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Chapter 4
Transport Layer
CIS 81 Networking Fundamentals
Rick ra!ianiCa"rillo College
gra!iani#ca"rillo$edu
Last %pdated& '('()**+
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Note
This presentation is not in the order o, the "ook or online curriculum$ This presentation also contains in,ormation "eyond the curriculum$
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Transport Layer -.er.iew
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4
Transport Layer
Transport Layer&
Responsi"le ,or creating and maintaining a logical connection
"etween the endpoints
/hat are the two protocols at the transport layer0 TCP Transmission Control 2rotocol
UDP %ser 3atagram 2rotocol
TCP UDP
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Application
Header + data
TCP Header UDP Header
or
23%& Segment
23%& Data
What is the application
PDU called?
What is the transport
PDU called?
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UDP
TCP
TCP/UDP TCP/UDP
The Layer 4 data stream is a& Logical connection "etween the endpoints
2ro.ides transport ser.ices
End-to-end service
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DataHTTPHeader
TCP
Header
P
Header
Data !in"
Header
Data !in"
Trailer
P Pac"etData !in"
Header
Data !in"
TrailerP Pac"etData !in"
Header
Data !in"
Trailer
P Pac"etData !in"Header
Data !in"
TrailerP Pac"etData !in"Header
Data !in"
Trailer
P Pac"etData !in"
Header
Data !in"
TrailerP Pac"etData !in"
Header
Data !in"
Trailer
DataHTTP
Header
TCP
Header
P
Header
Data !in"
Header
Data !in"
Trailer
Reminder o, encapsulation(decapsulation
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8
Focus on Transport Layer
TCP
TCP
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2rimary responsi"ilities&
Tracking the indi.idual communication "etween applications
/ho is the client0 /hich application0 /hich process0
Identi,ying the di,,erent applications 7TT29 FT29 etc$:
Segmenting data
;anaging each segment
Reassem"ling the segments
Transport Layer
www$cisco$com
TCP Segment
TCP Segment
TCP Segment
TCP Segment
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/hat two protocols are at the Transport Layer0 TCP
UDP
Pis a "est
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TC2 .s$ %32
TC2 pro.ides&
Relia"le deli.ery
Brror checking
Flow control
Congestion control
-rdered deli.ery
Connection esta"lishment
pplications&
TT2
FT2
Telnet
;SN messenger
%32 pro.ides&
%nrelia"le deli.ery
No error checking
No ,low control
No congestion control
No ordered deli.ery
No connectionesta"lishment
pplications
3NS 7usually:
S;T2
3C2
RT2 7Real
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1)
single clientmay ha.e multiple transport connections with multipleser.ers$
Notice that TCP is a connection-orientedser.ice 7two
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2ort Num"ers& TC2 and %32
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14
3oth TCP and UDPuse ports 7or sockets: num"ers to pass in,ormation to the
upper layers$
0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
32-bit Sequence Number
32 bit Acknowledgement Number
-bit !eader
"engt#
6-bit
$%eser&ed'
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16-bit ,*P *#ecksum 16-bit (rgent Pointer
tions $i4 an'
Data $i4 an'
TCP Header
HTTP is Port 45
UDP Header
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1
The application this TCP
segment came (rom
The application this TCP
segment is going to
The application this TCP
segment came (rom
The application this TCP
segment is going to
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15
Application
Header + dataPort n%m)ers are %sed )#
the receiver so it "no$s
$hich application it sho%ld
send the &Data' to
Application
Header + dataPort n%m)ers are %sed to
)# the sender to tell the
receiver $hich net$or"
application it sho%ld %se
(or the &Data'
Port 6%m)er
Port 6%m)er
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http&((www$iana$org(assignments(port
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Well 7no$n Ports 6%m)ers 5 to 859:
Reser.ed ,or common ser.ices and
applications
Client& TC2 destination port
Server& TC2 source port
Well 7no$n or ;egistered
Port 6%m)er
Well 7no$n or ;egistered
Port 6%m)er
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Well 7no$n or ;egisteredPort 6%m)er
Well 7no$n or ;egistered
Port 6%m)er
;egistered Ports 6%m)ers 859< to 8
ssigned to user processes orapplications$
Non
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D#namic or Private Ports 6%m)ers 9 to >>:>
lso known as Bphemeral 2orts
%sually assigned dynamically to client applications when initiating a
connection$
Client& TC2 source port
Server& TC2 destination port
;ay also include the range o, Registered 2orts 7Num"ers 1*)4 to
4+11:
Well 7no$n or ;egistered
Port 6%m)er
Private/D#namic Port
6%m)er
Well 7no$n or ;egistered
Port 6%m)er
Private/D#namic Port
6%m)er
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Client Server
Telnet
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))
Client sends TC2 segment with&
3estination 2ort& )'7/ell known port num"er:
Source 2ort& 1*)873ynamic 2ort assigned "y client:
Client TCP Header0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
32-bit Sequence Number
32 bit Acknowledgement Number
-bit !eader"engt#
6-bit$%eser&ed'
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tions $i4 an'
Data $i4 an'
9:8594
Data (or Telnet
Client Server
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)'
Ser.er responds with TC2 segment with&
3estination 2ort& 1*)873ynamic 2ort assigned "y client:
Source 2ort& )'7/ell known port num"er:
Server TCP Header0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
32-bit Sequence Number
32 bit Acknowledgement Number
-bit !eader"engt#
6-bit$%eser&ed'
(%
)
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tions $i4 an'
Data $i4 an'
85949:
Data (or Telnet
Client Server
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Notice the di,,erence in how source and destination port num"ers areused with clients and ser.ers&
Client initiating Telnet service@
3estination 2ort E )' 7telnet: Source 2ort E 1*)8 7dynamically assigned:
Server responding to Telnet service@
3estination 2ort E 1*)8 7source port o, client:
Source 2ort E )' 7telnet:
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Same client to same ser.er < Two di,,erent TT2 sessions
Client& Same destination port
Client& 3i,,erent source ports to uniuely identi,y this we" session$
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C:\Users\rigrazia>netstat -n
Active Connections
Proto Local Address Foreign Address State
TCP 192.16.1.1!1:"9 19.1##.219.2$:! T%&'()A%T
TCP 192.16.1.1!1:"99! 19.1##.219.2$:! T%&'()A%T
C:\Users\rigrazia>
TCP
or
UDP
So%rce Port
Destination P
Destination Port Connection State
So%rce P
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What makes each connection unique? How does the server knowwhich source port 49888 is who?
Connection de,ined "y the pair o, num"ers& So%rce P address9 So%rce port 7From Client to Ser.er: Destination P address9 Destination port7From Ser.er to
Client: 3i,,erent connections can use the same destination port on ser.er
host as long as the source ports or source I2s are di,,erent$
8=9848858
898>>
Destination
Port
45
45
45
So%rce
Port
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6ote@ /hen downloading a we" document and its o"Gects it is common that
there will "e se.eral TC2 sessions created$
netstat Bn$$$ciscocom$$$googlecom
TCP
or
UDP So%rce Port
Destination P
Destination Port
Connection StateSo%rce P
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%sing NetStat
-pen a we" "rowser$
-pen a command prompt window 7Start
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Connectionless Transport& %32
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'1
UDP
/hat do you notice looking at the %32 protocol0
No ,rills9 "are"ones transport protocol$
3estination and Source 2orts
Length and Checksum 7used ,or error checking:
RFC 658
Connectionless transport
No >handshaking? 7no connection esta"lishment: as with TC2 7coming:
%nrelia"le deli.ery
No error checking
No ,low control
No congestion control
No ordered deli.ery
0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
16-bit (DP "engt# 16-bit (DP *#ecksum
Data $i4 an'?
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%32
so%rce port
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%32
Why would an application developer choose UDP rather than TP? 0iner application-la#er control
TC2 will continue to resend segments that are not acknowledged$
pplications that use %32 can tolerate some data loss&
Streaming .ideo
DoI2 7Doice o.er I2:
pplication decides whether or not to resend entire ,ile& TFT2
0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
16-bit (DP "engt# 16-bit (DP *#ecksum
Data $i4 an'
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%32
6o connection esta)lishment
TC2 uses a three
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%32
6o connection state
%32 does not maintain connection state as does TC2 7coming:
%sed ,or relia"ility and ,low control$
Ser.er can support more acti.e clients when not maintaining state
in,ormation
Small pac"et header overhead
TC2 header has )* "ytes o, o.erhead$
%32 header has only 8 "ytes o, o.erhead
0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
16-bit (DP "engt# 16-bit (DP *#ecksum
Data $i4 an'
Time
%32segment
%32segment
%32segment
%32segment
Client Server
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Note on %32
6ote@ 1%ltimedia Applications and UDP
There is an issue 7contro.ersy: with multimedia applications o.er%32$
%32 o,,ers no congestion control 7as we will see with TC2:
Congestion control is needed to pre.ent the network ,rom entering andstaying in a congested state$
I, all applications were using %329 "ecause o, congestion9 .ery ,ew%32 packets would "e deli.ered and this would also cause TC2 tra,,icrates to dramatically decrease$
;any applications gi.e you a choice o, TC2 or %32$
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-nline aming
ame data Ser.er and client need make sure all data 7mo.es9
actions9 etc: reach the other end relia"ly$
Doice chat Some missing data can "e tolerated 7up to a point:$
Retransmission would cause delay$
uestion& 3o the /orld
o, /arcra,t ser.ers
use TC2 or %320
nswer& TC2 ,or game
data9 %32 ,or .oicechat$
/hy0
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%32 Checksum 7FJI:
%32 checksum pro.ides error detection9 any changed "its or missing segments$ Simpli,ied e@planation 7see RFC 1*61 ,or more details:& Sender
%32 adds 15 "it KwordsA keeping a c%m%lative s%m
2er,orms ones complemento, the sum o, all the 15
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%32 Checksum 7FJI:
/hat i, there is an error0 %32 does nothing to reco.er the error$ It is up to the application layer protocol 7e@ample TFT2: to decide what to do9
such as prompt the user to download(upload the entire ,ile again$
0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
16-bit (DP "engt# 16-bit (DP *#ecksum
Data $i4 an'
Time
%32segment
%32segment
%32segment
%32segment
Client Server
Cumulative Sum: 1100101011001010
1s complement: 0011000100110101
Total: 1111101111111111
FinalChecksum
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4*
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Connection
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4)
TCP
TC2 pro.ides relia"le deli.ery on top o, unrelia"le I2
TC2 pro.ides&
Relia"le deli.ery
Brror checking
Flow control
Congestion control
-rdered deli.ery
Connection esta"lishment
0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
32-bit Sequence Number
32 bit Acknowledgement Number
-bit !eader"engt#
6-bit$%eser&ed'
(%
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tions $i4 an'
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0 15 16 31
bi b bi i i b
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4'
TC2
so%rce port
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TCP@ Connection Esta)lishment
0or a connection to )e esta)lished9 the two end stations must synchroni!e
on each others TC2 initial se.%ence n%m)ers S6s
Se.%ence n%m)ers&
Track the order o, packets Bnsure that no packets are lost in transmission$
The initial se.%ence n%m)eris the starting num"er used when a TC2
connection is esta"lished$
B@changing "eginning seuence num"ers during the connection seuence
ensures that lost data can "e reco.ered$
0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
32-bit Sequence Number
32 bit Acknowledgement Number
-bit !eader"engt#
6-bit$%eser&ed'
(
%
)
A
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+
P
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!
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tions $i4 an'
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Three
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45
Three
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46
Three
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48 Step 8@ Client sends S6* SEFG4>: last (o%r digits
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4+ Step 9@ Server responds $ith AC7G4>
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Step :@ Client sends AC7G8=
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Client no$ sends HTTP ;e.%est ET to We) Server
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)
TC2& Connection Termination
1$ /hen the client has no more data to send in the stream9 it sends a segment
with the 06 (lag set$
)$ The ser.er sends an AC7to acknowledge the receipt o, the FIN to terminate
the session ,rom client to ser.er$'$ The ser.er sends a 06to the client9 to terminate the ser.er to client session$
4$ The client responds with an AC7to acknowledge the FIN ,rom the ser.er$
0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
32-bit Sequence Number
32 bit Acknowledgement Number
-bit !eader"engt#
6-bit$%eser&ed'
(
%
)
A
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+
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'
2acket Tracer B@ercise& TC2 Connection and
Termination
3ownload the ,ile& 2T
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4
0lo$ Control and ;elia)ilit#
;elia)ilit#
uaranteed deli.ery
0lo$ Control Flow control makes sure these "u,,ers do not recei.e more data than
the connection can handle$
0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
32-bit Sequence Number
32 bit Acknowledgement Number
-bit !eader"engt#
6-bit$%eser&ed'
(%
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0 15 16 31
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0lo$ Control and ;elia)ilit#
This $indo$ siIespeci,ies the num"er o, "ytes9 starting with the
acknowledgment num"er9 that the recei.ing hosts TC2 layer is currently
prepared to recei.e$
Included in e.ery TC2 segment starting with three
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5
;eceive Windo$ Sending host can send only that amount o, data "e,ore getting an acknowledgment and
window update ,rom this 7the recei.ing: host$
Send Windo$ not a TCP (ield
The TC2 Recei.e /indow si!e o, the other host$
Client Eample
Recei.e /indow Si!eE9*** "ytes Ser.er can only send 9*** "ytes "e,ore it recei.esan acknowledgement$
Send /indow Si!e E 1*9*** "ytes Ser.er told the client that it can send the ser.er1*9*** "ytes "e,ore recei.ing an acknowledgment$
ClientWindo$SiIeG>*555
ServerWin
do$
SiIeG85*55
5Ser.erAs Send
/indow& 1*9***
;y Recei.e
/indow& 1*9***;y Recei.e
/indow& 9***
ClientAs Send
/indow& 9***I can send 10,000
bytes without hearing
an AC, and I can
only receive !,000
bytes at a time."
I can send !,000
bytes without hearing
an AC, and I can
only receive 10,000
bytes at a time."
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Flow Control and Relia"ility
Application Data 855*555 )#tes
8-8555 8558-9555 9558-:555 :558-
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Se.%ence 6%m)er and Ac"no$ledgements Remote host sends TC2 segments with a Se.%ence 6%m)er$
Note& This is the ,irst "yte in the segment$
32-bit Sequence Number
32 bit Acknowledgement Number
-bit !eader"engt#
6-bit$%eser&ed'
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;SS , 1 *** " t
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+
This is known as a
Stop-and-Waitwindowing protocol$
Ser.er must wait ,or
acknowledgment
"e,ore continuing to
send data$
"etter method0
Sliding Windo$s
3JN;IC
Send /indow =yte&
This is the last "yte
that can "e sent
"e,ore recei.ing anCM
Client
SBE19**1 7to )9***:
We)
Server
SBE)9**1 7to '9***:
SBE'9**1 7to 49***:
SBE49**1 7to 9***:
CME9**1
SBE1 7to 19***:
SBE59**1 7to 69***:
SBE69**1 7to 89***:
SBE89**1 7to +9***:
SBE+9**1 7to 1*9***:
CME1*9**1
SBE9**1 7to 59***:
J/
ClientWindo$SiIeG>*555
ClientWindo$SiIeG>*555
ServerWindo
$
SiIeG85*555
ServerWindo$
SiIeG85*555
J
J
Send /indow&=yte 1*9***
Send /indow&
=yte 19***
ServerWindo
$
SiIeG85*555
J
SBE1*9**1 7to 119***:
Send
Windo$G>*555
Clienthas a
/indow Si!e o,
9*** "ytes
ClientWindo$SiIeG>*555
;SS o, 19*** "ytes
S d
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TCP Windo$ SiIe TC2 pro.ides (%ll-
d%ple service9which means data
can "e ,lowing ineach direction9independent o, theother direction$
Recei.er sendsaccepta"lewindow si!e to
sender duringeach segmenttransmission 7,lowcontrol:
I, too much data"eing sent9accepta"le
window si!e isreduced I, more data can
"e handled9accepta"lewindow si!e isincreased
Client
SBE19**1 )9***
We)
Server
SBE)9**1
'9***SBE'9**1
49***SBE49**1
9***
CME9**1
SBE1 19***1
SBE59**1
69***SBE69**1
89***SBE89**1
+9***SBE+9**1 1*9***
CME1*9**1
SBE9**1
59***
J/
ClientWindo$SiIeG>*555
ClientWindo$SiIeG>*555
ServerWindo
$
SiIeG85*555
ServerWindo$
SiIeG85*555
J
J
Send /indow&=yte 1*9***
Send /indow&
=yte 19***
ServerWindo
$
SiIeG85*555
J
SBE1*9**1 119***
Send
Windo$G>*555
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Sliding Windo$ Protocol
Sliding window algorithms are a method o, ,low control ,or network data trans,ers usingthe recei.ers /indow si!e$
The sender computes its usa"le window9 which is how much data it can immediatelysend$
-.er time9 this sliding window mo.es to the rights9 as the recei.er acknowledges data$
The recei.er sends acknowledgements as its TC2 recei.e "u,,er empties$
The terms used to descri"e the mo.ement o, the le,t and right edges o, this slidingwindow are&
1$ The le,t edge closes 7mo.es to the right: when data is sent and acknowledged$
)$ The right edge opens 7mo.es to the right: allowing more data to "e sent$ This happenswhen the recei.er acknowledges a certain num"er o, "ytes recei.ed$
'$ The middle edge open 7mo.es to the right: as data is sent9 "ut not yet acknowledged$
Kctets sent6ot AC7ed
Usa)le Windo$Can send ASAP
Wor"ing Windo$ siIe
Usa)le Windo$Can send ASAP
nitial Windo$ siIe
Sliding /indows 73ynamic or Scaling:
0 15 16 31
16-bit Source Port Number 16-bit Destination Port Number
32-bit Sequence Number
32 bit Acknowledgement Number
TCP Header
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5)
8 9 : < > 4 = 85 88 89 8:
9
:
ost = gi.es ost a window si!e o, 5 7octets:$ ost "egins "y sending octets to ost =& octets 19 )9 and ' and slides itAs
window o.er showing it has sent those ' octets$
ost will not increase its usa"le window si!e "y '9 until it recei.es an
CMnowldegement ,rom ost = that it has recei.ed some or all o, the octets$
ost =9 not waiting ,or all o, the 5 octets to arri.e9 a,ter recei.ing the third octet
sends an e@pectational CMnowledgement o, >4? to ost $
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8 9 : < > 4 = 85 88 89 8:
8 9 : < > 4 = 85 88 89 8:
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