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Linux TCP/IP Stack 

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1: Physical

Layer 

2: Data

Link 

4: Transport

3: Netork 

!: "pplication

#: Presentation

$: Session

Inter%ace Layer &'thernet( etc)*

Protocol Layer &TCP / IP*

Socket layer 

Process

  TCP / IP +s) ,SI -o.el

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TCP/IP Stack ,+er+ie

  Process

1: sosend &))) *

  Socket Layer 

2: tcp_output & ) *

  Protocol Layer &TCP Layer*

3: ip_output & ) *

  Inter%ace Layer &'thernet De+ice Dri+er*

,utput 0ueue

5: rec+%ro-&)*

Input 0ueue

3: ip_input & ))) *

4: tcp_input & ))) *

  Protocol Layer &IP Layer*

4: ethernet_output & ) * 2: ethernet_input & )) *

Physical e.ia

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Process Layer to TCP Layer 

sen. &int socket( const char u%( int lenth( int %las*

Process

5ernel sen.to &int socket( const char .ata6u%%er( int lenth( int %las( struct socka..r .estination( int .estination 6lenth*

sen.it &struct proc p( int socket( struct -sh.r -p( int %las( int return6si7e*

sosen. &struct socket s( struct -u% a..r( struct uio uio( struct -u% top( struct -u% control( int %las *

uipc6syscalls)c

uipc6socket)c

tcp6userre8 &struct socket s( int re8uest( struct -u% -( struct -u% na-( struct -u% control * tcp6userre8)c

tcp6output &struct tcpc tp* tcp6output)cTCP Layer 

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Socket Layer 

sen.to &int socket( const char .ata6u%%er( int lenth( int %las( struct socka..r .estination( int .estination 6lenth*

Data

Data

9nuse. Space

1$ ;ytes

Data

12< ;ytes

-;u% 

 1 ;ytes

 2< ;ytes

 

2 ;ytes

 

$ ;ytes

$< ;ytes

-6nextpkt = N9LL

-6next = N9LL-6next

-6nextpkt = N9LL

-6len = 1 -6len = $

-6.ata -6.ata

-6type = T6D"T" -6type = T6D"T"

-6%las = 6P5T>D? -6%las =

-6pkth.r)len = 1$

-6pkth.r)rec+i% =N9LL

.ata6u%%er 

MBUF Chain

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Socket Layer @sosen. passes .ata an. control in%or-ation to the protocol layersosen.&struct socket s( struct -u% a..r( struct uio uio( struct -u% .ata6u%%er( struct -u% control( int %las *

Initiali7e a ne -e-ory u%%er an.

  +ariales to hol. %las

yes

Is there enouh spacein the u%%er 

sspace&s@As6sn.*

no

Copy .ata6u%%er -u% 

Bree the -e-ory u%%ers

recei+e.1 ore u%%ers

to sen.

 yes

  no

int error = tcp_usrreq(s, flas, !"uf, addr, control#

 error 

?eturn +alue o% error to sen.to & *

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TCP Layer @ tcp6usrre8&struct socket s( int re8uest( struct -u% .ata6u%%er( -u% na-( -u% control*

Initiali7e internet protocol control lock inp an.

TCP control lock tp

to store in%or-ation use%ul %or TCP

  Con+ert Socket to

Internet Protocol Control ;lock 

  inp = sotoinpc&so*

Con+ert the internet protocol control lock 

  to a tcp control lock

tp = intopc&inp*

re8uest

P?96S'ND

int error = tcp6output&tp*return error 

to tcp6userre8& *

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Calle. y tcp6usrre8 %or one o% the %olloin reasons:

To sen. the initial SN

To sen. a %inishe.6sen.in -essae

To sen. .ata

To sen. a in.o up.ate a%ter .ata has een recei+e.)

tcp6ouput & * %unctionality:

 1) .eter-ines hether TCP can sen. a se-ent or not .epen.in on:

  %las in the .ata sent y the socket layer to sen. an "C5( etc)

Si7e o% in.o a.+ertise. y the recei+erEs en.)

"-ount o% .ata rea.y to sen.hether unacknole.e. .ata alrea.y exists %or the connection

2) Calculate the a-ount o% .ata to e sent .epen.in on:

si7e o% recei+erEs in.o

nu-er o% ytes in the sen. u%%er 

3) Check %or in.o shrink 

4) Sen. a se-ent"llocate a u%%er %or the TCP an. IP hea.er %ro- the hea.er te-plate

Copy the TCP an. IP hea.er te-plate into the the u%%er to e sent)

Bill the %iel.s in the TCP hea.er)

  Decre-ent the nu-er o% u%%ers to te sent( so that the en. can e checke.)

Set se8uencenu-er an. acknole.e-ent %iel.)

