diff --git a/internet/basicstack.go b/internet/basicstack.go
index 865b01d..197661e 100644
--- a/internet/basicstack.go
+++ b/internet/basicstack.go
@@ -61,7 +61,8 @@ func (sb *StackBasic) Recv(frame []byte) error {
 	totalLen := ifrm.TotalLength()
 	for i := range sb.handlers {
 		h := &sb.handlers[i]
-		if h.proto == ifrm.Protocol() {
+		proto := ifrm.Protocol()
+		if h.proto == proto {
 			return h.recv(frame[:totalLen], off)
 		}
 	}
@@ -92,6 +93,7 @@ func (sb *StackBasic) Handle(frame []byte) (int, error) {
 			ifrm.SetTotalLength(uint16(totalLen))
 			ifrm.SetFlags(dontFrag)
 			ifrm.SetTTL(64)
+			ifrm.SetProtocol(h.proto)
 			ifrm.SetCRC(ifrm.CalculateHeaderCRC())
 			if ifrm.Protocol() == lneto.IPProtoTCP {
 				tfrm, _ := tcp.NewFrame(ifrm.Payload())
diff --git a/internet/stackbasic_test.go b/internet/stackbasic_test.go
index 2ae839f..de9175a 100644
--- a/internet/stackbasic_test.go
+++ b/internet/stackbasic_test.go
@@ -14,9 +14,11 @@ func TestBasicStack(t *testing.T) {
 	var connCl, connSv TCPConn
 	setupClientServer(t, rng, &sbCl, &sbSv, &connCl, &connSv)
 	var buf [2048]byte
+	nextToSend := &sbCl
+	nextToRecv := &sbSv
 	exchangeAndExpectStates := func(clState, svState tcp.State) {
 		t.Helper()
-		expectExchange(t, &sbCl, &sbSv, buf[:])
+		expectExchange(t, nextToSend, nextToRecv, buf[:])
 		gotCl := connCl.State()
 		gotSv := connSv.State()
 		if gotCl != clState {
@@ -25,20 +27,25 @@ func TestBasicStack(t *testing.T) {
 		if gotSv != svState {
 			t.Errorf("want server state %s, got %s", svState, gotSv)
 		}
+		nextToSend, nextToRecv = nextToRecv, nextToSend
 	}
-	exchangeAndExpectStates(tcp.StateSynSent, tcp.StateListen)
+	exchangeAndExpectStates(tcp.StateSynSent, tcp.StateSynRcvd)         // Client sends over first SYN and server receives it.
+	exchangeAndExpectStates(tcp.StateEstablished, tcp.StateSynRcvd)     // server sends back SYNACK, establishing connection on client side.
+	exchangeAndExpectStates(tcp.StateEstablished, tcp.StateEstablished) // Client sends ACK, establishing connection in full.
 }
 
 func expectExchange(t *testing.T, from, to *StackBasic, buf []byte) {
+	t.Helper()
 	n, err := from.Handle(buf)
 	if err != nil {
-		t.Error(err)
+		t.Error("expectExchange:Handle:", err)
 	} else if n == 0 {
 		t.Error("expected data exchange")
+		return
 	}
 	err = to.Recv(buf[:n])
 	if err != nil {
-		t.Error(err)
+		t.Error("expectExchange:Recv:", err)
 	}
 }
 
diff --git a/ipv4/definitions.go b/ipv4/definitions.go
index 14c45ee..4097e58 100644
--- a/ipv4/definitions.go
+++ b/ipv4/definitions.go
@@ -4,7 +4,7 @@ const (
 	sizeHeader = 20
 )
 
-// ToS represents the Traffic Class (a.k.a Type of Service).
+// ToS represents the Traffic Class (a.k.a Type of Service). It is 8 bits long. 6 MSB are Differentiated Services; 2 LSB are Explicit Congenstion Notification.
 type ToS uint8
 
 // DS returns the top 6 bits of the IPv4 ToS holding the Differentiated Services field
@@ -14,7 +14,7 @@ func (tos ToS) DS() uint8 { return uint8(tos) >> 2 }
 // ECN is the Explicit Congestion Notification which provides congestion control and non-congestion control traffic.
 func (tos ToS) ECN() uint8 { return uint8(tos & 0b11) }
 
-// Flags holds fragmentation field data of an IPv4 header.
+// Flags holds fragmentation field data of an IPv4 header. It is 16 bits long.
 type Flags uint16
 
