[RFC] rework location update request

[Alexander Couzens]

Hi,

recently my modem with an already known SIM got a location update
reject, because the procedure timed out. So I looked more
into the location updates.

How openbsc location update works:

MS                   Network
LOC REQ ->
         <- Identity Req IMSI
Identity Resp IMSI ->
         <- Identity Req IMEI
Identity Resp IMEI ->
         <- LOC ACCEPT

OpenBSC is using a Timeout (5 sec) for the completion of location
update.

So let's say we trigger for unknown reason the timeout (e.g. RF
problem).

MS                   Network
LOC REQ ->
         <- Identity Req IMSI
(No response)
         <- LOC Reject! cause 13 / No roaming allowed.

If the MS receive it, it will put our network on it's blacklist and
won't try it again until power cycled.

We could change reject cause to something different [1], but
we might want to reject foreign SIMs not to try our network again.
The problem here is that openBSC use the same configuration to reject
unknown SIMs as well for timeouts.

But I think this timeout should be removed. 04.08 doesn't
describe a timeout on location update request on network side. But it
describes other timeouts:
04.08 describes:
 - T3260 auth req. sent, wait until response (12 sec)
 - T3270 identity req. sent, wait until response (12 sec)


I think we should implement both timers and drop the RR connection if
it fires.

I'm unsure if it's needed: But OpenBSC could count un-successful
location update req. Reset counter if it's succeeded. After a certain
amount of failures we send a location update reject. (unsure which
cause)

best,
lynxis

Related Specs:
 GSM 04.08 [1] Chapter 4.4 describing location update
 GSM 04.08 [1] Chapter 11.2 Timers
 GSM 04.08 [1] Chapter Annex G Reject Causes

[0] nitb.cfg: network: location updating reject cause
[1] ETSI TS 100 940
-- 
Alexander Couzens

mail: lynxis at fe80.eu
jabber: lynxis at fe80.eu
mobile: +4915123277221
gpg: 390D CF78 8BF9 AA50 4F8F  F1E2 C29E 9DA6 A0DF 8604
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