| SIP is a simple, ASCII-based protocol
that uses requests and responses to establish communication
among the various components in the network and to ultimately
establish a conference between two or more end points.
Users in a SIP network are identified by unique SIP addresses.
A SIP address is similar to an e-mail address and is in the
format of sip: userID@gateway.com. The user
ID can be either a user name or an E.164 address.
Users register with a registrar server using their assigned
SIP addresses. The registrar server provides this information
to the location server upon request.
When a user initiates a call, a SIP request is sent to a
SIP server (either a proxy or a redirect server). The request
includes the address of the caller (in the From header field)
and the address of the intended callee (in the To header field).
The following sections provide simple examples of successful,
point-to-point calls established using a proxy and a redirect
server.
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Over time, a SIP end user might move between end systems.
The location of the end user can be dynamically registered
with the SIP server. The location server can use one or more
protocols (including finger, rwhois, and LDAP) to locate the
end user. Because the end user can be logged in at more than
one station and because the location server can sometimes
have inaccurate information, it might return more than one
address for the end user.
If the request is coming through a SIP proxy server, the proxy
server will try each of the returned addresses until it locates
the end user. If the request is coming through a SIP redirect
server, the redirect server forwards all the addresses to
the caller in the Contact header field of the invitation response.
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