Why Security Should be needed to Initiate RRC Connection Re-establishment?

Question

Upon detecting Radio Link Failure UE will do RRC Connection re-establishment but to do RRC Connection re-establishment security should be activated in UE, otherwise UE will go Idle mode.So here(After detecting RLF) we are resetting all PHY,MAC,RLC,PDCP Configurations but why we are making security should be there to do RRC Connection re-establishment.

Answer 1

RRC Connection Reestablishment request can be sent by UE once the AS security is activated. After activating the security, if RLF occur or reconfiguration fails or handover fails, this procedure is triggered by UE to eNodeB. If security is activated then only eNodeB can checks for genuinity of UE. After getting information in rrc connection request message, eNodeB verifies against the inform it has. This way eNodeB does validation of UE.
Assume if security activation is not mandatory to trigger rrc re-establishment, then any UE can initiate rrc connection re-establishment for any other UE.

Comments

Thank you very much Harshitha

Answer 2

Hi reddy,

In detail after activating security algorithm both UE and ENb can able to calculate SHORT MAC-I (which is used to verify the UE at Enb side ). In re establishment procedure re establishment message is sent through SRB0 , This is not security protected so Enb will not verify the message in this channel, in order to validate the UE ENb will check the Short MAC-I which is sent as part of re establishment message.