Reduce heap allocs 2 (#46)

* reduce heap allocations in tcp logging; omit use of AppendFloat which allocates a metric sh*tton

* debugheaplog: better heap statistic logging

* heap: use string for pcap.Frame.Protocol

* add potential to eliminate Flags.String heap alloc, remove incorrect HEAP comments

* add StackAsync.DebugErr and httpraw.SetBytes

* many heap alloc reductions and replacement of bytes.Equal with internal.BytesEqual

arp/handler.go

@@ -1,7 +1,6 @@
 package arp
 
 import (
-	"bytes"
 	"errors"
 	"log/slog"
 
@@ -101,7 +100,7 @@ func (h *Handler) expectSize() int {
 
 func (h *Handler) QueryResult(protoAddr []byte) (hwAddr []byte, err error) {
 	for i := range h.queries {
-		if bytes.Equal(protoAddr, h.queries[i].protoaddr) {
+		if internal.BytesEqual(protoAddr, h.queries[i].protoaddr) {
 			if !h.queries[i].querysent {
 				return nil, errors.New("query not yet sent")
 			}
@@ -118,7 +117,7 @@ func (h *Handler) QueryResult(protoAddr []byte) (hwAddr []byte, err error) {
 func (h *Handler) DiscardQuery(protoAddr []byte) error {
 	for i := range h.queries {
 		q := &h.queries[i]
-		if bytes.Equal(protoAddr, q.protoaddr) {
+		if internal.BytesEqual(protoAddr, q.protoaddr) {
 			q.destroy()
 			return nil
 		}
@@ -240,7 +239,7 @@ func (h *Handler) Demux(ethFrame []byte, frameOffset int) error {
 	switch afrm.Operation() {
 	case OpRequest:
 		_, protoaddr := afrm.Target()
-		if !bytes.Equal(protoaddr, h.ourProtoAddr) {
+		if !internal.BytesEqual(protoaddr, h.ourProtoAddr) {
 			return nil // Not for us.
 		}
 		h.pendingResponse = h.pendingResponse[:len(h.pendingResponse)+1] // Extend pending buffer.
@@ -251,7 +250,7 @@ func (h *Handler) Demux(ethFrame []byte, frameOffset int) error {
 		for i := range h.queries {
 			q := &h.queries[i]
 			mac := q.response()
-			if mac == nil && bytes.Equal(q.protoaddr, protoaddr) {
+			if mac == nil && internal.BytesEqual(q.protoaddr, protoaddr) {
 				q.hwaddr = append(q.hwaddr, hwaddr...)
 				if q.dstHw != nil {
 					if !internal.IsZeroed(q.dstHw...) {

http/httpraw/cookie.go

@@ -2,7 +2,6 @@ package httpraw
 
 import (
 	"bytes"
-	"errors"
 )
 
 // Cookie implements cookie key-value parsing. Methods function similarly to eponymous [Header] methods.
@@ -55,7 +54,7 @@ func (dst *Cookie) CopyFrom(c Cookie) {
 // Parse parses the cookie's buffer in place.
 func (c *Cookie) Parse() error {
 	if len(c.kvs) > 0 {
-		return errors.New("cookies already parsed, reset before parsing again")
+		return errCookiesParsed
 	}
 	off := 0
 	for {

http/httpraw/header.go

@@ -2,9 +2,9 @@ package httpraw
 
 import (
 	"bytes"
-	"errors"
 	"io"
 	"slices"
+	"unsafe"
 )
 
 const (
@@ -75,7 +75,9 @@ func (h *Header) ParseBytes(asResponse bool, b []byte) error {
 // Parse parses accumulated data in-place with no copying. One can set HTTP header data buffer with [Header.Reset].
 // It fails if HTTP data is incomplete.
 func (h *Header) Parse(asResponse bool) error {
+	debuglog("http:parse:reset")
 	h.Reset(h.hbuf.buf)
+	debuglog("http:parse:start")
 	return h.parse(asResponse)
 }
 
@@ -97,7 +99,7 @@ func (h *Header) Parse(asResponse bool) error {
 //	}
 func (h *Header) TryParse(asResponse bool) (needMoreData bool, err error) {
 	if h.flags.hasAny(flagDoneParsingHeader) {
-		return false, errors.New("TryParse called after header parsed")
+		return false, errAlreadyParsed
 	} else if h.flags.hasAny(flagMangledBuffer) {
 		return false, errMangledBuffer
 	}
@@ -225,15 +227,18 @@ func (h *Header) Reset(buf []byte) {
 		panic("small buffer and flagNoBufferGrow set")
 	}
 	const persistentFlags = flagNoBufferGrow
+	debuglog("http:reset:hbuf")
 	h.hbuf.reset(buf)
 	*h = Header{
 		hbuf:  h.hbuf,
 		flags: h.flags & persistentFlags,
 	}
+	debuglog("http:reset:done")
 }
 
 // Body returns the surplus data following headers. It is only valid as long as Parse* or Reset methods are not called.
 func (h *Header) Body() ([]byte, error) {
+	debuglog("http:body")
 	if h.flags.hasAny(flagMangledBuffer) {
 		return nil, errMangledBuffer
 	} else if h.flags.hasAny(flagDoneParsingHeader) {
@@ -242,7 +247,14 @@ func (h *Header) Body() ([]byte, error) {
 	return nil, errUnparsed
 }
 
-// Set sets a key-value pair in the HTTP header. Calling Set mangles the buffer.
+// SetBytes is equivalent to [Header.Set] but with a []byte value. Does not keep reference to value slice.
+// Calling SetBytes Mangles the buffer.
+func (h *Header) SetBytes(key string, value []byte) {
+	h.Set(key, unsafe.String(&value[0], len(value)))
+}
+
+// Set sets a key-value pair in the HTTP header.
+// Calling Set mangles the buffer.
 func (h *Header) Set(key, value string) {
 	hb := &h.hbuf
 	var useKv *argsKV
@@ -269,10 +281,13 @@ func (h *Header) Set(key, value string) {
 
 // Get gets the first value of a key found in the headers. Use [Header.ForEach] to find multiple values corresponding to same key.
 func (h *Header) Get(key string) []byte {
+	debuglog("http:get:start")
 	kv := h.peekHeader(key)
 	if kv.isValid() {
+		debuglog("http:get:found")
 		return h.hbuf.musttoken(kv.value)
 	}
+	debuglog("http:get:notfound")
 	return nil
 }
 
@@ -338,7 +353,7 @@ func (h *Header) AppendRequest(dst []byte) ([]byte, error) {
 	if h.flags.hasAny(flagOOMReached) {
 		return dst, errOOM
 	} else if h.requestURI.len == 0 || h.method.len == 0 {
-		return dst, errors.New("need method/request URI to create request header")
+		return dst, errNeedMethodURI
 	} else if len(proto) == 0 {
 		return dst, errNoProto
 	}
@@ -368,7 +383,7 @@ func (h *Header) AppendResponse(dst []byte) ([]byte, error) {
 	if h.flags.hasAny(flagOOMReached) {
 		return dst, errOOM
 	} else if h.statusCode.len == 0 || h.statusText.len == 0 {
-		return dst, errors.New("invalid status code or text")
+		return dst, errBadStatusCodeTxt
 	} else if len(proto) == 0 {
 		return dst, errNoProto
 	}

http/httpraw/parse.go

@@ -3,8 +3,11 @@ package httpraw
 import (
 	"bytes"
 	"errors"
+	"log/slog"
 	"slices"
 	"unsafe"
+
+	"github.com/soypat/lneto/internal"
 )
 
 var (
@@ -16,8 +19,15 @@ var (
 	errOOM         = errors.New("httpraw: buffer out of memory")
 	// Header.Set and Header.Add mangles the buffer.
 	// Call them after retrieving the Body. Do not call them before parsing the header (why would you even do that?).
-	errMangledBuffer = errors.New("httpraw: mangled buffer")
-	errNoCookies     = errors.New("no cookie found")
+	errMangledBuffer    = errors.New("httpraw: mangled buffer")
+	errNoCookies        = errors.New("no cookie found")
+	errEmptyURI         = errors.New("empty URI")
+	errLongStatusCode   = errors.New("long status code")
+	errBadStatusCode    = errors.New("invalid status code")
+	errAlreadyParsed    = errors.New("TryParse called after header parsed")
+	errNeedMethodURI    = errors.New("need method/request URI to create request header")
+	errBadStatusCodeTxt = errors.New("invalid status code or text")
+	errCookiesParsed    = errors.New("cookies already parsed, reset before parsing again")
 )
 
 type headerBuf struct {
@@ -29,6 +39,20 @@ type headerBuf struct {
 	headers []argsKV
 }
 
