8.7. Multimedia-Auth-Request (MAR) Command

   The Multimedia-Auth-Request (MAR) command is indicated by the
   Command-Code set to 286 and the Command Flags' 'R' bit set.  The
   Diameter client in a SIP server sends this command to the Diameter
   server to request that the Diameter server authenticate and authorize
   a user attempt to use some SIP service (in this context, SIP service
   can be something as simple as a SIP subscription or using the proxy
   services for a SIP request).

   The MAR command may also register the SIP server's own URI to the
   Diameter server, so that future LIR/LIA messages can return this URI.
   If the SIP server is acting as a SIP registrar (see examples in
   Sections 6.2 and 6.3), its Diameter client MUST include a SIP-
   Server-URI AVP in the MAR command.  In any other cases (see example
   in Section 6.4), its Diameter client MUST NOT include a SIP-Server-
   URI AVP in the MAR command.

   The SIP-Method AVP MUST include the SIP method name of the SIP
   request that triggered this Diameter MAR message.  The Diameter
   server can use this AVP to authorize some SIP requests depending on
   the method.

   The Diameter MAR message MUST include a SIP-AOR AVP.  The SIP-AOR AVP
   indicates the target of the SIP request.  The value of the AVP is
   extracted from different places in SIP request, depending on the
   semantics of the SIP request.  For SIP REGISTER messages the SIP-AOR
   AVP value indicates the intended public user identity under
   registration, and it is the SIP or SIPS URI populated in the To
   header field value (addr-spec as per RFC 3261 [RFC3261]) of the SIP
   REGISTER request.  For other types of SIP requests, such as INVITE,
   SUBSCRIBE, MESSAGE, etc., the SIP-AOR AVP value indicates the
   intended destination of the request.  This is typically populated in
   the Request-URI of the SIP request.  Extracting the SIP-AOR AVP value

   from the proper SIP header field is the Diameter client's
   responsibility.  Extensions to SIP (new SIP methods or new semantics)
   may require the SIP-AOR to be extracted from other parts of the
   request.

   If the SIP request includes some sort of authentication information,
   the Diameter client MUST include the user name, extracted from the
   authentication information of the SIP request, in the User-Name AVP
   value.

   The Message Format of the MAR command is as follows:

       <MAR> ::= < Diameter Header: 286, REQ, PXY >
                 < Session-Id >
                 { Auth-Application-Id }
                 { Auth-Session-State }
                 { Origin-Host }
                 { Origin-Realm }
                 { Destination-Realm }
                 { SIP-AOR }
                 { SIP-Method }
                 [ Destination-Host ]
                 [ User-Name ]
                 [ SIP-Server-URI ]
                 [ SIP-Number-Auth-Items ]
                 [ SIP-Auth-Data-Item ]
               * [ Proxy-Info ]
               * [ Route-Record ]
               * [ AVP ]

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