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 provides
accuracies 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 correct
time 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 updates
across the network. Recent studies show a large number of stratum 1 time sources on the internet are bad time keepers. A reported 391 of
957 supposedly stratum 1 NTP time sources had time offsets of over 10 seconds. Incredibly, one time source was offset by a staggering
6 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 network
is dependant on the accuracy of the time source used by the time server. A helpful hint is to beware when obtaining the time from sources
that 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 address
and 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 anycast
servers 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 send
unicast requests in order to determine the network propagation delay between the server and client and then continue operation in multicast
In unicast mode, the client and server end-system addresses are assigned following the usual IPv4, IPv6 or OSI conventions. In multicast
mode, the server uses a designated local broadcast address or multicast group address. An IP local broadcast address has scope limited to
a single IP subnet, since routers do not propagate IP broadcast datagram's. On the other hand, an IP multicast group address has scope
extending to potentially the entire Internet. The scoping, routing and group membership procedures are determined by considerations beyond
the scope of this document. For IPv4, the IANA has assigned the multicast group address 184.108.40.206 for NTP, which is used both by multicast
servers 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 the
Internet Group Management Protocol (IGMP) [DEE89], in order that the local router joins the multicast group and relays messages to the
IPv4 or IPv6 multicast group addresses assigned by the IANA. Other than the IP addressing conventions and IGMP, there is no difference
in 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 anycast
client sends a request to the designated local broadcast or multicast group address as described below. For this purpose, the NTP multicast
group address assigned by the IANA is used. One or more anycast servers listen on the designated local broadcast address or multicast group
address. Each anycast server, upon receiving a request, sends a unicast reply message to the originating client. The client then binds to
the 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