Dual-funded channels and Splicing Project Tracking

[dunxen]

Overview

Project tracking and implementation status of interactive transaction construction, dual-funding (V2 channel establishment), channel quiescence, and splicing.

Specifications:

• BOLT 2: Interactive transaction construction
• BOLT 2: Dual-funding (a.k.a V2 channel establishment)
• BOLT 2: Channel quiescence
• BOLT 2: Splicing (PR).

Wire messages

• Dual funding and interactive tx construction wire messages #1794
• Remove OptionalField and make DataLossProtect fields mandatory #2253
• Add Splicing messages #2542

Interactive Transaction Construction

• Interactive Transaction Construction #2419
• Interactive TX negotiation tracks shared outputs #2989
• Support RBF for interactively constructed transactions #3281 (depends on Implement accepting dual-funded channels without contributing #3137)

Dual-funding (V2 channel establishment)

0. Refactoring

• Split prefunded Channel into Inbound/Outbound channels #2077
• Address outstanding 2077 feedback #2382

1. Accept dual-funded channels

• Implement accepting dual-funded channels without contributing #3137
• Follow-ups for PR 3137 #3416
• Introduce interactive signing state flags for funded states. #3637
• Interop tests and fixes
• Release in 0.2

2. Accept dual-funded channels with contributions

• Interop tests and fixes
• Release as alpha

3. Create & accept dual-funded channels

• Move funding signing after commitment #1571
• Implement V2 channel establishment #2302 (depends on Implement accepting dual-funded channels without contributing #3137)
• (Maybe) Allow initiator and acceptor to add other outputs
• Interop tests and fixes
• Release

4. RBF support

• Support RBF for interactively constructed transactions #3281

Dual-funding implementation phase feature table

🟢 = supported
🔴 = unsupported

Impl. phase	Accept V2	Create V2	RBF	Contribute funds
1. Accept V2 channels	🟢	🔴	🔴	🔴
2. Accept V2 channels w/con	🟢	🔴	🔴	🔴
3. Create V2 channels	🟢	🟢	🔴	🟢
3. RBF support	🟢	🟢	🟢	🟢

Channel quiescence

• Enable decoding HTLC onions when fully committed #2933
• WIP based on Enable decoding HTLC onions when fully committed #2933 (should be only one PR; cc: @wpaulino)

Splicing

Draft/Prototype PRs

• [Early Draft] Splicing prototype #3274: Very old baseline (v0.0.123); prototype (not for merging). Splicing use case works, from V2, from V1, also pay-splice-pay. Also prototype support for RBF.
• [Early Draft][Splicing] Partial, perform interactive transaction negotiation for new funding #3444: Obsolete solution, uses duplicate ChannelContext via cloning (not for merging). Currently splicing use case works up to exchange of tx_complete's, but not commitment_signed.
• [Early Draft][Splicing] Interactive tx negotiation, with a single ChannelContext #3715: Most recent version, no ChannelContext duplication. Currently splicing use case works up to the first tx_complete.

Use case phases status

Use case phase	branch or draft PR	merged/pending PRs
Initial handshake, splice_init, splice_ack	main	#3407 #3647
Tx negotiation, tx_add_input, tx_add_output	#3715	#3443
Tx negotiation completion, final tx_complete's	#3444	-
Commitment exchange	#3274	-
tx_signatures exchange, funding tx broadcast	#3274	-
splice_locked exchange, after confirmation	#3274	-

0. Preparations

• ([Splicing] Add splicing compile feature flag #3294 )
• [Splicing] Support shared signature in InteractiveTransaction #3295
• ([Splicing] Update Splicing msgs #3293)
• ([Splicing] Preserve funding_transaction for the later lifecycle of the channel #3317) ([Splicing] Make funding transaction available in funded channel  #3300 )
• ([Splicing] Signer extended with method to sign prev funding transaction input #3316) ([Splicing] Signing utility for splicing #3312 )
• ([Splicing] Clone for ChannelContext #3332)
• ([Splicing] Partial, handle splice_init & splice_ack messages #3407)
• (Dual funding extension: begin_interactive_funding_tx_construction #3443)
• [Splicing] Add reserve check to splicing #3641 (non-critical)
• [Draft] Combine FundedChannel and PendingV2Channel through traits #3702
• (more to come)

1. Basic implementation

• Basic Splicing #3298
• [Splicing] Partial, handle splice_init & splice_ack messages #3407

2. Additional features

• Integrate quiescence
• Splice on V1 channel
• RBF support
• Allow payment during pending splice
• Support splice-out
• Contributions from the acceptor
• Handle interruptions and restarts
• Work with other implementations

[npslaney]

Thinking through LSP onboarding strategies having the ability to start with a balanced channel is a really nice option. Definitely watching this one 👀

[ariard]

I think one of the first step could be to make OnchainTxHandler its own interface that way the reorg/fee-bumping/rebroadcast logic could be shared between our ChannelMonitor and MultipartyConstruction stuff. See : #606 (comment) for ideas.

There is also an open question on how much code for doing that is already available in BDK.

[dunxen]

I think one of the first step could be to make OnchainTxHandler its own interface that way the reorg/fee-bumping/rebroadcast logic could be shared between our ChannelMonitor and MultipartyConstruction stuff. See : #606 (comment) for ideas.

There is also an open question on how much code for doing that is already available in BDK.

Thanks for the comment link! I'll sync up with the BDK peeps on that open question too.

[ariard]

Available to review advances in dual-funded implementation, even if it's early refactor for now. I've reviewed back recently the state of the BOLT proposal and it's sounds to be mature enough to solidify around a v0.1.

[dunxen]

Available to review advances in dual-funded implementation, even if it's early refactor for now. I've reviewed back recently the state of the BOLT proposal and it's sounds to be mature enough to solidify around a v0.1.

Great, picking up refactor next! 🙏

[dunxen]

Available to review advances in dual-funded implementation, even if it's early refactor for now. I've reviewed back recently the state of the BOLT proposal and it's sounds to be mature enough to solidify around a v0.1.

Great, picking up refactor next! pray

Next is now. So picking up from now :)