ate? 200 OK 200 OK - Sure, like this ! Audio exchange - Let us get going! Figure 14.7 Example of the SIP protocol dialogue Control Network Informational Successful Redirection Request failure Server failure Global failure Figure 14.8 SIP responses 1. INVITE 2. INVITE 4. 2xx 3. 2xx 5. ACK 6. ACK user agent server user agent Figure 14.9 Proxy mode SIP operation Figure 14.8. The server keeps the client informed of the status of the call by means of these responses. There are two modes of operation in SIP: when servers are used with a proxy server or with a redirect server. The proxy server returns responses on behalf of the user. The redirect server informs the client of the current location of the user. Then, the client can reach the user directly. The proxy server takes care of the location of the user. Thus, the process is transparent to the client. There is another mode of operation when no servers are used. The user agent can send directly requests to the other user agent. Even when a SIP server is used in the first exchange of messages between the parties, the subsequent exchanges may be addressed directly to the user agent, without traversing any server. The normal exchange of SIP messages is illustrated in Figure 14.9. Translation between telephone number and SIP address is performed in a gateway at the PSTN-IP border. SIP uses other protocols for specific tasks, for example DNS for user-domain location and SDP for media selection. 3GPP is dedicated to using SIP for call control. 14.4.2 SESSION DESCRIPTION PROTOCOL (SDP) Session descriptions have a list format containing information about the session (see RFC 2327). Message headers are in plain text and look similar to e-mail headers. SDP is designed to convey session information to recipients and media information that pertain to the session. It allows more than one media stream to be associated with a session. SDP inform the receivers of its messages the existence of a session. It also conveys sufficient information to enable joining and participating in t