[BitTorrent] Re: bt2 protocol features

Justin Cormack justin at street-vision.com
Wed Jun 9 15:28:17 EDT 2004


On Wed, 2004-06-09 at 19:39, Elliott Mitchell wrote:
> > From: John Hoffman <theshadow at shambala.net>
> > 
> > >Personally I think the idea of retrieving blocks by their hash codes
> > >is quite beautiful (I was throwing this idea around in here a month or
> > >two ago, but my vast ignorance of things IP cost it all credibility).
> > 
> > I'd have to disagree; the overhead in requesting by offset would be 4 
> > bytes (2^32*16384 = 64 terabyte max torrent size), while requesting by 
> > hash would be 20 bytes.
> 
> Assuming you use the full hash value during the request (you need all of
> it to check the block, but not all of it is needed to make an
> unambiguous request). A much more important question is, "Is this a
> significant cost in bandwidth?" Figuring you might send that out 10 times
> (once for the request, once for the response; once for the entire block
> to announce you've got it), then the additional overhead is 160 bytes
> total, less than 1% overhead.
> 
> I've almost completed two experimental implementations of this mode. I
> carefully allowed for dropping part of the hash to save bandwidth, but
> I'm not even sure that is significant enough to even bother with.
> 

I wouldnt drop part of the hash. It might be nice if there was an option
to use different hashes in bt2, but dropping it is a bad thing, as
constructing collisions of partial hashes is apparently not too hard (ie
has been done).

Talking to peers, the overhead depends on how many have messages you
send. When you get a block you send out one per peer, so if these are
done by hash this is more noticeable than requests, as you might send
out a few hundred...

Justin





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