timeout Expires on     Send STR   Discon
             Access Device

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

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

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

   Discon    ASR Received                   Send ASA   Discon

   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 confirm   Send RAR   Pending
             authentication and/or
             authorization of the user

   Pending   Received RAA with a failed     Cleanup    Idle
             Result-Code

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

   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. AVPs may need to be inspected as a part of these tasks. The tasks can happen either immediately after record reception or in a post-processing phase. However, as these tasks are typically application or even policy dependent, they are not standardized by the Diameter specifications. Applications MAY define requirements on when to accept accounting records based on the used value of Accounting-Realtime-Required AVP, credit 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         Disconnect Idle
             buffer space available and     user/dev
             realtime 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         interim
             record can be overwritten)     record
             and 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         Disconnect Idle
             buffer space available and     user/dev
             realtime 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            user/dev
             realtime 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 server-initiated re-auth is supported, since some applications do not allow access devices to prompt the user for re-auth.

8.3.1.  Re-Auth-Request

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

 Message Format

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

8.3.2.  Re-Auth-Answer

The Re-Auth-Answer (RAA), indicated by the Command-Code set to 258 and the message flags' 'R' bit clear, is sent in response to the RAR. The Result-Code AVP MUST be present, and 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-Max-Cache-Time ]
               * [ Proxy-Info ]
               * [ AVP ]

8.4.  Session Termination

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

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

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

It is also possible that a session that was authorized is never actually started due to action of a proxy. For example, a proxy may 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 a Diameter client or by a Diameter proxy to inform the Diameter Server that an authenticated and/or authorized session is being terminated.

                Message Format

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

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 lack of credit or other reasons that were not anticipated when the session was first authorized.

An access device that receives an ASR with Session-ID equal to a currently active session MAY stop the session. Whether the access device stops the session or not is implementation- and/or configuration-dependent. For example, an access device may honor ASRs from certain agents only. In any case, the access device MUST respond with an Abort-Session-Answer, including a Result-Code AVP to indicate what action it took.

8.5.1.  Abort-Session-Request

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

                 Message Format

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

8.5.2.  Abort-Session-Answer

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

If the session identified by Session-Id in the ASR was successfully terminated, 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

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

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

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

The Diameter server may assume that all sessions that were active prior to detection of a client restart have been terminated. The Diameter server MAY clean up all session state associated with such lost sessions, and 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 in NTP format [RFC5905] (Mills, D., Martin, J., Burbank, J., and W. Kasch, “Network Time Protocol Version 4: Protocol and Algorithms Specification,” June 2010.), 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.example.com;1876543210;523

Example, in which there is an optional value:

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

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

8.9.  Authorization-Lifetime AVP

The Authorization-Lifetime AVP (AVP Code 291) is of type Unsigned32 and contains the maximum number of seconds of service to be provided to the user before the user is to be re-authenticated and/or re- authorized. Care should be taken when the Authorization- Lifetime value is determined, since a low, non-zero, value could create significant Diameter traffic, which could congest both the network and the agents.

A value of zero (0) means that immediate re-auth is necessary by the access device. The absence of this AVP, or a value of all ones (meaning all bits in the 32 bit field are set to one) means no re-auth is expected.

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. The server MUST return a value that is equal to, or smaller, than the one provided by the client.

8.10.  Auth-Grace-Period AVP

The Auth-Grace-Period AVP (AVP Code 276) is of type Unsigned32 and contains the number of seconds the Diameter server will wait following the expiration of the Authorization-Lifetime AVP before cleaning up resources for the session.

8.11.  Auth-Session-State AVP

The Auth-Session-State AVP (AVP Code 277) is of type Enumerated and specifies whether state is maintained for a particular session. The client MAY include this AVP in requests as a hint to the server, but the value in the server's answer message is binding. The following values are supported:

STATE_MAINTAINED 0

This value is used to specify that session state is being maintained, and the access device MUST issue a session termination message when service to the user is terminated. This is the default value.

NO_STATE_MAINTAINED 1

This value is used to specify that no session termination messages will be sent by the access device upon expiration of the Authorization-Lifetime.

8.12.  Re-Auth-Request-Type AVP

The Re-Auth-Request-Type AVP (AVP Code 285) is of type Enumerated and is included in application-specific auth answers to inform the client of the action expected upon expiration of the Authorization-Lifetime. If the answer message contains an Authorization-Lifetime AVP with a positive value, the Re-Auth-Request-Type AVP MUST be present in an answer message. The following values are defined:

AUTHORIZE_ONLY 0

An authorization only re-auth is expected upon expiration of the Authorization-Lifetime. This is the default value if the AVP is not present in answer messages that include the Authorization- Lifetime.

