Figure shows a complete, albeit simple, TCP client that fetches the current time and date from a specified server, and Figure shows a complete version of the server. These two examples introduce many of the terms and concepts that are expanded on throughout the rest of the book.

Our client was protocol-dependent on IPv4 and we modified it to use IPv6 instead. But this just gave us another protocol-dependent program. In Chapter 11, we will develop some functions to let us write protocol-independent code, which will be important as the Internet starts using IPv6.

Throughout the text, we will use the wrapper functions developed in Section 1.4 to reduce the size of our code, yet still check every function call for an error return. Our wrapper functions all begin with a capital letter.

The Single Unix Specification Version 3, known by several other names and called simply The POSIX Specification by us, is the confluence of two long-running standards efforts, finally drawn together by The Austin Group.

Readers interested in the history of Unix networking should consult [Salus 1994] for a description of Unix history, and [Salus 1995] for the history of TCP/IP and the Internet.

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