SR OS tracks each GTP-C peer for which it has at least a single GTP session or PDP context active. It tracks the peer’s operational state with the following mechanisms:
Regular GTP echo messages and parameters are configurable on a per-mgw-profile basis. When the echo mechanism fails, the peer is considered down.
Active route entries toward the peer are monitored. If no route toward the peer is available, the peer is considered down.
The Restart counter value of the peer is monitored. This is initially learned when the first active session or context is created. If the value is not available in regular messaging, an echo request is sent out immediately to learn the correct value. If the Restart counter is incremented during any later messaging exchange, the peer is considered rebooted.
When a peer is considered down or rebooted, all active GTP sessions and PDP contexts are forcefully removed.
SR OS also keeps a recovery counter in a persistent state, and increments this value on every reboot. This value is kept in the restcntr.txt file on CF3 and may not be modified or removed. This value is included in every control plane message.
SR OS responds to GTP echo messages for both active peers and unknown sources. This can be restricted using CPM filters if required. An incoming echo request from an unknown source does not create a peer; this can only be done by setting up GTP sessions or PDP contexts.