Allow SED to reattach to it's parent if the link was lost before timeout. (#1234)

* Note in API description that SED poll period is separated from child timeout.

* Allow reattach of SED that detached from it's parent but did not timeout.

include/openthread.h

@@ -1028,6 +1028,9 @@ OTAPI ThreadError OTCALL otSendPendingSet(otInstance *aInstance, const otOperati
 /**
  * Get the data poll period of sleepy end device.
  *
+ * @note This function updates only poll period of sleepy end device. To update child timeout the function
+ *       otGetChildTimeout() shall be called.
+ *
  * @param[in]  aInstance A pointer to an OpenThread instance.
  *
  * @returns  The data poll period of sleepy end device.

src/core/thread/mle_router.cpp

@@ -1670,7 +1670,16 @@ ThreadError MleRouter::HandleParentRequest(const Message &aMessage, const Ip6::M
     SuccessOrExit(error = Tlv::GetTlv(aMessage, Tlv::kChallenge, sizeof(challenge), challenge));
     VerifyOrExit(challenge.IsValid(), error = kThreadError_Parse);
 
-    if ((child = FindChild(macAddr)) == NULL)
+    child = FindChild(macAddr);
+
+    if (child != NULL && !(child->mMode & ModeTlv::kModeFFD))
+    {
+        // Parent Request from a MTD child means that the child had detached. It can be safely removed.
+        RemoveNeighbor(*child);
+        child = NULL;
+    }
+
+    if (child == NULL)
     {
         VerifyOrExit((child = NewChild()) != NULL, ;);