7.3 Transport of NAS messages

36.3003GPPEvolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN)Overall descriptionRelease 17Stage 2TS

The AS provides reliable in-sequence delivery of NAS messages in a cell. During handover, message loss or duplication of NAS messages can occur.

In E-UTRAN, NAS messages are either concatenated with RRC messages or carried in RRC without concatenation. Upon arrival of concurrent NAS messages for the same UE requiring both concatenation with RRC for the high priority queue and also without concatenation for the lower priority queue, the messages are first queued as necessary to maintain in-sequence delivery.

In downlink, when an EPS bearer (EPC) or PDU Session (5GC) establishment or release procedure is triggered, or for EDT in case of Control Plane CIoT EPS optimisation or Control Plane CIoT 5GS Optimisation, the NAS message should normally be concatenated with the associated RRC message. When the EPS bearer (EPC) or PDU Session (5GC) is modified and when the modification also depends on a modification of the radio bearer, the NAS message and associated RRC message should normally be concatenated. Concatenation of DL NAS with RRC message is not allowed otherwise. In uplink, concatenation of NAS messages with RRC message is used only for transferring the initial NAS message during connection setup and for EDT in case of Control Plane CIoT EPS optimisation or Control Plane CIoT 5GS Optimisation. Initial Direct Transfer is not used in E-UTRAN and no NAS message is concatenated with RRC connection request.

Multiple NAS messages can be sent in a single downlink RRC message during EPS bearer (EPC) or PDU Session (5GC) establishment or modification. In this case, the order of the NAS messages in the RRC message shall be kept the same as that in the corresponding S1-AP (EPC) or NG-AP (5GC) message in order to ensure the in-sequence delivery of NAS messages.

NOTE: NAS messages are integrity protected and ciphered by PDCP, in addition to the integrity protection and ciphering performed by NAS.