Diameter 协议

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

Diameter Base Protocol

Status of this Memo

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

Copyright Notice

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

Abstract

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

Conventions Used In This Document

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

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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

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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

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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

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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

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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.

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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.

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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

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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.

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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.

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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],

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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

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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.

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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.

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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

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

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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.

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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.

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- 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.

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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 [e-mail]. 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.

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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 [e-mail]. 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.

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RFC 3588 Diameter Based Protocol September 2003

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

Figure 3: Redirecting a Diameter Message

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

2.8.4. Translation Agents

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

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

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

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

Figure 4: Translation of RADIUS to Diameter

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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.

Calhoun, et al. Standards Track [Page 30]

RFC 3588 Diameter Based Protocol September 2003

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

As noted in Section 6.1.8, a relay or proxy agent MUST append a
Route-Record AVP to all requests forwarded. The AVP contains the
identity of the peer the request was received from.

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

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

Similarly, the local Diameter agent, on receiving a Diameter response
authorizing a session, MUST check the Route-Record AVPs to make sure
that the route traversed by the response is acceptable. At each
step, forwarding of an authorization response is considered evidence
of a willingness to take on financial risk relative to the session.
A local realm may wish to limit this exposure, for example, by
establishing credit limits for intermediate realms and refusing to
accept responses which would violate those limits. By issuing an
accounting request corresponding to the authorization response, the
local realm implicitly indicates its agreement to provide the service
indicated in the authorization response. If the service cannot be
provided by the local realm, then a DIAMETER_UNABLE_TO_COMPLY error
message MUST be sent within the accounting request; a Diameter client
receiving an authorization response for a service that it cannot
perform MUST NOT substitute an alternate service, and then send
accounting requests for the alternate service instead.

Calhoun, et al. Standards Track [Page 31]

RFC 3588 Diameter Based Protocol September 2003

3. Diameter Header

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

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

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

Message Length
The Message Length field is three octets and indicates the length
of the Diameter message including the header fields.

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

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

R(equest) - If set, the message is a request. If cleared, the
message is an answer.
P(roxiable) - If set, the message MAY be proxied, relayed or
redirected. If cleared, the message MUST be
locally processed.
E(rror) - If set, the message contains a protocol error,
and the message will not conform to the ABNF
described for this command. Messages with the 'E'

Calhoun, et al. Standards Track [Page 32]

RFC 3588 Diameter Based Protocol September 2003

bit set are commonly referred to as error
messages. This bit MUST NOT be set in request
messages. See Section 7.2.
T(Potentially re-transmitted message)
- This flag is set after a link failover procedure,
to aid the removal of duplicate requests. It is
set when resending requests not yet acknowledged,
as an indication of a possible duplicate due to a
link failure. This bit MUST be cleared when
sending a request for the first time, otherwise
the sender MUST set this flag. Diameter agents
only need to be concerned about the number of
requests they send based on a single received
request; retransmissions by other entities need
not be tracked. Diameter agents that receive a
request with the T flag set, MUST keep the T flag
set in the forwarded request. This flag MUST NOT
be set if an error answer message (e.g., a
protocol error) has been received for the earlier
message. It can be set only in cases where no
answer has been received from the server for a
request and the request is sent again. This flag
MUST NOT be set in answer messages.

r(eserved) - these flag bits are reserved for future use, and
MUST be set to zero, and ignored by the receiver.

Command-Code
The Command-Code field is three octets, and is used in order to
communicate the command associated with the message. The 24-bit
address space is managed by IANA (see Section 11.2.1).

Command-Code values 16,777,214 and 16,777,215 (hexadecimal values
FFFFFE -FFFFFF) are reserved for experimental use (See Section
11.3).

Application-ID
Application-ID is four octets and is used to identify to which
application the message is applicable for. The application can be
an authentication application, an accounting application or a
vendor specific application. See Section 11.3 for the possible
values that the application-id may use.

The application-id in the header MUST be the same as what is
contained in any relevant AVPs contained in the message.

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RFC 3588 Diameter Based Protocol September 2003

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

End-to-End Identifier
The End-to-End Identifier is an unsigned 32-bit integer field (in
network byte order) and is used to detect duplicate messages.
Upon reboot implementations MAY set the high order 12 bits to
contain the low order 12 bits of current time, and the low order
20 bits to a random value. Senders of request messages MUST
insert a unique identifier on each message. The identifier MUST
remain locally unique for a period of at least 4 minutes, even
across reboots. The originator of an Answer message MUST ensure
that the End-to-End Identifier field contains the same value that
was found in the corresponding request. The End-to-End Identifier
MUST NOT be modified by Diameter agents of any kind. The
combination of the Origin-Host (see Section 6.3) and this field is
used to detect duplicates. Duplicate requests SHOULD cause the
same answer to be transmitted (modulo the hop-by-hop Identifier
field and any routing AVPs that may be present), and MUST NOT
affect any state that was set when the original request was
processed. Duplicate answer messages that are to be locally
consumed (see Section 6.2) SHOULD be silently discarded.

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

Calhoun, et al. Standards Track [Page 34]

RFC 3588 Diameter Based Protocol September 2003

3.1. Command Codes

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

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

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

Calhoun, et al. Standards Track [Page 35]

RFC 3588 Diameter Based Protocol September 2003

3.2. Command Code ABNF specification

Every Command Code defined MUST include a corresponding ABNF
specification, which is used to define the AVPs that MUST or MAY be
present. The following format is used in the definition:

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

command-name = diameter-name

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

diameter-message = header [ *fixed] [ *required] [ *optional]
[ *fixed]

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

application-id = 1*DIGIT

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

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

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

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

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

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

Calhoun, et al. Standards Track [Page 36]

RFC 3588 Diameter Based Protocol September 2003

optional = [qual] "[" avp-name "]"
; The avp-name in the 'optional' rule cannot
; evaluate to any AVP Name which is included
; in a fixed or required rule. The AVP can
; appear anywhere in the message.

qual = [min] "*" [max]
; See ABNF conventions, RFC 2234 Section 6.6.
; The absence of any qualifiers depends on whether
; it precedes a fixed, required, or optional
; rule. If a fixed or required rule has no
; qualifier, then exactly one such AVP MUST
; be present. If an optional rule has no
; qualifier, then 0 or 1 such AVP may be
; present.
;
; NOTE: "[" and "]" have a different meaning
; than in ABNF (see the optional rule, above).
; These braces cannot be used to express
; optional fixed rules (such as an optional
; ICV at the end). To do this, the convention
; is '0*1fixed'.

min = 1*DIGIT
; The minimum number of times the element may
; be present. The default value is zero.

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

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

avp-name = avp-spec / "AVP"
; The string "AVP" stands for *any* arbitrary
; AVP Name, which does not conflict with the
; required or fixed position AVPs defined in
; the command code definition.

Calhoun, et al. Standards Track [Page 37]

RFC 3588 Diameter Based Protocol September 2003

The following is a definition of a fictitious command code:

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

3.3. Diameter Command Naming Conventions

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

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

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

- 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.

Calhoun, et al. Standards Track [Page 38]

RFC 3588 Diameter Based Protocol September 2003

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

4.1. AVP Header

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

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

AVP Code
The AVP Code, combined with the Vendor-Id field, identifies the
attribute uniquely. AVP numbers 1 through 255 are reserved for
backward compatibility with RADIUS, without setting the Vendor-Id
field. AVP numbers 256 and above are used for Diameter, which are
allocated by IANA (see Section 11.1).

AVP Flags
The AVP Flags field informs the receiver how each attribute must
be handled. The 'r' (reserved) bits are unused and SHOULD be set
to 0. Note that subsequent Diameter applications MAY define
additional bits within the AVP Header, and an unrecognized bit
SHOULD be considered an error. The 'P' bit indicates the need for
encryption for end-to-end security.

The 'M' Bit, known as the Mandatory bit, indicates whether support
of the AVP is required. If an AVP with the 'M' bit set is
received by a Diameter client, server, proxy, or translation agent
and either the AVP or its value is unrecognized, the message MUST
be rejected. Diameter Relay and redirect agents MUST NOT reject
messages with unrecognized AVPs.

Calhoun, et al. Standards Track [Page 39]

RFC 3588 Diameter Based Protocol September 2003

The 'M' bit MUST be set according to the rules defined for the AVP
containing it. In order to preserve interoperability, a Diameter
implementation MUST be able to exclude from a Diameter message any
Mandatory AVP which is neither defined in the base Diameter
protocol nor in any of the Diameter Application specifications
governing the message in which it appears. It MAY do this in one
of the following ways:

1) If a message is rejected because it contains a Mandatory AVP
which is neither defined in the base Diameter standard nor in
any of the Diameter Application specifications governing the
message in which it appears, the implementation may resend the
message without the AVP, possibly inserting additional standard
AVPs instead.

2) A configuration option may be provided on a system wide, per
peer, or per realm basis that would allow/prevent particular
Mandatory AVPs to be sent. Thus an administrator could change
the configuration to avoid interoperability problems.

Diameter implementations are required to support all Mandatory
AVPs which are allowed by the message's formal syntax and defined
either in the base Diameter standard or in one of the Diameter
Application specifications governing the message.

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

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

Unless otherwise noted, AVPs will have the following default AVP
Flags field settings:

The 'M' bit MUST be set. The 'V' bit MUST NOT be set.

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

Calhoun, et al. Standards Track [Page 40]

RFC 3588 Diameter Based Protocol September 2003

4.1.1. Optional Header Elements

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

Vendor-ID
The Vendor-ID field is present if the 'V' bit is set in the AVP
Flags field. The optional four-octet Vendor-ID field contains the
IANA assigned "SMI Network Management Private Enterprise Codes"
[ASSIGNNO] value, encoded in network byte order. Any vendor
wishing to implement a vendor-specific Diameter AVP MUST use their
own Vendor-ID along with their privately managed AVP address
space, guaranteeing that they will not collide with any other
vendor's vendor-specific AVP(s), nor with future IETF
applications.

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

4.2. Basic AVP Data Formats

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

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

Integer32
32 bit signed value, in network byte order. The AVP Length field
MUST be set to 12 (16 if the 'V' bit is enabled).

Integer64
64 bit signed value, in network byte order. The AVP Length field
MUST be set to 16 (20 if the 'V' bit is enabled).

Calhoun, et al. Standards Track [Page 41]

RFC 3588 Diameter Based Protocol September 2003

Unsigned32
32 bit unsigned value, in network byte order. The AVP Length
field MUST be set to 12 (16 if the 'V' bit is enabled).

Unsigned64
64 bit unsigned value, in network byte order. The AVP Length
field MUST be set to 16 (20 if the 'V' bit is enabled).

Float32
This represents floating point values of single precision as
described by [FLOATPOINT]. The 32-bit value is transmitted in
network byte order. The AVP Length field MUST be set to 12 (16 if
the 'V' bit is enabled).

Float64
This represents floating point values of double precision as
described by [FLOATPOINT]. The 64-bit value is transmitted in
network byte order. The AVP Length field MUST be set to 16 (20 if
the 'V' bit is enabled).

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

4.3. Derived AVP Data Formats

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

The below AVP Derived Data Formats are commonly used by applications.

Address
The Address format is derived from the OctetString AVP Base
Format. It is a discriminated union, representing, for example a
32-bit (IPv4) [IPV4] or 128-bit (IPv6) [IPV6] address, most
significant octet first. The first two octets of the Address

Calhoun, et al. Standards Track [Page 42]

RFC 3588 Diameter Based Protocol September 2003

AVP represents the AddressType, which contains an Address Family
defined in [IANAADFAM]. The AddressType is used to discriminate
the content and format of the remaining octets.

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

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

On 6h 28m 16s UTC, 7 February 2036 the time value will overflow.
SNTP [SNTP] describes a procedure to extend the time to 2104.
This procedure MUST be supported by all DIAMETER nodes.

UTF8String
The UTF8String format is derived from the OctetString AVP Base
Format. This is a human readable string represented using the
ISO/IEC IS 10646-1 character set, encoded as an OctetString using
the UTF-8 [UFT8] transformation format described in RFC 2279.

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

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

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

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

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

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

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

Calhoun, et al. Standards Track [Page 43]

RFC 3588 Diameter Based Protocol September 2003

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

DiameterIdentity = FQDN

DiameterIdentity value is used to uniquely identify a Diameter
node for purposes of duplicate connection and routing loop
detection.

The contents of the string MUST be the FQDN of the Diameter node.
If multiple Diameter nodes run on the same host, each Diameter
node MUST be assigned a unique DiameterIdentity. If a Diameter
node can be identified by several FQDNs, a single FQDN should be
picked at startup, and used as the only DiameterIdentity for that
node, whatever the connection it is sent on.

