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Promoting the Use of End-To-End Congestion Control in the Internet

IEEE/ACM Transaction on Networking

Vol.7 No.4 August 1999

Sally Floyd and Kevin Fall

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Abstract

• Potentially negative impacts of an increasing deployment of non-congestion-controlled best-effort traffic on the Internet

• These impacts range from extreme unfairness against competing TCP traffic to the potential for congestion collapse.

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Introduction

• Promote the end-to-end congestion control:– Router mechanisms– Restrict the bandwidth of selected high-bandwi

dth best-effort flows

• Approaches:– identify a high bnadwidth flow as Unresponsive– Not TCP-friendly– Using disproportionate bandwidth

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Possible Approaches

1. Involves the deployment of packet scheduling disciplines in routers that isolate each flow(per-flow scheduling mechanisms)

2. (in this paper)for routers to support the continued use of end-to-end congestion control to share scarce bandwidth

3. Rely on financial incentives or pricing mechanisms to control sharing

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TCP

UDP

Definition:

GoodPut: A Flow as the bandwidth delivered to the recerver,excluding duplicate packets.

Unresponsive flows: flows that do not use end-to-end congestion control,and do not reduce their load on the network when subjected to packet drops.

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25%

WRR:each flow assigned an equal weight in units of bytes/s

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Danger of Congestion Collapse

• Classical congestion collapse– Largely due to TCP connection unnecessarily re

transmitting packets– Corrected by the timer improvements and cong

estion-control mechanism in modern implementations of TCP

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(cont’d)

• Congestion collapse from undelivered packets– Bandwidth is wasted by delivering packet throu

gh the network that are dropped before reaching their ultimate destination

– (different from classical)the degraded condition is not stable,but returns to normal once the load is reduced

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Set bandwidth 128 kb/s, 9%

Bandwidth wasted by the packets that never reach dest.

UDP flow restricted to 25% of the link bandwidth

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35%

10%

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Prevent Congestion Collapse from undelivered packets

1. To Succeed,through incentives at routers,in maintaining an environment characterized by end-to-end congestion control

2. Maintain a virtual-circuit-style environment where packets are prevented from entering the network unless the network has sufficient reources to deliver

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Identify flow to regulate(A)

• TCP-friendly flows:if its arrival rate does not exceed the arrival of conformant TCP connection in the same circumstances.

• Assume a flow should not use more bandwidth than would the most aggressive conformant TCP implementation in the same circumstances.

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• Maximum sending rate:T bytes/s

• Maximum packet size:B bytes

• Minimum round-trip time R

• Packet drop rate p

• Can Calculate the max arrival rate

• Limitations:Can only be applied to a flow at the level of granularity of a single TCP connection (hard to determine B,R)

pR

BT

*

*3/25.1

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Identify flow to regulate(B)

• Unresponsive:its arrival rate decreases appropriately in response to an increased packet drop rate.

• If the packet drop rate of the connection increases by a factor of x, then the arrival rate from the source should decrease by a factor of roughly x

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(Cont’d)

• If steady-state drop rate increases by a factor x and the presented load for a high-bandwidth flow does not decrease by a factor reasonably close to or more,then the flow can be deemed not to be using congestion control(unresponsive)

x

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Conclusion

• In this paper has argued on the need for end-to-end congestion control, and further,on the need for mechanisms in the network to detect and restrict unreoponsive or high bandwidth best-effort flows in times of congestion

• In this paper have not yet oulined a specific proposal for mechanisms for identifying and controlling unresponsive flows.

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Future Work

• The most important issue is not the precise functioning of the mechanisms to restrict the bandwidth of unresponsive best-effort flows,but simply that such mechanisms be deployed.

• Mechanisms such as these would go a long way to making concrete the essential role played by congestion control for best-effort traffic in the Internet.