  A modular fluid-flow model for network congestion analysis and control is
proposed. The model is derived from an information conservation law stating
that the information is either in transit, lost or received. Mathematical
models of network elements such as queues, users, and transmission channels,
and network description variables, including sending/acknowledgement rates and
delays, are inferred from this law and obtained by applying this principle
locally. The modularity of the devised model makes it sufficiently generic to
describe any network topology, and appealing for building simulators. Previous
models in the literature are often not capable of capturing the transient
behavior of the network precisely, making the resulting analysis inaccurate in
practice. Those models can be recovered from exact reduction or approximation
of this new model. An important aspect of this particular modeling approach is
the introduction of new tight building blocks that implement mechanisms ignored
by the existing ones, notably at the queue and user levels. Comparisons with
packet-level simulations corroborate the proposed model.