DiameterURI

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

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

; No transport security

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

; Transport security used

FQDN = Fully Qualified Host Name

port = ":" 1*DIGIT

; One of the ports used to listen for
; incoming connections.
; If absent,
; the default Diameter port (3868) is
; assumed.

transport = ";transport=" transport-protocol

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

Calhoun, et al. Standards Track [Page 44]

RFC 3588 Diameter Based Protocol September 2003

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

protocol = ";protocol=" aaa-protocol

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

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

The following are examples of valid Diameter host identities:

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

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

IPFilterRule
The IPFilterRule format is derived from the OctetString AVP Base
Format. It uses the ASCII charset. Packets may be filtered based
on the following information that is associated with it:

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

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

Calhoun, et al. Standards Track [Page 45]

RFC 3588 Diameter Based Protocol September 2003

IPFilterRule filters MUST follow the format:

action dir proto from src to dst [options]

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

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

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

src and dst <address/mask> [ports]

The <address/mask> may be specified as:
ipno An IPv4 or IPv6 number in dotted-
quad or canonical IPv6 form. Only
this exact IP number will match the
rule.
ipno/bits An IP number as above with a mask
width of the form 1.2.3.4/24. In
this case, all IP numbers from
1.2.3.0 to 1.2.3.255 will match.
The bit width MUST be valid for the
IP version and the IP number MUST
NOT have bits set beyond the mask.
For a match to occur, the same IP
version must be present in the
packet that was used in describing
the IP address. To test for a
particular IP version, the bits part
can be set to zero. The keyword
"any" is 0.0.0.0/0 or the IPv6
equivalent. The keyword "assigned"
is the address or set of addresses
assigned to the terminal. For IPv4,
a typical first rule is often "deny
in ip! assigned"

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

Calhoun, et al. Standards Track [Page 46]

RFC 3588 Diameter Based Protocol September 2003

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

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

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

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

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

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

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

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

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

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

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

Calhoun, et al. Standards Track [Page 47]

RFC 3588 Diameter Based Protocol September 2003

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

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

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

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

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

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

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

Calhoun, et al. Standards Track [Page 48]

RFC 3588 Diameter Based Protocol September 2003

QoSFilterRule
The QosFilterRule format is derived from the OctetString AVP Base
Format. It uses the ASCII charset. Packets may be marked or
metered based on the following information that is associated with
it:

Direction (in or out)
Source and destination IP address (possibly masked)
Protocol
Source and destination port (lists or ranges)
DSCP values (no mask or range)

Rules for the appropriate direction are evaluated in order, with
the first matched rule terminating the evaluation. Each packet is
evaluated once. If no rule matches, the packet is treated as best
effort. An access device that is unable to interpret or apply a
QoS rule SHOULD NOT terminate the session.

QoSFilterRule filters MUST follow the format:

action dir proto from src to dst [options]

tag - Mark packet with a specific DSCP
[DIFFSERV]. The DSCP option MUST be
included.
meter - Meter traffic. The metering options
MUST be included.

dir The format is as described under IPFilterRule.

proto The format is as described under
IPFilterRule.

src and dst The format is as described under
IPFilterRule.

4.4. Grouped AVP Values

The Diameter protocol allows AVP values of type 'Grouped.' This
implies that the Data field is actually a sequence of AVPs. It is
possible to include an AVP with a Grouped type within a Grouped type,
that is, to nest them. AVPs within an AVP of type Grouped have the
same padding requirements as non-Grouped AVPs, as defined in Section
4.

Calhoun, et al. Standards Track [Page 49]

RFC 3588 Diameter Based Protocol September 2003

The AVP Code numbering space of all AVPs included in a Grouped AVP is
the same as for non-grouped AVPs. Further, if any of the AVPs
encapsulated within a Grouped AVP has the 'M' (mandatory) bit set,
the Grouped AVP itself MUST also include the 'M' bit set.

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

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

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

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

avp = header [ *fixed] [ *required] [ *optional]
[ *fixed]

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

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

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

4.4.1. Example AVP with a Grouped Data type

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

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

An Example-AVP with Grouped Data follows.

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

Origin-Host = "example.com".

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

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

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

optional AVPs included are

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

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

The data for the optional AVPs is represented in hex since the format
of these AVPs is neither known at the time of definition of the
Example-AVP group, nor (likely) at the time when the example instance
of this AVP is interpreted - except by Diameter implementations which
support the same set of AVPs. The encoding example illustrates how
padding is used and how length fields are calculated. Also note that
AVPs may be present in the Grouped AVP value which the receiver
cannot interpret (here, the Recover-Policy and Futuristic-Acct-Record
AVPs).

Calhoun, et al. Standards Track [Page 51]

RFC 3588 Diameter Based Protocol September 2003

This AVP would be encoded as follows:

0 1 2 3 4 5 6 7
+-------+-------+-------+-------+-------+-------+-------+-------+
0 | Example AVP Header (AVP Code = 999999), Length = 468 |
+-------+-------+-------+-------+-------+-------+-------+-------+
8 | Origin-Host AVP Header (AVP Code = 264), Length = 19 |
+-------+-------+-------+-------+-------+-------+-------+-------+
16 | 'e' | 'x' | 'a' | 'm' | 'p' | 'l' | 'e' | '.' |
+-------+-------+-------+-------+-------+-------+-------+-------+
24 | 'c' | 'o' | 'm' |Padding| Session-Id AVP Header |
+-------+-------+-------+-------+-------+-------+-------+-------+
32 | (AVP Code = 263), Length = 50 | 'g' | 'r' | 'u' | 'm' |
+-------+-------+-------+-------+-------+-------+-------+-------+
. . .
+-------+-------+-------+-------+-------+-------+-------+-------+
64 | 'A' | 'F' | '3' | 'B' | '8' | '1' |Padding|Padding|
+-------+-------+-------+-------+-------+-------+-------+-------+
72 | Session-Id AVP Header (AVP Code = 263), Length = 51 |
+-------+-------+-------+-------+-------+-------+-------+-------+
80 | 'g' | 'r' | 'u' | 'm' | 'p' | '.' | 'e' | 'x' |
+-------+-------+-------+-------+-------+-------+-------+-------+
. . .
+-------+-------+-------+-------+-------+-------+-------+-------+
104 | '0' | 'A' | 'F' | '3' | 'B' | '8' | '2' |Padding|
+-------+-------+-------+-------+-------+-------+-------+-------+
112 | Recovery-Policy Header (AVP Code = 8341), Length = 223 |
+-------+-------+-------+-------+-------+-------+-------+-------+
120 | 0x21 | 0x63 | 0xbc | 0x1d | 0x0a | 0xd8 | 0x23 | 0x71 |
+-------+-------+-------+-------+-------+-------+-------+-------+
. . .
+-------+-------+-------+-------+-------+-------+-------+-------+
320 | 0x2f | 0xd7 | 0x96 | 0x6b | 0x8c | 0x7f | 0x92 |Padding|
+-------+-------+-------+-------+-------+-------+-------+-------+
328 | Futuristic-Acct-Record Header (AVP Code = 15930), Length = 137|
+-------+-------+-------+-------+-------+-------+-------+-------+
336 | 0xfe | 0x19 | 0xda | 0x58 | 0x02 | 0xac | 0xd9 | 0x8b |
+-------+-------+-------+-------+-------+-------+-------+-------+
. . .
+-------+-------+-------+-------+-------+-------+-------+-------+
464 | 0x41 |Padding|Padding|Padding|
+-------+-------+-------+-------+

Calhoun, et al. Standards Track [Page 52]

RFC 3588 Diameter Based Protocol September 2003

4.5. Diameter Base Protocol AVPs

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

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

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RFC 3588 Diameter Based Protocol September 2003

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

Calhoun, et al. Standards Track [Page 54]

RFC 3588 Diameter Based Protocol September 2003

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

Calhoun, et al. Standards Track [Page 55]

RFC 3588 Diameter Based Protocol September 2003

5. Diameter Peers

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

5.1. Peer Connections

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

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

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

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

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

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

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

5.2. Diameter Peer Discovery

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

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RFC 3588 Diameter Based Protocol September 2003

on existing IETF standards. The first option (manual configuration)
MUST be supported by all DIAMETER nodes, while the latter two options
(SRVLOC and DNS) MAY be supported.

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

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

2. The Diameter implementation uses SLPv2 [SLP] to discover Diameter
services. The Diameter service template [TEMPLATE] is included in
Appendix A.

It is recommended that SLPv2 security be deployed (this requires
distributing keys to SLPv2 agents). This is discussed further in
Appendix A. SLPv2 security SHOULD be used (requiring distribution
of keys to SLPv2 agents) in order to ensure that discovered peers
are authorized for their roles. SLPv2 is discussed further in
Appendix A.

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

3.1 The services relevant for the task of transport protocol
selection are those with NAPTR service fields with values
"AAA+D2x", where x is a letter that corresponds to a transport
protocol supported by the domain. This specification defines
D2T for TCP and D2S for SCTP. We also establish an IANA
registry for NAPTR service name to transport protocol
mappings.

These NAPTR records provide a mapping from a domain, to the
SRV record for contacting a server with the specific transport
protocol in the NAPTR services field. The resource record
will contain an empty regular expression and a replacement
value, which is the SRV record for that particular transport
protocol. If the server supports multiple transport
protocols, there will be multiple NAPTR records, each with a
different service value. As per RFC 2915 [NAPTR], the client

Calhoun, et al. Standards Track [Page 57]

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discards any records whose services fields are not applicable.
For the purposes of this specification, several rules are
defined.

3.2 A client MUST discard any service fields that identify a
resolution service whose value is not "D2X", for values of X
that indicate transport protocols supported by the client.
The NAPTR processing as described in RFC 2915 will result in
discovery of the most preferred transport protocol of the
server that is supported by the client, as well as an SRV
record for the server.

The domain suffixes in the NAPTR replacement field SHOULD
match the domain of the original query.

4. If no NAPTR records are found, the requester queries for those
address records for the destination address,
'_diameter._sctp'.realm or '_diameter._tcp'.realm. Address
records include A RR's, AAAA RR's or other similar records, chosen
according to the requestor's network protocol capabilities. If
the DNS server returns no address records, the requestor gives up.

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

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

Authorization can be achieved for example, by configuration of a
Diameter Server CA. Alternatively this can be achieved by
definition of OIDs within TLS or IKE certificates so as to signify
Diameter Server authorization.

A dynamically discovered peer causes an entry in the Peer Table (see
Section 2.6) to be created. Note that entries created via DNS MUST
expire (or be refreshed) within the DNS TTL. If a peer is discovered

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RFC 3588 Diameter Based Protocol September 2003

outside of the local realm, a routing table entry (see Section 2.7)
for the peer's realm is created. The routing table entry's
expiration MUST match the peer's expiration value.

5.3. Capabilities Exchange

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

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

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

Similarly, a receiver of a Capabilities-Exchange-Req (CER) message
that does not have any security mechanisms in common with the sender
MUST return a Capabilities-Exchange-Answer (CEA) with the Result-Code
AVP set to DIAMETER_NO_COMMON_SECURITY, and SHOULD disconnect the
transport layer connection.

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

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

Since the CER/CEA messages cannot be proxied, it is still possible
that an upstream agent receives a message for which it has no
available peers to handle the application that corresponds to the
Command-Code. In such instances, the 'E' bit is set in the answer

Calhoun, et al. Standards Track [Page 59]

RFC 3588 Diameter Based Protocol September 2003

message (see Section 7.) with the Result-Code AVP set to
DIAMETER_UNABLE_TO_DELIVER to inform the downstream to take action
(e.g., re-routing request to an alternate peer).

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

5.3.1. Capabilities-Exchange-Request

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

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

Message Format

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

5.3.2. Capabilities-Exchange-Answer

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

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

Message Format

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

5.3.3. Vendor-Id AVP

The Vendor-Id AVP (AVP Code 266) is of type Unsigned32 and contains
the IANA "SMI Network Management Private Enterprise Codes" [ASSIGNNO]
value assigned to the vendor of the Diameter application. In
combination with the Supported-Vendor-Id AVP (Section 5.3.6), this
MAY be used in order to know which vendor specific attributes may be
sent to the peer. It is also envisioned that the combination of the
Vendor-Id, Product-Name (Section 5.3.7) and the Firmware-Revision
(Section 5.3.4) AVPs MAY provide very useful debugging information.

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

5.3.4. Firmware-Revision AVP

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

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

5.3.5. Host-IP-Address AVP

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

5.3.6. Supported-Vendor-Id AVP

The Supported-Vendor-Id AVP (AVP Code 265) is of type Unsigned32 and
contains the IANA "SMI Network Management Private Enterprise Codes"
[ASSIGNNO] value assigned to a vendor other than the device vendor.
This is used in the CER and CEA messages in order to inform the peer
that the sender supports (a subset of) the vendor-specific AVPs
defined by the vendor identified in this AVP.