AUTHORIZE_AUTHENTICATE 1

An authentication and authorization re-auth is expected upon expiration of the Authorization-Lifetime.

8.13.  Session-Timeout AVP

The Session-Timeout AVP (AVP Code 27) [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) 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 8.11).

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) [RFC2865] (Rigney, C., Willens, S., Rubens, A., and W. Simpson, “Remote Authentication Dial In User Service (RADIUS),” June 2000.) is of type UTF8String, which contains the User-Name, in a format consistent with the NAI specification [RFC4282] (Aboba, B., Beadles, M., Arkko, J., and P. Eronen, “The Network Access Identifier,” December 2005.).

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 and Session-Termination-Request messages for this session MUST be sent to the same authorization 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 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 [RFC2975] (Aboba, B., Arkko, J., and D. Harrington, “Introduction to Accounting Management,” October 2000.) 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 [RFC2975] (Aboba, B., Arkko, J., and D. Harrington, “Introduction to Accounting Management,” October 2000.), 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 Diameter server SHOULD collect accounting information for the session. The Accounting-Request message is used to transmit the accounting information to the Diameter server, which MUST reply with the Accounting-Answer message to confirm reception. The Accounting-Answer message includes the Result-Code AVP, which MAY indicate that an error was present in the accounting message. The value of the Accounting-Realtime-Required AVP received earlier for the session in question may indicate that the user's session has to be terminated when a rejected Accounting-Request message was received.

9.3.  Accounting Application Extension and Requirements

Each Diameter application (e.g., NASREQ, MobileIP), SHOULD 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 that section.

Applications have the option of using one or both of the following accounting application extension models:

Split Accounting Service

The accounting message will carry the Application Id of the Diameter base accounting application (see Section 2.4 (Application Identifiers)). Accounting messages maybe routed to Diameter nodes other than the corresponding Diameter application. These nodes might be centralized accounting servers that provide accounting service for multiple different Diameter applications. These nodes MUST advertise the Diameter base accounting Application Id during capabilities exchange.

Coupled Accounting Service

The accounting messages will carry the Application Id of the application that is using it. The application itself will process the received accounting records or forward them to an accounting server. There is no accounting application advertisement required during capabilities exchange and the accounting messages will be routed the same as any of the other application messages.

In cases where an application does not define its own accounting service, it is preferred that the split accounting model be used.

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 (Duplicate Detection) 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.

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

If an application uses accounting messages, it can correlate accounting records with a specific application session by using the Session-Id of the particular application session in the accounting messages. Accounting messages MAY also use a different Session-Id from that of the application sessions in which case other session related information is needed to perform correlation.

In cases where an application requires multiple accounting sub-session, an Accounting-Sub-Session-Id AVP is used to differentiate each sub-session. The Session-Id would remain constant for all sub-sessions and is be used to correlate all the sub-sessions to a particular application session. 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.

There are also cases where an application needs to correlate multiple application sessions into a single accounting record; the accounting record may span multiple different Diameter applications and sessions used by the same user at a given time. In such cases, the Acct-Multi-Session- Id AVP is used. The Acct-Multi-Session-Id AVP SHOULD be signaled by the server to the access device (typically during authorization) when it determines that a request belongs to an existing session. The access device MUST then include the Acct-Multi-Session-Id AVP in all subsequent accounting messages.

The Acct-Multi-Session-Id AVP MAY include the value of the original Session-Id. 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.

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 ]
                [ Destination-Host ]
                [ Accounting-Sub-Session-Id ]
                [ Acct-Session-Id ]
                [ Acct-Multi-Session-Id ]
                [ Acct-Interim-Interval ]
                [ Accounting-Realtime-Required ]
                [ Origin-State-Id ]
                [ Event-Timestamp ]
              * [ Proxy-Info ]
              * [ Route-Record ]
              * [ AVP ]

9.7.2.  Accounting-Answer

The Accounting-Answer (ACA) command, indicated by the Command-Code field set to 271 and the Command Flags' 'R' bit cleared, is used to acknowledge an Accounting-Request command. The Accounting-Answer command contains the same Session-Id as the corresponding request.

Only the target Diameter Server, known as the home Diameter Server, SHOULD respond with the Accounting-Answer command.

