Currently old LLMEs and MM contexts that haven't been explicitly
detached or cancelled are not removed until another request with the
same IMSI is made. These stale entries may accumulate over time and
severely compromise the operation of the SGSN.
This patch implements age based LLME expiry, when the maximum age has
been reached, the corresponding MM context is cancelled. If such an MM
context doesn't exist, the LLME is unassigned directly.
The implementation works as follows.
- llme->age_timestamp is reset on each received PTP LLC message
- sgsn_llme_check_cb is invoked periodically (each 30s)
- sgsn_llme_check_cb sets the age_timestamp to the current time if
it has been reset
- sgsn_llme_check_cb computes the age and expires the LLME if
it exceeds gprs_max_time_to_idle()
Ticket: OW#1364
Sponsored-by: On-Waves ehf
[hfreyther: Fix typo in comment LMME -> LLME]
81ffb740f7