lightning-bolts/02-peer-protocol.md

BOLT #2: Peer Protocol for Channel Management

The peer channel protocol has three phases: establishment, normal operation, and closing.

Table of Contents

• Channel
  • Channel Establishment
    • The open_channel Message
    • The accept_channel Message
    • The funding_created Message
    • The funding_signed Message
    • The funding_locked Message
  • Channel Close
    • Closing Initiation: shutdown
    • Closing Negotiation: closing_signed
  • Normal Operation
    • Forwarding HTLCs
    • cltv_expiry_delta Selection
    • Adding an HTLC: update_add_htlc
    • Removing an HTLC: update_fulfill_htlc, update_fail_htlc, and update_fail_malformed_htlc
    • Committing Updates So Far: commitment_signed
    • Completing the Transition to the Updated State: revoke_and_ack
    • Updating Fees: update_fee
  • Message Retransmission
• Authors

Channel

Channel Establishment

Channel establishment begins immediately after authentication and consists of the funding node (funder) sending an open_channel message, followed by the responding node (fundee) sending accept_channel. With the channel parameters locked in, the funder is able to create the funding transaction and both versions of the commitment transaction, as described in BOLT #3. The funder then sends the outpoint of the funding output with the funding_created message, along with the signature for the fundee's version of the commitment transaction. Once the fundee learns the funding outpoint, it's able to generate the funder's commitment for the commitment transaction and send it over using the funding_signed message.

Once the channel funder receives the funding_signed message, it must broadcast the funding transaction to the Bitcoin network. After the funding_signed message is sent/received, both sides should wait for the funding transaction to enter the blockchain and reach the specified depth (number of confirmations). After both sides have sent the funding_locked message, the channel is established and can begin normal operation. The funding_locked message includes information which will be used to construct channel authentication proofs.

    +-------+                              +-------+
    |       |--(1)---  open_channel  ----->|       |
    |       |<-(2)--  accept_channel  -----|       |
    |       |                              |       |
    |   A   |--(3)--  funding_created  --->|   B   |
    |       |<-(4)--  funding_signed  -----|       |
    |       |                              |       |
    |       |--(5)--- funding_locked  ---->|       |
    |       |<-(6)--- funding_locked  -----|       |
    +-------+                              +-------+

    - where node A is 'funder' and node B is 'fundee'

If this fails at any stage, or if one node decides the channel terms offered by the other node are not suitable, the channel establishment fails.

Note that multiple channels can operate in parallel, as all channel messages are identified by either a temporary_channel_id (before the funding transaction is created) or channel_id (derived from the funding transaction).

The open_channel Message

This message contains information about a node and indicates its desire to set up a new channel.

1. type: 32 (open_channel)
2. data:
   • [32:chain_hash]
   • [32:temporary_channel_id]
   • [8:funding_satoshis]
   • [8:push_msat]
   • [8:dust_limit_satoshis]
   • [8:max_htlc_value_in_flight_msat]
   • [8:channel_reserve_satoshis]
   • [8:htlc_minimum_msat]
   • [4:feerate_per_kw]
   • [2:to_self_delay]
   • [2:max_accepted_htlcs]
   • [33:funding_pubkey]
   • [33:revocation_basepoint]
   • [33:payment_basepoint]
   • [33:delayed_payment_basepoint]
   • [33:htlc_basepoint]
   • [33:first_per_commitment_point]
   • [1:channel_flags]
   • [2:shutdown_len] (option_upfront_shutdown_script)
   • [shutdown_len: shutdown_scriptpubkey] (option_upfront_shutdown_script)

The chain_hash value denotes the exact blockchain the opened channel will reside within. This is usually the genesis hash of the respective blockchain. The existence of the chain_hash allows nodes to open channels across many distinct blockchains as well as have channels within multiple blockchains opened to the same peer (if it supports the target chains).

The temporary_channel_id is used to identify this channel until the funding transaction is established. funding_satoshis is the amount the sender is putting into the channel. dust_limit_satoshis is the threshold below which outputs should not be generated for this node's commitment or HTLC transaction; i.e. HTLCs below this amount plus HTLC transaction fees are not enforceable on-chain. This reflects the reality that tiny outputs are not considered standard transactions and will not propagate through the Bitcoin network.

max_htlc_value_in_flight_msat is a cap on total value of outstanding HTLCs, which allows a node to limit its exposure to HTLCs; similarly, max_accepted_htlcs limits the number of outstanding HTLCs the other node can offer. channel_reserve_satoshis is the minimum amount that the other node is to keep as a direct payment. htlc_minimum_msat indicates the smallest value HTLC this node will accept.

feerate_per_kw indicates the initial fee rate by 1000-weight (i.e. 1/4 the more normally-used 'feerate per kilobyte') which this side will pay for commitment and HTLC transactions, as described in BOLT #3 (this can be adjusted later with an update_fee message). to_self_delay is the number of blocks that the other nodes to-self outputs must be delayed, using OP_CHECKSEQUENCEVERIFY delays; this is how long it will have to wait in case of breakdown before redeeming its own funds.

Only the least-significant bit of channel_flags is currently defined: announce_channel. This indicates whether the initiator of the funding flow wishes to advertise this channel publicly to the network as detailed within BOLT #7.

The funding_pubkey is the public key in the 2-of-2 multisig script of the funding transaction output. The revocation_basepoint is combined with the revocation preimage for this commitment transaction to generate a unique revocation key for this commitment transaction. The payment_basepoint, htlc_basepoint, and delayed_payment_basepoint are similarly used to generate a series of keys for any payments to this node: delayed_payment_basepoint is used for payments encumbered by a delay. Varying these keys ensures that the transaction ID of each commitment transaction is unpredictable by an external observer, even if one commitment transaction is seen — this property is very useful for preserving privacy when outsourcing penalty transactions to third parties.