5.3.7. Product-Name AVP

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

5.4. Disconnecting Peer connections

When a Diameter node disconnects one of its transport connections,
its peer cannot know the reason for the disconnect, and will most
likely assume that a connectivity problem occurred, or that the peer
has rebooted. In these cases, the peer may periodically attempt to
reconnect, as stated in Section 2.1. In the event that the
disconnect was a result of either a shortage of internal resources,
or simply that the node in question has no intentions of forwarding
any Diameter messages to the peer in the foreseeable future, a
periodic connection request would not be welcomed. The
Disconnection-Reason AVP contains the reason the Diameter node issued
the Disconnect-Peer-Request message.

The Disconnect-Peer-Request message is used by a Diameter node to
inform its peer of its intent to disconnect the transport layer, and
that the peer shouldn't reconnect unless it has a valid reason to do
so (e.g., message to be forwarded). Upon receipt of the message, the

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Disconnect-Peer-Answer is returned, which SHOULD contain an error if
messages have recently been forwarded, and are likely in flight,
which would otherwise cause a race condition.

The receiver of the Disconnect-Peer-Answer initiates the transport
disconnect.

5.4.1. Disconnect-Peer-Request

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

Message Format

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

5.4.2. Disconnect-Peer-Answer

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

Message Format

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

5.4.3. Disconnect-Cause AVP

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

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REBOOTING 0
A scheduled reboot is imminent.

BUSY 1
The peer's internal resources are constrained, and it has
determined that the transport connection needs to be closed.

DO_NOT_WANT_TO_TALK_TO_YOU 2
The peer has determined that it does not see a need for the
transport connection to exist, since it does not expect any
messages to be exchanged in the near future.

5.5. Transport Failure Detection

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

5.5.1. Device-Watchdog-Request

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

Message Format

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

5.5.2. Device-Watchdog-Answer

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

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Message Format

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

5.5.3. Transport Failure Algorithm

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

5.5.4. Failover and Failback Procedures

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

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

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

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

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

5.6. Peer State Machine

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

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

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

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

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

For TLS usage, a TLS handshake will begin when both ends are in the
open state. If the TLS handshake is successful, all further messages
will be sent via TLS. If the handshake fails, both ends move to the
closed state.

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

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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

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R-Peer-Disc R-Disc Closed
R-Rcv-CER R-Snd-CEA R-Open
R-Rcv-CEA Process-CEA R-Open

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

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

5.6.1. Incoming connections

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

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

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

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

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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).

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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.

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Process-DWA The DWA message is serviced.

Process A message is serviced.

5.6.4. The Election Process

The election is performed on the responder. The responder compares
the Origin-Host received in the CER sent by its peer with its own
Origin-Host. If the local Diameter entity's Origin-Host is higher
than the peer's, a Win-Election event is issued locally.

The comparison proceeds by considering the shorter OctetString to be
padded with zeros so that it length is the same as the length of the
longer, then performing an octet-by-octet unsigned comparison with
the first octet being most significant. Any remaining octets are
assumed to have value 0x80.

6. Diameter message processing

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

6.1. Diameter Request Routing Overview

A request is sent towards its final destination using a combination
of the Destination-Realm and Destination-Host AVPs, in one of these
three combinations:

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

- a request that needs to be sent to a home server serving a
specific realm, but not to a specific server (such as the first
request of a series of round-trips), MUST contain a Destination-
Realm AVP, but MUST NOT contain a Destination-Host AVP.

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

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

- Authentication requests that span multiple round trips

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

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

Note that an agent can forward a request to a host described in the
Destination-Host AVP only if the host in question is included in its
peer table (see Section 2.7). Otherwise, the request is routed based
on the Destination-Realm only (see Sections 6.1.6).

The Destination-Realm AVP MUST be present if the message is
proxiable. Request messages that may be forwarded by Diameter agents
(proxies, redirects or relays) MUST also contain an Acct-
Application-Id AVP, an Auth-Application-Id AVP or a Vendor-Specific-
Application-Id AVP. A message that MUST NOT be forwarded by Diameter
agents (proxies, redirects or relays) MUST not include the
Destination-Realm in its ABNF. The value of the Destination-Realm
AVP MAY be extracted from the User-Name AVP, or other application-
specific methods.

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

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

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

3. The procedures listed in Section 6.1.6 are followed, which is
known as Request Routing.

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

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

Note the processing rules contained in this section are intended to
be used as general guidelines to Diameter developers. Certain
implementations MAY use different methods than the ones described
here, and still comply with the protocol specification. See Section
7 for more detail on error handling.

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6.1.1. Originating a Request

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

- the Command-Code is set to the appropriate value

- the 'R' bit is set

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

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

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

- an Acct-Application-Id AVP, an Auth-Application-Id or a Vendor-
Specific-Application-Id AVP must be included if the request is
proxiable.

6.1.2. Sending a Request

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

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

- The message should be saved in the list of pending requests.

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

6.1.3. Receiving Requests

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

6.1.4. Processing Local Requests

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

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

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

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

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

6.1.5. Request Forwarding

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

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

6.1.6. Request Routing

Diameter request message routing is done via realms and applications.
A Diameter message that may be forwarded by Diameter agents (proxies,
redirects or relays) MUST include the target realm in the
Destination-Realm AVP and one of the application identification AVPs
Auth-Application-Id, Acct-Application-Id or Vendor-Specific-
Application-Id. The realm MAY be retrieved from the User-Name AVP,
which is in the form of a Network Access Identifier (NAI). The realm
portion of the NAI is inserted in the Destination-Realm AVP.

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

6.1.7. Redirecting requests

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

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RFC 3588 Diameter Based Protocol September 2003

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

Figure 5: Diameter Redirect Agent

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

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

6.1.8. Relaying and Proxying Requests

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

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

A relay or proxy agent MAY include the Proxy-Info AVP in requests if
it requires access to any local state information when the
corresponding response is received. Proxy-Info AVP has certain
security implications and SHOULD contain an embedded HMAC with a
node-local key. Alternatively, it MAY simply use local storage to
store state information.

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

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

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

Figure 6: Routing of Diameter messages

6.2. Diameter Answer Processing

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

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

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

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

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

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

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

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

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

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Note that the error messages (see Section 7.3) are also subjected to
the above processing rules.

6.2.1. Processing received Answers

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

6.2.2. Relaying and Proxying Answers

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

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

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

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

6.3. Origin-Host AVP

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

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

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

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

6.4. Origin-Realm AVP

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

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

6.5. Destination-Host AVP

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

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

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

6.6. Destination-Realm AVP

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

Request messages whose ABNF does not list the Destination-Realm AVP
as a mandatory AVP are inherently non-routable messages.

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

6.7. Routing AVPs

The AVPs defined in this section are Diameter AVPs used for routing
purposes. These AVPs change as Diameter messages are processed by
agents, and therefore MUST NOT be protected by end-to-end security.

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6.7.1. Route-Record AVP

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

6.7.2. Proxy-Info AVP

The Proxy-Info AVP (AVP Code 284) is of type Grouped. The Grouped
Data field has the following ABNF grammar:

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

6.7.3. Proxy-Host AVP

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

6.7.4. Proxy-State AVP

The Proxy-State AVP (AVP Code 33) is of type OctetString, and
contains state local information, and MUST be treated as opaque data.

6.8. Auth-Application-Id AVP

The Auth-Application-Id AVP (AVP Code 258) is of type Unsigned32 and
is used in order to advertise support of the Authentication and
Authorization portion of an application (see Section 2.4). The
Auth-Application-Id MUST also be present in all Authentication and/or
Authorization messages that are defined in a separate Diameter
specification and have an Application ID assigned.

6.9. Acct-Application-Id AVP

The Acct-Application-Id AVP (AVP Code 259) is of type Unsigned32 and
is used in order to advertise support of the Accounting portion of an
application (see Section 2.4). The Acct-Application-Id MUST also be
present in all Accounting messages. Exactly one of the Auth-
Application-Id and Acct-Application-Id AVPs MAY be present.

6.10. Inband-Security-Id AVP

The Inband-Security-Id AVP (AVP Code 299) is of type Unsigned32 and
is used in order to advertise support of the Security portion of the
application.

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Currently, the following values are supported, but there is ample
room to add new security Ids.

NO_INBAND_SECURITY 0
This peer does not support TLS. This is the default value, if the
AVP is omitted.

TLS 1
This node supports TLS security, as defined by [TLS].

6.11. Vendor-Specific-Application-Id AVP

The Vendor-Specific-Application-Id AVP (AVP Code 260) is of type
Grouped and is used to advertise support of a vendor-specific
Diameter Application. Exactly one of the Auth-Application-Id and
Acct-Application-Id AVPs MAY be present.

This AVP MUST also be present as the first AVP in all experimental
commands defined in the vendor-specific application.

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

AVP Format

<Vendor-Specific-Application-Id> ::= < AVP Header: 260 >
1* [ Vendor-Id ]
0*1{ Auth-Application-Id }
0*1{ Acct-Application-Id }

6.12. Redirect-Host AVP

One or more of instances of this AVP MUST be present if the answer
message's 'E' bit is set and the Result-Code AVP is set to
DIAMETER_REDIRECT_INDICATION.

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

6.13. Redirect-Host-Usage AVP

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

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

DONT_CACHE 0
The host specified in the Redirect-Host AVP should not be cached.
This is the default value.

ALL_SESSION 1
All messages within the same session, as defined by the same value
of the Session-ID AVP MAY be sent to the host specified in the
Redirect-Host AVP.

ALL_REALM 2
All messages destined for the realm requested MAY be sent to the
host specified in the Redirect-Host AVP.

REALM_AND_APPLICATION 3
All messages for the application requested to the realm specified
MAY be sent to the host specified in the Redirect-Host AVP.

ALL_APPLICATION 4
All messages for the application requested MAY be sent to the host
specified in the Redirect-Host AVP.

ALL_HOST 5
All messages that would be sent to the host that generated the
Redirect-Host MAY be sent to the host specified in the Redirect-
Host AVP.

ALL_USER 6
All messages for the user requested MAY be sent to the host
specified in the Redirect-Host AVP.

6.14. Redirect-Max-Cache-Time AVP

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

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

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6.15. E2E-Sequence AVP

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

7. Error Handling

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

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

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

Figure 7: Example of Protocol Error causing answer message

Figure 7 provides an example of a message forwarded upstream by a
Diameter relay. When the message is received by Relay 2, and it
detects that it cannot forward the request to the home server, an
answer message is returned with the 'E' bit set and the Result-Code
AVP set to DIAMETER_UNABLE_TO_DELIVER. Given that this error falls

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within the protocol error category, Relay 1 would take special
action, and given the error, attempt to route the message through its
alternate Relay 3.

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

Figure 8: Example of Application Error Answer message

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

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

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

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

- A command is received with an AVP that is omitted, yet is
mandatory according to the command's ABNF. The receiver issues an
answer with the Result-Code set to DIAMETER_MISSING_AVP, and
creates an AVP with the AVP Code and other fields set as expected
in the missing AVP. The created AVP is then added to the Failed-
AVP AVP.

The Result-Code AVP describes the error that the Diameter node
encountered in its processing. In case there are multiple errors,
the Diameter node MUST report only the first error it encountered

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(detected possibly in some implementation dependent order). The
specific errors that can be described by this AVP are described in
the following section.

7.1. Result-Code AVP

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

The Result-Code data field contains an IANA-managed 32-bit address
space representing errors (see Section 11.4). Diameter provides the
following classes of errors, all identified by the thousands digit in
the decimal notation:

- 1xxx (Informational)
- 2xxx (Success)
- 3xxx (Protocol Errors)
- 4xxx (Transient Failures)
- 5xxx (Permanent Failure)

A non-recognized class (one whose first digit is not defined in this
section) MUST be handled as a permanent failure.

7.1.1. Informational

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

DIAMETER_MULTI_ROUND_AUTH 1001
This informational error is returned by a Diameter server to
inform the access device that the authentication mechanism being
used requires multiple round trips, and a subsequent request needs
to be issued in order for access to be granted.

7.1.2. Success

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

DIAMETER_SUCCESS 2001
The Request was successfully completed.

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DIAMETER_LIMITED_SUCCESS 2002
When returned, the request was successfully completed, but
additional processing is required by the application in order to
provide service to the user.

7.1.3. Protocol Errors

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

DIAMETER_COMMAND_UNSUPPORTED 3001
The Request contained a Command-Code that the receiver did not
recognize or support. This MUST be used when a Diameter node
receives an experimental command that it does not understand.

DIAMETER_UNABLE_TO_DELIVER 3002
This error is given when Diameter can not deliver the message to
the destination, either because no host within the realm
supporting the required application was available to process the
request, or because Destination-Host AVP was given without the
associated Destination-Realm AVP.

DIAMETER_REALM_NOT_SERVED 3003
The intended realm of the request is not recognized.

