Types of SIP Devices

There are four types of SIP devices:

• SIP user agent (UA) is an endpoint in SIP end-to-end communication. A UA is a user agent client (UAC) when it initiates a request and waits to receive a response. A UA is a user agent server (UAS) when it receives a request and generates a response. A given UA will be a UAC or a UAS depending on whether it is initiating the request or receiving the request.
• A SIP proxy (or proxy server) is an 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’s primary role is routing. Its job is to ensure that a request is sent to another entity closer to the targeted user. A proxy interprets, and if necessary, rewrites specific parts of a request message before forwarding it.
• A SIP redirect server is a UAS that generates redirect responses to requests it receives, directing the client to contact an alternate set of targets. Unlike a proxy which forwards the request to the alternate set of targets, the redirect response tells the UAC to directly contact the alternate targets.
• A SIP registrar is a server that accepts REGISTER requests and places the information it receives in those requests into the location service for the domain it handles. Proxies and redirect servers can use the information from the location service to determine the location of the targeted user.

  A redirect server and a registrar are each a special type of UA because they act as the UAS for the requests they process.