Apply SWS avoidance to ACKs with window updates

When recovering from a zero-receive-window situation, and asked to
send out an ACK, ensure that we apply SWS avoidance in our window
updates.

Fixes #5984

PiperOrigin-RevId: 373689578

2 files changed, 13 insertions, 1 deletions

diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index 90edcfba6..50d39cbad 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -1601,7 +1601,7 @@ func (e *endpoint) selectWindow() (wnd seqnum.Size) {
 //
 // For large receive buffers, the threshold is aMSS - once reader reads more
 // than aMSS we'll send ACK. For tiny receive buffers, the threshold is half of
-// receive buffer size. This is chosen arbitrairly.
+// receive buffer size. This is chosen arbitrarily.
 // crossed will be true if the window size crossed the ACK threshold.
 // above will be true if the new window is >= ACK threshold and false
 // otherwise.
diff --git a/pkg/tcpip/transport/tcp/rcv.go b/pkg/tcpip/transport/tcp/rcv.go
index ee2c08cd6..133371455 100644
--- a/pkg/tcpip/transport/tcp/rcv.go
+++ b/pkg/tcpip/transport/tcp/rcv.go
@@ -148,6 +148,18 @@ func (r *receiver) getSendParams() (RcvNxt seqnum.Value, rcvWnd seqnum.Size) {
 		}
 		newWnd = curWnd
 	}
+
+	// Apply silly-window avoidance when recovering from zero-window situation.
+	// Keep advertising zero receive window up until the new window reaches a
+	// threshold.
+	if r.rcvWnd == 0 && newWnd != 0 {
+		r.ep.rcvQueueInfo.rcvQueueMu.Lock()
+		if crossed, above := r.ep.windowCrossedACKThresholdLocked(int(newWnd), int(r.ep.ops.GetReceiveBufferSize())); !crossed && !above {
+			newWnd = 0
+		}
+		r.ep.rcvQueueInfo.rcvQueueMu.Unlock()
+	}
+
 	// Stash away the non-scaled receive window as we use it for measuring
 	// receiver's estimated RTT.
 	r.rcvWnd = newWnd