Re: [Ntp] Antw: [EXT] Handling of future versions in historic implementations

On 9/23/2022 12:27 AM, Ulrich Windl wrote:
>>>> Miroslav Lichvar <mlichvar@redhat.com> schrieb am 22.09.2022 um 17:25 in
> Nachricht <Yyx+VOkuFCAuBBYb@localhost>:
>> I did a bit of archeology to see how the older NTP implementations
>> handled future versions, now that there are some concerns about NTPv5
>> making incompatible changes.
> 
> Thanks for the work!
> 
> ...
>> The NTPv1 implementation (ntpd) was written by Louis A. Mamakos and Mike
>> Petry in C. In the code I found (from May 1989) in the main() function
>> there is:
>>
>> 	if ((pkt‑>status & VERSIONMASK) != NTPVERSION_1)
>> 		continue;
>>
>> 	receive(dst, pkt, &tv, i);
>>
>> That is, future versions were ignored.
> 
> ...and "older versions"...
> 
>>
>> The NTPv2 implementation seems to be a new implementation written from
> ...
>> Again, future versions were ignored.
>>
>> The NTPv3 implementation was based on the NTPv2 xntpd and the NTPv3
> ...
>> This seems to be intentionally handling future versions as compatible
>> with NTPv3.
> 
> A bad idea as the version was not defined at the time of writing
> ...
> 
>>
>> The NTPv4 support was implemented in xntpd by David Mills and it was
>> renamed to ntpd. In the current version of the code there is:
>>
>> 	if (hisversion == NTP_VERSION) {
>> 		sys_newversion++;		/* new version */
>> 	} else if (   !(restrict_mask & RES_VERSION)
>> 		   && hisversion >= NTP_OLDVERSION) {
>> 		sys_oldversion++;		/* previous version */
> 
> I think the assumption was (at the time of writing) that there won't be a
> version > NTP_VERSION, so the oldversion test would apply to the interval
> [NTP_OLDVERSION...NTP_VERSION-1].

You would be wrong.

> As obvious in all example code, a comment explaining what the code is supposed
> to do would have been helpful.
> 
> ...
> 
>> The future versions are accepted as "previous version". This might
>> seem like a bug, but from what we have heard on this list recently,
>> it was intended.
> 
> I think that's just a "postmortem interpretation", but it never was the
> intention.

You would be wrong.

>>
>> So, it seems this idea of future versions being compatible with older
>> versions started with NTPv3, but for some reason it wasn't specified
>> in RFC 1305 or RFC 5905.
> 
> As stated before, I think "it's a bad idea".

You are certainly entitled to your opinion.

> Regards,
> Ulrich
> 
> 
> 
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-- 
Harlan Stenn <stenn@nwtime.org>
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