spicy offload chkpt

firewall/packet.go

@@ -65,3 +65,12 @@ func (fp Packet) MarshalJSON() ([]byte, error) {
 		"Fragment":   fp.Fragment,
 	})
 }
+
+// ParsedPacket is a Packet plus the parse byproducts the RX path reuses
+type ParsedPacket struct {
+	Packet
+	IPHdrLen int
+	// FragAny reports any fragmentation at all: MF flag or nonzero offset for IPv4, a fragment extension header for IPv6.
+	// Distinct from Packet.Fragment, which is true only for NON-FIRST fragments
+	FragAny bool
+}

inside.go

@@ -15,7 +15,7 @@ import (
 	"github.com/slackhq/nebula/routing"
 )
 
-func (f *Interface) consumeInsidePacket(pkt tio.Packet, fwPacket *firewall.Packet, nb []byte, sendBatch *batch.SendBatch, rejectBuf []byte, q int, localCache firewall.ConntrackCache) {
+func (f *Interface) consumeInsidePacket(pkt tio.Packet, fwPacket *firewall.ParsedPacket, nb []byte, sendBatch *batch.SendBatch, 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
@@ -74,7 +74,7 @@ func (f *Interface) consumeInsidePacket(pkt tio.Packet, fwPacket *firewall.Packe
 		return
 	}
 
-	hostinfo, ready := f.getOrHandshakeConsiderRouting(fwPacket, func(hh *HandshakeHostInfo) {
+	hostinfo, ready := f.getOrHandshakeConsiderRouting(&fwPacket.Packet, func(hh *HandshakeHostInfo) {
 		// 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.
@@ -105,7 +105,7 @@ func (f *Interface) consumeInsidePacket(pkt tio.Packet, fwPacket *firewall.Packe
 		return
 	}
 
-	dropReason := f.firewall.Drop(*fwPacket, false, hostinfo, f.pki.GetCAPool(), localCache)
+	dropReason := f.firewall.Drop(fwPacket.Packet, false, hostinfo, f.pki.GetCAPool(), localCache)
 	if dropReason == nil {
 		f.sendInsideMessage(hostinfo, pkt, nb, sendBatch)
 	} else {
@@ -371,7 +371,7 @@ func (f *Interface) getOrHandshakeConsiderRouting(fwPacket *firewall.Packet, cac
 }
 
 func (f *Interface) sendMessageNow(t header.MessageType, st header.MessageSubType, hostinfo *HostInfo, p, nb, out []byte) {
-	fp := &firewall.Packet{}
+	fp := &firewall.ParsedPacket{}
 	err := newPacket(p, false, fp)
 	if err != nil {
 		f.l.Warn("error while parsing outgoing packet for firewall check", "error", err)
@@ -379,7 +379,7 @@ func (f *Interface) sendMessageNow(t header.MessageType, st header.MessageSubTyp
 	}
 
 	// check if packet is in outbound fw rules
-	dropReason := f.firewall.Drop(*fp, false, hostinfo, f.pki.GetCAPool(), nil)
+	dropReason := f.firewall.Drop(fp.Packet, false, hostinfo, f.pki.GetCAPool(), nil)
 	if dropReason != nil {
 		if f.l.Enabled(context.Background(), slog.LevelDebug) {
 			f.l.Debug("dropping cached packet",

interface.go

@@ -360,7 +360,7 @@ func (f *Interface) listenOut(i int) {
 	ctCache := firewall.NewConntrackCacheTicker(f.ctx, f.l, f.conntrackCacheTimeout)
 	lhh := f.lightHouse.NewRequestHandler()
 	h := &header.H{}
-	fwPacket := &firewall.Packet{}
+	fwPacket := &firewall.ParsedPacket{}
 	nb := make([]byte, 12, 12)
 	scratch := make([]byte, mtu)
 
@@ -416,7 +416,7 @@ func (f *Interface) listenIn(queue tio.Queue, i int) {
 	rejectBuf := make([]byte, mtu)
 	arenaSize := batch.SendBatchCap * (udp.MTU + 32)
 	sb := batch.NewSendBatch(f.writers[i], batch.SendBatchCap, arenaSize)
-	fwPacket := &firewall.Packet{}
+	fwPacket := &firewall.ParsedPacket{}
 	nb := make([]byte, 12, 12)
 
 	conntrackCache := firewall.NewConntrackCacheTicker(f.ctx, f.l, f.conntrackCacheTimeout)

outside.go

@@ -26,7 +26,7 @@ var ErrOutOfWindow = errors.New("out of window packet")
 // readOutsidePackets processes one received underlay packet.
 // Message payloads are decrypted IN PLACE, so packet must stay untouched
 // by the caller until the batcher for queue q has been flushed
-func (f *Interface) readOutsidePackets(via ViaSender, scratch []byte, packet []byte, h *header.H, fwPacket *firewall.Packet, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache) {
+func (f *Interface) readOutsidePackets(via ViaSender, scratch []byte, packet []byte, h *header.H, fwPacket *firewall.ParsedPacket, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache) {
 	err := h.Parse(packet)
 	if err != nil {
 		// Hole punch packets are 0 or 1 byte big, so lets ignore printing those errors
@@ -186,7 +186,7 @@ func (f *Interface) readOutsidePackets(via ViaSender, scratch []byte, packet []b
 	}
 }
 
-func (f *Interface) handleOutsideRelayPacket(hostinfo *HostInfo, via ViaSender, scratch []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, scratch []byte, packet []byte, h *header.H, fwPacket *firewall.ParsedPacket, lhf *LightHouseHandler, nb []byte, q int, localCache firewall.ConntrackCache) {
 	// Successfully validated the thing. Get rid of the Relay header and the AEAD tag
 	signedPayload := packet[header.Len : len(packet)-hostinfo.ConnectionState.dKey.Overhead()]
 	// Pull the Roaming parts up here, and return in all call paths.
@@ -315,7 +315,11 @@ var (
 )
 
