Tcp.ip Trong 14 Ngay.pdf

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Teach Yourself TCP/IP in 14 Days

Second Edition

Pr efa ce t o Second Edit ion

About t he Aut hor

Overview

Int r oduct ion

1. Open Syst ems, St an dar ds, and Pr ot ocol s

2. TCP/IP an d t he Int er net

3. The Int er net Pr ot ocol (IP)

4. TCP an d UDP

5. Gat eway an d Rout ing Pr ot ocol s

6. Tel net an d FTP

7. TCP/IP Configur a t ion and Administ r a t ion Basics

8. TCP/IP and Net wor ks

9. Set t ing Up a Sampl e TCP/IP Net wor k: Ser ver s

10. Set t ing Up a Sampl e TCP/IP Net wor k: DOS and Windows Cl ien t s

11. Domain Name Ser vice

12. Net wor k Fil e Syst em an d Net wor k Inf or mat ion Ser vice

13. Managing an d Tr oubl eshoot ing TCP/IP

14. The Socket Pr ogr amming Int er fa ce

Appendix A: Acr onyms an d Abbreviat ions

Appen dix B: Gl ossar y

Appen dix C: Commands

Appendix D: Wel l -Known Por t Number s

Appendix E: RFCs

Appendix F: Answer s t o Quizz es

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Teach Yourself TCP/IP in 14 Days, Second

Edition

The second edit ion of Teach Yourself TCP/IP in 14 Daysexpan ds on t he very popul ar fir st

dit ion, br inging t he infor mat ion u p-t o-dat e an d adding n ew t opics t o compl et e t he

over age of TCP/IP. The book ha s been r eor gan ized t o make r eading an d l ear ning easier ,

s wel l as t o pr ovide a mor e l ogical appr oach t o t he subject .

New mat er ial in t his edit ion deal s wit h inst al l ing, configur ing, an d t est ing a TCP/IP

et work of ser vers an d cl ient s. You wil l see how t o easil y set up UNIX, Linu x, an d

Windows NT ser ver s for a l l popul ar TCP/IP ser vices, incl uding Tel net , FTP, DNS, NIS,nd NFS. On t he cl ien t side, you wil l see how t o set up DOS, Windows, Windows 95, and

WinSock t o int er act wit h a ser ver. Exampl es an d t ips t hr oughout t hese sect ions make

he pr ocess easy and cl ear .

Al so added in t his edit ion of Teach Yourself TCP/IP in 14 Daysar e new sect ions on DNS,

NFS, an d NIS. These net wor k ser vices ha ve become popul ar wit h t he gr owt h of l ar ge

TCP/IP net wor ks, so we show you h ow t o conf igur e an d use t hem al l . A new sect ion on

he l at est ver sion of IP updat es t he t r eat ment of t he base pr ot ocol s t o 1996 st an dar ds.

Tim Par ker

Mail :

Dean Mil l er

Comment s Depar t ment

ams Publ ish ing

01 W. 103r d St r eet

ndianapol is, IN 46290

Topics Cover ed in Det a il in t h is Edit ion

The TCP/IP Pr ot ocol Famil y

Transport

Rout ing

Net wor k Addr esses

User Ser vices Gat eway Pr ot ocol s

Others


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Topics Cover ed in Det a il in t h is Edit ion

St an dar ds an d t erminol ogy

Net work ar chit ect ur e

Hist or y of TCP/IP an d t he Int er net

IPng (IP ver sion 6)

Tel net an d FTP

Configur ing ser vers and cl ient s

Introduction

o you've just been t ol d you a r e on a TCP/IP net wor k, you a r e t he n ew TCP/IP syst em

dminist r at or , or you ha ve t o inst al l a TCP/IP syst em. But you don't know very much

bout TCP/IP. Tha t 's wher e t his book comes in . You don 't need any pr ogr amming skil l s,

n d fa mil iar it y wit h oper at ing syst ems is assumed. Even if you've never t ouch ed a

omput er befor e, you shoul d be abl e t o fol l ow t he mat erial .

This book is int ended for beginn ing t hr ough int er mediat e user s an d covers al l t he

r ot ocol s invol ved in TCP/IP. Each pr ot ocol is examined in a fa ir l evel of det ail t o showow it wor ks an d how it int era ct s wit h t he ot her pr ot ocol s in t he TCP/IP famil y. Al ong

he way, t his book shows you t he basic t ool s r equir ed t o inst al l , configur e, an d maint ain

TCP/IP net wor k. It al so shows you most of t he user ut il it ies t hat ar e avail abl e.

