Troubleshooting the network connection speed

[Marko Vojinovic]

Hi folks! :-)

Before I go complain to my ISP, I'd like to hear if anyone can give me
an idea what is going on with my networks... :-)

I have two machines, with following link properties:

 local --- 20Mbps/2Mbps (GSM wireless)
 remote --- 100Mbps/100Mbps (100Mbit LAN connected to optical uplink)

The remote machine is in another country, cca 2000 km away. It is
connected to a 10Gbit optical link, but only through a 100Mbps switch,
so that caps the bandwidth.

When transferring large files via wget from remote to local, the
maximum bandwidth that I get is 2Mbps. It *used to be* 20Mbps (couple
of days ago). Occasionally it drops down to 300Kbps (it just happened
as I write this), but after several minutes it gets back up to 2Mbps.

But it doesn't want to get back up to 20Mbps, which is the max download
throughput for the local machine.

To test the local link, I opened 15-20 random youtube links
simultaneously in Firefox. It easily capped the full 20Mbps, so the
local link apparently works as advertised.

Another test of the local link --- I went to

  http://fedoraproject.org/get-fedora

and clicked the big blue "download now" button, to download the Live
Desktop .iso --- the download manager in Firefox says it will complete
it in 17 hours, since it is downloading at 15 KBps (i.e. 150 Kbps).
This is of course ridiculously slow, for a 20Mbps link.

All speed numbers are consistently reported by jnettop, KDE network
widget, Firefox download manager and wget. If you suggest some other
tool to measure the throughput, I'll try it out too.

The remote machine appears to work ok --- I have downloaded and
uploaded (elsewhere) all sorts of things, and it consistently works at
100Mbps up/down. Downloaded Fedora DVD iso in a couple of minutes. I
can seed torrents from it at 100Mbps no problem (this is currently off
because I'm trying to pull something to the local machine).

So I believe something is wrong with my local link, but don't know
exactly what --- youtube works, but other things don't.

Any ideas how to troubleshoot this?

Also, any ideas what to tell to my ISP?

I could ask them to look into it, but they just might open a bunch of
youtube links, verify that the link works, and blame the remote machine.

Any suggestions appreciated.

TIA, :-)
Marko

P.S. Before anyone asks --- I *do* know the difference between bits and
bytes, Mbps and MBps, etc. I was careful to provide you with a
case-sensitive units, and I know what I'm talking about. :-)