GSO/GRO offloads, with TCP+ECN and UDP support

connection_state.go

@@ -10,7 +10,7 @@ import (
 	"github.com/slackhq/nebula/noiseutil"
 )
 
-const ReplayWindow = 1024
+const ReplayWindow = 8192
 
 type ConnectionState struct {
 	eKey           noiseutil.CipherState

cpupin_linux.go

@@ -0,0 +1,23 @@
+//go:build linux && !android && !e2e_testing
+
+package nebula
+
+import (
+	"runtime"
+
+	"golang.org/x/sys/unix"
+)
+
+// pinThreadToCPU restricts the calling OS thread to the given CPU via
+// sched_setaffinity(2). Combined with runtime.LockOSThread on the
+// goroutine, this prevents the kernel from migrating us across CPUs and
+// in turn keeps every sendmmsg from this goroutine going through the
+// same XPS-selected TX ring, eliminating the wire-side reorder that
+// otherwise fragments one nebula flow across multiple rings.
+func pinThreadToCPU(cpu int) error {
+	runtime.LockOSThread()
+	var set unix.CPUSet
+	set.Zero()
+	set.Set(cpu)
+	return unix.SchedSetaffinity(0, &set)
+}

cpupin_other.go

@@ -0,0 +1,11 @@
+//go:build !linux || android || e2e_testing
+
+package nebula
+
+// pinThreadToCPU is a no-op outside Linux: only Linux exposes a stable
+// per-thread CPU affinity API and only Linux has XPS-driven TX ring
+// selection in the first place. On every other platform there's nothing
+// to fix here.
+func pinThreadToCPU(_ int) error {
+	return nil
+}

ecn_inner_test.go

@@ -0,0 +1,125 @@
+package nebula
+
+import (
+	"io"
+	"log/slog"
+	"testing"
+)
+
+func TestInnerECN(t *testing.T) {
+	cases := []struct {
+		name string
+		pkt  []byte
+		want byte
+	}{
+		{"empty", nil, 0},
+		{"v4_NotECT", v4WithToS(0x00), 0x00},
+		{"v4_ECT0", v4WithToS(0x02), 0x02},
+		{"v4_ECT1", v4WithToS(0x01), 0x01},
+		{"v4_CE", v4WithToS(0x03), 0x03},
+		{"v4_DSCP_then_NotECT", v4WithToS(0x88 | 0x00), 0x00},
+		{"v4_DSCP_then_CE", v4WithToS(0x88 | 0x03), 0x03},
+		{"v6_NotECT", v6WithTC(0x00), 0x00},
+		{"v6_ECT0", v6WithTC(0x02), 0x02},
+		{"v6_CE", v6WithTC(0x03), 0x03},
+		{"v6_DSCP_then_CE", v6WithTC(0x88 | 0x03), 0x03},
+		{"unknown_version", []byte{0xa5, 0xff}, 0},
+	}
+	for _, c := range cases {
+		t.Run(c.name, func(t *testing.T) {
+			got := innerECN(c.pkt)
+			if got != c.want {
+				t.Errorf("innerECN=0x%02x want 0x%02x", got, c.want)
+			}
+		})
+	}
+}
+
+// v4WithToS returns a 2-byte slice tall enough for innerECN: byte 0 carries
+// version=4 in the high nibble, byte 1 is the full ToS so we exercise both
+// the DSCP and ECN portions through the byte 1 mask.
+func v4WithToS(tos byte) []byte {
+	return []byte{0x45, tos}
+}
+
+// v6WithTC builds a 2-byte slice that places a known traffic class value
+// across bytes 0 (high nibble of TC) and 1 (low nibble of TC). innerECN
+// extracts ECN as (b[1]>>4)&0x03, which corresponds to TC[1:0].
+func v6WithTC(tc byte) []byte {
+	return []byte{0x60 | (tc>>4)&0x0f, (tc & 0x0f) << 4}
+}
+
+func TestApplyOuterECN(t *testing.T) {
+	silent := slog.New(slog.NewTextHandler(io.Discard, nil))
+	hi := &HostInfo{}
+
+	// Build a v4 packet helper with a given inner ECN field.
+	v4 := func(innerECN byte) []byte {
+		// 20-byte minimal IPv4 header with ToS = innerECN (DSCP zeroed).
+		return []byte{
+			0x45, innerECN, 0, 28,
+			0, 0, 0x40, 0,
+			64, 6, 0, 0,
+			10, 0, 0, 1,
+			10, 0, 0, 2,
+		}
+	}
+	// Build a v6 packet helper with a given inner ECN field. ECN occupies
+	// TC[1:0] which sit at byte 1 mask 0x30.
+	v6 := func(innerECN byte) []byte {
+		// 40-byte minimal IPv6 header with TC[1:0] = innerECN.
+		pkt := make([]byte, 40)
+		pkt[0] = 0x60                   // version=6, TC[7:4]=0
+		pkt[1] = (innerECN & 0x03) << 4 // TC[3:0]: low 2 bits = ECN, top 2 = DSCP-low (0)
+		return pkt
+	}
+
+	type cell struct {
+		outer    byte
+		inner    byte
+		wantECN  byte
+		wantSame bool // expect inner unchanged (true => verify the byte didn't move)
+	}
+
+	// RFC 6040 normal-mode combine table. Only outer==CE causes mutation.
+	table := []cell{
+		{ecnNotECT, ecnNotECT, ecnNotECT, true},
+		{ecnNotECT, ecnECT0, ecnECT0, true},
+		{ecnNotECT, ecnECT1, ecnECT1, true},
+		{ecnNotECT, ecnCE, ecnCE, true},
+
+		{ecnECT0, ecnNotECT, ecnNotECT, true},
+		{ecnECT0, ecnECT0, ecnECT0, true},
+		{ecnECT0, ecnECT1, ecnECT1, true},
+		{ecnECT0, ecnCE, ecnCE, true},
+
+		{ecnECT1, ecnNotECT, ecnNotECT, true},
+		{ecnECT1, ecnECT0, ecnECT0, true},
+		{ecnECT1, ecnECT1, ecnECT1, true},
+		{ecnECT1, ecnCE, ecnCE, true},
+
+		{ecnCE, ecnNotECT, ecnNotECT, true}, // legacy: log, leave alone
+		{ecnCE, ecnECT0, ecnCE, false},      // CE folded in
+		{ecnCE, ecnECT1, ecnCE, false},
+		{ecnCE, ecnCE, ecnCE, true},
+	}
+
+	for _, c := range table {
+		t.Run("v4", func(t *testing.T) {
+			pkt := v4(c.inner)
+			applyOuterECN(pkt, c.outer, hi, silent)
+			got := pkt[1] & 0x03
+			if got != c.wantECN {
+				t.Errorf("v4 outer=0x%02x inner=0x%02x: got 0x%02x want 0x%02x", c.outer, c.inner, got, c.wantECN)
+			}
+		})
+		t.Run("v6", func(t *testing.T) {
+			pkt := v6(c.inner)
+			applyOuterECN(pkt, c.outer, hi, silent)
+			got := (pkt[1] >> 4) & 0x03
+			if got != c.wantECN {
+				t.Errorf("v6 outer=0x%02x inner=0x%02x: got 0x%02x want 0x%02x", c.outer, c.inner, got, c.wantECN)
+			}
+		})
+	}
+}

firewall/cache.go

@@ -5,6 +5,8 @@ import (
 	"log/slog"
 	"sync/atomic"
 	"time"
+
+	"github.com/slackhq/nebula/logging"
 )
 
 // ConntrackCache is used as a local routine cache to know if a given flow
@@ -56,8 +58,8 @@ func (c *ConntrackCacheTicker) Get() ConntrackCache {
 	if tick := c.cacheTick.Load(); tick != c.cacheV {
 		c.cacheV = tick
 		if ll := len(c.cache); ll > 0 {
-			if c.l.Enabled(context.Background(), slog.LevelDebug) {
-				c.l.Debug("resetting conntrack cache", "len", ll)
+			if c.l.Enabled(context.Background(), logging.LevelTrace) {
+				c.l.Log(context.Background(), logging.LevelTrace, "resetting conntrack cache", "len", ll)
 			}
 			c.cache = make(ConntrackCache, ll)
 		}

firewall/cache_test.go

@@ -6,6 +6,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/slackhq/nebula/logging"
 	"github.com/slackhq/nebula/test"
 	"github.com/stretchr/testify/assert"
 )
@@ -30,27 +31,27 @@ func newFixedTicker(t *testing.T, l *slog.Logger, cacheLen int) *ConntrackCacheT
 
 func TestConntrackCacheTicker_Get_TextFormat(t *testing.T) {
 	buf := &bytes.Buffer{}
-	l := test.NewLoggerWithOutputAndLevel(buf, slog.LevelDebug)
+	l := test.NewLoggerWithOutputAndLevel(buf, logging.LevelTrace)
 
 	c := newFixedTicker(t, l, 3)
 	c.Get()
 
-	assert.Equal(t, "level=DEBUG msg=\"resetting conntrack cache\" len=3\n", buf.String())
+	assert.Equal(t, "level=DEBUG-4 msg=\"resetting conntrack cache\" len=3\n", buf.String())
 }
 
 func TestConntrackCacheTicker_Get_JSONFormat(t *testing.T) {
 	buf := &bytes.Buffer{}
-	l := test.NewJSONLoggerWithOutput(buf, slog.LevelDebug)
+	l := test.NewJSONLoggerWithOutput(buf, logging.LevelTrace)
 
 	c := newFixedTicker(t, l, 2)
 	c.Get()
 
-	assert.JSONEq(t, `{"level":"DEBUG","msg":"resetting conntrack cache","len":2}`, strings.TrimSpace(buf.String()))
+	assert.JSONEq(t, `{"level":"DEBUG-4","msg":"resetting conntrack cache","len":2}`, strings.TrimSpace(buf.String()))
 }
 
-func TestConntrackCacheTicker_Get_QuietBelowDebug(t *testing.T) {
+func TestConntrackCacheTicker_Get_QuietBelowTrace(t *testing.T) {
 	buf := &bytes.Buffer{}
-	l := test.NewLoggerWithOutputAndLevel(buf, slog.LevelInfo)
+	l := test.NewLoggerWithOutputAndLevel(buf, slog.LevelDebug)
 
 	c := newFixedTicker(t, l, 5)
 	c.Get()
@@ -60,7 +61,7 @@ func TestConntrackCacheTicker_Get_QuietBelowDebug(t *testing.T) {
 
 func TestConntrackCacheTicker_Get_QuietWhenCacheEmpty(t *testing.T) {
 	buf := &bytes.Buffer{}
-	l := test.NewLoggerWithOutputAndLevel(buf, slog.LevelDebug)
+	l := test.NewLoggerWithOutputAndLevel(buf, logging.LevelTrace)
 
 	c := newFixedTicker(t, l, 0)
 	c.Get()

go.mod

@@ -43,6 +43,7 @@ require (
 	github.com/cespare/xxhash/v2 v2.3.0 // indirect
 	github.com/davecgh/go-spew v1.1.1 // indirect
 	github.com/google/btree v1.1.2 // indirect
+	github.com/guptarohit/asciigraph v0.9.0 // indirect
 	github.com/munnerz/goautoneg v0.0.0-20191010083416-a7dc8b61c822 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/prometheus/client_model v0.6.2 // indirect

go.sum

@@ -60,6 +60,8 @@ github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX
 github.com/google/gofuzz v1.0.0/go.mod h1:dBl0BpW6vV/+mYPU4Po3pmUjxk6FQPldtuIdl/M65Eg=
 github.com/google/gopacket v1.1.19 h1:ves8RnFZPGiFnTS0uPQStjwru6uO6h+nlr9j6fL7kF8=
 github.com/google/gopacket v1.1.19/go.mod h1:iJ8V8n6KS+z2U1A8pUwu8bW5SyEMkXJB8Yo/Vo+TKTo=
+github.com/guptarohit/asciigraph v0.9.0 h1:MvCSRRVkT2XvU1IO6n92o7l7zqx1DiFaoszOUZQztbY=
+github.com/guptarohit/asciigraph v0.9.0/go.mod h1:dYl5wwK4gNsnFf9Zp+l06rFiDZ5YtXM6x7SRWZ3KGag=
 github.com/jpillora/backoff v1.0.0/go.mod h1:J/6gKK9jxlEcS3zixgDgUAsiuZ7yrSoa/FX5e0EB2j4=
 github.com/json-iterator/go v1.1.6/go.mod h1:+SdeFBvtyEkXs7REEP0seUULqWtbJapLOCVDaaPEHmU=
 github.com/json-iterator/go v1.1.10/go.mod h1:KdQUCv79m/52Kvf8AW2vK1V8akMuk1QjK/uOdHXbAo4=

inside.go

@@ -10,10 +10,23 @@ import (
 	"github.com/slackhq/nebula/iputil"
 	"github.com/slackhq/nebula/noiseutil"
 	"github.com/slackhq/nebula/overlay/batch"
+	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/routing"
 )
 
-func (f *Interface) consumeInsidePacket(packet []byte, fwPacket *firewall.Packet, nb []byte, sendBatch batch.TxBatcher, rejectBuf []byte, q int, localCache firewall.ConntrackCache) {
+func (f *Interface) consumeInsidePacket(pkt tio.Packet, fwPacket *firewall.Packet, nb []byte, sendBatch batch.TxBatcher, rejectBuf []byte, q int, localCache firewall.ConntrackCache) {
+	// borrowed: pkt.Bytes is owned by the originating tio.Queue and is
+	// only valid until the next Read on that queue. Every consumer below
+	// (parse, self-forward, handshake cache, sendInsideMessage) reads it
+	// synchronously; do not retain pkt outside this call. If a future
+	// caller needs to keep the packet, use pkt.Clone() to detach it from
+	// the borrow.
+	//
+	// pkt.Bytes is either one IP datagram (GSO zero) or a TSO/USO
+	// superpacket. In both cases the L3+L4 headers at the start describe
+	// the same 5-tuple every segment will share, so a single newPacket /
+	// firewall check covers the whole superpacket.
+	packet := pkt.Bytes
 	err := newPacket(packet, false, fwPacket)
 	if err != nil {
 		if f.l.Enabled(context.Background(), slog.LevelDebug) {
@@ -38,7 +51,14 @@ func (f *Interface) consumeInsidePacket(packet []byte, fwPacket *firewall.Packet
 		// routes packets from the Nebula addr to the Nebula addr through the Nebula
 		// TUN device.
 		if immediatelyForwardToSelf {
-			_, err := f.readers[q].Write(packet)
+			// Write copies into the kernel queue synchronously, so seg's lifetime ends at return.
+			// A self-forwarded superpacket would be re-handed to the
+			// kernel as one giant blob; segment first so the loopback
+			// path sees one IP datagram per Write.
+			err := tio.SegmentSuperpacket(pkt, func(seg []byte) error {
+				_, werr := f.readers[q].Write(seg)
+				return werr
+			})
 			if err != nil {
 				f.l.Error("Failed to forward to tun", "error", err)
 			}
@@ -54,7 +74,19 @@ func (f *Interface) consumeInsidePacket(packet []byte, fwPacket *firewall.Packet
 	}
 
 	hostinfo, ready := f.getOrHandshakeConsiderRouting(fwPacket, func(hh *HandshakeHostInfo) {
-		hh.cachePacket(f.l, header.Message, 0, packet, f.sendMessageNow, f.cachedPacketMetrics)
+		// borrowed: SegmentSuperpacket builds each segment in the kernel-supplied pkt
+		// bytes underneath. cachePacket explicitly copies its argument (handshake_manager.go cachePacket),
+		// so retaining segments past the loop is safe.
+		err := tio.SegmentSuperpacket(pkt, func(seg []byte) error {
+			hh.cachePacket(f.l, header.Message, 0, seg, f.sendMessageNow, f.cachedPacketMetrics)
+			return nil
+		})
+		if err != nil && f.l.Enabled(context.Background(), slog.LevelDebug) {
+			f.l.Debug("Failed to segment superpacket for handshake cache",
+				"error", err,
+				"vpnAddr", fwPacket.RemoteAddr,
+			)
+		}
 	})
 
 	if hostinfo == nil {
@@ -74,7 +106,7 @@ func (f *Interface) consumeInsidePacket(packet []byte, fwPacket *firewall.Packet
 
 	dropReason := f.firewall.Drop(*fwPacket, false, hostinfo, f.pki.GetCAPool(), localCache)
 	if dropReason == nil {
-		f.sendInsideMessage(hostinfo, packet, nb, sendBatch, rejectBuf, q)
+		f.sendInsideMessage(hostinfo, pkt, nb, sendBatch, rejectBuf, q)
 	} else {
 		f.rejectInside(packet, rejectBuf, q)
 		if f.l.Enabled(context.Background(), slog.LevelDebug) {
@@ -86,39 +118,46 @@ func (f *Interface) consumeInsidePacket(packet []byte, fwPacket *firewall.Packet
 	}
 }
 
-// sendInsideMessage encrypts a firewall-approved inside packet into the
-// caller's batch slot for later sendmmsg flush. When hostinfo.remote is not
-// valid we fall through to the relay slow path via the unbatched sendNoMetrics
-// so relay behavior is unchanged.
-func (f *Interface) sendInsideMessage(hostinfo *HostInfo, p, nb []byte, sendBatch batch.TxBatcher, rejectBuf []byte, q int) {
+// sendInsideMessage encrypts a firewall-approved inside packet (or every
+// segment of a TSO/USO superpacket) into the caller's batch slot for
+// later sendmmsg flush. Segmentation is fused with encryption here so the
+// kernel-supplied superpacket bytes never get written into a separate
+// scratch arena: SegmentSuperpacket builds each segment's plaintext in
+// segScratch[:segLen] in turn, and we encrypt directly into a fresh
+// SendBatch slot.
+//
+// When hostinfo.remote is not valid we fall through to the relay slow
+// path via the unbatched sendNoMetrics so relay behavior is unchanged;
+// each segment of a superpacket goes through that path independently.
+// sendInsideMessage takes a borrowed pkt: pkt.Bytes is only valid until the
+// next Read on the originating tio.Queue. Each segment is encrypted into a
+// fresh sendBatch slot (Reserve returns owned scratch), so the borrow ends
+// inside the SegmentSuperpacket callback below. Do not retain pkt or any
+// seg slice past the callback's return.
+func (f *Interface) sendInsideMessage(hostinfo *HostInfo, pkt tio.Packet, nb []byte, sendBatch batch.TxBatcher, rejectBuf []byte, q int) {
 	ci := hostinfo.ConnectionState
 	if ci.eKey == nil {
 		return
 	}
 
-	if !hostinfo.remote.IsValid() {
+	remote := hostinfo.GetRemote()
+	if !remote.IsValid() {
 		// Slow path: relay fallback. Reuse rejectBuf as the ciphertext
 		// scratch; sendNoMetrics arranges header space for SendVia.
-		f.sendNoMetrics(header.Message, 0, ci, hostinfo, netip.AddrPort{}, p, nb, rejectBuf, q)
-		return
-	}
-
-	scratch := sendBatch.Next()
-	if scratch == nil {
-		// Batch full: bypass batching and send this packet directly so we
-		// never drop traffic on over-subscribed iterations.
-		f.sendNoMetrics(header.Message, 0, ci, hostinfo, netip.AddrPort{}, p, nb, rejectBuf, q)
+		// Segment any superpacket so each segment is sized to fit a
+		// single relay encap.
+		err := tio.SegmentSuperpacket(pkt, func(seg []byte) error {
+			f.sendNoMetrics(header.Message, 0, ci, hostinfo, netip.AddrPort{}, seg, nb, rejectBuf, q)
+			return nil
+		})
+		if err != nil {
+			hostinfo.logger(f.l).Error("Failed to segment superpacket for relay send",
+				"error", err,
+			)
+		}
 		return
 	}
 
-	if noiseutil.EncryptLockNeeded {
-		ci.writeLock.Lock()
-	}
-	c := ci.messageCounter.Add(1)
-
-	out := header.Encode(scratch, header.Version, header.Message, 0, hostinfo.remoteIndexId, c)
-	f.connectionManager.Out(hostinfo)
-
 	if hostinfo.lastRebindCount != f.rebindCount {
 		//NOTE: there is an update hole if a tunnel isn't used and exactly 256 rebinds occur before the tunnel is
 		// finally used again. This tunnel would eventually be torn down and recreated if this action didn't help.
@@ -131,20 +170,63 @@ func (f *Interface) sendInsideMessage(hostinfo *HostInfo, p, nb []byte, sendBatc
 		}
 	}
 
-	out, err := ci.eKey.EncryptDanger(out, out, p, c, nb)
-	if noiseutil.EncryptLockNeeded {
-		ci.writeLock.Unlock()
-	}
+	ecnEnabled := f.ecnEnabled.Load()
+
+	err := tio.SegmentSuperpacket(pkt, func(seg []byte) error {
+		// header + plaintext + AEAD tag (16 bytes for both AES-GCM and ChaCha20-Poly1305)
+		scratch := sendBatch.Reserve(header.Len + len(seg) + 16)
+
+		if noiseutil.EncryptLockNeeded {
+			ci.writeLock.Lock()
+		}
+		c := ci.messageCounter.Add(1)
+
+		out := header.Encode(scratch, header.Version, header.Message, 0, hostinfo.remoteIndexId, c)
+		f.connectionManager.Out(hostinfo)
+
+		out, encErr := ci.eKey.EncryptDanger(out, out, seg, c, nb)
+		if noiseutil.EncryptLockNeeded {
+			ci.writeLock.Unlock()
+		}
+		if encErr != nil {
+			hostinfo.logger(f.l).Error("Failed to encrypt outgoing packet",
+				"error", encErr,
+				"udpAddr", remote,
+				"counter", c,
+			)
+			// Skip this segment; the rest of the superpacket can still
+			// go out — TCP will retransmit anything we drop here.
+			return nil
+		}
+
+		var ecn byte
+		if ecnEnabled {
+			ecn = innerECN(seg)
+		}
+		sendBatch.Commit(out, remote, ecn)
+		return nil
+	})
 	if err != nil {
-		hostinfo.logger(f.l).Error("Failed to encrypt outgoing packet",
+		hostinfo.logger(f.l).Error("Failed to segment superpacket for send",
 			"error", err,
-			"udpAddr", hostinfo.remote,
-			"counter", c,
 		)
-		return
 	}
+}
 
-	sendBatch.Commit(len(out), hostinfo.remote)
+// innerECN returns the 2-bit IP-level ECN codepoint of an inner IPv4 or IPv6
+// packet, or 0 if pkt is too short or its IP version is unrecognized. Used at
+// encap to copy the inner codepoint onto the outer carrier per RFC 6040.
+func innerECN(pkt []byte) byte {
+	if len(pkt) < 2 {
+		return 0
+	}
+	switch pkt[0] >> 4 {
+	case 4:
+		return pkt[1] & 0x03
+	case 6:
+		return (pkt[1] >> 4) & 0x03
+	}
+	return 0
 }
 
 func (f *Interface) rejectInside(packet []byte, out []byte, q int) {

interface.go

@@ -6,6 +6,7 @@ import (
 	"fmt"
 	"log/slog"
 	"net/netip"
+	"runtime"
 	"slices"
 	"sync"
 	"sync/atomic"
@@ -50,7 +51,14 @@ type InterfaceConfig struct {
 	reQueryWait     time.Duration
 
 	ConntrackCacheTimeout time.Duration
-	l                     *slog.Logger
+
+	// CpuAffinity, when non-empty, names the CPUs each TUN reader goroutine
+	// should pin to. Queue i pins to CpuAffinity[i % len(CpuAffinity)] —
+	// shorter lists than `routines` cycle. Empty list keeps the default
+	// pin-to-(i % NumCPU) behavior.
+	CpuAffinity []int
+
+	l *slog.Logger
 }
 
 type Interface struct {
@@ -74,7 +82,16 @@ type Interface struct {
 	routines              int
 	disconnectInvalid     atomic.Bool
 	closed                atomic.Bool
-	relayManager          *relayManager
+	// cpuAffinity, when non-empty, names the CPUs each TUN reader goroutine
+	// should pin to. Queue i pins to cpuAffinity[i % len(cpuAffinity)].
+	// Empty falls back to the default pin-to-(i % NumCPU) behavior.
+	cpuAffinity []int
+	// ecnEnabled gates RFC 6040 underlay ECN propagation. When true,
+	// inside.go copies the inner ECN onto the outer carrier on encap and
+	// decryptToTun folds outer CE into the inner header on decap. Toggle
+	// via tunnels.ecn (default true).
+	ecnEnabled   atomic.Bool
+	relayManager *relayManager
 
 	tryPromoteEvery atomic.Uint32
 	reQueryEvery    atomic.Uint32
@@ -204,6 +221,7 @@ func NewInterface(ctx context.Context, c *InterfaceConfig) (*Interface, error) {
 		relayManager:          c.relayManager,
 		connectionManager:     c.connectionManager,
 		conntrackCacheTimeout: c.ConntrackCacheTimeout,
+		cpuAffinity:           c.CpuAffinity,
 
 		metricHandshakes: metrics.GetOrRegisterHistogram("handshakes", nil, metrics.NewExpDecaySample(1028, 0.015)),
 		messageMetrics:   c.MessageMetrics,
@@ -265,7 +283,16 @@ func (f *Interface) activate() error {
 	}
 	f.readers = f.inside.Readers()
 	for i := range f.readers {
-		f.batchers[i] = batch.NewPassthrough(f.readers[i])
+		caps := tio.QueueCapabilities(f.readers[i])
+		if caps.TSO || caps.USO {
+			// Multi-lane: TCP gets coalesced when TSO is on, UDP when USO
+			// is on, everything else (and either lane disabled) falls
+			// through to passthrough so non-IP / non-TCP-UDP traffic still
+			// reaches the TUN.
+			f.batchers[i] = batch.NewMultiCoalescer(f.readers[i], caps.TSO, caps.USO)
+		} else {
+			f.batchers[i] = batch.NewPassthrough(f.readers[i])
+		}
 	}
 
 	// On error the caller owns the cleanup, Control.Start cancels the service context
@@ -327,15 +354,13 @@ func (f *Interface) listenOut(i int) {
 	fwPacket := &firewall.Packet{}
 	nb := make([]byte, 12, 12)
 
-	coalescer := f.batchers[i]
-
-	listener := func(fromUdpAddr netip.AddrPort, payload []byte) {
+	listener := func(fromUdpAddr netip.AddrPort, payload []byte, meta udp.RxMeta) {
 		plaintext := f.batchers[i].Reserve(len(payload))
-		f.readOutsidePackets(ViaSender{UdpAddr: fromUdpAddr}, plaintext[:0], payload, h, fwPacket, lhh, nb, i, ctCache.Get())
+		f.readOutsidePackets(ViaSender{UdpAddr: fromUdpAddr}, plaintext[:0], payload, h, fwPacket, lhh, nb, i, ctCache.Get(), meta)
 	}
 
 	flusher := func() {
-		if err := coalescer.Flush(); err != nil {
+		if err := f.batchers[i].Flush(); err != nil {
 			f.l.Error("Failed to flush tun coalescer", "error", err)
 		}
 	}
@@ -354,8 +379,27 @@ func (f *Interface) listenOut(i int) {
 }
 
 func (f *Interface) listenIn(reader tio.Queue, i int) {
+	// Pin this goroutine to one CPU. LockOSThread alone keeps the goroutine
+	// on a single OS thread but the kernel can still migrate that thread
+	// across CPUs — XPS reads smp_processor_id() at sendmmsg time and picks
+	// the TX ring from the current CPU's xps_cpus map, so an unpinned
+	// thread bouncing between CPUs spreads one nebula flow's packets across
+	// multiple TX rings, which the rings then drain at independent rates
+	// and the wire delivers reordered.
+	//
+	// Pinning keeps every sendmmsg from this goroutine going through the
+	// same TX ring, so the wire sees per-flow order. Cost: less scheduler
+	// flexibility — if i % NumCPU collides between two TUN reader
+	// goroutines they share a CPU.
+	cpu := i % runtime.NumCPU()
+	if n := len(f.cpuAffinity); n > 0 {
+		cpu = f.cpuAffinity[i%n]
+	}
+	if err := pinThreadToCPU(cpu); err != nil {
+		f.l.Warn("failed to pin tun reader to CPU", "queue", i, "cpu", cpu, "err", err)
+	}
 	rejectBuf := make([]byte, mtu)
-	sb := batch.NewSendBatch(batch.SendBatchCap, udp.MTU+32)
+	sb := batch.NewSendBatch(f.writers[i], batch.SendBatchCap, udp.MTU+32)
 	fwPacket := &firewall.Packet{}
 	nb := make([]byte, 12, 12)
 
@@ -372,35 +416,24 @@ func (f *Interface) listenIn(reader tio.Queue, i int) {
 			break
 		}
 
-		sb.Reset()
 		for _, pkt := range pkts {
-			if sb.Len() >= sb.Cap() {
-				f.flushBatch(sb, i)
-				sb.Reset()
-			}
 			f.consumeInsidePacket(pkt, fwPacket, nb, sb, rejectBuf, i, conntrackCache.Get())
 		}
-		if sb.Len() > 0 {
-			f.flushBatch(sb, i)
+		if err := sb.Flush(); err != nil {
+			f.l.Error("Failed to write outgoing batch", "error", err, "writer", i)
 		}
 	}
 
 	f.l.Debug("overlay reader is done", "reader", i)
 }
 
-func (f *Interface) flushBatch(sb batch.TxBatcher, q int) {
-	bufs, dsts := sb.Get()
-	if err := f.writers[q].WriteBatch(bufs, dsts); err != nil {
-		f.l.Error("Failed to write outgoing batch", "error", err, "writer", q)
-	}
-}
-
 func (f *Interface) RegisterConfigChangeCallbacks(c *config.C) {
 	c.RegisterReloadCallback(f.reloadFirewall)
 	c.RegisterReloadCallback(f.reloadSendRecvError)
 	c.RegisterReloadCallback(f.reloadAcceptRecvError)
 	c.RegisterReloadCallback(f.reloadDisconnectInvalid)
 	c.RegisterReloadCallback(f.reloadMisc)
+	c.RegisterReloadCallback(f.reloadEcn)
 
 	for _, udpConn := range f.writers {
 		c.RegisterReloadCallback(udpConn.ReloadConfig)
@@ -533,6 +566,20 @@ func (f *Interface) reloadMisc(c *config.C) {
 	}
 }
 
+// reloadEcn syncs Interface.ecnEnabled with the tunnels.ecn config knob.
+// Default is enabled (RFC 6040 normal mode); set false on the rare path
+// where an underlay middlebox rewrites or drops ECN bits unpredictably.
+func (f *Interface) reloadEcn(c *config.C) {
+	initial := c.InitialLoad()
+	if initial || c.HasChanged("tunnels.ecn") {
+		v := c.GetBool("tunnels.ecn", true)
+		f.ecnEnabled.Store(v)
+		if !initial {
+			f.l.Info("tunnels.ecn changed", "enabled", v)
+		}
+	}
+}
+
 func (f *Interface) emitStats(ctx context.Context, i time.Duration) {
 	ticker := time.NewTicker(i)
 	defer ticker.Stop()

main.go

@@ -231,6 +231,7 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 		relayManager:          NewRelayManager(ctx, l, hostMap, c),
 		punchy:                punchy,
 		ConntrackCacheTimeout: conntrackCacheTimeout,
+		CpuAffinity:           parseCpuAffinity(c, l, routines),
 		l:                     l,
 	}
 
@@ -248,6 +249,7 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 		ifce.reloadDisconnectInvalid(c)
 		ifce.reloadSendRecvError(c)
 		ifce.reloadAcceptRecvError(c)
+		ifce.reloadEcn(c)
 
 		handshakeManager.f = ifce
 		go handshakeManager.Run(ctx)
@@ -282,6 +284,53 @@ func Main(c *config.C, configTest bool, buildVersion string, l *slog.Logger, dev
 	}, nil
 }
 
+// parseCpuAffinity reads `tun.cpu_affinity` from the config — a list of
+// integer CPU IDs, one per TUN reader goroutine. Empty / unset returns nil
+// (listenIn falls back to its default `i % NumCPU` pinning). Length
+// mismatch with `routines` is a warning, not an error: shorter lists are
+// modulo-cycled across queues, longer lists' tail is ignored. Invalid
+// entries (non-integer, out of range) are also a warning and disable the
+// override entirely so we don't silently pin to the wrong CPU.
+func parseCpuAffinity(c *config.C, l *slog.Logger, routines int) []int {
+	raw := c.Get("tun.cpu_affinity")
+	if raw == nil {
+		return nil
+	}
+	rv, ok := raw.([]any)
+	if !ok {
+		l.Warn("tun.cpu_affinity must be a list of integers; ignoring", "value", raw)
+		return nil
+	}
+	nCPU := runtime.NumCPU()
+	cpus := make([]int, 0, len(rv))
+	for i, e := range rv {
+		var cpu int
+		switch v := e.(type) {
+		case int:
+			cpu = v
+		case int64:
+			cpu = int(v)
+		case float64:
+			cpu = int(v)
+		default:
+			l.Warn("tun.cpu_affinity entry not an integer; ignoring affinity",
+				"index", i, "value", e)
+			return nil
+		}
+		if cpu < 0 || cpu >= nCPU {
+			l.Warn("tun.cpu_affinity entry out of range; ignoring affinity",
+				"index", i, "cpu", cpu, "num_cpu", nCPU)
+			return nil
+		}
+		cpus = append(cpus, cpu)
+	}
+	if len(cpus) != routines {
+		l.Warn("tun.cpu_affinity length doesn't match routines; queues will modulo-cycle through the list",
+			"affinity_len", len(cpus), "routines", routines)
+	}
+	return cpus
+}
+
 func moduleVersion() string {
 	info, ok := debug.ReadBuildInfo()
 	if !ok {

outside.go

@@ -13,6 +13,7 @@ import (
 
 	"github.com/slackhq/nebula/firewall"
 	"github.com/slackhq/nebula/header"
+	"github.com/slackhq/nebula/udp"
 	"golang.org/x/net/ipv4"
 )
 
@@ -22,7 +23,7 @@ const (
 
 var ErrOutOfWindow = errors.New("out of window packet")
 
-func (f *Interface) readOutsidePackets(via ViaSender, out []byte, packet []byte, h *header.H, fwPacket *firewall.Packet, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache) {
+func (f *Interface) readOutsidePackets(via ViaSender, out []byte, packet []byte, h *header.H, fwPacket *firewall.Packet, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache, meta udp.RxMeta) {
 	err := h.Parse(packet)
 	if err != nil {
 		// Hole punch packets are 0 or 1 byte big, so lets ignore printing those errors
@@ -135,7 +136,7 @@ func (f *Interface) readOutsidePackets(via ViaSender, out []byte, packet []byte,
 	case header.Message:
 		switch h.Subtype {
 		case header.MessageNone:
-			f.handleOutsideMessagePacket(hostinfo, out, packet, fwPacket, nb, q, localCache)
+			f.handleOutsideMessagePacket(hostinfo, out, packet, fwPacket, nb, q, localCache, meta)
 		default:
 			hostinfo.logger(f.l).Error("IsValidSubType was true, but unexpected message subtype seen", "from", via, "header", h)
 			return
@@ -169,7 +170,7 @@ func (f *Interface) readOutsidePackets(via ViaSender, out []byte, packet []byte,
 	}
 }
 
-func (f *Interface) handleOutsideRelayPacket(hostinfo *HostInfo, via ViaSender, out []byte, packet []byte, h *header.H, fwPacket *firewall.Packet, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache) {
+func (f *Interface) handleOutsideRelayPacket(hostinfo *HostInfo, via ViaSender, out []byte, packet []byte, h *header.H, fwPacket *firewall.Packet, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache, meta udp.RxMeta) {
 	// The entire body is sent as AD, not encrypted.
 	// The packet consists of a 16-byte parsed Nebula header, Associated Data-protected payload, and a trailing 16-byte AEAD signature value.
 	// The packet is guaranteed to be at least 16 bytes at this point, b/c it got past the h.Parse() call above. If it's
@@ -218,7 +219,7 @@ func (f *Interface) handleOutsideRelayPacket(hostinfo *HostInfo, via ViaSender,
 			relay:     relay,
 			IsRelayed: true,
 		}
-		f.readOutsidePackets(via, out[:0], signedPayload, h, fwPacket, lhf, nb, q, localCache)
+		f.readOutsidePackets(via, out[:0], signedPayload, h, fwPacket, lhf, nb, q, localCache, meta)
 	case ForwardingType:
 		// Find the target HostInfo relay object
 		targetHI, targetRelay, err := f.hostMap.QueryVpnAddrsRelayFor(hostinfo.vpnAddrs, relay.PeerAddr)
@@ -518,7 +519,61 @@ func (f *Interface) decrypt(hostinfo *HostInfo, mc uint64, out []byte, packet []
 	return out, nil
 }
 
