INVITE SIP: hop in the path. Bob@131.160.1.112 SIP/2.0 (3) INVITE SIP: Bob@university.com SIP/2.0 Status line Table 5-5 Several headers Format of an SIP Response Empty line Message body Status Line A status line has three elements: protocol version, status code, and a reason phrase. The current protocol version is written as SIP/2.0. The status code reports transaction status. As described earlier, status codes are integers from 100 to 699 and are grouped into six differ- ent classes (refer to Table 5-1). The reason phrase is meant for human eyes only. It is not meaningful for computers processing SIP responses. Below there is an example of a status line. SIP/2.0 180 Ringing Reliable Transmission of Responses Final responses are transmitted reliably between server and client, using retransmissions or a reliable transport protocol to ensure delivery. Provisional responses are not. They may either be received by the client or be lost in the network. SIP takes this approach because it is more concerned with whether a session was established or not, and the reasons why it wasn't, than with how the ses- sion setup is progressing. Ergo in a SIP call, for instance, callers are guaranteed notification that the call has been accepted, but might not know when the callee alert began (Figure 5-13). SIP can be extended for reliable delivery of provisional responses if necessary. Chapter 5 Laura Figure 5-13 SIP does not ensure that provisional (1) INVITE responses are (2) 180 Ringing received. (3) 200 OK (4) ACK Conversation SIP Headers SIP requests contain some SIP headers after the request line, whereas SIP responses put them after the status line. Headers provide information about the request (or response) and about the body it contains. Some head- ers can be used in both requests and responses, but others are specific to requests (or responses) alone. The header consists of the header name, fol- lowed by a colon, followed by the header value. For instance, the header called From, which identifies the originator of a partic