owed by the method name, INVITE in this case. This number is incremented for each new request sent. In this example, the command sequence number is initialized to 1, but it could start at another integer value. The Via header fields plus the Max-Forwards, To, From, Call- ID, and CSeq header fields represent the minimum required header field set in any SIP request message. Other header fields can be included as optional additional information, or information needed for a specific request type. A Contact header field is also required in this INVITE message, which contains the SIP URI of Tesla's communication device, known as a user agent (UA); this URI can be used to route messages directly to Tesla. The optional Subject header field is present in this example. It is not used by the protocol, but could be displayed during alerting to aid the called party in deciding whether to accept the call. The same sort of useful prioritization and screening commonly performed using the Subject and From header fields in an e-mail message is also possible with a SIP INVITE request. Additional header fields are present in this INVITE message, which contain the media information neces- sary to set up the call. The Content-Type and Content-Length header fields indicate that the message body is SDP [3] and contains 158 octets of data. The basis for the octet count of 158 is shown in Table 2.1, where the CR LF at the end of each line is shown as a © and the octet count for each line is shown on the right-hand side. A blank line separates the message body from the header field list, which ends with the Content-Length header field. In this case, there are seven lines of SDP data describing the media attributes that the caller Tesla desires for the call. This media information is needed because SIP makes no assumptions about the type of media session to be established-the caller must specify exactly what type of session (audio, video, gaming) that he wishes to establish. The SDP field names are listed in Table 2.2, and will be discussed