-func (f *Interface) handleOutsideMessagePacket(hostinfo *HostInfo, out []byte, packet []byte, fwPacket *firewall.Packet, nb []byte, q int, localCache firewall.ConntrackCache) {
+// 2-bit IP-level ECN codepoints (lower bits of IPv4 ToS / IPv6 TC).
+const (
+	ecnNotECT = 0x00
+	ecnECT1   = 0x01
+	ecnECT0   = 0x02
+	ecnCE     = 0x03
+)
+
+// applyOuterECN folds an outer CE mark from the underlay into the inner
+// IP header per RFC 6040 normal mode. It mutates pkt[1] in place. Other
+// codepoints are advisory only and leave the inner unchanged.
+//
+// Merge cases (outer × inner → action):
+//
+//	outer != CE                : no-op (inner is authoritative)
+//	outer == CE, inner Not-ECT : log; cannot propagate to a non-ECN host
+//	outer == CE, inner ECT/CE  : rewrite inner ECN to CE
+func applyOuterECN(pkt []byte, outerECN byte, hostinfo *HostInfo, l *slog.Logger) {
+	if outerECN&ecnCE != ecnCE || len(pkt) < 2 {
+		return
+	}
+	switch pkt[0] >> 4 {
+	case 4:
+		switch pkt[1] & 0x03 {
+		case ecnNotECT:
+			if l.Enabled(context.Background(), slog.LevelDebug) {
+				hostinfo.logger(l).Debug("RFC 6040: outer CE on inner Not-ECT, leaving inner unchanged")
+			}
+		case ecnCE:
+			// Already CE.
+		default:
+			pkt[1] = (pkt[1] &^ 0x03) | ecnCE
+		}
+	case 6:
+		switch (pkt[1] >> 4) & 0x03 {
+		case ecnNotECT:
+			if l.Enabled(context.Background(), slog.LevelDebug) {
+				hostinfo.logger(l).Debug("RFC 6040: outer CE on inner Not-ECT, leaving inner unchanged")
+			}
+		case ecnCE:
+			// Already CE.
+		default:
+			pkt[1] = (pkt[1] &^ 0x30) | (ecnCE << 4)
+		}
+	}
+}
+
+func (f *Interface) handleOutsideMessagePacket(hostinfo *HostInfo, out []byte, packet []byte, fwPacket *firewall.Packet, nb []byte, q int, localCache firewall.ConntrackCache, meta udp.RxMeta) {
+	// RFC 6040 normal-mode combine: fold any outer CE mark stamped by the
+	// underlay into the inner header before firewall + TUN write. Other
+	// outer codepoints are advisory only — we keep the inner unchanged.
+	if f.ecnEnabled.Load() {
+		applyOuterECN(out, meta.OuterECN, hostinfo, f.l)
+	}
+
 	err := newPacket(out, true, fwPacket)
 	if err != nil {
 		hostinfo.logger(f.l).Warn("Error while validating inbound packet",

overlay/batch/batch.go

@@ -14,20 +14,15 @@ type RxBatcher interface {
 }
 
 type TxBatcher interface {
-	// Next returns a zero-length slice with slotCap capacity over the next unused
-	// slot's backing bytes. The caller writes into the returned slice and then
-	// calls Commit with the final length and destination. Next returns nil when
-	// the batch is full.
-	Next() []byte
-	// Commit records the slot just returned by Next as a packet of length n
-	// destined for dst.
-	Commit(n int, dst netip.AddrPort)
-	// Reset clears committed slots; backing storage is retained for reuse.
-	Reset()
-	// Len returns the number of committed packets.
-	Len() int
-	// Cap returns the maximum number of slots in the batch.
-	Cap() int
-	// Get returns the buffers needed to send the batch
-	Get() ([][]byte, []netip.AddrPort)
+	// Reserve creates a pkt to borrow
+	Reserve(sz int) []byte
+	// Commit borrows pkt and records its destination plus the 2-bit
+	// IP-level ECN codepoint to set on the outer (carrier) header. The
+	// caller must keep pkt valid until the next Flush. Pass 0 (Not-ECT)
+	// to leave the outer ECN field unset.
+	Commit(pkt []byte, dst netip.AddrPort, outerECN byte)
+	// Flush emits every queued packet via the underlying batch writer in
+	// arrival order. Returns the first error observed. After Flush returns,
+	// borrowed payload slices may be recycled.
+	Flush() error
 }

overlay/batch/coalesce_core.go

@@ -0,0 +1,163 @@
+package batch
+
+import (
+	"bytes"
+	"encoding/binary"
+)
+
+// flowKey identifies a transport flow by {src, dst, sport, dport, family}.
+// Comparable, so map lookups and linear scans over the slot list stay tight.
+// Shared by the TCP and UDP coalescers; each coalescer keeps its own
+// openSlots map, so a TCP and UDP flow on the same 5-tuple-without-proto
+// never alias.
+type flowKey struct {
+	src, dst     [16]byte
+	sport, dport uint16
+	isV6         bool
+}
+
+// initialSlots is the starting capacity of the slot pool. One flow per
+// packet is the worst case so this matches a typical carrier-side
+// recvmmsg batch on the encrypted UDP socket.
+const initialSlots = 64
+
+// parsedIP is the IP-level result of parseIPPrologue. The caller layers
+// L4-specific parsing (TCP / UDP) on top.
+type parsedIP struct {
+	fk       flowKey
+	ipHdrLen int
+	// pkt is the original buffer trimmed to the IP-declared total length.
+	// Anything below the IP layer (transport parsers) should slice into
+	// pkt rather than the unbounded original.
+	pkt []byte
+}
+
+// parseIPPrologue extracts the IP-level fields the coalescers care about:
+// IHL/payload length, version, src/dst addresses, and the L4 protocol byte.
+// Returns ok=false for malformed input, IPv4 with options or fragmentation,
+// or IPv6 with extension headers (all rejected by both coalescers in
+// identical ways before this refactor).
+//
+// On success, p.pkt is len-trimmed to the IP-declared length so callers
+// don't have to repeat the trim. wantProto is the IANA protocol number to
+// require (6 for TCP, 17 for UDP); ok=false for any other value.
+func parseIPPrologue(pkt []byte, wantProto byte) (parsedIP, bool) {
+	var p parsedIP
+	if len(pkt) < 20 {
+		return p, false
+	}
+	v := pkt[0] >> 4
+	switch v {
+	case 4:
+		ihl := int(pkt[0]&0x0f) * 4
+		if ihl != 20 {
+			return p, false
+		}
+		if pkt[9] != wantProto {
+			return p, false
+		}
+		// Reject actual fragmentation (MF or non-zero frag offset).
+		if binary.BigEndian.Uint16(pkt[6:8])&0x3fff != 0 {
+			return p, false
+		}
+		totalLen := int(binary.BigEndian.Uint16(pkt[2:4]))
+		if totalLen > len(pkt) || totalLen < ihl {
+			return p, false
+		}
+		p.ipHdrLen = 20
+		p.fk.isV6 = false
+		copy(p.fk.src[:4], pkt[12:16])
+		copy(p.fk.dst[:4], pkt[16:20])
+		p.pkt = pkt[:totalLen]
+	case 6:
+		if len(pkt) < 40 {
+			return p, false
+		}
+		if pkt[6] != wantProto {
+			return p, false
+		}
+		payloadLen := int(binary.BigEndian.Uint16(pkt[4:6]))
+		if 40+payloadLen > len(pkt) {
+			return p, false
+		}
+		p.ipHdrLen = 40
+		p.fk.isV6 = true
+		copy(p.fk.src[:], pkt[8:24])
+		copy(p.fk.dst[:], pkt[24:40])
+		p.pkt = pkt[:40+payloadLen]
+	default:
+		return p, false
+	}
+	return p, true
+}
+
+// ipHeadersMatch compares the IP portion of two packet header prefixes for
+// byte-for-byte equality on every field that must be identical across
+// coalesced segments. Size/IPID/IPCsum and the 2-bit IP-level ECN field are
+// masked out — the appendPayload step merges CE into the seed.
+//
+// The transport (L4) portion of the header is checked separately by the
+// per-protocol matcher.
+func ipHeadersMatch(a, b []byte, isV6 bool) bool {
+	if isV6 {
+		// IPv6: byte 0 = version/TC[7:4], byte 1 = TC[3:0]/flow[19:16],
+		// bytes [2:4] = flow[15:0], [6:8] = next_hdr/hop, [8:40] = src+dst.
+		// ECN lives in TC[1:0] = byte 1 mask 0x30. Skip [4:6] payload_len.
+		if a[0] != b[0] {
+			return false
+		}
+		if a[1]&^0x30 != b[1]&^0x30 {
+			return false
+		}
+		if !bytes.Equal(a[2:4], b[2:4]) {
+			return false
+		}
+		if !bytes.Equal(a[6:40], b[6:40]) {
+			return false
+		}
+		return true
+	}
+	// IPv4: byte 0 = version/IHL, byte 1 = DSCP(6)|ECN(2),
+	// [6:10] flags/fragoff/TTL/proto, [12:20] src+dst.
+	// Skip [2:4] total len, [4:6] id, [10:12] csum.
+	if a[0] != b[0] {
+		return false
+	}
+	if a[1]&^0x03 != b[1]&^0x03 {
+		return false
+	}
+	if !bytes.Equal(a[6:10], b[6:10]) {
+		return false
+	}
+	if !bytes.Equal(a[12:20], b[12:20]) {
+		return false
+	}
+	return true
+}
+
+// mergeECNIntoSeed ORs the 2-bit IP-level ECN field of pkt's IP header
+// onto the seed's IP header, so a CE mark on any coalesced segment
+// propagates to the final superpacket. (CE is 0b11; ORing yields CE if
+// any segment carried it.) Used by both TCP and UDP coalescers, so the
+// invariant lives in one place.
+func mergeECNIntoSeed(seedHdr, pktHdr []byte, isV6 bool) {
+	if isV6 {
+		seedHdr[1] |= pktHdr[1] & 0x30
+	} else {
+		seedHdr[1] |= pktHdr[1] & 0x03
+	}
+}
+
+// reserveFromBacking implements the Reserve half of the RxBatcher contract
+// shared by TCP and UDP coalescers. The backing slice grows on demand;
+// already-committed slices reference the old array and remain valid until
+// Flush resets backing.
+func reserveFromBacking(backing *[]byte, sz int) []byte {
+	if len(*backing)+sz > cap(*backing) {
+		newCap := max(cap(*backing)*2, sz)
+		*backing = make([]byte, 0, newCap)
+	}
+	start := len(*backing)
+	*backing = (*backing)[:start+sz]
+	return (*backing)[start : start+sz : start+sz]
+}

overlay/batch/multi_coalesce.go

@@ -0,0 +1,133 @@
+package batch
+
+import (
+	"io"
+)
+
+// MultiCoalescer fans plaintext packets out to lane-specific batchers based
+// on the IP/L4 protocol of the packet, sharing a single Reserve arena
+// across lanes so the caller's allocation pattern is unchanged.
+//
+// Lanes are processed independently: the TCP coalescer only sees TCP, the
+// UDP coalescer only sees UDP, and the passthrough lane handles everything
+// else. Per-flow arrival order is preserved because a single 5-tuple only
+// ever lands in one lane and each lane preserves its own slot order.
+//
+// Cross-lane order is NOT preserved across the TCP/UDP/passthrough split.
+// This is acceptable because the carrier-side recvmmsg path already
+// stable-sorts by (peer, message counter) before delivering plaintext
+// here, so replay-window invariants are unaffected, and apps observe
+// correct per-flow ordering — which is all the IP layer guarantees anyway.
+// Do not "fix" this by interleaving lane outputs at flush time; that
+// negates the entire point of coalescing (each lane needs to see runs of
+// adjacent same-flow packets to coalesce them).
+type MultiCoalescer struct {
+	tcp *TCPCoalescer
+	udp *UDPCoalescer
+	pt  *Passthrough
+
+	// arena shared across all lanes so a single Reserve grows one backing
+	// slice; lane Commit calls borrow into this same arena.
+	backing []byte
+}
+
+// NewMultiCoalescer builds a multi-lane batcher. tcpEnabled lets the caller
+// opt out of TCP coalescing (e.g. when the queue can't do TSO); udpEnabled
+// likewise gates UDP coalescing (only enable when USO was negotiated).
+// Either lane disabled redirects its traffic into the passthrough lane.
+func NewMultiCoalescer(w io.Writer, tcpEnabled, udpEnabled bool) *MultiCoalescer {
+	m := &MultiCoalescer{
+		pt:      NewPassthrough(w),
+		backing: make([]byte, 0, initialSlots*65535),
+	}
+	if tcpEnabled {
+		m.tcp = NewTCPCoalescer(w)
+	}
+	if udpEnabled {
+		m.udp = NewUDPCoalescer(w)
+	}
+	return m
+}
+
+func (m *MultiCoalescer) Reserve(sz int) []byte {
+	if len(m.backing)+sz > cap(m.backing) {
+		newCap := max(cap(m.backing)*2, sz)
+		m.backing = make([]byte, 0, newCap)
+	}
+	start := len(m.backing)
+	m.backing = m.backing[:start+sz]
+	return m.backing[start : start+sz : start+sz]
+}
+
+// Commit dispatches pkt to the appropriate lane based on IP version + L4
+// proto. Borrowed slice contract is identical to the single-lane batchers
+// — pkt must remain valid until the next Flush.
+//
+// On the success path the IP/TCP-or-UDP parse happens here once and the
+// parsed struct is handed to the lane via commitParsed so the lane doesn't
+// re-walk the header. On a parse failure we fall through to the lane's
+// public Commit, which re-runs the parse before passthrough — that path
+// only fires for malformed/unsupported packets so the duplicated parse is
+// not on the hot path. The lane's public Commit still works for direct
+// callers.
+func (m *MultiCoalescer) Commit(pkt []byte) error {
+	if len(pkt) < 20 {
+		return m.pt.Commit(pkt)
+	}
+	v := pkt[0] >> 4
+	var proto byte
+	switch v {
+	case 4:
+		proto = pkt[9]
+	case 6:
+		if len(pkt) < 40 {
+			return m.pt.Commit(pkt)
+		}
+		proto = pkt[6]
+	default:
+		return m.pt.Commit(pkt)
+	}
+	switch proto {
+	case ipProtoTCP:
+		if m.tcp != nil {
+			info, ok := parseTCPBase(pkt)
+			if !ok {
+				// Malformed/unsupported TCP shape (IP options, fragments, ...)
+				// — the TCP lane handles this as passthrough.
+				return m.tcp.Commit(pkt)
+			}
+			return m.tcp.commitParsed(pkt, info)
+		}
+	case ipProtoUDP:
+		if m.udp != nil {
+			info, ok := parseUDP(pkt)
+			if !ok {
+				return m.udp.Commit(pkt)
+			}
+			return m.udp.commitParsed(pkt, info)
+		}
+	}
+	return m.pt.Commit(pkt)
+}
+
+// Flush drains every lane in a fixed order: TCP, UDP, passthrough. Errors
+// from a lane do not stop subsequent lanes from flushing — we keep
+// draining and return the first observed error so a single bad packet
+// doesn't strand the others.
+func (m *MultiCoalescer) Flush() error {
+	var first error
+	keep := func(err error) {
+		if err != nil && first == nil {
+			first = err
+		}
+	}
+	if m.tcp != nil {
+		keep(m.tcp.Flush())
+	}
+	if m.udp != nil {
+		keep(m.udp.Flush())
+	}
+	keep(m.pt.Flush())
+	m.backing = m.backing[:0]
+	return first
+}

overlay/batch/multi_coalesce_test.go

@@ -0,0 +1,94 @@
+package batch
+
+import (
+	"testing"
+)
+
+// TestMultiCoalescerRoutesByProto confirms TCP/UDP/other land in the right
+// lane: TCP and UDP get coalesced when their lanes are enabled, anything
+// else (ICMP here) falls through to plain Write.
+func TestMultiCoalescerRoutesByProto(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	m := NewMultiCoalescer(w, true, true)
+
+	tcpPay := make([]byte, 1200)
+	udpPay := make([]byte, 1200)
+	icmp := make([]byte, 28)
+	icmp[0] = 0x45
+	icmp[2] = 0
+	icmp[3] = 28
+	icmp[9] = 1
+
+	if err := m.Commit(buildTCPv4(1000, tcpAck, tcpPay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := m.Commit(buildTCPv4(2200, tcpAck, tcpPay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := m.Commit(buildUDPv4(2000, 53, udpPay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := m.Commit(buildUDPv4(2000, 53, udpPay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := m.Commit(icmp); err != nil {
+		t.Fatal(err)
+	}
+	if err := m.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// 1 TCP super (2 segments) + 1 UDP super (2 segments) = 2 gso writes.
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes (one TCP + one UDP), got %d", len(w.gsoWrites))
+	}
+	if len(w.writes) != 1 {
+		t.Fatalf("want 1 plain write (ICMP), got %d", len(w.writes))
+	}
+}
+
+// TestMultiCoalescerDisabledUDPFallsThrough verifies that when the UDP lane
+// is disabled (e.g. kernel doesn't support USO), UDP packets still reach
+// the kernel via the passthrough lane rather than being lost.
+func TestMultiCoalescerDisabledUDPFallsThrough(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	m := NewMultiCoalescer(w, true, false) // TSO on, USO off
+
+	if err := m.Commit(buildUDPv4(1000, 53, make([]byte, 800))); err != nil {
+		t.Fatal(err)
+	}
+	if err := m.Commit(buildUDPv4(1000, 53, make([]byte, 800))); err != nil {
+		t.Fatal(err)
+	}
+	if err := m.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 0 {
+		t.Errorf("UDP must NOT be coalesced when USO disabled, got %d gso writes", len(w.gsoWrites))
+	}
+	if len(w.writes) != 2 {
+		t.Errorf("UDP must pass through as 2 plain writes, got %d", len(w.writes))
+	}
+}
+
+// TestMultiCoalescerDisabledTCPFallsThrough mirrors the TSO=off case.
+func TestMultiCoalescerDisabledTCPFallsThrough(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	m := NewMultiCoalescer(w, false, true) // TSO off, USO on
+
+	pay := make([]byte, 1200)
+	if err := m.Commit(buildTCPv4(1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := m.Commit(buildTCPv4(2200, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := m.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 0 {
+		t.Errorf("TCP must NOT be coalesced when TSO disabled, got %d gso writes", len(w.gsoWrites))
+	}
+	if len(w.writes) != 2 {
+		t.Errorf("TCP must pass through as 2 plain writes, got %d", len(w.writes))
+	}
+}

overlay/batch/tcp_coalesce.go

@@ -0,0 +1,722 @@
+package batch
+
+import (
+	"bytes"
+	"encoding/binary"
+	"io"
+	"log/slog"
+	"net/netip"
+	"sort"
+
+	"github.com/slackhq/nebula/overlay/tio"
+)
+
+// ipProtoTCP is the IANA protocol number for TCP. Hardcoded instead of
+// reaching for golang.org/x/sys/unix — that package doesn't define the
+// constant on Windows, which would break cross-compiles even though this
+// file runs unchanged on every platform.
+const ipProtoTCP = 6
+
+// tcpCoalesceBufSize caps total bytes per superpacket. Mirrors the kernel's
+// sk_gso_max_size of ~64KiB; anything beyond this would be rejected anyway.
+const tcpCoalesceBufSize = 65535
+
+// tcpCoalesceMaxSegs caps how many segments we'll coalesce into a single
+// superpacket. Keeping this well below the kernel's TSO ceiling bounds
+// latency.
+const tcpCoalesceMaxSegs = 64
+
+// tcpCoalesceHdrCap is the scratch space we copy a seed's IP+TCP header
+// into. IPv6 (40) + TCP with full options (60) = 100 bytes.
+const tcpCoalesceHdrCap = 100
+
+// coalesceSlot is one entry in the coalescer's ordered event queue. When
+// passthrough is true the slot holds a single borrowed packet that must be
+// emitted verbatim (non-TCP, non-admissible TCP, or oversize seed). When
+// passthrough is false the slot is an in-progress coalesced superpacket:
+// hdrBuf is a mutable copy of the seed's IP+TCP header (we patch total
+// length and pseudo-header partial at flush), and payIovs are *borrowed*
+// slices from the caller's plaintext buffers — no payload is ever copied.
+// The caller (listenOut) must keep those buffers alive until Flush.
+type coalesceSlot struct {
+	passthrough bool
+	rawPkt      []byte // borrowed when passthrough
+
+	fk       flowKey
+	hdrBuf   [tcpCoalesceHdrCap]byte
+	hdrLen   int
+	ipHdrLen int
+	isV6     bool
+	gsoSize  int
+	numSeg   int
+	totalPay int
+	nextSeq  uint32
+	// psh closes the chain: set when the last-accepted segment had PSH or
+	// was sub-gsoSize. No further appends after that.
+	psh     bool
+	payIovs [][]byte
+}
+
+// TCPCoalescer accumulates adjacent in-flow TCP data segments across
+// multiple concurrent flows and emits each flow's run as a single TSO
+// superpacket via tio.GSOWriter. All output — coalesced or not — is
+// deferred until Flush so arrival order is preserved on the wire. Owns
+// no locks; one coalescer per TUN write queue.
+type TCPCoalescer struct {
+	plainW io.Writer
+	gsoW   tio.GSOWriter // nil when the queue doesn't support TSO
+
+	// slots is the ordered event queue. Flush walks it once and emits each
+	// entry as either a WriteGSO (coalesced) or a plainW.Write (passthrough).
+	slots []*coalesceSlot
+	// openSlots maps a flow key to its most recent non-sealed slot, so new
+	// segments can extend an in-progress superpacket in O(1). Slots are
+	// removed from this map when they close (PSH or short-last-segment),
+	// when a non-admissible packet for that flow arrives, or in Flush.
+	openSlots map[flowKey]*coalesceSlot
+	// lastSlot caches the most recently touched open slot. Steady-state
+	// bulk traffic is dominated by a single flow, so comparing the
+	// incoming key against the cached slot's own fk lets the hot path
+	// skip the map lookup (and the aeshash of a 38-byte key) entirely.
+	// Kept in lockstep with openSlots: nil whenever the slot it pointed
+	// at is removed/sealed.
+	lastSlot *coalesceSlot
+	pool     []*coalesceSlot // free list for reuse
+
+	backing []byte
+}
+
+func NewTCPCoalescer(w io.Writer) *TCPCoalescer {
+	c := &TCPCoalescer{
+		plainW:    w,
+		slots:     make([]*coalesceSlot, 0, initialSlots),
+		openSlots: make(map[flowKey]*coalesceSlot, initialSlots),
+		pool:      make([]*coalesceSlot, 0, initialSlots),
+		backing:   make([]byte, 0, initialSlots*65535),
+	}
+	if gw, ok := tio.SupportsGSO(w, tio.GSOProtoTCP); ok {
+		c.gsoW = gw
+	}
+	return c
+}
+
+// parsedTCP holds the fields extracted from a single parse so later steps
+// (admission, slot lookup, canAppend) don't re-walk the header.
+type parsedTCP struct {
+	fk        flowKey
+	ipHdrLen  int
+	tcpHdrLen int
+	hdrLen    int
+	payLen    int
+	seq       uint32
+	flags     byte
+}
+
+// parseTCPBase extracts the flow key and IP/TCP offsets for any TCP packet,
+// regardless of whether it's admissible for coalescing. Returns ok=false
+// for non-TCP or malformed input. Accepts IPv4 (no options, no fragmentation)
+// and IPv6 (no extension headers).
+func parseTCPBase(pkt []byte) (parsedTCP, bool) {
+	var p parsedTCP
+	ip, ok := parseIPPrologue(pkt, ipProtoTCP)
+	if !ok {
+		return p, false
+	}
+	pkt = ip.pkt
+	p.fk = ip.fk
+	p.ipHdrLen = ip.ipHdrLen
+
+	if len(pkt) < p.ipHdrLen+20 {
+		return p, false
+	}
+	tcpOff := int(pkt[p.ipHdrLen+12]>>4) * 4
+	if tcpOff < 20 || tcpOff > 60 {
+		return p, false
+	}
+	if len(pkt) < p.ipHdrLen+tcpOff {
+		return p, false
+	}
+	p.tcpHdrLen = tcpOff
+	p.hdrLen = p.ipHdrLen + tcpOff
+	p.payLen = len(pkt) - p.hdrLen
+	p.seq = binary.BigEndian.Uint32(pkt[p.ipHdrLen+4 : p.ipHdrLen+8])
+	p.flags = pkt[p.ipHdrLen+13]
+	p.fk.sport = binary.BigEndian.Uint16(pkt[p.ipHdrLen : p.ipHdrLen+2])
+	p.fk.dport = binary.BigEndian.Uint16(pkt[p.ipHdrLen+2 : p.ipHdrLen+4])
+	return p, true
+}
+
+// TCP flag bits (byte 13 of the TCP header). Only the bits actually consulted
+// by the coalescer are named; FIN/SYN/RST/URG/CWR are rejected via the
+// negative mask in coalesceable, not by name.
+const (
+	tcpFlagPsh = 0x08
+	tcpFlagAck = 0x10
+	tcpFlagEce = 0x40
+)
+
+// coalesceable reports whether a parsed TCP segment is eligible for
+// coalescing. Accepts ACK, ACK|PSH, ACK|ECE, ACK|PSH|ECE with a
+// non-empty payload. CWR is excluded because it marks a one-shot
+// congestion-window-reduced transition the receiver must observe at a
+// segment boundary.
+func (p parsedTCP) coalesceable() bool {
+	if p.flags&tcpFlagAck == 0 {
+		return false
+	}
+	if p.flags&^(tcpFlagAck|tcpFlagPsh|tcpFlagEce) != 0 {
+		return false
+	}
+	return p.payLen > 0
+}
+
+func (c *TCPCoalescer) Reserve(sz int) []byte {
+	return reserveFromBacking(&c.backing, sz)
+}
+
+// Commit borrows pkt. The caller must keep pkt valid until the next Flush,
+// whether or not the packet was coalesced — passthrough (non-admissible)
+// packets are queued and written at Flush time, not synchronously.
+func (c *TCPCoalescer) Commit(pkt []byte) error {
+	if c.gsoW == nil {
+		c.addPassthrough(pkt)
+		return nil
+	}
+	info, ok := parseTCPBase(pkt)
+	if !ok {
+		c.addPassthrough(pkt)
+		return nil
+	}
+	return c.commitParsed(pkt, info)
+}
+
+// commitParsed is the post-parse half of Commit. The caller must have
+// already verified parseTCPBase succeeded (info is a valid TCP parse).
+// Used by MultiCoalescer.Commit to avoid re-walking the IP/TCP header
+// after the dispatcher has already done so.
+func (c *TCPCoalescer) commitParsed(pkt []byte, info parsedTCP) error {
+	if c.gsoW == nil {
+		c.addPassthrough(pkt)
+		return nil
+	}
+	if !info.coalesceable() {
+		// TCP but not admissible (SYN/FIN/RST/URG/CWR or zero-payload).
+		// Seal this flow's open slot so later in-flow packets don't extend
+		// it and accidentally reorder past this passthrough.
+		if last := c.lastSlot; last != nil && last.fk == info.fk {
+			c.lastSlot = nil
+		}
+		delete(c.openSlots, info.fk)
+		c.addPassthrough(pkt)
+		return nil
+	}
+
+	// Single-flow fast path: with only one open flow the cache hits every
+	// packet, and len(openSlots)==1 lets us skip the 38-byte fk compare
+	// when there are multiple flows in flight (where the hit rate would
+	// be ~0 and the compare is pure overhead).
+	var open *coalesceSlot
+	if last := c.lastSlot; last != nil && len(c.openSlots) == 1 && last.fk == info.fk {
+		open = last
+	} else {
+		open = c.openSlots[info.fk]
+	}
+	if open != nil {
+		if c.canAppend(open, pkt, info) {
+			c.appendPayload(open, pkt, info)
+			if open.psh {
+				delete(c.openSlots, info.fk)
+				c.lastSlot = nil
+			} else {
+				c.lastSlot = open
+			}
+			return nil
+		}
+		// Can't extend — seal it and fall through to seed a fresh slot.
+		delete(c.openSlots, info.fk)
+		if c.lastSlot == open {
+			c.lastSlot = nil
+		}
+	}
+	c.seed(pkt, info)
+	return nil
+}
+
+// Flush emits every queued event in (per-flow) seq order. Coalesced slots
+// go out via WriteGSO; passthrough slots go out via plainW.Write.
+// reorderForFlush first sorts each flow's slots into TCP-seq order within
+// passthrough-bounded segments and merges contiguous adjacent slots, so
+// any wire-side reorder that crossed an rxOrder batch boundary doesn't
+// get amplified into kernel-visible reorder by the slot machinery.
+// Returns the first error observed; keeps draining so one bad packet
+// doesn't hold up the rest. After Flush returns, borrowed payload slices
+// may be recycled.
+func (c *TCPCoalescer) Flush() error {
+	c.reorderForFlush()
+	var first error
+	for _, s := range c.slots {
+		var err error
+		if s.passthrough {
+			_, err = c.plainW.Write(s.rawPkt)
+		} else {
+			err = c.flushSlot(s)
+		}
+		if err != nil && first == nil {
+			first = err
+		}
+		c.release(s)
+	}
+	for i := range c.slots {
+		c.slots[i] = nil
+	}
+	c.slots = c.slots[:0]
+	for k := range c.openSlots {
+		delete(c.openSlots, k)
+	}
+	c.lastSlot = nil
+
+	c.backing = c.backing[:0]
+	return first
+}
+
+func (c *TCPCoalescer) addPassthrough(pkt []byte) {
+	s := c.take()
+	s.passthrough = true
+	s.rawPkt = pkt
+	c.slots = append(c.slots, s)
+}
+
+func (c *TCPCoalescer) seed(pkt []byte, info parsedTCP) {
+	if info.hdrLen > tcpCoalesceHdrCap || info.hdrLen+info.payLen > tcpCoalesceBufSize {
+		// Pathological shape — can't fit our scratch, emit as-is.
+		c.addPassthrough(pkt)
+		return
+	}
+	s := c.take()
+	s.passthrough = false
+	s.rawPkt = nil
+	copy(s.hdrBuf[:], pkt[:info.hdrLen])
+	s.hdrLen = info.hdrLen
+	s.ipHdrLen = info.ipHdrLen
+	s.isV6 = info.fk.isV6
+	s.fk = info.fk
+	s.gsoSize = info.payLen
+	s.numSeg = 1
+	s.totalPay = info.payLen
+	s.nextSeq = info.seq + uint32(info.payLen)
+	s.psh = info.flags&tcpFlagPsh != 0
+	s.payIovs = append(s.payIovs[:0], pkt[info.hdrLen:info.hdrLen+info.payLen])
+	c.slots = append(c.slots, s)
+	if !s.psh {
+		c.openSlots[info.fk] = s
+		c.lastSlot = s
+	} else if last := c.lastSlot; last != nil && last.fk == info.fk {
+		// PSH-on-seed seals the slot immediately. Any prior cached open
+		// slot for this flow has just been sealed-and-replaced by this
+		// passthrough-shaped seed, so drop the cache too.
+		c.lastSlot = nil
+	}
+}
+
+// canAppend reports whether info's packet extends the slot's seed: same
+// header shape and stable contents, adjacent seq, not oversized, chain not
+// closed.
+func (c *TCPCoalescer) canAppend(s *coalesceSlot, pkt []byte, info parsedTCP) bool {
+	if s.psh {
+		return false
+	}
+	if info.hdrLen != s.hdrLen {
+		return false
+	}
+	if info.seq != s.nextSeq {
+		return false
+	}
+	if s.numSeg >= tcpCoalesceMaxSegs {
+		return false
+	}
+	if info.payLen > s.gsoSize {
+		return false
+	}
+	if s.hdrLen+s.totalPay+info.payLen > tcpCoalesceBufSize {
+		return false
+	}
+	// ECE state must be stable across a burst — receivers expect the
+	// flag set on every segment of a CE-echoing window or none.
+	seedFlags := s.hdrBuf[s.ipHdrLen+13]
+	if (seedFlags^info.flags)&tcpFlagEce != 0 {
+		return false
+	}
+	if !headersMatch(s.hdrBuf[:s.hdrLen], pkt[:info.hdrLen], s.isV6, s.ipHdrLen) {
+		return false
+	}
+	return true
+}
+
+func (c *TCPCoalescer) appendPayload(s *coalesceSlot, pkt []byte, info parsedTCP) {
+	s.payIovs = append(s.payIovs, pkt[info.hdrLen:info.hdrLen+info.payLen])
+	s.numSeg++
+	s.totalPay += info.payLen
+	s.nextSeq = info.seq + uint32(info.payLen)
+	if info.flags&tcpFlagPsh != 0 {
+		// Propagate PSH into the seed header so kernel TSO sets it on the
+		// last segment. Without this the sender's push signal is dropped.
+		s.hdrBuf[s.ipHdrLen+13] |= tcpFlagPsh
+	}
+	// Merge IP-level CE marks into the seed: headersMatch ignores ECN, so
+	// this is the one place the signal is preserved.
+	mergeECNIntoSeed(s.hdrBuf[:s.ipHdrLen], pkt[:s.ipHdrLen], s.isV6)
+	if info.payLen < s.gsoSize || info.flags&tcpFlagPsh != 0 {
+		s.psh = true
+	}
+}
+
+func (c *TCPCoalescer) take() *coalesceSlot {
+	if n := len(c.pool); n > 0 {
+		s := c.pool[n-1]
+		c.pool[n-1] = nil
+		c.pool = c.pool[:n-1]
+		return s
+	}
+	return &coalesceSlot{}
+}
+
+func (c *TCPCoalescer) release(s *coalesceSlot) {
+	s.passthrough = false
+	s.rawPkt = nil
+	for i := range s.payIovs {
+		s.payIovs[i] = nil
+	}
+	s.payIovs = s.payIovs[:0]
+	s.numSeg = 0
+	s.totalPay = 0
+	s.psh = false
+	c.pool = append(c.pool, s)
+}
+
+// flushSlot patches the header and calls WriteGSO. Does not remove the
+// slot from c.slots.
+func (c *TCPCoalescer) flushSlot(s *coalesceSlot) error {
+	total := s.hdrLen + s.totalPay
+	l4Len := total - s.ipHdrLen
+	hdr := s.hdrBuf[:s.hdrLen]
+
+	if s.isV6 {
+		binary.BigEndian.PutUint16(hdr[4:6], uint16(l4Len))
+	} else {
+		binary.BigEndian.PutUint16(hdr[2:4], uint16(total))
+		hdr[10] = 0
+		hdr[11] = 0
+		binary.BigEndian.PutUint16(hdr[10:12], ipv4HdrChecksum(hdr[:s.ipHdrLen]))
+	}
+
+	var psum uint32
+	if s.isV6 {
+		psum = pseudoSumIPv6(hdr[8:24], hdr[24:40], ipProtoTCP, l4Len)
+	} else {
+		psum = pseudoSumIPv4(hdr[12:16], hdr[16:20], ipProtoTCP, l4Len)
+	}
+	tcsum := s.ipHdrLen + 16
+	binary.BigEndian.PutUint16(hdr[tcsum:tcsum+2], foldOnceNoInvert(psum))
+
+	return c.gsoW.WriteGSO(hdr[:s.ipHdrLen], hdr[s.ipHdrLen:], s.payIovs, tio.GSOProtoTCP)
+}
+
+// headersMatch compares two IP+TCP header prefixes for byte-for-byte
+// equality on every field that must be identical across coalesced
+// segments. Size/IPID/IPCsum/seq/flags/tcpCsum are masked out, as is the
+// 2-bit IP-level ECN field — appendPayload merges CE into the seed.
+func headersMatch(a, b []byte, isV6 bool, ipHdrLen int) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	if !ipHeadersMatch(a, b, isV6) {
+		return false
+	}
+	// TCP: compare [0:4] ports, [8:13] ack+dataoff, [14:16] window,
+	// [18:tcpHdrLen] options (incl. urgent).
+	tcp := ipHdrLen
+	if !bytes.Equal(a[tcp:tcp+4], b[tcp:tcp+4]) {
+		return false
+	}
+	if !bytes.Equal(a[tcp+8:tcp+13], b[tcp+8:tcp+13]) {
+		return false
+	}
+	if !bytes.Equal(a[tcp+14:tcp+16], b[tcp+14:tcp+16]) {
+		return false
+	}
+	if !bytes.Equal(a[tcp+18:], b[tcp+18:]) {
+		return false
+	}
+	return true
+}
+
+// reorderForFlush neutralizes wire-side reorder that the rxOrder buffer
+// couldn't catch (anything crossing a recvmmsg batch boundary). Without
+// this pass a small wire reorder — counter 250 arriving in batch K when
+// 200..249 are coming in batch K+1 — would seed an out-of-seq slot first
+// and emit it ahead of the lower-seq slot, manifesting at the inner TCP
+// receiver as a much larger reorder than the wire actually had.
+//
+// Two phases:
+//  1. Sort each passthrough-bounded segment of c.slots by (flow, seq).
+//     Cross-flow ordering inside a segment isn't preserved (it never was
+//     and doesn't matter for any single flow's TCP correctness).
+//  2. Sweep once and merge adjacent same-flow slots whose ranges are now
+//     contiguous AND whose tail is gsoSize-aligned. The tail constraint
+//     matters because the kernel TSO splitter chops at gsoSize from the
+//     start of the merged payload — a short segment in the middle would
+//     desynchronize every later segment.
+//
+// Passthrough slots act as barriers: the merge check skips them on either
+// side, so a SYN/FIN/RST/CWR is never reordered relative to its flow's
+// data.
+func (c *TCPCoalescer) reorderForFlush() {
+	if len(c.slots) <= 1 {
+		return
+	}
+	runStart := 0
+	for i := 0; i <= len(c.slots); i++ {
+		if i < len(c.slots) && !c.slots[i].passthrough {
+			continue
+		}
+		c.sortRun(c.slots[runStart:i])
+		runStart = i + 1
+	}
+	out := c.slots[:0]
+	logged := false
+	for _, s := range c.slots {
+		if n := len(out); n > 0 {
+			prev := out[n-1]
+			if !prev.passthrough && !s.passthrough && prev.fk == s.fk {
+				// Same-flow neighbors after sort. If they aren't seq-
+				// contiguous it's a real gap — packets the wire reordered
+				// across batches, or actual loss before nebula. Log it so
+				// the operator can quantify how often it happens; the data
+				// itself still emits in seq order, kernel TCP handles the
+				// gap via its OOO queue.
+				if prev.nextSeq != slotSeedSeq(s) {
+					logged = true
+					gap := int64(slotSeedSeq(s)) - int64(prev.nextSeq)
+					slog.Default().Warn("tcp coalesce: cross-slot seq gap",
+						"src", flowKeyAddr(s.fk, false),
+						"dst", flowKeyAddr(s.fk, true),
+						"sport", s.fk.sport,
+						"dport", s.fk.dport,
+						"prev_seed_seq", slotSeedSeq(prev),
+						"prev_next_seq", prev.nextSeq,
+						"this_seed_seq", slotSeedSeq(s),
+						"gap_bytes", gap,
+						"prev_seg_count", prev.numSeg,
+						"prev_total_pay", prev.totalPay,
+					)
+				}
+				if canMergeSlots(prev, s) {
+					mergeSlots(prev, s)
+					c.release(s)
+					continue
+				}
+			}
+		}
+		out = append(out, s)
+	}
+	if logged {
+		slog.Default().Warn("==== end of batch ====")
+	}
+	c.slots = out
+}
+
+// flowKeyAddr returns the src or dst address from fk as a netip.Addr for
+// logging. Only used on the cold gap-log path so the netip allocation
+// doesn't matter.
+func flowKeyAddr(fk flowKey, dst bool) netip.Addr {
+	src := fk.src
+	if dst {
+		src = fk.dst
+	}
+	if fk.isV6 {
+		return netip.AddrFrom16(src)
+	}
+	var v4 [4]byte
+	copy(v4[:], src[:4])
+	return netip.AddrFrom4(v4)
+}
+
+// sortRun stable-sorts run by (flowKey, seedSeq) so each flow's slots
+// cluster together in seq order, ready for the merge sweep. Stable so
+// equal-key slots keep their original relative position (defensive — a
+// duplicate seedSeq would already mean something's wrong upstream).
+func (c *TCPCoalescer) sortRun(run []*coalesceSlot) {
+	if len(run) <= 1 {
+		return
+	}
+	sort.SliceStable(run, func(i, j int) bool {
+		a, b := run[i], run[j]
+		if cmp := flowKeyCompare(a.fk, b.fk); cmp != 0 {
+			return cmp < 0
+		}
+		return tcpSeqLess(slotSeedSeq(a), slotSeedSeq(b))
+	})
+}
+
+// slotSeedSeq returns the TCP seq of the slot's seed (first segment).
+// nextSeq tracks the seq just past the last appended byte; subtracting
+// totalPay walks back to the seed. uint32 wraparound is the right TCP
+// arithmetic so no special-casing is needed.
+func slotSeedSeq(s *coalesceSlot) uint32 {
+	return s.nextSeq - uint32(s.totalPay)
+}
+
+// tcpSeqLess reports whether a precedes b in TCP serial-number arithmetic
+// (RFC 1323 §2.3). The signed int32 cast turns the modular subtraction
+// into the right comparison even across the 2^32 wrap.
+func tcpSeqLess(a, b uint32) bool {
+	return int32(a-b) < 0
+}
+
+// flowKeyCompare orders flowKeys deterministically. The exact ordering
+// is irrelevant — only that same-flow slots cluster together so the
+// post-sort sweep can merge contiguous pairs.
+func flowKeyCompare(a, b flowKey) int {
+	if c := bytes.Compare(a.src[:], b.src[:]); c != 0 {
+		return c
+	}
+	if c := bytes.Compare(a.dst[:], b.dst[:]); c != 0 {
+		return c
+	}
+	if a.sport != b.sport {
+		if a.sport < b.sport {
+			return -1
+		}
+		return 1
+	}
+	if a.dport != b.dport {
+		if a.dport < b.dport {
+			return -1
+		}
+		return 1
+	}
+	if a.isV6 != b.isV6 {
+		if !a.isV6 {
+			return -1
+		}
+		return 1
+	}
+	return 0
+}
+
+// canMergeSlots reports whether s can fold into prev as one merged TSO
+// superpacket. Same flow, contiguous TCP byte range, equal gsoSize, and
+// fits within the kernel TSO limits. The tail-of-prev check rejects any
+// merge whose first slot ended on a sub-gsoSize segment — kernel TSO
+// would split the merged skb at gsoSize boundaries from the start, so a
+// short segment in the middle would corrupt every later segment. PSH and
+// ECE state must agree across both slots: PSH is a semantic delimiter
+// (preserving the sender's push boundary) and ECE state must be uniform
+// across a window (the same rule canAppend enforces for in-flow appends).
+//
+// Note: a slot sealed by reorder (canAppend returned false on seq
+// mismatch) keeps psh=false, so this restriction does not block the
+// reorder-fix merge — only legitimate PSH-set seals.
+func canMergeSlots(prev, s *coalesceSlot) bool {
+	if prev.psh {
+		return false
+	}
+	if prev.fk != s.fk {
+		return false
+	}
+	if prev.gsoSize != s.gsoSize {
+		return false
+	}
+	if prev.nextSeq != slotSeedSeq(s) {
+		return false
+	}
+	if prev.numSeg+s.numSeg > tcpCoalesceMaxSegs {
+		return false
+	}
+	if prev.hdrLen+prev.totalPay+s.totalPay > tcpCoalesceBufSize {
+		return false
+	}
+	if len(prev.payIovs[len(prev.payIovs)-1]) != prev.gsoSize {
+		return false
+	}
+	prevFlags := prev.hdrBuf[prev.ipHdrLen+13]
+	sFlags := s.hdrBuf[s.ipHdrLen+13]
+	if (prevFlags^sFlags)&tcpFlagEce != 0 {
+		return false
+	}
+	if !headersMatch(prev.hdrBuf[:prev.hdrLen], s.hdrBuf[:s.hdrLen], prev.isV6, prev.ipHdrLen) {
+		return false
+	}
+	return true
+}
+
+// mergeSlots folds src into dst in place: payIovs concatenated, counters
+// and totals updated, PSH and IP-level CE bits OR'd into the seed header
+// so neither the push signal nor a CE mark is lost. The seed header's
+// seq, gsoSize, and fk are unchanged. Caller is responsible for releasing
+// src (it's no longer in c.slots after this call).
+func mergeSlots(dst, src *coalesceSlot) {
+	dst.payIovs = append(dst.payIovs, src.payIovs...)
+	dst.numSeg += src.numSeg
+	dst.totalPay += src.totalPay
+	dst.nextSeq = src.nextSeq
+	if src.psh {
+		dst.psh = true
+		dst.hdrBuf[dst.ipHdrLen+13] |= tcpFlagPsh
+	}
+	mergeECNIntoSeed(dst.hdrBuf[:dst.ipHdrLen], src.hdrBuf[:src.ipHdrLen], dst.isV6)
+}
+
+// ipv4HdrChecksum computes the IPv4 header checksum over hdr (which must
+// already have its checksum field zeroed) and returns the folded/inverted
+// 16-bit value to store.
+func ipv4HdrChecksum(hdr []byte) uint16 {
+	var sum uint32
+	for i := 0; i+1 < len(hdr); i += 2 {
+		sum += uint32(binary.BigEndian.Uint16(hdr[i : i+2]))
+	}
+	if len(hdr)%2 == 1 {
+		sum += uint32(hdr[len(hdr)-1]) << 8
+	}
+	for sum>>16 != 0 {
+		sum = (sum & 0xffff) + (sum >> 16)
+	}
+	return ^uint16(sum)
+}
+
+// pseudoSumIPv4 / pseudoSumIPv6 build the L4 pseudo-header partial sum
+// expected by the virtio NEEDS_CSUM kernel path: the 32-bit accumulator
+// before folding. proto selects the L4 (TCP or UDP); the UDP coalescer
+// reuses these helpers.
+func pseudoSumIPv4(src, dst []byte, proto byte, l4Len int) uint32 {
+	var sum uint32
+	sum += uint32(binary.BigEndian.Uint16(src[0:2]))
+	sum += uint32(binary.BigEndian.Uint16(src[2:4]))
+	sum += uint32(binary.BigEndian.Uint16(dst[0:2]))
+	sum += uint32(binary.BigEndian.Uint16(dst[2:4]))
+	sum += uint32(proto)
+	sum += uint32(l4Len)
+	return sum
+}
+
+func pseudoSumIPv6(src, dst []byte, proto byte, l4Len int) uint32 {
+	var sum uint32
+	for i := 0; i < 16; i += 2 {
+		sum += uint32(binary.BigEndian.Uint16(src[i : i+2]))
+		sum += uint32(binary.BigEndian.Uint16(dst[i : i+2]))
+	}
+	sum += uint32(l4Len >> 16)
+	sum += uint32(l4Len & 0xffff)
+	sum += uint32(proto)
+	return sum
+}
+
+// foldOnceNoInvert folds the 32-bit accumulator to 16 bits and returns it
+// unchanged (no one's complement). This is what virtio NEEDS_CSUM wants in
+// the L4 checksum field — the kernel will add the payload sum and invert.
+func foldOnceNoInvert(sum uint32) uint16 {
+	for sum>>16 != 0 {
+		sum = (sum & 0xffff) + (sum >> 16)
+	}
+	return uint16(sum)
+}

