The number of unexpected GTP signalling messages received.
Trigger Condition: This counter is updated whenever an
unexpected Signalling message is received.
Condition includes,
a) A Response message for which there is no corresponding
outstanding Request.
b) A GTP control plane message a GSN is not expected to
handle (such as a PDU Notification Request received by
a GGSN).
c) Charging messages in GTP path and vice-versa.
d) LFN bit not being set when CG path uses short-header
and vice-versa.