closingd: configurable closing fee negotiation step

When negotiating the transaction fee for closing a channel [1], we used
to always pick the middle of the range between our proposal and the
peer's proposal.

Introduce a new option `fee_negotiation_step` to the close command, so
the peer who initiates the close can choose his back off step.

Partially resolves #3270

[1] https://github.com/lightningnetwork/lightning-rfc/blob/master/02-peer-protocol.md#closing-negotiation-closing_signed

Changelog-Added: New optional parameter to the `close` command to control the closing transaction fee negotiation back off step

closingd/closing_wire.csv

@@ -21,6 +21,8 @@ msgdata,closing_init,local_scriptpubkey_len,u16,
 msgdata,closing_init,local_scriptpubkey,u8,local_scriptpubkey_len
 msgdata,closing_init,remote_scriptpubkey_len,u16,
 msgdata,closing_init,remote_scriptpubkey,u8,remote_scriptpubkey_len
+msgdata,closing_init,fee_negotiation_step,u64,
+msgdata,closing_init,fee_negotiation_step_unit,u8,
 msgdata,closing_init,reconnected,bool,
 msgdata,closing_init,next_index_local,u64,
 msgdata,closing_init,next_index_remote,u64,

closingd/closingd.c

@@ -2,6 +2,7 @@
 #include <ccan/fdpass/fdpass.h>
 #include <closingd/gen_closing_wire.h>
 #include <common/close_tx.h>
+#include <common/closing_fee.h>
 #include <common/crypto_sync.h>
 #include <common/derive_basepoints.h>
 #include <common/htlc.h>
@@ -489,13 +490,14 @@ static void adjust_feerange(struct feerange *feerange,
 }
 
 /* Figure out what we should offer now. */
-static struct amount_sat adjust_offer(struct per_peer_state *pps,
-				      const struct channel_id *channel_id,
-				      const struct feerange *feerange,
-				      struct amount_sat remote_offer,
-				      struct amount_sat min_fee_to_accept)
+static struct amount_sat
+adjust_offer(struct per_peer_state *pps, const struct channel_id *channel_id,
+	     const struct feerange *feerange, struct amount_sat remote_offer,
+	     struct amount_sat min_fee_to_accept, u64 fee_negotiation_step,
+	     u8 fee_negotiation_step_unit)
 {
-	struct amount_sat min_plus_one, avg;
+	struct amount_sat min_plus_one, range_len, step_sat, result;
+	struct amount_msat step_msat;
 
 	/* Within 1 satoshi?  Agree. */
 	if (!amount_sat_add(&min_plus_one, feerange->min, AMOUNT_SAT(1)))
@@ -507,8 +509,15 @@ static struct amount_sat adjust_offer(struct per_peer_state *pps,
 	if (amount_sat_greater_eq(min_plus_one, feerange->max))
 		return remote_offer;
 
+	/* feerange has already been adjusted so that our new offer is ok to be
+	 * any number in [feerange->min, feerange->max] and after the following
+	 * min_fee_to_accept is in that range. Thus, pick a fee in
+	 * [min_fee_to_accept, feerange->max]. */
+	if (amount_sat_greater(feerange->min, min_fee_to_accept))
+		min_fee_to_accept = feerange->min;
+
 	/* Max is below our minimum acceptable? */
-	if (amount_sat_less(feerange->max, min_fee_to_accept))
+	if (!amount_sat_sub(&range_len, feerange->max, min_fee_to_accept))
 		peer_failed(pps, channel_id,
 			    "Feerange %s-%s"
 			    " below minimum acceptable %s",
@@ -519,18 +528,44 @@ static struct amount_sat adjust_offer(struct per_peer_state *pps,
 			    type_to_string(tmpctx, struct amount_sat,
 					   &min_fee_to_accept));
 
