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trampoline: fix off-by-one confusion of fees 

The convention is that edges (start_node -> edge_node) store
the policy/fees for the *start_node*.
This is what the non-trampoline edges were already using (for a long time),
but the trampoline ones were off-by-one (policy was for end_node),
which was then worked around in multiple places, to correct for...

i.e. I think because of all the workarounds, there was no actual bug,
but it was just very confusing.

Also note that the prior usage of trampoline edges would not work if
we (sender) were not directly connected to a TF (trampoline-forwarder)
but had extra edges in the route to even get to the first TF.
Having the policy corresponding to the start_node of the edge would work
even in that case.
master
SomberNight 2 years ago
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4 changed files with 45 additions and 42 deletions
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  1. 9
      electrum/lnonion.py
  2. 20
      electrum/lnrouter.py
  3. 2
      electrum/lnworker.py
  4. 56
      electrum/trampoline.py

9
electrum/lnonion.py
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@ -241,10 +241,6 @@ def calc_hops_data_for_payment(
# payloads, backwards from last hop (but excluding the first edge): # payloads, backwards from last hop (but excluding the first edge):
for edge_index in range(len(route) - 1, 0, -1): for edge_index in range(len(route) - 1, 0, -1):
route_edge = route[edge_index] route_edge = route[edge_index]
is_trampoline = route_edge.is_trampoline()
if is_trampoline:
amt += route_edge.fee_for_edge(amt)
cltv_abs += route_edge.cltv_delta
hop_payload = { hop_payload = {
"amt_to_forward": {"amt_to_forward": amt}, "amt_to_forward": {"amt_to_forward": amt},
"outgoing_cltv_value": {"outgoing_cltv_value": cltv_abs}, "outgoing_cltv_value": {"outgoing_cltv_value": cltv_abs},
@ -252,9 +248,8 @@ def calc_hops_data_for_payment(
} }
hops_data.append( hops_data.append(
OnionHopsDataSingle(payload=hop_payload)) OnionHopsDataSingle(payload=hop_payload))
if not is_trampoline: amt += route_edge.fee_for_edge(amt)
amt += route_edge.fee_for_edge(amt) cltv_abs += route_edge.cltv_delta
cltv_abs += route_edge.cltv_delta
hops_data.reverse() hops_data.reverse()
return hops_data, amt, cltv_abs return hops_data, amt, cltv_abs

20
electrum/lnrouter.py
Unescape View File

@ -73,10 +73,10 @@ class PathEdge:
@attr.s @attr.s
class RouteEdge(PathEdge): class RouteEdge(PathEdge):
fee_base_msat = attr.ib(type=int, kw_only=True) fee_base_msat = attr.ib(type=int, kw_only=True) # for start_node
fee_proportional_millionths = attr.ib(type=int, kw_only=True) fee_proportional_millionths = attr.ib(type=int, kw_only=True) # for start_node
cltv_delta = attr.ib(type=int, kw_only=True) cltv_delta = attr.ib(type=int, kw_only=True) # for start_node
node_features = attr.ib(type=int, kw_only=True, repr=lambda val: str(int(val))) # note: for end node! node_features = attr.ib(type=int, kw_only=True, repr=lambda val: str(int(val))) # note: for end_node!
def fee_for_edge(self, amount_msat: int) -> int: def fee_for_edge(self, amount_msat: int) -> int:
return fee_for_edge_msat(forwarded_amount_msat=amount_msat, return fee_for_edge_msat(forwarded_amount_msat=amount_msat,
@ -87,7 +87,7 @@ class RouteEdge(PathEdge):
def from_channel_policy( def from_channel_policy(
cls, cls,
*, *,
channel_policy: 'Policy', channel_policy: 'Policy', # for start_node
short_channel_id: bytes, short_channel_id: bytes,
start_node: bytes, start_node: bytes,
end_node: bytes, end_node: bytes,
@ -138,26 +138,26 @@ LNPaymentRoute = Sequence[RouteEdge]
LNPaymentTRoute = Sequence[TrampolineEdge] LNPaymentTRoute = Sequence[TrampolineEdge]
def is_route_sane_to_use(route: LNPaymentRoute, invoice_amount_msat: int, min_final_cltv_delta: int) -> bool: def is_route_sane_to_use(route: LNPaymentRoute, *, amount_msat_for_dest: int, cltv_delta_for_dest: int) -> bool:
"""Run some sanity checks on the whole route, before attempting to use it. """Run some sanity checks on the whole route, before attempting to use it.
called when we are paying; so e.g. lower cltv is better called when we are paying; so e.g. lower cltv is better
""" """
if len(route) > NUM_MAX_EDGES_IN_PAYMENT_PATH: if len(route) > NUM_MAX_EDGES_IN_PAYMENT_PATH:
return False return False
amt = invoice_amount_msat amt = amount_msat_for_dest
cltv_delta = min_final_cltv_delta cltv_delta = cltv_delta_for_dest
for route_edge in reversed(route[1:]): for route_edge in reversed(route[1:]):
if not route_edge.is_sane_to_use(amt): return False if not route_edge.is_sane_to_use(amt): return False
amt += route_edge.fee_for_edge(amt) amt += route_edge.fee_for_edge(amt)
cltv_delta += route_edge.cltv_delta cltv_delta += route_edge.cltv_delta
total_fee = amt - invoice_amount_msat total_fee = amt - amount_msat_for_dest
# TODO revise ad-hoc heuristics # TODO revise ad-hoc heuristics
if cltv_delta > NBLOCK_CLTV_DELTA_TOO_FAR_INTO_FUTURE: if cltv_delta > NBLOCK_CLTV_DELTA_TOO_FAR_INTO_FUTURE:
return False return False
# FIXME in case of MPP, the fee checks are done independently for each part, # FIXME in case of MPP, the fee checks are done independently for each part,
# which is ok for the proportional checks but not for the absolute ones. # which is ok for the proportional checks but not for the absolute ones.
# This is not that big of a deal though as we don't split into *too many* parts. # This is not that big of a deal though as we don't split into *too many* parts.
if not is_fee_sane(total_fee, payment_amount_msat=invoice_amount_msat): if not is_fee_sane(total_fee, payment_amount_msat=amount_msat_for_dest):
return False return False
return True return True

