Section: 13.3 Private User Identity
Content: The private user identity shall take the form of an NAI, and shall have the form username@realm as specified in clause 2.1 of IETF RFC 4282 [53]. NOTE 1: It is possible for a representation of the IMSI to be contained within the NAI for the private identity. For 3GPP systems, the private user identity used for the user shall be as specified in clause 4.2 of 3GPP TS 24.229[ IP multimedia call control protocol based on Session Initiation Protocol (SIP) and Session Description Protocol (SDP); Stage 3 ]  [81] and in 3GPP TS 23.228[ IP Multimedia Subsystem (IMS); Stage 2 ]  [24] Annex E.3.1. If the private user identity is not known, the private user identity shall be derived from the IMSI. The following steps show how to build the private user identity out of the IMSI: 1. Use the whole string of digits as the username part of the private user identity; and 2. convert the leading digits of the IMSI, i.e. MNC and MCC, into a domain name, as described in clause 13.2. The result will be a private user identity of the form "<IMSI>@ims.mnc<MNC>.mcc<MCC>.3gppnetwork.org". For example: If the IMSI is 234150999999999 (MCC = 234, MNC = 15), the private user identity then takes the form "234150999999999@ims.mnc015.mcc234.3gppnetwork.org".. The private user identity for a Stand-alone Non-Public Network (SNPN) subscriber with an IMSI-based SUPI type, shall use the IMSI-based derivation described above, and append nid<NID> of the SNPN, between the "ims." and "mnc<MNC>" labels. NOTE 2: The UE takes the NID from "list of subscriber data" as specified in 3GPP TS 23.122[ Non-Access-Stratum (NAS) functions related to Mobile Station (MS) in idle mode ]  [139], from the entry selected by the UE. For an SNPN, if IMSI is as above, and for a NID of 000007ed9d5, the private user identity takes the form "234150999999999@ims.nid000007ed9d5.mnc015.mcc234.3gppnetwork.org". The Private User Identity for a Stand-alone Non-Public Network (SNPN) subscriber identified by a SUPI containing a network-specific identifier that takes the form of an NAI is obtained as follows: the username is the same as the username in the NAI, and the realm portion is identical with the Home domain defined in clause 13.2. For 3GPP2 systems, if there is no IMC present, the UE shall derive the private user identity as described in Annex C of 3GPP2 X.S0013-004 [67].