Hierarchical state routing (HSR), proposed in Scalable Routing Strategies for Ad Hoc Wireless Networks by Iwata et al. (1999), is a typical example of a hierarchical routing protocol. HSR maintains a hierarchical topology, where elected clusterheads at the lowest level become members of the next higher level. On the higher level, superclusters are formed, and so on. Nodes which want to communicate to a node outside of their cluster ask their clusterhead to forward their packet to the next level, until a clusterhead of the other node is in the same cluster. The packet then travels down to the destination node.