
1/5/2007
EAG Gateway Series Administration Guidelines
MAG-06007 Rev. A
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:[email protected]. 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.
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.
For more information, see RFC 2543—SIP: Session Initiation Protocol, which can be found at
http://www.faqs.org/rfcs/.
Using a Proxy Server
If a proxy server is used, the caller UA sends an INVITE request to the proxy server, the proxy server
determines the path, and then forwards the request to the callee.
The callee responds to the proxy server, which in turn, forwards the response to the caller.
The proxy server forwards the acknowledgments of both parties. A session is then established
between the caller and callee. Real-time Transfer Protocol (RTP) is used for the communication
between the caller and the callee.
Using a Redirect Server
If a redirect server is used, the caller UA sends an INVITE request to the redirect server, the redirect
server contacts the location server to determine the path to the callee, and then the redirect server
sends that information back to the caller. The caller then acknowledges receipt of the information.
The caller then sends a request to the device indicated in the redirection information (which could be
the callee or another server that will forward the request). Once the request reaches the callee, it
sends back a response and the caller acknowledges the response. RTP is used for the communication
between the caller and the callee.
SIP Versus H.323
In addition to SIP, there are other protocols that facilitate voice transmission over IP. One such protocol
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