2
electrum/lnworker.py
Unescape View File

@ -2023,7 +2023,7 @@ class LNWallet(LNWorker):
if not route: if not route:
raise NoPathFound() raise NoPathFound()
# test sanity # test sanity
if not is_route_sane_to_use(route, amount_msat, min_final_cltv_delta): if not is_route_sane_to_use(route, amount_msat_for_dest=amount_msat, cltv_delta_for_dest=min_final_cltv_delta):
self.logger.info(f"rejecting insane route {route}") self.logger.info(f"rejecting insane route {route}")
raise NoPathFound() raise NoPathFound()
assert len(route) > 0 assert len(route) > 0

56
electrum/trampoline.py
Unescape View File

@ -5,7 +5,7 @@ import random
from typing import Mapping, DefaultDict, Tuple, Optional, Dict, List, Iterable, Sequence, Set from typing import Mapping, DefaultDict, Tuple, Optional, Dict, List, Iterable, Sequence, Set
from .lnutil import LnFeatures from .lnutil import LnFeatures
from .lnonion import calc_hops_data_for_payment, new_onion_packet from .lnonion import calc_hops_data_for_payment, new_onion_packet, OnionPacket
from .lnrouter import RouteEdge, TrampolineEdge, LNPaymentRoute, is_route_sane_to_use, LNPaymentTRoute from .lnrouter import RouteEdge, TrampolineEdge, LNPaymentRoute, is_route_sane_to_use, LNPaymentTRoute
from .lnutil import NoPathFound, LNPeerAddr from .lnutil import NoPathFound, LNPeerAddr
from . import constants from . import constants
@ -168,14 +168,19 @@ def trampoline_policy(
def _extend_trampoline_route( def _extend_trampoline_route(
route: List, route: List[TrampolineEdge],
start_node: bytes, *,
start_node: bytes = None,
end_node: bytes, end_node: bytes,
trampoline_fee_level: int, trampoline_fee_level: int,
pay_fees=True pay_fees: bool = True,
): ):
"""Extends the route and modifies it in place.""" """Extends the route and modifies it in place."""
if start_node is None:
assert route
start_node = route[-1].end_node
trampoline_features = LnFeatures.VAR_ONION_OPT trampoline_features = LnFeatures.VAR_ONION_OPT
# get policy for *start_node*
policy = trampoline_policy(trampoline_fee_level) policy = trampoline_policy(trampoline_fee_level)
route.append( route.append(
TrampolineEdge( TrampolineEdge(
@ -223,17 +228,19 @@ def create_trampoline_route(
# we build a route of trampoline hops and extend the route list in place # we build a route of trampoline hops and extend the route list in place
route = [] route = []
second_trampoline = None
# our first trampoline hop is decided by the channel we use # our first trampoline hop is decided by the channel we use
_extend_trampoline_route(route, my_pubkey, my_trampoline, trampoline_fee_level) _extend_trampoline_route(
route, start_node=my_pubkey, end_node=my_trampoline,
trampoline_fee_level=trampoline_fee_level, pay_fees=False,
)
if is_legacy: if is_legacy:
# we add another different trampoline hop for privacy # we add another different trampoline hop for privacy
if use_two_trampolines: if use_two_trampolines:
trampolines = trampolines_by_id() trampolines = trampolines_by_id()
second_trampoline = _choose_second_trampoline(my_trampoline, list(trampolines.keys()), failed_routes) second_trampoline = _choose_second_trampoline(my_trampoline, list(trampolines.keys()), failed_routes)
_extend_trampoline_route(route, my_trampoline, second_trampoline, trampoline_fee_level) _extend_trampoline_route(route, end_node=second_trampoline, trampoline_fee_level=trampoline_fee_level)
# the last trampoline onion must contain routing hints for the last trampoline # the last trampoline onion must contain routing hints for the last trampoline
# node to find the recipient # node to find the recipient
invoice_routing_info = encode_routing_info(r_tags) invoice_routing_info = encode_routing_info(r_tags)
@ -255,14 +262,17 @@ def create_trampoline_route(
add_trampoline = True add_trampoline = True
if add_trampoline: if add_trampoline:
second_trampoline = _choose_second_trampoline(my_trampoline, invoice_trampolines, failed_routes) second_trampoline = _choose_second_trampoline(my_trampoline, invoice_trampolines, failed_routes)
_extend_trampoline_route(route, my_trampoline, second_trampoline, trampoline_fee_level) _extend_trampoline_route(route, end_node=second_trampoline, trampoline_fee_level=trampoline_fee_level)
# final edge (not part of the route if payment is legacy, but eclair requires an encrypted blob) # Add final edge. note: eclair requires an encrypted t-onion blob even in legacy case.
_extend_trampoline_route(route, route[-1].end_node, invoice_pubkey, trampoline_fee_level, pay_fees=False) # Also needed for fees for last TF!
_extend_trampoline_route(route, end_node=invoice_pubkey, trampoline_fee_level=trampoline_fee_level)
# check that we can pay amount and fees # check that we can pay amount and fees
for edge in route[::-1]: if not is_route_sane_to_use(
amount_msat += edge.fee_for_edge(amount_msat) route=route,
if not is_route_sane_to_use(route, amount_msat, min_final_cltv_delta): amount_msat_for_dest=amount_msat,
cltv_delta_for_dest=min_final_cltv_delta,
):
raise NoPathFound("We cannot afford to pay the fees.") raise NoPathFound("We cannot afford to pay the fees.")
return route return route
@ -275,7 +285,7 @@ def create_trampoline_onion(
total_msat: int, total_msat: int,
payment_hash: bytes, payment_hash: bytes,
payment_secret: bytes, payment_secret: bytes,
): ) -> Tuple[OnionPacket, int, int]:
# all edges are trampoline # all edges are trampoline
hops_data, amount_msat, cltv_abs = calc_hops_data_for_payment( hops_data, amount_msat, cltv_abs = calc_hops_data_for_payment(
route, route,
@ -318,20 +328,21 @@ def create_trampoline_onion(
def create_trampoline_route_and_onion( def create_trampoline_route_and_onion(
*, *,
amount_msat, amount_msat: int, # that final receiver gets
total_msat, total_msat: int,
min_final_cltv_delta: int, min_final_cltv_delta: int,
invoice_pubkey, invoice_pubkey: bytes,
invoice_features, invoice_features,
my_pubkey: bytes, my_pubkey: bytes,
node_id, node_id: bytes,
r_tags, r_tags,
payment_hash: bytes, payment_hash: bytes,
payment_secret: bytes, payment_secret: bytes,
local_height: int, local_height: int,
trampoline_fee_level: int, trampoline_fee_level: int,
use_two_trampolines: bool, use_two_trampolines: bool,
failed_routes: Iterable[Sequence[str]]): failed_routes: Iterable[Sequence[str]],
) -> Tuple[LNPaymentTRoute, OnionPacket, int, int]:
# create route for the trampoline_onion # create route for the trampoline_onion
trampoline_route = create_trampoline_route( trampoline_route = create_trampoline_route(
amount_msat=amount_msat, amount_msat=amount_msat,
@ -343,7 +354,8 @@ def create_trampoline_route_and_onion(
r_tags=r_tags, r_tags=r_tags,
trampoline_fee_level=trampoline_fee_level, trampoline_fee_level=trampoline_fee_level,
use_two_trampolines=use_two_trampolines, use_two_trampolines=use_two_trampolines,
failed_routes=failed_routes) failed_routes=failed_routes,
)
# compute onion and fees # compute onion and fees
final_cltv_abs = local_height + min_final_cltv_delta final_cltv_abs = local_height + min_final_cltv_delta
trampoline_onion, amount_with_fees, bucket_cltv_abs = create_trampoline_onion( trampoline_onion, amount_with_fees, bucket_cltv_abs = create_trampoline_onion(
@ -354,8 +366,4 @@ def create_trampoline_route_and_onion(
payment_hash=payment_hash, payment_hash=payment_hash,
payment_secret=payment_secret) payment_secret=payment_secret)
bucket_cltv_delta = bucket_cltv_abs - local_height bucket_cltv_delta = bucket_cltv_abs - local_height
bucket_cltv_delta += trampoline_route[0].cltv_delta
# trampoline fee for this very trampoline
trampoline_fee = trampoline_route[0].fee_for_edge(amount_with_fees)
amount_with_fees += trampoline_fee
return trampoline_route, trampoline_onion, amount_with_fees, bucket_cltv_delta return trampoline_route, trampoline_onion, amount_with_fees, bucket_cltv_delta

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