Tuesday 28 January 2003

Pic of the day: Peering into the computer ... (Screenshot from The Sims.)

Peer-to-peer review

I've been using BitTorrent for a week now, since last Tuesday. I also had the pleasure of toying with the new alternative client, which allows you to change some of the settings while the program is running. It has given me a bit more insight in how the program works.

The tracker seems to be just a clearinghouse or a register where the various clients are made aware of each other. They then negotiate the file exchange with each other. The pieces seem to be fairly small. I find it hard to believe that they are 1MB. One megabit, perhaps. Anyway, it works like this:

When I open the BitTorrent file and select where to save the result file, it is created as an empty file in its full size. Then it is filled in piece by piece in random order, not from one end. This means that a half completed file is not a subset of a 75% finished file – they are both random subsets of the full file. This means that if there are a few download clients running, they can complete each other without the need for a full file to act as a seed. (It is still considered a good thing to have one full file, just in case.)

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When selecting who to upload to, the client will prefer those who themselves upload the most. This is considered for each file, so opening another upload window will not help, quite the opposite, as they have to share the same bandwidth. You will generally get the best results by running only one client program at a time.

This behavior is not just fair, it also gathers the clients in cliques based on their connection speed. Fast machines will tend to exchange chunks together at high speed, mid-range machines will also stick with their own, and dial-up modems will end up with their own slow kind most of the time. "Birds of a feather flock together." Worst off are those with new or almost new files, since they will have little or nothing to share. Luckily, one in four times a priority is assigned at random. So even if you are at the bottom of everyone's wish list, you will still get a packet some of the time.

My experience with a DSL line is that I upload noticeably more than I download, most of the time. I think this is a sign that most of my fellow BitTorrent users have a slower connection than I have. This will not shock me, as broadband is less common in the USA than here. In fact, I think it's mainly South Korea that has more broadband than Scandinavia. Perhaps the spread of peer-to-peer network and video download will give more people the incentive to upgrade too?

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I know this is horribly geeky, but sometimes I'll just sit and look at the download window and the numbers there and try to guess what is happening. Sometimes the download suddenly shoots up to 20 KB/sec or so, although that is really really rare. Evidently I have been connected to someone with a real broadband connection, perhaps on a university server or something. More often it's the other way around: I upload at 7-10, but only get 1-3 back. Then I've probably stumbled upon someone on a dial-up connection, and not too fast either.

I'll look at the AnimeSuki pages and see how many are uploading and downloading. There are usually a lot more people downloading, but that's the way it's supposed to be. That's the way it works ... the more people who are downloading, the better (at least up to some rather high number). And it's strange to know that they are there, and I don't know who they are. All I know is they like the same anime that I do. I guess it has been found worthy by ... a jury of peers.