 // newPacket validates and parses the interesting bits for the firewall out of the ip and sub protocol headers
-func newPacket(data []byte, incoming bool, fp *firewall.Packet) error {
+func newPacket(data []byte, incoming bool, fp *firewall.ParsedPacket) error {
+	// fp is reused across packets; reset the parse byproducts here so a
+	// parser's early-error return can't leak the previous packet's offsets.
+	fp.IPHdrLen = 0
+	fp.FragAny = false
 	if len(data) < 1 {
 		return ErrPacketTooShort
 	}
@@ -330,7 +334,7 @@ func newPacket(data []byte, incoming bool, fp *firewall.Packet) error {
 	return ErrUnknownIPVersion
 }
 
-func parseV6(data []byte, incoming bool, fp *firewall.Packet) error {
+func parseV6(data []byte, incoming bool, fp *firewall.ParsedPacket) error {
 	dataLen := len(data)
 	if dataLen < ipv6.HeaderLen {
 		return ErrIPv6PacketTooShort
@@ -356,6 +360,7 @@ func parseV6(data []byte, incoming bool, fp *firewall.Packet) error {
 		switch proto {
 		case layers.IPProtocolESP, layers.IPProtocolNoNextHeader:
 			fp.Protocol = uint8(proto)
+			fp.IPHdrLen = offset
 			fp.RemotePort = 0
 			fp.LocalPort = 0
 			fp.Fragment = false
@@ -366,6 +371,7 @@ func parseV6(data []byte, incoming bool, fp *firewall.Packet) error {
 				return ErrIPv6PacketTooShort
 			}
 			fp.Protocol = uint8(proto)
+			fp.IPHdrLen = offset
 			fp.LocalPort = 0 //incoming vs outgoing doesn't matter for icmpv6
 			icmptype := data[offset+1]
 			switch icmptype {
@@ -383,6 +389,9 @@ func parseV6(data []byte, incoming bool, fp *firewall.Packet) error {
 			}
 
 			fp.Protocol = uint8(proto)
+			// offset is the L4 header start: 40 for a plain packet, past the
+			// extension chain otherwise. The coalescer only accepts 40.
+			fp.IPHdrLen = offset
 			if incoming {
 				fp.RemotePort = binary.BigEndian.Uint16(data[offset : offset+2])
 				fp.LocalPort = binary.BigEndian.Uint16(data[offset+2 : offset+4])
@@ -400,6 +409,10 @@ func parseV6(data []byte, incoming bool, fp *firewall.Packet) error {
 				return ErrIPv6PacketTooShort
 			}
 
+			// Either way this packet is a fragment shape the coalescer must
+			// not touch, first fragment included.
+			fp.FragAny = true
+
 			// Check if this is the first fragment
 			fragmentOffset := binary.BigEndian.Uint16(data[offset+2:offset+4]) &^ uint16(0x7) // Remove the reserved and M flag bits
 			if fragmentOffset != 0 {
@@ -441,7 +454,7 @@ func parseV6(data []byte, incoming bool, fp *firewall.Packet) error {
 	return ErrIPv6CouldNotFindPayload
 }
 
-func parseV4(data []byte, incoming bool, fp *firewall.Packet) error {
+func parseV4(data []byte, incoming bool, fp *firewall.ParsedPacket) error {
 	// Do we at least have an ipv4 header worth of data?
 	if len(data) < ipv4.HeaderLen {
 		return ErrIPv4PacketTooShort
@@ -458,6 +471,10 @@ func parseV4(data []byte, incoming bool, fp *firewall.Packet) error {
 	// Check if this is the second or further fragment of a fragmented packet.
 	flagsfrags := binary.BigEndian.Uint16(data[6:8])
 	fp.Fragment = (flagsfrags & 0x1FFF) != 0
+	// Any fragmentation at all (MF or offset): first fragments have readable
+	// ports for the firewall but must never be coalesced.
+	fp.FragAny = (flagsfrags & 0x3fff) != 0
+	fp.IPHdrLen = ihl
 
 	// Firewall handles protocol checks
 	fp.Protocol = data[9]
@@ -501,7 +518,7 @@ func parseV4(data []byte, incoming bool, fp *firewall.Packet) error {
 	return nil
 }
 
-func (f *Interface) handleOutsideMessagePacket(hostinfo *HostInfo, messageCounter uint64, out []byte, scratch []byte, fwPacket *firewall.Packet, nb []byte, q int, localCache firewall.ConntrackCache) {
+func (f *Interface) handleOutsideMessagePacket(hostinfo *HostInfo, messageCounter uint64, out []byte, scratch []byte, fwPacket *firewall.ParsedPacket, nb []byte, q int, localCache firewall.ConntrackCache) {
 	err := newPacket(out, true, fwPacket)
 	if err != nil {
 		hostinfo.logger(f.l).Warn("Error while validating inbound packet",
@@ -511,7 +528,7 @@ func (f *Interface) handleOutsideMessagePacket(hostinfo *HostInfo, messageCounte
 		return
 	}
 
-	dropReason := f.firewall.Drop(*fwPacket, true, hostinfo, f.pki.GetCAPool(), localCache)
+	dropReason := f.firewall.Drop(fwPacket.Packet, true, hostinfo, f.pki.GetCAPool(), localCache)
 	if dropReason != nil {
 		f.rejectOutside(out, hostinfo.ConnectionState, hostinfo, nb, scratch, q)
 		if f.l.Enabled(context.Background(), slog.LevelDebug) {
@@ -523,7 +540,7 @@ func (f *Interface) handleOutsideMessagePacket(hostinfo *HostInfo, messageCounte
 		return
 	}
 
