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www.ietf.org/mail-archive/web/sip/current/msg10306.html

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Posted by crouse

The Proxy server—This server can perform call routing, authentication, authorization, address resolution, and loop detection. A UA sends its call setup messages through a proxy server. The proxy server can forward the messages if it knows where the called party is located, or it can query other servers to find that information. It then forwards the request to the next hop. When it receives a response to the request, it forwards that to the client UA. After the call is set up, the proxy server can elect to stay in the signaling path so that it also sees call change or termination messages, or it can withdraw from the path and let the UAs communicate directly. Cisco has a SIP proxy server product.
Redirect server—UAs and proxy servers can contact a redirect server to find the location of an endpoint. This is particularly useful in a network that has mobile users whose location changes. The redirect server can let its clients know that a user has moved either temporarily or permanently. It can also return multiple possible addresses for the user, if necessary. When a UA has multiple addresses, the proxy server can fork the call, sending it to each address either simultaneously or sequentially. This allows "Find Me/Follow Me" type services. Cisco routers can act as SIP redirect servers.

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Posted by HamidAliKhan

- A SIP server may either proxy or redirects a request - Which of the two methods applies is a configuration issue. It may be statically configured or dynamically determined (CPL).
- Redirection useful if a user moves or changes her provider (PSTN: “The number you have dialed is not available.”) -- Caller does not need to try the original server next time. Stateless.
- Proxy useful if forking, AAA, firewall control needed. In general, proxying grants more control to the server.

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Posted by yogendra

# Proxy server—This server can perform call routing, authentication, authorization, address resolution, and loop detection. A UA sends its call setup messages through a proxy server. The proxy server can forward the messages if it knows where the called party is located, or it can query other servers to find that information. It then forwards the request to the next hop. When it receives a response to the request, it forwards that to the client UA. After the call is set up, the proxy server can elect to stay in the signaling path so that it also sees call change or termination messages, or it can withdraw from the path and let the UAs communicate directly. Cisco has a SIP proxy server product.

# Redirect server—UAs and proxy servers can contact a redirect server to find the location of an endpoint. This is particularly useful in a network that has mobile users whose location changes. The redirect server can let its clients know that a user has moved either temporarily or permanently. It can also return multiple possible addresses for the user, if necessary. When a UA has multiple addresses, the proxy server can fork the call, sending it to each address either simultaneously or sequentially. This allows "Find Me/Follow Me" type services. Cisco routers can act as SIP redirect servers.

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Posted by sagitraz