The Gannet project proposes a functional programming approach for high-abstraction design of very large Systems-on-Chip (SoCs). Gannet is a distributed service-based SoC architecture, i.e. a network of services offered by hardware or software cores. The Gannet SoC performs tasks by executing functional task description programs on a distributed computing system, the Gannet machine. Communication in this distributed system is entirely packet-based. The talk introduces a packet-level operational semantics to formally describe the operation of the Gannet machine.