+// reset sets the buffer data and discards all parsed data.
+func (h *headerBuf) reset(buf []byte) {
+	if buf == nil {
+		buf = h.buf[:0] // Reuse buffer but discard raw data on nil input.
+	}
+	if cap(h.headers) == 0 {
+		h.headers = make([]argsKV, 16)
+	}
+	*h = headerBuf{
+		buf:     buf,
+		headers: h.headers[:0],
+	}
+}
+
 type tokint = uint16
 
 type headerSlice struct {
@@ -56,11 +80,16 @@ type scannerState struct {
 }
 
 func (h *Header) parse(asResponse bool) (err error) {
+	debuglog("http:firstline:start")
 	err = h.parseFirstLine(asResponse)
 	if err != nil {
+		debuglog("http:firstline:err")
 		return err
 	}
-	return h.parseNextHeaders()
+	debuglog("http:firstline:done")
+	err = h.parseNextHeaders()
+	debuglog("http:headers:done")
+	return err
 }
 
 func (h *Header) parseFirstLine(asResponse bool) (err error) {
@@ -90,9 +119,11 @@ func (hb *headerBuf) readFromBytes(b []byte) {
 func (hb *headerBuf) free() int { return cap(hb.buf) - len(hb.buf) }
 
 func (hb *headerBuf) parseNextHeaders(ss *scannerState) {
+	debuglog("http:nexthdr:loop")
 	for kv := hb.next(ss); kv.isValid(); kv = hb.next(ss) {
-		hb.headers = append(hb.headers, kv)
+		hb.headers = append(hb.headers, kv) // TODO(HEAP): inc=16B slice growth when capacity exceeded
 	}
+	debuglog("http:nexthdr:done")
 }
 
 func (hb *headerBuf) offBuf() []byte {
@@ -127,6 +158,7 @@ func (hb *headerBuf) scanUntilByte(c byte) []byte {
 }
 
 func (hb *headerBuf) parseFirstLineRequest(initFlags flags) (method, uri, proto headerSlice, flags flags, err error) {
+	debuglog("http:req:scan")
 	hb.off = 0 // Parsing first line resets offset.
 	var b []byte
 	hb.skipLeadingCRLF()
@@ -135,6 +167,7 @@ func (hb *headerBuf) parseFirstLineRequest(initFlags flags) (method, uri, proto
 	if len(b) < 5 {
 		return method, uri, proto, flags, errNeedMore
 	}
+	debuglog("http:req:parse")
 
 	methodEnd := max(0, bytes.IndexByte(b, ' '))
 	reqURIEnd := bytes.IndexByte(b[methodEnd+1:], ' ')
@@ -145,7 +178,7 @@ func (hb *headerBuf) parseFirstLineRequest(initFlags flags) (method, uri, proto
 			flags |= flagNoHTTP11
 		}
 	} else if reqURIEnd == 0 {
-		return method, uri, proto, flags, errors.New("empty URI")
+		return method, uri, proto, flags, errEmptyURI
 	} else {
 		// No version provided.
 		reqURIEnd = methodEnd + 1
@@ -158,6 +191,7 @@ func (hb *headerBuf) parseFirstLineRequest(initFlags flags) (method, uri, proto
 }
 
 func (hb *headerBuf) parseFirstLineResponse(initFlags flags) (statusCode, statusText headerSlice, flags flags, err error) {
+	debuglog("http:resp:scan")
 	hb.off = 0 // Parsing first line resets offset.
 	var b []byte
 	hb.skipLeadingCRLF()
@@ -166,23 +200,23 @@ func (hb *headerBuf) parseFirstLineResponse(initFlags flags) (statusCode, status
 	if len(b) < 5 {
 		return statusCode, statusText, flags, errNeedMore
 	}
+	debuglog("http:resp:parse")
 
 	statusCodeEnd := max(0, bytes.IndexByte(b, ' '))
-	if statusCodeEnd < 0 {
-		return statusCode, statusText, flags, errors.New("missing status code")
-	}
 	code := b[:statusCodeEnd]
 	text := b[statusCodeEnd:]
 	if len(code) > 3 {
-		return statusCode, statusText, flags, errors.New("long status code")
+		return statusCode, statusText, flags, errLongStatusCode
 	}
 	for i := range code {
 		if code[i] > '9' || code[i] < '0' {
-			return statusCode, statusText, flags, errors.New("invalid status code")
+			debuglog("http:resp:invalid-code")
+			return statusCode, statusText, flags, errBadStatusCode
 		}
 	}
 	statusCode = hb.slice(code)
 	statusText = hb.slice(text)
+	debuglog("http:resp:done")
 	return statusCode, statusText, flags, nil
 }
 
@@ -360,17 +394,6 @@ func (hb *headerBuf) next(ss *scannerState) argsKV {
 	return resultKV
 }
 
-// reset sets the buffer data and discards all parsed data.
-func (h *headerBuf) reset(buf []byte) {
-	if buf == nil {
-		buf = h.buf[:0] // Reuse buffer but discard raw data on nil input.
-	}
-	*h = headerBuf{
-		buf:     buf,
-		headers: h.headers[:0],
-	}
-}
-
 // ConnectionClose returns true if 'Connection: close' header is set or if a invalid header was found.
 func (h *Header) ConnectionClose() bool {
 	closed := h.flags.hasAny(flagConnClose) ||
@@ -402,3 +425,11 @@ func bytes2tok(buf, value []byte) headerSlice {
 		len:   tokint(len(value)),
 	}
 }
+
+const enableDebug = false
+
+func debuglog(msg string) {
+	if enableDebug {
+		internal.LogAttrs(nil, slog.LevelDebug, msg)
+	}
+}

internal/debug.go

@@ -1,7 +1,86 @@
 package internal
 
-import "log/slog"
+import (
+	"log/slog"
+	"runtime"
+	"strconv"
+	"sync"
+	"unsafe"
+)
 
 const (
 	LevelTrace slog.Level = slog.LevelDebug - 2
+
+	usePrintLogAllocs = true
 )
+
+var (
+	memstats    runtime.MemStats
+	lastAllocs  uint64
+	lastMallocs uint64
+	allocmu     sync.Mutex
+	allocbuf    [256]byte
+)
+
+func LogAllocs(msg string) {
+	allocmu.Lock()
+	runtime.ReadMemStats(&memstats)
+	if memstats.TotalAlloc == lastAllocs {
+		allocmu.Unlock()
+		return
+	}
+	if usePrintLogAllocs {
+		print("[ALLOC] ", msg)
+		print(" inc=", int64(memstats.TotalAlloc)-int64(lastAllocs))
+		print(" n=", int64(memstats.Mallocs)-int64(lastMallocs))
+		print(" heap=", memstats.HeapAlloc)
+		print(" tot=", memstats.TotalAlloc)
+		println()
+	} else {
+		n := copy(allocbuf[:], "[ALLOC] ")
+		n += copy(allocbuf[n:], msg)
+		n += copyValueInt(allocbuf[n:], "inc", int64(memstats.TotalAlloc)-int64(lastAllocs))
+		n += copyValueInt(allocbuf[n:], "n", int64(memstats.Mallocs)-int64(lastMallocs))
+		n += copyValueUint(allocbuf[n:], "heap", memstats.HeapAlloc)
+		n += copyValueUint(allocbuf[n:], "tot", memstats.TotalAlloc)
+		println(unsafe.String(&allocbuf[0], n))
+	}
+	lastAllocs = memstats.TotalAlloc
+	lastMallocs = memstats.Mallocs
+	allocmu.Unlock()
+}
+
+func copyValueInt(buf []byte, key string, v int64) int {
+	// ' ' + key + '=' + up to 20 chars for int64
+	if len(buf) < 2+len(key)+20 {
+		return 0
+	}
+	buf[0] = ' '
+	n := 1 + copy(buf[1:], key)
+	buf[n] = '='
+	n++
+	return n + copyInt(buf[n:], v)
+}
+
+func copyValueUint(buf []byte, key string, v uint64) int {
+	if len(buf) < 2+len(key)+20 {
+		return 0
+	}
+	buf[0] = ' '
+	n := 1 + copy(buf[1:], key)
+	buf[n] = '='
+	n++
+	return n + copyUint(buf[n:], v)
+}
+
+// copyInt formats v into buf and returns the number of bytes written.
+// Caller must ensure cap(buf) >= 20.
+func copyInt(buf []byte, v int64) int {
+	return len(strconv.AppendInt(buf[:0], v, 10))
+}
+
+// copyUint formats v into buf and returns the number of bytes written.
+// Caller must ensure cap(buf) >= 20.
+func copyUint(buf []byte, v uint64) int {
+	return len(strconv.AppendUint(buf[:0], v, 10))
+}

internal/debug_heaplog.go

@@ -15,9 +15,6 @@ const (
 )
 
 var (
-	memstats   runtime.MemStats
-	lastAllocs uint64
-
 	timebuf [len(timefmt) * 2]byte
 )
 
@@ -28,12 +25,7 @@ func LogEnabled(l *slog.Logger, lvl slog.Level) bool {
 func LogAttrs(_ *slog.Logger, level slog.Level, msg string, attrs ...slog.Attr) {
 	now := time.Now()
 	n := len(now.AppendFormat(timebuf[:0], timefmt))
-	runtime.ReadMemStats(&memstats)
-	if memstats.TotalAlloc != lastAllocs {
-		print("[ALLOC] inc=", int64(memstats.TotalAlloc)-int64(lastAllocs))
-		print(" tot=", memstats.TotalAlloc, " seqs")
-		println()
-	}
+	LogAllocs(msg)
 	print("time=", unsafe.String(&timebuf[0], n), " ")
 	if level == LevelTrace {
 		print("TRACE ")
@@ -57,8 +49,12 @@ func LogAttrs(_ *slog.Logger, level slog.Level, msg string, attrs ...slog.Attr)
 		}
 	}
 	println()
+	allocmu.Lock()
 	runtime.ReadMemStats(&memstats)
-	if memstats.TotalAlloc != lastAllocs {
-		lastAllocs = memstats.TotalAlloc
+	if lastAllocs != memstats.TotalAlloc {
+		print("alloc increase in heaplog")
 	}
+	lastAllocs = memstats.TotalAlloc
+	lastMallocs = memstats.Mallocs
+	allocmu.Unlock()
 }

internal/ltesto/ltesto.go

@@ -1,12 +1,12 @@
 package ltesto
 
 import (
-	"bytes"
 	"math"
 	"math/rand"
 
