This Iu timer is Osmocom specific, but is made to resemble T3314
timer from GERAN (also named READY timer).
The idea of this activity timer was to arm it whenever PMM state
transitions to CONNECTED, and then rearm it every time there's some
sort of activity, until there's none for some time, then we send a
Release Command to close the conn with the HNGBW/RNC. That's the
same principle as per spec-defined READY timer T3314.
However, there's still a fundamental problem with it: GTP-U in
GERAN passes through the SGSN, but in UTRAN, the GTP-U stream
goes directly from the HnodeB to the GGSN. Hence, there's no proper
way to re-arm this timer upon activity in UTRAN, basically because
the SGSN will never see (userplane data) activity. That explains why
the E_MM_PDU_RECEPTION event exists for mm_state_gb_fsm, but doesn't
exist for mm_state_iu_fsm.
As a result, the timer is currently never rearmed, which means it
will transition to IDLE always after 44 seconds (default value) once
it went into CONNECTED state.
In UTRAN, there is a SCCP connection for each subscriber between
RNC/hNB and SGSN. If the subscriber is no longer in the respective
state, the RNC/hNB should release that IuPS SCCP connection, whcih
in turn means the SGSN cleans up its state.
Furthermore, SCCP has a built-in IT (inactivity timer). So should
the RNC/hNB die, that timer would time out, and the SGSN-side local
SCCP stack (provider) wold send a RELEASE.ind for that connection
to the user (SGSN).
TLDR; this timer is not really needed and cannot be implemented
properly in UTRAN, so let's remove it.
Related: OS#5116
Change-Id: Ibc71829e417bf2dd0c27deb842369dd4f17010d6
Pau Espin
3caa7f6d97