114	SIP: Understanding the Session Initiation Protocol

Figure 5.1 Call queuing with a call processing center.

uses a 180 Ringing response instead, no media path will be established between the UAC and the gateway, and the caller will never hear a ring tone, busy tone, or recorded announcement (e.g., “The number you have dialed has changed, the new number is . . .”) since these are all heard in the media path prior to the call being answered. Figure 9.1 shows an example call flow with early media.

5.2 Success

Success class responses indicate that the request has succeeded or has been accepted.

5.2.1200 OK

The 200 OK response has two uses in SIP. When used to accept a session invitation, it will contain a message body containing the media properties of the UAS (called party). When used in response to other requests, it indicates a successful completion or receipt of the request. The response stops further retransmissions of the request. In response to an OPTIONS, the message body may contain the capabilities of the server. A message body may also be present in a response to a REGISTER request. For 200 OK responses to other methods, a message body is not permitted.