scope:

A typical function of a Session Initiation Protocol (SIP) Proxy is to insert a Record-Route header into initial, dialog-creating requests in order to make subsequent, in-dialog requests pass through it. This header contains a SIP Uniform Resource Identifier (URI) or SIPS (secure SIP) URI indicating where and how the subsequent requests should be sent to reach the proxy. These SIP or SIPS URIs can contain IPv4 or IPv6 addresses and URI parameters that could influence the routing such as the transport parameter (for example, transport=tcp), or a compression indication like "comp=sigcomp". When a proxy has to change some of those parameters between its incoming and outgoing interfaces (multi-homed proxies, transport protocol switching, or IPv4 to IPv6 scenarios, etc.), the question arises on what should be put in Record-Route header(s). It is not possible to make one header have the characteristics of both interfaces at the same time. This document aims to clarify these scenarios and fix bugs already identified on this topic; it formally recommends the use of the double Record-Route technique as an alternative to the current RFC 3261 text, which describes only a Record-Route rewriting solution.