 	"github.com/soypat/lneto"
 	"github.com/soypat/lneto/ethernet"
+	"github.com/soypat/lneto/internal"
 	"github.com/soypat/lneto/ipv4"
 	"github.com/soypat/lneto/ipv4/icmpv4"
 	"github.com/soypat/lneto/tcp"
@@ -134,9 +134,9 @@ func (gen *PacketGen) AppendRandomIPv4TCPPacket(dst []byte, rng *rand.Rand, seg
 	switch {
 	case gen.SrcTCP != tfrm.SourcePort():
 		panic("IP options overwrite TCP header")
-	case !bytes.Equal(ifrm.Options(), ipOpts):
+	case !internal.BytesEqual(ifrm.Options(), ipOpts):
 		panic("bad ip options written, ensure ip options length is multiple of 4")
-	case !bytes.Equal(tfrm.Options(), tcpOpts):
+	case !internal.BytesEqual(tfrm.Options(), tcpOpts):
 		panic("bad tcp options written, ensure tcp options length is multiple of 4")
 	case *ifrm.DestinationAddr() != gen.DstIPv4:
 		panic("IP options overwrite own header")

internal/slices.go

@@ -1,5 +1,7 @@
 package internal
 
+import "unsafe"
+
 // IsZeroed returns true if all arguments are set to their zero value.
 func IsZeroed[T comparable](a ...T) bool {
 	var z T
@@ -65,3 +67,15 @@ func SliceReclaim[T any](ptr *[]T) *T {
 	}
 	return &(*ptr)[n]
 }
+
+// BytesEqual is heapless replacement of [bytes.Equal] since it allocates in tinygo.
+// https://github.com/tinygo-org/tinygo/issues/4045
+func BytesEqual(a, b []byte) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	if len(a) == 0 {
+		return true
+	}
+	return unsafe.String(&a[0], len(a)) == unsafe.String(&b[0], len(b))
+}

internet/definitions_tinygo.go

@@ -21,6 +21,7 @@ func (s *cbnode) Encapsulate(carrierData []byte, offsetToIP, offsetToFrame int)
 }
 
 func (s *cbnode) Demux(carrierData []byte, frameOffset int) error {
+	debugLog("cbnode:pre-demux")
 	return s._demux(carrierData, frameOffset)
 }
 

internet/pcap/capture.go

@@ -6,6 +6,7 @@ import (
 	"encoding/binary"
 	"errors"
 	"fmt"
+	"log/slog"
 	"math"
 
 	"github.com/soypat/lneto"
@@ -29,12 +30,6 @@ var (
 	ErrFieldByClassNotFound = errors.New("pcap: field by class not found")
 )
 
-type proto string
-
-const (
-	ProtoEthernet proto = "Ethernet"
-)
-
 type PacketBreakdown struct {
 	hdr  httpraw.Header
 	dmsg dns.Message
@@ -66,8 +61,10 @@ func (pc *PacketBreakdown) initFrames() []Frame {
 }
 
 func (pc *PacketBreakdown) CaptureEthernet(dst []Frame, pkt []byte, bitOffset int) ([]Frame, error) {
+	debuglog("pcap:eth:start")
 	if dst == nil {
 		dst = pc.initFrames()
+		debuglog("pcap:eth:initframes")
 	}
 	if bitOffset%8 != 0 {
 		return dst, errors.New("ethernet must be parsed at byte boundary")
@@ -80,8 +77,10 @@ func (pc *PacketBreakdown) CaptureEthernet(dst []Frame, pkt []byte, bitOffset in
 	if pc.validator().HasError() {
 		return dst, pc.validator().ErrPop()
 	}
+	debuglog("pcap:eth:validated")
 
-	finfo := reclaimFrame(&dst, ProtoEthernet, bitOffset, baseEthernetFields[:])
+	finfo := reclaimFrame(&dst, "Ethernet", bitOffset, baseEthernetFields[:])
+	debuglog("pcap:eth:reclaimed")
 	etype := efrm.EtherTypeOrSize()
 	end := 14*octet + bitOffset
 	if etype.IsSize() {
@@ -102,12 +101,14 @@ func (pc *PacketBreakdown) CaptureEthernet(dst []Frame, pkt []byte, bitOffset in
 	case ethernet.TypeIPv6:
 		dst, err = pc.CaptureIPv6(dst, pkt, end)
 	default:
-		reclaimRemainingFrame(&dst, etype, FieldClassPayload, end, octet*len(pkt))
+		reclaimRemainingFrame(&dst, "Unknown Ethertype", FieldClassPayload, end, octet*len(pkt))
 	}
+	debuglog("pcap:eth:done")
 	return dst, err
 }
 
 func (pc *PacketBreakdown) CaptureARP(dst []Frame, pkt []byte, bitOffset int) ([]Frame, error) {
+	debuglog("pcap:arp:start")
 	if bitOffset%8 != 0 {
 		return dst, errors.New("ARP must be parsed at byte boundary")
 	}
@@ -120,7 +121,7 @@ func (pc *PacketBreakdown) CaptureARP(dst []Frame, pkt []byte, bitOffset int) ([
 		return dst, pc.validator().ErrPop()
 	}
 
-	finfo := reclaimFrame(&dst, ethernet.TypeARP, bitOffset, baseARPFields[:])
+	finfo := reclaimFrame(&dst, "ARP", bitOffset, baseARPFields[:])
 	const varstart = 8 * octet
 	_, hlen := afrm.Hardware()
 	_, plen := afrm.Protocol()
@@ -154,6 +155,7 @@ func (pc *PacketBreakdown) CaptureARP(dst []Frame, pkt []byte, bitOffset int) ([
 }
 
 func (pc *PacketBreakdown) CaptureIPv6(dst []Frame, pkt []byte, bitOffset int) ([]Frame, error) {
+	debuglog("pcap:ipv6:start")
 	if bitOffset%8 != 0 {
 		return dst, errors.New("IPv6 must be parsed at byte boundary")
 	}
@@ -165,7 +167,8 @@ func (pc *PacketBreakdown) CaptureIPv6(dst []Frame, pkt []byte, bitOffset int) (
 	if pc.validator().HasError() {
 		return dst, pc.validator().ErrPop()
 	}
-	reclaimFrame(&dst, ethernet.TypeIPv6, bitOffset, baseIPv6Fields[:])
+	reclaimFrame(&dst, "IPv6", bitOffset, baseIPv6Fields[:])
+	debuglog("pcap:ipv6:reclaimed")
 	proto := ifrm6.NextHeader()
 	end := bitOffset + 40*octet
 	var protoErrs []error
@@ -196,6 +199,7 @@ func (pc *PacketBreakdown) CaptureIPv6(dst []Frame, pkt []byte, bitOffset int) (
 }
 
 func (pc *PacketBreakdown) CaptureIPv4(dst []Frame, pkt []byte, bitOffset int) ([]Frame, error) {
+	debuglog("pcap:ipv4:start")
 	if bitOffset%8 != 0 {
 		return dst, errors.New("IPv4 must be parsed at byte boundary")
 	}
@@ -207,9 +211,11 @@ func (pc *PacketBreakdown) CaptureIPv4(dst []Frame, pkt []byte, bitOffset int) (
 	if pc.validator().HasError() {
 		return dst, pc.validator().ErrPop()
 	}
+	debuglog("pcap:ipv4:validated")
 	// limit packet to the actual IPv4 frame size
 	pkt = pkt[:bitOffset/8+int(ifrm4.TotalLength())]
-	finfo := reclaimFrame(&dst, ethernet.TypeIPv4, bitOffset, baseIPv4Fields[:])
+	finfo := reclaimFrame(&dst, "IPv4", bitOffset, baseIPv4Fields[:])
+	debuglog("pcap:ipv4:reclaimed")
 	options := ifrm4.Options()
 	if len(options) > 0 {
 		finfo.Fields = append(finfo.Fields, FrameField{
@@ -221,6 +227,7 @@ func (pc *PacketBreakdown) CaptureIPv4(dst []Frame, pkt []byte, bitOffset int) (
 	if ifrm4.CalculateHeaderCRC() != 0 {
 		finfo.Errors = append(finfo.Errors, lneto.ErrBadCRC)
 	}
+	debuglog("pcap:ipv4:crc-done")
 	proto := ifrm4.Protocol()
 	end := bitOffset + octet*ifrm4.HeaderLength()
 	var protoErrs []error
@@ -262,10 +269,12 @@ func (pc *PacketBreakdown) CaptureIPv4(dst []Frame, pkt []byte, bitOffset int) (
 			}
 		}
 	}
+	debuglog("pcap:ipv4:proto-crc-done")
 	return pc.captureIPProto(proto, dst, pkt, end, protoErrs...)
 }
 
 func (pc *PacketBreakdown) captureIPProto(proto lneto.IPProto, dst []Frame, pkt []byte, bitOffset int, ipProtoErrs ...error) (_ []Frame, err error) {
+	debuglog("pcap:ipproto:start")
 	nextFrame := len(dst)
 	switch proto {
 	case lneto.IPProtoTCP:
@@ -275,20 +284,22 @@ func (pc *PacketBreakdown) captureIPProto(proto lneto.IPProto, dst []Frame, pkt
 	case lneto.IPProtoUDPLite:
 		dst, err = pc.CaptureUDP(dst, pkt, bitOffset)
 		if len(dst) > nextFrame {
-			dst[nextFrame].Protocol = lneto.IPProtoUDPLite
+			dst[nextFrame].Protocol = "UDPLite"
 		}
 	case lneto.IPProtoICMP:
 		dst, err = pc.CaptureICMPv4(dst, pkt, bitOffset)
 	default:
-		reclaimRemainingFrame(&dst, proto, 0, bitOffset, octet*len(pkt))
+		reclaimRemainingFrame(&dst, "unknown proto", 0, bitOffset, octet*len(pkt))
 	}
 	if len(ipProtoErrs) > 0 && len(dst) > nextFrame {
 		dst[nextFrame].Errors = append(dst[nextFrame].Errors, ipProtoErrs...)
 	}
+	debuglog("pcap:ipproto:done")
 	return dst, err
 }
 
