TCP Connection Management and Congestion Control Computer Networking: A Top Down Approach 6 th edition Jim Kurose, Keith Ross Addison-Wesley March 2012.

Transport services and protocols provide logical

communication between app processes running on different hosts

transport protocols run in end systems send side breaks app

messages into segments passes to network layer

rcv side reassembles segments into messages passes to app layer

more than one transport protocol available to apps Internet TCP and UDP

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

logical end-end transport

Transport vs network layer

network layer logical communication between hosts

transport layer logical communication between processes relies on enhances

network layer services

Household analogy12 kids sending letters

to 12 kids processes = kids app messages =

letters in envelopes hosts = houses transport protocol =

Ann and Bill network-layer protocol

= postal service

Internet transport-layer protocols reliable in-order

delivery (TCP) congestion control flow control connection setup

unreliable unordered delivery UDP no-frills extension of

ldquobest-effortrdquo IP

services not available delay guarantees bandwidth guarantees

application

transportnetworkdata linkphysical network

data linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

networkdata linkphysical

application

transportnetworkdata linkphysical

logical end-end transport

TCP Overview RFCs 793 1122 1323 2018 2581

full duplex data bi-directional data flow

in same connection MSS maximum

segment size

connection-oriented handshaking (exchange

of control msgs) initrsquos sender receiver state before data exchange

flow controlled sender will not

overwhelm receiver

point-to-point one sender one

receiver

reliable in-order byte steam no ldquomessage

boundariesrdquo

pipelined TCP congestion and flow

control set window size

send amp receive bufferssocketdoor

T C Psend buffer

T C Preceive buffer

socketdoor

segm ent

applicationwrites data

applicationreads data

TCP segment structure

source port dest port

32 bits

applicationdata

(variable length)

sequence number

acknowledgement numberReceive window

Urg data pnterchecksum

FSRPAUheadlen

notused

Options (variable length)

URG urgent data (generally not used)

ACK ACK valid

PSH push data now(generally not used)

RST SYN FINconnection estab(setup teardown

commands)

bytes rcvr willingto accept

countingby bytes of data(not segments)

Internetchecksum

(as in UDP)

TCP Connection Management

Recall TCP sender receiver establish ldquoconnectionrdquo before exchanging data segments

initialize TCP variables seq s buffers flow control info

(eg RcvWindow) client connection initiator Socket clientSocket = new

Socket(hostnameport

number) server contacted by client Socket connectionSocket =

welcomeSocketaccept()

Three way handshake

Step 1 client host sends TCP SYN segment to server specifies initial seq no data

Step 2 server host receives SYN replies with SYNACK segment

server allocates buffers specifies server initial

seq Step 3 client receives SYNACK

replies with ACK segment which may contain data

TCP seq rsquos and ACKsSeq rsquos

byte stream ldquonumberrdquo of first byte in segmentrsquos data

ACKs seq of next byte

expected from other side

cumulative ACKQ how receiver handles

out-of-order segments A TCP spec doesnrsquot

say - up to implementor

Host A Host B

Seq=42 ACK=79 data = lsquoCrsquo

Seq=79 ACK=43 data = lsquoCrsquo

Seq=43 ACK=80

Usertypes

lsquoCrsquo

host ACKsreceipt

of echoedlsquoCrsquo

host ACKsreceipt of

lsquoCrsquo echoesback lsquoCrsquo

timesimple telnet scenario

TCP reliable data transfer

TCP creates rdt service on top of IPrsquos unreliable service

Pipelined segments Cumulative acks TCP uses single

retransmission timer

Retransmissions are triggered by timeout events duplicate acks

Initially consider simplified TCP sender ignore duplicate acks ignore flow control

congestion control

TCP sender eventsdata rcvd from app Create segment with

seq seq is byte-stream

number of first data byte in segment

start timer if not already running (think of timer as for oldest unacked segment)

expiration interval TimeOutInterval

timeout retransmit segment

that caused timeout restart timer Ack rcvd If acknowledges

previously unacked segments update what is known

to be acked start timer if there are

outstanding segments

TCP sender(simplified)

NextSeqNum = InitialSeqNum SendBase = InitialSeqNum

loop (forever) switch(event)

event data received from application above create TCP segment with sequence number NextSeqNum if (timer currently not running) start timer pass segment to IP NextSeqNum = NextSeqNum + length(data)

event timer timeout retransmit not-yet-acknowledged segment with smallest sequence number start timer

event ACK received with ACK field value of y if (y gt SendBase) SendBase = y if (there are currently not-yet-acknowledged segments) start timer

end of loop forever

Commentbull SendBase-1 last cumulatively ackrsquoed byteExamplebull SendBase-1 = 71y= 73 so the rcvrwants 73+ y gt SendBase sothat new data is acked

