To write a client-server application, you must create two programs: the client and the server. Within these programs, you must define how the two applications will communicate—the rules each program must follow. As you have learned, the rules the client and server program must follow to communicate are protocols. In short, protocols define how client-server programs interact.
Most programmers implement protocols using a series of LAYERS, where each layer deals with a particular task. For example, the lowest layer might deal with the actual communication across the NETWORK hardware. The next layer might CHECK the bits sent across the network for communication errors. Likewise, the next layer might package the bits into a MESSAGE format your program understands. Client-server applications use upper-layer protocols that are NORMALLY based on a specific application (such as printing a file or transferring a file). The upper-layer protocol defines a strict set of rules on how two applications exchange data.

To write a client-server application, you must create two programs: the client and the server. Within these programs, you must define how the two applications will communicate—the rules each program must follow. As you have learned, the rules the client and server program must follow to communicate are protocols. In short, protocols define how client-server programs interact.
Most programmers implement protocols using a series of layers, where each layer deals with a particular task. For example, the lowest layer might deal with the actual communication across the network hardware. The next layer might check the bits sent across the network for communication errors. Likewise, the next layer might package the bits into a message format your program understands. Client-server applications use upper-layer protocols that are normally based on a specific application (such as printing a file or transferring a file). The upper-layer protocol defines a strict set of rules on how two applications exchange data.