 func (pc *PacketBreakdown) CaptureTCP(dst []Frame, pkt []byte, bitOffset int) ([]Frame, error) {
+	debuglog("pcap:tcp:start")
 	if bitOffset%8 != 0 {
 		return dst, errors.New("TCP must be parsed at byte boundary")
 	}
@@ -300,8 +311,10 @@ func (pc *PacketBreakdown) CaptureTCP(dst []Frame, pkt []byte, bitOffset int) ([
 	if pc.validator().HasError() {
 		return dst, pc.validator().ErrPop()
 	}
+	debuglog("pcap:tcp:validated")
 	end := bitOffset + octet*tfrm.HeaderLength()
-	finfo := reclaimFrame(&dst, lneto.IPProtoTCP, bitOffset, baseTCPFields[:])
+	finfo := reclaimFrame(&dst, "TCP", bitOffset, baseTCPFields[:])
+	debuglog("pcap:tcp:reclaimed")
 	options := tfrm.Options()
 	if len(options) > 0 {
 		finfo.Fields = append(finfo.Fields, FrameField{
@@ -312,15 +325,19 @@ func (pc *PacketBreakdown) CaptureTCP(dst []Frame, pkt []byte, bitOffset int) ([
 	}
 	payload := tfrm.Payload()
 	if len(payload) > 0 {
+		debuglog("pcap:tcp:http-start")
 		dst, err = pc.CaptureHTTP(dst, pkt, end)
+		debuglog("pcap:tcp:http-done")
 		if err != nil {
 			reclaimRemainingFrame(&dst, unknownPayloadProto, FieldClassPayload, end, octet*len(pkt))
 		}
 	}
+	debuglog("pcap:tcp:done")
 	return dst, nil
 }
 
 func (pc *PacketBreakdown) CaptureUDP(dst []Frame, pkt []byte, bitOffset int) ([]Frame, error) {
+	debuglog("pcap:udp:start")
 	if bitOffset%8 != 0 {
 		return dst, errors.New("UDP must be parsed at byte boundary")
 	}
@@ -332,7 +349,8 @@ func (pc *PacketBreakdown) CaptureUDP(dst []Frame, pkt []byte, bitOffset int) ([
 	if pc.validator().HasError() {
 		return dst, pc.validator().ErrPop()
 	}
-	reclaimFrame(&dst, lneto.IPProtoUDP, bitOffset, baseUDPFields[:])
+	reclaimFrame(&dst, "UDP", bitOffset, baseUDPFields[:])
+	debuglog("pcap:udp:reclaimed")
 	end := bitOffset + 8*octet
 	payload := ufrm.Payload()
 	dstport := ufrm.DestinationPort()
@@ -347,10 +365,12 @@ func (pc *PacketBreakdown) CaptureUDP(dst []Frame, pkt []byte, bitOffset int) ([
 	if err != nil {
 		reclaimRemainingFrame(&dst, unknownPayloadProto, FieldClassPayload, end, octet*len(pkt))
 	}
+	debuglog("pcap:udp:done")
 	return dst, nil
 }
 
 func (pc *PacketBreakdown) CaptureICMPv4(dst []Frame, pkt []byte, bitOffset int) ([]Frame, error) {
+	debuglog("pcap:icmp:start")
 	if bitOffset%8 != 0 {
 		return dst, errors.New("ICMPv4 must be parsed at byte boundary")
 	}
@@ -360,7 +380,7 @@ func (pc *PacketBreakdown) CaptureICMPv4(dst []Frame, pkt []byte, bitOffset int)
 		return dst, err
 	}
 
-	finfo := reclaimFrame(&dst, lneto.IPProtoICMP, bitOffset, baseICMPv4Fields[:])
+	finfo := reclaimFrame(&dst, "ICMP", bitOffset, baseICMPv4Fields[:])
 
 	// Add type-specific fields.
 	switch ifrm.Type() {
@@ -541,9 +561,9 @@ func httpBodyClass(contentType, body []byte) FieldClass {
 		switch {
 		case bytes.HasPrefix(mediaType, []byte("text/")):
 			return FieldClassText
-		case bytes.Equal(mediaType, []byte("application/json")),
-			bytes.Equal(mediaType, []byte("application/javascript")),
-			bytes.Equal(mediaType, []byte("application/xml")):
+		case internal.BytesEqual(mediaType, []byte("application/json")),
+			internal.BytesEqual(mediaType, []byte("application/javascript")),
+			internal.BytesEqual(mediaType, []byte("application/xml")):
 			return FieldClassText
 		case bytes.HasSuffix(mediaType, []byte("+json")),
 			bytes.HasSuffix(mediaType, []byte("+xml")):
@@ -562,6 +582,7 @@ func httpBodyClass(contentType, body []byte) FieldClass {
 }
 
 func (pc *PacketBreakdown) CaptureHTTP(dst []Frame, pkt []byte, bitOffset int) ([]Frame, error) {
+	debuglog("pcap:http:start")
 	const httpProtocol = "HTTP"
 	if bitOffset%8 != 0 {
 		return dst, errors.New("HTTP must be parsed at byte boundary")
@@ -578,6 +599,7 @@ func (pc *PacketBreakdown) CaptureHTTP(dst []Frame, pkt []byte, bitOffset int) (
 	if err != nil {
 		return dst, err
 	}
+	debuglog("pcap:http:parsed")
 	hdrLen := pc.hdr.BufferParsed()
 	body, _ := pc.hdr.Body()
 	bodyClass := httpBodyClass(pc.hdr.Get("Content-Type"), body)
@@ -596,6 +618,7 @@ func (pc *PacketBreakdown) CaptureHTTP(dst []Frame, pkt []byte, bitOffset int) (
 			BitLength:      len(body) * octet,
 		},
 	)
+	debuglog("pcap:http:done")
 	return dst, nil
 }
 
@@ -626,7 +649,7 @@ func (ff Flags) IsRightAligned() bool { return ff&FlagRightAligned != 0 }
 
 type Frame struct {
 	PacketBitOffset int
-	Protocol        any
+	Protocol        string
 	Fields          []FrameField
 	Errors          []error
 }
@@ -1306,7 +1329,7 @@ var baseNTPFields = [...]FrameField{
 // reclaimFrame extends dst via [internal.SliceReclaim], resets the reclaimed Frame
 // with the given protocol and bit offset while preserving Fields and Errors
 // backing arrays, and appends baseFields. Returns the Frame for further modification.
-func reclaimFrame(dst *[]Frame, proto any, bitOffset int, baseFields []FrameField) *Frame {
+func reclaimFrame(dst *[]Frame, proto string, bitOffset int, baseFields []FrameField) *Frame {
 	finfo := internal.SliceReclaim(dst)
 	*finfo = Frame{
 		PacketBitOffset: bitOffset,
@@ -1319,10 +1342,18 @@ func reclaimFrame(dst *[]Frame, proto any, bitOffset int, baseFields []FrameFiel
 
 // reclaimRemainingFrame extends dst via SliceReclaim and populates the reclaimed
 // Frame as a single-field "remaining payload" frame, reusing the old Fields backing array.
-func reclaimRemainingFrame(dst *[]Frame, proto any, class FieldClass, pktBitOffset, pktBitLen int) {
+func reclaimRemainingFrame(dst *[]Frame, proto string, class FieldClass, pktBitOffset, pktBitLen int) {
 	finfo := reclaimFrame(dst, proto, pktBitOffset, nil)
 	finfo.Fields = append(finfo.Fields, FrameField{
 		Class:     class,
 		BitLength: pktBitLen - pktBitOffset,
 	})
 }
+
+const enableDebug = false
+
+func debuglog(msg string) {
+	if enableDebug {
+		internal.LogAttrs(nil, slog.LevelDebug, msg)
+	}
+}

internet/pcap/capture_test.go

@@ -209,7 +209,7 @@ func TestRightAlignedFields(t *testing.T) {
 	// Find IPv6 frame.
 	var ipv6Frame *Frame
 	for i := range frames {
-		if frames[i].Protocol == ethernet.TypeIPv6 {
+		if frames[i].Protocol == "IPv6" {
 			ipv6Frame = &frames[i]
 			break
 		}
@@ -476,15 +476,16 @@ func ExampleFormatter_dhcp() {
 	// Output:
 	// Ethernet len=14; destination=ff:ff:ff:ff:ff:ff; source=de:ad:be:ef:ca:fe; protocol=0x0800
 	// IPv4 len=20; version=0x04; (Header Length)=5; (Type of Service)=0x00; (Total Length)=312; identification=0x6043; flags=0x4000; (Time to live)=0x40; protocol=0x11; checksum=0xd972; source=0.0.0.0; destination=255.255.255.255
-	// UDP [RFC768] len=8; (Source port)=68; (Destination port)=67; size=292; checksum=0x0000
+	// UDP len=8; (Source port)=68; (Destination port)=67; size=292; checksum=0x0000
 	// DHCPv4 len=240; op=1; (Hardware Address Type)=0x01; (Hardware Address Length)=6; Hops=0x00; (Transaction ID)=0xdeadbeef; (Start Time)=0x0001; Flags=0x0000; (Client Address)=0.0.0.0; (Offered Address)=0.0.0.0; (Server Next Address)=255.255.255.255; (Relay Agent Address)=0.0.0.0; (Client Hardware Address)=de:ad:be:ef:ca:fe; options
-	//	(DHCP message type.)=1
-	//	(Parameter request list)=0x0102031a1c060f2a
-	//	(DHCP maximum message size)=558
-	//	(Requested IP address)=192.168.1.100
-	//	(Client identifier)="lneto-test"
-	//	(Hostname string)="myhost"
+	// 	(DHCP message type.)=1
+	// 	(Parameter request list)=0x0102031a1c060f2a
+	// 	(DHCP maximum message size)=558
+	// 	(Requested IP address)=192.168.1.100
+	// 	(Client identifier)="lneto-test"
+	// 	(Hostname string)="myhost"
 }
+
 func writeOpt(dst []byte, opt dhcpv4.OptNum, data ...byte) int {
 	dst[0] = byte(opt)
 	dst[1] = byte(len(data))

