  Byzantine Consensus is fundamental for building consistent and fault-tolerant
distributed systems. In traditional quorum-based consensus protocols, quorums
are defined using globally known assumptions shared among all participants.
Motivated by decentralized applications on open networks, the Stellar
blockchain relaxes these global assumptions by allowing each participant to
define its quorums using local information. A similar model called Consensus
with Unknown Participants (CUP) studies the minimal knowledge required to solve
consensus in ad-hoc networks where each participant knows only a subset of
other participants of the system. We prove that Stellar cannot solve consensus
using the initial knowledge provided to participants in the CUP model, even
though CUP can. We propose an oracle called sink detector that augments this
knowledge, enabling Stellar participants to solve consensus.