 // IsEvil returns true if evil bit set as per [RFC3514].
diff --git a/ipv4/frame_test.go b/ipv4/frame_test.go
new file mode 100644
index 0000000..88f1d25
--- /dev/null
+++ b/ipv4/frame_test.go
@@ -0,0 +1,109 @@
+package ipv4
+
+import (
+	"math"
+	"math/rand"
+	"testing"
+
+	"github.com/soypat/lneto"
+)
+
+func TestFrame(t *testing.T) {
+	var buf [1024]byte
+
+	ifrm, err := NewFrame(buf[:])
+	if err != nil {
+		t.Fatal(err)
+	}
+	rng := rand.New(rand.NewSource(1))
+	const wantVersion = 4
+	v := new(lneto.Validator)
+	for i := 0; i < 100; i++ {
+		// SET VALUES:
+		wantIHL := uint8(5 + rng.Intn(10))
+		wantToS := ToS(rng.Intn(4))
+		ifrm.SetVersionAndIHL(wantVersion, wantIHL)
+		wantPayloadLen := rng.Intn(6)
+		ifrm.SetToS(wantToS)
+		wantTotalLength := 4*uint16(wantIHL) + uint16(wantPayloadLen)
+		ifrm.SetTotalLength(wantTotalLength)
+		wantID := uint16(rng.Intn(math.MaxUint16))
+		ifrm.SetID(wantID)
+		wantFlags := Flags(rng.Intn(16))
+		ifrm.SetFlags(wantFlags)
+		wantTTL := uint8(rng.Intn(256))
+		ifrm.SetTTL(wantTTL)
+		wantProtocol := lneto.IPProto(rng.Intn(256))
+		ifrm.SetProtocol(wantProtocol)
+		wantCRC := uint16(rng.Intn(math.MaxUint16))
+		ifrm.SetCRC(wantCRC)
+		src := ifrm.SourceAddr()
+		rng.Read(src[:])
+		wantSrc := *src
+		dst := ifrm.DestinationAddr()
+		rng.Read(dst[:])
+		wantDst := *dst
+		ifrm.ValidateExceptCRC(v)
+		ifrm.ValidateSize(v)
+		if v.Err() != nil {
+			t.Error(v.Err())
+		}
+
+		// OPTION+PAYLOAD VALIDATION:
+		opts := ifrm.Options()
+		payload := ifrm.Payload()
+		payloadOff := int(wantIHL) * 4
+		wantOptions := buf[sizeHeader:payloadOff]
+		wantPayload := buf[payloadOff : payloadOff+wantPayloadLen]
+		if len(payload) != wantPayloadLen {
+			t.Errorf("want payload length %d, got %d", wantPayloadLen, len(payload))
+		}
+		if len(opts) != len(wantOptions) {
+			t.Errorf("want length of options %d, got %d", len(wantOptions), len(opts))
+		}
+		if len(opts) > 0 && &wantOptions[0] != &opts[0] {
+			t.Error("first byte of options unexpected pointer")
+		}
+		if len(payload) > 0 && &wantPayload[0] != &payload[0] {
+			t.Error("first byte of payload unexpected pointer")
+		}
+		if len(payload) > 0 {
+			payload[0] = byte(rng.Int()) // write over start of payload to catch field aliasing.
+		}
+		if len(opts) > 0 {
+			opts[0] = byte(rng.Int()) // Catch field aliasing.
+		}
+
+		// FIELD VALIDATION:
+		if ver, ihl := ifrm.VersionAndIHL(); ver != wantVersion || ihl != wantIHL {
+			t.Errorf("wanted IHL %d, got version,IHL %d,%d ", wantIHL, ver, ihl)
+		}
+		if tos := ifrm.ToS(); tos != wantToS {
+			t.Errorf("wanted ToS %d, got %d", wantToS, tos)
+		}
+		if tl := ifrm.TotalLength(); tl != wantTotalLength {
+			t.Errorf("wanted total length %d, got %d", wantTotalLength, tl)
+		}
+		if id := ifrm.ID(); id != wantID {
+			t.Errorf("want ID %d, got %d", wantID, id)
+		}
+		if flags := ifrm.Flags(); flags != wantFlags {
+			t.Errorf("want flags %d, got %d", wantFlags, flags)
+		}
+		if ttl := ifrm.TTL(); ttl != wantTTL {
+			t.Errorf("want TTL %d, got %d", wantTTL, ttl)
+		}
+		if proto := ifrm.Protocol(); proto != wantProtocol {
+			t.Errorf("want protocol %d, got %d", wantProtocol, proto)
+		}
+		if crc := ifrm.CRC(); crc != wantCRC {
+			t.Errorf("want crc %d, got %d", wantCRC, crc)
+		}
+		if wantDst != *dst {
+			t.Errorf("want dst addr %d, got %d", wantDst, dst)
+		}
+		if wantSrc != *src {
+			t.Errorf("want src addr %d, got %d", wantSrc, src)
+		}
+	}
+}