diff --git a/examples/stack/main.go b/examples/stack/main.go
index f2ca15a..7423a85 100644
--- a/examples/stack/main.go
+++ b/examples/stack/main.go
@@ -56,40 +56,39 @@ func main() {
 	if err != nil {
 		log.Fatal(err)
 	}
+
 	tap := NewHTTPTap("http://127.0.0.1:7070")
-	// tap, err := internal.NewTap("tap0", iface)
-	if err != nil {
-		log.Fatal(err)
-	}
 	defer tap.Close()
+
 	fmt.Println("hosting server at ", addrPort.String())
 	var buf [mtu]byte
 	for {
-		n, err := tap.Read(buf[:])
+		nread, err := tap.Read(buf[:])
 		if err != nil {
 			slogger.error("tap-err", slog.String("err", err.Error()))
 			log.Fatal(err)
-		} else if n > 0 {
-			err = lStack.RecvEth(buf[:n])
+		} else if nread > 0 {
+			err = lStack.RecvEth(buf[:nread])
 			if err != nil {
-				slogger.error("recv", slog.String("err", err.Error()), slog.Int("plen", n))
+				slogger.error("recv", slog.String("err", err.Error()), slog.Int("plen", nread))
 			} else {
-				slogger.info("recv", slog.Int("plen", n))
+				slogger.info("recv", slog.Int("plen", nread))
 			}
-		} else if n == 0 {
-			time.Sleep(250 * time.Millisecond)
 		}
-		n, err = lStack.HandleEth(buf[:])
+		nw, err := lStack.HandleEth(buf[:])
 		if err != nil {
 			slogger.error("handle", slog.String("err", err.Error()))
-		} else if n > 0 {
-			_, err = tap.Write(buf[:n])
+		} else if nw > 0 {
+			_, err = tap.Write(buf[:nw])
 			if err != nil {
 				log.Fatal(err)
 			} else {
-				slogger.info("write", slog.Int("plen", n))
+				slogger.info("write", slog.Int("plen", nw))
 			}
 		}
+		if nread == 0 && nw == 0 {
+			time.Sleep(5 * time.Millisecond)
+		}
 	}
 }
 
@@ -337,6 +336,10 @@ func (is *IPv4Stack) Handle(ethFrame []byte, ipOff int) (int, error) {
 			ifrm.SetFlags(dontFrag)
 			ifrm.SetTTL(64)
 			ifrm.SetCRC(ifrm.CalculateHeaderCRC())
+			if ifrm.Protocol() == lneto2.IPProtoTCP {
+				tfrm, _ := tcp.NewFrame(ifrm.Payload())
+				is.info("IPv4Stack:send", slog.String("ip", ifrm.String()), slog.String("tcp", tfrm.String()))
+			}
 			return totalLen, nil
 		}
 	}
@@ -347,7 +350,6 @@ type TCPStack struct {
 	validator lneto2.Validator
 	handlers  []handler
 	logger
-	crc lneto2.CRC791
 }
 
 func (ts *TCPStack) Protocol() uint32 { return uint32(lneto2.IPProtoTCP) }
@@ -389,20 +391,9 @@ func (ts *TCPStack) Recv(ipFrame []byte, tcpOff int) error {
 	if err = ts.validator.Err(); err != nil {
 		return err
 	}
-	ts.crc.Reset()
-	switch ipVersion {
-	case 4:
-		ifrm, _ := ipv4.NewFrame(ipFrame)
-		ifrm.CRCWriteTCPPseudo(&ts.crc)
-		ts.log.Info("tcpStack:recv", slog.String("ipfrm", ifrm.String()), slog.String("frame", tfrm.String()))
-	case 6:
-		i6frm, _ := ipv6.NewFrame(ipFrame)
-		i6frm.CRCWritePseudo(&ts.crc)
-	}
-	tfrm.CRCWrite(&ts.crc)
-	crc := ts.crc.Sum16()
+	crc := tcpChecksum(ipFrame, len(tfrm.RawData()))
 	gotCRC := tfrm.CRC()
-	if ts.crc.Sum16() != gotCRC {
+	if crc != gotCRC {
 		ts.error("TCPStack:Recv:crc-mismatch", slog.Uint64("lport", uint64(lport)), slog.Uint64("want", uint64(crc)), slog.Uint64("got", uint64(gotCRC)))
 		return errors.New("TCP crc mismatch")
 	}
@@ -428,6 +419,7 @@ func (ts *TCPStack) Handle(ipFrame []byte, tcpOff int) (n int, err error) {
 			}
 		}
 		if n > 0 {
+			ipFrame = ipFrame[:tcpOff+n]
 			break
 		}
 	}
@@ -435,24 +427,14 @@ func (ts *TCPStack) Handle(ipFrame []byte, tcpOff int) (n int, err error) {
 		return 0, err
 	}
 	// TCP packet written.
-	tfrm, _ := tcp.NewFrame(ipFrame[tcpOff : tcpOff+n])
+	tfrm, _ := tcp.NewFrame(ipFrame[tcpOff:])
 