DIAMETER_TOO_BUSY 3004
When returned, a Diameter node SHOULD attempt to send the message
to an alternate peer. This error MUST only be used when a
specific server is requested, and it cannot provide the requested
service.

DIAMETER_LOOP_DETECTED 3005
An agent detected a loop while trying to get the message to the
intended recipient. The message MAY be sent to an alternate peer,
if one is available, but the peer reporting the error has
identified a configuration problem.

DIAMETER_REDIRECT_INDICATION 3006
A redirect agent has determined that the request could not be
satisfied locally and the initiator of the request should direct
the request directly to the server, whose contact information has
been added to the response. When set, the Redirect-Host AVP MUST
be present.

DIAMETER_APPLICATION_UNSUPPORTED 3007
A request was sent for an application that is not supported.

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DIAMETER_INVALID_HDR_BITS 3008
A request was received whose bits in the Diameter header were
either set to an invalid combination, or to a value that is
inconsistent with the command code's definition.

DIAMETER_INVALID_AVP_BITS 3009
A request was received that included an AVP whose flag bits are
set to an unrecognized value, or that is inconsistent with the
AVP's definition.

DIAMETER_UNKNOWN_PEER 3010
A CER was received from an unknown peer.

7.1.4. Transient Failures

Errors that fall within the transient failures category are used
to inform a peer that the request could not be satisfied at the
time it was received, but MAY be able to satisfy the request in
the future.

DIAMETER_AUTHENTICATION_REJECTED 4001
The authentication process for the user failed, most likely due to
an invalid password used by the user. Further attempts MUST only
be tried after prompting the user for a new password.

DIAMETER_OUT_OF_SPACE 4002
A Diameter node received the accounting request but was unable to
commit it to stable storage due to a temporary lack of space.

ELECTION_LOST 4003
The peer has determined that it has lost the election process and
has therefore disconnected the transport connection.

7.1.5. Permanent Failures

Errors that fall within the permanent failures category are used
to inform the peer that the request failed, and should not be
attempted again.

DIAMETER_AVP_UNSUPPORTED 5001
The peer received a message that contained an AVP that is not
recognized or supported and was marked with the Mandatory bit. A
Diameter message with this error MUST contain one or more Failed-
AVP AVP containing the AVPs that caused the failure.

DIAMETER_UNKNOWN_SESSION_ID 5002
The request contained an unknown Session-Id.

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DIAMETER_AUTHORIZATION_REJECTED 5003
A request was received for which the user could not be authorized.
This error could occur if the service requested is not permitted
to the user.

DIAMETER_INVALID_AVP_VALUE 5004
The request contained an AVP with an invalid value in its data
portion. A Diameter message indicating this error MUST include
the offending AVPs within a Failed-AVP AVP.

DIAMETER_MISSING_AVP 5005
The request did not contain an AVP that is required by the Command
Code definition. If this value is sent in the Result-Code AVP, a
Failed-AVP AVP SHOULD be included in the message. The Failed-AVP
AVP MUST contain an example of the missing AVP complete with the
Vendor-Id if applicable. The value field of the missing AVP
should be of correct minimum length and contain zeroes.

DIAMETER_RESOURCES_EXCEEDED 5006
A request was received that cannot be authorized because the user
has already expended allowed resources. An example of this error
condition is a user that is restricted to one dial-up PPP port,
attempts to establish a second PPP connection.

DIAMETER_CONTRADICTING_AVPS 5007
The Home Diameter server has detected AVPs in the request that
contradicted each other, and is not willing to provide service to
the user. One or more Failed-AVP AVPs MUST be present, containing
the AVPs that contradicted each other.

DIAMETER_AVP_NOT_ALLOWED 5008
A message was received with an AVP that MUST NOT be present. The
Failed-AVP AVP MUST be included and contain a copy of the
offending AVP.

DIAMETER_AVP_OCCURS_TOO_MANY_TIMES 5009
A message was received that included an AVP that appeared more
often than permitted in the message definition. The Failed-AVP
AVP MUST be included and contain a copy of the first instance of
the offending AVP that exceeded the maximum number of occurrences

DIAMETER_NO_COMMON_APPLICATION 5010
This error is returned when a CER message is received, and there
are no common applications supported between the peers.

DIAMETER_UNSUPPORTED_VERSION 5011
This error is returned when a request was received, whose version
number is unsupported.

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DIAMETER_UNABLE_TO_COMPLY 5012
This error is returned when a request is rejected for unspecified
reasons.

DIAMETER_INVALID_BIT_IN_HEADER 5013
This error is returned when an unrecognized bit in the Diameter
header is set to one (1).

DIAMETER_INVALID_AVP_LENGTH 5014
The request contained an AVP with an invalid length. A Diameter
message indicating this error MUST include the offending AVPs
within a Failed-AVP AVP.

DIAMETER_INVALID_MESSAGE_LENGTH 5015
This error is returned when a request is received with an invalid
message length.

DIAMETER_INVALID_AVP_BIT_COMBO 5016
The request contained an AVP with which is not allowed to have the
given value in the AVP Flags field. A Diameter message indicating
this error MUST include the offending AVPs within a Failed-AVP
AVP.

DIAMETER_NO_COMMON_SECURITY 5017
This error is returned when a CER message is received, and there
are no common security mechanisms supported between the peers. A
Capabilities-Exchange-Answer (CEA) MUST be returned with the
Result-Code AVP set to DIAMETER_NO_COMMON_SECURITY.

7.2. Error Bit

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

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RFC 3588 Diameter Based Protocol September 2003

Message Format

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

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

7.3. Error-Message AVP

The Error-Message AVP (AVP Code 281) is of type UTF8String. It MAY
accompany a Result-Code AVP as a human readable error message. The
Error-Message AVP is not intended to be useful in real-time, and
SHOULD NOT be expected to be parsed by network entities.

7.4. Error-Reporting-Host AVP

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

7.5. Failed-AVP AVP

The Failed-AVP AVP (AVP Code 279) is of type Grouped and provides
debugging information in cases where a request is rejected or not
fully processed due to erroneous information in a specific AVP. The
value of the Result-Code AVP will provide information on the reason
for the Failed-AVP AVP.

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

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

AVP Format

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

7.6. Experimental-Result AVP

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

AVP Format

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

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

7.7. Experimental-Result-Code AVP

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

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

8. Diameter User Sessions

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

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

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

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

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

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

8.1. Authorization Session State Machine

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

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

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

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

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

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

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

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

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

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

Pending Failed Service-specific Cleanup Idle
authorization answer received

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

Open Successful Service-specific Provide Open
authorization answer received Service

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

Open Session-Timeout Expires on Send STR Discon
Access Device

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

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

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

Discon ASR Received Send ASA Discon

Discon STA Received Discon. Idle
user/device

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

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

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

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

Open Service-specific authorization Send Idle
request received, and user failed serv.
is not authorized specific
answer,
Cleanup

Open Home server wants to Send ASR Discon
terminate the service

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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

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

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

8.2. Accounting Session State Machine

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

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

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

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

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

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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

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PendingS Successful accounting Open
start answer received

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

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

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

PendingS Failed accounting start answer Open
received and realtime equal
to GRANT_AND_LOSE

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

PendingS User service terminated Store PendingS
stop
record

Open Interim interval elapses Send PendingI
accounting
interim
record
Open User service terminated Send PendingL
accounting
stop req.

PendingI Successful accounting interim Open
answer received

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

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

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

PendingI Failed accounting interim Open
answer received and realtime
equal to GRANT_AND_LOSE

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

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

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

PendingE Failure to send and no buffer Idle
space available

PendingE Failed accounting event answer Idle
received

PendingB Successful accounting answer Delete Idle
received record

PendingB Failure to send Idle

PendingB Failed accounting answer Delete Idle
received record

PendingL Successful accounting Idle
stop answer received

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

PendingL Failure to send and no buffer Idle
space available

PendingL Failed accounting stop answer Idle
received

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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

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

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

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

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

Open Session supervision timer Ts Stop Ts Idle
expired

8.3. Server-Initiated Re-Auth

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

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

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

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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.

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Message Format

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

8.4. Session Termination

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

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

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

It is also possible that a session that was authorized is never
actually started due to action of a proxy. For example, a proxy may
modify an authorization answer, converting the result from success to
failure, prior to forwarding the message to the access device. If
the answer did not contain an Auth-Session-State AVP with the value

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NO_STATE_MAINTAINED, a proxy that causes an authorized session not to
be started MUST issue an STR to the Diameter server that authorized
the session, since the access device has no way of knowing that the
session had been authorized.

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

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

8.4.1. Session-Termination-Request

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

Message Format

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

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8.4.2. Session-Termination-Answer

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

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

Message Format

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

8.5. Aborting a Session

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

For example, the Diameter server that originally authorized the
session may be required to cause that session to be stopped for
credit or other reasons that were not anticipated when the session
was first authorized. On the other hand, an operator may maintain a
management server for the purpose of issuing ASRs to administratively
remove users from the network.

An access device that receives an ASR with Session-ID equal to a
currently active session MAY stop the session. Whether the access

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

Note that if the access device does stop the session upon receipt of
an ASR, it issues an STR to the authorizing server (which may or may
not be the agent issuing the ASR) just as it would if the session
were terminated for any other reason.

8.5.1. Abort-Session-Request

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

Message Format

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

8.5.2. Abort-Session-Answer

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

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

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Message Format

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

8.6. Inferring Session Termination from Origin-State-Id

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

By including Origin-State-Id in CER/CEA messages, an access device
allows a next-hop server to determine immediately upon connection
whether the device has lost its sessions since the last connection.

By including Origin-State-Id in request messages, an access device
also allows a server with which it communicates via proxy to make
such a determination. However, a server that is not directly
connected with the access device will not discover that the access
device has been restarted unless and until it receives a new request
from the access device. Thus, use of this mechanism across proxies
is opportunistic rather than reliable, but useful nonetheless.

When a Diameter server receives an Origin-State-Id that is greater
than the Origin-State-Id previously received from the same issuer, it
may assume that the issuer has lost state since the previous message
and that all sessions that were active under the lower Origin-State-
Id have been terminated. The Diameter server MAY clean up all
session state associated with such lost sessions, and MAY also issues
STRs for all such lost sessions that were authorized on upstream
servers, to allow session state to be cleaned up globally.

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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):

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

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

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

Example, in which there is no optional value:
accesspoint7.acme.com;1876543210;523

Example, in which there is an optional value:
accesspoint7.acme.com;1876543210;523;mobile@200.1.1.88

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

8.9. Authorization-Lifetime AVP

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

A value of zero (0) means that immediate re-auth is necessary by the
access device. This is typically used in cases where multiple
authentication methods are used, and a successful auth response with
this AVP set to zero is used to signal that the next authentication
method is to be immediately initiated. The absence of this AVP, or a
value of all ones (meaning all bits in the 32 bit field are set to
one) means no re-auth is expected.

If both this AVP and the Session-Timeout AVP are present in a
message, the value of the latter MUST NOT be smaller than the
Authorization-Lifetime AVP.

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An Authorization-Lifetime AVP MAY be present in re-authorization
messages, and contains the number of seconds the user is authorized
to receive service from the time the re-auth answer message is
received by the access device.

This AVP MAY be provided by the client as a hint of the maximum
lifetime that it is willing to accept. However, the server MAY
return a value that is equal to, or smaller, than the one provided by
the client.

8.10. Auth-Grace-Period AVP

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

8.11. Auth-Session-State AVP

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

STATE_MAINTAINED 0
This value is used to specify that session state is being
maintained, and the access device MUST issue a session termination
message when service to the user is terminated. This is the
default value.

NO_STATE_MAINTAINED 1
This value is used to specify that no session termination messages
will be sent by the access device upon expiration of the
Authorization-Lifetime.

8.12. Re-Auth-Request-Type AVP

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

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AUTHORIZE_ONLY 0
An authorization only re-auth is expected upon expiration of the
Authorization-Lifetime. This is the default value if the AVP is
not present in answer messages that include the Authorization-
Lifetime.

AUTHORIZE_AUTHENTICATE 1
An authentication and authorization re-auth is expected upon
expiration of the Authorization-Lifetime.

8.13. Session-Timeout AVP

The Session-Timeout AVP (AVP Code 27) [RADIUS] is of type Unsigned32
and contains the maximum number of seconds of service to be provided
to the user before termination of the session. When both the
Session-Timeout and the Authorization-Lifetime AVPs are present in an
answer message, the former MUST be equal to or greater than the value
of the latter.

A session that terminates on an access device due to the expiration
of the Session-Timeout MUST cause an STR to be issued, unless both
the access device and the home server had previously agreed that no
session termination messages would be sent (see Section 8.9).

A Session-Timeout AVP MAY be present in a re-authorization answer
message, and contains the remaining number of seconds from the
beginning of the re-auth.

A value of zero, or the absence of this AVP, means that this session
has an unlimited number of seconds before termination.

