The future of software is distributed, and successful distributed systems (in all realms) depend on contracts, which means protocols. Yet protocol design is a black art, poorly understood and rarely taught. Protocols are typically laborious to develop, over-complex, and slow to adapt. Yet decentralised, large-scale reactive systems need decentralized, reactive protocol development. The high cost and red tape of protocol design is a major problem in our industry. Pieter Hintjens presents a decade of research and practice in tackling this problem. His approach depends on reducing the cost and latency of every aspect of protocol design. He focusses on cheap experimentation, and exploring of new avenues, and smooth accumulation of gradual change, driven by real use. His strategy of "contract-driven development" teaches how to negotiate contracts within communities, and between layers, and how to drive change through real need, rather than speculation and assumption.