-	err = f.batchers[q].Commit(out, batch.SortKey{Epoch: hostinfo.ConnectionState.epoch, Counter: messageCounter})
+	err = f.batchers[q].Commit(out, batch.SortKey{Epoch: hostinfo.ConnectionState.epoch, Counter: messageCounter}, fwPacket)
 	if err != nil {
 		f.l.Error("Failed to write to tun", "error", err)
 	}

outside_test.go

@@ -17,7 +17,7 @@ import (
 )
 
 func Test_newPacket(t *testing.T) {
-	p := &firewall.Packet{}
+	p := &firewall.ParsedPacket{}
 
 	// length fails
 	err := newPacket([]byte{}, true, p)
@@ -96,7 +96,7 @@ func Test_newPacket(t *testing.T) {
 }
 
 func Test_newPacket_v6(t *testing.T) {
-	p := &firewall.Packet{}
+	p := &firewall.ParsedPacket{}
 
 	// invalid ipv6
 	ip := layers.IPv6{
@@ -345,7 +345,7 @@ func Test_newPacket_v6(t *testing.T) {
 }
 
 func Test_newPacket_ipv6Fragment(t *testing.T) {
-	p := &firewall.Packet{}
+	p := &firewall.ParsedPacket{}
 
 	ip := &layers.IPv6{
 		Version:    6,
@@ -525,7 +525,7 @@ func BenchmarkParseV6(b *testing.B) {
 	secondFrag = append(secondFrag, fragHeader...)
 	secondFrag = append(secondFrag, []byte{0xde, 0xad, 0xbe, 0xef}...)
 
-	fp := &firewall.Packet{}
+	fp := &firewall.ParsedPacket{}
 
 	b.Run("Normal", func(b *testing.B) {
 		for i := 0; i < b.N; i++ {
@@ -649,7 +649,7 @@ func serializeAH(ah *layers.IPSecAH) []byte {
 // host OS parses the real header, a firewall port/proto bypass. The fix makes parseV6 land
 // on the same offset the host does.
 func Test_newPacket_v6ExtHeaderOverflow(t *testing.T) {
-	p := &firewall.Packet{}
+	p := &firewall.ParsedPacket{}
 
 	const (
 		hdrLen      = 40              // IPv6 header
@@ -675,3 +675,88 @@ func Test_newPacket_v6ExtHeaderOverflow(t *testing.T) {
 	// the host delivers to, not the forged 443 at the overflowed offset.
 	assert.Equal(t, uint16(22), p.LocalPort, "firewall must parse the real transport header, not the overflowed offset")
 }
+
+// Test_newPacket_parsedFields pins the ParsedPacket byproducts the RX
+// batcher consumes: IPHdrLen (the true L4 offset) and FragAny (any fragment
+// shape at all — unlike Packet.Fragment, which is port-oriented and true
+// only for non-first fragments).
+func Test_newPacket_parsedFields(t *testing.T) {
+	p := &firewall.ParsedPacket{}
+
+	// Plain IPv4 TCP, IHL 20: L4 offset 20, no fragment shape.
+	v4 := make([]byte, 28)
+	v4[0] = 0x45
+	v4[9] = firewall.ProtoTCP
+	binary.BigEndian.PutUint16(v4[6:8], 0x4000) // DF only
+	require.NoError(t, newPacket(v4, true, p))
+	assert.Equal(t, 20, p.IPHdrLen)
+	assert.False(t, p.FragAny)
+	assert.False(t, p.Fragment)
+
+	// IPv4 first fragment (MF set, offset 0): the firewall can read ports
+	// (Fragment false) but the coalescer must not touch it (FragAny true).
+	ff := make([]byte, 28)
+	ff[0] = 0x45
+	ff[9] = firewall.ProtoUDP
+	binary.BigEndian.PutUint16(ff[6:8], 0x2000) // MF, offset 0
+	require.NoError(t, newPacket(ff, true, p))
+	assert.False(t, p.Fragment)
+	assert.True(t, p.FragAny)
+	assert.Equal(t, 20, p.IPHdrLen)
+
+	// IPv4 non-first fragment (nonzero offset): both flags set.
+	nf := make([]byte, 28)
+	nf[0] = 0x45
+	nf[9] = firewall.ProtoUDP
+	binary.BigEndian.PutUint16(nf[6:8], 0x00b9)
+	require.NoError(t, newPacket(nf, true, p))
+	assert.True(t, p.Fragment)
+	assert.True(t, p.FragAny)
+
+	// IPv4 with options (IHL 24): IPHdrLen tracks the real L4 offset.
+	opts := make([]byte, 32)
+	opts[0] = 0x46
+	opts[9] = firewall.ProtoTCP
+	binary.BigEndian.PutUint16(opts[6:8], 0x4000)
+	require.NoError(t, newPacket(opts, true, p))
+	assert.Equal(t, 24, p.IPHdrLen)
+	assert.False(t, p.FragAny)
+
+	// Plain IPv6 TCP: L4 at 40.
+	v6 := make([]byte, 60)
+	v6[0] = 0x60
+	v6[6] = firewall.ProtoTCP
+	require.NoError(t, newPacket(v6, true, p))
+	assert.Equal(t, 40, p.IPHdrLen)
+	assert.False(t, p.FragAny)
+
+	// IPv6 hop-by-hop then TCP: IPHdrLen lands past the extension header.
+	hbh := make([]byte, 60)
+	hbh[0] = 0x60
+	hbh[6] = 0 // hop-by-hop
+	hbh[40] = firewall.ProtoTCP
+	hbh[41] = 0 // HdrExtLen 0 -> 8-byte header
+	require.NoError(t, newPacket(hbh, true, p))
+	assert.Equal(t, 48, p.IPHdrLen)
+	assert.False(t, p.FragAny)
+
+	// IPv6 first fragment: terminal proto resolved, FragAny set, Fragment not.
+	f6 := make([]byte, 60)
+	f6[0] = 0x60
+	f6[6] = 44 // fragment extension header
+	f6[40] = firewall.ProtoUDP
+	require.NoError(t, newPacket(f6, true, p))
+	assert.True(t, p.FragAny)
+	assert.False(t, p.Fragment)
+	assert.Equal(t, uint8(firewall.ProtoUDP), p.Protocol)
+
+	// IPv6 non-first fragment: both set, walk stops at the fragment header.
+	f6n := make([]byte, 60)
+	f6n[0] = 0x60
+	f6n[6] = 44
+	f6n[40] = firewall.ProtoUDP
+	binary.BigEndian.PutUint16(f6n[42:44], 0x0008)
+	require.NoError(t, newPacket(f6n, true, p))
+	assert.True(t, p.Fragment)
+	assert.True(t, p.FragAny)
+}

