4 files changed, 71 insertions, 10 deletions
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index dc4f5b3e7..026d330c4 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -171,6 +171,9 @@ type TCPSenderState struct {
 	// Outstanding is the number of packets in flight.
 	Outstanding int
 
+	// SackedOut is the number of packets which have been selectively acked.
+	SackedOut int
+
 	// SndWnd is the send window size in bytes.
 	SndWnd seqnum.Size
 
diff --git a/pkg/tcpip/transport/tcp/endpoint.go b/pkg/tcpip/transport/tcp/endpoint.go
index 7a37c10bb..bb0795f78 100644
--- a/pkg/tcpip/transport/tcp/endpoint.go
+++ b/pkg/tcpip/transport/tcp/endpoint.go
@@ -2989,6 +2989,7 @@ func (e *endpoint) completeState() stack.TCPEndpointState {
 		Ssthresh:         e.snd.sndSsthresh,
 		SndCAAckCount:    e.snd.sndCAAckCount,
 		Outstanding:      e.snd.outstanding,
+		SackedOut:        e.snd.sackedOut,
 		SndWnd:           e.snd.sndWnd,
 		SndUna:           e.snd.sndUna,
 		SndNxt:           e.snd.sndNxt,
diff --git a/pkg/tcpip/transport/tcp/snd.go b/pkg/tcpip/transport/tcp/snd.go
index baec762e1..cc991aba6 100644
--- a/pkg/tcpip/transport/tcp/snd.go
+++ b/pkg/tcpip/transport/tcp/snd.go
@@ -137,6 +137,9 @@ type sender struct {
 	// that have been sent but not yet acknowledged.
 	outstanding int
 
+	// sackedOut is the number of packets which are selectively acked.
+	sackedOut int
+
 	// sndWnd is the send window size.
 	sndWnd seqnum.Size
 
@@ -372,6 +375,7 @@ func (s *sender) updateMaxPayloadSize(mtu, count int) {
 		m = 1
 	}
 
+	oldMSS := s.maxPayloadSize
 	s.maxPayloadSize = m
 	if s.gso {
 		s.ep.gso.MSS = uint16(m)
@@ -394,6 +398,7 @@ func (s *sender) updateMaxPayloadSize(mtu, count int) {
 
 	// Rewind writeNext to the first segment exceeding the MTU. Do nothing
 	// if it is already before such a packet.
+	nextSeg := s.writeNext
 	for seg := s.writeList.Front(); seg != nil; seg = seg.Next() {
 		if seg == s.writeNext {
 			// We got to writeNext before we could find a segment
@@ -401,16 +406,22 @@ func (s *sender) updateMaxPayloadSize(mtu, count int) {
 			break
 		}
 
-		if seg.data.Size() > m {
+		if nextSeg == s.writeNext && seg.data.Size() > m {
 			// We found a segment exceeding the MTU. Rewind
 			// writeNext and try to retransmit it.
-			s.writeNext = seg
-			break
+			nextSeg = seg
+		}
+
+		if s.ep.sackPermitted && s.ep.scoreboard.IsSACKED(seg.sackBlock()) {
+			// Update sackedOut for new maximum payload size.
+			s.sackedOut -= s.pCount(seg, oldMSS)
+			s.sackedOut += s.pCount(seg, s.maxPayloadSize)
 		}
 	}
 
 	// Since we likely reduced the number of outstanding packets, we may be
 	// ready to send some more.
+	s.writeNext = nextSeg
 	s.sendData()
 }
 
@@ -629,13 +640,13 @@ func (s *sender) retransmitTimerExpired() bool {
 
 // pCount returns the number of packets in the segment. Due to GSO, a segment
 // can be composed of multiple packets.
-func (s *sender) pCount(seg *segment) int {
+func (s *sender) pCount(seg *segment, maxPayloadSize int) int {
 	size := seg.data.Size()
 	if size == 0 {
 		return 1
 	}
 
-	return (size-1)/s.maxPayloadSize + 1
+	return (size-1)/maxPayloadSize + 1
 }
 
 // splitSeg splits a given segment at the size specified and inserts the
@@ -1023,7 +1034,7 @@ func (s *sender) sendData() {
 			break
 		}
 		dataSent = true
-		s.outstanding += s.pCount(seg)
+		s.outstanding += s.pCount(seg, s.maxPayloadSize)
 		s.writeNext = seg.Next()
 	}
 
