[nzlug] puzzling network prob between two linux boxes on homenetwork

[Michael Field]

It may pay to check out stats in "netstat -s" and "ifconfig". You should
have no errors in the ifconfig stats.

In "netstat -s" the most important bits are "segments retransmitted" in
the Tcp stats. It this is high compared to the number of sengments
sent/received then you have some packet loss which will cause your
connections to 'burp' until a timer expires and the packet is
retransmitted.

Here is an example of reasonable well connected system for reference:

Tcp:
    9110 active connections openings
    5678 passive connection openings
    196 failed connection attempts
    2355 connection resets received
    121 connections established
    20493911 segments received
    10752815 segments send out
    1492 segments retransmited
    0 bad segments received.
    3686 resets sent

Attempts to connect to 'down' sites and other things will cause a low
level of segments to be retransmitted, so don't expect it to be 0!

If you want to watch this in action, use the ftp command line client and
use the "hash" command so you can see if the connection 'burps' due to
packet loss.

Mike

-----Original Message-----
From: nzlug-bounces at linux.net.nz [mailto:nzlug-bounces at linux.net.nz] On
Behalf Of stephen
Sent: Tuesday, 26 June 2007 9:53 p.m.
To: NZLUG Mailing List
Subject: [nzlug] puzzling network prob between two linux boxes on
homenetwork

I have a little networking mystery which is driving me nuts.

Box A is connected directly via ethernet to my (cheap Belkin) router.
The router and Box A both think they have a 100Mbps full duplex link (as
reported by ethtool eth0 on Box A, status lights on the router).

Box B is connected via ethernet to a cheap Dlink switch, which in turn
is connected to a LAN over power (you know, Homeplug) dongle, with the
remote dongle going to the router. Box B and the switch both think they
have a 100Mbps full duplex link. Homeplug is supposed to be 85 Mbps in
theory, and anecdotally I hear it should be good for 10 anyway.

Box B, despite the suboptimal adhoc set of links, can max out my 4 Mbps
cable connection, as reported by various DSL speed test sites. So can
Box A.

However, I can not get better that about 1.5 Mbps from box A to box B
(measured with a very large file served by Apache on box B, and wget on
box A). Sometimes it is as low as 0.9 Mbps.

I have swapped cables. I have tried wireless on Box A (the router is
also 801.11g capable). I have restarted the router, restarted the switch
and jiggled the Homeplug dongles. No change.

So, to recap, both these boxes can do at least 4 Mbps to the internet,
but they can't manage even half that to each other.

I had intended that Box B, which is in a nice quiet room upstairs,
become a server for media files and for backups, but I want much better
bandwidth for that.

Suggestions for poking around?

Stephen


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