LTE: Why MME does replay of UE Security Capabilities in the Security mode command message ?

Question

In attach request, UE sends UE network capability which contains the list of security algorithms can be used to enable security between UE and network node. My question is, when UE already knows its supported algorithms then why MME resend the same list of security algorithm to UE in security mode command ? What would be the reasoning behind it ?

Answer 1

UE will share list of security algorithms which were supported by UE in UE Network capability IE. then MME can select some security algorithms based on MME policies which were received in UE Network capability IE and send in security mode command. means, MME may not support all of the security algorithms which were received in UE Network capability.

Comments

Thanks to share your answer but I still did not get answer completely.

MME receives UE supported security algorithms in the "UE Network Capability IE" and  MME selects one encryption and one integrity algorithm from that list (considering at least one common encryption and integrity algorithm is supported at MME).
MME informs UE about the selected algorithm in Security Mode Command. Since UE already knows about its supported security algorithm then why UE security algorithm list is replayed to UE again in Security mode command message.

Is there any possibility list of supported security algorithms  may get changed at UE dynamically ?

Answer 2

i want security mode failure cases 1)security capabilities mismatch.
2)security mode rejected,unspecified.
,,,after failure what happening?