nses in this session or dialog will contain both the tag generated by Tesla and the tag generated by Marconi. The response also contains a Contact header field, which contains an address at which Marconi can be contacted directly once the session is established. When the called party Marconi decides to accept the call (i.e., the phone is answered), a 200 OK response is sent. This response also indicates that the type of media session proposed by the caller is acceptable. The 200 OK is an exam- ple of a "success class" response. The 200 OK message body contains Marconi's media information: SIP/2.0 200 OK Via: SIP/2.0/UDP lab.high-voltage.org:5060;branch=z9hG4bKfw19b ;received=100.101.102. To: G. Marconi ksip:marconi@radio.org>;tag=a53e42 From: Nikola Tesla <sip:n.tesla@high-voltage.org>;tag=76341 Call-ID: 123456789@lab.high-voltage.org CSeq: 1 INVITE Contact: <sip:marconi@tower.radio.org> Content-Type: application/sdp Content-Length: 155 o=Marconi 2890844528 2890844528 IN IP4 tower.radio.org s=Phone Call C=IN IP4 200.201.202.203 m=audio 60000 RTP/AVP 0 a=rtpmap 0 PCMU/8000 This response is constructed the same way as the 180 Ringing response and contains the same To tag and Contact URI. The media capa- bilities, however, must be communicated in a SDP message body added to the response. From the same SDP fields as Table 2.2, the SDP contains: End-point IP address (200.201.202.203); Media format (audio); Port number (60000); Media transport protocol (RTP); Media encoding (PCM u-Law); Sampling rate (8,000 Hz). The final step is to confirm the media session with an "acknowledgment" request. The confirmation means that Tesla has received successfully Marconi's SIP: Understanding the Session Initiation Protocol response. This exchange of media information allows the media session to be established using another protocol, RTP in this example. ACK sip: marconi@tower.radio.org SIP/2.0 Via: SIP/2.0/UDP lab.high-voltage.org:5060;branch=z9hG4bk321g Max-Forwards: 70 To: G. Marconi <sip:marconi@radio.org>;tag=a53e42 From: Nikol