services will include integration with the World Wide Web or other databases
of information. However, the first set of services implemented using SIP will
be PSTN telephony features, which are used as examples throughout this
chapter.
Services can reside in a number of locations in SIP. For example, many ser-
vices can reside exclusively in user agents, requiring no support or servers in
the network. Intelligent phones such as those shown in Figure 2.6 can well
support a variety of SIP services. Other services can reside in proxy or redirect
servers. The following simple example illustrates the implementation options
in SIP.Service Example
Consider a call-forward, no-answer service, in which a user wants an unan-
swered call to his or her SIP phone to automatically forward to a voicemail
server after a certain period of time or a certain number of “rings.” This service
could be implemented in either a proxy server, called user agent, or calling
user agent.Server Implementation
This service could be implemented in the proxy server that handles registra-
tions for the called party. The resulting flow is shown in Figure 7.1. The proxy
starts a timer when the INVITEis proxied to the latest registered address for
the SIP phone:sips:[email protected]Since the call is not answered (no 200 OKis sent by the phone), the proxy
sends a CANCELto stop the phone from ringing, then forks the INVITEto the
voicemail server, rewriting the Request-URIto the following:sips:[email protected]The voicemail system answers, plays a prompt, and records a message on
behalf of the called party.
If a SIP server wishes to provide services beyond the initial call setup
(INVITE/200 OK/ACKexchange), the proxy must insert a Record-Route
header into the INVITErequest. This ensures that all future requests, such as
re-INVITEs and other methods, will be routed through the proxy, giving the
proxy an opportunity to invoke a service.136 Chapter 7