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LDK Splicing Prototype

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About Splicing

Best starting point is Dusty Daemon's lightningsplice.com site.

What is Splicing?
Splicing can change the capacity of a channel, without closing it. It's possible to increase or decrease the capacity, by adding or removing some funds on-chain.

Why Splicing?
Splicing can be used for different goals. e.g.:

  • For a routing node, splicing allows to increase the capacity of a well-performing channel (without closing it). It is even possible to move some funds to one channel to another in one go.
  • For a LSP node – wallet setup, the channel can be upgraded/downgraded as needed (alternative is to open an additional or close & reopen).
  • For a LN + Onchain spending wallet experience it is possible to keep funds in LN, and send from it or receive to it on-chain using splicing (alternative is to keep them in separate 'pockets', or use Submarine Swap).

The main advantage is less on-chain operations (one instead of two in some use cases), but splicing also opens up new use cases (mentioned above).

The first implementation is/was in CLN, started by Dusty Daemon.

Status of Splicing

Specification

The splicing spec is quite mature, but not yet finalized. (bolts PR #863)

Implementations

Core Lightning
CLN has released splicing implementation, experimental.
Released with 23.08 (Aug'23).

Eclair/Phoenix (Acinq)
Released, used in production, between their LSP node and the wallets. Not interoperable with other implementations.
(Versions: Lightning-kmp lib: version 1.5.0, Jul'23, Phoenix Android wallet release 2.0.0, Aug'23.)

Splicing in LDK

It is work-in-progress, on the 2023 Roadmap.
Tracking issue: https://github.com/lightningdevkit/rust-lightning/issues/1621

News

  • Nov 13: Splicing message definitions got merged (PR #2544)
  • Oct 1: Phoenix iOS v2.0 released, bringing splicing to iOS as well.
  • Aug 23: CLN splicing implementation released, release 23.08.
  • Aug 16: New Phoenix Android version 2.0 released, uses splicing internally.
  • Aug 11: My Nostr post on successful splice using ldk proto
  • Jul'23: Client lib by Acinq, lightning-kmp, released with splicing support (v 1.5.0)

LDK-Splicing prototype (by optout)

Warning: This prototype is experimental code, using it will likely result in some funds lost!

The current (splicing-hapa2) prototype is a rudimentary splicing implementation in LDK, aimed to be the basis for the final implementation.
Some of the simplifying assumptions:

  • No quiscence is used/checked
  • Happy path only, no complex combinations, no error scenarios
  • TX negotiation messages are skipped, two simpler messages are used instead

Repositories:

Contact/Contribute

Please feel free to ask questions or give feedback:

Prototype Details

ldk-splicing

Contains the splice proto: required messages, new API splice_channel, and some events (e.g. SpliceAcked), and also a step-by-step functional test.

ldk-sample

Exposes the splice functionality, through a new splicein command (splicein <channel_id> <peer_pubkey> <add_amt_satoshis>). Works on an existing channel, and adds some more funds to it.

ldk-node

Currently not done, but it is planned to expose splicing functionality through ldk-node as well.

Building modified LDK

git clone https://github.com/ldksplicing/ldk-splicing
cd ldk-splicing

or alternatively you can use an existing LDK folder

cd rust-lightning
git remote add ldksplicing https://github.com/ldksplicing/ldk-splicing
git fetch ldksplicing
git checkout -b splicing -t ldksplicing/main

  • Build it, simply with cargo build

  • You can try or look a splicing use case test: cargo test splic

Building modified ldk-sample

git clone https://github.com/ldksplicing/ldk-sample
cd ldk-sample
  • Modify Cargo.toml to point to locally built modified LDK.
    Under [dependencies] replace the lightning... lines:
# To build against updated LDK built locally, you need to change the following 7 lines to the form:
# lightning = { path = "<RELATIVE_PATH_TO_LDK_FOLDER>/lightning", features = ["max_level_trace"] }
# instead of
# lightning = { version = "0.0.117", features = ["max_level_trace"] }

You need to do this for all lines (7 of them).

  • Build: cargo build

  • For running, you need a locally accessible Bitcoin Core node.

  • Run: cargo run <bitcoind-rpc-username>:<bitcoind-rpc-password>@<bitcoind-rpc-host>:<bitcoind-rpc-port> datadir1 19735 testnet
    After startup, type help command, you should see a splicein command:

COMMANDS:
  Channels:
      ...
      splicein <channel_id> <peer_pubkey> <add_amt_satoshis>

End-to-end test between two ldk-sample instances

Prerequisites:

  • A locally built modified LDK (rust-lightning); see above.
  • A locally built modified ldk-sample, using modifed LDK; see above.
  • a Bitcoin Core node on testnet, accessible through RPC, with a wallet loaded with some balance.

Steps

Start two shells (S1: and S2: indicate the shell):

S1: cd ldk-sample
S1: cargo run <bitcoind-rpc-username>:<bitcoind-rpc-password>@<bitcoind-rpc-host>:<bitcoind-rpc-port> datadir1 19735 testnet 

S2: cd ldk-sample
S2: cargo run <bitcoind-rpc-username>:<bitcoind-rpc-password>@<bitcoind-rpc-host>:<bitcoind-rpc-port> datadir2 19745 testnet 
--> note the 'Local Node ID' displayed

S1: openchannel <node-id-from-S2>@localhost:19745 8000
S1: listchannels
S1: splicein <channel-id-from-above> <node-id-from-S2> 2000
S1: listchannels

Sample output:

LDK startup successful. Enter "help" to view available commands. Press Ctrl-D to quit.
Local Node ID is 03b6d5..a0.
> 
> openchannel 020ab1..5c@localhost:19745 3500
EVENT: initiated channel with peer 020ab1..5c. 
EVENT: Channel 3a09ec..f3 with peer 020ab1..5c is pending awaiting funding lock-in!
EVENT: Channel 3a09ec..f3 with peer 020ab1..5c is ready to be used!
> 
> listchannels
[
	{
		channel_id: 3a09ec..f3,
		funding_txid: f2a439..3a,
		peer_pubkey: 020ab1..5c,
		is_channel_ready: true,
		channel_value_satoshis: 3500,
	},
]
> 
> splicein 3a09ec..f3 020ab1..5c 2000
EVENT: initiated splice-in with add'l amount 2000 on channel 3a09ec..f3.  
Debug SpliceAcked  channel_id 3a09ec..f3  pre-post 3500 5500  curr funding f2a4397..3a 1
EVENT: Channel 3a09ec..f3 splicing tx created, from 3500 to 5500, txid ae8f28d4843f2d24ae0558b05841d2555ee70df0e404e7b37a18c85c56a6054a
> 
> listchannels
[
	{
		channel_id: 3a09ec..f3,
		funding_txid: ae8f28..4a,
		peer_pubkey: 020ab1..5c,
		is_channel_ready: false,
		channel_value_satoshis: 5500,
	},
]
> 
> closechannel 3a09ec..f3 020ab1..5c
EVENT: initiating channel close 
EVENT: Channel 3a09ec..f3 with counterparty 020ab1..5c closed due to: CooperativeClosure
> 
> listchannels
[]
>
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