Internet Communications Using SIP - RIPHAH INFORMATION ...

Internet Communications Enabled by SIP 17Table 2.3(continued)FUNCTION DESCRIPTION COMMENTSQuery using a peer-to-peerprocedure such as Chord todetermine the IP address of thedestination peer.This is the model for basicP2P SIP and also requiresno central SIP proxyservers.Session setupThe INVITE message informsthe called party of the IP addressand ports of the caller and offersa choice of audio/video codecs.A provisional response message,such as 100 Trying or 180 Ringing,informs the caller of the progress.The message 200 OK confirms thecalled party is ready to receivemedia at its specified IP addressand port(s) and the selected codecfrom the choice.The ACK message from the calleracknowledges is readiness as well.RTP media packets containing audio,video or IM will now flow betweenthe SIP endpoints.SIP CapabilitiesSIP-enabled IP devices can call each other directly, if they know each other’sURL. Thus, an IP phone call can be placed directly between two or more SIPphones or PCs.Small conferences can be held by several users connecting to one device actingas the conference bridge, where one of the SIP phones can act as both conferenceparticipant and conference bridge.Besides SIP devices such as phones, PCs, IP telephony gateways, and mobiledevices, service providers also deploy SIP servers for a variety of additionalservices.Figure 2.2 illustrates how SIP servers perform a routing service that puts thecaller in contact with the called party in a step-by-step fashion, taking intoaccount the desired service and user preferences. We will show in the followingsections that the SIP service model provides users with all services knownfrom the circuit-switched telephone network, as well as new services thatresult from taking advantage of the Internet.