The shutdown_scriptpubkey allows the sending node to commit to where funds will go on mutual close, which the remote node should enforce even if a node is compromised later.

[ FIXME: Describe dangerous feature bit for larger channel amounts. ]

Requirements

The sending node:

• MUST ensure that the chain_hash value identifies the chain it wishes to open the channel within.
• MUST ensure temporary_channel_id is unique from any other channel ID with the same peer.
• MUST set funding_satoshis to less than 2^24 satoshi.
• MUST set push_msat to equal or less than 1000 * funding_satoshis.
• MUST set funding_pubkey, revocation_basepoint, htlc_basepoint, payment_basepoint, and delayed_payment_basepoint to valid DER-encoded, compressed, secp256k1 pubkeys.
• MUST set first_per_commitment_point to the per-commitment point to be used for the initial commitment transaction, derived as specified in BOLT #3.
• MUST set undefined bits in channel_flags to 0.
• if both nodes advertised the option_upfront_shutdown_script feature:
  • MUST include either a valid shutdown_scriptpubkey as required by shutdown scriptpubkey, or a zero-length shutdown_scriptpubkey.
• otherwise:
  • MAY include ashutdown_scriptpubkey.

The sending node SHOULD:

• set to_self_delay sufficient to ensure the sender can irreversibly spend a commitment transaction output, in case of misbehavior by the receiver.
• set feerate_per_kw to at least the rate it estimates would cause the transaction to be immediately included in a block.
• set dust_limit_satoshis to a sufficient value to allow commitment transactions to propagate through the Bitcoin network.
• set htlc_minimum_msat to the minimum value HTLC it is willing to accept from this peer.

The receiving node MUST:

• ignore undefined bits in channel_flags.
• if the connection has been re-established after receiving a previous open_channel but before receiving a funding_created message:
  • accept a new open_channel message.
  • discard the previous open_channel message.

The receiving node MAY fail the channel if:

• announce_channel is false (0), yet it wishes to publicly announce the channel.
• funding_satoshis is too small.
• it considers htlc_minimum_msat too large.
• it considers max_htlc_value_in_flight_msat too small.
• it considers channel_reserve_satoshis too large.
• it considers max_accepted_htlcs too small.

The receiving node MUST fail the channel if:

• push_msat is greater than funding_satoshis * 1000.
• to_self_delay is unreasonably large.
• max_accepted_htlcs is greater than 483.
• it considers feerate_per_kw too small for timely processing, or unreasonably large.
• funding_pubkey, revocation_basepoint, htlc_basepoint, payment_basepoint, or delayed_payment_basepoint are not valid DER-encoded compressed secp256k1 pubkeys.

The receiving node MUST NOT:

• consider funds received using push_msat to be received until the funding transaction has reached sufficient depth.

Rationale

The channel reserve is specified by the peer's channel_reserve_satoshis: 1% of the channel total is suggested. Each side of a channel maintains this reserve so it always has something to lose, if it were to try to broadcast an old, revoked commitment transaction. Initially, this reserve may not be met, as only one side has funds; but the protocol ensures that progress is always toward it being met, and once met, it is maintained.

The sender can unconditionally give initial funds to the receiver using a non-zero push_msat — this is one case where the normal reserve mechanism doesn't apply. However, like any other on-chain transaction, this payment is not certain until the funding transaction has been confirmed sufficiently (may be double-spent) and may require a separate method to prove payment via on-chain confirmation.

The feerate_per_kw is generally only a concern to the sender (who pays the fees), but there is also the feerate paid by HTLC transactions; thus, unreasonably large fee rates can also penalize the recipient.

Separating the htlc_basepoint from the payment_basepoint improves security: a node needs the secret associated with the htlc_basepoint to produce HTLC signatures for the protocol, but the secret for the payment_basepoint can be in cold storage.

Future

It would be easy to have a local feature bit which indicated that a receiving node was prepared to fund a channel, which would reverse this protocol.

The accept_channel Message

This message contains information about a node and indicates its acceptance of the new channel.

1. type: 33 (accept_channel)
2. data:
   • [32:temporary_channel_id]
   • [8:dust_limit_satoshis]
   • [8:max_htlc_value_in_flight_msat]
   • [8:channel_reserve_satoshis]
   • [8:htlc_minimum_msat]
   • [4:minimum_depth]
   • [2:to_self_delay]
   • [2:max_accepted_htlcs]
   • [33:funding_pubkey]
   • [33:revocation_basepoint]
   • [33:payment_basepoint]
   • [33:delayed_payment_basepoint]
   • [33:htlc_basepoint]
   • [33:first_per_commitment_point]
   • [2:shutdown_len] (option_upfront_shutdown_script)
   • [shutdown_len: shutdown_scriptpubkey] (option_upfront_shutdown_script)

Requirements

The temporary_channel_id MUST be the same as the temporary_channel_id in the open_channel message.

The sender:

• SHOULD set minimum_depth to a number of blocks it considers reasonable to avoid double-spending of the funding transaction.

The receiver:

• if the chain_hash value within the open_channel message is set to a hash of a chain unknown to the receiver:
  • MUST reject the channel.
• if minimum_depth is unreasonably large:
  • MAY reject the channel.

Other fields have the same requirements as their counterparts in open_channel.

The funding_created Message

This message describes the outpoint which the funder has created for the initial commitment transactions. After receiving the peer's signature, it will broadcast the funding transaction.

1. type: 34 (funding_created)
2. data:
   • [32:temporary_channel_id]
   • [32:funding_txid]
   • [2:funding_output_index]
   • [64:signature]

Requirements

The sender MUST set:

• temporary_channel_id the same as the temporary_channel_id in the open_channel message.
• funding_txid to the transaction ID of a non-malleable transaction, which it MUST NOT broadcast.
• funding_output_index to the output number of that transaction which corresponds the funding transaction output as defined in BOLT #3.
• signature to the valid signature using its funding_pubkey for the initial commitment transaction, as defined in BOLT #3.

