Computer Network Time Synchronization

How NTP Disciplines the Time (Computer Network Time Synchronization)

Every NTP host has a hardware system clock, usually implemented as a counter driven by a quartz crystal or surface acoustic wave (SAW) oscillator. The counter interrupts the processor at intervals of 10 ms or so and advances the software system clock by this interval. The software clock can be read by application programs using […]

How NTP Manages Associations (Computer Network Time Synchronization)

Recall that an NTP client has an association for each remote server and local reference clock. There are three types of associations: persistent, preemptable, and ephemeral. Persistent associations are explicitly configured and mobilized at startup. Preemptable associations are mobilized at startup or as the result of a server discovery scheme described later. Ephemeral associations are […]

How NTP Discovers Servers (Computer Network Time Synchronization)

For a newbie confronting NTP for the first time, the most irksome task to overcome is the selection of one or more servers appropriate to the geography at hand. Over the years, this has become something of a black art and the origin of urban legends. There are four schemes to discover candidate servers: ordinary […]

How NTP Deals with Stale Time Values (Computer Network Time Synchronization)

During the lifetime of an association, which can be the lifetime of the computer and operating system on which it runs, there may come an hour, a day, or a week when one or another server becomes unreachable or shows only stale time values. What is a client to do when it has no time […]

How NTP Manages Network Resources (Computer Network Time Synchronization)

Think of the global NTP subnet as a vast collection of coupled oscillators, some nudging others faster or slower to huddle around UTC. Whatever algorithm is used to wrangle the oscillator herd, it must be stable, not given to ugly stampedes and endure milling about as oscillators join and leave the herd or lurch in […]

How NTP Avoids Errors (Computer Network Time Synchronization)

Years of accumulated experience running NTP in the Internet suggest the most common cause of timekeeping errors is a malfunction somewhere on the NTP subnet path from the client to the primary server or its synchronization source. This could be due to broken hardware, software bugs, or configuration errors. Or, it could be an evil […]

How NTP Performance Is Determined (Computer Network Time Synchronization)

No discussion of NTP operation is complete without mention of where timekeeping errors originate and what NTP does about them.However, for the purposes of discussion here, it is necessary only to describe how the errors are interpreted and how the error budget is compiled and passed along the chain from the primary servers through intervening […]

How NTP Controls Access (Computer Network Time Synchronization)

Most NTP servers provide service to an intended client population usually defined by a geographic or organization affiliation. In the case of public NTP servers, this is specified in the lists of public servers maintained at http:// If all NTP clients obeyed the rules specified in these lists, server access controls would not be […]

How NTP Watches for Terrorists (Computer Network Time Synchronization)

That serious havoc can result if the computer clocks supporting stock trading, airline reservation, and transportation systems are terrorized is self-evident. A determined hacker or terrorist could crash an airplane, invalidate a corporate buyout, or dismantle the telephone system, not to mention steal credit cards and render online commerce unusable. When the AT&T telephone network […]

How NTP Clocks Are Watched (Computer Network Time Synchronization)

The NTP software distributions include several utility programs that provide remote monitoring, control, and configuration functions. Ordinarily, these programs are used to detect and repair broken servers, find and fix bugs in the software, and monitor timekeeping performance. Two monitoring protocols and programs have been developed: ntpq to monitor the operation and overall performance of […]