From 92b8abc921dd15b710f67335562210eb713fbb39 Mon Sep 17 00:00:00 2001
From: Guus Sliepen <guus@tinc-vpn.org>
Date: Sun, 11 Oct 2009 18:57:58 +0200
Subject: [PATCH] Use MTU probes to regularly ping other nodes over UDP.

This keeps NAT mappings for UDP alive, and will also detect when a node is not
reachable via UDP anymore or if the path MTU is decreasing. Tinc will fall back
to TCP if the node has become unreachable.

If UDP communication is impossible, we stop sending probes, but we retry if it
changes its keys.

We also decouple the UDP and TCP ping mechanisms completely, to ensure tinc
properly detects failure of either method.
---
 src/meta.c       |  2 --
 src/net_packet.c | 50 +++++++++++++++++++++++++++++++++++++-----------
 2 files changed, 39 insertions(+), 13 deletions(-)

diff --git a/src/meta.c b/src/meta.c
index 765baec..21400f2 100644
--- a/src/meta.c
+++ b/src/meta.c
@@ -225,7 +225,5 @@ bool receive_meta(connection_t *c) {
 		return false;
 	}
 
-	c->last_ping_time = now;
-
 	return true;
 }
diff --git a/src/net_packet.c b/src/net_packet.c
index 63e3592..7a73ef5 100644
--- a/src/net_packet.c
+++ b/src/net_packet.c
@@ -58,31 +58,57 @@ static void send_udppacket(node_t *, vpn_packet_t *);
 
 #define MAX_SEQNO 1073741824
 
+// mtuprobes == 1..30: initial discovery, send bursts with 1 second interval
+// mtuprobes ==    31: sleep pinginterval seconds
+// mtuprobes ==    32: send 1 burst, sleep pingtimeout second
+// mtuprobes ==    33: no response from other side, restart PMTU discovery process
+
 void send_mtu_probe(node_t *n) {
 	vpn_packet_t packet;
 	int len, i;
+	int timeout = 1;
 	
 	n->mtuprobes++;
 	n->mtuevent = NULL;
 
-	if(!n->status.reachable) {
+	if(!n->status.reachable || !n->status.validkey) {
 		logger(LOG_DEBUG, "Trying to send MTU probe to unreachable node %s (%s)", n->name, n->hostname);
+		n->mtuprobes = 0;
 		return;
 	}
 
+	if(n->mtuprobes > 32) {
+		ifdebug(TRAFFIC) logger(LOG_INFO, "%s (%s) did not respond to UDP ping, restarting PMTU discovery", n->name, n->hostname);
+		n->mtuprobes = 1;
+		n->minmtu = 0;
+		n->maxmtu = MTU;
+	}
+
 	if(n->mtuprobes >= 10 && !n->minmtu) {
 		ifdebug(TRAFFIC) logger(LOG_INFO, "No response to MTU probes from %s (%s)", n->name, n->hostname);
+		n->mtuprobes = 0;
 		return;
 	}
 
+	if(n->mtuprobes == 30 || (n->mtuprobes < 30 && n->minmtu >= n->maxmtu)) {
+		n->mtu = n->minmtu;
+		ifdebug(TRAFFIC) logger(LOG_INFO, "Fixing MTU of %s (%s) to %d after %d probes", n->name, n->hostname, n->mtu, n->mtuprobes);
+		n->mtuprobes = 31;
+	}
+
+	if(n->mtuprobes == 31) {
+		timeout = pinginterval;
+		goto end;
+	} else if(n->mtuprobes == 32) {
+		timeout = pingtimeout;
+	}
+
 	for(i = 0; i < 3; i++) {
-		if(n->mtuprobes >= 30 || n->minmtu >= n->maxmtu) {
-			n->mtu = n->minmtu;
-			ifdebug(TRAFFIC) logger(LOG_INFO, "Fixing MTU of %s (%s) to %d after %d probes", n->name, n->hostname, n->mtu, n->mtuprobes);
-			return;
-		}
+		if(n->maxmtu <= n->minmtu)
+			len = n->maxmtu;
+		else
+			len = n->minmtu + 1 + rand() % (n->maxmtu - n->minmtu);
 
-		len = n->minmtu + 1 + rand() % (n->maxmtu - n->minmtu);
 		if(len < 64)
 			len = 64;
 		
@@ -96,10 +122,11 @@ void send_mtu_probe(node_t *n) {
 		send_udppacket(n, &packet);
 	}
 
+end:
 	n->mtuevent = new_event();
 	n->mtuevent->handler = (event_handler_t)send_mtu_probe;
 	n->mtuevent->data = n;
-	n->mtuevent->time = now + 1;
+	n->mtuevent->time = now + timeout;
 	event_add(n->mtuevent);
 }
 
@@ -110,8 +137,12 @@ void mtu_probe_h(node_t *n, vpn_packet_t *packet, length_t len) {
 		packet->data[0] = 1;
 		send_packet(n, packet);
 	} else {
+		if(len > n->maxmtu)
+			len = n->maxmtu;
 		if(n->minmtu < len)
 			n->minmtu = len;
+		if(n->mtuprobes > 30)
+			n->mtuprobes = 30;
 	}
 }
 
@@ -279,9 +310,6 @@ static void receive_udppacket(node_t *n, vpn_packet_t *inpkt) {
 
 	inpkt->priority = 0;
 
-	if(n->connection)
-		n->connection->last_ping_time = now;
-
 	if(!inpkt->data[12] && !inpkt->data[13])
 		mtu_probe_h(n, inpkt, origlen);
 	else
-- 
2.25.1