Location Services
Alocation service is a general term used in RFC 2543 for a database. The data-
base may contain information about users such as URIs, IP addresses, scripts,
features, and other preferences. It also may contain routing information about
the SIP-enabled network, including the locations of proxies, gateways, and
other location services. User agents generally do not interact directly with a
location service, but go through a proxy, redirect, or registrar server. SIP
servers use a non-SIP protocol to query, update, and retrieve records from the
location service in the course of routing a SIP message.
The role of these elements will be discussed in terms of SIP functions in the
next section.
SIP Functions
The SIP protocol will be introduced in terms of some of the basic functions
of a communications network: address resolution, session-related functions
(including session setup, media negotiation, session modification, session ter-
mination, and cancellation), mid-call signaling, call control, QoS call setup,
and nonsession-related functions (such as mobility, message transport, event
subscription and notification, authentication, and extensibility). Each of these
will be discussed and explained in turn.
SIP Overview 107LOCATING SIP SERVERS
SIP servers can be located using a number of schemes. User agents are typically
configured with IP addresses of a primary and secondary SIP proxy server, in
much the same way that a web browser has a default web page that it loads
upon initialization. This proxy server is sometimes referred to as the outbound
proxy, since a user agent will route outgoing messages to that proxy.
Proxies can also be located using a DNS lookup, in which the domain name
from a SIP URI is extracted and the IP address of the proxy server supporting
that domain is found. This proxy is sometimes called an incoming proxy, since
it is used to route incoming calls for that particular domain.
A SIP registrar server can be hand-configured in the device or can be located
using IP multicasting. Registrar servers listen at the well-known SIP multicast
address (such as sip.mcast.net) and can receive registrations. A SIP registrar
server can often be located by sending a registration request to an outbound
proxy, which then proxies the request to a registrar server. In this way, SIP
servers can be located by sending requests to other SIP Servers, as part of the
address resolution process described for SIP in the next section and governed
by RFC 3263.