43 KiB
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
Channel establishment begins immediately after authentication, and
consists of the funding node sending an open_channel message,
followed by the responding node 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
03.
The funder then sends the outpoint of the funding output along with a
signature for the responder's version of the commitment transaction
with the funding_created message. Once the responder learns the
funding outpoint, she is able to generate the initiator's commitment
for the commitment transaction, and send it over using the
funding_signed message.
Once the channel funder receives the funding_signed message, they
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 their
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 -----| |
+-------+ +-------+
If this fails at any stage, or a node decides that 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.
- type: 32 (
open_channel) - 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] - [
4: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:first_per_commitment_point]
- [
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 channel
across many distinct blockchains as well as have channels within multiple
blockchains opened to the same peer (if they support 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 output should be generated for this node's commitment or HTLC transaction; ie. 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 (ie. 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.
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 and delayed_payment_basepoint are similarly used to generate a series of keys for any payments to this node: delayed_payment_basepoint is used to 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.
FIXME: Describe Dangerous feature bit for larger channel amounts.
Requirements
A sending node MUST ensure that the chain_hash value identifies the chain they
they wish to open the channel within. For the Bitcoin blockchain, the
chain_hash value MUST be (encoded in hex):
000000000019d6689c085ae165831e934ff763ae46a2a6c172b3f1b60a8ce26f.
A sending node MUST ensure temporary_channel_id is unique from any other
channel id with the same peer. The sender MUST set funding_satoshis
to less than 2^24 satoshi. The sender MUST set push_msat to
equal or less than to 1000 * funding_satoshis. The sender SHOULD set to_self_delay sufficient to ensure the sender
can irreversibly spend a commitment transaction output in case of
misbehavior by the receiver.
funding_pubkey, revocation_basepoint, payment_basepoint and delayed_payment_basepoint MUST be valid DER-encoded
compressed secp256k1 pubkeys. The sender SHOULD set feerate_per_kw
to at least the rate it estimates would cause the transaction to be
immediately included in a block.
The sender SHOULD set dust_limit_satoshis to a sufficient value to
allow commitment transactions to propagate through the Bitcoin
network. It SHOULD set htlc_minimum_msat to the minimum
amount HTLC it is willing to accept from this peer.
The receiving node MUST accept a new open_channel message if the
connection has been re-established after receiving a previous
open_channel and before receiving a funding_created message. In
this case, the receiving node MUST discard the previous open_channel
message.
The receiving node MUST fail the channel if to_self_delay is
unreasonably large. The receiver MAY fail the channel if
funding_satoshis is too small, and MUST fail the channel if
push_msat is greater than funding_satoshis * 1000.
The receiving node MAY fail the channel if it considers
htlc_minimum_msat too large, max_htlc_value_in_flight_msat too small, channel_reserve_satoshis too large, or max_accepted_htlcs too small. It MUST fail the channel if max_accepted_htlcs is greater than 483.
The receiver MUST fail the channel if it
considers feerate_per_kw too small for timely processing, or unreasonably large. The
receiver MUST fail the channel if funding_pubkey, revocation_basepoint, payment_basepoint or delayed_payment_basepoint
are not valid DER-encoded compressed secp256k1 pubkeys.
The receiver 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, and 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 that is also the feerate paid by HTLC transactions; thus unreasonably large fee rates can also penalize the recipient.
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.
- type: 33 (
accept_channel) - data:
- [
32:temporary_channel_id] - [
8:dust_limit_satoshis] - [
8:max_htlc_value_in_flight_msat] - [
8:channel_reserve_satoshis] - [
4:minimum_depth] - [
4:htlc_minimum_msat] - [
2:to_self_delay] - [
2:max_accepted_htlcs] - [
33:funding_pubkey] - [
33:revocation_basepoint] - [
33:payment_basepoint] - [
33:delayed_payment_basepoint] - [
33:first_per_commitment_point]
- [
Requirements
The receiving MUST reject the channel if the chain_hash value within the
open_channel message is set to a hash of a chain unknown to the receiver.
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 MAY reject the minimum_depth if it considers it unreasonably large.
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.
