Add support for zero-fee commitment format

docs/release-notes/eclair-vnext.md

@@ -64,6 +64,26 @@ To open a taproot channel with a node that supports the `option_simple_taproot`
 $ eclair-cli open --nodeId=<node_id> --fundingSatoshis=<funding_amount> --channelType=simple_taproot_channel --announceChannel=false
 ```
 
+### Zero-fee Channels (experimental)
+
+This release adds support for zero-fee commitments (using v3/TRUC transactions), as specified in [the BOLTs](https://github.com/lightning/bolts/pull/1228).
+With this new type of channels, commitment transactions don't pay any mining fee (in most cases), which gets rid of the `update_fee` mechanism and all of the related channel reserve issues.
+It also gets rid of the undesired channel force-closes that happen when the mempool feerate spikes and channel participants disagree on what feerate to use, which has been a major source of wasted on-chain space.
+It also offers better protection against pinning attacks (thanks to package relay) and reduces the on-chain footprint compared to anchor output channels.
+
+This feature is not actived by default, because we haven't tested yet that v3/TRUC transactions always propagate to miners.
+To enable it, add the following to your `eclair.conf`:
+
+```conf
+eclair.features.zero_fee_commitments = optional
+```
+
+To open a zero-fee channel with a node that supports the `zero_fee_commitments` feature, use the following command:
+
+```sh
+$ eclair-cli open --nodeId=<node_id> --fundingSatoshis=<funding_amount> --channelType=zero_fee_commitments
+```
+
 ### Remove support for non-anchor channels
 
 We remove the code used to support legacy channels that don't use anchor outputs or taproot.

eclair-core/src/main/resources/reference.conf

@@ -76,6 +76,7 @@ eclair {
     option_quiesce = optional
     option_attribution_data = optional
     option_onion_messages = optional
+    zero_fee_commitments = disabled
     // This feature should only be enabled when acting as an LSP for mobile wallets.
     // When activating this feature, the peer-storage section should be customized to match desired SLAs.
     option_provide_storage = disabled

eclair-core/src/main/scala/fr/acinq/eclair/Features.scala

@@ -359,6 +359,11 @@ object Features {
     val mandatory = 38
   }
 
+  case object ZeroFeeCommitments extends Feature with InitFeature with NodeFeature with ChannelTypeFeature {
+    val rfcName = "zero_fee_commitments"
+    val mandatory = 40
+  }
+
   case object ProvideStorage extends Feature with InitFeature with NodeFeature {
     val rfcName = "option_provide_storage"
     val mandatory = 42
@@ -479,6 +484,7 @@ object Features {
     Quiescence,
     AttributionData,
     OnionMessages,
+    ZeroFeeCommitments,
     ProvideStorage,
     ChannelType,
     ScidAlias,

eclair-core/src/main/scala/fr/acinq/eclair/NodeParams.scala

@@ -21,9 +21,9 @@ import fr.acinq.bitcoin.scalacompat.Crypto.PublicKey
 import fr.acinq.bitcoin.scalacompat.{Block, BlockHash, Crypto, Satoshi, SatoshiLong}
 import fr.acinq.eclair.Setup.Seeds
 import fr.acinq.eclair.blockchain.fee._
-import fr.acinq.eclair.channel.ChannelFlags
 import fr.acinq.eclair.channel.fsm.Channel
 import fr.acinq.eclair.channel.fsm.Channel.{BalanceThreshold, ChannelConf, UnhandledExceptionStrategy}
+import fr.acinq.eclair.channel.{ChannelFlags, ChannelTypes}
 import fr.acinq.eclair.crypto.Noise.KeyPair
 import fr.acinq.eclair.crypto.keymanager.{ChannelKeyManager, NodeKeyManager, OnChainKeyManager}
 import fr.acinq.eclair.db._
@@ -364,7 +364,7 @@ object NodeParams extends Logging {
     require(htlcMinimum > 0.msat, "channel.htlc-minimum-msat must be strictly greater than 0")
 