overlay/batch/batch.go

@@ -1,5 +1,7 @@
 package batch
 
+import "github.com/slackhq/nebula/firewall"
+
 // SortKey identifies a packet's position in its sender's transmission order.
 // Epoch is a receiver-local ordinal for the tunnel (ConnectionState) that
 // decrypted the packet. A re-handshake replaces the tunnel outright — new
@@ -15,9 +17,12 @@ type SortKey struct {
 
 type RxBatcher interface {
 	// Commit stages pkt to be flushed by the batch. key must carry the
-	// packet's session epoch and message counter. The caller must keep pkt
-	// valid until the next Flush, and not re-use it.
-	Commit(pkt []byte, key SortKey) error
+	// packet's session epoch and message counter; pp must be the firewall's
+	// parse of this same packet. The caller must keep pkt valid until the
+	// next Flush, and not re-use it. pp, by contrast, is borrowed only for
+	// the duration of the call — the caller reuses one ParsedPacket per
+	// receive loop — so implementations must copy what they need from it.
+	Commit(pkt []byte, key SortKey, pp *firewall.ParsedPacket) error
 	// Flush emits every staged packet. Packets are first sorted by key, so
 	// within each protocol lane emission follows the sender's transmission
 	// order regardless of arrival order. One shape may legally be overtaken

overlay/batch/coalesce_core.go

@@ -40,34 +40,19 @@ type parsedIP struct {
 // 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.
+// This is the standalone-lane-Commit entry; the dispatcher path uses
+// parseIPAt, where the protocol was already resolved upstream.
 func parseIPPrologue(pkt []byte, wantProto byte) (parsedIP, bool) {
 	var p parsedIP
 	if len(pkt) < 20 {
 		return p, false
 	}
-	v := pkt[0] >> 4
-	switch v {
+	switch pkt[0] >> 4 {
 	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]
+		return parseIPv4Prologue(pkt)
 	case 6:
 		if len(pkt) < 40 {
 			return p, false
@@ -75,18 +60,77 @@ func parseIPPrologue(pkt []byte, wantProto byte) (parsedIP, bool) {
 		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 parseIPv6Prologue(pkt)
+	}
+	return p, false
+}
+
+// parseIPAt is the dispatcher-path prologue: newPacket already resolved the
+// L4 protocol and header offset once for the firewall, so the proto sniff is
+// replaced by a cross-check of the caller's ipHdrLen. A plain header (v4:
+// IHL 20, v6: exactly 40 — no options, no extension headers) is the only
+// coalesceable shape, which is the same rule parseIPPrologue enforces
+// through its own reads.
+func parseIPAt(pkt []byte, ipHdrLen int) (parsedIP, bool) {
+	var p parsedIP
+	if len(pkt) < 20 {
 		return p, false
 	}
+	switch pkt[0] >> 4 {
+	case 4:
+		if ipHdrLen != 20 {
+			return p, false
+		}
+		return parseIPv4Prologue(pkt)
+	case 6:
+		if ipHdrLen != 40 || len(pkt) < 40 {
+			return p, false
+		}
+		return parseIPv6Prologue(pkt)
+	}
+	return p, false
+}
+
+// parseIPv4Prologue is the shared IPv4 tail of the two prologue entries.
+// The caller has verified len(pkt) >= 20 and either the protocol
+// (parseIPPrologue) or the upstream-resolved header length (parseIPAt).
+func parseIPv4Prologue(pkt []byte) (parsedIP, bool) {
+	var p parsedIP
+	ihl := int(pkt[0]&0x0f) * 4
+	if ihl != 20 {
+		return p, false
+	}
+	// Reject actual fragmentation (MF or non-zero frag offset). On the
+	// dispatcher path FragAny was already gated; kept as defense in depth —
+	// a fragment folded into a superpacket would corrupt reassembly.
+	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]
+	return p, true
+}
+
+// parseIPv6Prologue is the shared IPv6 tail; caller has verified
+// len(pkt) >= 40 and version/proto-or-offset.
+func parseIPv6Prologue(pkt []byte) (parsedIP, bool) {
+	var p parsedIP
+	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]
 	return p, true
 }
 

overlay/batch/multi_coalesce.go

@@ -6,7 +6,7 @@ import (
 	"log/slog"
 	"slices"
 
