Re: [Ntp] Antw: Re: Calls for Adoption -- NTP Extension Field drafts -- Four separate drafts

Miroslav Lichvar <mlichvar@redhat.com> Wed, 28 August 2019 10:37 UTC

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Date: Wed, 28 Aug 2019 12:37:52 +0200
From: Miroslav Lichvar <mlichvar@redhat.com>
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Subject: Re: [Ntp] Antw: Re: Calls for Adoption -- NTP Extension Field drafts -- Four separate drafts
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On Wed, Aug 28, 2019 at 12:07:53PM +0200, Heiko Gerstung wrote:
> Regarding backwards compatibility of NTPv5: I think it would be no problem to change the NTP packet format if there are requirements we want to meet that can be fulfilled more efficiently with a changed packet header. Any implementation that ignores the version field deserves whatever happens to it and, since a client mode packet contains the version number, an NTP server would most probably have to reply to a v4 client request with a v4 server response anyway. Only if an implementation sends a request with version=5 and then tries to decode the response based on a v4 packet format there will be a problem. And again, implementations doing this should go down in flames anyway IMHO. 

I think a good reason to keep v5 mostly compatible with v4, similarly
to how previous versions were compatible with one another, would be
avoiding extra complexity in both server and client implementations.

My suggestion would be to keep the NTP header 48 octets long and
change only two fields: the refid and reference timestamp. They are
ignored by current servers and most clients. That gives us 12 octets
of contiguous space in the header to work with. That's plenty for the
timescale negotiation and other metadata. Longer fields should be in
extension fields. No MACs allowed.

FWIW, I'm willing to help with the NTPv5 specification if people are
considering to start working on that.

-- 
Miroslav Lichvar