Why does uploading on Bittorrent slow my Comcast internet connection?

October 13, 2004 10:31 AM

TechSupportFilter: When I upload (perfectly legal) bittorrent files at speeds greater than about 13 kbps, my Comcast internet connection gets completely swamped, and I'm unable to surf the web or download email. Comcast tech support has been out three times over the past month, and they're still scratching their heads. But I get the feeling they don't exactly know what they're doing. Does anyone have any idea what might be going on, or what further tests I should run to find out?
posted by gd779 to computers & internet (14 answers total)

An extremely common problem. As you approach the limits of your upload speeds (which are quite likely limited at around that rate) it gets harder to download, because even when downloading, you must upload some information (for example, acknowledgements that you received pieces of data).

Also, interactivity goes to crap because ISP's usually use very long network queues. This is fine for throughput but bad for interactivity.

You can limit your upload rates to something below the maximum your connection has and this will help. If you are running linux or perhaps osX you can probably run some kind of traffic shaper. Whether there is such a thing for windows or not, I do not know.

Some bit torrent clients will allow you to set an overall max upload speed. The basic client only allows a per-torrent max rate.
posted by RustyBrooks at 10:37 AM on October 13, 2004

For windows, you can use Netlimiter to limit your upload speed on a per-program or per-IP basis. As RustyBrooks said, this is a very common problem. If your cable can only support 13 kbps upload (Which seems low to me -- mine can support 80-90kbps or so, but I don't know what internet access is like in the US), then you'll want to cap your uploads somewhere between 7 and 10 kbps.
posted by Jairus at 10:51 AM on October 13, 2004

Thanks for suggesting Netlimiter, that looks helpful. But Comcast advertises a cap of 256 kbps uploading. Obviously, location and what not will vary that quite a bit. But I would think that a maximum upload speed of 13 kbps indicates that something is wrong?
posted by gd779 at 10:58 AM on October 13, 2004

The Azareus client in particular has both upload and download rate caps.
posted by smackfu at 11:00 AM on October 13, 2004

I guess I figured you meant 13 k-bytes per second, rather than 13 k-bits (which is more like 1.5 kbytes per second, quite low). Which does the 13 kbps refer to?
posted by RustyBrooks at 11:03 AM on October 13, 2004

(I should clarify -- I get 80-90 KB per second, not 80-90 kbits per second)
posted by Jairus at 11:13 AM on October 13, 2004

Myself and a coworker had the same problem with Comcast. I solved it with an upgrade of Azareus. He went with Netlimiter. Both worked, but my solution is cheaper. I have gotten to about ~400kbps download since.
posted by yerfatma at 11:19 AM on October 13, 2004

Dammit, you're right RustyBrooks - I was being stupid for a second there, and forgot to convert Comcast's upload cap into kilobytes (to be clear, I am getting 13 Kilobytes upload). Even so, isn't 13 Kbps abnormally low compared to the advertised 32 Kbps cap?
posted by gd779 at 11:26 AM on October 13, 2004

Comcast advertises, according to your previous message, a 256 kbits/second rate = 32 kbytes/s in theory. If you're getting 13 kbytes/s from bit torrent then that is about 105 kbits/s. So there are a few questions:

* how accurate is bit torrent's assesment of the upload rate?
* what traffic is bit torrent using that is not included in the rate? For example, keeping in contact with the tracker and other clients, acknowledging packets, etc.
* What other traffic is going through the network connection at the same time?
* Is comcast lying about the upload bandwidth cap?

I wouldn't be surprised in bit torrent is using 50% more than the noted amount when you take into account miscellaneous communication. It's not a very efficient protocol on the individual user level. I find I can rarely do better than 16 kbytes/s upload even though I also am supposed to have 256 kbits/s = 32 kbytes/s upload rates. On the other hand I *can* easily upload 25 kbytes/s or more with FTP or even scp.

It's not an option for everyone but when I am trying to make files highly available I seed from as many sources as possible. I have a very simple scripted solution. The main bit torrent client (python) comes with btlaunchmany which you basically pass a directory name to. It will start downloading all the torrents in that directory. As a bonus, if you add or remove a torrent to the directory while the program is running, it'll start/stop downloading that torrent. This is awesome because then I can just right-click-save torrents into that directory and they magically start up. Or, in this case, I have a process that runs on all my upload machines that fetch all the torrents in a particular directory on a central server and starts downloading those files, which allows me to spready my available bandwidth out nicely. Anyway, this requires no more tools that the bit torrent client and a copy of rsync.
posted by RustyBrooks at 1:38 PM on October 13, 2004

The convention is to write "kbps" for kilobits per second, and "K/s" for kilobtyes.

Usually I figure a factor of ten to include overhead, so 13K/s would be about 130kbps (assuming a protocol like FTP, which would be a good way to test it).
posted by sfenders at 2:30 PM on October 13, 2004 [1 favorite]

I wouldn't be surprised in bit torrent is using 50% more than the noted amount when you take into account miscellaneous communication.

Interestingly, Netlimiter shows about the same level of outgoing traffic as bittorrent, and I would assume that Netlimiter doesn't distinguish between actual data and ack's and whatnot.

The convention is to write "kbps" for kilobits per second, and "K/s" for kilobtyes.

Thanks, I thought the difference was just capital K for kilobytes and lower case k for kilobits.

Well, now that I know that my internet connection probably isn't broken after all, Netlimiter solved the practical aspect of the problem. So thanks!

I will admit, though, that this upload limit seems remarkably stingy to me. Maybe I should move to Canada.
posted by gd779 at 2:50 PM on October 13, 2004

Yeah, "KBps" or "KB/s" are also is used to mean kilobytes. "Kbps" on the other hand, is ambiguous; it's often used for both, so therefore not advisable. It's the 'B' that should be capitalized, since it's the bit that distinguishes bytes. Oh, well, it's all so confusing.

Anyway, 13 of any of the above is way too slow. You might have better luck moving to Japan.
posted by sfenders at 3:45 PM on October 13, 2004

When you max out your upload then downloads suffer as well. In TCP/IP you are always trying to send a packet saying "hey I got your packet, send me another." If upload is swamped then those "hey" packets are slow to get out, thus slowing everything down.

There is a bittorrent client called bittornado which lets you limit upload speeds.
posted by skallas at 4:01 PM on October 13, 2004

On the Mac there's a useful GUI bandwidth throttle called CarraFix, just in case any Apple users are experiencing similar problems.
posted by ikkyu2 at 4:11 PM on October 13, 2004