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6.7. SIP Soft State Termination

   SIP can create soft states in SIP nodes based on events such as SIP
   registrations or SIP event subscriptions.  These states are
   periodically refreshed, and cease to exist if they are not refreshed.
   Additionally, an administrative action can be taken to terminate a
   SIP soft state, or the SIP UA can explicitly terminate a SIP soft
   state.

   The Diameter base protocol offers a mechanism to create and delete
   states in Diameter nodes.  These states are called Diameter user
   sessions.  The Diameter server decides whether to use a Diameter user
   session as a mechanism to map to a SIP soft state.  If the Diameter
   server decides to use Diameter user sessions, the termination of a
   Diameter user session implies the termination of the corresponding
   SIP soft state (e.g., registration, event subscription), and vice
   versa.  If the Diameter server does not use Diameter user sessions,
   this Diameter SIP application offers specific commands to manage the
   SIP soft states.  Implementations compliant with this specification
   MUST support both mechanisms of session management.

   We provide support for both Diameter client- and Diameter
   server-initiated session termination.  Depending on whether Diameter
   sessions are used, termination of a SIP soft state can be achieved by
   one of the following methods:

   o  When the Diameter client (SIP proxy) wants to terminate the SIP
      soft state and Diameter user sessions are not maintained (i.e.,
      the Auth-Session-State AVP has been previously set to
      NO_STATE_MAINTAINED), the Diameter client MUST send a
      Server-Assignment-Request (SAR) message with the
      SIP-Server-Assignment-Type AVP (Section 9.4) set to any of the
      deregistration values:  TIMEOUT_DEREGISTRATION,
      USER_DEREGISTRATION, TIMEOUT_DEREGISTRATION_STORE_SERVER_NAME,
      USER_DEREGISTRATION_STORE_SERVER_NAME,
      ADMINISTRATIVE_DEREGISTRATION, DEREGISTRATION_TOO_MUCH_DATA.

   o  When the Diameter client (SIP proxy) wants to terminate the SIP
      soft state and Diameter user sessions are maintained (i.e., the
      Auth-Session-State AVP has been previously set to
      STATE_MAINTAINED), the Diameter client MUST send a Session-
      Termination-Request (STR) message as per regular procedures
      according to RFC 3588 [RFC3588].

   o  When the Diameter server wants to terminate the SIP soft state and
      Diameter user sessions are not maintained (i.e., the
      Auth-Session-State AVP has been previously set to
      NO_STATE_MAINTAINED), the Diameter server MUST send a
      Registration-Termination-Request (RTR) message (see Section 8.9).

   o  When the Diameter server wants to terminate the SIP soft state and
      Diameter user sessions are maintained (i.e., the
      Auth-Session-State AVP has been previously set to
      STATE_MAINTAINED), the Diameter server MUST send an
      Abort-Session-Request (ASR) message as per regular procedures
      according to RFC 3588 [RFC3588].

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