Detect increases in PMTU.

[tinc] / src / net_packet.c

diff --git a/src/net_packet.c b/src/net_packet.c
index 2780f51..c209248 100644 (file)
--- a/src/net_packet.c
+++ b/src/net_packet.c
@@ -70,11 +70,15 @@ bool localdiscovery = false;
    mtuprobes ==    32: send 1 burst, sleep pingtimeout second
    mtuprobes ==    33: no response from other side, restart PMTU discovery process
 
-   Probes are sent in batches of three, with random sizes between the lower and
-   upper boundaries for the MTU thus far discovered.
+   Probes are sent in batches of at least three, with random sizes between the
+   lower and upper boundaries for the MTU thus far discovered.
 
-   In case local discovery is enabled, a fourth packet is added to each batch,
+   After the initial discovery, a fourth packet is added to each batch with a
+   size larger than the currently known PMTU, to test if the PMTU has increased.
+
+   In case local discovery is enabled, another packet is added to each batch,
    which will be broadcast to the local network.
+
 */
 
 void send_mtu_probe(node_t *n) {
@@ -126,11 +130,16 @@ void send_mtu_probe(node_t *n) {
                timeout = pingtimeout;
        }
 
-       for(i = 0; i < 3 + localdiscovery; i++) {
-               if(n->maxmtu <= n->minmtu)
+       for(i = 0; i < 4 + localdiscovery; i++) {
+               if(i == 0) {
+                       if(n->mtuprobes < 30 || n->maxmtu + 8 >= MTU)
+                               continue;
+                       len = n->maxmtu + 8;
+               } else if(n->maxmtu <= n->minmtu) {
                        len = n->maxmtu;
-               else
+               } else {
                        len = n->minmtu + 1 + rand() % (n->maxmtu - n->minmtu);
+               }
 
                if(len < 64)
                        len = 64;
@@ -138,7 +147,7 @@ void send_mtu_probe(node_t *n) {
                memset(packet.data, 0, 14);
                RAND_pseudo_bytes(packet.data + 14, len - 14);
                packet.len = len;
-               if(i >= 3 && n->mtuprobes <= 10)
+               if(i >= 4 && n->mtuprobes <= 10)
                        packet.priority = -1;
                else
                        packet.priority = 0;
@@ -164,6 +173,13 @@ void mtu_probe_h(node_t *n, vpn_packet_t *packet, length_t len) {
                send_udppacket(n, packet);
        } else {
                if(n->mtuprobes > 30) {
+                       if (len == n->maxmtu + 8) {
+                               ifdebug(TRAFFIC) logger(LOG_INFO, "Increase in PMTU to %s (%s) detected, restarting PMTU discovery", n->name, n->hostname);
+                               n->maxmtu = MTU;
+                               n->mtuprobes = 10;
+                               return;
+                       }
+
                        if(n->minmtu)
                                n->mtuprobes = 30;
                        else