[core] EID-to-RLOC cache map optimization (#4091)

This commit adds EID-to-RLOC map cache entries by inspecting packets
received, aiming to reduce RealmLocal multicast AQ when response is
expected.

src/core/thread/address_resolver.cpp

@@ -212,8 +212,10 @@ void AddressResolver::InvalidateCacheEntry(Cache &aEntry, InvalidationReason aRe
     aEntry.mState = Cache::kStateInvalid;
 }
 
-void AddressResolver::UpdateCacheEntry(const Ip6::Address &aEid, Mac::ShortAddress aRloc16)
+otError AddressResolver::UpdateCacheEntry(const Ip6::Address &aEid, Mac::ShortAddress aRloc16)
 {
+    otError error = OT_ERROR_NOT_FOUND;
+
     for (int i = 0; i < kCacheEntries; i++)
     {
         if (mCache[i].mState == Cache::kStateInvalid || mCache[i].mTarget != aEid)
@@ -240,11 +242,29 @@ void AddressResolver::UpdateCacheEntry(const Ip6::Address &aEid, Mac::ShortAddre
             otLogNoteArp("Cache entry updated (snoop): %s, 0x%04x", aEid.ToString().AsCString(), aRloc16);
         }
 
-        ExitNow();
+        error = OT_ERROR_NONE;
     }
 
+    return error;
+}
+
+otError AddressResolver::AddCacheEntry(const Ip6::Address &aEid, Mac::ShortAddress aRloc16)
+{
+    otError error = OT_ERROR_NONE;
+    Cache * entry = NewCacheEntry();
+
+    VerifyOrExit(entry != NULL, error = OT_ERROR_NO_BUFS);
+
+    entry->mTarget   = aEid;
+    entry->mRloc16   = aRloc16;
+    entry->mTimeout  = 0;
+    entry->mFailures = 0;
+    entry->mState    = Cache::kStateCached;
+
+    MarkCacheEntryAsUsed(*entry);
+
 exit:
-    return;
+    return error;
 }
 
 void AddressResolver::RestartAddressQueries(void)

src/core/thread/address_resolver.hpp

@@ -103,13 +103,28 @@ public:
     void Remove(uint8_t aRouterId);
 
     /**
-     * This method updates an existing cache entry for the EID, if one exists.
+     * This method updates an existing cache entry for the EID.
      *
-     * @param[in]  aEid     A reference to the EID.
-     * @param[in]  aRloc16  The RLOC16 corresponding to @p aEid.
+     * @param[in]  aEid               A reference to the EID.
+     * @param[in]  aRloc16            The RLOC16 corresponding to @p aEid.
+     *
+     * @retval OT_ERROR_NONE           Successfully updates an existing cache entry.
+     * @retval OT_ERROR_NOT_FOUND      No cache entry with @p aEid.
      *
      */
-    void UpdateCacheEntry(const Ip6::Address &aEid, Mac::ShortAddress aRloc16);
+    otError UpdateCacheEntry(const Ip6::Address &aEid, Mac::ShortAddress aRloc16);
+
+    /**
+     * This method adds one cache entry for the EID.
+     *
+     * @param[in]  aEid               A reference to the EID.
+     * @param[in]  aRloc16            The RLOC16 corresponding to @p aEid.
+     *
+     * @retval OT_ERROR_NONE           Successfully adds one cache entry.
+     * @retval OT_ERROR_NO_BUFS        Insufficient buffer space available to add one cache entry.
+     *
+     */
+    otError AddCacheEntry(const Ip6::Address &aEid, Mac::ShortAddress aRloc16);
 
     /**
      * This method returns the RLOC16 for a given EID, or initiates an Address Query if the mapping is not known.

src/core/thread/mesh_forwarder_ftd.cpp

@@ -593,7 +593,23 @@ void MeshForwarder::UpdateRoutes(uint8_t *           aFrame,
     VerifyOrExit(!aMeshDest.IsBroadcast() && aMeshSource.IsShort());
     SuccessOrExit(GetIp6Header(aFrame, aFrameLength, aMeshSource, aMeshDest, ip6Header));
 
-    Get<AddressResolver>().UpdateCacheEntry(ip6Header.GetSource(), aMeshSource.GetShort());
+    if (!ip6Header.GetSource().IsRoutingLocator() && !ip6Header.GetSource().IsAnycastRoutingLocator() &&
+        Get<NetworkData::Leader>().IsOnMesh(ip6Header.GetSource()) /* only for on mesh address which may require AQ */)
+    {
+        if (Get<AddressResolver>().UpdateCacheEntry(ip6Header.GetSource(), aMeshSource.GetShort()) ==
+            OT_ERROR_NOT_FOUND)
+        {
+            // Thread 1.1 Specification 5.5.2.2:
+            // FTDs MAY add/update EID-to-RLOC Map Cache entries by inspecting packets being received.
+            if ((Get<Mle::MleRouter>().IsFullThreadDevice() /* only for FTD */ &&
+                 !Get<Mle::MleRouter>().IsMinimalChild(aMeshSource.GetShort()) /* Exclude MTD child source */) &&
+                (aMeshDest.GetShort() == Get<Mac::Mac>().GetShortAddress() ||
+                 Get<Mle::MleRouter>().IsMinimalChild(aMeshDest.GetShort())) /* Received for itself or its MTD child */)
+            {
+                Get<AddressResolver>().AddCacheEntry(ip6Header.GetSource(), aMeshSource.GetShort());
+            }
+        }
+    }
 
     neighbor = Get<Mle::MleRouter>().GetNeighbor(ip6Header.GetSource());
     VerifyOrExit(neighbor != NULL && !neighbor->IsFullThreadDevice());