Move detection of PMTU decrease to try_mtu().

When we have fixed the PMTU, n->mtuprobes == -1. When we send MTU probes
when mtuprobes == -1, decrease mtuprobes, and reset it back to -1 in
mtu_probe_h(). If mtuprobes < -1, send MTU probes every second, until
mtuprobes <= -4, in which case we will restart MTU discovery.

diff --git a/src/net_packet.c b/src/net_packet.c
index 44f9ef4..b2c6f90 100644 (file)
--- a/src/net_packet.c
+++ b/src/net_packet.c
@@ -151,18 +151,20 @@ static void udp_probe_h(node_t *n, vpn_packet_t *packet, length_t len) {
                        timeout_add(&n->udp_ping_timeout, &udp_probe_timeout_handler, n, &(struct timeval){udp_discovery_timeout, 0});
                }
 
-               if(probelen >= n->maxmtu + 1) {
+               if(probelen > n->maxmtu) {
                        logger(DEBUG_TRAFFIC, LOG_INFO, "Increase in PMTU to %s (%s) detected, restarting PMTU discovery", n->name, n->hostname);
+                       n->minmtu = probelen;
                        n->maxmtu = MTU;
                        /* Set mtuprobes to 1 so that try_mtu() doesn't reset maxmtu */
                        n->mtuprobes = 1;
                        return;
+               } else if(n->mtuprobes < 0 && probelen == n->maxmtu) {
+                       /* We got a maxmtu sized packet, confirming the PMTU is still valid. */
+                       n->mtuprobes = -1;
                }
 
                /* If applicable, raise the minimum supported MTU */
 
-               if(probelen > n->maxmtu)
-                       probelen = n->maxmtu;
                if(n->minmtu < probelen) {
                        n->minmtu = probelen;
                        try_fix_mtu(n);
@@ -968,7 +970,9 @@ static void try_mtu(node_t *n) {
 
        /* mtuprobes == 0..19: initial discovery, send bursts with 1 second interval, mtuprobes++
           mtuprobes ==    20: fix MTU, and go to -1
-          mtuprobes ==    -1: send one >maxmtu probe every pinginterval */
+          mtuprobes ==    -1: send one maxmtu and one maxmtu+1 probe every pinginterval
+          mtuprobes ==-2..-3: send one maxmtu probe every second
+          mtuprobes ==    -4: maxmtu no longer valid, reset minmtu and maxmtu and go to 0 */
 
        struct timeval elapsed;
        timersub(&now, &n->mtu_ping_sent, &elapsed);
@@ -976,20 +980,33 @@ static void try_mtu(node_t *n) {
                if(n->mtuprobes != 0 && elapsed.tv_sec == 0 && elapsed.tv_usec < 333333)
                        return;
        } else {
-               if(elapsed.tv_sec < pinginterval)
-                       return;
+               if(n->mtuprobes < -1) {
+                       if(elapsed.tv_sec < 1)
+                               return;
+               } else {
+                       if(elapsed.tv_sec < pinginterval)
+                               return;
+               }
        }
 
        n->mtu_ping_sent = now;
 
        try_fix_mtu(n);
 
+       if(n->mtuprobes < -3) {
+               /* We lost three MTU probes, restart discovery */
+               logger(DEBUG_TRAFFIC, LOG_INFO, "Decrease in PMTU to %s (%s) detected, restarting PMTU discovery", n->name, n->hostname);
+               n->mtuprobes = 0;
+               n->minmtu = 0;
+       }
+
        if(n->mtuprobes < 0) {
                /* After the initial discovery, we only send one maxmtu and one
                   maxmtu+1 probe to detect PMTU increases. */
                send_udp_probe_packet(n, n->maxmtu);
-               if(n->maxmtu + 1 < MTU)
+               if(n->mtuprobes == -1 && n->maxmtu + 1 < MTU)
                        send_udp_probe_packet(n, n->maxmtu + 1);
+               n->mtuprobes--;
        } else {
                /* Before initial discovery begins, set maxmtu to the most likely value.
                   If it's underestimated, we will correct it after initial discovery. */