Set three %iel.s in the IP hea.er @ IP lenth( TTL an. Tos)

Pass the .atara- to IP

TCP Layer (tcp_output$c# @ tcp6output&struct tcpc tp*

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TCP Layer (tcp_output$c# @ tcp6output&struct tcpc tp*

struct socket so = tp @A t6inpc @A inp6socket

Initiali7e a tcp hea.er tcp_header 

i.le

I.le is true i% the -ax se8uence nu-ere8uals the ol.est unacknole.e. se8uence nu-er(

i% an "C5 is not expecte. %ro- the other en.)

int i.le = &tp @A sn.6-ax == tp @A sn.6una*

Check "C5 Bla

"cknole.e-ent isnot expecte.( set the

conestion in.o to

one se-ent

tp @A sn.6cn. =

tp @A t6-axseF

true

%alse

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TCP Layer @ tcp6output&struct tcpc tp*

Deter-ine lenth o% .ata that shoul.

 e trans-itte. an. the %las to e use.)

len is the -ini-u- nu-er o% ytes in thesen. u%%er(

in &the -ini-u- o% the recei+erEs in.o*

an. the conestion in.o)

len = -in&so @A so6sn.)s6cc( in* @ o%% 

"cknole.e-ent is

not expecte.( set the

conestion in.o to

one se-enttp @A sn.6cn. =

tp @A t6-axseF

o%% is the o%%set in ytes %ro- the einnin o%

the sen. u%%er o% the %irst .ata yte to sen.)

o%% ytes ha+e alrea.y een sent an.

acknole.e-enton those is aaite.)

int o%% = tp @A sn.6nxt @ tp @A sn.6una

Deter-ine the %las like T>6"C5( T>6BIN(

T>6?ST( T>6SN%las = tcp 6out%las G tp @A t6state H

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TCP Layer @ tcp6output&struct tcpc tp*

tp @A t6%las

TB6"C5N,JSen. acknole.e-ent

Deter-ine the %las like T>6"C5( T>6BIN(

T>6?ST( T>6SN

%las = tcp 6out%las G tp @A t6state H

tp @A t6%las

TB6SN KK T>6?ST

tp @A t6%las

T>6BIN

true

%alse

%alse

trueSen. se8uence nu-er 

or reset

Binishe. sen.in

true

%alse

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Ckeck %las to .eter-ine the type o% -essae:

in.o proe

retrans-ission

nor-al .ata trans-ission

Lenth o% .ata 44 ;ytes

1 @ 4 @ 1#

yes

Create a ne -u% chain(

copy the surplus .ata an.

 point it to the %irst -u% chain)

%llocate an !"uf for the &C' )' header and data if possi"le$

M'T>D? & -( 6D,NTJ"IT( T6>'"D?*

6D,NTJ"IT in.icates that i% -e-ory is not a+ailale %or

-u% then co-e out o% the routine an. return an error state)

no

Copy the .ata %ro- the socket sen. u%%er into the

ne packet hea.er -u% 

ip6output&-( tp@At6inpc @A inp6options( tp @A t6inpc @A inp6route(

  so @A so6options S,6D,N,T?,9T'( *

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Packets

.a-ae.

ip_output$c

ip_output(struct !"uf *!, struct !"uf *opt, struct route *ro, int flas, struct ip_!options *i!o#1) >ea.er initiali7ation

2) ?oute Selection

3) Source a..ress selection an. Bra-entation

1$ +eader initialiation

The +alue o% %lasO .eci.es hatEs to e .one ith the .ata IP6B,?J"?DINM : Borar. packet IP6?,9T'T,IB : ?oute .irectly to Inter%ace IP6"LL,J;?,"DC"ST : "llo roa.castin o% packet IP6?"J,9TP9T : Packet contains pre@constructe. hea.er 

yes '??,? 

i% &&%las == IP6B,?J"?DINM * KK 

&%las == IP6?"J,9TP9T **

no

Sa+e hea.er lenth in hlen

%or %ra-entation alorith-

Construct an. initiali7e IP hea.er 

set ip_v = 4( clear ip_off 

assin uni8ue i.enti%ier to ip_id 

length, offset, TTL, protocol, TOS etc

are set y hiher layers)

yes

noI% the packet has to e %orar.e. to another host( i)e i% the

-achine is actin as a router( then the IP hea.er %or %orar.e.

 packets shoul. not e -o.i%ie. y ip6output)

Check i% there ere any errors hile a..in hea.ers in hiher 

layers) ost o% the %iel.s o% the IP hea.er are pre .e%ine. y

hiher layer protocols)

I% the packet is not ein %orar.e. an. has to e sent to

another host then initiali7e the IP hea.er)

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2$ -oute .election

Qeri%y Cache. ?oute %or

.estination a..ress

Bin. the inter%ace on hich the

 packet has to e place.) Ifp points tothe inter%aceEs ifnet  structure)

I% &cache.6route == .estination*

Locate route : Call rtalloc(dst_ip) to

locate a route to the .estination) Bin.

the inter%ace on hich the packet has

to e place.) Ifp points to the

inter%aceEs ifnet  structure) I%rtalloc(dst_ip) %ails to %in. a route(

return host unreachale error)

yes

no

" cache. route -ay e pro+i.e. to ip6output as an

aru-ent) 9DP an. TCP -aintain a route cache

associate. ith each socket)

Check i% the cache. route is the correct .estination) I% a

route has not een pro+i.e.( ip6output sets a te-porary

route structure calle. iproute. 

