Chap 12 Modified PP2003

7/25/2019 Chap 12 Modified PP2003

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TCP/IP Protocol Suite 1

Chapter 12Chapter 12

Upon completion you will be able to:

TransmissionTransmissionControl ProtocolControl Protocol

Be able to name and understand the services offered by TCP

Understand TCPs flow and error control and congestion control

Be familiar with the fields in a TCP segmentUnderstand the phases in a connection-oriented connection

Understand the TCP transition state diagram

Be able to name and understand the timers used in TCP

Be familiar with the TCP options

Objectives


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Figure 12.1 TCP/P protocol suite

TCP provides a set of services. What are those services


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Table !"#!Table !"#! $ell-%nown ports used by TCP$ell-%nown ports used by TCP

TCP provides a process-to-process communication serviceusing port numers.


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Figure 12.2 &tream delivery


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Figure 12.4 TCP segments

TCP provides a stream deliver! service. It rea"s up the dstream into segments of variale si#e. $ach segment receia header and is handed o% to the IP la!er.

&& 'ecause the sending and the receiving processesma! not necessaril! (rite or read dataat the same rate) TCP needs u%ers for storage.


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Figure 12.4 TCP services and features

TCP can create a full-duple* service. +ata can ,o( inoth

directions at the same time u%ers on each side holdthedata to e transmitted and sent.

TCP provides a connection-oriented service the t(oTCPs

estalish a connection) data is e*changed) and theconnection isterminated.

TCP provides a reliale service.

urthermore) TCP has a numer of features.0ll !tes transferred are numered ! TCP. Thenumeringstarts (ith a random value -32452 -16.

&there are t(o 7elds called the sequence numberandthe


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&uppose a TCP connection is transferring a file of '((( bytes#The first byte is numbered !(((!# $hat are the se)uence

numbers for each segment if data is sent in five segments* each

carrying !((( bytes+

Example2

&olutionThe following shows the se)uence number for each segment:

&egment !&e)uence ,umber: !(*((! range: !(*((! to !!*(((.

&egment "&e)uence ,umber: !!*((! range: !!*((! to !"*(((.

&egment &e)uence ,umber: !"*((! range: !"*((! to !*(((.

&egment 0&e)uence ,umber: !*((! range: !*((! to !0*(((.

&egment '&e)uence ,umber: !0*((! range: !0*((! to !'*(((.


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The value in the se)uence numberfield of a

segment defines the number of the first data

byte contained in that segment#

The value of the ac%nowledgmentfield in asegment defines the number of the ne1t byte a

party e1pects to receive#

The ac%nowledgment numberis cumulative#


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Figure 12.4 TCP services and features

TCP also provides1.,o( control)2.error control) and5.Congestion control.We (ill e*amine each of these shortl!.

'efore (e do) let8s e*amine the TCP header a little moreclosel!.

The TCP pac"et is called a segment ut most people 9ustcall it a pac"et6.


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Figure 12.5 TCP segment format pac%ets in TCP are called segments.

Windo( si#e set ! receiver (ith ma* : ;


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Figure 12.6 Control field

=ore on these its later.


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Figure 12.7 Pseudoheader added to the TCP datagram to calculate

chec%sum


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The inclusion of the chec%sum in TCP

is mandatory#

Note:Note:


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Figure 12.8 2ncapsulation and decapsulation


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12.4 A TCP CONNECTION

TCP is connection-oriented#TCP is connection-oriented# 3 connection-oriented transport protocol3 connection-oriented transport protocolestablishes a virtual path between the source and destinationestablishes a virtual path between the source and destination# 3ll of the# 3ll of the

segments belonging to a message are then sent over this virtual path#segments belonging to a message are then sent over this virtual path# 33

connection-oriented transmission re)uires three phases:connection-oriented transmission re)uires three phases:

!#!#connection establishment*connection establishment*

"#"#data transfer*data transfer*##connection termination#connection termination#

The topics discussed in this section include:The topics discussed in this section include:

Connection 2stablishmentConnection 2stablishment

4ata Transfer4ata Transfer

Connection TerminationConnection Termination

Connection 5esetConnection 5eset


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Figure 12.9 Connection establishment using three-way handshaking

0 server tellsits TCP that itis read! to ma"ea connection -this is called a

passive open.

>

r(nd is thereceiver (indo(si#e) as (e (illsee later.

ote S@ itis set in 7rstpac"et Achosen randoml!

Should this e A1?

o ) ecause it is S@ segmentCannot carr in data


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3 &6, segment cannot carry data* but it

consumes one se)uence number#

3 &6, 7 3C8 segment cannot carry

data* but does consume one

se)uence number#

3n 3C8 segment* if carrying no data*

consumes no se)uence number#


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SYN fooding attack

Bad person -> arge n!"#er o$ SYNseg"ents %&it' $ake () addresses* to aser+er ->ser+er aocates t'e necessar,reso!rces %eg ./B ta#es and setting

ti"ers* -> ser+er sends SYN/seg"ents to $ake cients &'ic' are ost ($ t'e n!"#er o$ SYN seg"ents is arge

t'e ser+er e+ent!a, r!ns o!t o$reso!rces and "a, cras'

.o co!nter t'ese attacks so"e ser+erspostpone reso!rce aocation !nti t'eentire connection is set !p !sing acookie



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Figure 12.1 4ata transfer

otice ho( the 0CBandS$ D are

pigg!ac"ed.

Push ,ag meansdeliverthe data to thereceiveras soon as it is

receiveddon8t put it in au%erand hold until !ouhaveenough !tes for a

complete segment6.This feature isusuall!ignored.


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/an aso send rgent data #, setting t'e #it .'e sending ./) creates aseg"ent and inserts t'e !rgent data att'e #eginning o$ t'e seg"ent %t'eseg"ent can aso contain nor"a data*or ea"pe ,o! &ant to send a /tr-/to stop (n t'is case rgent )ointer ast !rgent #,te 1 :'en t'erecei+ing ./) recei+es a seg"ent &it't'e #it set it etracts t'e !rgentdata $ro" t'e seg"ent !sing t'e +a!eo$ pointer and dei+ers it o!t o$order to t'e recei+ing appicationprogra"



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Figure 12.11 Connection termination using three-way handsha%ing

0 I segment consumes one seEuence numer ifit does not carr! data. So should third segment eseE *>1?& a I segment can include the last chun" of data

sent ! the client


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Figure 12.12 9alf-close

Client is 7nished)ut Server is not !et7nished. So Server0CBs the Client8s I)ut does not signal itso(n I 9ust !et.

!-1

*>1

*>1

#>1


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Connection 5eset

Fsing the Geset ,ag GST6) one can

;+en! a reEuest for a connection

;0ort a current connection

;Terminate an idle connection


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12.5 STATE TRANSITION DIAGRAM

To %eep trac% of all the different events happeningTo %eep trac% of all the different events happening during connectionduring connectionestablishment* connection termination* and data transfer* the TCPestablishment* connection termination* and data transfer* the TCP

software is implemented as asoftware is implemented as afinite state machine#finite state machine###


&cenarios&cenarios


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Table !"#Table !"# &tates for TCP&tates for TCP


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Figure 12.1! &tate transition diagram


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Client states

Server states

Fi 12 14 C i bli h d i i


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Figure 12.14 Connection establishment and termination


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The common value for &; isbetween ( seconds and ! minute#

=a*imum segment lifetime

=SH6.

Note:Note:


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Figure 12.15 "#nnecti#n ter$inati#n usingthree-way handsha%e

Fi 12 16 &i lt


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Figure 12.16 &imultaneous open

Fi 12 17 &i lt l


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Figure 12.17 &imultaneous close

Figure 12 18 4 i ti


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Figure 12.18 4enying a connection

Figure 12 19 3borting a connection


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Figure 12.19 3borting a connection


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12.6 FLOW CONTROL


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Figure 12.2 &liding window

r(nd is the receiver (indo( si#e c(nd is the congestion(indo( si#e


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3 sliding window is used to ma%e

transmission more efficient as well as

to control the flow of data so that thedestination does not become

overwhelmed with data#

TCPs sliding windows are byte

oriented#

Note:Note:


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$hat is the value of the receiver window rwnd. for host 3 ifthe receiver* host B* has a buffer si=e of '*((( bytes and !*(((

bytes of received and unprocessed data+

Example3

&olution

The value of rwnd > '*((( ? !*((( > 0*(((# 9ost B can receive

only 0*((( bytes of data before overflowing its buffer# 9ost Badvertises this value in its ne1t segment to 3#


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$hat is the si=e of the window for host 3 if the value of rwnd is*((( bytes and the value of cwnd is *'(( bytes+

Example4

&olution

The si=e of the window is the smaller of rwnd and cwnd* which

is *((( bytes#


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Figure 12.22 21ample


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Figure 12.22 21ample

,e1t* the sender receives a pac%et with an ac%nowledgment

value of "( and an rwnd of !"# The host has not sent any new

bytes# The value of cwnd is still "(# &how the new window#

Figure 12.2! 21ample


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Figure 12.2! 21ample

3ssume the sender has sent bytes "( to "(@# The senders

window shrin%s accordingly#

,ow the sender receives a pac%et with an ac%nowledgmentvalue of "!( and an rwnd of '# The value of cwnd is still "(#

&how the new window#

Figure 12.24 21ample A


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Figure 12.24 21ample A


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9ow can the receiver avoid shrin%ing the window in theprevious e1ample+

Example9

&olution

The receiver needs to %eep trac% of the last ac%nowledgment

number and the last rwnd# f we add the ac%nowledgmentnumber to rwnd we get the byte number following the right

wall# f we want to prevent the right wall from moving to the

left shrin%ing.* we must always have the following

relationship#new ac% 7 new rwnd D last ac% 7 last rwnd

or

new rwnd D last ac% 7 last rwnd. ? new ac%


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To avoid shrinking the sender window, the receivermust wait until more space is available in its bufer.

:indo& s'!tdo&n .'e recei+er can te"porari, s'!t do&n t'e

&indo& #, sending an rwndo$ 0 i$ t'erecei+er does not &ant to recei+e an, data$ro" t'e sender $or a &'ie

.'e sender does not act!a, s'rink t'esi


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&ome points about TCPs sliding windows:

The si=e of the window is the lesser of rwndand cwnd#

The source does not have to send a full windows

worth of data#The window can be opened or closed by the receiver*

but should not be shrun%#

The destination can send an ac%nowledgment at any

time as long as it does not result in a shrin%ing window#

The receiver can temporarily shut down the windowE

the sender* however* can always send a segment of one

byte after the window is shut down#

Note:Note:

Figure 12.24 &illy $indow &yndrome


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g y y

Sill! Windo( S!ndrome Slo( sender or receiver6- What if TCsends segments that are onl! 1 !te long? @ou (ould have !tes of header) 1 !te of data) for a total of 1 !tes.Jer! (astefulK

TCP should (ait until it has more data efore it sends a 1-!t

segment. 'ut ho( long should it (ait to assemle data?

agle8s 0lgorithm1. The sending TCP sends the 7rst piece of data it receivesfrom the sending application even if it is onl! 1 !te.2. 0fter sending the 7rst segment) the sending TCP accumul

data in the output u%er and (aits until eitherthe receivingTCP sends 0CB or until enough data has accumulated to 7lla ma*imum-si#e segment. 0t this time) the sending TCP cansend the segment.5. Step 2 is repeated for the rest of the transmission.

Figure 12.24 &illy $indow &yndrome


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g y y

Sill! Windo( S!ndrome - What happens if the receiving TCPhas a u%er si#e of 1 !tes and the sending TCP has 9ustsent 1 !tes. The receiving u%er is no( full so the receitells the sender to stop (indo( si#e : 6.

The receiver no( reads 1 !te of data) processes it) and sen

a (indo( si#e of 1 ecause no( there is one space in theinput u%er6. The sender gets the (indo( si#e and sends 1!te. This procedure continues.

Clar"8s Solution - 0c"no(ledge receipt right a(a!) ut don8tchange the (indo( si#e until !ou have at least half the u%e

space availale.

Lr) dela! the ac" until there is a decent amount of u%er spaavailale. 0dv -3 reduces traMc. +isadv -3 the sender8s timeend efore it receives an 0CB.


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12.7 ERROR CONTROL

TCP provides reliability using error controlTCP provides reliability using error control* which detects* which detects corruptedcorrupted**lostlost** out-of-orderout-of-order* and* and duplicated segmentsduplicated segments##

2rror control in TCP is achieved through the use of :2rror control in TCP is achieved through the use of :

the chec%sumthe chec%sum** ac%nowledgmentac%nowledgment** and time-out#and time-out#


Chec%sumChec%sum

3c%nowledgment3c%nowledgment

3c%nowledgment Type3c%nowledgment Type

5etransmission5etransmission

Fut-of-Frder &egmentsFut-of-Frder &egments&ome &cenarios&ome &cenarios

TCP 2rror Control


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TCP supports asic error control.

It uses a 1;-it arithmetic chec"sum) similar to the ones(e have alread! seen.

TCP uses the 0CB message to con7rm receipt of segments.

There is a numer of asic rules pertaining to 0CBs

Gule 1 When one ends sends data) it must pigg!ac" the0CB for an! data received. $*ample in 9ust a moment6

Gule 2 If a receiver has no data to send and a segmentarrives) do not 0CB it immediatel!. Wait until t(o segmentsarrive) then 0CB. Lr (ait


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TCP 2rror Control


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Gule 5 When a segment arrives (ith an e*pected seEuence

numer and the previous in-order segment has not een0CBed) the receiver immediatel! sends an 0CB. $*ampleon previous slide6

Gule When a segment arrives (ith a seEuence numerhigher than e*pected) the receiver immediatel! sends an

0CB announcing the seEuence numer it e*pected. Thisleads to fast retransmission) (hich (e (ill see shortl!.

Gule


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TCP 2rror Control


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urthermore) a retransmission (ill occur if the retransmissio

timer GTL6 e*pires) or three duplicate 0CBs arrive in order.

or GTL e*ample) see previous slide.6

or three 0CBs) see ne*t slide.6

Figure 12.27 Three 3C8s in a row* fast retransmission


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Figure 12.28 ;ost ac%nowledgment


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Figure 12.29 ;ost ac%nowledgment corrected by resending a segment


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Delayed segment ea,ed ./) seg"ent are treated t'e

sa"e &a, as ost or corr!pted seg"ents#, t'e recei+er

.'e dea,ed seg"ent "a, arri+e a$ter it'as #een resent %a d!picate seg"ent*

Duplicate segment :'en a seg"ent arri+es t'at contains a

se?!ence n!"#er ess t'an t'epre+io!s, ackno&edged #,tes it isdiscarded



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3C8 segments do not consume

se)uence numbers and are not

ac%nowledged#

Note:Note:


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n modern implementations* a

retransmission occurs if theretransmission timer e1pires or three

duplicate 3C8 segments have arrived#

Note:Note:


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,o retransmission timer is set for an

3C8 segment#

Note:Note:


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4ata may arrive out of order and be

temporarily stored by the receiving TCP*but TCP guarantees that no out-of-order

segment is delivered to the process#

Note:Note:


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The receiver TCP delivers only ordereddata to the process#

Note:Note:


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;ost ac%nowledgments may create

deadloc% if they are not properlyhandled#

Note:Note:

d k t d # t


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eadock created #, ost/ ecei+er -> r&nd 0 -> / %i$ it

'as no data to send*

/ -> ost %pro#e"*

eadock -> eac' end is &aiting $or aresponse $ro" t'e ot'er end andnot'ing is 'appening

So!tion -> )ersistence ti"er is !sed%disc!ssed ater*


12 8 CONGESTION CONTROL


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12.8 CONGESTION CONTROL

Congestion control refers to the mechanisms and techni)uesCongestion control refers to the mechanisms and techni)ues

to %eep theto %eep the

load below the capacity#load below the capacity#

Congestion in a net(or" ma! occur if the load on thenet(or" numer of pac"ets sent to the net(or"6 isgreater than the capacit! of the net(or" the

numer of pac"ets a net(or" can handle6.

Congestion in a net(or" or internet(or" occursecause routers and s(itches have Eueues

Figure 12.! 5outer )ueues


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Figure 12.!1 Pac%et delay and networ% load


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Propagation dela! >processingdela!

Figure 12.!2 Throughput versus networ% load


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The reason is the

discarding of pac"ets !the routers


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/ongestion contro "ec'anis"s re$ers to tec'ni?!es and "ec'anis"s

t'at can eit'er pre+ent congestion#e$ore it 'appens or re"o+econgestion a$ter it 'as 'appened open-oop congestion contro %pre+ention*

and cosed oop congestion contro %re"o+a*


@ ti t


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@pen-oop congestion contro etrans"ission poic,

.'e retrans"ission poic, and t'e retrans"ission ti"ers "!st#e designed to opti"i


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/osed-oop congestion contro Back press!re

in$or"ing t'e pre+io!s !pstrea" ro!ter to red!ce t'e

rate o$ o!tgoing packets /'oke point

is a packet sent #, a ro!ter to t'e so!rce to in$or" it o$congestion

is si"iar to (/)Cs so!rce ?!enc' packet ("picit signaing

etecting an i"picit signa &arning o$ congestion and

so& do&n its sending rate =* recei+ing dea,ed /

=picit signaing o!ter eperiencing congestion can send an epicit

signa #, setting a #it in a packet to t'e sender or t'erecei+er

TCP Congestion Control


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0ctual (indo( si#e : minimum r(nd) c(nd6

TCP8s general polic! for handling congestion is asedonthree phases slo( start) congestion avoidanceandcongestion detection.

Slo( start algorithm - c(nd starts at 1. With each 0CBreceived) cwnddoules. So second time cwnd: 2) then) then A) etc.

The cwnd value does not increase forever. cwndstops

(hen it eEuals ssthresh (slow start threshold)) (hichhas a ma* si#e of ;


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Figure 12.!! &low start* e1ponential increase


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Figure 12.!4 Congestion avoidance* additive increase

TCP Congestion Control* Congestion detection


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What happens if congestion occurs?

1. If an GTL timer times out) there is proal! congestion.So cut the threshold in half)set c(nd ac" to 1)andrestart the slo( start phase again.

2.If three 0CBs in a ro( are received) there ma! econgestion. Socut the threshold in half)set c(nd to the value of the threshold) andrestart the avoidance phase.

Figure 12.!5 TCP congestion policy summary


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12 9 TCP TIMERS


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12.9 TCP TIMERS

To perform its operation smoothly* most TCP implementations use atTo perform its operation smoothly* most TCP implementations use at

least four timers:least four timers:

5etransmission Timer5etransmission Timer

Persistence TimerPersistence Timer

8eepalive Timer8eepalive TimerT2-$3T TimerT2-$3T Timer

5etransmission Timer


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To retransmit a lost segment) TCP emplo!s a retransmissiontimer that handles the retransmission time-out GTL6) the(aiting time for an 0CB of a segment.

If an 0CB is received efore the timer goes o%) toss the timer.

If timer goes o% efore 0CB is received) segment isretransmitted and timer is reset.

To calculate the GTL value) (e8ll need a couple other values.



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The 7rst value (e need to "no( is the roundtrip time GTT6 of sending a segment and then gettingthe 0CB.

To calculate GTT) (e could use the GTT =easured valuesimpl! time 0CB received minus time pac"et sent6)

ut this value varies greatl! on toda!8s Internet.

So instead) (e (ill calculate GTT Smoothed.

irst time GTT Smoothed : GTT =easured

0fter thatGTT Smoothed : 1-a6 * GTT Smoothed > a* GTT =easure

(here anormall! is set to 1/A.



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=ost implementations also use the GTT +eviation.

irst time GTT +eviation : GTT =easured / 2

0fter that

GTT +eviation : 1-b

6 * GTT +eviation >b* NSmoothed GTT - =easured GTTN

Where busuall! eEuals 1/.

inall!) GTL : GTT Smoothed > * GTT +eviation

Example


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;et us give a hypothetical e1ample#


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Example! (cont"nued)

5TT> !#' 5TT &> !#'5TT4> !#' / " > (#' 5TF > !#' 7 0 1 (#' > 0#'

#$hen the first data segment is sent* a new 5TT measurement

starts# ,ote that the sender does not start an 5TT measurement

when it sends the 3C8 segment* because it does not consume ase)uence number and there is no time-out# ,o 5TT

measurement starts for the second data segment because a

measurement is already in progress#

5TT> "#'

5TT&> /A !#'. 7 !/A "#'. > !#"'

5TT4> /0 (#'.7 !/0 G!#"' ? "#'G > (#A T6PF 9252 , BFF8HH

5TF > !#"' 7 0 (#A. > 0#0

Figure 12.!8 21ample !(


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8arns 3lgorithm


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When the sending TCP receives an 0CB for a segment) is thithe 0CB for the original segment) or for the retransmittedsegment?

+epending upon (hich one !ou choose can a%ect thecalculation of !our GTL timer.

So Barn sa!s do not consider the round trip time of aretransmitted segment in the calculation of the ne( GTT.

+o not update the value of GTT until !ou send a segmentand receive an 0CB (ithout the need for retransmission.

21ponential Bac%off


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What is the value of GTL if a retransmission occurs?

=ost TCP implementations use an e*ponential ac"o%strateg!.

The value of GTL is douled for each retransmission.

See the ne*t slide for an e*ample of this.

Example


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Persistence Timer


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What if a receiver sends !ou a (indo( si#e of ?@ou stop transmitting until !ou receive an 0CB (ith a ne( (indo( si

What happens if this 0CB is lost? +eadloc" occurs.

When the sending TCP receives an ac"no(ledgment (itha (indo( si#e of #ero) the persistence timer is started.

When persistence timer goes o%) the sending TCP sends

a special segment called aprobe.

The proe alerts the receiving TCP that the ac"no(ledgment(as lost and should e resent.

If a response is not received) the sender continues sending theproe segments and douling) and resetting the value of thepersistence timer until the value reaches a threshold usuall! ; sec

0fter that sender sends one proe segment ever! ;s until the (indis reopened.

The Fther Timers


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Beepalive Timer

If t(o sides transfer data and then go silent) is the connectistill valid? Lr did it die someho(?

$ach time a server hears something from the other side) setthe Beepalive Timer to sa!) 2 hours. If the server doesn8thear an!thing (ithin 2 hours) it sends a proe.

o response after 1 proes each of (hich is O


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12.10 OPTIONS

The TCP header can have up to 0( bytes of optional information#The TCP header can have up to 0( bytes of optional information#

Fptions convey additional information to the destination or align otherFptions convey additional information to the destination or align other

options#options#

Figure 12.4 Fptions


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Figure 12.4! a1imum-segment-si=e option


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This option set during connection estalishment6 de7nes

the ma*imum si#e of the data 7eld (ithin a segment.=a* si#e is ;


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In case a (indo( si#e of 1; its ;


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Can use this option to calculate round trip time GTT.

Example


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The receivers function is more involved# t %eeps trac% of the

last ac%nowledgment sent !"(((.# $hen the first segmentarrives* it contains the bytes !"((( to !"(@@# The first byte is

the same as the value of lastac%# t then copies the timestamp

value 0"(. into the tsrecent variable# The value of lastac% is

still !"((( no new ac%nowledgment has been sent.# $hen thesecond segment arrives* since none of the byte numbers in this

segment include the value of lastac%* the value of the

timestamp field is ignored# $hen the receiver decides to send

an accumulative ac%nowledgment with ac%nowledgment

!""((* it changes the value of lastac% to !""(( and inserts the

value of tsrecent in the echo reply field# The value of tsrecent

will not change until it isreplaced by a new segment that

carries byte !""(( ne1t segment.#

p

Example2 (#ont"nued)


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,ote that as the e1ample shows* the 5TT calculated is the time

difference between sending the first segment and receiving thethird segment# This is actually the meaning of 5TT: the time

difference between a pac%et sent and the ac%nowledgment

received# The third segment carries the ac%nowledgment for

the first and second segments#

p

Figure 12.46 21ample !"


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The time-stamp option can also e usedfor P0WS protection against (rapped

seEuence numers6

S/-per"itted


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S/ per"itted

sed on, d!ring connectionesta#is'"ent

.'e 'ost t'at sends t'e SYN seg"ent

adds t'is option to s'o& t'at it cans!pport t'e S/ option

($ t'e ot'er end in its SYN /

seg"ent aso inc!des t'is optiont'en t'e t&o ends can !se t'e S/option d!ring data trans$er


S/


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S/

(s !sed d!ring data trans$er on, i$ #ot'ends agree .'e option inc!des a ist $or #ocks

arri+ing o!t-o$-order

=ac' #ock occ!pies t&o 32 #it n!"#erst'at deDne t'e #eginning and t'e endo$ t'e #ocks

S/ option cannot deDne "ore t'an

4 #ocks .'e Drst #ock can #e !sed to report t'ed!picates .'is is !sed on, i$ t'ei"pe"entation ao&s t'is $eat!re


Figure 12.47 &3C8 &elective 3c%nowledgment.


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Example


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;et us see how the &3C8 option is used to list out-of-order bloc%s# n


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Example


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The e1ample in


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Example


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The e1ample in


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Documents

Chap 12 Modified PP2003