Why Identity Request Need to be perform by MME after successful Authentication and Security Mode Complete...?

Question

I saw a scenario where an Identity is requested by MME after successful Authentication and Security mode complete.. As a Response to this message UE is Sending IMEI? what is the need of checking IMEI number after successful authentication? If IMEI is in block list then ongoing Attach procedure is going to be Terminate...? Identity Request intention is mainly to check the IMEI belongs to any one of Black, while or Green list then why cannt MME check for this before Security Mode?

Comments

Is it defined somewhere in specifications or not ?

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I saw this behavior while analyzing a log file

Answer 1

In Attach request, UE sends GUTI information. MME/HSS stores GUTI to IMSI mapping.

When UE is attaching, if MME does not have mapping to GUTI received from UE, it will request for identity from UE .ie. it will send out Identity request message and UE will respond with IMSI if identity type requested is 1

If identity type requested is 2 then UE will respond with IMEI. I am not sure in what case IMEI is requested.

In case MME has mapping, you will not see identity request going out from MME

Why this is done after security mode procedure is because if IMSI is sent in attach request it is prone to interception because message is not encrypted and this is the reason GUTI is sent by UE

Spec:
Reference: 3GPP TS 24.301

Comments

With IMEI you can identify Ue type and therefore perform analysys of thr traffic, failures in terms of UE type.