internet/pcap/format.go

@@ -4,7 +4,6 @@ import (
 	"encoding/binary"
 	"encoding/hex"
 	"errors"
-	"fmt"
 	"math"
 	"net/netip"
 	"slices"
@@ -12,8 +11,8 @@ import (
 	"strings"
 	"sync"
 	_ "time"
+	"unsafe"
 
-	"github.com/soypat/lneto"
 	"github.com/soypat/lneto/ethernet"
 	"github.com/soypat/lneto/ntp"
 	"github.com/soypat/lneto/tcp"
@@ -38,6 +37,7 @@ type Formatter struct {
 // FormatFrames appends the formatted frame data to the destination buffer according the Formatter state.
 // Is equivalent to [Formatter.FormatFrame] called on each Frame with the FrameSep inserted between frames.
 func (f *Formatter) FormatFrames(dst []byte, frms []Frame, pkt []byte) (_ []byte, err error) {
+	debuglog("pcap:fmt:start")
 	sep := f.frameSep()
 	for ifrm := range frms {
 		if ifrm != 0 {
@@ -48,14 +48,16 @@ func (f *Formatter) FormatFrames(dst []byte, frms []Frame, pkt []byte) (_ []byte
 			return dst, err
 		}
 	}
+	debuglog("pcap:fmt:done")
 	return dst, nil
 }
 
 // FormatFrame formats a single frame's protocol, fields, and errors into dst.
 func (f *Formatter) FormatFrame(dst []byte, frm Frame, pkt []byte) (_ []byte, err error) {
+	debuglog("pcap:fmtframe:start")
 	sep := f.fieldSep()
 	bitlen := frm.LenBits()
-	dst = appendProtocol(dst, frm.Protocol)
+	dst = append(dst, frm.Protocol...)
 	if bitlen%8 == 0 {
 		dst = append(dst, " len="...)
 		dst = strconv.AppendInt(dst, int64(bitlen/8), 10)
@@ -70,7 +72,7 @@ func (f *Formatter) FormatFrame(dst []byte, frm Frame, pkt []byte) (_ []byte, er
 			continue
 		}
 		dst = append(dst, sep...)
-		if field.Class == FieldClassFlags && frm.Protocol == lneto.IPProtoTCP {
+		if field.Class == FieldClassFlags && frm.Protocol == "TCP" {
 			// TCP flags pretty print special case.
 			dst = append(dst, "flags="...)
 			v, err := f.fieldAsUint(pkt, frm.PacketBitOffset+field.FrameBitOffset, field.BitLength, field.Flags.IsRightAligned())
@@ -95,6 +97,7 @@ func (f *Formatter) FormatFrame(dst []byte, frm Frame, pkt []byte) (_ []byte, er
 		}
 		dst = append(dst, ')')
 	}
+	debuglog("pcap:fmtframe:done")
 	return dst, nil
 }
 
@@ -135,6 +138,9 @@ func (f *Formatter) formatField(dst []byte, pktStartOff int, field FrameField, p
 	if hasSpaces {
 		dst = append(dst, ')')
 	}
+	if field.BitLength == 0 {
+		return dst, nil
+	}
 	dst = append(dst, '=')
 	f.mubuf.Lock()
 	defer f.mubuf.Unlock()
@@ -142,6 +148,7 @@ func (f *Formatter) formatField(dst []byte, pktStartOff int, field FrameField, p
 	if err != nil {
 		return dst, err
 	}
+	debuglog("pcap:fmtfield:append-done")
 	fieldBitStart := pktStartOff + field.FrameBitOffset
 	switch field.Class {
 	default:
@@ -151,6 +158,7 @@ func (f *Formatter) formatField(dst []byte, pktStartOff int, field FrameField, p
 		dst = append(dst, "0x"...)
 		dst = hex.AppendEncode(dst, f.buf)
 	case FieldClassTimestamp:
+		debuglog("pcap:fmtfield:timestamp")
 		// inspired by [time.RFC3339]
 		const littlerfc3339 = "2006-01-02T15:04:05.9999"
 		if len(f.buf) != 8 {
@@ -158,8 +166,13 @@ func (f *Formatter) formatField(dst []byte, pktStartOff int, field FrameField, p
 		}
 		ts := ntp.TimestampFromUint64(binary.BigEndian.Uint64(f.buf))
 		dst = ts.Time().AppendFormat(dst, littlerfc3339)
+		debuglog("pcap:fmtfield:timestamp-done")
 	case FieldClassText:
-		dst = strconv.AppendQuote(dst, string(f.buf))
+		debuglog("pcap:fmtfield:text")
+		if len(f.buf) > 0 {
+			dst = strconv.AppendQuote(dst, unsafe.String(&f.buf[0], len(f.buf)))
+		}
+		debuglog("pcap:fmtfield:text-done")
 	case FieldClassDst, FieldClassSrc, FieldClassSize, FieldClassAddress, FieldClassOperation:
 		// IP, MAC addresses and ports.
 		if field.BitLength <= 16 {
@@ -219,20 +232,3 @@ func (f *Formatter) fieldAsUint(pkt []byte, fieldBitStart, bitlen int, rightAlig
 	}
 	return binary.BigEndian.Uint64(f.uintBuf[:]), nil
 }
-
-// appendProtocol appends the string representation of a Frame.Protocol value
-// to dst without going through fmt.Appendf reflect machinery.
-func appendProtocol(dst []byte, p any) []byte {
-	switch p := p.(type) {
-	case proto:
-		return append(dst, string(p)...)
-	case string:
-		return append(dst, p...)
-	case ethernet.Type:
-		return append(dst, p.String()...)
-	case lneto.IPProto:
-		return append(dst, p.String()...)
-	default:
-		return fmt.Appendf(dst, "%v", p)
-	}
-}

internet/stack-ip.go

@@ -232,3 +232,11 @@ func (l logger) debug(msg string, attrs ...slog.Attr) {
 func (l logger) trace(msg string, attrs ...slog.Attr) {
 	internal.LogAttrs(l.log, internal.LevelTrace, msg, attrs...)
 }
+
+const enableAllocLog = false
+
+func debugLog(msg string) {
+	if enableAllocLog {
+		internal.LogAllocs(msg)
+	}
+}

tcp/conn.go

@@ -1,7 +1,6 @@
 package tcp
 
 import (
-	"bytes"
 	"errors"
 	"log/slog"
 	"net"
@@ -20,6 +19,8 @@ var (
 	errNoRemoteAddr        = errors.New("tcp: no remote address established")
 	errInvalidIP           = errors.New("tcp: invalid IP")
 	errMismatchedIPVersion = errors.New("mismatched IP version")
+	errBadDemuxOffset      = errors.New("bad offset in TCPConn.Recv")
+	errIPAddrMismatch      = errors.New("IP addr mismatch on TCPConn")
 )
 
 // Conn builds on the [Handler] abstraction and adds IP header knowledge, time management, and familiar user facing API
@@ -329,14 +330,14 @@ func (conn *Conn) Demux(buf []byte, off int) (err error) {
 	conn.mu.Lock()
 	defer conn.mu.Unlock()
 	if off >= len(buf) {
-		return errors.New("bad offset in TCPConn.Recv")
+		return errBadDemuxOffset
 	}
 	raddr, _, id, _, err := internal.GetIPAddr(buf[:off])
 	if err != nil {
 		return err
 	}
-	if conn.isRaddrSet() && !bytes.Equal(conn.remoteAddr, raddr) {
-		return errors.New("IP addr mismatch on TCPConn")
+	if conn.isRaddrSet() && !internal.BytesEqual(conn.remoteAddr, raddr) {
+		return errIPAddrMismatch
 	}
 	conn.trace("tcpconn.Recv", slog.Uint64("lport", uint64(conn.h.LocalPort())), slog.Uint64("rport", uint64(conn.h.remotePort)))
 	err = conn.h.Recv(buf[off:])

tcp/definitions.go

@@ -183,6 +183,8 @@ func (flags Flags) HasAny(mask Flags) bool { return flags&mask != 0 }
 // Mask returns the flags with non-flag bits unset.
 func (flags Flags) Mask() Flags { return flags & flagMask }
 
