Fix 1 zero window advertisement bug and a TCP test flake.

In TestReceiveBufferAutoTuning we now send a keep-alive packet to measure the
current window rather than a 1 byte segment as the returned window value in the
latter case is reduced due to the 1 byte segment now being held in the receive
buffer and can cause the test to flake if the segment overheads were to change.

In getSendParams in rcv.go we were advertising a non-zero window even if
available window space was zero after we received the previous segment. In such
a case newWnd and curWnd will be the same and we end up advertising a tiny but
non-zero window and this can cause the next segment to be dropped.

PiperOrigin-RevId: 334314070

Author: hbhasker

pkg/tcpip/transport/tcp/rcv.go

@@ -88,6 +88,13 @@ func (r *receiver) acceptable(segSeq seqnum.Value, segLen seqnum.Size) bool {
 // segments to send.
 func (r *receiver) getSendParams() (rcvNxt seqnum.Value, rcvWnd seqnum.Size) {
 	avail := wndFromSpace(r.ep.receiveBufferAvailable())
+	if avail == 0 {
+		// We have no space available to accept any data, move to zero window
+		// state.
+		r.rcvWnd = 0
+		return r.rcvNxt, 0
+	}
+
 	acc := r.rcvNxt.Add(seqnum.Size(avail))
 	newWnd := r.rcvNxt.Size(acc)
 	curWnd := r.rcvNxt.Size(r.rcvAcc)

pkg/tcpip/transport/tcp/tcp_test.go

@@ -5435,8 +5435,8 @@ func TestListenNoAcceptNonUnicastV4(t *testing.T) {
 // TestListenNoAcceptMulticastBroadcastV6 makes sure that TCP segments with a
 // non unicast IPv6 address are not accepted.
 func TestListenNoAcceptNonUnicastV6(t *testing.T) {
-	multicastAddr := tcpip.Address("\xff\x0e\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x01")
-	otherMulticastAddr := tcpip.Address("\xff\x0e\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x02")
+	multicastAddr := tcpip.Address("\xff\x0e\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x01")
+	otherMulticastAddr := tcpip.Address("\xff\x0e\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x01\x02")
 
 	tests := []struct {
 		name    string
@@ -6182,7 +6182,9 @@ func TestReceiveBufferAutoTuning(t *testing.T) {
 
 	rawEP := c.CreateConnectedWithOptions(header.TCPSynOptions{TS: true, WS: 4})
 	tsVal := uint32(rawEP.TSVal)
-	rawEP.SendPacketWithTS([]byte{1}, tsVal)
+	rawEP.NextSeqNum--
+	rawEP.SendPacketWithTS(nil, tsVal)
+	rawEP.NextSeqNum++
 	pkt := rawEP.VerifyAndReturnACKWithTS(tsVal)
 	curRcvWnd := int(header.TCP(header.IPv4(pkt).Payload()).WindowSize()) << c.WindowScale
 	scaleRcvWnd := func(rcvWnd int) uint16 {