Gossip Protocol 八卦協議

channel_announcement structure:

Fields:
  node_signature_1: node 1 signature
  node_signature_2: node 2 signature
  bitcoin_signature_1: node 1 Bitcoin key signature
  bitcoin_signature_2: node 2 Bitcoin key signature
  features: channel feature bits
  chain_hash: blockchain identifier (mainnet/testnet)
  short_channel_id: 8 byte channel identifier
    block height (3 bytes)
    tx index (3 bytes)
    output index (2 bytes)
  node_id_1: node 1 pubkey (lexicographically smaller)
  node_id_2: node 2 pubkey (lexicographically larger)
  bitcoin_key_1: node 1 Bitcoin pubkey
  bitcoin_key_2: node 2 Bitcoin pubkey

Validation steps:
1. Check UTXO exists for short_channel_id
2. UTXO is P2WSH, locked to 2-of-2 multisig
3. Verify all 4 signatures
4. Confirm chain_hash matches

channel_update structure:

Fields:
  signature: publisher's signature
  chain_hash: blockchain identifier
  short_channel_id: channel identifier
  timestamp: Unix timestamp (update time)
  message_flags:
    bit 0: must_be_one
  channel_flags:
    bit 0: direction (0=node1->node2, 1=node2->node1)
    bit 1: disabled (is channel disabled)
  cltv_expiry_delta: HTLC timelock delta
  htlc_minimum_msat: minimum HTLC amount
  fee_base_msat: base routing fee
  fee_proportional_millionths: proportional routing fee
  htlc_maximum_msat: maximum HTLC amount (optional)

Fee calculation:
fee = fee_base_msat + (amount * fee_proportional_millionths / 1000000)

Update rules:
• timestamp must be greater than previous
• Each direction updates independently
• Can disable/enable channel

node_announcement structure:

Fields:
  signature: node signature
  features: node feature bits
  timestamp: update time
  node_id: node public key
  rgb_color: node color (3 bytes)
  alias: node alias (max 32 bytes UTF-8)
  addresses: network address list
    type 1: IPv4 (6 bytes)
    type 2: IPv6 (18 bytes)
    type 3: Tor v2 (12 bytes) [deprecated]
    type 4: Tor v3 (37 bytes)
    type 5: DNS hostname
    type 6: WebSocket

Prerequisites:
• Node must have at least one public channel
• Otherwise node_announcement is ignored
• Prevents spam node information

Gossip Query Mechanism:

Initial sync:

New Node                           Old Node
    |                                    |
    |---- query_channel_range ---------->|
    |     (request channels in block range)
    |                                    |
    |<--- reply_channel_range -----------|
    |     (returns short_channel_ids)    |
    |                                    |
    |---- query_short_channel_ids ------>|
    |     (request full channel info)    |
    |                                    |
    |<--- channel_announcement ----------|
    |<--- channel_update ----------------|
    |<--- node_announcement -------------|
    |                                    |
    v                                    v

Incremental sync:
• gossip_timestamp_filter: only receive updates after specific time
• Reduces duplicate message transmission

Private Channels (Unannounced):

Private channels:
• Do not broadcast channel_announcement
• Not in public graph
• Only known to channel parties

Use cases:
• Mobile wallets connecting to LSP (privacy)
• Merchants not exposing payment channels
• Reducing network graph size

Route Hints in Invoices:

Invoice:
  route_hints:
    - pubkey: 02abc...
      short_channel_id: 123x456x0
      fee_base_msat: 1000
      fee_proportional: 100
      cltv_expiry_delta: 40

This lets senders route to recipients with private channels,
without exposing the channel to the entire network.