The sender:

• when creating the funding transaction:
  • SHOULD use only BIP141 (Segregated Witness) inputs.

The recipient:

• if signature is incorrect:
  • MUST fail the channel.

Rationale

The funding_output_index can only be 2 bytes, since that's how it's packed into the channel_id and used throughout the gossip protocol. The limit of 65535 outputs should not be overly burdensome.

A transaction with all Segregated Witness inputs is not malleable, hence the funding transaction recommendation.

The funding_signed Message

This message gives the funder the signature it needs for the first commitment transaction, so it can broadcast signature knowing funds can be redeemed, if need be.

This message introduces the channel_id to identify the channel. It's derived from the funding transaction by combining the funding_txid and the funding_output_index, using big-endian exclusive-OR (i.e. funding_output_index alters the last 2 bytes).

1. type: 35 (funding_signed)
2. data:
   • [32:channel_id]
   • [64:signature]

Requirements

The sender MUST set:

• channel_id by exclusive-OR of the funding_txid and the funding_output_index from the funding_created message.
• signature to the valid signature, using its funding_pubkey for the initial commitment transaction, as defined in BOLT #3.

The recipient:

• if signature is incorrect:
  • MUST fail the channel.
• MUST NOT broadcast the funding transaction before receipt of a valid funding_signed.
• on receipt of a valid funding_signed:
  • SHOULD broadcast the funding transaction.

The funding_locked Message

This message indicates that the funding transaction has reached the minimum_depth asked for in accept_channel. Once both nodes have sent this, the channel enters normal operating mode.

1. type: 36 (funding_locked)
2. data:
   • [32:channel_id]
   • [33:next_per_commitment_point]

Requirements

The sender MUST:

• wait until the funding transaction has reached minimum_depth before sending this message.
• set next_per_commitment_point to the per-commitment point to be used for the following commitment transaction, derived as specified in BOLT #3.

A non-funding node (fundee) SHOULD:

• forget the channel if it does not see the funding transaction after a reasonable timeout.

From the point of waiting for funding_locked onward, either node MAY fail the channel if it does not receive a required response from the other node after a reasonable timeout.

Rationale

The non-funder can simply forget the channel ever existed, since no funds are at risk. If the fundee were to remember the channel forever, that would create a Denial of Service risk; therefore, forgetting it is recommended (even if the promise of push_msat is significant).

Future

An SPV proof could be added and block hashes could be routed in separate messages.

Channel Close

Nodes can negotiate a mutual close of the connection, which unlike a unilateral close, allows them to access their funds immediately and can be negotiated with lower fees.

Closing happens in two stages: 1) one side indicates it wants to clear the channel (and thus will accept no new HTLCs) 2) once all HTLCs are resolved, the final channel close negotiation begins.

    +-------+                              +-------+
    |       |--(1)-----  shutdown  ------->|       |
    |       |<-(2)-----  shutdown  --------|       |
    |       |                              |       |
    |       | <complete all pending HTLCs> |       |
    |   A   |                 ...          |   B   |
    |       |                              |       |
    |       |<-(3)-- closing_signed  F1----|       |
    |       |--(4)-- closing_signed  F2--->|       |
    |       |              ...             |       |
    |       |--(?)-- closing_signed  Fn--->|       |
    |       |<-(?)-- closing_signed  Fn----|       |
    +-------+                              +-------+

Closing Initiation: shutdown

Either node (or both) can send a shutdown message to initiate closing, along with the scriptpubkey it wants to be paid to.

1. type: 38 (shutdown)
2. data:
   • [32:channel_id]
   • [2:len]
   • [len:scriptpubkey]

Requirements

A sending node:

• if there are updates pending on the receiving node's commitment transaction:

  • MUST NOT send a shutdown.

• MUST NOT send an update_add_htlc after a shutdown.

• SHOULD fail to route any HTLC added after it sent shutdown.

• if it sent a non-zero-length shutdown_scriptpubkey in open_channel or accept_channel:

  • MUST send the same value in scriptpubkey.

• MUST set scriptpubkey in one of the following forms:

  1. OP_DUP OP_HASH160 20 20-bytes OP_EQUALVERIFY OP_CHECKSIG (pay to pubkey hash), OR
  2. OP_HASH160 20 20-bytes OP_EQUAL (pay to script hash), OR
  3. OP_0 20 20-bytes (version 0 pay to witness pubkey), OR
  4. OP_0 32 32-bytes (version 0 pay to witness script hash)

A receiving node:

• if the scriptpubkey is not in one of the above forms:
  • SHOULD fail the connection.
• once there are no outstanding updates on the peer:
  • MUST reply to a shutdown message with a shutdown, unless it has already sent a shutdown.
• if both nodes advertised the option_upfront_shutdown_script feature, and the receiving node received a non-zero-length shutdown_scriptpubkey in open_channel or accept_channel, and that shutdown_scriptpubkey is not equal to scriptpubkey:
  • MUST fail the connection.

Rationale

If channel state is always "clean" (no pending changes) when a shutdown starts, the question of how to behave if it wasn't is avoided: the sender always sends a commitment_signed first.

As shutdown implies a desire to terminate, it implies that no new HTLCs will be added or accepted.

The scriptpubkey forms include only standard forms accepted by the Bitcoin network, which ensures the resulting transaction will propagate to miners.

The option_upfront_shutdown_script feature means that the node wanted to pre-commit to shutdown_scriptpubkey in case it was compromised somehow. This is a weak commitment (a malevolent implementation tends to ignore specifications like this one!) but provides an incremental improvement in security by requiring cooperation of the receiving node to change the scriptpubkey.

