name: NAS-MAC
full_form: Message Authentication Code for NAS for Integrity (called MAC in TS24.301 [9])
context: and the target Cell-ID as the message to be protected to calculate NAS-MAC (see Annex B.2.1). The uplink NAS COUNT is increased by the UE in exactly the same way as if it had sent a NAS message. The first 16 bits of NAS-MAC form UL_NAS_MAC and the last 16 bits form XDL_NAS_MAC, which is stored by the UE. The UE shall send the RRCConntectionRestablishmentRequest message to the target eNB and shall include S-TMSI, the 5 least significant bits (LSB) of the NAS COUNT that was used to calculate NAS-MAC and UL_NAS_MAC in the message. The target eNB recognises the RRCConntectionRestablishmentRequest message sent by a UE relates to the Control Plane CIoT EPS optimisation based on the presence of the S-TMSI in the message. The target eNB shall send the S-TMSI, LSB of NAS COUNT, UL_NAS_MAC and target Cell-ID in the eNB CP Relocation Indication message to the MME that is serving the UE (this can be deteremined by the S-TMSI). The MME uses LSB of NAS COUNT to estimate the full uplink NAS COUNT
description: NAS-MAC is a Message Authentication Code used for integrity protection in NAS messages. It's specifically mentioned in the context of RRCConnectionRe-establishment Procedure for Control Plane CIoT EPS optimisation.  NAS-MAC involves calculating a MAC using the target Cell-ID as the message to be protected and is further divided into UL_NAS_MAC (Uplink) and DL_NAS_MAC (Downlink).
explanation: The text explicitly states that NAS-MAC is a 'Message Authentication Code' used for 'integrity protection in NAS messages'. This directly classifies it as a Message Authentication Code.
section_title: 7.4.4	RRCConnectionRe-establishment Procedure for Control Plane CIoT EPS optimisation
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