This AVP MAY be provided by the client as a hint of the maximum
timeout that it is willing to accept. However, the server MAY return
a value that is equal to, or smaller, than the one provided by the
client.

8.14. User-Name AVP

The User-Name AVP (AVP Code 1) [RADIUS] is of type UTF8String, which
contains the User-Name, in a format consistent with the NAI
specification [NAI].

8.15. Termination-Cause AVP

The Termination-Cause AVP (AVP Code 295) is of type Enumerated, and
is used to indicate the reason why a session was terminated on the
access device. The following values are defined:

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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.

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Typically, Origin-State-Id is used by an access device that always
starts up with no active sessions; that is, any session active prior
to restart will have been lost. By including Origin-State-Id in a
message, it allows other Diameter entities to infer that sessions
associated with a lower Origin-State-Id are no longer active. If an
access device does not intend for such inferences to be made, it MUST
either not include Origin-State-Id in any message, or set its value
to 0.

8.17. Session-Binding AVP

The Session-Binding AVP (AVP Code 270) is of type Unsigned32, and MAY
be present in application-specific authorization answer messages. If
present, this AVP MAY inform the Diameter client that all future
application-specific re-auth messages for this session MUST be sent
to the same authorization server. This AVP MAY also specify that a
Session-Termination-Request message for this session MUST be sent to
the same authorizing server.

This field is a bit mask, and the following bits have been defined:

RE_AUTH 1
When set, future re-auth messages for this session MUST NOT
include the Destination-Host AVP. When cleared, the default
value, the Destination-Host AVP MUST be present in all re-auth
messages for this session.

STR 2
When set, the STR message for this session MUST NOT include the
Destination-Host AVP. When cleared, the default value, the
Destination-Host AVP MUST be present in the STR message for this
session.

ACCOUNTING 4
When set, all accounting messages for this session MUST NOT
include the Destination-Host AVP. When cleared, the default
value, the Destination-Host AVP, if known, MUST be present in all
accounting messages for this session.

8.18. Session-Server-Failover AVP

The Session-Server-Failover AVP (AVP Code 271) is of type Enumerated,
and MAY be present in application-specific authorization answer
messages that either do not include the Session-Binding AVP or
include the Session-Binding AVP with any of the bits set to a zero
value. If present, this AVP MAY inform the Diameter client that if a

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re-auth or STR message fails due to a delivery problem, the Diameter
client SHOULD issue a subsequent message without the Destination-Host
AVP. When absent, the default value is REFUSE_SERVICE.

The following values are supported:

REFUSE_SERVICE 0
If either the re-auth or the STR message delivery fails, terminate
service with the user, and do not attempt any subsequent attempts.

TRY_AGAIN 1
If either the re-auth or the STR message delivery fails, resend
the failed message without the Destination-Host AVP present.

ALLOW_SERVICE 2
If re-auth message delivery fails, assume that re-authorization
succeeded. If STR message delivery fails, terminate the session.

TRY_AGAIN_ALLOW_SERVICE 3
If either the re-auth or the STR message delivery fails, resend
the failed message without the Destination-Host AVP present. If
the second delivery fails for re-auth, assume re-authorization
succeeded. If the second delivery fails for STR, terminate the
session.

8.19. Multi-Round-Time-Out AVP

The Multi-Round-Time-Out AVP (AVP Code 272) is of type Unsigned32,
and SHOULD be present in application-specific authorization answer
messages whose Result-Code AVP is set to DIAMETER_MULTI_ROUND_AUTH.
This AVP contains the maximum number of seconds that the access
device MUST provide the user in responding to an authentication
request.

8.20. Class AVP

The Class AVP (AVP Code 25) is of type OctetString and is used to by
Diameter servers to return state information to the access device.
When one or more Class AVPs are present in application-specific
authorization answer messages, they MUST be present in subsequent
re-authorization, session termination and accounting messages. Class
AVPs found in a re-authorization answer message override the ones
found in any previous authorization answer message. Diameter server
implementations SHOULD NOT return Class AVPs that require more than
4096 bytes of storage on the Diameter client. A Diameter client that
receives Class AVPs whose size exceeds local available storage MUST
terminate the session.

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8.21. Event-Timestamp AVP

The Event-Timestamp (AVP Code 55) is of type Time, and MAY be
included in an Accounting-Request and Accounting-Answer messages to
record the time that the reported event occurred, in seconds since
January 1, 1900 00:00 UTC.

9. Accounting

This accounting protocol is based on a server directed model with
capabilities for real-time delivery of accounting information.
Several fault resilience methods [ACCMGMT] have been built in to the
protocol in order minimize loss of accounting data in various fault
situations and under different assumptions about the capabilities of
the used devices.

9.1. Server Directed Model

The server directed model means that the device generating the
accounting data gets information from either the authorization server
(if contacted) or the accounting server regarding the way accounting
data shall be forwarded. This information includes accounting record
timeliness requirements.

As discussed in [ACCMGMT], real-time transfer of accounting records
is a requirement, such as the need to perform credit limit checks and
fraud detection. Note that batch accounting is not a requirement,
and is therefore not supported by Diameter. Should batched
accounting be required in the future, a new Diameter application will
need to be created, or it could be handled using another protocol.
Note, however, that even if at the Diameter layer accounting requests
are processed one by one, transport protocols used under Diameter
typically batch several requests in the same packet under heavy
traffic conditions. This may be sufficient for many applications.

The authorization server (chain) directs the selection of proper
transfer strategy, based on its knowledge of the user and
relationships of roaming partnerships. The server (or agents) uses
the Acct-Interim-Interval and Accounting-Realtime-Required AVPs to
control the operation of the Diameter peer operating as a client.
The Acct-Interim-Interval AVP, when present, instructs the Diameter
node acting as a client to produce accounting records continuously
even during a session. Accounting-Realtime-Required AVP is used to
control the behavior of the client when the transfer of accounting
records from the Diameter client is delayed or unsuccessful.

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The Diameter accounting server MAY override the interim interval or
the realtime requirements by including the Acct-Interim-Interval or
Accounting-Realtime-Required AVP in the Accounting-Answer message.
When one of these AVPs is present, the latest value received SHOULD
be used in further accounting activities for the same session.

9.2. Protocol Messages

A Diameter node that receives a successful authentication and/or
authorization messages from the Home AAA server MUST collect
accounting information for the session. The Accounting-Request
message is used to transmit the accounting information to the Home
AAA server, which MUST reply with the Accounting-Answer message to
confirm reception. The Accounting-Answer message includes the
Result-Code AVP, which MAY indicate that an error was present in the
accounting message. A rejected Accounting-Request message MAY cause
the user's session to be terminated, depending on the value of the
Accounting-Realtime-Required AVP received earlier for the session in
question.

Each Diameter Accounting protocol message MAY be compressed, in order
to reduce network bandwidth usage. If IPsec and IKE are used to
secure the Diameter session, then IP compression [IPComp] MAY be used
and IKE [IKE] MAY be used to negotiate the compression parameters.
If TLS is used to secure the Diameter session, then TLS compression
[TLS] MAY be used.

9.3. Application document requirements

Each Diameter application (e.g., NASREQ, MobileIP), MUST define their
Service-Specific AVPs that MUST be present in the Accounting-Request
message in a section entitled "Accounting AVPs". The application
MUST assume that the AVPs described in this document will be present
in all Accounting messages, so only their respective service-specific
AVPs need to be defined in this section.

9.4. Fault Resilience

Diameter Base protocol mechanisms are used to overcome small message
loss and network faults of temporary nature.

Diameter peers acting as clients MUST implement the use of failover
to guard against server failures and certain network failures.
Diameter peers acting as agents or related off-line processing
systems MUST detect duplicate accounting records caused by the
sending of same record to several servers and duplication of messages

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in transit. This detection MUST be based on the inspection of the
Session-Id and Accounting-Record-Number AVP pairs. Appendix C
discusses duplicate detection needs and implementation issues.

Diameter clients MAY have non-volatile memory for the safe storage of
accounting records over reboots or extended network failures, network
partitions, and server failures. If such memory is available, the
client SHOULD store new accounting records there as soon as the
records are created and until a positive acknowledgement of their
reception from the Diameter Server has been received. Upon a reboot,
the client MUST starting sending the records in the non-volatile
memory to the accounting server with appropriate modifications in
termination cause, session length, and other relevant information in
the records.

A further application of this protocol may include AVPs to control
how many accounting records may at most be stored in the Diameter
client without committing them to the non-volatile memory or
transferring them to the Diameter server.

The client SHOULD NOT remove the accounting data from any of its
memory areas before the correct Accounting-Answer has been received.
The client MAY remove oldest, undelivered or yet unacknowledged
accounting data if it runs out of resources such as memory. It is an
implementation dependent matter for the client to accept new sessions
under this condition.

9.5. Accounting Records

In all accounting records, the Session-Id AVP MUST be present; the
User-Name AVP MUST be present if it is available to the Diameter
client. If strong authentication across agents is required, end-to-
end security may be used for authentication purposes.

Different types of accounting records are sent depending on the
actual type of accounted service and the authorization server's
directions for interim accounting. If the accounted service is a
one-time event, meaning that the start and stop of the event are
simultaneous, then the Accounting-Record-Type AVP MUST be present and
set to the value EVENT_RECORD.

If the accounted service is of a measurable length, then the AVP MUST
use the values START_RECORD, STOP_RECORD, and possibly,
INTERIM_RECORD. If the authorization server has not directed interim
accounting to be enabled for the session, two accounting records MUST
be generated for each service of type session. When the initial

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Accounting-Request for a given session is sent, the Accounting-
Record-Type AVP MUST be set to the value START_RECORD. When the last
Accounting-Request is sent, the value MUST be STOP_RECORD.

If the authorization server has directed interim accounting to be
enabled, the Diameter client MUST produce additional records between
the START_RECORD and STOP_RECORD, marked INTERIM_RECORD. The
production of these records is directed by Acct-Interim-Interval as
well as any re-authentication or re-authorization of the session. The
Diameter client MUST overwrite any previous interim accounting
records that are locally stored for delivery, if a new record is
being generated for the same session. This ensures that only one
pending interim record can exist on an access device for any given
session.

A particular value of Accounting-Sub-Session-Id MUST appear only in
one sequence of accounting records from a DIAMETER client, except for
the purposes of retransmission. The one sequence that is sent MUST
be either one record with Accounting-Record-Type AVP set to the value
EVENT_RECORD, or several records starting with one having the value
START_RECORD, followed by zero or more INTERIM_RECORD and a single
STOP_RECORD. A particular Diameter application specification MUST
define the type of sequences that MUST be used.

9.6. Correlation of Accounting Records

The Diameter protocol's Session-Id AVP, which is globally unique (see
Section 8.8), is used during the authorization phase to identify a
particular session. Services that do not require any authorization
still use the Session-Id AVP to identify sessions. Accounting
messages MAY use a different Session-Id from that sent in
authorization messages. Specific applications MAY require different
a Session-ID for accounting messages.

However, there are certain applications that require multiple
accounting sub-sessions. Such applications would send messages with
a constant Session-Id AVP, but a different Accounting-Sub-Session-Id
AVP. In these cases, correlation is performed using the Session-Id.
It is important to note that receiving a STOP_RECORD with no
Accounting-Sub-Session-Id AVP when sub-sessions were originally used
in the START_RECORD messages implies that all sub-sessions are
terminated.

Furthermore, there are certain applications where a user receives
service from different access devices (e.g., Mobile IPv4), each with
their own unique Session-Id. In such cases, the Acct-Multi-Session-
Id AVP is used for correlation. During authorization, a server that

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determines that a request is for an existing session SHOULD include
the Acct-Multi-Session-Id AVP, which the access device MUST include
in all subsequent accounting messages.

The Acct-Multi-Session-Id AVP MAY include the value of the original
Session-Id. It's contents are implementation specific, but MUST be
globally unique across other Acct-Multi-Session-Id, and MUST NOT
change during the life of a session.

A Diameter application document MUST define the exact concept of a
session that is being accounted, and MAY define the concept of a
multi-session. For instance, the NASREQ DIAMETER application treats
a single PPP connection to a Network Access Server as one session,
and a set of Multilink PPP sessions as one multi-session.

9.7. Accounting Command-Codes

This section defines Command-Code values that MUST be supported by
all Diameter implementations that provide Accounting services.

9.7.1. Accounting-Request

The Accounting-Request (ACR) command, indicated by the Command-Code
field set to 271 and the Command Flags' 'R' bit set, is sent by a
Diameter node, acting as a client, in order to exchange accounting
information with a peer.

One of Acct-Application-Id and Vendor-Specific-Application-Id AVPs
MUST be present. If the Vendor-Specific-Application-Id grouped AVP
is present, it must have an Acct-Application-Id inside.

The AVP listed below SHOULD include service specific accounting AVPs,
as described in Section 9.3.

