Components of SIP

Entities interacting in a SIP scenario are called User Agents (UA)
User Agents may operate in two fashions -

• User Agent Client (UAC) : It generates requests and send those to servers.
• User Agent Server (UAS) : It gets requests, processes those requests and generate responses.

Note: A single UA may function as both.

Clients:

In general we associate the notion of clients to the end users i.e. the applications running on the systems used by people. It may be a softphone application running on your PC or a messaging device in your IP phone. It generates a request when you try to call another person over the network and sends the request to a server (generally a proxy server). We will go through the format of requests and proxy servers in more detail later.

Servers:

Servers are in general part of the network. They possess a predefined set of rules to handle the requests sent by clients.
Servers can be of several types -

• 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 know in advance. So the client sends the request to a proxy server. The server on behalf of the client (as if giving a proxy for it) forwards the request to another proxy server or the recipient itself.
• Redirect Server: A redirect server redirects the request back to the client indicating that the client needs to try a different route to get to the recipient. It generally happens when a recipient has moved from its original position either temporarily or permanently.
• Registrar: As you might have guessed already, one of the prime jobs of the servers is to detect the location of an user in a network. How do they know the location? If you are thinking that users have to register their locations to a Registrar server, you are absolutely right. Users from time to time refreshes their locations by registering (sending a special type of message) to a Registrar server.
• Location Server: The addresses registered to a Registrar are stored in a Location Server.

Now that the components are ready, we need the SIP commands to make them work.

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