 detail in Section 7.1, but a quick review of the lines shows the basic information necessary to establish a session. Table 2.2 includes the: Connection IP address (100.101.102.103); Media format (audio); Port number (49170); Media transport protocol (RTP); Media encoding (PCM Law); Introduction to SIP Table 2.1 Content-Length Calculation Example TOTAL v=IO® o=Tesla 2890844526 2890844526 IN IP4 lab.high-voltage.orgO s=Phone Call R c=IN IP4 100.101.102.103© t=0 I © ® m=audio 49170 RTP/AVP IC R a=rtpmap:0 PCMU/8000© R Table 2.2 SDP Data from Example SDP Parameter Parameter Name Version number o=Tesla 2890844526 2890844526 Origin containing name IN IP4 lab.high-voltage.org s=Phone Call Subject C=IN IP4 100.101.102.103 Connection t=0 0 m=audio 49170 RTP/AVP 0 Media a=rtpmap 0 PCMU/8000 Attributes Sampling rate (8,000 Hz). INVITE is an example of a SIP request message. There are five other methods or types of SIP requests currently defined in the SIP specification RFC 3261 and others in extension RFCs. The next message in Figure 2.1 is a 180 Ringing message sent in response to the INVITE. This message indi- cates that the called party Marconi has received the INVITE and that alerting is taking place. The alerting could be ringing a phone, flashing a message on a screen, or any other method of attracting the attention of the called party, Marconi. SIP: Understanding the Session Initiation Protocol The 180 Ringing is an example of a SIP response message. Responses are numerical and are classified by the first digit of the number. A 180 response is an "informational class" response, identified by the first digit being a 1. Infor- mational responses are used to convey noncritical information about the prog- ress of the call. Many SIP response codes were based on HTTP version 1.1 response codes with some extensions and additions. Anyone who has ever browsed the World Wide Web has likely received a "404 Not Found" response from a Web server when a requested page was not found. 404 Not Found is also a valid SIP "client error