This paper discusses the use of Explicit Congestion Notification (ECN) mechanisms in the TCP/IP protocol. The first part proposes new guidelines for TCP’s response to ECN mechanisms (e.g., Source Quench packets, ECN fields in packet headers). Next, using simulations, we explore the benefits and drawbacks of ECN in TCP/IP networks. Our simulations use RED gateways modified to set an ECN bit in the IP packet header as an indication of congestion, with Reno-style TCP modified to respond to ECN as well as to packet drops as indications of congestion. The simulations show that one advantage of ECN mechanisms is in avoiding unnecessary packet drops, and therefore avoiding unnecessary delay for packets from low-bandwidth delay-sensitive TCP connections. A second advantage of ECN mechanisms is in networks (generally LANs) where the effectiveness of TCP retransmit timers is limited by the coarse granularity of the TCP clock. The paper also discusses some implementation issues concerning specific ECN mechanisms in TCP/IP networks.

Congestion control mechanisms for ATM networks as selected by the ATM Forum traffic management group are described. Reasons behind these selections are explained. In particular, selection criterion for selection between rate-based and credit-based approach and the key points of the debate between these two approaches are presented. The approach that was finally selected and several other schemes that were considered are described.

Congestion control plays an important role in the effective and stable operation of ATM networks. This paper first gives a historical overview of rate-based congestion control algorithms developed in the ATM Forum, showing how the current ATM Forum standard regarding the traffic management control methods is exploited, by these algorithms. Then, analytical approach is used to quantitatively evaluate their performance and show the effectiveness of the rate-based approach. In presenting the numerical examples, we emphasize that appropriate control parameter settings are essential for proper traffic management in an ATM network environment. 1

In ATM networks congestion control plays an essential role to support different quality of service guarantees for a large variety of traffic types. While for real-time communications, such as CBR and VBR services, a preventive open-loop congestion control mechanism is applied, a reactive closed-loop mechanism has been suggested for the ABR service class, which was introduced aiming at data communication applications. This paper presents a performance analysis of the ratebased congestion control mechanism developed by the ATM Forum. Using a differential equation approach for the computation of the evolution of the allowed cell rate and the buffer content during steady state, we derive closed-form expressions to estimate the maximum buffer lengths. These expressions can be formulated for steady-state conditions as well as for transient phases. In our approach, scenarios with and without priority for the resource management cells are considered. From numerical examples we conclude that an...

this paper we present the argument in support of an integrated approach (while remaining cognizant of the fact that rate-based control was selected by the ATM Forum in their September meeting). Why Rate in the Wide Area

In ATM network environments, congestion control plays an important role to support different quality of service guarantees for a large variety of traffic types. While for real-time communications, like CBR and VBR services, a preventive open-loop congestion control mechanism is applied, a reactive closed-loop mechanism has been suggested for the ABR service class, which was introduced mainly for data communications. This paper presents an analysis to evaluate the performance of the rate-based congestion control mechanism developed by the ATM Forum. We derive formulae to compute the evolution of the allowed cell rate and the resulting buffer content during steady state. Therefore, we focus on traffic scenarios consisting of single sources as well as of homogeneous mixes of ABR sources. From numerical examples we can conclude that an appropriate setting of the control parameters is important to achieve a reasonable performance of the rate-based control mechanism, especially to obtain low...

Recently, there has been much interest in using active queue management in routers in order to protect users from connections that are not very responsive to congestion notification. A recent Internet draft recommends schemes based on random early detection for achieving the above goals, to the extent possible, in a system without "per-flow" state. However, a "stateless" system with FIFO queueing is very much handicapped in the degree to which flow isolation and fairness are achieved. Starting with the observation that a "stateless" system is but one extreme in a spectrum of design choices and that per-flow queueing for a large number of flows is possible, we present active queue management mechanisms that are tailored to provide a high degree of isolation and fairness for TCP connections in a gigabit IP router using per-flow queueing. We show that IP flow state in a router can be bounded if the scheduling discipline used has finite memory and investigate the performance implications o...

We study the performance of TCP/IP on a multi-hop ATM network by simulation in an effort to evaluate the effectiveness of various congestion-control schemes at the ATM layer. The congestion control schemes considered are the ATM-Early Packet Discard (ATM-EPD) and link-level flow control. The performance of these schemes are compared against that of an ATM layer with no congestion control, as well as TCP/IP over a datagram network. We compare the schemes in terms of the throughputs obtained by long and short connections, number of retransmissions, and fairness in bandwidth allocation between connections with unequal round-trip delays. Our results show that significant unfairness in the amount of bandwidth allocated to connections may result in a large ATM network if no congestion control policies are used at the ATM layer. The ATM-EPD scheme is able to remove some of the unfairness, providing performance close to that of a datagram network, but can still provide unacceptable performance...

In this paper, we address the problem of optimal decentralized traffic engineering when multiple paths are available for each call. More precisely, given a set of possible paths for each call, we aim at distributing the traffic among the available paths in order to maximize a given utility function. To solve this problem, we propose a large family of decentralized sending rate control laws having the property that each of the members of this family "steers" the traffic allocation to an optimal operation point. The approach taken relies on the control theory concept of Sliding Modes. These control laws allow each ingress node to independently adjust its traffic sending rates and/or redistribute its sending rates among multiple paths. The only nonlocal information needed is binary feedback from each congested node in the path. The control laws presented are applicable to a large class of utility functions, namely, utility functions that can be expressed as the sum of concave functions of the sending rates. We show that the technique can be applied not only to usual rate adaptive traffic with multiple paths, but also to rate adaptive traffic with minimum service requirements and/or maximum allowed sending rate and to assured service traffic, always allowing for multiple paths. It is also shown that these control laws are robust with respect to failures; i.e., they automatically reroute traffic if a link failure occurs. Finally, we provide some insight on how to choose the "right" control law. In particular, we provide a way of choosing a member of the family of control laws that reduces the sending rate oscillation caused by implementation constraints like delays and quantization. An example of application of the approach delineated in this paper is also presented. This...

This paper discusses and compares the performance of three easy-to-implement congestion detection schemes for ATM switches supporting the ABR service category.