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

1. 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

2. Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion /30

3. Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion /30

4. 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 theirInternet applications. • The network itself must participate in controllingits own resource utilization. /30

5. 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 /30

6. 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. /30

7. Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion /30

8. 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 /30

9. Problem of Unfairness • When TCPflows 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 /30 1 UDP flow

10. 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 /30

11. Problem of Unfairness • No unfairness with weighted round-robin (WRR) scheduling in routers Aggregate goodput TCP goodput UDP arrival rate UDP goodput /30

12. 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 /30

13. Congestion collapse • Simulations with one TCP flow and three UDP flows, showing congestion collapse with FIFO scheduling. /30

14. Congestion collapse • Simulations with one TCP flow and three UDP flows, showing congestion collapse with WRR scheduling. /30

15. Congestion collapse • Congestion collapse as the number of UDP flows increases. /30

16. Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion /30

17. 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. /30

18. 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— /30

19. 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. /30

20. Identify Not TCP-friendly flow • Response by the router: • Restrict the bandwidth of best-effort flows determinednot 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. /30

21. 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. /30

22. 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 /30

23. 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/OFFor 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. /30

24. 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 timeor larger packet sizes /30

25. 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 . /30

26. 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. /30

27. Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion /30

28. 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. /30

29. 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. /30

30. Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion /30

31. 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. /30