Re-synchronization Failure

Re-Synchronization mechanism is a rare.It happens when difference of sequence number at HSS and UE/SIM is not in valid range. Before we go in detail of it, it would be better we just scroll through 3G Authentication.


Here we shall explain Re-synchronization with example. As we know Sequence Number is of 48 bits
SQN=(SEQ (43-Bits)+ IND(3-Bits))

Ideally HSS should have high sequence number(SEQ) at any (IND)index. Lets assume if sequence number difference is equal to and more than 2, then it is needed to synchronize again with HSS. This range is called DELTA range that is defined by SIM vendor. 

During re-synchronization process UE/SIM send it's own sequence number to HSS, HSS updates it own end, generates fresh Authentication Vectors (AV) on updated Sequence Number, then to UE to authenticate Identity and Network again.

Re-synchronization is a very useful process by which we can Sync sequence number at HSS with SIM because we can not update sequence number at SIM because it is stored in non-volatile memory. Ideally both HSS and SIM should have Identical Sequence Number for an Index, but it is not always true because of following causes that could create Re-Synchronization.

1) As we know controlling entity (MME/SGSN) can ask for multiple Authentication Vectors. There are chances that some vectors are not used and got staled, this will increase the sequence number for some IND (Index) at HSS but not at SIM shall result in Re-Synchronization if difference reaches to 2. I.e. Sequence Number for a Index at HSS very high.

2) Sequence Number at SIM/UE for a Index is Higher than the HSS. Only possible to simulate in LAB network. 



Your Comments /Suggestions and Questions are always welcome,  shall clarify with best of our knowledge. So feel free to put Questions.

2 comments:

  1. Can you please let me know like how to simulate the above scenario in the lab? any views? thanks.

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    Replies
    1. Hi,
      We have no idea of your lab setup, Still here are some of the ways you can simulate

      1) Alter HSS database.
      If you have access to HSS database then first do normal attach from real UE then set Authentication Sequence number to Zero (By directly into Database or Delete Subscriber make sure CLR in not triggered and then create it again)then attach again from real UE(Phone). Then UE shall trigger Re-Sync Request as Sequence Number is miss-Matched.

      2) With 2 Real HSS in LAB
      Create subscriber in Both HSS, Then Attach with Real-UE to any of the HSS make it successful. Now Re-attach again with same Real-UE now make sure that request shall go to other HSS(i.e. Not used yet). then UE shall trigger Re-sync request as Sequence Number is miss-Matched.

      3)Resetting Sequence Number at SIM using Simulator.

      Thanks for your query.

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