htlcswitch: cap fee updates to max fee allocation

In this commit, we begin to enforce a maximum channel commitment fee for
channel initiators when attempting to update their commitment fee. Now,
if the new commitment fee happens to exceed their maximum, then a fee
update of the maximum fee allocation will be proposed instead if needed.

A default of up to 50% of the channel initiator's balance is enforced
for the maximum channel commitment fee. It can be modified through the
`--max-channel-fee-allocation` CLI flag.

config.go

@@ -317,6 +317,8 @@ type config struct {
 
 	MaxOutgoingCltvExpiry uint32 `long:"max-cltv-expiry" description:"The maximum number of blocks funds could be locked up for when forwarding payments."`
 
+	MaxChannelFeeAllocation float64 `long:"max-channel-fee-allocation" description:"The maximum percentage of total funds that can be allocated to a channel's commitment fee. This only applies for the initiator of the channel. Valid values are within [0.1, 1]."`
+
 	net tor.Net
 
 	Routing *routing.Conf `group:"routing" namespace:"routing"`
@@ -432,7 +434,8 @@ func loadConfig() (*config, error) {
 		Watchtower: &lncfg.Watchtower{
 			TowerDir: defaultTowerDir,
 		},
-		MaxOutgoingCltvExpiry: htlcswitch.DefaultMaxOutgoingCltvExpiry,
+		MaxOutgoingCltvExpiry:   htlcswitch.DefaultMaxOutgoingCltvExpiry,
+		MaxChannelFeeAllocation: htlcswitch.DefaultMaxLinkFeeAllocation,
 	}
 
 	// Pre-parse the command line options to pick up an alternative config
@@ -591,6 +594,13 @@ func loadConfig() (*config, error) {
 		return nil, err
 	}
 
+	// Ensure a valid max channel fee allocation was set.
+	if cfg.MaxChannelFeeAllocation <= 0 || cfg.MaxChannelFeeAllocation > 1 {
+		return nil, fmt.Errorf("invalid max channel fee allocation: "+
+			"%v, must be within [0.1, 1]",
+			cfg.MaxChannelFeeAllocation)
+	}
+
 	// Validate the Tor config parameters.
 	socks, err := lncfg.ParseAddressString(
 		cfg.Tor.SOCKS, strconv.Itoa(defaultTorSOCKSPort),

htlcswitch/link.go

@@ -4,6 +4,7 @@ import (
 	"bytes"
 	"crypto/sha256"
 	"fmt"
+	"math"
 	prand "math/rand"
 	"sync"
 	"sync/atomic"
@@ -50,6 +51,11 @@ const (
 	// DefaultMaxLinkFeeUpdateTimeout represents the maximum interval in
 	// which a link should propose to update its commitment fee rate.
 	DefaultMaxLinkFeeUpdateTimeout = 60 * time.Minute
+
+	// DefaultMaxLinkFeeAllocation is the highest allocation we'll allow
+	// a channel's commitment fee to be of its balance. This only applies to
+	// the initiator of the channel.
+	DefaultMaxLinkFeeAllocation float64 = 0.5
 )
 
 // ForwardingPolicy describes the set of constraints that a given ChannelLink
@@ -250,6 +256,11 @@ type ChannelLinkConfig struct {
 	// accept for a forwarded HTLC. The value is relative to the current
 	// block height.
 	MaxOutgoingCltvExpiry uint32
+
+	// MaxFeeAllocation is the highest allocation we'll allow a channel's
+	// commitment fee to be of its balance. This only applies to the
+	// initiator of the channel.
+	MaxFeeAllocation float64
 }
 
 // channelLink is the service which drives a channel's commitment update
@@ -995,22 +1006,27 @@ out:
 			// If we are the initiator, then we'll sample the
 			// current fee rate to get into the chain within 3
 			// blocks.
-			feePerKw, err := l.sampleNetworkFee()
+			netFee, err := l.sampleNetworkFee()
 			if err != nil {
 				log.Errorf("unable to sample network fee: %v", err)
 				continue
 			}
 
 			// We'll check to see if we should update the fee rate
-			// based on our current set fee rate.
+			// based on our current set fee rate. We'll cap the new
+			// fee rate to our max fee allocation.
 			commitFee := l.channel.CommitFeeRate()
-			if !shouldAdjustCommitFee(feePerKw, commitFee) {
+			maxFee := l.channel.MaxFeeRate(l.cfg.MaxFeeAllocation)
+			newCommitFee := lnwallet.SatPerKWeight(
+				math.Min(float64(netFee), float64(maxFee)),
+			)
+			if !shouldAdjustCommitFee(newCommitFee, commitFee) {
 				continue
 			}
 
