diff --git a/src/Makefile.am b/src/Makefile.am index 52f38e48..ea53ea26 100644 --- a/src/Makefile.am +++ b/src/Makefile.am @@ -250,7 +250,7 @@ endif tinc_LDADD = $(READLINE_LIBS) $(CURSES_LIBS) sptps_speed_LDADD = -lrt -LIBS = @LIBS@ +LIBS = @LIBS@ -lm if TUNEMU LIBS += -lpcap diff --git a/src/have.h b/src/have.h index 85479f75..80f405e9 100644 --- a/src/have.h +++ b/src/have.h @@ -40,6 +40,7 @@ #include #include #include +#include #ifdef HAVE_MINGW #include diff --git a/src/net_packet.c b/src/net_packet.c index 828e193b..2f15bb4e 100644 --- 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++; }