Lightning Routing

Lightning is a source-routed network: the sender computes the full path through intermediaries and constructs the onion accordingly. Each hop sees only its forwarding instructions, not the full route.

Channel graph

Built from channel_announcement + channel_update gossip:

Edge {
  short_channel_id,
  node_a, node_b,
  capacity (from on-chain funding tx),
  per-direction:
    fee_base_msat,
    fee_proportional_millionths (PPM),
    cltv_expiry_delta,
    htlc_minimum_msat, htlc_maximum_msat,
    enabled flag,
}

Each edge has TWO directions, each with potentially different fees and CLTV deltas.

Cost function

Standard scoring (LND, CLN, LDK):

cost(edge, amount) =
    base_fee_msat
  + (proportional_millionths × amount) / 10^6
  + cltv_delta × cltv_factor
  + amount × probability_factor / 10^6 × ln(1 / p_success)

p_success (apriori probability of success) often defaults to 95% for unknown edges, decreasing with recent failures.

Pathfinder: Dijkstra or Yen's k-shortest-paths over the cost function.

Multi-Path Payments (MPP)

Split a payment across multiple paths; each part has the same payment hash. Receiver claims when all parts arrive.

payment_secret (BOLT 11 's' tag) marks parts as belonging to one payment.

max_parts typically 16 in LND, configurable. Smaller parts = higher likelihood of finding routes; more onion overhead.

Atomic Multi-Path (AMP)

Each "part" has a different payment hash, computed deterministically from a shared secret + per-part nonce. Receiver computes secret from any t-of-n parts (Shamir-like). Variant of MPP, less common in production.

Trampoline routing

Solves: pathfinding requires complete network gossip → mobile clients can't keep up.

Mechanism:

• Sender constructs onion targeting trampoline nodes.
• Each trampoline does its own pathfinding to the next trampoline / destination.
• Sender doesn't need full graph; just knows about trampoline nodes.

CLN, LDK support; LND partial.

Blinded paths (BOLT 4 update, BOLT12)

Receiver constructs a "blinded path" — a partial route that hides the final destination. Sender's pathfinder routes to the blinded path's introduction node, then onion delivers via blinded routing data.

Privacy benefit: sender doesn't learn receiver's node id.

Pickhardt payments (probabilistic)

Research direction (Pickhardt-Nowostawski 2021):

• Treat routing as min-cost-flow problem.
• Cost incorporates probability of channel having sufficient liquidity at the moment.
• Optimization: maximize success probability under some fee budget.

CLN and LDK have implementations; experimental in production.

CLTV calculation

For a 3-hop route Alice → Bob → Carol → Dave:

final_cltv = current_height + min_final_cltv_expiry_delta + payment_secret_buffer
carol_cltv = final_cltv + carol.cltv_expiry_delta
bob_cltv   = carol_cltv + bob.cltv_expiry_delta
alice_cltv = bob_cltv   + alice's_safety_margin

alice_cltv is what's set in Alice's first HTLC.

Failure handling

When a hop fails:

• Returns update_fail_htlc with encrypted error message.
• Sender decrypts, learns:
  • Which hop failed.
  • Failure type (channel disabled, fee insufficient, expiry mismatch, no liquidity, etc.).
• Sender's pathfinder updates its scoring (decrease p_success on failed edge) and retries with different path.

Common failure types (BOLT 4)

• temporary_channel_failure — try again later.
• permanent_channel_failure — drop this edge.
• fee_insufficient — your fee assumption was off; retry with more fee.
• channel_disabled — peer marked channel disabled, drop edge.
• expiry_too_soon / incorrect_cltv_expiry — CLTV mismatch.
• incorrect_or_unknown_payment_details — destination doesn't recognize this payment hash (BOLT 11 invoice expired or wrong).
• final_incorrect_htlc_amount — receiver doesn't recognize amount (MPP not assembled correctly).

Implementations

Common bugs

• Hardcoding cltv_factor → routes prefer few-hop high-fee paths vs many-hop low-fee.
• Treating htlc_maximum_msat = 0 as "infinite" → all routes rejected (it means "no max" only when set to channel capacity).
• Stale channel_update cached → routing through a channel that's been disabled an hour ago.
• Privacy: pathfinder leaking through timing attacks (test all hops but route only first part).

See also

• bolts/SKILL.md
• onion/SKILL.md
• gossip/SKILL.md
• trampoline/SKILL.md
• amp-mpp/SKILL.md