-	"github.com/slackhq/nebula/iputil"
+	"github.com/slackhq/nebula/firewall"
 )
 
 // MultiCoalescer stages plaintext packets with their (epoch, counter) sort
@@ -47,9 +47,15 @@ type MultiCoalescer struct {
 	staged []stagedPacket
 }
 
+// stagedPacket also carries the scalars dispatch needs from the firewall's
+// ParsedPacket: pp itself is reused by the caller per packet and must not be
+// retained past Commit, so the relevant fields are copied by value here.
 type stagedPacket struct {
-	pkt []byte
-	key SortKey
+	pkt      []byte
+	key      SortKey
+	proto    byte
+	fragAny  bool
+	ipHdrLen uint16
 }
 
 // NewMultiCoalescer builds a multi-lane batcher over w, based on available
@@ -65,32 +71,18 @@ func NewMultiCoalescer(w io.Writer, l *slog.Logger) RxBatcher {
 	return m
 }
 
-// IANA protocol numbers for the IPv6 extension headers
-// iputil.IPv6FindUpperProtocol can step over. The set here must match what
-// that walker walks: it is the hot path's cheap pre-guard, so the walk is
-// only paid when it can actually make progress.
-const (
-	ipProtoHopByHop = 0
-	ipProtoRouting  = 43
-	ipProtoFragment = 44
-	ipProtoAH       = 51
-	ipProtoDestOpts = 60
-)
-
-// isIPv6ExtHeader reports whether nh is an extension header the terminal-
-// protocol walk knows how to step over.
-func isIPv6ExtHeader(nh byte) bool {
-	switch nh {
-	case ipProtoHopByHop, ipProtoRouting, ipProtoFragment, ipProtoAH, ipProtoDestOpts:
-		return true
-	}
-	return false
-}
-
-// Commit stages pkt for the next Flush. All parsing and lane dispatch is
-// deferred to Flush so it runs on packets already in transmission order.
-func (m *MultiCoalescer) Commit(pkt []byte, key SortKey) error {
-	m.staged = append(m.staged, stagedPacket{pkt: pkt, key: key})
+// Commit stages pkt for the next Flush. All lane dispatch is deferred to
+// Flush so it runs on packets already in transmission order. pp is the
+// firewall's parse of pkt — the single source of truth for the packet's
+// protocol and L4 offset — and is only borrowed for this call.
+func (m *MultiCoalescer) Commit(pkt []byte, key SortKey, pp *firewall.ParsedPacket) error {
+	m.staged = append(m.staged, stagedPacket{
+		pkt:      pkt,
+		key:      key,
+		proto:    pp.Protocol,
+		fragAny:  pp.FragAny,
+		ipHdrLen: uint16(pp.IPHdrLen),
+	})
 	return nil
 }
 
@@ -114,59 +106,43 @@ func compareStaged(a, b stagedPacket) int {
 	return 1
 }
 
-// dispatch routes one packet to the appropriate lane based on IP version +
-// L4 proto. 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.
-func (m *MultiCoalescer) dispatch(pkt []byte) error {
-	if len(pkt) < 20 {
-		return m.pt.enqueue(pkt)
-	}
-	v := pkt[0] >> 4
-	var proto byte
-	switch v {
-	case 4:
-		proto = pkt[9]
-	case 6:
-		if len(pkt) < 40 {
-			return m.pt.enqueue(pkt)
-		}
-		proto = pkt[6]
-		if isIPv6ExtHeader(proto) {
-			// Walk to the terminal protocol so the packet routes to its flow's protocol lane.
-			// This protects flow ordering.
-			proto, _, _ = iputil.IPv6FindUpperProtocol(pkt)
-		}
-	default:
-		return m.pt.enqueue(pkt)
-	}
-	switch proto {
+// dispatch routes one staged packet to its lane.
+// The protocol and L4 offset come from the firewall's parse of the same packet.
+// Any shape a lane can't coalesce seals every open chain in its lane
+func (m *MultiCoalescer) dispatch(sp stagedPacket) error {
+	switch sp.proto {
 	case ipProtoTCP:
 		if m.tcp != nil {
-			info, ok := parseTCPBase(pkt)
-			if !ok {
-				// Unsupported TCP shape (IP options, fragments, ...). Its flow
-				// key is unknowable, so seal every open chain: dispatch runs in
-				// transmission order, and sealing is what keeps later data from
-				// extending a chain that would emit ahead of this packet.
+			if sp.fragAny {
 				m.tcp.sealAllOpen()
-				m.tcp.addVerbatim(pkt)
+				m.tcp.addVerbatim(sp.pkt)
 				return nil
 			}
-			return m.tcp.commitParsed(pkt, info)
+			info, ok := parseTCPAt(sp.pkt, int(sp.ipHdrLen))
+			if !ok {
+				m.tcp.sealAllOpen()
+				m.tcp.addVerbatim(sp.pkt)
+				return nil
+			}
+			return m.tcp.commitParsed(sp.pkt, info)
 		}
 	case ipProtoUDP:
 		if m.udp != nil {
-			info, ok := parseUDP(pkt)
-			if !ok {
+			if sp.fragAny {
 				m.udp.sealAllOpen()
-				m.udp.addVerbatim(pkt)
+				m.udp.addVerbatim(sp.pkt)
 				return nil
 			}
-			return m.udp.commitParsed(pkt, info)
+			info, ok := parseUDPAt(sp.pkt, int(sp.ipHdrLen))
+			if !ok {
+				m.udp.sealAllOpen()
+				m.udp.addVerbatim(sp.pkt)
+				return nil
+			}
+			return m.udp.commitParsed(sp.pkt, info)
 		}
 	}
-	return m.pt.enqueue(pkt)
+	return m.pt.enqueue(sp.pkt)
 }
 
