Diameter (protocol)

Diameter is an authentication, authorization, and accounting protocol for computer networks. It evolved from and replaces the much less capable RADIUS protocol that preceded it.

Diameter Applications extend the base protocol by adding new commands and/or attributes, such as those for use with the Extensible Authentication Protocol (EAP).

Comparison with RADIUS

The name is a play on words, derived from the RADIUS protocol, which is the predecessor (a diameter is twice the radius). Diameter is not directly backwards compatible but provides an upgrade path for RADIUS. The main features provided by Diameter but lacking in RADIUS are:


A Diameter Application is not a software application but is a protocol based on the Diameter base protocol defined in RFC 6733 (Obsoletes: RFC 3588). Each application is defined by an application identifier and can add new command codes and/or new mandatory AVPs (Attribute-Value Pair). Adding a new optional AVP does not require a new application.

Examples of Diameter applications:

Both the HSS and the SLF communicate using the Diameter protocol.

(Generic Bootstrapping Architecture): Bootstrapping Server Function


The Diameter protocol was initially developed by Pat R. Calhoun, Glen Zorn, and Ping Pan in 1998 to provide a framework for authentication, authorization and accounting (AAA) that could overcome the limitations of RADIUS. RADIUS had issues with reliability, scalability, security and flexibility. RADIUS cannot deal effectively with remote access, IP mobility and policy control. The Diameter protocol defines a policy protocol used by clients to perform policy, AAA, and resource control. This allows a single server to handle policies for many services.[1]

Like RADIUS, Diameter provides AAA functionality, but it is using TCP and SCTP instead of UDP, therefore logic for detection of communication problems is not included in the Diameter protocol itself. The Diameter protocol is enhanced further by the development of the 3rd Generation Partnership Project (3GPP) IP Multimedia Subsystem (IMS). The Cx, Dh, Dx, Rf, Ro, and Sh interfaces are supported by Diameter applications.[2] Through the use of extensions, the protocol was designed to be extensible to support proxies, brokers, strong security, mobile IP, network-access servers (NASREQ), accounting and resource management.

Protocol Description

The Diameter base protocol is defined by RFC 6733 (Obsoletes: RFC 3588) and defines the minimum requirements for an AAA protocol. Diameter Applications can extend the base protocol by adding new commands, attributes, or both. Diameter security is provided by IPsec or TLS. The IANA has assigned TCP and SCTP port number 3868 to Diameter.

Packet Format

The packet consists of a Diameter header and a variable number of Attribute-Value Pairs, or AVPs, for encapsulating information relevant to the Diameter message.

Diameter Header
Bit offset  0 1 2 3 4 5 6 7 8 910111213141516171819202122232425262728293031
0 version message length
32 R P E T         command code
64 application ID
96 hop-by-hop ID
128 end-to-end ID


This field indicates the version of the Diameter Base Protocol. As of 2014, the only value supported is 1.[3]

Message Length

The Message Length field indicates the length of the Diameter message including the header fields and the padded AVPs.


The "R" (Request) bit – If set, the message is a request. If cleared, the message is an answer.

The "P" (Proxiable) bit – If set, the message MAY be proxied, relayed or redirected. If cleared, the message MUST be locally processed.

The "E" (Error) bit – If set, the message contains a protocol error, and the message will not conform to the CCF described for this command. Messages with the "E" bit set are commonly referred to as error messages. This bit MUST NOT be set in request messages.

The "T" (Potentially re-transmitted message) bit – 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.


Each command Request/Answer pair is assigned a command code, and the request or answer is identified via the ’R’ bit in the Command Flags field of the header.

The values 0-255 are reserved for RADIUS backward compatibility. The values 256-16777213 are for permanent, standard commands allocated by IANA. The values 16777214 and 16777215 (hex 0xFFFFFE and 0xFFFFFF) are reserved for experimental and testing purposes.

