NTP glossary

A number of terms describe important entities or activities that are part of NTP. These are defined here.

client mode

The mode in which a host polls a time server that it might synchronize with, but it will not respond to polls from that time server. When a host is operating in this mode, the time server it is polling is said to operate in ``server mode''.

Coordinated Universal Time

The international standard reference time. It also corresponds to the local time at zero longitude. The standards pertaining to the definition and maintenance of Coordinated Universal Time, which effectively replaced Greenwich Mean Time (GMT) on 1 January 1972, are promulgated in Recommendation 460 of the International Consultative Committee for Radio (Comite Consultatif International de Radiodiffusion or CCIR) of the International Telecommunications Union. The CCIR is located at 2, rue de Varembe, 1211 Geneva 20, Switzerland. Responsibility for time standards in the United States rests with the Time and Frequency Division of the National Institute of Standards and Technology (NIST) in Boulder, Colorado.

dispersion

A measure, in seconds, of how scattered the time offsets have been from a given time server.

drift

A measure, in hertz per second, of how quickly the skew of a clock is changing. See also ``skew''.

host

Any computer connected to the network.

Internet

The collection of interconnected networks that grew out of ARPANET, the Defense Advanced Research Projects Agency network, and that use TCP/IP to communicate to function as a single cooperative network.

poll

The sending of an NTP packet from a host to an NTP time server to request the current time. The server responds by recording the current time in the packet, then sending it back to the originating host. See also ``NTP packet''.

NTP packet

A message sent over the network that conforms to the Network Time Protocol format. This format includes space for recording the current time. See also ``poll''.

primary server

Another name for a stratum 1 server. See also ``stratum''.

roundtrip delay

The time it takes for a host to send an NTP packet to another host and get an NTP packet back from that host in reply.

secondary server

Another name for a stratum 2 server.

server mode

The mode in which a time server allows itself to be polled by a host (the client) that wishes to synchronize with it. In this mode, if the time server polls the client to try to synchronize with it, the client does not respond. In this case, the client is said to operate in ``client mode''.

skew

A measure, in hertz, of the difference between the actual frequency of a clock and what its frequency should be to keep perfect time. See also ``drift''.

slew

To adjust gradually the time of a clock until it tells the correct time. Compare with ``step''.

step

To change the time of a clock to the correct time with no intermediate adjustments. Compare with ``slew''.

stratum

The distance a host running the xntpd time daemon is from an external source of Coordinated Universal Time (UTC). A stratum 1 server has direct access to an external source of UTC, such as a radio clock synchronized to a standard time signal broadcast. In general, a stratum n server is n-1 network hops away from a stratum 1 server. For example, a stratum 4 server is 3 hops away from a stratum 1 server. Also, a stratum n server is at a higher stratum than a stratum n-1 server. For example, a stratum 3 server is at a higher stratum than a stratum 2 server, and at a lower stratum than a stratum 4 server. See also ``time daemon''.

symmetric active mode

The mode in which a host has configured itself to poll a time server that it might synchronize with. In this mode, the host also allows itself to be polled by that time server.

symmetric passive mode

The mode in which a time server is polled by a host that has configured itself in ``symmetric active mode''. In this mode, the time server can also poll that host.

synchronization subnet

A collection of hosts that synchronize time with each other. The top layer of the subnet consists of stratum 1 servers.

synchronize clocks

To set two clocks to the same time and ensure that they are running at the same ``speed''. The speed at which a clock runs is determined by its frequency, that is, how often it ``ticks'' to the next fraction of a second. The design of a particular clock determines how small that fraction is.

synchronize with a host

To synchronize the local clock with another host's clock, either by stepping or slewing the local clock to the time reported in the NTP packet received from the most recent poll of that host.

time client

A host running the xntpd time daemon that requests a time server to furnish it with that server's best estimate of Coordinated Universal Time. Hosts running at a stratum higher than 1, except for those at the highest stratum, typically function as both time clients (polling same-stratum or lower-stratum servers) and time servers (furnishing the current time to other hosts). Compare with ``time server''.

time daemon

The program running on a host that synchronizes the host's hardware clock to Coordinated Universal Time in accordance with the protocols known as the Network Time Protocol. The name of this program is xntpd.

time server

A host running the xntpd time daemon that, upon request, furnishes its best estimate of Coordinated Universal Time. Compare with ``time client''.

trap receiver

A program that listens for NTP mode 6 packets sent from another host. The host that ``springs the trap'' by sending the mode 6 packet to the trap receiver does so in response to the occurrence of an exception. These exceptions include the following events:

• The host reboots.

• The host detects that its clock has been reset.

• The host detects that the stratum of some other host in the synchronization subnet has changed.

• The host detects that a new host has been added to the synchronization subnet.

• The host detects that some other host in the synchronization subnet has become unreachable.

• The host detects that some other host in the synchronization subnet has been authenticated.

• The host selects another host to synchronize with.

• The host detects that a system call has failed.

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