Roxen Community: RFC 3588 Diameter Base Protocol (Standards Track)

[Text version]

Network Working Group
Request for Comments: 3588
Category: Standards Track P. Calhoun
Airespace, Inc.
J. Loughney
Nokia
E. Guttman
Sun Microsystems, Inc.
G. Zorn
Cisco Systems, Inc.
J. Arkko
Ericsson
September 2003
Status of this Memo

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

Copyright Notice

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

Abstract

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

Conventions Used In This Document

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14, RFC 2119 [KEYWORD].

Table of Contents
1. Introduction................................................. 6 1.1. Diameter Protocol..................................... 9 1.1.1. Description of the Document Set.............. 10 1.2. Approach to Extensibility............................. 11 1.2.1. Defining New AVP Values...................... 11 1.2.2. Creating New AVPs............................ 11 1.2.3. Creating New Authentication Applications..... 11 1.2.4. Creating New Accounting Applications......... 12 1.2.5. Application Authentication Procedures........ 14 1.3. Terminology........................................... 14 2. Protocol Overview............................................ 18 2.1. Transport............................................. 20 2.1.1. SCTP Guidelines.............................. 21 2.2. Securing Diameter Messages............................ 21 2.3. Diameter Application Compliance....................... 21 2.4. Application Identifiers............................... 22 2.5. Connections vs. Sessions.............................. 22 2.6. Peer Table............................................ 23 2.7. Realm-Based Routing Table............................. 24 2.8. Role of Diameter Agents............................... 25 2.8.1. Relay Agents................................. 26 2.8.2. Proxy Agents................................. 27 2.8.3. Redirect Agents.............................. 28 2.8.4. Translation Agents........................... 29 2.9. End-to-End Security Framework......................... 30 2.10. Diameter Path Authorization........................... 30 3. Diameter Header.............................................. 32 3.1. Command Codes......................................... 35 3.2. Command Code ABNF specification....................... 36 3.3. Diameter Command Naming Conventions................... 38 4. Diameter AVPs................................................ 38 4.1. AVP Header............................................ 39 4.1.1. Optional Header Elements..................... 41 4.2. Basic AVP Data Formats................................ 41 4.3. Derived AVP Data Formats.............................. 42 4.4. Grouped AVP Values.................................... 49 4.4.1. Example AVP with a Grouped Data Type......... 50 4.5. Diameter Base Protocol AVPs........................... 53 5. Diameter Peers............................................... 56 5.1. Peer Connections...................................... 56 5.2. Diameter Peer Discovery............................... 56 5.3. Capabilities Exchange................................. 59 5.3.1. Capabilities-Exchange-Request................ 60 5.3.2. Capabilities-Exchange-Answer................. 60 5.3.3. Vendor-Id AVP................................ 61 5.3.4. Firmware-Revision AVP........................ 61
5.3.5. Host-IP-Address AVP.......................... 62 5.3.6. Supported-Vendor-Id AVP...................... 62 5.3.7. Product-Name AVP............................. 62 5.4. Disconnecting Peer Connections........................ 62 5.4.1. Disconnect-Peer-Request...................... 63 5.4.2. Disconnect-Peer-Answer....................... 63 5.4.3. Disconnect-Cause AVP......................... 63 5.5. Transport Failure Detection........................... 64 5.5.1. Device-Watchdog-Request...................... 64 5.5.2. Device-Watchdog-Answer....................... 64 5.5.3. Transport Failure Algorithm.................. 65 5.5.4. Failover and Failback Procedures............. 65 5.6. Peer State Machine.................................... 66 5.6.1. Incoming connections......................... 68 5.6.2. Events....................................... 69 5.6.3. Actions...................................... 70 5.6.4. The Election Process......................... 71 6. Diameter Message Processing.................................. 71 6.1. Diameter Request Routing Overview..................... 71 6.1.1. Originating a Request........................ 73 6.1.2. Sending a Request............................ 73 6.1.3. Receiving Requests........................... 73 6.1.4. Processing Local Requests.................... 73 6.1.5. Request Forwarding........................... 74 6.1.6. Request Routing.............................. 74 6.1.7. Redirecting Requests......................... 74 6.1.8. Relaying and Proxying Requests............... 75 6.2. Diameter Answer Processing............................ 76 6.2.1. Processing Received Answers.................. 77 6.2.2. Relaying and Proxying Answers................ 77 6.3. Origin-Host AVP....................................... 77 6.4. Origin-Realm AVP...................................... 78 6.5. Destination-Host AVP.................................. 78 6.6. Destination-Realm AVP................................. 78 6.7. Routing AVPs.......................................... 78 6.7.1. Route-Record AVP............................. 79 6.7.2. Proxy-Info AVP............................... 79 6.7.3. Proxy-Host AVP............................... 79 6.7.4. Proxy-State AVP.............................. 79 6.8. Auth-Application-Id AVP............................... 79 6.9. Acct-Application-Id AVP............................... 79 6.10. Inband-Security-Id AVP................................ 79 6.11. Vendor-Specific-Application-Id AVP.................... 80 6.12. Redirect-Host AVP..................................... 80 6.13. Redirect-Host-Usage AVP............................... 80 6.14. Redirect-Max-Cache-Time AVP........................... 81 6.15. E2E-Sequence AVP...................................... 82
7. Error Handling............................................... 82 7.1. Result-Code AVP....................................... 84 7.1.1. Informational................................ 84 7.1.2. Success...................................... 84 7.1.3. Protocol Errors.............................. 85 7.1.4. Transient Failures........................... 86 7.1.5. Permanent Failures........................... 86 7.2. Error Bit............................................. 88 7.3. Error-Message AVP..................................... 89 7.4. Error-Reporting-Host AVP.............................. 89 7.5. Failed-AVP AVP........................................ 89 7.6. Experimental-Result AVP............................... 90 7.7. Experimental-Result-Code AVP.......................... 90 8. Diameter User Sessions....................................... 90 8.1. Authorization Session State Machine................... 92 8.2. Accounting Session State Machine...................... 96 8.3. Server-Initiated Re-Auth.............................. 101 8.3.1. Re-Auth-Request.............................. 102 8.3.2. Re-Auth-Answer............................... 102 8.4. Session Termination................................... 103 8.4.1. Session-Termination-Request.................. 104 8.4.2. Session-Termination-Answer................... 105 8.5. Aborting a Session.................................... 105 8.5.1. Abort-Session-Request........................ 106 8.5.2. Abort-Session-Answer......................... 106 8.6. Inferring Session Termination from Origin-State-Id.... 107 8.7. Auth-Request-Type AVP................................. 108 8.8. Session-Id AVP........................................ 108 8.9. Authorization-Lifetime AVP............................ 109 8.10. Auth-Grace-Period AVP................................. 110 8.11. Auth-Session-State AVP................................ 110 8.12. Re-Auth-Request-Type AVP.............................. 110 8.13. Session-Timeout AVP................................... 111 8.14. User-Name AVP......................................... 111 8.15. Termination-Cause AVP................................. 111 8.16. Origin-State-Id AVP................................... 112 8.17. Session-Binding AVP................................... 113 8.18. Session-Server-Failover AVP........................... 113 8.19. Multi-Round-Time-Out AVP.............................. 114 8.20. Class AVP............................................. 114 8.21. Event-Timestamp AVP................................... 115 9. Accounting................................................... 115 9.1. Server Directed Model................................. 115 9.2. Protocol Messages..................................... 116 9.3. Application Document Requirements..................... 116 9.4. Fault Resilience...................................... 116 9.5. Accounting Records.................................... 117 9.6. Correlation of Accounting Records..................... 118
9.7. Accounting Command-Codes.............................. 119 9.7.1. Accounting-Request........................... 119 9.7.2. Accounting-Answer............................ 120 9.8. Accounting AVPs....................................... 121 9.8.1. Accounting-Record-Type AVP................... 121 9.8.2. Acct-Interim-Interval AVP.................... 122 9.8.3. Accounting-Record-Number AVP................. 123 9.8.4. Acct-Session-Id AVP.......................... 123 9.8.5. Acct-Multi-Session-Id AVP.................... 123 9.8.6. Accounting-Sub-Session-Id AVP................ 123 9.8.7. Accounting-Realtime-Required AVP............. 123 10. AVP Occurrence Table......................................... 124 10.1. Base Protocol Command AVP Table....................... 124 10.2. Accounting AVP Table.................................. 126 11. IANA Considerations.......................................... 127 11.1. AVP Header............................................ 127 11.1.1. AVP Code..................................... 127 11.1.2. AVP Flags.................................... 128 11.2. Diameter Header....................................... 128 11.2.1. Command Codes................................ 128 11.2.2. Command Flags................................ 129 11.3. Application Identifiers............................... 129 11.4. AVP Values............................................ 129 11.4.1. Result-Code AVP Values....................... 129 11.4.2. Accounting-Record-Type AVP Values............ 130 11.4.3. Termination-Cause AVP Values................. 130 11.4.4. Redirect-Host-Usage AVP Values............... 130 11.4.5. Session-Server-Failover AVP Values........... 130 11.4.6. Session-Binding AVP Values................... 130 11.4.7. Disconnect-Cause AVP Values.................. 130 11.4.8. Auth-Request-Type AVP Values................. 130 11.4.9. Auth-Session-State AVP Values................ 130 11.4.10. Re-Auth-Request-Type AVP Values.............. 131 11.4.11. Accounting-Realtime-Required AVP Values...... 131 11.5. Diameter TCP/SCTP Port Numbers........................ 131 11.6. NAPTR Service Fields.................................. 131 12. Diameter Protocol Related Configurable Parameters............ 131 13. Security Considerations...................................... 132 13.1. IPsec Usage........................................... 133 13.2. TLS Usage............................................. 134 13.3. Peer-to-Peer Considerations........................... 134 14. References................................................... 136 14.1. Normative References.................................. 136 14.2. Informative References................................ 138 15. Acknowledgements............................................. 140 Appendix A. Diameter Service Template........................... 141 Appendix B. NAPTR Example....................................... 142 Appendix C. Duplicate Detection................................. 143
Appendix D. Intellectual Property Statement..................... 145 Authors' Addresses............................................... 146 Full Copyright Statement......................................... 147
1. Introduction

