3rd Edition, Chapter 5

Transport Layer

11-1

2016 session 1TELE3118: Network TechnologiesWeek 11: Transport LayerTCP

Some slides have been taken from:

rComputer Networking: A Top Down Approach Featuring the Internet,3rd edition. Jim Kurose, Keith Ross. Addison-Wesley, July 2004. Allmaterial copyright 1996-2004. J.F Kurose and K.W. Ross, All RightsReserved.

Transport Layer

11-2

TCP Connection Management: Setup

Recall: TCP sender, receiverestablish “connection”before exchanging datasegments

rinitialize TCP variables:

mseq. #s

buffers, flow controlinfo (e.g. RcvWindow)

rclient: connection initiator

mconnect()

rserver: contacted by client

maccept()

Three way handshake:

Step 1: client host sends TCPSYN segment to server

mspecifies initial seq #

mno data

Step 2: server host receivesSYN, replies with SYNACKsegment

mserver allocates buffers

mspecifies server initialseq. #

Step 3: client receives SYNACK,replies with ACK segment,which may contain data

Transport Layer

11-3

TCP Connection Management: Teardown

Closing a connection:

Step 1: client end systemsends TCP FIN controlsegment to server

Step 2: server receives FIN,replies with ACK. Closesconnection, sends FIN.

Step 3: client receives FIN,replies with ACK.

mEnters “timed wait” -will respond with ACKto received FINs

Step 4: server, receives ACK.Connection closed.

client

FIN

server

ACK

FIN

closed

timed wait

Transport Layer

11-4

TCP Connection Management (cont)

TCP client

lifecycle

TCP server

lifecycle

Transport Layer

11-5

TCP Flow Control

rreceive side of TCPconnection has areceive buffer:

rspeed-matchingservice: matching thesend rate to thereceiving app’s drainrate

rapp process may beslow at reading frombuffer

sender won’t overflow

receiver’s buffer by

transmitting too much,

too fast

flow control

Transport Layer

11-6

TCP Flow control: how it works

(Suppose TCP receiverdiscards out-of-ordersegments)

spare room in buffer

= RcvWindow

= RcvBuffer-[LastByteRcvd -LastByteRead]

Rcvr advertises spareroom by including valueof RcvWindow insegments

Sender limits unACKeddata to RcvWindow

mguarantees receivebuffer doesn’t overflow

Transport Layer

11-7

Congestion Control

Congestion:

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

rdifferent from flow control!

rmanifestations:

mlost packets (buffer overflow at routers)

mlong delays (queueing in router buffers)

ra top-10 problem!

Transport Layer

11-8

Approaches towards congestion control

End-end congestioncontrol:

rno explicit feedback fromnetwork

rcongestion inferred fromend-system observed loss,delay

rapproach taken by TCP

Network-assistedcongestion control:

rrouters provide feedbackto end systems

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

mexplicit rate sendershould send at

Two broad approaches towards congestion control:

Transport Layer

11-9

TCP Congestion Control

rend-end control (no networkassistance)

rsender limits transmission:

LastByteSent-LastByteAcked

 CongWin

rRoughly,

CongWin is dynamic, functionof perceived networkcongestion

How does senderperceive congestion?

rloss event = timeout or3 duplicate acks

TCP sender reducesrate (CongWin) afterloss event

three mechanisms:

mAIMD

mslow start

mconservative aftertimeout events

rate =

CongWin

RTT

Bytes/sec

Transport Layer

11-10

TCP AIMD

multiplicative decrease:cut CongWin in halfafter loss event

additive increase:increase CongWin by1 MSS every RTT inthe absence of lossevents: probing

Long-lived TCP connection

Transport Layer

11-11

TCP Slow Start

When connection begins,CongWin = 1 MSS. E.g.:

•MSS = 500 bytes

•RTT = 200 msec

•initial rate = 20 kbps

ravailable bandwidth may be >>MSS/RTT

mRamp-up takes too long

rWhen connection begins,increase rate exponentiallyuntil first loss event:

double CongWin every RTT

done by incrementing CongWinfor every ACK received

rSummary: initial rate slow butramps up exponentially fast

Host A

one segment

RTT

Host B

time

two segments

four segments

Transport Layer

11-12

Refinement

rAfter 3 dup ACKs:

CongWin is cut in half

mwindow then growslinearly

rBut after timeout event:

CongWin instead set to1 MSS;

mwindow then growsexponentially

mto a threshold, thengrows linearly

• 3 dup ACKs indicatesnetwork capable ofdelivering some segments

• timeout before 3 dupACKs is “more alarming”

Philosophy:

Transport Layer

11-13

Refinement (more)

Q: When should theexponential increaseswitch to linear?

A: When CongWin gets to1/2 of its value beforetimeout.

Implementation:

rVariable Threshold

rAt loss event, Threshold isset to 1/2 of CongWin justbefore loss event

Transport Layer

11-14

Summary: TCP Congestion Control

When CongWin is below Threshold, sender inslow-start phase, window grows exponentially.

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

When a triple duplicate ACK occurs, Thresholdset to CongWin/2 and CongWin set toThreshold.

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

Transport Layer

11-15

TCP sender congestion control

Event

State

TCP Sender Action

Commentary

ACK receiptfor previouslyunackeddata

Slow Start(SS)

CongWin = CongWin + MSS,

If (CongWin > Threshold)

set state to “CongestionAvoidance”

Resulting in a doubling ofCongWin every RTT

ACK receiptfor previouslyunackeddata

Congestion

Avoidance(CA)

CongWin = CongWin+MSS *(MSS/CongWin)

Additive increase, resultingin increase of CongWin by1 MSS every RTT

Loss eventdetected bytripleduplicateACK

SS or CA

Threshold = CongWin/2,

CongWin = Threshold,

Set state to “CongestionAvoidance”

Fast recovery,implementing multiplicativedecrease. CongWin will notdrop below 1 MSS.

Timeout

SS or CA

Threshold = CongWin/2,

CongWin = 1 MSS,

Set state to “Slow Start”

Enter slow start

DuplicateACK

SS or CA

Increment duplicate ACK countfor segment being acked

CongWin and Threshold notchanged

Transport Layer

11-16

TCP throughput

rWhat’s the average throughout of TCP as afunction of window size and RTT?

mIgnore slow start

rLet W be the window size when loss occurs.

rWhen window is W, throughput is W/RTT

rJust after loss, window drops to W/2,throughput to W/2RTT.

rAverage throughout: .75 W/RTT

Transport Layer

11-17

TCP Futures

rExample: 1500 byte segments, 100ms RTT, want 10Gbps throughput

rRequires window size W = 83,333 in-flightsegments

rThroughput in terms of loss rate:

➜ L = 2·11-10 Wow

rNew versions of TCP for high-speed needed!

Transport Layer

11-18

Fairness goal: if K TCP sessions share samebottleneck link of bandwidth R, each should haveaverage rate of R/K

TCP connection 1

bottleneck

router

capacity R

TCP

connection 2

TCP Fairness

Transport Layer

11-19

Why is TCP fair?

Two competing sessions:

rAdditive increase gives slope of 1, as throughout increases

rmultiplicative decrease decreases throughput proportionally

equal bandwidth share

Connection 1 throughput

Connection 2 throughput

congestion avoidance: additive increase

loss: decrease window by factor of 2

congestion avoidance: additive increase

loss: decrease window by factor of 2

Transport Layer

11-20

Fairness (more)

Fairness and UDP

rMultimedia apps oftendo not use TCP

mdo not want ratethrottled by congestioncontrol

rInstead use UDP:

mpump audio/video atconstant rate, toleratepacket loss

rResearch area: TCPfriendly

Fairness and parallel TCPconnections

rnothing prevents app fromopening parallel cnctionsbetween 2 hosts.

rWeb browsers do this

rExample: link of rate Rsupporting 9 cnctions;

mnew app asks for 1 TCP, getsrate R/10

mnew app asks for 11 TCPs,gets R/2 !