  Minimizing end-to-end latency in geo-replicated systems usually makes it
necessary to compromise on resilience, resource efficiency, or throughput
performance, because existing approaches either tolerate only crashes, require
additional replicas, or rely on a global leader for consensus. In this paper,
we eliminate the need for such tradeoffs by presenting Isos, a leaderless
replication protocol that tolerates up to $f$ Byzantine faults with a minimum
of $3f+1$ replicas. To reduce latency in wide-area environments, Isos relies on
an efficient consensus algorithm that allows all participating replicas to
propose new requests and thereby enables clients to avoid delays by submitting
requests to their nearest replica. In addition, Isos minimizes overhead by
limiting message ordering to requests that conflict with each other (e.g., due
to accessing the same state parts) and by already committing them after three
communication steps if at least $f+1$ replicas report each conflict. Our
experimental evaluation with a geo-replicated key-value store shows that these
properties allow Isos to provide lower end-to-end latency than existing
protocols, especially for use-case scenarios in which the clients of a system
are distributed across multiple locations.
