[BitTorrent] Re: bt2 protocol features

Elliott Mitchell ehem at m5p.com
Mon Jun 7 16:23:00 EDT 2004


> From: Olaf van der Spek <OvdSpek at LIACS.NL>
> dpmott at sep.com wrote:
> > --- In BitTorrent at yahoogroups.com, Bram Cohen <bram at b...> wrote:
> > 
> >>>Related thing, how about peers refering to pieces *strictly* by 
> >>>hash?
> >>
> >>It turns out that the size of those hashes can be fairly 
> >>significant, and the program logic winds up doing a lookup to 
> >>translate back from hash to index on the other side anyway.

> > Let me know if any of this is wrong...
> > 
> > If you were to eliminate the "offset" and "length" fields when 
> > requesting a chunk, and make the chunksize appropriately small, then 
> > you could just request a chunk by offset.
> > 
> > And, you can request a validation hash for each chunk.
> > 
> > And, this approach would elminate that whole "chunk" and "piece" 
> > distinction...
> > 
> > Is any of that wrong, or undesirable for some reason?
> 
> Yes, a piece size of between 16 kb and 64 kb would be required and that 
> would significantly increase the protocol overhead compared with 2 mb 
> pieces.

Really?

The mainline client by default already requests chunks in mere 32KB
chunks and will ignore requests larger than 256KB. Even if this size has
to be shrunk pipelining on the client is a trivial solution (and again,
already implemented on the mainline client). The use of Merkle trees will
also force hashes to cover smaller pieces (which is a good thing because
a client will no longer be able to get credit for uploading garbage).

Next argument please.

> And at such sizes, you're running into limitations of current HDDs when 
> it comes to random IO, especially on high-bandwidth links.

Hmm, we can already cause this by requesting random chunks of random
pieces with the existing protocol. The solution of course is to request
chunks that are clustered together, and possibly have a client impose a
slight penalty when discontiguous chunks are requested.


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