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*

Post a reply to this message

Orchid XP v8 wrote:
> And BT software rates me on how much data I upload, 

You're aware that the share ratio isn't actually part of the protocol, 
right?  Nobody is keeping track of that except you and perhaps some private 
trackers out there?

-- 
Darren New, San Diego CA, USA (PST)
   Forget "focus follows mouse." When do
   I get "focus follows gaze"?

Post a reply to this message

Orchid XP v8 wrote:
> - 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.
> 
> - 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?

The wikipedia article mentions some aspects of BT that specifically take 
this into account.

> - 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.

But in all technicality, you've helped exactly the same, relative to the 
whole torrent.  Which is the only relevant measure.

--
Tim Cook
http://empyrean.freesitespace.net

Post a reply to this message

BTW, it was also designed for things like game patches, where you have 
gazillions of people all downloading at once, with only a trickle of 
interest after the initial flood of downloads.

Post a reply to this message

Darren New wrote:

> You're aware that the share ratio isn't actually part of the protocol, 
> right?

Sure. I just feel like a bit of an arsehole downloading all this stuff 
and then not giving anything back, that's all. :-}

...OK, I am officially weird.

[But also, the mathematics of the system is non-optimal. I hate 
non-optimal!]

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

Post a reply to this message

Orchid XP v8 wrote:
> Darren New wrote:
>> You're aware that the share ratio isn't actually part of the protocol,
>> right?
> 
> Sure. I just feel like a bit of an arsehole downloading all this stuff
> and then not giving anything back, that's all. :-}

I have a 900MB torrent up to share ratio 12.

Why not? I have no monthly bandwidth caps.

Post a reply to this message

On Sun, 17 Jan 2010 18:18:53 +0000, Orchid XP v8 wrote:

> Sure. I just feel like a bit of an arsehole downloading all this stuff
> and then not giving anything back, that's all. :-}

Just leave it run; I usually let my torrents run until either there are 0 
peers for an extended period of time or until I hit a share ratio of 
1:1.  Sometimes I'll let them go longer, but I give priority to those 
with a share ratio below 1:1.

Jim

Post a reply to this message

Orchid XP v8 wrote:
> - 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.

The download speed isn't tied to the upload speed like that.  They are 
independent.

Besides, the system works well for asymmetric connections like yours. 
If a typical ratio of upload to download speed is 10x, then with 10 
seeds you can top out your download speed, despite each seeder's upload 
speed being low.

> - 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?

The file is broken up into blocks, you know, rather than transferred 
sequentially.

So, the client will download one block from you, while downloading other 
blocks from other seeds.  You'll still be helping.

...Chambers

Post a reply to this message

>> - 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?
> 
> The file is broken up into blocks, you know, rather than transferred 
> sequentially.
> 
> So, the client will download one block from you, while downloading other 
> blocks from other seeds.  You'll still be helping.

Yes, but the BT algorithm is to try to select the peers which give you 
the fastest download rate - to reduce leaching, but mainly so you 
download from whichever peers are topologically "nearest" to you.

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

Post a reply to this message

Nicolas Alvarez wrote:

> I have a 900MB torrent up to share ratio 12.
> 
> Why not?

Becuase it might take several human lifetimes to reach that ratio?

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

Post a reply to this message