ies of the end users. It also supports the fundamental security services: authentication, access control, confidentiality and integrity. SIP is a client-server protocol. Clients issue requests and servers answer with responses. A limited number of request types, also referred to as methods are defined for SIP: INVITE is used to ask for the presence of a certain party in a multimedia session. The ACK method is sent to acknowledge a new connection. OPTIONS are used to get information about the capabilities of a server. The REGISTER method informs a server about the current location of a user. A client sends a BYE method to leave a session. The CANCEL method terminates parallel searches. A SIP dialogue can look like Figure 14.7. When a server receives a request, it sends back a response. A code number identi- fies each type of response. There are six main types of responses. They are listed in Caller Callee INVITE INVITE - Would you like to communicate? 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 subseque