The shutdown response requirement implies that the node sends commitment_signed to commit any outstanding changes before replying; however, it could theoretically reconnect instead, which would simply erase all outstanding uncommitted changes.

Closing Negotiation: closing_signed

Once shutdown is complete and the channel is empty of HTLCs, the final current commitment transactions will have no HTLCs, and closing fee negotiation begins. Each node chooses a fee it thinks is fair, and signs the close transaction with the scriptpubkey fields from the shutdown messages (along with its chosen fee) and sends the signature. The process terminates when both agree on the same fee, or one side fails the channel.

1. type: 39 (closing_signed)
2. data:
   • [32:channel_id]
   • [8:fee_satoshis]
   • [64:signature]

Requirements

A sending node:

• after shutdown has been received and no HTLCs remain in either commitment transaction:
  • SHOULD send a closing_signed message.
• MUST set fee_satoshis lower than or equal to the base fee of the final commitment transaction, as calculated in BOLT #3.
• SHOULD set the initial fee_satoshis according to its estimate of cost of inclusion in a block.
• MUST set signature to the Bitcoin signature of the close transaction, as specified in BOLT #3.

The receiving node:

• after shutdown has been received and no HTLCs remain in either commitment transaction:
  • SHOULD send a closing_signed message.
• if the signature is not valid for either variant of close transaction specified in BOLT #3:
  • MUST fail the connection.
• if fee_satoshis is equal to its previously sent fee_satoshis:
  • SHOULD sign and broadcast the final closing transaction
  • MAY close the connection.
• otherwise:
  • MUST fail the connection if fee_satoshis is greater than the base fee of the final commitment transaction as calculated in BOLT #3
• SHOULD fail the connection if fee_satoshis is not strictly between its last-sent fee_satoshis and its previously-received fee_satoshis, unless it has reconnected since then.
• if the receiver agrees with the fee:
  • SHOULD reply with a closing_signed with the same fee_satoshis value.
• otherwise:
  • MUST propose a value "strictly between" the received fee_satoshis and its previously-sent fee_satoshis.

Rationale

The "strictly between" requirement ensures that forward progress is made, even if only by a single satoshi at a time. To avoid keeping state and to handle the corner case, where fees have shifted between disconnection and reconnection, negotiation restarts on reconnection.

Note there is limited risk if the closing transaction is delayed, but it will be broadcast very soon; so there is usually no reason to pay a premium for rapid processing.

Normal Operation

Once both nodes have exchanged funding_locked (and optionally announcement_signatures), the channel can be used to make payments via Hash TimeLocked Contracts.

Changes are sent in batches: one or more update_ messages are sent before a commitment_signed message, as in the following diagram:

    +-------+                            +-------+
    |       |--(1)---- add_htlc   ------>|       |
    |       |--(2)---- add_htlc   ------>|       |
    |       |<-(3)---- add_htlc   -------|       |
    |       |                            |       |
    |       |--(4)----   commit   ------>|       |
    |   A   |                            |   B   |
    |       |<-(5)--- revoke_and_ack-----|       |
    |       |<-(6)----   commit   -------|       |
    |       |                            |       |
    |       |--(7)--- revoke_and_ack---->|       |
    +-------+                            +-------+

Counter-intuitively, these updates apply to the other node's commitment transaction; the node only adds those updates to its own commitment transaction when the remote node acknowledges it has applied them via revoke_and_ack.

Thus each update traverses through the following states:

1. Pending on the receiver
2. In the receiver's latest commitment transaction,
3. ... and the receiver's previous commitment transaction has been revoked, and the HTLC is pending on the sender.
4. ... and in the sender's latest commitment transaction
5. ... and the sender's previous commitment transaction has been revoked

As the two nodes' updates are independent, the two commitment transactions may be out of sync indefinitely. This is not concerning: what matters is whether both sides have irrevocably committed to a particular HTLC or not (the final state, above).

Forwarding HTLCs

In general, a node offers HTLCs for two reasons: to initiate a payment of its own, or to forward another node's payment. In the forwarding case, care must be taken to ensure the outgoing HTLC cannot be redeemed unless the incoming HTLC can be redeemed. The following requirements ensure this is always true:

The respective addition/removal of an HTLC is considered irrevocably committed when:

1. the commitment transaction with/without it is committed by both nodes, and any previous commitment transaction without/with it has been revoked, OR
2. the commitment transaction with/without it has been irreversibly committed to the blockchain.

Requirements

A node:

• until the incoming HTLC has been irrevocably committed:
  • MUST NOT offer an HTLC (update_add_htlc) in response to an incoming HTLC.
• until the removal of the outgoing HTLC is irrevocably committed, OR until the outgoing on-chain HTLC output has been spent via the HTLC-timeout transaction (with sufficient depth):
  • MUST NOT fail an incoming HTLC (update_fail_htlc) for which it has committed to an outgoing HTLC.
• once its cltv_expiry has been reached, OR if cltv_expiry - current_height < cltv_expiry_delta for the outgoing channel:
  • MUST fail an incoming HTLC (update_fail_htlc).
• if an incoming HTLC's cltv_expiry is unreasonably far in the future:
  • SHOULD fail that incoming HTLC (update_fail_htlc).
• upon receiving an update_fulfill_htlc for the outgoing HTLC, OR upon discovering the payment_preimage from an on-chain HTLC spend:
  • MUST fulfill an incoming HTLC for which it has committed to an outgoing HTLC.

Rationale

In general, one side of the exchange needs to be dealt with before the other. Fulfilling an HTLC is different: knowledge of the preimage is, by definition, irrevocable and the incoming HTLC should be fulfilled as soon as possible to reduce latency.

