  The Transmission Control Protocol (TCP) utilizes congestion avoidance and
control mechanisms as a preventive measure against congestive collapse and as
an adaptive measure in the presence of changing network conditions. The set of
available congestion control algorithms is diverse, and while many have been
studied from empirical and simulation perspectives, there is a notable lack of
analytical work for some variants. To gain more insight into the dynamics of
these algorithms, we: (1) propose a general modeling scheme consisting of a set
of functional differential equations of retarded type (RFDEs) and of the
congestion window as a function of time; (2) apply this scheme to TCP Reno and
demonstrate its equivalence to a previous, well known model for TCP Reno; (3)
show an application of the new framework to the widely-deployed congestion
control algorithm TCP CUBIC, for which analytical models are few and limited;
and (4) validate the model using simulations. Our modeling framework yields a
fluid model for TCP CUBIC. From a theoretical analysis of this model, we
discover that TCP CUBIC is locally uniformly asymptotically stable -- a
property of the algorithm previously unknown.