-	/* Bisect between our minimum and max. */
-	if (amount_sat_greater(feerange->min, min_fee_to_accept))
-		min_fee_to_accept = feerange->min;
+	if (fee_negotiation_step_unit ==
+	    CLOSING_FEE_NEGOTIATION_STEP_UNIT_SATOSHI) {
+		/* -1 because the range boundary has already been adjusted with
+		 * one from our previous proposal. So, if the user requested a
+		 * step of 1 satoshi at a time we should just return our end of
+		 * the range from this function. */
+		amount_msat_from_u64(&step_msat,
+				     (fee_negotiation_step - 1) * MSAT_PER_SAT);
+	} else {
+		/* fee_negotiation_step is e.g. 20 to designate 20% from
+		 * range_len (which is in satoshi), so:
+		 * range_len * fee_negotiation_step / 100 [sat]
+		 * is equivalent to:
+		 * range_len * fee_negotiation_step * 10 [msat] */
+		amount_msat_from_u64(&step_msat,
+				     range_len.satoshis /* Raw: % calc */ *
+					 fee_negotiation_step * 10);
+	}
 
-	if (!amount_sat_add(&avg, feerange->max, min_fee_to_accept))
-		peer_failed(pps, channel_id,
-			    "Fee offer %s max too large",
-			    type_to_string(tmpctx, struct amount_sat,
-					   &feerange->max));
+	step_sat = amount_msat_to_sat_round_down(step_msat);
+
+	if (feerange->higher_side == LOCAL) {
+		if (!amount_sat_sub(&result, feerange->max, step_sat))
+			/* step_sat > feerange->max, unlikely */
+			return min_fee_to_accept;
+
+		if (amount_sat_less_eq(result, min_fee_to_accept))
+			return min_fee_to_accept;
+	} else {
+		if (!amount_sat_add(&result, min_fee_to_accept, step_sat))
+			/* overflow, unlikely */
+			return feerange->max;
 
-	avg.satoshis /= 2; /* Raw: average calculation */
-	return avg;
+		if (amount_sat_greater_eq(result, feerange->max))
+			return feerange->max;
+	}
+
+	return result;
 }
 
 #if DEVELOPER
@@ -570,6 +605,9 @@ int main(int argc, char *argv[])
 	struct feerange feerange;
 	enum side funder;
 	u8 *scriptpubkey[NUM_SIDES], *funding_wscript;
+	u64 fee_negotiation_step;
+	u8 fee_negotiation_step_unit;
+	char fee_negotiation_step_str[32]; /* fee_negotiation_step + "sat" */
 	struct channel_id channel_id;
 	bool reconnected;
 	u64 next_index[NUM_SIDES], revocations_received;
@@ -597,6 +635,8 @@ int main(int argc, char *argv[])
 				   &offer[LOCAL],
 				   &scriptpubkey[LOCAL],
 				   &scriptpubkey[REMOTE],
+				   &fee_negotiation_step,
+				   &fee_negotiation_step_unit,
 				   &reconnected,
 				   &next_index[LOCAL],
 				   &next_index[REMOTE],
@@ -609,13 +649,21 @@ int main(int argc, char *argv[])
 	/* stdin == requests, 3 == peer, 4 = gossip, 5 = gossip_store, 6 = hsmd */
 	per_peer_state_set_fds(pps, 3, 4, 5);
 
+	snprintf(fee_negotiation_step_str, sizeof(fee_negotiation_step_str),
+		 "%" PRIu64 "%s", fee_negotiation_step,
+		 fee_negotiation_step_unit ==
+			 CLOSING_FEE_NEGOTIATION_STEP_UNIT_PERCENTAGE
+		     ? "%"
+		     : "sat");
+
 	status_debug("out = %s/%s",
 		     type_to_string(tmpctx, struct amount_sat, &out[LOCAL]),
 		     type_to_string(tmpctx, struct amount_sat, &out[REMOTE]));
 	status_debug("dustlimit = %s",
 		     type_to_string(tmpctx, struct amount_sat, &our_dust_limit));
 	status_debug("fee = %s",
 		     type_to_string(tmpctx, struct amount_sat, &offer[LOCAL]));
+	status_debug("fee negotiation step = %s", fee_negotiation_step_str);
 	derive_channel_id(&channel_id, &funding_txid, funding_txout);
 
