[mle] implement hysteresis for establishing links and merging (#2409)

This commit applies a hysteresis to establishing new links or merging
partitions.  In particular, this commit introduces new minimum link
margin thresholds that must be met before attempting to establish a
new link or merging to a different partition.  Enforcing a mininmum
link margin threshold before establishing new links helps avoid links
that will quickly become invalid due to normal time-varying link
qualities.

include/openthread/types.h

@@ -272,6 +272,11 @@ typedef enum otError
      */
     OT_ERROR_DISABLED_FEATURE = 33,
 
+    /**
+     * The link margin was too low.
+     */
+    OT_ERROR_LINK_MARGIN_LOW = 34,
+
     /**
      * Generic error (should not use).
      */

src/core/common/logging.cpp

@@ -410,6 +410,10 @@ const char *otThreadErrorToString(otError aError)
         retval = "GenericError";
         break;
 
+    case OT_ERROR_LINK_MARGIN_LOW:
+        retval = "LinkMarginLow";
+        break;
+
     default:
         retval = "UnknownErrorType";
         break;

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

@@ -1079,7 +1079,27 @@
 #define OPENTHREAD_CONFIG_MLE_SEND_LINK_REQUEST_ON_ADV_TIMEOUT  0
 #endif
 
-/*
+/**
+ * @def OPENTHREAD_CONFIG_MLE_LINK_REQUEST_MARGIN_MIN
+ *
+ * Specifies the minimum link margin in dBm required before attempting to establish a link with a neighboring router.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_MLE_LINK_REQUEST_MARGIN_MIN
+#define OPENTHREAD_CONFIG_MLE_LINK_REQUEST_MARGIN_MIN           10
+#endif
+
+/**
+ * @def OPENTHREAD_CONFIG_MLE_PARTITION_MERGE_MARGIN_MIN
+ *
+ * Specifies the minimum link margin in dBm required before attempting to merge to a different partition.
+ *
+ */
+#ifndef OPENTHREAD_CONFIG_MLE_PARTITION_MERGE_MARGIN_MIN
+#define OPENTHREAD_CONFIG_MLE_PARTITION_MERGE_MARGIN_MIN        10
+#endif
+
+/**
  * @def OPENTHREAD_CONFIG_ENABLE_DEBUG_UART
  *
  * Enable the "Debug Uart" platform feature.

src/core/thread/mle_router.cpp

@@ -1305,6 +1305,7 @@ otError MleRouter::HandleAdvertisement(const Message &aMessage, const Ip6::Messa
     ThreadNetif &netif = GetNetif();
     otError error = OT_ERROR_NONE;
     const otThreadLinkInfo *linkInfo = static_cast<const otThreadLinkInfo *>(aMessageInfo.GetLinkInfo());
+    uint8_t linkMargin = LinkQualityInfo::ConvertRssToLinkMargin(GetNetif().GetMac().GetNoiseFloor(), linkInfo->mRss);
     Mac::ExtAddress macAddr;
     SourceAddressTlv sourceAddress;
     LeaderDataTlv leaderData;
@@ -1343,6 +1344,8 @@ otError MleRouter::HandleAdvertisement(const Message &aMessage, const Ip6::Messa
         otLogInfoMle(GetInstance(), "Different partition (peer:%d, local:%d)",
                      leaderData.GetPartitionId(), mLeaderData.GetPartitionId());
 
+        VerifyOrExit(linkMargin >= OPENTHREAD_CONFIG_MLE_PARTITION_MERGE_MARGIN_MIN, error = OT_ERROR_LINK_MARGIN_LOW);
+
         if ((mDeviceMode & ModeTlv::kModeFFD) &&
             (mLastPartitionIdTimeout > 0) &&
             (partitionId == mLastPartitionId))
@@ -1528,7 +1531,9 @@ otError MleRouter::HandleAdvertisement(const Message &aMessage, const Ip6::Messa
         }
 
         // Send link request if no link to router
-        if ((router->GetState() != Neighbor::kStateValid) && (router->GetState() != Neighbor::kStateLinkRequest))
+        if ((router->GetState() != Neighbor::kStateValid) &&
+            (router->GetState() != Neighbor::kStateLinkRequest) &&
+            (linkMargin >= OPENTHREAD_CONFIG_MLE_LINK_REQUEST_MARGIN_MIN))
         {
             router->SetExtAddress(macAddr);
             router->GetLinkInfo().Clear();