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1.8: Wrapping Up Networks

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    This chapter has introduced a lot of concepts and techniques for designing and dealing with data communication networks. A natural question arises: "Is all of this stuff really needed?"

    The answer, of course, is "It depends." It obviously depends on the application, which may not require all of the features that the various network layers provide. It also depends on several lower-layer aspects.

    For example, if at the link layer the entire network consists of just a single point-to-point link, there is no need for a network layer at all. There may still be a requirement to multiplex the link, but multiplexing does not require any of the routing function of a network layer because everything that goes in one end of the link is destined for whatever is attached at the other end. In addition, there is probably no need for some of the transport services of the end-to-end layer because frames, segments, streams, or messages come out of the link in the same order they went in. A short link is sometimes quite reliable, in which case the end-to-end layer may not need to provide a duplicate-generating resend mechanism and in turn can omit duplicate suppression. What remains in the end-to-end function is session services (such as authenticating the identity of the user and encrypting the communication for privacy) and presentation services (marshaling application data into a form that can be transmitted as a message or a stream.)

    Similarly, if at the link layer the entire network consists of just a single broadcast link, a network layer is needed, but it is vestigial: it consists of just enough intelligence at each receiver to discard packets addressed to different targets. For example, a backplane bus described is a reliable broadcast network with an end-to-end layer that provides only presentation services. For another example, an Ethernet, which is less reliable, needs a healthier set of end-to-end services because it exhibits greater variations in delay. On the other hand, packet loss is still rare enough that it may be possible to ignore it, and reordered packet delivery is not a problem.

    As with all aspects of computer system design, good judgment and careful consideration of trade-offs are required for a design that works well and also is economical.

    This summary completes our conceptual material about networks. In the remaining sections of this chapter are a case study of a popular network design, the Ethernet, and a collection of network-related war stories.

    This page titled 1.8: Wrapping Up Networks is shared under a CC BY-NC-SA license and was authored, remixed, and/or curated by Jerome H. Saltzer & M. Frans Kaashoek (MIT OpenCourseWare) .