Use a smarter algorithm for choosing MTU discovery probe sizes.

[tinc] / src / net_packet.c

diff --git a/src/net_packet.c b/src/net_packet.c
index 828e193..2f15bb4 100644 (file)
--- a/src/net_packet.c
+++ b/src/net_packet.c
@@ -922,13 +922,30 @@ static void try_mtu(node_t *n) {
                if(n->maxmtu + 8 < MTU)
                        send_udp_probe_packet(n, n->maxmtu + 8);
        } else {
-               /* Probes are sent with random sizes between the
-                  lower and upper boundaries for the MTU thus far discovered. */
-               int len = n->maxmtu;
-               if(n->minmtu < n->maxmtu)
-                       len = n->minmtu + 1 + rand() % (n->maxmtu - n->minmtu);
-               send_udp_probe_packet(n, MAX(len, 64));
-
+               /* Decreasing the number of probes per cycle might make the algorithm react faster to lost packets,
+                  but it will typically increase convergence time in the no-loss case. */
+               const length_t probes_per_cycle = 8;
+
+               /* This magic value was determined using math simulations.
+                  It will result in a 1339-byte first probe, followed (if there was a reply) by a 1417-byte probe.
+                  Since 1417 is just below the range of tinc MTUs over typical networks,
+                  this fine-tuning allows tinc to cover a lot of ground very quickly. */
+               const float multiplier = 0.982;
+
+               const float cycle_position = probes_per_cycle - (n->mtuprobes % probes_per_cycle) - 1;
+               const length_t minmtu = MAX(n->minmtu, 64);
+               const float interval = n->maxmtu - minmtu;
+
+               /* The core of the discovery algorithm is this exponential.
+                  It produces very large probes early in the cycle, and then it very quickly decreases the probe size.
+                  This reflects the fact that in the most difficult cases, we don't get any feedback for probes that
+                  are too large, and therefore we need to concentrate on small offsets so that we can quickly converge
+                  on the precise MTU as we are approaching it.
+                  The last probe of the cycle is always 1 byte in size - this is to make sure we'll get at least one
+                  reply per cycle so that we can make progress. */
+               const length_t offset = powf(interval, multiplier * cycle_position / (probes_per_cycle - 1));
+
+               send_udp_probe_packet(n, minmtu + offset);
                if(n->mtuprobes >= 0)
                        n->mtuprobes++;
        }