Add support for setting preferred Router Id (#716)

This commit adds a new ot API to set preferred Router Id for the
node. Upon becoming a router/leader the node attempts to use the
preferred Router Id. If it is not set or if it can not be used,
a randomly generated Router Id is picked.

include/openthread.h

@@ -932,6 +932,23 @@ uint32_t otGetPollPeriod(otInstance *aInstance);
  */
 void otSetPollPeriod(otInstance *aInstance, uint32_t aPollPeriod);
 
+/**
+ * Set the preferred Router Id.
+ *
+ * Upon becoming a router/leader the node attempts to use this Router Id. If the
+ * preferred Router Id is not set or if it can not be used, a randomly generated
+ * router id is picked. This property can be set only when the device role is
+ * either detached or disabled.
+ *
+ * @param[in]  aInstance    A pointer to an OpenThread instance.
+ * @param[in]  aRouterId    The preferred Router Id.
+ *
+ * @retval kThreadError_None         Successfully set the preferred Router Id.
+ * @retval kThreadError_InvalidState Could not set (role is not detached or disabled)
+ *
+ */
+ThreadError otSetPreferredRouterId(otInstance *aInstance, uint8_t aRouterId);
+
 /**
  * @}
  */

src/core/openthread.cpp

@@ -938,6 +938,11 @@ void otSetPollPeriod(otInstance *, uint32_t aPollPeriod)
     sThreadNetif->GetMeshForwarder().SetAssignPollPeriod(aPollPeriod);
 }
 
+ThreadError otSetPreferredRouterId(otInstance *, uint8_t aRouterId)
+{
+    return sThreadNetif->GetMle().SetPreferredRouterId(aRouterId);
+}
+
 #ifdef OPENTHREAD_MULTIPLE_INSTANCE
 
 otInstance *otInstanceInit(void *aInstanceBuffer, uint64_t *aInstanceBufferSize)

src/core/thread/mle_router.cpp

@@ -245,7 +245,8 @@ ThreadError MleRouter::BecomeLeader(void)
 
     routerId = IsRouterIdValid(mPreviousRouterId) ? AllocateRouterId(mPreviousRouterId) : AllocateRouterId();
     VerifyOrExit(IsRouterIdValid(routerId), error = kThreadError_NoBufs);
-    mRouterId = static_cast<uint8_t>(routerId);
+
+    mRouterId = routerId;
     mPreviousRouterId = mRouterId;
 
     memcpy(&mRouters[mRouterId].mMacAddr, mMac.GetExtAddress(), sizeof(mRouters[mRouterId].mMacAddr));
@@ -2721,6 +2722,21 @@ void MleRouter::SetLeaderPartitionId(uint32_t aPartitionId)
     mFixedLeaderPartitionId = aPartitionId;
 }
 
+ThreadError MleRouter::SetPreferredRouterId(uint8_t aRouterId)
+{
+    ThreadError error = kThreadError_None;
+
+    VerifyOrExit(
+        (mDeviceState == kDeviceStateDetached) || (mDeviceState == kDeviceStateDisabled),
+        error = kThreadError_InvalidState
+    );
+
+    mPreviousRouterId = aRouterId;
+
+exit:
+    return error;
+}
+
 void MleRouter::HandleMacDataRequest(const Child &aChild)
 {
     static const uint8_t tlvs[] = {Tlv::kLeaderData, Tlv::kNetworkData};

src/core/thread/mle_router.hpp

@@ -175,6 +175,20 @@ public:
      */
     void SetLeaderPartitionId(uint32_t aPartitionId);
 
+    /**
+     * This method sets the preferred Router Id. Upon becoming a router/leader the node
+     * attempts to use this Router Id. If the preferred Router Id is not set or if it
+     * can not be used, a randomly generated router Id is picked.
+     * This property can be set when he device role is detached or disabled.
+     *
+     * @param[in]  aRouterId             The preferred Router Id.
+     *
+     * @retval kThreadError_None         Successfully set the preferred Router Id.
+     * @retval kThreadError_InvalidState Could not set (role is other than detached and disabled)
+     *
+     */
+    ThreadError SetPreferredRouterId(uint8_t aRouterId);
+
     /**
      * This method returns the next hop towards an RLOC16 destination.
      *