if(n->mtuprobes < 0)
return;
- if(n->mtuprobes == 90 || n->minmtu >= n->maxmtu) {
+ if(n->mtuprobes == 20 || n->minmtu >= n->maxmtu) {
if(n->minmtu > n->maxmtu)
n->minmtu = n->maxmtu;
else
timeout_add(&n->udp_ping_timeout, &udp_probe_timeout_handler, n, &(struct timeval){udp_discovery_timeout, 0});
}
- if(probelen >= n->maxmtu + 8) {
+ if(probelen >= n->maxmtu + 1) {
logger(DEBUG_TRAFFIC, LOG_INFO, "Increase in PMTU to %s (%s) detected, restarting PMTU discovery", n->name, n->hostname);
n->maxmtu = MTU;
- n->mtuprobes = 30;
+ n->mtuprobes = 0;
return;
}
return;
}
- /* mtuprobes == 0..89: initial discovery, send bursts with 1 second interval, mtuprobes++
- mtuprobes == 90: fix MTU, and go to -1
+ /* 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 pingtimeout */
struct timeval now;
if(n->mtuprobes < 0) {
/* After the initial discovery, we only send one >maxmtu probe
to detect PMTU increases. */
- if(n->maxmtu + 8 < MTU)
- send_udp_probe_packet(n, n->maxmtu + 8);
+ if(n->maxmtu + 1 < MTU)
+ send_udp_probe_packet(n, n->maxmtu + 1);
} 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 1329-byte first probe, followed (if there was a reply) by a 1407-byte probe.
+ Since 1407 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 = 0.97;
+
+ const float cycle_position = probes_per_cycle - (n->mtuprobes % probes_per_cycle) - 1;
+ const length_t minmtu = MAX(n->minmtu, 512);
+ 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++;
}