j3d1/tinc
1
0
Fork 0
Code Issues 1 Pull requests Releases Wiki Activity

Add IP_MTU-based maxmtu estimation.

Browse source 
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/
This commit is contained in:
Etienne Dechamps 2014-12-31 16:12:11 +00:00
parent c1532035e2
commit bce17c83e8
1 changed files with 70 additions and 1 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

71
src/net_packet.c
View file

@ -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;