1220 SIP is not a vertically integrated communications system.

 

SIP is rather a component that can be used with other IETF protocols to

   build a complete multimedia architecture.  Typically, these

   architectures will include protocols such as the Real-time Transport

   Protocol (RTP) (RFC 1889 [28]) for transporting real-time data and

   providing QoS feedback, the Real-Time streaming protocol (RTSP) (RFC

   2326 [29]) for controlling delivery of streaming media, the Media

   Gateway Control Protocol (MEGACO) (RFC 3015 [30]) for controlling

   gateways to the Public Switched Telephone Network (PSTN), and the

   Session Description Protocol (SDP) (RFC 2327 [1]) for describing

   multimedia sessions.  Therefore, SIP should be used in conjunction

   with other protocols in order to provide complete services to the

   users.  However, the basic functionality and operation of SIP does

   not depend on any of these protocols.

Max-Forwards serves to limit the number of hops a request can make on

   the way to its destination.  It consists of an integer that is

   decremented by one at each hop.

 

   Content-Type contains a description of the message body (not shown).

 

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

 

   The complete set of SIP header fields is defined in Section 20.