In SIP, a proxy or proxy server is

RFC 3261, section 6

[a]n intermediary entity that acts as both a server and a client for the purpose of making requests on behalf of other clients. A proxy server primarily plays the role of routing, which means its job is to ensure that a request is sent to another entity "closer" to the targeted user. Proxies are also useful for enforcing policy (for example, making sure a user is allowed to make a call). A proxy interprets, and, if necessary, rewrites specific parts of a request message before forwarding it.

In a strict sense, the term proxy is missleading. A proxy does not act as a substitute on behalf of the actual target (the location service is much closer to that function) but rather as a mere router. People not fluent in SIP often use the term server for a proxy (see the discussion for that issue in the glossary entry for server).

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