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-Message ]
                [ Error-Reporting-Host ]
                [ Failed-AVP ]
                [ Acct-Interim-Interval ]
                [ Accounting-Realtime-Required ]
                [ Origin-State-Id ]
                [ Event-Timestamp ]
              * [ Proxy-Info ]
              * [ AVP ]

9.8.  Accounting AVPs

This section contains AVPs that describe accounting usage information related to a specific session.

9.8.1.  Accounting-Record-Type AVP

The Accounting-Record-Type AVP (AVP Code 480) is of type Enumerated and contains the type of accounting record being sent. The following values are currently defined for the Accounting-Record-Type AVP:

EVENT_RECORD 1

An Accounting Event Record is used to indicate that a one-time event has occurred (meaning that the start and end of the event are simultaneous). This record contains all information relevant to the service, and 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 AVP

The Acct-Interim-Interval AVP (AVP Code 85) is of type Unsigned32 and is sent from the Diameter home authorization server to the Diameter client. The client uses information in this AVP to decide how and when to produce accounting records. With different values in this AVP, service sessions can result in one, two, or two+N accounting records, based on the needs of the home-organization. The following accounting record production behavior is directed by the inclusion of this AVP:

1. The omission of the Acct-Interim-Interval AVP or its inclusion with Value field set to 0 means that EVENT_RECORD, START_RECORD, and STOP_RECORD are produced, as appropriate for the service.
   
   
2. The inclusion of the AVP with Value field set to a non-zero value means that INTERIM_RECORD records MUST be produced between the START_RECORD and STOP_RECORD records. The Value field of this AVP is the nominal interval between these records in seconds. The Diameter node that originates the accounting information, known as the client, MUST produce the first INTERIM_RECORD record roughly at the time when this nominal interval has elapsed from 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 cannot 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 be present or not. AVPs that occur only inside a Grouped AVP are not shown 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  |1  |0  |1  |
Re-Auth-Request-Type|0  |0  |0  |0  |0  |0  |1  |0  |0  |0  |0  |0  |
Route-Record        |0  |0  |0  |0  |0  |0  |0+ |0  |0+ |0  |0+ |0  |
Session-Binding     |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
Session-Id          |0  |0  |0  |0  |0  |0  |1  |1  |1  |1  |1  |1  |
Session-Server-     |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
  Failover          |   |   |   |   |   |   |   |   |   |   |   |   |
Session-Timeout     |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
Supported-Vendor-Id |0+ |0+ |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
Termination-Cause   |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |1  |0  |
User-Name           |0  |0  |0  |0  |0  |0  |0-1|0-1|0-1|0-1|0-1|0-1|
Vendor-Id           |1  |1  |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
Vendor-Specific-    |0+ |0+ |0  |0  |0  |0  |0  |0  |0  |0  |0  |0  |
  Application-Id    |   |   |   |   |   |   |   |   |   |   |   |   |
--------------------+---+---+---+---+---+---+---+---+---+---+---+---+

10.2.  Accounting AVP Table

The table in this section is used to represent which AVPs defined in this document are to be present in the Accounting messages. These AVP occurrence requirements are guidelines, which may be expanded, and/or overridden by application-specific requirements in the Diameter applications documents.

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

11.  IANA Considerations

This section provides guidance to the Internet Assigned Numbers Authority (IANA) regarding registration of values related to the Diameter protocol, in accordance with BCP 26 [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.). The following policies are used here with the meanings defined in BCP 26: "Private Use", "First Come First Served", "Expert Review", "Specification Required", "IETF Review", "Standards Action".

This section explains the criteria to be used by the IANA for assignment of numbers within namespaces defined within this document.

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 DIME 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 DIME 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] (IANA,, “RADIUS Types,” .). 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 [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.). Release of blocks of AVPs (more than 3 at a time for a given purpose) should require IETF Review.

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 [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

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 (0x00-0xff) are reserved for RADIUS backward compatibility, and are defined as "RADIUS Packet Type Codes" in [RADTYPE] (IANA,, “RADIUS Types,” .). Values 256 - 8,388,607 (0x100 to 0x7fffff) are for permanent, standard commands, allocated by IETF Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.). 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 8,388,608 - 16,777,213 (0x800000 - 0xfffffd) are reserved for vendor-specific command codes, to be allocated on a First Come, First Served basis by IANA [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.). The request to IANA for a Vendor-Specific Command Code SHOULD include a reference to a publicly available specification which documents the command in sufficient detail to aid in interoperability between independent implementations. If the specification cannot be made publicly available, the request for a vendor-specific command code MUST include the contact information of persons and/or entities responsible for authoring and maintaining the command.

