## 3 Proposed Scheme: FRBP
### Local Real-Time Failure Avoidance and Recovery  
Subtree or whole tree reconstruction naturally incurs a high cost. Thus, intermediate nodes avoid real-time broadcast faults by local monitoring. In this section, we describe the local avoidance and recovery mechanism to deal with this issue. In firm-based real-time routing, destinations maintain their DBPs with the history of success or failure of real-time data delivery. firm real-time unicast maintains DBP at sources and destinations, and forwarding nodes on the paths between both are aware of the related information for real-time data delivery. Meanwhile, in real-time broadcasting, each destination, i.e., each sensor node in the case of network broadcast, maintains its own DBP. That is to say, it is possible to monitor the surroundings at all times. Thus, if DBP values are negative, the relevant node starts the fault recovery process. In the tree structure, real-time failures of intermediate nodes could lead to a number of failures downstream. Intermediate real-time failures are caused by node failure, link failure and increased delay at their parents. In the last case, even though the intermediate node receives data packets within the deadline, it could lead to real-time failures of downstream nodes. Intermediate nodes therefore monitor local states related to real-time data delivery including their parents. Note that intermediate nodes cannot obtain DBPs from downstream nodes in the normal case. An intermediate node regards its parent as real-time failure in three cases: (i) it does not receive a periodic beacon from the parent; (ii) it does not receive the data packet; and (iii) the transmission speed of the parent becomes lower than the required transmission speed. If a parent node is temporarily down or occupied by a full data packet queue due to congestion, it could not send a beacon or data. However, since this kind of failure might be recovered shortly, the child nodes replace the parent with the second-best node in their parent candidate set until the parent becomes stable. If the parent is permanently depleted, the second-best node continues its parent role. The transmission speed of the parent could be obtained from both the beacon message and the header of the data packet. The intermediate node then re-elects another parent. The reelection rules are exactly the same as the parent election rules in real-time broadcast tree construction. If the re-elected parent is a leaf node, the parent operates as an intermediate node after election. The new parent may not cover all leaf nodes of the previous parent, but this problem could be solved by DBP-based local failure recovery. This local monitoring and recovery process by intermediate nodes could result in real-time failure avoidance or resilient real-time failure recovery on the condition that intermediate nodes cannot obtain DBPs of downstream nodes. When this avoidance mechanism works normally, the DBP value of each node could maintain a fine condition. Though the value is shown to be negative, it would return to the normal condition by substitution of intermediate nodes. Only in the case of an abnormal state is which there is no qualified nodes for substitution parents or some area suffers from bad conditions due to the congestion repeats after the repetition of fault avoidance, DBP values would be worse. That is to say, if the avoidance mechanism works properly, such situations would rarely occur. Since leaf nodes have no child nodes, they focus on their own real-time data reception. Thus, they measure only their own DBP, and the fault avoidance mechanism works based on DBP. In the case of a DBP value falling down to a negative one, a relevant leaf node designates a parent as in the case of intermediate nodes. However, since it might recover shortly after due to the DBP characteristics, it causes an unnecessary reporting process whenever the values are negative. When a leaf node shows temporal fluctuation, DBP reflects its historical characteristics, and we expect that will be recovered after a while.