Time Servers and NTP

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Time servers and NTP - what are they?

Time Servers and NTP are widely used to synchronise the time on computer networks. NTP provides the ability to access time servers,organize the time Synchronisation subnet and adjust the local clock in each participating subnet computer. Typically, NTP providesaccuracies of between 1 and 50 milliseconds depending on the time source and network paths.

Network Time Protocol can be used to synchronise the time on computers across a network. A NTP time server is used to obtain the correcttime from a time source and adjust the local time in each connecting computer.

It is extremely important choosing the right time source to work with the time server, as this forms the basis of all time updatesacross the network. Recent studies show a large number of stratum 1 time sources on the internet are bad time keepers. A reported 391 of957 supposedly stratum 1 NTP time sources had time offsets of over 10 seconds. Incredibly, one time source was offset by a staggering6 years. these facts are based on research by Nelson Minar, MIT Media Lab Cambridge, MA USA .

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The major reason for this time inaccuaracy on the internet appears to be Mis-configuration

The quality of the time source used by the time server cannot be stressed more highly. The accuracy of each computer time on the networkis dependant on the accuracy of the time source used by the time server. A helpful hint is to beware when obtaining the time from sourcesthat cannot be validated, i.e. from an unknown third party across the internet.

Operating Modes And Addressing

NTP Version 4 can operate in either unicast (point to point), multicast (point to multipoint) or anycast (multipoint to point) modes.

A unicast client sends a request to a designated server at its unicast address and expects a reply from which it can determine the time and, optionally, the roundtrip delay and local clock offset relative to the server.A multicast server periodically sends a unsolicited message to a designated IPv4 or IPv6 local broadcast address or multicast group addressand ordinarily expects no requests from clients. A multicast client listens on this address and ordinarily sends no requests.

An anycast client sends a request to a designated IPv4 or IPv6 local broadcast address or multicast group address. One or more anycastservers reply with their individual unicast addresses. The client binds to the first one received, then continues operation in unicast mode.

Multicast servers should respond to client unicast requests, as well as send unsolicited multicast messages. Multicast clients may sendunicast requests in order to determine the network propagation delay between the server and client and then continue operation in multicastmode.

In unicast mode, the client and server end-system addresses are assigned following the usual IPv4, IPv6 or OSI conventions. In multicastmode, the server uses a designated local broadcast address or multicast group address. An IP local broadcast address has scope limited toa single IP subnet, since routers do not propagate IP broadcast datagram's. On the other hand, an IP multicast group address has scopeextending to potentially the entire Internet. The scoping, routing and group membership procedures are determined by considerations beyondthe scope of this document. For IPv4, the IANA has assigned the multicast group address for NTP, which is used both by multicastservers and anycast clients. NTP multicast addresses for IPv6 and OSI have yet to be determined.

Multicast clients listen on the designated local broadcast address or multicast group address. In the case of local broadcast addresses,no further provisions are necessary. In the case of IP multicast addresses, the multicast client and anycast server must implement theInternet Group Management Protocol (IGMP) [DEE89], in order that the local router joins the multicast group and relays messages to theIPv4 or IPv6 multicast group addresses assigned by the IANA. Other than the IP addressing conventions and IGMP, there is no differencein server or client operations with either the local broadcast address or multicast group address.

Anycast mode is designed for use with a set of co-operating servers whose addresses are not known beforehand by the client. An anycastclient sends a request to the designated local broadcast or multicast group address as described below. For this purpose, the NTP multicastgroup address assigned by the IANA is used. One or more anycast servers listen on the designated local broadcast address or multicast groupaddress. Each anycast server, upon receiving a request, sends a unicast reply message to the originating client. The client then binds tothe first such message received and continues operation in unicast mode. Subsequent replies from other anycast servers are ignored.


Nelson Minar. MIT Media Lab E15-305 20 Ames Street Cambridge, MA 02139 USA

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