BOLT7: broadcast htlc_maximum_msat in `channel_update s

Have c-lightning nodes send out the largest value for
`htlc_maximum_msat` that makes sense, ie the lesser of
the peer's max_inflight_htlc value or the total channel
capacity minus the total channel reserve.

channeld/channeld.c

@@ -60,6 +60,7 @@
 #define PEER_FD 3
 #define GOSSIP_FD 4
 #define HSM_FD 5
+#define min(x, y) ((x) < (y) ? (x) : (y))
 
 struct commit_sigs {
 	struct peer *peer;
@@ -227,6 +228,25 @@ static const u8 *hsm_req(const tal_t *ctx, const u8 *req TAKES)
 	return msg;
 }
 
+/*
+ * The maximum msat that this node will accept for an htlc.
+ * It's flagged as an optional field in `channel_update`.
+ *
+ * We advertise the maximum value possible, defined as the smaller
+ * of the remote's maximum in-flight HTLC or the total channel
+ * capacity minus the cumulative reserve.
+ * FIXME: does this need fuzz?
+ */
+static const u64 advertised_htlc_max(const u64 funding_msat,
+		const struct channel_config *our_config,
+		const struct channel_config *remote_config)
+{
+	u64 cumulative_reserve_msat = (our_config->channel_reserve_satoshis +
+		remote_config->channel_reserve_satoshis) * 1000;
+	return min(remote_config->max_htlc_value_in_flight_msat,
+			funding_msat - cumulative_reserve_msat);
+}
+
 /* Create and send channel_update to gossipd (and maybe peer) */
 static void send_channel_update(struct peer *peer, int disable_flag)
 {
@@ -247,7 +267,11 @@ static void send_channel_update(struct peer *peer, int disable_flag)
 						 peer->cltv_delta,
 						 peer->conf[REMOTE].htlc_minimum_msat,
 						 peer->fee_base,
-						 peer->fee_per_satoshi);
+						 peer->fee_per_satoshi,
+						 advertised_htlc_max(
+							 peer->channel->funding_msat,
+							 &peer->conf[LOCAL],
+							 &peer->conf[REMOTE]));
 	wire_sync_write(GOSSIP_FD, take(msg));
 }
 
@@ -2306,7 +2330,6 @@ static void handle_offer_htlc(struct peer *peer, const u8 *inmsg)
 
 	/* FIXME: Fuzz the boundaries a bit to avoid probing? */
 	case CHANNEL_ERR_MAX_HTLC_VALUE_EXCEEDED:
-		/* FIXME: We should advertise this? */
 		failcode = WIRE_TEMPORARY_CHANNEL_FAILURE;
 		failmsg = tal_fmt(inmsg, "Maximum value exceeded");
 		goto failed;

gossipd/gossip_wire.csv

@@ -130,6 +130,7 @@ gossip_local_channel_update,,cltv_expiry_delta,u16
 gossip_local_channel_update,,htlc_minimum_msat,u64
 gossip_local_channel_update,,fee_base_msat,u32
 gossip_local_channel_update,,fee_proportional_millionths,u32
+gossip_local_channel_update,,htlc_maximum_msat,u64
 
 gossip_local_channel_close,3027
 gossip_local_channel_close,,short_channel_id,struct short_channel_id

gossipd/gossipd.c

@@ -942,17 +942,14 @@ static void update_local_channel(struct daemon *daemon,
 				 u64 htlc_minimum_msat,
 				 u32 fee_base_msat,
 				 u32 fee_proportional_millionths,
+				 u64 htlc_maximum_msat,
 				 const char *caller)
 {
 	secp256k1_ecdsa_signature dummy_sig;
 	u8 *update, *msg;
 	u32 timestamp = time_now().ts.tv_sec;
 	u8 message_flags, channel_flags;
 
-	/* `message_flags` are optional.
-	 * Currently, not set by c-lightning */
-	message_flags = 0;
-
 	/* So valgrind doesn't complain */
 	memset(&dummy_sig, 0, sizeof(dummy_sig));
 
@@ -965,15 +962,19 @@ static void update_local_channel(struct daemon *daemon,
 	if (disable)
 		channel_flags |= ROUTING_FLAGS_DISABLED;
 
-	update = towire_channel_update(tmpctx, &dummy_sig,
+	// We set the htlc_maximum_msat value
+	message_flags = 0 | ROUTING_OPT_HTLC_MAX_MSAT;
+
+	update = towire_channel_update_option_channel_htlc_max(tmpctx, &dummy_sig,
 				       &daemon->rstate->chain_hash,
 				       &chan->scid,
 				       timestamp,
 				       message_flags, channel_flags,
 				       cltv_expiry_delta,
 				       htlc_minimum_msat,
 				       fee_base_msat,
-				       fee_proportional_millionths);
+				       fee_proportional_millionths,
+				       htlc_maximum_msat);
 
 	if (!wire_sync_write(HSM_FD,
 			     towire_hsm_cupdate_sig_req(tmpctx, update))) {
@@ -1026,6 +1027,7 @@ static void maybe_update_local_channel(struct daemon *daemon,
 			     hc->htlc_minimum_msat,
 			     hc->base_fee,
 			     hc->proportional_fee,
+			     hc->htlc_maximum_msat,
 			     __func__);
 }
 
