Add metric to count number of segments acknowledged by DSACK.

- Creates new metric "/tcp/segments_acked_with_dsack" to count the number of
segments acked with DSACK.
- Added check to verify the metric is getting incremented when a DSACK is sent
in the unit tests.

PiperOrigin-RevId: 386135949

pkg/sentry/socket/netstack/netstack.go

@@ -274,6 +274,7 @@ var Metrics = tcpip.Stats{
 		Timeouts:                           mustCreateMetric("/netstack/tcp/timeouts", "Number of times RTO expired."),
 		ChecksumErrors:                     mustCreateMetric("/netstack/tcp/checksum_errors", "Number of segments dropped due to bad checksums."),
 		FailedPortReservations:             mustCreateMetric("/netstack/tcp/failed_port_reservations", "Number of time TCP failed to reserve a port."),
+		SegmentsAckedWithDSACK:             mustCreateMetric("/netstack/tcp/segments_acked_with_dsack", "Number of segments for which DSACK was received."),
 	},
 	UDP: tcpip.UDPStats{
 		PacketsReceived:          mustCreateMetric("/netstack/udp/packets_received", "Number of UDP datagrams received via HandlePacket."),

pkg/tcpip/tcpip.go

@@ -1845,6 +1845,10 @@ type TCPStats struct {
 	// FailedPortReservations is the number of times TCP failed to reserve
 	// a port.
 	FailedPortReservations *StatCounter
+
+	// SegmentsAckedWithDSACK is the number of segments acknowledged with
+	// DSACK.
+	SegmentsAckedWithDSACK *StatCounter
 }
 
 // UDPStats collects UDP-specific stats.

pkg/tcpip/transport/tcp/snd.go

@@ -1154,6 +1154,13 @@ func (s *sender) walkSACK(rcvdSeg *segment) {
 	idx := 0
 	n := len(rcvdSeg.parsedOptions.SACKBlocks)
 	if checkDSACK(rcvdSeg) {
+		dsackBlock := rcvdSeg.parsedOptions.SACKBlocks[0]
+		numDSACK := uint64(dsackBlock.End-dsackBlock.Start) / uint64(s.MaxPayloadSize)
+		// numDSACK can be zero when DSACK is sent for subsegments.
+		if numDSACK < 1 {
+			numDSACK = 1
+		}
+		s.ep.stack.Stats().TCP.SegmentsAckedWithDSACK.IncrementBy(numDSACK)
 		s.rc.setDSACKSeen(true)
 		idx = 1
 		n--

pkg/tcpip/transport/tcp/tcp_rack_test.go

@@ -540,6 +540,28 @@ func TestRACKDetectDSACK(t *testing.T) {
 	case invalidDSACKDetected:
 		t.Fatalf("RACK DSACK detected when there is no duplicate SACK")
 	}
+
+	metricPollFn := func() error {
+		tcpStats := c.Stack().Stats().TCP
+		stats := []struct {
+			stat *tcpip.StatCounter
+			name string
+			want uint64
+		}{
+			// Check DSACK was received for one segment.
+			{tcpStats.SegmentsAckedWithDSACK, "stats.TCP.SegmentsAckedWithDSACK", 1},
+		}
+		for _, s := range stats {
+			if got, want := s.stat.Value(), s.want; got != want {
+				return fmt.Errorf("got %s.Value() = %d, want = %d", s.name, got, want)
+			}
+		}
+		return nil
+	}
+
+	if err := testutil.Poll(metricPollFn, 1*time.Second); err != nil {
+		t.Error(err)
+	}
 }
 
 // TestRACKDetectDSACKWithOutOfOrder tests that RACK detects DSACK with out of
@@ -680,6 +702,28 @@ func TestRACKDetectDSACKSingleDup(t *testing.T) {
 	case invalidDSACKDetected:
 		t.Fatalf("RACK DSACK detected when there is no duplicate SACK")
 	}
+
+	metricPollFn := func() error {
+		tcpStats := c.Stack().Stats().TCP
+		stats := []struct {
+			stat *tcpip.StatCounter
+			name string
+			want uint64
+		}{
+			// Check DSACK was received for a subsegment.
+			{tcpStats.SegmentsAckedWithDSACK, "stats.TCP.SegmentsAckedWithDSACK", 1},
+		}
+		for _, s := range stats {
+			if got, want := s.stat.Value(), s.want; got != want {
+				return fmt.Errorf("got %s.Value() = %d, want = %d", s.name, got, want)
+			}
+		}
+		return nil
+	}
+
+	if err := testutil.Poll(metricPollFn, 1*time.Second); err != nil {
+		t.Error(err)
+	}
 }
 
 // TestRACKDetectDSACKDupWithCumulativeACK tests DSACK for two non-contiguous