+func (flags Flags) Invalid() bool { return flags&flagMask != flags }
+
 // StringFlags returns human readable flag string. i.e:
 //
 //	"[SYN,ACK]"
@@ -201,6 +203,8 @@ func (flags Flags) String() string {
 		return "[FIN,ACK]"
 	case pshack:
 		return "[PSH,ACK]"
+	case FlagFIN | FlagPSH | FlagACK:
+		return "[FIN,PSH,ACK]"
 	case FlagACK:
 		return "[ACK]"
 	case FlagSYN:
@@ -210,41 +214,48 @@ func (flags Flags) String() string {
 	case FlagRST:
 		return "[RST]"
 	}
-	if flags&flagMask != flags {
+	if flags.Invalid() {
 		return strInvalidTCPFlags
 	}
-	buf := make([]byte, 0, 2+3*bits.OnesCount16(uint16(flags)))
-	buf = append(buf, '[')
-	buf = flags.AppendFormat(buf)
-	buf = append(buf, ']')
-	return string(buf)
+	// Since Go 1.26 this should not allocate if returned string does not escape and is smaller than 32 bytes.
+	// https://go.dev/blog/allocation-optimizations
+	var buf [2 + 4*9]byte
+	buf[0] = '['
+	n := flags.format((*[36]byte)(buf[1:]))
+	buf[1+n] = ']'
+	return string(buf[:2+n])
+}
+
+// AppendFormat appends a human readable flag string to b returning the extended buffer.
+func (flags Flags) AppendFormat(b []byte) []byte {
+	var buf [36]byte
+	n := flags.format(&buf)
+	return append(b, buf[:n]...)
 }
 
 const strInvalidTCPFlags = "<invalid TCP flags>"
 
-// AppendFormat appends a human readable flag string to b returning the extended buffer.
-func (flags Flags) AppendFormat(b []byte) []byte {
+func (flags Flags) format(buf *[4 * 9]byte) (n int) {
 	if flags == 0 {
-		return b
-	} else if flags&flagMask != flags {
-		return append(b, strInvalidTCPFlags...)
+		return 0
+	} else if flags.Invalid() {
+		return copy(buf[:], strInvalidTCPFlags)
 	}
-
-	// String Flag const
 	const flaglen = 3
 	const strflags = "FINSYNRSTPSHACKURGECECWRNS "
 	var addcommas bool
 	for flags != 0 { // written by Github Copilot- looks OK.
 		i := bits.TrailingZeros16(uint16(flags))
 		if addcommas {
-			b = append(b, ',')
+			buf[n] = ','
+			n++
 		} else {
 			addcommas = true
 		}
-		b = append(b, strflags[i*flaglen:i*flaglen+flaglen]...)
+		n += copy(buf[n:], strflags[i*flaglen:i*flaglen+flaglen])
 		flags &= ^(1 << i)
 	}
-	return b
+	return n
 }
 
 // State enumerates states a TCP connection progresses through during its lifetime as per RFC9293.

tcp/handler.go

@@ -216,7 +216,13 @@ func (h *Handler) Recv(incomingPacket []byte) error {
 		}
 	}
 	if h.logenabled(internal.LevelTrace) {
-		h.trace("tcp.Handler:rx-done", slog.Uint64("port", uint64(h.localPort)), slog.Uint64("remoteport", uint64(remotePort)), slog.String("seg", segIncoming.String()))
+		h.trace("tcp.Handler:rx-done",
+			slog.Uint64("lport", uint64(h.localPort)),
+			slog.Uint64("rport", uint64(remotePort)),
+			slog.Uint64("seg.seq", uint64(segIncoming.SEQ)),
+			slog.Uint64("seg.ack", uint64(segIncoming.ACK)),
+			slog.Uint64("seg.datalen", uint64(segIncoming.DATALEN)),
+		)
 	}
 	return nil
 }

tcp/listener.go

@@ -1,7 +1,6 @@
 package tcp
 
 import (
-	"bytes"
 	"errors"
 	"log/slog"
 	"net"
@@ -293,7 +292,7 @@ func getConn(conns []handler, remotePort uint16, remoteAddr []byte) int {
 		}
 		gotPort := conn.RemotePort()
 		gotaddr := conn.RemoteAddr()
-		if remotePort == gotPort && bytes.Equal(remoteAddr, gotaddr) {
+		if remotePort == gotPort && internal.BytesEqual(remoteAddr, gotaddr) {
 			return i
 		}
 	}

tcp/tcp_test.go

@@ -788,3 +788,99 @@ func FuzzTCBActions(f *testing.F) {
 		// hasPanicked = false
 	})
 }
+
+func TestFlagsString(t *testing.T) {
+	tests := []struct {
+		flags tcp.Flags
+		want  string
+	}{
+		{flags: 0, want: "[]"},
+		{flags: tcp.FlagSYN, want: "[SYN]"},
+		{flags: tcp.FlagACK, want: "[ACK]"},
+		{flags: tcp.FlagFIN, want: "[FIN]"},
+		{flags: tcp.FlagRST, want: "[RST]"},
+		{flags: tcp.FlagSYN | tcp.FlagACK, want: "[SYN,ACK]"},
+		{flags: tcp.FlagFIN | tcp.FlagACK, want: "[FIN,ACK]"},
+		{flags: tcp.FlagPSH | tcp.FlagACK, want: "[PSH,ACK]"},
+		// Multi-flag combinations.
+		{flags: tcp.FlagFIN | tcp.FlagSYN | tcp.FlagACK, want: "[FIN,SYN,ACK]"},
+		{flags: tcp.FlagURG | tcp.FlagACK, want: "[ACK,URG]"},
+		{flags: tcp.FlagECE | tcp.FlagCWR, want: "[ECE,CWR]"},
+		{flags: tcp.FlagNS | tcp.FlagACK, want: "[ACK,NS ]"},
+		// All flags set.
+		{flags: tcp.FlagFIN | tcp.FlagSYN | tcp.FlagRST | tcp.FlagPSH | tcp.FlagACK | tcp.FlagURG | tcp.FlagECE | tcp.FlagCWR | tcp.FlagNS, want: "[FIN,SYN,RST,PSH,ACK,URG,ECE,CWR,NS ]"},
+		// Invalid flags (bits above mask).
+		{flags: 0xFFFF, want: "<invalid TCP flags>"},
+	}
+	for _, tc := range tests {
+		got := tc.flags.String()
+		if got != tc.want {
+			t.Errorf("Flags(%#x).String() = %q; want %q", uint16(tc.flags), got, tc.want)
+		}
+	}
+}
+
+func TestFlagsAppendFormat(t *testing.T) {
+	tests := []struct {
+		flags tcp.Flags
+		want  string
+	}{
+		{flags: 0, want: ""},
+		{flags: tcp.FlagSYN, want: "SYN"},
+		{flags: tcp.FlagACK, want: "ACK"},
+		{flags: tcp.FlagFIN, want: "FIN"},
+		{flags: tcp.FlagRST, want: "RST"},
+		{flags: tcp.FlagSYN | tcp.FlagACK, want: "SYN,ACK"},
+		{flags: tcp.FlagFIN | tcp.FlagACK, want: "FIN,ACK"},
+		{flags: tcp.FlagPSH | tcp.FlagACK, want: "PSH,ACK"},
+		{flags: tcp.FlagFIN | tcp.FlagSYN | tcp.FlagACK, want: "FIN,SYN,ACK"},
+		{flags: tcp.FlagURG | tcp.FlagACK, want: "ACK,URG"},
+		{flags: tcp.FlagECE | tcp.FlagCWR, want: "ECE,CWR"},
+		{flags: tcp.FlagNS | tcp.FlagACK, want: "ACK,NS "},
+		{flags: tcp.FlagFIN | tcp.FlagSYN | tcp.FlagRST | tcp.FlagPSH | tcp.FlagACK | tcp.FlagURG | tcp.FlagECE | tcp.FlagCWR | tcp.FlagNS, want: "FIN,SYN,RST,PSH,ACK,URG,ECE,CWR,NS "},
+		{flags: 0xFFFF, want: "<invalid TCP flags>"},
+	}
+	for _, tc := range tests {
+		got := string(tc.flags.AppendFormat(nil))
+		if got != tc.want {
+			t.Errorf("Flags(%#x).AppendFormat(nil) = %q; want %q", uint16(tc.flags), got, tc.want)
+		}
+	}
+
+	// Test that AppendFormat appends to existing data.
+	prefix := []byte("flags=")
+	got := string(SYNACK.AppendFormat(prefix))
+	if want := "flags=SYN,ACK"; got != want {
+		t.Errorf("AppendFormat with prefix = %q; want %q", got, want)
+	}
+}
+
+func TestFlagsStringAppendFormatConsistency(t *testing.T) {
+	// Verify String() and AppendFormat() produce consistent results
+	// for all single-flag and common multi-flag values.
+	allFlags := []tcp.Flags{
+		0,
+		tcp.FlagFIN, tcp.FlagSYN, tcp.FlagRST, tcp.FlagPSH,
+		tcp.FlagACK, tcp.FlagURG, tcp.FlagECE, tcp.FlagCWR, tcp.FlagNS,
+		SYNACK, FINACK, PSHACK,
+		tcp.FlagFIN | tcp.FlagSYN | tcp.FlagRST | tcp.FlagPSH | tcp.FlagACK | tcp.FlagURG | tcp.FlagECE | tcp.FlagCWR | tcp.FlagNS,
+	}
+	for _, flags := range allFlags {
+		str := flags.String()
+		appended := string(flags.AppendFormat(nil))
+		// String() wraps in brackets, AppendFormat() does not.
+		if flags == 0 {
+			if str != "[]" {
+				t.Errorf("Flags(0).String() = %q; want %q", str, "[]")
+			}
+			if appended != "" {
+				t.Errorf("Flags(0).AppendFormat() = %q; want empty", appended)
+			}
+			continue
+		}
+		wantStr := "[" + appended + "]"
+		if str != wantStr {
+			t.Errorf("Flags(%#x): String()=%q inconsistent with AppendFormat()=%q (want %q)", uint16(flags), str, appended, wantStr)
+		}
+	}
+}

tcp/txqueue.go

@@ -7,7 +7,12 @@ import (
 )
 
 var (
-	errPacketQueueFull = errors.New("packet queue full")
+	errPacketQueueFull  = errors.New("packet queue full")
+	errQueuedPacketsLEZ = errors.New("queued packets <=0")
+	errInvalidBufSize   = errors.New("invalid buffer size")
+	errSeqLessThanLast  = errors.New("sequence number less than last sequence number")
+	errNoPacketToAck    = errors.New("no packet to ack")
+	errAckUnsent        = errors.New("ack of unsent packet")
 )
 
 const (
@@ -55,9 +60,9 @@ type ringidx struct {
 func (rtx *ringTx) Reset(buf []byte, maxqueuedPackets int, iss Value) error {
 	buf = buf[:len(buf):len(buf)] // safely omit capacity section.
 	if maxqueuedPackets <= 0 {
-		return errors.New("queued packets <=0")
+		return errQueuedPacketsLEZ
 	} else if len(buf) < minBufferSize || len(buf) < maxqueuedPackets {
-		return errors.New("invalid buffer size")
+		return errInvalidBufSize
 	}
 
