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COM210: Com uter Networks CNCOM210: Com uter Networks CN

Dr Ahmad Al-Zubi

Dr Ahmad Al-Zubi

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TCP: Overview

Point-to-point:

,

Reliable, in-order byte steam:

But TCP chops it up into segments for

Pipelined (window) flow control:

network

Send & receive buffers

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TCP: Overview

application

writes dataapplication

doorTCP

send buffer

TCP

receive buffer

socketdoor

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TCP: Overview

Full duplex data: -

MSS: maximum segment size

-

handshaking (exchange of control msgs)

exchange

Flow & Con estion Control:sender will not overwhelm receiver or the

network

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TCP segment structure

source port # dest port #

32 bits

URG: urgent data(generally not used)

countingby bytes

acknowledgement number

rcvr window sizeFSRPAUheadlen

notused

ACK: ACK #valid

PSH: ush data now

of data

(not segments!)

ptr urgent datachecksum

Options (variable length)

(generally not used)

RST, SYN, FIN:

ytesrcvr willing

to accept

application

(setup, teardowncommands)

(variable length)nternetchecksum(as in UDP)

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TCP seq. #s and ACKs (I)

Sequence Numbers:

b te stream number of first b te in

segments data

ACKs:

seq # of next byte expected from other sidecumulative ACK

Q: how receiver handles out-of-order segmentsA: TCP spec doesnt say, - up to

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TCP Seq. #s and ACKs (II)

Host A Host B

Usertypes

Chost ACKsreceipt of

C, echoesback C

ost sreceipt

of echoedC

sim le telnet scenario

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Temporal Redundancy Model

Packets Sequence Numbers CRC or Checksum

Status Reports ACKs

Timeout

, SACKs

Bitmaps

Packets

Retransmissions

n orma on

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Status Report Design

Cumulativeacks:

Can work with go-back-Nretransmission

lost

because the receiver would generatedu licate acks

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TCP: reliable data transfer (I)event: data received

from application above

create, send segment one way data transfer

waitforwait

event: timer timeout for

segment with seq # y

no flow, congestion

event

event retransmit segment

event: ACK received,with ACK # y

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ACK processing

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00 sendbase = initial_sequence number01 nextseqnum = initial_sequence number02

TCP:

oop orever 04 switch(event)05 event: data received from application above06 create TCP segment with sequence number nextseqnum07 start timer for segment nextseqnum

reliable09 nextseqnum = nextseqnum + length(data)10 event: timer timeout for segment with sequence number y11 retransmit segment with sequence number y12 compute new timeout interval for segment y

transfer (II)

14 event: ACK received, with ACK field value of y15 if (y > sendbase) { /* cumulative ACK of all data up to y */16 cancel all timers for segments with sequence numbers < y

17 sendbase = y

19 else { /* a duplicate ACK for already ACKed segment */20 increment number of duplicate ACKs received for y21 if (number of duplicate ACKS received for y == 3) {22 /* TCP fast retransmit */Simplified

24 restart timer for segment y25 }26 } /* end of loop forever */

TCPsender

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TCP ACK generation

Event TCP Receiver action

in-order segment arrival,no gaps,

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

in-order segment arrival,

no gaps,

immediately send single

cumulative ACKone delayed ACK pending

out-of-order segment arrival- -

send duplicate ACK, indicating seq. #

.

gap detected!

arrival of segment that

immediate ACK if segment starts

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part a y or comp ete y s gap at ower en o gap

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TCP: retransmission scenariosHost A Host B Host A Host B

ut

timeout

out

losstime

XSeq=92

q=100

tim

S

time lost AC scenariotime remature timeout,

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cumulative ACKs

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TCP Flow Control

receiver: explicitlyinforms sender ofsender wont overrun

receivers buffers b

flow control

RcvWindowfield in TCP

transmitting too much,too fast

segmen

sender: keeps theamount of

RcvBuffer = size or TCP Receive Buffer

RcvWindow = amount of spare room in Buffer

transmitted,unACKed data lessthan most recentl

receivedRcvWindow

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receiver buffering

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Timeout and RTT Estimation

Timeout: for robust detection of

Problem: How long should timeoutbe ?

=

Too short => wasteful

Solution: adaptive timeout:based on

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es ma e o max

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How to estimate max RTT?

RTT = prop + queuing delay

So, different samples of RTTs will givedifferent random values of ueuin dela

Cheb shevs Theorem:MaxRTT = Avg RTT + k*Deviation

Error probability is less than 1/(k**2)

Result true for ANY distribution ofsamples

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Round Trip Time and Timeout (II)

Q: how to estimate RTT?