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Message Format

<ACR> ::= < Diameter Header: 271, REQ, PXY >
< Session-Id >
{ Origin-Host }
{ Origin-Realm }
{ Destination-Realm }
{ Accounting-Record-Type }
{ Accounting-Record-Number }
[ Acct-Application-Id ]
[ Vendor-Specific-Application-Id ]
[ User-Name ]
[ Accounting-Sub-Session-Id ]
[ Acct-Session-Id ]
[ Acct-Multi-Session-Id ]
[ Acct-Interim-Interval ]
[ Accounting-Realtime-Required ]
[ Origin-State-Id ]
[ Event-Timestamp ]
* [ Proxy-Info ]
* [ Route-Record ]
* [ AVP ]

9.7.2. Accounting-Answer

The Accounting-Answer (ACA) command, indicated by the Command-Code
field set to 271 and the Command Flags' 'R' bit cleared, is used to
acknowledge an Accounting-Request command. The Accounting-Answer
command contains the same Session-Id and includes the usage AVPs only
if CMS is in use when sending this command. Note that the inclusion
of the usage AVPs when CMS is not being used leads to unnecessarily
large answer messages, and can not be used as a server's proof of the
receipt of these AVPs in an end-to-end fashion. If the Accounting-
Request was protected by end-to-end security, then the corresponding
ACA message MUST be protected by end-to-end security.

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

One of Acct-Application-Id and Vendor-Specific-Application-Id AVPs
MUST be present. If the Vendor-Specific-Application-Id grouped AVP
is present, it must have an Acct-Application-Id inside.

The AVP listed below SHOULD include service specific accounting AVPs,
as described in Section 9.3.

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Message Format

<ACA> ::= < Diameter Header: 271, PXY >
< Session-Id >
{ Result-Code }
{ Origin-Host }
{ Origin-Realm }
{ Accounting-Record-Type }
{ Accounting-Record-Number }
[ Acct-Application-Id ]
[ Vendor-Specific-Application-Id ]
[ User-Name ]
[ Accounting-Sub-Session-Id ]
[ Acct-Session-Id ]
[ Acct-Multi-Session-Id ]
[ Error-Reporting-Host ]
[ Acct-Interim-Interval ]
[ Accounting-Realtime-Required ]
[ Origin-State-Id ]
[ Event-Timestamp ]
* [ Proxy-Info ]
* [ AVP ]

9.8. Accounting AVPs

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

9.8.1. Accounting-Record-Type AVP

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

EVENT_RECORD 1
An Accounting Event Record is used to indicate that a one-time
event has occurred (meaning that the start and end of the event
are simultaneous). This record contains all information relevant
to the service, and is the only record of the service.

START_RECORD 2
An Accounting Start, Interim, and Stop Records are used to
indicate that a service of a measurable length has been given. An
Accounting Start Record is used to initiate an accounting session,
and contains accounting information that is relevant to the
initiation of the session.

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INTERIM_RECORD 3
An Interim Accounting Record contains cumulative accounting
information for an existing accounting session. Interim
Accounting Records SHOULD be sent every time a re-authentication
or re-authorization occurs. Further, additional interim record
triggers MAY be defined by application-specific Diameter
applications. The selection of whether to use INTERIM_RECORD
records is done by the Acct-Interim-Interval AVP.

STOP_RECORD 4
An Accounting Stop Record is sent to terminate an accounting
session and contains cumulative accounting information relevant to
the existing session.

9.8.2. Acct-Interim-Interval

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

1. The omission of the Acct-Interim-Interval AVP or its inclusion
with Value field set to 0 means that EVENT_RECORD, START_RECORD,
and STOP_RECORD are produced, as appropriate for the service.

2. The inclusion of the AVP with Value field set to a non-zero value
means that INTERIM_RECORD records MUST be produced between the
START_RECORD and STOP_RECORD records. The Value field of this AVP
is the nominal interval between these records in seconds. The
Diameter node that originates the accounting information, known as
the client, MUST produce the first INTERIM_RECORD record roughly
at the time when this nominal interval has elapsed from the
START_RECORD, the next one again as the interval has elapsed once
more, and so on until the session ends and a STOP_RECORD record is
produced.

The client MUST ensure that the interim record production times
are randomized so that large accounting message storms are not
created either among records or around a common service start
time.

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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.

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DELIVER_AND_GRANT 1
The AVP with Value field set to DELIVER_AND_GRANT means that the
service MUST only be granted as long as there is a connection to
an accounting server. Note that the set of alternative accounting
servers are treated as one server in this sense. Having to move
the accounting record stream to a backup server is not a reason to
discontinue the service to the user.

GRANT_AND_STORE 2
The AVP with Value field set to GRANT_AND_STORE means that service
SHOULD be granted if there is a connection, or as long as records
can still be stored as described in Section 9.4.

This is the default behavior if the AVP isn't included in the
reply from the authorization server.

GRANT_AND_LOSE 3
The AVP with Value field set to GRANT_AND_LOSE means that service
SHOULD be granted even if the records can not be delivered or
stored.

10. AVP Occurrence Table

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

The table uses the following symbols:

0 The AVP MUST NOT be present in the message.
0+ Zero or more instances of the AVP MAY be present in the
message.
0-1 Zero or one instance of the AVP MAY be present in the
message. It is considered an error if there are more than
one instance of the AVP.
1 One instance of the AVP MUST be present in the message.
1+ At least one instance of the AVP MUST be present in the
message.

10.1. Base Protocol Command AVP Table

The table in this section is limited to the non-accounting Command
Codes defined in this specification.

Calhoun, et al. Standards Track [Page 124]

RFC 3588 Diameter Based Protocol September 2003

+-----------------------------------------------+
| Command-Code |
+---+---+---+---+---+---+---+---+---+---+---+---+
Attribute Name |CER|CEA|DPR|DPA|DWR|DWA|RAR|RAA|ASR|ASA|STR|STA|
--------------------+---+---+---+---+---+---+---+---+---+---+---+---+
Acct-Interim- |0 |0 |0 |0 |0 |0 |0-1|0 |0 |0 |0 |0 |
Interval | | | | | | | | | | | | |
Accounting-Realtime-|0 |0 |0 |0 |0 |0 |0-1|0 |0 |0 |0 |0 |
Required | | | | | | | | | | | | |
Acct-Application-Id |0+ |0+ |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Auth-Application-Id |0+ |0+ |0 |0 |0 |0 |1 |0 |1 |0 |1 |0 |
Auth-Grace-Period |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Auth-Request-Type |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Auth-Session-State |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Authorization- |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Lifetime | | | | | | | | | | | | |
Class |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0+ |0+ |
Destination-Host |0 |0 |0 |0 |0 |0 |1 |0 |1 |0 |0-1|0 |
Destination-Realm |0 |0 |0 |0 |0 |0 |1 |0 |1 |0 |1 |0 |
Disconnect-Cause |0 |0 |1 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Error-Message |0 |0-1|0 |0-1|0 |0-1|0 |0-1|0 |0-1|0 |0-1|
Error-Reporting-Host|0 |0 |0 |0 |0 |0 |0 |0-1|0 |0-1|0 |0-1|
Failed-AVP |0 |0+ |0 |0+ |0 |0+ |0 |0+ |0 |0+ |0 |0+ |
Firmware-Revision |0-1|0-1|0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Host-IP-Address |1+ |1+ |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Inband-Security-Id |0+ |0+ |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Multi-Round-Time-Out|0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Origin-Host |1 |1 |1 |1 |1 |1 |1 |1 |1 |1 |1 |1 |
Origin-Realm |1 |1 |1 |1 |1 |1 |1 |1 |1 |1 |1 |1 |
Origin-State-Id |0-1|0-1|0 |0 |0-1|0-1|0-1|0-1|0-1|0-1|0-1|0-1|
Product-Name |1 |1 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Proxy-Info |0 |0 |0 |0 |0 |0 |0+ |0+ |0+ |0+ |0+ |0+ |
Redirect-Host |0 |0 |0 |0 |0 |0 |0 |0+ |0 |0+ |0 |0+ |
Redirect-Host-Usage |0 |0 |0 |0 |0 |0 |0 |0-1|0 |0-1|0 |0-1|
Redirect-Max-Cache- |0 |0 |0 |0 |0 |0 |0 |0-1|0 |0-1|0 |0-1|
Time | | | | | | | | | | | | |
Result-Code |0 |1 |0 |1 |0 |1 |0 |1 |0 |0 |0 |1 |
Re-Auth-Request-Type|0 |0 |0 |0 |0 |0 |1 |0 |0 |0 |0 |0 |
Route-Record |0 |0 |0 |0 |0 |0 |0+ |0 |0+ |0 |0+ |0 |
Session-Binding |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Session-Id |0 |0 |0 |0 |0 |0 |1 |1 |1 |1 |1 |1 |
Session-Server- |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Failover | | | | | | | | | | | | |
Session-Timeout |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Supported-Vendor-Id |0+ |0+ |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Termination-Cause |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |1 |0 |
User-Name |0 |0 |0 |0 |0 |0 |0-1|0-1|0-1|0-1|0-1|0-1|
Vendor-Id |1 |1 |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |

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Vendor-Specific- |0+ |0+ |0 |0 |0 |0 |0 |0 |0 |0 |0 |0 |
Application-Id | | | | | | | | | | | | |
--------------------+---+---+---+---+---+---+---+---+---+---+---+---+

10.2. Accounting AVP Table

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

+-----------+
| Command |
| Code |
+-----+-----+
Attribute Name | ACR | ACA |
------------------------------+-----+-----+
Acct-Interim-Interval | 0-1 | 0-1 |
Acct-Multi-Session-Id | 0-1 | 0-1 |
Accounting-Record-Number | 1 | 1 |
Accounting-Record-Type | 1 | 1 |
Acct-Session-Id | 0-1 | 0-1 |
Accounting-Sub-Session-Id | 0-1 | 0-1 |
Accounting-Realtime-Required | 0-1 | 0-1 |
Acct-Application-Id | 0-1 | 0-1 |
Auth-Application-Id | 0 | 0 |
Class | 0+ | 0+ |
Destination-Host | 0-1 | 0 |
Destination-Realm | 1 | 0 |
Error-Reporting-Host | 0 | 0+ |
Event-Timestamp | 0-1 | 0-1 |
Origin-Host | 1 | 1 |
Origin-Realm | 1 | 1 |
Proxy-Info | 0+ | 0+ |
Route-Record | 0+ | 0+ |
Result-Code | 0 | 1 |
Session-Id | 1 | 1 |
Termination-Cause | 0-1 | 0-1 |
User-Name | 0-1 | 0-1 |
Vendor-Specific-Application-Id| 0-1 | 0-1 |
------------------------------+-----+-----+

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11. IANA Considerations

This section provides guidance to the Internet Assigned Numbers
Authority (IANA) regarding registration of values related to the
Diameter protocol, in accordance with BCP 26 [IANA]. The following
policies are used here with the meanings defined in BCP 26: "Private
Use", "First Come First Served", "Expert Review", "Specification
Required", "IETF Consensus", "Standards Action".

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

Diameter is not intended as a general purpose protocol, and
allocations SHOULD NOT be made for purposes unrelated to
authentication, authorization or accounting.

For registration requests where a Designated Expert should be
consulted, the responsible IESG area director should appoint the
Designated Expert. For Designated Expert with Specification
Required, the request is posted to the AAA WG mailing list (or, if it
has been disbanded, a successor designated by the Area Director) for
comment and review, and MUST include a pointer to a public
specification. Before a period of 30 days has passed, the Designated
Expert will either approve or deny the registration request and
publish a notice of the decision to the AAA WG mailing list or its
successor. A denial notice must be justified by an explanation and,
in the cases where it is possible, concrete suggestions on how the
request can be modified so as to become acceptable.

11.1. AVP Header

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

11.1.1. AVP Codes

The AVP Code namespace is used to identify attributes. There are
multiple namespaces. Vendors can have their own AVP Codes namespace
which will be identified by their Vendor-ID (also known as
Enterprise-Number) and they control the assignments of their vendor-
specific AVP codes within their own namespace. The absence of a
Vendor-ID or a Vendor-ID value of zero (0) identifies the IETF IANA
controlled AVP Codes namespace. The AVP Codes and sometimes also
possible values in an AVP are controlled and maintained by IANA.

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AVP Code 0 is not used. AVP Codes 1-255 are managed separately as
RADIUS Attribute Types [RADTYPE]. This document defines the AVP
Codes 257-274, 276-285, 287, 291-300, 480, 483 and 485-486. See
Section 4.5 for the assignment of the namespace in this
specification.

AVPs may be allocated following Designated Expert with Specification
Required [IANA]. Release of blocks of AVPs (more than 3 at a time
for a given purpose) should require IETF Consensus.