 // Flush sorts the staged batch into transmission order, replays it into the
@@ -178,7 +154,7 @@ func (m *MultiCoalescer) Flush() error {
 
 	var errs []error
 	for _, sp := range m.staged {
-		if err := m.dispatch(sp.pkt); err != nil {
+		if err := m.dispatch(sp); err != nil {
 			errs = append(errs, err)
 		}
 	}

overlay/batch/multi_coalesce_test.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"testing"
 
+	"github.com/slackhq/nebula/firewall"
 	"github.com/slackhq/nebula/test"
 )
 
@@ -48,19 +49,19 @@ func TestMultiCoalescerRoutesByProto(t *testing.T) {
 	icmp[3] = 28
 	icmp[9] = 1
 
-	if err := m.Commit(buildTCPv4(1000, tcpAck, tcpPay), k.next()); err != nil {
+	if err := m.Commit(buildTCPv4(1000, tcpAck, tcpPay), k.next(), testPP(buildTCPv4(1000, tcpAck, tcpPay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildTCPv4(2200, tcpAck, tcpPay), k.next()); err != nil {
+	if err := m.Commit(buildTCPv4(2200, tcpAck, tcpPay), k.next(), testPP(buildTCPv4(2200, tcpAck, tcpPay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildUDPv4(2000, 53, udpPay), k.next()); err != nil {
+	if err := m.Commit(buildUDPv4(2000, 53, udpPay), k.next(), testPP(buildUDPv4(2000, 53, udpPay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildUDPv4(2000, 53, udpPay), k.next()); err != nil {
+	if err := m.Commit(buildUDPv4(2000, 53, udpPay), k.next(), testPP(buildUDPv4(2000, 53, udpPay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(icmp, k.next()); err != nil {
+	if err := m.Commit(icmp, k.next(), testPP(icmp)); err != nil {
 		t.Fatal(err)
 	}
 	if err := m.Flush(); err != nil {
@@ -89,13 +90,13 @@ func TestMultiCoalescerRestoresTransmissionOrder(t *testing.T) {
 
 	// Transmission order: seq 1000 (c1), 2200 (c2), 3400 (c3).
 	// Arrival order: 3400, 1000, 2200.
-	if err := m.Commit(buildTCPv4(3400, tcpAck, pay), SortKey{Epoch: 1, Counter: 3}); err != nil {
+	if err := m.Commit(buildTCPv4(3400, tcpAck, pay), SortKey{Epoch: 1, Counter: 3}, testPP(buildTCPv4(3400, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildTCPv4(1000, tcpAck, pay), SortKey{Epoch: 1, Counter: 1}); err != nil {
+	if err := m.Commit(buildTCPv4(1000, tcpAck, pay), SortKey{Epoch: 1, Counter: 1}, testPP(buildTCPv4(1000, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildTCPv4(2200, tcpAck, pay), SortKey{Epoch: 1, Counter: 2}); err != nil {
+	if err := m.Commit(buildTCPv4(2200, tcpAck, pay), SortKey{Epoch: 1, Counter: 2}, testPP(buildTCPv4(2200, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
 	if err := m.Flush(); err != nil {
@@ -115,10 +116,10 @@ func TestMultiCoalescerRestoresTransmissionOrder(t *testing.T) {
 
 	// Retransmit: seq 1000 again but counter 4 — sorts after seq 4600 (c3).
 	w.writes, w.gsoWrites, w.order = nil, nil, nil
-	if err := m.Commit(buildTCPv4(1000, tcpAck, pay), SortKey{Epoch: 1, Counter: 4}); err != nil {
+	if err := m.Commit(buildTCPv4(1000, tcpAck, pay), SortKey{Epoch: 1, Counter: 4}, testPP(buildTCPv4(1000, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildTCPv4(4600, tcpAck, pay), SortKey{Epoch: 1, Counter: 3}); err != nil {
+	if err := m.Commit(buildTCPv4(4600, tcpAck, pay), SortKey{Epoch: 1, Counter: 3}, testPP(buildTCPv4(4600, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
 	if err := m.Flush(); err != nil {
@@ -144,16 +145,16 @@ func TestMultiCoalescerRestoresOrderAcrossFlows(t *testing.T) {
 
 	// Transmission: A.100 (c1), B.500 (c2), A.1300 (c3), B.1700 (c4).
 	// Arrival: A.1300, B.1700, A.100, B.500.
-	if err := m.Commit(buildTCPv4Ports(1000, 2000, 1300, tcpAck, pay), SortKey{Epoch: 1, Counter: 3}); err != nil {
+	if err := m.Commit(buildTCPv4Ports(1000, 2000, 1300, tcpAck, pay), SortKey{Epoch: 1, Counter: 3}, testPP(buildTCPv4Ports(1000, 2000, 1300, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildTCPv4Ports(3000, 2000, 1700, tcpAck, pay), SortKey{Epoch: 1, Counter: 4}); err != nil {
+	if err := m.Commit(buildTCPv4Ports(3000, 2000, 1700, tcpAck, pay), SortKey{Epoch: 1, Counter: 4}, testPP(buildTCPv4Ports(3000, 2000, 1700, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildTCPv4Ports(1000, 2000, 100, tcpAck, pay), SortKey{Epoch: 1, Counter: 1}); err != nil {
+	if err := m.Commit(buildTCPv4Ports(1000, 2000, 100, tcpAck, pay), SortKey{Epoch: 1, Counter: 1}, testPP(buildTCPv4Ports(1000, 2000, 100, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildTCPv4Ports(3000, 2000, 500, tcpAck, pay), SortKey{Epoch: 1, Counter: 2}); err != nil {
+	if err := m.Commit(buildTCPv4Ports(3000, 2000, 500, tcpAck, pay), SortKey{Epoch: 1, Counter: 2}, testPP(buildTCPv4Ports(3000, 2000, 500, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
 	if err := m.Flush(); err != nil {
@@ -195,10 +196,10 @@ func TestMultiCoalescerEpochOrdersAcrossRehandshake(t *testing.T) {
 	pay := make([]byte, 1200)
 
