This paper studies partial differential equation
(PDE) models for the dynamics of peer-to-peer (P2P) filesharing
networks. Using as independent variables time and a
fluid measure of residual work, our PDE model tracks the
population profile of the P2P swarm, allowing for general filesize
distributions. Focusing on the processor sharing discipline,
which we validate as an accurate model of homogeneous P2P
networks, we provide a series of analytical results invoking
tools of feedback control theory. We establish local stability
of the equilibrium, analyze variability around this equilibrium,
and compute transient response times, all of which are shown
to match tightly with simulation results for a full packet-level
implementation of the BitTorrent protocol. We also extend our
model to heterogeneous bandwidth scenarios, and to the case of
peers contributing to the system after they finish download.