Wumbo Channels 大額通道

Traditional Limits:

Maximum channel capacity: 2^24 - 1 satoshis
                        = 16,777,215 sats
                        = ~0.16777215 BTC

Maximum HTLC size: A portion of channel capacity

Reasons:
• Limit risk during early stages
• Using 3-byte encoding for amount field
• Encourage risk distribution across multiple channels

Wumbo Channels (option_support_large_channel):

Maximum channel capacity: 2^64 - 1 satoshis (theoretical)
                        = Actually limited by Bitcoin total supply

Practical limits:
• Node's available funds
• Risk counterparty is willing to accept
• Software implementation limits

Common large channels: 1-10 BTC

option_support_large_channel (Feature Bits 18/19)

Feature Negotiation:

Alice                                 Bob
  |                                    |
  |---- init ----------------------->  |
  |     features: ...18/19...          |
  |                                    |
  |<--- init -------------------------  |
  |     features: ...18/19...          |
  |                                    |
  | [Both support Wumbo]               |
  |                                    |
  |---- open_channel ----------------> |
  |     funding_satoshis: 500000000    |  (5 BTC)
  |                                    |

Requirements:
• Both nodes must support option_support_large_channel
• Feature bits exchanged in init message
• Can only open large channel when both support it

Compatibility:
• Bit 18 (even): Compulsory
• Bit 19 (odd): Optional support
• Most modern nodes use optional mode

各實現的配置：

LND（lnd.conf）：
[Application Options]
# Wumbo 預設已啟用（v0.11.0+）
# 可以設置最大通道大小
maxchansize=1000000000  # 10 BTC（以 satoshis 計）

# 開設 Wumbo 通道
lncli openchannel --node_key <pubkey> \
  --local_amt 500000000  # 5 BTC

CLN（config）：
# 啟用大額通道支援
large-channels

# 設置最大通道大小
max-channel-size-sat=1000000000

# 開設通道
lightning-cli fundchannel <node_id> 5btc

Eclair（eclair.conf）：
eclair.max-funding-satoshis=1000000000

檢查是否支援：
# LND
lncli getinfo | grep -i wumbo

# 查看對方節點是否支援
lncli getnodeinfo <pubkey> | jq '.node.features["19"]'

Risk Management for Large Channels

Counterparty Selection:
• Choose reliable, well-known nodes
• Confirm good uptime history
• Avoid large channels with unfamiliar nodes
• Consider distributing across multiple counterparties

Operational Requirements:
• Continuous channel state monitoring
• Use watchtower protection
• Regular channel state backups
• Ensure high node availability
• Prepare sufficient emergency funds for force closes

Risk Quantification:

Assume 5 BTC channel, counterparty acts maliciously or goes offline:

Worst case: Force close

Losses:
• Force close fees: ~0.001-0.01 BTC
• Funds lock time: Up to 2 weeks
• Potential HTLC losses: Depends on unsettled amounts
• Opportunity cost: Cannot use during lock period

If counterparty publishes old state and you have no watchtower:
• May lose all channel funds

Suitable Scenarios for Wumbo Channels

1. Exchanges/Large Merchants
   • High transaction volume needs
   • Professional operations capability
   • Can tolerate higher risk
   • Reduce channel management overhead

2. Routing Node Operators
   • Provide large payment routing
   • Become network backbone
   • Earn more routing fees
   • Interconnect with other large nodes

3. LSP (Lightning Service Provider)
   • Provide liquidity for users
   • Reduce channel opening costs
   • Batch process user needs
   • Professional liquidity management

4. Enterprise Settlement
   • B2B large payments
   • Regular settlement needs
   • Trust relationship established
   • Reduce frequent channel opens/closes

Not Suitable For:
• Individual small users
• Beginner node operators
• No 24/7 monitoring capability
• With unfamiliar nodes

Wumbo + MPP Strategy

Without Wumbo (Traditional Method):
Sending 1 BTC payment

Channel 1 (0.15 BTC) --+
Channel 2 (0.15 BTC) --+--> Use MPP to split into 7+ parts
Channel 3 (0.15 BTC) --+    High complexity
Channel 4 (0.15 BTC) --+    Parts may fail
...                  --+

Using Wumbo:
Sending 1 BTC payment

Wumbo channel (5 BTC) --> Single path completion
                          Simple, fast

Hybrid Strategy (Recommended):
• 1-2 Wumbo channels for large payments
• Multiple small channels for redundancy and privacy
• MPP provides extra capacity when needed
• Balance risk and convenience