  Byzantine Reliable Broadcast (BRB) is a fundamental distributed computing
primitive, with applications ranging from notifications to asynchronous payment
systems. Motivated by practical consideration, we study Client-Server Byzantine
Reliable Broadcast (CSB), a multi-shot variant of BRB whose interface is split
between broadcasting clients and delivering servers. We present Draft, an
optimally resilient implementation of CSB. Like most implementations of BRB,
Draft guarantees both liveness and safety in an asynchronous environment. Under
good conditions, however, Draft achieves unparalleled efficiency. In a moment
of synchrony, free from Byzantine misbehaviour, and at the limit of infinitely
many broadcasting clients, a Draft server delivers a $b$-bits payload at an
asymptotic amortized cost of $0$ signature verifications, and $\log_2(c) + b$
bits exchanged, where $c$ is the number of clients in the system. This is the
information-theoretical minimum number of bits required to convey the payload
($b$ bits, assuming it is compressed), along with an identifier for its sender
($\log_2(c)$ bits, necessary to enumerate any set of $c$ elements, and optimal
if broadcasting frequencies are uniform or unknown). These two achievements
have profound practical implications. Real-world BRB implementations are often
bottlenecked either by expensive signature verifications, or by communication
overhead. For Draft, instead, the network is the limit: a server can deliver
payloads as quickly as it would receive them from an infallible oracle.
