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122 TCP Congestion Control Ion Stoica TAs Junda Liu DK Moon David Zats http insteecsberkeleyeduee122 Materials with thanks to Vern Paxson Jennifer Rexford and colleagues at UC Berkeley ID: 532765 Download Presentation

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EE 122:TCP Congestion Control

Ion StoicaTAs: Junda Liu, DK Moon, David Zatshttp://inst.eecs.berkeley.edu/~ee122/(Materials with thanks to Vern Paxson, Jennifer Rexford,and colleagues at UC Berkeley)Slide2

2

Goals of Today’s LecturePrinciples of congestion control

Learning that congestion is occurringAdapting to alleviate the congestionTCP congestion controlAdditive-increase, multiplicative-decrease (AIMD)How to begin transmitting: Slow StartSlide3

3

What We KnowWe know:

How to process packets in a switchHow to route packets in the networkHow to send packets reliablyWe don’t know:How fast to sendSlide4

4

It’s Not Just The Sender & ReceiverFlow control

keeps one fast sender from overwhelming a slow receiverCongestion control keeps a set of senders from overloading the networkThree congestion control problems:Adjusting to bottleneck bandwidthWithout any a priori knowledgeCould be a Gbps link; could be a modemAdjusting to variations in bandwidthSharing bandwidth between flowsSlide5

5

Congestion is Unavoidable

Two packets arrive at the same timeThe node can only transmit one… and either buffers or drops the otherIf many packets arrive in a short period of timeThe node cannot keep up with the arriving traffic… and the buffer may eventually overflowSlide6

6

Congestion Collapse

Definition: Increase in network load results in a decrease of useful work doneDue to:Undelivered packetsPackets consume resources and are dropped later in networkSpurious retransmissions of packets still in flightUnnecessary retransmissions lead to more load!Pouring gasoline on a fireMid-1980s: Internet grinds to a haltUntil Jacobson/Karels (Berkeley!) devise TCP congestion controlSlide7

7

View from a Single Flow

Knee – point after which Throughput increases very slowlyDelay increases quicklyCliff – point after whichThroughput starts to decrease very fast to zero (congestion collapse)Delay approaches infinity

Load

Load

Throughput

Delay

knee

cliff

congestion

collapse

packet

lossSlide8

8

General ApproachesSend without care

Many packet drops(1) ReservationsPre-arrange bandwidth allocationsRequires negotiation before sending packetsLow utilization(2) PricingDon’t drop packets for the high-biddersRequires payment modelSlide9

9

General Approaches (cont’d)(3) Dynamic Adjustment

Probe network to test level of congestionSpeed up when no congestionSlow down when congestionSuboptimal, messy dynamics, simple to implementAll three techniques have their placeBut for generic Internet usage, dynamic adjustment is the most appropriateDue to pricing structure, traffic characteristics, and good citizenshipSlide10

10

TCP Congestion ControlTCP connection has window

Controls number of unacknowledged packetsSending rate: ~Window/RTTVary window size to control sending rateSlide11

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Sizing the Windows

cwnd (Congestion Windows) How many bytes can be sent without overflowing routersComputed by congestion control algorithmAdvertisedWindow How many bytes can be sent without overflowing the senderDetermined by the receiverSlide12

12

EffectiveWindow

Limits how much data can be in transitImplemented as # of bytesDescribed as # packets (segments) in this lecture

EffectiveWindow

= MaxWindow – (LastByteSent – LastByteAcked)

MaxWindow = min(cwnd, AdvertisedWindow)

LastByteAcked

LastByteSent

sequence number increases

MaxWindow

EffectiveWindowSlide13

13

Two Basic ComponentsDetecting congestion

Rate adjustment algorithmDepends on congestion or notThree subproblems within adjustment problemFinding fixed bandwidthAdjusting to bandwidth variationsSharing bandwidthSlide14

14

Detecting CongestionPacket dropping is best sign of congestion

Delay-based methods are hard and riskyHow do you detect packet drops? ACKsTCP uses ACKs to signal receipt of dataACK denotes last contiguous byte receivedActually, ACKs indicate next segment expectedTwo signs of packet dropsNo ACK after certain time interval: time-outSeveral duplicate ACKs (ignore for now)Slide15

15

Rate AdjustmentBasic structure:

Upon receipt of ACK (of new data): increase rateUpon detection of loss: decrease rateBut what increase/decrease functions should we use?Depends on what problem we are solvingSlide16

16

Problem #1: Single Flow, Fixed BW

Want to get a first-order estimate of the available bandwidthAssume bandwidth is fixedIgnore presence of other flowsWant to start slow, but rapidly increase rate until packet drop occurs (“slow-start”)Adjustment: cwnd initially set to 1cwnd++ upon receipt of ACK Slide17

17

Slow-Start

cwnd increases exponentially: cwnd doubles every time a full cwnd of packets has been sentEach ACK releases two packetsSlow-start is called “slow” because of starting point

segment 1

cwnd = 1

cwnd = 2

segment 2

segment 3

cwnd = 4

segment 4

segment 5

segment 6

segment 7

cwnd = 8

cwnd = 3Slide18

18

5 Minute BreakQuestions Before We Proceed?Slide19

19

Problems with Slow-StartSlow-start can result in many losses

Roughly the size of cwnd ~ BW*RTTExample:At some point, cwnd is enough to fill “pipe”After another RTT, cwnd is double its previous valueAll the excess packets are dropped!Need a more gentle adjustment algorithm once have rough estimate of bandwidthSlide20

