Thanks Neels, that was informative On Tuesday, 22 May 2018, Neels Hofmeyr <[email protected]> wrote:
> On Sun, May 20, 2018 at 05:14:41AM -0400, loay abdelrazek wrote: > > Hi Harald > > > > Yes yes I totally understand , so what could be the cases for sending the > > IMSI and what would be the possible remediation for such thing > > It sounds like you actually don't "totally" understand and are asking for > a 101 > on IMSI and TMSI? > > The subscriber is identified to the network by its IMSI, which stays > constant. > > A new TMSI is given on every Location Update, meaning it changes all the > time, > which is good for privacy. The network does its best to remember the last > TMSI > the phone has been given. > > But before the network can know who that subscriber is, things have to > start > out with the IMSI. That's normally when the phone asks for a Location > Update > for the first time. Usually the new TMSI is sent to the phone ciphered, so > that > the IMSI <-> TMSI mapping is essentially secret. > > A TMSI is short-lived and an MSC may decide not to store IMSI<->TMSI > mappings > forever, for various/unknown reasons. So as soon as there is no TMSI left > in > the MSC's subscriber state, the only way to page is by IMSI. > > Also, the phone may have forgotten its TMSI, e.g. after roaming to a > different > network. > > But: AFAICT, under "proper" operations, a subscriber should have a TMSI > after a > Location Update, and either detach when it leaves or be implicitly > detached by > timeout. So as long as it is Page-able by the MSC, it should have a TMSI as > well. In other words, the MSC should either have a TMSI to page for, or the > subscriber's state should be "not attached", i.e. can't page anyway. Bottom > line, whenever paging goes out by IMSI, the MSC has actively decided to not > bother about privacy (by implementation, config or by licensing details). > > IIRC, the only way to get an IMSI paging out of osmo-msc is to globally > switch > off TMSI allocation. > > ~N >
