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List: ntp-hackers
Subject: [ntp:hackers] NTP interleaved mode
From: "David L. Mills" <mills () udel ! edu>
Date: 2008-06-04 17:39:50
Message-ID: 4846D366.7050905 () udel ! edu
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Guys,
Some time back I proposed a method with with NTP could do something like
the IEEE 1588 two-step protocol, where the transmit timestamp is
captured in hardware and transmitted to the client in a following
packet. The mechanics of the timestamping process itself is explored in
a white paper on the NTP project page previously announced. The paper
was written to help resolve the issue of how to define how timestamps
are struck in the NTPv4 specification.
I've worked out the details on how to do this as an option in NTP
without changing the packet header format. It is done using an
interleaved approach which takes two basic rounds to deliver two sets of
timestamps. While the timestamps are delayed for two rounds, the overall
rate of timestamps is the same as the basic protocol and the symmetric
model is preserved. In addition, the protocol is backwards compatible
and automatically adapts to the basic or interleaved modes, so current
peers won't notice any change.
The protocol makes sense only for symmetric modes, as state must be
preserved at least for the transmit timestamp. While it recovers from
lost, duplicate or bogus packets, the recovery is not a smooth as in
basic mode. Largely for that reason, it is most suitable for fast LANs
and relatively small poll intervals. It is not intended as an
alternative to 1588, but it has potentially comparable accuracy. My
objective is that it could provide performance equivalent to the PPS
signal as used today.
There is a white paper and briefing on NTP On-Wire Protocols on the NTP
Project Page at www.eecis.udel.edu/~mills/ntp.html. In addition, there
is a simulation tool program that can be used to explore behavior under
various normal and error conditions. Comments would be relished. The
protocol is simple and requires only a couple of additional state
variables, but truly intricate. If you are interested and challenged,
figure out how to avoid a protocol reset when a peer sends more than one
poll during a single round, which can happen is symmetric modes.
Dave
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