EEC 682/782

EEC-484/584Computer NetworksEEC-484/584Computer Networks

Lecture 14Lecture 14

Wenbing ZhaoWenbing Zhao

wenbing@ieee.org wenbing@ieee.org

(Part of the slides are based on Drs. Kurose &Ross’s slides for their Computer Networking book)(Part of the slides are based on Drs. Kurose &Ross’s slides for their Computer Networking book)

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing 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

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

TCP ACK GenerationTCP ACK Generation

Event at Receiver

Arrival of in-order segment with

expected seq #. All data up to

expected seq # already ACKed

Arrival of in-order segment with

expected seq #. One other

segment has ACK pending

Arrival of out-of-order segment

higher-than-expect seq. # .

Gap detected

Arrival of segment that

partially or completely fills gap

TCP Receiver action

Delayed ACK. Wait up to 500ms

for 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 that

segment starts at lower end of gap

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

TCP Flow ControlTCP Flow Control

•Receive side of TCPconnection has areceive buffer:

••Speed-matchingservice: matchingthe send rate to thereceiving app’s drainrate

•App process may beslow at reading frombuffer

Flow control:

sender won’t overflow

receiver’s buffer by

transmitting too much,

too fast

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

TCP Flow ControlTCP Flow Control

(Suppose TCP receiver discardsout-of-order segments)

Spare room in buffer

= RcvWindow

= RcvBuffer-[LastByteRcvd- LastByteRead]

••Rcvr advertisesspare room byincluding value ofRcvWindow insegments

••Sender limitsunACKed data toRcvWindow

–guarantees receivebuffer doesn’toverflow

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

Principles of Congestion ControlPrinciples of Congestion Control

Congestion:

•Informally: “too many sources sending too muchdata too fast for network to handle”

•Different from flow control!

•Manifestations:

–lost packets (buffer overflow at routers)

–long delays (queueing in router buffers)

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

Approaches towards Congestion ControlApproaches towards Congestion Control

End-end congestioncontrol:

•no explicit feedback fromnetwork

•congestion inferred fromend-system observedloss, delay

•approach taken by TCP

Network-assistedcongestion control:

••routers provide feedbackto end systems

–single bit indicatingcongestion (SNA,DECbit, TCP/IP ECN,ATM)

–explicit rate sendershould send at

Two broad approaches towards congestion control

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

TCP Congestion Control:Additive Increase, Multiplicative DecreaseTCP Congestion Control:Additive Increase, Multiplicative Decrease

•Approach: increase transmission rate (window size),probing for usable bandwidth, until loss occurs

–Additive increase: increase cwnd every RTT untilloss detected

–Multiplicative decrease: cut cwnd after loss

Saw tooth

behavior: probing

for bandwidth

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

TCP Congestion ControlTCP Congestion Control

•Sender limits transmission:

LastByteSent-LastByteAcked

 cwnd

•Roughly,

•cwnd is dynamic, function ofperceived networkcongestion

How does senderperceivecongestion?

••loss event = timeoutor 3 duplicate acks

••TCP sender reducesrate (cwnd) afterloss event

rate =

cwnd

RTT

Bytes/sec

1010

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

TCP Slow StartTCP Slow Start

•When connection begins,cwnd = 1 MSS

–Example: MSS = 500 bytes& RTT = 200 msec

–Initial rate = 25 kBps

•Available bandwidth maybe >> MSS/RTT

–Desirable to quickly rampup to respectable rate

•When connectionbegins, increase rateexponentially fast untilfirst loss event

1111

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

TCP Slow StartTCP Slow Start

•When connectionbegins, increase rateexponentially until firstloss event:

–Double cwnd every RTT

–Done by incrementingcwnd for every ACKreceived

•Summary: initial rate isslow but ramps upexponentially fast

Host A

one segment

RTT

Host B

time

two segments

four segments

1212

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

Congestion AvoidanceCongestion Avoidance

Q: When should theexponential increaseswitch to linear?

A: When cwnd gets to1/2 of its valuebefore timeout

Implementation:

••Variable Threshold

••At loss event, Thresholdis set to 1/2 of cwnd justbefore loss event

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

1313

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

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 growsexponentially

–to a threshold, then growslinearly

 3 dup ACKs indicatesnetwork capable ofdelivering some segments

 timeout indicates a“more alarming”congestion scenario

Philosophy:

1414

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

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 incongestion-avoidance phase, window grows linearly

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

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

1515

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

TCP Sender Congestion ControlTCP Sender Congestion Control

State

Event

TCP Sender Action

Commentary

Slow Start(SS)

ACK receiptfor previouslyunackeddata

CongWin = CongWin + MSS,

If (CongWin > Threshold)

set state to “CongestionAvoidance”

Resulting in a doublingof CongWin every RTT

Congestion

Avoidance(CA)

ACK receiptfor previouslyunackeddata

CongWin = CongWin+ MSS * (MSS/CongWin)

Additive increase,resulting in increase ofCongWin by 1 MSSevery RTT

1616

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

TCP Sender Congestion ControlTCP Sender Congestion Control

State

Event

TCP Sender Action

Commentary

SS or CA

Losseventdetectedby tripleduplicateACK

Threshold = CongWin/2,

CongWin = Threshold,

Set state to “CongestionAvoidance”

Fast recovery,implementingmultiplicativedecrease. CongWinwill not drop below 1MSS.

SS or CA

Timeout

Threshold = CongWin/2,

CongWin = 1 MSS,

Set state to “Slow Start”

Enter slow start

SS or CA

DuplicateACK

Increment duplicate ACKcount for segment beingacked

CongWin andThreshold notchanged

1717

Spring Semester 2008

EEC-484/584: Computer Networks

Wenbing Zhao

TCP Congestion ControlTCP Congestion Control

Segment lost

Repeated acks

Slow start