5.3.3 Temporary identities

A temporary user identity for 5GS-based services, the 5G globally unique temporary identity (5G-GUTI), is used for identification within the signalling procedures. In case of PLMN the 5G-GUTI is globally unique and in case of SNPN the 5G-GUTI is unique within an SNPN. When the UE is registered to the same PLMN or SNPN over 3GPP and non-3GPP access, the UE and the AMF maintain one 5G-GUTI that is common to both 3GPP and non-3GPP access. When the UE is required to delete the 5G-GUTI according to a NAS procedure, the UE shall delete the 5G-GUTI only if it is not registered to the same PLMN or SNPN through other access. When the UE is registered to different PLMNs or SNPNs over 3GPP access and non-3GPP access, the UE maintains two 5G-GUTIs, a 5G-GUTI for the registration with a PLMN or SNPN over the 3GPP access and another 5G-GUTI for the registration with another PLMN or SNPN over the non-3GPP access. In the paging and service request procedures, a shortened form of the 5G-GUTI, the 5G S-temporary mobile subscriber identity (5G-S-TMSI), is used to enable more efficient radio signalling. The purpose of the 5G-GUTI and 5G-S-TMSI is to provide identity confidentiality, i.e. to protect a user from being identified and located by an intruder. The structure of the 5G-GUTI and its derivatives are specified in 3GPP TS 23.003 [4]. The 5G-GUTI has two main components (see 3GPP TS 23.501 [8]):

• the GUAMI; and
• the 5G-TMSI that provides an unambiguous identity of the UE within the AMF(s) identified by the GUAMI.

The 5G-S-TMSI has three main components:

• the AMF set ID that uniquely identifies the AMF set within the AMF region;
• the AMF pointer that identifies one or more AMFs within the AMF set; and
• the 5G-TMSI.

A UE supporting N1 mode includes a valid 5G-GUTI, if any is available, in the REGISTRATION REQUEST and DEREGISTRATION REQUEST messages. In the SERVICE REQUEST message, the UE includes a valid 5G-S-TMSI as user identity. The AMF shall assign a new 5G-GUTI for a particular UE:

• during a successful initial registration procedure;
• during a successful registration procedure for mobility registration update;
• after a successful service request procedure invoked as a response to a paging request from the network and before the:
• release of the N1 NAS signalling connection; or
• suspension of the N1 NAS signalling connection due to user plane CIoT 5GS optimization i.e. before the UE and the AMF enter 5GMM-IDLE mode with suspend indication;

as specified in clause 5.4.4.1; and

• after the AMF receives an indication from the lower layers that it has received the NGAP UE context resume request message or NGAP path switch request message as specified in 3GPP TS 38.413 [31] for a UE in 5GMM-IDLE mode with suspend indication and this resumption is a response to a paging request from the network, and before the:
• release of the N1 NAS signalling connection; or
• suspension of the N1 NAS signalling connection due to user plane CIoT 5GS optimization i.e. before the UE and the AMF enter 5GMM-IDLE mode with suspend indication;

as specified in clause 5.4.4.1.

The AMF should assign a new 5G-GUTI for a particular UE during a successful registration procedure for periodic registration update. The AMF may assign a new 5G-GUTI at any time for a particular UE by performing the generic UE configuration update procedure.

If a new 5G-GUTI is assigned by the AMF, the UE and the AMF handle the 5G-GUTI as follows:

• Upon receipt of a 5GMM message containing a new 5G-GUTI, the UE considers the new 5G-GUTI as valid and the old 5G-GUTI as invalid, stops timer T3519 if running, and deletes any stored SUCI. The new 5G-GUTI is stored in a non-volatile memory in the USIM if the corresponding file is present in the USIM, else in the non-volatile memory in the ME, as described in annex C.
• The AMF considers the old 5G-GUTI as invalid as soon as an acknowledgement for a registration or generic UE configuration update procedure is received.