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Transport LayerTransport Layer(contd..)

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Approaches towards congestion controlApproaches towards congestion control

End-end congestion control:

no explicit feedback from networkcongestion inferred from end-

system observed loss, delayapproach taken by TCP

Network-assisted congestioncontrol:

routers provide feedback to endsystems

single bit indicating congestion(SNA, DECbit, TCP/IP ECN, ATM)

explicit rate sender should send at

Two broad approaches towards congestion control:

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Case study: ATM ABR congestion controlCase study: ATM ABR congestion control

ABR:ABR: available bit rate:elastic serviceif senders path

underloaded:

sender should use availablebandwidthif senders path congested:sender throttled to minimumguaranteed rate

RMRM (resource management)cells:

sent by sender, interspersed withdata cells

bits in RM cell set by switches(network-assisted)

NI bit:NI bit:no increase in rate (mildcongestion)

CI bit:CI bit: congestion indication

RM cells returned to sender byreceiver, with bits intact

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Case study: ATM ABR congestionCase study: ATM ABR congestion

controlcontrol

Two-byte ER (explicit rate) field in RM cell

congested switch may lower ER value in cell

sender send rate thus maximum supportable rate on path

EFCI bit in data cells: set to 1 in congested switch

if data cell preceding RM cell has EFCI set, sender sets CI bit in returnedRM cell

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TCP congestion control:TCP congestion control:-- additive increase, multiplicative decreaseadditive increase, multiplicative decrease

8 K b y t e s

1 6 K b y t e s

2 4 K b y t e s

t im e

c o n g e s t i o n

w i n d o w

Approach: increase transmission rate (window size),

probing for usable bandwidth, until loss occurs

additive increase:increase CongWin by 1 MSS

every RTT until loss detected multiplicative decrease: cut CongWin in half after

loss

time

congestionwindowsize

Saw tooth

behavior: probing

for bandwidth

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TCP Congestion Control: detailsTCP Congestion Control: details

sender limits transmission:LastByteSent-LastByteAcked

CongWinRoughly,

CongWin is dynamic, function ofperceived network congestion

How does senderperceive congestion?

loss event = timeout or3duplicate acks

TCP sender reduces rate(CongWin) after lossevent

three mechanisms:

AIMDslow startconservative after timeoutevents

rate =CongWin

RTTBytes/sec

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TCP Slow StartTCP Slow Start

When connection begins,CongWin = 1 MSSExample: MSS = 500 bytes &RTT = 200 msec

initial rate = 20 kbpsavailable bandwidth may be >>MSS/RTT

desirable to quickly ramp up torespectable rate

When connection begins,increase rateexponentially fast untilfirst loss event

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TCP Slow Start (more)TCP Slow Start (more)

When connection begins,increase rate exponentiallyuntil first loss event:-double CongWin every RTT

-done by incrementing CongWinfor every ACK received

Summary:initial rate is slowbut ramps up exponentially

fast

Host A

onesegmentRTT

Host B

time

twosegments

foursegments

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RefinementRefinementQ: When should the

exponential increase switchto linear?

A: When CongWin gets to 1/2of its value before timeout.

Implementation:Variable Threshold

At loss event, Threshold is setto 1/2 of CongWin just beforeloss event

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Refinement: inferring lossRefinement: inferring loss

After 3 dup ACKs:CongWin is cut in half

window then grows linearly

But after timeout event:

CongWin instead set to 1 MSS;

window then grows exponentially

to a threshold, then grows linearly

3 dup ACKs indicatesnetwork capable ofdelivering some segments timeout indicates amore alarmingcongestion scenario

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Summary: TCP Congestion ControlSummary: TCP Congestion Control

When CongWin is below Threshold, sender in slow-start phase, window grows exponentially.

When CongWin is above Threshold, sender is incongestion-avoidance phase, window grows linearly.

When a triple duplicate ACK occurs, Threshold setto CongWin/2 and CongWin set to Threshold.

When timeout occurs, Threshold set to CongWin/2and CongWin is set to 1 MSS.