[bitcoin-dev] A Replacement for RBF and CPFP: Non-Destructive TXID Dependencies for Fee Sponsoring

[Jeremy <jlrubin@mit.edu>]

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Hi Bitcoin Devs,


I'd like to share with you a draft proposal for a mechanism to replace
CPFP and RBF for
increasing fees on transactions in the mempool that should be more
robust against attacks.

A reference implementation demonstrating these rules is available
[here](https://github.com/bitcoin/bitcoin/compare/master...JeremyRubin:subsidy-tx)
for those who
prefer to not read specs.

Should the mailing list formatting be bungled, it is also available as
a gist [here](https://gist.github.com/JeremyRubin/92a9fc4c6531817f66c2934282e71fdf).

Non-Destructive TXID Dependencies for Fee Sponsoring
====================================================

This BIP proposes a general purpose mechanism for expressing
non-destructive (i.e., not requiring
the spending of a coin) dependencies on specific transactions being in
the same block that can be
used to sponsor fees of remote transactions.

Motivation
==========

The mempool has a variety of protections and guards in place to ensure
that miners are economic and
to protect the network from denial of service.

The rough surface of these policies has some unintended consequences
for second layer protocol
developers. Applications are either vulnerable to attacks (such as
transaction pinning) or must go
through great amounts of careful protocol engineering to guard against
known mempool attacks.

This is insufficient because if new attacks are found, there is
limited ability to deploy fixes for
them against deployed contract instances (such as open lightning
channels). What is required is a
fully abstracted primitive that requires no special structure from an
underlying transaction in
order to increase fees to confirm the transactions.

Consensus Specification
=======================

If a transaction's last output's scripPubKey is of the form OP_VER
followed by n*32 bytes, where
n>1, it is interpreted as a vector of TXIDs (Sponsor Vector). The
Sponsor Vector TXIDs  must also be
in the block the transaction is validated in, with no restriction on
order or on specifying a TXID
more than once. This can be accomplished simply with the following patch:


```diff
+
+    // Extract all required fee dependencies
+    std::unordered_set<uint256, SaltedTxidHasher> dependencies;
+
+    const bool dependencies_enabled = VersionBitsState(pindex->pprev,
chainparams.GetConsensus(),
Consensus::DeploymentPos::DEPLOYMENT_TXID_DEPENDENCY,
versionbitscache) == ThresholdState::ACTIVE;
+    if (dependencies_enabled) {
+        for (const auto& tx : block.vtx) {
+            // dependency output is if the last output of a txn is
OP_VER followed by a sequence of 32*n
+            // bytes
+            // vout.back() must exist because it is checked in CheckBlock
+            const CScript& dependencies_script = tx->vout.back().scriptPubKey;
+            // empty scripts are valid, so be sure we have at least one byte
+            if (dependencies_script.size() && dependencies_script[0]
== OP_VER) {
+                const size_t size = dependencies_script.size() - 1;
+                if (size % 32 == 0 && size > 0) {
+                    for (auto start = dependencies_script.begin() +1,
stop = start + 32; start < dependencies_script.end(); start = stop,
stop += 32) {
+                        uint256 txid;
+                        std::copy(start, stop, txid.begin());
+                        dependencies.emplace(txid);
+                    }
+                }
+                // No rules applied otherwise, open for future upgrades
+            }
+        }
+        if (dependencies.size() > block.vtx.size()) {
+            return
state.Invalid(BlockValidationResult::BLOCK_CONSENSUS,
"bad-dependencies-too-many-target-txid");
+        }
+    }
+
     for (unsigned int i = 0; i < block.vtx.size(); i++)
     {
         const CTransaction &tx = *(block.vtx[i]);
+        if (!dependencies.empty()) {
+            dependencies.erase(tx.GetHash());
+        }

         nInputs += tx.vin.size();

@@ -2190,6 +2308,9 @@ bool CChainState::ConnectBlock(const CBlock&
block, BlockValidationState& state,
         }
         UpdateCoins(tx, view, i == 0 ? undoDummy :
blockundo.vtxundo.back(), pindex->nHeight);
     }
+    if (!dependencies.empty()) {
+        return state.Invalid(BlockValidationResult::BLOCK_CONSENSUS,
"bad-dependency-missing-target-txid");
+    }
```

### Design Motivation
The final output of a transaction is an unambiguous location to attach
metadata to a transaction
such that the data is available for transaction validation. This data
could be committed to anywhere,
with added implementation complexity, or in the case of Taproot
annexes, incompatibility with
non-Taproot addresses (although this is not a concern for sponsoring a
transaction that does not use
Taproot).

