The Diameter Sip Application : General 

The Diameter SIP application can be used in a SIP environment where
   an interface to a AAA infrastructure is required to authenticate and
   authorize the usage of SIP resources.  This application provides
   support for SIP User Agents and proxies that implement and use HTTP
   Digest authentication [RFC2617], which is the authentication
   mechanism mandated by SIP [RFC3261].  The application is extensible
   and, if need arises, it can be extended to provide support for other
   authentication mechanisms or extensions to HTTP Digest authentication
   when they occur.

   This application provides limited support for accounting services as
   follows: the Diameter server is able to provide the addresses of
   accounting severs to the Diameter client.  Figure 1, below, shows a
   general overview of the integration of the SIP architecture with the
   AAA architecture.

   According to Figure 1, there are one or more SIP User Agents (UAs)
   that initiate or terminate SIP traffic through one or more SIP
   servers.  Both SIP servers implement a Diameter client that supports
   the Diameter application described in this specification.

                              +--------+
                  UAR/UAA +--->|Diameter|<----+ PPR/PPA
                  LIR/LIA |    | server |     | MAR/MAA
                          |    +--------+     | SAR/SAA
                          |                   | RTR/RTA
                          |                   |
                          v                   v
   +------+   SIP     +--------+    SIP   +--------+   SIP     +------+
   | SIP  |<--------->|  SIP   |<-------->|  SIP   |<--------->| SIP  |
   |  UA  |           |server 1|          |server 2|           |  UA  |
   +------+           +--------+          +--------+           +------+
                          ^                   ^
                  UAR/UAA |                   |
                  LIR/LIA |                   | MAR/MAA
                          |    +--------+     | SAR/SAA
                          +--->|Diameter|<----+
                               |   SL   |
                               +--------+

        Figure 1: Architecture of the Diameter application for SIP

   In Figure 1, it can be seen that SIP server 1 sends different
   Diameter commands and receives different responses than those sent
   and received by SIP server 2.  This is because SIP server 1 in
   Figure 1 is located at the edge of a network, and its main task is to
   locate SIP server 2.  SIP server 2 is requesting and receiving
   authentication and authorization data from the Diameter server and is
   not located at the edge of the network.

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