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Problem #2: Single Flow, Varying BW

Want to be able to track available bandwidth, oscillating around its current valuePossible variations: (in terms of RTTs)Multiplicative increase or decrease: cwnd a*cwndAdditive increase or decrease: cwnd cwnd + bFour alternatives:AIAD: gentle increase, gentle decreaseAIMD: gentle increase, drastic decreaseMIAD: drastic increase, gentle decrease (too many losses)MIMD: drastic increase and decreaseSlide21

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Problem #3: Multiple FlowsWant steady state to be “fair”

Many notions of fairness, but here all we require is that two identical flows end up with the same bandwidthThis eliminates MIMD and AIADAIMD is the only remaining solution!Slide22

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Buffer and Window Dynamics

No congestion  x increases by one packet/RTT every RTTCongestion  decrease x by factor 2A

B

C = 50 pkts/RTT

xSlide23

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AIMD Sharing Dynamics

AB

x

1

D

E

No congestion

rate increases by one packet/RTT every RTT

Congestion

 decrease rate by factor 2

Rates equalize

 fair share

x

2Slide24

24

AIAD Sharing Dynamics

AB

x

1

D

E

No congestion

x increases by one packet/RTT every RTT

Congestion

 decrease x by 1

x

2Slide25

25

Efficient Allocation: Challenges of Congestion Control

Too slowFail to take advantage of available bandwidth  underloadToo fastOvershoot knee  overload, high delay, lossEveryone’s doing itMay all under/over shoot  large oscillationsOptimal:xi=

X

goal

Efficiency = 1 - distance from efficiency line

User 1: x

1

User 2: x

2

Efficiency

line

2 user example

overload

underloadSlide26

26

Example

User 1: x1User 2: x2

fairness

line

efficiency

line

1

1

Total bandwidth 1

Inefficient:

x

1

+x

2

=0.7

(0.2, 0.5)

Congested:

x

1

+x

2

=1.2

(0.7, 0.5)

Efficient

:

x

1

+x

2

=1

Not fair

(0.7, 0.3)

Efficient

:

x

1

+x

2

=1

Fair

(0.5, 0.5)Slide27

27

MIAD

User 1: x1User 2: x2

fairness

line

efficiency

line

(x

1h

,x

2h

)

(x

1h

-a

D

,x

2h

-a

D

)

(b

I

(x

1h

-a

D

), b

I

(x

2h

-a

D

))

Increase:

x*b

I

Decrease:

x - a

D

Does not converge to fairness

Does not converges to efficiencySlide28

28

AIAD

User 1: x1User 2: x2

fairness

line

efficiency

line

(x

1h

,x

2h

)

(x

1h

-a

D

,x

2h

-a

D

)

(x

1h

-a

D

+a

I

),

x

2h

-a

D

+a

I

))

Increase:

x + a

I

Decrease:

x - a

D

Does not converge to fairness

Does not converge to efficiencySlide29

29

MIMD

User 1: x1User 2: x2

fairness

line

efficiency

line

(x

1h

,x

2h

)

(b

d

x

1h

,b

d

x

2h

)

(b

I

b

D

x

1h

,

b

I

b

D

x

2h

)

Increase:

x*b

I

Decrease:

x*b

D

Does not converge to fairness

Converges to efficiency iffSlide30

30

(b

Dx1h+aI

,

b

D

x

2h

+a

I

)

AIMD

User 1: x

1

User 2: x

2

fairness

line

efficiency

line

(x

1h

,x

2h

)

(b

D

x

1h

,b

D

x

2h

)

Increase:

x+a

D

Decrease:

x*b

D

Converges to fairness

Converges to efficiency

Increments smaller as fairness increasesSlide31

31

Implementing AIMDAfter each ACK

Increment cwnd by 1/cwnd (cwnd += 1/cwnd)As a result, cwnd is increased by one only if all segments in a cwnd have been acknowledged But need to decide when to leave slow-start and enter AIMDUse ssthresh variable Slide32

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Slow Start/AIMD Pseudocode

Initially: cwnd = 1; ssthresh = infinite;New ack received: if (cwnd < ssthresh)

/* Slow Start*/

cwnd = cwnd + 1;

else

/* Congestion Avoidance */

cwnd = cwnd + 1/cwnd;

Timeout:

/* Multiplicative decrease */

ssthresh = cwnd/2;

cwnd = 1;Slide33

33

The big picture (with timeouts)

Time

cwnd

Timeout

Slow

Start

AIMD

ssthresh

Timeout

Slow

Start

Slow

Start

AIMDSlide34

34

SummaryCongestion is inevitable

Internet does not reserve resources in advanceTCP actively tries to grab capacityCongestion control critical for avoiding collapseAIMD: Additive Increase, Multiplicative DecreaseCongestion detected via packet loss (fail-safe)Slow start to find initial sending rate & to restart after timeoutNext classAdvanced congestion control

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