BIP431: Opt In Topologically Restricted Until Confirmation Transactions For More Robust Fee-bumping

[glozow]

This is a BIP for Topologically Restricted Until Confirmation (TRUC) Transactions. It's also called "v3 transaction policy" since the marker is nVersion=3.

A specification is useful for coordination between node impls that want to implement the same policy and applications that want to use it. For those that are not interested in the details of v3 policy, this also serves as a writeup of the specific pinning problems we aim to address. There has been discussion of using this in other protocol design and multiple requests for its documentation to exist in the BIPs repository, so I'm opening a PR here.

Implementation:

• v3 transaction policy for anti-pinning bitcoin#28948
• policy: restrict all TRUC (v3) transactions to 10kvB bitcoin#29873
• policy: bump TX_MAX_STANDARD_VERSION to 3 bitcoin#29496

Example usage and things built on top:

• Package RBF
  • Cluster size 2 package rbf bitcoin#28984
• Ephemeral Anchors
  • Ephemeral Anchors bitcoin#29001
  • Ephemeral anchors #1524
• LN commitment transactions
  • https://delvingbitcoin.org/t/lightning-transactions-with-v3-and-ephemeral-anchors/418
  • https://github.com/instagibbs/bolts/commits/zero_fee_commitment
• LN-Symmetry
  • https://delvingbitcoin.org/t/ln-symmetry-project-recap/359
  • https://github.com/instagibbs/lightning/tree/eltoo_support

Discussion and history:

• https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-January/019817.html
• https://gist.github.com/glozow/25d9662c52453bd08b4b4b1d3783b9ff
• https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2022-September/020937.html
• https://delvingbitcoin.org/t/v3-transaction-policy-for-anti-pinning/340
• policy: nVersion=3 and Package RBF bitcoin#25038

[t-bast]

FWIW, concept ACK :)

[murchandamus]

ACK 4bd12d5

[glozow]

Thanks @murchandamus, took all your suggestions (4bd12d5...af8e903)

[ismaelsadeeq]

Concept ACK

[glozow]

@luke-jr this has been open for a month, would you mind taking a look?

[ariard]

I think this should document that 1000 vb child limit is experimental and it cannot be relied on by downstream projects.
This opt-in policy is not robust towards NTA pinning (cf. “The Good, The Bad, The Ugly” 2020 mail) and “loophole” pinning exposed in Core’s #28948. This can note that additional opt-in policy might be applied on top of nversion=3 see Core’s #29454.

[murchandamus]

ACK 63e8a71

[glozow]

Thanks @murchandamus! Accepted your suggestions.

[luke-jr]

As with #1524, node policy is not a standardizable subject matter in itself - every node decides its own policy. BIP 125 already defined a way for wallets to indicate they prefer RBF-like treatment of their transactions, and I don't see why the same signal can't be used for this.

[vostrnad]

Every node deciding its own policy is not mutually exclusive with it being standardizable – each node decides which (if any) standards to follow. Policy standards make just as much sense as e.g. wallet standards or P2P protocol standards.

[Roasbeef]

BIP 125 already defined a way for wallets to indicate they prefer RBF-like treatment of their transactions, and I don't see why the same signal can't be used for this.

BIP 125 defines a set of rules w.r.t transaction replacement. Some of those rules have edge cases that can lead to defects where transactions that should otherwise be replaced, can't be replaced. This class of replacement defects loosely falls under the umbrella of "transaction pinning". The TRUC rules resolve those issues, creating a replacement semantics that better serve off-chain protocols, and address some known pinning vectors. For many off-chain protocols, TRUC will supersede base RBF. As the semantics are distinct, it cannot use the existing sequence fields carved out by BIP 125 (the rules are also incompatible). Instead, it uses transaction v3 (no longer non standard) as a way to signal replacement under a distinct set of rules.

[luke-jr]

BIP 125 defines a signalling mechanism, not policy (which is again outside the scope of BIPs) even if it describes a particular policy. The intent of the signal is clear, and it can be implemented in other ways such as the one that seems to be desired here.