 	*rtx = ringTx{
@@ -126,7 +131,7 @@ func (rtx *ringTx) MakePacket(b []byte, currentSeq Value) (int, error) {
 	}
 	endSeq, ok := rtx.sentEndSeq()
 	if ok && currentSeq.LessThan(endSeq) {
-		return 0, errors.New("sequence number less than last sequence number")
+		return 0, errSeqLessThanLast
 	}
 	// Reading unsent ring consumes unsent and converts it to "sent".
 	unsent, _ := rtx.unsentRing()
@@ -316,11 +321,11 @@ func (sl *sentlist) AddPacket(datalen, off, bufsize int, seq Value) *ringidx {
 func (sl *sentlist) RecvAck(ack Value, bufsize int) error {
 	newest := sl.Newest()
 	if newest == nil {
-		return errors.New("no packet to ack")
+		return errNoPacketToAck
 	}
 	endseq := newest.endSeq()
 	if endseq.LessThan(ack) {
-		return errors.New("ack of unsent packet")
+		return errAckUnsent
 	}
 	// Mark fully acked.
 	for i := 0; i < len(sl.pkts); i++ {

x/xnet/pcap-printer.go

@@ -62,7 +62,33 @@ func (stack *CapturePrinter) PrintPacket(prefix string, pkt []byte) {
 	if err == nil {
 		if useTimestamps {
 			diff := captime.Sub(stack.origin)
-			fmtbuf = strconv.AppendFloat(fmtbuf, diff.Seconds(), 'f', stack.timeprec, 32)
+			ms := diff.Milliseconds()
+			fmtbuf = strconv.AppendInt(fmtbuf, ms/1000, 10)
+			fmtbuf = append(fmtbuf, '.')
+			frac := ms % 1000
+			if frac < 0 {
+				frac = -frac
+			}
+			// Pad fractional part to timeprec digits (up to 3).
+			switch {
+			case stack.timeprec >= 3:
+				if frac < 100 {
+					fmtbuf = append(fmtbuf, '0')
+				}
+				if frac < 10 {
+					fmtbuf = append(fmtbuf, '0')
+				}
+				fmtbuf = strconv.AppendInt(fmtbuf, frac, 10)
+			case stack.timeprec == 2:
+				frac /= 10 // truncate to centiseconds
+				if frac < 10 {
+					fmtbuf = append(fmtbuf, '0')
+				}
+				fmtbuf = strconv.AppendInt(fmtbuf, frac, 10)
+			case stack.timeprec == 1:
+				frac /= 100 // truncate to deciseconds
+				fmtbuf = strconv.AppendInt(fmtbuf, frac, 10)
+			}
 			fmtbuf = append(fmtbuf, ' ')
 		}
 		fmtbuf = append(fmtbuf, prefix...)

x/xnet/stack-async.go

@@ -2,6 +2,7 @@ package xnet
 
 import (
 	"errors"
+	"log/slog"
 	"net/netip"
 	"sync"
 	"time"
@@ -11,6 +12,7 @@ import (
 	"github.com/soypat/lneto/dhcpv4"
 	"github.com/soypat/lneto/dns"
 	"github.com/soypat/lneto/ethernet"
+	"github.com/soypat/lneto/internal"
 	"github.com/soypat/lneto/internet"
 	"github.com/soypat/lneto/ntp"
 	"github.com/soypat/lneto/tcp"
@@ -566,3 +568,28 @@ func addr4(addr [4]byte, ok bool) netip.Addr {
 	}
 	return netip.AddrFrom4(addr)
 }
+
+// Debug prints debugging information. Very useful for users when coupled with
+// the debugheaplog build tag. See [internal.LogAttrs] debugheaplog version.
+//
+//	go build -tags=debugheaplog ./yourprogram
+func (s *StackAsync) Debug(msg string) {
+	internal.LogAttrs(slog.Default(), slog.LevelDebug, "stackasync",
+		slog.String("umsg", msg),
+		slog.Uint64("sent", s.totalsent),
+		slog.Uint64("recv", s.totalrecv),
+	)
+}
+
+// DebugErr prints debugging and error info. Very useful for users when coupled with
+// the debugheaplog build tag. See [internal.LogAttrs] debugheaplog version.
+//
+//	go build -tags=debugheaplog ./yourprogram
+func (s *StackAsync) DebugErr(msg, err string) {
+	internal.LogAttrs(slog.Default(), slog.LevelError, "stackasync",
+		slog.String("umsg", msg),
+		slog.String("err", err),
+		slog.Uint64("sent", s.totalsent),
+		slog.Uint64("recv", s.totalrecv),
+	)
+}

x/xnet/xnet_test.go

@@ -439,22 +439,22 @@ func (tst *tester) TCPExchange(expect tcpExpectExchange, stack1, stack2 *StackAs
 	}
 	srcEth := src.HardwareAddress()
 	dstEth := dst.HardwareAddress()
-	if !bytes.Equal(srcEth[:], tst.getData(pcap.ProtoEthernet, pcap.FieldClassSrc)) {
-		t.Errorf("mismatched ethernet src addr %x", tst.getData(pcap.ProtoEthernet, pcap.FieldClassSrc))
+	if !bytes.Equal(srcEth[:], tst.getData(protoEthernet, pcap.FieldClassSrc)) {
+		t.Errorf("mismatched ethernet src addr %x", tst.getData(protoEthernet, pcap.FieldClassSrc))
 	}
-	if !bytes.Equal(dstEth[:], tst.getData(pcap.ProtoEthernet, pcap.FieldClassDst)) {
-		t.Errorf("mismatched ethernet dst addr %x", tst.getData(pcap.ProtoEthernet, pcap.FieldClassDst))
+	if !bytes.Equal(dstEth[:], tst.getData(protoEthernet, pcap.FieldClassDst)) {
+		t.Errorf("mismatched ethernet dst addr %x", tst.getData(protoEthernet, pcap.FieldClassDst))
 	}
-	if tst.getInt(ethernet.TypeIPv4, pcap.FieldClassVersion) != 4 {
-		t.Errorf("did not get IP version=4, got=%d", tst.getInt(ethernet.TypeIPv4, pcap.FieldClassVersion))
+	if tst.getInt(protoIPv4, pcap.FieldClassVersion) != 4 {
+		t.Errorf("did not get IP version=4, got=%d", tst.getInt(protoIPv4, pcap.FieldClassVersion))
 	}
 	srcAddr := src.Addr()
 	dstAddr := dst.Addr()
-	if !bytes.Equal(srcAddr.AsSlice(), tst.getData(ethernet.TypeIPv4, pcap.FieldClassSrc)) {
-		t.Errorf("mismatched ip src addr %d", tst.getData(ethernet.TypeIPv4, pcap.FieldClassSrc))
+	if !bytes.Equal(srcAddr.AsSlice(), tst.getData(protoIPv4, pcap.FieldClassSrc)) {
+		t.Errorf("mismatched ip src addr %d", tst.getData(protoIPv4, pcap.FieldClassSrc))
 	}
-	if !bytes.Equal(dstAddr.AsSlice(), tst.getData(ethernet.TypeIPv4, pcap.FieldClassDst)) {
-		t.Errorf("mismatched ip dst addr %d", tst.getData(ethernet.TypeIPv4, pcap.FieldClassDst))
+	if !bytes.Equal(dstAddr.AsSlice(), tst.getData(protoIPv4, pcap.FieldClassDst)) {
+		t.Errorf("mismatched ip dst addr %d", tst.getData(protoIPv4, pcap.FieldClassDst))
 	}
 	tfrm := tst.getTCPFrame()
 
@@ -502,11 +502,11 @@ func (tst *tester) ARPExchangeOnly(querying, target *StackAsync) {
 	tgtIP := target.Addr()
 
