Chapter 2: Networking the ClientServer Way

For most applications, using Windows Server 2003 in a networked environment means buying into the client/server model. To help you understand this networking model, which best explains why it's necessary for Windows Server 2003 to exist, we explore the client/server model in detail in this chapter. Along the way, you discover more about the types of capabilities and services that make client/server networks work and the various ways that clients and servers interact on such networks.

Clients Request Services

In Chapter 1, we explain that clients ask for services and that both hardware and software are necessary to make networking work on any computer. In this chapter, we take a closer look at the various pieces and parts involved in a client/server relationship to help you understand what happens when a client requests a service from a server.

At the most basic level, a client must have a network connection available to transmit a request for services. Likewise, the client must have the correct software installed to formulate an intelligible request and pass it to the network, which is where a server can notice and respond to such a request.

Making the connection

To request network services, a client must have the following hardwar:

• Network interface card (NIC): A NIC (also called a network adapter or a network board) allows a computer to interact with the network. Before a NIC can transmit signals onto the network medium and receive signals from that network medium, you must configure it.

• Physical connection: The link between the computer and its network must work properly. This means that clients can transmit outgoing signals and receive incoming signals thorough their network connections. Likewise, the network cabling itself - also known as the network medium - must be properly configured and interconnected for signals to travel from sender to receiver.

This takes care of the connections part in a three-part simple model for networking, which requires that connections, communications, and services all be available and working.

Software uses the connection

The software on the client computer handles the communications and services necessary for the network to operate . Here's a list of software that you normally find on a networked client computer, starting from the hardware level (or as close as software can get to hardware) and working up to the applications that request network services:

• Network driver: A special-purpose piece of software that enables a computer to send data from the computer's central processing unit (CPU) to the NIC when an outgoing message is ready to be sent. The network driver also forwards a request for immediate attention (called an interrupt ) to the CPU when an incoming message arrives. You might say that the driver allows the PC to communicate with the NIC, which in turn communicates with the network.

• Protocol stack: A collection of communications software that provides the type of "shared language" necessary for successful networking. The protocol stack governs which formats network messages can assume, and it defines a set of rules for how to interpret their contents. Two computers must use the same protocol stack to communicate. We cover protocol stacks throughly in Chapter 3.

• Redirector: A redirector, or equivalent software, issues requests for remote resources or services to the protocol stack and receives the incoming replies from the protocol stack. With a redirector running in the background, applications don't need to be explicitly network aware, because the redirector handles network connections.

• Network-aware application: Network-aware applications understand when service requests can be satisfied locally or must be satisfied remotely. In the latter case, a redirector may be present, but it may not necessarily handle certain types of network services (such as e-mail or Web-page access). However, the redirector can handle other types of network services, such as providing access to a file stored elsewhere on the network that's applied as an attachment to an e-mail message. In such a case, the redirector grabs a copy of that file across the network and attaches it to the outgoing e-mail message.

When a client makes a request for a resource or service that requires access to the network, either the application (if it's network aware) or a redirector (if the application isn't network aware) formulates a formal request for a remote service. Satisfying the request may involve the transfer of a small amount of data (as when requesting a listing of a directory on a machine elsewhere on the network). However, it may also involve transferring a large amount of data (as when sending a large file off to be printed or when copying a large file from the client machine to a server).

The request is ferried through the protocol stack that the client and server have in common. For short requests, a handful of short messages travel from the client and are reassembled and handled by the server. For large information transfers, the client breaks up the file into hundreds or thousands of small information packages, each of which is shipped across the network separately and then reassembled on the receiving end.

KEY CONCEPT 

The protocol stack tells the network driver to send little packages of data (called frames or packets ) from the computer, through the NIC, and across the network to the intended recipient (the server). On the receiving end, the same thing happens in reverse, with a few additional considerations that you find out about in the following section.