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IMS R2 signalling flow - Re-Registration: User currently registered - IMS R2-1a. SIP REGISTER request: from UE1 to P-CSCF1
  • The registration expires in UE1. UE1 reregisters by sending a new REGISTER request. This request is sent to the same P-CSCF with which UE1 initially registered.
  • Authorization: It carries authentication information. The private user identity (user1_private@home1.net) is carried in the username field of the Digest AKA protocol. As this is a re-registration process, the cached information (realm, nonce, algorithm, uri, response) is also sent.
    NOTE: The actual nonce value in the WWW-Authenticate header field is encoded in base64, and it may look like: nonce="A34Cm+Fva37UYWpGNB34JP"
  • Security-Client: Lists the supported algorithm(s) by UE1. The values spi-c, spi-s, port-c and port-s are new parameter values and will not be used for security association setup in case the request is answered with 200 (OK).
  • Security-Verify: Contains the security agreement as represented by the previously received Security-Server header.
  • Upon receiving this request P-CSCF1 will detect that it already has a registration record for UE1 and will reset its SIP registration timer for UE1 to the Expires time in this request.