Pn=555-1111 100 Trying 180 Ringing One way voice path No media path established Telephone switch plays recorded annoucement "The number you have dialed has changed. The new number is 555-2222." 502 Bad gateway fail?" Figure 5.2 PSTN interworking without early media. Caller Gateway Telephone switch INVITE sip:555-2222@gateway.com;user=phone sdp Caller IAM CdPn=555-2222 100 Trying 183 Session Progress sdp Gateway Media path One way voice path Caller hears Telephone switch ring tone in RTP plays ring tone 200 OK sdp Gateway Media path Two way voice path Figure 5.3 PSTN interworking with early media. SIP: Understanding the Session Initiation Protocol 5.2 Success Success class responses indicate that the request has succeeded or has been accepted. 5.2.1 200 OK The 200 OK response has two uses in SIP. When used to accept a session invi- tation, it will contain a message body containing the media properties of the UAS (called party). When used in response to other requests, it indicates suc- cessful completion or receipt of the request. The response stops further retrans- missions 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 CANCEL, INFO, MESSAGE, SUBSCRIBE, NOTIFY, and PRACK, a message body is not permitted. 5.2.2 202 Accepted The 202 Accepted response [3] indicates that the UAS has received and understood the request, but that the request may not have been authorized or processed by the server. It is commonly used in responses to SUBSCRIBE (see Section 4.1.8) and REFER (see Section 4.1.7), and sometimes MESSAGE (see Section 4.1.10) methods. 5.3 Redirection Redirection class responses are generally sent by a SIP server acting as a redi- rect server in response to an INVITE, as described in Section 3.3.2. A UAS, however, can also send a redirection class response to implement certain types of call forwarding features. There is no requirement that a UAC receiving a red