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Re: [Ntp] Follow-up to yesterday's mic comment about PTP security

Daniel Franke <dfoxfranke@gmail.com> Thu, 25 July 2019 11:48 UTC

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From: Daniel Franke <dfoxfranke@gmail.com>
Date: Thu, 25 Jul 2019 07:48:23 -0400
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To: Ulrich Windl <Ulrich.Windl@rz.uni-regensburg.de>
Cc: kristof.teichel@ptb.de, "ntp@ietf.org" <ntp@ietf.org>
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Subject: Re: [Ntp] Follow-up to yesterday's mic comment about PTP security
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> The probem is the "distance" here, as it definitely refers to "cable length",
> or more specifically "route length". Back in the time when the WWW started,
> connections inside Germany were routed via USA sometimes (that was the time
> whan a "good" transferrate was near 5kB/s). To make matters worse, you cannot
> safely assume that the route (length) from A to B will be the same as from B to
> A.

For the purposes of determining an error bound using the formula I
gave, assuming a distance that is too short will give you a more
conservative bound. So the safest assumption is that you are
communicating in a straight line in both directions, through a vacuum
medium. Making more "realistic" assumptions will give you a tighter
bound, but then an adversary who can violate those assumptions can
violate your error bound.

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