SIP Components | SIP Architecture

Although SIP works in conjunction with other technologies and protocols, there are two fundamental components that are used by the Session Initiation Protocol:

• User agents, which are endpoints of a call (i.e., each of the participants in a call)
• SIP servers, which are computers on the network that service requests from clients, and send back responses

User Agents

User agents are both the computer that is being used to make a call, and the target computer that is being called. These make the two endpoints of the communication session. There are two components to a user agent: a client and a server. When a user agent makes a request (such as initiating a session), it is the User Agent Client (UAC), and the user agent responding to the request is the User Agent Server (UAS). Because the user agent will send a message, and then respond to another, it will switch back and forth between these roles throughout a session.

Even though other devices that we’ll discuss are optional to various degrees, User Agents must exist for a SIP session to be established. Without them, it would be like trying to make a phone call without having another person to call. One UA will invite the other into a session, and SIP can then be used to manage and tear down the session when it is complete. During this time, the UAC will use SIP to send requests to the UAS, which will acknowledge the request and respond to it. Just as a conversation between two people on the phone consists of conveying a message or asking a question and then waiting for a response, the UAC and UAS will exchange messages and swap roles in a similar manner throughout the session. Without this interaction, communication couldn’t exist.

Although a user agent is often a software application installed on a computer, it can also be a PDA, USB phone that connects to a computer, or a gateway that connects the network to the Public Switched Telephone Network. In any of these situations however, the user agent will continue to act as both a client and a server, as it sends and responds to messages.

SIP Server

The SIP server is used to resolve usernames to IP addresses, so that requests sent from one user agent to another can be directed properly. A user agent registers with the SIP server, providing it with their username and current IP address, thereby establishing their current location on the network. This also verifies that they are online, so that other user agents can see whether they’re available and invite them into a session. Because the user agent probably wouldn’t know the IP address of another user agent, a request is made to the SIP server to invite another user into a session. The SIP server then identifies whether the person is currently online, and if so, compares the username to their IP address to determine their location. If the user isn’t part of that domain, and thereby uses a different SIP server, it will also pass on requests to other servers.

In performing these various tasks of serving client requests, the SIP server will act in any of several different roles:

• Registrar server
• Proxy server
• Redirect server

Registrar Server

Registrar servers are used to register the location of a user agent who has logged onto the network. It obtains the IP address of the user and associates it with their username on the system. This creates a directory of all those who are currently logged onto the network, and where they are located. When someone wishes to establish a session with one of these users, the Registrar server’s information is referred to, thereby identifying the IP addresses of those involved in the session.

Proxy Server

Proxy servers are computers that are used to forward requests on behalf of other computers. If a SIP server receives a request from a client, it can forward the request onto another SIP server on the network. While functioning as a proxy server, the SIP server can provide such functions as network access control, security, authentication, and authorization.

Redirect Server

The Redirect servers are used by SIP to redirect clients to the user agent they are attempting to contact. If a user agent makes a request, the Redirect server can respond with the IP address of the user agent being contacted. This is different from a Proxy server, which forwards the request on your behalf, as the Redirect server essentially tells you to contact them yourself.

The Redirect server also has the ability to “fork” a call, by splitting the call to several locations. If a call was made to a particular user, it could be split to a number of different locations, so that it rang at all of them at the same time. The first of these locations to answer the call would receive it, and the other locations would stop ringing.

Note

RFC 3261 defines the different types of SIP servers as logical devices, meaning that they can be implemented as separate servers or as part of a single application that resides on a single physical server. In other words, a single physical server may act in all or one of these roles. In addition to this, the SIP servers can interact with other servers and applications on your network to provide additional services, such as authentication or billing. The SIP servers could access Lightweight Directory Access Protocol (LDAP) servers, database applications, or other applications to access back-end services.

Stateful versus Stateless

The servers used by SIP can run in one of two modes: stateful or stateless. When a server runs in stateful mode, it will keep track of all requests and responses it sends and receives. A server that operates in a stateless mode won’t remember this information, but will instead forget about what it has done once it has processed a request. A server running in stateful mode generally is found in a domain where the user agents resides, whereas stateless servers are often found as part of the backbone, receiving so many requests that it would be difficult to keep track of them.

Location Service

The location service is used to keep a database of those who have registered through a SIP server, and where they are located. When a user agent registers with a Registrar server, a REGISTER request is made (which we’ll discuss in the later section). If the Registrar accepts the request, it will obtain the SIP-address and IP address of the user agent, and add it to the location service for its domain. This database provides an up-to-date catalog of everyone who is online, and where they are located, which Redirect servers and Proxy servers can then use to acquire information about user agents. This allows the servers to connect user agents together or forward requests to the proper location.