http://www.rfc-editor.org/rfc/rfc3420.txt
RFC 3420: Internet Media Type message/sipfrag, R. Sparks2002-11-01T23:00:00-00:00This document registers the message/sipfrag Multipurpose Internet Mail Extensions (MIME) media type. This type is similar to message/sip, but allows certain subsets of well formed Session Initiation Protocol (SIP) messages to be represented instead of requiring a complete SIP message. In addition to end-to-end security uses, message/sipfrag is used with the REFER method to convey information about the status of a referenced request. [STANDARDS-TRACK]