2.2 Message Syntax

This protocol relies on the SIP message format, as specified in [RFC3261] section 25.1, and extends definitions of authentication-related header fields, specifically Authentication-Info, Authorization, Proxy-Authenticate, Proxy-Authorization, and WWW-Authenticate, as well as the Referred-By header field, which is specified in [RFC3892]. This protocol defines two new header fields, Proxy-Authentication-Info and p-session-on-behalf-of, and makes use of endpoint identification extensions that are specified in [MS-SIPRE].

All of the message syntax specified in this protocol is described in both prose and an Augmented Backus-Naur Form (ABNF) defined in [RFC5234].