Note that Diameter defines a mechanism for Vendor-Specific AVPs,
where the Vendor-Id field in the AVP header is set to a non-zero
value. Vendor-Specific AVPs codes are for Private Use and should be
encouraged instead of allocation of global attribute types, for
functions specific only to one vendor's implementation of Diameter,
where no interoperability is deemed useful. Where a Vendor-Specific
AVP is implemented by more than one vendor, allocation of global AVPs
should be encouraged instead.

11.1.2. AVP Flags

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

11.2. Diameter Header

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

11.2.1. Command Codes

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

The values 16,777,214 and 16,777,215 (hexadecimal values 0xfffffe -
0xffffff) are reserved for experimental commands. As these codes are
only for experimental and testing purposes, no guarantee is made for
interoperability between Diameter peers using experimental commands,
as outlined in [IANA-EXP].

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11.2.2. Command Flags

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

11.3. Application Identifiers

As defined in Section 2.4, the Application Identifier is used to
identify a specific Diameter Application. There are standards-track
application ids and vendor specific application ids.

IANA [IANA] has assigned the range 0x00000001 to 0x00ffffff for
standards-track applications; and 0x01000000 - 0xfffffffe for vendor
specific applications, on a first-come, first-served basis. The
following values are allocated.

Diameter Common Messages 0
NASREQ 1 [NASREQ]
Mobile-IP 2 [DIAMMIP]
Diameter Base Accounting 3
Relay 0xffffffff

Assignment of standards-track application IDs are by Designated
Expert with Specification Required [IANA].

Both Application-Id and Acct-Application-Id AVPs use the same
Application Identifier space.

Vendor-Specific Application Identifiers, are for Private Use.
Vendor-Specific Application Identifiers are assigned on a First Come,
First Served basis by IANA.

11.4. AVP Values

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

11.4.1. Result-Code AVP Values

As defined in Section 7.1, the Result-Code AVP (AVP Code 268) defines
the values 1001, 2001-2002, 3001-3010, 4001-4002 and 5001-5017.

All remaining values are available for assignment via IETF Consensus
[IANA].

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11.4.2. Accounting-Record-Type AVP Values

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

11.4.3. Termination-Cause AVP Values

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

11.4.4. Redirect-Host-Usage AVP Values

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

11.4.5. Session-Server-Failover AVP Values

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

11.4.6. Session-Binding AVP Values

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

11.4.7. Disconnect-Cause AVP Values

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

11.4.8. Auth-Request-Type AVP Values

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

11.4.9. Auth-Session-State AVP Values

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

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11.4.10. Re-Auth-Request-Type AVP Values

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

11.4.11. Accounting-Realtime-Required AVP Values

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

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

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

11.5. Diameter TCP/SCTP Port Numbers

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

11.6. NAPTR Service Fields

The registration in the RFC MUST include the following information:

Service Field: The service field being registered. An example for a
new fictitious transport protocol called NCTP might be "AAA+D2N".

Protocol: The specific transport protocol associated with that
service field. This MUST include the name and acronym for the
protocol, along with reference to a document that describes the
transport protocol. For example - "New Connectionless Transport
Protocol (NCTP), RFC 5766".

Name and Contact Information: The name, address, email address and
telephone number for the person performing the registration.

The following values have been placed into the registry:

Services Field Protocol
AAA+D2T TCP
AAA+D2S SCTP

12. Diameter protocol related configurable parameters

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

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RFC 3588 Diameter Based Protocol September 2003

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

Realm Routing Table
A Diameter proxy server routes messages based on the realm portion
of a Network Access Identifier (NAI). The server MUST have a
table of Realm Names, and the address of the peer to which the
message must be forwarded to. The routing table MAY also include
a "default route", which is typically used for all messages that
cannot be locally processed.

Tc timer
The Tc timer controls the frequency that transport connection
attempts are done to a peer with whom no active transport
connection exists. The recommended value is 30 seconds.

13. Security Considerations

The Diameter base protocol assumes that messages are secured by using
either IPSec or TLS. This security mechanism is acceptable in
environments where there is no untrusted third party agent. In other
situations, end-to-end security is needed.

Diameter clients, such as Network Access Servers (NASes) and Mobility
Agents MUST support IP Security [SECARCH] and MAY support TLS [TLS].
Diameter servers MUST support TLS and IPsec. Diameter
implementations MUST use transmission-level security of some kind
(IPsec or TLS) on each connection.

If a Diameter connection is not protected by IPsec, then the CER/CEA
exchange MUST include an Inband-Security-ID AVP with a value of TLS.
For TLS usage, a TLS handshake will begin when both ends are in the
open state, after completion of the CER/CEA exchange. If the TLS
handshake is successful, all further messages will be sent via TLS.
If the handshake fails, both ends move to the closed state.

It is suggested that IPsec be used primarily at the edges for intra-
domain exchanges. For NAS devices without certificate support, pre-
shared keys can be used between the NAS and a local AAA proxy.

For protection of inter-domain exchanges, TLS is recommended. See
Sections 13.1 and 13.2 for more details on IPsec and TLS usage.

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RFC 3588 Diameter Based Protocol September 2003

13.1. IPsec Usage

All Diameter implementations MUST support IPsec ESP [IPsec] in
transport mode with non-null encryption and authentication algorithms
to provide per-packet authentication, integrity protection and
confidentiality, and MUST support the replay protection mechanisms of
IPsec.

Diameter implementations MUST support IKE for peer authentication,
negotiation of security associations, and key management, using the
IPsec DOI [IPSECDOI]. Diameter implementations MUST support peer
authentication using a pre-shared key, and MAY support certificate-
based peer authentication using digital signatures. Peer
authentication using the public key encryption methods outlined in
IKE's Sections 5.2 and 5.3 [IKE] SHOULD NOT be used.

Conformant implementations MUST support both IKE Main Mode and
Aggressive Mode. When pre-shared keys are used for authentication,
IKE Aggressive Mode SHOULD be used, and IKE Main Mode SHOULD NOT be
used. When digital signatures are used for authentication, either
IKE Main Mode or IKE Aggressive Mode MAY be used.

When digital signatures are used to achieve authentication, an IKE
negotiator SHOULD use IKE Certificate Request Payload(s) to specify
the certificate authority (or authorities) that are trusted in
accordance with its local policy. IKE negotiators SHOULD use
pertinent certificate revocation checks before accepting a PKI
certificate for use in IKE's authentication procedures.

The Phase 2 Quick Mode exchanges used to negotiate protection for
Diameter connections MUST explicitly carry the Identity Payload
fields (IDci and IDcr). The DOI provides for several types of
identification data. However, when used in conformant
implementations, each ID Payload MUST carry a single IP address and a
single non-zero port number, and MUST NOT use the IP Subnet or IP
Address Range formats. This allows the Phase 2 security association
to correspond to specific TCP and SCTP connections.

Since IPsec acceleration hardware may only be able to handle a
limited number of active IKE Phase 2 SAs, Phase 2 delete messages may
be sent for idle SAs, as a means of keeping the number of active
Phase 2 SAs to a minimum. The receipt of an IKE Phase 2 delete
message SHOULD NOT be interpreted as a reason for tearing down a
Diameter connection. Rather, it is preferable to leave the
connection up, and if additional traffic is sent on it, to bring up
another IKE Phase 2 SA to protect it. This avoids the potential for
continually bringing connections up and down.

Calhoun, et al. Standards Track [Page 133]

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13.2. TLS Usage

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

Diameter nodes MUST be able to negotiate the following TLS cipher
suites:

TLS_RSA_WITH_RC4_128_MD5
TLS_RSA_WITH_RC4_128_SHA
TLS_RSA_WITH_3DES_EDE_CBC_SHA

Diameter nodes SHOULD be able to negotiate the following TLS cipher
suite:

TLS_RSA_WITH_AES_128_CBC_SHA

Diameter nodes MAY negotiate other TLS cipher suites.

13.3. Peer-to-Peer Considerations

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

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

Calhoun, et al. Standards Track [Page 134]

RFC 3588 Diameter Based Protocol September 2003

trusted to protect SNMP. These restrictions can be awkward at best.
Since TLS supports application-level granularity in certificate
policy, TLS SHOULD be used to protect Diameter connections between
administrative domains. IPsec is most appropriate for intra-domain
usage when pre-shared keys are used as a security mechanism.

When pre-shared key authentication is used with IPsec to protect
Diameter, unique pre-shared keys are configured with Diameter peers,
who are identified by their IP address (Main Mode), or possibly their
FQDN (Aggressive Mode). As a result, it is necessary for the set of
Diameter peers to be known beforehand. Therefore, peer discovery is
typically not necessary.

The following is intended to provide some guidance on the issue.

It is recommended that a Diameter peer implement the same security
mechanism (IPsec or TLS) across all its peer-to-peer connections.
Inconsistent use of security mechanisms can result in redundant
security mechanisms being used (e.g., TLS over IPsec) or worse,
potential security vulnerabilities. When IPsec is used with
Diameter, a typical security policy for outbound traffic is "Initiate
IPsec, from me to any, destination port Diameter"; for inbound
traffic, the policy would be "Require IPsec, from any to me,
destination port Diameter".

This policy causes IPsec to be used whenever a Diameter peer
initiates a connection to another Diameter peer, and to be required
whenever an inbound Diameter connection occurs. This policy is
attractive, since it does not require policy to be set for each peer
or dynamically modified each time a new Diameter connection is
created; an IPsec SA is automatically created based on a simple
static policy. Since IPsec extensions are typically not available to
the sockets API on most platforms, and IPsec policy functionality is
implementation dependent, use of a simple static policy is the often
the simplest route to IPsec-enabling a Diameter implementation.

One implication of the recommended policy is that if a node is using
both TLS and IPsec, there is not a convenient way in which to use
either TLS or IPsec, but not both, without reserving an additional
port for TLS usage. Since Diameter uses the same port for TLS and
non-TLS usage, where the recommended IPsec policy is put in place, a
TLS-protected connection will match the IPsec policy, and both IPsec
and TLS will be used to protect the Diameter connection. To avoid
this, it would be necessary to plumb peer-specific policies either
statically or dynamically.

Calhoun, et al. Standards Track [Page 135]

RFC 3588 Diameter Based Protocol September 2003

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

14. References

14.1. Normative References

[AAATRANS] Aboba, B. and J. Wood, "Authentication, Authorization
and Accounting (AAA) Transport Profile", RFC 3539,
June 2003.

[ABNF] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.

[ASSIGNNO] Reynolds, J., "Assigned Numbers: RFC 1700 is Replaced
by an On-line Database", RFC 3232, January 2002.

[DIFFSERV] Nichols, K., Blake, S., Baker, F. and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.

[DIFFSERVAF] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,
"Assured Forwarding PHB Group", RFC 2597, June 1999.

[DIFFSERVEF] Davie, B., Charny, A., Bennet, J., Benson, K., Le
Boudec, J., Courtney, W., Davari, S., Firoiu, V. and
D. Stiliadis, "An Expedited Forwarding PHB", RFC 3246,
March 2002.

[DNSSRV] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR
for specifying the location of services (DNS SRV)",
RFC 2782, February 2000.

[EAP] Blunk, L. and J. Vollbrecht, "PPP Extensible
Authentication Protocol (EAP)", RFC 2284, March 1998.

[FLOATPOINT] Institute of Electrical and Electronics Engineers,
"IEEE Standard for Binary Floating-Point Arithmetic",
ANSI/IEEE Standard 754-1985, August 1985.

[IANA] Narten, T. and H. Alvestrand, "Guidelines for Writing
an IANA Considerations Section in RFCs", BCP 26, RFC
2434, October 1998.

Calhoun, et al. Standards Track [Page 136]

RFC 3588 Diameter Based Protocol September 2003

[IANAADFAM] IANA; "Address Family Numbers",
http://www.iana.org/assignments/address-family-numbers

[IANAWEB] IANA, "Number assignment", http://www.iana.org

[IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.

[IPComp] Shacham, A., Monsour, R., Pereira, R. and M. Thomas,
"IP Payload Compression Protocol (IPComp)", RFC 3173,
September 2001.

[IPSECDOI] Piper, D., "The Internet IP Security Domain of
Interpretation for ISAKMP", RFC 2407, November 1998.

[IPV4] Postel, J., "Internet Protocol", STD 5, RFC 791,
September 1981.

[IPV6] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.

[KEYWORDS] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.

[NAI] Aboba, B. and M. Beadles, "The Network Access
Identifier", RFC 2486, January 1999.

[NAPTR] Mealling, M. and R. Daniel, "The naming authority
pointer (NAPTR) DNS resource record," RFC 2915,
September 2000.

[RADTYPE] IANA, "RADIUS Types",
http://www.iana.org/assignments/radius-types

[SCTP] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M.,
Zhang, L. and V. Paxson, "Stream Control Transmission
Protocol", RFC 2960, October 2000.

