1.3.1. SIP User Agents and Servers

User agents (UAs) are appliances or applications, such as, SIP phones, residential gateways and software that initiate and receive calls over a SIP network.

Servers are application programs that accept requests, service requests and return responses to those requests. Examples of the different types of servers are:

• Location Server - Used by a SIP redirect or proxy server to obtain information about the location of the called party.
• Proxy Server - An intermediate program that operates as a server and a client and which makes requests on behalf of the client. A proxy server does not initiate new requests, it interprets and possibly modifies a request message before forwarding it to the destination.
• Redirect Server - Accepts a request from a client and maps the address to zero or more new addresses and returns the new addresses to the client. The server does not accept calls or generate SIP requests on behalf of clients.
• Registrar Server - Accepts REGISTER requests from clients. Often, the registrar server is located on the same physical server as the proxy server or redirect server.

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