[mle] maintain best parent candidate during router-only search (#3803)

* maintain best parent candidate during router-only search
* add margin for duplicate parent request check

src/core/thread/mle.cpp

@@ -1606,8 +1606,13 @@ void Mle::HandleAttachTimer(void)
         // to which the device is attached. This ensures that the
         // new parent candidate is compared with the current parent
         // and that it is indeed preferred over the current one.
+        // If we are in kAttachStateParentRequestRouter and cannot
+        // find a parent with best link quality(3), we will keep
+        // the candidate and forward to REED stage to find a better
+        // parent.
 
-        if (mParentCandidate.GetState() == Neighbor::kStateParentResponse &&
+        if ((mParentCandidate.GetLinkInfo().GetLinkQuality() == 3 || mAttachState != kAttachStateParentRequestRouter) &&
+            mParentCandidate.GetState() == Neighbor::kStateParentResponse &&
             (mRole != OT_DEVICE_ROLE_CHILD || mReceivedResponseFromParent || mParentRequestMode == kAttachBetter) &&
             SendChildIdRequest() == OT_ERROR_NONE)
         {
@@ -3133,8 +3138,6 @@ otError Mle::HandleParentResponse(const Message &aMessage, const Ip6::MessageInf
 
     linkQuality = LinkQualityInfo::ConvertLinkMarginToLinkQuality(linkMargin);
 
-    VerifyOrExit(mAttachState != kAttachStateParentRequestRouter || linkQuality == 3);
-
     // Connectivity
     SuccessOrExit(error = Tlv::GetTlv(aMessage, Tlv::kConnectivity, sizeof(connectivity), connectivity));
     VerifyOrExit(connectivity.IsValid(), error = OT_ERROR_PARSE);

src/core/thread/mle_constants.hpp

@@ -60,6 +60,7 @@ enum
     kThreadVersion                  = 2,     ///< Thread Version
     kUdpPort                        = 19788, ///< MLE UDP Port
     kParentRequestRouterTimeout     = 750,   ///< Router Parent Request timeout
+    kParentRequestDuplicateMargin   = 50,    ///< Margin for duplicate parent request
     kParentRequestReedTimeout       = 1250,  ///< Router and REEDs Parent Request timeout
     kAttachStartJitter              = 50,    ///< Maximum jitter time added to start of attach.
     kAnnounceProcessTimeout         = 250,   ///< Timeout after receiving Announcement before channel/pan-id change

src/core/thread/mle_router.cpp

@@ -1649,7 +1649,7 @@ otError MleRouter::HandleParentRequest(const Message &aMessage, const Ip6::Messa
         }
 #endif
     }
-    else if (TimerMilli::Elapsed(child->GetLastHeard()) < kParentRequestRouterTimeout)
+    else if (TimerMilli::Elapsed(child->GetLastHeard()) < kParentRequestRouterTimeout - kParentRequestDuplicateMargin)
     {
         ExitNow(error = OT_ERROR_DUPLICATED);
     }