Zero Fee Commitments

[carlaKC]

High level tracking issue for implementing zero fee commitments lightning/bolts#1228.

In the interest of keeping things reviewable + maintaining velocity, I propose breaking the feature up into the parts outlined below. Checklist is a summary of the requirements outlined in the spec PR.

Add Feature Bit

Already completed in #3656.

• Support a zero_fee_commitments feature bit (40/41):
• Allow channel type using even feature (40)

Channel Establishment

Implemented in #3792

• Negotiation available in tests
• open_channel:
  • Sender: MUST set feerate_per_kw to zero
  • Receiver: MUST fail if max_accepted_htlcs > 114
  • Receiver: MUST fail if feerate_per_kw != 0
• open_channel_v2 if channel_type includes zero_fee_commitments:
  • Sender: MUST set commitment_feerate_per_kw to 0
  • Receiver: MUST fail if commitment_feerate_per_kw =! 0
• funding_signed:
  • If no channel type was negotiated assume it to be zero_fee_commitments if it was negotiated

Note: channel type does not include static_remote_key.

Fee Handling

• update_fee:
  • Sender: MUST NOT send update_fee
• Fee paying node for zero_fee_commitment channels:
  • Does not need to maintain a fee spike buffer
  • Does not need to account for fees for offered htlcs
  • Does not need to be able to pay for anchors
• Non-fee paying node for zero_fee_commitments:
  • Does not need to maintain a fee spike buffer

Dependency TxBuilder

Depend on new TxBuilder trait in #3775 to build new commitment transaction format.

Commitment Transaction

• Commitment transaction:
  • Set version to 3
  • Add a zero value P2A output
  • Fee must be zero if P2A is dust; may be non-zero if non-dust
  • Can add trimmed outputs up to 240 sats to shared anchor, then goes to fees
  • If to_local of tx owner < holder's dust_limit_satoshis MUST add to shared_anchor
  • If to_remote of tx owner < holder's dust_limit_satoshis MUST add to shared_anchor
  • Trimmed offered and received HTLCs go to shared_anchor
  • Add shared anchor last, then BIP 69 sort
• Timeout/Success txns:
  • Use SINGLE|ACP sighash on signatures from peer
  • Set sequence = 0 for txin[0]
  • Fee must be zero
• Commitment must be paid for using CPFP (and must submitpackage)
• MUST spend shared_anchor on broadcast to incentivize mining

Enable

• Negotiation available (outside of tests)