[Roasbeef]

BIP 125 defines a signalling mechanism, not policy

It clearly defines both:

The opt-in full Replace-by-Fee (opt-in full-RBF) signaling policy described here allows spenders to add a signal to a transaction indicating that they want to be able to replace that transaction in the future

This policy specifies two ways a transaction can signal that it is replaceable.

Inherited signaling: Transactions that don't explicitly signal replaceability are replaceable under this policy for as long as any one of their ancestors signals replaceability and remains unconfirmed.

Because descendant transactions may also be replaceable under this policy through inherited signaling, any method used to process opt-in full-RBF transactions should be inherited by any descendant transactions for as long as any ancestor opt-in full-RBF transactions remain unconfirmed.

It also defines a precise set of rules for the policy. If an implementation deviates from those rules, then it isn't BIP 125. From the OP, there's a clear need to define a new policy and signalling mechanism, which this document does.

[luke-jr]

It may be poorly worded, but that isn't the point.

[luke-jr]

(Note that BIP 125's relationship with policy was a matter of discussion back when it was submitted, and it was nearly rejected - the only reason it was accepted in the end, was as a signalling BIP. And even if you want to insist it defines policy, mistakes made in the past would not be a reason to repeat them in the future, and thus would actually work against accepting this proposal)

[jonatack]

Looks pretty close. Would need a README entry and a BIP number assignment, if deemed to be in-scope.

[glozow]

Thanks for the review @jonatack! I'll wait for a number assignment.

[jonatack]

Assigned number 431.

Per BIP 2: The BIP process exists for standardisation between independent projects. While policy is a per-node decision, R&D related to node relay policy and a degree of interop standardization have been and continue to be worked on to try to solve fundamental long-standing issues in bitcoin and across the LN implementations. This R&D documentation, along with signaling v3 transactions and opt-in interop standardization, seem in-scope here.

[glozow]

Thanks @jonatack @murchandamus!

I've added the number, and used that to update the file name, readme table, etc. Also took all of your suggestions.

[carlaKC]

ACK 32e4e44

With the tiny non-blocking nitpick that a few places in the doc use markdown _ for italics (I think) where they should use '' for mediawiki italics.

[murchandamus]

This looks good to me and ready to go except for a couple nits (which I don’t think would need to hold up a merge) and a formatting issue:

1. The JEDEC standard specifies that uppercase K (kilo) refers to 1024¹ (and M (mega), G (giga) as 1024² and 1024³ respectively). However, since the numbers in your example are referring to a factor of 1000¹, they should be using lowercase k, the symbol of the metric prefix kilo referring to a factor of 1000¹. Therefore, it would be preferable if the units were updated to read "kvB" instead of "KvB", and "ksats" instead of "Ksats".

2. AFAIU, technical writing standards generally recommend a space between the numerical value and unit symbol.

E.g. using a space is recommended by the SI Standard,
^{via https://www.nist.gov/pml/special-publication-330/sp-330-section-5}

by NIST,
^{via https://physics.nist.gov/cuu/Units/checklist.html}

as well as IEEE:
^{via https://academia.stackexchange.com/q/54885/8305}

My personal preference is to separate the value and symbol with a Narrow No-Break Space (U+202F) as that provides the expected visual offset, renders as a smaller gap than a full length space, but still prevents unfortunate line breaks. My suggested changes already make use of NNBSPs.

[murchandamus]

Oops, I overlooked a couple more values

[glozow]

Thanks @carlaKC @murchandamus! I've fixed the italics, spacing, and "k".

[carlaKC]

re-ACK 04d3a06 🤩

[glozow]

(I think this is ready, please lmk if there's anything left to do on my end)

[jonatack]

ACK 04d3a06

Re-checked the RBF rules vs description headers.

[jonatack]

Updated for bitcoin/bitcoin#29873 👍

[jonatack]

With bitcoin/bitcoin#29496 on the table, note to possibly consider updating/appending to https://github.com/bitcoin/bitcoin/blob/master/doc/policy/mempool-replacements.md#current-replace-by-fee-policy with TRUC at some point (and making sure the multiple links to it in this BIP remain valid).