VLR / 3G: periodic LU expiry by T3212
pre-VLR code had T3212 subscriber expiry if it missed a periodic LU.
libvlr has stubs ready, but it is not yet enforced nor fully implemented.
Add T3212 expiry with an msc_vlr_test_periodic_lu unit test (using fake time).
Some pointers and related things to balance out in a nice (TM) implementation:
- We so far don't, but we should have a mechanism to cache unused auth tuples for a subscriber even past IMSI Detach.
(An idea to optimise lookups and iterate less subscribers for <insert action here> would be to keep inactive subscribers for which we merely cache auth tuples in a separate llist.)
- TS 23.012 2.4.2 "Implicit IMSI detach" describes periodic location updating timer
- TS 23.012 3.7.3 Extended periodic LAU Signalling (the possibility to send extended timers during Location Updating Accept)
- TS 23.012 18.104.22.168 "Process Detach_IMSI_VLR"
- Are we required to notify the HLR of an IMSI Detach? (not sure, so far we don't)