Channel Update Gossip 通道更新廣播

channel_update Message Format:

Field Definitions:

signature: 64 bytes
  Schnorr/ECDSA signature of entire message

chain_hash: 32 bytes
  Blockchain identifier (Bitcoin mainnet/testnet)

short_channel_id: 8 bytes
  Channel identifier (block_height:tx_index:output_index)

timestamp: 4 bytes
  Unix timestamp for ordering updates

message_flags: 1 byte
  • bit 0: must_be_one (compatibility)
  • Other bits reserved

channel_flags: 1 byte
  • bit 0: direction (0=node1->node2, 1=node2->node1)
  • bit 1: disable (1=channel temporarily disabled)

cltv_expiry_delta: 2 bytes
  CLTV delta required for forwarding HTLCs

htlc_minimum_msat: 8 bytes
  Minimum accepted HTLC amount

fee_base_msat: 4 bytes
  Base fee (millisatoshis)

fee_proportional_millionths: 4 bytes
  Proportional fee (per million)

htlc_maximum_msat: 8 bytes (optional, if message_flags bit 0)
  Maximum accepted HTLC amount

Routing Fee Formula:

fee = fee_base_msat + (amount_msat * fee_proportional_millionths / 1,000,000)

Example:

Channel Policy:
  • fee_base_msat: 1000 (1 sat)
  • fee_proportional_millionths: 100 (0.01%)

Forwarding 1,000,000 msat (1000 sats):
  fee = 1000 + (1,000,000 * 100 / 1,000,000)
      = 1000 + 100
      = 1100 msat (1.1 sats)

Forwarding 10,000,000 msat (10,000 sats):
  fee = 1000 + (10,000,000 * 100 / 1,000,000)
      = 1000 + 1000
      = 2000 msat (2 sats)

Common Fee Settings:

1. Zero-fee Channel:
   • base: 0, ppm: 0
   • Used for connections between own nodes

2. Low-fee Node:
   • base: 0-10, ppm: 1-10
   • Prioritizes routing volume

3. Standard Node:
   • base: 1000, ppm: 100-500
   • Balances revenue and routing volume

4. High-fee Node:
   • base: 1000+, ppm: 1000+
   • Popular routes or scarce liquidity

Channel Update Directions:

One channel, two directions, two updates:

  Node A (pubkey: 02aa...)       Node B (pubkey: 03bb...)
        |                                |
        |     short_channel_id: 700000x1x0
        |                                |
        |---- channel_update (dir=0) ----|
        |     A's policy for A->B        |
        |     fee_base: 1000             |
        |     fee_ppm: 100               |
        |                                |
        |---- channel_update (dir=1) ----|
        |     B's policy for B->A        |
        |     fee_base: 500              |
        |     fee_ppm: 50                |

Direction Determination (channel_flags bit 0):

direction = 0:
  Publisher is node1 (smaller pubkey)
  Defines policy from node1 to node2

direction = 1:
  Publisher is node2 (larger pubkey)
  Defines policy from node2 to node1

Determining node1/node2:
  if pubkey_a < pubkey_b:
      node1 = A, node2 = B
  else:
      node1 = B, node2 = A

Channel Update Propagation:

Broadcast Flow:

1. Node updates channel policy
   lncli updatechanpolicy --base_fee_msat 1000 ...

2. Node creates and signs channel_update
   timestamp = current_time

3. Sends to all connected peers
   peer.send(channel_update)

4. Peers validate
   • Signature correct?
   • Timestamp newer than known?
   • Channel exists?

5. After validation, forward to other peers
   -> Gradually propagates across network

Rate Limiting:

Preventing gossip flood attacks:
  • timestamp must be increasing
  • Too frequent updates may be ignored
  • Nodes can set minimum update interval
  • Recommended interval: at least 1 hour

LND Implementation:
  • By default rejects too frequent updates
  • gossip.channel-update-interval config

Gossip Queries (Sync):

When new node joins network:

  query_channel_range:
    Request all channels in specified block range

  reply_channel_range:
    Reply with channel list

  query_short_channel_ids:
    Request details for specific channels

  reply_short_channel_ids_end:
    Reply with channel_announcement and channel_update

Channel Disable Flag:

channel_flags bit 1 (disable):

  disable = 0: Channel normally available
  disable = 1: Channel temporarily disabled

Disable Reasons:
  • Peer offline
  • Channel liquidity exhausted
  • Under maintenance
  • Manual disable

Auto-disable Behavior (LND):

When peer disconnects:
  1. Wait for period (gossip.inactive-channel-ticker)
  2. Send channel_update with disable=1
  3. Other nodes stop routing to this channel

When peer reconnects:
  1. Send channel_update with disable=0
  2. Channel available for routing again

Config:
  [gossip]
  inactive-channel-ticker = 20m (default)