An HTLC with an unreasonably long expiry is a denial-of-service vector and therefore is not allowed. Note that the exact value of "unreasonable" is currently unclear and may depend on network topology.

cltv_expiry_delta Selection

Once an HTLC has timed out, it can either be fulfilled or timed-out; care must be taken around this transition both for offered and received HTLCs.

Consider the following scenario, where A sends an HTLC to B, who forwards to C, who delivers the goods as soon as the payment is received.

1. C needs to be sure that the HTLC from B cannot time out, even if B becomes unresponsive; i.e. C can fulfill the incoming HTLC on-chain before B can time it out on-chain.

2. B needs to be sure that if C fulfills the HTLC from B, it can fulfill the incoming HTLC from A; i.e. B can get the preimage from C and fulfill the incoming HTLC on-chain before A can time it out on-chain.

The critical settings here are the cltv_expiry_delta in BOLT #7 and the related min_final_cltv_expiry in BOLT #11. cltv_expiry_delta is the minimum difference in HTLC CLTV timeouts, in the forwarding case (B). min_final_ctlv_expiry is the minimum difference between HTLC CLTV timeout and the current block height, for the terminal case (C).

Note that if this value is too low for a channel, the risk is only to the node accepting the HTLC, not the node offering it. For this reason, the cltv_expiry_delta for the outgoing channel is used as the delta across a node.

The worst-case number of blocks between outgoing and incoming HTLC resolution can be derived, given a few assumptions:

• A worst-case reorganization depth R blocks.
• A grace-period G blocks after HTLC timeout before giving up on an unresponsive peer and dropping to chain.
• A number of blocks S between transaction broadcast and the transaction being included in a block.

The worst case is for a forwarding node (B) that takes the longest possible time to spot the outgoing HTLC fulfillment and also takes the longest possible time to redeem it on-chain:

1. The B->C HTLC times out at block N, and B waits G blocks until it gives up waiting for C. B or C commits to the blockchain, and B spends HTLC, which takes S blocks to be included.
2. Bad case: C wins the race (just) and fulfills the HTLC, B only sees that transaction when it sees block N+G+S+1.
3. Worst case: There's reorganization R deep in which C wins and fulfills. B only sees transaction at N+G+S+R.
4. B now needs to fulfill the incoming A->B HTLC, but A is unresponsive: B waits G more blocks before giving up waiting for A. A or B commits to the blockchain.
5. Bad case: B sees A's commitment transaction in block N+G+S+R+G+1 and has to spend the HTLC output, which takes S blocks to be mined.
6. Worst case: there's another reorganization R deep which A uses to spend the commitment transaction, so B sees A's commitment transaction in block N+G+S+R+G+R and has to spend the HTLC output, which takes S blocks to be mined.
7. B's HTLC spend needs to be at least R deep before it times out, otherwise another reorganization could allow A to timeout the transaction.

Thus, the worst case is 3R+2G+2S assuming R is at least 1. Note that the chances of three reorganizations in which the other node wins all of them is low for R of 2 or more. Since high fees are used (and HTLC spends can use almost arbitrary fees), S should be small; although, given that block times are irregular and empty blocks still occur, S = 2 should be considered a minimum. Similarly, the grace period G can be low (1 or 2), as nodes are required to timeout or fulfill as soon as possible; but if G is too low it increases the risk of unnecessary channel closure due to networking delays.

There are four values that need be derived:

1. The cltv_expiry_delta for channels, 3R+2G+2S: if in doubt, a cltv_expiry_delta of 12 is reasonable (R=2, G=1, S=2).

2. For sent HTLCs: the timeout deadline after which the channel has to be failed and timed out on-chain. This is G blocks after the HTLC's cltv_expiry: 1 block is reasonable.

3. For received HTLCs (with a preimage): the fulfillment deadline after which the channel has to be failed and the HTLC fulfilled on-chain before its cltv_expiry. See steps 4-7 above, which imply a deadline of 2R+G+S blocks before cltv_expiry: 7 blocks is reasonable.

4. The minimum cltv_expiry accepted for terminal payments: this is the same calculation as above. The default in BOLT #11 is 9, which is slightly more conservative than the 7 this calculation suggests.

Requirements

An offering node:

• MUST estimate a timeout deadline for each HTLC it offers.
• MUST NOT offer an HTLC with a timeout deadline before its cltv_expiry.
• if an HTLC which it offered is in either node's current commitment transaction is past this timeout deadline:
  • MUST fail the channel.

A fulfilling node:

• for each HTLC it is attempting to fulfill:
  • MUST estimate a fulfillment deadline.
• MUST fail (and not forward) an HTLC whose fulfillment deadline is already past.
• if a HTLC it has fulfilled is in either node's current commitment transaction and is past this fulfillment deadline:
  • MUST fail the connection.

Adding an HTLC: update_add_htlc

Either node can send update_add_htlc to offer a HTLC to the other, which is redeemable in return for a payment preimage. Amounts are in millisatoshi, though on-chain enforcement is only possible for whole satoshi amounts greater than the dust limit (in commitment transactions these are rounded down as specified in BOLT #3).

The format of the onion_routing_packet portion, which indicates where the payment is destined, is described in BOLT #4.

1. type: 128 (update_add_htlc)
2. data:
   • [32:channel_id]
   • [8:id]
   • [8:amount_msat]
   • [32:payment_hash]
   • [4:cltv_expiry]
   • [1366:onion_routing_packet]

Requirements

A sending node:

• MUST NOT offer amount_msat it cannot pay for in the remote commitment transaction at the current feerate_per_kw (see "Updating Fees") while maintaining its channel reserve.
• MUST offer amount_msat greater than 0.
• MUST NOT offer amount_msat below the receiving node's htlc_minimum_msat
• MUST set cltv_expiry less than 500000000.
• for channels with chain_hash identifying the Bitcoin blockchain:
  • MUST set the four most significant bytes of amount_msat to 0.
• if result would be offering more than the remote's max_accepted_htlcs HTLCs, in the remote commitment transaction:
  • MUST NOT add an HTLC.
• if the sum of total offered HTLCs would exceed the remote's max_htlc_value_in_flight_msat:
  • MUST NOT add an HTLC.
• for the first HTLC it offers:
  • MUST set id to 0.
• MUST increase the value of id by 1 for each successive offer.

