CS 356: Computer Network Architectures Lecture 19: Congestion - - PowerPoint PPT Presentation

CS 356: Computer Network Architectures Lecture 19: Congestion Avoidance

• Chap. 6.4 and related papers

Xiaowei Yang xwy@cs.duke.edu

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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

Administrivia

• Midterm summary

– Will discussion solutions in next lecture – Return exams

Two Modes of Congestion Control

• 1. Probing for the available bandwidth

– slow start (cwnd < ssthresh)

• 2. Avoid overloading the network

– congestion avoidance (cwnd >= ssthresh)

Slow Start

• Initial value:

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?

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.

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The Sawtooth behavior of TCP

• For every ACK received

– Cwnd += 1/cwnd *MSS

• For every packet lost

– Cwnd /= 2

RTT Cwnd

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Why does it work? [Chiu-Jain]

– A feedback control system – The network uses feedback y to adjust users’ load åx_i

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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

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Metrics to measure convergence

• Responsiveness
• Smoothness

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Model the system as a linear control system

• Four sample types of controls
• AIAD, AIMD, MIAD, MIMD

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Phase plot

x1 x2

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TCP congestion control is AIMD

• 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 Cwnd

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Macroscopic behavior of TCP

p RTT MSS

• 5

. 1

• Throughput is inversely proportional to RTT:
• In a steady state, total packets sent in one sawtooth cycle:

– S = w + (w+1) + … (w+w) = 3/2 w2

• the maximum window size is determined by the loss rate

– 1/S = p – w =

• The length of one cycle: w * RTT
• Average throughput: 3/2 w * MSS / RTT

1 1.5p

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

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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

Design Space for resource allocation

• Router-based vs. Host-based
• Reservation-based vs. Feedback-based
• Window-based vs. Rate-based

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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

Queuing mechanisms

• Router-enforced resource allocation
• Default

– First come first serve (FIFO)

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

Weighted Fair Queuing

• Different queues get different weights

– Take wi amount of bits from a queue in each round – Fi = Si + Pi / wi

w=2 w=1

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

• Unused quantum saved for the next round
• How to set quantum size?

– Too small – Too large

Congestion Avoidance

Slow down before packet loss happens

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

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

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 0.875

• AIMD

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

RED algorithm

• 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

avg_qlen p min_thresh 1 max_thresh

Even out packet drops

• 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

avg_qlen TempP min_thresh 1 max_thresh

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

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

X CE=1 ECE=1 CWR=1

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

cwnd

Summary

• The problem of network resource allocation

– Case studies

• TCP congestion control
• Fair queuing
• Congestion avoidance

– Active queue management – Source-based congestion avoidance