Re: [chrony-users] NTP bogus timestamps - Chrony on openSUSE 15.1

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On Wed, 21 Aug 2019, James Knott wrote:

On 2019-08-21 09:44 AM, Miroslav Lichvar wrote:
It has no impact on accuracy.

Maybe not on my local network, but what if the server was some distance
away?  I realize NTP was developed back in the days when a 56 Kb/s
connection was really something, but even with today's high bandwidth
connections there is some latency that would cause the client to be
slightly behind the server.  The calculations based on those time stamps
were meant to determine that latency and correct for it.

NOTHING changes in the ntp protocol. The protocol does NOT use the sent
timestamp, but the time from a saved table in which that sent timestamp is
associated with the correct time that the packet was sent. The timestamp acts
purely as an identifier of the packet.

Incidentally, at work a few months ago, there was some discussion about
NTP on a major LRT project I was working on, though I wasn't directly
involved with the NTP servers.  On this system, they have 2 GPS/NTP
servers, at different locations, that were to be synced with 2 other
servers.  This system runs over a fibre backbone, that's 11 Km long and
they're somewhat fussy about NTP.  I had to explain, to one of my
co-workers, how NTP worked.

Great. But in your explanation, just insert a line which says "When the client
receives back the packet from the server, it uses the T1 timestamp to look up
in a local table what time it was when that first client packet was sent to
the server and uses that, instead of the timestamp in the packet as the T1 in
the ntp protocol. Remember that the server does NOTHING with that T1
timestamp. All the server does is to fill in the the next two slots, when the
server received the packet and when it sent it out again. Only the client, which can have ( and does if the client is designed to
impliment that "NTP Client Data Minimization" note. ) the needed data, ever
does anything with that first timestamp.

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