  Cryptocurrencies such as Bitcoin are realized using distributed systems and
hence critically rely on the performance and security of the interconnecting
network. The requirements on these networks and their usage, however can differ
significantly from traditional communication networks, with implications on all
layers of the protocol stack. This paper is motivated by these differences, and
in particular by the observation that many fundamental design aspects of these
networks are not well-understood today. In order to support the networking
community to contribute to this emerging application domain, we present a
structured overview of the field, from topology and neighbor discovery to block
and transaction propagation. In particular, we provide the context,
highlighting differences and commonalities with traditional networks, review
the state-of-the-art, and identify open research challenges. Our paper can
hence also be seen as a call-to-arms to improve the foundation on top of which
cryptocurrencies are built.
