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disco,net/tstun,wgengine/magicsock: probe peer MTU

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Automatically probe the path MTU to a peer when peer MTU is enabled, but do not
use the MTU information for anything yet.

Updates #311

Signed-off-by: Val <valerie@tailscale.com>
This commit is contained in:
Val
2023-10-05 20:05:19 +02:00
committed by valscale
parent b4e587c3bd
commit 893bdd729c
7 changed files with 91 additions and 40 deletions
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33
net/tstun/mtu.go
View File

@@ -79,14 +79,16 @@ const (
safeTUNMTU TUNMTU = 1280
)
// MaxProbedWireMTU is the largest MTU we will test for path MTU
// discovery.
var MaxProbedWireMTU WireMTU = 9000
func init() {
if MaxProbedWireMTU > WireMTU(maxTUNMTU) {
MaxProbedWireMTU = WireMTU(maxTUNMTU)
}
// WireMTUsToProbe is a list of the on-the-wire MTUs we want to probe. Each time
// magicsock discovery begins, it will send a set of pings, one of each size
// listed below.
var WireMTUsToProbe = []WireMTU{
WireMTU(safeTUNMTU), // Tailscale over Tailscale :)
TUNToWireMTU(safeTUNMTU), // Smallest MTU allowed for IPv6, current default
1400, // Most common MTU minus a few bytes for tunnels
1500, // Most common MTU
8000, // Should fit inside all jumbo frame sizes
9000, // Most jumbo frames are this size or larger
}
// wgHeaderLen is the length of all the headers Wireguard adds to a packet
@@ -125,7 +127,7 @@ func WireToTUNMTU(w WireMTU) TUNMTU {
// MTU. It is also the path MTU that we default to if we have no
// information about the path to a peer.
//
// 1. If set, the value of TS_DEBUG_MTU clamped to a maximum of MaxTunMTU
// 1. If set, the value of TS_DEBUG_MTU clamped to a maximum of MaxTUNMTU
// 2. If TS_DEBUG_ENABLE_PMTUD is set, the maximum size MTU we probe, minus wg overhead
// 3. If TS_DEBUG_ENABLE_PMTUD is not set, the Safe MTU
func DefaultTUNMTU() TUNMTU {
@@ -135,12 +137,23 @@ func DefaultTUNMTU() TUNMTU {
debugPMTUD, _ := envknob.LookupBool("TS_DEBUG_ENABLE_PMTUD")
if debugPMTUD {
return WireToTUNMTU(MaxProbedWireMTU)
// TODO: While we are just probing MTU but not generating PTB,
// this has to continue to return the safe MTU. When we add the
// code to generate PTB, this will be:
//
// return WireToTUNMTU(maxProbedWireMTU)
return safeTUNMTU
}
return safeTUNMTU
}
// SafeWireMTU returns the wire MTU that is safe to use if we have no
// information about the path MTU to this peer.
func SafeWireMTU() WireMTU {
return TUNToWireMTU(safeTUNMTU)
}
// DefaultWireMTU returns the default TUN MTU, adjusted for wireguard
// overhead.
func DefaultWireMTU() WireMTU {

13
net/tstun/mtu_test.go
View File

@@ -39,15 +39,18 @@ func TestDefaultTunMTU(t *testing.T) {
t.Errorf("default TUN MTU = %d, want %d, clamping failed", DefaultTUNMTU(), maxTUNMTU)
}
// If PMTUD is enabled, the MTU should default to the largest probed
// MTU, but only if the user hasn't requested a specific MTU.
// If PMTUD is enabled, the MTU should default to the safe MTU, but only
// if the user hasn't requested a specific MTU.
//
// TODO: When PMTUD is generating PTB responses, this will become the
// largest MTU we probe.
os.Setenv("TS_DEBUG_MTU", "")
os.Setenv("TS_DEBUG_ENABLE_PMTUD", "true")
if DefaultTUNMTU() != WireToTUNMTU(MaxProbedWireMTU) {
t.Errorf("default TUN MTU = %d, want %d", DefaultTUNMTU(), WireToTUNMTU(MaxProbedWireMTU))
if DefaultTUNMTU() != safeTUNMTU {
t.Errorf("default TUN MTU = %d, want %d", DefaultTUNMTU(), safeTUNMTU)
}
// TS_DEBUG_MTU should take precedence over TS_DEBUG_ENABLE_PMTUD.
mtu = WireToTUNMTU(MaxProbedWireMTU - 1)
mtu = WireToTUNMTU(MaxPacketSize - 1)
os.Setenv("TS_DEBUG_MTU", strconv.Itoa(int(mtu)))
if DefaultTUNMTU() != mtu {
t.Errorf("default TUN MTU = %d, want %d", DefaultTUNMTU(), mtu)