- type: 34 (
funding_created) - 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. The sender MUST set funding_txid to the transaction ID of a non-malleable transaction, which it MUST NOT broadcast, and MUST set funding_output_index to the output number of that transaction which corresponds the funding transaction output as defined in BOLT #3, and MUST set signature to the valid signature using its funding_pubkey for the initial commitment transaction as defined in BOLT #3. The sender SHOULD use only BIP141 (Segregated Witness) inputs when creating the funding transaction.
The recipient MUST fail the channel if signature is incorrect.
Rationale
The funding_output_index can only be 2 bytes, since that's how we'll pack it into the channel_id 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 recommendation for the funding transaction.
The funding_signed message
This message gives the funder the signature they need for the first commitment transaction, so they can broadcast it knowing they can redeem their funds if they need to.
This message introduces the channel_id to identify the channel, which is derived from the funding transaction by combining the funding_txid and the funding_output_index using big-endian exclusive-OR (ie. funding_output_index alters the last two bytes).
- type: 35 (
funding_signed) - 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, and MUST set signature to the valid signature using its funding_pubkey for the initial commitment transaction as defined in BOLT #3.
The recipient MUST fail the channel if signature is incorrect.
The recipient SHOULD broadcast the funding transaction on receipt of a valid funding_signed and MUST NOT broadcast the funding transaction earlier.
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.
- type: 36 (
funding_locked) - 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.
The sender MUST 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 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, a 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; even if push_msat is significant, if it remembers
the channel forever on the promise of the funding transaction finally
appearing, there is a denial of service risk.
Future
We could add an SPV proof, and route block hashes in separate messages.
Channel Close
Nodes can negotiate a mutual close for 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: the first is by one side indicating that it wants to clear the channel (and thus will accept no new HTLCs), and 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,
and indicating the scriptpubkey it wants to be paid to.
- type: 38 (
shutdown) - data:
- [
32:channel_id] - [
2:len] - [
len:scriptpubkey]
- [
Requirements
A node MUST NOT send a shutdown if there are updates pending
on the receiving node's commitment transaction.
A node MUST NOT send an update_add_htlc after a shutdown. A sending node
SHOULD fail to route any HTLC added after it sent shutdown.
A sending node MUST set scriptpubkey to one of the following forms:
OP_DUPOP_HASH1602020-bytesOP_EQUALVERIFYOP_CHECKSIG(pay to pubkey hash), OROP_HASH1602020-bytesOP_EQUAL(pay to script hash), OROP_02020-bytes (version 0 pay to witness pubkey), OROP_03232-bytes (version 0 pay to witness script hash)
A receiving node SHOULD fail the connection if the scriptpubkey is not one
of those forms.
A receiving node MUST reply to a shutdown message with a shutdown once there are no outstanding updates on the peer, unless it has already sent a shutdown.
Rationale
If channel state is always "clean" (no pending changes) when a
shutdown starts, we avoid the question of how to behave if it wasn't;
the sender always send an 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, ensuring that the resulting transaction will
propagate to miners.
The shutdown response requirement implies that the node sends commitment_signed to commit any outstanding changes before replying, but it could theoretically reconnect instead, which simply erases 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 and that fee, and sends the signature. The
process terminates when both agree on the same fee, or one side fails
the channel.
- type: 39 (
closing_signed) - data:
- [
32:channel_id] - [
8:fee_satoshis] - [
64:signature]
- [
Requirements
Nodes SHOULD send a closing_signed message after shutdown has
been received and no HTLCs remain in either commitment transaction.
A sending node MUST set fee_satoshis lower than or equal to the
fee of the final commitment transaction.
The sender SHOULD set the initial fee_satoshis according to its
estimate of cost of inclusion in a block.
The sender MUST set signature to the Bitcoin signature of the close
transaction with the node responsible for paying the bitcoin fee
paying fee_satoshis, without populating any output which is below
its own dust_limit_satoshis. The sender MAY also eliminate its own
output from the mutual close transaction.
The receiver MUST check signature is valid for either the close
transaction with the given fee_satoshis as detailed above and its
own dust_limit_satoshis OR that same transaction with the sender's
output eliminated, and MUST fail the connection if it is not.