overlay/batch/tcp_coalesce_bench_test.go

@@ -0,0 +1,173 @@
+package batch
+
+import (
+	"encoding/binary"
+	"testing"
+
+	"github.com/slackhq/nebula/overlay/tio"
+)
+
+// nopTunWriter is a zero-alloc tio.GSOWriter for benchmarks. Discards
+// everything but satisfies the interface the coalescer detects.
+type nopTunWriter struct{}
+
+func (nopTunWriter) Write(p []byte) (int, error) { return len(p), nil }
+func (nopTunWriter) WriteGSO(hdr []byte, transportHdr []byte, pays [][]byte, _ tio.GSOProto) error {
+	return nil
+}
+func (nopTunWriter) Capabilities() tio.Capabilities {
+	return tio.Capabilities{TSO: true, USO: true}
+}
+
+// buildTCPv4BulkFlow returns a slice of N adjacent ACK-only TCP segments
+// on a single 5-tuple, each carrying payloadLen bytes. Seq numbers are
+// contiguous so every packet is coalesceable onto the previous one.
+func buildTCPv4BulkFlow(n, payloadLen int) [][]byte {
+	pkts := make([][]byte, n)
+	pay := make([]byte, payloadLen)
+	seq := uint32(1000)
+	for i := range n {
+		pkts[i] = buildTCPv4(seq, tcpAck, pay)
+		seq += uint32(payloadLen)
+	}
+	return pkts
+}
+
+// buildTCPv4Interleaved returns nFlows * perFlow packets with per-flow
+// seq continuity but round-robin across flows — worst case for any
+// "last-slot" cache.
+func buildTCPv4Interleaved(nFlows, perFlow, payloadLen int) [][]byte {
+	pay := make([]byte, payloadLen)
+	seqs := make([]uint32, nFlows)
+	for i := range seqs {
+		seqs[i] = uint32(1000 + i*1000000)
+	}
+	pkts := make([][]byte, 0, nFlows*perFlow)
+	for range perFlow {
+		for f := range nFlows {
+			sport := uint16(10000 + f)
+			pkts = append(pkts, buildTCPv4Ports(sport, 2000, seqs[f], tcpAck, pay))
+			seqs[f] += uint32(payloadLen)
+		}
+	}
+	return pkts
+}
+
+// buildICMPv4 returns a minimal non-TCP packet that takes the passthrough
+// branch in Commit.
+func buildICMPv4() []byte {
+	pkt := make([]byte, 28)
+	pkt[0] = 0x45
+	binary.BigEndian.PutUint16(pkt[2:4], 28)
+	pkt[9] = 1 // ICMP
+	copy(pkt[12:16], []byte{10, 0, 0, 1})
+	copy(pkt[16:20], []byte{10, 0, 0, 2})
+	return pkt
+}
+
+// runCommitBench drives Commit over pkts batchSize at a time, flushing
+// between batches, and reports per-packet cost.
+func runCommitBench(b *testing.B, pkts [][]byte, batchSize int) {
+	b.Helper()
+	c := NewTCPCoalescer(nopTunWriter{})
+	b.ReportAllocs()
+	b.SetBytes(int64(len(pkts[0])))
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		pkt := pkts[i%len(pkts)]
+		if err := c.Commit(pkt); err != nil {
+			b.Fatal(err)
+		}
+		if (i+1)%batchSize == 0 {
+			if err := c.Flush(); err != nil {
+				b.Fatal(err)
+			}
+		}
+	}
+	// Drain any trailing partial batch so slot state doesn't leak across runs.
+	_ = c.Flush()
+}
+
+// BenchmarkCommitSingleFlow is the bulk-TCP steady state: one flow,
+// contiguous seq, 1200-byte payloads. Every packet past the seed should
+// append onto the open slot. This is the case we most care about.
+func BenchmarkCommitSingleFlow(b *testing.B) {
+	pkts := buildTCPv4BulkFlow(tcpCoalesceMaxSegs, 1200)
+	runCommitBench(b, pkts, tcpCoalesceMaxSegs)
+}
+
+// BenchmarkCommitInterleaved4 has 4 concurrent bulk flows round-robined.
+// A single-entry fast-path cache will miss on every packet; an N-way
+// cache or map lookup carries the weight.
+func BenchmarkCommitInterleaved4(b *testing.B) {
+	pkts := buildTCPv4Interleaved(4, tcpCoalesceMaxSegs, 1200)
+	runCommitBench(b, pkts, len(pkts))
+}
+
+// BenchmarkCommitInterleaved16 stresses the map at higher flow counts.
+func BenchmarkCommitInterleaved16(b *testing.B) {
+	pkts := buildTCPv4Interleaved(16, tcpCoalesceMaxSegs, 1200)
+	runCommitBench(b, pkts, len(pkts))
+}
+
+// BenchmarkCommitPassthrough exercises the non-TCP branch: parseTCPBase
+// bails early and addPassthrough is the only work.
+func BenchmarkCommitPassthrough(b *testing.B) {
+	pkt := buildICMPv4()
+	pkts := make([][]byte, 64)
+	for i := range pkts {
+		pkts[i] = pkt
+	}
+	runCommitBench(b, pkts, 64)
+}
+
+// BenchmarkCommitNonCoalesceableTCP sends SYN|ACK packets on one flow.
+// Each packet takes the "TCP but not admissible" branch which does a
+// map delete + passthrough. Measures the seal-without-slot cost.
+func BenchmarkCommitNonCoalesceableTCP(b *testing.B) {
+	pay := make([]byte, 0)
+	pkts := make([][]byte, 64)
+	for i := range pkts {
+		pkts[i] = buildTCPv4(uint32(1000+i), tcpSyn|tcpAck, pay)
+	}
+	runCommitBench(b, pkts, 64)
+}
+
+// runMultiCommitBench drives MultiCoalescer.Commit. The dispatcher does
+// the IP/L4 parse once and passes the parsed struct to the lane, so this
+// is the bench that shows the savings of skipping the lane's re-parse.
+func runMultiCommitBench(b *testing.B, pkts [][]byte, batchSize int) {
+	b.Helper()
+	m := NewMultiCoalescer(nopTunWriter{}, true, true)
+	b.ReportAllocs()
+	b.SetBytes(int64(len(pkts[0])))
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		pkt := pkts[i%len(pkts)]
+		if err := m.Commit(pkt); err != nil {
+			b.Fatal(err)
+		}
+		if (i+1)%batchSize == 0 {
+			if err := m.Flush(); err != nil {
+				b.Fatal(err)
+			}
+		}
+	}
+	_ = m.Flush()
+}
+
+// BenchmarkMultiCommitSingleFlow is the multi-lane analogue of
+// BenchmarkCommitSingleFlow — same workload but routed through the
+// dispatcher. The delta vs the single-lane bench measures dispatcher
+// overhead.
+func BenchmarkMultiCommitSingleFlow(b *testing.B) {
+	pkts := buildTCPv4BulkFlow(tcpCoalesceMaxSegs, 1200)
+	runMultiCommitBench(b, pkts, tcpCoalesceMaxSegs)
+}
+
+// BenchmarkMultiCommitInterleaved4 mirrors BenchmarkCommitInterleaved4
+// through the dispatcher.
+func BenchmarkMultiCommitInterleaved4(b *testing.B) {
+	pkts := buildTCPv4Interleaved(4, tcpCoalesceMaxSegs, 1200)
+	runMultiCommitBench(b, pkts, len(pkts))
+}

overlay/batch/tcp_coalesce_test.go

@@ -0,0 +1,1022 @@
+package batch
+
+import (
+	"encoding/binary"
+	"testing"
+
+	"github.com/slackhq/nebula/overlay/tio"
+)
+
+// fakeTunWriter records plain Writes and WriteGSO calls without touching a
+// real TUN fd. WriteGSO records the IP header, transport header, and
+// borrowed payload fragments separately so tests can inspect each.
+type fakeTunWriter struct {
+	gsoEnabled bool
+	writes     [][]byte
+	gsoWrites  []fakeGSOWrite
+}
+
+// fakeGSOWrite captures one WriteGSO call. hdr is the concatenation of the
+// IP and transport headers (in that order), gsoSize / isV6 / csumStart are
+// derived from the call so existing assertions keep working unchanged.
+type fakeGSOWrite struct {
+	hdr       []byte
+	pays      [][]byte
+	gsoSize   uint16
+	isV6      bool
+	csumStart uint16
+}
+
+// total returns hdrLen + sum of pay lens.
+func (g fakeGSOWrite) total() int {
+	n := len(g.hdr)
+	for _, p := range g.pays {
+		n += len(p)
+	}
+	return n
+}
+
+// payLen sums the pays.
+func (g fakeGSOWrite) payLen() int {
+	var n int
+	for _, p := range g.pays {
+		n += len(p)
+	}
+	return n
+}
+
+func (w *fakeTunWriter) Write(p []byte) (int, error) {
+	buf := make([]byte, len(p))
+	copy(buf, p)
+	w.writes = append(w.writes, buf)
+	return len(p), nil
+}
+
+func (w *fakeTunWriter) WriteGSO(hdr []byte, transportHdr []byte, pays [][]byte, _ tio.GSOProto) error {
+	hcopy := make([]byte, len(hdr)+len(transportHdr))
+	copy(hcopy, hdr)
+	copy(hcopy[len(hdr):], transportHdr)
+	paysCopy := make([][]byte, len(pays))
+	for i, p := range pays {
+		pc := make([]byte, len(p))
+		copy(pc, p)
+		paysCopy[i] = pc
+	}
+	var gsoSize uint16
+	if len(pays) > 1 {
+		gsoSize = uint16(len(pays[0]))
+	}
+	isV6 := len(hdr) > 0 && hdr[0]>>4 == 6
+	w.gsoWrites = append(w.gsoWrites, fakeGSOWrite{
+		hdr:       hcopy,
+		pays:      paysCopy,
+		gsoSize:   gsoSize,
+		isV6:      isV6,
+		csumStart: uint16(len(hdr)),
+	})
+	return nil
+}
+
+func (w *fakeTunWriter) Capabilities() tio.Capabilities {
+	return tio.Capabilities{TSO: w.gsoEnabled, USO: w.gsoEnabled}
+}
+
+// buildTCPv4 constructs a minimal IPv4+TCP packet with the given payload,
+// seq, and flags. Assumes no IP options and a 20-byte TCP header.
+func buildTCPv4(seq uint32, flags byte, payload []byte) []byte {
+	return buildTCPv4Ports(1000, 2000, seq, flags, payload)
+}
+
+// buildTCPv4Ports is buildTCPv4 with caller-specified ports so tests can
+// build distinct flows.
+func buildTCPv4Ports(sport, dport uint16, seq uint32, flags byte, payload []byte) []byte {
+	const ipHdrLen = 20
+	const tcpHdrLen = 20
+	total := ipHdrLen + tcpHdrLen + len(payload)
+	pkt := make([]byte, total)
+
+	pkt[0] = 0x45
+	pkt[1] = 0x00
+	binary.BigEndian.PutUint16(pkt[2:4], uint16(total))
+	binary.BigEndian.PutUint16(pkt[4:6], 0)
+	binary.BigEndian.PutUint16(pkt[6:8], 0x4000)
+	pkt[8] = 64
+	pkt[9] = ipProtoTCP
+	copy(pkt[12:16], []byte{10, 0, 0, 1})
+	copy(pkt[16:20], []byte{10, 0, 0, 2})
+
+	binary.BigEndian.PutUint16(pkt[20:22], sport)
+	binary.BigEndian.PutUint16(pkt[22:24], dport)
+	binary.BigEndian.PutUint32(pkt[24:28], seq)
+	binary.BigEndian.PutUint32(pkt[28:32], 12345)
+	pkt[32] = 0x50
+	pkt[33] = flags
+	binary.BigEndian.PutUint16(pkt[34:36], 0xffff)
+
+	copy(pkt[40:], payload)
+	return pkt
+}
+
+const (
+	tcpAck    = 0x10
+	tcpPsh    = 0x08
+	tcpSyn    = 0x02
+	tcpFin    = 0x01
+	tcpAckPsh = tcpAck | tcpPsh
+)
+
+func TestCoalescerPassthroughWhenGSOUnavailable(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: false}
+	c := NewTCPCoalescer(w)
+	pkt := buildTCPv4(1000, tcpAck, []byte("hello"))
+	if err := c.Commit(pkt); err != nil {
+		t.Fatal(err)
+	}
+	// No sync write — passthrough is deferred to Flush.
+	if len(w.writes) != 0 || len(w.gsoWrites) != 0 {
+		t.Fatalf("no Add-time writes: got writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.writes) != 1 || len(w.gsoWrites) != 0 {
+		t.Fatalf("want single plain write, got writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}
+
+func TestCoalescerNonTCPPassthrough(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pkt := make([]byte, 28)
+	pkt[0] = 0x45
+	binary.BigEndian.PutUint16(pkt[2:4], 28)
+	pkt[9] = 1
+	copy(pkt[12:16], []byte{10, 0, 0, 1})
+	copy(pkt[16:20], []byte{10, 0, 0, 2})
+	if err := c.Commit(pkt); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.writes) != 1 || len(w.gsoWrites) != 0 {
+		t.Fatalf("ICMP should pass through unchanged")
+	}
+}
+
+func TestCoalescerSeedThenFlushAlone(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pkt := buildTCPv4(1000, tcpAck, make([]byte, 1000))
+	if err := c.Commit(pkt); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.writes) != 0 || len(w.gsoWrites) != 0 {
+		t.Fatalf("unexpected output before flush")
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// Single-segment flush goes through WriteGSO with GSO_NONE
+	// (virtio NEEDS_CSUM lets the kernel fill in the L4 csum).
+	if len(w.gsoWrites) != 1 || len(w.writes) != 0 {
+		t.Fatalf("single-seg flush: writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+	g := w.gsoWrites[0]
+	if g.total() != 40+1000 {
+		t.Errorf("super total=%d want %d", g.total(), 40+1000)
+	}
+	if g.payLen() != 1000 {
+		t.Errorf("payLen=%d want 1000", g.payLen())
+	}
+}
+
+func TestCoalescerCoalescesAdjacentACKs(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	if err := c.Commit(buildTCPv4(1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(2200, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(3400, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 1 {
+		t.Fatalf("want 1 gso write, got %d (plain=%d)", len(w.gsoWrites), len(w.writes))
+	}
+	g := w.gsoWrites[0]
+	if g.gsoSize != 1200 {
+		t.Errorf("gsoSize=%d want 1200", g.gsoSize)
+	}
+	if len(g.hdr) != 40 {
+		t.Errorf("hdrLen=%d want 40", len(g.hdr))
+	}
+	if g.csumStart != 20 {
+		t.Errorf("csumStart=%d want 20", g.csumStart)
+	}
+	if len(g.pays) != 3 {
+		t.Errorf("pay count=%d want 3", len(g.pays))
+	}
+	if g.total() != 40+3*1200 {
+		t.Errorf("superpacket len=%d want %d", g.total(), 40+3*1200)
+	}
+	if tot := binary.BigEndian.Uint16(g.hdr[2:4]); int(tot) != g.total() {
+		t.Errorf("ip total_length=%d want %d", tot, g.total())
+	}
+}
+
+func TestCoalescerRejectsSeqGap(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	if err := c.Commit(buildTCPv4(1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(3000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// Each packet flushes as its own single-segment WriteGSO.
+	if len(w.gsoWrites) != 2 || len(w.writes) != 0 {
+		t.Fatalf("seq gap: want 2 gso writes got writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}
+
+func TestCoalescerRejectsFlagMismatch(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	if err := c.Commit(buildTCPv4(1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	// SYN|ACK is non-admissible. Must flush matching flow's slot (gso)
+	// and then plain-write the SYN packet itself.
+	syn := buildTCPv4(2200, tcpSyn|tcpAck, pay)
+	if err := c.Commit(syn); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.writes) != 1 || len(w.gsoWrites) != 1 {
+		t.Fatalf("flag mismatch: want 1 plain + 1 gso, got writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}
+
+func TestCoalescerRejectsFIN(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	fin := buildTCPv4(1000, tcpAck|tcpFin, []byte("x"))
+	if err := c.Commit(fin); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// FIN isn't admissible — passthrough as plain, no slot, no gso.
+	if len(w.writes) != 1 || len(w.gsoWrites) != 0 {
+		t.Fatalf("FIN should be passthrough, got writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}
+
+func TestCoalescerShortLastSegmentClosesChain(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	full := make([]byte, 1200)
+	half := make([]byte, 500)
+	if err := c.Commit(buildTCPv4(1000, tcpAck, full)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(2200, tcpAck, half)); err != nil {
+		t.Fatal(err)
+	}
+	// Chain now closed; next packet seeds a new slot on the same flow
+	// after flushing the old one.
+	if err := c.Commit(buildTCPv4(2700, tcpAck, full)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// Expect two gso writes: the first two packets coalesced, then the
+	// third flushed alone (single-seg via GSO_NONE).
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes got %d", len(w.gsoWrites))
+	}
+	if len(w.writes) != 0 {
+		t.Fatalf("want 0 plain writes got %d", len(w.writes))
+	}
+	if w.gsoWrites[0].gsoSize != 1200 {
+		t.Errorf("gsoSize=%d want 1200", w.gsoWrites[0].gsoSize)
+	}
+	if got, want := w.gsoWrites[0].total(), 40+1200+500; got != want {
+		t.Errorf("super len=%d want %d", got, want)
+	}
+}
+
+func TestCoalescerPSHFinalizesChain(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	if err := c.Commit(buildTCPv4(1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(2200, tcpAckPsh, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(3400, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// First two coalesce; the third seeds a fresh slot that flushes alone.
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes got %d", len(w.gsoWrites))
+	}
+	if len(w.writes) != 0 {
+		t.Fatalf("want 0 plain writes got %d", len(w.writes))
+	}
+}
+
+// TestCoalescerPropagatesPSHFromAppended ensures that when an appended
+// segment carries PSH (or is short, sealing the chain), the PSH bit ends
+// up in the emitted superpacket's TCP flags. The kernel TSO path keeps
+// PSH only on the last segment iff the input header has it set; if the
+// coalescer drops it the sender's push signal never reaches the receiver.
+func TestCoalescerPropagatesPSHFromAppended(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	// Seed has no PSH; second segment carries PSH and seals the chain.
+	if err := c.Commit(buildTCPv4(1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(2200, tcpAckPsh, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 1 {
+		t.Fatalf("want 1 gso write got %d", len(w.gsoWrites))
+	}
+	g := w.gsoWrites[0]
+	const ipHdrLen = 20
+	flags := g.hdr[ipHdrLen+13]
+	if flags&tcpPsh == 0 {
+		t.Fatalf("PSH lost from coalesced superpacket: flags=0x%02x", flags)
+	}
+	if flags&tcpAck == 0 {
+		t.Fatalf("ACK missing from coalesced superpacket: flags=0x%02x", flags)
+	}
+}
+
+func TestCoalescerRejectsDifferentFlow(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	p1 := buildTCPv4(1000, tcpAck, pay)
+	p2 := buildTCPv4(2200, tcpAck, pay)
+	binary.BigEndian.PutUint16(p2[20:22], 9999)
+	if err := c.Commit(p1); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(p2); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// Two independent flows, each flushes its own single-segment WriteGSO.
+	if len(w.gsoWrites) != 2 || len(w.writes) != 0 {
+		t.Fatalf("diff flow: want 2 gso writes got writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}
+
+func TestCoalescerRejectsIPOptions(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 500)
+	pkt := buildTCPv4(1000, tcpAck, pay)
+	// Bump IHL to 6 to simulate 4 bytes of IP options. Don't actually add
+	// bytes — parser should bail before it matters.
+	pkt[0] = 0x46
+	if err := c.Commit(pkt); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// Non-admissible parse → passthrough as plain.
+	if len(w.writes) != 1 || len(w.gsoWrites) != 0 {
+		t.Fatalf("IP options should passthrough, got writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}
+
+func TestCoalescerCapBySegments(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 512)
+	seq := uint32(1000)
+	for i := 0; i < tcpCoalesceMaxSegs+5; i++ {
+		if err := c.Commit(buildTCPv4(seq, tcpAck, pay)); err != nil {
+			t.Fatal(err)
+		}
+		seq += uint32(len(pay))
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	for _, g := range w.gsoWrites {
+		segs := len(g.pays)
+		if segs > tcpCoalesceMaxSegs {
+			t.Fatalf("super exceeded seg cap: %d > %d", segs, tcpCoalesceMaxSegs)
+		}
+	}
+}
+
+// TestCoalescerMultipleFlowsInSameBatch proves two interleaved bulk TCP
+// flows coalesce independently in a single Flush.
+func TestCoalescerMultipleFlowsInSameBatch(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+
+	// Flow A: sport 1000. Flow B: sport 3000.
+	if err := c.Commit(buildTCPv4Ports(1000, 2000, 100, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(3000, 2000, 500, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(1000, 2000, 1300, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(3000, 2000, 1700, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(1000, 2000, 2500, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(3000, 2000, 2900, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes (one per flow), got %d", len(w.gsoWrites))
+	}
+	if len(w.writes) != 0 {
+		t.Fatalf("want no plain writes, got %d", len(w.writes))
+	}
+	// Each superpacket should carry 3 segments.
+	for i, g := range w.gsoWrites {
+		if len(g.pays) != 3 {
+			t.Errorf("gso[%d]: segs=%d want 3", i, len(g.pays))
+		}
+		if g.gsoSize != 1200 {
+			t.Errorf("gso[%d]: gsoSize=%d want 1200", i, g.gsoSize)
+		}
+	}
+	// Verify each superpacket carries the source port it was seeded with.
+	seenSports := map[uint16]bool{}
+	for _, g := range w.gsoWrites {
+		sp := binary.BigEndian.Uint16(g.hdr[20:22])
+		seenSports[sp] = true
+	}
+	if !seenSports[1000] || !seenSports[3000] {
+		t.Errorf("expected superpackets for sports 1000 and 3000, got %v", seenSports)
+	}
+}
+
+// TestCoalescerPreservesArrivalOrder confirms that with passthrough and
+// coalesced events both queued, Flush emits them in Add order rather than
+// writing passthrough packets synchronously.
+func TestCoalescerPreservesArrivalOrder(t *testing.T) {
+	w := &orderedFakeWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	// Sequence: coalesceable TCP, ICMP (passthrough), coalesceable TCP on
+	// a different flow. Expected emit order: gso(X), plain(ICMP), gso(Y).
+	pay := make([]byte, 1200)
+	if err := c.Commit(buildTCPv4Ports(1000, 2000, 100, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	icmp := make([]byte, 28)
+	icmp[0] = 0x45
+	binary.BigEndian.PutUint16(icmp[2:4], 28)
+	icmp[9] = 1
+	copy(icmp[12:16], []byte{10, 0, 0, 1})
+	copy(icmp[16:20], []byte{10, 0, 0, 3})
+	if err := c.Commit(icmp); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(3000, 2000, 500, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	// Nothing should have hit the writer synchronously.
+	if len(w.events) != 0 {
+		t.Fatalf("Add emitted events synchronously: %v", w.events)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if got, want := w.events, []string{"gso", "plain", "gso"}; !stringSliceEq(got, want) {
+		t.Fatalf("flush order=%v want %v", got, want)
+	}
+}
+
+// orderedFakeWriter records only the sequence of call types so tests can
+// assert arrival order without inspecting bytes.
+type orderedFakeWriter struct {
+	gsoEnabled bool
+	events     []string
+}
+
+func (w *orderedFakeWriter) Write(p []byte) (int, error) {
+	w.events = append(w.events, "plain")
+	return len(p), nil
+}
+
+func (w *orderedFakeWriter) WriteGSO(hdr []byte, transportHdr []byte, pays [][]byte, _ tio.GSOProto) error {
+	w.events = append(w.events, "gso")
+	return nil
+}
+
+func (w *orderedFakeWriter) Capabilities() tio.Capabilities {
+	return tio.Capabilities{TSO: w.gsoEnabled, USO: w.gsoEnabled}
+}
+
+func stringSliceEq(a, b []string) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	for i := range a {
+		if a[i] != b[i] {
+			return false
+		}
+	}
+	return true
+}
+
+// TestCoalescerInterleavedFlowsPreserveOrdering checks that a non-admissible
+// packet (SYN) mid-flow only flushes its own flow, not others.
+func TestCoalescerInterleavedFlowsPreserveOrdering(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+
+	// Flow A two segments.
+	if err := c.Commit(buildTCPv4Ports(1000, 2000, 100, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(1000, 2000, 1300, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	// Flow B two segments.
+	if err := c.Commit(buildTCPv4Ports(3000, 2000, 500, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(3000, 2000, 1700, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	// Flow A SYN (non-admissible) — must flush only flow A's slot.
+	syn := buildTCPv4Ports(1000, 2000, 9999, tcpSyn|tcpAck, pay)
+	if err := c.Commit(syn); err != nil {
+		t.Fatal(err)
+	}
+	// Flow B continues — should still be coalesced with its seed.
+	if err := c.Commit(buildTCPv4Ports(3000, 2000, 2900, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+
+	// Expected:
+	//   - 1 gso for flow A (first 2 segments)
+	//   - 1 plain for flow A SYN
+	//   - 1 gso for flow B (3 segments)
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes, got %d", len(w.gsoWrites))
+	}
+	if len(w.writes) != 1 {
+		t.Fatalf("want 1 plain write (SYN), got %d", len(w.writes))
+	}
+	// Find the 3-segment gso (flow B) and the 2-segment gso (flow A).
+	var segCounts []int
+	for _, g := range w.gsoWrites {
+		segCounts = append(segCounts, len(g.pays))
+	}
+	if !(segCounts[0] == 2 && segCounts[1] == 3) && !(segCounts[0] == 3 && segCounts[1] == 2) {
+		t.Errorf("unexpected segment counts: %v (want 2 and 3)", segCounts)
+	}
+}
+
+// ECN test helpers and constants.
+
+const (
+	tcpEce = 0x40
+	tcpCwr = 0x80
+
+	// 2-bit IP-level ECN codepoints (lower 2 bits of IPv4 ToS / IPv6 TC).
+	ecnNotECT = 0x00
+	ecnECT1   = 0x01
+	ecnECT0   = 0x02
+	ecnCE     = 0x03
+)
+
+// buildTCPv4WithToS is buildTCPv4 with caller-specified IPv4 ToS so tests can
+// drive DSCP and ECN bits.
+func buildTCPv4WithToS(tos byte, seq uint32, flags byte, payload []byte) []byte {
+	pkt := buildTCPv4(seq, flags, payload)
+	pkt[1] = tos
+	return pkt
+}
+
+// buildTCPv6 mirrors buildTCPv4 for IPv6. tcLow is the low 4 bits of Traffic
+// Class, which carries the ECN codepoint (mask 0x03) and the bottom 2 DSCP
+// bits — enough to drive the ECN paths under test.
+func buildTCPv6(tcLow byte, seq uint32, flags byte, payload []byte) []byte {
+	const ipHdrLen = 40
+	const tcpHdrLen = 20
+	pkt := make([]byte, ipHdrLen+tcpHdrLen+len(payload))
+
+	pkt[0] = 0x60                // version=6, TC[7:4]=0
+	pkt[1] = (tcLow & 0x0f) << 4 // TC[3:0] in high nibble; flow=0
+	binary.BigEndian.PutUint16(pkt[4:6], uint16(tcpHdrLen+len(payload)))
+	pkt[6] = ipProtoTCP
+	pkt[7] = 64
+	copy(pkt[8:24], []byte{0xfd, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 1})
+	copy(pkt[24:40], []byte{0xfd, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2})
+
+	binary.BigEndian.PutUint16(pkt[40:42], 1000)
+	binary.BigEndian.PutUint16(pkt[42:44], 2000)
+	binary.BigEndian.PutUint32(pkt[44:48], seq)
+	binary.BigEndian.PutUint32(pkt[48:52], 12345)
+	pkt[52] = 0x50
+	pkt[53] = flags
+	binary.BigEndian.PutUint16(pkt[54:56], 0xffff)
+
+	copy(pkt[60:], payload)
+	return pkt
+}
+
+// TestCoalescerCoalescesEceFlow confirms that ECN-Echo-marked ACKs (an
+// ECN-aware flow under congestion) keep getting coalesced into a TSO
+// superpacket instead of falling out to passthrough, and that the seed
+// retains ECE on the wire.
+func TestCoalescerCoalescesEceFlow(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	flags := byte(tcpAck | tcpEce)
+	if err := c.Commit(buildTCPv4(1000, flags, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(2200, flags, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 1 {
+		t.Fatalf("want 1 gso write, got %d (plain=%d)", len(w.gsoWrites), len(w.writes))
+	}
+	g := w.gsoWrites[0]
+	if len(g.pays) != 2 {
+		t.Errorf("pay count=%d want 2", len(g.pays))
+	}
+	if seedFlags := g.hdr[20+13]; seedFlags&tcpEce == 0 {
+		t.Errorf("seed flags=0x%02x want ECE preserved", seedFlags)
+	}
+}
+
+// TestCoalescerCwrSealsFlow confirms that a CWR-bearing segment in the
+// middle of a flow goes to passthrough and seals the open slot, so a later
+// in-flow segment seeds a new slot rather than extending the prior burst.
+func TestCoalescerCwrSealsFlow(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	if err := c.Commit(buildTCPv4(1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(2200, tcpAck|tcpCwr, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(3400, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.writes) != 1 {
+		t.Fatalf("want 1 plain write (CWR), got %d", len(w.writes))
+	}
+	// Two GSO writes: the first seed before CWR, and a fresh seed after.
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes, got %d", len(w.gsoWrites))
+	}
+	for i, g := range w.gsoWrites {
+		if len(g.pays) != 1 {
+			t.Errorf("gso %d pay count=%d want 1", i, len(g.pays))
+		}
+	}
+}
+
+// TestCoalescerEceMismatchReseeds confirms that toggling ECE mid-flow does
+// not silently merge — receivers expect ECE either set on every segment of
+// a CE-echoing window or none.
+func TestCoalescerEceMismatchReseeds(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	if err := c.Commit(buildTCPv4(1000, tcpAck|tcpEce, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(2200, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 separate seeds, got %d gso writes", len(w.gsoWrites))
+	}
+	for i, g := range w.gsoWrites {
+		if len(g.pays) != 1 {
+			t.Errorf("gso %d pay count=%d want 1", i, len(g.pays))
+		}
+	}
+}
+
+// TestCoalescerMergesCEMark confirms that an ECT(0) burst with a single
+// CE-marked packet still coalesces, and the merged superpacket carries CE.
+func TestCoalescerMergesCEMark(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	if err := c.Commit(buildTCPv4WithToS(ecnECT0, 1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	// Router along the path stamped CE on this one.
+	if err := c.Commit(buildTCPv4WithToS(ecnCE, 2200, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4WithToS(ecnECT0, 3400, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 1 {
+		t.Fatalf("want 1 merged gso write, got %d (plain=%d)", len(w.gsoWrites), len(w.writes))
+	}
+	g := w.gsoWrites[0]
+	if len(g.pays) != 3 {
+		t.Errorf("pay count=%d want 3", len(g.pays))
+	}
+	if got := g.hdr[1] & 0x03; got != ecnCE {
+		t.Errorf("seed ECN=0x%02x want CE 0x%02x", got, ecnCE)
+	}
+}
+
+// TestCoalescerDscpMismatchReseeds confirms that the new ECN-mask in
+// headersMatch did not also relax DSCP — different DSCP must still split.
+func TestCoalescerDscpMismatchReseeds(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	// Same ECN (Not-ECT), different DSCP (0x10 vs 0x20 in upper 6 bits).
+	tosA := byte(0x10<<2) | ecnNotECT
+	tosB := byte(0x20<<2) | ecnNotECT
+	if err := c.Commit(buildTCPv4WithToS(tosA, 1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4WithToS(tosB, 2200, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 separate seeds (different DSCP), got %d", len(w.gsoWrites))
+	}
+}
+
+// TestCoalescerIPv6CoalescesEceFlow is the IPv6 analogue of
+// TestCoalescerCoalescesEceFlow.
+func TestCoalescerIPv6CoalescesEceFlow(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	flags := byte(tcpAck | tcpEce)
+	if err := c.Commit(buildTCPv6(0, 1000, flags, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv6(0, 2200, flags, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 1 {
+		t.Fatalf("want 1 gso write, got %d", len(w.gsoWrites))
+	}
+	g := w.gsoWrites[0]
+	if seedFlags := g.hdr[40+13]; seedFlags&tcpEce == 0 {
+		t.Errorf("seed flags=0x%02x want ECE preserved", seedFlags)
+	}
+}
+
+// TestCoalescerSortsReorderedSeedsAndMerges feeds three same-flow MSS
+// segments out of TCP-seq order (mimicking a wire reorder that escaped
+// the rxOrder per-batch sort). Without the reorderForFlush sort+merge,
+// each out-of-seq arrival would seed its own slot and the slots would
+// emit in arrival order, producing a kernel-visible TCP reorder. With
+// the sort+merge, the three slots are sorted by seq and folded back into
+// one in-order TSO superpacket — same shape the receiver TCP would have
+// seen had the wire never reordered.
+func TestCoalescerSortsReorderedSeedsAndMerges(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	// Arrival order: seq 1000, 3400, 2200. The 3400 seeds a separate slot
+	// because 3400 != nextSeq=2200, then 2200 fails to extend the 3400 slot
+	// and seeds its own. Three slots end up in c.slots; reorderForFlush
+	// should sort them into [1000,2200,3400] and merge them back into one.
+	if err := c.Commit(buildTCPv4(1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(3400, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(2200, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 1 {
+		t.Fatalf("want 1 merged gso write got %d", len(w.gsoWrites))
+	}
+	g := w.gsoWrites[0]
+	if len(g.pays) != 3 {
+		t.Fatalf("merged segs=%d want 3", len(g.pays))
+	}
+	const ipHdrLen = 20
+	if seedSeq := binary.BigEndian.Uint32(g.hdr[ipHdrLen+4 : ipHdrLen+8]); seedSeq != 1000 {
+		t.Errorf("merged seed seq=%d want 1000 (lowest)", seedSeq)
+	}
+}
+
+// TestCoalescerSortAcrossFlowsMergesEachIndependently checks that two
+// flows interleaved with reorder are each sorted-and-merged in isolation
+// without any cross-flow contamination.
+func TestCoalescerSortAcrossFlowsMergesEachIndependently(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	// Flow A (sport 1000) seq 100, 1300; flow B (sport 3000) seq 500, 1700.
+	// Arrival: A.1300, B.1700, A.100, B.500 — every flow reordered.
+	if err := c.Commit(buildTCPv4Ports(1000, 2000, 1300, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(3000, 2000, 1700, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(1000, 2000, 100, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4Ports(3000, 2000, 500, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes (one per flow merged), got %d", len(w.gsoWrites))
+	}
+	for i, g := range w.gsoWrites {
+		if len(g.pays) != 2 {
+			t.Errorf("gso[%d] segs=%d want 2", i, len(g.pays))
+		}
+		const ipHdrLen = 20
+		seedSeq := binary.BigEndian.Uint32(g.hdr[ipHdrLen+4 : ipHdrLen+8])
+		sport := binary.BigEndian.Uint16(g.hdr[ipHdrLen : ipHdrLen+2])
+		// Each flow's merged seed should be the LOWER of its two seqs.
+		switch sport {
+		case 1000:
+			if seedSeq != 100 {
+				t.Errorf("flow A seed seq=%d want 100", seedSeq)
+			}
+		case 3000:
+			if seedSeq != 500 {
+				t.Errorf("flow B seed seq=%d want 500", seedSeq)
+			}
+		default:
+			t.Errorf("unexpected sport %d", sport)
+		}
+	}
+}
+
+// TestCoalescerSortKeepsPSHBoundary verifies that a PSH-sealed slot is
+// not folded into a later seq-contiguous slot — PSH placement is part of
+// the wire signal and merging across it would shift the receiver's push
+// boundary by an arbitrary number of segments.
+func TestCoalescerSortKeepsPSHBoundary(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	// Seq 1000 (no PSH) + 2200 (PSH) → seal one slot with PSH set.
+	// Seq 3400 (no PSH) is contiguous to 3400 from seq 2200+1200; without
+	// the PSH check it would merge in.
+	if err := c.Commit(buildTCPv4(1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(2200, tcpAckPsh, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(3400, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes (PSH-sealed and fresh seed), got %d", len(w.gsoWrites))
+	}
+}
+
+// TestCoalescerSortKeepsPassthroughBarrier confirms a passthrough slot in
+// the middle of the queue prevents the post-sort merge from folding
+// across it. Reordered same-flow data on either side of the passthrough
+// is sorted/merged independently.
+func TestCoalescerSortKeepsPassthroughBarrier(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	// First two segments seed S1 (then a 3400 reorder seeds S2).
+	if err := c.Commit(buildTCPv4(1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv4(3400, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	// Non-coalesceable packet (SYN+ACK) flushes S1's openSlots entry and
+	// becomes a passthrough barrier in c.slots.
+	if err := c.Commit(buildTCPv4(9999, tcpSyn|tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	// Post-barrier same-flow data: should never end up before the SYN.
+	if err := c.Commit(buildTCPv4(2200, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// We expect: gso(merged 1000+3400 ranges sorted but not contiguous so 2
+	// gso writes), plain(SYN), gso(2200 alone). The pre-barrier sort should
+	// land 1000 before 3400, and the post-barrier 2200 stays after the SYN.
+	if len(w.writes) != 1 {
+		t.Fatalf("want 1 plain SYN passthrough, got %d", len(w.writes))
+	}
+}
+
+// TestCoalescerIPv6MergesCEMark is the IPv6 analogue of
+// TestCoalescerMergesCEMark. ECN bits live in TC[1:0] = byte 1 mask 0x30.
+func TestCoalescerIPv6MergesCEMark(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewTCPCoalescer(w)
+	pay := make([]byte, 1200)
+	// tcLow is the low 4 bits of TC; ECN occupies the bottom 2 of those.
+	if err := c.Commit(buildTCPv6(ecnECT0, 1000, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildTCPv6(ecnCE, 2200, tcpAck, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 1 {
+		t.Fatalf("want 1 merged gso write, got %d", len(w.gsoWrites))
+	}
+	g := w.gsoWrites[0]
+	// Byte 1 high nibble holds TC[3:0]; ECN is the low 2 bits of that nibble,
+	// which appears in byte 1 mask 0x30 (>>4 to read the codepoint value).
+	if got := (g.hdr[1] >> 4) & 0x03; got != ecnCE {
+		t.Errorf("seed v6 ECN=0x%02x want CE 0x%02x", got, ecnCE)
+	}
+}

overlay/batch/tx_batch.go

@@ -4,58 +4,63 @@ import "net/netip"
 
 const SendBatchCap = 128
 
-// SendBatch accumulates encrypted UDP packets for potential TX offloading.
+// batchWriter is the minimal subset of udp.Conn needed by SendBatch to flush.
+type batchWriter interface {
+	WriteBatch(bufs [][]byte, addrs []netip.AddrPort, outerECNs []byte) error
+}
+
+// SendBatch accumulates encrypted UDP packets and flushes them via WriteBatch.
 // One SendBatch is owned by each listenIn goroutine; no locking is needed.
-// The backing storage holds up to batchCap packets of slotCap bytes each;
-// bufs and dsts are parallel slices of committed slots.
+// The backing arena grows on demand: when there isn't room for the next slot
+// we allocate a fresh backing array. Already-committed slices keep referencing
+// the old array and remain valid until Flush drops them.
 type SendBatch struct {
-	bufs     [][]byte
-	dsts     []netip.AddrPort
-	backing  []byte
-	slotCap  int
-	batchCap int
-	nextSlot int
+	out     batchWriter
+	bufs    [][]byte
+	dsts    []netip.AddrPort
+	ecns    []byte
+	backing []byte
 }
 
-func NewSendBatch(batchCap, slotCap int) *SendBatch {
+func NewSendBatch(out batchWriter, batchCap, slotCap int) *SendBatch {
 	return &SendBatch{
-		bufs:     make([][]byte, 0, batchCap),
-		dsts:     make([]netip.AddrPort, 0, batchCap),
-		backing:  make([]byte, batchCap*slotCap),
-		slotCap:  slotCap,
-		batchCap: batchCap,
+		out:     out,
+		bufs:    make([][]byte, 0, batchCap),
+		dsts:    make([]netip.AddrPort, 0, batchCap),
+		ecns:    make([]byte, 0, batchCap),
+		backing: make([]byte, 0, batchCap*slotCap),
 	}
 }
 
