lightningd: new option for htlc dust limit

To reduce the surface area of amount of a channel balance that can be
eaten up as htlc dust, we introduce a new config
'--max-dust-htlc-exposure-msat', which sets the max amount that any
channel's balance can be added as dust

Changelog-Added: config: new option --max-dust-htlc-exposure-msat, which limits the total amount of sats to be allowed as dust on a channel

Author: niftynei

channeld/test/run-full_channel.c

@@ -406,6 +406,8 @@ int main(int argc, const char *argv[])
 
 	local_config->max_htlc_value_in_flight = AMOUNT_MSAT(-1ULL);
 	remote_config->max_htlc_value_in_flight = AMOUNT_MSAT(-1ULL);
+	local_config->max_dust_htlc_exposure_msat = AMOUNT_MSAT(-1ULL);
+	remote_config->max_dust_htlc_exposure_msat = AMOUNT_MSAT(-1ULL);
 	local_config->channel_reserve = AMOUNT_SAT(0);
 	remote_config->channel_reserve = AMOUNT_SAT(0);
 	local_config->htlc_minimum = AMOUNT_MSAT(0);

common/channel_config.c

@@ -9,6 +9,7 @@ void towire_channel_config(u8 **pptr, const struct channel_config *config)
 	towire_amount_msat(pptr, config->htlc_minimum);
 	towire_u16(pptr, config->to_self_delay);
 	towire_u16(pptr, config->max_accepted_htlcs);
+	towire_amount_msat(pptr, config->max_dust_htlc_exposure_msat);
 }
 
 void fromwire_channel_config(const u8 **ptr, size_t *max,
@@ -20,4 +21,5 @@ void fromwire_channel_config(const u8 **ptr, size_t *max,
 	config->htlc_minimum = fromwire_amount_msat(ptr, max);
 	config->to_self_delay = fromwire_u16(ptr, max);
 	config->max_accepted_htlcs = fromwire_u16(ptr, max);
+	config->max_dust_htlc_exposure_msat = fromwire_amount_msat(ptr, max);
 }

common/channel_config.h

@@ -71,6 +71,12 @@ struct channel_config {
 	 * similarly, `max_accepted_htlcs` limits the number of outstanding
 	 * HTLCs the other node can offer. */
 	u16 max_accepted_htlcs;
+
+	/* BOLT-TBD #X
+	 *
+	 * maximum dust exposure allowed for this channel
+	 */
+	struct amount_msat max_dust_htlc_exposure_msat;
 };
 
 void towire_channel_config(u8 **pptr, const struct channel_config *config);

devtools/mkcommit.c

@@ -149,6 +149,7 @@ static int parse_config(char *argv[],
 	config->max_htlc_value_in_flight = AMOUNT_MSAT(-1ULL);
 	config->htlc_minimum = AMOUNT_MSAT(0);
 	config->max_accepted_htlcs = 483;
+	config->max_dust_htlc_exposure_msat = AMOUNT_MSAT(-1ULL);
 
 	config->to_self_delay = atoi(argv[argnum]);
 	argnum++;

doc/lightning-listconfigs.7.md

@@ -71,6 +71,7 @@ On success, an object is returned, containing:
 - **rescan** (integer, optional): `rescan` field from config or cmdline, or default
 - **fee-per-satoshi** (u32, optional): `fee-per-satoshi` field from config or cmdline, or default
 - **max-concurrent-htlcs** (u32, optional): `max-concurrent-htlcs` field from config or cmdline, or default
+- **max-dust-htlc-exposure-msat** (msat, optional): `max-dust-htlc-exposure-mast` field from config or cmdline, or default
 - **min-capacity-sat** (u64, optional): `min-capacity-sat` field from config or cmdline, or default
 - **addr** (string, optional): `addr` field from config or cmdline (can be more than one)
 - **announce-addr** (string, optional): `announce-addr` field from config or cmdline (can be more than one)
@@ -206,4 +207,4 @@ RESOURCES
 ---------
 
 Main web site: <https://github.com/ElementsProject/lightning>
-[comment]: # ( SHA256STAMP:47c067588120e0f9a71206313685cebb2a8c515e9b04b688b202d2772c8f8146)
+[comment]: # ( SHA256STAMP:71a911b67203f75e7c1f717be611f505713fce4e8113fc4a84c89bc50730d2bf)

doc/schemas/listconfigs.schema.json

@@ -183,6 +183,10 @@
 	    "type": "u32",
 	    "description": "`max-concurrent-htlcs` field from config or cmdline, or default"
 	},
+	"max-dust-htlc-exposure-msat": {
+	    "type": "msat",
+	    "description": "`max-dust-htlc-exposure-mast` field from config or cmdline, or default"
+	},
 	"min-capacity-sat": {
 	    "type": "u64",
 	    "description": "`min-capacity-sat` field from config or cmdline, or default"

lightningd/lightningd.h

@@ -7,6 +7,8 @@
 #include <sys/stat.h>
 #include <wallet/wallet.h>
 
+struct amount_msat;
+
 /* Various adjustable things. */
 struct config {
 	/* How long do we want them to lock up their funds? (blocks) */
@@ -31,6 +33,9 @@ struct config {
 	/* htlcs per channel */
 	u32 max_concurrent_htlcs;
 
