Anchor outputs
Also covering Simplified commitments
Anchor outputs are special outputs in LN commitment transactions that are designed to allow the transaction to be fee bumped. An earlier name for the proposal was simplified commitments.
Each time the balance changes in an LN channel, a commitment transaction is created and signed by the participating parties. The transaction is only broadcast if one party decides to unilaterally close the channel (e.g. because the other party has become unresponsive). Because the broadcast of the commitment transaction may occur a long time after it was created, the commitment transaction may pay too much or too little in transaction fees. Paying a too-low feerate may prevent the commitment transaction from confirming before any timelocks contained within it expire, allowing funds to be stolen.
The solution to this is for the commitment transaction to pay a minimal amount of fees and then to allow either channel participant to fee bump the transaction. Early designs to accomplish this using Replace-by-Fee (RBF) fee bumping ran into problems with transaction pinning. Later designs used Child Pays For Parent (CPFP) fee bumping and came to depend on CPFP carve-out to circumvent the pinning problem.
As of this writing, the most recent versions of the design add two
outputs to the commitment transaction—one for each LN party—and
require all other outputs in the commitment transaction to have their
scripts encumbered by a 1 OP_CHECKSEQUENCEVERIFY (CSV) condition
that prevents them from being spent for at least one block.
To be fully effective, the protocol also depends on Bitcoin full nodes implementing package relay so that there’s a way to CPFP fee bump commitment transactions even if their feerates are below a node’s minimum relay fee. But until package relay is available, it’s possible for LN nodes to just pay a somewhat higher feerate on their commitment transactions to ensure that they’ll be accepted by nodes.
Primary code and documentation
Optech newsletter and website mentions
2024
- Research about historic use of anchor outputs for possibly imbuing them with v3 properties
- Eclair #2816 allows the node operator to choose the max fee for an anchor output
- Core Lightning #6785 makes anchor-style channels the default
- Discussion about LN anchors and v3 transaction relay proposal
2023
- Core Lightning #6334 updates and expands CLN’s experimental support for anchor outputs
- LDK #2368 allows manually accepting new channels created by a peer that use anchor outputs
- LDK #1841 implements a solution for the fee-stealing attack described in BOLTs #824
- LDK #1860 adds support for channels using anchor outputs
2022
- Implementation of ephemeral anchors built on v3 transactions
- Proposal to use multiple pre-committed fees to avoid needing anchor outputs in many cases
- Proposed relay of v3 transactions attempts to improve on anchor outputs
- Eclair #2134 enables anchor outputs by default
2021
- Rust-Lightning #1176 adds initial support for anchor outputs-style fee bumping
- Complications for HD wallet recovery in zero HTLC fee anchor outputs protocol
- Eclair #1932 implements the zero HTLC fee anchor outputs protocol
- BOLTs #824 adds a variation on the anchor outputs protocol that prevents a fee-stealing attack
- LND 0.13.0-beta begins using anchor outputs by default for all new channels
- LND #5274 limits the maximum funds reserved for fee bumping anchor outputs
- Request for feedback on using anchor outputs by default in LND
- Eclair 0.5.1 adds and updates features preparing for anchor outputs
- LND 0.12.0-beta adds watchtower support for channels using anchor outputs
- LND #4908 reserves a balance for fee bumping when using anchor outputs
2020
- 2020 year in review: anchor outputs
- LND #4779 allows sweeping HTLCs from anchor outputs
- BOLT5 updated to prevent a pinning attack against anchor outputs
- LND #4782 adds watchtower client support for channels using anchor outputs
- Eclair 0.4.2 adds experimental support for anchor outputs
- Why do anchor outputs need to enforce an nSequence of 1?
- LND #4576 starts adding support for anchor outputs to LND’s watchtower
- Eclair #1501 adds support for unilateral closes of channels using anchors
- LND #4606 & #4592 improve LND’s effectiveness at fee bumping anchor outputs
- LND #4558 updates LND to the latest anchor outputs specification
- BOLTs #688 adds support for anchor outputs to the LN protocol
- C-Lightning #3830 adds experimental support for anchor outputs
- Eclair #1491 adds support for anchor outputs
- Discussion of solutions for attacks against LN, including anchor outputs
- Eclair #1484 adds basic support for anchor outputs
- Anchor outputs may help mitigate LN fee ransom attack
- LND 0.10 presentation: anchor outputs
- Using anchor outputs to ensure HTLC sends can be fee bumped
- LND #3821 adds draft anchor commitments support
- LND #3829 updates code & documentation to simplify adding anchor outputs
2019
- 2019 year-in-review: anchor outputs
- Continued discussion of LN anchor outputs
- LN simplified commitments discussion
2018