SampleRTT: measured time from segmenttransmission until ACK receipt

ignore retransmissions, cumulatively

ACKed segments amp e w vary w y => wan

estimated RTT smoother

,just current SampleRTT to calculate

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TCP Round Trip Time and

Avera eRTT = 1-x *Avera eRTT + x*Sam leRTT

Exponential weighted moving average (EWMA) influence of iven sam le decreases

exponentially fast; x = 0.1

Setting the timeout

AverageRTT plus safety margin proportional tovariation

Timeout = AverageRTT + 4*Deviation

Deviation = (1-x)*Deviation + x*|SampleRTT- AverageRTT|

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TCP Connection Management - 1

Recall: TCP sender, receiver establishconnection before exchan in data se ments

initialize TCP variables:seq. #s

buffers, flow control info (e.g. RcvWindow) c ent :connect on n t ator

Socket clientSocket = new Socket("hostname","port

" server:contacted by client

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.

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TCP Connection Management - 2

Three way handshake:

Step 1: client end system sends TCP SYNcontrol segment to serverspecifies initial seq #

,replies with SYNACK control segment

allocates buffers

-

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TCP Connection Management - 3

client closes socket: clientSocket.close();

Step 1: client end system sends TCPFIN control segment to server

ep : server receives FIN, replies withACK. Closes connection, sends FIN.

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TCP Connection Management - 4

Fddfdf

close

close

wait

closedtimed

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TCP Connection Management - 5

, .

Enters timed wait - will respond witho rece ve s

closed.

ote: with small modification, can handlesimultaneous FINs.

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TCP Connection Management - 6

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c en ecyc e

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TCP Connection Management - 7

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Recap: Stability of a Multiplexed System

Average Input Rate > Average Output Rate=> system is unstable!

How to ensure stability ?1. Reserve enough capacity so that

eman s ess an reserve capac y2. Dynamically detect overload and adapt

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overload

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Congestion Problem in Packet Switching

A C10 MbsEthernet

statistical multiplexing

B 1.5 Mbs

swaiting for output

link

D E Cost: self-descriptive header per-packet,

buffering and delays for applications.

Need to either reserve resources or

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dynamically detect/adapt to overload for stability

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iThe Congestion Problem

1 ro em: eman ou s r ps ava a e capac y

CapacityDemand

If information about and isn

known in a central location where

i with zero time delays,King Saud University - Dr Ahmad Al-Zubi 28

the congestion problem is solved!

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The Congestion Problem

Problems:

Incomplete information (eg: lossindications)

Distributed solution required

Con estion andcontrol/measurement locationsdifferent

Time-varying, heterogeneous time-delay

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The Congestion Problem

Static fixes may notsolve congestion

a Memor becomes chea infinite memor

No buffer Too late

All links 19.2 kb/s Replace with 1 Mb/s

b)Linksbecome cheap (high speed links)?

S S S S S S S S

=

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The Congestion Problem

(fast routers & switches)

A

SS

C

Scenario: All links 1 Gb/s.

=> high-speed congestion!!(lose more packets faster!)

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Principles of Congestion Control

Congestion: informally: too many sources sending too

much data too fast for networkto handle

erent rom ow contro rece verover oa

manifestations:lost packets (buffer overflow at routers)

long delays (queuing in router buffers)

a top-10 problem!

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Causes/costs of congestion:

two senders, tworece vers

one router,

no

retransmission large delays

when

maximumachievable

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throughput

C / f i

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Causes/costs of congestion:

one router, finitebuffers

sender retransmission of lost packet

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C / t f ti

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Causes/costs of congestion:

Costs of congestion:

Unneeded retransmissions: link carriesmulti le co ies of kt due to s urious

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timeouts

C / t f ti

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Causes/costs of congestion:

Another cost of congestion:

when packet dropped, any upstreamtransmission capacity used for that packet

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A h t d

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Approaches towards

Two broad approaches towardscon estion control:

End-endcongestion control:

no exp c ee ac rom ne wor

congestion inferred from end-system

observed loss, delay a roach taken b TCP

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A h t d

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Approaches towards

Network assistedcongestioncontrol:

routers provide feedback to end

single bit indicating congestion, , ,

explicit rate sender should send at

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TCP congestion control - 1

end-end control no network assistance

transmission rate limited by congestionwindow size, Congwin, over segments:

