The non-access stratum (NAS) described in the present document forms the highest stratum of the control plane between the UE and the AMF (reference point "N1" see TS 23.501) for both 3GPP and non-3GPP access. Main functions of the protocols that are part of the NAS are:

1. support of mobility of the user equipment (UE) including also common procedures such as authentication, identification, generic UE configuration update and security mode control procedures;
2. support of session management procedures to establish and maintain data connectivity between the UE and the data network; and
3. NAS transport procedure to provide a transport of SMS, LPP, SLPP, LCS, UPP-CMI container, UE policy container, SOR transparent container and UE parameters update information payload.

Principles for the handing of 5GS security contexts and for the activation of ciphering and integrity protection, when a NAS signalling connection is established, are provided in subclause 4.4. For the support of the above functions, the following procedures are supplied within this specification:

1. elementary procedures for 5GS mobility management in clause 5; and
2. elementary procedures for 5GS session management in clause 6.

Signalling procedures for the control of NAS security are described as part of the 5GMM common procedures in subclause 5.4. Complete NAS transactions consist of specific sequences of elementary procedures. Examples of such specific sequences can be found in TS 23.502. The NAS for 5GS follows the protocol architecture model for layer 3 as described in TS 24.007.

A 5GS session management (5GSM) message is piggybacked in specific 5GS mobility management (5GMM) transport messages. To this purpose, the 5GSM messages can be transmitted in an information element in the 5GMM transport messages. In this case, the UE, the AMF and the SMF execute the 5GMM procedure and the 5GSM procedure in parallel. The success of the 5GMM procedure is not dependent on the success of the piggybacked 5GSM procedure. The UE can only initiate the 5GSM procedure when there is a 5GMM context established at the UE. During 5GMM procedures, the UE and the AMF shall suspend the transmission of 5GSM messages, except when:

1. the 5GMM procedure is piggybacking 5GSM messages; or
2. the UE is in 5GMM-CONNECTED mode and a service request procedure for re-establishing user-plane resources of PDU session(s) is initiated without including PDU session status IE or Allowed PDU session status IE. In this case, the UE and the AMF need not suspend the transmission of 5GSM messages related to other PDU session(s) than the one(s) for which the user- plane resources re-establishment is requested.

If the UE determines to locally release the N1 NAS signalling connection upon receiving an SOR transparent container during a registration procedure as specified in TS 23.122 Annex C.2, the UE shall suspend the transmission of 5GSM messages after sending the REGISTRATION COMPLETE message and until the N1 NAS signalling connection is released to obtain service on a higher priority PLMN, with the exception of the case when the UE has an emergency PDU session. A 5GMM message piggybacking a 5GSM message for a PDU session shall be delivered via the access associated with the PDU session, if any, with the following exceptions:

1. the AMF shall send, via 3GPP access, a DL NAS TRANSPORT message piggybacking a downlink 5GSM message of a network-requested 5GSM procedure for a PDU session associated with non-3GPP access if the conditions specified in subclause 5.5.1.3.4 or subclause 5.6.1.4 are met;
2. the UE shall send an UL NAS TRANSPORT message piggybacking a response message to the 5GSM message described in a) via either:
   1. 3GPP access; or
   2. non-3GPP access if the UE is in 5GMM-CONNECTED mode over non-3GPP access; and
3. the UE shall send, via the target access, an UL NAS TRANSPORT message piggybacking a 5GSM message associated with a request type set to "existing PDU session" or "existing emergency PDU session" for handover of an existing PDU session between 3GPP access and non-3GPP access.

A 5GMM message piggybacking a 5GSM message as a response message to a request message associated with an MA PDU session, shall be delivered via the same access that the initial message was received.

The network operator may restrict subscriber's access to certain RAT. For this purpose, the network may send the RAT utilization control IE to the UE via the REGISTRATION ACCEPT message (see subclause 5.5.1.2). Additionally, the UE shall not consider any PLMN and RAT combination with RAT restriction as a candidate PLMN for PLMN selection purpose, as specified in TS 23.122.