 	funding_wscript = bitcoin_redeem_2of2(ctx,
@@ -628,13 +676,14 @@ int main(int argc, char *argv[])
 			     channel_reestablish, scriptpubkey[LOCAL],
 			     &last_remote_per_commit_secret);
 
-	peer_billboard(true, "Negotiating closing fee between %s"
-		       " and %s satoshi (ideal %s)",
-		       type_to_string(tmpctx, struct amount_sat,
-				      &min_fee_to_accept),
-		       type_to_string(tmpctx, struct amount_sat,
-				      &commitment_fee),
-		       type_to_string(tmpctx, struct amount_sat, &offer[LOCAL]));
+	peer_billboard(
+	    true,
+	    "Negotiating closing fee between %s and %s satoshi (ideal %s) "
+	    "using step %s",
+	    type_to_string(tmpctx, struct amount_sat, &min_fee_to_accept),
+	    type_to_string(tmpctx, struct amount_sat, &commitment_fee),
+	    type_to_string(tmpctx, struct amount_sat, &offer[LOCAL]),
+	    fee_negotiation_step_str);
 
 	/* BOLT #2:
 	 *
@@ -692,7 +741,9 @@ int main(int argc, char *argv[])
 			offer[LOCAL] = adjust_offer(pps,
 						    &channel_id,
 						    &feerange, offer[REMOTE],
-						    min_fee_to_accept);
+						    min_fee_to_accept,
+						    fee_negotiation_step,
+						    fee_negotiation_step_unit);
 			send_offer(pps, chainparams, &channel_id,
 				   funding_pubkey,
 				   scriptpubkey, &funding_txid, funding_txout,

common/Makefile

@@ -73,6 +73,7 @@ COMMON_SRC_NOGEN :=				\
 COMMON_SRC_GEN := common/gen_status_wire.c common/gen_peer_status_wire.c
 
 COMMON_HEADERS_NOGEN := $(COMMON_SRC_NOGEN:.c=.h)	\
+	common/closing_fee.h				\
 	common/ecdh.h					\
 	common/errcode.h				\
 	common/gossip_constants.h			\

common/closing_fee.h

@@ -0,0 +1,16 @@
+#ifndef LIGHTNING_COMMON_CLOSING_FEE_H
+#define LIGHTNING_COMMON_CLOSING_FEE_H
+
+#include "config.h"
+
+#include <ccan/short_types/short_types.h>
+
+/** During closing fee negotiation give up N% of the range between our
+ * proposal and the peer's proposal on each step. */
+static const u8 CLOSING_FEE_NEGOTIATION_STEP_UNIT_PERCENTAGE = 0;
+
+/** During closing fee negotiation give up N satoshi of the range between our
+ * proposal and the peer's proposal on each step. */
+static const u8 CLOSING_FEE_NEGOTIATION_STEP_UNIT_SATOSHI = 1;
+
+#endif /* LIGHTNING_COMMON_CLOSING_FEE_H */

contrib/pyln-client/pyln/client/lightning.py

@@ -412,11 +412,12 @@ def close(self, peer_id, *args, **kwargs):
                 if args[0] is None and isinstance(args[1], int):
                     return self._deprecated_close(peer_id, *args, **kwargs)
 
-        def _close(peer_id, unilateraltimeout=None, destination=None):
+        def _close(peer_id, unilateraltimeout=None, destination=None, fee_negotiation_step=None):
             payload = {
                 "id": peer_id,
                 "unilateraltimeout": unilateraltimeout,
-                "destination": destination
+                "destination": destination,
+                "fee_negotiation_step": fee_negotiation_step
             }
             return self.call("close", payload)
 

doc/lightning-close.7.md

@@ -4,7 +4,7 @@ lightning-close -- Command for closing channels with direct peers
 SYNOPSIS
 --------
 