A receiving node:

• receiving an amount_msat equal to 0, OR less than its own htlc_minimum_msat:
  • SHOULD fail the channel.
• receiving an amount_msat that the sending node cannot afford at the current feerate_per_kw (while maintaining its channel reserve):
  • SHOULD fail the channel.
• if a sending node adds more than its max_accepted_htlcs HTLCs to its local commitment transaction, OR adds more than its max_htlc_value_in_flight_msat worth of offered HTLCs to its local commitment transaction:
  • SHOULD fail the channel.
• if sending node sets cltv_expiry to greater or equal to 500000000:
  • SHOULD fail the channel.
• for channels with chain_hash identifying the Bitcoin blockchain, if the four most significant bytes of amount_msat are not 0:
  • MUST fail the channel.
• MUST allow multiple HTLCs with the same payment_hash.
• if the sender did not previously acknowledge the commitment of that HTLC:
  • MUST ignore a repeated id value after a reconnection.
• if other id violations occur:
  • MAY fail the channel.

The onion_routing_packet contains an obfuscated list of hops and instructions for each hop along the path. It commits to the HTLC by setting the payment_hash as associated data, i.e. includes the payment_hash in the computation of HMACs. This prevents replay attacks that would reuse a previous onion_routing_packet with a different payment_hash.

Rationale

Invalid amounts are a clear protocol violation and indicate a breakdown.

If a node did not accept multiple HTLCs with the same payment hash, an attacker could probe to see if a node had an existing HTLC. This requirement, to deal with duplicates, leads us to use a separate identifier; its assumed a 64-bit counter never wraps.

Retransmissions of unacknowledged updates are explicitly allowed for reconnection purposes; allowing them at other times simplifies the recipient code (though strict checking may help debugging).

max_accepted_htlcs is limited to 483 to ensure that, even if both sides send the maximum number of HTLCs, the commitment_signed message will still be under the maximum message size. It also ensures that a single penalty transaction can spend the entire commitment transaction, as calculated in BOLT #5.

cltv_expiry values equal to or greater than 500000000 would indicate a time in seconds, and the protocol only supports an expiry in blocks.

amount_msat is deliberately limited for this version of the specification; larger amounts are not necessary, nor wise, during the bootstrap phase of the network.

Removing an HTLC: update_fulfill_htlc, update_fail_htlc, and update_fail_malformed_htlc

For simplicity, a node can only remove HTLCs added by the other node. There are three reasons for removing an HTLC: it has timed out, it has failed to route, or the payment preimage is supplied.

The reason field is an opaque encrypted blob for the benefit of the original HTLC initiator as defined in BOLT #4; however, there's a special malformed failure variant for the case where our peer couldn't parse it: in this case the current node encrypts it into a update_fail_htlc for relaying.

1. type: 130 (update_fulfill_htlc)
2. data:
   • [32:channel_id]
   • [8:id]
   • [32:payment_preimage]

For a timed out or route-failed HTLC:

1. type: 131 (update_fail_htlc)
2. data:
   • [32:channel_id]
   • [8:id]
   • [2:len]
   • [len:reason]

For an unparsable HTLC:

1. type: 135 (update_fail_malformed_htlc)
2. data:
   • [32:channel_id]
   • [8:id]
   • [32:sha256_of_onion]
   • [2:failure_code]

Requirements

A node:

• SHOULD remove an HTLC as soon as it can.
• SHOULD fail an HTLC which has timed out.
• until the corresponding HTLC is irrevocably committed in both sides' commitment transactions:
  • MUST NOT send an update_fulfill_htlc, update_fail_htlc, or update_fail_malformed_htlc.

A receiving node:

• if the id does not correspond to an HTLC in its current commitment transaction:
  • MUST fail the channel.
• if the payment_preimage value in update_fulfill_htlc doesn't SHA256 hash to the corresponding HTLC payment_hash:
  • MUST fail the channel.
• if the BADONION bit in failure_code is not set for update_fail_malformed_htlc:
  • MUST fail the channel.
• if the sha256_of_onion in update_fail_malformed_htlc doesn't match the onion it sent:
  • MAY retry or choose an alternate error response.
• otherwise, a receiving node which has an outgoing HTLC canceled by update_fail_malformed_htlc:
  • MUST return an error in the update_fail_htlc sent to the link which originally sent the HTLC, using the failure_code given and setting the data to sha256_of_onion.

Rationale

A node that doesn't time out HTLCs risks channel failure (see cltv_expiry_delta Selection).

A node that sends update_fulfill_htlc, before the sender, is also committed to the HTLC and risks losing funds.

If the onion is malformed, the upstream node won't be able to extract a key to generate a response — hence the special failure message which makes this node do it.

The node can check that the SHA256 the upstream is complaining about does match the onion it sent, which may allow it to detect random bit errors. However, without re-checking the actual encrypted packet sent, it won't know whether the error was its own or the remote's; so such detection is left as an option.

Committing Updates So Far: commitment_signed

When a node has changes for the remote commitment, it can apply them, sign the resulting transaction (as defined in BOLT #3), and send a commitment_signed message.

