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... summarizes the requirements, analytical models, algorithmic analysis and
performance under typical Internet conditions. Another document [MIL91b]
describes the NTP ...
... NTP Version 2 described in RFC-1119(-> 1305draft) [MIL89]. It is built on the Internet
Protocol (IP) [DAR81a] and User Datagram Protocol ...
... DAR81b], but is
specifically designed to maintain accuracy and robustness, even when
used over typical Internet paths involving multiple gateways, highly
dispersive delays and unreliable nets.
...
... Details specific to NTP packet formats used with the Internet Protocol
(IP) and User Datagram Protocol ...
...
Other mechanisms have been specified in the Internet protocol suite to
record and transmit the time at which an event takes place, including
the Daytime protocol ...
... SU81]. Experimental results on
measured clock offsets and roundtrip delays in the Internet are
discussed in [MIL83a], [MIL85b ...
... NTP places heavy emphasis on the accuracy
and stability of the service operated in an unmanaged, global-internet
environment. In DTS a synchronization subnet consists of clerks,
...
... can be seen that the design of Hellospeak strongly influenced the design
of NTP, Hellospeak itself is not an Internet protocol and is unsuited
for use outside its local-net environment.
...
... process requires a broadcast capability, which is not a ubiquitous
feature of the Internet. While this model has been extended to support
hierarchical configurations in which a slave on one network serves as a
...
... broadcast
and would result in unacceptable overhead in an internet environment. In
addition, for reasons given in Section 4 of this paper, the statistical
properties of the FTA algorithm ...
... addition, for reasons given in Section 4 of this paper, the statistical
properties of the FTA algorithm are not likely to be optimal in an
internet environment with highly dispersive delays.
...
... algorithms require large numbers of messages, especially
when large numbers of clocks are involved, and are designed to detect
faults that have rarely been found in the Internet experience. For these
reasons they are not considered further in this document.
...
... In practice it is not possible to determine the truechimers from the
falsetickers on other than a statistical basis, especially with
hierarchical configurations and a statistically noisy Internet. While it
is possible to bound the maximum errors in the time-transfer procedures,
assuming sufficiently generous tolerances are adopted for the hardware ...
... model unique is the adaptive configuration, polling,
filtering, selection and correctness mechanisms which tailor the
dynamics of the system to fit the ubiquitous Internet environment.
...
... 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
failures. Some number of local-net hosts ...
... Port (peer.peerport,
pkt.peerport): These are the 32-bit Internet address and 16-bit port
number of the peer.
...
... Port (peer.hostport,
pkt.hostport): These are the 32-bit Internet address and 16-bit port
number of the host ...
...
In the case of stratum 2 and greater (secondary reference) this is the
four-octet Internet address of the peer selected for synchronization.
...
...
Table 4<$&tab4> shows the parameters assumed for all implementations
operating in the Internet system. It is necessary to agree on the values
for these parameters in order to avoid unnecessary network overheads ...
... .PORT): This is the port number (123) assigned by the
Internet Assigned Numbers Authority to NTP.
...
... Minimum Poll Interval (NTP.MINPOLL): This is the minimum poll interval
allowed by any peer of the Internet system, in seconds to a power of
two.
...
... Maximum Poll Interval (NTP.MAXPOLL): This is the maximum poll interval
allowed by any peer of the Internet system, in seconds to a power of
two.
...
... described in Appendix B are implemented and the packet mode is
6 (control), the control-message procedure is called. The source and
destination Internet addresses and ports in the IP and UDP ...
... NTP specification, since there may be other algorithms that work
well in practice. However, one found to work well in the Internet
environment is described in Section 4 and its use is recommended.
...
... performance. However, a clock-
selection algorithm found to work well in the Internet environment is
described in Section 4, while a local-clock algorithm is described in
...
... restricting non-preauthorized accesses, unless the intent is to hide the
existence of the time server itself. Well-behaved Internet hosts are
expected to return an ICMP ...
... is required, the design can be based
on an access-control list with each entry consisting of a 32-bit
Internet address, 32-bit mask and three-bit mode. If the logical AND of
...
... algorithms
suggested in this section were developed and refined over several years
of operation in the Internet under widely varying topologies, speeds and
traffic regimes. While these ...
... represents the logarithm of the integration interval since the last
update. The clamps insure stable operation under typical conditions
encountered in the Internet. Then, compute new values for the Clock-
Adjust and Skew-Compensation registers
...
...
Considerable experience with the Internet environment suggests the
values of CLOCK.MAX tabulated in Table 6 as appropriate. In practice,
these values are exceeded with a single time-server source only under
...
... Defense Advanced Research Projects Agency. Internet Protocol. DARPANetwork Working Group Report RFC-791std5, USC Information SciencesInstitute, September 1981. ...
... Defense Advanced Research Projects Agency. Internet Control MessageProtocol. DARPA Network Working Group ...
... Time Synchronization in DCNET Hosts. DARPA Internet ProjectReport IEN-173, COMSAT Laboratories, February 1981. ...
...
In the case of stratum 2 and greater (secondary reference) this is the
four-octet Internet address of the primary reference host.
...
... ASCII nonprinting format effectors) can be added to improve
readability for simple monitoring programs that do not reformat the data
field. Internet addresses are represented as four octets in the form
[n.n.n.n], where n is in decimal notation and the brackets are optional.
Timestamps, including reference, originate, receive and transmit values,
...
... monitoring systems and even bring down major portions of the network in
the fashion of the classic Internet Worm.
...
... However, while it may eventually be the case that ubiquitous, widely
applicable authentication methodology may be adopted by the Internet
community and effectively overtake the mechanism described here, it does
not appear that specific standards and implementations will happen
within the lifetime ...
... to improve frequency stability to the order of a millisecond per day.
The various parameters were retuned for optimum loop stability using
measured data over typical Internet paths and with typical local-clock
hardware. In version 3 the phase-lock loop model was further refined to
...
... model and design example which has been refined with the aid of
feedback-control analysis and extensive simulation using data collected
over ordinary Internet paths. Section 5 of RFC-1119(-> 1305draft) on the NTP local
...
... overall NTP subnet stability in the Internet, it is essential that the
local-clock characteristics of all NTP time servers conform to the
...
... can be used in an NTP implementation to improve accuracy and stability
and reduce errors due to asymmetric paths in the Internet. The new
algorithm has been simulated using data collected over ordinary Internet
...
... and reduce errors due to asymmetric paths in the Internet. The new
algorithm has been simulated using data collected over ordinary Internet
paths and, along with the new local-clock algorithm, implemented and
...
... algorithm, implemented and
tested in the Fuzzball time servers now running in the Internet. Note
that this algorithm is not integral to the NTP ...
... crystal oscillators is probably in the range of 10-4 (100 ppm). Many if
not most Internet timekeeping systems can tolerate jitter to at least
the order of the intrinsic local-clock resolution, called precision in
the NTP ...