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] (Narten, T., “Assigning Experimental and Testing Numbers Considered Useful, Work in Progress.,” .).

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 [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.3.  Application Identifiers

As defined in Section 2.4 (Application Identifiers), the Application Id is used to identify a specific Diameter Application. There are standards-track Application Ids and vendor specific Application Ids.

IANA [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.) 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
      Diameter Base Accounting            3
      Relay                               0xffffffff

Assignment of standards-track Application Ids are by Designated Expert with Specification Required [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

Both Auth-Application-Id and Acct-Application-Id AVPs use the same Application Id space. A Diameter node advertising itself as a relay agent MUST set either Application-Id or Acct-Application-Id to 0xffffffff.

Vendor-Specific Application Ids, are for Private Use. Vendor-Specific Application Ids 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. This section lists such attributes in the Diameter base protocol and their IANA allocation rules.

Allocation of Application Ids was discussed in Section 2.4 (Application Identifiers). Other attributes in the base protocol do not take enumerated values or bit masks or employ existing name spaces such as SMI Network Management Private Enterprise Codes [RFC3232] (Reynolds, J., “Assigned Numbers: RFC 1700 is Replaced by an On-line Database,” January 2002.) or IP addresses. The allocation of new values for these existing name spaces is done in accordance with the rules already defined for them.

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, 3001-3012, 4001-4003 and 5001-5021.

All remaining values are available for assignment via IETF Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.2.  Experimental-Result-Code AVP

Values for this AVP are purely local to the indicated vendor, and no IANA registry is maintained for them.

11.4.3.  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 Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.4.  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 Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.5.  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 Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.6.  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 Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.7.  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 Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.8.  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 Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.9.  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 Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.10.  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 Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.11.  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 Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.12.  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 Review [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.4.13.  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 Review. [RFC5226] (Narten, T. and H. Alvestrand, “Guidelines for Writing an IANA Considerations Section in RFCs,” May 2008.).

11.5.  Diameter TCP, SCTP and TLS/TCP Port Numbers

The IANA has assigned port number 3868 for TCP and SCTP. The port number [TBD] has been assigned for TLS/TCP.

11.6.  S-NAPTR Parameters

This document registers a S-NAPTR Application Service Tag value of "aaa".

This document also registers the following S-NAPTR Application Protocol Tags:

  Tag                | Protocol
  -------------------|---------
  diameter.tcp       | TCP
  diameter.sctp      | SCTP
  diameter.tls.tcp   | TLS/TCP

12.  Diameter protocol related configurable parameters

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

Diameter Peer

A Diameter entity MAY communicate with peers that are statically configured. A statically configured Diameter peer would require that either the IP address or the fully qualified domain name (FQDN) be supplied, which would then be used to resolve through DNS.

Routing Table

A Diameter proxy server routes messages based on the realm portion of a Network Access Identifier (NAI). The server MUST have a table of Realm Names, and the address of the peer to which the message must be forwarded 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 messages SHOULD be secured by using TLS [RFC5246] (Dierks, T. and E. Rescorla, “The Transport Layer Security (TLS) Protocol Version 1.2,” August 2008.). Additional security mechanisms such as IPsec [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.) MAY also be deployed to secure connections between peers. However, all Diameter base protocol implementations MUST support the use of TLS and the Diameter protocol MUST NOT be used without any security mechanism.

If a Diameter connection is to be protected via TLS or IPsec, then TLS or IPsec handshake SHOULD begin prior to any Diameter message exchange. All security parameters for TLS or IPsec are configured independent of the Diameter protocol. All Diameter message will be sent through the TLS or IPsec connection after a successful setup.

For TLS connections to be established in the open state, the CER/CEA exchange MUST include an Inband-Security-ID AVP with a value of TLS. The TLS handshake will begin when both ends successfully reached 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. See Sections 13.1 for more details.

13.1.  TLS Usage

Diameter nodes using 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.

13.2.  Peer-to-Peer Considerations

As with any peer-to-peer protocol, proper configuration of the trust model within a Diameter peer is essential to security. When certificates are used, it is necessary to configure the root certificate authorities trusted by the Diameter peer. These root CAs are likely to be unique to Diameter usage and distinct from the root CAs that might be trusted for other purposes such as Web browsing. In general, it is expected that those root CAs will be configured so as to reflect the business relationships between the organization hosting the Diameter peer and other organizations. As a result, a Diameter peer will typically not be configured to allow connectivity with any arbitrary peer. With certificate authentication, Diameter peers may not be known beforehand and therefore peer discovery may be required.

