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"SIP provides a registration function that allows users to upload their current locations for use by proxy servers. "
"Since registrations play an important role in SIP, a User Agent Server that handles a REGISTER is given the special name registrar."

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Posted by HamidAliKhan

An intermediate node that acts on behalf of the Client. The Proxy Server has a function that can receive requests. Until authenticated by the Proxy, all User Agents are considered to be non-trusted. The Proxy authenticates both the User Agent Client and Users Agent Server. It has a server function that can modify the header section of the SIP request then pass the modified request to another server (either a Network Server or a User Agent Server)

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Posted by cdmaguru

"SIP makes use of elements called proxy servers to help route requests to the user's current location, authenticate and authorize users for services, implement provider call-routing policies, and provide features to users."
"SIP also provides a registration function that allows users to upload their current locations for use by proxy servers. "
"Since registrations play an important role in SIP, a User Agent Server that handles a REGISTER is given the special name registrar."
"It is an important concept that the distinction between types of SIP servers is logical, not physical."

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Posted by avloginov

In computer networks, a proxy server is a server (a computer system or an application program) which services the requests of its clients by forwarding requests to other servers. A client connects to the proxy server, requesting some service, such as a file, connection, web page, or other resource, available from a different server. The proxy server provides the resource by connecting to the specified server and requesting the service on behalf of the client. A proxy server may optionally alter the client's request or the server's response, and sometimes it may serve the request without contacting the specified server.

"A server that sits between a client application, such as a Web browser, and a real server. It intercepts all requests to the real server to see if it can fulfill the requests itself. If not, it forwards the request to the real server."

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Posted by sagitraz