Because of t he compl ex nat ur e of TCP/IP an d t he l ack of a fr iendl y user int erf ace,

here is a l ot of infor mat ion t o l ook at . Thr oughout t he book, t he r ol e of each pr ot ocol

s shown separ at el y, as is t he way it works on n et works of a l l sizes. The r el at ionship

wit h l ar ge int ern et wor ks (l ike t he Int ern et ) is al so cover ed.

Each ch apt er in t he book adds t o t he compl exit y of t he syst em, buil ding on t he mat er ial

n t he ear l ier chapt er s. Al t hough some chapt er s seem t o be unr el at ed t o TCP/IP at f ir st


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l an ce, al l t he mat erial is invol ved in an int egr al man ner wit h t he TCP/IP pr ot ocol

amil y. The l ast few chapt ers cover t he inst al l at ion and t r oubl eshoot ing of a n et wor k.

By t he t ime you f inish t his book, you wil l un der st an d t he diff er ent component s of a

TCP/IP syst em, as wel l as t he compl ex acr onym-hea vy jar gon u sed. Fol l owing t he

xampl es pr esent ed, you shoul d be a bl e t o inst al l an d configur e a compl et e TCP/IP

et wor k for any opera t ing syst em an d har dwar e pl at for m.

The TCP/IP Protocol Family

Transport

Tr ansmission Cont r ol Pr ot ocol (TCP): connect ion-based ser vices

User Dat agr am Pr ot ocol (UDP): conn ect ionl ess ser vices

Routing

Int er net Pr ot ocol (IP): ha ndl es t r an smission of infor mat ion

Int er net Cont r ol Message Prot ocol (ICMP): ha ndl es st at us messages for IP

Rout ing Inf or mat ion Pr ot ocol (RIP): det er mines r out ing

Open Shor t est Pat h Fir st (OSPF): al t er nat e pr ot ocol for

det er mining r out ing

Network Addresses

Addr ess Resol u t ion Pr ot ocol (ARP): det er mines addr esses


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Domain Name Syst em (DNS): det er mines addr esses fr om mach ine names

Rever se Addr ess Resol u t ion Pr ot ocol (RARP): - det er mines

addresses

User Services

Boot Pr ot ocol (BOOTP): st ar t s up a net work mach ine

Fil e Tr an sfer Pr ot ocol (FTP): t r an sfer s fil es

Tel net : al l ows r emot e l ogins

Gateway Protocols

Ext erior Gat eway Pr ot ocol (EGP): t r an sfer s r out ing infor mat ion for

ext ernal net works

Gat eway-t o-Gat eway Pr ot ocol (GGP): t r an sfer s r out ing inf or mat ion

bet ween gat eways

Int erior Gat eway Pr ot ocol (IGP): t r an sfer s r out ing infor mat ion

for int ernal net works

Others

Net wor k Fil e Syst em (NFS): enabl es dir ect or ies on on e machine t o be

mount ed on anot her

Net wor k Inf or mat ion Ser vice (NIS): maint ains user accou nt s acr oss

net wor ks


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Remot e Pr ocedur e Cal l (RPC): ena bl es r emot e appl icat ions t o commun icat e

Simpl e Mail Tr an sfer Pr ot ocol (SMTP): t r an sfer s el ect r onic mail

Simpl e Net wor k Man agement Pr ot ocol (SNMP): sends st at us

messages about t he net work


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The TCP/IP Pr ot ocol Famil y

The TCP/IP Protocol Family

Transport

TCP (Tr ansmission Cont r ol Pr ot ocol ) Connect ion-based ser vices (Day 4)

UDP (User Dat agr am Pr ot ocol ) Connect ionl ess ser vices (Day 4)

Routing

IP (Int er net Pr ot ocol ) Han dl es t r an smission of inf or mat ion (Day 3)

ICMP (Int er net Cont r ol Message

Pr ot ocol )Handl es st a t us messages for IP (Day 3)

RIP (Rout ing Inf or mat ion Pr ot ocol ) Det er mines r out ing (Day 5)

OSPF (Open Shor t est Pat h Fir st )

Al t erna t e pr ot ocol for det ermining rout ing

(Day 5)

Network Addresses

ARP (Addr ess Resol u t ion Pr ot ocol ) Det er mines addr esses (Day 2)

DNS (Domain Name Syst em)Det er mines addr esses f r om mach ine names

(Day 2 and Day 11)

RARP (Rever se Addr ess Resol u t ion

Pr ot ocol )Det er mines addr esses (Day 2)

User Services

BOOTP (Boot Pr ot ocol ) St ar t s up a net wor k mach ine (Day 11)

