What is the use of eNodeB direct transfer message and when it is used ?

Question

It may be an easy question but still I have some doubt.
How RRC Connection setup procedure gets impacted with the presence of "S-TMSI" as an UE-Identity within the RRC Connection Request message ?
Note: RRC Connection request message can carry either 5 bytes long (S-TMSI) or 5 bytes long random value.

Answer 1

UE sends Direct transfer message to eNB to deliver the NAS message to EPC Core - MME.

During the attach procedure:
1. MME sends (to eNB/UE) the NAS messages - Attach accept + Activate Default Bearer Request piggybacked with RRC Connection Reconfiguration message.
2. Now UE reply back with RRC Connection Reconfiguration Complete to eNB and UE also sends a DIRECT TRANSFER message to MME which contains Attach complete.

(Note: This direct transfer message is destined to MME and eNB simply forwards (direct transfer) this message to MME)