Promoting the Use of End-to-End Congestion Control in the Internet Sally Floyd and Kevin Fall IEEE-ACAM Transactions on Networking, 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 1 UDP flow 8 /30

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 9 /30

Problem of Unfairness No unfairness with weighted round-robin (WRR) scheduling in routers Aggregate goodput UDP goodput UDP arrival rate TCP goodput 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 round- trip times. So, verify that a high-bandwidth flow was responsive. 20 /30

Identify Unresponsive Flows 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 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 sizes 23 /30

Identify Disproportionate- bandwidth flow 24 /30

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. 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