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IMS S1 signalling flow - Session Initiation: Mobile origination and termination roaming, with different network operators - IMS S1-5a. SIP UPDATE request (UE1 to UE2): from UE1 to P-CSCF1
  • When the resource reservation is completed, UE1 sends the UPDATE request to the terminating endpoint, via the signalling path established by the INVITE request.
  • Request-URI: Takes the value of the Contact header of the received 183 Session Progress response.
  • Via: Takes the value of either the IP address or FQDN of the originating UE.
  • CSeq: Takes a higher value than that in the previous request.
  • Security-Verify: Contains the security agreement as represented by the received Security-Server header.