This document defines an extension to the URL MIME External-Body Access-Type to satisfy the content indirection requirements for the Session Initiation Protocol (SIP). These extensions are aimed at allowing any MIME part in a SIP message to be referred to indirectly via a URI.
RFC 4483-2006 history
2006RFC 4483-2006 A Mechanism for Content Indirection in Session Initiation Protocol (SIP) Messages