 	// New session's first data arrives before the old session's last data.
-	if err := m.Commit(buildTCPv4(2200, tcpAck, pay), SortKey{Epoch: 8, Counter: 1}); err != nil {
+	if err := m.Commit(buildTCPv4(2200, tcpAck, pay), SortKey{Epoch: 8, Counter: 1}, testPP(buildTCPv4(2200, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildTCPv4(1000, tcpAck, pay), SortKey{Epoch: 7, Counter: 9_000_000}); err != nil {
+	if err := m.Commit(buildTCPv4(1000, tcpAck, pay), SortKey{Epoch: 7, Counter: 9_000_000}, testPP(buildTCPv4(1000, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
 	if err := m.Flush(); err != nil {
@@ -227,10 +228,10 @@ func TestMultiCoalescerNoUSOFallsThrough(t *testing.T) {
 		t.Fatal("UDP lane must not come up without USO")
 	}
 
-	if err := m.Commit(buildUDPv4(1000, 53, make([]byte, 800)), k.next()); err != nil {
+	if err := m.Commit(buildUDPv4(1000, 53, make([]byte, 800)), k.next(), testPP(buildUDPv4(1000, 53, make([]byte, 800)))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildUDPv4(1000, 53, make([]byte, 800)), k.next()); err != nil {
+	if err := m.Commit(buildUDPv4(1000, 53, make([]byte, 800)), k.next(), testPP(buildUDPv4(1000, 53, make([]byte, 800)))); err != nil {
 		t.Fatal(err)
 	}
 	if err := m.Flush(); err != nil {
@@ -261,7 +262,7 @@ func TestMultiCoalescerNoOffloadsStillSorts(t *testing.T) {
 	}
 	// Committed in reverse transmission order; keys carry the truth.
 	for i := len(pkts) - 1; i >= 0; i-- {
-		if err := m.Commit(pkts[i], SortKey{Epoch: 1, Counter: uint64(i + 1)}); err != nil {
+		if err := m.Commit(pkts[i], SortKey{Epoch: 1, Counter: uint64(i + 1)}, testPP(pkts[i])); err != nil {
 			t.Fatal(err)
 		}
 	}
@@ -325,13 +326,13 @@ func TestMultiCoalescerIPv6FragmentStaysInLane(t *testing.T) {
 	m := newTestMultiCoalescer(t, w)
 	k := &keySeq{epoch: 1}
 
-	if err := m.Commit(buildUDPv6Fragment(2000, 53, make([]byte, 512)), k.next()); err != nil {
+	if err := m.Commit(buildUDPv6Fragment(2000, 53, make([]byte, 512)), k.next(), testPP(buildUDPv6Fragment(2000, 53, make([]byte, 512)))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next()); err != nil {
+	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next(), testPP(buildUDPv6(2000, 53, make([]byte, 800)))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next()); err != nil {
+	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next(), testPP(buildUDPv6(2000, 53, make([]byte, 800)))); err != nil {
 		t.Fatal(err)
 	}
 	if err := m.Flush(); err != nil {
@@ -358,19 +359,19 @@ func TestMultiCoalescerFragmentSealsUDPChains(t *testing.T) {
 	m := newTestMultiCoalescer(t, w)
 	k := &keySeq{epoch: 1}
 
-	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next()); err != nil {
+	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next(), testPP(buildUDPv6(2000, 53, make([]byte, 800)))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next()); err != nil {
+	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next(), testPP(buildUDPv6(2000, 53, make([]byte, 800)))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildUDPv6Fragment(2000, 53, make([]byte, 512)), k.next()); err != nil {
+	if err := m.Commit(buildUDPv6Fragment(2000, 53, make([]byte, 512)), k.next(), testPP(buildUDPv6Fragment(2000, 53, make([]byte, 512)))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next()); err != nil {
+	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next(), testPP(buildUDPv6(2000, 53, make([]byte, 800)))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next()); err != nil {
+	if err := m.Commit(buildUDPv6(2000, 53, make([]byte, 800)), k.next(), testPP(buildUDPv6(2000, 53, make([]byte, 800)))); err != nil {
 		t.Fatal(err)
 	}
 	if err := m.Flush(); err != nil {
@@ -398,10 +399,10 @@ func TestMultiCoalescerNoTSOFallsThrough(t *testing.T) {
 	}
 