@@ -1093,15 +1095,17 @@ static void handle_get_update(struct peer *peer, const u8 *msg)
 /* Return true if the information has changed. */
 static bool halfchan_new_info(const struct half_chan *hc,
 			      u16 cltv_delta, u64 htlc_minimum_msat,
-			      u32 fee_base_msat, u32 fee_proportional_millionths)
+			      u32 fee_base_msat, u32 fee_proportional_millionths,
+			      u64 htlc_maximum_msat)
 {
 	if (!is_halfchan_defined(hc))
 		return true;
 
 	return hc->delay != cltv_delta
 		|| hc->htlc_minimum_msat != htlc_minimum_msat
 		|| hc->base_fee != fee_base_msat
-		|| hc->proportional_fee != fee_proportional_millionths;
+		|| hc->proportional_fee != fee_proportional_millionths
+		|| hc->htlc_maximum_msat != htlc_maximum_msat;
 }
 
 static void handle_local_channel_update(struct peer *peer, const u8 *msg)
@@ -1111,6 +1115,7 @@ static void handle_local_channel_update(struct peer *peer, const u8 *msg)
 	bool disable;
 	u16 cltv_expiry_delta;
 	u64 htlc_minimum_msat;
+	u64 htlc_maximum_msat;
 	u32 fee_base_msat;
 	u32 fee_proportional_millionths;
 	int direction;
@@ -1121,7 +1126,8 @@ static void handle_local_channel_update(struct peer *peer, const u8 *msg)
 						  &cltv_expiry_delta,
 						  &htlc_minimum_msat,
 						  &fee_base_msat,
-						  &fee_proportional_millionths)) {
+						  &fee_proportional_millionths,
+						  &htlc_maximum_msat)) {
 		status_broken("peer %s bad local_channel_update %s",
 			      type_to_string(tmpctx, struct pubkey, &peer->id),
 			      tal_hex(tmpctx, msg));
@@ -1151,7 +1157,8 @@ static void handle_local_channel_update(struct peer *peer, const u8 *msg)
 	 * Or, if it's an unannounced channel (only sending to peer). */
 	if (halfchan_new_info(&chan->half[direction],
 			      cltv_expiry_delta, htlc_minimum_msat,
-			      fee_base_msat, fee_proportional_millionths)
+			      fee_base_msat, fee_proportional_millionths,
+			      htlc_maximum_msat)
 	    || ((chan->half[direction].channel_flags & ROUTING_FLAGS_DISABLED)
 		&& !disable)
 	    || !is_chan_public(chan)) {
@@ -1161,6 +1168,7 @@ static void handle_local_channel_update(struct peer *peer, const u8 *msg)
 				     htlc_minimum_msat,
 				     fee_base_msat,
 				     fee_proportional_millionths,
+				     htlc_maximum_msat,
 				     __func__);
 	}
 
@@ -1748,6 +1756,7 @@ static void gossip_send_keepalive_update(struct daemon *daemon,
 			     hc->htlc_minimum_msat,
 			     hc->base_fee,
 			     hc->proportional_fee,
+			     hc->htlc_maximum_msat,
 			     __func__);
 }
 

hsmd/hsmd.c

@@ -653,6 +653,7 @@ static struct io_plan *handle_channel_update_sig(struct io_conn *conn,
 	struct short_channel_id scid;
 	u32 timestamp, fee_base_msat, fee_proportional_mill;
 	u64 htlc_minimum_msat;
+	u64 htlc_maximum_msat;
 	u8 message_flags, channel_flags;
 	u16 cltv_expiry_delta;
 	struct bitcoin_blkid chain_hash;
@@ -661,10 +662,11 @@ static struct io_plan *handle_channel_update_sig(struct io_conn *conn,
 	if (!fromwire_hsm_cupdate_sig_req(tmpctx, msg_in, &cu))
 		return bad_req(conn, c, msg_in);
 
-	if (!fromwire_channel_update(cu, &sig, &chain_hash,
-				     &scid, &timestamp, &message_flags, &channel_flags,
-				     &cltv_expiry_delta, &htlc_minimum_msat,
-				     &fee_base_msat, &fee_proportional_mill)) {
+	if (!fromwire_channel_update_option_channel_htlc_max(cu, &sig,
+			&chain_hash, &scid, &timestamp, &message_flags,
+			&channel_flags, &cltv_expiry_delta,
+			&htlc_minimum_msat, &fee_base_msat,
+			&fee_proportional_mill, &htlc_maximum_msat)) {
 		return bad_req_fmt(conn, c, msg_in, "Bad inner channel_update");
 	}
 	if (tal_count(cu) < offset)
@@ -676,10 +678,11 @@ static struct io_plan *handle_channel_update_sig(struct io_conn *conn,
 
 	sign_hash(&node_pkey, &hash, &sig);
 
-	cu = towire_channel_update(tmpctx, &sig, &chain_hash,
+	cu = towire_channel_update_option_channel_htlc_max(tmpctx, &sig, &chain_hash,
 				   &scid, timestamp, message_flags, channel_flags,
 				   cltv_expiry_delta, htlc_minimum_msat,
-				   fee_base_msat, fee_proportional_mill);
+				   fee_base_msat, fee_proportional_mill,
+				   htlc_maximum_msat);
 	return req_reply(conn, c, take(towire_hsm_cupdate_sig_reply(NULL, cu)));
 }
 

tests/test_pay.py

@@ -150,7 +150,7 @@ def test_pay_get_error_with_update(node_factory):
     # channel_update, and it should patch it to include a type prefix. The
     # prefix 0x0102 should be in the channel_update, but not in the
     # onionreply (negation of 0x0102 in the RE)
-    l1.daemon.wait_for_log(r'Extracted channel_update 0102.*from onionreply 10070080(?!.*0102)')
+    l1.daemon.wait_for_log(r'Extracted channel_update 0102.*from onionreply 10070088[0-9a-fA-F]{88}')
 
     # And now monitor for l1 to apply the channel_update we just extracted
     l1.daemon.wait_for_log('Received channel_update for channel {}\(.\) now DISABLED was ACTIVE \(from error\)'.format(chanid2))