[Ntp] Handling of future versions in historic implementations

[Miroslav Lichvar <[e-mail]>]

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.

>From what I understand, NTPv0 was implemented by David Mills in assembly
for the Fuzzball router. I found a disk image that contains some text
related to NTP, but I didn't find an easy way to disassemble it and
even if I did I'd probably not understand it well enough to find the
instructions working with the reserved fiel which become the version
field in later specifications. If anyone would like to try that,
please let me know.

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.

The NTPv2 implementation seems to be a new implementation written from
scratch by Dennis Fergusson. This is the origin of "xntpd". In the code
that is available on www.eecis.udel.edu (from Nov 1989) there is:

	if (PKT_VERSION(pkt->li_vn_mode) == NTP_VERSION) {
		sys_newversionpkt++;
	} else if (PKT_VERSION(pkt->li_vn_mode) == NTP_OLDVERSION) {
		sys_oldversionpkt++;
	} else {
		sys_unknownversion++;
		return;
	}

Again, future versions were ignored.

The NTPv3 implementation was based on the NTPv2 xntpd and the NTPv3
support was implemented by Lars H. Mathiesen (according to the
COPYRIGHT file). In the code (from June 1994) there is:

	if (PKT_VERSION(pkt->li_vn_mode) >= NTP_VERSION) {
		sys_newversionpkt++;
	} else if (PKT_VERSION(pkt->li_vn_mode) >= NTP_OLDVERSION) {
		sys_oldversionpkt++;
	} else {
		sys_unknownversion++;
		return;
	}

This seems to be intentionally handling future versions as compatible
with NTPv3.

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 */
	} else {
		DPRINTF(2, ("receive: drop: RES_VERSION\n"));
		sys_badlength++;
		return;				/* old version */
	}

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.

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.

-- 
Miroslav Lichvar