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lnpeer: modern fee negotiation
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Updates the closing fee negotiation to comply with most recent spec
changes, see lightning/bolts#847
The closing negotiation is backwards compatible with the old
negotiation.
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@bitromortac @ecdsa
bitromortac authored and ecdsa committed Mar 7, 2022
1 parent 95f42df commit ec740d4
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Showing 3 changed files with 152 additions and 17 deletions.
154 changes: 137 additions & 17 deletions electrum/lnpeer.py
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Original file line number Diff line number Diff line change
Expand Up @@ -51,6 +51,7 @@
from .lnutil import ln_dummy_address
from .json_db import StoredDict
from .invoices import PR_PAID
from .simple_config import FEE_LN_ETA_TARGET

if TYPE_CHECKING:
from .lnworker import LNGossip, LNWallet
Expand Down Expand Up @@ -1840,30 +1841,68 @@ async def _shutdown(self, chan: Channel, payload, *, is_local: bool):
assert our_scriptpubkey
# estimate fee of closing tx
our_sig, closing_tx = chan.make_closing_tx(our_scriptpubkey, their_scriptpubkey, fee_sat=0)
fee_rate = self.network.config.fee_per_kb()
our_fee = fee_rate * closing_tx.estimated_size() // 1000
fee_rate_per_kb = self.network.config.eta_target_to_fee(FEE_LN_ETA_TARGET)
if not fee_rate_per_kb: # fallback
fee_rate_per_kb = self.network.config.fee_per_kb()
our_fee = fee_rate_per_kb * closing_tx.estimated_size() // 1000
# TODO: anchors: remove this, as commitment fee rate can be below chain head fee rate?
# BOLT2: The sending node MUST set fee less than or equal to the base fee of the final ctx
max_fee = chan.get_latest_fee(LOCAL if is_local else REMOTE)
our_fee = min(our_fee, max_fee)
drop_to_remote = False

drop_to_remote = False # does the peer drop its to_local output or not?
def send_closing_signed():
MODERN_FEE = True
if MODERN_FEE:
nonlocal fee_range_sent # we change fee_range_sent in outer scope
fee_range_sent = fee_range
closing_signed_tlvs = {'fee_range': fee_range}
else:
closing_signed_tlvs = {}

our_sig, closing_tx = chan.make_closing_tx(our_scriptpubkey, their_scriptpubkey, fee_sat=our_fee, drop_remote=drop_to_remote)
self.send_message('closing_signed', channel_id=chan.channel_id, fee_satoshis=our_fee, signature=our_sig)
self.logger.info(f"Sending fee range: {closing_signed_tlvs} and fee: {our_fee}")
self.send_message(
'closing_signed',
channel_id=chan.channel_id,
fee_satoshis=our_fee,
signature=our_sig,
closing_signed_tlvs=closing_signed_tlvs,
)
def verify_signature(tx, sig):
their_pubkey = chan.config[REMOTE].multisig_key.pubkey
preimage_hex = tx.serialize_preimage(0)
pre_hash = sha256d(bfh(preimage_hex))
return ecc.verify_signature(their_pubkey, sig, pre_hash)

# this is the fee range we initially try to enforce
# we aim at a fee between next block inclusion and some lower value
fee_range = {'min_fee_satoshis': our_fee // 2, 'max_fee_satoshis': our_fee * 2}
their_fee = None
fee_range_sent = {}
is_initiator = chan.constraints.is_initiator
# the funder sends the first 'closing_signed' message
if chan.constraints.is_initiator:
if is_initiator:
send_closing_signed()

# negotiate fee
while True:
# FIXME: the remote SHOULD send closing_signed, but some don't.
cs_payload = await self.wait_for_message('closing_signed', chan.channel_id)
try:
cs_payload = await self.wait_for_message('closing_signed', chan.channel_id)
except asyncio.exceptions.TimeoutError:
if not is_initiator and not their_fee: # we only force close if a peer doesn't reply
self.lnworker.schedule_force_closing(chan.channel_id)
raise Exception("Peer didn't reply with closing signed, force closed.")
else:
# situation when we as an initiator send a fee and the recipient
# already agrees with that fee, but doens't tell us
raise Exception("Peer didn't reply, probably already closed.")

their_previous_fee = their_fee
their_fee = cs_payload['fee_satoshis']
if their_fee > max_fee:
raise Exception(f'the proposed fee exceeds the base fee of the latest commitment transaction {is_local, their_fee, max_fee}')

# 0. integrity checks
# determine their closing transaction
their_sig = cs_payload['signature']
# verify their sig: they might have dropped their output
our_sig, closing_tx = chan.make_closing_tx(our_scriptpubkey, their_scriptpubkey, fee_sat=their_fee, drop_remote=False)
Expand All @@ -1885,17 +1924,98 @@ def verify_signature(tx, sig):
to_remote_amount = closing_tx.outputs()[to_remote_idx].value
transaction.check_scriptpubkey_template_and_dust(their_scriptpubkey, to_remote_amount)

# Agree if difference is lower or equal to one (see below)
if abs(our_fee - their_fee) < 2:
# 1. check fees
# if fee_satoshis is equal to its previously sent fee_satoshis:
if our_fee == their_fee:
# SHOULD sign and broadcast the final closing transaction.
break # we publish

