burst/iburst

[Gary E. Miller]

Yo Hal!

On Fri, 17 Jun 2016 04:05:40 -0700
Hal Murray <hmurray at megapathdsl.net> wrote:

> gem at rellim.com said:
> > Gack.  Something about how burst works was bugging me.  So I put
> > tcpdump on it.  I don't see any difference between burst and not
> > having burst... I'm not saying it does not do anything, just not
> > anything immediately obvious.   
> 
> burst only matters if the polling interval is greater than 6.  The
> burst size is limited to 8 and also limited so that the average
> polling rate doesn't exceed the non-burst rate with a polling
> interval of 6.

The burst (not iburst) size is this calculation:

	    peer->burst = min(1 << (peer->hpoll -
		peer->minpoll), NTP_SHIFT) - 1;

I'm not sure what that really means...  NTP_SHIFT is 8, so the max is 8.
But that 8 is an exponent, not an integer!  That makes max burst 256 I
think.

> > iburst does have an affect.  I see six, (not eight) time requests,
> > spaced 2 seconds apart.  Sorta like the doc says.  
> 
> 6 is what I expect after reading the documentation recently.  I may
> have said 8 previously.  If so, sorry for the confusion.

The doc also used to say 8, so you had the wrong answer on the good
authority.

> > But a second peer ignores the iburst, maybe some rate limit thing?  
> 
> Are you using "peer" or "server" in your ntp.conf?

Both, depending on the remote server type.  The doc said not to
burst/iburst with a peer. Eric added a change so ntpd enforces that, so
now iburst/burst is ony for servers.

> I can't think of any reason for the second (or third...) server
> to do anything differently from the first.  I'd expect them to be
> interleaved.  You do have to put iburst on each server line.

I had a mix of types for a while, so I could see the differences.  I can't
say that I see any differences in performance, bu my network is not the
kind burst/iburst is supposed to help.

RGDS
GARY
---------------------------------------------------------------------------
Gary E. Miller Rellim 109 NW Wilmington Ave., Suite E, Bend, OR 97703
	gem at rellim.com  Tel:+1 541 382 8588
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