RRC connection procedure is a part of UEs core network attach procedure. RRC connection procedure is required to make radio connection between UE and EnodeB. Either UE is just switched on or UE wants to change state from RRC-IDLE to RRC-Connected, "RRC connection setup procedure need to be executed". In RRC Connection setup procedure. UE and EnodeB establish SRB1 (Dedicated signalling radio bearer) which creates a path for UE to send initial NAS messages (like attach request/service request etc) to MME node of core network.
There are three main messages exchanged while executing rrc connection setup procedure.
1. RRC Conneciton Request ( UE ----> eNodeB).
>> carry initial UE identifier (S-TMSI or 5 bytes longs random value)
2. RRC Connection Setup (eNodeB -----> UE).
>> carry radio resource config dedicated with SRB1 configuraitons.
3. RRC Connection setup complete (UE-------> eNodeB)
>> carry initial NAS message and selected plmnId - identity (one out of 6).
For more details:
Please go through 3GPP 36.331 specification.