The TCP/IP model and protocols have their problems too. First, the model does not clearly distinguish the
concepts of service, interface, and protocol. Good software engineering practice requires differentiating between
the specification and the implementation, something that OSI does very carefully, and TCP/IP does not.
Consequently, the TCP/IP model is not much of a guide for designing new networks using new technologies.
Second, the TCP/IP model is not at all general and is poorly suited to describing any protocol stack other than TCP/IP. Trying to use the TCP/IP model to describe Bluetooth, for example, is completely impossible.
Third, the host-to-network layer is not really a layer at all in the normal sense of the term as used in the context of layered protocols. It is an interface (between the network and data link layers). The distinction between an interface and a layer is crucial, and one should not be sloppy about it.
Fourth, the TCP/IP model does not distinguish (or even mention) the physical and data link layers. These are completely different. The physical layer has to do with the transmission characteristics of copper wire, fiber optics, and wireless communication. The data link layer's job is to delimit the start and end of frames and get them from one side to the other with the desired degree of reliability. A proper model should include both as separate layers. The TCP/IP model does not do this.
Finally, although the IP and TCP protocols were carefully thought out and well implemented, many of the other
protocols were ad hoc, generally produced by a couple of graduate students hacking away until they got tired.
The protocol implementations were then distributed free, which resulted in their becoming widely used, deeply
entrenched, and thus hard to replace. Some of them are a bit of an embarrassment now. The virtual terminal
protocol, TELNET, for example, was designed for a ten-character per second mechanical Teletype terminal. It
knows nothing of graphical user interfaces and mice. Nevertheless, 25 years later, it is still in widespread use.
In summary, despite its problems, the OSI model (minus the session and presentation layers) has proven to be exceptionally useful for discussing computer networks. In contrast, the OSI protocols have not become popular. The reverse is true of TCP/IP: the model is practically nonexistent, but the protocols are widely used. Since computer scientists like to have their cake and eat it, too, in this book we will use a modified OSI model but concentrate primarily on the TCP/IP and related protocols, as well as newer ones such as 802, SONET, and Bluetooth. In effect.
A Critique of the TCP/IP Reference Model
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