[BitTorrent] hash-based requests

Justin Cormack justin at street-vision.com
Wed Feb 16 06:58:42 EST 2005


> 
> 
> >From: Justin Cormack <justin at street-vision.com>
> > > 
> > > >From: Justin Cormack <justin at street-vision.com>
> > > >  
> > > > > If HAVE messages are somehow indexed to tree location, then it works
> > > > > pretty well.
> > > > 
> > > > You can encode the path down a binary tree (left = 0, 1 = right) as the
> > > > HAVE message payload. Thats fairly independent of how (if at all) you
> > > > actually index your nodes. You can with non binary trees too if you insist...
> > > 
> > > Problem is this turns back into a variable length string.
> > 
> > But log N bounded, which isnt very much. But it doesnt buy you anything.
> 
> I already suggested a numbering structure which would work. Keeps the
> HAVE messages smaller.

They will be the same size regardless, they have to be able to count all the
nodes.
 
> > > > > > The other alternative is a two-phase protocol where you first obtain the Merkle
> > > > > > tree before you can do anything else.
> > > > > 
> > > > > I'm working on a sample implementation which computes the hashes, and
> > > > > uses a midsize branching factor. Turns out working with n-ary trees isn't
> > > > > always easy.  %-)
> > > > 
> > > > Its almost never worth it. Possbly never... Whats your reason for not using
> > > > binary?
> > > 
> > > I'm working with block size divided by hash size as the branching factor.
> > > This results in a node fitting into a single message. At the same time
> > > a block of hashes can be accounted for like a payload block. Should also
> > > mollify the folks looking for a flatish tree (2TB with only 3 levels).
> > > 
> > > Problem is computing the indicies correctly.  %-)
> 
> > Well you can get that right (and padding the tree with empty leaves (length 0,
> > hash to match) might help.
> > 
> > However why not have a binary tree and send more children and granchildren
> > with a request if you want to get the message size up? It pretty much amounts
> > to the same thing and is simpler. With a 32k message, and 20 byte hashes, 
> > each request will get 10 levels of a binary tree in one message (slightly
> > smaller, so only 1TB fits in 3 levels...)
> 
> You wouldn't bother sending the direct children, only the grandchildren.
> At this point it effectively reverts to an n-ary tree. Notably converting
> to bitfield indicies once again becomes annoying. This does limit things
> to power of 2 branching, but the computations are still annoying.
> 
> With things turning into an n-ary tree no matter what, I once again have
> to question the wisdom to sticking to "THEX". Just seems completely
> inappropriate here.

Explain why you need the grandchildren not the children?

It would be really nice to have a system that didnt have numbering, as then
you can just store everything in hash tables, and the protocol wouldnt 
enforce numbering. Bloom filters would do instead of bitmaps (with occasional
problem of false positives, but you could build this into the protocol, just
make a small list of responses which come back dont have when you request).
Then you can add have messages into the bloom filter without having to know
anything about them or create an entry for them. Only problem is the size
is larger than a bitmap to send.

The false positives are easy to deal with, there wont be many, just check
against them before sending out requests, if get back a donthave message
you know you had a false positive, add to list. If you get a have check if
it is in the table of false positives first and delete it if it is, else
add to Bloom filter.

To save on sending Bloom filters you could 
(a) add a seed message that means I have everything. Always help the seeds.
(b) if the representation of your filter is longer than the have messages
that make it up (ie you have nothing or very little) send haves instead.

Justin


 
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