From: Etienne Dechamps Date: Wed, 31 Dec 2014 16:12:11 +0000 (+0000) Subject: Add IP_MTU-based maxmtu estimation. X-Git-Tag: release-1.1pre12~232 X-Git-Url: https://git.tinc-vpn.org/git/browse?a=commitdiff_plain;h=bce17c83e871cb8a8c9158045eaf13f1be4b3d13;p=tinc Add IP_MTU-based maxmtu estimation. Linux provides a getsockopt() option, IP_MTU, to get the kernel's best guess at a connection MTU. In practice, it seems to return the MTU of the physical interface the socket is using. This patch uses this option to initialize maxmtu to a better value when MTU discovery starts. Unfortunately, this is not supported on Windows. Winsock has options such as SO_MAX_MSG_SIZE, SO_MAXDG and SO_MAXPATHDG but they seem useless as they always return absurdly large values (typically, 65507), as confirmed by http://support.microsoft.com/kb/822061/ --- diff --git a/src/net_packet.c b/src/net_packet.c index 626114c8..34bdd269 100644 --- a/src/net_packet.c +++ b/src/net_packet.c @@ -37,6 +37,8 @@ #include "digest.h" #include "device.h" #include "ethernet.h" +#include "ipv4.h" +#include "ipv6.h" #include "graph.h" #include "logger.h" #include "net.h" @@ -144,7 +146,8 @@ static void udp_probe_h(node_t *n, vpn_packet_t *packet, length_t len) { 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 = 0; + /* Set mtuprobes to 1 so that try_mtu() doesn't reset maxmtu */ + n->mtuprobes = 1; return; } @@ -883,6 +886,67 @@ static void try_udp(node_t* n) { } } +static length_t choose_initial_maxmtu(node_t *n) { +#ifdef IP_MTU + + int sock = -1; + + const sockaddr_t *sa = NULL; + int sockindex; + choose_udp_address(n, &sa, &sockindex); + if(!sa) + return MTU; + + sock = socket(sa->sa.sa_family, SOCK_DGRAM, IPPROTO_UDP); + if(sock < 0) { + logger(DEBUG_TRAFFIC, LOG_ERR, "Creating MTU assessment socket for %s (%s) failed: %s", n->name, n->hostname, sockstrerror(sockerrno)); + return MTU; + } + + if(connect(sock, &sa->sa, SALEN(sa->sa))) { + logger(DEBUG_TRAFFIC, LOG_ERR, "Connecting MTU assessment socket for %s (%s) failed: %s", n->name, n->hostname, sockstrerror(sockerrno)); + close(sock); + return MTU; + } + + int ip_mtu; + socklen_t ip_mtu_len = sizeof ip_mtu; + if(getsockopt(sock, IPPROTO_IP, IP_MTU, &ip_mtu, &ip_mtu_len)) { + logger(DEBUG_TRAFFIC, LOG_ERR, "getsockopt(IP_MTU) on %s (%s) failed: %s", n->name, n->hostname, sockstrerror(sockerrno)); + close(sock); + return MTU; + } + + close(sock); + + /* getsockopt(IP_MTU) returns the MTU of the physical interface. + We need to remove various overheads to get to the tinc MTU. */ + length_t mtu = ip_mtu; + mtu -= (sa->sa.sa_family == AF_INET6) ? sizeof(struct ip6_hdr) : sizeof(struct ip); + mtu -= 8; /* UDP */ + if(n->status.sptps) { + mtu -= SPTPS_DATAGRAM_OVERHEAD; + if((n->options >> 24) >= 4) + mtu -= sizeof(node_id_t) + sizeof(node_id_t); + } + + if (mtu < 512) { + logger(DEBUG_TRAFFIC, LOG_ERR, "getsockopt(IP_MTU) on %s (%s) returned absurdly small value: %d", n->name, n->hostname, ip_mtu); + return MTU; + } + if (mtu > MTU) + return MTU; + + logger(DEBUG_TRAFFIC, LOG_INFO, "Using system-provided maximum tinc MTU for %s (%s): %hd", n->name, n->hostname, mtu); + return mtu; + +#else + + return MTU; + +#endif +} + // This function tries to determines the MTU of a node. // By calling this function repeatedly, n->minmtu will be progressively increased, and at some point, n->mtu will be fixed to n->minmtu. // If the MTU is already fixed, this function checks if it can be increased. @@ -922,6 +986,11 @@ static void try_mtu(node_t *n) { if(n->maxmtu + 1 < MTU) send_udp_probe_packet(n, n->maxmtu + 1); } else { + /* Before initial discovery begins, set maxmtu to the most likely value. + If it's underestimated, we will correct it after initial discovery. */ + if(n->mtuprobes == 0) + n->maxmtu = choose_initial_maxmtu(n); + /* 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;