8/9/2019 TCP Review

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TCP REVIEW

Slide content courtesy of Erich Nahum (IBM Research)

8/9/2019 TCP Review

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THE TCP PROTOCOL

Connection-oriented, point-to-point protocol: Connection establishment and teardown phases

Phone-like circ!it abstraction "application-la#er $iew%

One sender& one recei$er

Called a 'reliable b#te stream( protocol

)eneral p!rpose "*or an# network en$ironment%

Originally optimized for certain kinds oftransfer:

Telnet "interacti$e remote lo+in%

,TP "lon+& slow trans*ers%

Web is like neither o* these

CPSC 641 IN!ER "#11 "

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TCP PROTOCOL "CO.T%

Provides a reliable, in-order, byte streamabstraction:

Reco$er lost packets and detect/drop d!plicates 0etect and drop corr!pted packets Preser$e order in b#te stream& no 'messa+e bo!ndaries( ,!ll-d!ple12 bi-directional data 3ow in same connection

Flow and congestion control: ,low control2 sender will not o$erwhelm recei$er Con+estion control2 sender will not o$erwhelm the network 4lidin+ window 3ow control 4end and recei$e b!5ers Con+estion control done $ia adapti$e 3ow control window si6e

CPSC 641 IN!ER "#11 $

soc%et

layer

!CP

send &uffer

a''lication

rites data

!CP

receie &uffer

soc%et

layer

a''lication

reads data

data se*ment

+C, se*ment

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THE TCP HE70ER

Fields enable thefollowing:

Uni!ely identifying each"CP connection

"8-t!ple2 client IP and port&

ser$er IP and port%#dentifying a byte range

within that connection

Checks!m val!e to detectcorr!ption

Flags to identify protocolstate transitions $%&',F#', (%")

#nforming other side ofyo!r state $*C+)

CPSC 641 IN!ER "#11 4

source port # dest port #

32 bits

applicationdata

(variable length)

sequence number

acknowledgement numberrcvr window size

ptr urgent datachecksum

FSR

!"headlen

notused

ptions (variable length)

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E4T79LI4HI.) 7 TCP CO..ECTIO.

Client sends %&' withinitial se!encen!mber $#%' )

%erver responds withits own %&' w.sen!mber & and *C+ ofclient #%' with /0$ne1t e1pected byte)

Client *C+s server2s #%'with &/0

"he 34-way handshake5

, & randomly chosen

*ll mod!lo 46-bitarithmetic

CPSC 641 IN!ER "#11 -

client

S.N(/)

server

S.N(.)0

+C,(/01

)

+C,(.01)

connect()listen()port $%

accept()

read()

time

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4E.0I.) 07T7

%ender "CP passes segments to #P totransmit:

:eeps a cop# in b!5er at send side in case o* loss

Called a 'reliable b#te stream( protocol

4ender m!st obe# recei$er ad$ertised window

(eceiver sends acknowledgments $*C+s)

7C:s can be pi++#backed on data +oin+ the other wa# Protocol allows recei$er to 7C: e$er#otherpacket in

attempt to red!ce 7C: tra;c "dela#ed 7C:s%

0ela# sho!ld not be more than "t#picall# ?== ms%

Well see how this ca!ses problems later

CPSC 641 IN!ER "#11 6

soc%et

layer

!CP

send &uffer

a''lication

rites data

!CP

receie &uffer

soc%et

layer

a''lication

reads data

data se*ment

+C, se*ment

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PREVE.TI.) CO.)E4TIO.

%ender may not only overr!n receiver, b!tmay also overr!n intermediate ro!ters:

.o wa# to e1plicitl# know ro!ter b!5er occ!panc#&

so we need to in*erit *rom packet losses 7ss!mption is that losses stem *rom con+estion& namel#&

that intermediate ro!ters ha$e no a$ailable b!5ers%ender maintains a congestionwindow: .e$er ha$e more than CW o* !n-acknowled+ed data

o!tstandin+ "or RWI. data@ min o* the two% 4!ccessi$e 7C:s *rom recei$er ca!se CW to +row>

7ow C8 grows based on which of 6 phases: 4low-start2 initial state> Con+estion a$oidance2 stead#-state> 4witch between the two when CW A slow-start threshold

CPSC 641 IN!ER "#11

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CO.)E4TIO. CO.TROL PRI.CIPLE4

9ack of congestion control wo!ld lead to congestioncollapse $acobson ;;)