Re: Network Performance Issues.
On Sunday 15 April 2007 02:03, Mike Bird wrote:
> On Saturday 14 April 2007 15:02, Alan Chandler wrote:
> > The problem I have is this. I have a linux desktop (machine D)
> > siting on a LAN in my home. This is connected to a linux
> > firewall/nat router (machine S) with two ethernet cards. One links
> > out to the internet, the other connects to the internal lan.
> > Connections to the internet from machine D go through machine S,
> > which acts as a NAT translation. I do all the control and
> > firewalling using IPTABLES in machine S.
> >
> > Downloading a video from youtube onto Machine D's desktop I get a
> > download speed of about 7Kbytes/sec. Which is very low. If I try
> > to download the same one by ssh'ing into mahcine S and running wget
> > on the same url, I get a download speed of 80Kbytes/sec (ie more
> > than 10 times faster).
>
> Alan,
>
> I would try wget on machine D to verify that the problem is network
> rather than your browser on D. If that doesn't answer your question
> I would run wireshark (or tshark from the command line) for a few
> seconds while machine D was downloading via S, being sure to capture
> on all interfaces. The result will show what is happening to packets
> trying to cross S.
Thank you for all the people who did respond to this. Just to say I AM
USING wget for this.
However, the suggestion to run wireshark was important because I think
it is giving me a clue of the problem. This will need a TCP expert to
help me - I think the problem seems to be in the sequence numbers. Let
me try and explain. (wireshark is running on the gateway originally
was looking on all interfaces - but this didn't help - so I limited it
to the WAN interface)
Firstly the case of direct downloading from my gateway. This is a
summary of a three protocol exchanges using wireshark
youtube->me http continuation seq 189688 next seq 191136
youtube->me http continuation seq 191136 next seq 192584
me->youtube tcp ack seq no 192584
youtube->me http continuation seq 192584 next seq 194032
Now when I look at a similar exchange when I am using the gateway
machine just as a hop and there is a machine behind the gateway I get a
different pattern
youtube->me http cont seq 4344 next seq 5792
me->youtube tcp ack seq 5792
youtube->me http cont seq 7240 next seq 8688
me->youtube dup ack seq 5792
youtube->me http cont seq 10136 next seq 11584
me->youtube dup ack seq 5792
youtube->me http cont seq 5790 next seq 7240
me->youtube tcp ack seq 8688
youtube->me http cont seq 8688 next seq 10136
As you can see, in this case, it appears that some packets are being
lost and have to repeated via a dup ack.
One can presume that youtube is actually sending the packets since it is
clear in the earlier case that it does so (and pretty quickly too), so
I think it might come down some reason that the packets are getting
dropped before wireshark sees them.
I have an IPTABLES firewall, but I can't see why that should effect
things. Can someone tell me
a) Whether wireshark sees the packets before they are processed by
IPTABLES?
b) what other reason could I have for lost packets?
--
Alan Chandler
http://www.chandlerfamily.org.uk
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