 Understanding Changing Telecommunications - Building a Successful Telecom Business description protocol (SDP), which is specified in RFC 2327, for channel and terminal capability handling, and the session initiation protocol (SIP) for registration, admission, status and Q.931 messages. SIP uses a client server model similar to the HTTP. It is used in conjunction with other protocols such as SDP, RTP and RSVP. SIP can set up all types of sessions, e.g. voice, video, text or combinations, and manage any of these types of sessions, regardless of the media type. SIP is a direct competitor to H.323. An IP telephone call is considered a kind multimedia session in which voice is exchanged between the parties. The voice path is independent of whether SIP or H.323 is used for signalling. SIP is independent of the transport layer and only requires an unreliable datagram service, as it provides its own reliability mechanism. It is typically transported over UDP or TCP, but could also use frame relay, ATM AAL5 and X.25. SIP supports personal mobility and negotiation of the capabilities 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 communic