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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 Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion 1 /30 Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion 2 /30 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. 3 /30 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 4 /30 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. 5 /30 Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion 6 /30 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 7 /30 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 8 /30 1 UDP flow Problem of Unfairness Aggregate goodput UDP arrival rate UDP goodput TCP goodput • Unfairness with FCFS scheduling in routers. • Goodput: a goodput of a flow is as the bandwidth delivered to the receiver, excluding duplicate packets. 9 /30 Problem of Unfairness Aggregate goodput TCP goodput UDP arrival rate UDP goodput • No unfairness with weighted round-robin (WRR) scheduling in routers 10/30 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 11/30 Congestion collapse • Simulations with one TCP flow and three UDP flows, showing congestion collapse with FIFO scheduling. 12/30 Congestion collapse • Simulations with one TCP flow and three UDP flows, showing congestion collapse with WRR scheduling. 13/30 Congestion collapse • Congestion collapse as the number of UDP flows increases. 14/30 Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion 15/30 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. 16/30 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— 17/30 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. 18/30 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. 19/30 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 roundtrip times. • So, verify that a high-bandwidth flow was responsive. 20/30 Identify Unresponsive Flows • Responsive flow: its arrival rate decreases appropriately in response to an increased packet drop rate. • Drop rate increase by a factor 𝑥, the presented load for a high bandwidth flow should decrease by a factor 𝑥. • Require estimates of • Flow’s arrival rate • Packet drop rate 21/30 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. 22/30 Identify Disproportionatebandwidth 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 sizes 23/30 Identify Disproportionatebandwidth flow • Two components of the disproportionatebandwidth 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 log(3𝑛)/ 𝑛 ,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 𝑐/ 𝑝 Bps for some constant 𝑐. 24/30 Identify Disproportionatebandwidth 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. 25/30 Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion 26/30 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. 27/30 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. 28/30 Outline • Introduction • The Problem of Unresponsive Flows • Identifying Flows to Regulate • Alternate Approach • Conclusion 29/30 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. 30/30