[Bitcoin-development] Proposal: SPV Fee Discovery mechanism

[Nathan Wilcox]

On Thu, Jun 11, 2015 at 7:10 AM, Peter Todd <pete at petertodd.org> wrote:

> On Wed, Jun 10, 2015 at 02:18:30PM -0700, Aaron Voisine wrote:
> > The other complication is that this will tend to be a lagging indicator
> > based on network congestion from the last time you connected. If we
> assume
> > that transactions are being dropped in an unpredictable way when blocks
> are
> > full, knowing the network congestion *right now* is critical, and even
> then
> > you just have to hope that someone who wants that space more than you do
> > doesn't show up after you disconnect.
>
> Hence the need for ways to increase fees on transactions after initial
> broadcast like replace-by-fee and child-pays-for-parent.
>
>
I haven't looked closely at replace-by-fee yet, but I assume this is a
non-consensus change to mempool mechanics. To me, this seems like the
"actuator" side of fee mechanics: it provides a transaction sender a way to
influence the system.  By contrast, learning about fees is the "sensor"
side of fee mechanics.

Consider how a replace-by-fee wallet makes fee decisions. When does it
replace by fee?  It needs feedback in one of two forms:

a. Direct feedback from a trnasaction relay service, or:

b. Information in the blockchain, which is verified by all verifying nodes
and refined by all miners.

The first kind of information is quite acceptable and practical for many
use cases, but leave the wallet vulnerable to fabrications by that service.
This vulnerability is precisely what SPV security intends to mitigate,
right?

With only information type a, a (non-SPV) wallet can "shop around" to find
competing services, and this should work pretty well, provided the wallet
can discover those competing services. If, OTOH, the wallet has access to
information type b, it now has "perfect competition" across all such
services, even when it can't discover the low-priced services directly.

This actual-fees-in-the-actual-block-chain information of type b seem like
a powerful source of pricing information, and if SPV security is already
valuable for other reasons, then it seems natural to leverage that value.


Re: "dropped in an unpredictable way" - transactions would be dropped
> lowest fee/KB first, a completely predictable way.
>
> --
> 'peter'[:-1]@petertodd.org
> 0000000000000000124bae79afdcee9267b4e6f8137758b8b4135455cd8e3bfd
>
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-- 
Nathan Wilcox
Least Authoritarian

email: nathan at leastauthority.com
twitter: @least_nathan
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