-func (b *SendBatch) Next() []byte {
-	if b.nextSlot >= b.batchCap {
-		return nil
+func (b *SendBatch) Reserve(sz int) []byte {
+	if len(b.backing)+sz > cap(b.backing) {
+		// Grow: allocate a fresh backing. Already-committed slices still
+		// reference the old array and remain valid until Flush drops them.
+		newCap := max(cap(b.backing)*2, sz)
+		b.backing = make([]byte, 0, newCap)
 	}
-	start := b.nextSlot * b.slotCap
-	return b.backing[start : start : start+b.slotCap] //set len to 0 but cap to slotCap
+	start := len(b.backing)
+	b.backing = b.backing[:start+sz]
+	return b.backing[start : start+sz : start+sz]
 }
 
-func (b *SendBatch) Commit(n int, dst netip.AddrPort) {
-	start := b.nextSlot * b.slotCap
-	b.bufs = append(b.bufs, b.backing[start:start+n])
+func (b *SendBatch) Commit(pkt []byte, dst netip.AddrPort, outerECN byte) {
+	b.bufs = append(b.bufs, pkt)
 	b.dsts = append(b.dsts, dst)
-	b.nextSlot++
+	b.ecns = append(b.ecns, outerECN)
 }
 
-func (b *SendBatch) Reset() {
+func (b *SendBatch) Flush() error {
+	var err error
+	if len(b.bufs) > 0 {
+		err = b.out.WriteBatch(b.bufs, b.dsts, b.ecns)
+	}
+	for i := range b.bufs {
+		b.bufs[i] = nil
+	}
 	b.bufs = b.bufs[:0]
 	b.dsts = b.dsts[:0]
-	b.nextSlot = 0
-}
-
-func (b *SendBatch) Len() int {
-	return len(b.bufs)
-}
-
-func (b *SendBatch) Cap() int {
-	return b.batchCap
-}
-
-func (b *SendBatch) Get() ([][]byte, []netip.AddrPort) {
-	return b.bufs, b.dsts
+	b.ecns = b.ecns[:0]
+	b.backing = b.backing[:0]
+	return err
 }

overlay/batch/tx_batch_test.go

@@ -5,65 +5,120 @@ import (
 	"testing"
 )
 
-func TestSendBatchBookkeeping(t *testing.T) {
-	b := NewSendBatch(4, 32)
-	if b.Len() != 0 || b.Cap() != 4 {
-		t.Fatalf("fresh batch: len=%d cap=%d", b.Len(), b.Cap())
+type fakeBatchWriter struct {
+	bufs  [][]byte
+	addrs []netip.AddrPort
+	ecns  []byte
+}
+
+func (w *fakeBatchWriter) WriteBatch(bufs [][]byte, addrs []netip.AddrPort, ecns []byte) error {
+	// Snapshot — SendBatch.Flush nils its slot pointers right after WriteBatch
+	// returns, so tests must capture data before that happens.
+	w.bufs = make([][]byte, len(bufs))
+	for i, b := range bufs {
+		cp := make([]byte, len(b))
+		copy(cp, b)
+		w.bufs[i] = cp
 	}
+	w.addrs = append(w.addrs[:0], addrs...)
+	w.ecns = append(w.ecns[:0], ecns...)
+	return nil
+}
+
+func TestSendBatchReserveCommitFlush(t *testing.T) {
+	fw := &fakeBatchWriter{}
+	b := NewSendBatch(fw, 4, 32)
 
 	ap := netip.MustParseAddrPort("10.0.0.1:4242")
 	for i := 0; i < 4; i++ {
-		slot := b.Next()
-		if slot == nil {
-			t.Fatalf("slot %d: Next returned nil before cap", i)
-		}
-		if cap(slot) != 32 || len(slot) != 0 {
-			t.Fatalf("slot %d: got len=%d cap=%d want len=0 cap=32", i, len(slot), cap(slot))
+		slot := b.Reserve(32)
+		if cap(slot) != 32 {
+			t.Fatalf("slot %d: cap=%d want 32", i, cap(slot))
 		}
-		// Write a marker byte.
-		slot = append(slot, byte(i), byte(i+1), byte(i+2))
-		b.Commit(len(slot), ap)
+		pkt := append(slot[:0], byte(i), byte(i+1), byte(i+2))
+		b.Commit(pkt, ap, 0)
 	}
-	if b.Next() != nil {
-		t.Fatalf("Next should return nil when full")
+	if err := b.Flush(); err != nil {
+		t.Fatalf("Flush: %v", err)
 	}
-	if b.Len() != 4 {
-		t.Fatalf("Len=%d want 4", b.Len())
+	if len(fw.bufs) != 4 {
+		t.Fatalf("WriteBatch got %d bufs want 4", len(fw.bufs))
 	}
-	for i, buf := range b.bufs {
+	for i, buf := range fw.bufs {
 		if len(buf) != 3 || buf[0] != byte(i) {
 			t.Errorf("buf %d: %x", i, buf)
 		}
-		if b.dsts[i] != ap {
-			t.Errorf("dst %d: got %v want %v", i, b.dsts[i], ap)
+		if fw.addrs[i] != ap {
+			t.Errorf("addr %d: got %v want %v", i, fw.addrs[i], ap)
 		}
 	}
 
-	// Reset returns empty and Next works again.
-	b.Reset()
-	if b.Len() != 0 {
-		t.Fatalf("after Reset Len=%d want 0", b.Len())
+	// Flush again with nothing committed — should be a no-op.
+	fw.bufs = nil
+	if err := b.Flush(); err != nil {
+		t.Fatalf("empty Flush: %v", err)
 	}
-	slot := b.Next()
-	if slot == nil || cap(slot) != 32 {
-		t.Fatalf("after Reset Next nil or wrong cap: %v cap=%d", slot == nil, cap(slot))
+	if fw.bufs != nil {
+		t.Fatalf("empty Flush triggered WriteBatch")
+	}
+
+	// Reuse after Flush.
+	slot := b.Reserve(32)
+	if cap(slot) != 32 {
+		t.Fatalf("after Flush Reserve wrong cap: %d", cap(slot))
 	}
 }
 
 func TestSendBatchSlotsDoNotOverlap(t *testing.T) {
-	b := NewSendBatch(3, 8)
+	fw := &fakeBatchWriter{}
+	b := NewSendBatch(fw, 3, 8)
 	ap := netip.MustParseAddrPort("10.0.0.1:80")
 
-	// Fill three slots, each with its own sentinel byte.
 	for i := 0; i < 3; i++ {
-		s := b.Next()
-		s = append(s, byte(0xA0+i), byte(0xB0+i))
-		b.Commit(len(s), ap)
+		s := b.Reserve(8)
+		pkt := append(s[:0], byte(0xA0+i), byte(0xB0+i))
+		b.Commit(pkt, ap, 0)
+	}
+	if err := b.Flush(); err != nil {
+		t.Fatalf("Flush: %v", err)
 	}
 
-	for i, buf := range b.bufs {
+	for i, buf := range fw.bufs {
 		if buf[0] != byte(0xA0+i) || buf[1] != byte(0xB0+i) {
 			t.Errorf("slot %d corrupted: %x", i, buf)
 		}
 	}
 }
+
+func TestSendBatchGrowPreservesCommitted(t *testing.T) {
+	fw := &fakeBatchWriter{}
+	// Tiny initial backing forces a grow on the second Reserve.
+	b := NewSendBatch(fw, 1, 4)
+	ap := netip.MustParseAddrPort("10.0.0.1:80")
+
+	s1 := b.Reserve(4)
+	pkt1 := append(s1[:0], 0x11, 0x22, 0x33, 0x44)
+	b.Commit(pkt1, ap, 0)
+
+	s2 := b.Reserve(8) // exceeds remaining cap, triggers grow
+	pkt2 := append(s2[:0], 0xA, 0xB, 0xC, 0xD, 0xE)
+	b.Commit(pkt2, ap, 0)
+
+	// pkt1 must still be intact even though backing reallocated.
+	if pkt1[0] != 0x11 || pkt1[3] != 0x44 {
+		t.Fatalf("first packet corrupted by grow: %x", pkt1)
+	}
+
+	if err := b.Flush(); err != nil {
+		t.Fatalf("Flush: %v", err)
+	}
+	if len(fw.bufs) != 2 {
+		t.Fatalf("got %d bufs want 2", len(fw.bufs))
+	}
+	if fw.bufs[0][0] != 0x11 || fw.bufs[0][3] != 0x44 {
+		t.Errorf("first packet on the wire: %x", fw.bufs[0])
+	}
+	if fw.bufs[1][0] != 0xA || fw.bufs[1][4] != 0xE {
+		t.Errorf("second packet on the wire: %x", fw.bufs[1])
+	}
+}

overlay/batch/udp_coalesce.go

@@ -0,0 +1,342 @@
+package batch
+
+import (
+	"encoding/binary"
+	"io"
+
+	"github.com/slackhq/nebula/overlay/tio"
+)
+
+// ipProtoUDP is the IANA protocol number for UDP.
+const ipProtoUDP = 17
+
+// udpCoalesceBufSize caps total bytes per UDP superpacket. Mirrors the
+// kernel's gso_max_size; payloads beyond this are emitted as-is.
+const udpCoalesceBufSize = 65535
+
+// udpCoalesceMaxSegs caps how many segments we'll coalesce. Kernel UDP-GSO
+// accepts up to 64 segments per skb (UDP_MAX_SEGMENTS); stay under that.
+const udpCoalesceMaxSegs = 64
+
+// udpCoalesceHdrCap is the scratch space we copy a seed's IP+UDP header
+// into. IPv6 (40) + UDP (8) = 48; round up for safety.
+const udpCoalesceHdrCap = 64
+
+// udpSlot is one entry in the UDPCoalescer's ordered event queue. Same
+// passthrough-vs-coalesced shape as the TCP coalescer's slot, but no
+// seq/PSH/CWR bookkeeping — UDP segments only need 5-tuple + length
+// matching to coalesce.
+type udpSlot struct {
+	passthrough bool
+	rawPkt      []byte // borrowed when passthrough
+
+	fk       flowKey
+	hdrBuf   [udpCoalesceHdrCap]byte
+	hdrLen   int
+	ipHdrLen int
+	isV6     bool
+	gsoSize  int // per-segment UDP payload length
+	numSeg   int
+	totalPay int
+	// sealed closes the chain: set when a sub-gsoSize segment is appended
+	// (kernel UDP-GSO requires every segment but the last to be exactly
+	// gsoSize) or when limits are hit. No further appends after.
+	sealed  bool
+	payIovs [][]byte
+}
+
+// UDPCoalescer accumulates adjacent in-flow UDP datagrams across multiple
+// concurrent flows and emits each flow's run as a single GSO_UDP_L4
+// superpacket via tio.GSOWriter. Falls back to per-packet writes when the
+// underlying writer doesn't support USO.
+//
+// All output — coalesced or not — is deferred until Flush so per-flow
+// arrival order is preserved on the wire. Cross-flow order is NOT preserved
+// across the TCP/UDP/passthrough split when this coalescer runs alongside
+// others — see multi_coalesce.go. Per-flow order is preserved because a
+// single 5-tuple only ever lands in one lane and each lane preserves its
+// own slot order.
+//
+// Owns no locks; one coalescer per TUN write queue.
+type UDPCoalescer struct {
+	plainW io.Writer
+	gsoW   tio.GSOWriter // nil when the queue can't accept GSO_UDP_L4
+
+	slots     []*udpSlot
+	openSlots map[flowKey]*udpSlot
+	pool      []*udpSlot
+
+	backing []byte
+}
+
+// NewUDPCoalescer wraps w. The caller is responsible for only constructing
+// this when the underlying Queue's Capabilities advertise USO; otherwise
+// the kernel may reject GSO_UDP_L4 writes. If w does not implement
+// tio.GSOWriter at all (single-packet Queue), the coalescer degrades to
+// plain Writes — same defensive shape as the TCP coalescer.
+func NewUDPCoalescer(w io.Writer) *UDPCoalescer {
+	c := &UDPCoalescer{
+		plainW:    w,
+		slots:     make([]*udpSlot, 0, initialSlots),
+		openSlots: make(map[flowKey]*udpSlot, initialSlots),
+		pool:      make([]*udpSlot, 0, initialSlots),
+		backing:   make([]byte, 0, initialSlots*udpCoalesceBufSize),
+	}
+	if gw, ok := tio.SupportsGSO(w, tio.GSOProtoUDP); ok {
+		c.gsoW = gw
+	}
+	return c
+}
+
+// parsedUDP holds the fields extracted from a single parse so later steps
+// (admission, slot lookup, canAppend) don't re-walk the header.
+type parsedUDP struct {
+	fk       flowKey
+	ipHdrLen int
+	hdrLen   int // ipHdrLen + 8
+	payLen   int
+}
+
+// parseUDP extracts the flow key and IP/UDP offsets for a UDP packet.
+// Returns ok=false for non-UDP, malformed, or unsupported header shapes
+// (IPv4 with options/fragmentation, IPv6 with extension headers).
+func parseUDP(pkt []byte) (parsedUDP, bool) {
+	var p parsedUDP
+	ip, ok := parseIPPrologue(pkt, ipProtoUDP)
+	if !ok {
+		return p, false
+	}
+	pkt = ip.pkt
+	p.fk = ip.fk
+	p.ipHdrLen = ip.ipHdrLen
+
+	if len(pkt) < p.ipHdrLen+8 {
+		return p, false
+	}
+	p.hdrLen = p.ipHdrLen + 8
+	// UDP `length` field: must equal IP-derived length-of-UDP-header-plus-payload.
+	udpLen := int(binary.BigEndian.Uint16(pkt[p.ipHdrLen+4 : p.ipHdrLen+6]))
+	if udpLen < 8 || udpLen > len(pkt)-p.ipHdrLen {
+		return p, false
+	}
+	p.payLen = udpLen - 8
+	p.fk.sport = binary.BigEndian.Uint16(pkt[p.ipHdrLen : p.ipHdrLen+2])
+	p.fk.dport = binary.BigEndian.Uint16(pkt[p.ipHdrLen+2 : p.ipHdrLen+4])
+	return p, true
+}
+
+func (c *UDPCoalescer) Reserve(sz int) []byte {
+	return reserveFromBacking(&c.backing, sz)
+}
+
+// Commit borrows pkt. The caller must keep pkt valid until the next Flush.
+func (c *UDPCoalescer) Commit(pkt []byte) error {
+	if c.gsoW == nil {
+		c.addPassthrough(pkt)
+		return nil
+	}
+	info, ok := parseUDP(pkt)
+	if !ok {
+		c.addPassthrough(pkt)
+		return nil
+	}
+	return c.commitParsed(pkt, info)
+}
+
+// commitParsed is the post-parse half of Commit. The caller must have
+// already verified parseUDP succeeded. Used by MultiCoalescer.Commit to
+// avoid re-walking the IP/UDP header.
+func (c *UDPCoalescer) commitParsed(pkt []byte, info parsedUDP) error {
+	if c.gsoW == nil {
+		c.addPassthrough(pkt)
+		return nil
+	}
+	if open := c.openSlots[info.fk]; open != nil {
+		if c.canAppend(open, pkt, info) {
+			c.appendPayload(open, pkt, info)
+			if open.sealed {
+				delete(c.openSlots, info.fk)
+			}
+			return nil
+		}
+		// Can't extend — seal it and fall through to seed a fresh slot.
+		delete(c.openSlots, info.fk)
+	}
+	c.seed(pkt, info)
+	return nil
+}
+
+func (c *UDPCoalescer) Flush() error {
+	var first error
+	for _, s := range c.slots {
+		var err error
+		if s.passthrough {
+			_, err = c.plainW.Write(s.rawPkt)
+		} else {
+			err = c.flushSlot(s)
+		}
+		if err != nil && first == nil {
+			first = err
+		}
+		c.release(s)
+	}
+	for i := range c.slots {
+		c.slots[i] = nil
+	}
+	c.slots = c.slots[:0]
+	for k := range c.openSlots {
+		delete(c.openSlots, k)
+	}
+	c.backing = c.backing[:0]
+	return first
+}
+
+func (c *UDPCoalescer) addPassthrough(pkt []byte) {
+	s := c.take()
+	s.passthrough = true
+	s.rawPkt = pkt
+	c.slots = append(c.slots, s)
+}
+
+func (c *UDPCoalescer) seed(pkt []byte, info parsedUDP) {
+	if info.hdrLen > udpCoalesceHdrCap || info.hdrLen+info.payLen > udpCoalesceBufSize {
+		c.addPassthrough(pkt)
+		return
+	}
+	s := c.take()
+	s.passthrough = false
+	s.rawPkt = nil
+	copy(s.hdrBuf[:], pkt[:info.hdrLen])
+	s.hdrLen = info.hdrLen
+	s.ipHdrLen = info.ipHdrLen
+	s.isV6 = info.fk.isV6
+	s.fk = info.fk
+	s.gsoSize = info.payLen
+	s.numSeg = 1
+	s.totalPay = info.payLen
+	s.sealed = false
+	s.payIovs = append(s.payIovs[:0], pkt[info.hdrLen:info.hdrLen+info.payLen])
+	c.slots = append(c.slots, s)
+	c.openSlots[info.fk] = s
+}
+
+// canAppend reports whether info's packet extends the slot's seed.
+// Kernel UDP-GSO requires every segment except possibly the last to be
+// exactly gsoSize, and the last may be shorter (≤ gsoSize).
+func (c *UDPCoalescer) canAppend(s *udpSlot, pkt []byte, info parsedUDP) bool {
+	if s.sealed {
+		return false
+	}
+	if info.hdrLen != s.hdrLen {
+		return false
+	}
+	if s.numSeg >= udpCoalesceMaxSegs {
+		return false
+	}
+	if info.payLen > s.gsoSize {
+		return false
+	}
+	if s.hdrLen+s.totalPay+info.payLen > udpCoalesceBufSize {
+		return false
+	}
+	if !udpHeadersMatch(s.hdrBuf[:s.hdrLen], pkt[:info.hdrLen], s.isV6, s.ipHdrLen) {
+		return false
+	}
+	return true
+}
+
+func (c *UDPCoalescer) appendPayload(s *udpSlot, pkt []byte, info parsedUDP) {
+	s.payIovs = append(s.payIovs, pkt[info.hdrLen:info.hdrLen+info.payLen])
+	s.numSeg++
+	s.totalPay += info.payLen
+	// Merge IP-level CE marks into the seed (same trick TCP coalescer uses).
+	mergeECNIntoSeed(s.hdrBuf[:s.ipHdrLen], pkt[:s.ipHdrLen], s.isV6)
+	if info.payLen < s.gsoSize {
+		// Last-segment-can-be-shorter: this seals the chain.
+		s.sealed = true
+	}
+}
+
+func (c *UDPCoalescer) take() *udpSlot {
+	if n := len(c.pool); n > 0 {
+		s := c.pool[n-1]
+		c.pool[n-1] = nil
+		c.pool = c.pool[:n-1]
+		return s
+	}
+	return &udpSlot{}
+}
+
+func (c *UDPCoalescer) release(s *udpSlot) {
+	s.passthrough = false
+	s.rawPkt = nil
+	for i := range s.payIovs {
+		s.payIovs[i] = nil
+	}
+	s.payIovs = s.payIovs[:0]
+	s.numSeg = 0
+	s.totalPay = 0
+	s.sealed = false
+	c.pool = append(c.pool, s)
+}
+
+// flushSlot patches the IP header total length / IPv6 payload length and
+// the UDP length to the *total* across all coalesced segments, then seeds
+// the UDP checksum field with the pseudo-header partial (single-fold, not
+// inverted) per virtio NEEDS_CSUM. The kernel's ip_rcv_core (v4) and
+// ip6_rcv_core (v6) trim the skb to those length fields, so per-segment
+// values would silently drop everything but the first segment. The kernel
+// then walks each segment in __udp_gso_segment, recomputing per-segment
+// uh->len / iph->tot_len / IPv6 plen and adjusting the checksum via
+// `check = csum16_add(csum16_sub(uh->check, uh->len), newlen)` — meaning
+// our seed's uh->check must be consistent with the seed's uh->len, which
+// is what passing the total to both pseudoSum and the UDP length field
+// guarantees.
+func (c *UDPCoalescer) flushSlot(s *udpSlot) error {
+	hdr := s.hdrBuf[:s.hdrLen]
+	total := s.hdrLen + s.totalPay // full IP+UDP+all_payloads bytes
+	l4Len := total - s.ipHdrLen    // total UDP (8 + sum of payloads)
+
+	if s.isV6 {
+		binary.BigEndian.PutUint16(hdr[4:6], uint16(l4Len))
+	} else {
+		binary.BigEndian.PutUint16(hdr[2:4], uint16(total))
+		hdr[10] = 0
+		hdr[11] = 0
+		binary.BigEndian.PutUint16(hdr[10:12], ipv4HdrChecksum(hdr[:s.ipHdrLen]))
+	}
+
+	// UDP length field (offset 4 inside the UDP header) = total UDP size.
+	binary.BigEndian.PutUint16(hdr[s.ipHdrLen+4:s.ipHdrLen+6], uint16(l4Len))
+
+	var psum uint32
+	if s.isV6 {
+		psum = pseudoSumIPv6(hdr[8:24], hdr[24:40], ipProtoUDP, l4Len)
+	} else {
+		psum = pseudoSumIPv4(hdr[12:16], hdr[16:20], ipProtoUDP, l4Len)
+	}
+	udpCsumOff := s.ipHdrLen + 6
+	binary.BigEndian.PutUint16(hdr[udpCsumOff:udpCsumOff+2], foldOnceNoInvert(psum))
+
+	return c.gsoW.WriteGSO(hdr[:s.ipHdrLen], hdr[s.ipHdrLen:], s.payIovs, tio.GSOProtoUDP)
+}
+
+// udpHeadersMatch compares two IP+UDP header prefixes for byte-equality on
+// every field that must be identical across coalesced segments. Length
+// fields and the ECN bits in IP TOS/TC are masked out — appendPayload
+// merges CE into the seed; flushSlot rewrites lengths.
+func udpHeadersMatch(a, b []byte, isV6 bool, ipHdrLen int) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	if !ipHeadersMatch(a, b, isV6) {
+		return false
+	}
+	// UDP: compare sport+dport ([0:4]). Skip length [4:6] and checksum [6:8] —
+	// length varies (we rewrite at flush) and the checksum will be redone.
+	udp := ipHdrLen
+	if a[udp] != b[udp] || a[udp+1] != b[udp+1] || a[udp+2] != b[udp+2] || a[udp+3] != b[udp+3] {
+		return false
+	}
+	return true
+}

overlay/batch/udp_coalesce_test.go

@@ -0,0 +1,383 @@
+package batch
+
+import (
+	"encoding/binary"
+	"testing"
+)
+
+// buildUDPv4 builds a minimal IPv4+UDP packet with the given payload and ports.
+func buildUDPv4(sport, dport uint16, payload []byte) []byte {
+	const ipHdrLen = 20
+	const udpHdrLen = 8
+	total := ipHdrLen + udpHdrLen + len(payload)
+	pkt := make([]byte, total)
+
+	pkt[0] = 0x45
+	pkt[1] = 0x00
+	binary.BigEndian.PutUint16(pkt[2:4], uint16(total))
+	binary.BigEndian.PutUint16(pkt[4:6], 0)
+	binary.BigEndian.PutUint16(pkt[6:8], 0x4000)
+	pkt[8] = 64
+	pkt[9] = ipProtoUDP
+	copy(pkt[12:16], []byte{10, 0, 0, 1})
+	copy(pkt[16:20], []byte{10, 0, 0, 2})
+
+	binary.BigEndian.PutUint16(pkt[20:22], sport)
+	binary.BigEndian.PutUint16(pkt[22:24], dport)
+	binary.BigEndian.PutUint16(pkt[24:26], uint16(udpHdrLen+len(payload)))
+	binary.BigEndian.PutUint16(pkt[26:28], 0)
+
+	copy(pkt[28:], payload)
+	return pkt
+}
+
+// buildUDPv6 builds a minimal IPv6+UDP packet.
+func buildUDPv6(sport, dport uint16, payload []byte) []byte {
+	const ipHdrLen = 40
+	const udpHdrLen = 8
+	total := ipHdrLen + udpHdrLen + len(payload)
+	pkt := make([]byte, total)
+
+	pkt[0] = 0x60
+	binary.BigEndian.PutUint16(pkt[4:6], uint16(udpHdrLen+len(payload)))
+	pkt[6] = ipProtoUDP
+	pkt[7] = 64
+	pkt[8] = 0xfe
+	pkt[9] = 0x80
+	pkt[23] = 1
+	pkt[24] = 0xfe
+	pkt[25] = 0x80
+	pkt[39] = 2
+
+	binary.BigEndian.PutUint16(pkt[40:42], sport)
+	binary.BigEndian.PutUint16(pkt[42:44], dport)
+	binary.BigEndian.PutUint16(pkt[44:46], uint16(udpHdrLen+len(payload)))
+	binary.BigEndian.PutUint16(pkt[46:48], 0)
+
+	copy(pkt[48:], payload)
+	return pkt
+}
+
+func TestUDPCoalescerPassthroughWhenGSOUnavailable(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: false}
+	c := NewUDPCoalescer(w)
+	pkt := buildUDPv4(1000, 53, make([]byte, 100))
+	if err := c.Commit(pkt); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.writes) != 0 || len(w.gsoWrites) != 0 {
+		t.Fatalf("no Add-time writes: writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.writes) != 1 || len(w.gsoWrites) != 0 {
+		t.Fatalf("want single plain write, got writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}
+
+func TestUDPCoalescerNonUDPPassthrough(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	// ICMP packet
+	pkt := make([]byte, 28)
+	pkt[0] = 0x45
+	binary.BigEndian.PutUint16(pkt[2:4], 28)
+	pkt[9] = 1
+	copy(pkt[12:16], []byte{10, 0, 0, 1})
+	copy(pkt[16:20], []byte{10, 0, 0, 2})
+	if err := c.Commit(pkt); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.writes) != 1 || len(w.gsoWrites) != 0 {
+		t.Fatalf("ICMP must pass through unchanged: writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}
+
+func TestUDPCoalescerSeedThenFlushAlone(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	pkt := buildUDPv4(1000, 53, make([]byte, 800))
+	if err := c.Commit(pkt); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// Single-segment flush goes through WriteGSO; the writer infers GSO_NONE
+	// from len(pays)==1 and the kernel fills in the UDP csum (NEEDS_CSUM).
+	if len(w.gsoWrites) != 1 || len(w.writes) != 0 {
+		t.Fatalf("single-seg flush: writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}
+
+func TestUDPCoalescerCoalescesEqualSized(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	pay := make([]byte, 1200)
+	for i := 0; i < 3; i++ {
+		if err := c.Commit(buildUDPv4(1000, 53, pay)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 1 {
+		t.Fatalf("want 1 gso write, got %d (plain=%d)", len(w.gsoWrites), len(w.writes))
+	}
+	g := w.gsoWrites[0]
+	if g.gsoSize != 1200 {
+		t.Errorf("gsoSize=%d want 1200", g.gsoSize)
+	}
+	if len(g.pays) != 3 {
+		t.Errorf("pay count=%d want 3", len(g.pays))
+	}
+	if g.csumStart != 20 {
+		t.Errorf("csumStart=%d want 20", g.csumStart)
+	}
+	// IP totalLen and UDP length must be the TOTAL across all segments —
+	// the kernel's ip_rcv_core trims skbs to iph->tot_len, so a per-segment
+	// value would silently drop everything but the first segment. Total =
+	// IP(20) + UDP(8) + 3*1200 = 3628.
+	gotTotalLen := binary.BigEndian.Uint16(g.hdr[2:4])
+	if gotTotalLen != 3628 {
+		t.Errorf("ipv4 total_len=%d want 3628 (must be total across segments)", gotTotalLen)
+	}
+	gotUDPLen := binary.BigEndian.Uint16(g.hdr[20+4 : 20+6])
+	if gotUDPLen != 8+3*1200 {
+		t.Errorf("udp len=%d want %d", gotUDPLen, 8+3*1200)
+	}
+}
+
+// Last segment may be shorter, sealing the chain.
+func TestUDPCoalescerShortLastSegmentSeals(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	full := make([]byte, 1200)
+	tail := make([]byte, 600)
+	if err := c.Commit(buildUDPv4(1000, 53, full)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildUDPv4(1000, 53, full)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildUDPv4(1000, 53, tail)); err != nil {
+		t.Fatal(err)
+	}
+	// A 4th packet, even same-sized, must NOT join — chain is sealed.
+	if err := c.Commit(buildUDPv4(1000, 53, full)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes (sealed + new seed), got %d", len(w.gsoWrites))
+	}
+	if len(w.gsoWrites[0].pays) != 3 {
+		t.Errorf("first super: want 3 pays, got %d", len(w.gsoWrites[0].pays))
+	}
+	if len(w.gsoWrites[1].pays) != 1 {
+		t.Errorf("second super: want 1 pay (re-seed), got %d", len(w.gsoWrites[1].pays))
+	}
+}
+
+// A larger-than-gsoSize packet cannot extend the slot — it reseeds.
+func TestUDPCoalescerLargerThanSeedReseeds(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	if err := c.Commit(buildUDPv4(1000, 53, make([]byte, 800))); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildUDPv4(1000, 53, make([]byte, 1200))); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 separate seeds, got %d", len(w.gsoWrites))
+	}
+}
+
+// Different 5-tuples must not coalesce.
+func TestUDPCoalescerDifferentFlowsKeepSeparate(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	pay := make([]byte, 800)
+	if err := c.Commit(buildUDPv4(1000, 53, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildUDPv4(2000, 53, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildUDPv4(1000, 53, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(buildUDPv4(2000, 53, pay)); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// Two flows × 2 datagrams each = 2 superpackets of 2 segments.
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes (one per flow), got %d", len(w.gsoWrites))
+	}
+	for i, g := range w.gsoWrites {
+		if len(g.pays) != 2 {
+			t.Errorf("super %d: want 2 pays, got %d", i, len(g.pays))
+		}
+	}
+}
+
+// Caps at udpCoalesceMaxSegs.
+func TestUDPCoalescerCapsAtMaxSegs(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	pay := make([]byte, 100)
+	for i := 0; i < udpCoalesceMaxSegs+5; i++ {
+		if err := c.Commit(buildUDPv4(1000, 53, pay)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	// First superpacket holds udpCoalesceMaxSegs segments; the spillover
+	// reseeds a new one.
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 gso writes (cap then reseed), got %d", len(w.gsoWrites))
+	}
+	if len(w.gsoWrites[0].pays) != udpCoalesceMaxSegs {
+		t.Errorf("first super: pays=%d want %d", len(w.gsoWrites[0].pays), udpCoalesceMaxSegs)
+	}
+	if len(w.gsoWrites[1].pays) != 5 {
+		t.Errorf("second super: pays=%d want 5", len(w.gsoWrites[1].pays))
+	}
+}
+
+// CE marks on appended segments must be merged into the seed's IP TOS.
+func TestUDPCoalescerMergesCEMark(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	pay := make([]byte, 800)
+	pkt0 := buildUDPv4(1000, 53, pay) // ECN=00
+	pkt1 := buildUDPv4(1000, 53, pay)
+	pkt1[1] = 0x03 // CE
+	pkt2 := buildUDPv4(1000, 53, pay)
+	if err := c.Commit(pkt0); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(pkt1); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(pkt2); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 1 {
+		t.Fatalf("want 1 merged gso write, got %d (plain=%d)", len(w.gsoWrites), len(w.writes))
+	}
+	if w.gsoWrites[0].hdr[1]&0x03 != 0x03 {
+		t.Errorf("CE not merged into seed (tos=%#x)", w.gsoWrites[0].hdr[1])
+	}
+}
+
+// IPv6 path: same flow, equal-sized → coalesced.
+func TestUDPCoalescerIPv6Coalesces(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	pay := make([]byte, 1200)
+	for i := 0; i < 3; i++ {
+		if err := c.Commit(buildUDPv6(1000, 53, pay)); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 1 {
+		t.Fatalf("want 1 gso write, got %d", len(w.gsoWrites))
+	}
+	g := w.gsoWrites[0]
+	if !g.isV6 {
+		t.Errorf("expected v6 write")
+	}
+	if g.csumStart != 40 {
+		t.Errorf("csumStart=%d want 40", g.csumStart)
+	}
+	// IPv6 payload_len and UDP length must be TOTAL — kernel's
+	// ip6_rcv_core trims to payload_len + ipv6 hdr size. Total UDP = 8 +
+	// 3*1200 = 3608.
+	gotPlen := binary.BigEndian.Uint16(g.hdr[4:6])
+	if gotPlen != 8+3*1200 {
+		t.Errorf("ipv6 payload_len=%d want %d (must be total)", gotPlen, 8+3*1200)
+	}
+	gotUDPLen := binary.BigEndian.Uint16(g.hdr[40+4 : 40+6])
+	if gotUDPLen != 8+3*1200 {
+		t.Errorf("udp len=%d want %d", gotUDPLen, 8+3*1200)
+	}
+}
+
+// DSCP differences must reseed (headers don't match outside ECN).
+func TestUDPCoalescerDSCPMismatchReseeds(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	pay := make([]byte, 800)
+	pkt0 := buildUDPv4(1000, 53, pay)
+	pkt1 := buildUDPv4(1000, 53, pay)
+	pkt1[1] = 0xb8 // EF DSCP, ECN=0
+	if err := c.Commit(pkt0); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Commit(pkt1); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.gsoWrites) != 2 {
+		t.Fatalf("want 2 separate seeds (different DSCP), got %d", len(w.gsoWrites))
+	}
+}
+
+// Fragmented IPv4 must not be coalesced.
+func TestUDPCoalescerFragmentedIPv4PassesThrough(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	pkt := buildUDPv4(1000, 53, make([]byte, 200))
+	binary.BigEndian.PutUint16(pkt[6:8], 0x2000) // MF=1
+	if err := c.Commit(pkt); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.writes) != 1 || len(w.gsoWrites) != 0 {
+		t.Fatalf("frag must pass through plain, got writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}
+
+// IPv4 with options is not admissible (we require IHL=5).
+func TestUDPCoalescerIPv4WithOptionsPassesThrough(t *testing.T) {
+	w := &fakeTunWriter{gsoEnabled: true}
+	c := NewUDPCoalescer(w)
+	pkt := buildUDPv4(1000, 53, make([]byte, 200))
+	pkt[0] = 0x46 // IHL = 6 (24-byte IPv4 header — has options)
+	if err := c.Commit(pkt); err != nil {
+		t.Fatal(err)
+	}
+	if err := c.Flush(); err != nil {
+		t.Fatal(err)
+	}
+	if len(w.writes) != 1 || len(w.gsoWrites) != 0 {
+		t.Fatalf("ipv4-with-options must pass through plain, got writes=%d gso=%d", len(w.writes), len(w.gsoWrites))
+	}
+}

overlay/device.go

@@ -18,7 +18,7 @@ type Device interface {
 	Networks() []netip.Prefix
 	Name() string
 	RoutesFor(netip.Addr) routing.Gateways
-	SupportsMultiqueue() bool //todo remove?
+	SupportsMultiqueue() bool
 	NewMultiQueueReader() error
 	Readers() []tio.Queue
 }

overlay/overlaytest/noop.go

@@ -31,7 +31,7 @@ func (NoopTun) Name() string {
 	return "noop"
 }
 
-func (NoopTun) Read() ([][]byte, error) {
+func (NoopTun) Read() ([]tio.Packet, error) {
 	return nil, nil
 }
 

overlay/tio/queueset_gso_linux.go

@@ -0,0 +1,79 @@
+package tio
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+
+	"golang.org/x/sys/unix"
+)
+
+type offloadQueueSet struct {
+	pq []*Offload
+	// pqi is exactly the same as pq, but stored as the interface type
+	pqi        []Queue
+	shutdownFd int
+	// usoEnabled is true when newTun successfully negotiated TUN_F_USO4|6
+	// with the kernel. Queues created by Add inherit this and surface it
+	// via Offload.USOSupported so coalescers can gate USO emission.
+	usoEnabled bool
+}
+
+// NewOffloadQueueSet creates a QueueSet that uses virtio_net_hdr to do
+// TSO segmentation in userspace. usoEnabled tells downstream queues whether
+// the kernel agreed to deliver/accept GSO_UDP_L4 superpackets — coalescers
+// should fall back to per-packet writes when this is false.
+func NewOffloadQueueSet(usoEnabled bool) (QueueSet, error) {
+	shutdownFd, err := unix.Eventfd(0, unix.EFD_NONBLOCK|unix.EFD_CLOEXEC)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create eventfd: %w", err)
+	}
+
+	out := &offloadQueueSet{
+		pq:         []*Offload{},
+		pqi:        []Queue{},
+		shutdownFd: shutdownFd,
+		usoEnabled: usoEnabled,
+	}
+
+	return out, nil
+}
+
+func (c *offloadQueueSet) Queues() []Queue {
+	return c.pqi
+}
+
+func (c *offloadQueueSet) Add(fd int) error {
+	x, err := newOffload(fd, c.shutdownFd, c.usoEnabled)
+	if err != nil {
+		return err
+	}
+	c.pq = append(c.pq, x)
+	c.pqi = append(c.pqi, x)
+
+	return nil
+}
+
+func (c *offloadQueueSet) wakeForShutdown() error {
+	var buf [8]byte
+	binary.NativeEndian.PutUint64(buf[:], 1)
+	_, err := unix.Write(c.shutdownFd, buf[:])
+	return err
+}
+
+func (c *offloadQueueSet) Close() error {
+	errs := []error{}
+
+	// Signal all readers blocked in poll to wake up and exit
+	if err := c.wakeForShutdown(); err != nil {
+		errs = append(errs, err)
+	}
+
+	for _, x := range c.pq {
+		if err := x.Close(); err != nil {
+			errs = append(errs, err)
+		}
+	}
+
+	return errors.Join(errs...)
+}

overlay/tio/container_poll_linux.go → overlay/tio/queueset_poll_linux.go

@@ -8,20 +8,20 @@ import (
 	"golang.org/x/sys/unix"
 )
 
-type pollContainer struct {
+type pollQueueSet struct {
 	pq []*Poll
 	// pqi is exactly the same as pq, but stored as the interface type
 	pqi        []Queue
 	shutdownFd int
 }
 
-func NewPollContainer() (Container, error) {
+func NewPollQueueSet() (QueueSet, error) {
 	shutdownFd, err := unix.Eventfd(0, unix.EFD_NONBLOCK|unix.EFD_CLOEXEC)
 	if err != nil {
 		return nil, fmt.Errorf("failed to create eventfd: %w", err)
 	}
 
-	out := &pollContainer{
+	out := &pollQueueSet{
 		pq:         []*Poll{},
 		pqi:        []Queue{},
 		shutdownFd: shutdownFd,
@@ -30,11 +30,11 @@ func NewPollContainer() (Container, error) {
 	return out, nil
 }
 
-func (c *pollContainer) Queues() []Queue {
+func (c *pollQueueSet) Queues() []Queue {
 	return c.pqi
 }
 
-func (c *pollContainer) Add(fd int) error {
+func (c *pollQueueSet) Add(fd int) error {
 	x, err := newPoll(fd, c.shutdownFd)
 	if err != nil {
 		return err
@@ -45,14 +45,14 @@ func (c *pollContainer) Add(fd int) error {
 	return nil
 }
 
-func (c *pollContainer) wakeForShutdown() error {
+func (c *pollQueueSet) wakeForShutdown() error {
 	var buf [8]byte
 	binary.NativeEndian.PutUint64(buf[:], 1)
 	_, err := unix.Write(int(c.shutdownFd), buf[:])
 	return err
 }
 
