  We propose uBFT, the first State Machine Replication (SMR) system to achieve
microsecond-scale latency in data centers, while using only $2f{+}1$ replicas
to tolerate $f$ Byzantine failures. The Byzantine Fault Tolerance (BFT)
provided by uBFT is essential as pure crashes appear to be a mere illusion with
real-life systems reportedly failing in many unexpected ways. uBFT relies on a
small non-tailored trusted computing base -- disaggregated memory -- and
consumes a practically bounded amount of memory. uBFT is based on a novel
abstraction called Consistent Tail Broadcast, which we use to prevent
equivocation while bounding memory. We implement uBFT using RDMA-based
disaggregated memory and obtain an end-to-end latency of as little as 10us.
This is at least 50$\times$ faster than MinBFT , a state of the art $2f{+}1$
BFT SMR based on Intel's SGX. We use uBFT to replicate two KV-stores (Memcached
and Redis), as well as a financial order matching engine (Liquibook). These
applications have low latency (up to 20us) and become Byzantine tolerant with
as little as 10us more. The price for uBFT is a small amount of reliable
disaggregated memory (less than 1 MiB), which in our prototype consists of a
small number of memory servers connected through RDMA and replicated for fault
tolerance.