14.  References

14.1. Normative References

14.2. Informational References

Appendix A.  Acknowledgements

A.1.  RFC3588bis

The authors would like to thank the following people that have provided proposals and contributions to this document:

To Vishnu Ram and Satendra Gera for their contributions on Capabilities Updates, Predictive Loop Avoidance as well as many other technical proposals. To Tolga Asveren for his insights and contributions on almost all of the proposed solutions incorporated into this document. To Timothy Smith for helping on the Capabilities Updates and other topics. To Tony Zhang for providing fixes to loop holes on composing Failed-AVPs as well as many other issues and topics. To Jan Nordqvist for clearly stating the usage of Application Ids. To Anders Kristensen for providing needed technical opinions. To David Frascone for providing invaluable review of the document. To Mark Jones for providing clarifying text on vendor command codes and other vendor specific indicators.

Special thanks to the Diameter extensibility design team which helped resolve the tricky question of mandatory AVPs and ABNF semantics. The members of this team are as follows:

Avi Lior, Jari Arkko, Glen Zorn, Lionel Morand, Mark Jones, Tolga Asveren Jouni Korhonen, Glenn McGregor.

Special thanks also to people who have provided invaluable comments and inputs especially in resolving controversial issues:

Glen Zorn, Yoshihiro Ohba, Marco Stura, and Pasi Eronen.

Finally, we would like to thank the original authors of this document:

Pat Calhoun, John Loughney, Jari Arkko, Erik Guttman and Glen Zorn.

Their invaluable knowledge and experience has given us a robust and flexible AAA protocol that many people have seen great value in adopting. We greatly appreciate their support and stewardship for the continued improvements of Diameter as a protocol. We would also like to extend our gratitude to folks aside from the authors who have assisted and contributed to the original version of this document. Their efforts significantly contributed to the success of Diameter.

A.2.  RFC3588

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 B.  S-NAPTR Example

As an example, consider a client that wishes to resolve aaa:example1.com. The client performs a NAPTR query for that domain, and the following NAPTR records are returned:

 ;;        order pref flags service   regexp replacement
 IN NAPTR  50    50   "s"   "aaa:diameter.tls.tcp" ""
              _diameter._tls.example1.com
 IN NAPTR  100   50   "s"   "aaa:diameter.tcp" 	   ""
              _aaa._tcp.example1.com
 IN NAPTR  150   50   "s"   "aaa:diameter.sctp"    ""
              _diameter._sctp.example1.com

This indicates that the server supports TLS, TCP and SCTP in that order. If the client supports TLS, TLS will be used, targeted to a host determined by an SRV lookup of _diameter._tls.example1.com. That lookup would return:

 ;;       Priority  Weight  Port    Target
 IN SRV   0         1       5060    server1.example1.com
 IN SRV   0         2       5060    server2.example1.com

As an alternative example, a client that wishes to resolve aaa:example2.com. The client performs a NAPTR query for that domain, and the following NAPTR records are returned:

 ;;        order pref flags service   regexp replacement
 IN NAPTR  150   50   "a"   "aaa:diameter.tls.tcp"  ""
              server1.example2.com
 IN NAPTR  150   50   "a"   "aaa:diameter.tls.tcp"  ""
              server2.example2.com

This indicates that the server supports TCP available at the returned host names.

Appendix C.  Duplicate Detection

As described in Section 9.4, accounting record duplicate detection is based on session identifiers. Duplicates can appear for various reasons:

• 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.  Internationalized Domain Names

To be compatible with the existing DNS infrastructure and simplify host and domain name comparison, Diameter identities (FQDNs) are represented in ASCII form. This allows the Diameter protocol to fall in-line with the DNS strategy of being transparent from the effects of Internationalized Domain Names (IDNs) by following the recommendations in [RFC4690] (Klensin, J., Faltstrom, P., Karp, C., and IAB, “Review and Recommendations for Internationalized Domain Names (IDNs),” September 2006.) and [RFC5890] (Klensin, J., “Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework,” August 2010.). Applications that provide support for IDNs outside of the Diameter protocol but interacting with it SHOULD use the representation and conversion framework described in [RFC5890] (Klensin, J., “Internationalized Domain Names for Applications (IDNA): Definitions and Document Framework,” August 2010.), [RFC5891] (Klensin, J., “Internationalized Domain Names in Applications (IDNA): Protocol,” August 2010.) and [RFC3492] (Costello, A., “Punycode: A Bootstring encoding of Unicode for Internationalized Domain Names in Applications (IDNA),” March 2003.).

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