-func (c *pollContainer) Close() error {
+func (c *pollQueueSet) Close() error {
 	errs := []error{}
 
 	if err := c.wakeForShutdown(); err != nil {

overlay/tio/segment_bench_test.go

@@ -0,0 +1,65 @@
+//go:build linux && !android && !e2e_testing
+
+package tio
+
+import "testing"
+
+// fakeBatch stands in for batch.TxBatcher inside the bench — same shape
+// of pointer-capturing closure that sendInsideMessage builds.
+type fakeBatch struct{ buf [65536]byte }
+
+func (b *fakeBatch) Reserve(sz int) []byte { return b.buf[:sz] }
+func (b *fakeBatch) Commit([]byte)         {}
+
+type fakeHostInfo struct {
+	remoteIndexId uint32
+	counter       uint64
+}
+type fakeIface struct {
+	rebindCount uint8
+	hi          *fakeHostInfo
+}
+
+// BenchmarkSegmentSuperpacketAllocsTSO measures allocation per
+// SegmentSuperpacket call when a closure captures pointer-bearing
+// receivers — the realistic shape of sendInsideMessage's closure.
+func BenchmarkSegmentSuperpacketAllocsTSO(b *testing.B) {
+	const mss = 1400
+	const numSeg = 32
+	pkt := buildTSOv6(mss*numSeg, mss)
+	gso := GSOInfo{
+		Size:      mss,
+		HdrLen:    60, // 40 (IPv6) + 20 (TCP)
+		CsumStart: 40,
+		Proto:     GSOProtoTCP,
+	}
+	p := Packet{Bytes: pkt, GSO: gso}
+
+	hi := &fakeHostInfo{remoteIndexId: 0xdeadbeef}
+	f := &fakeIface{rebindCount: 7, hi: hi}
+	fb := &fakeBatch{}
+
+	// SegmentSuperpacket consumes pkt destructively; refresh from a master
+	// copy each iter (matches the production pattern where every TUN read
+	// hands the segmenter a fresh kernel-supplied buffer).
+	master := append([]byte(nil), pkt...)
+	work := make([]byte, len(pkt))
+	p.Bytes = work
+
+	b.ReportAllocs()
+	b.ResetTimer()
+	for i := 0; i < b.N; i++ {
+		copy(work, master)
+		err := SegmentSuperpacket(p, func(seg []byte) error {
+			out := fb.Reserve(16 + len(seg) + 16)
+			out[0] = byte(f.rebindCount)
+			out[1] = byte(hi.counter)
+			hi.counter++
+			fb.Commit(out)
+			return nil
+		})
+		if err != nil {
+			b.Fatalf("SegmentSuperpacket: %v", err)
+		}
+	}
+}

overlay/tio/segment_other.go

@@ -0,0 +1,18 @@
+//go:build !linux || android || e2e_testing
+
+package tio
+
+import "fmt"
+
+// SegmentSuperpacket invokes fn once per segment of pkt. On non-Linux
+// builds (and Android/e2e_testing) this package does not provide a Queue
+// implementation, so any caller that does construct a Packet here can only
+// be operating on non-superpacket bytes and the stub forwards them
+// directly. A non-zero GSO field is a programming error from the caller
+// and returns an explicit error rather than silently misbehaving.
+func SegmentSuperpacket(pkt Packet, scratch []byte, fn func(seg []byte) error) error {
+	if pkt.GSO.IsSuperpacket() {
+		return fmt.Errorf("tio: GSO superpacket on platform without segmentation support")
+	}
+	return fn(pkt.Bytes)
+}

overlay/tio/tio.go

@@ -1,56 +1,170 @@
 package tio
 
-import "io"
+import (
+	"io"
+)
 
-// defaultBatchBufSize is the per-Queue scratch size for Read on backends
-// that don't do TSO segmentation. 65535 covers any single IP packet.
-const defaultBatchBufSize = 65535
-
-// Container holds one or many Queue objects and helps close them in an orderly way
-type Container interface {
+// QueueSet holds one or many Queue objects and helps close them in an orderly way.
+type QueueSet interface {
 	io.Closer
 	Queues() []Queue
 
-	// Add takes a tun fd, adds it to the container, and prepares it for use as a Queue
+	// Add takes a tun fd, adds it to the set, and prepares it for use as a Queue.
 	Add(fd int) error
 }
 
+// Capabilities advertises which kernel offload features a Queue
+// successfully negotiated. Callers consult this to decide which coalescers
+// to wire onto the write path — a Queue without TSO can't usefully accept a
+// TCPCoalescer, and a Queue without USO can't accept a UDPCoalescer.
+type Capabilities struct {
+	// TSO means the FD was opened with IFF_VNET_HDR and the kernel agreed
+	// to TUN_F_TSO4|TSO6 — i.e. WriteGSO with GSOProtoTCP is safe.
+	TSO bool
+	// USO means the kernel additionally agreed to TUN_F_USO4|USO6, so
+	// WriteGSO with GSOProtoUDP is safe. Linux ≥ 6.2.
+	USO bool
+}
+
 // Queue is a readable/writable Poll queue. One Queue is driven by a single
-// read goroutine plus concurrent writers (see Write / WriteReject below).
+// read goroutine plus a single writer (see Write below).
 type Queue interface {
 	io.Closer
 
-	// Read returns one or more packets. The returned slices are borrowed
-	// from the Queue's internal buffer and are only valid until the next
-	// Read or Close on this Queue - callers must encrypt or copy each
-	// slice before the next call. Not safe for concurrent Reads.
-	Read() ([][]byte, error)
+	// Read returns one or more packets. The returned Packet.Bytes slices
+	// are borrowed from the Queue's internal buffer and are only valid
+	// until the next Read or Close on this Queue - callers must encrypt
+	// or copy each slice before the next call. A Packet may carry a
+	// GSO/USO superpacket (see GSOInfo); when GSO.IsSuperpacket() is
+	// true the caller must segment Bytes before treating it as a single
+	// IP datagram. Not safe for concurrent Reads.
+	Read() ([]Packet, error)
 
 	// Write emits a single packet on the plaintext (outside→inside)
 	// delivery path. Not safe for concurrent Writes.
 	Write(p []byte) (int, error)
 }
 
-// GSOWriter is implemented by Queues that can emit a TCP TSO superpacket
+// Packet is the unit Queue.Read returns. Bytes points into the queue's
+// internal buffer and is only valid until the next Read or Close on the
+// queue that produced it. GSO is the zero value for an already-segmented
+// IP datagram; when non-zero it describes a kernel-supplied TSO/USO
+// superpacket the caller must segment before consuming.
+type Packet struct {
+	Bytes []byte
+	GSO   GSOInfo
+}
+
+// GSOInfo describes a kernel-supplied superpacket sitting in Packet.Bytes.
+// The zero value means "not a superpacket" — Bytes is one regular IP
+// datagram and no segmentation is required.
+type GSOInfo struct {
+	// Size is the GSO segment size: max payload bytes per segment
+	// (== TCP MSS for TSO, == UDP payload chunk for USO). Zero means
+	// not a superpacket.
+	Size uint16
+	// HdrLen is the total L3+L4 header length within Bytes (already
+	// corrected via correctHdrLen, so safe to slice on).
+	HdrLen uint16
+	// CsumStart is the L4 header offset inside Bytes (== L3 header
+	// length).
+	CsumStart uint16
+	// Proto picks the L4 protocol (TCP or UDP) so the segmenter knows
+	// which checksum/header layout to apply.
+	Proto GSOProto
+}
+
+// IsSuperpacket reports whether g describes a multi-segment GSO/USO
+// superpacket that needs segmentation before its bytes can be encrypted
+// and sent on the wire.
+func (g GSOInfo) IsSuperpacket() bool { return g.Size > 0 }
+
+// Clone returns a Packet whose Bytes is a freshly allocated copy of p.Bytes,
+// safe to retain past the next Read or Close on the originating Queue.
+// GSO metadata is copied verbatim. Use this only when a caller genuinely
+// needs to outlive the borrowed-slice contract — the hot path reads should
+// continue to consume the borrow synchronously to avoid the allocation.
+func (p Packet) Clone() Packet {
+	if p.Bytes == nil {
+		return p
+	}
+	cp := make([]byte, len(p.Bytes))
+	copy(cp, p.Bytes)
+	return Packet{Bytes: cp, GSO: p.GSO}
+}
+
+// CapsProvider is an optional interface implemented by Queues that
+// successfully negotiated kernel offload features at open time. Callers
+// pick a write-path coalescer based on the result. Queues that don't
+// implement it are treated as having no offload capability — callers must
+// fall back to plain per-packet writes.
+type CapsProvider interface {
+	Capabilities() Capabilities
+}
+
+// QueueCapabilities returns q's negotiated offload capabilities, or the
+// zero value when q does not advertise any.
+func QueueCapabilities(q Queue) Capabilities {
+	if cp, ok := q.(CapsProvider); ok {
+		return cp.Capabilities()
+	}
+	return Capabilities{}
+}
+
+// GSOProto selects the L4 protocol for a GSO superpacket. Determines which
+// VIRTIO_NET_HDR_GSO_* type the writer stamps and which checksum offset
+// inside the transport header virtio NEEDS_CSUM expects.
+type GSOProto uint8
+
+const (
+	GSOProtoTCP GSOProto = iota
+	GSOProtoUDP
+)
+
+// GSOWriter is implemented by Queues that can emit a TCP or UDP superpacket
 // assembled from a header prefix plus one or more borrowed payload
 // fragments, in a single vectored write (writev with a leading
 // virtio_net_hdr). This lets the coalescer avoid copying payload bytes
 // between the caller's decrypt buffer and the TUN. Backends without GSO
-// support return false from GSOSupported and coalescing is skipped.
-//
-// hdr contains the IPv4/IPv6 + TCP header prefix (mutable - callers will
-// have filled in total length and pseudo-header partial). pays are
-// non-overlapping payload fragments whose concatenation is the full
-// superpacket payload; they are read-only from the writer's perspective
-// and must remain valid until the call returns. gsoSize is the MSS:
-// every segment except possibly the last is exactly that many bytes.
-// csumStart is the byte offset where the TCP header begins within hdr.
+// support do not implement this interface and coalescing is skipped.
 //
-// # TODO fold into Queue
+// hdr contains the IPv4/IPv6 header prefix (mutable - callers will have
+// filled in total length and IP csum). transportHdr is the TCP or UDP
+// header (mutable - the L4 checksum field must hold the pseudo-header
+// partial, single-fold not inverted, per virtio NEEDS_CSUM semantics).
+// pays are non-overlapping payload fragments whose concatenation is the
+// full superpacket payload; they are read-only from the writer's
+// perspective and must remain valid until the call returns. Every segment
+// in pays except possibly the last is exactly the same size. proto picks
+// the L4 protocol so the writer knows which GSOType / CsumOffset to set.
 //
-// hdr's TCP checksum field must already hold the pseudo-header partial
-// sum (single-fold, not inverted), per virtio NEEDS_CSUM semantics.
+// Callers should also consult CapsProvider (via SupportsGSO or
+// QueueCapabilities) for the per-protocol negotiated capability; an
+// implementation of GSOWriter is necessary but not sufficient since USO
+// may not have been negotiated even when TSO was.
 type GSOWriter interface {
-	WriteGSO(hdr []byte, pays [][]byte, gsoSize uint16, isV6 bool, csumStart uint16) error
-	GSOSupported() bool
+	WriteGSO(hdr []byte, transportHdr []byte, pays [][]byte, proto GSOProto) error
+}
+
+// SupportsGSO reports whether w implements GSOWriter and the underlying
+// queue advertises the negotiated capability for `want`. A writer that
+// implements GSOWriter but not CapsProvider is treated as permissive
+// (used by tests and fakes that don't negotiate).
+func SupportsGSO(w any, want GSOProto) (GSOWriter, bool) {
+	gw, ok := w.(GSOWriter)
+	if !ok {
+		return nil, false
+	}
+	cp, ok := w.(CapsProvider)
+	if !ok {
+		return gw, true
+	}
+	caps := cp.Capabilities()
+	switch want {
+	case GSOProtoTCP:
+		return gw, caps.TSO
+	case GSOProtoUDP:
+		return gw, caps.USO
+	}
+	return gw, false
 }

overlay/tio/tio_gso_linux.go

@@ -0,0 +1,461 @@
+package tio
+
+import (
+	"fmt"
+	"io"
+	"log/slog"
+	"os"
+	"sync"
+	"sync/atomic"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+
+	"github.com/slackhq/nebula/overlay/tio/virtio"
+)
+
+// tunRxBufSize is the per-Read worst-case footprint inside rxBuf: one
+// kernel-supplied packet body, which is at most ~64 KiB (tunReadBufSize).
+// Segmentation happens at encrypt time on a per-routine MTU-sized scratch
+// (see SegmentSuperpacket), so rxBuf only holds raw kernel-supplied bytes.
+// We round up to give comfortable margin for the drain headroom check
+// below.
+const tunRxBufSize = 64 * 1024
+
+// tunRxBufCap is the total size we allocate for the per-reader rx
+// buffer. With reads landing directly in rxBuf, each drain iteration
+// consumes up to tunRxBufSize of headroom for the kernel-supplied bytes.
+// Sized to two such iterations so the initial blocking read plus one
+// drain read both fit without partial-drop.
+const tunRxBufCap = tunRxBufSize * 2
+
+// tunDrainCap caps how many packets a single Read will accumulate via
+// the post-wake drain loop. Sized to soak up a burst of small ACKs while
+// bounding how much work a single caller holds before handing off.
+const tunDrainCap = 64
+
+// gsoMaxIovs caps the iovec budget WriteGSO assembles per call: 3 fixed
+// entries (virtio_net_hdr, IP hdr, transport hdr) plus up to gsoMaxIovs-3
+// payload fragments. Sized comfortably above the typical kernel GSO
+// segment cap (Linux UDP_GRO is 64) so realistic coalesced bursts never
+// touch the limit. iovecs are tiny (16 bytes), so the entire scratch is
+// 4 KiB — fine to keep resident on every queue. WriteGSO returns an error
+// rather than reallocating when a caller exceeds this budget.
+const gsoMaxIovs = 256
+
+// validVnetHdr is the 10-byte virtio_net_hdr we prepend to every non-GSO TUN
+// write. Only flag set is VIRTIO_NET_HDR_F_DATA_VALID, which marks the skb
+// CHECKSUM_UNNECESSARY so the receiving network stack skips L4 checksum
+// verification. All packets that reach the plain Write paths already carry
+// a valid L4 checksum (either supplied by a remote peer whose ciphertext we
+// AEAD-authenticated, produced by segmentTCPYield/segmentUDPYield during
+// superpacket segmentation, or built locally by CreateRejectPacket), so
+// trusting them is safe.
+var validVnetHdr = [virtio.Size]byte{unix.VIRTIO_NET_HDR_F_DATA_VALID}
+
+// Offload wraps a TUN file descriptor with poll-based reads. The FD provided will be changed to non-blocking.
+// A shared eventfd allows Close to wake all readers blocked in poll.
+type Offload struct {
+	fd         int
+	shutdownFd int
+	readPoll   [2]unix.PollFd
+	writePoll  [2]unix.PollFd
+	// writeLock serializes blockOnWrite's read+clear of writePoll[*].Revents.
+	// Any goroutine that calls Write may end up parked in poll(2); without
+	// the lock concurrent waiters could race the Revents reset and lose
+	// events.
+	writeLock sync.Mutex
+	closed    atomic.Bool
+	rxBuf     []byte   // backing store for kernel-handed packets read this drain
+	rxOff     int      // cursor into rxBuf for the current Read drain
+	pending   []Packet // packets returned from the most recent Read
+
+	// readVnetScratch holds the 10-byte virtio_net_hdr split off the front of
+	// every TUN read via readv(2). Decoupling the header from the packet body
+	// lets us read the body directly into rxBuf at the current rxOff with
+	// no userspace copy on the GSO_NONE fast path.
+	readVnetScratch [virtio.Size]byte
+	// readIovs is the readv(2) iovec scratch wired once at construction —
+	// iovec[0] points at readVnetScratch; iovec[1].Base/Len is updated per
+	// read to address the current rxBuf slot.
+	readIovs [2]unix.Iovec
+
+	// usoEnabled records whether the kernel agreed to TUN_F_USO* on this FD,
+	// so writers can decide whether emitting GSO_UDP_L4 superpackets is safe.
+	usoEnabled bool
+
+	// gsoHdrBuf is a per-queue 10-byte scratch for the virtio_net_hdr emitted
+	// by WriteGSO. Kept separate from the read-only package-level validVnetHdr
+	// so non-GSO Writes can ship that constant directly while WriteGSO
+	// rewrites this scratch on every call.
+	gsoHdrBuf [virtio.Size]byte
+	// gsoIovs is the writev iovec scratch for WriteGSO. Pre-sized to
+	// gsoMaxIovs at construction; never grown. WriteGSO returns an error
+	// (and drops the call) if a caller hands it more fragments than fit.
+	gsoIovs []unix.Iovec
+}
+
+func newOffload(fd int, shutdownFd int, usoEnabled bool) (*Offload, error) {
+	if err := unix.SetNonblock(fd, true); err != nil {
+		return nil, fmt.Errorf("failed to set tun fd non-blocking: %w", err)
+	}
+
+	out := &Offload{
+		fd:         fd,
+		shutdownFd: shutdownFd,
+		usoEnabled: usoEnabled,
+		closed:     atomic.Bool{},
+		readPoll: [2]unix.PollFd{
+			{Fd: int32(fd), Events: unix.POLLIN},
+			{Fd: int32(shutdownFd), Events: unix.POLLIN},
+		},
+		writePoll: [2]unix.PollFd{
+			{Fd: int32(fd), Events: unix.POLLOUT},
+			{Fd: int32(shutdownFd), Events: unix.POLLIN},
+		},
+		writeLock: sync.Mutex{},
+
+		rxBuf:   make([]byte, tunRxBufCap),
+		gsoIovs: make([]unix.Iovec, 2, gsoMaxIovs),
+	}
+
+	out.gsoIovs[0].Base = &out.gsoHdrBuf[0]
+	out.gsoIovs[0].SetLen(virtio.Size)
+
+	// readIovs[0] is wired once to the virtio_net_hdr scratch; per-read we
+	// only repoint readIovs[1] at the next rxBuf slot (see readPacket).
+	out.readIovs[0].Base = &out.readVnetScratch[0]
+	out.readIovs[0].SetLen(virtio.Size)
+
+	return out, nil
+}
+
+func (r *Offload) blockOnRead() error {
+	const problemFlags = unix.POLLHUP | unix.POLLNVAL | unix.POLLERR
+	var err error
+	for {
+		_, err = unix.Poll(r.readPoll[:], -1)
+		if err != unix.EINTR {
+			break
+		}
+	}
+	//always reset these!
+	tunEvents := r.readPoll[0].Revents
+	shutdownEvents := r.readPoll[1].Revents
+	r.readPoll[0].Revents = 0
+	r.readPoll[1].Revents = 0
+	//do the err check before trusting the potentially bogus bits we just got
+	if err != nil {
+		return err
+	}
+	if shutdownEvents&(unix.POLLIN|problemFlags) != 0 {
+		return os.ErrClosed
+	} else if tunEvents&problemFlags != 0 {
+		return os.ErrClosed
+	}
+	return nil
+}
+
+func (r *Offload) blockOnWrite() error {
+	const problemFlags = unix.POLLHUP | unix.POLLNVAL | unix.POLLERR
+	var err error
+	for {
+		_, err = unix.Poll(r.writePoll[:], -1)
+		if err != unix.EINTR {
+			break
+		}
+	}
+	//always reset these!
+	r.writeLock.Lock()
+	tunEvents := r.writePoll[0].Revents
+	shutdownEvents := r.writePoll[1].Revents
+	r.writePoll[0].Revents = 0
+	r.writePoll[1].Revents = 0
+	r.writeLock.Unlock()
+	//do the err check before trusting the potentially bogus bits we just got
+	if err != nil {
+		return err
+	}
+	if shutdownEvents&(unix.POLLIN|problemFlags) != 0 {
+		return os.ErrClosed
+	} else if tunEvents&problemFlags != 0 {
+		return os.ErrClosed
+	}
+	return nil
+}
+
+// readPacket issues a single readv(2) splitting the virtio_net_hdr off
+// into readVnetScratch and reading the packet body directly into rxBuf at
+// the current rxOff. Returns the body length (zero virtio header bytes,
+// just the IP packet/superpacket). block controls whether EAGAIN is
+// retried via poll: the initial read of a drain blocks; subsequent drain
+// reads do not.
+//
+// The body iovec capacity is always tunReadBufSize; callers (the Read
+// drain loop) gate entry on tunRxBufCap-rxOff >= tunRxBufSize, sized to
+// hold one worst-case kernel-supplied packet body. Without that gate the
+// body iovec could be smaller than the next inbound packet and the
+// kernel would truncate.
+func (r *Offload) readPacket(block bool) (int, error) {
+	for {
+		r.readIovs[1].Base = &r.rxBuf[r.rxOff]
+		r.readIovs[1].SetLen(tunReadBufSize)
+		n, _, errno := syscall.Syscall(unix.SYS_READV, uintptr(r.fd), uintptr(unsafe.Pointer(&r.readIovs[0])), uintptr(len(r.readIovs)))
+		if errno == 0 {
+			if int(n) < virtio.Size {
+				return 0, io.ErrShortWrite
+			}
+			return int(n) - virtio.Size, nil
+		}
+		if errno == unix.EAGAIN {
+			if !block {
+				return 0, errno
+			}
+			if err := r.blockOnRead(); err != nil {
+				return 0, err
+			}
+			continue
+		}
+		if errno == unix.EINTR {
+			continue
+		}
+		if errno == unix.EBADF {
+			return 0, os.ErrClosed
+		}
+		return 0, errno
+	}
+}
+
+// Read returns one or more packets from the tun. Each Packet either
+// carries a single ready-to-use IP datagram (GSO zero) or a TSO/USO
+// superpacket plus the GSOInfo a caller needs to segment it (see
+// SegmentSuperpacket). The first read blocks via poll; once the fd is
+// known readable we drain additional packets non-blocking until the
+// kernel queue is empty (EAGAIN), we've collected tunDrainCap packets,
+// or we're out of rxBuf headroom. This amortizes the poll wake over
+// bursts of small packets (e.g. TCP ACKs). Packet.Bytes slices point
+// into the Offload's internal buffer and are only valid until the next
+// Read or Close on this Queue.
+func (r *Offload) Read() ([]Packet, error) {
+	r.pending = r.pending[:0]
+	r.rxOff = 0
+
+	// Initial (blocking) read. Retry on decode errors so a single bad
+	// packet does not stall the reader.
+	for {
+		n, err := r.readPacket(true)
+		if err != nil {
+			return nil, err
+		}
+		if err := r.decodeRead(n); err != nil {
+			// Drop and read again — a bad packet should not kill the reader.
+			continue
+		}
+		break
+	}
+
+	// Drain: non-blocking reads until the kernel queue is empty, the drain
+	// cap is reached, or rxBuf no longer has room for another worst-case
+	// kernel-supplied packet (tunRxBufSize).
+	for len(r.pending) < tunDrainCap && tunRxBufCap-r.rxOff >= tunRxBufSize {
+		n, err := r.readPacket(false)
+		if err != nil {
+			// EAGAIN / EINTR / anything else: stop draining. We already
+			// have a valid batch from the first read.
+			break
+		}
+		if n <= 0 {
+			break
+		}
+		if err := r.decodeRead(n); err != nil {
+			// Drop this packet and stop the drain; we'd rather hand off
+			// what we have than keep spinning here.
+			break
+		}
+	}
+
+	return r.pending, nil
+}
+
+// decodeRead processes the packet sitting in rxBuf at rxOff (length
+// pktLen). The bytes stay in rxBuf — for GSO_NONE we slice them as a
+// regular IP datagram (running finishChecksum if NEEDS_CSUM is set);
+// for TSO/USO superpackets we attach the corrected GSO metadata so the
+// caller can segment lazily at encrypt time. rxOff advances past the
+// kernel-supplied body and nothing else, since segmentation no longer
+// writes back into rxBuf.
+func (r *Offload) decodeRead(pktLen int) error {
+	if pktLen <= 0 {
+		return fmt.Errorf("short tun read: %d", pktLen)
+	}
+	var hdr virtio.Hdr
+	hdr.Decode(r.readVnetScratch[:])
+
+	body := r.rxBuf[r.rxOff : r.rxOff+pktLen]
+
+	if hdr.GSOType == unix.VIRTIO_NET_HDR_GSO_NONE {
+		if hdr.Flags&unix.VIRTIO_NET_HDR_F_NEEDS_CSUM != 0 {
+			if err := virtio.FinishChecksum(body, hdr); err != nil {
+				return err
+			}
+		}
+		r.pending = append(r.pending, Packet{Bytes: body})
+		r.rxOff += pktLen
+		return nil
+	}
+
+	// GSO superpacket: validate, fix the kernel-supplied HdrLen on the
+	// FORWARD path (CorrectHdrLen), pick the L4 protocol, and attach
+	// the metadata. The bytes stay in rxBuf untouched, segmentation
+	// happens in SegmentSuperpacket at encrypt time.
+	if err := virtio.CheckValid(body, hdr); err != nil {
+		return err
+	}
+	if err := virtio.CorrectHdrLen(body, &hdr); err != nil {
+		return err
+	}
+	proto, err := protoFromGSOType(hdr.GSOType)
+	if err != nil {
+		return err
+	}
+	r.pending = append(r.pending, Packet{
+		Bytes: body,
+		GSO: GSOInfo{
+			Size:      hdr.GSOSize,
+			HdrLen:    hdr.HdrLen,
+			CsumStart: hdr.CsumStart,
+			Proto:     proto,
+		},
+	})
+	r.rxOff += pktLen
+	return nil
+}
+
+func (r *Offload) Write(buf []byte) (int, error) {
+	iovs := [2]unix.Iovec{
+		{Base: &validVnetHdr[0]},
+		{Base: &buf[0]},
+	}
+	iovs[0].SetLen(virtio.Size)
+	iovs[1].SetLen(len(buf))
+	return r.writeWithScratch(buf, &iovs)
+}
+
+func (r *Offload) writeWithScratch(buf []byte, iovs *[2]unix.Iovec) (int, error) {
+	if len(buf) == 0 {
+		return 0, nil
+	}
+	iovs[1].Base = &buf[0]
+	iovs[1].SetLen(len(buf))
+	return r.rawWrite(unsafe.Slice(&iovs[0], len(iovs)))
+}
+
+func (r *Offload) rawWrite(iovs []unix.Iovec) (int, error) {
+	for {
+		n, _, errno := syscall.Syscall(unix.SYS_WRITEV, uintptr(r.fd), uintptr(unsafe.Pointer(&iovs[0])), uintptr(len(iovs)))
+		if errno == 0 {
+			if int(n) < virtio.Size {
+				return 0, io.ErrShortWrite
+			}
+			return int(n) - virtio.Size, nil
+		}
+		if errno == unix.EAGAIN {
+			if err := r.blockOnWrite(); err != nil {
+				return 0, err
+			}
+			continue
+		}
+		if errno == unix.EINTR {
+			continue
+		}
+		if errno == unix.EBADF {
+			return 0, os.ErrClosed
+		}
+		return 0, errno
+	}
+}
+
+// Capabilities reports the offload features negotiated for this Queue. TSO
+// is always true for Offload (we only construct it on IFF_VNET_HDR FDs);
+// USO is true only when the kernel agreed to TUN_F_USO4|6 at open time
+// (Linux ≥ 6.2).
+func (r *Offload) Capabilities() Capabilities {
+	return Capabilities{TSO: true, USO: r.usoEnabled}
+}
+
+func (r *Offload) WriteGSO(hdr []byte, transportHdr []byte, pays [][]byte, proto GSOProto) error {
+	if len(hdr) == 0 || len(pays) == 0 || len(transportHdr) == 0 {
+		return nil
+	}
+	// L4 checksum offset inside transportHdr: TCP=16 (the `check` field after
+	// seq/ack/dataoff/flags/window), UDP=6 (after sport/dport/length).
+	var csumOff uint16
+	switch proto {
+	case GSOProtoUDP:
+		csumOff = 6
+	default:
+		csumOff = 16
+	}
+	vhdr := virtio.Hdr{
+		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
+		HdrLen:     uint16(len(hdr) + len(transportHdr)),
+		GSOSize:    uint16(len(pays[0])),
+		CsumStart:  uint16(len(hdr)),
+		CsumOffset: csumOff,
+	}
+	if len(pays) > 1 {
+		ipVer := hdr[0] >> 4
+		switch {
+		case proto == GSOProtoUDP && (ipVer == 4 || ipVer == 6):
+			vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_UDP_L4
+		case ipVer == 6:
+			vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_TCPV6
+		case ipVer == 4:
+			vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_TCPV4
+		default:
+			vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_NONE
+			vhdr.GSOSize = 0
+		}
+	} else {
+		vhdr.GSOType = unix.VIRTIO_NET_HDR_GSO_NONE
+		vhdr.GSOSize = 0
+	}
+	vhdr.Encode(r.gsoHdrBuf[:])
+
+	// Build the iovec array: [virtio_hdr, hdr, transportHdr, pays...]. r.gsoIovs[0] is
+	// wired to gsoHdrBuf at construction and never changes.
+	need := 3 + len(pays)
+	if need > cap(r.gsoIovs) {
+		slog.Default().Warn("tio: WriteGSO iovec budget exceeded; dropping superpacket",
+			"need", need, "cap", cap(r.gsoIovs), "segments", len(pays))
+		return fmt.Errorf("tio: WriteGSO needs %d iovecs but cap is %d", need, cap(r.gsoIovs))
+	}
+	r.gsoIovs = r.gsoIovs[:need]
+	r.gsoIovs[1].Base = &hdr[0]
+	r.gsoIovs[1].SetLen(len(hdr))
+	r.gsoIovs[2].Base = &transportHdr[0]
+	r.gsoIovs[2].SetLen(len(transportHdr))
+	for i, p := range pays {
+		r.gsoIovs[3+i].Base = &p[0]
+		r.gsoIovs[3+i].SetLen(len(p))
+	}
+
+	_, err := r.rawWrite(r.gsoIovs)
+	return err
+}
+
+func (r *Offload) Close() error {
+	if r.closed.Swap(true) {
+		return nil
+	}
+
+	//shutdownFd is owned by the container, so we should not close it
+	var err error
+	if r.fd >= 0 {
+		err = unix.Close(r.fd)
+		r.fd = -1
+	}
+
+	return err
+}

overlay/tio/tio_poll_linux.go

@@ -21,7 +21,7 @@ type Poll struct {
 	closed    atomic.Bool
 
 	readBuf  []byte
-	batchRet [1][]byte
+	batchRet [1]Packet
 }
 
 func newPoll(fd int, shutdownFd int) (*Poll, error) {
@@ -97,12 +97,12 @@ func (t *Poll) blockOnWrite() error {
 	return nil
 }
 
-func (t *Poll) Read() ([][]byte, error) {
+func (t *Poll) Read() ([]Packet, error) {
 	n, err := t.readOne(t.readBuf)
 	if err != nil {
 		return nil, err
 	}
-	t.batchRet[0] = t.readBuf[:n]
+	t.batchRet[0] = Packet{Bytes: t.readBuf[:n]}
 	return t.batchRet[:], nil
 }
 

overlay/tio/tun_file_linux_test.go

@@ -15,7 +15,7 @@ import (
 )
 
 // newReadPipe returns a read fd. The matching write fd is registered for cleanup.
-// The caller takes ownership of the read fd (pass it to newOffload / newFriend).
+// The caller takes ownership of the read fd (pass it into a QueueSet).
 func newReadPipe(t *testing.T) int {
 	t.Helper()
 	var fds [2]int
@@ -29,7 +29,7 @@ func newReadPipe(t *testing.T) int {
 func TestPoll_WakeForShutdown_WakesFriends(t *testing.T) {
 	pipe1 := newReadPipe(t)
 	pipe2 := newReadPipe(t)
-	parent, err := NewPollContainer()
+	parent, err := NewPollQueueSet()
 	require.NoError(t, err)
 	require.NoError(t, parent.Add(pipe1))
 	require.NoError(t, parent.Add(pipe2))

overlay/tio/tun_linux_offload.go

@@ -0,0 +1,51 @@
+//go:build linux && !android && !e2e_testing
+// +build linux,!android,!e2e_testing
+
+package tio
+
+import (
+	"fmt"
+
+	"golang.org/x/sys/unix"
+
+	"github.com/slackhq/nebula/overlay/tio/virtio"
+)
+
+// protoFromGSOType maps a virtio_net_hdr GSOType to the GSOProto value the
+// segment-time helpers use. Returns an error for GSO_NONE or any unknown
+// value — the caller should only invoke this on a confirmed superpacket.
+func protoFromGSOType(t uint8) (GSOProto, error) {
+	switch t {
+	case unix.VIRTIO_NET_HDR_GSO_TCPV4, unix.VIRTIO_NET_HDR_GSO_TCPV6:
+		return GSOProtoTCP, nil
+	case unix.VIRTIO_NET_HDR_GSO_UDP_L4:
+		return GSOProtoUDP, nil
+	default:
+		return 0, fmt.Errorf("unsupported virtio gso type: %d", t)
+	}
+}
+
+// SegmentSuperpacket invokes fn once per segment of pkt. For non-GSO pkts
+// fn is called once with pkt.Bytes (no segmentation, no copy). For GSO/USO
+// superpackets fn is called once per segment with a slice of pkt.Bytes
+// holding that segment's plaintext (a freshly-patched L3+L4 header sliced
+// in front of the original payload chunk). The slide is destructive: pkt is
+// consumed by this call and its bytes are in an undefined state when
+// SegmentSuperpacket returns. Callers must not retain pkt or any earlier
+// seg slice past fn's return for that segment. The scratch parameter is
+// unused on the destructive path and kept only for cross-platform
+// signature compatibility. Aborts and returns the first error from fn or
+// from per-segment construction.
+func SegmentSuperpacket(pkt Packet, fn func(seg []byte) error) error {
+	if !pkt.GSO.IsSuperpacket() {
+		return fn(pkt.Bytes)
+	}
+	switch pkt.GSO.Proto {
+	case GSOProtoTCP:
+		return virtio.SegmentTCP(pkt.Bytes, pkt.GSO.HdrLen, pkt.GSO.CsumStart, pkt.GSO.Size, fn)
+	case GSOProtoUDP:
+		return virtio.SegmentUDP(pkt.Bytes, pkt.GSO.HdrLen, pkt.GSO.CsumStart, pkt.GSO.Size, fn)
+	default:
+		return fmt.Errorf("unsupported gso proto: %d", pkt.GSO.Proto)
+	}
+}

