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The TCP/IP Guide  9  The Open System Interconnection (OSI) Reference Model       9  Key OSI Reference Model Concepts

"N" Notation and Other OSI Model Layer Terminology 1 2 Protocols: Horizontal (Corresponding Layer) Communication

Modularity and Inter-Layer Interactions

One of the primary goals of the OSI Reference Model is to allow the interconnection of different implementations of various layers. Thus, the intention is to have somewhat autonomous individual layers that you can “mix and match”—to a point. The only way to make this work is to have well-defined ways that the layers connect together, and that brings us back to the matter of interfaces. Each layer must present a consistent, well-documented interface to the layers above it so that any upper layer implementation can use the lower layer properly.

I'll provide an example from the world of TCP/IP to illustrate what I mean. The heart of the TCP/IP protocol suite is the Internet Protocol (IP). Whenever you use any application on the Internet—email, Web sites, FTP, chat rooms, and so on—you are indirectly using IP.

However, you never use IP directly—you generally use one of two transport layer (layer 4) protocols: the Transmission Control Protocol (TCP) or the User Datagram Protocol (UDP). A standard interface exists between the network layer and the transport layer in the TCP/IP protocol stack, which defines how IP is to be used by upper layer protocols; this enables TCP and UDP to interface to it. Similarly, both TCP and UDP present a particular interface to the hundreds of higher-layer protocols and applications that use them at higher layers.

Many different types of communication actually take place between layers. Control information is passed to enable the higher layers to utilize the lower ones, and for the lower ones to pass status and results information back to the higher ones. Data is also passed in both directions across the interface. For transmission, it flows down to the lower layer, each time normally resulting in data encapsulation. Upon reception, the process is reversed, with data being sent back up across the interface from lower to higher layer.

"N" Notation and Other OSI Model Layer Terminology 1 2 Protocols: Horizontal (Corresponding Layer) Communication