33 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 version of commitment transaction as described in
BOLT
03.
The funder then sends the outpoint of the funding output along with a
signature for the responser'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 ensure the blockchain and reach their
specified depth (number of confirmations). After both sides have sent
the funding_locked message, the channel is estimated 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.
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:
- [8: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 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-inflight-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 kiloweight (ie. 4 times by 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 set the most significant bit in
temporary-channel-id, and MUST ensure it 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. The sender SHOULD set minimum-depth to
an amount where the sender considers reorganizations to be low risk.
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 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-amount * 1000.
The receiving node MAY fail the channel if it considers
htlc-minimum-msat too large, max-htlc-value-in-flight 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 511.
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 unresonably large fee rates can also penalize the reciepient.
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:
- [8: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 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:
- [8:temporary-channel-id]
- [32:txid]
- [1: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 txid to the transaction ID of a non-malleable transaction, which it MUST NOT broadcast, and MUST set output-index to the output number of that transaction which corresponds the funding transaction output as defined in BOLT #3, MUST set pad to zero, 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, and MUST ignore pad.
Rationale
The output-index can only be 1 byte, since that's how we'll pack it into the channel-id used throughout the protocol. The limit of 255 outputs should not be overly burdensome.
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.
- type: 35 (
funding_signed) - data:
- [8:temporary-channel-id]
- [64:signature]
Requirements
The sender MUST set temporary-channel-id the same as the temporary-channel-id in the open_channel 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 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:
- [8:temporary-channel-id]
- [8:channel-id]
- [64:announcement-node-signature]
- [64:announcement-bitcoin-signature]
- [33:next-per-commitment-point]
The channel-id is the unique description of the funding transaction.
It is constructed with the most significant 3 bytes as the block
height, the next 3 bytes indicating the transaction index within the
block, and the least significant two bytes indicating the output
index which pays to the channel.
This channel-id is used in all messages referring to the channel
from this point onwards.
announcement-node-signature and announcement-bitcoin-signature are the optional signature for channel_announcement as described in BOLT #7.
Requirements
The sender MUST wait until the funding transaction has reached
minimum-depth before sending this message. The sender MUST encode
the block position of the funding transaction into channel-id. If
the sender has already received funding_locked from the other node,
it MUST fail the channel if its own channel-id does not match the
received. 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.
If the recipient has already sent funding_locked it MUST fail the
channel if channel-id does not match the channel-id it sent.
The sender MUST set announcement-node-signature and announcement-bitcoin-signature to the signatures for the
channel_announcement message, or all zeroes if it does not want the
channel announced.
The recipient SHOULD fail the channel if the announcement-node-signature and announcement-bitcoin-signatures are incorrect (and not all zeroes).
The recipient SHOULD queue the channel_announcement message for its
peers if it has sent and received a non-zero announcement-node-signature and announcement-bitcoin-signature.
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:
- [8: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 which are not yet added to the sender’s commitment.
A node MUST NOT send an update_add_htlc after a shutdown,
and MUST NOT send more than one 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 update_commit 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 update_commit 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 script_pubkey 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:
- [8: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
the receiver's dust-limit-satoshis.
The receiver MUST check signature is valid for the close transaction
with the given fee-satoshis as detailed above, 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
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, 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
commit_sig 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).
Risks With HTLC Timeouts
HTLCs tend to be chained across the network. 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.
This difference in timeouts 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 expiry, plus some
additional delay for the transaction which redeems the HTLC output to
be irreversibly committed to the blockchain.
Thus a node MUST estimate the deadline for successful redemption for each HTLC it offers. 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.
Adding an 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 route portion, which indicates where the payment
is destined, is described in [BOLT #4].
- type: 128 (
update_add_htlc) - data:
- [8:channel-id]
- [8:id]
- [4:amount-msat]
- [4:expiry]
- [32:payment-hash]
- [1254:onion-routing-packet]
Requirements
A sending node MUST NOT offer amount-msat it cannot pay for in the
remote commitment transaction at the current fee-rate (see "Updating
Fees") while maintaining its channel reserve, and MUST offer
amount-msat greater than 0, and MUST NOT offer amount-msat below
the receiving node's htlc-minimum-msat.
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.
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-sat equal to zero, below its own htlc-minimum-msat, or
which the sending node cannot afford at the current fee-rate 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.
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 it has the sender did not previously acknowledge the
commitment of that HTLC. A receiving node MAY fail the channel if
other id violations occur.
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 511, to ensure that even if both
sides send the maximum number of HTLCs, the commit_sig 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.
Removing an HTLC: update_fulfill_htlc and update_fail_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].
- type: 130 (
update_fulfill_htlc) - data:
- [8:channel-id]
- [8:id]
- [32:payment-preimage]
For a timed out or route-failed HTLC:
- type: 131 (
update_fail_htlc) - data:
- [8:channel-id]
- [8:id]
- [154:reason]
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 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 which closes an incoming HTLC in response to an
update-fail-htlc message on an offered HTLC MUST copy the reason
field to the outgoing update-fail-htlc.
Rationale
A node which doesn't time out HTLCs risks channel failure (see "Risks With HTLC Timeouts").
Committing Updates So Far: commitsig
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
commitsig message.
- type: 132 (
commit_sig) - data:
- [8:channel-id]
- [64:signature]
- [2:num-htlcs]
- [num-htlcs*64:htlc-signature]
Requirements
A node MUST NOT send a commitsig message which does not include any
updates. Note that a node MAY send a commitsig message which only
alters the fee, and MAY send a commitsig 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 exactly 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 commitsig 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 acknowledgement of receipt
of the commitsig, so it's a logical time for the commitsig sender
to apply to its own commitment, any pending updates it sent before
that commitsig.
This message also supplies the signatures for the sender's HTLC-timeout transactions. See BOLT #5 for how this is used with a penalty transaction.
The description of key derivation is in BOLT #3.
- type: 133 (
revoke_and_ack) - data:
- [8:channel-id]
- [32:per-commitment-secret]
- [33:next-per-commitment-point]
- [1:padding]
- [2:num-htlc-timeouts]
- [num-htlc-timeouts*64:htlc-timeout-signature]
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, and MUST set padding to all zeroes.
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 MUST ignore the value of padding. A receiving node MAY fail if the per-commitment-secret was not generated by the protocol in BOLT #3.
A receiving node MUST fail the channel if any htlc-timeout-signature is not valid, or if num-htlc-timeout is not equal to the number of outputs in the sending node's commitment transaction corresponding to HTLCs offered by the sending node.
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:
- [8: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.
Authors
FIXME
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