Winter 2003 CS244a Handout 7 1

CS492B Project #2 TCP Tutorial #2

2003. 9.22Jin Hyun Ju

Winter 2003 CS244a Handout 7 2

CS244a: An Introduction to Computer Networks

Handout 7: Congestion Control

Nick McKeownProfessor of Electrical Engineering and Computer Science, Stanford University

nickm@stanford.eduhttp://www.stanford.edu/~nickm

Winter 2003 CS244a Handout 7 3

Main points Congestion is inevitable TCP sources detect congestion and, co-

operatively, reduce the rate at which they transmit.

The rate is controlled using the TCP window size.

TCP modifies the rate according to “Additive Increase, Multiplicative Decrease (AIMD)”.

To jump start flows, TCP uses a fast restart mechanism (called “slow start”!).

Winter 2003 CS244a Handout 7 4

CongestionH1

H2

R1 H3

A1(t)10Mb/s

D(t)1.5Mb/s

A2(t)100Mb/s

A1(t)

A2(t)X(t)

D(t)

A1(t)

A2(t)

D(t)

X(t)

Cumulativebytes

t

Winter 2003 CS244a Handout 7 5

Congestion is unavoidableArguably it’s good!

We use packet switching because it makes efficient use of the links. Therefore, buffers in the routers are frequently occupied.

If buffers are always empty, delay is low, but our usage of the network is low.

If buffers are always occupied, delay is high, but we are using the network more efficiently.

So how much congestion is too much?

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Detecting Congestion

duplicated ACK time out without congestion control…

Winter 2003 CS244a Handout 7 7

TCP Congestion Control

TCP implements host-based, feedback-based, window-based congestion control.

TCP sources attempts to determine how much capacity is available

TCP sends packets, then reacts to observable events (loss).

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TCP Congestion Control

TCP sources change the sending rate by modifying the window size:Window = min{Advertized window, Congestion Window}

In other words, send at the rate of the slowest component: network or receiver.

“cwnd” follows additive increase/multiplicative decrease On receipt of Ack: cwnd += 1 On packet loss (timeout): cwnd *= 0.5

Receiver Transmitter (“cwnd”)

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Additive Increase

D A D D A A D D A AD A

Src

Dest

When ACK is received:

cwnd += 1/cwnd

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Leads to the TCP “sawtooth”

t

Rate

halved

Timeouts

Could take a long time to get started!

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“Slow Start”Designed to cold-start connection quickly at startup or if

a connection has been halted (e.g. window dropped to zero,or window full, but ACK is lost).

How it works: increase cwnd by 1 for each ACK received.

D A D D A A D D

A A

D

A

Src

Dest

D

A

1 2 4 8

Winter 2003 CS244a Handout 7 12

Fast Recovery

Upon 3 duplicate ACKs, TCP retransmits. Does not enter slow-start: there are

probably ACKs in the pipe that will continue correct AIMD operation.

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TCP Reno Congestion Control

halved

Timeout

Exponential “slow start” t

Rate

Why is it called slow-start? Because TCP originally hadno congestion control mechanism. The source would just

start by sending a whole window’s worth of data.

Slow start in operation until it reaches half of

previous cwnd.

Triple duplicated ACK