http://www.rfc-editor.org/rfc/rfc4483.txt
RFC 4483: A Mechanism for Content Indirection in Session Initiation Protocol (SIP) Messages, E. Burger2006-05-01T23:00:00-00:00This 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. [STANDARDS-TRACK]