Use packet size before decompression to calculate path MTU.

Since compression can either grow or shrink a packet, the size of an MTU probe
after decompression might not reflect the real path MTU. Now we use the size
before decompression, which is independent of the compression algorithm, and
substract a safety margin such that the calculated path MTU will be safe even
for packets which grow as much as possible after compression.

diff --git a/src/net_packet.c b/src/net_packet.c
index 0126d41..28cf161 100644 (file)
--- a/src/net_packet.c
+++ b/src/net_packet.c
@@ -103,15 +103,15 @@ void send_mtu_probe(node_t *n)
        event_add(n->mtuevent);
 }
 
-void mtu_probe_h(node_t *n, vpn_packet_t *packet) {
+void mtu_probe_h(node_t *n, vpn_packet_t *packet, length_t len) {
        ifdebug(TRAFFIC) logger(LOG_INFO, _("Got MTU probe length %d from %s (%s)"), packet->len, n->name, n->hostname);
 
        if(!packet->data[0]) {
                packet->data[0] = 1;
                send_packet(n, packet);
        } else {
-               if(n->minmtu < packet->len)
-                       n->minmtu = packet->len;
+               if(n->minmtu < len)
+                       n->minmtu = len;
        }
 }
 
@@ -270,6 +270,8 @@ static void receive_udppacket(node_t *n, vpn_packet_t *inpkt)
 
        /* Decompress the packet */
 
+       length_t origlen = inpkt->len;
+
        if(n->incompression) {
                outpkt = pkt[nextpkt++];
 
@@ -280,6 +282,8 @@ static void receive_udppacket(node_t *n, vpn_packet_t *inpkt)
                }
 
                inpkt = outpkt;
+
+               origlen -= MTU/64 + 20;
        }
 
        inpkt->priority = 0;
@@ -288,7 +292,7 @@ static void receive_udppacket(node_t *n, vpn_packet_t *inpkt)
                n->connection->last_ping_time = now;
 
        if(!inpkt->data[12] && !inpkt->data[13])
-               mtu_probe_h(n, inpkt);
+               mtu_probe_h(n, inpkt, origlen);
        else
                receive_packet(n, inpkt);
 }