diff --git a/stack/socket_tcp.go b/stack/socket_tcp.go
index e0e10ad..54a7cc4 100644
--- a/stack/socket_tcp.go
+++ b/stack/socket_tcp.go
@@ -149,6 +149,11 @@ func (t *TCPSocket) handleRecv(response []byte, pkt *TCPPacket) (n int, err erro
 	// if segIncoming.SEQ != t.scb.RecvNext() {
 	// 	return 0, ErrDroppedPacket // SCB does not admit out-of-order packets.
 	// }
+
+	t.scb.Recv(segIncoming)
+	// if err != nil {
+	// 	return 0, err
+	// }
 	if segIncoming.Flags.HasAny(seqs.FlagPSH) {
 		if len(payload) != int(segIncoming.DATALEN) {
 			return 0, errors.New("segment data length does not match payload length")
@@ -163,11 +168,9 @@ func (t *TCPSocket) handleRecv(response []byte, pkt *TCPPacket) (n int, err erro
 			return 0, err
 		}
 	}
-	err = t.scb.Recv(segIncoming)
-	if err != nil {
-		return 0, err
+	if t.tx.Buffered() > 0 {
+		return t.handleUser(response, pkt) // Yield to handleUser.
 	}
-
 	segOut, ok := t.scb.PendingSegment(0)
 	if !ok {
 		return 0, nil // No pending control segment. Yield to handleUser.
@@ -294,7 +297,7 @@ func (r *ring) Read(b []byte) (int, error) {
 	if r.end >= r.off {
 		// start       off       end      len(buf)
 		//   |  sfree   |  used   |  efree   |
-		n := copy(b, r.buf[r.off:])
+		n := copy(b, r.buf[r.off:r.end])
 		r.off += n
 		r.onReadEnd()
 		return n, nil
@@ -304,7 +307,7 @@ func (r *ring) Read(b []byte) (int, error) {
 	n := copy(b, r.buf[r.off:])
 	r.off += n
 	if n < len(b) {
-		n2 := copy(b[n:], r.buf)
+		n2 := copy(b[n:], r.buf[:r.end])
 		r.off = n2
 		n += n2
 	}
diff --git a/stack/stack_test.go b/stack/stack_test.go
index 9e6c61c..44f2442 100644
--- a/stack/stack_test.go
+++ b/stack/stack_test.go
@@ -36,7 +36,7 @@ func TestStackEstablish(t *testing.T) {
 	}
 }
 
-func TestStackSendReceive(t *testing.T) {
+func TestStackSendReceive_simplex(t *testing.T) {
 	client, server := createTCPClientServerPair(t)
 
 	// 3 way handshake needs2 exchanges to complete.
@@ -47,23 +47,42 @@ func TestStackSendReceive(t *testing.T) {
 
 	// Send data from client to server.
 	const data = "hello world"
-	err := client.Send([]byte(data))
-	if err != nil {
-		t.Fatal(err)
+	socketSendString(client, data)
+	txStacks(t, 1, client.PortStack(), server.PortStack())
+	if client.State() != seqs.StateEstablished || server.State() != seqs.StateEstablished {
+		t.Fatal("not established")
+	}
+	got := socketReadAllString(server)
+	if got != data {
+		t.Error("got", got, "want", data)
 	}
+}
 
-	txStacks(t, 1, client.PortStack(), server.PortStack())
+func TestStackSendReceive_duplex(t *testing.T) {
+	client, server := createTCPClientServerPair(t)
+	cstack, sstack := client.PortStack(), server.PortStack()
+	// 3 way handshake needs2 exchanges to complete.
+	txStacks(t, 2, cstack, sstack)
 	if client.State() != seqs.StateEstablished || server.State() != seqs.StateEstablished {
 		t.Fatal("not established")
 	}
 
-	var buf [len(data)]byte
-	n, err := server.Recv(buf[:])
-	if err != nil {
-		t.Fatal(err)
+	// Send data from client to server.
+	const data = "hello world"
+	socketSendString(client, data)
+	socketSendString(server, data)
+	tx, bytes := txStacks(t, 2, cstack, sstack)
+	if client.State() != seqs.StateEstablished || server.State() != seqs.StateEstablished {
+		t.Fatal("not established")
 	}
-	if string(buf[:n]) != data {
-		t.Error("got", string(buf[:n]), "want", data)
+	t.Logf("tx=%d bytes=%d", tx, bytes)
+	clientstr := socketReadAllString(client)
+	serverstr := socketReadAllString(server)
+	if clientstr != data {
+		t.Error("got", clientstr, "want", data)
+	}
+	if serverstr != data {
+		t.Error("got", serverstr, "want", data)
 	}
 }
 
@@ -173,3 +192,23 @@ func createPortStacks(t *testing.T, n int) (stacks []*stack.PortStack) {
 	}
 	return stacks
 }
+
+func socketReadAllString(s *stack.TCPSocket) string {
+	var str strings.Builder
+	var buf [1024]byte
+	for {
+		n, err := s.Recv(buf[:])
+		str.Write(buf[:n])
+		if n == 0 || err != nil {
+			break
+		}
+	}
+	return str.String()
+}
+
+func socketSendString(s *stack.TCPSocket, str string) {
+	err := s.Send([]byte(str))
+	if err != nil {
+		panic(err)
+	}
+}