# 2. at start, adapt our fee range if we are not the channel initiator
fee_range_received = cs_payload['closing_signed_tlvs'].get('fee_range')
self.logger.info(f"Received fee range: {fee_range_received} and fee: {their_fee}")
# The sending node: if it is not the funder:
if fee_range_received and not is_initiator and not fee_range_sent:
# SHOULD set max_fee_satoshis to at least the max_fee_satoshis received
fee_range['max_fee_satoshis'] = max(fee_range_received['max_fee_satoshis'], fee_range['max_fee_satoshis'])
# SHOULD set min_fee_satoshis to a fairly low value
# TODO: what's fairly low value? allows the initiator to go to low values
fee_range['min_fee_satoshis'] = fee_range['max_fee_satoshis'] // 2

# 3. if fee_satoshis matches its previously sent fee_range:
if fee_range_sent and (fee_range_sent['min_fee_satoshis'] <= their_fee <= fee_range_sent['max_fee_satoshis']):
# SHOULD reply with a closing_signed with the same fee_satoshis value if it is different from its previously sent fee_satoshis
if our_fee != their_fee:
our_fee = their_fee
send_closing_signed() # peer publishes
break
# SHOULD use `fee_satoshis` to sign and broadcast the final closing transaction
else:
our_fee = their_fee
break # we publish

# 4. if the message contains a fee_range
if fee_range_received:
overlap_min = max(fee_range['min_fee_satoshis'], fee_range_received['min_fee_satoshis'])
overlap_max = min(fee_range['max_fee_satoshis'], fee_range_received['max_fee_satoshis'])
# if there is no overlap between that and its own fee_range
if overlap_min > overlap_max:
raise Exception("There is no overlap between between their and our fee range.")
# TODO: MUST fail the channel if it doesn't receive a satisfying fee_range after a reasonable amount of time
# otherwise:
else:
if is_initiator:
# if fee_satoshis is not in the overlap between the sent and received fee_range:
if not (overlap_min <= their_fee <= overlap_max):
# MUST fail the channel
self.lnworker.schedule_force_closing(chan.channel_id)
raise Exception("Their fee is not in the overlap region, we force closed.")
# otherwise:
else:
our_fee = their_fee
# MUST reply with the same fee_satoshis.
send_closing_signed() # peer publishes
break
# otherwise (it is not the funder):
else:
# if it has already sent a closing_signed:
if fee_range_sent:
# if fee_satoshis is not the same as the value it sent:
if their_fee != our_fee:
# MUST fail the channel
self.lnworker.schedule_force_closing(chan.channel_id)
raise Exception("Expected the same fee as ours, we force closed.")
# otherwise:
else:
# MUST propose a fee_satoshis in the overlap between received and (about-to-be) sent fee_range.
our_fee = (overlap_min + overlap_max) // 2
send_closing_signed()
continue
# otherwise, if fee_satoshis is not strictly between its last-sent fee_satoshis
# and its previously-received fee_satoshis, UNLESS it has since reconnected:
elif their_previous_fee and not (min(our_fee, their_previous_fee) < their_fee < max(our_fee, their_previous_fee)):
# SHOULD fail the connection.
raise Exception('Their fee is not between our last sent and their last sent fee.')
# otherwise, if the receiver agrees with the fee:
elif abs(their_fee - our_fee) <= 1: # we cannot have another strictly in-between value
# SHOULD reply with a closing_signed with the same fee_satoshis value.
our_fee = their_fee
send_closing_signed() # peer publishes
break
# this will be "strictly between" (as in BOLT2) previous values because of the above
our_fee = (our_fee + their_fee) // 2
# another round
send_closing_signed()
# the non-funder replies
if not chan.constraints.is_initiator:
# otherwise:
else:
# MUST propose a value "strictly between" the received fee_satoshis and its previously-sent fee_satoshis.
our_fee_proposed = (our_fee + their_fee) // 2
if not (min(our_fee, their_fee) < our_fee_proposed < max(our_fee, their_fee)):
our_fee_proposed += (their_fee - our_fee) // 2
else:
our_fee = our_fee_proposed
send_closing_signed()

# reaching this part of the code means that we have reached agreement; to make
# sure the peer doesn't force close, send a last closing_signed
if not is_initiator:
send_closing_signed()

# add signatures
closing_tx.add_signature_to_txin(
txin_idx=0,
Expand Down
3 changes: 3 additions & 0 deletions electrum/tests/regtest.py
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Original file line number Diff line number Diff line change
Expand Up @@ -44,6 +44,9 @@ def test_unixsockets(self):
class TestLightningAB(TestLightning):
agents = ['alice', 'bob']

def test_collaborative_close(self):
self.run_shell(['collaborative_close'])

def test_backup(self):
self.run_shell(['backup'])

Expand Down
12 changes: 12 additions & 0 deletions electrum/tests/regtest/regtest.sh
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Original file line number Diff line number Diff line change
Expand Up @@ -158,6 +158,18 @@ if [[ $1 == "backup" ]]; then
fi


if [[ $1 == "collaborative_close" ]]; then
wait_for_balance alice 1
echo "alice opens channel"
bob_node=$($bob nodeid)
channel=$($alice open_channel $bob_node 0.15)
new_blocks 3
wait_until_channel_open alice
echo "alice closes channel"
request=$($bob close_channel $channel)
fi


if [[ $1 == "extract_preimage" ]]; then
# instead of settling bob will broadcast
$bob enable_htlc_settle false
Expand Down

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