FTP (Fil e Tr an sfer Pr ot ocol ) Tr an sfer s fil es (Day 6)

Tel net Ena bl es r emot e l ogins (Day 6)

TFTP (Tr ivial Fil e Tr an sfer Pr ot ocol ) Ena bl es r emot e fil e t r an sfer s (Day 6)

Gateway Protocols


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EGP (Ext er ior Gat eway Pr ot ocol )Tr ansfers rout ing infor mat ion for ext erna l

net wor ks (Day 3 an d Day 5)

GGP (Gat eway-t o-Gat eway Pr ot ocol )Tr an sfer s rout ing informat ion bet ween

gat eways (Day 3 and Day 5)

IGP (Int er ior Gat eway Pr ot ocol )Tr ansfers rout ing infor mat ion for int erna l

net wor ks (Day 5)Others

NFS (Net wor k Fil e Syst em)Ena bl es dir ect or ies on on e mach ine t o be

moun t ed on a not her (Day 12)

NIS (Net wor k Infor mat ion Ser vice)Maint ains user a ccoun t s acr oss net works

(Day 12)

NTP (Net wor k Time Pr ot ocol ) Synchr onizes cl ocks (Day 11)

PING (Packet Int er net Gr oper ) Checks conn ect ivit y (Day 7)

RPC (Remot e Pr ocedur e Cal l )Ena bl es r emot e appl icat ions t o commun icat e

(Day 12)

SNMP (Simpl e Net wor k Management

Pr ot ocol )

Sends st at us messages about t he net work

(Day 13)


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Open Syst ems

What Is an Open Syst em?

Net wor k Ar chit ect ur es

Local Ar ea Net wor ks

The Bus Net wor k

The Ring Net wor k

The Hub Net wor k

Wide Ar ea Net wor ks

Layers

The Appl ica t ion Layer

The Pr esent at ion Layer

The Session Layer

The Tr an spor t Layer

The Net work Layer

The Dat a Link Layer

The Physical Layer

Ter minol ogy an d Not at ions

Packet s

Subsyst ems

Ent it ies

N Not at ion

N-Fun ct ions

N-Facil it ies

Services

Making Sense of t he Jar gon

Queu es an d Conn ect ions Standards

Set t ing St andards

In t erne t St andards

Pr ot ocol s

Br eaking Dat a Apar t

Pr ot ocol Headers

Summar y

Q&A Quiz


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1

Open Systems, Standards, and Protocols

Today I st ar t l ooking a t t he subject of TCP/IP by cover ing some backgr oun d inf or mat ion

ou wil l need t o put TCP/IP in per spect ive, an d t o un der st an d why t he TCP/IP pr ot ocol s

wer e designed t he way t hey a r e. This cha pt er cover s some impor t an t inf or mat ion,

ncl uding t he fol l owing:

What an open syst em is

How an open syst em ha ndl es net wor king

Why st an dar ds ar e requir ed

How st an dar ds for pr ot ocol s l ike TCP/IP ar e devel oped

Wha t a pr ot ocol is

The OSI pr ot ocol s

You might be eager t o get st ar t ed wit h t he nit t y-grit t y of t he TCP/IP pr ot ocol s, or t o

ind ou t how t o use t he bet t er -known ser vices l ike FTP an d Tel net . If you ha ve a specific

equir ement t o sat isfy (such a s how t o t r an sfer a f il e fr om one syst em t o anot her ), by

l l means use t he Tabl e of Cont ent s t o f ind t he sect ion you want . But if you want t o

eal l y underst and TCP/IP, you wil l need t o wade t hr ough t he mat erial in t his chapt er.

t 's not compl icat ed, al t hou gh t her e ar e quit e a few subject s t o be cover ed. Luckil y,

one of it r equir es memorizat ion; more oft en t ha n n ot it is a mat t er of set t ing t he st age

or somet hing el se I discuss in t he n ext week or so. So don't get t oo over whel med by t his

hapt er!

Open Systems

This is a book about a f amil y of pr ot ocol s cal l ed TCP/IP, so why bot her l ooking a t open

yst ems an d st an dar ds at al l ? Pr imar il y because TCP/IP gr ew out of t he n eed t o devel opst an dar dized commun icat ions pr ocedur e t ha t woul d inevit abl y be used on a var iet y of

l at forms. The need for a st andar d, and one t hat was r eadil y avail abl e t o anyone

hence open), was vit al l y impor t an t t o TCP/IP's success. Ther efor e, a l it t l e backgr ound


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el ps put t he design of TCP/IP in t o per spect ive.

