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From: Mick <michaelkintzios@gmail.com>
To: gentoo-user@lists.gentoo.org
Subject: Re: [gentoo-user] {OT} Cable latency & Skype
Date: Tue, 13 Nov 2007 08:57:41 +0000
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On Monday 12 November 2007, Grant wrote:
> I just switched from DSL to cable and I'm noticing a significant delay
> when using Skype, even when nothing else is happening on my network.
> Has anyone else noticed this and had success "fixing" it? I'm using a
> Gentoo router so I can try just about anything.
If this is a MTU problem (it may very well not be, see below) I would go ab=
out=20
it as follows.
Do you know what is the recommended MTU of your ISP? What MTU your modem i=
s=20
set at? If not and they won't tell you you could try to find out by=20
yourself:
Set the MTU of *all* nodes between your PC & your ISP's modem to 1500 (PC,=
=20
router, etc). Then run something like this:
$ ping -c 3 -v -M do -s 1452 www.yahoo.com (better if you use the IP addre=
ss=20
of yahoo.com, of even better if you use one of your ISP's gateways, DNS=20
servers, etc. to avoid problems with network congestion upstream - in that=
=20
sense ebay on a Sunday afternoon may not be a good choice ;).
Work your way up to greater packet values until you cannot ping anymore=20
because packet fragmentation occurs. Once you find the value at which it=20
switches over then add 28 (for the packet headers) to the maximum value at=
=20
which ping works. This should be the value at which your modem is set.
Set all other network components to the same MTU (inc. all PCs in your=20
network) and you should be good - BUT only if the modem's MTU was correct i=
n=20
the first place.
If your modem has the wrong MTU for the ISPs network assuming they don't te=
ll=20
you what that is, then try altering it from 1500 downwards to 1492, 1480,=20
1458, 1430, 1400, while you test for maximum line speed. Set all your=20
network components to the same MTU and check with a broadband speed tester=
=20
(e.g. www.speetest.net) to see when you get maximum speed. Alternatively,=
=20
try downloading some iso image from a main mirror near you (warning - not=
=20
all mirrors are born equal in terms of bandwidth, so some experimentation i=
s=20
required).
Eventually, you'll find some max MTU value at which you get the best=20
comparative performance. For the tests to be meaningful they should be=20
better performed during quiet times (see contention below). BTW, from the=
=20
little I know about cable the MTU is usually set at 1500, but others could=
=20
advise better.
If all that doesn't fix it, then it may have nothing to do with MTU and it=
=20
could be related to faulty components in your network (cables, switches,=20
modem, etc.); ISP throttling (is poor performance occurring only some=20
times/days or is it universal); busy Skype servers(?), or good old high lev=
el=20
pipe contention, because the world and his wife are on the Internet these=20
days and the infrastructure has not grown enough for it.
HTH.
=2D-=20
Regards,
Mick
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