[mle] inform previous parent on attach to a new parent (#2181)

This commit adds a new feature for a child to inform its previous
parent when it attaches to a new parent. OpenThread config option
`CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH` controls the behavior of
this feature (default is disabled). If this feature is enabled, after
a device attaches to a new parent, it sends an IP message (with empty
payload) to its previous parent.

src/core/openthread-core-default-config.h

@@ -850,6 +850,19 @@
 #define OPENTHREAD_CONFIG_SUPERVISION_MSG_NO_ACK_REQUEST        0
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH
+ *
+ * Define as 1 for a child to inform its previous parent when it attaches to a new parent.
+ *
+ * If this feature is enabled, when a device attaches to a new parent, it will send an IP message (with empty payload
+ * and mesh-local IP address as the source address) to its previous parent.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH
+#define OPENTHREAD_CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH    0
+#endif
+
 /**
  * @def OPENTHREAD_CONFIG_NCP_ENABLE_PEEK_POKE
  *
@@ -873,7 +886,6 @@
 #define OPENTHREAD_CONFIG_STAY_AWAKE_BETWEEN_FRAGMENTS          0
 #endif
 
-
 /*
  * @def OPENTHREAD_CONFIG_ENABLE_DEBUG_UART
  *

src/core/thread/mle.cpp

@@ -87,6 +87,9 @@ Mle::Mle(ThreadNetif &aThreadNetif) :
     mDiscoverContext(NULL),
     mIsDiscoverInProgress(false),
     mEnableEui64Filtering(false),
+#if OPENTHREAD_CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH
+    mPreviousParentRloc(Mac::kShortAddrInvalid),
+#endif
     mAnnounceChannel(OT_RADIO_CHANNEL_MIN),
     mPreviousChannel(0),
     mPreviousPanId(Mac::kPanIdBroadcast)
@@ -315,6 +318,10 @@ otError Mle::Restore(void)
                               ModeTlv::kModeSecureDataRequest);
         mParent.SetRloc16(GetRloc16(GetRouterId(networkInfo.mRloc16)));
         mParent.SetState(Neighbor::kStateRestored);
+
+#if OPENTHREAD_CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH
+        mPreviousParentRloc = mParent.GetRloc16();
+#endif
     }
     else
     {
@@ -627,6 +634,11 @@ otError Mle::SetStateChild(uint16_t aRloc16)
         netif.GetAnnounceBeginServer().SendAnnounce(1 << mPreviousChannel);
     }
 
+#if OPENTHREAD_CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH
+    InformPreviousParent();
+    mPreviousParentRloc = mParent.GetRloc16();
+#endif
+
     otLogInfoMle(GetInstance(), "Mode -> Child");
     return OT_ERROR_NONE;
 }
@@ -3267,6 +3279,45 @@ exit:
     return error;
 }
 
+#if OPENTHREAD_CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH
+otError Mle::InformPreviousParent(void)
+{
+    ThreadNetif &netif = GetNetif();
+    otError error = OT_ERROR_NONE;
+    Message *message = NULL;
+    Ip6::MessageInfo messageInfo;
+
+    VerifyOrExit((mPreviousParentRloc != Mac::kShortAddrInvalid)  && (mPreviousParentRloc != mParent.GetRloc16()));
+
+    VerifyOrExit((message = netif.GetIp6().NewMessage(0)) != NULL, error = OT_ERROR_NO_BUFS);
+    SuccessOrExit(error = message->SetLength(0));
+
+    messageInfo.SetSockAddr(GetMeshLocal64());
+    messageInfo.SetPeerAddr(GetMeshLocal16());
+    messageInfo.GetPeerAddr().mFields.m16[7] = HostSwap16(mPreviousParentRloc);
+    messageInfo.SetInterfaceId(netif.GetInterfaceId());
+
+    SuccessOrExit(error = netif.GetIp6().SendDatagram(*message, messageInfo, Ip6::kProtoNone));
+    message = NULL;
+
+    otLogInfoMle(GetInstance(), "Sending message to inform previous parent 0x%04x", mPreviousParentRloc);
+
+exit:
+
+    if (message != NULL)
+    {
+        message->Free();
+    }
+
+    if (error != OT_ERROR_NONE)
+    {
+        otLogWarnMle(GetInstance(), "Failed to inform previous parent, error:%s", otThreadErrorToString(error));
+    }
+
+    return error;
+}
+#endif // OPENTHREAD_CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH
+
 Mle &Mle::GetOwner(const Context &aContext)
 {
 #if OPENTHREAD_ENABLE_MULTIPLE_INSTANCES

src/core/thread/mle.hpp

@@ -1371,6 +1371,10 @@ private:
     bool IsBetterParent(uint16_t aRloc16, uint8_t aLinkQuality, ConnectivityTlv &aConnectivityTlv);
     void ResetParentCandidate(void);
 
+#if OPENTHREAD_CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH
+    otError InformPreviousParent(void);
+#endif
+
     static Mle &GetOwner(const Context &aContext);
 
     MessageQueue mDelayedResponses;
@@ -1407,6 +1411,10 @@ private:
     bool mIsDiscoverInProgress;
     bool mEnableEui64Filtering;
 
+#if OPENTHREAD_CONFIG_INFORM_PREVIOUS_PARENT_ON_REATTACH
+    uint16_t mPreviousParentRloc;
+#endif
+
     uint8_t mAnnounceChannel;
     uint8_t mPreviousChannel;
     uint16_t mPreviousPanId;