It shall be possible to implicitly register impus see

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It shall be possible to implicitly register IMPU(s) (see clause 4.3.3.4 in TS 23.228 [3]). All the IMPU(s) being implicitly registered shall be delivered by the HSS to the S-CSCF and subsequently to the P-CSCF. The S-CSCF shall regard all implicitly registered IMPU(s) as registered IMPU(s). When an IMPU has been registered this registration will be valid for some period of time. Both the UE and the S-CSCF will keep track of a timer for this purpose but the expiration time in the UE is smaller than the one in the S-CSCF in order to make it possible for the UE to be registered and reachable without interruptions. A successful registration of a previously registered IMPU (including implicitly registered IMPUs) means the expiry time of the registration is refreshed. If the user has been successfully authenticated, the S-CSCF sends a SM10 SIP 2xx Auth_OK message to the I-CSCF indicating that the registration was successful. The 2xx Auth_OK message contains the Authentication-Info header with a response digest as specified in RFC 2617 [12]. The response digest allows the UE to authenticate the HN. In SM11 the I-CSCF forwards the SIP 2xx Auth_OK towards the P-CSCF. The P-CSCF associates the UE's packet source IP address along with the "sent-by" parameter of the Via header, cf. RFC 3261 [6], of the REGISTER message with the IMPI and all the successfully registered IMPUs related to that IMPI. If managing of client-initiated connections as defined in RFC 5626 [32] is used then the P-CSCF shall also include the UE's packet source port of the REGISTER message as part of the association. The P-CSCF stores the associated parameters in an IP address check table. If managing of client-initiated connections is not used then the P-CSCF shall overwrite any existing entry in the IP address check table which has the same IP address, but a different IMPI. If managing of client-initiated connections is used then the P-CSCF shall overwrite any existing entry in the IP address check table which has the same (IP address, port) pair, but a different IMPI. The P-CSCF forwards the SIP 2xx AUTH_OK towards the UE. NOTE 5: If a P-CSCF associated the port with the IMPI even when managing of client-initiated connections was not used then the UE would be unnecessarily restricted in opening new connections during a registration. The restriction is unavoidable in the presence of NAT. Upon receiving SM12, the UE shall calculate the expected response from the HN as described in RFC 2617 [12]. To authenticate the HN, the UE shall compare its expected response to the response provided by the HN. If the comparison fails the UE shall abort the communication. N.2.1.2 Authentication Requirements for Non-registration Messages For the purposes of this subsection, the name "authentication" is used synonymously with "message origin authentication".
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