IETF RFC 8497 PDF

SIP networks use signaling monitoring tools to diagnose user-reported problems and to perform regression testing if network or user agent (UA) software is upgraded. As networks grow and become interconnected, including connection via transit networks, it becomes impractical to predict the path that SIP signaling will take between user agents and therefore impractical to monitor SIP signaling end to end.

This document describes an indicator for the SIP protocol that can be used to mark signaling as being of interest to logging. Such marking will typically be applied as part of network testing controlled by the network operator and is not used in normal user agent signaling. Operators of all networks on the signaling path can agree to carry such marking end to end, including the originating and terminating SIP user agents, even if a session originates and terminates in different networks.