[BitTorrent] BT2 & hash trees

Olaf van der Spek OvdSpek at LIACS.NL
Thu Jul 22 06:51:46 EDT 2004


> Whenever I request a part of the hash tree, I have to be able to verify it
> against the root hash instantly. I cannot do that, if I start with only he
root
> hash and request directly some hash for a 32KB piece. I would either have
to
> work my way through n levels of the hash tree first, requesting 2*n hashes
that
> allow me to verify the hash of that first piece - or I could request the
nth
> level as a whole to verify all hashes of the nth level against the root
hash at
> once.
> The latter solution should clearly be preferred because otherwise you
would have
> to request twice as many hashes and send a lot more requests producing
more

You don't need to transfer the entire path to the root each time.
After some time, you already have part of the tree and you only need the
first part of the path (looking from the bottom).
That way, you'll transfer n, not 2n.



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