 	// Validate Ethernet layer (request is broadcast)
-	if !bytes.Equal(qHw[:], tst.getData(pcap.ProtoEthernet, pcap.FieldClassSrc)) {
-		t.Errorf("request: mismatched ethernet src addr %x", tst.getData(pcap.ProtoEthernet, pcap.FieldClassSrc))
+	if !bytes.Equal(qHw[:], tst.getData(protoEthernet, pcap.FieldClassSrc)) {
+		t.Errorf("request: mismatched ethernet src addr %x", tst.getData(protoEthernet, pcap.FieldClassSrc))
 	}
-	if !bytes.Equal(broadcast[:], tst.getData(pcap.ProtoEthernet, pcap.FieldClassDst)) {
-		t.Errorf("request: expected broadcast ethernet dst addr, got %x", tst.getData(pcap.ProtoEthernet, pcap.FieldClassDst))
+	if !bytes.Equal(broadcast[:], tst.getData(protoEthernet, pcap.FieldClassDst)) {
+		t.Errorf("request: expected broadcast ethernet dst addr, got %x", tst.getData(protoEthernet, pcap.FieldClassDst))
 	}
 
 	// Validate ARP request fields
@@ -515,13 +515,13 @@ func (tst *tester) ARPExchangeOnly(querying, target *StackAsync) {
 	if tst.getARPOperation() != arp.OpRequest {
 		t.Errorf("request: expected ARP OpRequest, got %d", tst.getARPOperation())
 	}
-	if !bytes.Equal(qHw[:], tst.getFieldByClassLen(ethernet.TypeARP, pcap.FieldClassSrc, 6, 0)) {
+	if !bytes.Equal(qHw[:], tst.getFieldByClassLen(protoARP, pcap.FieldClassSrc, 6, 0)) {
 		t.Errorf("request: mismatched ARP sender HW")
 	}
-	if !bytes.Equal(qIP.AsSlice(), tst.getFieldByClassLen(ethernet.TypeARP, pcap.FieldClassSrc, 4, 0)) {
+	if !bytes.Equal(qIP.AsSlice(), tst.getFieldByClassLen(protoARP, pcap.FieldClassSrc, 4, 0)) {
 		t.Errorf("request: mismatched ARP sender proto")
 	}
-	if !bytes.Equal(tgtIP.AsSlice(), tst.getFieldByClassLen(ethernet.TypeARP, pcap.FieldClassSrc, 4, 1)) {
+	if !bytes.Equal(tgtIP.AsSlice(), tst.getFieldByClassLen(protoARP, pcap.FieldClassSrc, 4, 1)) {
 		t.Errorf("request: mismatched ARP target proto")
 	}
 
@@ -549,27 +549,27 @@ func (tst *tester) ARPExchangeOnly(querying, target *StackAsync) {
 	tst.buf = tst.buf[:n]
 
 	// Validate Ethernet layer (reply is unicast to querying)
-	if !bytes.Equal(tgtHw[:], tst.getData(pcap.ProtoEthernet, pcap.FieldClassSrc)) {
-		t.Errorf("reply: mismatched ethernet src addr %x", tst.getData(pcap.ProtoEthernet, pcap.FieldClassSrc))
+	if !bytes.Equal(tgtHw[:], tst.getData(protoEthernet, pcap.FieldClassSrc)) {
+		t.Errorf("reply: mismatched ethernet src addr %x", tst.getData(protoEthernet, pcap.FieldClassSrc))
 	}
-	if !bytes.Equal(qHw[:], tst.getData(pcap.ProtoEthernet, pcap.FieldClassDst)) {
-		t.Errorf("reply: expected unicast to querying, got %x", tst.getData(pcap.ProtoEthernet, pcap.FieldClassDst))
+	if !bytes.Equal(qHw[:], tst.getData(protoEthernet, pcap.FieldClassDst)) {
+		t.Errorf("reply: expected unicast to querying, got %x", tst.getData(protoEthernet, pcap.FieldClassDst))
 	}
 
 	// Validate ARP reply fields
 	if tst.getARPOperation() != arp.OpReply {
 		t.Errorf("reply: expected ARP OpReply, got %d", tst.getARPOperation())
 	}
-	if !bytes.Equal(tgtHw[:], tst.getFieldByClassLen(ethernet.TypeARP, pcap.FieldClassSrc, 6, 0)) {
+	if !bytes.Equal(tgtHw[:], tst.getFieldByClassLen(protoARP, pcap.FieldClassSrc, 6, 0)) {
 		t.Errorf("reply: mismatched ARP sender HW (should be target's MAC)")
 	}
-	if !bytes.Equal(tgtIP.AsSlice(), tst.getFieldByClassLen(ethernet.TypeARP, pcap.FieldClassSrc, 4, 0)) {
+	if !bytes.Equal(tgtIP.AsSlice(), tst.getFieldByClassLen(protoARP, pcap.FieldClassSrc, 4, 0)) {
 		t.Errorf("reply: mismatched ARP sender proto (should be target's IP)")
 	}
-	if !bytes.Equal(qHw[:], tst.getFieldByClassLen(ethernet.TypeARP, pcap.FieldClassSrc, 6, 1)) {
+	if !bytes.Equal(qHw[:], tst.getFieldByClassLen(protoARP, pcap.FieldClassSrc, 6, 1)) {
 		t.Errorf("reply: mismatched ARP target HW (should be querying's MAC)")
 	}
-	if !bytes.Equal(qIP.AsSlice(), tst.getFieldByClassLen(ethernet.TypeARP, pcap.FieldClassSrc, 4, 1)) {
+	if !bytes.Equal(qIP.AsSlice(), tst.getFieldByClassLen(protoARP, pcap.FieldClassSrc, 4, 1)) {
 		t.Errorf("reply: mismatched ARP target proto (should be querying's IP)")
 	}
 
@@ -593,7 +593,7 @@ func (tst *tester) ARPExchangeOnly(querying, target *StackAsync) {
 func (tst *tester) getTCPFrame() tcp.Frame {
 	tst.t.Helper()
 	// Find the IP frame's position in the captured packet buffer.
-	ipFrm := getProtoFrame(tst.frmbuf, ethernet.TypeIPv4)
+	ipFrm := getProtoFrame(tst.frmbuf, protoIPv4)
 	if ipFrm == nil {
 		tst.t.Fatal("no IP frame in capture")
 	}
@@ -615,7 +615,7 @@ func (tst *tester) getTCPFrame() tcp.Frame {
 	return frame
 }
 
-func (tst *tester) getPayload(proto any) []byte {
+func (tst *tester) getPayload(proto string) []byte {
 	tst.t.Helper()
 	i := 0
 	for i = 0; i < len(tst.frmbuf); i++ {
@@ -633,7 +633,7 @@ func (tst *tester) getPayload(proto any) []byte {
 	return tst.getData(proto, pcap.FieldClassPayload)
 }
 
-func (tst *tester) getData(proto any, field pcap.FieldClass) []byte {
+func (tst *tester) getData(proto string, field pcap.FieldClass) []byte {
 	tst.t.Helper()
 	frm := getProtoFrame(tst.frmbuf, proto)
 	if frm == nil {
@@ -654,7 +654,7 @@ func (tst *tester) getData(proto any, field pcap.FieldClass) []byte {
 	return tst.buf[bitoff/8 : bitoff/8+bitlen/8]
 }
 
-func (tst *tester) getInt(proto any, field pcap.FieldClass) uint64 {
+func (tst *tester) getInt(proto string, field pcap.FieldClass) uint64 {
 	tst.t.Helper()
 	frm := getProtoFrame(tst.frmbuf, proto)
 	if frm == nil {
@@ -671,7 +671,7 @@ func (tst *tester) getInt(proto any, field pcap.FieldClass) uint64 {
 	return v
 }
 
-func getProtoFrame(frms []pcap.Frame, proto any) *pcap.Frame {
+func getProtoFrame(frms []pcap.Frame, proto string) *pcap.Frame {
 	for i := range frms {
 		if frms[i].Protocol == proto {
 			return &frms[i]
@@ -690,7 +690,7 @@ func setzero[T ~[]E, E any](s T) {
 // getFieldByClassLen finds a field by protocol, class, and octet length.
 // occurrence specifies which match to return (0 = first, 1 = second, etc.)
 // This is needed for ARP where sender and target fields share the same class.
-func (tst *tester) getFieldByClassLen(proto any, class pcap.FieldClass, octetLen, occurrence int) []byte {
+func (tst *tester) getFieldByClassLen(proto string, class pcap.FieldClass, octetLen, occurrence int) []byte {
 	tst.t.Helper()
 	frm := getProtoFrame(tst.frmbuf, proto)
 	if frm == nil {
@@ -712,7 +712,7 @@ func (tst *tester) getFieldByClassLen(proto any, class pcap.FieldClass, octetLen
 
 func (tst *tester) getARPOperation() arp.Operation {
 	tst.t.Helper()
-	return arp.Operation(tst.getInt(ethernet.TypeARP, pcap.FieldClassOperation))
+	return arp.Operation(tst.getInt(protoARP, pcap.FieldClassOperation))
 }
 
 // TestTCPConn_BufferNotClearedOnPassiveClose tests that data remains readable after
@@ -981,7 +981,7 @@ func TestStackAsync_ICMPEchoChecksum(t *testing.T) {
 		SequenceNumber: 4,
 		Payload:        icmpPayload,
 	})
-	pkt[len(pkt)-1] ^= 0xFF // Corrupt ICMP payload.
+	pkt[len(pkt)-1] ^= 0xFF                   // Corrupt ICMP payload.
 	pkt = append(pkt, 0xDE, 0xAD, 0xBE, 0xEF) // Simulate Ethernet FCS.
 	err = stack.Demux(pkt, 0)
 	if err == nil {
@@ -989,3 +989,10 @@ func TestStackAsync_ICMPEchoChecksum(t *testing.T) {
 	}
 
 }
+
+const (
+	protoEthernet = "Ethernet"
+	protoARP      = "ARP"
+	protoIPv4     = "IPv4"
+	protoTCP      = "TCP"
+)