Authentication, Authorization and Accounting (AAA) protocols such as TACACS [TACACS] and RADIUS [RADIUS] were initially deployed to provide dial-up PPP [PPP] and terminal server access. Over time, with the growth of the Internet and the introduction of new access technologies, including wireless, DSL, Mobile IP and Ethernet, routers and network access servers (NAS) have increased in complexity and density, putting new demands on AAA protocols.

Network access requirements for AAA protocols are summarized in [AAAREQ]. These include:
Failover [RADIUS] does not define failover mechanisms, and as a result, failover behavior differs between implementations. In order to provide well defined failover behavior, Diameter supports application-layer acknowledgements, and defines failover algorithms and the associated state machine. This is described in Section 5.5 and [AAATRANS].
Transmission-level security [RADIUS] defines an application-layer authentication and integrity scheme that is required only for use with Response packets. While [RADEXT] defines an additional authentication and integrity mechanism, use is only required during Extensible Authentication Protocol (EAP) sessions. While attribute-hiding is supported, [RADIUS] does not provide support for per-packet confidentiality. In accounting, [RADACCT] assumes that replay protection is provided by the backend billing server, rather than within the protocol itself.
While [RFC3162] defines the use of IPsec with RADIUS, support for IPsec is not required. Since within [IKE] authentication occurs only within Phase 1 prior to the establishment of IPsec SAs in Phase 2, it is typically not possible to define separate trust or authorization schemes for each application. This limits the usefulness of IPsec in inter-domain AAA applications (such as roaming) where it may be desirable to define a distinct certificate hierarchy for use in a AAA deployment. In order to provide universal support for transmission-level security, and enable both intra- and inter-domain AAA deployments, IPsec support is mandatory in Diameter, and TLS support is optional. Security is discussed in Section 13.
Reliable transport RADIUS runs over UDP, and does not define retransmission behavior; as a result, reliability varies between implementations. As described in [ACCMGMT], this is a major issue in accounting, where packet loss may translate directly into revenue loss. In order to provide well defined transport behavior, Diameter runs over reliable transport mechanisms (TCP, SCTP) as defined in [AAATRANS].
Agent support [RADIUS] does not provide for explicit support for agents, including Proxies, Redirects and Relays. Since the expected behavior is not defined, it varies between implementations. Diameter defines agent behavior explicitly; this is described in Section 2.8.
Server-initiated messages While RADIUS server-initiated messages are defined in [DYNAUTH], support is optional. This makes it difficult to implement features such as unsolicited disconnect or reauthentication/reauthorization on demand across a heterogeneous deployment. Support for server-initiated messages is mandatory in Diameter, and is described in Section 8.
Auditability RADIUS does not define data-object security mechanisms, and as a result, untrusted proxies may modify attributes or even packet headers without being detected. Combined with lack of support for capabilities negotiation, this makes it very difficult to determine what occurred in the event of a dispute. While implementation of data object security is not mandatory within Diameter, these capabilities are supported, and are described in [AAACMS].
Transition support While Diameter does not share a common protocol data unit (PDU) with RADIUS, considerable effort has been expended in enabling backward compatibility with RADIUS, so that the two protocols may be deployed in the same network. Initially, it is expected that Diameter will be deployed within new network devices, as well as within gateways enabling communication between legacy RADIUS devices and Diameter agents. This capability, described in [NASREQ], enables Diameter support to be added to legacy networks, by addition of a gateway or server speaking both RADIUS and Diameter.
In addition to addressing the above requirements, Diameter also provides support for the following:
Capability negotiation RADIUS does not support error messages, capability negotiation, or a mandatory/non-mandatory flag for attributes. Since RADIUS clients and servers are not aware of each other's capabilities, they may not be able to successfully negotiate a mutually acceptable service, or in some cases, even be aware of what service has been implemented. Diameter includes support for error handling (Section 7), capability negotiation (Section 5.3), and mandatory/non-mandatory attribute-value pairs (AVPs) (Section 4.1).
Peer discovery and configuration RADIUS implementations typically require that the name or address of servers or clients be manually configured, along with the corresponding shared secrets. This results in a large administrative burden, and creates the temptation to reuse the RADIUS shared secret, which can result in major security vulnerabilities if the Request Authenticator is not globally and temporally unique as required in [RADIUS]. Through DNS, Diameter enables dynamic discovery of peers. Derivation of dynamic session keys is enabled via transmission-level security.
Roaming support The ROAMOPS WG provided a survey of roaming implementations [ROAMREV], detailed roaming requirements [ROAMCRIT], defined the Network Access Identifier (NAI) [NAI], and documented existing implementations (and imitations) of RADIUS-based roaming [PROXYCHAIN]. In order to improve scalability, [PROXYCHAIN] introduced the concept of proxy chaining via an intermediate server, facilitating roaming between providers. However, since RADIUS does not provide explicit support for proxies, and lacks auditability and transmission-level security features, RADIUS- based roaming is vulnerable to attack from external parties as well as susceptible to fraud perpetrated by the roaming partners themselves. As a result, it is not suitable for wide-scale deployment on the Internet [PROXYCHAIN]. By providing explicit support for inter-domain roaming and message routing (Sections 2.7 and 6), auditability [AAACMS], and transmission-layer security (Section 13) features, Diameter addresses these limitations and provides for secure and scalable roaming.
In the decade since AAA protocols were first introduced, the capabilities of Network Access Server (NAS) devices have increased substantially. As a result, while Diameter is a considerably more sophisticated protocol than RADIUS, it remains feasible to implement within embedded devices, given improvements in processor speeds and the widespread availability of embedded IPsec and TLS implementations.

1.1. Diameter Protocol

The Diameter base protocol provides the following facilities:
- Delivery of AVPs (attribute value pairs) - Capabilities negotiation - Error notification - Extensibility, through addition of new commands and AVPs (required in [AAAREQ]). - Basic services necessary for applications, such as handling of user sessions or accounting
All data delivered by the protocol is in the form of an AVP. Some of these AVP values are used by the Diameter protocol itself, while others deliver data associated with particular applications that employ Diameter. AVPs may be added arbitrarily to Diameter messages, so long as the required AVPs are included and AVPs that are explicitly excluded are not included. AVPs are used by the base Diameter protocol to support the following required features:
- Transporting of user authentication information, for the purposes of enabling the Diameter server to authenticate the user.
- Transporting of service specific authorization information, between client and servers, allowing the peers to decide whether a user's access request should be granted.
- Exchanging resource usage information, which MAY be used for accounting purposes, capacity planning, etc.
- Relaying, proxying and redirecting of Diameter messages through a server hierarchy.
The Diameter base protocol provides the minimum requirements needed for a AAA protocol, as required by [AAAREQ]. The base protocol may be used by itself for accounting purposes only, or it may be used with a Diameter application, such as Mobile IPv4 [DIAMMIP], or network access [NASREQ]. It is also possible for the base protocol to be extended for use in new applications, via the addition of new commands or AVPs. At this time the focus of Diameter is network access and accounting applications. A truly generic AAA protocol used by many applications might provide functionality not provided by Diameter. Therefore, it is imperative that the designers of new applications understand their requirements before using Diameter. See Section 2.4 for more information on Diameter applications.
Any node can initiate a request. In that sense, Diameter is a peer- to-peer protocol. In this document, a Diameter Client is a device at the edge of the network that performs access control, such as a Network Access Server (NAS) or a Foreign Agent (FA). A Diameter client generates Diameter messages to request authentication, authorization, and accounting services for the user. A Diameter agent is a node that does not authenticate and/or authorize messages locally; agents include proxies, redirects and relay agents. A Diameter server performs authentication and/or authorization of the user. A Diameter node MAY act as an agent for certain requests while acting as a server for others.
The Diameter protocol also supports server-initiated messages, such as a request to abort service to a particular user.

