Lower CLAT max MTU from 65536 to 1528.

Android's clat is for reaching the internet, which in general
never has an ipv4 L3 mtu higher than 1500.

(We could probably hit this on a jumboframe enabled IPv6-only
wifi network [ http://b/292057969#comment18

 ] where RA claims
MTU of 9K [9170])

Bug: 292057969
Test: TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I7a32455571458b8a3f1121ad980d54323b2ef06b

service/src/com/android/server/connectivity/ClatCoordinator.java

@@ -78,7 +78,7 @@ public class ClatCoordinator {
     @VisibleForTesting
     static final int MTU_DELTA = 28;
     @VisibleForTesting
-    static final int CLAT_MAX_MTU = 65536;
+    static final int CLAT_MAX_MTU = 1500 + MTU_DELTA;
 
     // This must match the interface prefix in clatd.c.
     private static final String CLAT_PREFIX = "v4-";
@@ -673,7 +673,7 @@ public class ClatCoordinator {
             throw new IOException("Detect MTU on " + tunIface + " failed: " + e);
         }
         final int mtu = adjustMtu(detectedMtu);
-        Log.i(TAG, "ipv4 mtu is " + mtu);
+        Log.i(TAG, "detected ipv4 mtu of " + detectedMtu + " adjusted to " + mtu);
 
         // Config tun interface mtu, address and bring up.
         try {

tests/unit/java/com/android/server/connectivity/ClatCoordinatorTest.java

@@ -508,10 +508,10 @@ public class ClatCoordinatorTest {
         // Expected mtu is that the detected mtu minus MTU_DELTA(28).
         assertEquals(1372, ClatCoordinator.adjustMtu(1400));
         assertEquals(1472, ClatCoordinator.adjustMtu(ETHER_MTU));
-        assertEquals(65508, ClatCoordinator.adjustMtu(CLAT_MAX_MTU));
+        assertEquals(1500, ClatCoordinator.adjustMtu(CLAT_MAX_MTU));
 
-        // Expected mtu is that CLAT_MAX_MTU(65536) minus MTU_DELTA(28).
-        assertEquals(65508, ClatCoordinator.adjustMtu(CLAT_MAX_MTU + 1 /* over maximum mtu */));
+        // Expected mtu is that CLAT_MAX_MTU(1528) minus MTU_DELTA(28).
+        assertEquals(1500, ClatCoordinator.adjustMtu(CLAT_MAX_MTU + 1 /* over maximum mtu */));
     }
 
     private void verifyDump(final ClatCoordinator coordinator, boolean clatStarted) {