 			// If we do, then we'll send a new UpdateFee message to
 			// the remote party, to be locked in with a new update.
-			if err := l.updateChannelFee(feePerKw); err != nil {
+			if err := l.updateChannelFee(newCommitFee); err != nil {
 				log.Errorf("unable to update fee rate: %v", err)
 				continue
 			}

htlcswitch/link_test.go

@@ -1686,6 +1686,7 @@ func newSingleLinkTestHarness(chanAmt, chanReserve btcutil.Amount) (
 		MinFeeUpdateTimeout:   30 * time.Minute,
 		MaxFeeUpdateTimeout:   40 * time.Minute,
 		MaxOutgoingCltvExpiry: DefaultMaxOutgoingCltvExpiry,
+		MaxFeeAllocation:      DefaultMaxLinkFeeAllocation,
 	}
 
 	const startingHeight = 100
@@ -3736,9 +3737,10 @@ func TestChannelLinkUpdateCommitFee(t *testing.T) {
 	// First, we'll create our traditional three hop network. We'll only be
 	// interacting with and asserting the state of two of the end points
 	// for this test.
+	const aliceInitialBalance = btcutil.SatoshiPerBitcoin * 3
 	channels, cleanUp, _, err := createClusterChannels(
-		btcutil.SatoshiPerBitcoin*3,
-		btcutil.SatoshiPerBitcoin*5)
+		aliceInitialBalance, btcutil.SatoshiPerBitcoin*5,
+	)
 	if err != nil {
 		t.Fatalf("unable to create channel: %v", err)
 	}
@@ -3757,8 +3759,15 @@ func TestChannelLinkUpdateCommitFee(t *testing.T) {
 		{"alice", "bob", &lnwire.FundingLocked{}, false},
 		{"bob", "alice", &lnwire.FundingLocked{}, false},
 
+		// First fee update.
 		{"alice", "bob", &lnwire.UpdateFee{}, false},
+		{"alice", "bob", &lnwire.CommitSig{}, false},
+		{"bob", "alice", &lnwire.RevokeAndAck{}, false},
+		{"bob", "alice", &lnwire.CommitSig{}, false},
+		{"alice", "bob", &lnwire.RevokeAndAck{}, false},
 
+		// Second fee update.
+		{"alice", "bob", &lnwire.UpdateFee{}, false},
 		{"alice", "bob", &lnwire.CommitSig{}, false},
 		{"bob", "alice", &lnwire.RevokeAndAck{}, false},
 		{"bob", "alice", &lnwire.CommitSig{}, false},
@@ -3779,7 +3788,7 @@ func TestChannelLinkUpdateCommitFee(t *testing.T) {
 
 	// triggerFeeUpdate is a helper closure to determine whether a fee
 	// update was triggered and completed properly.
-	triggerFeeUpdate := func(newFeeRate lnwallet.SatPerKWeight,
+	triggerFeeUpdate := func(feeEstimate, newFeeRate lnwallet.SatPerKWeight,
 		shouldUpdate bool) {
 
 		t.Helper()
@@ -3795,7 +3804,7 @@ func TestChannelLinkUpdateCommitFee(t *testing.T) {
 
 		// Next, we'll send the first fee rate response to Alice.
 		select {
-		case n.feeEstimator.byteFeeIn <- newFeeRate:
+		case n.feeEstimator.byteFeeIn <- feeEstimate:
 		case <-time.After(time.Second * 5):
 			t.Fatalf("alice didn't query for the new network fee")
 		}
@@ -3830,11 +3839,20 @@ func TestChannelLinkUpdateCommitFee(t *testing.T) {
 
 	// Triggering the link to update the fee of the channel with the same
 	// fee rate should not send a fee update.
-	triggerFeeUpdate(startingFeeRate, false)
+	triggerFeeUpdate(startingFeeRate, startingFeeRate, false)
 
 	// Triggering the link to update the fee of the channel with a much
 	// larger fee rate _should_ send a fee update.
-	triggerFeeUpdate(startingFeeRate*3, true)
+	newFeeRate := startingFeeRate * 3
+	triggerFeeUpdate(newFeeRate, newFeeRate, true)
+
+	// Triggering the link to update the fee of the channel with a fee rate
+	// that exceeds its maximum fee allocation should result in a fee rate
+	// corresponding to the maximum fee allocation.
+	maxFeeRate := channels.aliceToBob.MaxFeeRate(
+		n.aliceChannelLink.cfg.MaxFeeAllocation,
+	)
+	triggerFeeUpdate(aliceInitialBalance, maxFeeRate, true)
 }
 
 // TestChannelLinkAcceptDuplicatePayment tests that if a link receives an
@@ -4236,6 +4254,7 @@ func (h *persistentLinkHarness) restartLink(
 		// Set any hodl flags requested for the new link.
 		HodlMask:              hodl.MaskFromFlags(hodlFlags...),
 		MaxOutgoingCltvExpiry: DefaultMaxOutgoingCltvExpiry,
+		MaxFeeAllocation:      DefaultMaxLinkFeeAllocation,
 	}
 
 	const startingHeight = 100

htlcswitch/test_utils.go

@@ -1116,6 +1116,7 @@ func (h *hopNetwork) createChannelLink(server, peer *mockServer,
 			OnChannelFailure:        func(lnwire.ChannelID, lnwire.ShortChannelID, LinkFailureError) {},
 			OutgoingCltvRejectDelta: 3,
 			MaxOutgoingCltvExpiry:   DefaultMaxOutgoingCltvExpiry,
+			MaxFeeAllocation:        DefaultMaxLinkFeeAllocation,
 		},
 		channel,
 	)

peer.go

@@ -585,6 +585,7 @@ func (p *peer) addLink(chanPoint *wire.OutPoint,
 		OutgoingCltvRejectDelta: p.outgoingCltvRejectDelta,
 		TowerClient:             p.server.towerClient,
 		MaxOutgoingCltvExpiry:   cfg.MaxOutgoingCltvExpiry,
+		MaxFeeAllocation:        cfg.MaxChannelFeeAllocation,
 	}
 
 	link := htlcswitch.NewChannelLink(linkCfg, lnChan)