Mor e impor t an t l y, open syst ems ha ve become de rigueur in t he cu r r ent compet it ive

mar ket . The t er m open systemis ban died ar oun d by man y peopl e as a sol ut ion f or al l

r obl ems (t o be r epl aced occasiona l l y by t he t er m client/server), but neit her t er m is

sual l y pr oper l y used or u nder st ood by t he peopl e spout ing t hem. Under st an ding what

n open syst em r eal l y is an d what it impl ies l eads t o a bet t er awar eness of TCP/IP's r ol e

n a net wor k and across l ar ge int ernet wor ks l ike t he Int ernet .

n a simil ar vein, t he u se of st an dar ds ensur es t ha t a pr ot ocol such as TCP/IP is t he same

n ea ch syst em. This mean s t ha t your PC can t al k t o a minicomput er r un ning TCP/IP

wit hout special t r ansl at ion or conver sion r out ines. It means t hat an ent ir e net wor k of

iffer ent ha r dwar e and opera t ing syst ems can wor k wit h t he same net wor k pr ot ocol s.

Devel oping a st an dar d is not a t r ivial pr ocess. Oft en a singl e st an dar d invol ves mor e

ha n a singl e document descr ibing a soft war e syst em. A st an dar d oft en invol ves t he

nt er r el at ionship of man y diff er ent pr ot ocol s, as does TCP/IP. Knowing t he int er act ionset ween TCP/IP and t he ot her componen t s of a commun icat ions syst em is impor t an t for

r oper configur at ion an d opt imiza t ion, an d t o ensur e t ha t al l t he services you n eed ar e

vail abl e and int erwor king pr operl y.

What Is an Open System?

Ther e ar e man y definit ions of open syst ems, an d a singl e, concise definit ion t ha t

ver yone is ha ppy wit h is fa r fr om being accept ed. For most peopl e, an open syst em is best

oosel y defined as one for which t he ar chit ect ur e is not a secret . The descript ion of t he

r chit ect ur e has been publ ished or is r eadil y avail abl e t o anyone who want s t o buil d

r oduct s for a ha r dwar e or soft war e pl at for m. This definit ion of an open syst em appl ies

qual l y wel l t o har dwar e and sof t ware.

When mor e t ha n a singl e vendor begins pr oducing pr oduct s for a pl at for m, cust omer s

a ve a choice. You don't par t icul ar l y l ike Nocr ash Soft war e's net wor k monit oring

oft war e? No pr obl em, because Fau l t Fr ee Soft war e's pr oduct r un s on t he Nocra sh

a r dwar e, an d you l ike it s fan cy int erf ace much bet t er. You need a more col or ful

r aphical fr ont -end t o your Whizban g PC t ha n t he one Whizban g pr ovides? Downl oad

ne fr om Super Soft war e t hr ough t he Int ern et , and it wor ks perf ect l y. The pr imar y

dea, of cour se, is a move away f r om pr opr iet ar y pl at for ms t o one t ha t is mul t ivendor.

A decade ago, open syst ems wer e vir t ua l l y nonexist ent . Each ha r dwar e manu fact ur er

ad a pr oduct l ine, and you were pr act ical l y bound t o t hat manufact ur er for a l l youroft war e an d ha r dwar e needs. Some compan ies t ook advant age of t he ca pt ive mar ket ,

ha r ging out r ageous pr ices or f or cing un wan t ed configur at ions on t heir cust omer s. The

roun dswel l of r esent ment grew t o t he point t hat cust omers began for cing t he issue.


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The l ack of choice in soft war e and ha r dwar e pur cha ses is why sever al dedicat ed

minicomput er an d mainf r ame compan ies eit her went ban kr upt or ha d t o accept open

yst em pr incipl es: t heir cust omer s got fed up wit h r el ying on a singl e vendor . A good

xampl e of a compan y t ha t made t he a dapt at ion is Digit al Equipment Cor por at ion (DEC).

They moved fr om a pr opr iet ar y oper a t ing syst em on t heir VMS minicomput er s t o a UNIX-

t an dar d open oper at ing syst em. By doing t ha t , t hey kept t heir cust omer s ha ppy, an d

hey sol d mor e mach ines. Tha t 's one of t he pr imar y r easons DEC is st il l in business t oday.

UNIX is a cl assic exampl e of an open soft war e pl at for m. UNIX has been a r oun d for 30

ear s. The sour ce code for t he UNIX oper at ing syst em was made a vail abl e t o an yone who

want ed it , al most fr om t he st ar t . UNIX's sour ce code is wel l un der st ood an d easy t o

wor k wit h, t he r esul t of 30 year s of devel opment an d impr ovement . UNIX can be por t ed

o ru n on pr act ical l y any har dwar e pl at for m, el imina t ing al l pr opr iet ar y dependencies.