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TCP congestion control - 2

w segments, each with MSS

w * MSSt roug put =RTT

Bytes/sec

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TCP congestion control - 3

Probing for usable bandwidth:

overrun

avoid/control network overrun

IncreaseCongwin until loss (congestion)

,probing (increasing) again

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Additive Increase/Multiplicative

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Additive Increase/Multiplicative

For stability:

- - - -

Decrease performed enough times as

AIMD policy satisfies this condition,

provided packet loss is congestionindicator

window

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Fairness

Fairness oal: if N TCP sessions

share same bottleneck link, each

TCP connection 1

bottleneckrouter

capacity R

TCPconnection 2

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Fairness Analysis

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AIMD Converges to Fairness

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TCP congestion control - 4

TCP uses AIMD policy in steady state

Transient phase: aka Slow start

Important variables:

threshold: defines threshold between twoslow start hase con estion avoidance

phase

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TCP Slowstart - 1

Slowstart algorithm

n a ze: ongw n =

for (each segment ACKed)Congwin++

until (loss event ORCongWin > threshold)

Exponential increase (per RTT)in windowsize (not so slow!)

Loss event:timeout (Tahoe TCP) and/orthree duplicate ACKs (Reno TCP)

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TCP Slowstart - 2

asdf Host A Host B

RTT

time

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TCP D i

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TCP Dynamics

1st RTT 2nd RTT 3rd RTT 4th RTT

Rate of acks determines rate of .

100 Mbps 10 Mbps

RouterQ

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TCP C i A id 1

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TCP Congestion Avoidance - 1

/* slowstart is over */

Congestion avoidance

/* Congwin > threshold */

Until (loss event) {ever w se ments ACKed:

Congwin++}

=Congwin = 1perform slowstart

1

1: TCP Reno ski s slowstart aka fast recover after

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three duplicate ACKs and performs close to AIMD

TCP C ti A id 2

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TCP Congestion Avoidance - 2

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TCP i d d i ( )

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TCP window dynamics (more)

Receiver Window

Window

(cwnd)

Idle

Interval

ssthresh

1

me un s o s

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TCP l t d li 1

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TCP latency modeling - 1

Q: How long does it take to

server after sending a request?

connec on es a s men

data transfer delay

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TCP latency modeling 2

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TCP latency modeling - 2

Assume one link between client andserver of rate R

Assume: fixed congestion window, Wsegments

S: MSS (bits)

O: object size (bits)

no re ransm ss ons no oss, nocorruption)

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TCP latency modeling 3

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TCP latency modeling - 3

Two cases to consider:

segment in window returns

sent

WS/R < RTT + S/R: wait for ACK

after sending windows worth ofdata sent

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TCP latency modeling 4

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TCP latency modeling - 4

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= +

TCP latency modeling 5

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TCP latency modeling - 5

K = O/WS

Case 2: latency = 2RTT + O/R

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+ - + -

TCP latency modeling:

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slow start. Will show that the latenc of one ob ect of

size O is:

RRRTTP

RRTTLatency )12(2

+++=

w ere s e num er o mes s a s aserver:

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TCP latency modeling:

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}1,{min = KQP

- where Q is the number of times the server

would stall if the ob ect were of infinitesize.

- an s t e num er o w n ows t at coverthe object.

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TCP latency modeling:

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initiate TCPconnection

O/S = 15 segments

requestobject

first window= S/R

K = 4 windows

second window= 2S/R

third window= 4S/R

Q = 2fourth window

= 8S/R

= - , =

Server stalls P=2 times.complete

transmissionobjectdelivered

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time atclient

server

TCP latency modeling:

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S=

until server receives acknowledgement

R

1

= Sk

R

windowthafter thetimestall2 1 kRSRTT

RS k = +

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TCP latency modeling:

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stallTimeRTTOP

2latency ++=

SRTT

SRTT

O kP

p

221

1

+++=

=

SSO

RRR

P

k 1

=

=

RRR

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Sample Results

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Sample Results

R O/R P Minimum Latency:O/R + 2 RTT

Latency with slowstart

28 Kbps 28.6 sec 1 28.8 sec 28.9 sec

100 Kbps 8 sec 2 8.2 sec 8.4 sec

1 Mbps 800msec

5 1 sec 1.5 sec

10 Mb s 80 msec 7 0.28 sec 0.98 sec

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Summary: Chapter 3

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Su a y C apte 3

Principles behind transport layer services:

reliable data transfer

congestion control

ns an a on an mp emen a on n eInternet

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,