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Time Server
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... NTP) Version 3, which is used to synchronize timekeeping among
a set of distributed time servers and clients. It defines the
architectures, ...
... of nanoseconds while preserving a non-ambiguous date well into the next
century. The protocol includes provisions to specify the characteristics
and estimate the error of the local clock and the time server to which
it may be synchronized. It also includes provisions for operation with a
number of mutually suspicious, hierarchically distributed primary
...
... scheme is intended for multi-server LANs where each of a set of possibly
many time servers determines its local-time offset relative to each of
the other servers in the set using periodic timestamped messages, then
...
... resources, such as backbone gateways, and operated as primary time
servers. The purpose of NTP is to convey timekeeping information from
these servers to other time servers ...
... time
servers. The purpose of NTP is to convey timekeeping information from
these servers to other time servers via the Internet and also to cross-
check clocks and mitigate errors due to equipment or propagation
...
... hosts or gateways, acting as
secondary time servers, run NTP with one or more of the primary servers.
In order to reduce the protocol overhead ...
... specified time. Dispersion represents the maximum error of the local
clock relative to the reference clock. Since most host time servers will
synchronize via another peer time server, there are two components in
...
... host time servers will
synchronize via another peer time server, there are two components in
each of these three products, those determined by the peer relative to
the primary reference source of standard time and those measured by the
...
... subnet is a connected network of primary and
secondary time servers, clients and interconnecting transmission paths.
A primary time server ...
... time servers, clients and interconnecting transmission paths.
A primary time server is directly synchronized to a primary reference
source, usually a radio clock. A secondary time server derives
...
... A primary time server is directly synchronized to a primary reference
source, usually a radio clock. A secondary time server derives
synchronization, possibly via other secondary servers, from a primary
server over ...
... topology should be organized to produce the highest accuracy, but
must never be allowed to form a loop. An additional factor is that each
increment in stratum involves a potentially unreliable time server which
introduces additional measurement errors. The selection algorithm used
...
... mode the host, usually a LAN time server, announces its willingness to
synchronize, but not to be synchronized by the peer.
...
... operating in this mode the host, usually a LAN time server operating on
a high-speed broadcast ...
... client/server distinction (almost)
disappears. Symmetric passive mode is intended for use by time servers
operating near the root nodes (lowest stratum) of the synchronization ...
...
Symmetric active mode is intended for use by time servers operating near
the end nodes (highest stratum) of the synchronization ...
... LANs with
numerous workstations and where the highest accuracies are not required.
In the typical scenario one or more time servers on the LAN send
periodic broadcasts ...
... considerably simplified in this mode; however, a modified form of the
clock selection algorithm may prove useful in cases where multiple time
servers are used for enhanced reliability.
...
... work to build a minimum-weight spanning tree relative to the primary
reference time servers and thus a hierarchical-master-slave
synchronization subnet.
...
... The NTP design is such that accidental or malicious data modification
(tampering) or destruction (jamming) at a time server should not in
general result in timekeeping errors elsewhere in the synchronization
...
... general result in timekeeping errors elsewhere in the synchronization
subnet. However, the success of this approach depends on redundant time
servers and diverse network paths, together with the assumption that
tampering or jamming will not occur at many time servers ...
... time
servers and diverse network paths, together with the assumption that
tampering or jamming will not occur at many time servers throughout the
synchronization subnet at the same time. In principle, the ...
... at the same time. In principle, the subnet
vulnerability can be engineered through the selection of time servers
known to be trusted and allowing only those time servers to become the
...
... vulnerability can be engineered through the selection of time servers
known to be trusted and allowing only those time servers to become the
synchronization source. The authentication procedures described in
...
... It does not seem useful to maintain a secret clock, as would result from
restricting non-preauthorized accesses, unless the intent is to hide the
existence of the time server itself. Well-behaved Internet hosts are
...
... list while minimizing wander, especially on high-speed LANs with many
time servers. Wander causes needless network overhead, since the poll
...
... addition, the protocol environment resists jamming attacks by employing
redundant time servers and diverse network paths. Resistance to
stochastic disruptions, actual or manufactured, are minimized by careful
...
... schemes; although several possibilities are immediately obvious. In one
scenario a group of time servers peers among themselves using symmetric
modes and shares one secret key, say key 1, while another group ...
... optional features, (1) an algorithm to combine the offsets of a number
of peer time servers in order to enhance accuracy and (2) improved
local-clock algorithms which allow the poll intervals on all
...
... a new algorithm to combine the offsets of a number of peer
time servers is presented in Appendix F. This algorithm is modelled on
those used by national standards laboratories to combine the weighted
...
... paths and, along with the new local-clock algorithm, implemented and
tested in the Fuzzball time servers now running in the Internet. Note
that this algorithm ...
... designed so that these bits can be set manually or by the radio timecode
at the primary time servers and then automatically distributed
throughout the synchronization subnet ...
... fifteen seconds late relative to UTC. However, NTP primary time servers
resolve the epoch using the broadcast timecode, so that the NTP ...
... satellite/landline paths. It
has been found experimentally that these errors can be considerably
reduced by combining the apparent offsets of a number of time servers to
produce a more accurate working offset. Following is a description of
the combining method ...
... topology is that of a tree rooted at the
primary server(s). Thus, there is an unbroken path from every time
server to the primary reference source. Accuracy and stability are
proportional to synchronization distance <$ELAMBDA>, defined as
...