1. type: 132 (commitment_signed)
2. data:
   • [32:channel_id]
   • [64:signature]
   • [2:num_htlcs]
   • [num_htlcs*64:htlc_signature]

Requirements

A sending node:

• MUST NOT send a commitment_signed message which does not include any updates.
• MAY send a commitment_signed message which only alters the fee.
• MAY send a commitment_signed message which doesn't change the commitment transaction aside from the new revocation hash (due to dust, identical HTLC replacement, or insignificant or multiple fee changes).
• MUST include one htlc_signature for every HTLC transaction corresponding to BIP69 lexicographic ordering of the commitment transaction.

A receiving node:

• if signature is not valid for its local commitment transaction once all pending updates are applied:
  • MUST fail the channel.
• if num_htlcs is not equal to the number of HTLC outputs in the local commitment transaction once all pending updates are applied:
  • MUST fail the channel.
• if any htlc_signature is not valid for the corresponding HTLC transaction:
  • MUST fail the channel.
• MUST respond with a revoke_and_ack message.

Rationale

There's little point offering spam updates: it implies a bug.

The num_htlcs field is redundant, but makes the packet length check fully self-contained.

Completing the Transition to the Updated State: revoke_and_ack

Once the recipient of commitment_signed checks the signature and knows it has a valid new commitment transaction, it replies with the commitment preimage for the previous commitment transaction in a revoke_and_ack message.

This message also implicitly serves as an acknowledgment of receipt of the commitment_signed, so this is a logical time for the commitment_signed sender to apply (to its own commitment) any pending updates it sent before that commitment_signed.

The description of key derivation is in BOLT #3.

1. type: 133 (revoke_and_ack)
2. data:
   • [32:channel_id]
   • [32:per_commitment_secret]
   • [33:next_per_commitment_point]

Requirements

A sending node:

• MUST set per_commitment_secret to the secret used to generate keys for the previous commitment transaction.
• MUST set next_per_commitment_point to the values for its next commitment transaction.

A receiving node:

• if per_commitment_secret does not generate the previous per_commitment_point:
  • MUST fail the channel.
• if the per_commitment_secret was not generated by the protocol in BOLT #3:
  • MAY fail the channel.

A node:

• MUST NOT broadcast old (revoked) commitment transactions: doing so will allow the other node to seize all channel funds.
• SHOULD NOT sign commitment transactions, unless it's about to broadcast them (due to a failed connection), to reduce the above risk.

Updating Fees: update_fee

An update_fee message is sent by the node which is paying the Bitcoin fee. Like any update, it is first committed to the receiver's commitment transaction and then (once acknowledged) committed to the sender's. Unlike an HTLC, update_fee is never closed but simply replaced.

There is a possibility of a race, as the recipient can add new HTLCs before it receives the update_fee. Under this circumstance, the sender may not be able to afford the fee on its own commitment transaction, once the update_fee is finally acknowledged by the recipient. In this case, the fee will be less than the fee rate, as described in BOLT #3.

The exact calculation used for deriving the fee from the fee rate is given in BOLT #3.

1. type: 134 (update_fee)
2. data:
   • [32:channel_id]
   • [4:feerate_per_kw]

Requirements

The node responsible for paying the Bitcoin fee:

• SHOULD send update_fee to ensure the current fee rate is sufficient (by a significant margin) for timely processing of the commitment transaction.

The node not responsible for paying the Bitcoin fee:

• MUST NOT send update_fee.

A receiving node:

• if the update_fee is too low for timely processing, OR is unreasonably large:
  • SHOULD fail the channel.
• if the sender is not responsible for paying the Bitcoin fee:
  • MUST fail the channel.
• if the sender cannot afford the new fee rate on the receiving node's current commitment transaction:
  • SHOULD fail the channel,
    • but MAY delay this check until the update_fee is committed.

Rationale

Bitcoin fees are required for unilateral closes to be effective — particularly since there is no general method for the broadcasting node to use child-pays-for-parent to increase its effective fee.

Given the variance in fees, and the fact that the transaction may be spent in the future, it's a good idea for the fee payer to keep a good margin (say 5x the expected fee requirement); but, due to differing methods of fee estimation, an exact value is not specified.

Since the fees are currently one-sided (the party which requested the channel creation always pays the fees for the commitment transaction), it's simplest to only allow it to set fee levels; however, as the same fee rate applies to HTLC transactions, the receiving node must also care about the reasonableness of the fee.

Message Retransmission

Because communication transports are unreliable, and may need to be re-established from time to time, the design of the transport has been explicitly separated from the protocol.

Nonetheless, it's assumed our transport is ordered and reliable. Reconnection introduces doubt as to what has been received, so there are explicit acknowledgments at that point.

This is fairly straightforward in the case of channel establishment and close where messages have an explicit order, but during normal operation acknowledgments of updates are delayed until the commitment_signed / revoke_and_ack exchange; so it cannot be assumed the updates have been received. This also means that the receiving node only needs to store updates upon receipt of commitment_signed.

Note that messages described in BOLT #7 are independent of particular channels; their transmission requirements are covered there, and besides being transmitted after init (as all messages are), they are independent of requirements here.

1. type: 136 (channel_reestablish)
2. data:
   • [32:channel_id]
   • [8:next_local_commitment_number]
   • [8:next_remote_revocation_number]
   • [32:your_last_per_commitment_secret] (option-data-loss-protect)
   • [33:my_current_per_commitment_point] (option-data-loss-protect)

Requirements

A funding node:

• upon disconnection:
  • if it has broadcast the funding transaction:
    • MUST remember the channel for reconnection.
  • otherwise:
    • SHOULD NOT remember the channel for reconnection.

A non-funding node:

• upon disconnection:
  • if it has sent the funding_signed message:
    • MUST remember the channel for reconnection.
  • otherwise:
    • SHOULD NOT remember the channel for reconnection.

