(per RFC3261)

Media type name: message
Media subtype name: sip
Required parameters: none

Optional parameters: version
   version: The SIP-Version number of the enclosed message (e.g.,
   "2.0").  If not present, the version defaults to "2.0".
Encoding scheme: SIP messages consist of an 8-bit header
   optionally followed by a binary MIME data object.  As such, SIP
   messages must be treated as binary.  Under normal circumstances
   SIP messages are transported over binary-capable transports, no
   special encodings are needed.

Security considerations: see below
   Motivation and examples of this usage as a security mechanism
   in concert with S/MIME are given in 23.4.