[Bitcoin-development] Squashing redundant tx data in blocks on the wire

Emin Gün Sirer el33th4x0r at gmail.com
Sat Jul 19 00:54:14 UTC 2014


I thought I'd chime in and point out some research results that might help.
Even if they don't, there is a cool underlying technique that some of you
might find interesting.

The problem being tackled here is very similar to "set reconciliation,"
where
peer A thinks that the set of transactions that should be in the block is
S_A,
and peer B has actually included set S_B, and S_A and S_B are expected
to not differ much. Ideally, one would like the communication complexity
between A and B to be O(delta), not O(S_B) as it is right now. And ideally,
one would like B to send a single message to A, and for A to figure out the
difference between the two sets, without any lengthy back and forth
communication. In essence, I would like to give you some magical packet
that is pretty small and communicates just the delta between what you and
I know.

This paper from Cornell describes a scheme for achieving this:
   Yaron Minsky, Ari Trachtenberg, Richard Zippel: Set reconciliation with
nearly optimal communication complexity. IEEE Transactions on Information
Theory 49(9): 2213-2218 (2003)
   http://ipsit.bu.edu/documents/ieee-it3-web.pdf

Those of you looking for a TL;DR should read the intro and then skip to
page 8 for the example. The underlying trick is very cool, comes from the
peer-to-peer/gossip literature, and it is underused. It'd be really cool if
it
could be applied to this problem to reduce the size of the packets.

This approach has three benefits over the Bloom filter approach (if I
understand the Bloom filter idea correctly):

(1) Bloom filters require packets that are still O(S_A),

(2) Bloom filters are probabilistic, so require extra complications
when there is a hash collision. In the worst case, A might get confused
about which transaction B actually included, which would lead to a
fork. (I am not sure if I followed the Bloom filter idea fully -- this may
not happen with the proposal, but it's a possibility with a naive Bloom
filter implementation)

(3) Bloom filters are interactive, so when A detects that B has included
some transactions that A does not know about, it has to send a message
to figure out what those transactions are.

Set reconciliation is O(delta), non-probabilistic, and non-interactive. The
naive version requires that one have some idea of the size of the delta,
but I think the paper has some discussion of how to handle the delta
estimate.

I have not gone through the full exercise of actually applying this trick to
the Bitcoin p2p protocol yet, but wanted to draw your attention to it.
If someone is interested in applying this stuff to Bitcoin, I'd be happy
to communicate further off list.

- egs



On Fri, Jul 18, 2014 at 12:55 PM, Jeff Garzik <jgarzik at bitpay.com> wrote:

> Related:  We must handle some legitimate miner-privately-mined cases,
> such as miner payout TXs (outside coinbase) or side chain conditional
> TXs[1].
>
> [1] https://bitcointalk.org/index.php?topic=676703.msg7682680#msg7682680
>
> On Fri, Jul 18, 2014 at 3:51 PM, Kaz Wesley <keziahw at gmail.com> wrote:
> > I've updated the gist, and added an additional proposal that I think
> > meshes well:
> >
> https://gist.github.com/kazcw/43c97d3924326beca87d#ultra-fast-block-validation
> >
> > sparseblocks + UFBV would tighten the new-block process to this (when
> > txes have been received in advance):
> > - receive block (~2kB for 1000 tx)
> > - check whether block contains txes known to belong to conflict-sets,
> > and if so whether more than one tx from a single conflict-set has been
> > included (a few operations on very small sets)
> > - relay block (~2kB)
> >
> > The benefits of these changes only occur when the transactions have
> > been seen in advance, but incentivizing ahead-of-block transaction
> > propogation is a plus, as Jeff mentioned; working on a block without
> > first ensuring peers have its transactions would be very expensive
> > from a miner's point of view.
> >
> >
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>
>
> --
> Jeff Garzik
> Bitcoin core developer and open source evangelist
> BitPay, Inc.      https://bitpay.com/
>
>
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