1.1.1. Description of the Document Set

Currently, the Diameter specification consists of a base specification (this document), Transport Profile [AAATRANS] and applications: Mobile IPv4 [DIAMMIP], and NASREQ [NASREQ].

The Transport Profile document [AAATRANS] discusses transport layer issues that arise with AAA protocols and recommendations on how to overcome these issues. This document also defines the Diameter failover algorithm and state machine.
The Mobile IPv4 [DIAMMIP] application defines a Diameter application that allows a Diameter server to perform AAA functions for Mobile IPv4 services to a mobile node.
The NASREQ [NASREQ] application defines a Diameter Application that allows a Diameter server to be used in a PPP/SLIP Dial-Up and Terminal Server Access environment. Consideration was given for servers that need to perform protocol conversion between Diameter and RADIUS.
In summary, this document defines the base protocol specification for AAA, which includes support for accounting. The Mobile IPv4 and the NASREQ documents describe applications that use this base specification for Authentication, Authorization and Accounting.

1.2. Approach to Extensibility

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

1.2.1. Defining New AVP Values

New applications should attempt to reuse AVPs defined in existing applications when possible, as opposed to creating new AVPs. For AVPs of type Enumerated, an application may require a new value to communicate some service-specific information.

In order to allocate a new AVP value, a request MUST be sent to IANA [IANA], along with an explanation of the new AVP value. IANA considerations for Diameter are discussed in Section 11.

1.2.2. Creating New AVPs

When no existing AVP can be used, a new AVP should be created. The new AVP being defined MUST use one of the data types listed in Section 4.2.

In the event that a logical grouping of AVPs is necessary, and multiple "groups" are possible in a given command, it is recommended that a Grouped AVP be used (see Section 4.4).
In order to create a new AVP, a request MUST be sent to IANA, with a specification for the AVP. The request MUST include the commands that would make use of the AVP.

1.2.3. Creating New Authentication Applications

Every Diameter application specification MUST have an IANA assigned Application Identifier (see Section 2.4) or a vendor specific Application Identifier.
Should a new Diameter usage scenario find itself unable to fit within an existing application without requiring major changes to the specification, it may be desirable to create a new Diameter application. Major changes to an application include:
- Adding new AVPs to the command, which have the "M" bit set.
- Requiring a command that has a different number of round trips to satisfy a request (e.g., application foo has a command that requires one round trip, but new application bar has a command that requires two round trips to complete).
- Adding support for an authentication method requiring definition of new AVPs for use with the application. Since a new EAP authentication method can be supported within Diameter without requiring new AVPs, addition of EAP methods does not require the creation of a new authentication application.
Creation of a new application should be viewed as a last resort. An implementation MAY add arbitrary non-mandatory AVPs to any command defined in an application, including vendor-specific AVPs without needing to define a new application. Please refer to Section 11.1.1 for details.
In order to justify allocation of a new application identifier, Diameter applications MUST define one Command Code, or add new mandatory AVPs to the ABNF.
The expected AVPs MUST be defined in an ABNF [ABNF] grammar (see Section 3.2). If the Diameter application has accounting requirements, it MUST also specify the AVPs that are to be present in the Diameter Accounting messages (see Section 9.3). However, just because a new authentication application id is required, does not imply that a new accounting application id is required.
When possible, a new Diameter application SHOULD reuse existing Diameter AVPs, in order to avoid defining multiple AVPs that carry similar information.

1.2.4. Creating New Accounting Applications

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

1.2.5. Application Authentication Procedures

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

1.3. Terminology
AAA Authentication, Authorization and Accounting.
Accounting The act of collecting information on resource usage for the purpose of capacity planning, auditing, billing or cost allocation.
Accounting Record An accounting record represents a summary of the resource consumption of a user over the entire session. Accounting servers creating the accounting record may do so by processing interim accounting events or accounting events from several devices serving the same user.
Authentication The act of verifying the identity of an entity (subject).
Authorization The act of determining whether a requesting entity (subject) will be allowed access to a resource (object).
AVP The Diameter protocol consists of a header followed by one or more Attribute-Value-Pairs (AVPs). An AVP includes a header and is used to encapsulate protocol-specific data (e.g., routing information) as well as authentication, authorization or accounting information.
Broker A broker is a business term commonly used in AAA infrastructures. A broker is either a relay, proxy or redirect agent, and MAY be operated by roaming consortiums. Depending on the business model, a broker may either choose to deploy relay agents or proxy agents.
Diameter Agent A Diameter Agent is a Diameter node that provides either relay, proxy, redirect or translation services.
Diameter Client A Diameter Client is a device at the edge of the network that performs access control. An example of a Diameter client is a Network Access Server (NAS) or a Foreign Agent (FA).
Diameter Node A Diameter node is a host process that implements the Diameter protocol, and acts either as a Client, Agent or Server.
Diameter Peer A Diameter Peer is a Diameter Node to which a given Diameter Node has a direct transport connection.
Diameter Security Exchange A Diameter Security Exchange is a process through which two Diameter nodes establish end-to-end security.
Diameter Server A Diameter Server is one that handles authentication, authorization and accounting requests for a particular realm. By its very nature, a Diameter Server MUST support Diameter applications in addition to the base protocol.
Downstream Downstream is used to identify the direction of a particular Diameter message from the home server towards the access device.
End-to-End Security TLS and IPsec provide hop-by-hop security, or security across a transport connection. When relays or proxy are involved, this hop-by-hop security does not protect the entire Diameter user session. End-to-end security is security between two Diameter nodes, possibly communicating through Diameter Agents. This security protects the entire Diameter communications path from the originating Diameter node to the terminating Diameter node.
Home Realm A Home Realm is the administrative domain with which the user maintains an account relationship.
Home Server See Diameter Server.
Interim accounting An interim accounting message provides a snapshot of usage during a user's session. It is typically implemented in order to provide for partial accounting of a user's session in the case of a device reboot or other network problem prevents the reception of a session summary message or session record.
Local Realm A local realm is the administrative domain providing services to a user. An administrative domain MAY act as a local realm for certain users, while being a home realm for others.
Multi-session A multi-session represents a logical linking of several sessions. Multi-sessions are tracked by using the Acct-Multi-Session-Id. An example of a multi-session would be a Multi-link PPP bundle. Each leg of the bundle would be a session while the entire bundle would be a multi-session.
Network Access Identifier The Network Access Identifier, or NAI [NAI], is used in the Diameter protocol to extract a user's identity and realm. The identity is used to identify the user during authentication and/or authorization, while the realm is used for message routing purposes.
Proxy Agent or Proxy In addition to forwarding requests and responses, proxies make policy decisions relating to resource usage and provisioning. This is typically accomplished by tracking the state of NAS devices. While proxies typically do not respond to client Requests prior to receiving a Response from the server, they may originate Reject messages in cases where policies are violated. As a result, proxies need to understand the semantics of the messages passing through them, and may not support all Diameter applications.
Realm The string in the NAI that immediately follows the '@' character. NAI realm names are required to be unique, and are piggybacked on the administration of the DNS namespace. Diameter makes use of the realm, also loosely referred to as domain, to determine whether messages can be satisfied locally, or whether they must be routed or redirected. In RADIUS, realm names are not necessarily piggybacked on the DNS namespace but may be independent of it.
Real-time Accounting Real-time accounting involves the processing of information on resource usage within a defined time window. Time constraints are typically imposed in order to limit financial risk.
Relay Agent or Relay Relays forward requests and responses based on routing-related AVPs and realm routing table entries. Since relays do not make policy decisions, they do not examine or alter non-routing AVPs. As a result, relays never originate messages, do not need to understand the semantics of messages or non-routing AVPs, and are capable of handling any Diameter application or message type. Since relays make decisions based on information in routing AVPs and realm forwarding tables they do not keep state on NAS resource usage or sessions in progress.
Redirect Agent Rather than forwarding requests and responses between clients and servers, redirect agents refer clients to servers and allow them to communicate directly. Since redirect agents do not sit in the forwarding path, they do not alter any AVPs transiting between client and server. Redirect agents do not originate messages and are capable of handling any message type, although they may be configured only to redirect messages of certain types, while acting as relay or proxy agents for other types. As with proxy agents, redirect agents do not keep state with respect to sessions or NAS resources.
Roaming Relationships Roaming relationships include relationships between companies and ISPs, relationships among peer ISPs within a roaming consortium, and relationships between an ISP and a roaming consortium.
Security Association A security association is an association between two endpoints in a Diameter session which allows the endpoints to communicate with integrity and confidentially, even in the presence of relays and/or proxies.
Session A session is a related progression of events devoted to a particular activity. Each application SHOULD provide guidelines as to when a session begins and ends. All Diameter packets with the same Session-Identifier are considered to be part of the same session.
Session state A stateful agent is one that maintains session state information, by keeping track of all authorized active sessions. Each authorized session is bound to a particular service, and its state is considered active either until it is notified otherwise, or by expiration.
Sub-session A sub-session represents a distinct service (e.g., QoS or data characteristics) provided to a given session. These services may happen concurrently (e.g., simultaneous voice and data transfer during the same session) or serially. These changes in sessions are tracked with the Accounting-Sub-Session-Id.
Transaction state The Diameter protocol requires that agents maintain transaction state, which is used for failover purposes. Transaction state implies that upon forwarding a request, the Hop-by-Hop identifier is saved; the field is replaced with a locally unique identifier, which is restored to its original value when the corresponding answer is received. The request's state is released upon receipt of the answer. A stateless agent is one that only maintains transaction state.
Translation Agent A translation agent is a stateful Diameter node that performs protocol translation between Diameter and another AAA protocol, such as RADIUS.
Transport Connection A transport connection is a TCP or SCTP connection existing directly between two Diameter peers, otherwise known as a Peer- to-Peer Connection.
Upstream Upstream is used to identify the direction of a particular Diameter message from the access device towards the home server.
User The entity requesting or using some resource, in support of which a Diameter client has generated a request.
2. Protocol Overview