overlay/tio/tun_linux_offload_test.go

@@ -0,0 +1,794 @@
+//go:build linux && !android && !e2e_testing
+// +build linux,!android,!e2e_testing
+
+package tio
+
+import (
+	"encoding/binary"
+	"os"
+	"testing"
+
+	"golang.org/x/sys/unix"
+	"gvisor.dev/gvisor/pkg/tcpip/checksum"
+
+	"github.com/slackhq/nebula/overlay/tio/virtio"
+)
+
+// testSegScratchSize is a generous segmentation scratch sized to fit any
+// of the synthetic TSO/USO superpackets these tests generate (one
+// worst-case 64 KiB superpacket plus replicated per-segment headers).
+const testSegScratchSize = 192 * 1024
+
+// verifyChecksum confirms that the one's-complement sum across `b`, seeded
+// with a folded pseudo-header sum, equals all-ones (valid).
+func verifyChecksum(b []byte, pseudo uint16) bool {
+	return checksum.Checksum(b, pseudo) == 0xffff
+}
+
+// segmentForTest is the test-only counterpart to the production
+// SegmentSuperpacket path. It handles GSO_NONE (with optional
+// finishChecksum) inline and dispatches GSO superpackets through
+// SegmentSuperpacket, draining each yielded segment into a
+// freshly-copied [][]byte slot so callers can iterate after the call
+// returns. Tests pre-set hdr.HdrLen correctly, so correctHdrLen is not
+// invoked here.
+func segmentForTest(pkt []byte, hdr virtio.Hdr, out *[][]byte, scratch []byte) error {
+	if hdr.GSOType == unix.VIRTIO_NET_HDR_GSO_NONE {
+		cp := append([]byte(nil), pkt...)
+		if hdr.Flags&unix.VIRTIO_NET_HDR_F_NEEDS_CSUM != 0 {
+			if err := virtio.FinishChecksum(cp, hdr); err != nil {
+				return err
+			}
+		}
+		*out = append(*out, cp)
+		return nil
+	}
+	proto, err := protoFromGSOType(hdr.GSOType)
+	if err != nil {
+		return err
+	}
+	gso := GSOInfo{
+		Size:      hdr.GSOSize,
+		HdrLen:    hdr.HdrLen,
+		CsumStart: hdr.CsumStart,
+		Proto:     proto,
+	}
+	return SegmentSuperpacket(Packet{Bytes: pkt, GSO: gso}, func(seg []byte) error {
+		*out = append(*out, append([]byte(nil), seg...))
+		return nil
+	})
+}
+
+// pseudoHeaderIPv4 returns the folded pseudo-header sum used to verify a
+// TCP/UDP segment's checksum in tests. src/dst are 4 bytes each.
+func pseudoHeaderIPv4(src, dst []byte, proto byte, l4Len int) uint16 {
+	s := uint32(checksum.Checksum(src, 0)) + uint32(checksum.Checksum(dst, 0))
+	s += uint32(proto) + uint32(l4Len)
+	s = (s & 0xffff) + (s >> 16)
+	s = (s & 0xffff) + (s >> 16)
+	return uint16(s)
+}
+
+// pseudoHeaderIPv6 returns the folded pseudo-header sum used to verify a
+// TCP/UDP segment's checksum in tests. src/dst are 16 bytes each.
+func pseudoHeaderIPv6(src, dst []byte, proto byte, l4Len int) uint16 {
+	s := uint32(checksum.Checksum(src, 0)) + uint32(checksum.Checksum(dst, 0))
+	s += uint32(l4Len>>16) + uint32(l4Len&0xffff) + uint32(proto)
+	s = (s & 0xffff) + (s >> 16)
+	s = (s & 0xffff) + (s >> 16)
+	return uint16(s)
+}
+
+// buildTSOv4 builds a synthetic IPv4/TCP TSO superpacket with a payload of
+// `payLen` bytes split at `mss`.
+func buildTSOv4(t *testing.T, payLen, mss int) ([]byte, virtio.Hdr) {
+	t.Helper()
+	const ipLen = 20
+	const tcpLen = 20
+	pkt := make([]byte, ipLen+tcpLen+payLen)
+
+	// IPv4 header
+	pkt[0] = 0x45 // version 4, IHL 5
+	// total length is meaningless for TSO but set it anyway
+	binary.BigEndian.PutUint16(pkt[2:4], uint16(ipLen+tcpLen+payLen))
+	binary.BigEndian.PutUint16(pkt[4:6], 0x4242) // original ID
+	pkt[8] = 64                                  // TTL
+	pkt[9] = unix.IPPROTO_TCP
+	copy(pkt[12:16], []byte{10, 0, 0, 1}) // src
+	copy(pkt[16:20], []byte{10, 0, 0, 2}) // dst
+
+	// TCP header
+	binary.BigEndian.PutUint16(pkt[20:22], 12345) // sport
+	binary.BigEndian.PutUint16(pkt[22:24], 80)    // dport
+	binary.BigEndian.PutUint32(pkt[24:28], 10000) // seq
+	binary.BigEndian.PutUint32(pkt[28:32], 20000) // ack
+	pkt[32] = 0x50                                // data offset 5 words
+	pkt[33] = 0x18                                // ACK | PSH
+	binary.BigEndian.PutUint16(pkt[34:36], 65535) // window
+
+	// payload
+	for i := 0; i < payLen; i++ {
+		pkt[ipLen+tcpLen+i] = byte(i & 0xff)
+	}
+
+	return pkt, virtio.Hdr{
+		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
+		GSOType:    unix.VIRTIO_NET_HDR_GSO_TCPV4,
+		HdrLen:     uint16(ipLen + tcpLen),
+		GSOSize:    uint16(mss),
+		CsumStart:  uint16(ipLen),
+		CsumOffset: 16,
+	}
+}
+
+func TestSegmentTCPv4(t *testing.T) {
+	const mss = 100
+	const numSeg = 3
+	pkt, hdr := buildTSOv4(t, mss*numSeg, mss)
+
+	scratch := make([]byte, testSegScratchSize)
+	var out [][]byte
+	if err := segmentForTest(pkt, hdr, &out, scratch); err != nil {
+		t.Fatalf("segmentForTest: %v", err)
+	}
+	if len(out) != numSeg {
+		t.Fatalf("expected %d segments, got %d", numSeg, len(out))
+	}
+
+	for i, seg := range out {
+		if len(seg) != 40+mss {
+			t.Errorf("seg %d: unexpected len %d", i, len(seg))
+		}
+		totalLen := binary.BigEndian.Uint16(seg[2:4])
+		if totalLen != uint16(40+mss) {
+			t.Errorf("seg %d: total_len=%d want %d", i, totalLen, 40+mss)
+		}
+		id := binary.BigEndian.Uint16(seg[4:6])
+		if id != 0x4242+uint16(i) {
+			t.Errorf("seg %d: ip id=%#x want %#x", i, id, 0x4242+uint16(i))
+		}
+		seq := binary.BigEndian.Uint32(seg[24:28])
+		wantSeq := uint32(10000 + i*mss)
+		if seq != wantSeq {
+			t.Errorf("seg %d: seq=%d want %d", i, seq, wantSeq)
+		}
+		flags := seg[33]
+		wantFlags := byte(0x10) // ACK only, PSH cleared
+		if i == numSeg-1 {
+			wantFlags = 0x18 // ACK | PSH preserved on last
+		}
+		if flags != wantFlags {
+			t.Errorf("seg %d: flags=%#x want %#x", i, flags, wantFlags)
+		}
+		// IPv4 header checksum must verify against itself.
+		if !verifyChecksum(seg[:20], 0) {
+			t.Errorf("seg %d: bad IPv4 header checksum", i)
+		}
+		// TCP checksum must verify against the pseudo-header.
+		psum := pseudoHeaderIPv4(seg[12:16], seg[16:20], unix.IPPROTO_TCP, 20+mss)
+		if !verifyChecksum(seg[20:], psum) {
+			t.Errorf("seg %d: bad TCP checksum", i)
+		}
+	}
+}
+
+func TestSegmentTCPv4OddTail(t *testing.T) {
+	// Payload of 250 bytes with MSS 100 → segments of 100, 100, 50.
+	pkt, hdr := buildTSOv4(t, 250, 100)
+	scratch := make([]byte, testSegScratchSize)
+	var out [][]byte
+	if err := segmentForTest(pkt, hdr, &out, scratch); err != nil {
+		t.Fatalf("segmentForTest: %v", err)
+	}
+	if len(out) != 3 {
+		t.Fatalf("want 3 segments, got %d", len(out))
+	}
+	wantPayLens := []int{100, 100, 50}
+	for i, seg := range out {
+		if len(seg)-40 != wantPayLens[i] {
+			t.Errorf("seg %d: pay len %d want %d", i, len(seg)-40, wantPayLens[i])
+		}
+		if !verifyChecksum(seg[:20], 0) {
+			t.Errorf("seg %d: bad IPv4 header checksum", i)
+		}
+		psum := pseudoHeaderIPv4(seg[12:16], seg[16:20], unix.IPPROTO_TCP, 20+wantPayLens[i])
+		if !verifyChecksum(seg[20:], psum) {
+			t.Errorf("seg %d: bad TCP checksum", i)
+		}
+	}
+}
+
+func TestSegmentTCPv6(t *testing.T) {
+	const ipLen = 40
+	const tcpLen = 20
+	const mss = 120
+	const numSeg = 2
+	payLen := mss * numSeg
+	pkt := make([]byte, ipLen+tcpLen+payLen)
+
+	// IPv6 header
+	pkt[0] = 0x60 // version 6
+	binary.BigEndian.PutUint16(pkt[4:6], uint16(tcpLen+payLen))
+	pkt[6] = unix.IPPROTO_TCP
+	pkt[7] = 64
+	// src/dst fe80::1 / fe80::2
+	pkt[8] = 0xfe
+	pkt[9] = 0x80
+	pkt[23] = 1
+	pkt[24] = 0xfe
+	pkt[25] = 0x80
+	pkt[39] = 2
+
+	// TCP header
+	binary.BigEndian.PutUint16(pkt[40:42], 12345)
+	binary.BigEndian.PutUint16(pkt[42:44], 80)
+	binary.BigEndian.PutUint32(pkt[44:48], 7)
+	binary.BigEndian.PutUint32(pkt[48:52], 99)
+	pkt[52] = 0x50
+	pkt[53] = 0x19 // FIN | ACK | PSH — exercise FIN clearing too
+	binary.BigEndian.PutUint16(pkt[54:56], 65535)
+
+	for i := 0; i < payLen; i++ {
+		pkt[ipLen+tcpLen+i] = byte(i)
+	}
+
+	hdr := virtio.Hdr{
+		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
+		GSOType:    unix.VIRTIO_NET_HDR_GSO_TCPV6,
+		HdrLen:     uint16(ipLen + tcpLen),
+		GSOSize:    uint16(mss),
+		CsumStart:  uint16(ipLen),
+		CsumOffset: 16,
+	}
+
+	scratch := make([]byte, testSegScratchSize)
+	var out [][]byte
+	if err := segmentForTest(pkt, hdr, &out, scratch); err != nil {
+		t.Fatalf("segmentForTest: %v", err)
+	}
+	if len(out) != numSeg {
+		t.Fatalf("want %d segments, got %d", numSeg, len(out))
+	}
+
+	for i, seg := range out {
+		if len(seg) != ipLen+tcpLen+mss {
+			t.Errorf("seg %d: len %d want %d", i, len(seg), ipLen+tcpLen+mss)
+		}
+		pl := binary.BigEndian.Uint16(seg[4:6])
+		if pl != uint16(tcpLen+mss) {
+			t.Errorf("seg %d: payload_length=%d want %d", i, pl, tcpLen+mss)
+		}
+		seq := binary.BigEndian.Uint32(seg[44:48])
+		if seq != uint32(7+i*mss) {
+			t.Errorf("seg %d: seq=%d want %d", i, seq, 7+i*mss)
+		}
+		flags := seg[53]
+		// Original flags = 0x19 (FIN|ACK|PSH). FIN(0x01)+PSH(0x08) should be
+		// cleared on all but the last; ACK(0x10) always preserved.
+		wantFlags := byte(0x10)
+		if i == numSeg-1 {
+			wantFlags = 0x19
+		}
+		if flags != wantFlags {
+			t.Errorf("seg %d: flags=%#x want %#x", i, flags, wantFlags)
+		}
+		psum := pseudoHeaderIPv6(seg[8:24], seg[24:40], unix.IPPROTO_TCP, tcpLen+mss)
+		if !verifyChecksum(seg[ipLen:], psum) {
+			t.Errorf("seg %d: bad TCP checksum", i)
+		}
+	}
+}
+
+func TestSegmentGSONonePassesThrough(t *testing.T) {
+	pkt, hdr := buildTSOv4(t, 100, 100)
+	hdr.GSOType = unix.VIRTIO_NET_HDR_GSO_NONE
+	hdr.Flags = 0 // no NEEDS_CSUM, leave packet untouched
+
+	scratch := make([]byte, testSegScratchSize)
+	var out [][]byte
+	if err := segmentForTest(pkt, hdr, &out, scratch); err != nil {
+		t.Fatalf("segmentForTest: %v", err)
+	}
+	if len(out) != 1 {
+		t.Fatalf("want 1 segment, got %d", len(out))
+	}
+	if len(out[0]) != len(pkt) {
+		t.Fatalf("unexpected length: %d vs %d", len(out[0]), len(pkt))
+	}
+}
+
+// TestSegmentRejectsLegacyUDPGSO ensures the legacy GSO_UDP (UFO) marker is
+// still rejected; only modern GSO_UDP_L4 (USO) is supported.
+func TestSegmentRejectsLegacyUDPGSO(t *testing.T) {
+	hdr := virtio.Hdr{GSOType: unix.VIRTIO_NET_HDR_GSO_UDP}
+	var out [][]byte
+	if err := segmentForTest(nil, hdr, &out, nil); err == nil {
+		t.Fatalf("expected rejection for legacy UDP GSO")
+	}
+}
+
+// buildUSOv4 builds a synthetic IPv4/UDP USO superpacket with payload of
+// payLen bytes, segmented at gsoSize.
+func buildUSOv4(t *testing.T, payLen, gsoSize int) ([]byte, virtio.Hdr) {
+	t.Helper()
+	const ipLen = 20
+	const udpLen = 8
+	pkt := make([]byte, ipLen+udpLen+payLen)
+
+	// IPv4 header
+	pkt[0] = 0x45 // version 4, IHL 5
+	binary.BigEndian.PutUint16(pkt[2:4], uint16(ipLen+udpLen+payLen))
+	binary.BigEndian.PutUint16(pkt[4:6], 0x4242)
+	pkt[8] = 64
+	pkt[9] = unix.IPPROTO_UDP
+	copy(pkt[12:16], []byte{10, 0, 0, 1})
+	copy(pkt[16:20], []byte{10, 0, 0, 2})
+
+	// UDP header (length + checksum filled in per segment by segmentUDPYield)
+	binary.BigEndian.PutUint16(pkt[20:22], 12345) // sport
+	binary.BigEndian.PutUint16(pkt[22:24], 53)    // dport
+
+	for i := 0; i < payLen; i++ {
+		pkt[ipLen+udpLen+i] = byte(i & 0xff)
+	}
+
+	return pkt, virtio.Hdr{
+		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
+		GSOType:    unix.VIRTIO_NET_HDR_GSO_UDP_L4,
+		HdrLen:     uint16(ipLen + udpLen),
+		GSOSize:    uint16(gsoSize),
+		CsumStart:  uint16(ipLen),
+		CsumOffset: 6,
+	}
+}
+
+func TestSegmentUDPv4(t *testing.T) {
+	const gso = 100
+	const numSeg = 3
+	pkt, hdr := buildUSOv4(t, gso*numSeg, gso)
+
+	scratch := make([]byte, testSegScratchSize)
+	var out [][]byte
+	if err := segmentForTest(pkt, hdr, &out, scratch); err != nil {
+		t.Fatalf("segmentForTest: %v", err)
+	}
+	if len(out) != numSeg {
+		t.Fatalf("expected %d segments, got %d", numSeg, len(out))
+	}
+
+	for i, seg := range out {
+		if len(seg) != 28+gso {
+			t.Errorf("seg %d: len %d want %d", i, len(seg), 28+gso)
+		}
+		totalLen := binary.BigEndian.Uint16(seg[2:4])
+		if totalLen != uint16(28+gso) {
+			t.Errorf("seg %d: total_len=%d want %d", i, totalLen, 28+gso)
+		}
+		// kernel UDP-GSO does NOT bump the IPv4 ID across segments; every
+		// segment carries the same ID as the seed.
+		id := binary.BigEndian.Uint16(seg[4:6])
+		if id != 0x4242 {
+			t.Errorf("seg %d: ip id=%#x want %#x", i, id, 0x4242)
+		}
+		udpLen := binary.BigEndian.Uint16(seg[24:26])
+		if udpLen != uint16(8+gso) {
+			t.Errorf("seg %d: udp len=%d want %d", i, udpLen, 8+gso)
+		}
+		if !verifyChecksum(seg[:20], 0) {
+			t.Errorf("seg %d: bad IPv4 header checksum", i)
+		}
+		psum := pseudoHeaderIPv4(seg[12:16], seg[16:20], unix.IPPROTO_UDP, 8+gso)
+		if !verifyChecksum(seg[20:], psum) {
+			t.Errorf("seg %d: bad UDP checksum", i)
+		}
+	}
+}
+
+func TestSegmentUDPv4OddTail(t *testing.T) {
+	// 250 bytes payload, gsoSize=100 → segments of 100, 100, 50.
+	pkt, hdr := buildUSOv4(t, 250, 100)
+	scratch := make([]byte, testSegScratchSize)
+	var out [][]byte
+	if err := segmentForTest(pkt, hdr, &out, scratch); err != nil {
+		t.Fatalf("segmentForTest: %v", err)
+	}
+	if len(out) != 3 {
+		t.Fatalf("want 3 segments, got %d", len(out))
+	}
+	wantPay := []int{100, 100, 50}
+	for i, seg := range out {
+		if len(seg)-28 != wantPay[i] {
+			t.Errorf("seg %d: pay len %d want %d", i, len(seg)-28, wantPay[i])
+		}
+		udpLen := binary.BigEndian.Uint16(seg[24:26])
+		if udpLen != uint16(8+wantPay[i]) {
+			t.Errorf("seg %d: udp len=%d want %d", i, udpLen, 8+wantPay[i])
+		}
+		if !verifyChecksum(seg[:20], 0) {
+			t.Errorf("seg %d: bad IPv4 header checksum", i)
+		}
+		psum := pseudoHeaderIPv4(seg[12:16], seg[16:20], unix.IPPROTO_UDP, 8+wantPay[i])
+		if !verifyChecksum(seg[20:], psum) {
+			t.Errorf("seg %d: bad UDP checksum", i)
+		}
+	}
+}
+
+func TestSegmentUDPv6(t *testing.T) {
+	const ipLen = 40
+	const udpLen = 8
+	const gso = 120
+	const numSeg = 2
+	payLen := gso * numSeg
+	pkt := make([]byte, ipLen+udpLen+payLen)
+
+	// IPv6 header
+	pkt[0] = 0x60
+	binary.BigEndian.PutUint16(pkt[4:6], uint16(udpLen+payLen))
+	pkt[6] = unix.IPPROTO_UDP
+	pkt[7] = 64
+	pkt[8] = 0xfe
+	pkt[9] = 0x80
+	pkt[23] = 1
+	pkt[24] = 0xfe
+	pkt[25] = 0x80
+	pkt[39] = 2
+
+	binary.BigEndian.PutUint16(pkt[40:42], 12345)
+	binary.BigEndian.PutUint16(pkt[42:44], 53)
+
+	for i := 0; i < payLen; i++ {
+		pkt[ipLen+udpLen+i] = byte(i)
+	}
+
+	hdr := virtio.Hdr{
+		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
+		GSOType:    unix.VIRTIO_NET_HDR_GSO_UDP_L4,
+		HdrLen:     uint16(ipLen + udpLen),
+		GSOSize:    uint16(gso),
+		CsumStart:  uint16(ipLen),
+		CsumOffset: 6,
+	}
+
+	scratch := make([]byte, testSegScratchSize)
+	var out [][]byte
+	if err := segmentForTest(pkt, hdr, &out, scratch); err != nil {
+		t.Fatalf("segmentForTest: %v", err)
+	}
+	if len(out) != numSeg {
+		t.Fatalf("want %d segments, got %d", numSeg, len(out))
+	}
+
+	for i, seg := range out {
+		if len(seg) != ipLen+udpLen+gso {
+			t.Errorf("seg %d: len %d want %d", i, len(seg), ipLen+udpLen+gso)
+		}
+		pl := binary.BigEndian.Uint16(seg[4:6])
+		if pl != uint16(udpLen+gso) {
+			t.Errorf("seg %d: payload_length=%d want %d", i, pl, udpLen+gso)
+		}
+		ul := binary.BigEndian.Uint16(seg[ipLen+4 : ipLen+6])
+		if ul != uint16(udpLen+gso) {
+			t.Errorf("seg %d: udp len=%d want %d", i, ul, udpLen+gso)
+		}
+		psum := pseudoHeaderIPv6(seg[8:24], seg[24:40], unix.IPPROTO_UDP, udpLen+gso)
+		if !verifyChecksum(seg[ipLen:], psum) {
+			t.Errorf("seg %d: bad UDP checksum", i)
+		}
+	}
+}
+
+// TestSegmentUDPCEPropagates confirms IP-level CE marks on the seed appear on
+// every segment. UDP has no transport-level CWR/ECE: the IP TOS/TC byte is
+// copied verbatim into every segment by the segment-prefix copy.
+func TestSegmentUDPCEPropagates(t *testing.T) {
+	pkt, hdr := buildUSOv4(t, 200, 100)
+	pkt[1] = 0x03 // CE codepoint in IP-ECN
+
+	scratch := make([]byte, testSegScratchSize)
+	var out [][]byte
+	if err := segmentForTest(pkt, hdr, &out, scratch); err != nil {
+		t.Fatalf("segmentForTest: %v", err)
+	}
+	if len(out) != 2 {
+		t.Fatalf("want 2 segments, got %d", len(out))
+	}
+	for i, seg := range out {
+		if seg[1]&0x03 != 0x03 {
+			t.Errorf("seg %d: CE missing (tos=%#x)", i, seg[1])
+		}
+		if !verifyChecksum(seg[:20], 0) {
+			t.Errorf("seg %d: bad IPv4 header checksum", i)
+		}
+	}
+}
+
+// TestSegmentTCPCwrFirstSegmentOnly confirms RFC 3168 §6.1.2: when a TSO
+// burst's seed has CWR set, only the first emitted segment carries CWR.
+// ECE is preserved on every segment (different signal, persistent state).
+func TestSegmentTCPCwrFirstSegmentOnly(t *testing.T) {
+	const mss = 100
+	const numSeg = 3
+	pkt, hdr := buildTSOv4(t, mss*numSeg, mss)
+	// Seed flags: CWR | ECE | ACK | PSH.
+	pkt[33] = 0x80 | 0x40 | 0x10 | 0x08
+
+	scratch := make([]byte, testSegScratchSize)
+	var out [][]byte
+	if err := segmentForTest(pkt, hdr, &out, scratch); err != nil {
+		t.Fatalf("segmentForTest: %v", err)
+	}
+	if len(out) != numSeg {
+		t.Fatalf("expected %d segments, got %d", numSeg, len(out))
+	}
+	for i, seg := range out {
+		flags := seg[33]
+		hasCwr := flags&0x80 != 0
+		hasEce := flags&0x40 != 0
+		hasPsh := flags&0x08 != 0
+		wantCwr := i == 0
+		wantPsh := i == numSeg-1
+		if hasCwr != wantCwr {
+			t.Errorf("seg %d: CWR=%v want %v (flags=%#x)", i, hasCwr, wantCwr, flags)
+		}
+		if !hasEce {
+			t.Errorf("seg %d: ECE missing (flags=%#x)", i, flags)
+		}
+		if hasPsh != wantPsh {
+			t.Errorf("seg %d: PSH=%v want %v (flags=%#x)", i, hasPsh, wantPsh, flags)
+		}
+		// IP and TCP checksums must still verify after the flag rewrite.
+		if !verifyChecksum(seg[:20], 0) {
+			t.Errorf("seg %d: bad IPv4 header checksum", i)
+		}
+		psum := pseudoHeaderIPv4(seg[12:16], seg[16:20], unix.IPPROTO_TCP, 20+mss)
+		if !verifyChecksum(seg[20:], psum) {
+			t.Errorf("seg %d: bad TCP checksum", i)
+		}
+	}
+}
+
+func BenchmarkSegmentTCPv4(b *testing.B) {
+	sizes := []struct {
+		name   string
+		payLen int
+		mss    int
+	}{
+		{"64KiB_MSS1460", 65000, 1460},
+		{"16KiB_MSS1460", 16384, 1460},
+		{"4KiB_MSS1460", 4096, 1460},
+	}
+	for _, sz := range sizes {
+		b.Run(sz.name, func(b *testing.B) {
+			const ipLen = 20
+			const tcpLen = 20
+			pkt := make([]byte, ipLen+tcpLen+sz.payLen)
+			pkt[0] = 0x45
+			binary.BigEndian.PutUint16(pkt[2:4], uint16(ipLen+tcpLen+sz.payLen))
+			binary.BigEndian.PutUint16(pkt[4:6], 0x4242)
+			pkt[8] = 64
+			pkt[9] = unix.IPPROTO_TCP
+			copy(pkt[12:16], []byte{10, 0, 0, 1})
+			copy(pkt[16:20], []byte{10, 0, 0, 2})
+			binary.BigEndian.PutUint16(pkt[20:22], 12345)
+			binary.BigEndian.PutUint16(pkt[22:24], 80)
+			binary.BigEndian.PutUint32(pkt[24:28], 10000)
+			binary.BigEndian.PutUint32(pkt[28:32], 20000)
+			pkt[32] = 0x50
+			pkt[33] = 0x18
+			binary.BigEndian.PutUint16(pkt[34:36], 65535)
+			for i := 0; i < sz.payLen; i++ {
+				pkt[ipLen+tcpLen+i] = byte(i)
+			}
+			hdr := virtio.Hdr{
+				Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
+				GSOType:    unix.VIRTIO_NET_HDR_GSO_TCPV4,
+				HdrLen:     uint16(ipLen + tcpLen),
+				GSOSize:    uint16(sz.mss),
+				CsumStart:  uint16(ipLen),
+				CsumOffset: 16,
+			}
+
+			scratch := make([]byte, testSegScratchSize)
+			out := make([][]byte, 0, 64)
+
+			// SegmentSuperpacket consumes its input destructively; restore
+			// pkt from a master copy each iteration. The restore mirrors the
+			// kernel→userspace copy that hands a fresh GSO blob to the
+			// segmenter in production, so it's representative cost rather
+			// than bench overhead.
+			master := append([]byte(nil), pkt...)
+			work := make([]byte, len(pkt))
+
+			b.SetBytes(int64(len(pkt)))
+			b.ResetTimer()
+			for i := 0; i < b.N; i++ {
+				copy(work, master)
+				out = out[:0]
+				if err := segmentForTest(work, hdr, &out, scratch); err != nil {
+					b.Fatal(err)
+				}
+			}
+		})
+	}
+}
+
+// TestTunFileWriteVnetHdrNoAlloc verifies the IFF_VNET_HDR fast-path write is
+// allocation-free. We write to /dev/null so every call succeeds synchronously.
+func TestTunFileWriteVnetHdrNoAlloc(t *testing.T) {
+	fd, err := unix.Open("/dev/null", os.O_WRONLY, 0)
+	if err != nil {
+		t.Fatalf("open /dev/null: %v", err)
+	}
+	t.Cleanup(func() { _ = unix.Close(fd) })
+
+	tf := &Offload{fd: fd}
+
+	payload := make([]byte, 1400)
+	// Warm up (first call may trigger one-time internal allocations elsewhere).
+	if _, err := tf.Write(payload); err != nil {
+		t.Fatalf("Write: %v", err)
+	}
+
+	allocs := testing.AllocsPerRun(1000, func() {
+		if _, err := tf.Write(payload); err != nil {
+			t.Fatalf("Write: %v", err)
+		}
+	})
+	if allocs != 0 {
+		t.Fatalf("Write allocated %.1f times per call, want 0", allocs)
+	}
+}
+
+// buildTSOv6 builds a synthetic IPv6/TCP TSO superpacket with payLen bytes
+// of payload, segmented at gso. Returns the packet bytes only; the
+// virtio_net_hdr is the caller's responsibility.
+func buildTSOv6(payLen, gso int) []byte {
+	const ipLen = 40
+	const tcpLen = 20
+	pkt := make([]byte, ipLen+tcpLen+payLen)
+
+	pkt[0] = 0x60 // version 6
+	binary.BigEndian.PutUint16(pkt[4:6], uint16(tcpLen+payLen))
+	pkt[6] = unix.IPPROTO_TCP
+	pkt[7] = 64
+	pkt[8] = 0xfe
+	pkt[9] = 0x80
+	pkt[23] = 1
+	pkt[24] = 0xfe
+	pkt[25] = 0x80
+	pkt[39] = 2
+
+	binary.BigEndian.PutUint16(pkt[40:42], 12345)
+	binary.BigEndian.PutUint16(pkt[42:44], 80)
+	binary.BigEndian.PutUint32(pkt[44:48], 7)
+	binary.BigEndian.PutUint32(pkt[48:52], 99)
+	pkt[52] = 0x50
+	pkt[53] = 0x10 // ACK only
+	binary.BigEndian.PutUint16(pkt[54:56], 65535)
+
+	for i := 0; i < payLen; i++ {
+		pkt[ipLen+tcpLen+i] = byte(i)
+	}
+	return pkt
+}
+
+// TestDecodeReadFitsMaxTSOAtDrainThreshold proves the rxBuf sizing is
+// correct: when rxOff is at the maximum value the drain headroom check
+// allows, decodeRead must still be able to absorb a worst-case 64KiB
+// TSO superpacket without dropping the burst. With segmentation deferred
+// to encrypt time, decodeRead writes only the kernel-supplied bytes into
+// rxBuf, so the size requirement is just "fit one worst-case input."
+//
+// Regression history: in a prior layout the rx buffer doubled as the
+// segmentation output, a near-threshold drain read returned "scratch too
+// small", the whole 45-segment TSO burst was dropped, and the remote's TCP
+// fast-retransmit collapsed cwnd. Keeping this test in the new layout
+// guards against re-introducing a drain headroom shortfall.
+func TestDecodeReadFitsMaxTSOAtDrainThreshold(t *testing.T) {
+	const ipv6HdrLen = 40
+	const tcpHdrLen = 20
+	const headerLen = ipv6HdrLen + tcpHdrLen
+	// Maximum TUN read body. The tunReadBufSize cap on readv's body iovec
+	// is what bounds the kernel's superpacket length.
+	pktLen := tunReadBufSize
+	payLen := pktLen - headerLen
+	const targetSegs = 64
+	gsoSize := (payLen + targetSegs - 1) / targetSegs
+
+	pkt := buildTSOv6(payLen, gsoSize)
+	if len(pkt) != pktLen {
+		t.Fatalf("buildTSOv6 produced %d bytes, want %d", len(pkt), pktLen)
+	}
+
+	o := &Offload{
+		rxBuf: make([]byte, tunRxBufCap),
+	}
+	// rxOff at the maximum value the drain headroom check permits before
+	// it would refuse another read. Any drain-time read up to this
+	// threshold MUST still process correctly.
+	o.rxOff = tunRxBufCap - tunRxBufSize
+
+	// Stage the body in rxBuf as if readv(2) just placed it there.
+	copy(o.rxBuf[o.rxOff:], pkt)
+
+	// Encode the matching virtio_net_hdr.
+	hdr := virtio.Hdr{
+		Flags:      unix.VIRTIO_NET_HDR_F_NEEDS_CSUM,
+		GSOType:    unix.VIRTIO_NET_HDR_GSO_TCPV6,
+		HdrLen:     uint16(headerLen),
+		GSOSize:    uint16(gsoSize),
+		CsumStart:  uint16(ipv6HdrLen),
+		CsumOffset: 16,
+	}
+	hdr.Encode(o.readVnetScratch[:])
+
+	startRxOff := o.rxOff
+	if err := o.decodeRead(pktLen); err != nil {
+		t.Fatalf("decodeRead at drain threshold returned %v — rxBuf sizing regression: "+
+			"tunRxBufSize=%d must hold one worst-case input (%d)",
+			err, tunRxBufSize, pktLen)
+	}
+
+	if len(o.pending) != 1 {
+		t.Fatalf("got %d packets, want 1 superpacket entry", len(o.pending))
+	}
+	got := o.pending[0]
+	if !got.GSO.IsSuperpacket() {
+		t.Fatalf("expected superpacket GSO metadata, got %+v", got.GSO)
+	}
+	if got.GSO.Proto != GSOProtoTCP {
+		t.Errorf("GSO.Proto=%d want TCP", got.GSO.Proto)
+	}
+	if got.GSO.Size != uint16(gsoSize) {
+		t.Errorf("GSO.Size=%d want %d", got.GSO.Size, gsoSize)
+	}
+	if got.GSO.HdrLen != uint16(headerLen) {
+		t.Errorf("GSO.HdrLen=%d want %d", got.GSO.HdrLen, headerLen)
+	}
+	if got.GSO.CsumStart != uint16(ipv6HdrLen) {
+		t.Errorf("GSO.CsumStart=%d want %d", got.GSO.CsumStart, ipv6HdrLen)
+	}
+	if len(got.Bytes) != pktLen {
+		t.Errorf("len(Bytes)=%d want %d", len(got.Bytes), pktLen)
+	}
+
+	// rxOff advances exactly by the kernel-supplied body length — no
+	// segmentation output to account for any more.
+	if o.rxOff != startRxOff+pktLen {
+		t.Errorf("rxOff=%d want %d", o.rxOff, startRxOff+pktLen)
+	}
+	if o.rxOff > tunRxBufCap {
+		t.Fatalf("rxOff=%d overran rxBuf (cap=%d)", o.rxOff, tunRxBufCap)
+	}
+
+	// Validate that segmenting the returned superpacket reproduces the
+	// expected per-segment IPv6 payload length and TCP checksum.
+	wantSegs := (payLen + gsoSize - 1) / gsoSize
+	gotSegs := 0
+	if err := SegmentSuperpacket(got, func(seg []byte) error {
+		defer func() { gotSegs++ }()
+		if len(seg) < headerLen+1 {
+			t.Errorf("seg %d too short: %d", gotSegs, len(seg))
+			return nil
+		}
+		if seg[0]>>4 != 6 {
+			t.Errorf("seg %d: bad IP version %#x", gotSegs, seg[0])
+		}
+		segPay := len(seg) - headerLen
+		gotPL := binary.BigEndian.Uint16(seg[4:6])
+		if gotPL != uint16(tcpHdrLen+segPay) {
+			t.Errorf("seg %d: payload_len=%d want %d", gotSegs, gotPL, tcpHdrLen+segPay)
+		}
+		psum := pseudoHeaderIPv6(seg[8:24], seg[24:40], unix.IPPROTO_TCP, tcpHdrLen+segPay)
+		if !verifyChecksum(seg[ipv6HdrLen:], psum) {
+			t.Errorf("seg %d: bad TCP checksum", gotSegs)
+		}
+		return nil
+	}); err != nil {
+		t.Fatalf("SegmentSuperpacket: %v", err)
+	}
+	if gotSegs != wantSegs {
+		t.Fatalf("got %d segments, want %d", gotSegs, wantSegs)
+	}
+}

overlay/tio/virtio/header_linux.go

@@ -0,0 +1,43 @@
+//go:build linux && !android && !e2e_testing
+// +build linux,!android,!e2e_testing
+
+package virtio
+
+import "encoding/binary"
+
+// Size is the on-wire length of struct virtio_net_hdr the kernel
+// prepends/expects on a TUN opened with IFF_VNET_HDR (TUNSETVNETHDRSZ
+// not set).
+const Size = 10
+
+// Hdr is the Go view of the legacy virtio_net_hdr.
+type Hdr struct {
+	Flags      uint8
+	GSOType    uint8
+	HdrLen     uint16
+	GSOSize    uint16
+	CsumStart  uint16
+	CsumOffset uint16
+}
+
+// Decode reads a virtio_net_hdr in host byte order (TUN default; we never
+// call TUNSETVNETLE so the kernel matches our endianness).
+func (h *Hdr) Decode(b []byte) {
+	h.Flags = b[0]
+	h.GSOType = b[1]
+	h.HdrLen = binary.NativeEndian.Uint16(b[2:4])
+	h.GSOSize = binary.NativeEndian.Uint16(b[4:6])
+	h.CsumStart = binary.NativeEndian.Uint16(b[6:8])
+	h.CsumOffset = binary.NativeEndian.Uint16(b[8:10])
+}
+
+// Encode is the inverse of Decode: writes the virtio_net_hdr fields into b
+// (must be at least Size bytes). Used to emit a TSO superpacket on egress.
+func (h *Hdr) Encode(b []byte) {
+	b[0] = h.Flags
+	b[1] = h.GSOType
+	binary.NativeEndian.PutUint16(b[2:4], h.HdrLen)
+	binary.NativeEndian.PutUint16(b[4:6], h.GSOSize)
+	binary.NativeEndian.PutUint16(b[6:8], h.CsumStart)
+	binary.NativeEndian.PutUint16(b[8:10], h.CsumOffset)
+}

overlay/tio/virtio/segment_linux.go

@@ -0,0 +1,401 @@
+//go:build linux && !android && !e2e_testing
+// +build linux,!android,!e2e_testing
+
+// Package virtio implements the pure validation, header-correction, and
+// per-segment slicing logic for kernel-supplied TSO/USO superpackets on
+// IFF_VNET_HDR TUN devices. It is FD-free and depends only on the byte
+// layout of the virtio_net_hdr and the IP/TCP/UDP headers it describes,
+// so it can be unit-tested in isolation from the tio Queue runtime.
+package virtio
+
+import (
+	"encoding/binary"
+	"errors"
+	"fmt"
+
+	"golang.org/x/sys/unix"
+	"gvisor.dev/gvisor/pkg/tcpip/checksum"
+)
+
+// Protocol header size bounds used to validate / cap kernel-supplied offsets.
+const (
+	ipv4HeaderMinLen = 20 // IHL=5, no options
+	ipv4HeaderMaxLen = 60 // IHL=15, max options
+	ipv6FixedLen     = 40 // IPv6 base header; extensions would extend this
+	tcpHeaderMinLen  = 20 // data-offset=5, no options
+	tcpHeaderMaxLen  = 60 // data-offset=15, max options
+)
+
+// Byte offsets inside an IPv4 header.
+const (
+	ipv4TotalLenOff = 2
+	ipv4IDOff       = 4
+	ipv4ChecksumOff = 10
+	ipv4SrcOff      = 12
+	ipv4AddrsEnd    = 20 // end of dst address (ipv4SrcOff + 2*4)
+)
+
+// Byte offsets inside an IPv6 header.
+const (
+	ipv6PayloadLenOff = 4
+	ipv6SrcOff        = 8
+	ipv6AddrsEnd      = 40 // end of dst address (ipv6SrcOff + 2*16)
+)
+
+// Byte offsets inside a TCP header (relative to its start, i.e. csumStart).
+const (
+	tcpSeqOff      = 4
+	tcpDataOffOff  = 12 // upper nibble is header len in 32-bit words
+	tcpFlagsOff    = 13
+	tcpChecksumOff = 16
+)
+
+// UDP header is fixed at 8 bytes: {sport, dport, length, checksum}.
+const (
+	udpHeaderLen   = 8
+	udpLengthOff   = 4
+	udpChecksumOff = 6
+)
+
+// tcpFinPshMask is cleared on every segment except the last of a TSO burst.
+const tcpFinPshMask = 0x09 // FIN(0x01) | PSH(0x08)
+
+// tcpCwrFlag is cleared on every segment except the first. Per RFC 3168
+// §6.1.2 the CWR bit signals a one-shot transition (the sender just halved
+// its window) and must appear on the first segment of a TSO burst only.
+const tcpCwrFlag = 0x80
+
+// CheckValid rejects packets whose virtio_net_hdr/IP combination would
+// cause a downstream miscompute. The TUN should never emit RSC_INFO and
+// the GSO type must agree with the IP version nibble.
+func CheckValid(pkt []byte, hdr Hdr) error {
+	// When RSC_INFO is set the csum_start/csum_offset fields are repurposed to
+	// carry coalescing info rather than checksum offsets. A TUN writing via
+	// IFF_VNET_HDR should never emit this, but if it did we would silently
+	// miscompute the segment checksums — refuse the packet instead.
+	if hdr.Flags&unix.VIRTIO_NET_HDR_F_RSC_INFO != 0 {
+		return fmt.Errorf("virtio RSC_INFO flag not supported on TUN reads")
+	}
+	if len(pkt) < ipv4HeaderMinLen {
+		return fmt.Errorf("packet too short")
+	}
+	ipVersion := pkt[0] >> 4
+	switch hdr.GSOType {
+	case unix.VIRTIO_NET_HDR_GSO_TCPV4:
+		if ipVersion != 4 {
+			return fmt.Errorf("invalid IP version %d for GSO type %d", ipVersion, hdr.GSOType)
+		}
+	case unix.VIRTIO_NET_HDR_GSO_TCPV6:
+		if ipVersion != 6 {
+			return fmt.Errorf("invalid IP version %d for GSO type %d", ipVersion, hdr.GSOType)
+		}
+	case unix.VIRTIO_NET_HDR_GSO_UDP_L4:
+		// USO carries either v4 or v6; the leading nibble disambiguates.
+		if !(ipVersion == 4 || ipVersion == 6) {
+			return fmt.Errorf("invalid IP version %d for GSO type %d", ipVersion, hdr.GSOType)
+		}
+	default:
+		if !(ipVersion == 6 || ipVersion == 4) {
+			return fmt.Errorf("invalid IP version %d for GSO type %d", ipVersion, hdr.GSOType)
+		}
+	}
+
+	return nil
+}
+
+// CorrectHdrLen rewrites hdr.HdrLen based on the actual transport header
+// length read out of pkt. The kernel's hdr.HdrLen on the FORWARD path can
+// be the length of the entire first packet, so we don't trust it.
+func CorrectHdrLen(pkt []byte, hdr *Hdr) error {
+	// Thank you wireguard-go for documenting these edge-cases
+	// Don't trust hdr.hdrLen from the kernel as it can be equal to the length
+	// of the entire first packet when the kernel is handling it as part of a
+	// FORWARD path. Instead, parse the transport header length and add it onto
+	// csumStart, which is synonymous for IP header length.
+
+	if hdr.GSOType == unix.VIRTIO_NET_HDR_GSO_UDP_L4 {
+		hdr.HdrLen = hdr.CsumStart + 8
+	} else {
+		if len(pkt) <= int(hdr.CsumStart+tcpDataOffOff) {
+			return errors.New("packet is too short")
+		}
+
+		tcpHLen := uint16(pkt[hdr.CsumStart+tcpDataOffOff] >> 4 * 4)
+		if tcpHLen < 20 || tcpHLen > 60 {
+			// A TCP header must be between 20 and 60 bytes in length.
+			return fmt.Errorf("tcp header len is invalid: %d", tcpHLen)
+		}
+		hdr.HdrLen = hdr.CsumStart + tcpHLen
+	}
+
+	if len(pkt) < int(hdr.HdrLen) {
+		return fmt.Errorf("length of packet (%d) < virtioNetHdr.HdrLen (%d)", len(pkt), hdr.HdrLen)
+	}
+
+	if hdr.HdrLen < hdr.CsumStart {
+		return fmt.Errorf("virtioNetHdr.HdrLen (%d) < virtioNetHdr.CsumStart (%d)", hdr.HdrLen, hdr.CsumStart)
+	}
+	cSumAt := int(hdr.CsumStart + hdr.CsumStart)
+	if cSumAt+1 >= len(pkt) {
+		return fmt.Errorf("end of checksum offset (%d) exceeds packet length (%d)", cSumAt+1, len(pkt))
+	}
+	return nil
+}
+
+// SegmentTCP walks a TSO superpacket pkt, yielding each segment as a
+// slice into pkt itself. Per-segment plaintext is laid out by sliding a
+// freshly-patched copy of the L3+L4 header into pkt at offset i*gsoSize,
+// where it sits immediately before that segment's payload chunk in the
+// original buffer. The slide is destructive: iter i's header write overwrites
+// the last hdrLen bytes of seg_{i-1}'s payload, which is dead by the time
+// the next iteration begins. pkt is consumed by this call and must not be
+// inspected by the caller after the final yield.
+func SegmentTCP(pkt []byte, hdrLenU, csumStartU, gsoSizeU uint16, yield func(seg []byte) error) error {
+	if gsoSizeU == 0 {
+		return fmt.Errorf("gso_size is zero")
+	}
+	if csumStartU == 0 {
+		return fmt.Errorf("csum_start is zero")
+	}
+
+	headerLen := int(hdrLenU)
+	csumStart := int(csumStartU)
+	isV4 := pkt[0]>>4 == 4
+
+	tcpHdrLen := int(pkt[csumStart+tcpDataOffOff]>>4) * 4
+	payLen := len(pkt) - headerLen
+	gsoSize := int(gsoSizeU)
+	numSeg := (payLen + gsoSize - 1) / gsoSize
+	if numSeg == 0 {
+		numSeg = 1
+	}
+
+	origSeq := binary.BigEndian.Uint32(pkt[csumStart+tcpSeqOff : csumStart+tcpSeqOff+4])
+	origFlags := pkt[csumStart+tcpFlagsOff]
+
+	var tmp [tcpHeaderMaxLen]byte
+	copy(tmp[:tcpHdrLen], pkt[csumStart:headerLen])
+	tmp[tcpSeqOff], tmp[tcpSeqOff+1], tmp[tcpSeqOff+2], tmp[tcpSeqOff+3] = 0, 0, 0, 0
+	tmp[tcpFlagsOff] = 0
+	tmp[tcpChecksumOff], tmp[tcpChecksumOff+1] = 0, 0
+	baseTcpHdrSum := uint32(checksum.Checksum(tmp[:tcpHdrLen], 0))
+
+	var baseProtoSum uint32
+	if isV4 {
+		baseProtoSum = uint32(checksum.Checksum(pkt[ipv4SrcOff:ipv4AddrsEnd], 0))
+	} else {
+		baseProtoSum = uint32(checksum.Checksum(pkt[ipv6SrcOff:ipv6AddrsEnd], 0))
+	}
+	baseProtoSum += uint32(unix.IPPROTO_TCP)
+
+	var origIPID uint16
+	var baseIPHdrSum uint32
+	if isV4 {
+		origIPID = binary.BigEndian.Uint16(pkt[ipv4IDOff : ipv4IDOff+2])
+		ihl := int(pkt[0]&0x0f) * 4
+		if ihl < ipv4HeaderMinLen || ihl > csumStart {
+			return fmt.Errorf("bad IPv4 IHL: %d", ihl)
+		}
+		var ipTmp [ipv4HeaderMaxLen]byte
+		copy(ipTmp[:ihl], pkt[:ihl])
+		ipTmp[ipv4TotalLenOff], ipTmp[ipv4TotalLenOff+1] = 0, 0
+		ipTmp[ipv4IDOff], ipTmp[ipv4IDOff+1] = 0, 0
+		ipTmp[ipv4ChecksumOff], ipTmp[ipv4ChecksumOff+1] = 0, 0
+		baseIPHdrSum = uint32(checksum.Checksum(ipTmp[:ihl], 0))
+	}
+
+	for i := 0; i < numSeg; i++ {
+		segStart := i * gsoSize
+		segEnd := segStart + gsoSize
+		if segEnd > payLen {
+			segEnd = payLen
+		}
+		segPayLen := segEnd - segStart
+		segLen := headerLen + segPayLen
+		headerOff := i * gsoSize
+
+		// Slide the header into place immediately before this segment's
+		// payload. Iter 0's header is already at pkt[:headerLen]; for
+		// i ≥ 1 we copy from there. The constant-byte fields of pkt[:headerLen]
+		// survive iter 0's in-place patches (only seq/flags/cksum/totalLen/id
+		// are touched), and iter 0's stale variable-field values are
+		// overwritten by the per-segment patches below.
+		if i > 0 {
+			copy(pkt[headerOff:headerOff+headerLen], pkt[:headerLen])
+		}
+		seg := pkt[headerOff : headerOff+segLen]
+
+		segSeq := origSeq + uint32(segStart)
+		segFlags := origFlags
+		if i != 0 {
+			segFlags &^= tcpCwrFlag
+		}
+		if i != numSeg-1 {
+			segFlags &^= tcpFinPshMask
+		}
+		totalLen := segLen
+
+		if isV4 {
+			segID := origIPID + uint16(i)
+			binary.BigEndian.PutUint16(seg[ipv4TotalLenOff:ipv4TotalLenOff+2], uint16(totalLen))
+			binary.BigEndian.PutUint16(seg[ipv4IDOff:ipv4IDOff+2], segID)
+			ipSum := baseIPHdrSum + uint32(totalLen) + uint32(segID)
+			binary.BigEndian.PutUint16(seg[ipv4ChecksumOff:ipv4ChecksumOff+2], foldComplement(ipSum))
+		} else {
+			binary.BigEndian.PutUint16(seg[ipv6PayloadLenOff:ipv6PayloadLenOff+2], uint16(headerLen-ipv6FixedLen+segPayLen))
+		}
+
+		binary.BigEndian.PutUint32(seg[csumStart+tcpSeqOff:csumStart+tcpSeqOff+4], segSeq)
+		seg[csumStart+tcpFlagsOff] = segFlags
+
+		tcpLen := tcpHdrLen + segPayLen
+		// Payload bytes still live at their original offset in pkt. The
+		// header slide above only writes into pkt[i*G : i*G+H], which is
+		// the tail of seg_{i-1}'s payload (already consumed) and never
+		// overlaps seg_i's own payload at pkt[H+i*G : H+(i+1)*G].
+		paySum := uint32(checksum.Checksum(pkt[headerLen+segStart:headerLen+segEnd], 0))
+		wide := uint64(baseTcpHdrSum) + uint64(paySum) + uint64(baseProtoSum)
+		wide += uint64(segSeq) + uint64(segFlags) + uint64(tcpLen)
+		wide = (wide & 0xffffffff) + (wide >> 32)
+		wide = (wide & 0xffffffff) + (wide >> 32)
+		binary.BigEndian.PutUint16(seg[csumStart+tcpChecksumOff:csumStart+tcpChecksumOff+2], foldComplement(uint32(wide)))
+
+		if err := yield(seg); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// SegmentUDP walks a USO superpacket, sliding a per-segment-patched
+// L3+L4 header into pkt at offset i*gsoSize and yielding pkt[i*G:i*G+segLen]
+// to the caller. Per-segment patches are total_len + IPv4 csum (or IPv6
+// payload_len) plus the UDP length and checksum. pkt is consumed
+// destructively; see SegmentTCP for the layout reasoning.
+//
+// UDP-GSO leaves the IPv4 ID identical across segments (the kernel does not
+// bump it), which is why the IP-level per-segment work is limited to
+// total_len + IPv4 header checksum (v4) or payload_len (v6).
+func SegmentUDP(pkt []byte, hdrLenU, csumStartU, gsoSizeU uint16, yield func(seg []byte) error) error {
+	if gsoSizeU == 0 {
+		return fmt.Errorf("gso_size is zero")
+	}
+	if csumStartU == 0 {
+		return fmt.Errorf("csum_start is zero")
+	}
+
+	isV4 := pkt[0]>>4 == 4
+	headerLen := int(hdrLenU)
+	csumStart := int(csumStartU)
+	if headerLen-csumStart != udpHeaderLen {
+		return fmt.Errorf("udp header len mismatch: %d", headerLen-csumStart)
+	}
+
+	payLen := len(pkt) - headerLen
+	gsoSize := int(gsoSizeU)
+	numSeg := (payLen + gsoSize - 1) / gsoSize
+	if numSeg == 0 {
+		numSeg = 1
+	}
+
+	var udpTmp [udpHeaderLen]byte
+	copy(udpTmp[:], pkt[csumStart:headerLen])
+	udpTmp[udpLengthOff], udpTmp[udpLengthOff+1] = 0, 0
+	udpTmp[udpChecksumOff], udpTmp[udpChecksumOff+1] = 0, 0
+	baseUDPHdrSum := uint32(checksum.Checksum(udpTmp[:], 0))
+
+	var baseProtoSum uint32
+	if isV4 {
+		baseProtoSum = uint32(checksum.Checksum(pkt[ipv4SrcOff:ipv4AddrsEnd], 0))
+	} else {
+		baseProtoSum = uint32(checksum.Checksum(pkt[ipv6SrcOff:ipv6AddrsEnd], 0))
+	}
+	baseProtoSum += uint32(unix.IPPROTO_UDP)
+
+	var baseIPHdrSum uint32
+	if isV4 {
+		ihl := int(pkt[0]&0x0f) * 4
+		if ihl < ipv4HeaderMinLen || ihl > csumStart {
+			return fmt.Errorf("bad IPv4 IHL: %d", ihl)
+		}
+		var ipTmp [ipv4HeaderMaxLen]byte
+		copy(ipTmp[:ihl], pkt[:ihl])
+		ipTmp[ipv4TotalLenOff], ipTmp[ipv4TotalLenOff+1] = 0, 0
+		ipTmp[ipv4ChecksumOff], ipTmp[ipv4ChecksumOff+1] = 0, 0
+		baseIPHdrSum = uint32(checksum.Checksum(ipTmp[:ihl], 0))
+	}
+
+	for i := 0; i < numSeg; i++ {
+		segStart := i * gsoSize
+		segEnd := segStart + gsoSize
+		if segEnd > payLen {
+			segEnd = payLen
+		}
+		segPayLen := segEnd - segStart
+		segLen := headerLen + segPayLen
+		headerOff := i * gsoSize
+
+		if i > 0 {
+			copy(pkt[headerOff:headerOff+headerLen], pkt[:headerLen])
+		}
+		seg := pkt[headerOff : headerOff+segLen]
+
+		totalLen := segLen
+		udpLen := udpHeaderLen + segPayLen
+
+		if isV4 {
+			binary.BigEndian.PutUint16(seg[ipv4TotalLenOff:ipv4TotalLenOff+2], uint16(totalLen))
+			ipSum := baseIPHdrSum + uint32(totalLen)
+			binary.BigEndian.PutUint16(seg[ipv4ChecksumOff:ipv4ChecksumOff+2], foldComplement(ipSum))
+		} else {
+			binary.BigEndian.PutUint16(seg[ipv6PayloadLenOff:ipv6PayloadLenOff+2], uint16(headerLen-ipv6FixedLen+segPayLen))
+		}
+
+		binary.BigEndian.PutUint16(seg[csumStart+udpLengthOff:csumStart+udpLengthOff+2], uint16(udpLen))
+
+		paySum := uint32(checksum.Checksum(pkt[headerLen+segStart:headerLen+segEnd], 0))
+		wide := uint64(baseUDPHdrSum) + uint64(paySum) + uint64(baseProtoSum)
+		wide += uint64(udpLen) + uint64(udpLen)
+		wide = (wide & 0xffffffff) + (wide >> 32)
+		wide = (wide & 0xffffffff) + (wide >> 32)
+		csum := foldComplement(uint32(wide))
+		if csum == 0 {
+			csum = 0xffff
+		}
+		binary.BigEndian.PutUint16(seg[csumStart+udpChecksumOff:csumStart+udpChecksumOff+2], csum)
+
+		if err := yield(seg); err != nil {
+			return err
+		}
+	}
+
+	return nil
+}
+
+// FinishChecksum computes the L4 checksum for a non-GSO packet that the kernel
+// handed us with NEEDS_CSUM set. csum_start / csum_offset point at the 16-bit
+// checksum field; we zero it, fold a full sum (the field was pre-loaded with
+// the pseudo-header partial sum by the kernel), and store the result.
+func FinishChecksum(seg []byte, hdr Hdr) error {
+	cs := int(hdr.CsumStart)
+	co := int(hdr.CsumOffset)
+	if cs+co+2 > len(seg) {
+		return fmt.Errorf("csum offsets out of range: start=%d offset=%d len=%d", cs, co, len(seg))
+	}
+	// The kernel stores a partial pseudo-header sum at [cs+co:]; sum over the
+	// L4 region starting at cs, folding the prior partial in as the seed.
+	partial := binary.BigEndian.Uint16(seg[cs+co : cs+co+2])
+	seg[cs+co] = 0
+	seg[cs+co+1] = 0
+	binary.BigEndian.PutUint16(seg[cs+co:cs+co+2], ^checksum.Checksum(seg[cs:], partial))
+	return nil
+}
+
+// foldComplement folds a 32-bit one's-complement partial sum to 16 bits and
+// complements it, yielding the on-wire Internet checksum value.
+func foldComplement(sum uint32) uint16 {
+	sum = (sum & 0xffff) + (sum >> 16)
+	sum = (sum & 0xffff) + (sum >> 16)
+	return ^uint16(sum)
+}

