http://www.rfc-editor.org/rfc/rfc8123.txt
RFC 8123: Requirements for Marking SIP Messages to be Logged, P. Dawes, C. Arunachalam2017-03-01T23:00:00-00:00This document describes the requirements for adding an indicator to the SIP Protocol Data Unit (PDU) or a SIP message that marks the PDU as a candidate for logging. Such a marking will typically be applied as part of network testing controlled by the network operator and not used in regular client signaling. However, such a marking can be carried end-to-end, including the SIP terminals, even if a session originates and terminates in different networks.