A Command Code is used to determine the action that is to be taken for a particular message. Some common Diameter commands defined in the protocol (base and applications) are:

AA-RequestAAR265Diameter NAS Application - RFC 7155
AA-AnswerAAA265Diameter NAS Application - RFC 7155
Diameter-EAP-RequestDER268Diameter EAP Application - RFC 4072
Diameter-EAP-AnswerDEA268Diameter EAP Application - RFC 4072
Abort-Session-RequestASR274Diameter base
Abort-Session-AnswerASA274Diameter base
Accounting-RequestACR271Diameter base
Accounting-AnswerACA271Diameter base
Credit-Control-RequestCCR272Diameter Credit-Control Application - RFC 4006
Credit-Control-AnswerCCA272Diameter Credit-Control Application - RFC 4006
Capabilities-Exchange-RequestCER257Diameter base
Capabilities-Exchange-AnswerCEA257Diameter base
Device-Watchdog-RequestDWR280Diameter base
Device-Watchdog-AnswerDWA280Diameter base
Disconnect-Peer-RequestDPR282Diameter base
Disconnect-Peer-AnswerDPA282Diameter base
Re-Auth-RequestRAR258Diameter base
Re-Auth-AnswerRAA258Diameter base
Session-Termination-RequestSTR275Diameter base
Session-Termination-AnswerSTA275Diameter base
User-Authorization-RequestUAR283Diameter SIP Application - RFC 4740
User-Authorization-AnswerUAA283Diameter SIP Application - RFC 4740
Server-Assignment-RequestSAR284Diameter SIP Application - RFC 4740
Server-Assignment-AnswerSAA284Diameter SIP Application - RFC 4740
Location-Info-RequestLIR285Diameter SIP Application - RFC 4740
Location-Info-AnswerLIA285Diameter SIP Application - RFC 4740
Multimedia-Auth-RequestMAR286Diameter SIP Application - RFC 4740
Multimedia-Auth-AnswerMAA286Diameter SIP Application - RFC 4740
Registration-Termination-RequestRTR287Diameter SIP Application - RFC 4740
Registration-Termination-AnswerRTA287Diameter SIP Application - RFC 4740
Push-Profile-RequestPPR288Diameter SIP Application - RFC 4740
Push-Profile-AnswerPPA288Diameter SIP Application - RFC 4740
User-Authorization-RequestUAR300Diameter base ( 3GPP ) RFC 3589
User-Authorization-AnswerUAA300Diameter base ( 3GPP ) RFC 3589
Server-Assignment-RequestSAR301Diameter base ( 3GPP ) RFC 3589
Server-Assignment-AnswerSAA301Diameter base ( 3GPP ) RFC 3589
Location-Info-RequestLIR302Diameter base ( 3GPP ) RFC 3589
Location-Info-AnswerLIA302Diameter base ( 3GPP ) RFC 3589
Multimedia-Auth-RequestMAR303Diameter base ( 3GPP ) RFC 3589
Multimedia-Auth-AnswerMAA303Diameter base ( 3GPP ) RFC 3589
Registration-Termination-RequestRTR304Diameter base ( 3GPP ) RFC 3589
Registration-Termination-AnswerRTA304Diameter base ( 3GPP ) RFC 3589
Push-Profile-RequestPPR305Diameter base ( 3GPP ) RFC 3589
Push-Profile-AnswerPPA305Diameter base ( 3GPP ) RFC 3589
User-Data-RequestUDR306Diameter base ( 3GPP ) RFC 3589
User-Data-AnswerUDA306Diameter base ( 3GPP ) RFC 3589
Profile-Update-RequestPUR307Diameter base ( 3GPP ) RFC 3589
Profile-Update-AnswerPUA307Diameter base ( 3GPP ) RFC 3589
Subscribe-Notifications-RequestSNR308Diameter base ( 3GPP ) RFC 3589
Subscribe-Notifications-AnswerSNA308Diameter base ( 3GPP ) RFC 3589
Push-Notification-RequestPNR309Diameter base ( 3GPP ) RFC 3589
Push-Notification-AnswerPNA309Diameter base ( 3GPP ) RFC 3589
Bootstrapping-Info-RequestBIR310Diameter base ( 3GPP ) RFC 3589
Bootstrapping-Info-AnswerBIA310Diameter base ( 3GPP ) RFC 3589
Message-Process-RequestMPR311Diameter base ( 3GPP ) RFC 3589
Message-Process-AnswerMPA311Diameter base ( 3GPP ) RFC 3589
Update-Location-RequestULR316 3GPP TS 29.272 [RFC 5516]
Update-Location-AnswerULA316 3GPP TS 29.272 [RFC 5516]
Cancel-Location-RequestCLR317 3GPP TS 29.272 [RFC 5516]
Cancel-Location-AnswerCLA317 3GPP TS 29.272 [RFC 5516]
Authentication-Information-RequestAIR318 3GPP TS 29.272 [RFC 5516]
Authentication-Information-AnswerAIA318 3GPP TS 29.272 [RFC 5516]
Insert-Subscriber-Data-RequestISD319 3GPP TS 29.272 [RFC 5516]
Insert-Subscriber-Data-AnswerISD319 3GPP TS 29.272 [RFC 5516]
Delete-Subscriber-Data-RequestDSD320 3GPP TS 29.272 [RFC 5516]
Delete-Subscriber-Data-AnswerDSD320 3GPP TS 29.272 [RFC 5516]
Purge-UE-RequestPER321 3GPP TS 29.272 [RFC 5516]
Purge-UE-AnswerPEA321 3GPP TS 29.272 [RFC 5516]
Notify-RequestNR323 3GPP TS 29.272 [RFC 5516]
Notify-AnswerNA323 3GPP TS 29.272 [RFC 5516]
Provide-Location-RequestPLR83886203GPP-LCS-SLg (Application-ID 16777255)
Provide-Location-AnswerPLA83886203GPP-LCS-SLg (Application-ID 16777255)
Routing-Info-RequestRIR83886223GPP-LCS-SLh (Application-ID 16777291)
Routing-Info-AnswerRIA83886223GPP-LCS-SLh (Application-ID 16777291)
AA-Mobile-Node-RequestAMR260 Diameter Mobile IPv4 - RFC 4004
AA-Mobile-Node-AnswerAMA260 Diameter Mobile IPv4 - RFC 4004
Home-Agent-MIP-RequestHAR262 Diameter Mobile IPv4 - RFC 4004
Home-Agent-MIP-AnswerHAA262 Diameter Mobile IPv4 - RFC 4004


