Subject: [bitcoin-dev] Rebatable fees & incentive-safe fee markets

From: Mark Friedenbach <[email-masked]>
Date: Thu, 28 Sep 2017 18:06:29 -0700

This article by Ron Lavi, Or Sattath, and Aviv Zohar was forwarded to
me and is of interest to this group:

    "Redesigning Bitcoin's fee market"
    https://arxiv.org/abs/1709.08881

I'll briefly summarize before providing some commentary of my own,
including transformation of the proposed mechanism into a relatively
simple soft-fork.  The article points out that bitcoin's auction
model for transaction fees / inclusion in a block is broken in the
sense that it does not achieve maximum clearing price* and to prevent
strategic bidding behavior.

(* Maximum clearing price meaning highest fee the user is willing to
   pay for the amount of time they had to wait.  In other words, miner
   income.  While this is a common requirement of academic work on
   auction protocols, it's not obvious that it provides intrinsic
   benefit to bitcoin for miners to extract from users the maximum
   amount of fee the market is willing to support.  However strategic
   bidding behavior (e.g. RBF and CPFP) does have real network and
   usability costs, which a more "correct" auction model would reduce
   in some use cases.)

Bitcoin is a "pay your bid" auction, where the user makes strategic
calculations to determine what bid (=fee) is likely to get accepted
within the window of time in which they want confirmation.  This bid
can then be adjusted through some combination of RBF or CPFP.

The authors suggest moving to a "pay lowest winning bid" model where
all transactions pay only the smallest fee rate paid by any
transaction in the block, for which the winning strategy is to bid the
maximum amount you are willing to pay to get the transaction
confirmed:

> Users can then simply set their bids truthfully to exactly the
> amount they are willing to pay to transact, and do not need to
> utilize fee estimate mechanisms, do not resort to bid shading and do
> not need to adjust transaction fees (via replace-by-fee mechanisms)
> if the mempool grows.

Unlike other proposed fixes to the fee model, this is not trivially
broken by paying the miner out of band.  If you pay out of band fee
instead of regular fee, then your transaction cannot be included with
other regular fee paying transactions without the miner giving up all
regular fee income.  Any transaction paying less fee in-band than the
otherwise minimum fee rate needs to also provide ~1Mvbyte * fee rate
difference fee to make up for that lost income.  So out of band fee is
only realistically considered when it pays on top of a regular feerate
paying transaction that would have been included in the block anyway.
And what would be the point of that?

As an original contribution, I would like to note that something
strongly resembling this proposal could be soft-forked in very easily.
The shortest explanation is:

    For scriptPubKey outputs of the form "<max-42-byte-push>", where
    the pushed data evaluates as true, a consensus rule is added that
    the coinbase must pay any fee in excess of the minimum fee rate
    for the block to the push value, which is a scriptPubKey.

Beyond fixing the perceived problems of bitcoin's fee auction model
leading to costly strategic behavior (whether that is a problem is a
topic open to debate!), this would have the additional benefits of:

    1. Allowing pre-signed transactions, of payment channel close-out
       for example, to provide sufficient fee for confirmation without
       knowledge of future rates or overpaying or trusting a wallet to
       be online to provide CPFP fee updates.

    2. Allowing explicit fees in multi-party transaction creation
       protocols where final transaction sizes are not known prior to
       signing by one or more of the parties, while again not
       overpaying or trusting on CPFP, etc.

    3. Allowing applications with expensive network access to pay
       reasonable fees for quick confirmation, without overpaying or
       querying a trusted fee estimator.  Blockstream Satellite helps
       here, but rebateable fees provides an alternative option when
       full block feeds are not available for whatever reason.

Using a fee rebate would carry a marginal cost of 70-100 vbytes per
instance.  This makes it a rather expensive feature, and therefore in
my own estimation not something that is likely to be used by most
transactions today.  However the cost is less than CPFP, and so I
expect that it would be a heavily used feature in things like payment
channel refund and uncooperative close-out transactions.

Here is a more worked out proposal, suitable for critiquing:

1. A transaction is allowed to specify an _Implicit Fee_, as usual, as
   well as one or more explicit _Rebateable Fees_.  A rebateable fee
   is an ouput with a scriptPubKey that consists of a single, minimal,
   nonzero push of up to 42 bytes.  Note that this is an always-true
   script that requires no signature to spend.

2. The _Fee Rate_ of a transaction is a fractional number equal to the
   combined implicit and rebateable fee divided by the size/weight of
   the transaction.

   (A nontrivial complication of this, which I will punt on for the
    moment, is how to group transactions for fee rate calculation such
    that CPFP doesn't bring down the minimum fee rate of the block,
    but to do so with rules that are both simple, because this is
    consensus code; and fair, so as to prevent unintended use of a
    rebate fee by children or siblings.)

3. The smallest fee rate of any non-coinbase transaction (or
   transaction group) is the _Marginal Fee Rate_ for the block and is
   included in the witness for the block.

4. The verifier checks that each transaction or transaction grouping
   provides a fee greater than or equal to the threshold fee rate, and
   at least one is exactly equal to the marginal rate (which proves
   the marginal rate is the minimum for the block).

This establishes the marginal fee rate, which alternatively expressed
is epsilon less than the fee rate that would have been required to get
into the block, assuming there was sufficient space.

5. A per-block _Dust Threshold_ is calculated using the marginal fee
   rate and reasonable assumptions about transaction size.

6. For each transaction (or transaction group), the _Required Fee_ is
   calculated to be the marginal fee rate times the size/weight of the
   transaction.  Implicit fee is applied towards this required fee and
   added to the _Miner's Fee Tally_.  Any excess implicit fee
   remaining is added to the _Implicit Fee Tally_.

7. For each transaction (group), the rebateable fees contribute
   proportionally towards towards meeting the remaining marginal fee
   requirement, if the implicit fee failed to do so.  Of what's left,
   one of two things can happen based on how much is remaining:

     A. If greater than or equal to the dust threshold is remaining in
        a specific rebateable fee, a requirement is added that an
        output be provided in the coinbase paying the remaining fee to
        a scriptPubKey equal to the push value (see #1 above).

        (With due consideration for what happens if a script is reused
         in multiple explicit fees, of course.)

     B. Otherwise, add remaining dust to the implicit fee tally.

8. For the very last transaction in the block, the miner builds a
   transaction claiming ALL of these explicit fees, and with a single
   zero-valued null/data output, thereby forwarding the fees on to the
   coinbase, as far as old clients are concerned.  This is only
   concerns consensus in that this final transaction does not change
   any of the previously mentioned tallies.

   (Aside: the zero-valued output is merely required because all
    transactions must have at least one output. It does however make a
    great location to put commitments for extensions to the block
    header, as being on the right-most path of the Merkle tree can
    mean shorter proofs.)

9. The miner is allowed to claim subsidy + the miner's fee tally + the
   explicit fee tally for themselves in the coinbase.  The coinbase is
   also required to contain all rebated fees above the dust threshold.

In summary, all transactions have the same actual fee rate equal to
the minimum fee rate that went into the creation of the block, which
is basically the marginal fee rate for transaction inclusion.

A variant of this proposal is that instead of giving the implicit fee
tally to the miner (the excess non-rebateable fees beyond the required
minimum), it is carried forward from block to block in the final
transaction and the miner is allowed to claim some average of past
fees, thereby smoothing out fees or providing some other incentive.