Table 2.3 (continued)
FUNCTION DESCRIPTION COMMENTS
Query using a peer-to-peer This is the model for basic
procedure such as Chord to P2P SIP and also requires
determine the IP address of the no central SIP proxy
destination peer. servers.
Session setup The INVITEmessage informs
the called party of the IP address
and ports of the caller and offers
a 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 the
called party is ready to receive
media at its specified IP address
and port(s) and the selected codec
from the choice.
The ACKmessage from the caller
acknowledges is readiness as well.
RTP media packets containing audio,
video or IM will now flow between
the SIP endpoints.SIP Capabilities
SIP-enabled IP devices can call each other directly, if they know each other’s
URL. Thus, an IP phone call can be placed directly between two or more SIP
phones or PCs.
Small conferences can be held by several users connecting to one device act-
ing as the conference bridge, where one of the SIP phones can act as both con-
ference participant and conference bridge.
Besides SIP devices such as phones, PCs, IP telephony gateways, and mobile
devices, service providers also deploy SIP servers for a variety of additional
services.
Figure 2.2 illustrates how SIP servers perform a routing service that puts the
caller in contact with the called party in a step-by-step fashion, taking into
account the desired service and user preferences. We will show in the follow-
ing sections that the SIP service model provides users with all services known
from the circuit-switched telephone network, as well as new services that
result from taking advantage of the Internet.
Internet Communications Enabled by SIP 17