# Do we need two forms of feedback in the Rate Control Protocol (RCP)?

## TL;DR

This paper investigates whether the Rate Control Protocol (RCP) benefits from including queue size feedback, using control theory and bifurcation analysis, and finds that excluding queue feedback leads to more stable system behavior.

## Contribution

The study provides a bifurcation-theoretic analysis showing that omitting queue size feedback in RCP results in more desirable stability properties compared to including it.

## Key findings

- Queue feedback can cause sub-critical Hopf bifurcation leading to instability.
- Without queue feedback, RCP exhibits stable, small-amplitude limit cycles.
- Numerical and simulation results support the analytical conclusions.

## Abstract

There is considerable interest in the networking community in explicit congestion control as it may allow the design of a fair, stable, low loss, low delay, and high utilization network. The Rate Control Protocol (RCP) is an example of such a congestion control protocol. The current design of RCP suggests that it should employ two forms of feedback; i.e. rate mismatch and queue size, in order to manage its flow control algorithms. An outstanding design question in RCP is whether the presence of queue size feedback is useful or not, given feedback based on rate mismatch. In this paper, we address this question using tools from control and bifurcation theory. We linearize the actual non-linear system and analyze the local asymptotic stability, robust stability and rate of convergence of both the design choices, i.e., with and without queue size feedback. But such analyses do not offer clear design recommendations on whether the queue feedback is useful or not. This motivates a bifurcation-theoretic analysis where we have to take non-linear terms into consideration, which helps to learn additional dynamical properties of the RCP system. In particular, we proceed to analyze two non-linear properties, namely, the type of Hopf bifurcation and the asymptotic stability of the bifurcating limit cycles. Analytical results reveal that the presence of queue feedback in RCP can induce a sub-critical Hopf bifurcation, which can lead to undesirable system behavior. Whereas, in the absence of queue feedback, the Hopf bifurcation is always super-critical where the bifurcating limit cycles are stable and of small amplitude. The analysis is corroborated by numerical computations and some packet-level simulations as well. Based on our work, the suggestion for RCP is to only include feedback based on rate mismatch in the design of the protocol.