overlay/tun_android.go

@@ -27,15 +27,15 @@ type tun struct {
 	l           *slog.Logger
 
 	readBuf  []byte
-	batchRet [1][]byte
+	batchRet [1]tio.Packet
 }
 
-func (t *tun) Read() ([][]byte, error) {
+func (t *tun) Read() ([]tio.Packet, error) {
 	n, err := t.rwc.Read(t.readBuf)
 	if err != nil {
 		return nil, err
 	}
-	t.batchRet[0] = t.readBuf[:n]
+	t.batchRet[0] = tio.Packet{Bytes: t.readBuf[:n]}
 	return t.batchRet[:], nil
 }
 

overlay/tun_darwin.go

@@ -33,7 +33,7 @@ type tun struct {
 	l           *slog.Logger
 
 	readBuf  []byte
-	batchRet [1][]byte
+	batchRet [1]tio.Packet
 }
 
 type ifReq struct {
@@ -554,12 +554,12 @@ func (t *tun) readOne(to []byte) (int, error) {
 	return n - 4, nil
 }
 
-func (t *tun) Read() ([][]byte, error) {
+func (t *tun) Read() ([]tio.Packet, error) {
 	n, err := t.readOne(t.readBuf)
 	if err != nil {
 		return nil, err
 	}
-	t.batchRet[0] = t.readBuf[:n]
+	t.batchRet[0] = tio.Packet{Bytes: t.readBuf[:n]}
 	return t.batchRet[:], nil
 }
 

overlay/tun_disabled.go

@@ -23,23 +23,41 @@ type disabledTun struct {
 	rx         metrics.Counter
 	l          *slog.Logger
 	numReaders int
+}
 
-	batchRet [1][]byte
+// disabledQueue is one tio.Queue view onto a shared disabledTun. Each queue
+// owns a private batchRet so concurrent Read calls from different reader
+// goroutines do not race on the returned slice.
+type disabledQueue struct {
+	parent   *disabledTun
+	batchRet [1]tio.Packet
 }
 
-func (t *disabledTun) Read() ([][]byte, error) {
-	r, ok := <-t.read
+func (q *disabledQueue) Read() ([]tio.Packet, error) {
+	r, ok := <-q.parent.read
 	if !ok {
 		return nil, io.EOF
 	}
 
-	t.tx.Inc(1)
-	if t.l.Enabled(context.Background(), slog.LevelDebug) {
-		t.l.Debug("Write payload", "raw", prettyPacket(r))
+	q.parent.tx.Inc(1)
+	if q.parent.l.Enabled(context.Background(), slog.LevelDebug) {
+		q.parent.l.Debug("Write payload", "raw", prettyPacket(r))
 	}
 
-	t.batchRet[0] = r
-	return t.batchRet[:], nil
+	q.batchRet[0] = tio.Packet{Bytes: r}
+	return q.batchRet[:], nil
+}
+
+// Write on a queue forwards to the underlying disabledTun. All queues share
+// one ICMP-handling/log path so this is a thin pass-through.
+func (q *disabledQueue) Write(b []byte) (int, error) {
+	return q.parent.Write(b)
+}
+
+// Close on a queue is a no-op. The shared channel and metrics are owned by
+// the disabledTun; Close on the device tears them down once for everybody.
+func (q *disabledQueue) Close() error {
+	return nil
 }
 
 func newDisabledTun(vpnNetworks []netip.Prefix, queueLen int, metricsEnabled bool, l *slog.Logger) *disabledTun {
@@ -120,7 +138,7 @@ func (t *disabledTun) NewMultiQueueReader() error {
 func (t *disabledTun) Readers() []tio.Queue {
 	out := make([]tio.Queue, t.numReaders)
 	for i := range t.numReaders {
-		out[i] = t
+		out[i] = &disabledQueue{parent: t}
 	}
 	return out
 }

overlay/tun_freebsd.go

@@ -104,7 +104,7 @@ type tun struct {
 	closed    atomic.Bool
 
 	readBuf  []byte
-	batchRet [1][]byte
+	batchRet [1]tio.Packet
 }
 
 // blockOnRead waits until the tun fd is readable or shutdown has been signaled.
@@ -159,12 +159,12 @@ func (t *tun) blockOnWrite() error {
 	return nil
 }
 
-func (t *tun) Read() ([][]byte, error) {
+func (t *tun) Read() ([]tio.Packet, error) {
 	n, err := t.readOne(t.readBuf)
 	if err != nil {
 		return nil, err
 	}
-	t.batchRet[0] = t.readBuf[:n]
+	t.batchRet[0] = tio.Packet{Bytes: t.readBuf[:n]}
 	return t.batchRet[:], nil
 }
 

overlay/tun_ios.go

@@ -30,15 +30,15 @@ type tun struct {
 	l           *slog.Logger
 
 	readBuf  []byte
-	batchRet [1][]byte
+	batchRet [1]tio.Packet
 }
 
-func (t *tun) Read() ([][]byte, error) {
+func (t *tun) Read() ([]tio.Packet, error) {
 	n, err := t.rwc.Read(t.readBuf)
 	if err != nil {
 		return nil, err
 	}
-	t.batchRet[0] = t.readBuf[:n]
+	t.batchRet[0] = tio.Packet{Bytes: t.readBuf[:n]}
 	return t.batchRet[:], nil
 }
 

overlay/tun_linux.go

@@ -25,7 +25,7 @@ import (
 )
 
 type tun struct {
-	readers     tio.Container
+	readers     tio.QueueSet
 	closeLock   sync.Mutex
 	Device      string
 	vpnNetworks []netip.Prefix
@@ -34,6 +34,14 @@ type tun struct {
 	TXQueueLen  int
 	deviceIndex int
 	ioctlFd     uintptr
+	vnetHdr     bool
+	// routeFeatureECN, when true, sets RTAX_FEATURE_ECN on every route we
+	// install for the tun. The kernel then actively negotiates ECN for
+	// connections destined to those prefixes (equivalent to `ip route
+	// change ... features ecn`) regardless of net.ipv4.tcp_ecn, so flows
+	// across the nebula mesh use ECN even when the host default is the
+	// passive setting (=2). Disable via tunnels.ecn=false.
+	routeFeatureECN bool
 
 	Routes                    atomic.Pointer[[]Route]
 	routeTree                 atomic.Pointer[bart.Table[routing.Gateways]]
@@ -72,7 +80,9 @@ type ifreqQLEN struct {
 }
 
 func newTunFromFd(c *config.C, l *slog.Logger, deviceFd int, vpnNetworks []netip.Prefix) (*tun, error) {
-	t, err := newTunGeneric(c, l, deviceFd, vpnNetworks)
+	// We don't know what flags the caller opened this fd with and can't turn
+	// on IFF_VNET_HDR after TUNSETIFF, so skip offload on inherited fds.
+	t, err := newTunGeneric(c, l, deviceFd, false, false, vpnNetworks)
 	if err != nil {
 		return nil, err
 	}
@@ -117,6 +127,18 @@ func tunSetIff(fd int, name string, flags uint16) (string, error) {
 	return strings.Trim(string(req.Name[:]), "\x00"), nil
 }
 
+// tsoOffloadFlags are the TUN_F_* bits we ask the kernel to enable when a
+// TSO-capable TUN is available. CSUM is required as a prerequisite for TSO.
+// TSO_ECN tells the kernel we propagate ECN correctly through coalesce and
+// segmentation, so it can deliver superpackets whose seed has CWR/ECE set
+// or whose IP-level codepoint is CE.
+const tsoOffloadFlags = unix.TUN_F_CSUM | unix.TUN_F_TSO4 | unix.TUN_F_TSO6 | unix.TUN_F_TSO_ECN
+
+// usoOffloadFlags adds UDP Segmentation Offload to tsoOffloadFlags. Requires
+// Linux ≥ 6.2; older kernels reject it and we fall back to TCP-only TSO via
+// tsoOffloadFlags.
+const usoOffloadFlags = tsoOffloadFlags | unix.TUN_F_USO4 | unix.TUN_F_USO6
+
 func newTun(c *config.C, l *slog.Logger, vpnNetworks []netip.Prefix, multiqueue bool) (*tun, error) {
 	baseFlags := uint16(unix.IFF_TUN | unix.IFF_NO_PI)
 	if multiqueue {
@@ -124,17 +146,51 @@ func newTun(c *config.C, l *slog.Logger, vpnNetworks []netip.Prefix, multiqueue
 	}
 	nameStr := c.GetString("tun.dev", "")
 
+	// First try to enable IFF_VNET_HDR via TUNSETIFF and negotiate TUN_F_*
+	// offloads via TUNSETOFFLOAD so we can receive TSO/USO superpackets.
+	// We try TSO+USO first, fall back to TSO-only on kernels without USO
+	// (Linux < 6.2), and finally give up on virtio headers entirely and
+	// reopen as a plain TUN if neither offload mask is accepted.
 	fd, err := openTunDev()
 	if err != nil {
 		return nil, err
 	}
-	name, err := tunSetIff(fd, nameStr, baseFlags)
+	vnetHdr := true
+	usoEnabled := false
+	name, err := tunSetIff(fd, nameStr, baseFlags|unix.IFF_VNET_HDR)
 	if err != nil {
 		_ = unix.Close(fd)
-		return nil, &NameError{Name: nameStr, Underlying: err}
+		vnetHdr = false
+	} else {
+		// Try TSO+USO first. On kernels without USO support (Linux < 6.2)
+		// the ioctl returns EINVAL; fall back to the TCP-only mask before
+		// giving up on VNET_HDR entirely.
+		if err = ioctl(uintptr(fd), unix.TUNSETOFFLOAD, uintptr(usoOffloadFlags)); err == nil {
+			usoEnabled = true
+		} else if err = ioctl(uintptr(fd), unix.TUNSETOFFLOAD, uintptr(tsoOffloadFlags)); err != nil {
+			l.Warn("Failed to enable TUN offload (TSO); proceeding without virtio headers", "error", err)
+			_ = unix.Close(fd)
+			vnetHdr = false
+		}
+	}
+
+	if !vnetHdr {
+		fd, err = openTunDev()
+		if err != nil {
+			return nil, err
+		}
+		name, err = tunSetIff(fd, nameStr, baseFlags)
+		if err != nil {
+			_ = unix.Close(fd)
+			return nil, &NameError{Name: nameStr, Underlying: err}
+		}
+	}
+
+	if vnetHdr {
+		l.Info("TUN offload enabled", "tso", true, "uso", usoEnabled)
 	}
 
-	t, err := newTunGeneric(c, l, fd, vpnNetworks)
+	t, err := newTunGeneric(c, l, fd, vnetHdr, usoEnabled, vpnNetworks)
 	if err != nil {
 		return nil, err
 	}
@@ -145,25 +201,34 @@ func newTun(c *config.C, l *slog.Logger, vpnNetworks []netip.Prefix, multiqueue
 }
 
 // newTunGeneric does all the stuff common to different tun initialization paths. It will close your files on error.
-func newTunGeneric(c *config.C, l *slog.Logger, fd int, vpnNetworks []netip.Prefix) (*tun, error) {
-	container, err := tio.NewPollContainer()
+func newTunGeneric(c *config.C, l *slog.Logger, fd int, vnetHdr, usoEnabled bool, vpnNetworks []netip.Prefix) (*tun, error) {
+	var qs tio.QueueSet
+	var err error
+	if vnetHdr {
+		qs, err = tio.NewOffloadQueueSet(usoEnabled)
+	} else {
+		qs, err = tio.NewPollQueueSet()
+	}
+
 	if err != nil {
 		_ = unix.Close(fd)
 		return nil, err
 	}
-	err = container.Add(fd)
+	err = qs.Add(fd)
 	if err != nil {
 		_ = unix.Close(fd)
 		return nil, err
 	}
 
 	t := &tun{
-		readers:                   container,
+		readers:                   qs,
 		closeLock:                 sync.Mutex{},
+		vnetHdr:                   vnetHdr,
 		vpnNetworks:               vpnNetworks,
 		TXQueueLen:                c.GetInt("tun.tx_queue", 500),
 		useSystemRoutes:           c.GetBool("tun.use_system_route_table", false),
 		useSystemRoutesBufferSize: c.GetInt("tun.use_system_route_table_buffer_size", 0),
+		routeFeatureECN:           c.GetBool("tunnels.ecn", true),
 		routesFromSystem:          map[netip.Prefix]routing.Gateways{},
 		l:                         l,
 	}
@@ -271,11 +336,21 @@ func (t *tun) NewMultiQueueReader() error {
 	}
 
 	flags := uint16(unix.IFF_TUN | unix.IFF_NO_PI | unix.IFF_MULTI_QUEUE)
+	if t.vnetHdr {
+		flags |= unix.IFF_VNET_HDR
+	}
 	if _, err = tunSetIff(fd, t.Device, flags); err != nil {
 		_ = unix.Close(fd)
 		return err
 	}
 
+	if t.vnetHdr {
+		if err = ioctl(uintptr(fd), unix.TUNSETOFFLOAD, uintptr(tsoOffloadFlags)); err != nil {
+			_ = unix.Close(fd)
+			return fmt.Errorf("failed to enable offload on multiqueue tun fd: %w", err)
+		}
+	}
+
 	err = t.readers.Add(fd)
 	if err != nil {
 		_ = unix.Close(fd)
@@ -450,6 +525,18 @@ func (t *tun) setDefaultRoute(cidr netip.Prefix) error {
 		Table:     unix.RT_TABLE_MAIN,
 		Type:      unix.RTN_UNICAST,
 	}
+	// Match the metric the kernel uses for its auto-installed connected
+	// route, so RouteReplace overwrites it in place instead of adding a
+	// second route at a worse metric. IPv6 connected routes are installed
+	// at metric 256 (IP6_RT_PRIO_KERN); IPv4 uses 0. Without this, the
+	// kernel route wins lookups and our MTU / AdvMSS / Features never
+	// apply on v6.
+	if cidr.Addr().Is6() {
+		nr.Priority = 256
+	}
+	if t.routeFeatureECN {
+		nr.Features |= unix.RTAX_FEATURE_ECN
+	}
 	err := netlink.RouteReplace(&nr)
 	if err != nil {
 		t.l.Warn("Failed to set default route MTU, retrying", "error", err, "cidr", cidr)
@@ -499,6 +586,9 @@ func (t *tun) addRoutes(logErrors bool) error {
 		if r.Metric > 0 {
 			nr.Priority = r.Metric
 		}
+		if t.routeFeatureECN {
+			nr.Features |= unix.RTAX_FEATURE_ECN
+		}
 
 		err := netlink.RouteReplace(&nr)
 		if err != nil {

overlay/tun_netbsd.go

@@ -68,15 +68,15 @@ type tun struct {
 	fd          int
 
 	readBuf  []byte
-	batchRet [1][]byte
+	batchRet [1]tio.Packet
 }
 
-func (t *tun) Read() ([][]byte, error) {
+func (t *tun) Read() ([]tio.Packet, error) {
 	n, err := t.readOne(t.readBuf)
 	if err != nil {
 		return nil, err
 	}
-	t.batchRet[0] = t.readBuf[:n]
+	t.batchRet[0] = tio.Packet{Bytes: t.readBuf[:n]}
 	return t.batchRet[:], nil
 }
 

overlay/tun_openbsd.go

@@ -59,15 +59,15 @@ type tun struct {
 	fd          int
 
 	readBuf  []byte
-	batchRet [1][]byte
+	batchRet [1]tio.Packet
 }
 
-func (t *tun) Read() ([][]byte, error) {
+func (t *tun) Read() ([]tio.Packet, error) {
 	n, err := t.readOne(t.readBuf)
 	if err != nil {
 		return nil, err
 	}
-	t.batchRet[0] = t.readBuf[:n]
+	t.batchRet[0] = tio.Packet{Bytes: t.readBuf[:n]}
 	return t.batchRet[:], nil
 }
 

overlay/tun_tester.go

@@ -30,7 +30,7 @@ type TestTun struct {
 	rxPackets chan []byte // Packets to receive into nebula
 	TxPackets chan []byte // Packets transmitted outside by nebula
 
-	batchRet [1][]byte
+	batchRet [1]tio.Packet
 }
 
 func newTun(c *config.C, l *slog.Logger, vpnNetworks []netip.Prefix, _ bool) (*TestTun, error) {
@@ -51,7 +51,9 @@ func newTun(c *config.C, l *slog.Logger, vpnNetworks []netip.Prefix, _ bool) (*T
 		l:           l,
 		rxPackets:   make(chan []byte, 10),
 		TxPackets:   make(chan []byte, 10),
-		batchRet: [1][]byte{make([]byte, udp.MTU)},
+		batchRet: [1]tio.Packet{
+			tio.Packet{Bytes: make([]byte, udp.MTU)},
+		},
 	}, nil
 }
 
@@ -166,13 +168,13 @@ func (t *TestTun) Close() error {
 	return nil
 }
 
-func (t *TestTun) Read() ([][]byte, error) {
-	t.batchRet[0] = t.batchRet[0][:udp.MTU]
-	n, err := t.read(t.batchRet[0])
+func (t *TestTun) Read() ([]tio.Packet, error) {
+	t.batchRet[0].Bytes = t.batchRet[0].Bytes[:udp.MTU]
+	n, err := t.read(t.batchRet[0].Bytes)
 	if err != nil {
 		return nil, err
 	}
-	t.batchRet[0] = t.batchRet[0][:n]
+	t.batchRet[0].Bytes = t.batchRet[0].Bytes[:n]
 	return t.batchRet[:], nil
 }
 

overlay/tun_windows.go

@@ -47,15 +47,15 @@ type winTun struct {
 	tun *wintun.NativeTun
 
 	readBuf  []byte
-	batchRet [1][]byte
+	batchRet [1]tio.Packet
 }
 
-func (t *winTun) Read() ([][]byte, error) {
+func (t *winTun) Read() ([]tio.Packet, error) {
 	n, err := t.tun.Read(t.readBuf, 0)
 	if err != nil {
 		return nil, err
 	}
-	t.batchRet[0] = t.readBuf[:n]
+	t.batchRet[0] = tio.Packet{Bytes: t.readBuf[:n]}
 	return t.batchRet[:], nil
 }
 

overlay/user.go

@@ -39,10 +39,10 @@ type UserDevice struct {
 	inboundWriter *io.PipeWriter
 
 	readBuf  []byte
-	batchRet [1][]byte
+	batchRet [1]tio.Packet
 }
 
-func (d *UserDevice) Read() ([][]byte, error) {
+func (d *UserDevice) Read() ([]tio.Packet, error) {
 	if d.readBuf == nil {
 		d.readBuf = make([]byte, defaultBatchBufSize)
 	}
@@ -50,7 +50,7 @@ func (d *UserDevice) Read() ([][]byte, error) {
 	if err != nil {
 		return nil, err
 	}
-	d.batchRet[0] = d.readBuf[:n]
+	d.batchRet[0] = tio.Packet{Bytes: d.readBuf[:n]}
 	return d.batchRet[:], nil
 }
 

udp/conn.go

@@ -11,12 +11,25 @@ const MTU = 9001
 // MaxWriteBatch is the largest batch any Conn.WriteBatch implementation is
 // required to accept. Callers SHOULD NOT pass more than this per call; Linux
 // backends preallocate sendmmsg scratch sized to this value, so exceeding it
-// only costs a chunked retry.
+// only costs additional sendmmsg chunks within a single WriteBatch call.
 const MaxWriteBatch = 128
 
+// RxMeta carries per-packet metadata extracted from the RX path (ancillary
+// data, kernel offload state, etc.) and passed to EncReader callbacks.
+// Backends that do not produce a particular signal leave its zero value.
+//
+// OuterECN is the 2-bit IP-level ECN codepoint stamped on the carrier
+// datagram (extracted from IP_TOS / IPV6_TCLASS cmsg on Linux). Zero
+// means Not-ECT, which is also the value backends without ECN RX support
+// supply on every packet.
+type RxMeta struct {
+	OuterECN byte
+}
+
 type EncReader func(
 	addr netip.AddrPort,
 	payload []byte,
+	meta RxMeta,
 )
 
 type Conn interface {
@@ -30,11 +43,14 @@ type Conn interface {
 	ListenOut(r EncReader, flush func()) error
 	WriteTo(b []byte, addr netip.AddrPort) error
 	// WriteBatch sends a contiguous batch of packets, each with its own
-	// destination. bufs and addrs must have the same length. Linux uses
-	// sendmmsg(2) for a single syscall; other backends fall back to a
-	// WriteTo loop. Returns on the first error; callers may observe a
-	// partial send if some packets went out before the error.
-	WriteBatch(bufs [][]byte, addrs []netip.AddrPort) error
+	// destination. bufs and addrs must have the same length. outerECNs may
+	// be nil (treated as all-zero / Not-ECT); when non-nil it must have the
+	// same length as bufs, and outerECNs[i] is the 2-bit IP-level ECN
+	// codepoint to set on packet i's outer header. Linux uses sendmmsg(2)
+	// for a single syscall and attaches the value as IP_TOS / IPV6_TCLASS
+	// cmsg; other backends ignore it. Returns on the first error; callers
+	// may observe a partial send if some packets went out before the error.
+	WriteBatch(bufs [][]byte, addrs []netip.AddrPort, outerECNs []byte) error
 	ReloadConfig(c *config.C)
 	SupportsMultipleReaders() bool
 	Close() error
@@ -57,7 +73,7 @@ func (NoopConn) SupportsMultipleReaders() bool {
 func (NoopConn) WriteTo(_ []byte, _ netip.AddrPort) error {
 	return nil
 }
-func (NoopConn) WriteBatch(_ [][]byte, _ []netip.AddrPort) error {
+func (NoopConn) WriteBatch(_ [][]byte, _ []netip.AddrPort, _ []byte) error {
 	return nil
 }
 func (NoopConn) ReloadConfig(_ *config.C) {

udp/raw_sendmmsg_linux.go

@@ -0,0 +1,62 @@
+//go:build !android && !e2e_testing
+// +build !android,!e2e_testing
+
+package udp
+
+import (
+	"net"
+	"syscall"
+	"unsafe"
+
+	"golang.org/x/sys/unix"
+)
+
+// rawSendmmsg performs sendmmsg(2) over a syscall.RawConn without
+// allocating a closure per call. The struct holds preallocated in/out
+// scratch (chunk/sent/errno) and a method-value bound at construction so
+// rawConn.Write receives a stable function pointer instead of a fresh
+// closure on every send.
+type rawSendmmsg struct {
+	msgs     []rawMessage
+	chunk    int
+	sent     int
+	errno    syscall.Errno
+	callback func(fd uintptr) bool
+}
+
+// bind wires r.callback to r.run. Must be called once after r.msgs is set;
+// subsequent send calls invoke r.callback without rebinding.
+func (r *rawSendmmsg) bind() { r.callback = r.run }
+
+// run is the preallocated callback rawConn.Write invokes. It reads its
+// input (r.chunk) and writes its outputs (r.sent, r.errno) through the
+// rawSendmmsg fields so the method value does not capture per-call locals
+// and therefore does not heap-allocate.
+func (r *rawSendmmsg) run(fd uintptr) bool {
+	r1, _, errno := unix.Syscall6(unix.SYS_SENDMMSG, fd,
+		uintptr(unsafe.Pointer(&r.msgs[0])), uintptr(r.chunk),
+		0, 0, 0,
+	)
+	if errno == syscall.EAGAIN || errno == syscall.EWOULDBLOCK {
+		return false
+	}
+	r.sent = int(r1)
+	r.errno = errno
+	return true
+}
+
+// send issues sendmmsg over rc against the first n entries of r.msgs.
+// Returns the number of entries the kernel processed and any error;
+// matches the original sendmmsg helper's contract.
+func (r *rawSendmmsg) send(rc syscall.RawConn, n int) (int, error) {
+	r.chunk = n
+	r.sent = 0
+	r.errno = 0
+	if err := rc.Write(r.callback); err != nil {
+		return r.sent, err
+	}
+	if r.errno != 0 {
+		return r.sent, &net.OpError{Op: "sendmmsg", Err: r.errno}
+	}
+	return r.sent, nil
+}

udp/rx_reorder_linux.go

@@ -0,0 +1,86 @@
+//go:build !android && !e2e_testing
+// +build !android,!e2e_testing
+
+package udp
+
+import (
+	"cmp"
+	"net/netip"
+	"slices"
+)
+
+// rxSegment is one nebula packet pulled out of a recvmmsg entry — either a
+// lone datagram or one segment of a GRO superpacket. cnt is the big-endian
+// uint64 message counter at bytes [8:16] of the nebula header; 0 if the
+// segment is too short to contain a header. ecn is the 2-bit IP-level ECN
+// codepoint stamped on the carrier (one value per slot, since GRO requires
+// equal ECN across coalesced datagrams).
+type rxSegment struct {
+	src netip.AddrPort
+	cnt uint64
+	buf []byte
+	ecn byte
+}
+
+// rxReorderBuffer accumulates one recvmmsg batch worth of segments,
+// splits any GRO superpackets at gso_size boundaries, stable-sorts by
+// (src, port, counter), then delivers in order. The reorder distance is
+// bounded by len(buf), which the caller sizes to stay well within the
+// receiver's ReplayWindow so older arrivals are not rejected as replays.
+type rxReorderBuffer struct {
+	buf []rxSegment
+}
+
+func newRxReorderBuffer(initialCap int) *rxReorderBuffer {
+	return &rxReorderBuffer{buf: make([]rxSegment, 0, initialCap)}
+}
+
+// reset prepares the buffer for the next recvmmsg batch.
+func (r *rxReorderBuffer) reset() { r.buf = r.buf[:0] }
+
+// addEntry expands one recvmmsg slot into rxSegments. When segSize <= 0 or
+// segSize >= len(payload) the payload is appended as a single segment;
+// otherwise the kernel-coalesced GRO superpacket is split at segSize
+// boundaries (the kernel guarantees every segment is exactly segSize bytes
+// except for the final one, which may be short). ecn applies uniformly to
+// every produced segment because GRO requires equal ECN across coalesced
+// datagrams.
+func (r *rxReorderBuffer) addEntry(from netip.AddrPort, payload []byte, segSize int, ecn byte) {
+	if segSize <= 0 || segSize >= len(payload) {
+		r.buf = append(r.buf, rxSegment{from, headerCounter(payload), payload, ecn})
+		return
+	}
+	for off := 0; off < len(payload); off += segSize {
+		end := off + segSize
+		if end > len(payload) {
+			end = len(payload)
+		}
+		seg := payload[off:end]
+		r.buf = append(r.buf, rxSegment{from, headerCounter(seg), seg, ecn})
+	}
+}
+
+// sortStable orders the accumulated segments by (src addr, src port,
+// counter). Same-source segments are reordered into counter order;
+// cross-source relative order is determined by a stable address compare so
+// the sort is total and predictable.
+func (r *rxReorderBuffer) sortStable() {
+	slices.SortStableFunc(r.buf, func(a, b rxSegment) int {
+		if c := a.src.Addr().Compare(b.src.Addr()); c != 0 {
+			return c
+		}
+		if c := cmp.Compare(a.src.Port(), b.src.Port()); c != 0 {
+			return c
+		}
+		return cmp.Compare(a.cnt, b.cnt)
+	})
+}
+
+// deliver invokes fn once per segment in sorted order, then nils the
+// per-entry buf reference so the next batch's append doesn't alias it.
+func (r *rxReorderBuffer) deliver(fn EncReader) {
+	for k := range r.buf {
+		fn(r.buf[k].src, r.buf[k].buf, RxMeta{OuterECN: r.buf[k].ecn})
+		r.buf[k].buf = nil
+	}
+}

udp/rx_reorder_linux_test.go

@@ -0,0 +1,203 @@
+//go:build !android && !e2e_testing
+// +build !android,!e2e_testing
+
+package udp
+
+import (
+	"encoding/binary"
+	"net/netip"
+	"testing"
+)
+
+// makeNebulaPkt returns a buffer whose [8:16] bytes encode the given
+// counter big-endian, the rest left zero. Anything shorter than 16 bytes
+// would yield counter 0; tests use this to simulate well-formed nebula
+// headers (the rxReorderBuffer doesn't care about anything else).
+func makeNebulaPkt(cnt uint64, payLen int) []byte {
+	if payLen < 16 {
+		payLen = 16
+	}
+	b := make([]byte, payLen)
+	binary.BigEndian.PutUint64(b[8:16], cnt)
+	return b
+}
+
+func srcOf(addr string, port uint16) netip.AddrPort {
+	return netip.AddrPortFrom(netip.MustParseAddr(addr), port)
+}
+
+func TestRxReorderBuffer_LonePassesThrough(t *testing.T) {
+	r := newRxReorderBuffer(8)
+	pkt := makeNebulaPkt(42, 100)
+	r.addEntry(srcOf("1.1.1.1", 4242), pkt, 0, 0x02)
+
+	if got := len(r.buf); got != 1 {
+		t.Fatalf("want 1 entry, got %d", got)
+	}
+	if r.buf[0].cnt != 42 {
+		t.Errorf("counter=%d want 42", r.buf[0].cnt)
+	}
+	if r.buf[0].ecn != 0x02 {
+		t.Errorf("ecn=%#x want 0x02", r.buf[0].ecn)
+	}
+	if len(r.buf[0].buf) != 100 {
+		t.Errorf("buf len=%d want 100", len(r.buf[0].buf))
+	}
+}
+
+func TestRxReorderBuffer_SegSizeGEPayloadIsLone(t *testing.T) {
+	// segSize >= len(payload) means the kernel did not coalesce this slot.
+	r := newRxReorderBuffer(8)
+	pkt := makeNebulaPkt(7, 50)
+	r.addEntry(srcOf("1.1.1.1", 1), pkt, 50, 0)
+	if got := len(r.buf); got != 1 {
+		t.Fatalf("segSize==len: want 1 entry, got %d", got)
+	}
+	r.reset()
+	r.addEntry(srcOf("1.1.1.1", 1), pkt, 60, 0)
+	if got := len(r.buf); got != 1 {
+		t.Fatalf("segSize>len: want 1 entry, got %d", got)
+	}
+}
+
+func TestRxReorderBuffer_GROSplitExactMultiple(t *testing.T) {
+	// 3 segments of 80 bytes each, packed into one 240-byte GRO superpacket.
+	const segSize = 80
+	const numSeg = 3
+	pkt := make([]byte, segSize*numSeg)
+	for i := range numSeg {
+		off := i * segSize
+		binary.BigEndian.PutUint64(pkt[off+8:off+16], uint64(100+i))
+	}
+
+	r := newRxReorderBuffer(8)
+	r.addEntry(srcOf("2.2.2.2", 5555), pkt, segSize, 0x03)
+	if got := len(r.buf); got != numSeg {
+		t.Fatalf("want %d segments, got %d", numSeg, got)
+	}
+	for i, seg := range r.buf {
+		if seg.cnt != uint64(100+i) {
+			t.Errorf("seg %d: cnt=%d want %d", i, seg.cnt, 100+i)
+		}
+		if len(seg.buf) != segSize {
+			t.Errorf("seg %d: buf len=%d want %d", i, len(seg.buf), segSize)
+		}
+		if seg.ecn != 0x03 {
+			t.Errorf("seg %d: ecn=%#x want 0x03 (uniform across GRO)", i, seg.ecn)
+		}
+	}
+}
+
+func TestRxReorderBuffer_GROSplitShortFinal(t *testing.T) {
+	// 200-byte payload, segSize=80 → segments of 80, 80, 40.
+	const segSize = 80
+	pkt := make([]byte, 200)
+	binary.BigEndian.PutUint64(pkt[8:16], 1)
+	binary.BigEndian.PutUint64(pkt[80+8:80+16], 2)
+	binary.BigEndian.PutUint64(pkt[160+8:160+16], 3)
+
+	r := newRxReorderBuffer(8)
+	r.addEntry(srcOf("3.3.3.3", 1), pkt, segSize, 0)
+	if got := len(r.buf); got != 3 {
+		t.Fatalf("want 3 segments, got %d", got)
+	}
+	wantLens := []int{80, 80, 40}
+	for i, seg := range r.buf {
+		if len(seg.buf) != wantLens[i] {
+			t.Errorf("seg %d: len=%d want %d", i, len(seg.buf), wantLens[i])
+		}
+	}
+}
+
+func TestRxReorderBuffer_SortGroupsBySrcThenCounter(t *testing.T) {
+	r := newRxReorderBuffer(8)
+	a := srcOf("1.1.1.1", 1)
+	b := srcOf("2.2.2.2", 1)
+	// Insert deliberately scrambled.
+	r.addEntry(a, makeNebulaPkt(3, 16), 0, 0)
+	r.addEntry(b, makeNebulaPkt(1, 16), 0, 0)
+	r.addEntry(a, makeNebulaPkt(1, 16), 0, 0)
+	r.addEntry(b, makeNebulaPkt(2, 16), 0, 0)
+	r.addEntry(a, makeNebulaPkt(2, 16), 0, 0)
+
+	r.sortStable()
+
+	want := []struct {
+		src netip.AddrPort
+		cnt uint64
+	}{
+		{a, 1}, {a, 2}, {a, 3}, {b, 1}, {b, 2},
+	}
+	if got := len(r.buf); got != len(want) {
+		t.Fatalf("len=%d want %d", got, len(want))
+	}
+	for i, w := range want {
+		if r.buf[i].src != w.src || r.buf[i].cnt != w.cnt {
+			t.Errorf("idx %d: got %v/%d want %v/%d",
+				i, r.buf[i].src, r.buf[i].cnt, w.src, w.cnt)
+		}
+	}
+}
+
+func TestRxReorderBuffer_SortStableAcrossPorts(t *testing.T) {
+	// Same source addr but different ports — must group by port.
+	r := newRxReorderBuffer(8)
+	addr := netip.MustParseAddr("4.4.4.4")
+	p1 := netip.AddrPortFrom(addr, 1)
+	p2 := netip.AddrPortFrom(addr, 2)
+	r.addEntry(p2, makeNebulaPkt(10, 16), 0, 0)
+	r.addEntry(p1, makeNebulaPkt(20, 16), 0, 0)
+	r.addEntry(p2, makeNebulaPkt(5, 16), 0, 0)
+
+	r.sortStable()
+
+	// Expect: p1/20 then p2/5 then p2/10.
+	if r.buf[0].src.Port() != 1 || r.buf[1].src.Port() != 2 || r.buf[2].src.Port() != 2 {
+		t.Fatalf("port order broken: %v %v %v",
+			r.buf[0].src.Port(), r.buf[1].src.Port(), r.buf[2].src.Port())
+	}
+	if r.buf[1].cnt != 5 || r.buf[2].cnt != 10 {
+		t.Errorf("counter order in p2: %d %d (want 5 10)", r.buf[1].cnt, r.buf[2].cnt)
+	}
+}
+
+func TestRxReorderBuffer_DeliverInOrderAndNilsRefs(t *testing.T) {
+	r := newRxReorderBuffer(4)
+	a := srcOf("5.5.5.5", 1)
+	r.addEntry(a, makeNebulaPkt(2, 32), 0, 0x01)
+	r.addEntry(a, makeNebulaPkt(1, 32), 0, 0x01)
+	r.sortStable()
+
+	var seenCnts []uint64
+	var seenECN []byte
+	r.deliver(func(src netip.AddrPort, buf []byte, meta RxMeta) {
+		seenCnts = append(seenCnts, binary.BigEndian.Uint64(buf[8:16]))
+		seenECN = append(seenECN, meta.OuterECN)
+	})
+
+	if len(seenCnts) != 2 || seenCnts[0] != 1 || seenCnts[1] != 2 {
+		t.Errorf("delivery order broken: %v", seenCnts)
+	}
+	if seenECN[0] != 0x01 || seenECN[1] != 0x01 {
+		t.Errorf("ecn passed wrong: %v", seenECN)
+	}
+	for i := range r.buf {
+		if r.buf[i].buf != nil {
+			t.Errorf("buf[%d].buf not nil after deliver", i)
+		}
+	}
+}
+
+func TestRxReorderBuffer_ResetIsReusable(t *testing.T) {
+	r := newRxReorderBuffer(2)
+	r.addEntry(srcOf("6.6.6.6", 1), makeNebulaPkt(1, 16), 0, 0)
+	r.addEntry(srcOf("6.6.6.6", 1), makeNebulaPkt(2, 16), 0, 0)
+	r.reset()
+	if got := len(r.buf); got != 0 {
+		t.Fatalf("after reset len=%d want 0", got)
+	}
+	r.addEntry(srcOf("6.6.6.6", 1), makeNebulaPkt(7, 16), 0, 0)
+	if r.buf[0].cnt != 7 {
+		t.Errorf("after reset+add: cnt=%d want 7", r.buf[0].cnt)
+	}
+}

udp/udp_darwin.go

@@ -140,7 +140,7 @@ func (u *StdConn) WriteTo(b []byte, ap netip.AddrPort) error {
 	}
 }
 