     val maxAcceptedHtlcs = config.getInt("channel.max-accepted-htlcs")
-    require(maxAcceptedHtlcs <= Channel.MAX_ACCEPTED_HTLCS, s"channel.max-accepted-htlcs must be lower than ${Channel.MAX_ACCEPTED_HTLCS}")
+    require(maxAcceptedHtlcs <= Channel.maxAcceptedHtlcs(ChannelTypes.AnchorOutputsZeroFeeHtlcTx()), s"channel.max-accepted-htlcs must be lower than ${Channel.maxAcceptedHtlcs(ChannelTypes.AnchorOutputsZeroFeeHtlcTx())}")
 
     val maxToLocalCLTV = CltvExpiryDelta(config.getInt("channel.max-to-local-delay-blocks"))
     val offeredCLTV = CltvExpiryDelta(config.getInt("channel.to-remote-delay-blocks"))

eclair-core/src/main/scala/fr/acinq/eclair/blockchain/fee/OnChainFeeConf.scala

@@ -93,6 +93,9 @@ case class OnChainFeeConf(feeTargets: FeeTargets,
       case Transactions.ZeroFeeHtlcTxAnchorOutputsCommitmentFormat | Transactions.ZeroFeeHtlcTxSimpleTaprootChannelCommitmentFormat =>
         // If the fee has a large enough change, we update the fee.
         currentFeeratePerKw.toLong == 0 || Math.abs((currentFeeratePerKw.toLong - nextFeeratePerKw.toLong).toDouble / currentFeeratePerKw.toLong) > updateFeeMinDiffRatio
+      case Transactions.ZeroFeeCommitmentFormat =>
+        // We never send update_fee when using zero-fee commitments.
+        false
     }
   }
 
@@ -111,6 +114,7 @@ case class OnChainFeeConf(feeTargets: FeeTargets,
     val networkFeerate = feerates.fast
     val networkMinFee = feerates.minimum
     commitmentFormat match {
+      case Transactions.ZeroFeeCommitmentFormat => FeeratePerKw(0 sat)
       case Transactions.UnsafeLegacyAnchorOutputsCommitmentFormat | Transactions.PhoenixSimpleTaprootChannelCommitmentFormat =>
         // Since Bitcoin Core v28, 1-parent-1-child package relay has been deployed: it should be ok if the commit tx
         // doesn't propagate on its own.

eclair-core/src/main/scala/fr/acinq/eclair/channel/ChannelFeatures.scala

@@ -104,6 +104,15 @@ object ChannelTypes {
     override def commitmentFormat: CommitmentFormat = ZeroFeeHtlcTxSimpleTaprootChannelCommitmentFormat
     override def toString: String = s"simple_taproot_channel${if (scidAlias) "+scid_alias" else ""}${if (zeroConf) "+zeroconf" else ""}"
   }
+  case class ZeroFeeCommitments(scidAlias: Boolean = false, zeroConf: Boolean = false) extends SupportedChannelType {
+    override def features: Set[ChannelTypeFeature] = Set(
+      if (scidAlias) Some(Features.ScidAlias) else None,
+      if (zeroConf) Some(Features.ZeroConf) else None,
+      Some(Features.ZeroFeeCommitments)
+    ).flatten
+    override def commitmentFormat: CommitmentFormat = ZeroFeeCommitmentFormat
+    override def toString: String = s"zero_fee_commitments${if (scidAlias) "+scid_alias" else ""}${if (zeroConf) "+zeroconf" else ""}"
+  }
 
   case class UnsupportedChannelType(featureBits: Features[InitFeature]) extends ChannelType {
     override def features: Set[InitFeature] = featureBits.activated.keySet
@@ -131,6 +140,10 @@ object ChannelTypes {
     SimpleTaprootChannel(zeroConf = true),
     SimpleTaprootChannel(scidAlias = true),
     SimpleTaprootChannel(scidAlias = true, zeroConf = true),
+    ZeroFeeCommitments(),
+    ZeroFeeCommitments(zeroConf = true),
+    ZeroFeeCommitments(scidAlias = true),
+    ZeroFeeCommitments(scidAlias = true, zeroConf = true),
     SimpleTaprootChannelsPhoenix,
   ).map {
     channelType => Features(channelType.features.map(_ -> FeatureSupport.Mandatory).toMap) -> channelType
@@ -154,6 +167,8 @@ object ChannelTypes {
     if (!announceChannel && Features.canUseFeature(localFeatures, remoteFeatures, Features.SimpleTaprootChannels)) {
       // We currently only support unannounced taproot channels.
