RRC and DL ,UL information transfer procedure in LTE

what is a rrc transcation identifier ? Why it is not present in uplink information transfer message which is a message, flows from UE to MME?
but rrc transcation identifier is present in downlink information transfer which flows from MME to enode B and then to UE
And downlink information transfer carries the NAS info or non 3gpp related information like CDMA related .