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)