If the receiver agrees with the fee, it SHOULD reply with a
closing_signed with the same fee_satoshis value, otherwise it
SHOULD propose a value strictly between the received fee_satoshis
and its previously-sent fee_satoshis.
Once a node has sent or received a closing_signed with matching
fee_satoshis it SHOULD close the connection and SHOULD sign and
broadcast the final closing transaction.
Rationale
There is a possibility of irreparable differences on closing if one
node considers the other's output too small to allow propagation on
the bitcoin network (aka "dust"), and that other node instead
considers that output to be too valuable to discard. This is why each
side uses its own dust_limit_satoshis, and the result can be a
signature validation failure, if they disagree on what the closing
transaction should look like.
However, if one side chooses to eliminate its own output, there's no reason for the other side to fail the closing protocol, so this is explicitly allowed.
Note that there is limited risk if the closing transaction is delayed, and 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---->| |
+-------+ +-------+
Counterintuitively, 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:
- Pending on the receiver
- In the receiver's latest commitment transaction,
- ... and the receiver's previous commitment transaction has been revoked, and the HTLC is pending on the sender.
- ... and in the sender's latest commitment transaction
- ... 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 a payment coming from another node. In the forwarding case, care must be taken to ensure that the outgoing HTLC cannot be redeemed unless the incoming HTLC can be redeemed; these requirements ensure that is always true.
The addition/removal of an HTLC is considered irrevocably committed when:
- the commitment transaction with/without it it is committed by both nodes, and any previous commitment transaction which without/with it has been revoked, OR
- the commitment transaction with/without it has been irreversibly committed to the blockchain.
Requirements
A node MUST NOT offer an HTLC (update_add_htlc) in response to an incoming HTLC until
the incoming HTLC has been irrevocably committed.
A node MUST NOT fail an incoming HTLC (update_fail_htlc) for which it has committed
to an outgoing HTLC, until the removal of the outgoing HTLC is irrevocably committed.
A node SHOULD fulfill an incoming HTLC for which it has committed to an outgoing HTLC,
as soon as it receives update_fulfill_htlc for the outgoing HTLC.
Rationale
In general, we need to complete one side of the exchange before dealing with the other. Fulfilling an HTLC is different: knowledge of the preimage is by definition irrevocable, so we should fulfill the incoming HTLC as soon as we can to reduce latency.
Risks With HTLC Timeouts
Once an HTLC has timed out where it could either be fulfilled or timed-out; care must be taken around this transition both for offered and received HTLCs.
As a result of forwarding an HTLC from node A to node C, B will end up having an incoming HTLC from A and an outgoing HTLC to C. B will make sure that the incoming HTLC has a greater timeout than the outgoing HTLC, so that B can get refunded from C sooner than it has to refund A if the payment does not complete.
For example, node A might offer node B an HTLC with a timeout of 3 days, and node B might offer node C the same HTLC with a timeout of 2 days:
3 days timeout 2 days timeout
A ------------------> B ------------------> C
The difference in timeouts is called cltv_expiry_delta in
BOLT #7.
This difference is important: after 2 days B can try to remove the offer to C even if C is unresponsive, by broadcasting the commitment transaction it has with C and spending the HTLC output. Even though C might race to try to use its payment preimage at that point to also spend the HTLC, it should be resolved well before the 3 day deadline so B can either redeem the HTLC off A or close it.
If the timing is too close, there is a risk of "one-sided redemption", where the payment preimage received from an offered HTLC is too late to be used for an incoming HTLC, leaving the node with unexpected liability.
Thus the effective timeout of the HTLC is the cltv_expiry, plus some
additional delay for the transaction which redeems the HTLC output to
be irreversibly committed to the blockchain.
The fulfillment risk is similar: if a node C fulfills an HTLC after its timeout, B might broadcast the commitment transaction and immediately broadcast the HTLC timeout transaction. In this scenario, B would gain knowledge of the preimage without paying C.
Requirements
A node MUST estimate the deadline for successful redemption for each HTLC. A node MUST NOT offer a HTLC after this deadline, and MUST fail the channel if an HTLC which it offered is in either node's current commitment transaction past this deadline.
A node MUST NOT fulfill an HTLC after this deadline, and MUST fail the connection if a HTLC it has fulfilled is in either node's current commitment transaction past this deadline.
