2.10 Summary

In this chapter we've studied both the conceptual and the implementation aspects of network applications. We've learned about the ubiquitous client-server paradigm adopted by Internet applications and seen its use in the HTTP, FTP, SMTP, POP3 and DNS protocols. We've studied these important application-level protocols, and their associated applications (the Web, file transfer, e-mail, and the domain name system) in some detail. We've examined how the socket API can be used to build network applications and walked through not only the use of sockets over connection-oriented (TCP) and connectionless (UDP) end-to-end transport services, but also built a simple web server using this API. The first step in our top-down journey "down" the layered network architecture is complete.

At the very beginning of this book, in section 1.3, we gave a rather vague, bare bones definition of a protocol as defining "the format and the order of messages exchanged between two communicating entities, as well as the actions taken on the transmission and/or receipt of a message." The material in this chapter, and in particular the detailed study of the HTTP, FTP, SMTP, POP3 and DNS protocols, has now added considerable substance to this definition. Protocols are a key concept in networking; our study of applications protocols has now given us the opportunity to develop a more intuitive feels for what protocols are all about.

In Section 2.1 we described the service models that TCP and UDP offer to applications that invoke them. We took an even closer look at these service models when we developed simple applications that run over TCP and UDP in Sections 2.6-2.7. However, we have said little about how TCP and UDP provide these service models. For example, we have said very little about how TCP provides a reliable data transfer service to its applications. In the next chapter we shall take a careful look at not only the what, but also the how and why, of transport protocols.

Armed with a knowledge about Internet application structure and application-level protocols, we're now ready to head further down the protocol stack and examine the transport layer in Chapter 3.

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