Congestion Avoidance and Control

V. Jacobson, SIGCOMM, 1988Multimedia Communications LAB at HUFS Pyoung-jae, Lee (whitewyrm@hufs.ac.kr)

Topics on Computer Network

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Congestion Problem

Congestion Load exceeds the capacity of the network Overflow at router queues

Problem Wasted bandwidth(retransmission required) Unpredictable delay

October 1986: first “congestion collapse” Caused by TCP retransmissions

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Congestion Problem

Congestion problem has become more severe as the computer network grows

New algorithm forcing the “packet conservation” can be used to achieve network stability i) round-trip-time variance estimation ii) exponential retransmit timer backoff iii) slow-start iv) more aggressive receiver ack policy v) dynamic window sizing on congestion

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“Conservation of packets” principle

Equilibrium = “stable, full window of data in transit” “Conservation” packet flow

New is not put into network until an old leaves Possible problems with packet conservation

The connection doesn’t get to equilibrium A sender injects a new packet before and old packet has ex-

ited The equilibrium can’t be reached because of resource limits

along the path

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Getting to equilibrium: Slow-Start

Self-clocking Automatically adjusts to BW & delay variations Sender uses ACK as a ‘clock’ Hard to start

Packet size(area) = BW x time

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Getting to equilibrium: Slow-Start

Add a congestion window to the per-connection state When starting or restarting after a loss, set congestion window

to on packet Cwnd = 1 packet

On each ack for new data, increase congestion window by one packet Cwnd += 1 packet

When sending, send the minimum of the receiver’s advertised window and congestion window Send window = min(rcv window, cwnd)

“Slow” is misnomer Rate grows exponentially R log2 W

R: round trip time W: window size in packets

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Getting to equilibrium: Slow-Start

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Conservation at equilibrium (round-trip timing)

Need good estimate of RTT mean estimate( ) RTT variation estimate

Increase quickly with load

TCP Estimating mean round trip time(RFC793)

Next packet sent

RFC813 suggests: = 2

MRR )1(

R

R

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Conservation at equilibrium (round-trip timing)

RTT estimation, theory

• A := (1-g)A + gM, 0 < g < 1

• A := A + g(M-A)(after rearranging)M-A = Er + Ee, whereEr (random error): due to noise in measurement

Ee (estimation error): due to bad choice of A

• A := A + gEr + gEe, we want large g to get most of Ee, but small g to reduce Er• Usually take 0.1 ≤ g ≤ 0.2

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Conservation at equilibrium (round-trip timing)

RTT estimation

• Goal: estimate variance of M

• σ2 = Σ |M-A|2– Squaring can cause overflow– Use mean deviation instead– mdev = Σ | M – A |

• mdev2 = (Σ | M – A |) 2 ≤ Σ | M – A |2 = σ2

• For normal distribution:mdev = sqrt( π/2 ) sdev ≈ 1.25 sdev

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Conservation at equilibrium (round-trip timing)

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Adapting to the path: congestion avoidance

Reasons that cause time out Packets damaged in transit Packets lost due to insufficient buffer

Congestion avoidance The network must be able to signal the transport endpoints

that congestion is occurring The endpoints must have a policy that decreases utilization

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Adapting to the path: congestion avoidance

Network congestion model

System stabilizes only if traffic is reduced as quickly as queues are growing

Li = N + λ Li-1,Li – load at interval i, average queue lengthN – constant

λ ≈ 0, no congestionλ > 1, congestion– Ln = λn L0, grows exponentially

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Adapting to the path: congestion avoidance

On congestion:

If no congestion detected

Wi = d Wi-1 (d < 1)

Take d = 0.5, Wi = Wi-1 / 2

Wi = Wi-1 + u (u << Wmax)Take u = 1, so Wi = Wi-1 + 1

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Adapting to the path: congestion avoidance

On timeout: cwnd = send window / 2

On ack: cwnd += 1 / cwnd

send window = min (rcv window, cwnd)

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Adapting to the path: congestion avoidance

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Result