On Wed, Jun 10, 2015 at 1:19 PM, Aaron Voisine <voisine@gmail.com> wrote:
It could be done by agreeing on a data format and encoding it in an op_return output in the coinbase transaction. If it catches on it could later be enforced with a soft fork.


Sounds plausible, except SPV protocols would need to include this coinbase txn if it's going to help SPV clients. (Until a softfork is activated, SPV clients should not rely on this encoding, since until that time the results can be fabricated by individual miners.)
 
For real up-to-the-minute fee calculations you're also going to want to look at the current mempool, how many transactions are waiting, what fees they're paying, etc, but of course that information is susceptible to sybil attack.

Hm, when you mention Sybil attack, I don't quite follow.

When a client relies on any report of a mempool [*], this is already outside the realm of locally-verifiable SPV information, so they are already susceptible to the service making false claims. If that's acceptable (and in many cases it may be) then this whole mechanism is moot, because the client can ask the service for fee statistics for past blocks.


In practice what we're doing for now is using services like blockcypher who's business is improving reliability of zero-conf to tell us what fee-per-kb is needed, and then putting a hard coded range around it to protect against the service being compromised.

This is interesting for me, because I had previously believed fees were fairly static presently, and also because I like hearing about real life wallet implementations.

So if this "SPV Fee Stats" feature were added, a wallet might rely on an API for timely stats (aka "block height < 1") then verify that the API isn't lying after doing SPV verification of fee stats for confirmed blocks.
 

This is also the kind of thing being done for exchange rate data which is probably the bigger security risk until bitcoin becomes the standard unit of account for the planet.


That makes sense, although there's no SPV equivalent for exchange data.


Aaron Voisine
co-founder and CEO
breadwallet.com

On Wed, Jun 10, 2015 at 10:37 AM, Nathan Wilcox <nathan@leastauthority.com> wrote:
[I'm currently wading through bitcoin-development. I'm still about a month behind, so I apologize in advance for any noisy redundancy in this post.]

While reading about blocksize, I've just finished Mike Hearn's blog post describing expected systemic behavior as actual blocks approach the current limit (with or without non-protocol-changing implementation improvements):
One detail Mike uses to argue against the "fee's will save us" line of reasoning is that wallets have no good way to learn fee information.

So, here's a proposal to fix that: put fee and (and perhaps block size, UTXO, etc...) statistics into the locally-verifiable data available to SPV clients (ie: block headers).


It's easy to imagine a hard fork that places details like per-block total fees, transaction count, fee variance, UTXO delta, etc... in a each block header. This would allow SPV clients to rely on this data with the same PoW-backed assurances as all other header data.

This mechanism seems valuable regardless of the outcome of blocksize debate. So long as fees are interesting or important, SPV clients should know about them. (Same for other stats such as UTXO count.)

Upgrading the protocol without a hard-fork may be possible and is left as an exercise for the reader. ;-)

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Nathan Wilcox
Least Authoritarian

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Nathan Wilcox
Least Authoritarian

email: nathan@leastauthority.com
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