Congestion Control - Supplementary

Slides are adapted on Jean Walrand’s Slides

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TOC: Congestion Control Congestion control for bandwidth sharing Cheating TCP Buffer Size ECN RED TCP Unfairness TCP Vegas

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Congestion Control: Questions

How to avoid network congestion?How to recover from congestion? Congestion occurs at access link and access

network

How to share the links bandwidth? If bandwidth is allocated and enforced

properly, no congestion should occur. Can improve treatment of flows

E.g., one flow should not get a much smaller fraction of bandwidth

Some flows might need some guaranteed bandwidth

Discovering available bandwidth What is fair?

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Questions: Available bandwidth?

Example:

A B

C E D F

1010 10 10 10

103 36

x

zy

= router = host

3 = link with bandwidth of 3Mbps(same for 6 and 10)

x, y, z = throughput of flows

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Questions: Available bandwidth?

Example:

A B

C E D F

1010 10 10 10

103 36

x

zy

• What is available bandwidth? (See later)• How does A “discover” the available bandwidth to B?• Some approaches:

1. Reservation: e.g., Telephone2. Adapt to congestion: TCP3. Pricing congestion: research topic

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Available bandwidth: Reservation

A B

C E D F

1010 10 10 10

103 36

x

zy

1. Routers (or manager) keep track of reserved rates

2. A requests a rate R to B from the network3. The network figures out if R is available4. If R is available, routers (or manager) update

reservations and confirm to A5. Note: complex, slow, requires connection

setup and enforcement

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Available bandwidth: Adapt

A B

C E D F

1010 10 10 10

103 36

x

zy

1. Transmit and slow down if congestion occur2. Example:

• Initially: x= 0, y = 3, z = 3• Then A increases its rate; C and E notice

congestion and slow down

• Later, C stops: A and E increase rates3. Notes:

• No guarantees: throughput may drop• Key question: how to adapt rates, e.g., TCP

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Available bandwidth: Pricing

Example:

A B

C E D F

1010 10 10 10

103 36

x

zy

• When they get saturated, routers mark packets

• If a flow with rate R uses saturated links, it gets marks with rate R

• Each mark costs one unit• Source slows down if price becomes

excessive• x= 1+, y = 2+, z = 2+

pA = 1 + 1; pC = pE = 2• x = 2+, y = 1+, z = 1+

pA = 2 + 2; pC = pE = 1

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Questions: What is Fair?

Example:

A B

C E D F

1010 10 10 10

103 36

x

zy

• x = y = z = 1.5: fair in max-min sense

• x = 0, y = z = 3: maximizes x + y + z

• 5x = 4y = 4z: equalizes resources flows use with x = 1.33, y = z = 1.67

• What if AB needs 2Mbps?(and is willing to pay for it)

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Cheating: Increase Faster

A Bx

C

D Ey

Limit rates:x = 2y

C

x

yx increases by 2 MSS/RTT per RTTy increases by 1 MSS/RTT per RTT

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Cheating: Increase Faster

A Bx

C = 50

D Ey

0

10

20

30

40

50

60

1 28 55 82 109

136

163

190

217

244

271

298

325

352

379

406

433

460

487

time

Rate

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Cheating: Start SS with CongWin > 1

A Bx

C

D Ey

x starts SS with CongWin = 4y starts SS with CongWin = 1

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Cheating: Open Many Connections

A Bx

C

D Ey

Assume • A starts 10 connections to B• D starts 1 connection to E• Each connection gets about the same throughput

Then A gets 10 times more throughput than D

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ECN: Explicit Congestion Notification

Standard TCP: Losses needed to detect congestion Wasteful and unnecessary

ECN: When congested, routers mark (IP) packets instead

of dropping them Destination marks ACKs of marked packets Source set CongWin = CongWin/2 when it sees mark

Advantages: No time wasted to retransmit Link errors not confused with congestion

Example: without ECN, at wireless link, packets can get corrupted by noise and interference, and get dropped eventually. Sender will think congestion has occurred.

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Explicit Congestion Notification

Illustration:

A B

CongWin =CongWin/ 2

A B

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Explicit Congestion NotificationBackward Compatibility:

Unused bits from TCP and IP headers are used for ECN.

One bit in IP header indicates if hosts implement ECN

If it does, router marks packet

If it does not, router drops packet

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RED

Random Early Detection:As queue builds up, drop or mark packets with increasing probability (before queue gets full)Advantages: Avoids penalizing streams with large bursts

This is done by keeping the queue size smaller than the buffer size

Earlier marking reduces dropped packets De-synchronizes the source behaviors

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RED: IllustrationC Mbps

PACKETS

• Calculate recent average of queue length: Qav Qav(n+1) = (1 – b) Qav(n) + b Q(n)

0 < b < 1• Determine drop or mark probability p(Qav):

1

0k

L H

Qav

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Router Buffers: Rule of Thumb

Imagine that all connections on input port with rate R “burst” for RTT seconds (until stopped by RED)

Router must store RxRTT for each port

Example: 40 Gbps throughput (sum of port rates) RTT = 200 ms (worst case?) Then storage = 8 Gbits = (about) 1 GByte

Question: Is this reasonable?

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Unfairness

Fact: TCP favors connections with short RTT

Cause: Increase rate is 1 MSS/RTT, so that it is faster for

connections with small RTT Recall our discussion of “Cheating RTT”

It is quite possible for a connection to get only a few percent of its fair shareSolutions: Modify TCP to increase in proportion to RTT?

Problem: Estimate of RTT is noisy TCP Vegas (see next)

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TCP Vegas

Consider one queue:

A Bx

C

D Ey

Assume both connections have the same backlogThen they have the same throughput …Vegas Algorithm: Estimate backlog by

Q = Outstanding – Rate Base_RTTRate = Bytes_Sent_Successfully / Current_RTT

If Q > 3 MSS, then slow down; otherwise, speed upProblem: Reno clobbers Vegas (unlike in real life)