[ntp:questions] ntpd wedged again

unruh unruh at invalid.ca
Sun Feb 26 15:27:02 UTC 2012


On 2012-02-26, Danny Mayer <mayer at ntp.org> wrote:
> On 2/26/2012 12:17 AM, Dave Hart wrote:
>> On Sun, Feb 26, 2012 at 03:37, Danny Mayer <mayer at ntp.org> wrote:
>>> It could well be that rackety is sending KOD packets and this server is
>>> not recognizing them as such. rackety has a heavy load under normal
>>> circumstances so it may well do so. When it does so the ntp timestamps
>>> are going to be totally wrong, deliberately, so they should have been
>>> discarded immediately. I don't remember whether or not KOD had been
>>> introduced back when 4.2.2 was shipped so it may not be ignoring it when
>>> it should.
>>>
>>> I suggest you pick a different server with a more modern version of ntpd
>>> and see if that stabilizes the situation.
>> 
>> While I'm all in favor of running modern ntpd, it's not going to
>> affect KoD handling in terms of timekeeping.  KoD responses only occur
>> when a source IP exceeds relatively generous rate limits, and they
>> carry timestamps that are totally useless, deliberately, which is a
>> different beast than totally wrong.  KoD timestamps are set to match
>> the sender's originate timestamps, so if they attempt to use them for
>> time, they learn nothing their local oscillator wasn't already telling
>> them.
>
> It's worse than that. I analyzed the KOD code at the time and in fact
> what the reference code does it copy the originator starting timestamp
> to the receiving and sending server timestamp and sends it back. Now if
> you calculate the resulting offset you will get a nasty surprise. After
> a short discussion with Dave Mills he agreed with me that it results in
> the result shifting and increasing the resulting offset. Check the
> results for yourself!

It sets the offset to half the round trip time.

>
> Danny



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