Consistent asymetric latency on monitoring?

Perry Lorier perry at coders.net
Thu Oct 22 01:10:29 CDT 2009


Rick Ernst wrote:
> Resent, since I responded from the wrong address:
> ---
> The basic operation of IP SLA is as surmised; payload with timestamps
> and other telemetry data is sent to a 'responder' which manipulates
> the payload, including adding its own timestamps, and returns the
> altered payload.
>   

Yup :) It's the obvious way to do it :)

> I had to do a mental walk-through, but I think I see how drift can
> cause this. I'm going to generate some artificial data, graph it, and
> see if it matches the general waveshape I'm seeing.
>
> I purposefully have the traffic generators ntp syncing against the
> responders. I thought that would keep the clocks more closely in sync.
> I don't necessarily care if the time is 'right', just that it's the
> same. 

This causes major problems.  What you're actually measuring here is how 
well ntp can keep the clock sync'd under assymetric latency.  ntp is 
trying to do it's own measurements of one way delay, without the help of 
clocks to measure clock drift as well.   As you can see from your graphs 
ntp is not coping[1].

You are far better to have each end sync to a local stratum 1 or stratum 
2 ntp source, preferably one over a different link to the one under 
test.  If you don't have a local stratum 1/2 time source at each end,  
you might be able find one over a local exchange or other less congested 
link.  If this is very important to you then you should consider looking 
at running your own stratum 1 clocks at each end syncronised off 
something like GPS, CDMA or a T1 clock.

> What kind of difference should I expect if I sync both
> generators and responders against the same source, or not sync the
> responder? I'm thinking that having one source with constant drift may
> be better than both devices trying to walk/correct the time.
>   

Most hardware clocks in PC's/routers/switches etc have pretty atrocious 
amounts of drift if left to free run[2], sometimes in the order of 
seconds or occasionally minutes per week.  To get useful numbers you 
really do need to syncronise them to /something/.  Synchronising them to 
each other causes problems as ntp I think (I could be wrong) assumes 
mostly symmetrical latency, and if the latency isn't symmetric assumes 
it's because one clock is running fast/slow and will alter the clock's 
speed to account for it.  The great thing about ntp stratum 1 servers is 
that by definition they have more or less the same time no matter where 
they are, so synchronising each against a local ntp server will be a 
much much better solution.  If possible you should consider peering with 
at least 3 upstreams, preferably 4(!)[3] other ntp servers.

[1]: To be fair it's a hard problem.  Anything that involves time just 
gets more and more complicated the more you look at it, ntp is extremely 
clever and probably knows more about time than I'd ever want to know, 
but you're making it's job hard.

[2]: http://vancouver-webpages.com/time/ / 
http://vancouver-webpages.com/time/ltmhist.png

[3]: 
http://twiki.ntp.org/bin/view/Support/SelectingOffsiteNTPServers#Section_5.3.3.




More information about the NANOG mailing list