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Unformatted text preview: 600 IEEE TRANSACTIONS ON MULTIMEDIA, VOL. 8, NO. 3, JUNE 2006 TCP Smoothness and Window Adjustment Strategy Chi Zhang , Member, IEEE , and Vassilis Tsaoussidis , Senior Member, IEEE Abstract— We observe that even when the system throughput is relatively stable, end users of media-streaming applications do not necessarily experience smooth throughput, due to the unsyn- chronized window adjustments triggered by random congestion indications. We analyze and evaluate the negative impact of random window adjustments on smoothness, short-term fairness, and long-term fairness. We further propose an experimental congestion avoidance mechanism, namely TCP( , , , ), based on coordinated window adjustments. The flow-level smoothness is enhanced significantly for media-streaming applications, without a cost on fairness and responsiveness. Responsiveness is even boosted when bandwidth is underutilized. Index Terms— Congestion control, fairness, multimedia commu- nication, smoothness, transport protocols. I. INTRODUCTION T RANSMISSION control of standard TCP [1] is based on the additive increase/multiplicative decrease (AIMD) window adjustment strategy [2] that exploits available band- width, avoids persistent congestion, and achieves system fairness. Traditional AIMD is a somewhat “blind” mechanism, in the sense that the congestion window increases steadily until congestive packet loss is detected, which necessitates the subsequent error recovery. In contrast, window adjustments of TCP Vegas [3] take a congestion avoidance approach. The congestion window increases linearly if the sampled RTT is relatively low, or decreases linearly if the RTT is relatively high. Although Vegas may achieve higher throughput than standard TCP [3], it cannot guarantee fairness [4]. While TCP congestion control is basically appropriate for bulk data transfers, some real-time applications such as media-streaming find the standard multiplicative decrease by a factor of 2 upon congestion to be unnecessarily severe, as it can cause throughput oscillations and even transmission gaps [5]. Throughput smoothness is crucial to the subjective perfor- mance of media-streaming. TCP-friendly protocols [6]–[10] therefore have been proposed with two fundamental goals: i) to achieve smooth downward adjustments; this is done by increasing the window decrease ratio during congestion and ii) to compete fairly with TCP flows; this is accomplished by reducing the window increase step according to a steady-state TCP throughput equation [11]. That is, TCP friendly protocols favor smoothness for multimedia applications by using a gentle Manuscript received July 17, 2004; revised June 9, 2005. The associate editor coordinating the review of this manuscript and approving it for publication was Prof. Madjid Merabti....
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This note was uploaded on 11/30/2011 for the course CIS 6930 taught by Professor Staff during the Fall '08 term at University of Florida.

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