Congestion Avoidance Created by M Bateman, A Ruddle & C Allison As part of the TCP View project

Congestion Avoidance Congestion control: reduce sending rate after bottleneck link is congested and its packet buffers overflow Congestion avoidance: reduce sending rate when bottleneck link is saturated, but before a queue builds up at the packet buffers TCP Tahoe/Reno do congestion control, not avoidance Comparison of congestion control and avoidance Suppose a “congestion control” connection competes for network bandwidth with a “congestion avoidance” connection. Who would get more bandwidth? What is better for the network: avoidance or control?

The DECbit Approach Developed for Digital Network Architecture (DNA) in late 80’s Packets carry “Congestion Experienced” (CE) bit in their header Routers set the CE flag when they have an average queue size of at least one packet –When would a router have a positive average queue size? –Why monitor average instead of instantaneous queue? –How can we estimate the average queue size? –Why wait for an average queue of at least one packet? Receivers return the CE bit to the sources using their ACKs Sources monitor fraction of packets that have CE bit set If more than 50% of packets have seen CE=1, sources reduces sending window by certain fraction; otherwise, the window is increased by a packet (AIMD) Note: this approach requires some support by the routers

Average queue estimation What do we mean by “busy period” for a link? Average instantaneous queue size over last busy period, plus current busy period Why to include the last busy period? Queue length Current time Time Current cycle Previous cycle Averaging interval

Random Early Detection Designed for use with TCP’s congestion control algorithms Similar objectives with DECbit, but routers send implicit feedback to sources, by dropping packets early, instead of setting a special bit “Early drop” means that router starts dropping packets before its buffers overflow, at the early onset of congestion –Specifically, RED drops packets probabilistically, based on relation of average queue size AvgLen to queuing MinThreshold and MaxThreshold –If AvgLen < MinThreshold, enqueue arriving packet –If MinThreshold < AvgLen <MaxThreshold, Calculate probability P Drop arriving packet with probability P –If MaxThreshold < AvgLen, drop arriving packet

Queue size estimation First-order weighted running average: –AvgLen = (1-weight) * AvgLen + weight * SampleLen –Weight is normally around 0.1 (and often a negative power of two) –How often should we sample the queue size? –What if packets have not arrived for a while?

Queue thresholds in RED MaxThresholdMinThreshold AvgLen Objective: maintain an average queue size between Min-to-MaxThreshold –How does this compare to DEC’s “at-least-one-packet on the average” scheme –Trade-offs in selecting these two thresholds –How large should the total buffer space be?

Drop Probability in RED Why to drop packets probabilistically? What is the relation between the number of dropped packets and the throughput of a connection?

Source based congestion control What if we don’t have any support from routers? –Source-based solutions Basic idea: sources watch for some sign from the network that congestion will soon happen Different ways to do this: –Reduce sending rate upon gradual increase of the Round-Trip Times –Increase or decrease window based on sign of : (LastWindow-PrevWindow) * (LastRTT-PrevRTT) –Decrease window when sending rate starts flattening out, even though window has been increasing –Compare expected throughput and measured throughput (used in TCP Vegas)

Basic idea of TCP Vegas

TCP Vegas BaseRTT: minimum of all measured RTTs ExpectedRate = CongestionWindow/ BaseRTT ActualRate = UnAckedBytes / LastRTT –Measured once per RTT at sender Diff = ExpectedRate – ActualRate –If Diff > X, decrease congestion window –If Diff < Y, increase congestion window –How would you choose the thresholds X and Y? How does TCP Vegas compare with TCP Reno? –Would you prefer to use TCP Vegas if everyone else uses Reno? –Would it be fair to use TCP Reno if everyone else uses Vegas?