[BitTorrent] Re: bt2 protocol features

Elliott Mitchell ehem at m5p.com
Wed Jun 9 14:39:01 EDT 2004

> 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.

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