I% the cache. route is pro+i.e.( %in. the inter%ace on

hich the %ra-e has to e sent)

I% the packet is ein route.( rtalloc locates a route to

the a..ress speci%ie. y dst ) I% rtalloc %ails( an

'>,ST9N?'"C> error is enerate.) I% ip6%orar. calle.

ip6output the error is con+erte. to an ICP error)

I% the a..ress is %oun. then ifp is -a.e to point to thr

ifnet structure %or the inter%ace) I% the next hop is not the packets %inal .estination( then dst is chane. to point to

the next hop router)

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3$ .ource address selection and Fra!entation

Check i% +ali. source

a..ress is speci%ie.)

Select the IP a..ress o% the outoin

inter%ace as the source a..ress)

Does the packet ha+eto e %ra-ente.

Bra-ent the packet i% itEs si7e is

reater than the T9)

I% there are no check6su- errors( sen.

the .ata to if_output  %unction o% the

selecte. inter%ace)

no

yes

yes

no

The %inal section o% the ip6output ensures that the

IP hea.er has a +ali. source IP a..ress) This

coul.nEt ha+e een .one earlier ecause the route

ha.nEt een selecte. yet) I% there is no source IP then

the IP a..ress o% the outoin inter%ace is use. as the

source IP)

Larer packets &packets that excee. the T9* -ust

 e %ra-ente. e%ore they can e sent)

In either case &%ra-ente. or not* the checksu- is

co-pute. &in_cksum*) I% no errors are %oun.( the

.ata is sent to if_output %unction o% the output

inter%ace)

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Inter%ace Layer &if_ethersu"r$c#

ether_output(struct ifnet *ifp, struct !"uf *!"uf, struct soc/addr *destination, struct rtentr0 *routin_entr0#1) Qeri%ication

2) Protocol@Speci%ic Processin

3) Bra-e Construction4) Inter%ace 0ueuin)

sen.err &'N'TD,JN*'thernet port

up an. runnin

i%p @A i%6%las

&IB69P K IB6?9NNINM *

no

yes

1$ erification

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Inter%ace Layer (if_ethersu"r$c# @ ether6output&struct i%net i%p( struct -u% -u%(struct socka..r .estination( struct rtentry rt6entry*

Bunction: Takes the .ata portion o% an 'thernet %ra-e ans encapsulates it ith a 14@yte hea.er an. places it on the inter%ace sen.68ueue)

Phases: Qeri%ication( Protocol@Speci%ic Processin( Bra-e Construction( Inter%ace 0ueuin)

%ru!ents

i%p points to outoin inter%aceEs i%net structure

-u% is the .ata to e sent

.estination is the .estination a..ress

rt6entry points o the routin entry

)nitialie

'thernet hea.er @ struct eth6hea.er eh

sen.err &'N'TD,JN*'thernet port

up an. runnin

i%p @A i%6%las

&IB69P K IB6?9NNINM *

no

yes

erification

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sen.err &'>,ST9N?'"C>*

?oute +ali.

rt6entry = rtalloc1 &.estination( 1*

  1

 Next hop a ateay

rt = rt @A rt6route

  1

Destination respon.in

to "?P re8uests

I% not then .o not sen. -ore packets to a+oi. %loo.in)

rt @A rt6%las

?TB6?'R'CT

no

erification

'rotocol .pecific 'rocessin

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'rotocol .pecific 'rocessinBunctionality: Bin.s 'thernet a..ress correspon.in to the IP a..ress o% the .estination)

9se -6copy& * to keep the packet till

an ack) Is rec+.)

.estination @A sa6%a-ily

  "B6IN'T

Sen. "?P roa.cast to %in. the

ethernet a..ress correspon.in to the.estination IP a..ress

Fra!e 'reparartion

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ake sure there is roo- %or the 14 yte

ethernet hea.er 

6P?'P'ND & -( si7eo%&ethernet6hea.er*(

6D,N,TJ"IT*

Fra!e 'reparartion

'rotocol .pecific 'rocessin

Bor- the 'thernet hea.er %ro-

ethernet %ra-e type(

ethernet "C a..ress(unicast ethernet a..ress associate.

ith the output inter%ace)

e)) the .e%ault ateay %or a host

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)nterface ueuin

Fra!e 'reparartion

Is the output 8ueue %ull

no

yes Discar. the %ra-e

Bree the -e-ory u%% 

sen.err & 'N,;9BS *

Place the %ra-e on the

inter%aceEs sen. 8ueue

lestart & i%p *

  i%6sn.

lestart & i%p *

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Inter%ace Layer (if_le$c# @ lestart&struct i%net i%p*

Bunction: De8ueues %ra-es %ro- the inter%ace output 8ueue an. arranes %or the- to e trans-itte. y the 'thernet Car.)

le @A sc6i%)i%6%las

IBB6?9NNINM

struct le6so%tc le = le6so%tcl G i%p @A i%6unit H

return error 

  1

Copy the the %ra-e in -u% to the

har.are u%%er 

Set the IBB6,"CTIQ' on to in.icate that the

.e+ice is usy trans-ittin)