Subject: [Ntp] Review of draft-ietf-ntp-chronos-01

From: Watson Ladd <[email-masked]>
Date: Thu, 17 Sep 2020 20:15:34 -0400

Dear NTP WG,

I've taken a look at the draft and I have some concerns: some
editorial, some substantial.
Editorial:

The first is with the description of timeshifting attacks. NTS doesn't
stop a server from lying, but does mean that you know your
communication to the server can only get delayed, not massively
fiddled with the way unauthenticated ones can be. I think some
revision can keep it clear.

NTPv4 doesn't specify any means of dealing with the pool: that's a
later development and varies per implementation.

Roughtime is cited, but I don't see the citation in the text anywhere.
Likewise the reference inclusions of RFC5226 and RFC3552 seem out of
place.

Substantive:

As envisioned in the draft chronos is a watchdog: if the NTP update
goes too far from Chronos, we use Chronos. This is not a continuous
function of the inputs. This discontinuity needs some analysis,
particularly as it can lead to oscillation.

We need to know what the parameters are. Some suggested values are in the text.

I find the relative security analysis of Marzullo's algorithm vs
Chronos unconvincing. Chronos takes the average of the middle 1/3,
assuming the middle 1/3 is small. Marzullo's algorithm finds the
smallest interval compatible with at least 1/2 the measurements.
Chasing through the citations I find experiments done with KoD
elicitations, not falseticking.

Sincerely,
Watson Ladd