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subnet
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... does not require reliable message delivery. The synchronization subnet
uses a self-organizing, hierarchical-master-slave configuration, with
synchronization paths determined by a minimum-weight spanning ...
... MIL85b] and further evolved under typical
operating conditions over the last three years. In addition, as the
result of experience in operating multiple-server subnets including
radio clocks at several sites in the U.S. and with clients in the U.S.
...
... service operated in an unmanaged, global-internet
environment. In DTS a synchronization subnet consists of clerks,
servers, couriers and time providers. With respect to the NTP
...
... selection algorithms are designed so that the clock synchronization
subnet self-organizes into a hierarchical-master-slave configuration
[MIT80]. With respect to timekeeping accuracy and stability, the
...
... relative to the peer. Each of these components are maintained
separately in the protocol in order to facilitate error control and
management of the subnet itself. They provide not only precision
measurements of offset and delay, but also definitive maximum error
bounds, so that the user interface ...
... services. Under
normal circumstances it is intended that the synchronization subnet of
primary and secondary servers assumes a hierarchical-master-slave
configuration with the primary servers at the root ...
... root of
the synchronization subnet. Appendix H contains an analysis of errors,
including a derivation of maximum error as a function of delay and
dispersion, where the latter quantity depends on the precision of the
...
... time within known accuracies, this provides a reliable, determistic
specification on timekeeping accuracies throughout the synchronization
subnet.
...
... learned such lessons at considerable cost [ABA89], the synchronization
subnet topology should be organized to produce the highest accuracy, but
must never be allowed to form a loop. An additional factor is that each
...
...
As a result of this design, the subnet reconfigures automatically in a
hierarchical-master-slave configuration to produce the most accurate and
reliable time, even when one or more primary or secondary servers or the
...
... primary servers (e.g., highly accurate WWVB radio clock operating at the
lowest synchronization distances) on a possibly partitioned subnet fail,
but one or more backup primary servers (e.g., less accurate WWV radio
clock operating at higher synchronization ...
... clock operating at higher synchronization distances) continue operation.
However, should all primary servers throughout the subnet fail, the
remaining secondary servers will synchronize among themselves while
distances ratchet upwards to a preselected maximum <169>infinity<170>
...
... well-known properties of the Bellman-Ford algorithm. Upon
reaching the maximum on all paths, a server will drop off the subnet and
free-run using its last determined time and frequency. Since these
computations are expected to be very precise, especially in frequency,
...
... primary reference source at the root of the synchronization subnet, in
seconds. Note that this variable can take on both positive and negative
values, depending on clock precision and skew.
...
... root of
the synchronization subnet, in seconds. Only positive values greater
than zero are possible.
...
... NTP.MAXSTRATUM): This is the maximum stratum value that
can be encoded as a packet variable, also interpreted as
<169>infinity<170> or unreachable by the subnet routing algorithm.
...
... operating near the root nodes (lowest stratum) of the synchronization
subnet and with a relatively large number of peers on an intermittent
basis. In this mode the identity ...
... the end nodes (highest stratum) of the synchronization subnet. Reliable
time service can usually be maintained with two peers at the next lower
...
... peer to the root of the synchronization subnet. Subscripts will be used
to identify the particular peer when this is not clear from context. The
...
... procedure below). The variables relative to the root of the
synchronization subnet via peer i are determined as follows:
...
... the peer to the root of the synchronization subnet. The host will not
synchronize to the selected peer if the distance is greater than
...
... NTP.MAXDISTANCE. The reason for the minimum clamp at NTP.MINDISPERSE is
to discourage subnet route flaps that can happen with Bellman-Ford
algorithms and small roundtrip delays.
...
... should not in
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 throughout the
synchronization subnet at the same time. In principle, the subnet
vulnerability can be engineered through the selection of ...
... throughout the
synchronization subnet at the same time. In principle, the subnet
vulnerability can be engineered through the selection of time servers
...
... is returned to the requestor. Through appropriate choice of
mask, it is possible to restrict requests by mode to individual
addresses, a particular subnet or net addresses, or have no restriction
at all. The access-control list would then serve as a filter ...
... distribution is the complex of algorithms used to reduce the effect of
statistical errors and falsetickers due to failure of various subnet
components, reference sources or propagation media. The algorithms
...
... in an NTP synchronization subnet must implement these algorithms. For
instance, simple workstations may dispense with one or both of them in
...
... connectivity. The parameters have been engineered for reliable operation
in a multi-level hierarchical subnet where unstable operation at one
level can disrupt possibly many other levels.
...
... accurate, than the NTP synchronization subnet, the recommended approach
at initialization is to set the Clock register ...
... synchronization with the broadcast timecode and only after the
majority of them have resynchronized will the subnet settle down. The
CLOCK.MINSTEP delay is designed to cope with this problem by forcing a
minimum interval since the last gradual adjustment was made before
...
...
paths exist between a trusted, stratum-one server in a particular
synchronization subnet and all other servers in that subnet. It employs
a crypto-checksum ...
... paths exist between a trusted, stratum-one server in a particular
synchronization subnet and all other servers in that subnet. It employs
a crypto-checksum, computed by the sender ...
... versions or existing implementations; however, in order to insure
overall NTP subnet stability in the Internet, it is essential that the
local-clock characteristics of all NTP time ...
... time servers and then automatically distributed
throughout the synchronization subnet to all other time servers.
Calendar Systems
...
...
A common problem in synchronization subnets is systematic time-offset
errors resulting from asymmetric transmission paths, where the networks
...
... NTP time-server
model. Timestamps exchanged with possibly many other subnet peers are
used to determine individual roundtrip delays and clock offsets relative
to each peer as described in the NTP ...
... delay and dispersion to the root (primary reference source) of the
synchronization subnet. It also discusses correctness assertions about
these error bounds and the time-transfer, filtering and selection
...
... primary reference source at the root of the synchronization subnet.
Exceptions will be noted as they arise.
...
... at the root of the synchronization subnet. The values of these variables
are either included in each update message or can be derived as
...
... as well as various error accumulations as described below. The following
discussion establishes how errors inherent in the time-transfer process
accumulate within the subnet and contribute to the overall error budget
at each server.
...
... <$EEPSILON> (sys.rootdispersion) relative to the root of the
synchronization subnet, as shown in Figure 15. Note the inclusion of the
selected peer dispersion and skew accumulation since the dispersion was
last updated, as well as the select dispersion <$Eepsilon sub xi>
...
... algorithm itself. Also, note that, in order
to preserve overall synchronization subnet stability, the final clock
offset <$ETHETA> is in fact determined from the offset of the local
clock relative to the peer clock, rather than the root ...
... offset <$ETHETA> is in fact determined from the offset of the local
clock relative to the peer clock, rather than the root of the subnet.
Finally, note that the packet variables <$EDELTA prime> and <$EEPSILON
prime> are in fact determined from the latest message received, not at
...
... sub i> represent the values at peer i relative to the root of the
synchronization subnet, the values
...
... clock can be determined directly, the offset relative to the root of the
synchronization subnet is not directly determinable, except on a
probabilistic basis and within the bounds established in this and the
previous section.
...
... distance path from the root of the synchronization subnet to a given
server i are the clock offset <$ETHETA sub i>, roundtrip delay <$EDELTA
sub i> and dispersion <$EEPSILON sub i> inherited by and characteristic
...
... development it was shown that, if the primary reference source at the
root of the synchronization subnet is in fact a correct clock, then the
true offset <$Etheta sub 0> relative to that clock must be contained in
the interval
...