add TCP keepalive: TCB methods for detecting and creating TCP keepali…

…ves; fix TCPConn.Close bug that would make an unopened connection enter FinWait1 forever until opened

control_user.go

@@ -251,3 +251,23 @@ func (tcb *ControlBlock) SetRecvWindow(wnd Size) {
 func (tcb *ControlBlock) SetLogger(log *slog.Logger) {
 	tcb.log = log
 }
+
+// IncomingIsKeepalive checks if an incoming segment is a keepalive segment.
+// Segments which are keepalives should not be passed into Recv or Send methods.
+func (tcb *ControlBlock) IncomingIsKeepalive(incomingSegment Segment) bool {
+	return incomingSegment.SEQ == tcb.rcv.NXT-1 &&
+		incomingSegment.Flags == FlagACK &&
+		incomingSegment.ACK == tcb.snd.NXT && incomingSegment.DATALEN == 0
+}
+
+// MakeKeepalive creates a TCP keepalive segment. This segment
+// should not be passed into Recv or Send methods.
+func (tcb *ControlBlock) MakeKeepalive() Segment {
+	return Segment{
+		SEQ:     tcb.snd.NXT - 1,
+		ACK:     tcb.rcv.NXT,
+		Flags:   FlagACK,
+		WND:     tcb.rcv.WND,
+		DATALEN: 0,
+	}
+}

stacks/tcpconn.go

@@ -300,13 +300,13 @@ func (sock *TCPConn) open(state seqs.State, localPortNum uint16, iss seqs.Value,
 
 func (sock *TCPConn) Close() error {
 	toSend := sock.tx.Buffered()
-	sock.closing = true
 	if toSend == 0 {
 		err := sock.scb.Close()
 		if err != nil {
 			return err
 		}
 	}
+	sock.closing = true
 	sock.stack.FlagPendingTCP(sock.localPort)
 	return nil
 }
@@ -342,7 +342,10 @@ func (sock *TCPConn) recv(pkt *TCPPacket) (err error) {
 	// By this point we know that the packet is valid and contains data, we process it.
 	payload := pkt.Payload()
 	segIncoming := pkt.TCP.Segment(len(payload))
-
+	if sock.scb.IncomingIsKeepalive(segIncoming) {
+		sock.trace("TCPConn.recv:keepalive")
+		return nil
+	}
 	err = sock.scb.Recv(segIncoming)
 	if err != nil {
 		if sock.scb.State() == seqs.StateClosed {