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Re: [Ntp] Antw: [EXT] Re: Quick review of WGLC for status change for draft‑ietf‑ntp‑update‑registries

Miroslav Lichvar <mlichvar@redhat.com> Tue, 09 August 2022 08:13 UTC

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Date: Tue, 09 Aug 2022 10:13:01 +0200
From: Miroslav Lichvar <mlichvar@redhat.com>
To: Harlan Stenn <stenn@nwtime.org>
Cc: Hal Murray <halmurray@sonic.net>, "ntp@ietf.org" <ntp@ietf.org>
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Subject: Re: [Ntp] Antw: [EXT] Re: Quick review of WGLC for status change for draft‑ietf‑ntp‑update‑registries
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On Mon, Aug 08, 2022 at 10:33:12PM -0700, Harlan Stenn wrote:
> > NIST servers respond to NTPV4 requests with NTPv3.
> 
> which works perfectly with V4 clients, and it works because of the way the
> protocol was designed and has worked for ntp V1-V4.

It works only with clients that support NTPv3. Not all clients do
that. And v1 is not compatible with v2-v4. There is no mode field yet.

Responding with NTPv3 to an NTPv4 request is ok when you know that the
NTPv4 header is compatible with NTPv3.

The bigger issue is that the NIST servers are responding with NTPv3
even to NTPv5 requests, which is not specified yet. There is nothing
in RFC1305 or RFC5905 that would guarantee future compatibility.

NTPv5 needs to be designed in such a way that if it is misinterpreted
as NTPv3 or NTPv4, it will produce an invalid response that will be
ignored by the client. The current proposal has that property.

> It sure looks to me like the NTPv5 work will be breaking this.

Well, if NTPv5 is expected to have incompatible changes like shorter
extension fields, it cannot be compabile with NTPv4. If it was
compatible with NTPv4, it wouldn't have to change to NTPv5.

-- 
Miroslav Lichvar

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