-func (u *StdConn) WriteBatch(bufs [][]byte, addrs []netip.AddrPort) error {
+func (u *StdConn) WriteBatch(bufs [][]byte, addrs []netip.AddrPort, _ []byte) error {
 	for i, b := range bufs {
 		if err := u.WriteTo(b, addrs[i]); err != nil {
 			return err
@@ -188,7 +188,7 @@ func (u *StdConn) ListenOut(r EncReader, flush func()) error {
 			continue
 		}
 
-		r(netip.AddrPortFrom(rua.Addr().Unmap(), rua.Port()), buffer[:n])
+		r(netip.AddrPortFrom(rua.Addr().Unmap(), rua.Port()), buffer[:n], RxMeta{})
 		flush()
 	}
 }

udp/udp_ecn_outer_linux_test.go

@@ -0,0 +1,61 @@
+//go:build linux && !android && !e2e_testing
+
+package udp
+
+import (
+	"net/netip"
+	"testing"
+)
+
+// TestPlanRunBreaksOnECNChange confirms that two same-destination, same-size
+// packets with different outer ECN end up in separate sendmmsg entries (the
+// kernel stamps one outer codepoint per entry, so a run that straddled the
+// boundary would silently lose information).
+func TestPlanRunBreaksOnECNChange(t *testing.T) {
+	u := &StdConn{gsoSupported: true}
+	dst := netip.MustParseAddrPort("10.0.0.1:4242")
+
+	bufs := [][]byte{
+		make([]byte, 1200),
+		make([]byte, 1200),
+		make([]byte, 1200),
+	}
+	addrs := []netip.AddrPort{dst, dst, dst}
+
+	t.Run("uniform_ecn_runs_together", func(t *testing.T) {
+		ecns := []byte{0x02, 0x02, 0x02}
+		runLen, segSize := u.planRun(bufs, addrs, ecns, 0, 64)
+		if runLen != 3 {
+			t.Errorf("runLen=%d want 3 (uniform ECT(0))", runLen)
+		}
+		if segSize != 1200 {
+			t.Errorf("segSize=%d want 1200", segSize)
+		}
+	})
+
+	t.Run("ecn_change_truncates_run", func(t *testing.T) {
+		// 0,0,3: first two run together, CE seeds a fresh entry.
+		ecns := []byte{0x00, 0x00, 0x03}
+		runLen, _ := u.planRun(bufs, addrs, ecns, 0, 64)
+		if runLen != 2 {
+			t.Errorf("runLen=%d want 2 (ECN changes at index 2)", runLen)
+		}
+	})
+
+	t.Run("nil_ecns_runs_full", func(t *testing.T) {
+		runLen, _ := u.planRun(bufs, addrs, nil, 0, 64)
+		if runLen != 3 {
+			t.Errorf("runLen=%d want 3 (nil ecns means no break)", runLen)
+		}
+	})
+
+	t.Run("first_ecn_is_singleton", func(t *testing.T) {
+		// Second packet has different ECN from the first → run halts at 1
+		// (the first packet alone forms the run).
+		ecns := []byte{0x00, 0x03, 0x03}
+		runLen, _ := u.planRun(bufs, addrs, ecns, 0, 64)
+		if runLen != 1 {
+			t.Errorf("runLen=%d want 1 (different ECN immediately)", runLen)
+		}
+	})
+}

udp/udp_generic.go

@@ -44,7 +44,7 @@ func (u *GenericConn) WriteTo(b []byte, addr netip.AddrPort) error {
 	return err
 }
 
-func (u *GenericConn) WriteBatch(bufs [][]byte, addrs []netip.AddrPort) error {
+func (u *GenericConn) WriteBatch(bufs [][]byte, addrs []netip.AddrPort, _ []byte) error {
 	for i, b := range bufs {
 		if _, err := u.UDPConn.WriteToUDPAddrPort(b, addrs[i]); err != nil {
 			return err
@@ -102,7 +102,7 @@ func (u *GenericConn) ListenOut(r EncReader, flush func()) error {
 			continue
 		}
 
-		r(netip.AddrPortFrom(rua.Addr().Unmap(), rua.Port()), buffer[:n])
+		r(netip.AddrPortFrom(rua.Addr().Unmap(), rua.Port()), buffer[:n], RxMeta{})
 		flush()
 	}
 }

udp/udp_linux.go

@@ -24,6 +24,58 @@ type StdConn struct {
 	isV4    bool
 	l       *slog.Logger
 	batch   int
+
+	// sendmmsg scratch. Each queue has its own StdConn, so no locking is
+	// needed. Sized to MaxWriteBatch at construction; WriteBatch chunks
+	// larger inputs.
+	writeMsgs  []rawMessage
+	writeIovs  []iovec
+	writeNames [][]byte
+
+	// Per-entry cmsg scratch. writeCmsg is one contiguous slab of
+	// MaxWriteBatch * writeCmsgSpace bytes; each entry holds two cmsg
+	// headers (UDP_SEGMENT then IP_TOS / IPV6_TCLASS) pre-filled once in
+	// prepareWriteMessages. WriteBatch only rewrites the per-call data
+	// payloads and toggles Hdr.Control / Hdr.Controllen to point at
+	// whichever subset of the two cmsgs applies.
+	writeCmsg         []byte
+	writeCmsgSpace    int
+	writeCmsgSegSpace int
+	writeCmsgEcnSpace int
+
+	// writeEntryEnd[e] is the bufs index *after* the last packet packed
+	// into mmsghdr entry e. Used to rewind `i` on partial sendmmsg success.
+	writeEntryEnd []int
+
+	// rawSend wraps the sendmmsg(2) callback in a closure-free helper so
+	// the hot path doesn't heap-allocate a fresh closure per call.
+	rawSend rawSendmmsg
+
+	// UDP GSO (sendmsg with UDP_SEGMENT cmsg) support. gsoSupported is
+	// probed once at socket creation. When true, WriteBatch packs same-
+	// destination consecutive packets into a single sendmmsg entry with a
+	// UDP_SEGMENT cmsg; otherwise each packet is its own entry.
+	gsoSupported bool
+
+	// UDP GRO (recvmsg with UDP_GRO cmsg) support. groSupported is probed
+	// once at socket creation. When true, listenOutBatch allocates larger
+	// RX buffers and a per-entry cmsg slot so the kernel can coalesce
+	// consecutive same-flow datagrams into a single recvmmsg entry; the
+	// delivered cmsg carries the gso_size used to split them back apart.
+	groSupported bool
+
+	// ecnRecvSupported is true when IP_RECVTOS / IPV6_RECVTCLASS was
+	// successfully enabled — the kernel will deliver the outer IP-ECN of
+	// each arriving datagram as a per-slot cmsg, and listenOutBatch passes
+	// the parsed value to the EncReader callback for RFC 6040 combine.
+	ecnRecvSupported bool
+
+	// rxOrder is the per-batch scratch listenOutBatch uses to gather every
+	// segment in a recvmmsg call (after splitting GRO superpackets) and
+	// stable-sort by (source, message-counter) before delivery. Reordering
+	// fits within the receiver's replay window so briefly out-of-order
+	// arrivals do not get rejected as replays.
+	rxOrder *rxReorderBuffer
 }
 
 func setReusePort(network, address string, c syscall.RawConn) error {
@@ -70,9 +122,196 @@ func NewListener(l *slog.Logger, ip netip.Addr, port int, multi bool, batch int)
 	}
 	out.isV4 = af == unix.AF_INET
 
+	out.prepareWriteMessages(MaxWriteBatch)
+	out.rawSend.msgs = out.writeMsgs
+	out.rawSend.bind()
+
+	out.prepareGSO()
+	// GRO delivers coalesced superpackets that need a cmsg to split back
+	// into segments. The single-packet RX path uses ReadFromUDPAddrPort
+	// and cannot see that cmsg, so only enable GRO for the batch path.
+	if batch > 1 {
+		out.prepareGRO()
+	}
+	// Best-effort: ask the kernel to deliver outer IP-ECN as ancillary data
+	// on every recvmmsg slot so the decap side can apply RFC 6040 combine.
+	// On older kernels these may not exist; failing here just means we get
+	// 0 (Not-ECT) on every slot, which is the same as ecn_mode=disable.
+	out.prepareECNRecv()
+
 	return out, nil
 }
 
+// prepareWriteMessages allocates one mmsghdr/iovec/sockaddr/cmsg scratch
+// slot per sendmmsg entry. The iovec slab is sized to n so all entries'
+// iovecs share one allocation; per-entry fan-out is further capped at
+// maxGSOSegments. Hdr.Iov / Hdr.Iovlen / Hdr.Control / Hdr.Controllen are
+// wired per call since each entry can span a variable number of iovecs
+// and may or may not carry a cmsg.
+//
+// Per-mmsghdr cmsg layout. Each entry's slot of length writeCmsgSpace holds
+// up to two cmsg headers placed at fixed offsets:
+//
+//	[0 .. writeCmsgSegSpace)              UDP_SEGMENT (gso_size, uint16)
+//	[writeCmsgSegSpace .. writeCmsgSpace) IP_TOS or IPV6_TCLASS (int32)
+//
+// Both headers are pre-filled once here; per-call we only rewrite the data
+// payload and toggle Hdr.Control / Hdr.Controllen to point at whichever
+// subset applies (none / segment-only / ecn-only / both).
+func (u *StdConn) prepareWriteMessages(n int) {
+	u.writeMsgs = make([]rawMessage, n)
+	u.writeIovs = make([]iovec, n)
+	u.writeNames = make([][]byte, n)
+	u.writeEntryEnd = make([]int, n)
+
+	u.writeCmsgSegSpace = unix.CmsgSpace(2)
+	u.writeCmsgEcnSpace = unix.CmsgSpace(4)
+	u.writeCmsgSpace = u.writeCmsgSegSpace + u.writeCmsgEcnSpace
+	u.writeCmsg = make([]byte, n*u.writeCmsgSpace)
+
+	ecnLevel := int32(unix.IPPROTO_IP)
+	ecnType := int32(unix.IP_TOS)
+	if !u.isV4 {
+		ecnLevel = unix.IPPROTO_IPV6
+		ecnType = unix.IPV6_TCLASS
+	}
+
+	for k := 0; k < n; k++ {
+		base := k * u.writeCmsgSpace
+		seg := (*unix.Cmsghdr)(unsafe.Pointer(&u.writeCmsg[base]))
+		seg.Level = unix.SOL_UDP
+		seg.Type = unix.UDP_SEGMENT
+		setCmsgLen(seg, unix.CmsgLen(2))
+
+		ecn := (*unix.Cmsghdr)(unsafe.Pointer(&u.writeCmsg[base+u.writeCmsgSegSpace]))
+		ecn.Level = ecnLevel
+		ecn.Type = ecnType
+		setCmsgLen(ecn, unix.CmsgLen(4))
+	}
+
+	for i := range u.writeMsgs {
+		u.writeNames[i] = make([]byte, unix.SizeofSockaddrInet6)
+		u.writeMsgs[i].Hdr.Name = &u.writeNames[i][0]
+	}
+}
+
+// maxGSOSegments caps the per-sendmsg GSO fan-out. Linux kernels have
+// historically capped UDP_MAX_SEGMENTS at 64; newer kernels raise it to 128.
+// We stay one below 64 because the kernel's check is
+//
+//	if (cork->length > cork->gso_size * UDP_MAX_SEGMENTS) return -EINVAL;
+//
+// and cork->length includes the 8-byte UDP header (udp_sendmsg passes
+// ulen = len + sizeof(udphdr) to ip_append_data). Packing exactly 64
+// same-size segments puts cork->length at gso_size*64 + 8, which is one
+// UDP-header over the bound and the kernel rejects the whole sendmmsg
+// with EINVAL. 63 leaves room for the header for any segSize >= 8.
+const maxGSOSegments = 63
+
+// maxGSOBytes bounds the total payload per sendmsg() when UDP_SEGMENT is
+// set. The kernel stitches all iovecs into a single skb whose length the
+// UDP length field can represent, and also enforces sk_gso_max_size (which
+// on most devices is 65536). We use 65000 to leave headroom under the
+// 65535 UDP-length cap, avoiding EMSGSIZE on large TSO superpackets.
+const maxGSOBytes = 65000
+
+// prepareGSO probes UDP_SEGMENT support and sets u.gsoSupported on success.
+// Best-effort; failure leaves it false.
+func (u *StdConn) prepareGSO() {
+	var probeErr error
+	if err := u.rawConn.Control(func(fd uintptr) {
+		probeErr = unix.SetsockoptInt(int(fd), unix.IPPROTO_UDP, unix.UDP_SEGMENT, 0)
+	}); err != nil {
+		u.l.Info("udp: GSO disabled", "reason", "rawconn control failed", "error", err)
+		recordCapability("udp.gso.enabled", false)
+		return
+	}
+	if probeErr != nil {
+		u.l.Info("udp: GSO disabled", "reason", "kernel rejected probe", "error", probeErr)
+		recordCapability("udp.gso.enabled", false)
+		return
+	}
+	u.gsoSupported = true
+	u.l.Info("udp: GSO enabled")
+	recordCapability("udp.gso.enabled", true)
+}
+
+// udpGROBufferSize sizes the per-entry recvmmsg buffer when UDP_GRO is on.
+// The kernel stitches a run of same-flow datagrams into a single skb whose
+// length is bounded by sk_gso_max_size (typically 65535); anything larger
+// would be MSG_TRUNCed. We use the maximum representable UDP length so a
+// full superpacket always lands intact.
+const udpGROBufferSize = 65535
+
+// udpGROCmsgPayload is the size of the UDP_GRO cmsg data delivered by the
+// kernel: a single int (gso_size in bytes). See udp_cmsg_recv() in
+// net/ipv4/udp.c.
+const udpGROCmsgPayload = 4
+
+// prepareGRO turns on UDP_GRO so the kernel coalesces consecutive same-flow
+// datagrams into one recvmmsg entry, with a cmsg carrying the gso_size used
+// to split them back apart on the application side.
+func (u *StdConn) prepareGRO() {
+	var probeErr error
+	if err := u.rawConn.Control(func(fd uintptr) {
+		probeErr = unix.SetsockoptInt(int(fd), unix.IPPROTO_UDP, unix.UDP_GRO, 1)
+	}); err != nil {
+		u.l.Info("udp: GRO disabled", "reason", "rawconn control failed", "error", err)
+		recordCapability("udp.gro.enabled", false)
+		return
+	}
+	if probeErr != nil {
+		u.l.Info("udp: GRO disabled", "reason", "kernel rejected probe", "error", probeErr)
+		recordCapability("udp.gro.enabled", false)
+		return
+	}
+	u.groSupported = true
+	u.l.Info("udp: GRO enabled")
+	recordCapability("udp.gro.enabled", true)
+}
+
+// prepareECNRecv turns on IP_RECVTOS / IPV6_RECVTCLASS so the outer IP-ECN
+// field of each arriving datagram is delivered as ancillary data alongside
+// the payload. listenOutBatch reads it via parseRecvCmsg and passes the
+// codepoint through the EncReader for RFC 6040 combine on the decap side.
+// Best-effort: we keep going on failure.
+func (u *StdConn) prepareECNRecv() {
+	var probeErr error
+	if err := u.rawConn.Control(func(fd uintptr) {
+		if u.isV4 {
+			probeErr = unix.SetsockoptInt(int(fd), unix.IPPROTO_IP, unix.IP_RECVTOS, 1)
+		} else {
+			probeErr = unix.SetsockoptInt(int(fd), unix.IPPROTO_IPV6, unix.IPV6_RECVTCLASS, 1)
+		}
+	}); err != nil {
+		u.l.Info("udp: outer-ECN RX disabled", "reason", "rawconn control failed", "error", err)
+		recordCapability("udp.ecn_rx.enabled", false)
+		return
+	}
+	if probeErr != nil {
+		u.l.Info("udp: outer-ECN RX disabled", "reason", "kernel rejected probe", "error", probeErr)
+		recordCapability("udp.ecn_rx.enabled", false)
+		return
+	}
+	u.ecnRecvSupported = true
+	u.l.Info("udp: outer-ECN RX enabled")
+	recordCapability("udp.ecn_rx.enabled", true)
+}
+
+// recordCapability registers (or updates) a boolean gauge for one of the
+// kernel-feature probes. Gauges go to 1 when the feature is enabled, 0 when
+// it is not — dashboards can show degraded state on partially-supported
+// kernels at a glance. Calling repeatedly with the same name updates the
+// existing gauge rather than registering a duplicate.
+func recordCapability(name string, enabled bool) {
+	g := metrics.GetOrRegisterGauge(name, nil)
+	if enabled {
+		g.Update(1)
+	} else {
+		g.Update(0)
+	}
+}
+
 func (u *StdConn) SupportsMultipleReaders() bool {
 	return true
 }
@@ -183,7 +422,10 @@ func (u *StdConn) listenOutSingle(r EncReader, flush func()) error {
 			return err
 		}
 		from = netip.AddrPortFrom(from.Addr().Unmap(), from.Port())
-		r(from, buffer[:n])
+		// listenOutSingle uses ReadFromUDPAddrPort which discards cmsgs,
+		// so the outer ECN field is not visible on this path. Zero RxMeta
+		// (Not-ECT) means RFC 6040 combine is a no-op.
+		r(from, buffer[:n], RxMeta{})
 		flush()
 	}
 }
@@ -194,7 +436,22 @@ func (u *StdConn) listenOutBatch(r EncReader, flush func()) error {
 	var operr error
 
 	bufSize := MTU
-	msgs, buffers, names := u.PrepareRawMessages(u.batch, bufSize)
+	cmsgSpace := 0
+	if u.groSupported {
+		bufSize = udpGROBufferSize
+		cmsgSpace = unix.CmsgSpace(udpGROCmsgPayload)
+	}
+	if u.ecnRecvSupported {
+		// IP_TOS arrives as 1 byte; IPV6_TCLASS arrives as a 4-byte int.
+		// Reserve enough for the wider of the two so the same buffer fits
+		// either family alongside any UDP_GRO cmsg.
+		cmsgSpace += unix.CmsgSpace(4)
+	}
+	msgs, buffers, names, _ := u.PrepareRawMessages(u.batch, bufSize, cmsgSpace)
+
+	if u.rxOrder == nil {
+		u.rxOrder = newRxReorderBuffer(u.batch * 64)
+	}
 
 	//reader needs to capture variables from this function, since it's used as a lambda with rawConn.Read
 	//defining it outside the loop so it gets re-used
@@ -204,6 +461,11 @@ func (u *StdConn) listenOutBatch(r EncReader, flush func()) error {
 	}
 
 	for {
+		if cmsgSpace > 0 {
+			for i := range msgs {
+				setMsgControllen(&msgs[i].Hdr, cmsgSpace)
+			}
+		}
 		err := u.rawConn.Read(reader)
 		if err != nil {
 			return err
@@ -212,6 +474,9 @@ func (u *StdConn) listenOutBatch(r EncReader, flush func()) error {
 			return operr
 		}
 
+		// Phase 1: gather every segment from this recvmmsg into rxOrder,
+		// splitting GRO superpackets into their constituent segments.
+		u.rxOrder.reset()
 		for i := 0; i < n; i++ {
 			// Its ok to skip the ok check here, the slicing is the only error that can occur and it will panic
 			if u.isV4 {
@@ -222,14 +487,77 @@ func (u *StdConn) listenOutBatch(r EncReader, flush func()) error {
 			from := netip.AddrPortFrom(ip.Unmap(), binary.BigEndian.Uint16(names[i][2:4]))
 			payload := buffers[i][:msgs[i].Len]
 
-			r(from, payload)
+			segSize := 0
+			outerECN := byte(0)
+			if cmsgSpace > 0 {
+				segSize, outerECN = parseRecvCmsg(&msgs[i].Hdr, u.groSupported, u.ecnRecvSupported, u.isV4)
+			}
+			u.rxOrder.addEntry(from, payload, segSize, outerECN)
 		}
+
+		// Phase 2 + 3: stable-sort by (src, port, counter), then deliver in
+		// order. Reorder distance is bounded by len(u.rxOrder.buf), which
+		// stays well within the receiver's ReplayWindow (currently 8192) so
+		// older arrivals are not rejected as replays.
+		u.rxOrder.sortStable()
+		u.rxOrder.deliver(r)
 		// End-of-batch: let callers (e.g. TUN write coalescer) flush any
 		// state they accumulated across this batch.
 		flush()
 	}
 }
 
+// headerCounter returns the big-endian uint64 message counter at bytes
+// [8:16] of a nebula packet, or 0 if the buffer is too short.
+func headerCounter(buf []byte) uint64 {
+	if len(buf) < 16 {
+		return 0
+	}
+	return binary.BigEndian.Uint64(buf[8:16])
+}
+
+// parseRecvCmsg walks the per-slot ancillary buffer once and extracts up to
+// two values of interest in a single pass: the UDP_GRO gso_size (when
+// wantGRO is true) and the outer IP-level ECN codepoint stamped on the
+// carrier (when wantECN is true). Returns zeros for whichever field is not
+// requested or not present. isV4 selects between IP_TOS (1-byte) and
+// IPV6_TCLASS (4-byte int) cmsg payloads.
+func parseRecvCmsg(hdr *msghdr, wantGRO, wantECN bool, isV4 bool) (gso int, ecn byte) {
+	controllen := int(hdr.Controllen)
+	if controllen < unix.SizeofCmsghdr || hdr.Control == nil {
+		return 0, 0
+	}
+	ctrl := unsafe.Slice(hdr.Control, controllen)
+	off := 0
+	for off+unix.SizeofCmsghdr <= len(ctrl) {
+		ch := (*unix.Cmsghdr)(unsafe.Pointer(&ctrl[off]))
+		clen := int(ch.Len)
+		if clen < unix.SizeofCmsghdr || off+clen > len(ctrl) {
+			return gso, ecn
+		}
+		dataOff := off + unix.CmsgLen(0)
+		switch {
+		case wantGRO && ch.Level == unix.SOL_UDP && ch.Type == unix.UDP_GRO:
+			if dataOff+udpGROCmsgPayload <= len(ctrl) {
+				gso = int(int32(binary.NativeEndian.Uint32(ctrl[dataOff : dataOff+udpGROCmsgPayload])))
+			}
+		case wantECN && isV4 && ch.Level == unix.IPPROTO_IP && ch.Type == unix.IP_TOS:
+			// IP_TOS arrives as a single byte; only the low 2 bits are ECN.
+			if dataOff+1 <= len(ctrl) {
+				ecn = ctrl[dataOff] & 0x03
+			}
+		case wantECN && !isV4 && ch.Level == unix.IPPROTO_IPV6 && ch.Type == unix.IPV6_TCLASS:
+			// IPV6_TCLASS arrives as a 4-byte int; ECN is the low 2 bits.
+			if dataOff+4 <= len(ctrl) {
+				ecn = byte(binary.NativeEndian.Uint32(ctrl[dataOff:dataOff+4])) & 0x03
+			}
+		}
+		// Advance by the aligned cmsg space.
+		off += unix.CmsgSpace(clen - unix.CmsgLen(0))
+	}
+	return gso, ecn
+}
+
 func (u *StdConn) ListenOut(r EncReader, flush func()) error {
 	if u.batch == 1 {
 		return u.listenOutSingle(r, flush)
@@ -243,19 +571,255 @@ func (u *StdConn) WriteTo(b []byte, ip netip.AddrPort) error {
 	return err
 }
 
-func (u *StdConn) WriteBatch(bufs [][]byte, addrs []netip.AddrPort) error {
+// WriteBatch sends bufs via sendmmsg(2) using the preallocated scratch on
+// StdConn. Consecutive packets to the same destination with matching segment
+// sizes (all but possibly the last) are coalesced into a single mmsghdr entry
+// carrying a UDP_SEGMENT cmsg, so one syscall can mix runs of GSO superpackets
+// with plain one-off datagrams. Without GSO support every packet is its own
+// entry, matching the prior behaviour.
+//
+// Chunks larger than the scratch are processed across multiple syscalls. If
+// sendmmsg returns an error AND zero entries went out we fall back to
+// per-packet WriteTo for that chunk so the caller still gets best-effort
+// delivery; on a partial-success error we just replay the remainder.
+func (u *StdConn) WriteBatch(bufs [][]byte, addrs []netip.AddrPort, ecns []byte) error {
 	if len(bufs) != len(addrs) {
 		return fmt.Errorf("WriteBatch: len(bufs)=%d != len(addrs)=%d", len(bufs), len(addrs))
 	}
-	//todo use sendmmsg
-	for i := 0; i < len(bufs); i++ {
-		if _, err := u.udpConn.WriteToUDPAddrPort(bufs[i], addrs[i]); err != nil {
-			return err
+	if ecns != nil && len(ecns) != len(bufs) {
+		return fmt.Errorf("WriteBatch: len(ecns)=%d != len(bufs)=%d", len(ecns), len(bufs))
+	}
+
+	// Callers deliver same-destination packets contiguously and in counter
+	// order, so we run the GSO planner directly without a pre-sort. A
+	// sorting pass measurably hurt throughput in microbenchmarks while
+	// providing no observed reordering benefit.
+
+	i := 0
+	for i < len(bufs) {
+		baseI := i
+		entry := 0
+		iovIdx := 0
+		for entry < len(u.writeMsgs) && i < len(bufs) {
+			iovBudget := len(u.writeIovs) - iovIdx
+			if iovBudget < 1 {
+				break
+			}
+			runLen, segSize := u.planRun(bufs, addrs, ecns, i, iovBudget)
+			if runLen == 0 {
+				break
+			}
+
+			for k := 0; k < runLen; k++ {
+				b := bufs[i+k]
+				if len(b) == 0 {
+					u.writeIovs[iovIdx+k].Base = nil
+					setIovLen(&u.writeIovs[iovIdx+k], 0)
+				} else {
+					u.writeIovs[iovIdx+k].Base = &b[0]
+					setIovLen(&u.writeIovs[iovIdx+k], len(b))
+				}
+			}
+
+			nlen, err := writeSockaddr(u.writeNames[entry], addrs[i], u.isV4)
+			if err != nil {
+				return err
+			}
+
+			hdr := &u.writeMsgs[entry].Hdr
+			hdr.Iov = &u.writeIovs[iovIdx]
+			setMsgIovlen(hdr, runLen)
+			hdr.Namelen = uint32(nlen)
+
+			var ecn byte
+			if ecns != nil {
+				ecn = ecns[i]
+			}
+			u.writeEntryCmsg(entry, runLen, segSize, ecn)
+
+			i += runLen
+			iovIdx += runLen
+			u.writeEntryEnd[entry] = i
+			entry++
+		}
+
+		if entry == 0 {
+			return fmt.Errorf("sendmmsg: no progress")
+		}
+
+		sent, serr := u.sendmmsg(entry)
+		if serr != nil && sent <= 0 {
+			// Nothing went out for this chunk; fall back to WriteTo for each
+			// packet that was queued this iteration. We only enter this path
+			// when sendmmsg returned an error AND zero entries succeeded —
+			// otherwise the partial-success advance below replays only the
+			// remainder, avoiding duplicates of already-sent packets.
+			//
+			// sent=-1 from sendmmsg means message 0 itself failed (partial
+			// success returns the count instead), so log entry 0's parameters
+			// — that's the entry the kernel rejected.
+			hdr0 := &u.writeMsgs[0].Hdr
+			runLen0 := u.writeEntryEnd[0] - baseI
+			seg0 := len(bufs[baseI])
+			ecn0 := byte(0)
+			if ecns != nil {
+				ecn0 = ecns[baseI]
+			}
+			u.l.Warn("sendmmsg had problem",
+				"sent", sent, "err", serr,
+				"entries", entry,
+				"entry0_runLen", runLen0,
+				"entry0_segSize", seg0,
+				"entry0_iovlen", hdr0.Iovlen,
+				"entry0_controllen", hdr0.Controllen,
+				"entry0_namelen", hdr0.Namelen,
+				"entry0_ecn", ecn0,
+				"entry0_dst", addrs[baseI],
+				"isV4", u.isV4,
+				"gso", u.gsoSupported,
+				"gro", u.groSupported,
+			)
+			for k := baseI; k < i; k++ {
+				if werr := u.WriteTo(bufs[k], addrs[k]); werr != nil {
+					return werr
+				}
+			}
+			continue
 		}
+		if sent == 0 {
+			return fmt.Errorf("sendmmsg made no progress")
+		}
+		// Rewind i to the end of the last successfully sent entry. For a
+		// full-success send this leaves i unchanged; for a partial send it
+		// replays the remainder on the next outer-loop iteration.
+		i = u.writeEntryEnd[sent-1]
 	}
 	return nil
 }
 
+// planRun groups consecutive packets starting at `start` that can be sent as
+// a single UDP GSO superpacket (one sendmmsg entry with UDP_SEGMENT cmsg).
+// A run of length 1 means the entry carries no UDP_SEGMENT cmsg and the
+// kernel treats it as a plain datagram. Returns the run length and the
+// per-segment size (which equals len(bufs[start])). Without GSO support
+// every call returns runLen=1. Outer ECN (when ecns != nil) is also a run
+// boundary — the kernel stamps one outer codepoint per sendmsg entry, so
+// mixing values inside a run would lose information.
+func (u *StdConn) planRun(bufs [][]byte, addrs []netip.AddrPort, ecns []byte, start, iovBudget int) (int, int) {
+	if start >= len(bufs) || iovBudget < 1 {
+		return 0, 0
+	}
+	segSize := len(bufs[start])
+	if !u.gsoSupported || segSize == 0 || segSize > maxGSOBytes {
+		return 1, segSize
+	}
+	dst := addrs[start]
+	var ecn byte
+	if ecns != nil {
+		ecn = ecns[start]
+	}
+	maxLen := maxGSOSegments
+	if iovBudget < maxLen {
+		maxLen = iovBudget
+	}
+	runLen := 1
+	total := segSize
+	for runLen < maxLen && start+runLen < len(bufs) {
+		nextLen := len(bufs[start+runLen])
+		if nextLen == 0 || nextLen > segSize {
+			break
+		}
+		if addrs[start+runLen] != dst {
+			break
+		}
+		if ecns != nil && ecns[start+runLen] != ecn {
+			break
+		}
+		if total+nextLen > maxGSOBytes {
+			break
+		}
+		total += nextLen
+		runLen++
+		if nextLen < segSize {
+			// A short packet must be the last in the run.
+			break
+		}
+	}
+	return runLen, segSize
+}
+
+// writeEntryCmsg sets up the per-mmsghdr Hdr.Control / Hdr.Controllen for one
+// entry. It writes the UDP_SEGMENT payload when runLen >= 2 and the
+// IP_TOS/IPV6_TCLASS payload when ecn != 0, then points hdr.Control at the
+// smallest contiguous span that covers whichever cmsg(s) actually apply.
+func (u *StdConn) writeEntryCmsg(entry, runLen, segSize int, ecn byte) {
+	hdr := &u.writeMsgs[entry].Hdr
+	useSeg := runLen >= 2
+	useEcn := ecn != 0
+	base := entry * u.writeCmsgSpace
+
+	if useSeg {
+		dataOff := base + unix.CmsgLen(0)
+		binary.NativeEndian.PutUint16(u.writeCmsg[dataOff:dataOff+2], uint16(segSize))
+	}
+	if useEcn {
+		dataOff := base + u.writeCmsgSegSpace + unix.CmsgLen(0)
+		binary.NativeEndian.PutUint32(u.writeCmsg[dataOff:dataOff+4], uint32(ecn))
+	}
+
+	switch {
+	case useSeg && useEcn:
+		hdr.Control = &u.writeCmsg[base]
+		setMsgControllen(hdr, u.writeCmsgSpace)
+	case useSeg:
+		hdr.Control = &u.writeCmsg[base]
+		setMsgControllen(hdr, u.writeCmsgSegSpace)
+	case useEcn:
+		hdr.Control = &u.writeCmsg[base+u.writeCmsgSegSpace]
+		setMsgControllen(hdr, u.writeCmsgEcnSpace)
+	default:
+		hdr.Control = nil
+		setMsgControllen(hdr, 0)
+	}
+}
+
+// sendmmsg issues sendmmsg(2) over u.rawConn against the first n entries
+// of u.writeMsgs. Routes through u.rawSend so the per-call kernel callback
+// stays alloc-free.
+func (u *StdConn) sendmmsg(n int) (int, error) {
+	return u.rawSend.send(u.rawConn, n)
+}
+
+// writeSockaddr encodes addr into buf (which must be at least
+// SizeofSockaddrInet6 bytes). Returns the number of bytes used. If isV4 is
+// true and addr is not a v4 (or v4-in-v6) address, returns an error.
+func writeSockaddr(buf []byte, addr netip.AddrPort, isV4 bool) (int, error) {
+	ap := addr.Addr().Unmap()
+	if isV4 {
+		if !ap.Is4() {
+			return 0, ErrInvalidIPv6RemoteForSocket
+		}
+		// struct sockaddr_in: { sa_family_t(2), in_port_t(2, BE), in_addr(4), zero(8) }
+		// sa_family is host endian.
+		binary.NativeEndian.PutUint16(buf[0:2], unix.AF_INET)
+		binary.BigEndian.PutUint16(buf[2:4], addr.Port())
+		ip4 := ap.As4()
+		copy(buf[4:8], ip4[:])
+		for j := 8; j < 16; j++ {
+			buf[j] = 0
+		}
+		return unix.SizeofSockaddrInet4, nil
+	}
+	// struct sockaddr_in6: { sa_family_t(2), in_port_t(2, BE), flowinfo(4), in6_addr(16), scope_id(4) }
+	binary.NativeEndian.PutUint16(buf[0:2], unix.AF_INET6)
+	binary.BigEndian.PutUint16(buf[2:4], addr.Port())
+	binary.NativeEndian.PutUint32(buf[4:8], 0)
+	ip6 := addr.Addr().As16()
+	copy(buf[8:24], ip6[:])
+	binary.NativeEndian.PutUint32(buf[24:28], 0)
+	return unix.SizeofSockaddrInet6, nil
+}
+
 func (u *StdConn) ReloadConfig(c *config.C) {
 	b := c.GetInt("listen.read_buffer", 0)
 	if b > 0 {

udp/udp_linux_32.go

@@ -30,11 +30,16 @@ type rawMessage struct {
 	Len uint32
 }
 
-func (u *StdConn) PrepareRawMessages(n, bufSize int) ([]rawMessage, [][]byte, [][]byte) {
+func (u *StdConn) PrepareRawMessages(n, bufSize, cmsgSpace int) ([]rawMessage, [][]byte, [][]byte, []byte) {
 	msgs := make([]rawMessage, n)
 	buffers := make([][]byte, n)
 	names := make([][]byte, n)
 
+	var cmsgs []byte
+	if cmsgSpace > 0 {
+		cmsgs = make([]byte, n*cmsgSpace)
+	}
+
 	for i := range msgs {
 		buffers[i] = make([]byte, bufSize)
 		names[i] = make([]byte, unix.SizeofSockaddrInet6)
@@ -48,9 +53,14 @@ func (u *StdConn) PrepareRawMessages(n, bufSize int) ([]rawMessage, [][]byte, []
 
 		msgs[i].Hdr.Name = &names[i][0]
 		msgs[i].Hdr.Namelen = uint32(len(names[i]))
+
+		if cmsgSpace > 0 {
+			msgs[i].Hdr.Control = &cmsgs[i*cmsgSpace]
+			msgs[i].Hdr.Controllen = uint32(cmsgSpace)
+		}
 	}
 
-	return msgs, buffers, names
+	return msgs, buffers, names, cmsgs
 }
 
 func setIovLen(v *iovec, n int) {

udp/udp_linux_64.go

@@ -33,11 +33,16 @@ type rawMessage struct {
 	Pad0 [4]byte
 }
 
-func (u *StdConn) PrepareRawMessages(n, bufSize int) ([]rawMessage, [][]byte, [][]byte) {
+func (u *StdConn) PrepareRawMessages(n, bufSize, cmsgSpace int) ([]rawMessage, [][]byte, [][]byte, []byte) {
 	msgs := make([]rawMessage, n)
 	buffers := make([][]byte, n)
 	names := make([][]byte, n)
 
+	var cmsgs []byte
+	if cmsgSpace > 0 {
+		cmsgs = make([]byte, n*cmsgSpace)
+	}
+
 	for i := range msgs {
 		buffers[i] = make([]byte, bufSize)
 		names[i] = make([]byte, unix.SizeofSockaddrInet6)
@@ -51,9 +56,14 @@ func (u *StdConn) PrepareRawMessages(n, bufSize int) ([]rawMessage, [][]byte, []
 
 		msgs[i].Hdr.Name = &names[i][0]
 		msgs[i].Hdr.Namelen = uint32(len(names[i]))
+
+		if cmsgSpace > 0 {
+			msgs[i].Hdr.Control = &cmsgs[i*cmsgSpace]
+			msgs[i].Hdr.Controllen = uint64(cmsgSpace)
+		}
 	}
 
-	return msgs, buffers, names
+	return msgs, buffers, names, cmsgs
 }
 
 func setIovLen(v *iovec, n int) {

udp/udp_rio_windows.go

@@ -161,7 +161,7 @@ func (u *RIOConn) ListenOut(r EncReader, flush func()) error {
 			continue
 		}
 
-		r(netip.AddrPortFrom(netip.AddrFrom16(rua.Addr).Unmap(), (rua.Port>>8)|((rua.Port&0xff)<<8)), buffer[:n])
+		r(netip.AddrPortFrom(netip.AddrFrom16(rua.Addr).Unmap(), (rua.Port>>8)|((rua.Port&0xff)<<8)), buffer[:n], RxMeta{})
 		flush()
 	}
 }
@@ -317,7 +317,7 @@ func (u *RIOConn) WriteTo(buf []byte, ip netip.AddrPort) error {
 	return winrio.SendEx(u.rq, dataBuffer, 1, nil, addressBuffer, nil, nil, 0, 0)
 }
 
-func (u *RIOConn) WriteBatch(bufs [][]byte, addrs []netip.AddrPort) error {
+func (u *RIOConn) WriteBatch(bufs [][]byte, addrs []netip.AddrPort, _ []byte) error {
 	for i, b := range bufs {
 		if err := u.WriteTo(b, addrs[i]); err != nil {
 			return err

udp/udp_tester.go

@@ -157,7 +157,7 @@ func (u *TesterConn) WriteTo(b []byte, addr netip.AddrPort) error {
 		return nil
 	}
 }
-func (u *TesterConn) WriteBatch(bufs [][]byte, addrs []netip.AddrPort) error {
+func (u *TesterConn) WriteBatch(bufs [][]byte, addrs []netip.AddrPort, _ []byte) error {
 	for i, b := range bufs {
 		if err := u.WriteTo(b, addrs[i]); err != nil {
 			return err
@@ -172,7 +172,7 @@ func (u *TesterConn) ListenOut(r EncReader, flush func()) error {
 		case <-u.done:
 			return os.ErrClosed
 		case p := <-u.RxPackets:
-			r(p.From, p.Data)
+			r(p.From, p.Data, RxMeta{})
 			p.Release()
 			flush()
 		}