The a t t r act ion of UNIX is not t he oper at ing syst em's feat ur es t hemsel ves but simpl y

ha t a UNIX user can r un soft war e fr om ot her UNIX pl at for ms, t ha t fil es ar e

ompat ibl e fr om one UNIX syst em t o an ot her (except for disk for mat s), an d t ha t a wide

ar iet y of vendor s sel l pr oduct s for UNIX.

The gr owt h of UNIX pushed t he l ar ge har dwar e man ufa ct ur ers t o t he open syst ems

r incipl e, r esul t ing in most man uf act ur er s l icensing t he r ight t o pr oduce a UNIX version

or t heir own ha r dwar e. This st ep l et cust omer s combine diff er ent ha r dwar e syst ems

nt o l ar ger net wor ks, al l r un ning UNIX an d wor king t oget her . Users coul d move

et ween machines al most t r anspar ent l y, ignor ant of t he act ual har dwar e pl at form

hey were on. Open syst ems, or igina l l y of pr ime impor t an ce onl y t o t he l ar gest

or por at ions an d gover nment s, is now a key el ement in even t he smal l est compan y'somput er st r at egy.

Al t hou gh UNIX is a copyr ight ed wor k now owned by

X/Open, t he det ail s of t he oper at ing syst em ha ve been publ ished

and ar e readil y avail abl e t o any devel oper who want s t o

pr oduce appl icat ions or ha r dwar e t hat wor k wit h t he opera t ingsyst em. UNIX is un ique in t h is r espect .

The t er m open system networkingmeans many t h ings, depending on whom you ask. In it s

r oadest definit ion, open syst em net working r efer s t o a net work based on a wel l -known

n d under st ood pr ot ocol (such as TCP/IP) t ha t ha s it s st an dar ds publ ished an d r eadil y

vail abl e t o anyone who wan t s t o use t hem. Open syst em net wor king al so r efers t o t he

r ocess of n et wor king open syst ems (mach aine-specific ha r dwar e a nd soft war e) using a

et work pr ot ocol . It is easy t o see why peopl e want open syst ems net working, t hough .

Thr ee services ar e widel y used and accoun t for t he highest percent age of net wor k

r af fic: fil e t r an sfer , el ect r onic mail , an d remot e l ogin. Wit hou t open syst ems

et working, set t ing up an y of t hese t hr ee ser vices woul d be a night mar e.


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il e t r an sfer s enabl e users t o sha r e fil es quickl y an d efficient l y, wit hout excessive

upl icat ion or concer ns about t he t r ansport met hod. Net wor k fil e t r ansfers ar e much

ast er t han an over night cour ier cr ossing t he count r y, and usual l y fast er t han copying

f il e on a disk an d car r ying it acr oss t he r oom. Fil e t r an sfer is al so ext r emel y

onvenient , which not onl y pl eases user s but al so el imina t es t ime del ays whil e wait ing

or mat er ial . A common open syst em gover ning fil e t r an sfer s mean s t ha t an y

ncompat ibil it ies bet ween t he t wo mach ines t r an sfer r ing fil es can be over come easil y.

El ect r onic mail ha s mushr oomed t o a phenomena l l y l ar ge ser vice, not just wit hin a

ingl e business but wor l dwide. The Int er net car r ies mil l ions of messages fr om peopl e in

over nment , pr ivat e indust r y, educat ional inst it ut ions, and pr ivat e int erest s.

El ect r onic mail is chea p (no paper , envel ope, or st amp) an d fa st (ar oun d t he wor l d in 60

econds or so). It is al so an obvious ext ension of t he comput er -based worl d we wor k in.

Wit hou t an open mail syst em, you woul dn't ha ve anywher e near t he capabil it ies you

ow enjoy.

ina l l y, r emot e l ogins ena bl e a user who is based on one syst em t o conn ect t hr ough a

et wor k t o any ot her syst em t ha t accept s him as a user. This can be in t he n ext

wor kgr oup, t he next st at e, or in an ot her count r y. Remot e l ogins enabl e users t o t ake

dvant age of par t icul ar har dwar e and sof t ware in anot her l ocat ion, as wel l as t o run

ppl icat ions on an ot her mach ine. Once aga in, wit hou t an open st an dar d, t his woul d be

l most impossibl e.

