If one machine has a WWV receiver, the goal becomes keeping all the other machines synchronized to it. If no machines have WWV receivers, each machine keeps track of its own time, and the goal is to keep all the machines together as well as possible. Many algorithms have been proposed for doing this synchronization (e.g., Cristian, 1989; Drummond and Babaoglu, 1993; and Kopetz and Ochsenreiter, 1987). A survey is given in (Ramanathan et al., 1990b).

All the algorithms have the same underlying model of the system, which we will now describe. Each machine is assumed to have a timer that causes an interrupt H times a second. When this timer goes off, the interrupt handler adds 1 to a software clock that keeps track of the number of ticks (interrupts) since some agreed-upon time in the past. Let us call the value of this clock C. More specifically, when the UTC time is t, the value of the clock on machine p is C_p(t). In a perfect world, we would have C_p(t)=t for all p and all t. In other words, dC/dt ideally should be 1.

Real timers do not interrupt exactly H times a second. Theoretically, a timer with H = 60 should generate 216,000 ticks per hour. In practice, the relative error obtainable with modern timer chips is about 10^-5, meaning that a particular machine can get a value in the range 215,998 to 216,002 ticks per hour. More precisely, if there exists some constant  such that

the timer can be said to be working within its specification. The constant is specified by the manufacturer and is known as the maximum drift rate. Slow, perfect, and fast clocks are shown in Fig. 3-5.

                                                         Fig. 1 . Not all clocks tick precisely at the correct rate.
If two clocks are drifting from UTC in the opposite direction, at a time t after they were synchronized, they may be as much as 2 t apart. If the operating system designers want to guarantee that no two clocks ever differ by more than , clocks must be resynchronized (in software) at least every /2 seconds. The various algorithms differ in precisely how this resynchronization is done.