TCP IP Illustrated

To measure anything like the ping time on a lan which

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Unformatted text preview: tem call time, about 8.5 times slower than the SPARC, seems about right. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/append_b.htm (2 of 3) [12/09/2001 14.48.00] Appendix B: Computer Clocks BSD/386 Version 1.0 provides microsecond resolution similar to the SPARC. It reads the 8253 clock register and calculates the number of microseconds since the last clock tick. This is made available to processes that call gettimeofday and to kernel modules such as the BSD Packet Filter. In relation to tcpdump these numbers mean that we can believe the millisecond and submillisecond values that are printed on the SPARC and BSD/386 systems, but the values printed by tcpdump under SVR4/386 will always be a multiple of 10 ms. For other programs that print round-trip times, such as ping (Chapter 7) and traceroute (Chapter 8), on the SPARC and BSD/386 systems we can believe the millisecond values that are output, but the values printed under SVR4/386 will always be multiples of 10. To measure anything like the ping time on a LAN, which we show in Chapter 7 to be around 3 ms, requires running ping on the SPARC or BSD/386. Some of the examples in this text were run under BSD/386 Version 0.9.4, which was similar to SVR4 in that it provided only 10-ms clock resolution. When we show tcpdump output from this system, we show only two numbers to the right of the decimal point, since that's the resolution provided. file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/append_b.htm (3 of 3) [12/09/2001 14.48.00] Appendix C: The sock Program The sock Program A simple test program named sock is used throughout the book to generate TCP and UDP data. It is used as both a client and server process. Having a test program like this, which is executable from a shell prompt, prevents us from having to write new client and server C programs for each specific feature that we want to examine. Since the purpose of this book is to understand the networ...
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