[mle] send a unicast MLE Announce in response to a received Announce (#2669)

This commit changes how MLE Announce messages are sent in response to
a received MLE Announce. In addition to a multicast MLE Announce
message, a unicast MLE Announce is also sent. This behavior can be
controlled using `OPENTHREAD_CONFIG_SEND_UNICAST_ANNOUNCE_RESPONSE`
config option. By default this is enabled.

This commit also changes actions taken after receiving an MLE Announce
in `Mle::HandleAnnounce()` to handle the situation where a device may
receive multiple MLE Announce messages with same info. No action is
taken if the device is detached, and current channel and panid match
the values from the received MLE Announce message.

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

@@ -1327,6 +1327,19 @@
 #define OPENTHREAD_CONFIG_PARENT_SEARCH_RSS_THRESHOLD -65
 #endif
 
+/**
+ * @def OPENTHREAD_CONFIG_SEND_UNICAST_ANNOUNCE_RESPONSE
+ *
+ * Define as 1 to enable sending of a unicast MLE Announce message in response to a received Announce message from
+ * a device.
+ *
+ * @note The unicast MLE announce message is sent in addition to (and after) the multicast MLE Announce.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_SEND_UNICAST_ANNOUNCE_RESPONSE
+#define OPENTHREAD_CONFIG_SEND_UNICAST_ANNOUNCE_RESPONSE 1
+#endif
+
 /**
  * @def OPENTHREAD_CONFIG_NCP_ENABLE_PEEK_POKE
  *

src/core/thread/mle.cpp

@@ -1979,13 +1979,23 @@ exit:
 }
 
 otError Mle::SendAnnounce(uint8_t aChannel, bool aOrphanAnnounce)
+{
+    Ip6::Address destination;
+
+    memset(&destination, 0, sizeof(destination));
+    destination.mFields.m16[0] = HostSwap16(0xff02);
+    destination.mFields.m16[7] = HostSwap16(0x0001);
+
+    return SendAnnounce(aChannel, aOrphanAnnounce, destination);
+}
+
+otError Mle::SendAnnounce(uint8_t aChannel, bool aOrphanAnnounce, const Ip6::Address &aDestination)
 {
     ThreadNetif &      netif = GetNetif();
     otError            error = OT_ERROR_NONE;
     ChannelTlv         channel;
     PanIdTlv           panid;
     ActiveTimestampTlv activeTimestamp;
-    Ip6::Address       destination;
     Message *          message;
 
     VerifyOrExit((message = NewMleMessage()) != NULL, error = OT_ERROR_NO_BUFS);
@@ -2016,11 +2026,7 @@ otError Mle::SendAnnounce(uint8_t aChannel, bool aOrphanAnnounce)
     panid.Init();
     panid.SetPanId(netif.GetMac().GetPanId());
     SuccessOrExit(error = message->Append(&panid, sizeof(panid)));
-
-    memset(&destination, 0, sizeof(destination));
-    destination.mFields.m16[0] = HostSwap16(0xff02);
-    destination.mFields.m16[7] = HostSwap16(0x0001);
-    SuccessOrExit(error = SendMessage(*message, destination));
+    SuccessOrExit(error = SendMessage(*message, aDestination));
 
     otLogInfoMle(GetInstance(), "Send Announce on channel %d", aChannel);
 
@@ -3188,36 +3194,53 @@ otError Mle::HandleAnnounce(const Message &aMessage, const Ip6::MessageInfo &aMe
 {
     ThreadNetif &             netif = GetNetif();
     otError                   error = OT_ERROR_NONE;
-    ChannelTlv                channel;
+    ChannelTlv                channelTlv;
     ActiveTimestampTlv        timestamp;
     const MeshCoP::Timestamp *localTimestamp;
-    PanIdTlv                  panid;
+    PanIdTlv                  panIdTlv;
+    uint8_t                   channel;
+    uint16_t                  panId;
 
     LogMleMessage("Receive Announce", aMessageInfo.GetPeerAddr());
 
-    SuccessOrExit(error = Tlv::GetTlv(aMessage, Tlv::kChannel, sizeof(channel), channel));
-    VerifyOrExit(channel.IsValid(), error = OT_ERROR_PARSE);
+    SuccessOrExit(error = Tlv::GetTlv(aMessage, Tlv::kChannel, sizeof(channelTlv), channelTlv));
+    VerifyOrExit(channelTlv.IsValid(), error = OT_ERROR_PARSE);
+    channel = static_cast<uint8_t>(channelTlv.GetChannel());
 
     SuccessOrExit(error = Tlv::GetTlv(aMessage, Tlv::kActiveTimestamp, sizeof(timestamp), timestamp));
     VerifyOrExit(timestamp.IsValid(), error = OT_ERROR_PARSE);
 
-    SuccessOrExit(error = Tlv::GetTlv(aMessage, Tlv::kPanId, sizeof(panid), panid));
-    VerifyOrExit(panid.IsValid(), error = OT_ERROR_PARSE);
+    SuccessOrExit(error = Tlv::GetTlv(aMessage, Tlv::kPanId, sizeof(panIdTlv), panIdTlv));
+    VerifyOrExit(panIdTlv.IsValid(), error = OT_ERROR_PARSE);
+    panId = panIdTlv.GetPanId();
 
     localTimestamp = netif.GetActiveDataset().GetTimestamp();
 
     if (localTimestamp == NULL || localTimestamp->Compare(timestamp) > 0)
     {
+        uint8_t  curChannel = netif.GetMac().GetChannel();
+        uint16_t curPanId   = netif.GetMac().GetPanId();
+
+        // No action is required if device is detached, and current
+        // channel and pan-id match the values from the received MLE
+        // Announce message.
+
+        VerifyOrExit((mRole != OT_DEVICE_ROLE_DETACHED) || (curChannel != channel) || (curPanId != panId));
+
         Stop(false);
-        mPreviousChannel = netif.GetMac().GetChannel();
-        mPreviousPanId   = netif.GetMac().GetPanId();
-        netif.GetMac().SetChannel(static_cast<uint8_t>(channel.GetChannel()));
-        netif.GetMac().SetPanId(panid.GetPanId());
+        mPreviousChannel = curChannel;
+        mPreviousPanId   = curPanId;
+        netif.GetMac().SetChannel(channel);
+        netif.GetMac().SetPanId(panId);
         Start(false, true);
     }
     else if (localTimestamp->Compare(timestamp) < 0)
     {
-        SendAnnounce(static_cast<uint8_t>(channel.GetChannel()), false);
+        SendAnnounce(channel, false);
+
+#if OPENTHREAD_CONFIG_SEND_UNICAST_ANNOUNCE_RESPONSE
+        SendAnnounce(channel, false, aMessageInfo.GetPeerAddr());
+#endif
     }
     else
     {

src/core/thread/mle.hpp

@@ -1483,6 +1483,7 @@ private:
     otError SendParentRequest(void);
     otError SendChildIdRequest(void);
     void    SendOrphanAnnounce(void);
+    otError SendAnnounce(uint8_t aChannel, bool aOrphanAnnounce, const Ip6::Address &aDestination);
 
     bool IsBetterParent(uint16_t aRloc16, uint8_t aLinkQuality, uint8_t aLinkMargin, ConnectivityTlv &aConnectivityTlv);
     void ResetParentCandidate(void);