CS 356: Computer Network Architectures Lecture 19: Congestion Avoidance Chap. 6.4 and related papers. Xiaowei Yang

Transcription

1 CS 356: Computer Network Architectures Lecture 19: Congestion Avoidance Chap. 6.4 and related papers Xiaowei Yang

2 Overview More on TCP congestion control Theory Macroscopic behavior TCP Cubic Queuing mechanisms DropTail Weighted fair queuing Deficit round robin Congestion Avoidance Random Early Detection (RED) Explicit Congestion Notification 2

3 Administrivia Midterm summary Will discussion solutions in next lecture Return exams

4 Two Modes of Congestion Control 1. Probing for the available bandwidth slow start (cwnd < ssthresh) 2. Avoid overloading the network congestion avoidance (cwnd >= ssthresh)

5 Initial value: Slow Start Set cwnd = 1 MSS Modern TCP implementation may set initial cwnd to 2 When receiving an ACK, cwnd+= 1 MSS If an ACK acknowledges two segments, cwnd is still increased by only 1 segment. Even if ACK acknowledges a segment that is smaller than MSS bytes long, cwnd is increased by 1. Question: how can you accelerate your TCP download?

6 Congestion Avoidance If cwnd >= ssthresh then each time an ACK is received, increment cwnd as follows: cwnd += MSS * (MSS / cwnd) (cwnd measured in bytes) So cwnd is increased by one MSS only if all cwnd/mss segments have been acknowledged.

7 The Sawtooth behavior of TCP Cwnd For every ACK received Cwnd += 1/cwnd *MSS For every packet lost Cwnd /= 2 RTT 7

8 Why does it work? [Chiu-Jain] A feedback control system The network uses feedback y to adjust users load åx_i 8

9 Goals of Congestion Avoidance Efficiency: the closeness of the total load on the resource ot its knee Fairness: When all x_i s are equal, F(x) = 1 When all x_i s are zero but x_j = 1, F(x) = 1/n Distributedness A centralized scheme requires complete knowledge of the state of the system Convergence The system approach the goal state from any starting state 9

10 Metrics to measure convergence Responsiveness Smoothness 10

11 Model the system as a linear control system Four sample types of controls AIAD, AIMD, MIAD, MIMD 11

12 Phase plot x 2 x 1 12

13 TCP congestion control is AIMD Cwnd Problems: Each source has to probe for its bandwidth Congestion occurs first before TCP backs off Unfair: long RTT flows obtain smaller bandwidth shares RTT 13

14 Macroscopic behavior of TCP Throughput is inversely proportional to RTT: 1.5 RTT MSS p In a steady state, total packets sent in one sawtooth cycle: S = w + (w+1) + (w+w) = 3/2 w 2 the maximum window size is determined by the loss rate 1/S = p w = 1 1.5p The length of one cycle: w * RTT Average throughput: 3/2 w * MSS / RTT 14

15 TCP Cubic CUBIC: a new TCP-friendly high-speed TCP variant by S. HaNorth, I. Rhee, and L. Xu Implemented in Linux kernel and Windows 10

16 Overview More on TCP congestion control Theory Macroscopic behavior TCP Cubic Queuing mechanisms DropTail Weighted fair queuing Deficit round robin Congestion Avoidance Random Early Detection (RED) Explicit Congestion Notification 16

17 Design Space for resource allocation Router-based vs. Host-based Reservation-based vs. Feedback-based Window-based vs. Rate-based

18 Overview More on TCP congestion control Theory Macroscopic behavior TCP Cubic Queuing mechanisms DropTail Weighted fair queuing Deficit round robin Congestion Avoidance Random Early Detection (RED) Explicit Congestion Notification 18

19 Queuing mechanisms Router-enforced resource allocation Default First come first serve (FIFO)

20 Properties of Fair Queuing Work conserving Link busy if there is traffic to send Max-min fair Cannot increase without decreasing any flow with a no-greater share

21 Weighted Fair Queuing w=1 w=2 Different queues get different weights Take w i amount of bits from a queue in each round F i = S i + P i / w i

22 Deficit Round Robin (DRR) WFQ: extracting min is O(log Q) DRR: O(1) rather than O(log Q) Each queue is allowed to send Q bytes per round If Q bytes are not sent (because packet is too large) deficit counter of queue keeps track of unused portion If queue is empty, deficit counter is reset to 0 Similar behavior as FQ but computationally simpler

23 Unused quantum saved for the next round How to set quantum size? Too small Too large

24 Congestion Avoidance Slow down before packet loss happens

25 Design goals Predict when congestion is going to happen Reduce sending rate before buffer overflows Not widely deployed Reducing queuing delay and packet loss are not essential

26 Mechanisms Router+host joint control Router: Early signaling of congestion Host: react to congestion signals Case studies: DECbit, Random Early Detection Host: Source-based congestion avoidance Host detects early congestion Case study: TCP Vegas

27 DECbit Add a congestion bit to a packet header A router sets the bit if its average queue length is non-zero Queue length is measured over a busy+idle interval If less than 50% of packets in one window do not have the bit set A host increases its congest window by 1 packet Otherwise Decreases by AIMD

28 Random Early Detection Random early detection (Floyd93) Goal: operate at the knee Problem: very hard to tune (why) RED is generalized by Active Queue Managment (AQM) A router measures average queue length using exponential weighted averaging algorithm: AvgLen = (1-Weight) * AvgLen + Weight * SampleQueueLen

29 RED algorithm p 1 min_thresh max_thresh avg_qlen If AvgLen MinThreshold Enqueue packet If MinThreshold < AvgLen < MaxThreshold Calculate dropping probability P Drop the arriving packet with probability P If MaxThreshold AvgLen Drop the arriving packet

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31 Even out packet drops TempP 1 min_thresh max_thresh avg_qlen TempP = MaxP x (AvgLen Min)/(Max-Min) P = TempP / (1 count * TempP) Count keeps track of how many newly arriving packets have been queued when min < Avglen < max It keeps drop evenly distributed over time, even if packets arrive in burst

32 An example MaxP = 0.02 AvgLen is half way between min and max thresholds TempP = 0.01 A burst of 1000 packets arrive With TempP, 10 packets may be discarded uniformly randomly among the 1000 packets With P, they are likely to be more evently spaced out, as P gradually increases if previous packets are not discarded

33 Explicit Congestion Notification A new IETF standard Two bits in IP header 00: No ECN support 01/10: ECN enabled transport 11: Congestion experienced Two TCP flags ECE: congestion experienced CWR: cwnd reduced ECE=1 CWR=1 X CE=1

34 Source-based congestion avoidance TCP Vegas Detect increases in queuing delay Reduces sending rate Details Record basertt (minimum seen) Compute ExpectedRate = cwnd/basertt Diff = ExpectedRate - ActualRate When Diff < α, incr cwnd linearly, when Diff > β, decr cwnd linearly α < β

35 cwnd

36 Summary The problem of network resource allocation Case studies TCP congestion control Fair queuing Congestion avoidance Active queue management Source-based congestion avoidance