The base Diameter protocol may be used by itself for accounting applications, but for use in authentication and authorization it is always extended for a particular application. Two Diameter applications are defined by companion documents: NASREQ [NASREQ],
Mobile IPv4 [DIAMMIP]. These applications are introduced in this document but specified elsewhere. Additional Diameter applications MAY be defined in the future (see Section 11.3).
Diameter Clients MUST support the base protocol, which includes accounting. In addition, they MUST fully support each Diameter application that is needed to implement the client's service, e.g., NASREQ and/or Mobile IPv4. A Diameter Client that does not support both NASREQ and Mobile IPv4, MUST be referred to as "Diameter X Client" where X is the application which it supports, and not a "Diameter Client".
Diameter Servers MUST support the base protocol, which includes accounting. In addition, they MUST fully support each Diameter application that is needed to implement the intended service, e.g., NASREQ and/or Mobile IPv4. A Diameter Server that does not support both NASREQ and Mobile IPv4, MUST be referred to as "Diameter X Server" where X is the application which it supports, and not a "Diameter Server".
Diameter Relays and redirect agents are, by definition, protocol transparent, and MUST transparently support the Diameter base protocol, which includes accounting, and all Diameter applications.
Diameter proxies MUST support the base protocol, which includes accounting. In addition, they MUST fully support each Diameter application that is needed to implement proxied services, e.g., NASREQ and/or Mobile IPv4. A Diameter proxy which does not support also both NASREQ and Mobile IPv4, MUST be referred to as "Diameter X Proxy" where X is the application which it supports, and not a "Diameter Proxy".
The base Diameter protocol concerns itself with capabilities negotiation, how messages are sent and how peers may eventually be abandoned. The base protocol also defines certain rules that apply to all exchanges of messages between Diameter nodes.
Communication between Diameter peers begins with one peer sending a message to another Diameter peer. The set of AVPs included in the message is determined by a particular Diameter application. One AVP that is included to reference a user's session is the Session-Id.
The initial request for authentication and/or authorization of a user would include the Session-Id. The Session-Id is then used in all subsequent messages to identify the user's session (see Section 8 for more information). The communicating party may accept the request, or reject it by returning an answer message with the Result-Code AVP set to indicate an error occurred. The specific behavior of the Diameter server or client receiving a request depends on the Diameter application employed.
Session state (associated with a Session-Id) MUST be freed upon receipt of the Session-Termination-Request, Session-Termination- Answer, expiration of authorized service time in the Session-Timeout AVP, and according to rules established in a particular Diameter application.

2.1. Transport

Transport profile is defined in [AAATRANS].

The base Diameter protocol is run on port 3868 of both TCP [TCP] and SCTP [SCTP] transport protocols.
Diameter clients MUST support either TCP or SCTP, while agents and servers MUST support both. Future versions of this specification MAY mandate that clients support SCTP.
A Diameter node MAY initiate connections from a source port other than the one that it declares it accepts incoming connections on, and MUST be prepared to receive connections on port 3868. A given Diameter instance of the peer state machine MUST NOT use more than one transport connection to communicate with a given peer, unless multiple instances exist on the peer in which case a separate connection per process is allowed.
When no transport connection exists with a peer, an attempt to connect SHOULD be periodically made. This behavior is handled via the Tc timer, whose recommended value is 30 seconds. There are certain exceptions to this rule, such as when a peer has terminated the transport connection stating that it does not wish to communicate.
When connecting to a peer and either zero or more transports are specified, SCTP SHOULD be tried first, followed by TCP. See Section 5.2 for more information on peer discovery.
Diameter implementations SHOULD be able to interpret ICMP protocol port unreachable messages as explicit indications that the server is not reachable, subject to security policy on trusting such messages. Diameter implementations SHOULD also be able to interpret a reset from the transport and timed-out connection attempts. If Diameter receives data up from TCP that cannot be parsed or identified as a Diameter error made by the peer, the stream is compromised and cannot be recovered. The transport connection MUST be closed using a RESET call (send a TCP RST bit) or an SCTP ABORT message (graceful closure is compromised).

2.1.1. SCTP Guidelines

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

Diameter clients, such as Network Access Servers (NASes) and Mobility Agents MUST support IP Security [SECARCH], and MAY support TLS [TLS]. Diameter servers MUST support TLS and IPsec. The Diameter protocol MUST NOT be used without any security mechanism (TLS or IPsec).

It is suggested that IPsec can be used primarily at the edges and in intra-domain traffic, such as using pre-shared keys between a NAS a local AAA proxy. This also eases the requirements on the NAS to support certificates. It is also suggested that inter-domain traffic would primarily use TLS. See Sections 13.1 and 13.2 for more details on IPsec and TLS usage.

2.3. Diameter Application Compliance

Application Identifiers are advertised during the capabilities exchange phase (see Section 5.3). For a given application, advertising support of an application implies that the sender supports all command codes, and the AVPs specified in the associated ABNFs, described in the specification.

An implementation MAY add arbitrary non-mandatory AVPs to any command defined in an application, including vendor-specific AVPs. Please refer to Section 11.1.1 for details.

2.4. Application Identifiers

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

The following Application Identifier values are defined:
Diameter Common Messages 0 NASREQ 1 [NASREQ] Mobile-IP 2 [DIAMMIP] Diameter Base Accounting 3 Relay 0xffffffff
Relay and redirect agents MUST advertise the Relay Application Identifier, while all other Diameter nodes MUST advertise locally supported applications. The receiver of a Capabilities Exchange message advertising Relay service MUST assume that the sender supports all current and future applications.
Diameter relay and proxy agents are responsible for finding an upstream server that supports the application of a particular message. If none can be found, an error message is returned with the Result-Code AVP set to DIAMETER_UNABLE_TO_DELIVER.

