This chapter describes the terms Client
in context with SIP.
In a typical SIP scenario, the devices interacting are called UA (User Agents). User Agents may operate as:
a UAC (User Agent Client) which initiates requests and send those to servers.Note:
a UAS (User Agent Server) which receives requests, processes them, and generate responses.
You may find that a single User Agent may hand both functions.
: Typically, the idea of clients are associated to the end users which may be applications running on the systems used by people. The applications may be a softclient running on a computer. Or it may be a telephone running a SIP application. In any case, this phone or application generates a request when you try to call another person over the network and sends the request to a server — generally a proxy server.
: Servers are part of the network and they are setup to handle the requests sent by clients. These Servers may be:
Proxy Server: These are the most common type of server in a SIP environment. When a request is generated, the exact address of the recipient is not known in advance. So the client sends the request to a proxy server. The server, on behalf of the client, forwards the request to another proxy server or the recipient itself.Parent topic: SIP Overview
Redirect Server: A redirect server redirects or sends the request back to the client indicating that the client has to try a different route to get to the recipient. This happens when a recipient has moved from its original position either temporarily or permanently.
Registrar: One of the key jobs of the servers is to determine the location of an user in a network. Users must register their locations to a Registrar. The users must periodically refresh their locations by registering, that is, by sending a special type of message to a Registrar server.
Location Server: The addresses that users send to a Registrar are stored in a Location Server.