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.
It carries authentication information. The private user identity (email@example.com) 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"
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).
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.