 	ts.validator.ResetErr()
 	tfrm.ValidateSize(&ts.validator) // Perform basic validation.
 	if err = ts.validator.Err(); err != nil {
 		return 0, err
 	}
-	ts.crc.Reset()
-	switch ipVersion {
-	case 4:
-		ifrm, _ := ipv4.NewFrame(ipFrame)
-		ifrm.CRCWriteTCPPseudo(&ts.crc)
-		ts.log.Info("tcpStack:send", slog.String("ipfrm", ifrm.String()), slog.String("frame", tfrm.String()))
-	case 6:
-		i6frm, _ := ipv6.NewFrame(ipFrame)
-		i6frm.CRCWritePseudo(&ts.crc)
-	}
-	crc := ts.crc.Sum16()
+	crc := tcpChecksum(ipFrame, n)
 	tfrm.SetCRC(crc)
 	return n, nil
 }
@@ -554,6 +536,12 @@ func NewHTTPTap(baseURL string) *HTTPTap {
 	return &h
 }
 
+type TAPNop struct{}
+
+func (h *TAPNop) Read(b []byte) (int, error)  { return 0, nil }
+func (h *TAPNop) Write(b []byte) (int, error) { return 0, nil }
+func (h *TAPNop) Close() error                { return nil }
+
 type HTTPTap struct {
 	c       http.Client
 	recvurl string
@@ -590,3 +578,30 @@ func (h *HTTPTap) Write(b []byte) (int, error) {
 }
 
 func (h *HTTPTap) Close() error { return nil }
+
+func tcpChecksum(ipFrame []byte, tcpPayload int) uint16 {
+	version := ipFrame[0] >> 4
+	var tfrm tcp.Frame
+	var crc lneto2.CRC791
+	switch version {
+	case 4:
+		ifrm, _ := ipv4.NewFrame(ipFrame)
+		crc.Write(ifrm.SourceAddr()[:])
+		crc.Write(ifrm.DestinationAddr()[:])
+		crc.AddUint16(uint16(tcpPayload))
+		crc.AddUint16(6)
+		tfrm, _ = tcp.NewFrame(ifrm.Payload())
+	case 6:
+		i6frm, _ := ipv6.NewFrame(ipFrame)
+		crc.Write(i6frm.SourceAddr()[:])
+		crc.Write(i6frm.DestinationAddr()[:])
+		crc.AddUint32(uint32(tcpPayload))
+		crc.AddUint32(6)
+		i6frm.CRCWritePseudo(&crc)
+		tfrm, _ = tcp.NewFrame(i6frm.Payload())
+	default:
+		panic("invalid IP version")
+	}
+	tfrm.CRCWrite(&crc)
+	return crc.Sum16()
+}
diff --git a/examples/stack/main_test.go b/examples/stack/main_test.go
new file mode 100644
index 0000000..caf07e3
--- /dev/null
+++ b/examples/stack/main_test.go
@@ -0,0 +1,41 @@
+package main
+
+import (
+	"testing"
+
+	"github.com/soypat/lneto/ethernet"
+	"github.com/soypat/lneto/ipv4"
+	"github.com/soypat/lneto/tcp"
+)
+
+func TestChecksum(t *testing.T) {
+	for _, epacket := range ethpackets {
+		efrm, _ := ethernet.NewFrame(epacket)
+		ifrm, _ := ipv4.NewFrame(efrm.Payload())
+		ipPayload := ifrm.Payload()
+		tfrm, _ := tcp.NewFrame(ipPayload)
+
+		crc := tcpChecksum(ifrm.RawData(), len(ipPayload))
+		wantCRC := tfrm.CRC()
+		if crc != wantCRC {
+			t.Fatalf("crc mismatch, got %x, want %x", crc, wantCRC)
+		}
+	}
+}
+
+var ethpackets = [][]byte{
+
+	{
+		0xc0, 0xff, 0xee, 0x00, 0xde, 0xad, 0x3a, 0xd1, 0x6d, 0x82, 0x6b, 0x1a, 0x08, 0x00, 0x45, 0x00,
+		0x00, 0x3c, 0xe3, 0xc6, 0x40, 0x00, 0x40, 0x06, 0xc1, 0xa1, 0xc0, 0xa8, 0x0a, 0x01, 0xc0, 0xa8,
+		0x0a, 0x02, 0xd5, 0x70, 0x00, 0x50, 0xc4, 0x10, 0x30, 0x49, 0x00, 0x00, 0x00, 0x00, 0xa0, 0x02,
+		0xfa, 0xf0, 0x2c, 0x80, 0x00, 0x00, 0x02, 0x04, 0x05, 0xb4, 0x04, 0x02, 0x08, 0x0a, 0xe8, 0x22,
+		0xd8, 0xfd, 0x00, 0x00, 0x00, 0x00, 0x01, 0x03, 0x03, 0x07,
+	},
+	{
+		0xc0, 0xff, 0xee, 0x00, 0xde, 0xad, 0xc0, 0xff, 0xee, 0x00, 0xde, 0xad, 0x08, 0x00, 0x45, 0x00,
+		0x00, 0x28, 0x00, 0x00, 0x40, 0x00, 0x40, 0x06, 0x70, 0x26, 0xc0, 0xa8, 0x0a, 0x02, 0x00, 0x00,
+		0x00, 0x00, 0x00, 0x50, 0xa5, 0xb8, 0x00, 0x00, 0x00, 0x64, 0x2f, 0x46, 0x4c, 0xe5, 0x50, 0x12,
+		0x08, 0x00, 0xBA, 0x90, 0x00, 0x00,
+	},
+}
diff --git a/go.mod b/go.mod
index 219f4f0..d1678ee 100644
--- a/go.mod
+++ b/go.mod
@@ -1,3 +1,3 @@
 module github.com/soypat/lneto
 
