Client/Server versus Peer-to-Peer Architecture

In looking at the components of SIP, you can see that requests are processed in different ways. When user agents communicate with one another, they send requests and responses to one another. In doing so, one acts as a User Agent Client, and the other fulfills the request acts as a User Agent Server. When dealing with SIP servers however, they simply send requests that are processed by a specific server. This reflects two different types of architectures used in network communications:

• Client/Server
• Peer-to-peer

Client/Server

In a client/server architecture, the relationship of the computers are separated into two roles:

• The client, which requests specific services or resources
• The server, which is dedicated to fulfilling requests by responding (or attempting to respond) with requested services or resources

An easy-to-understand example of a client/server relationship is seen when using the Internet. When using an Internet browser to access a Web site, the client would be the computer running the browser software, which would request a Web page from a Web server. The Web server receives this request and then responds to it by sending the Web page to the client computer. In VoIP, this same relationship can be seen when a client sends a request to register with a Registrar server, or makes a request to a Proxy Server or Redirect Server that allows it to connect with another user agent. In all these cases, the client’s role is to request services and resources, and the server’s role is to listen to the network and await requests that it can process or pass onto other servers.

The servers that are used on a network acquire their abilities to service requests by the programs installed on it. Because a server may run a number of services or have multiple server applications installed on it, a computer dedicated to the role of being a server may provide several functions on a network. For example, a Web server might also act as an e-mail server. In the same way, SIP servers also may provide different services. A Registrar can register clients and also run the location service that allows clients and other servers to locate other users who have registered on the network. In this way, a single server may provide diverse functionality to a network that would otherwise be unavailable.

Another important function of the server is that, unlike clients that may be disconnected from the Internet or shutdown on a network when the person using it is done, a server is generally active and awaiting client requests. Problems and maintenance aside, a dedicated server is up and running, so that it is accessible. The IP address of the server generally doesn’t change, meaning that clients can always find it on a network, making it important for such functions as finding other computers on the network.

Peer to Peer

A peer-to-peer (P2P) architecture is different from the client/server model, as the computers involved have similar capabilities, and can initiate sessions with one another to make and service requests from one another. Each computer provides services and resources, so if one becomes unavailable, another can be contacted to exchange messages or access resources. In this way, the user agents act as both client and server, and are considered peers.

Once a user agent is able to establish a communication session with another user agent, a P2P architecture is established where each machine makes requests and responds to the other. One machine acting as the User Agent client will make a request, while the other acting as the User Agent server will respond to it. Each machine can then swap roles, allowing them to interact as equals on the network. For example, if the applications being used allowed file sharing, a UAC could request a specific file from the UAS and download it. During this time, the peers could also be exchanging messages or talking using VoIP, and once these activities are completed, one could send a request to terminate the session to end the communications between them. As seen by this, the computers act in the roles of both client and server, but are always peers by having the same functionality of making and responding to requests.