6 files changed, 53 insertions, 5 deletions
diff --git a/pkg/tcpip/transport/tcp/connect.go b/pkg/tcpip/transport/tcp/connect.go
index 4695b66d6..34a631b53 100644
--- a/pkg/tcpip/transport/tcp/connect.go
+++ b/pkg/tcpip/transport/tcp/connect.go
@@ -333,7 +333,9 @@ func (h *handshake) synRcvdState(s *segment) tcpip.Error {
 	// number and "After sending the acknowledgment, drop the unacceptable
 	// segment and return."
 	if !s.sequenceNumber.InWindow(h.ackNum, h.rcvWnd) {
-		h.ep.sendRaw(buffer.VectorisedView{}, header.TCPFlagAck, h.iss+1, h.ackNum, h.rcvWnd)
+		if h.ep.allowOutOfWindowAck() {
+			h.ep.sendRaw(buffer.VectorisedView{}, header.TCPFlagAck, h.iss+1, h.ackNum, h.rcvWnd)
+		}
 		return nil
 	}
 
@@ -1185,8 +1187,7 @@ func (e *endpoint) handleSegment(s *segment) (cont bool, err tcpip.Error) {
 		// endpoint MUST terminate its connection.  The local TCP endpoint
 		// should then rely on SYN retransmission from the remote end to
 		// re-establish the connection.
-
-		e.snd.sendAck()
+		e.snd.maybeSendOutOfWindowAck(s)
 	} else if s.flagIsSet(header.TCPFlagAck) {
 		// Patch the window size in the segment according to the
 		// send window scale.
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index e645aa194..4e5a6089f 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -688,6 +688,10 @@ type endpoint struct {
 
 	// ops is used to get socket level options.
 	ops tcpip.SocketOptions
+
+	// lastOutOfWindowAckTime is the time at which the an ACK was sent in response
+	// to an out of window segment being received by this endpoint.
+	lastOutOfWindowAckTime time.Time `state:".(unixTime)"`
 }
 
 // UniqueID implements stack.TransportEndpoint.UniqueID.
@@ -3125,3 +3129,19 @@ func GetTCPSendBufferLimits(s tcpip.StackHandler) tcpip.SendBufferSizeOption {
 		Max:     ss.Max,
 	}
 }
+
+// allowOutOfWindowAck returns true if an out-of-window ACK can be sent now.
+func (e *endpoint) allowOutOfWindowAck() bool {
+	var limit stack.TCPInvalidRateLimitOption
+	if err := e.stack.Option(&limit); err != nil {
+		panic(fmt.Sprintf("e.stack.Option(%+v) failed with error: %s", limit, err))
+	}
+
+	now := time.Now()
+	if now.Sub(e.lastOutOfWindowAckTime) < time.Duration(limit) {
+		return false
+	}
+
+	e.lastOutOfWindowAckTime = now
+	return true
+}
diff --git a/pkg/tcpip/transport/tcp/endpoint_state.go b/pkg/tcpip/transport/tcp/endpoint_state.go
index c21dbc682..e4368026f 100644
--- a/pkg/tcpip/transport/tcp/endpoint_state.go
+++ b/pkg/tcpip/transport/tcp/endpoint_state.go
@@ -308,6 +308,16 @@ func (e *endpoint) loadRecentTSTime(unix unixTime) {
 	e.recentTSTime = time.Unix(unix.second, unix.nano)
 }
 
+// saveLastOutOfWindowAckTime is invoked by stateify.
+func (e *endpoint) saveLastOutOfWindowAckTime() unixTime {
+	return unixTime{e.lastOutOfWindowAckTime.Unix(), e.lastOutOfWindowAckTime.UnixNano()}
+}
+
+// loadLastOutOfWindowAckTime is invoked by stateify.
+func (e *endpoint) loadLastOutOfWindowAckTime(unix unixTime) {
+	e.lastOutOfWindowAckTime = time.Unix(unix.second, unix.nano)
+}
+
 // saveMeasureTime is invoked by stateify.
 func (r *rcvBufAutoTuneParams) saveMeasureTime() unixTime {
 	return unixTime{r.measureTime.Unix(), r.measureTime.UnixNano()}
diff --git a/pkg/tcpip/transport/tcp/rcv.go b/pkg/tcpip/transport/tcp/rcv.go
index 7a7c402c4..a5c82b8fa 100644
--- a/pkg/tcpip/transport/tcp/rcv.go
+++ b/pkg/tcpip/transport/tcp/rcv.go
@@ -385,7 +385,7 @@ func (r *receiver) handleRcvdSegmentClosing(s *segment, state EndpointState, clo
 		// fails, we ignore the packet:
 		// https://github.com/torvalds/linux/blob/v5.8/net/ipv4/tcp_input.c#L5591
 		if r.ep.snd.sndNxt.LessThan(s.ackNumber) {
-			r.ep.snd.sendAck()
+			r.ep.snd.maybeSendOutOfWindowAck(s)
 			return true, nil
 		}
 
@@ -454,7 +454,7 @@ func (r *receiver) handleRcvdSegment(s *segment) (drop bool, err tcpip.Error) {
 	// send an ACK and stop further processing of the segment.
 	// This is according to RFC 793, page 68.
 	if !r.acceptable(segSeq, segLen) {
-		r.ep.snd.sendAck()
+		r.ep.snd.maybeSendOutOfWindowAck(s)
 		return true, nil
 	}
 
diff --git a/pkg/tcpip/transport/tcp/snd.go b/pkg/tcpip/transport/tcp/snd.go
index dfc8fd248..538edd6cf 100644
--- a/pkg/tcpip/transport/tcp/snd.go
+++ b/pkg/tcpip/transport/tcp/snd.go
@@ -1548,3 +1548,13 @@ func (s *sender) sendSegmentFromView(data buffer.VectorisedView, flags byte, seq
 
 	return s.ep.sendRaw(data, flags, seq, rcvNxt, rcvWnd)
 }
+
+// maybeSendOutOfWindowAck sends an ACK if we are not being rate limited
+// currently.
+func (s *sender) maybeSendOutOfWindowAck(seg *segment) {
+	// Data packets are unlikely to be part of an ACK loop. So always send
+	// an ACK for a packet w/ data.
+	if seg.payloadSize() > 0 || s.ep.allowOutOfWindowAck() {
+		s.sendAck()
+	}
+}
diff --git a/pkg/tcpip/transport/tcp/tcp_test.go b/pkg/tcpip/transport/tcp/tcp_test.go
index da2730e27..cd3c4a027 100644
--- a/pkg/tcpip/transport/tcp/tcp_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_test.go
@@ -6302,6 +6302,13 @@ func TestReceiveBufferAutoTuning(t *testing.T) {
 
 	// Enable Auto-tuning.
 	stk := c.Stack()
+	// Disable out of window rate limiting for this test by setting it to 0 as we
+	// use out of window ACKs to measure the advertised window.
+	var tcpInvalidRateLimit stack.TCPInvalidRateLimitOption
+	if err := stk.SetOption(tcpInvalidRateLimit); err != nil {
+		t.Fatalf("e.stack.SetOption(%#v) = %s", tcpInvalidRateLimit, err)
+	}
+
 	const receiveBufferSize = 80 << 10 // 80KB.
 	const maxReceiveBufferSize = receiveBufferSize * 10
 	{