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Date: Mon, 17 Jun 2013 13:39:42 -0400
From: Peter Todd <pete@petertodd.org>
To: Jeff Garzik <jgarzik@bitpay.com>
Message-ID: <20130617173942.GA26623@petertodd.org>
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Cc: bitcoin-development@lists.sourceforge.net
Subject: Re: [Bitcoin-development] Decentralizing mining
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On Mon, Jun 17, 2013 at 11:16:01AM -0400, Jeff Garzik wrote:
> > Part of the broader issue that when we send peers INV advertisements we
> > should be telling them what the fee/kb is so our peers can prioritize
> > properly. That'd also help for the case where you want to broadcast two
> > transactions in a row, but the pair is only profitable because the
> > second is paying the fee for the first.
>=20
> Interesting proposals, particularly this last.  The net result impact
> is, however, that which was criticized in at least one forum thread:
> replace-with-higher-fee.

Actually the two are orthogonal: a low-priority no-fee tx might result
because it was from a customer paying a merchant via the payment
protocol. The merchant can then respend that tx with a fee to cover
both, but with the current mempool arrangement if the no-fee tx load is
high actually getting that first tx to propagate so the second can will
be difficult.

A nice way to do this would be to accept tx's into your mempool
indiscriminately but delay broadcasting INV messages until you find
child tx's that make the low-profit ones worth mining. When you do find
a child with a sufficiently high fee, send an INVGROUP message to notify
your peers of the new opportunity. Different nodes will have different
ideas of what priority TX deserves to be broadcast, but here provided
the group meets the threshold a peer will always find out.

> > Speaking of, the way we tell peers about new blocks is really
> > suboptimal: we tell every peer, in no particular order, about a new
> > block via a block INV message, and then we give them the new block in
> > parallel. I was looking through comp-sci papers on optimal
> > flood-fill/gossip algorithms for random graph networks and it appears
> > that optimal is to spend all your bandwidth to send the message to your
> > fastest peer first, followed by your next fastest and so on. This works
> > best because you get the exponential growth scaling faster by
> > propagating the message as "deep" as possible in the network, and it
> > then can flood outwards from there. Just sorting the peer list by
> > #inv-recevied/time when doing INV pushes and when attending to incoming
> > messages would probably be a big improvement.
>=20
> In terms of packet size, I would like to look into the network-wide
> costs of simply broadcasting block header + coinbase TX + TX list.  I
> bet miners would love to opt into that.

Whether or not that is a improvement is a really complex question, even
without taking failure into account. If you agressively prioritize peers
that are the most connected and keep your # of peers reasonably low you
can afford the memory to keep track of what tx's your peers already know
about so to save on round trips for TX hash's they don't have. On the
other hand if you have a large number of peers and can't do that, or
need to cut down on bandwidth used up by the INV floods and have a
probabalistic scheme, you are risking more round-trip latency.

Not to mention the nasty problem of how *relying* on TX hashes to keep
your bandwidth down means that anything disrupting that system suddenly
has a big impact on the network. I don't think we really understand all
the nuances of that - look at how few people realize that you need
multiples of average bandwidth to have sufficient emergency bandwidth
available to catch up in the event of a chain fork.

--=20
'peter'[:-1]@petertodd.org
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