hi, acc. to TS36.331, we know that "DL messages on CCCH that move UE to RRC-Idle should not include the RRC transaction identifier"
So, question is how does the UE knows that the message(connection reject) is meant for it.
UE receives RRC connection reject message along with contention resolution ID in MAC control element. Contention resolution ID is echo back of UE’s RRC connection request message. With contention resolution ID, UE can identify RRC connection reject message.