Re: [Ntp] Robert Wilton's Discuss on draft-ietf-ntp-interleaved-modes-05: (with DISCUSS and COMMENT)

Miroslav Lichvar <mlichvar@redhat.com> Tue, 29 June 2021 08:53 UTC

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Date: Tue, 29 Jun 2021 10:53:03 +0200
From: Miroslav Lichvar <mlichvar@redhat.com>
To: Robert Wilton <rwilton@cisco.com>
Cc: The IESG <iesg@ietf.org>, draft-ietf-ntp-interleaved-modes@ietf.org, ntp-chairs@ietf.org, ntp@ietf.org, odonoghue@isoc.org
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Subject: Re: [Ntp] Robert Wilton's Discuss on draft-ietf-ntp-interleaved-modes-05: (with DISCUSS and COMMENT)
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Thanks for the review.

On Mon, Jun 28, 2021 at 08:55:52AM -0700, Robert Wilton via Datatracker wrote:
> Thanks for this document, and whilst I agree with Eric that the mechanism is
> clever, I am less convinced that it is wise.  Specifically, I have a deep
> concern with repurposing fields to have a different meaning with no indication
> other than heuristics on the field to deduce their true meaning.  It feels to
> me that this pseudo-extension will operationally make NTP harder to manage and
> to debug issues.

I agree it might feel that way, but as far as I can tell it's not a
problem in practice. At least I have not heard any complains from
people who use it.

> I note that Daniel Franke suggested that this extra information be carried in
> extension fields, but the authors are concerned with the increase in packet
> size causing problems.  I didn't really understand the explanation as to why
> this would be a problem, in that it is comparing the length between basic and
> extended packets, but if the extension was negotiated then could all packets
> use the extension fields and be of the same length?

That's a good point. The problem is that there is no good way to
negotiate it. Some widely-used server implementations drop requests
with unknown extension fields. From a missing response the client
cannot tell if the server doesn't support it, or if it's under heavy
load or there is a packet loss in the network somewhere.

> Alternatively, would it at least be possible to use an extension field to flag
> that the fields now have a different meaning?  Would that allow receivers to
> discard the packets with an "unknown extension" warning rather than a "the peer
> seems to be sending me garbage" error.

I think this would have the same problem as the negotiation.

-- 
Miroslav Lichvar