2.5. Connections vs. Sessions

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

A connection is a transport level connection between two peers, used to send and receive Diameter messages. A session is a logical concept at the application layer, and is shared between an access device and a server, and is identified via the Session-Id AVP
+--------+ +-------+ +--------+ | Client | | Relay | | Server | +--------+ +-------+ +--------+ <----------> <----------> peer connection A peer connection B
<-----------------------------> User session x
Figure 1: Diameter connections and sessions
In the example provided in Figure 1, peer connection A is established between the Client and its local Relay. Peer connection B is established between the Relay and the Server. User session X spans from the Client via the Relay to the Server. Each "user" of a service causes an auth request to be sent, with a unique session identifier. Once accepted by the server, both the client and the server are aware of the session. It is important to note that there is no relationship between a connection and a session, and that Diameter messages for multiple sessions are all multiplexed through a single connection.

2.6. Peer Table

The Diameter Peer Table is used in message forwarding, and referenced by the Realm Routing Table. A Peer Table entry contains the following fields:
Host identity Following the conventions described for the DiameterIdentity derived AVP data format in Section 4.4. This field contains the contents of the Origin-Host (Section 6.3) AVP found in the CER or CEA message.
StatusT This is the state of the peer entry, and MUST match one of the values listed in Section 5.6.
Static or Dynamic Specifies whether a peer entry was statically configured, or dynamically discovered.
Expiration time Specifies the time at which dynamically discovered peer table entries are to be either refreshed, or expired.
TLS Enabled Specifies whether TLS is to be used when communicating with the peer.
Additional security information, when needed (e.g., keys, certificates)

2.7. Realm-Based Routing Table

All Realm-Based routing lookups are performed against what is commonly known as the Realm Routing Table (see Section 12). A Realm Routing Table Entry contains the following fields:
Realm Name This is the field that is typically used as a primary key in the routing table lookups. Note that some implementations perform their lookups based on longest-match-from-the-right on the realm rather than requiring an exact match.
Application Identifier An application is identified by a vendor id and an application id. For all IETF standards track Diameter applications, the vendor id is zero. A route entry can have a different destination based on the application identification AVP of the message. This field MUST be used as a secondary key field in routing table lookups.
Local Action The Local Action field is used to identify how a message should be treated. The following actions are supported:
1. LOCAL - Diameter messages that resolve to a route entry with the Local Action set to Local can be satisfied locally, and do not need to be routed to another server.
2. RELAY - All Diameter messages that fall within this category MUST be routed to a next hop server, without modifying any non-routing AVPs. See Section 6.1.8 for relaying guidelines
3. PROXY - All Diameter messages that fall within this category MUST be routed to a next hop server. The local server MAY apply its local policies to the message by including new AVPs to the message prior to routing. See Section 6.1.8 for proxying guidelines.
4. REDIRECT - Diameter messages that fall within this category MUST have the identity of the home Diameter server(s) appended, and returned to the sender of the message. See Section 6.1.7 for redirect guidelines.
Server Identifier One or more servers the message is to be routed to. These servers MUST also be present in the Peer table. When the Local Action is set to RELAY or PROXY, this field contains the identity of the server(s) the message must be routed to. When the Local Action field is set to REDIRECT, this field contains the identity of one or more servers the message should be redirected to.
Static or Dynamic Specifies whether a route entry was statically configured, or dynamically discovered.
Expiration time Specifies the time which a dynamically discovered route table entry expires.
It is important to note that Diameter agents MUST support at least one of the LOCAL, RELAY, PROXY or REDIRECT modes of operation. Agents do not need to support all modes of operation in order to conform with the protocol specification, but MUST follow the protocol compliance guidelines in Section 2. Relay agents MUST NOT reorder AVPs, and proxies MUST NOT reorder AVPs.

The routing table MAY include a default entry that MUST be used for any requests not matching any of the other entries. The routing table MAY consist of only such an entry.
When a request is routed, the target server MUST have advertised the Application Identifier (see Section 2.4) for the given message, or have advertised itself as a relay or proxy agent. Otherwise, an error is returned with the Result-Code AVP set to DIAMETER_UNABLE_TO_DELIVER.

2.8. Role of Diameter Agents

In addition to client and servers, the Diameter protocol introduces relay, proxy, redirect, and translation agents, each of which is defined in Section 1.3. These Diameter agents are useful for several reasons:
- They can distribute administration of systems to a configurable grouping, including the maintenance of security associations.
- They can be used for concentration of requests from an number of co-located or distributed NAS equipment sets to a set of like user groups.
- They can do value-added processing to the requests or responses. - They can be used for load balancing.
- A complex network will have multiple authentication sources, they can sort requests and forward towards the correct target.
The Diameter protocol requires that agents maintain transaction state, which is used for failover purposes. Transaction state implies that upon forwarding a request, its Hop-by-Hop identifier is saved; the field is replaced with a locally unique identifier, which is restored to its original value when the corresponding answer is received. The request's state is released upon receipt of the answer. A stateless agent is one that only maintains transaction state.
The Proxy-Info AVP allows stateless agents to add local state to a Diameter request, with the guarantee that the same state will be present in the answer. However, the protocol's failover procedures require that agents maintain a copy of pending requests.
A stateful agent is one that maintains session state information; by keeping track of all authorized active sessions. Each authorized session is bound to a particular service, and its state is considered active either until it is notified otherwise, or by expiration. Each authorized session has an expiration, which is communicated by Diameter servers via the Session-Timeout AVP.
Maintaining session state MAY be useful in certain applications, such as:
- Protocol translation (e.g., RADIUS <-> Diameter)
- Limiting resources authorized to a particular user
- Per user or transaction auditing
A Diameter agent MAY act in a stateful manner for some requests and be stateless for others. A Diameter implementation MAY act as one type of agent for some requests, and as another type of agent for others.

2.8.1. Relay Agents

Relay Agents are Diameter agents that accept requests and route messages to other Diameter nodes based on information found in the messages (e.g., Destination-Realm). This routing decision is performed using a list of supported realms, and known peers. This is known as the Realm Routing Table, as is defined further in Section 2.7.
Relays MAY be used to aggregate requests from multiple Network Access Servers (NASes) within a common geographical area (POP). The use of Relays is advantageous since it eliminates the need for NASes to be configured with the necessary security information they would otherwise require to communicate with Diameter servers in other realms. Likewise, this reduces the configuration load on Diameter servers that would otherwise be necessary when NASes are added, changed or deleted.
Relays modify Diameter messages by inserting and removing routing information, but do not modify any other portion of a message. Relays SHOULD NOT maintain session state but MUST maintain transaction state.
+------+ ---------> +------+ ---------> +------+ | | 1. Request | | 2. Request | | | NAS | | DRL | | HMS | | | 4. Answer | | 3. Answer | | +------+ <--------- +------+ <--------- +------+ example.net example.net example.com
Figure 2: Relaying of Diameter messages
The example provided in Figure 2 depicts a request issued from NAS, which is an access device, for the user bob@example.com. Prior to issuing the request, NAS performs a Diameter route lookup, using "example.com" as the key, and determines that the message is to be relayed to DRL, which is a Diameter Relay. DRL performs the same route lookup as NAS, and relays the message to HMS, which is example.com's Home Diameter Server. HMS identifies that the request can be locally supported (via the realm), processes the authentication and/or authorization request, and replies with an answer, which is routed back to NAS using saved transaction state.
Since Relays do not perform any application level processing, they provide relaying services for all Diameter applications, and therefore MUST advertise the Relay Application Identifier.

2.8.2. Proxy Agents

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

It is important to note that although proxies MAY provide a value-add function for NASes, they do not allow access devices to use end-to- end security, since modifying messages breaks authentication.

Proxies MAY be used in call control centers or access ISPs that provide outsourced connections, they can monitor the number and types of ports in use, and make allocation and admission decisions according to their configuration.
Proxies that wish to limit resources MUST maintain session state. All proxies MUST maintain transaction state.
Since enforcing policies requires an understanding of the service being provided, Proxies MUST only advertise the Diameter applications they support.