Network Architectures

To un der st an d net wor king pr ot ocol s, it is useful t o know a l it t l e about net wor ks. A

uick l ook at t he most common net wor k ar chit ect ur es wil l hel p l at er in t his book when

ou r ead about net wor k oper at ions an d rout ing. The t erm networkusua l l y means a set of

omput er s and per ipher al s (pr int er s, modems, pl ot t er s, scan ner s, an d so on) t ha t ar eonnect ed t oget her by some medium. The conn ect ion can be dir ect (t hr ough a cabl e) or

ndir ect (t hr ough a modem). The diff er ent devices on t he n et work commun icat e wit h

ach ot her t hr ough a pr edefined set of r ul es (t he pr ot ocol ).

The devices on a net work ca n be in t he same r oom or scat t er ed t hr ough a buil ding. They

an be separ at ed by man y mil es t hr ough t he use of dedicat ed t el ephone l ines, micr owave,

r a simil ar syst em. They can even be scat t er ed ar oun d t he wor l d, aga in conn ect ed by a

ong-dist an ce commun icat ions medium. The l ayout of t he n et work (t he a ct ua l devices

nd t he mann er in which t hey are connect ed t o each ot her) is cal l ed t he networkopology.


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Usual l y, if t he devices on a net wor k ar e in a singl e l ocat ion such a s a buil ding or a

roup of r ooms, t hey ar e cal l ed a l ocal ar ea net wor k, or LAN. LANs usua l l y have al l

he devices on t he net work connect ed by a singl e t ype of n et work cabl e. If t he devices

r e scat t ered widel y, such a s in differ ent buil dings or differ ent cit ies, t hey ar e usual l y

et up int o severa l LANs t hat ar e joined t oget her int o a l ar ger st r uct ur e cal l ed a wide

r ea net wor k, or WAN. A WAN is composed of t wo or mor e LANs. Each LAN has it s own

et wor k cabl e connect ing al l t he devices in t ha t LAN. The LANs ar e joined t oget her by

n ot her conn ect ion met hod, oft en h igh-speed t el ephone l ines or ver y fa st dedicat edet wor k cabl es cal l ed backbones, which I discuss in a moment .

One l ast point about WANs: t hey are of t en t r eat ed as a singl e ent it y for

r gan iza t iona l pur poses. For exampl e, t he ABC Soft war e compan y might ha ve br an ches

n f our diff er ent cit ies, wit h a LAN in ea ch cit y. Al l four LANs ar e joined t oget her by

igh-speed t el ephone l ines. However, as fa r as t he Int er net an d anyone out side t he ABC

oft war e compan y ar e concer ned, t he ABC Soft war e WAN is a singl e ent it y. (It ha s a

ingl e domain na me for t he Int er net . Dont worr y if you dont known wha t a domain is

t t his point in t ime; it r efers t o a singl e ent it y for or ganiza t ional pur poses on t he

nt ernet , as you wil l see l at er .)

Local Area Networks

TCP/IP wor ks acr oss LANs and WANs, and t her e ar e sever a l impor t ant aspect s of LANn d WAN t opol ogies you shoul d know about . You can st ar t wit h LANs an d l ook at t heir

opol ogies. Al t hough t her e ar e man y t opol ogies for LANs, t hr ee t opol ogies ar e

ominant : bus, r ing, an d hub.

The Bus Network

The bus net wor k is t he simpl est , compr ising a singl e main commun ica t ions pa t hway wit h

ach device at t ached t o t he main cabl e (bus) t hr ough a device cal l ed a t r an sceiver or

u nct ion box. The bu s is al so ca l l ed a backbone because it r esembl es a human spine wit h

ibs eman at ing fr om it . Fr om each t r an sceiver on t he bus, an ot her cabl e (oft en ver y

hor t ) r un s t o t he device's net work a dapt er . An exampl e of a bus net work is shown in

igu r e 1.1.

igur e 1.1. A schemat ic of a bus net wor k showingt h e back bo ne wit h t r ansce ive r sead ing t o ne t wo r k d evices.

The pr imar y advant age of a bus net work is t ha t it al l ows for a h igh-speed bus. Anot her
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dvant age of t he bus net wor k is t hat it is usua l l y immun e t o pr obl ems wit h a ny singl e

et wor k car d wit hin a device on t he n et wor k. This is becau se t he t r an sceiver al l ows

r aff ic t hr ough t he backbone whet her a device is at t ached t o t he junct ion box or n ot .

Each en d of t he bus is t er mina t ed wit h a bl ock of r esist or s or a simil ar el ect r ical device

o mar k t he end of t he cabl e el ect r ical l y. Each device on t he pat hway has a special

dent ifying n umber , or addr ess, t ha t l et s t he device know t ha t incoming inf or mat ion is

or t hat device.

