What is the use of "Re-establish UE-Identity" IE within the Re-establishment Request message?

Question

RRC Connection Request message carries "Initial UE identity" which is just 5 bytes long identifier. Why does so many information elements send within the "Re-establish UE-Identity" like C-RNTI, physcellId etc ?

Answer 1

RRC connection request message is sent by an UE when there is no radio connection with eNodeB it has.
But RRC Reestablishment request message is sent when existing radio connection goes down.
That's why RRC Connection Request just needs a single identifier as an initial identity i.e. either 5 bytes long random value or S-TMSI. But in case of Re-establishment where already existing connection broke down some other type of information required like C-RNTI, PhysCellId and shortMAC-I.

C-RNTI and PhysCellId is not sufficient to identify an UE. To make sure that Re-establishment Request message is received from a valid or genuine user, UE includes shortMAC-I IE within the RRC Connection Reestablishment Request message.

Comments

Thanks to share your thoughts. I have an additional query. Does the shortMAC-I within the RRC Re-establishment Request message, is integrity protected or just a plane information ?

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As RRC re-establishment request and rrc- re-establishment message goes over SRB0, and No Security applied to SRB0 messages, so the messages are never integrity protected.  

But to authenticate UE and the RRC-reestablishment request message UE sends the shortMAC-I is included in the message.

But after connection again established SRB1 is resumed and rrc-re-establishment complete will go through SRB1 and which will be integrity protected.