2.8.3. Redirect Agents

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

Since redirect agents do not relay messages, and only return an answer with the information necessary for Diameter agents to communicate directly, they do not modify messages. Since redirect agents do not receive answer messages, they cannot maintain session state. Further, since redirect agents never relay requests, they are not required to maintain transaction state.
The example provided in Figure 3 depicts a request issued from the access device, NAS, for the user bob@example.com. The message is forwarded by the NAS to its relay, DRL, which does not have a routing entry in its Diameter Routing Table for example.com. DRL has a default route configured to DRD, which is a redirect agent that returns a redirect notification to DRL, as well as HMS' contact information. Upon receipt of the redirect notification, DRL establishes a transport connection with HMS, if one doesn't already exist, and forwards the request to it.
+------+ | | | DRD | | | +------+ ^ | 2. Request | | 3. Redirection | | Notification | v +------+ ---------> +------+ ---------> +------+ | | 1. Request | | 4. Request | | | NAS | | DRL | | HMS | | | 6. Answer | | 5. Answer | | +------+ <--------- +------+ <--------- +------+ example.net example.net example.com
Figure 3: Redirecting a Diameter Message
Since redirect agents do not perform any application level processing, they provide relaying services for all Diameter applications, and therefore MUST advertise the Relay Application Identifier.

2.8.4. Translation Agents

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

Given that the Diameter protocol introduces the concept of long-lived authorized sessions, translation agents MUST be session stateful and MUST maintain transaction state.
Translation of messages can only occur if the agent recognizes the application of a particular request, and therefore translation agents MUST only advertise their locally supported applications.
+------+ ---------> +------+ ---------> +------+ | | RADIUS Request | | Diameter Request | | | NAS | | TLA | | HMS | | | RADIUS Answer | | Diameter Answer | | +------+ <--------- +------+ <--------- +------+ example.net example.net example.com
Figure 4: Translation of RADIUS to Diameter
2.9. End-to-End Security Framework

End-to-end security services include confidentiality and message origin authentication. These services are provided by supporting AVP integrity and confidentiality between two peers, communicating through agents.

End-to-end security is provided via the End-to-End security extension, described in [AAACMS]. The circumstances requiring the use of end-to-end security are determined by policy on each of the peers. Security policies, which are not the subject of standardization, may be applied by next hop Diameter peer or by destination realm. For example, where TLS or IPsec transmission- level security is sufficient, there may be no need for end-to-end security.
End-to-end security policies include:
- Never use end-to-end security.
- Use end-to-end security on messages containing sensitive AVPs. Which AVPs are sensitive is determined by service provider policy. AVPs containing keys and passwords should be considered sensitive. Accounting AVPs may be considered sensitive. Any AVP for which the P bit may be set or which may be encrypted may be considered sensitive.
- Always use end-to-end security.
It is strongly RECOMMENDED that all Diameter implementations support end-to-end security.

2.10. Diameter Path Authorization

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

In addition to authenticating each connection, each connection as well as the entire session MUST also be authorized. Before initiating a connection, a Diameter Peer MUST check that its peers are authorized to act in their roles. For example, a Diameter peer may be authentic, but that does not mean that it is authorized to act as a Diameter Server advertising a set of Diameter applications. Prior to bringing up a connection, authorization checks are performed at each connection along the path. Diameter capabilities negotiation (CER/CEA) also MUST be carried out, in order to determine what Diameter applications are supported by each peer. Diameter sessions MUST be routed only through authorized nodes that have advertised support for the Diameter application required by the session.
As noted in Section 6.1.8, a relay or proxy agent MUST append a Route-Record AVP to all requests forwarded. The AVP contains the identity of the peer the request was received from.
The home Diameter server, prior to authorizing a session, MUST check the Route-Record AVPs to make sure that the route traversed by the request is acceptable. For example, administrators within the home realm may not wish to honor requests that have been routed through an untrusted realm. By authorizing a request, the home Diameter server is implicitly indicating its willingness to engage in the business transaction as specified by the contractual relationship between the server and the previous hop. A DIAMETER_AUTHORIZATION_REJECTED error message (see Section 7.1.5) is sent if the route traversed by the request is unacceptable.
A home realm may also wish to check that each accounting request message corresponds to a Diameter response authorizing the session. Accounting requests without corresponding authorization responses SHOULD be subjected to further scrutiny, as should accounting requests indicating a difference between the requested and provided service.
Similarly, the local Diameter agent, on receiving a Diameter response authorizing a session, MUST check the Route-Record AVPs to make sure that the route traversed by the response is acceptable. At each step, forwarding of an authorization response is considered evidence of a willingness to take on financial risk relative to the session. A local realm may wish to limit this exposure, for example, by establishing credit limits for intermediate realms and refusing to accept responses which would violate those limits. By issuing an accounting request corresponding to the authorization response, the local realm implicitly indicates its agreement to provide the service indicated in the authorization response. If the service cannot be provided by the local realm, then a DIAMETER_UNABLE_TO_COMPLY error message MUST be sent within the accounting request; a Diameter client receiving an authorization response for a service that it cannot perform MUST NOT substitute an alternate service, and then send accounting requests for the alternate service instead.

3. Diameter Header

A summary of the Diameter header format is shown below. The fields are transmitted in network byte order.
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Version | Message Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | command flags | Command-Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Application-ID | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Hop-by-Hop Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | End-to-End Identifier | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVPs ... +-+-+-+-+-+-+-+-+-+-+-+-+-
Version This Version field MUST be set to 1 to indicate Diameter Version 1.
Message Length The Message Length field is three octets and indicates the length of the Diameter message including the header fields.
Command Flags The Command Flags field is eight bits. The following bits are assigned:
0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |R P E T r r r r| +-+-+-+-+-+-+-+-+
R(equest) - If set, the message is a request. If cleared, the message is an answer. P(roxiable) - If set, the message MAY be proxied, relayed or redirected. If cleared, the message MUST be locally processed. E(rror) - If set, the message contains a protocol error, and the message will not conform to the ABNF described for this command. Messages with the 'E'
bit set are commonly referred to as error messages. This bit MUST NOT be set in request messages. See Section 7.2. T(Potentially re-transmitted message) - This flag is set after a link failover procedure, to aid the removal of duplicate requests. It is set when resending requests not yet acknowledged, as an indication of a possible duplicate due to a link failure. This bit MUST be cleared when sending a request for the first time, otherwise the sender MUST set this flag. Diameter agents only need to be concerned about the number of requests they send based on a single received request; retransmissions by other entities need not be tracked. Diameter agents that receive a request with the T flag set, MUST keep the T flag set in the forwarded request. This flag MUST NOT be set if an error answer message (e.g., a protocol error) has been received for the earlier message. It can be set only in cases where no answer has been received from the server for a request and the request is sent again. This flag MUST NOT be set in answer messages.
r(eserved) - these flag bits are reserved for future use, and MUST be set to zero, and ignored by the receiver.
Command-Code The Command-Code field is three octets, and is used in order to communicate the command associated with the message. The 24-bit address space is managed by IANA (see Section 11.2.1).
Command-Code values 16,777,214 and 16,777,215 (hexadecimal values FFFFFE -FFFFFF) are reserved for experimental use (See Section 11.3).
Application-ID Application-ID is four octets and is used to identify to which application the message is applicable for. The application can be an authentication application, an accounting application or a vendor specific application. See Section 11.3 for the possible values that the application-id may use.
The application-id in the header MUST be the same as what is contained in any relevant AVPs contained in the message.
Hop-by-Hop Identifier The Hop-by-Hop Identifier is an unsigned 32-bit integer field (in network byte order) and aids in matching requests and replies. The sender MUST ensure that the Hop-by-Hop identifier in a request is unique on a given connection at any given time, and MAY attempt to ensure that the number is unique across reboots. The sender of an Answer message MUST ensure that the Hop-by-Hop Identifier field contains the same value that was found in the corresponding request. The Hop-by-Hop identifier is normally a monotonically increasing number, whose start value was randomly generated. An answer message that is received with an unknown Hop-by-Hop Identifier MUST be discarded.
End-to-End Identifier The End-to-End Identifier is an unsigned 32-bit integer field (in network byte order) and is used to detect duplicate messages. Upon reboot implementations MAY set the high order 12 bits to contain the low order 12 bits of current time, and the low order 20 bits to a random value. Senders of request messages MUST insert a unique identifier on each message. The identifier MUST remain locally unique for a period of at least 4 minutes, even across reboots. The originator of an Answer message MUST ensure that the End-to-End Identifier field contains the same value that was found in the corresponding request. The End-to-End Identifier MUST NOT be modified by Diameter agents of any kind. The combination of the Origin-Host (see Section 6.3) and this field is used to detect duplicates. Duplicate requests SHOULD cause the same answer to be transmitted (modulo the hop-by-hop Identifier field and any routing AVPs that may be present), and MUST NOT affect any state that was set when the original request was processed. Duplicate answer messages that are to be locally consumed (see Section 6.2) SHOULD be silently discarded.
AVPs AVPs are a method of encapsulating information relevant to the Diameter message. See Section 4 for more information on AVPs.
3.1. Command Codes