[SLP] Veizades, J., Guttman, E., Perkins, C. and M. Day,
"Service Location Protocol, Version 2", RFC 2165, June
1999.

[SNTP] Mills, D., "Simple Network Time Protocol (SNTP)
Version 4 for IPv4, IPv6 and OSI", RFC 2030, October
1996.

Calhoun, et al. Standards Track [Page 137]

RFC 3588 Diameter Based Protocol September 2003

[TCP] Postel, J. "Transmission Control Protocol", STD 7, RFC
793, January 1981.

[TEMPLATE] Guttman, E., Perkins, C. and J. Kempf, "Service
Templates and Service: Schemes", RFC 2609, June 1999.

[TLS] Dierks, T. and C. Allen, "The TLS Protocol Version
1.0", RFC 2246, January 1999.

[TLSSCTP] Jungmaier, A., Rescorla, E. and M. Tuexen, "Transport
Layer Security over Stream Control Transmission
Protocol", RFC 3436, December 2002.

[URI] Berners-Lee, T., Fielding, R. and L. Masinter,
"Uniform Resource Identifiers (URI): Generic Syntax",
RFC 2396, August 1998.

[UTF8] Yergeau, F., "UTF-8, a transformation format of ISO
10646", RFC 2279, January 1998.

14.2. Informative References

[AAACMS] P. Calhoun, W. Bulley, S. Farrell, "Diameter CMS
Security Application", Work in Progress.

[AAAREQ] Aboba, B., Calhoun, P., Glass, S., Hiller, T., McCann,
P., Shiino, H., Zorn, G., Dommety, G., Perkins, C.,
Patil, B., Mitton, D., Manning, S., Beadles, M.,
Walsh, P., Chen, X., Sivalingham, S., Hameed, A.,
Munson, M., Jacobs, S., Lim, B., Hirschman, B., Hsu,
R., Xu, Y., Campbell, E., Baba, S. and E. Jaques,
"Criteria for Evaluating AAA Protocols for Network
Access", RFC 2989, November 2000.

[ACCMGMT] Aboba, B., Arkko, J. and D. Harrington. "Introduction
to Accounting Management", RFC 2975, October 2000.

[CDMA2000] Hiller, T., Walsh, P., Chen, X., Munson, M., Dommety,
G., Sivalingham, S., Lim, B., McCann, P., Shiino, H.,
Hirschman, B., Manning, S., Hsu, R., Koo, H., Lipford,
M., Calhoun, P., Lo, C., Jaques, E., Campbell, E., Xu,
Y., Baba, S., Ayaki, T., Seki, T. and A. Hameed,
"CDMA2000 Wireless Data Requirements for AAA", RFC
3141, June 2001.

[DIAMMIP] P. Calhoun, C. Perkins, "Diameter Mobile IP
Application", Work in Progress.

Calhoun, et al. Standards Track [Page 138]

RFC 3588 Diameter Based Protocol September 2003

[DYNAUTH] Chiba, M., Dommety, G., Eklund, M., Mitton, D. and B.
Aboba, "Dynamic Authorization Extensions to Remote
Authentication Dial In User Service (RADIUS)", RFC
3576, July 2003.

[IANA-EXP] T. Narten, "Assigning Experimental and Testing Numbers
Considered Useful", Work in Progress.

[MIPV4] Perkins, C., "IP Mobility Support for IPv4", RFC 3344,
August 2002.

[MIPREQ] Glass, S., Hiller, T., Jacobs, S. and C. Perkins,
"Mobile IP Authentication, Authorization, and
Accounting Requirements", RFC 2977, October 2000.

[NASNG] Mitton, D. and M. Beadles, "Network Access Server
Requirements Next Generation (NASREQNG) NAS Model",
RFC 2881, July 2000.

[NASREQ] P. Calhoun, W. Bulley, A. Rubens, J. Haag, "Diameter
NASREQ Application", Work in Progress.

[NASCRIT] Beadles, M. and D. Mitton, "Criteria for Evaluating
Network Access Server Protocols", RFC 3169, September
2001.

[PPP] Simpson, W., "The Point-to-Point Protocol (PPP)", STD
51, RFC 1661, July 1994.

[PROXYCHAIN] Aboba, B. and J. Vollbrecht, "Proxy Chaining and
Policy Implementation in Roaming", RFC 2607, June
1999.

[RADACCT] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.

[RADEXT] Rigney, C., Willats, W. and P. Calhoun, "RADIUS
Extensions", RFC 2869, June 2000.

[RADIUS] Rigney, C., Willens, S., Rubens, A. and W. Simpson,
"Remote Authentication Dial In User Service (RADIUS)",
RFC 2865, June 2000.

[ROAMREV] Aboba, B., Lu, J., Alsop, J., Ding, J. and W. Wang,
"Review of Roaming Implementations", RFC 2194,
September 1997.

[ROAMCRIT] Aboba, B. and G. Zorn, "Criteria for Evaluating
Roaming Protocols", RFC 2477, January 1999.

Calhoun, et al. Standards Track [Page 139]

RFC 3588 Diameter Based Protocol September 2003

[SECARCH] Kent, S. and R. Atkinson, "Security Architecture for
the Internet Protocol", RFC 2401, November 1998.

[TACACS] Finseth, C., "An Access Control Protocol, Sometimes
Called TACACS", RFC 1492, July 1993.

15. Acknowledgements

The authors would like to thank Nenad Trifunovic, Tony Johansson and
Pankaj Patel for their participation in the pre-IETF Document Reading
Party. Allison Mankin, Jonathan Wood and Bernard Aboba provided
invaluable assistance in working out transport issues, and similarly
with Steven Bellovin in the security area.

Paul Funk and David Mitton were instrumental in getting the Peer
State Machine correct, and our deep thanks go to them for their time.

Text in this document was also provided by Paul Funk, Mark Eklund,
Mark Jones and Dave Spence. Jacques Caron provided many great
comments as a result of a thorough review of the spec.

The authors would also like to acknowledge the following people for
their contribution in the development of the Diameter protocol:

Allan C. Rubens, Haseeb Akhtar, William Bulley, Stephen Farrell,
David Frascone, Daniel C. Fox, Lol Grant, Ignacio Goyret, Nancy
Greene, Peter Heitman, Fredrik Johansson, Mark Jones, Martin Julien,
Bob Kopacz, Paul Krumviede, Fergal Ladley, Ryan Moats, Victor Muslin,
Kenneth Peirce, John Schnizlein, Sumit Vakil, John R. Vollbrecht and
Jeff Weisberg.

Finally, Pat Calhoun would like to thank Sun Microsystems since most
of the effort put into this document was done while he was in their
employ.

Calhoun, et al. Standards Track [Page 140]

RFC 3588 Diameter Based Protocol September 2003

Appendix A. Diameter Service Template

The following service template describes the attributes used by
Diameter servers to advertise themselves. This simplifies the
process of selecting an appropriate server to communicate with. A
Diameter client can request specific Diameter servers based on
characteristics of the Diameter service desired (for example, an AAA
server to use for accounting.)

Name of submitter: "Erik Guttman" <[e-mail]> Language of
service template: en

Security Considerations:
Diameter clients and servers use various cryptographic mechanisms
to protect communication integrity, confidentiality as well as
perform end-point authentication. It would thus be difficult if
not impossible for an attacker to advertise itself using SLPv2 and
pose as a legitimate Diameter peer without proper preconfigured
secrets or cryptographic keys. Still, as Diameter services are
vital for network operation it is important to use SLPv2
authentication to prevent an attacker from modifying or
eliminating service advertisements for legitimate Diameter
servers.

Template text:
-------------------------template begins here-----------------------
template-type=service:diameter

template-version=0.0

template-description=
The Diameter protocol is defined by RFC 3588.

template-url-syntax=
url-path= ; The Diameter URL format is described in Section 2.9.
; Example: 'aaa://aaa.example.com:1812;transport=tcp
supported-auth-applications= string L M
# This attribute lists the Diameter applications supported by the
# AAA implementation. The applications currently defined are:
# Application Name Defined by
# ---------------- -----------------------------------
# NASREQ Diameter Network Access Server Application
# MobileIP Diameter Mobile IP Application
#
# Notes:
# . Diameter implementations support one or more applications.
# . Additional applications may be defined in the future.
# An updated service template will be created at that time.

Calhoun, et al. Standards Track [Page 141]

RFC 3588 Diameter Based Protocol September 2003

#
NASREQ,MobileIP

supported-acct-applications= string L M
# This attribute lists the Diameter applications supported by the
# AAA implementation. The applications currently defined are:
# Application Name Defined by
# ---------------- -----------------------------------
# NASREQ Diameter Network Access Server Application
# MobileIP Diameter Mobile IP Application
#
# Notes:
# . Diameter implementations support one or more applications.
# . Additional applications may be defined in the future.
# An updated service template will be created at that time.
#
NASREQ,MobileIP

supported-transports= string L M
SCTP
# This attribute lists the supported transports that the Diameter
# implementation accepts. Note that a compliant Diameter
# implementation MUST support SCTP, though it MAY support other
# transports, too.
SCTP,TCP

-------------------------template ends here-----------------------

Appendix B. NAPTR Example

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

;; order pref flags service regexp replacement
IN NAPTR 50 50 "s" "AAA+D2S" ""
_diameter._sctp.example.com IN NAPTR 100 50 "s" "AAA+D2T"
"" _aaa._tcp.example.com

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

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

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RFC 3588 Diameter Based Protocol September 2003

Appendix C. Duplicate Detection

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

- 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.

Calhoun, et al. Standards Track [Page 143]

RFC 3588 Diameter Based Protocol September 2003

In general, only generation of duplicates due to failover or re-
sending of records in non-volatile storage can be reliably detected
by Diameter clients or agents. In such cases the Diameter client or
agents can mark the message as possible duplicate by setting the T
flag. Since the Diameter server is responsible for duplicate
detection, it can choose to make use of the T flag or not, in order
to optimize duplicate detection. Since the T flag does not affect
interoperability, and may not be needed by some servers, generation
of the T flag is REQUIRED for Diameter clients and agents, but MAY be
implemented by Diameter servers.

As an example, it can be usually be assumed that duplicates appear
within a time window of longest recorded network partition or device
fault, perhaps a day. So only records within this time window need
to be looked at in the backward direction. Secondly, hashing
techniques or other schemes, such as the use of the T flag in the
received messages, may be used to eliminate the need to do a full
search even in this set except for rare cases.

The following is an example of how the T flag may be used by the
server to detect duplicate requests.

A Diameter server MAY check the T flag of the received message to
determine if the record is a possible duplicate. If the T flag is
set in the request message, the server searches for a duplicate
within a configurable duplication time window backward and
forward. This limits database searching to those records where
the T flag is set. In a well run network, network partitions and
device faults will presumably be rare events, so this approach
represents a substantial optimization of the duplicate detection
process. During failover, it is possible for the original record
to be received after the T flag marked record, due to differences
in network delays experienced along the path by the original and
duplicate transmissions. The likelihood of this occurring
increases as the failover interval is decreased. In order to be
able to detect out of order duplicates, the Diameter server should
use backward and forward time windows when performing duplicate
checking for the T flag marked request. For example, in order to
allow time for the original record to exit the network and be
recorded by the accounting server, the Diameter server can delay
processing records with the T flag set until a time period
TIME_WAIT + RECORD_PROCESSING_TIME has elapsed after the closing
of the original transport connection. After this time period has
expired, then it may check the T flag marked records against the
database with relative assurance that the original records, if
sent, have been received and recorded.

Calhoun, et al. Standards Track [Page 144]

RFC 3588 Diameter Based Protocol September 2003

Appendix D. Intellectual Property Statement

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

The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights which may cover technology that may be required to practice
this standard. Please address the information to the IETF Executive
Director.

Calhoun, et al. Standards Track [Page 145]

RFC 3588 Diameter Based Protocol September 2003

Authors' Addresses

Pat R. Calhoun
Airespace, Inc.
110 Nortech Parkway
San Jose, California, 95134
USA

Phone: [phone]
Fax: [phone]
EMail: [e-mail]

John Loughney
Nokia Research Center
Itamerenkatu 11-13
00180 Helsinki
Finland

Phone: [phone]
EMail: [e-mail]

Jari Arkko
Ericsson
02420 Jorvas
Finland

Phone: [phone]
EMail: [e-mail]

Erik Guttman
Sun Microsystems, Inc.
Eichhoelzelstr. 7
74915 Waibstadt
Germany

Phone: [phone]
EMail: [e-mail]

Glen Zorn
Cisco Systems, Inc.
500 108th Avenue N.E., Suite 500
Bellevue, WA 98004
USA

Phone: [phone]

Calhoun, et al. Standards Track [Page 146]

RFC 3588 Diameter Based Protocol September 2003

Full Copyright Statement

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

This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
kind, provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.

The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

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

Calhoun, et al. Standards Track [Page 147]