Proxy Servers Definition
Proxy servers are computing devices (typically a server) that interface between data processing devices (e.g. computers) and other devices within a communications network. These devices may be located on the same local area network or an external network (e.g. the Internet). A proxy server usually has access to at least two communication interfaces. One interface communicates with a device requesting services (e.g. a client) and a device that is being requested for a service (the server).
The SIP proxy server performs call processing tasks on the behalf (as a proxy) of another device. Proxy servers receive requests (such as invite requests to start a communication session) and perform processes to assist in the establishment of the communication system. This process may only involve the forwarding of requests or it may involve the changing (processing) of information as it passes through the proxy server. SIP proxies may modify or create new SIP messages based on the requirements of the communication session and the setup of the services authorized for the proxy.
Proxy servers may forward call requests to more than one user agent (called forking proxy). This forwarding may be sequential (such as searching through a list of numbers or addresses) or they may be in parallel (such as ringing several extensions at the same time).
This figure shows how a SIP proxy server is responsible for establishing communication connections between devices within a specific domain. In this example, when the proxy server receives a request message from a User Agent (UA) inviting another party to join a session, the proxy server forwards this invitation onwards (it acts as a proxy) to the designated User Agent. If the designated User Agent is unavailable, the proxy server may direct the connection request to the second User Agent or the connection request may be forwarded via one (or more) proxy servers.
The proxy servers can communicate with elements other than User Agent and other proxies, for example to determine the IP address of another server a proxy may need to call on the services of the Domain Name System (DNS) to resolve a domain name into a IP address. Also the proxies are able to interact with an element known as a Location Service that is used to identify the present location of a particular user.Session Initiation Protocol - SIP Books
Introduction to SIP IP Telephony Systems)
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