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

Sally Floyd and Kevin Fall IEEE-ACAM Transactions on Networking,1999

101062804 馬儀蔓

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Outline

• Introduction• The Problem of Unresponsive Flows• Identifying Flows to Regulate• Alternate Approach• Conclusion

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Outline

• Introduction• The Problem of Unresponsive Flows• Identifying Flows to Regulate• Alternate Approach• Conclusion

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Introduction

• The Internet is more and more bigger.• No longer rely on • All end-nodes to use end-to-end congestion control

for best-effort traffic.• All developers to incorporate end-to-end congestion

control in their Internet applications.

• The network itself must participate in controlling its own resource utilization.

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Controlling Best-Effort Traffic

• Assume the Internet will continue to become congested due to a scarcity of bandwidth, three approaches• Per-flow scheduling• Separately regulate the bandwidth used by each

best-effort flow.• Incentives for end-to-end congestions control• Restrict the bandwidth of unresponsive best-effort

flows.• Pricing mechanisms 5

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Purpose

• As the Internet expands to an even larger community• Recognize the essential role of end-to-end

congestion control.• Strengthening incentives for using end-to-end

congestion control is critical issues.

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Outline

• Introduction• The Problem of Unresponsive Flows• Identifying Flows to Regulate• Alternate Approach• Conclusion

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Unresponsive Flows

• Unresponsive flows are high bandwidth flows that • Do not use end-to-end congestion control • Do not reduce their load on the network when

subjected to packet drops.

• Two problems• Unfairness• Congestion collapse 8

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Problem of Unfairness

• When TCP flows competing with unresponsive UDP flows for scare bandwidth• TCP flows reduce their sending rates.• The uncoorperate UDP flows use the available

bandwidth.

3 TCP flows

1 UDP flow9

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Problem of Unfairness

• Unfairness with FCFS scheduling in routers.• Goodput: a goodput of a flow is as the bandwidth delivered to the receiver,

excluding duplicate packets.

Aggregate goodput UDP goodput UDP arrival rate

TCP goodput

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Problem of Unfairness

• No unfairness with weighted round-robin (WRR) scheduling in routers

Aggregate goodput

UDP goodput UDP arrival rate

TCP goodput

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Congestion collapse

• Congestion collapse occurs when an increase in the network load results in a decrease in the useful work done by the network.• Classical congestion collapse• Fragment-based congestion collapse• Congestion collapse from • Undelivered packets• Bandwidth is wasted by delivering packets through the network

that are dropped before reaching their ultimate destination• Increased control traffic• Stale packets 12

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Congestion collapse

• Simulations with one TCP flow and three UDP flows, showing congestion collapse with FIFO scheduling.

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Congestion collapse

• Simulations with one TCP flow and three UDP flows, showing congestion collapse with WRR scheduling. 14

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Congestion collapse

• Congestion collapse as the number of UDP flows increases. 15

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Outline

• Introduction• The Problem of Unresponsive Flows• Identifying Flows to Regulate• Alternate Approach• Conclusion

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Flows to be Detected

• Identify a high bandwidth flow in times of congestion as• Not TCP-friendly flow• Applied to a single flow.

• Unresponsive flow• Applied to a single flow.

• Disproportionate-bandwidth flow• Applied to both a single flow and aggregates of flows.

• Based on IP and port number to distinguish different flows. 17

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Identify Not TCP-friendly flow

• Definition: TCP-friendly flows• Reducing its congestion window at least by

half upon indications of congestion.• Increasing its congestion window by a constant

rate of at most one packet per roundtrip time.• Sending rate for a TCP connection—

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Identify Not TCP-friendly flow

• Limitation• Only can test a flow of a single TCP

connection.• Difficult to determine the maximum packet

size B in bytes or the minimum RTT. • Measurements should be taken over a

sufficiently large time interval.• Only applies for non-bursty packet drop

behavior.19

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Identify Not TCP-friendly flow

• Response by the router:• Restrict the bandwidth of best-effort flows

determined not to be TCP-friendly in times of congestion.• Remove restriction when • There is no longer any significant link congestion.• It has been shown to reduce its arrival rate

appropriately in response to congestion.

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Identify Unresponsive Flows

• The TCP-friendly test is • Based on the specific congestion control

responses of TCP. • Not very useful for routers unable to assume

strong bounds on TCP packet sizes and round-trip times.

• So, verify that a high-bandwidth flow was responsive.

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Identify Unresponsive Flows

• Responsive flow: its arrival rate decreases appropriately in response to an increased packet drop rate. • Drop rate increase by a factor , the presented

load for a high bandwidth flow should decrease by a factor .• Require estimates of • Flow’s arrival rate • Packet drop rate

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Identify Unresponsive Flows

• Limitation: less straightforward for a flow with a variable demand.• Possible end-to-end congestion mechanisms.• The original data source itself could be ON/OFF or

have strong rate variations over time.

• Response by the router:• Restrict the bandwidth of best-effort flows

determined to be unresponsive in times of congestion.• Can apply test actively.

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Identify Disproportionate-bandwidth flow• Identify flows that use a disproportionate share of

the bandwidth in times of high congestion.• A disproportionate share is defined as a

significantly larger share than other flows.• TCP flow could use a “disproportionate share” of

bandwidth, if TCP• With persistent demand• Using large windows• With a significantly smaller roundtrip time or

larger packet sizes24

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Identify Disproportionate-bandwidth flow• Two components of the disproportionate-

bandwidth test• Check if a flow is using a disproportionate

share of the bandwidth.• The fraction of the flow’s aggregate arrival rate is

more than / ,for n is the number of flows with packet drops.

• Takes into account the level of congestion itself• Define a flow as having a high arrival rate relative

to the level of congestion if its arrival rate is greater than Bps for some constant .

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Identify Disproportionate-bandwidth flow• Limitation: gauging the level of unsatisfied demand

is problematic.• A large RRT TCP flow• A short busty web transfer

• Response by the router:• Limit the restriction of a high-bandwidth

responsive flow.• So, over the long run, each such flow receives as much

bandwidth as the highest-bandwidth unrestricted flow. 26

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Outline

• Introduction• The Problem of Unresponsive Flows• Identifying Flows to Regulate• Alternate Approach• Conclusion

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Per-flow scheduling

• Per-flow scheduling separately regulate the bandwidth used by each best-effort flow.• Indeed care of many of the fairness issues

concerning competing best-effort flows.• However, • It can not prevent congestion collapse from

undelivered packets.• May encourage flows make sure that “their”

queue in congested router never goes empty. 28

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Pricing mechanisms

• Pricing mechanisms use pricing as a way to • Share transmission resources.• Control and prevent the network congestion.

• However, the deployment of pricing structures is sensitive to the behavior of each flow in the global Internet.

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Outline

• Introduction• The Problem of Unresponsive Flows• Identifying Flows to Regulate• Alternate Approach• Conclusion

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Conclusion

• In the Internet,• Need for end-to-end congestion control.• Need mechanisms to detect and restrict

unresponsive or high-bandwidth best-effort flows in times of congestion control.

• Not yet outlined a specific proposal for mechanisms for identifying and controlling unresponsive flows.

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