Requirements for End-to-Middle Security for the Session Initiation Protocol (SIP)

A Session Initiation Protocol (SIP) User Agent (UA) does not always trust all intermediaries in its request path to inspect its message bodies and/or headers contained in its message. The UA might want to protect the message bodies and/or headers from intermediaries, except those that provide services based on its content. This situation requires a mechanism called "end-to-middle security" to secure the information passed between the UA and intermediaries, which does not interfere with end-to-end security. This document defines a set of requirements for a mechanism to achieve end-to-middle security.