A bus net wor k is sel dom a st r aight cabl e. Inst ead, it is usua l l y t wist ed ar ound wal l s

n d buil dings as needed. It does ha ve a singl e pat hway f r om one end t o t he ot her , wit h

ach end t er mina t ed in some way (usual l y wit h a r esist or ). Figur e 1.1 shows a l ogical

epr esent at ion of t he net work, mean ing it ha s simpl ified t he act ua l physical appear an ce

f t he net wor k int o a schemat ic wit h st r aight l ines and no real scal e t o t he

onnect ions. A physical r epr esent at ion of t he net wor k woul d show how it goes t hr ough

wal l s, ar ound desks, an d so on. Most devices on t he bus net work ca n send or r eceive

at a al ong t he bus by packaging a message wit h t he int ended r ecipient 's address.

A var iat ion of t he bus net work t opol ogy is foun d in man y smal l LANs t ha t use Thin

Et her net cabl e (which l ooks l ike t el evision coaxial cabl e) or t wist ed-pair cabl e (which

esembl es t el ephon e cabl es). This t ype of net work consist s of a l engt h of coaxial cabl e

ha t sna kes fr om mach ine t o mach ine. Unl ike t he bus net work in Figur e 1.1, t her e ar e no

r an sceivers on t he bus. Inst ead, each device is connect ed int o t he bus dir ect l y using a T-

ha ped connect or on t he net wor k int er face car d, oft en using a connect or cal l ed a BNC.

The conn ect or connect s t he machine t o t he t wo neighbors t hr ough t wo cabl es, one t o

ach n eighbor . At t he ends of t he n et work, a simpl e r esist or is added t o one side of t he T-onnect or t o t erminat e t he net wor k el ect r ical l y .

A schemat ic of t his t ype of n et wor k is shown in Figur e 1.2. Each net wor k device ha s a T-

onnect or a t t ached t o t he net wor k int erfa ce card, l eading t o it s t wo neighbors. The

wo ends of t he bus ar e t erminat ed wit h r esist ors.

igur e 1.2. A sche mat ic of a mac hine-t o-mac hine busne t wo r k .

This mach ine-t o-mach ine (al so cal l ed peer -t o-peer ) net wor k is not capabl e of sust aining

he higher speeds of t he backbone-based bus net wor k, pr imar il y becau se of t he medium of

he n et work ca bl e. A backbone n et work ca n use ver y high-speed cabl es such as fiber

pt ics, wit h smal l er (an d sl ower ) cabl es fr om each t r an sceiver t o t he device. A mach ine-

o-mach ine net work is usual l y buil t using t wist ed-pair or coaxial cabl e because t hese

abl es ar e much cheaper an d easier t o wor k wit h. Unt il r ecent l y, mach ine-t o-mach ine

et works wer e l imit ed t o a t hr oughput of a bout 10 Mbps (megabit s per second), al t hou gh

ecent devel opment s cal l ed 100VG AnyLAN an d Fast Et her net al l ow 100 Mbps on t his

ype of net wor k.

The a dvant age of t h is machine-t o-machine bus net wor k is it s simpl icit y. Adding new

machines t o t he net wor k means inst al l ing a net wor k car d and connect ing t he new
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mach ine int o a l ogical pl ace on t he backbone. One major advan t age of t he mach ine-t o-

mach ine bus net work is al so it s cost : it is pr obabl y t he l owest cost LAN t opol ogy

va il abl e. The pr obl em wit h t his t ype of bus net work is t ha t if one machine is t aken off

he net wor k cabl e, or t he net wor k int erf ace car d mal fun ct ions, t he backbone is br oken

n d must be t ied t oget her aga in wit h a jumper of some sor t or t he net work might cease

o funct ion pr operl y.

The Ring Network

A r ing n et wor k t opol ogy is oft en dr awn as it s na me suggest s, sha ped l ike a r ing. A

ypical r ing n et wor k schemat ic is shown in Figur e 1.3. You might ha ve hea r d of a token

ing networkbefor e, which is a r ing t opol ogy net wor k. You might be disappoint ed t o find

o physical r ing ar chit ect ur e in a r ing net wor k, t hough.

igur e 1.3. A schemat ic of a r ing net wor k.

Despit e t he al most au t omat ic assumpt ion t ha t a r ing

net wor k has a backbone wit h t he ends of t he cabl e joined t o

for m a l oop, t her e is no r eal cabl ing r ing at al l . The r ing na me

der ives fr om t he const r uct ion of t he cent r al cont r ol unit .