-go 1.20
+go 1.21
\ No newline at end of file
diff --git a/tcp/control.go b/tcp/control.go
index 0077a23..b5a076f 100644
--- a/tcp/control.go
+++ b/tcp/control.go
@@ -153,17 +153,18 @@ func (tcb *ControlBlock) Open(iss Value, wnd Size) (err error) {
 		tcb.logerr("tcb:open", slog.String("err", err.Error()))
 		return err
 	}
-	tcb._state = StateListen
-	tcb.prepareToHandshake(iss, wnd)
+
+	tcb.prepareToHandshake(iss, wnd, StateListen)
 	tcb.trace("tcb:open-server")
 	return nil
 }
 
 // prepareToHandshake initializes the TCB send/receive spaces with initial send sequence number and local window.
-func (tcb *ControlBlock) prepareToHandshake(iss Value, wnd Size) {
+func (tcb *ControlBlock) prepareToHandshake(iss Value, wnd Size, newState State) {
+	tcb.reset()
 	tcb.resetRcv(wnd, 0)
 	tcb.resetSnd(iss, 1)
-	tcb.pending = [2]Flags{}
+	tcb._state = newState
 }
 
 // HasPending returns true if there is a pending control segment to send. Calls to Send will advance the pending queue.
@@ -256,7 +257,7 @@ func (tcb *ControlBlock) Recv(seg Segment) (err error) {
 	case StateCloseWait:
 	case StateLastAck:
 		if seg.Flags.HasAny(FlagACK) {
-			tcb.close()
+			tcb.Abort()
 		}
 	case StateClosing:
 		// Thanks to @knieriem for finding and reporting this bug.
@@ -309,11 +310,10 @@ func (tcb *ControlBlock) Send(seg Segment) error {
 	switch tcb._state {
 	case StateClosed:
 		if seg.Flags == FlagSYN {
-			tcb._state = StateSynSent
-			tcb.prepareToHandshake(seg.SEQ, seg.WND)
+			tcb.prepareToHandshake(seg.SEQ, seg.WND, StateSynSent)
 			tcb.trace("tcb:open-client")
 		}
-	case StateSynRcvd:
+	case StateSynRcvd, StateEstablished:
 		if hasFIN {
 			tcb._state = StateFinWait1 // RFC 9293: 3.10.4 CLOSE call.
 		}
@@ -321,10 +321,6 @@ func (tcb *ControlBlock) Send(seg Segment) error {
 		if hasACK {
 			tcb._state = StateTimeWait
 		}
-	case StateEstablished:
-		if hasFIN {
-			tcb._state = StateFinWait1
-		}
 	case StateCloseWait:
 		if hasFIN {
 			tcb._state = StateLastAck
@@ -494,7 +490,7 @@ func (tcb *ControlBlock) handleRST(seq Value) error {
 		tcb.resetSnd(tcb.snd.ISS+tcb.rstJump(), tcb.snd.WND)
 		tcb.resetRcv(tcb.rcv.WND, 3_14159_2653^tcb.rcv.IRS)
 	} else {
-		tcb.close() // Enter closed state and return.
+		tcb.Abort() // Enter closed state and return.
 		return net.ErrClosed
 	}
 	return errDropSegment
@@ -504,13 +500,17 @@ func (tcb *ControlBlock) rstJump() Value {
 	return 100
 }
 