A node:

• MUST handle continuation of a previous channel on a new encrypted transport.
• upon disconnection:
  • MUST reverse any uncommitted updates sent by the other side (i.e. all messages beginning with update_ for which no commitment_signed has been received).
    • Note: a node MAY have already use the payment_preimage value from the update_fulfill_htlc, so the effects of update_fulfill_htlc are not completely reversed.
• upon reconnection:
  • if a channel is in an error state,
    • SHOULD retransmit the error packet and ignore any other packets for that channel.
  • otherwise:
    • MUST transmit channel_reestablish for each channel.
    • MUST wait for to receive the other node's channel_reestablish message before sending any other messages for that channel.

The sending node:

• MUST set next_local_commitment_number to the commitment number of the next commitment_signed it expects to receive.
• MUST set next_remote_revocation_number to the commitment number of the next revoke_and_ack message it expects to receive.

A node:

• if next_local_commitment_number is 1 in both the channel_reestablish it sent and received:
  • MUST retransmit funding_locked.
• otherwise:
  • MUST NOT retransmit funding_locked.
• upon reconnection:
  • MUST ignore any redundant funding_locked it receives.
• if next_local_commitment_number is equal to the commitment number of the last commitment_signed message the receiving node has sent:
  • MUST reuse the same commitment number for its next commitment_signed.
• otherwise, if next_local_commitment_number is not 1 greater than the commitment number of the last commitment_signed message the receiving node has sent:
  • SHOULD fail the channel.
• if next_remote_revocation_number is equal to the commitment number of the last revoke_and_ack the receiving node sent, AND the receiving node hasn't already received a closing_signed:
  • MUST re-send the revoke_and_ack.
• otherwise:
  • if next_remote_revocation_number is not equal to 1 greater than the commitment number of the last revoke_and_ack the receiving node has sent:
    • SHOULD fail the channel.
  • if it has sent no revoke_and_ack, AND next_remote_revocation_number is equal to 0:
    • SHOULD fail the channel.

A receiving node:

• if it supports option-data-loss-protect, AND the option-data-loss-protect fields are present:
  • if next_remote_revocation_number is greater than expected above, AND your_last_per_commitment_secret is correct for that next_remote_revocation_number minus 1:
    • MUST NOT broadcast its commitment transaction.
    • SHOULD fail the channel.
    • SHOULD store my_current_per_commitment_point to retrieve funds should the sending node broadcast its commitment transaction onchain.
  • otherwise (your_last_per_commitment_secret or my_current_per_commitment_point do not match the expected values):
    • SHOULD fail the channel.

A node:

• MUST not assume that previously-transmitted messages were lost,
  • if it has sent a previous commitment_signed message:
    • MUST handle the case where the corresponding commitment transaction is broadcast by the other side at any time. This is particularly important if the node does not simply retransmit the exact update_ messages as previously sent.
• upon reconnection:
  • if it has sent a previous closing_signed:
    • MUST send another closing_signed.
  • otherwise, if it has sent a previous shutdown:
    • MUST retransmit shutdown.

Rationale

The requirements above ensure that the opening phase is nearly atomic: if it doesn't complete, it starts again. The only exception is if the funding_signed message is sent but not received. In this case, the funder will forget the channel, and presumably open a new one upon reconnection; meanwhile, the other node will eventually forget the original channel, due to never receiving funding_locked or seeing the funding transaction on-chain.

There's no acknowledgment for error, so if a reconnect occurs it's polite to retransmit before disconnecting again; however, it's not a MUST, because there are also occasions where a node can simply forget the channel altogether.

closing_signed also has no acknowledgment so must be retransmitted upon reconnection (though negotiation restarts on reconnection, so it need not be an exact retransmission). The only acknowledgment for shutdown is closing_signed, so one or the other needs to be retransmitted.

The handling of updates is similarly atomic: if the commit is not acknowledged (or wasn't sent) the updates are re-sent. However, it's not insisted they be identical: they could be in a different order, involve different fees, or even be missing HTLCs which are now too old to be added. Requiring they be identical would effectively mean a write to disk by the sender upon each transmission, whereas the scheme here encourages a single persistent write to disk for each commitment_signed sent or received.

A re-transmittal of revoke_and_ack should never be asked for, after a closing_signed has been received, since that would imply a shutdown has been completed — which can only occur after the revoke_and_ack has been received by the remote node.

Note that the next_local_commitment_number starts at 1, since commitment number 0 is created during opening. next_remote_revocation_number will be 0 until the commitment_signed for commitment number 1 is received, at which point the revocation for commitment number 0 is sent.

funding_locked is implicitly acknowledged by the start of normal operation, which is known to have begun after a commitment_signed has been received — hence, the test for a next_local_commitment_number greater than 1.

A previous draft insisted that the funder "MUST remember ...if it has broadcast the funding transaction, otherwise it MUST NOT": this was in fact an impossible requirement; because, a node must either firstly commit to disk and secondly broadcast the transaction or vice versa. The new language reflects this reality: it's surely better to remember a channel which hasn't been broadcast than to forget one which has! Similarly, for the fundee's funding_signed message: it's better to remember a channel that never opens (and times out) than to let the funder open it while the fundee has forgotten it.

option-data-loss-protect was added to allow a node, which has somehow fallen behind (e.g. restored from old backup), to detect that it's fallen-behind. A fallen-behind node must know it cannot broadcast its current commitment transaction — which would lead to total loss of funds — as the remote node can prove it knows the revocation preimage. The error returned by the fallen-behind node (or simply the invalid numbers in the channel_reestablish it has sent) should make the other node drop its current commitment transaction to the chain. This will, at least, allow the fallen-behind node to recovery non-HTLC funds, if the my_current_per_commitment_point is valid. However, this also means the fallen-behind node has revealed this fact (though not provably: it could be lying), and the other node could use this to broadcast a previous state.

Authors

[ FIXME: Insert Author List ]

This work is licensed under a Creative Commons Attribution 4.0 International License.