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by zamadatix 958 days ago
This implies the reason most sparsely seeded torrents have chunk availability is because the transient leechers just happen to arrive consistently in time to hand off to one another. I think reality is sparse torrents tend to be kept alive by a few seeders slowly kicking out data (most likely because they are seeding a ton of other stuff too). Sequential or not, if you and another guy are planning on dumping a few seconds after finishing the torrent then it doesn't really matter if that is a chunk near the end or a random chunk - either way you're both trying to squeeze the last chunk out of the seeder.

Densely seeded torrents may be slightly different and play out more like you describe, but it doesn't really matter as much there as the first parts can be sourced from those watching sequentially and the end parts left to those that actually seed after getting a full file.
1 comments
JohnBooty 958 days ago 

    This implies the reason most sparsely seeded torrents 
    have chunk availability is because the transient leechers 
    just happen to arrive consistently in time to hand off to 
    one another.
No. I'm not sure where you got that idea.

This is a pathological and unrealistic example but please try to extrapolate to more realistic scenarios. Imagine a file with three pieces. 200 torrent users have zero pieces, 100 torrent users have the first piece, 50 have the first and second piece, and "Joe" is the sole seeder that has all three.

Obviously, assuming Joe doesn't have godlike bandwidth, there are 350 users who want that third piece and only a single Joe and there is going to be contention. In this pathological situation things will work itself out as soon as supply of the third piece increases, assuming people don't kill their clients as soon as they hit 100%, but things will be slower than they need to be at first.

What I think you're trying to point out is that there are situations where sequential torrent downloads aren't harmful to download speed or overall availability. Yes, that is true. If seeder bandwidth for a given piece is more than sufficient for leecher demand for that piece then... yes, no harm done.

As another user posted, http://bittorrent.org/bittorrentecon.pdf section 2.4.2 "Rarest first"

You're free to disagree with the design of bittorrent, obviously.

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