TCP/IP Illustrated, Volume 1: The Protocols

Appendix B. Computer Clocks

Since most of the examples in this text measure a time interval, we need to describe in more detail the type of timekeeping used by current Unix systems. The following description applies to the systems being used for the examples in this book, and for most Unix systems. Additional details are given in Sections 3.4 and 3.5 of [Leffler et al. 1989].

The hardware generates a clock interrupt at some frequency. For Sun SPARCs and Intel 80386s the interrupts occur every 10 ms.

It should be noted that most computers use an uncompensated crystal oscillator to generate these interrupts. As noted in Table 7 of RFC 1305 [Mills 1992], you don't want to ask what the drift per day of such an oscillator is. This means few computers ...