Re: [gentoo-user] {OT} Cable latency & Skype

[Mick <michaelkintzios@gmail.com>]

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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 about 
it as follows.

Do you know what is the recommended MTU of your ISP?  What MTU your modem is 
set at?  If not and they won't tell you you could try to find out by 
yourself:

Set the MTU of *all* nodes between your PC & your ISP's modem to 1500 (PC, 
router, etc).  Then run something like this:

$ ping -c 3 -v -M do -s 1452 www.yahoo.com  (better if you use the IP address 
of yahoo.com, of even better if you use one of your ISP's gateways, DNS 
servers, etc. to avoid problems with network congestion upstream - in that 
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 
because packet fragmentation occurs.  Once you find the value at which it 
switches over then add 28 (for the packet headers) to the maximum value at 
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 
network) and you should be good - BUT only if the modem's MTU was correct in 
the first place.

If your modem has the wrong MTU for the ISPs network assuming they don't tell 
you what that is, then try altering it from 1500 downwards to 1492, 1480, 
1458, 1430, 1400, while you test for maximum line speed.  Set all your 
network components to the same MTU and check with a broadband speed tester 
(e.g. www.speetest.net) to see when you get maximum speed.  Alternatively, 
try downloading some iso image from a main mirror near you (warning -  not 
all mirrors are born equal in terms of bandwidth, so some experimentation is 
required).

Eventually, you'll find some max MTU value at which you get the best 
comparative performance.  For the tests to be meaningful they should be 
better performed during quiet times (see contention below).  BTW, from the 
little I know about cable the MTU is usually set at 1500, but others could 
advise better.

If all that doesn't fix it, then it may have nothing to do with MTU and it 
could be related to faulty components in your network (cables, switches, 
modem, etc.); ISP throttling (is poor performance occurring only some 
times/days or is it universal); busy Skype servers(?), or good old high level 
pipe contention, because the world and his wife are on the Internet these 
days and the infrastructure has not grown enough for it.

HTH.
-- 
Regards,
Mick

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