Throttle the rate of MTU_INFO messages.

This makes sure MTU_INFO messages are only sent at the maximum rate of
5 per second (by default). As usual with these "probe" mechanisms, the
rate of these messages cannot be higher than the rate of data packets
themselves, since they are sent from the RX path.

bash_completion.d/tinc

@@ -4,7 +4,7 @@ _tinc() {
 	cur="${COMP_WORDS[COMP_CWORD]}"
 	prev="${COMP_WORDS[COMP_CWORD-1]}"
 	opts="-c -d -D -K -n -o -L -R -U --config --no-detach --debug --net --option --mlock --logfile --pidfile --chroot --user --help --version"
-	confvars="Address AddressFamily BindToAddress BindToInterface Broadcast BroadcastSubnet Cipher ClampMSS Compression ConnectTo DecrementTTL Device DeviceStandby DeviceType Digest DirectOnly Ed25519PrivateKeyFile Ed25519PublicKey Ed25519PublicKeyFile ExperimentalProtocol Forwarding GraphDumpFile Hostnames IffOneQueue IndirectData Interface KeyExpire ListenAddress LocalDiscovery MACExpire MACLength MaxOutputBufferSize MaxTimeout Mode Name PMTU PMTUDiscovery PingInterval PingTimeout Port PriorityInheritance PrivateKeyFile ProcessPriority Proxy PublicKeyFile ReplayWindow StrictSubnets Subnet TCPOnly TunnelServer UDPDiscovery UDPDiscoveryKeepaliveInterval UDPDiscoveryInterval UDPDiscoveryTimeout UDPInfoInterval UDPRcvBuf UDPSndBuf VDEGroup VDEPort Weight"
+	confvars="Address AddressFamily BindToAddress BindToInterface Broadcast BroadcastSubnet Cipher ClampMSS Compression ConnectTo DecrementTTL Device DeviceStandby DeviceType Digest DirectOnly Ed25519PrivateKeyFile Ed25519PublicKey Ed25519PublicKeyFile ExperimentalProtocol Forwarding GraphDumpFile Hostnames IffOneQueue IndirectData Interface KeyExpire ListenAddress LocalDiscovery MACExpire MACLength MaxOutputBufferSize MaxTimeout Mode MTUInfoInterval Name PMTU PMTUDiscovery PingInterval PingTimeout Port PriorityInheritance PrivateKeyFile ProcessPriority Proxy PublicKeyFile ReplayWindow StrictSubnets Subnet TCPOnly TunnelServer UDPDiscovery UDPDiscoveryKeepaliveInterval UDPDiscoveryInterval UDPDiscoveryTimeout UDPInfoInterval UDPRcvBuf UDPSndBuf VDEGroup VDEPort Weight"
 	commands="add connect debug del disconnect dump edit export export-all generate-ed25519-keys generate-keys generate-rsa-keys get help import info init invite join log network pcap pid purge reload restart retry set start stop top version"
 
 	case ${prev} in

doc/tinc.conf.5.in

@@ -571,6 +571,8 @@ This option controls the initial path MTU to this node.
 .It Va PMTUDiscovery Li = yes | no Po yes Pc
 When this option is enabled, tinc will try to discover the path MTU to this node.
 After the path MTU has been discovered, it will be enforced on the VPN.
+.It Va MTUInfoInterval Li = Ar seconds Pq 5
+The minimum amount of time between sending periodic updates about relay path MTU. Useful for quickly determining MTU to indirect nodes.
 .It Va Port Li = Ar port Pq 655
 The port number on which this tinc daemon is listening for incoming connections,
 which is used if no port number is specified in an

doc/tinc.texi

@@ -1333,6 +1333,10 @@ This option controls the initial path MTU to this node.
 When this option is enabled, tinc will try to discover the path MTU to this node.
 After the path MTU has been discovered, it will be enforced on the VPN.
 
+@cindex MTUInfoInterval
+@item MTUInfoInterval = <seconds> (5)
+The minimum amount of time between sending periodic updates about relay path MTU. Useful for quickly determining MTU to indirect nodes.
+
 @cindex Port
 @item Port = <@var{port}> (655)
 This is the port this tinc daemon listens on.

src/net.h

@@ -143,6 +143,7 @@ extern int udp_discovery_keepalive_interval;
 extern int udp_discovery_interval;
 extern int udp_discovery_timeout;
 
+extern int mtu_info_interval;
 extern int udp_info_interval;
 
 extern listen_socket_t listen_socket[MAXSOCKETS];

src/net_setup.c

@@ -518,6 +518,7 @@ bool setup_myself_reloadable(void) {
 	get_config_int(lookup_config(config_tree, "UDPDiscoveryInterval"), &udp_discovery_interval);
 	get_config_int(lookup_config(config_tree, "UDPDiscoveryTimeout"), &udp_discovery_timeout);
 
+	get_config_int(lookup_config(config_tree, "MTUInfoInterval"), &mtu_info_interval);
 	get_config_int(lookup_config(config_tree, "UDPInfoInterval"), &udp_info_interval);
 
 	get_config_bool(lookup_config(config_tree, "DirectOnly"), &directonly);

src/node.h

@@ -95,6 +95,7 @@ typedef struct node_t {
 
 	struct timeval mtu_ping_sent;           /* Last time a MTU probe was sent */
 
+	struct timeval mtu_info_sent;           /* Last time a MTU_INFO message was sent */
 	struct timeval udp_info_sent;           /* Last time a UDP_INFO message was sent */
 
 	length_t maxrecentlen;			/* Maximum size of recently received packets */

src/protocol_misc.c

@@ -31,6 +31,7 @@
 #include "xalloc.h"
 
 int maxoutbufsize = 0;
+int mtu_info_interval = 5;
 int udp_info_interval = 5;
 
 /* Status and error notification routines */
@@ -261,8 +262,15 @@ bool send_mtu_info(node_t *from, node_t *to, int mtu) {
 	if(!to->status.reachable)
 		return true;
 
-	if(from == myself && to->connection)
-		return true;
+	if(from == myself) {
+		if(to->connection)
+			return true;
+
+		struct timeval elapsed;
+		timersub(&now, &to->mtu_info_sent, &elapsed);
+		if(elapsed.tv_sec < mtu_info_interval)
+			return true;
+	}
 
 	if((to->nexthop->options >> 24) < 6)
 		return true;
@@ -284,6 +292,9 @@ bool send_mtu_info(node_t *from, node_t *to, int mtu) {
 		mtu = MIN(mtu, via->nexthop->minmtu);
 	}
 
+	if(from == myself)
+		to->mtu_info_sent = now;
+
 	/* If none of the conditions above match in the steady state, it means we're using TCP,
 	   so the MTU is irrelevant. That said, it is still important to honor the MTU that was passed in,
 	   because other parts of the relay path might be able to use UDP, which means they care about the MTU. */

src/tincctl.c

@@ -1371,6 +1371,7 @@ const var_t variables[] = {
 	{"UDPDiscoveryKeepaliveInterval", VAR_SERVER},
 	{"UDPDiscoveryInterval", VAR_SERVER},
 	{"UDPDiscoveryTimeout", VAR_SERVER},
+	{"MTUInfoInterval", VAR_SERVER},
 	{"UDPInfoInterval", VAR_SERVER},
 	{"UDPRcvBuf", VAR_SERVER},
 	{"UDPSndBuf", VAR_SERVER},