-// close sets ControlBlock state to closed and resets all sequence numbers and pending flag.
-func (tcb *ControlBlock) close() {
-	tcb._state = StateClosed
-	tcb.pending = [2]Flags{}
-	tcb.resetRcv(0, 0)
-	tcb.resetSnd(0, 0)
-	tcb.debug("tcb:close")
+// Abort sets ControlBlock state to Closed and resets all sequence numbers and pending flag.
+// No more data can be sent nor received after the connection is aborted until opened again.
+func (tcb *ControlBlock) Abort() {
+	tcb.reset()
+	tcb.debug("tcb:abort")
+}
+
+func (tcb *ControlBlock) reset() {
+	*tcb = ControlBlock{
+		logger: tcb.logger,
+	}
 }
 
 // Close implements a passive/active closing of a connection. It does not immediately
@@ -521,15 +521,18 @@ func (tcb *ControlBlock) Close() (err error) {
 	// See RFC 9293: 3.10.4 CLOSE call.
 	switch tcb._state {
 	case StateClosed:
-		err = errConnNotexist
+		err = errConnNotExist
 	case StateCloseWait:
 		tcb._state = StateLastAck
 		tcb.pending = [2]Flags{FlagFIN, FlagACK}
 	case StateListen, StateSynSent:
-		tcb.close()
+		// In Listen State there is no established connection.
+		// In SynSent the remote endpoint is not yet synchronized and upon receiving an RST will abort connection.
+		tcb.Abort()
 	case StateSynRcvd, StateEstablished:
 		// We suppose user has no more pending data to send, so we flag FIN to be sent.
 		// Users of this API should call Close only when they have no more data to send.
+		// When FIN is sent SCB will transition to FinWait1.
 		tcb.pending[0] = (tcb.pending[0] & FlagACK) | FlagFIN
 	case StateFinWait2, StateTimeWait:
 		err = errConnectionClosing
diff --git a/tcp/definitions.go b/tcp/definitions.go
index a0d928d..86a75b1 100644
--- a/tcp/definitions.go
+++ b/tcp/definitions.go
@@ -19,7 +19,7 @@ var (
 	errBufferTooSmall        = errors.New("buffer too small")
 	errNeedClosedTCBToOpen   = errors.New("need closed TCB to call open")
 	errInvalidState          = errors.New("invalid state")
-	errConnNotexist          = errors.New("connection does not exist")
+	errConnNotExist          = errors.New("connection does not exist")
 	errConnectionClosing     = errors.New("connection closing")
 	errExpectedSYN           = errors.New("seqs:expected SYN")
 	errBadSegack             = errors.New("seqs:bad segack")
@@ -286,7 +286,7 @@ func (s State) IsPreestablished() bool {
 // IsClosing returns true if the connection is in a closing state but not yet terminated (relieved of remote connection state).
 // Returns false for Closed pseudo state.
 func (s State) IsClosing() bool {
-	return !(s <= StateEstablished)
+	return s == StateFinWait1 || s == StateFinWait2 || s == StateClosing || s == StateLastAck || s == StateCloseWait
 }
 
 // IsClosed returns true if the connection closed and can possibly relieved of
@@ -297,7 +297,19 @@ func (s State) IsClosed() bool {
 
 // IsSynchronized returns true if the connection has gone through the Established state.
 func (s State) IsSynchronized() bool {
-	return s >= StateEstablished
+	return s >= StateEstablished && !s.IsClosed()
+}
+
+// txOpen returns true if the TCP state machine allows data to be sent by user.
+func (s State) txOpen() bool {
+	// In CloseWait state the remote endpoint has closed
+	// our receive hald of the connection but we can still transmit indefinitely.
+	return s == StateEstablished || s == StateCloseWait
+}
+
+// rxOpen returns true if the TCP state machine allows data to be received from remote endpoint.
+func (s State) rxOpen() bool {
+	return s == StateEstablished || s == StateFinWait1 || s == StateFinWait2
 }
 
 // IsDataOpen returns true if the connection allows sending and receiving of data.
diff --git a/tcp/frame.go b/tcp/frame.go
index 6ba67ff..f917693 100644
--- a/tcp/frame.go
+++ b/tcp/frame.go
@@ -128,7 +128,7 @@ func (tfrm Frame) Payload() []byte {
 
 // Segment returns the [Segment] representation of the TCP header and data length.
 func (tfrm Frame) Segment(payloadSize int) Segment {
-	if payloadSize > math.MaxUint32 {
+	if payloadSize > math.MaxInt32 {
 		panic("TCP overflow payload size")
 	}
 	return Segment{