Hi sripathi,
Actually that is needed because , In initial attach UE wont be having any C_RNTI so this will not hit. Take an example RRC re-establishment , Enb will not know UE is in out of sync, so he will be scheduling data normally, but when UE is in out of sync then UE will do rach to get synch then he will send re establishment context to ENB , then ENB will stop scheduling data and re establish UE context. so in the meanwhile of Re-establishing UE is not needed to decode any C_RNTI or SPS-RNTI because how ever after re-establishing UE will get those missed PDU back by sending missing SN through PDCP. If UE decodes those PDU it's of no use because UE is re-establishing its context but the data is processed by ENB with old context (like in PDCP security and other stuff, In RLC AM mode, MAC TB with UE channel condition), So these things has to be changed as per current UE condition.
Even in scenario like TA timer expires then UE will do rach to get timing advance at that time UE should not process any UL data until TA comes from ENB.
