netstack: enable SACK by default

There's no reason not to turn this on. Leaving it off makes it possible for
users to miss it.

PiperOrigin-RevId: 634026749

pkg/tcpip/tests/integration/istio_test.go

@@ -113,7 +113,7 @@ var (
 
 func newTestContext(t *testing.T) *testContext {
 	t.Helper()
-	localNIC, remoteNIC := pipe.New("" /* linkAddr1 */, "" /* linkAddr2 */, header.IPv4MinimumMTU)
+	localNIC, remoteNIC := pipe.New("" /* linkAddr1 */, "" /* linkAddr2 */, 1500)
 
 	localStack := stack.New(stack.Options{
 		NetworkProtocols:   []stack.NetworkProtocolFactory{ipv4.NewProtocol},

pkg/tcpip/transport/tcp/protocol.go

@@ -535,6 +535,7 @@ func NewProtocol(s *stack.Stack) stack.TransportProtocol {
 			Default: DefaultReceiveBufferSize,
 			Max:     MaxBufferSize,
 		},
+		sackEnabled:                true,
 		congestionControl:          ccReno,
 		availableCongestionControl: []string{ccReno, ccCubic},
 		moderateReceiveBuffer:      true,