Application-ID is used to identify for which Diameter application the message is applicable. The application can be an authentication application, an accounting application, or a vendor-specific application.

Diameter agents conforming to a certain Diameter extension publicize its support by including a specific value of in the Auth-Application-Id Attribute of the Capabilities-Exchange-Request (CER) and Capabilities-Exchange-Answer (CEA) command.

The value of the Application-ID field in the header is the same as any relevant Application-Id AVPs contained in the message. For instance, the value of the Application-ID and of the Auth-Application-Id Attribute in the Credit-Control-Request (CCR) and Credit-Control-Answer (CCA) Command for the Diameter Credit-Control Application is 4.[4]

Application-IDAbbr.Full nameUsage
0BaseDiameter Common MessagesDiameter protocol association establishment/teardown/maintenance
16777217Sh3GPP ShVoIP/IMS SIP Application Server to HSS interface
16777251S6a/S6d3GPP S6a/S6dLTE Roaming signaling
16777255SLg3GPP LCS SLgLocation services

Hop-by-Hop Identifier

The Hop-by-Hop Identifier is an unsigned 32-bit integer field (in network byte order) that is used to match the requests with their answers as the same value in the request is used in the response.

The Diameter protocol requires that relaying and proxying 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. Received answers that do not match a known Hop-by-Hop Identifier are ignored by the Diameter agent.

In case of redirecting agents, the Hop-by-Hop Identifier is maintained in the header as the Diameter agent responds with an answer message.

End-to-End Identifier

The End-to-End Identifier is an unsigned 32-bit integer field (in network byte order) that is used to detect duplicate messages along with the combination of the Origin-Host AVP.

