Reduce MTU to 1280 bytes (#1032)

* Reduce MTU to 1280 bytes

* Update 5.1.07 test to use 1232 byte ping

examples/drivers/windows/otLwf/eventprocessing.c

@@ -681,11 +681,11 @@ otLwfEventWorkerThread(
     KWAIT_BLOCK WaitBlocks[ARRAYSIZE(WaitEvents)] = { 0 };
 
     // Space to processing buffers
-    const ULONG MessageBufferSize = 1500;
+    const ULONG MessageBufferSize = 1280;
     PUCHAR MessageBuffer = FILTER_ALLOC_MEM(pFilter->FilterHandle, MessageBufferSize);
     if (MessageBuffer == NULL)
     {
-        LogError(DRIVER_DATA_PATH, "Failed to allocate 1500 bytes for MessageBuffer!");
+        LogError(DRIVER_DATA_PATH, "Failed to allocate 1280 bytes for MessageBuffer!");
         return;
     }
 

src/core/net/ip6.hpp

@@ -100,7 +100,7 @@ public:
     enum
     {
         kDefaultHopLimit = 64,
-        kMaxDatagramLength = 1500,
+        kMaxDatagramLength = 1280,
     };
 
     /**

tests/scripts/thread-cert/Cert_5_1_07_MaxChildCount.py

@@ -87,7 +87,7 @@ class Cert_5_1_07_MaxChildCount(unittest.TestCase):
         ipaddrs = self.nodes[SED1].get_addrs()
         for addr in ipaddrs:
             if addr[0:4] != 'fe80' and 'ff:fe00' not in addr:
-                self.assertTrue(self.nodes[LEADER].ping(addr, size=1280))
+                self.assertTrue(self.nodes[LEADER].ping(addr, size=1232))
                 break
 
         for i in range(3, 13):