Content-Type contains a description of the message body.

   Content-Length contains an octet (byte) count of the message body.

   The details of the session, such as the type of media, codec, or
   sampling rate, are not described using SIP.  Rather, the body of a
   SIP message contains a description of the session, encoded in some
   other protocol format.  One such format is the Session Description
   Protocol (SDP) (RFC 2327 [1]).  This SDP message (not shown in the
   example) is carried by the SIP message in a way that is analogous to
   a document attachment being carried by an email message, or a web
   page being carried in an HTTP message.