 	pay := make([]byte, 1200)
-	if err := m.Commit(buildTCPv4(1000, tcpAck, pay), k.next()); err != nil {
+	if err := m.Commit(buildTCPv4(1000, tcpAck, pay), k.next(), testPP(buildTCPv4(1000, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
-	if err := m.Commit(buildTCPv4(2200, tcpAck, pay), k.next()); err != nil {
+	if err := m.Commit(buildTCPv4(2200, tcpAck, pay), k.next(), testPP(buildTCPv4(2200, tcpAck, pay))); err != nil {
 		t.Fatal(err)
 	}
 	if err := m.Flush(); err != nil {
@@ -414,3 +415,29 @@ func TestMultiCoalescerNoTSOFallsThrough(t *testing.T) {
 		t.Errorf("TCP must pass through as 2 plain writes, got %d", len(w.writes))
 	}
 }
+
+// testPP derives the ParsedPacket newPacket would produce for the packet
+// shapes the tests build: plain v4/v6, v4 with options or fragment bits set,
+// and the single-fragment-header v6 shape from buildUDPv6Fragment. Anything
+// unrecognizable stays zero (proto 0 routes to the passthrough lane).
+func testPP(pkt []byte) *firewall.ParsedPacket {
+	pp := &firewall.ParsedPacket{}
+	if len(pkt) < 20 {
+		return pp
+	}
+	switch pkt[0] >> 4 {
+	case 4:
+		pp.Protocol = pkt[9]
+		pp.IPHdrLen = int(pkt[0]&0x0f) * 4
+		pp.FragAny = binary.BigEndian.Uint16(pkt[6:8])&0x3fff != 0
+	case 6:
+		pp.Protocol = pkt[6]
+		pp.IPHdrLen = 40
+		if pp.Protocol == 44 { // fragment extension header
+			pp.Protocol = pkt[40]
+			pp.IPHdrLen = 48
+			pp.FragAny = true
+		}
+	}
+	return pp
+}

overlay/batch/passthrough.go

@@ -2,6 +2,8 @@ package batch
 
 import (
 	"io"
+
+	"github.com/slackhq/nebula/firewall"
 )
 
 // Passthrough is a RxBatcher that doesn't batch anything, it just accumulates and then sends packets.
@@ -17,9 +19,7 @@ func NewPassthrough(w io.Writer) *Passthrough {
 	}
 }
 
-// Commit ignores the sort key: a bare Passthrough (no MultiCoalescer in
-// front) emits in arrival order, exactly as before keys existed.
-func (p *Passthrough) Commit(pkt []byte, _ SortKey) error {
+func (p *Passthrough) Commit(pkt []byte, _ SortKey, _ *firewall.ParsedPacket) error {
 	return p.enqueue(pkt)
 }
 

overlay/batch/tcp_coalesce.go

@@ -117,12 +117,27 @@ type parsedTCP struct {
 // regardless of whether it's admissible for coalescing. Returns ok=false for non-TCP or malformed input.
 // Accepts IPv4 (no options or 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
+		return parsedTCP{}, false
 	}
-	pkt = ip.pkt
+	return parseTCPTail(ip)
+}
+
+// parseTCPAt is parseTCPBase for the dispatcher path: the packet is already
+// known to be TCP and ipHdrLen is the upstream-resolved L4 offset (see parseIPAt).
+func parseTCPAt(pkt []byte, ipHdrLen int) (parsedTCP, bool) {
+	ip, ok := parseIPAt(pkt, ipHdrLen)
+	if !ok {
+		return parsedTCP{}, false
+	}
+	return parseTCPTail(ip)
+}
+
+// parseTCPTail layers the TCP-header parse on a validated IP prologue.
+func parseTCPTail(ip parsedIP) (parsedTCP, bool) {
+	var p parsedTCP
+	pkt := ip.pkt
 	p.fk = ip.fk
 	p.ipHdrLen = ip.ipHdrLen
 

overlay/batch/tcp_coalesce_bench_test.go

@@ -4,6 +4,7 @@ import (
 	"encoding/binary"
 	"testing"
 
+	"github.com/slackhq/nebula/firewall"
 	"github.com/slackhq/nebula/overlay/tio"
 	"github.com/slackhq/nebula/test"
 )
@@ -169,16 +170,22 @@ func BenchmarkCommitNonCoalesceableTCP(b *testing.B) {
 
 // runMultiCommitBench drives MultiCoalescer.Commit with in-order keys, so
 // it includes the staging sort's already-sorted fast path plus the
-// dispatch-time parse — the full steady-state cost of the batcher.
+// dispatch-time parse — the full steady-state cost of the batcher. The
+// ParsedPackets are precomputed: in production they fall out of the
+// firewall's newPacket, which this bench does not model.
 func runMultiCommitBench(b *testing.B, pkts [][]byte, batchSize int) {
 	b.Helper()
 	m := NewMultiCoalescer(nopTunWriter{}, test.NewLogger())
+	pps := make([]*firewall.ParsedPacket, len(pkts))
+	for i, p := range pkts {
+		pps[i] = testPP(p)
+	}
 	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, SortKey{Epoch: 1, Counter: uint64(i + 1)}); err != nil {
+		j := i % len(pkts)
+		if err := m.Commit(pkts[j], SortKey{Epoch: 1, Counter: uint64(i + 1)}, pps[j]); err != nil {
 			b.Fatal(err)
 		}
 		if (i+1)%batchSize == 0 {

overlay/batch/udp_coalesce.go

@@ -87,12 +87,27 @@ type parsedUDP struct {
 // 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
+		return parsedUDP{}, false
 	}
-	pkt = ip.pkt
+	return parseUDPTail(ip)
+}
+
+// parseUDPAt is parseUDP for the dispatcher path: the packet is already
+// known to be UDP and ipHdrLen is the upstream-resolved L4 offset (see parseIPAt).
+func parseUDPAt(pkt []byte, ipHdrLen int) (parsedUDP, bool) {
+	ip, ok := parseIPAt(pkt, ipHdrLen)
+	if !ok {
+		return parsedUDP{}, false
+	}
+	return parseUDPTail(ip)
+}
+
+// parseUDPTail layers the UDP-header parse on a validated IP prologue.
+func parseUDPTail(ip parsedIP) (parsedUDP, bool) {
+	var p parsedUDP
+	pkt := ip.pkt
 	p.fk = ip.fk
 	p.ipHdrLen = ip.ipHdrLen