Section: 5.5.6 Attach request message (for N1 mode only)
Content: The attach procedure is used to construct an ATTACH REQUEST message for a UE performing a registration procedure for initial registration when a) the UE: 1) previously was registered in S1 mode before entering state EMM-DEREGISTERED; and 2) has received an "interworking without N26 interface not supported" indication from the network; and b) EPS security context and a valid 4G-GUTI are available. The ATTACH REQUEST message is created by EMM by request of 5GMM which further includes the message in the REGISTRATION REQUEST message as described in 3GPP TS 24.501[ Non-Access-Stratum (NAS) protocol for 5G System (5GS); Stage 3 ]  [54]. The ATTACH REQUEST message shall contain only mandatory information elements. The UE shall set the EPS attach type IE in the ATTACH REQUEST message to "EPS attach". The UE shall include the eKSI (KSIASME) in the NAS Key Set Identifier IE in the ATTACH REQUEST message. The UE shall integrity protect the ATTACH REQUEST message with the current EPS security context and increase the uplink NAS COUNT by one. The UE shall set associated GUTI in the EPS mobile identity IE. The UE shall set the UE network capability IE according to its capabilities. The UE shall include an ESM DUMMY MESSAGE in the ESM message container IE.