[ntp:questions] Thoughts on KOD

Rob nomail at example.com
Mon Jul 7 16:11:40 UTC 2014

Danny Mayer <mayer at ntp.org> wrote:
> On 7/6/2014 2:42 AM, Rob wrote:
>> Harlan Stenn <stenn at ntp.org> wrote:
>>> Discussion appreciated.
>> I think it is best to remove KOD from ntpd.
>> It does not serve a useful purpose, because precisely the kind of
>> clients that you want to say goodbye to, do not support it.
>> In real life it has either no effect at all, or it even has a negative
>> effect because the client does not understand it and re-tries the
>> request sooner than it would when no reply was sent at all.
> You haven't read the code. Any client that ignores the KOD flag will
> find (if they ever looked) that their clock will be drifting away
> further and further from the proper time. When KOD is set the value of
> the received and sent timestamps are the same as the initial client sent
> timestamp. It doesn't use the system time for the returned packet.
> Calculate what this does to the resulting clock.
> Please also note that there is more than one type of KOD packet. See RFC
> 5905 Section 7.4. See also Figure 13. You need to clearly distinguish
> the different ones when talking about them. Most of this discussion
> seems to be about action a. As discussed above this is an extremely
> useful feature because any client ignoring the KOD flag and using the
> packet any way will get pushed way of the actual time that they would
> normally expect regardless of the client software used.

All that does not matter much because the client will usually not notice
it and will continue to send the requests you don't like.

When KOD would have been part of the standard from the beginning, one
could expect that most clients would handle it and exit or warn the
admin.  However, it was added later, and clients do exist that treat
KOD as a mishap that can be corrected by re-trying the request.
Immediately, not after 60 seconds or so.
So when you reply KOD to every request, you end up getting as many
requests as the ping-pong time to the client allows.

Of course, that has already been remedied by not replying with KOD
every time, but the basic issue still remains.

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