PiLi : A Simple , Fast , and Robust Family of Blockchain Protocols ∗

In every epoch, an eligible proposer proposes a next block extending from the freshest notarized chain it has seen. Everyone votes on the first proposal heard if 1) the proposed block extends from a parent block that is not too “stale”, and 2) no conflicting notarizations have been observed in the recent past. When a block collects majority votes, it becomes notarized but not yet final. If a notarized chain ends at 13 consecutive epochs the trailing 8 blocks and the prefix is final. If a block of the present epoch collects 3/4 fraction (and not just majority) votes, a node may advance to the next epoch immediately. Otherwise a node sends a timeout message when an epoch exipres and advances to the next epoch when majority timeout messages have been heard. This very simple protocol can achieve the following properties where n denotes the total number of nodes: 1) during a period in which 3n/4 honest nodes remain online, we can confirm transactions at raw network speed (i.e., asynchronously) as soon as we rotate to an honest and online proposer; 2) if more than n/2 of honest nodes have online presence for sufficiently long (w.r.t. the protocol’s confirmation delay), transactions get confirmed in expected constant number of synchronous rounds; and 3) we guarantee consistency for all honest nodes — including those who might have unstable network connections and drop offline every now and then — as long as at any point of time the total number of offline and corrupt nodes is less than n/2. In this sense, our protocol is strictly more robust than classical, synchronous honest-majority consensus.