Towards Stability Analysis of Data Transport Mechanisms: a Fluid Model and an Application

TL;DR

This paper introduces a unified fluid modeling framework for TCP congestion control algorithms, applies it to TCP Reno and CUBIC, and reveals the local stability of TCP CUBIC through analytical methods.

Contribution

It develops a general RFDE-based modeling scheme for TCP algorithms and demonstrates its application to TCP CUBIC, providing new stability insights.

Findings

TCP CUBIC is locally uniformly asymptotically stable.

The model accurately matches simulation results.

The framework unifies analysis of different TCP variants.

Abstract

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.