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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.