HPP: A Reliable Causal Broadcast Protocol for Large-Scale Replication in Wide Area Networks

This paper describes a fast, reliable, scalable and efficient broadcast protocol called HPP (hierarchical propagation protocol) for weak-consistency replica management. It is based on organizing the nodes in a network into a logical hierarchy and maintaining a limited amount of state information at each node. It ensures that messages are not lost due to failures or partitions and minimizes redundancy. Furthermore, the protocol allows messages to be diffused while nodes are down provided the parent and child nodes of a failed node are alive. Moreover, the protocol allows nodes to be moved in the logical hierarchy and the network to be restructured dynamically in order to improve performance, while still ensuring that no messages are lost while the switch takes place and without disturbing normal operation. A performance study of the protocol in terms of availability and propagation delay indicates that the protocol reduces the delay by a factor of four compared to a protocol that does not diffuse messages past failed nodes.