Iu: flimsy TMSI

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Neels Hofmeyr nhofmeyr at sysmocom.de
Thu Mar 17 01:05:32 UTC 2016


Hi Daniel and 3G folks,

about our difficulty in maintaining a prolonged link with the UE, I found
a bit of discrepancy concerning the TMSIs used.

When looking at a pcap where we authenticate and MM LU / GMM Attach
successfully on CS / PS, and with TMSI assignment enabled in CSCN, I see
TMSIs in these places:

1) UE sends TMSI in "(GMM) Attach Request"     (PS)
2) SGSN sends TMSI in "(GMM) Attach Accept"    (PS)
3) CSCN sends TMSI in "Location Update Accept" (CS)

All these TMSIs differ.

When I look in the RRC log of the hnb, it is logging a TMSI for pretty
much every received message on CS and PS domains. But it is always and
only logging the TMSI matching 1), i.e. the TMSI originally sent on the PS
domain. To rephrase, the same TMSI is logged for PS and CS domain
messages; for the MM LU Accept as well as the GMM Attach Accept, the hnb
logs TMSI 1) even though the CN sent TMSIs 3) and 2), respectively.

For 1) and 2), it seems that the SGSN should accept and use the TMSI sent
by the UE during the Attach Request, right?

For 3), according to the RRC log, it seems that the UE expects to see the
very same TMSI in CS that it sent to PS -- that'd be a problem. On the
other hand, when noting that, in the Location Updating Request, the UE
identified itself using the IMSI and not a TMSI, it may be indeed the
correct choice to disable TMSI use for CS?

I must admit that I'm not firm in the IMSI/TMSI realm, and staring at 3GPP
04.08 doesn't clarify it right away, so I'd appreciate some feedback from
this list. I can stare some more, but maybe you guys are quicker.

Thanks!

~Neels

-- 
- Neels Hofmeyr <nhofmeyr at sysmocom.de>          http://www.sysmocom.de/
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