SIP is based on an HTTP-like request/response transaction model.
   Each transaction consists of a request that invokes a particular
   method, or function, on the server and at least one response.  In
   this example, the transaction begins with Alice's softphone sending
   an INVITE request addressed to Bob's SIP URI.  INVITE is an example
   of a SIP method that specifies the action that the requestor (Alice)
   wants the server (Bob) to take.  The INVITE request contains a number
   of header fields.  Header fields are named attributes that provide
   additional information about a message.  The ones present in an
   INVITE include a unique identifier for the call, the destination
   address, Alice's address, and information about the type of session
   that Alice wishes to establish with Bob.  The INVITE (message F1 in
   Figure 1) might look like this:

                     atlanta.com  . . . biloxi.com
                 .      proxy              proxy     .
               .                                       .
       Alice's  . . . . . . . . . . . . . . . . . . . .  Bob's
      softphone                                        SIP Phone
         |                |                |                |
         |    INVITE F1   |                |                |
         |--------------->|    INVITE F2   |                |
         |  100 Trying F3 |--------------->|    INVITE F4   |
         |<---------------|  100 Trying F5 |--------------->|
         |                |<-------------- | 180 Ringing F6 |
         |                | 180 Ringing F7 |<---------------|
         | 180 Ringing F8 |<---------------|     200 OK F9  |
         |<---------------|    200 OK F10  |<---------------|
         |    200 OK F11  |<---------------|                |
         |<---------------|                |                |
         |                       ACK F12                    |
         |------------------------------------------------->|
         |                   Media Session                  |
         |<================================================>|
         |                       BYE F13                    |
         |<-------------------------------------------------|
         |                     200 OK F14                   |
         |------------------------------------------------->|
         |                                                  |

         Figure 1: SIP session setup example with SIP trapezoid

      INVITE sip:bob@biloxi.com SIP/2.0
      Via: SIP/2.0/UDP pc33.atlanta.com;branch=z9hG4bK776asdhds
      Max-Forwards: 70
      To: Bob <sip:bob@biloxi.com>
      From: Alice <sip:alice@atlanta.com>;tag=1928301774
      Call-ID: a84b4c76e66710@pc33.atlanta.com
      CSeq: 314159 INVITE
      Contact: <sip:alice@pc33.atlanta.com>
      Content-Type: application/sdp
      Content-Length: 142

      (Alice's SDP not shown)

   The first line of the text-encoded message contains the method name
   (INVITE).  The lines that follow are a list of header fields.  This
   example contains a minimum required set. (...)