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Ti+  for  TS 24.228§7.2.3§7.3.5§7.4.3
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IMS S2 signalling flow - Session Initiation: mobile origination and termination in home network - IMS S2-3a. SIP PRACK request (UE1 to UE2): from UE1 to P-CSCF1
  • UE1 determines which media flows should be used for this session, and which codecs should be used for each of those media flows. If there was any change in media flows, or if there was more than one choice of codec for a media flow, then UE1 includes a new SDP offer in the PRACK message sent to UE2. For this example, assume UE1 chooses H.263 as the codec to use for the single video stream. Therefore, UE1 sends a new SDP offer in the PRACK 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.
  • UE1 initiates the reservation procedures for the resources needed for this session.