-**close** *id* \[*unilateraltimeout*\] \[*destination*\]
+**close** *id* \[*unilateraltimeout*\] \[*destination*\] \[*fee_negotiation_step*\]
 
 DESCRIPTION
 -----------
@@ -28,6 +28,22 @@ The default is 2 days (172800 seconds).
 The *destination* can be of any Bitcoin accepted type, including bech32.
 If it isn't specified, the default is a c-lightning wallet address.
 
+The *fee_negotiation_step* parameter controls how closing fee
+negotiation is performed assuming the peer proposes a fee that is
+different than our estimate. On every negotiation step we must give up
+some amount from our proposal towards the peer's proposal. This parameter
+can be an integer in which case it is interpreted as number of satoshis
+to step at a time. Or it can be an integer followed by "%s" to designate
+a percentage of the interval to give up. A few examples, assuming the peer
+proposes a closing fee of 3000 satoshi and our estimate shows it must be 4000:
+* "10": our next proposal will be 4000-10=3990.
+* "10%": our next proposal will be 4000-(10% of (4000-3000))=3900.
+* "1": our next proposal will be 3999. This is the most extreme case when we
+insist on our fee as much as possible.
+* "100%": our next proposal will be 3000. This is the most relaxed case when
+we quickly accept the peer's proposal.
+The default is "50%".
+
 The peer needs to be live and connected in order to negotiate a mutual
 close. The default of unilaterally closing after 48 hours is usually a
 reasonable indication that you can no longer contact the peer.

lightningd/channel.c

@@ -1,6 +1,7 @@
 #include <bitcoin/script.h>
 #include <ccan/crypto/hkdf_sha256/hkdf_sha256.h>
 #include <ccan/tal/str/str.h>
+#include <common/closing_fee.h>
 #include <common/fee_states.h>
 #include <common/json_command.h>
 #include <common/jsonrpc_errors.h>
@@ -242,6 +243,9 @@ struct channel *new_channel(struct peer *peer, u64 dbid,
 	channel->shutdown_scriptpubkey[REMOTE]
 		= tal_steal(channel, remote_shutdown_scriptpubkey);
 	channel->final_key_idx = final_key_idx;
+	channel->closing_fee_negotiation_step = 50;
+	channel->closing_fee_negotiation_step_unit
+		= CLOSING_FEE_NEGOTIATION_STEP_UNIT_PERCENTAGE;
 	if (local_shutdown_scriptpubkey)
 		channel->shutdown_scriptpubkey[LOCAL]
 			= tal_steal(channel, local_shutdown_scriptpubkey);

lightningd/channel.h

@@ -94,6 +94,12 @@ struct channel {
 	/* Address for any final outputs */
 	u64 final_key_idx;
 
+	/* Amount to give up on each step of the closing fee negotiation. */
+	u64 closing_fee_negotiation_step;
+
+	/* Whether closing_fee_negotiation_step is in satoshi or %. */
+	u8 closing_fee_negotiation_step_unit;
+
 	/* Reestablishment stuff: last sent commit and revocation details. */
 	bool last_was_revoke;
 	struct changed_htlc *last_sent_commit;

lightningd/closing_control.c

@@ -290,6 +290,8 @@ void peer_start_closingd(struct channel *channel,
 				      minfee, feelimit, startfee,
 				      channel->shutdown_scriptpubkey[LOCAL],
 				      channel->shutdown_scriptpubkey[REMOTE],
+				      channel->closing_fee_negotiation_step,
+				      channel->closing_fee_negotiation_step_unit,
 				      reconnected,
 				      channel->next_index[LOCAL],
 				      channel->next_index[REMOTE],