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.
- type: 128 (
update_add_htlc) - data:
- [
32:channel_id] - [
8:id] - [
4:amount_msat] - [
4:cltv_expiry] - [
32:payment_hash] - [
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, and MUST set cltv_expiry less
than 500000000.
A sending node MUST NOT add an HTLC if it would result in it offering
more than the remote's max_accepted_htlcs HTLCs in the remote commitment
transaction, or if the sum of total offered HTLCs would exceed the remote's
max_htlc_value_in_flight_msat.
A sending node MUST set id to 0 for the first HTLC it offers, and
increase the value by 1 for each successive offer.
A receiving node SHOULD fail the channel if it receives an
amount_msat equal to zero, below its own htlc_minimum_msat, or
which the sending node cannot afford at the current feerate_per_kw while
maintaining its channel reserve. A receiving node 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, or
sets cltv_expiry to greater or equal to 500000000.
A receiving node MUST allow multiple HTLCs with the same payment hash.
A receiving node MUST ignore a repeated id value after a
reconnection if the sender did not previously acknowledge the
commitment of that HTLC. A receiving node MAY fail the channel if
other id violations occur.
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., including the payment_hash in the computation of HMACs.
This prevents replay attacks that'd 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 deal with duplicates leads us to using a separate identifier; we assume 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 or above 500000000 would indicate a time in
seconds, and the protocol only supports an expiry in blocks.
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,
but 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.
- type: 130 (
update_fulfill_htlc) - data:
- [
32:channel_id] - [
8:id] - [
32:payment_preimage]
- [
For a timed out or route-failed HTLC:
- type: 131 (
update_fail_htlc) - data:
- [
32:channel_id] - [
8:id] - [
2:len] - [
len:reason]
- [
For a unparsable HTLC:
- type: 135 (
update_fail_malformed_htlc) - 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; in particular, a node SHOULD fail an HTLC which has timed out.
A node MUST NOT send update_fulfill_htlc until an HTLC is
irrevocably committed in both sides' commitment transactions.
A receiving node MUST check that id corresponds to an HTLC in its
current commitment transaction, and MUST fail the channel if it does
not.
A receiving node MUST check that the payment_preimage value in
update_fulfill_htlc SHA256 hashes to the corresponding HTLC
payment_hash, and MUST fail the channel if it does not.
A receiving node MUST fail the channel if the BADONION bit in
failure_code is not set for update_fail_malformed_htlc.
A receiving node MAY check the sha256_of_onion in
update_fail_malformed_htlc and MAY retry or choose an alternate
error response if it does not match the onion it sent.
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 which doesn't time out HTLCs risks channel failure (see "Risks With HTLC Timeouts").
A node which sends update_fulfill_htlc before the sender is also
committed to the HTLC 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. Without re-checking the actual encrypted packet sent, however, it won't know whether the error was its own or on the remote side, 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.
- type: 132 (
commitment_signed) - data:
- [
32:channel_id] - [
64:signature] - [
2:num_htlcs] - [
num_htlcs*64:htlc_signature]
- [
Requirements
A node MUST NOT send a commitment_signed message which does not include any
updates. Note that a node MAY send a commitment_signed message which only
alters the fee, and MAY send a commitment_signed message which doesn't
change the commitment transaction other than the new revocation hash
(due to dust, identical HTLC replacement, or insignificant or multiple
fee changes). A node MUST include one htlc_signature for every HTLC
transaction corresponding to BIP69 lexicographic ordering of the commitment
transaction.
A receiving node MUST fail the channel if signature is not valid for
its local commitment transaction once all pending updates are applied.
A receiving node 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. A receiving node MUST fail the channel if
any htlc_signature is not valid for the corresponding HTLC transaction.
A receiving node 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, it knows that
it has a valid new commitment transaction, 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 it's 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.
- type: 133 (
revoke_and_ack) - 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 MUST check that per_commitment_secret generates the previous per_commitment_point, and MUST fail if it does not. A receiving node MAY fail if the per_commitment_secret was not generated by the protocol in BOLT #3.
