[ntp:questions] very slow convergence of ntp to correct time.

David L. Mills mills at udel.edu
Mon Jan 28 22:09:11 UTC 2008


Good suggestion. In either burst mode the code sends a single packet at 
each poll interval, but sends the remaining packets only after receiving 
a response; packets in the burst are sent at 2-s intervals. The most 
cautious can set the headway to 512 s, in which a single packet is sent 
at that interval or less, but two packets at 1024 only upon reeiving 
aresponse for the first.

Burst mode is of course not intended for busy servers, much less the 
national standards servers. It is inended for paths involving lossy, low 
speed nets with poll intervals 1024 s or more. The young folks among us 
might not remember (or even be alive) when the Internet was new and 
paths to Canada had delays up to several seconds and loss rates up to 
ten percent.

However, you give me an idea. Why not shut down the burst when the clock 
filter delivers the first sample? Gotta think about that.


Eric wrote:

> On Mon, 28 Jan 2008 19:19:12 +0000, "David L. Mills" <mills at udel.edu> wrote
> for the entire planet to see:
>>Many years ago the Proteon routers dropped the first packet after the 
>>cache timed out; that was a disaster. That case and the ones you 
>>describe are exactly what the NTP burst mode is designed for. The first 
>>packet in the burst carves the caches all along the route and back. The 
>>clock filter algorithm tosses it out in favor of the remaining packets 
>>in the burst. No ICMP is needed or wanted.
> I agree about ICMP.  UDP would be better.
> And BURST / IBURST are nice, but conventional wisdom has it that BURST
> really shouldn't be used towards servers that you don't administer, and
> IBURST will of course not handle the ongoing case.  
> In considering this more, I think a great option or tinker value would be
> one that simply sends an extra packet, rather than eight of them, and only
> if the previous poll for that association was sent more than x seconds ago.
> In other words, as long as the poll value is say 7 or less, nothing new is
> needed.  When the poll exceeds 7, then ten seconds before a poll is due to
> be sent an "explorer" poll is sent (and any response would likely be
> discarded).  EBURST, or maybe PAVE.
> - Eric

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