       Some(SimpleTaprootChannel(scidAlias = useScidAlias))
+    } else if (Features.canUseFeature(localFeatures, remoteFeatures, Features.ZeroFeeCommitments)) {
+      Some(ZeroFeeCommitments(scidAlias = useScidAlias))
     } else if (Features.canUseFeature(localFeatures, remoteFeatures, Features.AnchorOutputsZeroFeeHtlcTx)) {
       Some(AnchorOutputsZeroFeeHtlcTx(scidAlias = useScidAlias))
     } else {

eclair-core/src/main/scala/fr/acinq/eclair/channel/Commitments.scala

@@ -740,8 +740,8 @@ object Commitment {
                    commitmentInput: InputInfo,
                    commitmentFormat: CommitmentFormat,
                    spec: CommitmentSpec): (CommitTx, Seq[UnsignedHtlcTx]) = {
-    val outputs = makeCommitTxOutputs(localFundingKey.publicKey, remoteFundingPubKey, commitKeys.publicKeys, channelParams.localParams.paysCommitTxFees, commitParams.dustLimit, commitParams.toSelfDelay, spec, commitmentFormat)
-    val commitTx = makeCommitTx(commitmentInput, commitTxNumber, commitKeys.ourPaymentBasePoint, channelParams.remoteParams.paymentBasepoint, channelParams.localParams.isChannelOpener, outputs)
+    val outputs = makeCommitTxOutputs(localFundingKey.publicKey, remoteFundingPubKey, commitmentInput, commitKeys.publicKeys, channelParams.localParams.paysCommitTxFees, commitParams.dustLimit, commitParams.toSelfDelay, spec, commitmentFormat)
+    val commitTx = makeCommitTx(commitmentInput, commitTxNumber, commitKeys.ourPaymentBasePoint, channelParams.remoteParams.paymentBasepoint, channelParams.localParams.isChannelOpener, commitmentFormat, outputs)
     val htlcTxs = makeHtlcTxs(commitTx.tx, outputs, commitmentFormat)
     (commitTx, htlcTxs)
   }
@@ -755,8 +755,8 @@ object Commitment {
                     commitmentInput: InputInfo,
                     commitmentFormat: CommitmentFormat,
                     spec: CommitmentSpec): (CommitTx, Seq[UnsignedHtlcTx]) = {
-    val outputs = makeCommitTxOutputs(remoteFundingPubKey, localFundingKey.publicKey, commitKeys.publicKeys, !channelParams.localParams.paysCommitTxFees, commitParams.dustLimit, commitParams.toSelfDelay, spec, commitmentFormat)
-    val commitTx = makeCommitTx(commitmentInput, commitTxNumber, channelParams.remoteParams.paymentBasepoint, commitKeys.ourPaymentBasePoint, !channelParams.localParams.isChannelOpener, outputs)
+    val outputs = makeCommitTxOutputs(remoteFundingPubKey, localFundingKey.publicKey, commitmentInput, commitKeys.publicKeys, !channelParams.localParams.paysCommitTxFees, commitParams.dustLimit, commitParams.toSelfDelay, spec, commitmentFormat)
+    val commitTx = makeCommitTx(commitmentInput, commitTxNumber, channelParams.remoteParams.paymentBasepoint, commitKeys.ourPaymentBasePoint, !channelParams.localParams.isChannelOpener, commitmentFormat, outputs)
     val htlcTxs = makeHtlcTxs(commitTx.tx, outputs, commitmentFormat)
     (commitTx, htlcTxs)
   }