Question

In LTE Gx (Diameter) Interface how would transactions like Re-Auth Req/Ans mapped to IMSI? these transactions don't have Subscription ID and User-Name APN and same thing for other Diameter Interfaces transactions that are missing Subscription ID and User-Name APN.
If you can give me an insight I will greatly appreciated it!

Answer 1

Gx is a Diameter based interface where Session-Id AVP is used to map UE signalling.
If you closely observe CCR (Credit-Control-Request) message which goes from PCEF/PGW to PCRF during attach request, PGW includes Subscription-Id AVP (IMSI) in CCR message.
This message onward, PCRF maintains mapping between session id and IMSI. If PCRF initiates RAR towards PCEF, it just needs to add sessionid only.

I am not sure but logically it means per default bearer one CCR is triggered by PGW/PCEF to PCRF.
Please confirm to me if someone has knowledge on this.

Comments

I tried that approach I didn't see any common Session IDs between CCR and RAR, afaik Session IDs are unique per transaction.

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Hi Lava,
Even I have the same understanding what Harshita has in this case. I have Gx interface wireshark logs in which CCR and RAR both messages use the same value for Session-Id AVP.

For the time being if I agree with you that Session-Id AVP is not used to identify the UE then on which parameter basis , UE will be identified at the receiving side. Do u  know any other AVP that carries UE information in RAR ?

If you want I can share the wireshark logs with you for reference.

Thanks,
Vimal

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hi Vimal,
Thank you for your input, I think it depends on the vendor, in my case I tried filtering for session ID and I only got 1 CCR transactions or RAR transaction [depend on the session ID I filtered for], as per Identifying UE I know User-Name, and Subscription ID AVPs are used for that, but when these are not present in any Diameter Transactions then not sure how DB is recognizing the UE, would like some insight on that.

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By exchanging CCR/CCA a session is created between PCEF and PCRF for an UE. In case of any bearer establishment or termination, CCR-Type "UPDATE_REQUEST" is used. In case of session establishment CCR-Type would be "INITIAL_REQUEST" and for termination it would be "TERMINATION_REQUEST". If CCR-Type is assigned to "EVENT_REQUEST" then session would be not maintained.

Logically if a session is maintained between two nodes then using other AVPs like User-Name, Subscription Id to identify an UE is not efficient implementation.