TCP retransmission scenarios

Host A

Seq=100 20 bytes data

ACK=100

timepremature timeout

Host B

Seq=92 8 bytes data

ACK=120

Seq=92 8 bytes data

Seq=

92

tim

eout

ACK=120

Host A

Seq=92 8 bytes data

ACK=100

loss

tim

eout

lost ACK scenario

Host B

X

Seq=92 8 bytes data

ACK=100

time

Seq=

92

tim

eout

SendBase= 100

SendBase= 120

SendBase= 120

Sendbase= 100

TCP retransmission scenarios (more)

Host A

Seq=92 8 bytes data

ACK=100

loss

tim

eout

Cumulative ACK scenario

Host B

X

Seq=100 20 bytes data

ACK=120

time

SendBase= 120

TCP ACK generation [RFC 1122 RFC 2581]

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 segmentsend 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

Fast Retransmit

Time-out period often relatively long long delay before

resending lost packet

Detect lost segments via duplicate ACKs Sender often sends

many segments back-to-back

If segment is lost there will likely be many duplicate ACKs

If sender receives 3 ACKs for the same data it supposes that segment after ACKed data was lost fast retransmit resend

segment before timer expires

Host A

tim

eout

Host B

time

X

resend 2nd segment

Figure 337 Resending a segment after triple duplicate ACK

TCP Connection Management (cont)

Closing a connection

client closes socket clientSocketclose()

Step 1 client end system sends TCP FIN control segment to server

Step 2 server receives FIN replies with ACK Closes connection sends FIN

client

FIN

server

ACK

ACK

FIN

close

close

closed

tim

ed w

ait

TCP Connection Management (cont)

Step 3 client receives FIN replies with ACK

Enters ldquotimed waitrdquo - will respond with ACK to received FINs

Step 4 server receives ACK Connection closed

Note with small modification can handle simultaneous FINs

client

FIN

server

ACK

ACK

FIN

closing

closing

closed

tim

ed w

ait

closed

Principles of Congestion Control

Congestion informally ldquotoo many sources sending too

much data too fast for network to handlerdquo different from flow control manifestations

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

a top-10 problem

Approaches 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 TCPIP ECN ATM)

explicit rate sender should send at

Two broad approaches towards congestion control

TCP Congestion Control details

sender limits transmission LastByteSent-LastByteAcked

minCongWinRcvWindow Roughly

CongWin is dynamic function of perceived network congestion

How does sender perceive congestion

loss event = timeout or 3 duplicate acks

TCP sender reduces rate (CongWin) after loss event

three mechanisms AIMD slow start conservative after

timeout events

rate = CongWin

RTT Bytessec

TCP congestion control additive increase multiplicative decrease

8 Kbytes

16 Kbytes

24 Kbytes

time

congestionwindow

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

timecong

estio

n w

indo

w s

ize

Saw toothbehavior probing

for bandwidth

TCP Slow Start

When connection begins CongWin = 1 MSS Example MSS = 500

bytes amp RTT = 200 msec

initial rate = 20 kbps

available bandwidth may be gtgt MSSRTT desirable to quickly

ramp up to respectable rate

When connection begins increase rate exponentially fast until first loss event

TCP Slow Start (more)

When connection begins increase rate exponentially until first loss event double CongWin every

RTT done by incrementing CongWin 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

Refinement 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 indicates

network capable of delivering some segments timeout indicates a ldquomore alarmingrdquo congestion scenario

Philosophy

RefinementQ When should the

exponential increase switch to linear

A When CongWin gets to 12 of its value before timeout

Implementation Variable Threshold At loss event Threshold

is set to 12 of CongWin just before loss event

slow-start

fast recoverycongestion-avoidance

Summary TCP Congestion Control

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

When CongWin is above Threshold sender is in congestion-avoidance phase window grows linearly

When a triple duplicate ACK occurs Threshold set to CongWin2 and CongWin set to Threshold

When timeout occurs Threshold set to CongWin2 and CongWin is set to 1 MSS

TCP sender congestion control

State Event TCP Sender Action Commentary

Slow Start (SS)

ACK receipt for previously unacked data

CongWin = CongWin + MSS If (CongWin gt Threshold) set state to ldquoCongestion Avoidancerdquo

Resulting in a doubling of CongWin every RTT

CongestionAvoidance (CA)

ACK receipt for previously unacked data

CongWin = CongWin+MSS (MSSCongWin)

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

SS or CA Loss event detected by triple duplicate ACK

Threshold = CongWin2 CongWin = ThresholdSet state to ldquoCongestion Avoidancerdquo

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

SS or CA Timeout Threshold = CongWin2 CongWin = 1 MSSSet state to ldquoSlow Startrdquo

Enter slow start

SS or CA Duplicate ACK

Increment duplicate ACK count for segment being acked

CongWin and Threshold not changed

congestion control algorithmTh = CongWin = 1 MSS slow start or exponential increase While (No Packet Loss and CongWin lt Th)

send CongWin TCP segmentsfor each ACK increase CongWin by 1

congestion avoidance or linear increase While (No Packet Loss)

send CongWin TCP segmentsfor CongWin ACKs increase CongWin by 1

Th = CongWin2If (3 Dup ACKs) CongWin = ThIf (timeout) CongWin=1