When creating a request, the End-to-End Identifier is set to a locally unique value. The End-to-End Identifier is not modified by Diameter agents of any kind, and the same value in the corresponding request is used in the answer.

Attribute-Value Pairs (AVP)

AVP Header
Bit offset  0 1 2 3 4 5 6 7 8 910111213141516171819202122232425262728293031
0 AVP code
32 VMP      AVP length
64 vendor ID (optional)

For simplicity, "V" bit Means Vendor Specific; "M" bit means Mandatory; "P" bit means Protected.

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.

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

The "P" bit indicates the need for encryption for end-to-end security.

Attribute-NameCodeData Type

State machines

The RFC 3588 defines a core state machine for maintaining connections between peers and processing messages. This is part of the basic protocol functionality and all stacks should support it and as such abstract from the connectivity related operations.

Additionally, application specific state machines can be introduced either later or at a higher abstraction layer. The RFC 3588 defines an authorization and an accounting state machine.

Message flows

The communication between two diameter peers starts with the establishment of a transport connection (TCP or SCTP). The initiator then sends a Capabilities-Exchange-Request (CER) to the other peer, which responds with a Capabilities-Exchange-Answer (CEA). For RFC3588 compliant peers TLS (Transport Layer Security) may optionally be negotiated. For RFC6733 compliant peers TLS negotiation may optionally happen before the CER/CEA.

The connection is then ready for exchanging application messages.

If no messages have been exchanged for some time either side may send a Device-Watchdog-Request (DWR) and the other peer must respond with Device-Watchdog-Answer.

Either side may terminate the communication by sending a Disconnect-Peer-Request (DPR) which the other peer must respond to with Disconnect-Peer-Answer. After that the transport connection can be disconnected.


The Diameter protocol is currently defined in the following IETF RFCs: Obsolete RFCs are indicated with strikethrough text.

# Title Date published Related article Obsoleted by Notes
RFC 3588 Diameter Base Protocol. September 2003 RFC 6733
RFC 3589 Diameter Command Codes for Third Generation Partnership Project (3GPP) Release 5. September 2003
RFC 4004 Diameter Mobile IPv4 Application. August 2005
RFC 4005 Diameter Network Access Server Application. August 2005 RFC 7155
RFC 4006 Diameter Credit-Control Application. August 2005 Diameter Credit-Control Application
RFC 4072 Diameter Extensible Authentication Protocol (EAP) Application. August 2005
RFC 4740 Diameter Session Initiation Protocol (SIP) Application. M. November 2006
RFC 5224 Diameter Policy Processing Application. March 2008
RFC 5431 Diameter ITU-T Rw Policy Enforcement Interface Application. March 2009
RFC 5447 Diameter Mobile IPv6: Support for Network Access Server to Diameter Server Interaction. February 2009
RFC 5516 Diameter Command Code Registration for the Third Generation Partnership Project (3GPP) Evolved Packet System (EPS). April 2009 -
RFC 5624 Quality of Service Parameters for Usage with Diameter. August 2009
RFC 5719 Updated IANA Considerations for Diameter Command Code Allocations. January 2010 RFC 6733
RFC 6733 Diameter Base Protocol. October 2012
RFC 6737 The Diameter Capabilities Update Application. October 2012
RFC 7155 Diameter Network Access Server Application. April 2014

See also


  1. Pat R. Calhoun, Glen Zorn and Ping Pan (February 2001). "DIAMETER Framework Document". IETF. Retrieved 30 April 2009.
  2. Naman Mehta (20 March 2009). "Introduction to Diameter Protocol - What is Diameter Protocol?". Sun Microsystems. Retrieved 30 April 2009.
  3. "RFC 6733 - Diameter Base Protocol". PROPOSED STANDARD. Standards Track. ISSN 2070-1721. Retrieved 12 October 2014.
  4. "RFC 4006 - Diameter Credit-Control Application". PROPOSED STANDARD. Standards Track.

External links

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