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

Every Diameter message MUST contain a command code in its header's Command-Code field, which is used to determine the action that is to be taken for a particular message. The following Command Codes are defined in the Diameter base protocol:
Command-Name Abbrev. Code Reference -------------------------------------------------------- Abort-Session-Request ASR 274 8.5.1 Abort-Session-Answer ASA 274 8.5.2 Accounting-Request ACR 271 9.7.1 Accounting-Answer ACA 271 9.7.2 Capabilities-Exchange- CER 257 5.3.1 Request Capabilities-Exchange- CEA 257 5.3.2 Answer Device-Watchdog-Request DWR 280 5.5.1 Device-Watchdog-Answer DWA 280 5.5.2 Disconnect-Peer-Request DPR 282 5.4.1 Disconnect-Peer-Answer DPA 282 5.4.2 Re-Auth-Request RAR 258 8.3.1 Re-Auth-Answer RAA 258 8.3.2 Session-Termination- STR 275 8.4.1 Request Session-Termination- STA 275 8.4.2 Answer
3.2. Command Code ABNF specification

Every Command Code defined MUST include a corresponding ABNF specification, which is used to define the AVPs that MUST or MAY be present. The following format is used in the definition:
command-def = command-name "::=" diameter-message
command-name = diameter-name
diameter-name = ALPHA *(ALPHA / DIGIT / "-")
diameter-message = header [ *fixed] [ *required] [ *optional] [ *fixed]
header = "<" Diameter-Header:" command-id [r-bit] [p-bit] [e-bit] [application-id]">"
application-id = 1*DIGIT
command-id = 1*DIGIT ; The Command Code assigned to the command
r-bit = ", REQ" ; If present, the 'R' bit in the Command ; Flags is set, indicating that the message ; is a request, as opposed to an answer.
p-bit = ", PXY" ; If present, the 'P' bit in the Command ; Flags is set, indicating that the message ; is proxiable.
e-bit = ", ERR" ; If present, the 'E' bit in the Command ; Flags is set, indicating that the answer ; message contains a Result-Code AVP in ; the "protocol error" class.
fixed = [qual] "<" avp-spec ">" ; Defines the fixed position of an AVP
required = [qual] "{" avp-spec "}" ; The AVP MUST be present and can appear ; anywhere in the message.
optional = [qual] "[" avp-name "]" ; The avp-name in the 'optional' rule cannot ; evaluate to any AVP Name which is included ; in a fixed or required rule. The AVP can ; appear anywhere in the message.
qual = [min] "*" [max] ; See ABNF conventions, RFC 2234 Section 6.6. ; The absence of any qualifiers depends on whether ; it precedes a fixed, required, or optional ; rule. If a fixed or required rule has no ; qualifier, then exactly one such AVP MUST ; be present. If an optional rule has no ; qualifier, then 0 or 1 such AVP may be ; present. ; ; NOTE: "[" and "]" have a different meaning ; than in ABNF (see the optional rule, above). ; These braces cannot be used to express ; optional fixed rules (such as an optional ; ICV at the end). To do this, the convention ; is '0*1fixed'.
min = 1*DIGIT ; The minimum number of times the element may ; be present. The default value is zero.
max = 1*DIGIT ; The maximum number of times the element may ; be present. The default value is infinity. A ; value of zero implies the AVP MUST NOT be ; present.
avp-spec = diameter-name ; The avp-spec has to be an AVP Name, defined ; in the base or extended Diameter ; specifications.
avp-name = avp-spec / "AVP" ; The string "AVP" stands for *any* arbitrary ; AVP Name, which does not conflict with the ; required or fixed position AVPs defined in ; the command code definition.
The following is a definition of a fictitious command code:
Example-Request ::= < "Diameter-Header: 9999999, REQ, PXY > { User-Name } * { Origin-Host } * [ AVP
3.3. Diameter Command Naming Conventions

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

An example is a message set used to terminate a session. The command name is Session-Terminate-Request and Session-Terminate-Answer, while the acronyms are STR and STA, respectively.
Both the request and the answer for a given command share the same command code. The request is identified by the R(equest) bit in the Diameter header set to one (1), to ask that a particular action be performed, such as authorizing a user or terminating a session. Once the receiver has completed the request it issues the corresponding answer, which includes a result code that communicates one of the following:
- The request was successful
- The request failed
- An additional request must be sent to provide information the peer requires prior to returning a successful or failed answer.
- The receiver could not process the request, but provides information about a Diameter peer that is able to satisfy the request, known as redirect.
Additional information, encoded within AVPs, MAY also be included in answer messages.

4. Diameter AVPs

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

Some AVPs MAY be listed more than once. The effect of such an AVP is specific, and is specified in each case by the AVP description. Each AVP of type OctetString MUST be padded to align on a 32-bit boundary, while other AVP types align naturally. A number of zero- valued bytes are added to the end of the AVP Data field till a word boundary is reached. The length of the padding is not reflected in the AVP Length field.

4.1. AVP Header

The fields in the AVP header MUST be sent in network byte order. The format of the header is:
0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | AVP Code | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |V M P r r r r r| AVP Length | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Vendor-ID (opt) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Data ... +-+-+-+-+-+-+-+-+
AVP Code The AVP Code, combined with the Vendor-Id field, identifies the attribute uniquely. AVP numbers 1 through 255 are reserved for backward compatibility with RADIUS, without setting the Vendor-Id field. AVP numbers 256 and above are used for Diameter, which are allocated by IANA (see Section 11.1).
AVP Flags The AVP Flags field informs the receiver how each attribute must be handled. The 'r' (reserved) bits are unused and SHOULD be set to 0. Note that subsequent Diameter applications MAY define additional bits within the AVP Header, and an unrecognized bit SHOULD be considered an error. The 'P' bit indicates the need for encryption for end-to-end security.
The 'M' Bit, known as the Mandatory bit, indicates whether support of the AVP is required. If an AVP with the 'M' bit set is received by a Diameter client, server, proxy, or translation agent and either the AVP or its value is unrecognized, the message MUST be rejected. Diameter Relay and redirect agents MUST NOT reject messages with unrecognized AVPs.
The 'M' bit MUST be set according to the rules defined for the AVP containing it. In order to preserve interoperability, a Diameter implementation MUST be able to exclude from a Diameter message any Mandatory AVP which is neither defined in the base Diameter protocol nor in any of the Diameter Application specifications governing the message in which it appears. It MAY do this in one of the following ways:
1) If a message is rejected because it contains a Mandatory AVP which is neither defined in the base Diameter standard nor in any of the Diameter Application specifications governing the message in which it appears, the implementation may resend the message without the AVP, possibly inserting additional standard AVPs instead.
2) A configuration option may be provided on a system wide, per peer, or per realm basis that would allow/prevent particular Mandatory AVPs to be sent. Thus an administrator could change the configuration to avoid interoperability problems.
Diameter implementations are required to support all Mandatory AVPs which are allowed by the message's formal syntax and defined either in the base Diameter standard or in one of the Diameter Application specifications governing the message.
AVPs with the 'M' bit cleared are informational only and a receiver that receives a message with such an AVP that is not supported, or whose value is not supported, MAY simply ignore the AVP.
The 'V' bit, known as the Vendor-Specific bit, indicates whether the optional Vendor-ID field is present in the AVP header. When set the AVP Code belongs to the specific vendor code address space.
Unless otherwise noted, AVPs will have the following default AVP Flags field settings:
The 'M' bit MUST be set. The 'V' bit MUST NOT be set.
AVP Length The AVP Length field is three octets, and indicates the number of octets in this AVP including the AVP Code, AVP Length, AVP Flags, Vendor-ID field (if present) and the AVP data. If a message is received with an invalid attribute length, the message SHOULD be rejected.
4.1.1. Optional Header Elements

The AVP Header contains one optional field. This field is only present if the respective bit-flag is enabled.
Vendor-ID The Vendor-ID field is present if the 'V' bit is set in the AVP Flags field. The optional four-octet Vendor-ID field contains the IANA assigned "SMI Network Management Private Enterprise Codes" [ASSIGNNO] value, encoded in network byte order. Any vendor wishing to implement a vendor-specific Diameter AVP MUST use their own Vendor-ID along with their privately managed AVP address space, guaranteeing that they will not collide with any other vendor's vendor-specific AVP(s), nor with future IETF applications.
A vendor ID value of zero (0) corresponds to the IETF adopted AVP values, as managed by the IANA. Since the absence of the vendor ID field implies that the AVP in question is not vendor specific, implementations MUST NOT use the zero (0) vendor ID.
4.2. Basic AVP Data Formats

