Re: Tor Network response slowing to zero

[Janne Snabb <snabb@xxxxxxxx>]

On Wed, 2 Feb 2005 clifnor@xxxxxxxxxxxx wrote:

> Can other users confirm that network response is degrading badly during
> the past 2-3 weeks?

Hi,

I have noticed the same thing as well. Few weeks ago the network
was amazingly fast, but nowadays transfers are slow and connections
time out often.

My speculation is that this is a combination of three different
factors which all affect the network simultaneously:


1) The network usage has gone clearly up. This has triggered the
two other problems (which used to be there even before, but they
didn't manifest themselves because the network load was so low
compared to the available resources). This wouldn't be a problem
if the infrastructure scales well and some of the users contribute
more servers instead of just becoming end-users.

2) The file descriptor limit issue (see Roger's post on 28th Jan).

3) Introduction of low-bandwidth (*DSL) servers which are not set
up to throttle the bandwidth usage appropriately. This creates too
much traffic to completely saturate those low-speed links occasionally
and that leads to massive packet loss. And we know how TCP handles
packet loss greater than a couple of percent: not very well (makes
everything in the end stuck due to re-transmissions and recovery
timeouts).

The default torrc template does not include sample entries nor any
explanations for "BandwidthRate" nor "BandwidthBurst". Also the
installation documentation does not even mention setting these up
properly for low-speed links.

I had initially set the "BandwidthRate" (which I doubt most people
do) to a reasonable level compared to my available uplink bandwidth.
But I still noticed some times massive packet loss. After investigating
I realized that the default value for "BandwidthBurst" is so high
that it takes ages on a low-speed link for the bandwidth throttling
to kick in. I added a very tight value ("100 KB" instead of the
default "48 MB"). I haven't seen any packet loss after that.

According to the Tor FAQ 4.8 bandwidth throttling isn't really
needed, but instead:

  "Tor will automatically detect a lower capacity and scale back
  the attention you get from Tor clients."

... but I do not fully agree with this based on my experiences.

It only takes one saturated link within the tor cloud between the
user and the destination web (or whatever) server to mess things
up. This is not very unlikely, because there are lots of potentially
saturated communications links per one connection (as there are
multiple hops in each tor circuit).


Any other thoughts?

--
Janne Snabb
snabb@xxxxxx