@@ -1038,6 +1049,7 @@ func (s *sender) enterRecovery() {
 	// We inflate the cwnd by 3 to account for the 3 packets which triggered
 	// the 3 duplicate ACKs and are now not in flight.
 	s.sndCwnd = s.sndSsthresh + 3
+	s.sackedOut = 0
 	s.fr.first = s.sndUna
 	s.fr.last = s.sndNxt - 1
 	s.fr.maxCwnd = s.sndCwnd + s.outstanding
@@ -1207,6 +1219,7 @@ func (s *sender) walkSACK(rcvdSeg *segment) {
 				s.rc.update(seg, rcvdSeg, s.ep.tsOffset)
 				s.rc.detectReorder(seg)
 				seg.acked = true
+				s.sackedOut += s.pCount(seg, s.maxPayloadSize)
 			}
 			seg = seg.Next()
 		}
@@ -1380,10 +1393,10 @@ func (s *sender) handleRcvdSegment(rcvdSeg *segment) {
 			datalen := seg.logicalLen()
 
 			if datalen > ackLeft {
-				prevCount := s.pCount(seg)
+				prevCount := s.pCount(seg, s.maxPayloadSize)
 				seg.data.TrimFront(int(ackLeft))
 				seg.sequenceNumber.UpdateForward(ackLeft)
-				s.outstanding -= prevCount - s.pCount(seg)
+				s.outstanding -= prevCount - s.pCount(seg, s.maxPayloadSize)
 				break
 			}
 
@@ -1399,11 +1412,13 @@ func (s *sender) handleRcvdSegment(rcvdSeg *segment) {
 
 			s.writeList.Remove(seg)
 
-			// If SACK is enabled then Only reduce outstanding if
+			// If SACK is enabled then only reduce outstanding if
 			// the segment was not previously SACKED as these have
 			// already been accounted for in SetPipe().
 			if !s.ep.sackPermitted || !s.ep.scoreboard.IsSACKED(seg.sackBlock()) {
-				s.outstanding -= s.pCount(seg)
+				s.outstanding -= s.pCount(seg, s.maxPayloadSize)
+			} else {
+				s.sackedOut -= s.pCount(seg, s.maxPayloadSize)
 			}
 			seg.decRef()
 			ackLeft -= datalen
diff --git a/pkg/tcpip/transport/tcp/tcp_sack_test.go b/pkg/tcpip/transport/tcp/tcp_sack_test.go
index ef7f5719f..faf0c0ad7 100644
--- a/pkg/tcpip/transport/tcp/tcp_sack_test.go
+++ b/pkg/tcpip/transport/tcp/tcp_sack_test.go
@@ -590,3 +590,45 @@ func TestSACKRecovery(t *testing.T) {
 		expected++
 	}
 }
+
+// TestSACKUpdateSackedOut tests the sacked out field is updated when a SACK
+// is received.
+func TestSACKUpdateSackedOut(t *testing.T) {
+	c := context.New(t, uint32(mtu))
+	defer c.Cleanup()
+
+	probeDone := make(chan struct{})
+	ackNum := 0
+	c.Stack().AddTCPProbe(func(state stack.TCPEndpointState) {
+		// Validate that the endpoint Sender.SackedOut is what we expect.
+		if state.Sender.SackedOut != 2 && ackNum == 0 {
+			t.Fatalf("SackedOut got updated to wrong value got: %v want: 2", state.Sender.SackedOut)
+		}
+
+		if state.Sender.SackedOut != 0 && ackNum == 1 {
+			t.Fatalf("SackedOut got updated to wrong value got: %v want: 0", state.Sender.SackedOut)
+		}
+		if ackNum > 0 {
+			close(probeDone)
+		}
+		ackNum++
+	})
+	setStackSACKPermitted(t, c, true)
+	createConnectedWithSACKAndTS(c)
+
+	sendAndReceive(t, c, 8)
+
+	// ACK for [3-5] packets.
+	seq := seqnum.Value(context.TestInitialSequenceNumber).Add(1)
+	start := c.IRS.Add(seqnum.Size(1 + 3*maxPayload))
+	bytesRead := 2 * maxPayload
+	end := start.Add(seqnum.Size(bytesRead))
+	c.SendAckWithSACK(seq, bytesRead, []header.SACKBlock{{start, end}})
+
+	bytesRead += 3 * maxPayload
+	c.SendAck(seq, bytesRead)
+
+	// Wait for the probe function to finish processing the ACK before the
+	// test completes.
+	<-probeDone
+}