  Modern networks exhibit a high degree of variability in link rates. Cellular
network bandwidth inherently varies with receiver motion and orientation, while
class-based packet scheduling in datacenter and service provider networks
induces high variability in available capacity for network tenants. Recent work
has proposed numerous congestion control protocols to cope with this
variability, offering different tradeoffs between link utilization and queuing
delay. In this paper, we develop a formal model of congestion control over
time-varying links, and we use this model to derive a bound on the performance
of any congestion control protocol running over a time-varying link with a
given distribution of rate variation. Using the insights from this analysis, we
derive an optimal control law that offers a smooth tradeoff between link
utilization and queuing delay. We compare the performance of this control law
to several existing control algorithms on cellular link traces to show that
there is significant room for optimization.
