[mle] verify hop limit is 255 (#2167)

src/core/thread/mle.cpp

@@ -1942,7 +1942,7 @@ otError Mle::SendMessage(Message &aMessage, const Ip6::Address &aDestination)
     messageInfo.SetSockAddr(mLinkLocal64.GetAddress());
     messageInfo.SetPeerPort(kUdpPort);
     messageInfo.SetInterfaceId(netif.GetInterfaceId());
-    messageInfo.SetHopLimit(255);
+    messageInfo.SetHopLimit(kMleHopLimit);
 
     SuccessOrExit(error = mSocket.SendTo(aMessage, messageInfo));
 
@@ -2008,6 +2008,7 @@ void Mle::HandleUdpReceive(Message &aMessage, const Ip6::MessageInfo &aMessageIn
     Neighbor *neighbor;
 
     VerifyOrExit(aMessageInfo.GetLinkInfo() != NULL);
+    VerifyOrExit(aMessageInfo.GetHopLimit() == kMleHopLimit);
 
     length = aMessage.Read(aMessage.GetOffset(), sizeof(header), &header);
     VerifyOrExit(header.IsValid() && header.GetLength() <= length);

src/core/thread/mle.hpp

@@ -1336,6 +1336,7 @@ private:
     enum
     {
         kMleMessagePriority = Message::kPriorityHigh,
+        kMleHopLimit        = 255,
     };
 
     void GenerateNonce(const Mac::ExtAddress &aMacAddr, uint32_t aFrameCounter, uint8_t aSecurityLevel,