Paper 2021/632

Internet Computer Consensus

Jan Camenisch, Manu Drijvers, Timo Hanke, Yvonne-Anne Pignolet, Victor Shoup, and Dominic Williams

Abstract

We present the Internet Computer Consensus (ICC) family of protocols for atomic broadcast (a.k.a., consensus), which underpin the Byzantine fault-tolerant replicated state machines of the Internet Computer. The ICC protocols are leader-based protocols that assume partial synchrony, and that are fully integrated with a blockchain. The leader changes probabilistically in every round. These protocols are extremely simple and robust: in any round where the leader is corrupt (which itself happens with probability less than $1/3$), each ICC protocol will effectively allow another party to take over as leader for that round, with very little fuss, to move the protocol forward to the next round in a timely fashion. Unlike in many other protocols, there are no complicated subprotocols (such as "view change'' in PBFT) or unspecified subprotocols (such as "pacemaker'' in HotStuff). Moreover, unlike in many other protocols (such as PBFT and HotStuff), the task of reliably disseminating the blocks to all parties is an integral part the protocol, and not left to some other unspecified subprotocol. An additional property enjoyed by the ICC protocols (just like PBFT and HotStuff, and unlike others, such as Tendermint) is optimistic responsiveness, which means that when the leader is honest, the protocol will proceed at the pace of the actual network delay, rather than some upper bound on the network delay. We present three different protocols (along with various minor variations on each). One of these protocols (ICC1) is designed to be integrated with a peer-to-peer gossip sub-layer, which reduces the bottleneck created at the leader for disseminating large blocks, a problem that all leader-based protocols, like PBFT and HotStuff, must address, but typically do not. Our Protocol ICC2 addresses the same problem by substituting a low-communication reliable broadcast subprotocol (which may be of independent interest) for the gossip sub-layer.