End-to-end congestion control for tcp-friendly flows with variable packet size

Current TCP-friendly congestion control mechanisms adjust the packet rate in order to adapt to network conditions and obtain a throughput not exceeding that of a TCP connection operating under the same conditions. In an environment where the bottleneck resource is packet processing, this is the correct behavior. However, if the bottleneck resource is bandwidth, and flows may use packets of different size, resource sharing depends on packet size and is no longer fair. For some applications, such as Internet telephony, it is more natural to adjust the packet size, while keeping the packet rate as constant as possible. In this paper we study the impact of variations in packet size on equation-based congestion control and propose methods to remove the resulting throughput bias. We investigate the design space in detail and propose a number of possible designs. We evaluate these designs through simulation and conclude with some concrete proposals. Our findings can be used to design a TCP-friendly congestion control mechanism for applications that adjust packet size rather than packet rate, or applications that are forced to use a small packet size.

End-to-end Congestion Control for Flows with Variable Packet Size

Catherine Boutremans, Jean-Yves Le Boudec, Jörg Widmer

Current TCP-friendly congestion control mechanisms such as those used in TFRC adjust the packet rate in order to adapt to network conditions and obtain a throughput not exceeding that of a TCP connection operating under the same conditions. In an environment where the bottleneck resource is packet processing, this is the correct behavior. However, if the bottleneck resource is bandwidth, and flows may use packets of different sizes, resource sharing then depends on packet size and is no longer fair. Now for some applications, such as Internet telephony, it is more natural to adjust the packet size, while keeping the packet rate as constant as possible. In this paper we study the impact of variations in packet size on equation-based congestion control and propose methods to remove the throughput bias resulting from the use of different packet sizes. We investigate in detail the design space of the approaches by means of mathematical analysis and propose a number of possible designs. We evaluate these designs through simulation and conclude with some concrete proposals. Our findings can be used to design a TCP-friendly congestion control mechanism for applications that adjust packet size rather than rate, or that are forced to use a small packet size. We base our analysis on the TFRC protocol, but similar considerations also hold for other congestion control mechanisms.

2002

Global Fairness of additive-increase and multiplicative-decrease with heterogeneous round-trip times

Catherine Boutremans, Jean-Yves Le Boudec, Milan Vojnovic

Consider a network with an arbitrary topology and arbitrary communication delays, in which congestion control is based on additive-increase and multiplicative-decrease. We show that the source rates tend to be distributed in order to maximize an objective function called

F_A^h

(``

F_A^h

fairness). We derive this result under the assumption of rate proportional negative feedback and for the regime of rare negative feedback. This applies to TCP in moderately loaded networks, and to those TCP implementations that are designed to interpret multiple packet losses within one RTT as a single congestion indication and do not rely on re-transmission timeout. This result provides some insight into the distribution of rates, and hence of packet loss ratios, which can be expected in a given network with a number of competing TCP or TCP-friendly sources. We validate our findings by analyzing the parking lot scenario, and comparing with previous results \cite{floyd-91-b,mathis-97-a}, and an extensive numerical simulation with realistic parameter settings. We apply

F_A^h

fairness to gain a more accurate understanding of the bias of TCP against long round trip times.

1999

Global fairness of additive-increase and multiplicative-decrease with heterogeneous round-trip times

Catherine Boutremans, Jean-Yves Le Boudec, Milan Vojnovic

Consider a network with an arbitrary topology and arbitrary communication delays, in which congestion control is based on additive--increase and multiplicative--decrease. We show that the source rates tend to be distributed in order to maximize an objective function called

F_A^h

(``

F_A^h

fairness`). We derive this result under the assumption of rate proportional negative feedback and for the regime of rare negative feedback. This applies to TCP in moderately loaded networks, and to those TCP implementations that are designed to interpret multiple packet losses within one RTT as a single congestion indication and do not rely on re-transmission timeout. This result provides some insight into the distribution of rates, and hence of packet loss ratios, which can be expected in a given network with a number of competing TCP or TCP-friendly sources. We validate our findings by analyzing the parking lot scenario, and comparing with previous results \cite{floyd-91-b,mathis-97-a}, and an extensive numerical simulation with realistic parameter settings. We apply