■■ Proxy—SIP proxy servers will forward the request of the user to another
server that can provide the requested service, such as voicemail, confer-
encing, or presence information.
■■ Forking—A request from a user can be forwarded in several directions
simultaneously, as, for example, when trying various locations where
the called party may be found.
■■ Rendezvous and presence—The active form of rendezvous consists of
routing a request for call setup to another server or endpoint where the
desired service may be performed, such as communication with an
individual, or with a machine. The passive form of rendezvous consists
of presence information (that is, letting someone know that a party of
interest is connected to the network and its communication state, such
as available or busy).
■■ Mobility—Users may have many communication devices such as
phones, fax machines, computers, palm computers, and pagers, at
home, at work, and while traveling. User devices can be attached to
various types of networks, if proper gateways are provided: IP, PSTN,
mobile telephony, wireless mobile data, or paging. SIP call setup can
proceed without regard to the type of network or type of device the
parties may use at a certain instance.
■■ User preferences—Callers can specify how servers and the network
should handle their requests, and also specify what type of service is
desired or acceptable, whom they would like to reach, and whom they
would like to avoid (for example, to avoid making calls to busy lines or
to speak to machines). Called parties can specify how to handle incom-
ing calls, depending on a very large set of criteria, such as who the
caller is, from where the call is coming from, time of day, the communi-
cation device, and others.
■■ Routing control—The route taken by SIP messages can be specified and
recorded for various services.
Some (but not all) of the preceding features are known from the AIN and
other from e-mail and the web. It is the combination of all these features that
makes SIP unique. Last, but not least, SIP has unique features of interest to
developers and service providers, features not available in telecommunication
networks.
The similarity of SIP to HTTP facilitates easy service creation by a very
large community of software developers that may be familiar with web site
development.
SIP is text-based and easy to debug without using specialized test equip-
ment. SIP messages are seen on standard data analyzers in the very form
shown in this introduction.100 Chapter 6