Add support for rate limiting out of window ACKs.

Netstack today will send dupACK's with no rate limit for incoming out of
window segments. This can result in ACK loops for example if a TCP socket
connects to itself (actually permitted by TCP). Where the ACK sent in
response to packets being out of order itself gets considered as an out
of window segment resulting in another ACK being generated.

PiperOrigin-RevId: 355206877

1 files changed, 4 insertions, 3 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.