EEC-484/584 Computer Networks

EEC-484/584EEC-484/584Computer NetworksComputer Networks

Lecture 14Lecture 14

Wenbing ZhaoWenbing Zhao

[email protected](Part of the slides are based on Drs. Kurose & (Part of the slides are based on Drs. Kurose &

RossRoss’’s slides for their s slides for their Computer Networking Computer Networking book)book)

22

Spring Semester 2008Spring Semester 2008 EEC-484/584: Computer NetworksEEC-484/584: Computer Networks Wenbing ZhaoWenbing Zhao

OutlineOutline

• Reminder: – Wiki project#2 due: 4/16 Wednesday– Quiz 4: 4/21 Monday

• TCP– Reliable data transfer (ack generation)– Flow control– Congestion control

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TCP ACK GenerationTCP ACK Generation

Event at Receiver

Arrival of in-order segment withexpected seq #. All data up toexpected seq # already ACKed

Arrival of in-order segment withexpected seq #. One other segment has ACK pending

Arrival of out-of-order segmenthigher-than-expect seq. # .Gap detected

Arrival of segment that partially or completely fills gap

TCP Receiver action

Delayed ACK. Wait up to 500msfor next segment. If no next segment,send ACK

Immediately send single cumulative ACK, ACKing both in-order segments

Immediately send duplicate ACK, indicating seq. # of next expected byte

Immediate send ACK, provided thatsegment starts at lower end of gap

44


TCP Flow ControlTCP Flow Control• Receive side of TCP

connection has a receive buffer:

• Speed-matching service: matching the send rate to the receiving app’s drain rate

• App process may be slow at reading from buffer

Flow control:sender won’t overflow

receiver’s buffer bytransmitting too much,

too fast

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TCP Flow ControlTCP Flow Control

(Suppose TCP receiver discards out-of-order segments)

Spare room in buffer= RcvWindow

= RcvBuffer-[LastByteRcvd - LastByteRead]

• Rcvr advertises spare room by including value of RcvWindow in segments

• Sender limits unACKed data to RcvWindow– guarantees receive

buffer doesn’t overflow

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Principles of Congestion ControlPrinciples of Congestion Control

Congestion:• Informally: “too many sources sending too much

data too fast for network to handle”• Different from flow control!• Manifestations:

– lost packets (buffer overflow at routers)– long delays (queueing in router buffers)

77


Approaches towards Congestion ControlApproaches towards Congestion Control

End-end congestion control:

• no explicit feedback from network

• congestion inferred from end-system observed loss, delay

• approach taken by TCP

Network-assisted congestion control:

• routers provide feedback to end systems– 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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TCP Congestion Control: TCP Congestion Control: Additive Increase, Multiplicative DecreaseAdditive Increase, Multiplicative Decrease

• Approach: increase transmission rate (window size), probing for usable bandwidth, until loss occurs– Additive increase: increase cwnd every RTT until

loss detected– Multiplicative decrease: cut cwnd after loss

Saw toothbehavior: probing

for bandwidth

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TCP Congestion ControlTCP Congestion Control

• Sender limits transmission: LastByteSent-LastByteAcked cwnd

• Roughly,

• cwnd is dynamic, function of perceived network congestion

How does sender perceive congestion?

• loss event = timeout or 3 duplicate acks

• TCP sender reduces rate (cwnd) after loss event

rate = cwnd

RTT Bytes/sec

1010


TCP Slow StartTCP Slow Start

• When connection begins, cwnd = 1 MSS– Example: MSS = 500 bytes

& RTT = 200 msec– Initial rate = 25 kBps

• Available bandwidth may be >> MSS/RTT– Desirable to quickly ramp

up to respectable rate

• When connection begins, increase rate exponentially fast until first loss event

1111


TCP Slow StartTCP Slow Start• When connection

begins, increase rate exponentially until first loss event:– Double cwnd every RTT– Done by incrementing cwnd for every ACK received

• Summary: initial rate is slow but ramps up exponentially fast

Host A

one segment

RTT

Host B

time

two segments

four segments

1212


Congestion AvoidanceCongestion AvoidanceQ: When should the

exponential increase switch to linear?

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

Implementation:• Variable Threshold • At loss event, Threshold is

set to 1/2 of cwnd just before loss event

How to increase cwnd linearly:cwnd (new) = cwnd + mss*mss/cwnd

1313


Congestion ControlCongestion Control

• After 3 duplicated ACKs:– cwnd is cut in half– window then grows linearly

• But after timeout event:– cwnd instead set to 1 MSS– window then grows

exponentially– to a threshold, then grows

linearly

3 dup ACKs indicates

network capable of delivering some segments timeout indicates a “more alarming” congestion scenario

Philosophy:

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Summary: TCP Congestion ControlSummary: TCP Congestion Control

• When cwnd is below Threshold, sender in slow-start phase, window grows exponentially

• When cwnd is above Threshold, sender is in congestion-avoidance phase, window grows linearly

• When a triple duplicate ACK occurs, Threshold set to cwnd/2 and cwnd set to Threshold

• When timeout occurs, Threshold set to cwnd/2 and cwnd is set to 1 MSS

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TCP Sender Congestion ControlTCP Sender Congestion Control

State Event TCP Sender Action Commentary

Slow Start (SS)

ACK receipt for previously unacked data

CongWin = CongWin + MSS,

If (CongWin > Threshold) set state to “Congestion Avoidance”

Resulting in a doubling of CongWin every RTT

CongestionAvoidance (CA)

ACK receipt for previously unacked data

CongWin = CongWin+ MSS * (MSS/CongWin)

Additive increase, resulting in increase of CongWin by 1 MSS every RTT

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TCP Sender Congestion ControlTCP Sender Congestion Control

State Event TCP Sender Action Commentary

SS or CA Loss event detected by triple duplicate ACK

Threshold = CongWin/2, CongWin = Threshold,Set state to “Congestion Avoidance”

Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS.

SS or CA Timeout Threshold = CongWin/2, CongWin = 1 MSS,Set state to “Slow Start”

Enter slow start

SS or CA Duplicate ACK

Increment duplicate ACK count for segment being acked

CongWin and Threshold not changed

1717


TCP Congestion ControlTCP Congestion Control

Segment lost Repeated acks

Slow start

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