[ILUG] download hiccups after 750 KB or so

[Gavin McCullagh]

Hi,

On Wed, 12 Sep 2007, Joerg Baach wrote:

> came back from my holidays, hooked up my laptop (x31 / Debian / testing)
> up to the internet, and now notice that downloads
> (wget/firefox/whatever) get hiccups every 750 KB or so.

What is your definition of hiccup?  Do you mean the connection dies
completely, slows down radically or something else?

Did you apply upgrades in the meantime to say, the kernel?  What kernel are
you running?

> I booted windows, and had no hiccups.
> 
> With using Linux, I tested:
> 
> a) downloading from different servers - hiccups
> b) using ubuntu 7.04 instead of debian - hiccups
> c) cable instead of wlan - hiccups
> d) pluging in directly into the modem - hiccups
> 
> Wireshark shows a TCP Previous segment lost around that time, followed
> by lots of tcp dup ack. Too bad that my tcp knowledge is rather low, so
> I am not sure what to do with that info.

I guess it means some packet got dropped (possibly by your router?), then
you're machine sent dup acks to request retransmit of the dropped packet
(it would probably send one dup ack every packet it receives).  If the
retransmitted packet (or the dup ack) also got dropped, etc. all would then
grind to a halt as that packet would never be received.

There are some DSL routers which don't like certain tcp features such as
window scaling.  I'm not sure if you'd expect this behaviour, but you could
try turning it off to see if it makes any odds.

	echo 0 > /proc/sys/net/ipv4/tcp_window_scaling

Gavin