I know, that this thread is old one, but I am having the same issue. It does not seem to me to be the problem of single torrent. It is more the way how much do I use my connection. If I have only 1-2 torrents, everything is ok. If I have 10, I get this problem. It is also followed by aprox. 30 secs of no internet connection.

I use Deluge for downloading and my limits are set like this:
max connections: 200
max upload: 300 KiB/s
max upload slots: 5
max connection attempts per sec: 20

The connection outage is really annoying.

raqua, I've moved your post from where you originally posted to a thread of its own. I've also placed this thread in the Networking forum, so that it may get more adequate exposure. In the future, please try to let dead threads rest in peace.

What is your maximum upload/download speed? Its possible the line is getting so saturated that it seems like its not functioning.

Who is your ISP? Comcast for example throttles BT traffic.

What type of router do you have? Some older routers do not have enough resources to handle alot of connections and/or nat translations, and belly up when they hit the limit (BT traffic especially causes alot of connections and nat translations due to its inherit nature)

Mediacom in my area also throttles torrenting, but they aren't very good at it. If I just forward and use any port other than the default, they seem to not notice.

I see you throttled the upload speed to 300 kB/s. Is it really kB/s (kByte/s) or more like 300 kbit/s?

I'm asking this because when you throttle the upstream to 300 Kbyte/s that would mean you have some
sort of vDSL/fibreglass line with a download speed of > 3000 kByte/s, hence a 24+ megabit line (presuming your ISP gives you a tenth
of your downstream as upload like they do mostly).
I assume you throttled the upload to keep some bandwith for browsing etc..
If you throttled the upload for that reason but you have for example an ordinary 3 megabit aDSL line (375 kB/s down, 37,5 kB/s up),
throttling the upload to 300 kB/s will have no effect, the torrents might clog your upstream making browsing painfully slow
or even impossible.

Secondly I agree with the others that it might be your router.
I had a similar problem with torrents once.
At a certain point the router closed all connections.
I found out that my router considered a certain number of inbound connections (can't remember the exact value) a DoS attack, so he clipped all of them.
Increasing the max number of connections fixed that.

Thank you for your responses.

My internet link is 30Mbit/4Mbit. I connect to it using wifi. It is not a problem of my internet provider because when my connection breaks, I can not ping router, other PC's connected to the same router work fine.
As was mentioned in the original post I reacted to before this has been moved, when this happens, I can see message this message in kernel log:
Possible SYN flooding on port XXXXX. Sending cookies. (Port number is the one I run torrents on)

My router is DLink DIR-655 which I bought especially because it can handle a lot of connections. According to review at smallnetbuilder.com, it performed great in this regard and was able to handle 200 connections at a time. Maybe even more, but the measurement tools used in the test did not allowed to test more. Anyway, limiting max. connections to 150 did not help.

The thing is, that I had this message in dmesg before, but no connection interruption. Booting older kernel seems to help in this regard, so maybe this is some bug. I will check it next time I download so much at once which I do not do very often.

If you can't ping the router when this happens, then my first two suggestions are out... and if your other PCs can still access the network then it's not your router either. It almost has to be an issue with your local PC. Maybe your TCP/IP buffer is exceeding its limit? That possibility would be consistent with the SYN logs you're seeing. Google TCP tuning if you're interested. Another thing you want to try, but only when the issue occurs again, is ping your local NIC's IP and ping localhost. Verify both are successful.