Nodes MUST NOT broadcast old (revoked) commitment transactions; doing so will allow the other node to seize all the funds. Nodes SHOULD NOT sign commitment transactions unless it is about to broadcast them (due to a failed connection), to reduce this 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, then (once acknowledged) committed to the
sender's. Unlike an HTLC, update_fee is never closed, simply
replaced.
There is a possibility of a race: the recipient can add new HTLCs
before it receives the update_fee, and the sender may not be able to
afford the fee on its own commitment transaction once the update_fee
is 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.
- type: 134 (
update_fee) - data:
- [
32:channel_id] - [
4:feerate_per_kw]
- [
Requirements
The node which is responsible for paying the bitcoin fee SHOULD send
update_fee to ensure the current fee rate is sufficient for
timely processing of the commitment transaction by a significant
margin.
The node which is not responsible for paying the bitcoin fee MUST NOT
send update_fee.
A receiving node SHOULD fail the channel if the update_fee is too
low for timely processing, or unreasonably large.
A receiving node MUST fail the channel if the sender is not responsible for paying the bitcoin fee.
A receiving node SHOULD fail the channel if the sender cannot afford
the new fee rate on the receiving node's current commitment
transaction, but it 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 node which broadcasts it 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 differing methods of fee estimation mean we don't specify an exact value.
Since the fees are currently one-sided (the party which requested the channel creation always pays the fees for the commitment transaction), it is simplest to only allow them to set fee levels, but 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, we assume that our transport is ordered and reliable; reconnection introduces doubt as to what has been received, so we retransmit any channel messages which may not have been.
This is fairly straightforward in the case of channel establishment
and close where messages have an explicit order, but in normal
operation acknowledgments of updates are delayed until the
commitment_signed / revoke_and_ack exchange, so we cannot assume
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 other than being transmitted after init (like
any message), they are independent of requirements here.
Requirements
A node MUST handle continuing a previous channel on a new encrypted transport.
On disconnection, the funder MUST remember the channel for reconnection if it has broadcast the funding transaction, otherwise it MUST NOT.
On disconnection, the non-funding node MUST remember the channel for
reconnection if it has sent the funding_signed message, otherwise
it MUST NOT.
On disconnection, a node MUST reverse any uncommitted updates sent by
the other side (ie. all messages beginning with update_ for which no
commitment_signed has been received). Note that a node MAY have
already use the payment_preimage value from the update_fulfill_htlc,
so the effects of update_fulfill_htlc is not completely reversed.
On reconnection, if a channel is in an error state, the node SHOULD
retransmit the error packet and ignore any other packets for that
channel, or if the channel has entered closing negotiation, the node
MUST retransmit the last closing_signed.
Otherwise, on reconnection, a node MUST retransmit old messages after funding_signed which may not
have been received, and MUST NOT retransmit old messages which have
been explicitly or implicitly acknowledged. The following table
lists the acknowledgment conditions for each message:
funding_locked: acknowledged byupdate_messages,commitment_signed,revoke_and_ackorshutdownmessages.update_messages: acknowledged byrevoke_and_ack.commitment_signed: acknowledged byrevoke_and_ack.revoke_and_ack: acknowledged bycommitment_signedorclosing_signedshutdown: acknowledged byclosing_signed.
Before retransmitting commitment_signed, the node MUST send
appropriate update_ messages (the other node will have forgotten
them, as required above).
A node MAY simply retransmit messages which are identical to the
previous transmission. A node MUST not assume that
previously-transmitted messages were lost: in particular, if it has
sent a previous commitment_signed message, a node MUST handle the
case where the corresponding commitment transaction is broadcast by
the other side at any time. This is particularly important if a node
does not simply retransmit the exact same update_ messages as
previously sent.
A receiving node MAY ignore spurious message retransmission, or MAY fail the channel if they occur.
Rationale
The effect of requirements above are that the opening phase is almost
atomic: if it doesn't complete, it starts again. The only exception
is where the funding_signed message is sent and not received: in
this case, the funder will forget the channel and presumably open
a new one on reconnect; 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, but it's not a MUST
because there are also occasions where a node can simply forget the
channel altogether.
There is similarly no acknowledgment for closing_signed, so it
is also retransmitted on reconnection.
Authors
FIXME
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