[AG-TECH] Packet Loss

[Chris Kendrick]

Hi Markus

On Thu, 2006-08-24 at 13:08 +1000, Markus Buchhorn wrote:
At 10:53 AM 24/08/2006 +1000, Chris Kendrick wrote:
> >For some reason I was getting ~5sec patches of huge packet loss (as
> >shown in the RAT reception quality matrix) roughly every 3 minutes
from
> >them (voice went extremely choppy), but they were always receiving
good
> >audio from me.
> 
> Was it a regular interruption? i.e. every 180 seconds or whatever?
> 

Hmm, I didn't have my stopwatch out.  But I see how it can give
significant insight so I will take closer attention to that next time.


> Packet loss can be very uni-directional, and is usually unlikely to be
the software or tools (until you stress the bandwidth or cpu - e.g. vic
claims packet loss when it fails to service the network buffer quickly
enough due to cpu load; it's local packet loss, not network packet
loss). The bridge might be suspect, so testing with another bridge would
be good. Also, check by time of day.
> 

How do I check if it is the local machine having packet loss rather than
network packet loss?

If I happens again, I will do some testing with the local AG Admin and
test if it is a time of day thing.  

I did try a different Virtual Venue, but upon looking at the bridge that
is available for selection when on unicast at each VV I have since
noticed that they both use the same bridge AGSC/Manchester.   If it
happens again I will try to set up a testing session with the AG Admin
at the other end to try another bridge, unfortunately the people running
the AG at 8am there time are not technicians so I couldn't check this
straight away.


> >My question is what tools tecniques can I use to further diagnose
what
> >is going wrong if it happens next time?
> 
> I'd start with monitoring the traffic very carefully. Did vic show
similar drops? You can watch its traffic graphs and generate a nice
picture to take to your network folks. You can run things like iperf if
you're really keen.
> 

aahh, ok, I'll take that as, "Use iperf to take a look at network goings
on"

Yes sorry, I forgot to mention that the picture when very blocky and
choppy as well, when the audio suffered packet loss.

VIC has traffic graphs? how do you view them?



> What it sounds like to me is a cpu-bound router on the path. It might
or might not be actual congestion, but if it's regular then somebody is
either generating a significant burst of traffic at that interval, or a
burst of cpu load. I've had some experiences like that, most recently
with the AARNet router in Seattle, which was being probed (legitimately)
for its routing tables every 60 seconds. That SNMP query tipped the poor
router over the edge :-/ so I saw massive packet loss every 60ish
seconds. Took me a while to convince the network folks to chase it down,
but they monitored it at the edge of campus and saw what I saw, and I
could then show it was happening off-campus.
> 
> You really need to get friendly with the network engineers on the
path. Odds are it'll be close to Melbourne or Bristol, so I'd start
there. It's still possible that it's on the broader backbone, but that
means getting even more people looking at it. If it's not on the campus
network at either end, I'd start with the AARNet folks in Oz, and the
JANET folks in the UK.
> 
> Cheers,
>         Markus
> 

Thanks heaps for you feedback Markus

I will be better prepared to sort out such AG problems if it arrives
again.

Chris Kendrick
VPAC


> 
> Markus Buchhorn, ANU Internet Futures             |Ph: +61 2 61258810
> Markus.Buchhorn at anu.edu.au                        |Fx: +61 2 61259805
> The Australian National University, Canberra 0200 |Mob: 0417 281429
>