wgengine,net,ipn,disco: split up and define different types of MTU

Prepare for path MTU discovery by splitting up the concept of
DefaultMTU() into the concepts of the Tailscale TUN MTU, MTUs of
underlying network interfaces, minimum "safe" TUN MTU, user configured
TUN MTU, probed path MTU to a peer, and maximum probed MTU. Add a set
of likely MTUs to probe.

Updates #311

Signed-off-by: Val <valerie@tailscale.com>

net/tstun/mtu_test.go

@@ -4,25 +4,93 @@ package tstun
 
 import (
 	"os"
+	"strconv"
 	"testing"
 )
 
-func TestDefaultMTU(t *testing.T) {
-	orig := os.Getenv("TS_DEBUG_MTU")
-	defer os.Setenv("TS_DEBUG_MTU", orig)
+// Test the default MTU in the presence of various envknobs.
+func TestDefaultTunMTU(t *testing.T) {
+	// Save and restore the envknobs we will be changing.
 
+	// TS_DEBUG_MTU sets the MTU to a specific value.
+	defer os.Setenv("TS_DEBUG_MTU", os.Getenv("TS_DEBUG_MTU"))
 	os.Setenv("TS_DEBUG_MTU", "")
-	if DefaultMTU() != 1280 {
-		t.Errorf("DefaultMTU() = %d, want 1280", DefaultMTU())
+
+	// TS_DEBUG_ENABLE_PMTUD enables path MTU discovery.
+	defer os.Setenv("TS_DEBUG_ENABLE_PMTUD", os.Getenv("TS_DEBUG_ENABLE_PMTUD"))
+	os.Setenv("TS_DEBUG_ENABLE_PMTUD", "")
+
+	// With no MTU envknobs set, we should get the conservative MTU.
+	if DefaultTUNMTU() != safeTUNMTU {
+		t.Errorf("default TUN MTU = %d, want %d", DefaultTUNMTU(), safeTUNMTU)
 	}
 
-	os.Setenv("TS_DEBUG_MTU", "9000")
-	if DefaultMTU() != 9000 {
-		t.Errorf("DefaultMTU() = %d, want 9000", DefaultMTU())
+	// If set, TS_DEBUG_MTU should set the MTU.
+	mtu := maxTUNMTU - 1
+	os.Setenv("TS_DEBUG_MTU", strconv.Itoa(int(mtu)))
+	if DefaultTUNMTU() != mtu {
+		t.Errorf("default TUN MTU = %d, want %d, TS_DEBUG_MTU ignored", DefaultTUNMTU(), mtu)
 	}
 
-	os.Setenv("TS_DEBUG_MTU", "123456789")
-	if DefaultMTU() != maxMTU {
-		t.Errorf("DefaultMTU() = %d, want %d", DefaultMTU(), maxMTU)
+	// MTU should be clamped to maxTunMTU.
+	mtu = maxTUNMTU + 1
+	os.Setenv("TS_DEBUG_MTU", strconv.Itoa(int(mtu)))
+	if DefaultTUNMTU() != maxTUNMTU {
+		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.
+	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))
+	}
+	// TS_DEBUG_MTU should take precedence over TS_DEBUG_ENABLE_PMTUD.
+	mtu = WireToTUNMTU(MaxProbedWireMTU - 1)
+	os.Setenv("TS_DEBUG_MTU", strconv.Itoa(int(mtu)))
+	if DefaultTUNMTU() != mtu {
+		t.Errorf("default TUN MTU = %d, want %d", DefaultTUNMTU(), mtu)
+	}
+}
+
+// Test the conversion of wire MTU to/from Tailscale TUN MTU corner cases.
+func TestMTUConversion(t *testing.T) {
+	tests := []struct {
+		w WireMTU
+		t TUNMTU
+	}{
+		{w: 0, t: 0},
+		{w: wgHeaderLen - 1, t: 0},
+		{w: wgHeaderLen, t: 0},
+		{w: wgHeaderLen + 1, t: 1},
+		{w: 1360, t: 1280},
+		{w: 1500, t: 1420},
+		{w: 9000, t: 8920},
+	}
+
+	for _, tt := range tests {
+		m := WireToTUNMTU(tt.w)
+		if m != tt.t {
+			t.Errorf("conversion of wire MTU %v to TUN MTU = %v, want %v", tt.w, m, tt.t)
+		}
+	}
+
+	tests2 := []struct {
+		t TUNMTU
+		w WireMTU
+	}{
+		{t: 0, w: wgHeaderLen},
+		{t: 1, w: wgHeaderLen + 1},
+		{t: 1280, w: 1360},
+		{t: 1420, w: 1500},
+		{t: 8920, w: 9000},
+	}
+
+	for _, tt := range tests2 {
+		m := TUNToWireMTU(tt.t)
+		if m != tt.w {
+			t.Errorf("conversion of TUN MTU %v to wire MTU = %v, want %v", tt.t, m, tt.w)
+		}
 	}
 }