+	/* Max amount of dust allowed per channel */
+	struct amount_msat max_dust_htlc_exposure_msat;
+
 	/* How long between changing commit and sending COMMIT message. */
 	u32 commit_time_ms;
 

lightningd/opening_common.c

@@ -132,6 +132,9 @@ void channel_config(struct lightningd *ld,
 	ours->dust_limit = chainparams->dust_limit;
 	ours->max_htlc_value_in_flight = AMOUNT_MSAT(UINT64_MAX);
 
+	ours->max_dust_htlc_exposure_msat
+		= ld->config.max_dust_htlc_exposure_msat;
+
 	/* Don't care */
 	ours->htlc_minimum = AMOUNT_MSAT(0);
 

lightningd/options.c

@@ -10,6 +10,7 @@
 #include <common/features.h>
 #include <common/hsm_encryption.h>
 #include <common/json_command.h>
+#include <common/json_helpers.h>
 #include <common/json_tok.h>
 #include <common/param.h>
 #include <common/type_to_string.h>
@@ -671,6 +672,9 @@ static const struct config testnet_config = {
 	/* Testnet blockspace is free. */
 	.max_concurrent_htlcs = 483,
 
+	/* Max amount of dust allowed per channel (50ksat) */
+	.max_dust_htlc_exposure_msat = AMOUNT_MSAT(50000000),
+
 	/* Be aggressive on testnet. */
 	.cltv_expiry_delta = 6,
 	.cltv_final = 10,
@@ -717,6 +721,9 @@ static const struct config mainnet_config = {
 	/* While up to 483 htlcs are possible we do 30 by default (as eclair does) to save blockspace */
 	.max_concurrent_htlcs = 30,
 
+	/* Max amount of dust allowed per channel (50ksat) */
+	.max_dust_htlc_exposure_msat = AMOUNT_MSAT(50000000),
+
 	/* BOLT #2:
 	 *
 	 * 1. the `cltv_expiry_delta` for channels, `3R+2G+2S`: if in doubt, a
@@ -842,6 +849,14 @@ static char *opt_start_daemon(struct lightningd *ld)
 	errx(1, "Died with signal %u", WTERMSIG(exitcode));
 }
 
+static char *opt_set_msat(const char *arg, struct amount_msat *amt)
+{
+	if (!parse_amount_msat(amt, arg, strlen(arg)))
+		return tal_fmt(NULL, "Unable to parse millisatoshi '%s'", arg);
+
+	return NULL;
+}
+
 static char *opt_set_wumbo(struct lightningd *ld)
 {
 	feature_set_or(ld->our_features,
@@ -1005,6 +1020,9 @@ static void register_opts(struct lightningd *ld)
 	opt_register_arg("--max-concurrent-htlcs", opt_set_u32, opt_show_u32,
 			 &ld->config.max_concurrent_htlcs,
 			 "Number of HTLCs one channel can handle concurrently. Should be between 1 and 483");
+	opt_register_arg("--max-dust-htlc-exposure-msat", opt_set_msat,
+			 NULL, &ld->config.max_dust_htlc_exposure_msat,
+			 "Max HTLC amount that can be trimmed");
 	opt_register_arg("--min-capacity-sat", opt_set_u64, opt_show_u64,
 			 &ld->config.min_capacity_sat,
 			 "Minimum capacity in satoshis for accepting channels");
@@ -1496,6 +1514,8 @@ static void add_config(struct lightningd *ld,
 			   || opt->cb_arg == (void *)plugin_opt_flag_set) {
 			/* FIXME: We actually treat it as if they specified
 			 * --plugin for each one, so ignore these */
+		} else if (opt->cb_arg == (void *)opt_set_msat) {
+			json_add_amount_msat_only(response, name0, ld->config.max_dust_htlc_exposure_msat);
 #if EXPERIMENTAL_FEATURES
 		} else if (opt->cb_arg == (void *)opt_set_accept_extra_tlv_types) {
                         /* TODO Actually print the option */

openingd/dualopend.c

@@ -134,6 +134,9 @@ static struct tx_state *new_tx_state(const tal_t *ctx)
 	tx_state->lease_chan_max_msat = 0;
 	tx_state->lease_chan_max_ppt = 0;
 
+	/* no max_htlc_dust_exposure on remoteconf, we exclusively use the local's */
+	tx_state->remoteconf.max_dust_htlc_exposure_msat = AMOUNT_MSAT(0);
+
 	for (size_t i = 0; i < NUM_TX_MSGS; i++)
 		tx_state->tx_msg_count[i] = 0;