Smn stands for sip message n and cmm stands for cx

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SMn stands for SIP Message n and CMm stands for Cx message m which has a relation to the authentication process: SM1: REGISTER(IMPI*, IMPU) “IMPI*” in SM1 means that the inclusion of the IMPI is optional in SM1. NOTE 2a:When a registering UE omits the IMPI from the REGISTER request, the IMPI for the registration is derived from the registering IMPU. Since there can be only one registered instance of an IMPI at any point in time, the registering IMPU in this case cannot be shared across multiple UEs. In SM2 and SM3 the P-CSCF and the I-CSCF respectively forwards the SIP REGISTER towards the S-CSCF. If SM1 does not contain an IMPI, the P-CSCF shall behave according to Annex P.3 and forward the message as SM2 to the I- CSCF. NOTE 2b: Annex P.3 formulates conditions depending on the presence of an Authorization header. Note that, if SM1 does not contain an IMPI, then SM1 does not contain an Authorization header. 3GPP 3GPP TS 33.203 V12.67.0 (2014-0609) 76 Release 12
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The I-CSCF queries the HSS to find the address of the S-CSCF. If SM2 does not contain an IMPI the I-CSCF shall derive the IMPI from the IMPU in the REGISTER request as described in 3GPP TS 24.229 [8]. Then the I-CSCF forwards the message as SM3 to the S-CSCF. After receiving SM3, if the IMPU is not currently registered at the S-CSCF, the S-CSCF needs to set the registration flag at the HSS to initial registration pending. This is done in order to handle UE terminated calls while the initial registration is in progress and not successfully completed. The registration flag is stored in the HSS together with the S-CSCF name and user identity, and is used to indicate whether a particular IMPU of the user is unregistered or registered at a particular S-CSCF or if the initial registration at a particular S-CSCF is pending. The registration flag is set by the S-CSCF sending a Cx-Put to the HSS. If the IMPU is currently registered, the S-CSCF shall leave the registration flag set to registered. At this stage the HSS has performed a check that the IMPI and the IMPU belong to the same user. The S-CSCF shall determine the type of authentication based on the rules in Annex P. If SM3 does not contain an IMPI the S-CSCF shall derive the IMPI from the IMPU in the REGISTER request as described in 3GPP TS 24.229 [8]. If the IMS registration request is related to SIP Digest, then the procedures below apply. Upon receiving the SIP REGISTER the S-CSCF shall use a SIP Digest Authentication Vector (SD-AV) for authenticating the user. If the S-CSCF has no valid SD-AV for the specific IMPI, then the S-CSCF shall send a request for SD-AV(s) to the HSS in CM1 where the number m of SD-AVs wanted is equal to 1. CM1: Cx-AV-Req(IMPI, m) Upon receipt of a request from the S-CSCF, the HSS sends one SD-AV to the S-CSCF using CM2. The SD-AV consists of the qop (quality of protection) value, the authentication algorithm, realm, and a hash, called H(A1), of the IMPI, realm, and password. Refer to RFC 2617 [12] for additional information on the values in the authentication vector for SIP Digest based authentication.
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