Ah yes, Bit Torrent. The idea is simple; every download client is also 
an upload server. And by sharing the load in this way, each client has 
hundreds or even thousands of possible servers to choose from, and can 
select the one with the best transfer rate from their location.

There's a big long paper about how all this works, but we can summarise 
as follows:

- Every downloader is also an uploader.

- Each downloader connects to multiple uploaders.

- Downloaders try to download from the fastest uploader [from their 
location on the network].

- Uploaders upload fastest to the peers they download fastest from. 
Thus, hypothetically, the faster you upload, the faster you can 
download. This encourages fairness.

- In the interests of fairness, once you've fully downloaded a file, 
you're supposed to leave the torrent running for a while so that others 
can download it from you.

Tools like Azureus even keep a track of your "share rating" for each 
file, and a total rating. The better your share rating, the more you're 
helping to seed the torrents you've downloaded.

Now, here's where it all falls apart:

- I have ADSL. That stands for ASYMMETRIC Digital Subscriber Line. In 
laymen's terms, by download rate is about 5x faster than my upload rate. 
I can download at the speedy rate of around 310 KB/sec, but I can only 
*upload* at a maximum of about 46 KB/sec.

- This means that it is physically impossible for me to upload 
particularly fast. That should hypothetically result in low download speeds.

- As it happens, seeders by definition don't download anything. So they 
can't upload to others based on how fast they download form them. They 
therefore upload to the fastest clients. So I still get a nice download 
rate.

- However, this means that by the time I've downloaded a file, by share 
rating for that file is about 1%.

- No problem, I can just leave the torrent running for a while...

- But wait! This torrent has 450 seeders and 3 downloaders. And I 
sometimes see 200 KB/sec from a single seeder, so... why THE HELL is any 
peer going to download from me at a piffling 40 KB/sec when there are 
peers out there at least 5x faster?

- In short, I typically get upload rates of about 2 KB/sec (while 
downloading or seeding). [No, I'm not firewalled.]

- The whole share rating thing is a bit short-signed too. If I upload 
10MB to a 100MB torrent, I get a 10% rating. If I upload 100MB to a 
1,000MB torrent... I get a 10% rating. In one case I've helped way, way 
more, but I still get the same rating.

- More grave: If I upload 100MB to a 500MB torrent with 100 seeders and 
3 downloaders, I get a 20% rating. If I upload 100MB to a 500MB torrent 
with 3 seeders and 100 downloaders, I have helped out way, way more. But 
I still only get a 20% rating.

In short, my ability to give is vastly smaller than my ability to take. 
Not because I'm greedy, but because of the underlying technology. And BT 
software rates me on how much data I upload, rather than on how helpful 
I'm actually being. So Azureus is doomed to sit there for weeks 
uploading 2 KB/sec to torrents that probably don't need the help anyway 
just so my global share rating can approach unity...

-- 
http://blog.orphi.me.uk/
http://www.zazzle.com/MathematicalOrchid*

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