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Anchor Outputs 錨定輸出
了解 Anchor Outputs 如何讓閃電網路通道在強制關閉時動態調整手續費，解決費率預估難題。
12 分鐘
什麼是 Anchor Outputs？
Anchor Outputs（錨定輸出）是閃電網路承諾交易的一項重要改進。它在承諾交易中 添加兩個小額「錨定」輸出，讓任一方都可以通過 CPFP（Child-Pays-For-Parent） 在廣播時動態增加交易手續費，解決了預估未來費率的難題。
核心問題： 傳統承諾交易的手續費在簽名時就固定了。如果之後網路擁堵導致費率飆升， 你的通道關閉交易可能卡住無法確認，HTLC 可能超時，資金面臨風險。
費率問題
Traditional Commitment TX Fee Rate Dilemma: Time T0: Open channel • Current fee rate: 10 sat/vB • Commitment tx signed at 20 sat/vB (buffer) • Looks fine Time T0 + 6 months: Need to close channel • Current fee rate: 200 sat/vB (network congestion) • Your commitment tx only has 20 sat/vB • Transaction stuck in mempool! • HTLC may timeout, funds at risk! Or the opposite: Time T0: Open channel • Anticipating future congestion • Set high fee rate: 100 sat/vB • Sacrificing channel capacity for fees Time T0 + 6 months: Close channel • Current fee rate: 5 sat/vB • Overpaid by 95 sat/vB! • Money wasted Neither approach works!
Anchor Outputs 解決方案
Anchor Outputs Commitment Transaction Structure:
Commitment Transaction
+---------------------------------------------------+
| Inputs: |
| Funding TX output |
+---------------------------------------------------+
| Outputs: |
| to_local (Alice's balance) |
| to_remote (Bob's balance) |
| HTLC outputs (if any) |
| anchor_alice (330 sats) <- anchor output |
| anchor_bob (330 sats) <- anchor output |
+---------------------------------------------------+
| Fee: minimum rate (e.g., 1 sat/vB) |
+---------------------------------------------------+
Using CPFP to accelerate:
+--------------------+
| Commitment TX |------> low fee rate
| (parent) |
+--------+-----------+
|
| spend anchor_alice
v
+--------------------+
| CPFP TX |------> high fee rate
| (child) | covers total fee
+--------------------+
Miners will include parent and child together! Anchor 腳本
Anchor Output Script: <anchor_pubkey> CHECKSIG IFDUP NOTIF 16 CHECKSEQUENCEVERIFY ENDIF Explanation: 1. Owner can immediately spend with anchor_pubkey 2. If signature fails (someone else tries to spend) -> Must wait 16 blocks 3. This allows anyone to clean up unused anchors after 16 blocks Why 330 sats? • This is the dust limit • Smaller outputs would be rejected by nodes • Small enough to not waste channel capacity • Large enough to be recognized as valid output Feature bits: • option_anchor_outputs (bit 20/21) - original • option_anchors_zero_fee_htlc_tx (bit 22/23) - improved
Zero-Fee HTLC Anchors
改進版的 Anchor Outputs 還解決了 HTLC 交易的費率問題：
Zero-Fee HTLC Transactions:
Problem: HTLC-Success and HTLC-Timeout txs also need fees
Traditional approach:
Commitment TX --> HTLC Output
|
v
HTLC-Success TX (fixed fee rate)
or HTLC-Timeout TX (fixed fee rate)
These second-stage tx fee rates are also pre-signed!
Zero-Fee HTLC approach:
Commitment TX --> HTLC Output
|
v
HTLC TX (0 fee rate)
|
| add extra input to pay fee
v
Complete tx with CPFP
Benefits:
• HTLC txs can also dynamically adjust fee rate
• More flexibility in HTLC timeout races
• Can use wallet UTXOs to pay fees UTXO 需求
新的運營需求
使用 Anchor Outputs 的節點需要維護額外的鏈上 UTXO 用於 CPFP。 如果沒有可用 UTXO，可能無法及時加速交易。
UTXO 管理
- • LND 有專用的「anchor 錢包」
- • 自動維護最低餘額
- • 監控 UTXO 可用性
與 Watchtower 的關係
Watchtower + Anchor Outputs: Traditional Watchtower: • Stores pre-signed penalty transactions • Fee rate fixed at signing time • If fee rate too low, penalty may fail! Anchor-aware Watchtower: • Stores penalty tx + has CPFP capability • Needs its own UTXO pool • Can dynamically adjust fee rate • Increases Watchtower costs Challenges for Watchtower: 1. Maintain UTXO pool 2. Monitor fee rate market 3. Ready to pay CPFP fees 4. May need to charge users to cover costs
版本演進
原版 (option_anchor_outputs)
第一版 Anchor Outputs，承諾交易有 anchors，但 HTLC 交易仍使用預簽費率。
改進版 (option_anchors_zero_fee_htlc_tx)
當前推薦版本。HTLC 交易使用零費率，也可以通過 CPFP 加速。
static_remotekey 是必需的。
Simple Taproot Channels（未來）
Taproot 通道會使用不同的 anchor 設計，可能只需要單個共享 anchor。
實現狀態
LND 默認啟用
LND 0.12+ 默認使用 anchor channels，自動管理 anchor 錢包。
Core Lightning 支持
CLN 支持 anchor channels，可通過配置啟用。
Eclair 支持
Eclair 和 Phoenix 支持 anchor channels。
相關資源
下一步： 了解 通道儲備金 如何保護雙方免受欺詐。
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