The Data field is zero or more octets and contains information specific to the Attribute. The format and length of the Data field is determined by the AVP Code and AVP Length fields. The format of the Data field MUST be one of the following base data types or a data type derived from the base data types. In the event that a new Basic AVP Data Format is needed, a new version of this RFC must be created.
OctetString The data contains arbitrary data of variable length. Unless otherwise noted, the AVP Length field MUST be set to at least 8 (12 if the 'V' bit is enabled). AVP Values of this type that are not a multiple of four-octets in length is followed by the necessary padding so that the next AVP (if any) will start on a 32-bit boundary.
Integer32 32 bit signed value, in network byte order. The AVP Length field MUST be set to 12 (16 if the 'V' bit is enabled).
Integer64 64 bit signed value, in network byte order. The AVP Length field MUST be set to 16 (20 if the 'V' bit is enabled).
Unsigned32 32 bit unsigned value, in network byte order. The AVP Length field MUST be set to 12 (16 if the 'V' bit is enabled).
Unsigned64 64 bit unsigned value, in network byte order. The AVP Length field MUST be set to 16 (20 if the 'V' bit is enabled).
Float32 This represents floating point values of single precision as described by [FLOATPOINT]. The 32-bit value is transmitted in network byte order. The AVP Length field MUST be set to 12 (16 if the 'V' bit is enabled).
Float64 This represents floating point values of double precision as described by [FLOATPOINT]. The 64-bit value is transmitted in network byte order. The AVP Length field MUST be set to 16 (20 if the 'V' bit is enabled).
Grouped The Data field is specified as a sequence of AVPs. Each of these AVPs follows - in the order in which they are specified - including their headers and padding. The AVP Length field is set to 8 (12 if the 'V' bit is enabled) plus the total length of all included AVPs, including their headers and padding. Thus the AVP length field of an AVP of type Grouped is always a multiple of 4.
4.3. Derived AVP Data Formats

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

The below AVP Derived Data Formats are commonly used by applications.
Address The Address format is derived from the OctetString AVP Base Format. It is a discriminated union, representing, for example a 32-bit (IPv4) [IPV4] or 128-bit (IPv6) [IPV6] address, most significant octet first. The first two octets of the Address
AVP represents the AddressType, which contains an Address Family defined in [IANAADFAM]. The AddressType is used to discriminate the content and format of the remaining octets.
Time The Time format is derived from the OctetString AVP Base Format. The string MUST contain four octets, in the same format as the first four bytes are in the NTP timestamp format. The NTP Timestamp format is defined in chapter 3 of [SNTP].
This represents the number of seconds since 0h on 1 January 1900 with respect to the Coordinated Universal Time (UTC).
On 6h 28m 16s UTC, 7 February 2036 the time value will overflow. SNTP [SNTP] describes a procedure to extend the time to 2104. This procedure MUST be supported by all DIAMETER nodes.
UTF8String The UTF8String format is derived from the OctetString AVP Base Format. This is a human readable string represented using the ISO/IEC IS 10646-1 character set, encoded as an OctetString using the UTF-8 [UFT8] transformation format described in RFC 2279.
Since additional code points are added by amendments to the 10646 standard from time to time, implementations MUST be prepared to encounter any code point from 0x00000001 to 0x7fffffff. Byte sequences that do not correspond to the valid encoding of a code point into UTF-8 charset or are outside this range are prohibited.
The use of control codes SHOULD be avoided. When it is necessary to represent a new line, the control code sequence CR LF SHOULD be used.
The use of leading or trailing white space SHOULD be avoided.
For code points not directly supported by user interface hardware or software, an alternative means of entry and display, such as hexadecimal, MAY be provided.
For information encoded in 7-bit US-ASCII, the UTF-8 charset is identical to the US-ASCII charset.
UTF-8 may require multiple bytes to represent a single character / code point; thus the length of an UTF8String in octets may be different from the number of characters encoded.
Note that the AVP Length field of an UTF8String is measured in octets, not characters.
DiameterIdentity The DiameterIdentity format is derived from the OctetString AVP Base Format.
DiameterIdentity = FQDN
DiameterIdentity value is used to uniquely identify a Diameter node for purposes of duplicate connection and routing loop detection.
The contents of the string MUST be the FQDN of the Diameter node. If multiple Diameter nodes run on the same host, each Diameter node MUST be assigned a unique DiameterIdentity. If a Diameter node can be identified by several FQDNs, a single FQDN should be picked at startup, and used as the only DiameterIdentity for that node, whatever the connection it is sent on.
DiameterURI
The DiameterURI MUST follow the Uniform Resource Identifiers (URI) syntax [URI] rules specified below:
"aaa://" FQDN [ port ] [ transport ] [ protocol ]
; No transport security
"aaas://" FQDN [ port ] [ transport ] [ protocol ]
; Transport security used
FQDN = Fully Qualified Host Name
port = ":" 1*DIGIT
; One of the ports used to listen for ; incoming connections. ; If absent, ; the default Diameter port (3868) is ; assumed.
transport = ";transport=" transport-protocol
; One of the transports used to listen ; for incoming connections. If absent, ; the default SCTP [SCTP] protocol is ; assumed. UDP MUST NOT be used when ; the aaa-protocol field is set to ; diameter.
transport-protocol = ( "tcp" / "sctp" / "udp" )
protocol = ";protocol=" aaa-protocol
; If absent, the default AAA protocol ; is diameter.
aaa-protocol = ( "diameter" / "radius" / "tacacs+" )
The following are examples of valid Diameter host identities:
aaa://host.example.com;transport=tcp aaa://host.example.com:6666;transport=tcp aaa://host.example.com;protocol=diameter aaa://host.example.com:6666;protocol=diameter aaa://host.example.com:6666;transport=tcp;protocol=diameter aaa://host.example.com:1813;transport=udp;protocol=radius
Enumerated Enumerated is derived from the Integer32 AVP Base Format. The definition contains a list of valid values and their interpretation and is described in the Diameter application introducing the AVP.
IPFilterRule The IPFilterRule format is derived from the OctetString AVP Base Format. It uses the ASCII charset. Packets may be filtered based on the following information that is associated with it:
Direction (in or out) Source and destination IP address (possibly masked) Protocol Source and destination port (lists or ranges) TCP flags IP fragment flag IP options ICMP types
Rules for the appropriate direction are evaluated in order, with the first matched rule terminating the evaluation. Each packet is evaluated once. If no rule matches, the packet is dropped if the last rule evaluated was a permit, and passed if the last rule was a deny.
IPFilterRule filters MUST follow the format:
action dir proto from src to dst [options]
action permit - Allow packets that match the rule. deny - Drop packets that match the rule.
dir "in" is from the terminal, "out" is to the terminal.
proto An IP protocol specified by number. The "ip" keyword means any protocol will match.
src and dst <address/mask> [ports]
The <address/mask> may be specified as: ipno An IPv4 or IPv6 number in dotted- quad or canonical IPv6 form. Only this exact IP number will match the rule. ipno/bits An IP number as above with a mask width of the form 1.2.3.4/24. In this case, all IP numbers from 1.2.3.0 to 1.2.3.255 will match. The bit width MUST be valid for the IP version and the IP number MUST NOT have bits set beyond the mask. For a match to occur, the same IP version must be present in the packet that was used in describing the IP address. To test for a particular IP version, the bits part can be set to zero. The keyword "any" is 0.0.0.0/0 or the IPv6 equivalent. The keyword "assigned" is the address or set of addresses assigned to the terminal. For IPv4, a typical first rule is often "deny in ip! assigned"
The sense of the match can be inverted by preceding an address with the not modifier (!), causing all other addresses to be matched instead. This does not affect the selection of port numbers.
With the TCP, UDP and SCTP protocols, optional ports may be specified as:
{port/port-port}[,ports[,...]]
The '-' notation specifies a range of ports (including boundaries).
Fragmented packets that have a non-zero offset (i.e., not the first fragment) will never match a rule that has one or more port specifications. See the frag option for details on matching fragmented packets.
options: frag Match if the packet is a fragment and this is not the first fragment of the datagram. frag may not be used in conjunction with either tcpflags or TCP/UDP port specifications.
ipoptions spec Match if the IP header contains the comma separated list of options specified in spec. The supported IP options are: