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1

The TCP Protocol

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

– ‘Phone-like’ circuit abstraction (application-layer iew!

– "ne sender, one receier

– Called a #reliable byte stream$ protocol– %eneral purpose (&or any network enironment!

• "ri'inally optimied &or certain kinds o& trans&er:– )elnet (interactie remote lo'in!

– *)P (lon', slow trans&ers!

– +eb is like neither o& these

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TCP Protocol (cont)

• Proides a reliable, in-order, byte stream abstraction:– .ecoer lost packets and detect/drop duplicates

– 0etect and drop corrupted packets– Presere order in byte stream, no #messa'e boundaries$– *ull-duple: bi-directional data &low in same connection

• *low and con'estion control:

– *low control: sender will not oerwhelm receier– Con'estion control: sender will not oerwhelm the network– 2lidin' window &low control– 2end and receie bu&&ers– Con'estion control done ia adaptie &low control window sie

socket

layer TCP

send buffer 

application

writes data

TCP

receive buffer 

socket

layer 

application

reads data

data segment

 ACK segment

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3

The TCP Header

*ields enable the &ollowin':

• 4ni5uely identi&yin' aconnection(6-tuple o& client/serer 7P

address and portnumbers!

• 7denti&yin' a byte ran'ewithin that connection

• Checksum alue to detectcorruption

• *la's to identi&y protocol

state transitions (289,*79, .2)!

• 7n&ormin' other side o& your state (C;!

source port < dest port <

3 bits

application

data(ariable len'th!

se5uence number

acknowled'ement number

rcr window sie

ptr ur'ent datachecksum

*2.P4headlen

notused

"ptions (ariable len'th!

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6

Establishing a TCP Connection

• Client sends 289 withinitial se5uence number(729 = >!

• 2erer responds with itsown 289 w/se5 number

8 and C; o& client 729with >?1 (net epectedbyte!

• Client C;s serer@s729 with 8?1

• )he ‘3-way handshake’• >, 8 randomly chosen

• ll modulo 3-bitarithmetic

client

S Y N  (  ! 

serer

 S Y N  (  Y !  "

  A C K  (   " #

 !

 AC K  ( Y " #! 

connect(!listen(!port AB

accept(!

read(!

time

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Sending Data

• 2ender )CP passes se'ments to 7P to transmit:

– ;eeps a copy in bu&&er at send side in case o& loss– Called a #reliable byte stream$ protocol

– 2ender must obey receier adertised window

• .eceier sends acknowled'ments (C;s!

– C;s can be pi''ybacked on data 'oin' the other way– Protocol allows receier to C; eery other packet in

attempt to reduce C; tra&&ic (delayed C;s!

– 0elay should not be more than BB msD (typically BB ms!

– +e’ll see how this causes problems later

socket

layer TCP

send buffer 

application

writes data

TCP

receive buffer 

socket

layer 

application

reads data

data segment

 ACK segment

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E

Preventing Congestion

• 2ender may not only oerrun receier, but may also oerrunintermediate routers:– 9o way to eplicitly know router bu&&er occupancy,

so we need to in&er it &rom packet losses– ssumption is that losses stem &rom con'estion, namely, that

intermediate routers hae no aailable bu&&ers

• 2ender maintains a con'estion window:– 9eer hae more than C+ o& un-acknowled'ed data outstandin' (or

.+79 dataF min o& the two!– 2uccessie C;s &rom receier cause C+ to 'rowD

• Gow C+ 'rows based on which o& phases:– 2low-start: initial stateD– Con'estion aoidance: steady-stateD– 2witch between the two when C+ H slow-start threshold

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Congestion Control Principles

• Jack o& con'estion control would lead to con'estioncollapse (Kacobson AA!D

• 7dea is to be a #'ood network citien$D• +ould like to transmit as &ast as possible without lossD• Probe network to &ind aailable bandwidthD

• 7n steady-state: linear increase in C+ per .))D• &ter loss eent: C+ is haledD• )his is called additie increase /multiplicatie decrease

(7L0!D• Marious papers on why 7L0 leads to network stabilityD

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Slow Start

• 7nitial C+ = 1D• &ter each C;, C+ ?= 1F• Continue until:

– Joss occurs ".– C+ H slow start threshold

• )hen switch to con'estionaoidance

• 7& we detect loss, cut C+in hal&

• Nponential increase inwindow sie per .)) 

sender

o ne  se gme nt 

   .   )   )

receier

time

t w o  se gme nt s

f o ur  se gme nt s

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Congestion Avoidance

4ntil (loss!  a&ter C+ packets C;ed:  C+ ?= 1FQssthresh = C+/F

0ependin' on loss type:  2C;/*ast .etransmit:

C+/= F continueF  Course 'rained timeout:  C+ = 1F 'o to slow startD

()his is &or )CP .eno/2C;: )CP)ahoe always sets C+=1 a&ter a loss!

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How are losses recovered?

2ay packet is lost (data or C;!• Coarse-'rained )imeout:– 2ender does not receie C; a&ter

some period o& time– Nent is called a retransmission time-

out (.)"!

– .)" alue is based on estimatedround-trip time (.))!

– .)) is adRusted oer time usin'eponential wei'hted moin' aera'e:.)) = (1-!S.)) ? (!Ssample( is typically BD1!

*irst done in )CP )ahoe

S e $ % & '  )  b y t e s  d a t a 

 A C K %

 # * *

loss   t   i  m  e  o  u   t

lost C; scenario

S e $ % & '  )  b y t e s  d a t a 

 A C K % # * *

sender receier

time

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Fast Retransit

• .eceier epects 9, 'ets 9?1:– 7mmediately sends C;(9!– )his is called a duplicate C;– 0oes 9") delay C;s here– Continue sendin' dup C;s &or

each subse5uent packet (not 9!• 2ender 'ets 3 duplicate C;s:

– 7n&ers 9 is lost and resends– 3 chosen so out-o&-order packets

don’t tri''er *ast .etransmit

accidentally– Called #&ast$ since we don’t need towait &or a &ull .)) 

sender receier

time

S+ , %-* * * ( si . e %#* * * 

 AC K  - * * *

S+ , %/* * * S+ , %0 * * * 

S+ , %1 * * * 

 AC K  - * * *

 AC K  - * * *

 AC K  - * * *

S+ , %-* * * ( si . e %#* * * 

7ntroduced in )CP .eno

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!ther loss recover" ethods

• 2electie cknowled'ements (2C;!:– .eturned C;s contain option w/2C; block– Tlock says, U'ot up 9-1 90 'ot 9?1 throu'h 9?3U– sin'le C; can 'enerate a retransmission

• 9ew .eno partial C;s:– 9ew C; durin' &ast retransmit may not C; alloutstandin' dataD N:

• Gae C; o& 1, waitin' &or -E, 'et 3 dup acks o& 1• .etransmit , 'et C; o& 3, can now in&er 6 lost as well

• "ther schemes eist (eD'D, Me'as!• .eno has been prealentF 2C; now catchin' on

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How abo#t Connection Teardown?

• Nither side may terminate a

connectionD ( 7n &act,connection can stay hal&-closedD! Jet@s say theserer closes (typical in+++!

• 2erer sends *79 with se59umber (29?1! (iDeD, *79 isa byte in se5uence!

• Client C;@s the *79 with29? (Unet epectedU!

• Client sends it@s own *79when ready

• 2erer C;@s client *79 aswell with 29?1D

client

2 3 N ( Y ! 

serer

 A C K (  Y " #

 !

 AC K (  " #! 

 2 3 N (   !

close(!

close(!

closed   t   i  m  e   d  w

  a   i   t

time

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The TCP State $achine

• )CP uses a *inite 2tate Lachine, kept by each side o& a connection, to keep track o& whatstate a connection is inD• 2tate transitions re&lect inherent races that can happen in the network, eD'D, two *79@s

passin' each other in the networkD• Certain thin's can 'o wron' alon' the way, iDeD, packets can be dropped or corruptedD 7n

&act, machine is not per&ectF certain problems can arise not anticipated in the ori'inal.*CD

• )his is where timers will come in, which we will discuss more laterD

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TCP State $achine%Connection Establishent

N2)TJ72GN0

289V.CM0

289V2N9) 

CJ"2N0

J72)N9

client applicationcalls connect()

send S&'

receive S&'send S&' AC

server applicationcalls listen()

receive S&' * ACsend AC

receive AC

• CJ"2N0: more implied thanactual, iDeD, no connection

• J72)N9: willin' to receieconnections (accept call!

• 289-2N9): sent a 289,waitin' &or 289-C;

• 289-.NCN7MN0: receied a289, waitin' &or an C; o&our 289

•N2)TJ72GN0: connectionready &or data trans&er

receive S&'send AC

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TCP State $achine%Connection Teardown

N2)TJ72GN0

*79V+7)V

)7LNV+7) 

*79V+7)V1

J2)VC;

CJ"2NV+7) 

CJ"2N0

wait +,$S-(+./ seconds)

receive AC

receive F0'send AC

receive AC

o1 F0'

close() calledsend F0'

receive F0'send AC

• *79-+7)-1: we closed &irst,waitin' &or C; o& our *79(actie close!

• *79-+7)-: we closed &irst,other side has C;N0 our *79,but not yet *79@ed

• CJ"279%: other side closedbe&ore it receied our *79

• )7LN-+7): we closed, otherside closed, 'ot C; o& our *79

• CJ"2N-+7): other side sent*79 &irst, not us (passie close!

• J2)-C;: other side sent*79, then we did, now waitin'&or C;

CJ"279%

receive F0'

send AC

receive ACo1 F0'

close() calledsend F0'

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S#ar"% TCP Protocol

• Protocol proides reliability in &ace o& complenetwork behaior

• )ries to trade o&& e&&iciency with bein' U'ood

network citienU• Mast maRority o& bytes trans&erred on 7nternet

today are )CP-based:– +eb

– Lail– 9ews

– Peer-to-peer (9apster, %nutella, *ree9et, ;aWaa!