The Transmission Control Protocol (TCP) utilizes a congestion avoidance and control mechanism 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 applications of the new framework to the widely-deployed congestion control algorithm TCP CUBIC, for which analytical models are few and limited; as well as to H-TCP, another high-speed congestion control algorithm; and (4) validate the model using simulations. Our modeling framework yields a fluid model for window- or rate-based congestion control variants. From a theoretical analysis of this model with TCP CUBIC, we discover that CUBIC is locally uniformly asymptotically stable - a property of the algorithm previously unknown. Through further analysis, we derive a sufficient condition for H-TCP's stability, but observe via a numerical analysis and simulations that H-TCP rarely converges to an equilibrium and is usually not asymptotically stable in practical high-speed settings.

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.

There is growing interest in designing high speed routers with small buffers that store only tens of packets. Recent studies suggest that TCP NewReno, with the addition of a pacing mechanism, can interact with such routers without sacrificing link utilization. Unfortunately, as we show in this paper, as workload requirements grow and connection bandwidths increase, the interaction between the congestion control protocol and small buffer routers produce link utilizations that tend to zero. This is a simple consequence of the inverse square root dependence of TCP throughput on loss probability. In this paper we present a new congestion controller that avoids this problem by allowing a TCP connection to achieve arbitrarily large bandwidths without demanding the loss probability go to zero. We show that this controller produces stable behavior and, through simulation, we show its performance to be superior to TCP NewReno in a variety of environments. Lastly, because of its advantages in high bandwidth environments, we compare our controller's performance to some of the recently proposed high performance versions of TCP including HSTCP, STCP, and FAST. Simulations illustrate the superior performance of the proposed controller in a small buffer environment.

In pricing-based congestion control protocols it is common to assume that the rate of congestion feedback from the network is limited to a single bit per packet. To obtain a precise estimate of available bandwidth (as summarized by the congestion price) under the single-bit constraint, a session must consider feedback contained in a number of recently received packets. As more packets are considered, however, the estimate includes increasingly older information about the network state. We study this tradeoff between the quality and timeliness of feedback using control-theoretic approach, modeling the 'memory' incorporated into the price estimate as additional feedback delay. We show through analysis that obtaining arbitrary precision in the estimated price causes control instability, making it more difficult for a session to track its targeted optimal rate. Through continuous-time simulation of our model and packet-level simulations, we find that crude estimates of congestion price based on very few packets can yield good performance while allowing the session to operate far away from the boundary of instability. We also investigate the impact of estimation bias on protocol performance, showing that protocols use a form of integral control can compensate for biased price estimates.

In this paper we introduce an innovation in differentiated services architecture consisting of adaptive two-level coloring at the edge and preferential marking at the core. We identify general properties of these two processes which, when met, guarantee a desirable fixed point for the network; i.e., one where aggregated flow rates meet or exceed given targets in an over-provisioned network. Specific mechanisms realizing the aforementioned properties lead to so-called active rate management controllers for edge coloring, and a preferentially-marking, active queue management controller at the core. We discuss stability of the fixed point for this network, and validate results using ns simulations.

In this work we develop stability conditions for congestion control of the present Internet characterized by TCP-controlled sources and buffer-based active queue management (AQM) schemes. Prevailing stability results are geared towards rate-based AQMs and are not applicable to the class of networks considered here. Our new conditions can be expressed entirely in terms of network parameters, i.e., routing, which make them useful in the design of buffer-based AQMs such as RED, REM and PI.

Congestion control in TCP/AQM networks is expected to perform well for a wide-range of conditions, but recent advances in modeling and analysis indicate that present AQM (active queue management) schemes need an extra dose of adaptability to cope. The paper answers the call and proposes a self-tuning structure wherein AQM parameters are automatically tuned in response to on-line estimation of link capacity and traffic load. This approach is applicable to any AQM scheme that is parameterizable in terms of link capacity and TCP load. We describe this self-tuning structure, illustrate its application to PI (proportional-integral) and RED (random early detection) AQMs, provide stability analysis, and conduct ns simulations to compare with both fixed AQM schemes and the recently proposed adaptive RED.

Routers handle data packets from sources unresponsive to TCP's congestion avoidance feedback. We are interested in the impact these sources have on active queue management (AQM) control of long-lived TCP traffic. In this paper, we combine models of TCP/AQM dynamics with models of unresponsive traffic to analyze the effects on AQM performance.

Congestion control in TCP/AQM networks is expected to perform well for a wide-range of conditions, but recent advances in modeling and analysis indicate that present AQM schemes need an extra dose of adaptability to cope. This paper answers the call and proposes a self-tuning structure wherein AQM parameters are automatically tuned in response to on-line estimation of link capacity and traffic load. This approach is applicable to any AQM scheme that is parameterizable in terms of link capacity and TCP load. In this paper, we will describe this self-tuning structure, illustrate its application to PI and RED AQMs, provide stability analysis, and conduct ns simulations to compare with both fixed AQM schemes and the recently proposed adaptive RED.

In this paper we propose a new paradigm for a Differentiated Service (DiffServ) network consisting of two-color marking at the edges of the network using token buckets coupled with differential treatment in the core. Using fluid-flow modelling, we present existence conditions for token-bucket rates and differential marking probabilities at the core that result in all edges receiving at least their minimum guaranteed rates. We then present an integrated DiffServ architecture comprising of an active rate management controller at the marking edge and a two-level active queue management controller at the core. The validity of the fluid flow model and performance of this new scheme are verified using ns simulations.

We use a previously developed nonlinear dynamic model of TCP to analyze and design active queue management (AQM) control systems using random early detection (RED). First, we linearize the interconnection of TCP and a bottlenecked queue and discuss its feedback properties in terms of network parameters such as link capacity, load and round-trip time. Using this model, we next design an AQM control system using the RED scheme by relating its free parameters such as the low-pass filter break point and loss probability profile to the network parameters. We present guidelines for designing linearly stable systems subject to network parameters like propagation delay and load level. Robustness to variations in system loads is a prime objective. We present no simulations to support our analysis.

In this paper we study a previously developed linearized model of TCP and active queue management (AQM). We use classical control system techniques to develop controllers well suited for the application. The controllers are shown to have better theoretical properties than the well known RED controller. We present guidelines for designing stable controllers subject to network parameters like load level propagation delay etc. We also present simple implementation techniques which require a minimal change to RED implementations. The performance of the controllers are verified and compared with RED using ns simulations. The second of our designs, the proportional integral (PI) controller is shown to outperform RED significantly.