A bare scriptPubKey prefixed with OP_VER is defined to be invalid in
any context, and is trivially
provably unspendable and therefore pruneable.

If there is another convenient place to put the TXID vector, that's fine too.

As the output type is non-standard, unupgraded nodes will by default
not include Transactions
containing them in the mempool, limiting risk of an upgrade via this mechanism.

Policy Specification
====================

The mechanism proposed above is a general specification for
inter-transaction dependencies.

In this BIP, we only care to ensure a subset of behavior sufficient to
replace CPFP and RBF for fee
bumping.

Thus we restrict the mempool policy such that:

1. No Transaction with a Sponsor Vector may have any child spends; and
1. No Transaction with a Sponsor Vector may have any unconfirmed parents; and
1. The Sponsor Vector must have exactly 1 entry; and
1. The Sponsor Vector's entry must be present in the mempool; and
1. Every Transaction may have exactly 1 sponsor in the mempool; except
1. Transactions with a Sponsor Vector may not be sponsored.


The mempool treats ancestors and descendants limits as follows:

1. Sponsors are counted as children transactions for descendants; but
1. Sponsoring transactions are exempted from any limits saturated at
the time of submission.

This ensures that within a given package, every child transaction may
have a sponsor, but that the
mempool prefers to not accept new true children while there are
parents that can be cleared.

To prevent garbage sponsors, we also require that:

1. The Sponsor's feerate must be greater than the Sponsored's ancestor fee rate

We allow one Sponsor to replace another subject to normal replacement
policies, they are treated as
conflicts.


### Design Motivation

There are a few other ways to use OP_VER sponsors that are not
included. For instance, one could
make child chains that are only valid if their parent is in the same
block (this is incompatible
with CTV, exercise left to reader). These use cases are in a sense
incidental to the motivation
of this mechanism, and add a lot of implementation complexity.

What is wanted is a minimal mechanism that allows arbitrary
unconnected third parties to attach
fees to an arbitrary transaction. The set of rules given tightly
bounds how much extra work the
mempool might have to do to account for the new sponsors in the worst
case, while providing a "it
always works" API for end users that is not subject to traditional
issues around pinning.

Eventually, rational miners may wish to permit multiple sponsor
targets, or multiple sponsoring
transactions, but they are not required for the mechanism to work.
This is a benefit of the
minimality of the consensus rule, it is compatible with future policy
should it be implemented.


#### Attack Analysis of new Policy

In the worst case the new policy can lead to a 1/2 reduction in the
number of children allowed
(e.g., if there are 13 children submitted, then 12 sponsors, the 25
child limit will saturate
before) and a 2x increase in the maximum children (e.g., if there are
25 children submitted, and
then each are sponsored). Importantly, even in the latter attack
scenario, the DoS surface is not
great because the sponsor transactions have no children nor parents.

#### Package Relay/Orphan Pool

Future policy work might be able to insert sponsors into a special
sponsor pool with an eviction
policy that would enable sponsors to be queried and tracked for
transactions that have too low fee
to enter the mempool in the first place. This is treated as a separate
concern, as any strides on
package relay generally should be able to support sponsors trivially.

Reference Implementation
========================
A reference implementation demonstrating these rules is available
[here](https://github.com/bitcoin/bitcoin/compare/master...JeremyRubin:subsidy-tx).
This is a best
effort implementation, but has not been carefully audited for
correctness and likely diverges from
this document in ways that should either be reflected in this document
or amended in the code.


Best,

Jeremy



--
@JeremyRubin <https://twitter.com/JeremyRubin>
<https://twitter.com/JeremyRubin>

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