2010/3/16 Iñaki Baz Castillo ibc@aliax.net:
http://tools.ietf.org/html/rfc3261#section-20.38
RFC 3261 - 20.38 Timestamp
The Timestamp header field describes when the UAC sent the request to the UAS.
See Section 8.2.6 for details on how to generate a response to a request that contains the header field. Although there is no normative behavior defined here that makes use of the header, it allows for extensions or SIP applications to obtain RTT estimates.
8.2.6.1 Sending a Provisional Response [...] When a 100 (Trying) response is generated, any Timestamp header field present in the request MUST be copied into this 100 (Trying) response. If there is a delay in generating the response, the UAS SHOULD add a delay value into the Timestamp value in the response. This value MUST contain the difference between the time of sending of the response and receipt of the request, measured in seconds.