The t er m ringis a misnomer because ring net works don't ha ve an un ending ca bl e l ike a

us net wor k wit h t he t wo t erminat ors joined t oget her . Inst ead, t he r ing r efer s t o t he

esign of t he cent r al unit t hat ha ndl es t he net wor k's message passing. In a t oken r ing

et work, t he cent r al cont r ol un it is cal l ed a Media Access Unit , or MAU. The MAU ha s

r ing cir cuit inside it (for which t he n et work t opol ogy is na med). The r ing inside t heMAU ser ves as t he bus for devices t o obt a in messages.

The Hub Network

A hu b net work u ses a main cabl e much l ike t he bus net work, which is cal l ed t he

ackplane.The hu b t opol ogy is shown in Figur e 1.4. Fr om t he backpl an e, a set of ca bl eseads t o a h ub, which is a box cont aining severa l por t s int o which devices ar e pl ugged.

The cabl es t o a connect ion point ar e of t en cal l ed drops, because t hey dr op fr om t he

ackpl an e t o t he port s.
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igur e 1.4. A schemat ic of a hub net wor k.

Hub net works can be very l ar ge, using a high-speed fiber opt ic backpl an e an d sl ight l y

l ower Et her net dr ops t o hu bs fr om which a workgr oup can be suppor t ed. The h ub

et work can al so be smal l , wit h a coupl e of hu bs suppor t ing a few devices connect ed

oget her by st an dar d Et her net cabl es. The hu b net wor k is scal eabl e (meaning you can

t ar t smal l an d expan d as you n eed t o), which is par t of it s at t r act ion.

Hub net wor ks ha ve become popul ar for l ar ge inst al l at ions, in par t because t hey ar e

asy t o set up an d maint ain. They a l so can be t he l east expensive syst em in man y l ar ger

nst al l at ions, which adds t o t heir at t r act ion. The backpl an e can ext end across a

onsider abl e dist an ce just l ike a bus net work, wher eas t he por t s, or connect ion point s,

r e usua l l y grou ped in a set pl aced in a box or pan el . Ther e can be man y pan el s or

onnect ion boxes at t ached t o t he backpl an e.

Wide Area Networks

As I ment ioned ea r l ier , LANs can be combined int o a l ar ge ent it y cal l ed a WAN. WANs

r e usual l y composed of LANs joined t oget her by a h igh-speed l ink (such as a t el ephon e

ine or dedicat ed cabl e). At t he ent r an ce t o each LAN, one or mor e mach ines act as t he

ink bet ween t he LAN an d WAN: t hese ar e cal l ed gat eways. I t al k about gat eways andhe t ypes of gat eways used in a WAN in mor e det ail on man y of t he f ol l owing days, but

or now you need t o know onl y t hat a gat eway is t he int erfa ce bet ween a LAN and a

WAN. The same appl ies for an y LAN t ha t accesses t he Int er net : one mach ine u sua l l y

ct s as t he gat eway fr om t he LAN t o t he Int ernet (which is r eal l y just a very l ar ge

WAN).

Many t erms ot her t han gatewayar e al so used. You wil l hear t erms l ike routerand bridge.

They ar e al l gat eways, but t hey perfor m sl ight l y different t asks. To underst and t heir

ol es (which I ment ion man y t imes in t he n ext week's mat er ial ), you need t o t ake a qu ick

ook at how WANs ar e l aid out .

LANs can be t ied t o a WAN t hr ough a gat eway t ha t ha ndl es t he passage of dat a

et ween t he LAN an d WAN backbone. In a simpl e l ayou t , a r out er is used t o per for m t his

u nct ion . This is shown in Figur e 1.5.

igur e 1.5. A r out er connect s a LAN t o t hebackbone.

Anot her gat eway device, cal l ed a br idge, is used t o connect LANs using t he same

et wor k pr ot ocol . Br idges ar e used onl y when t he same net wor k pr ot ocol (such a s

TCP/IP) is on bot h LANs. The br idge does not ca r e which physica l media is used. Br idges


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an connect t wist ed-pair LANs t o coaxial LANs, for exampl e, or act as an int er fa ce t o a

iber opt ic net work. As l ong as t he net work pr ot ocol is t he same, t he br idge fu nct ions

r oper l y.

f t wo or mor e LANs ar e invol ved in on e or gan iza t ion a nd t her e is t he possibil it y of a

ot of t r aff ic bet ween t hem, it is bet t er t o connect t he t wo LANs dir ect l y wit h a br idge

nst ead of l oading t he backbone wit h t he cr oss-t r af fic. This is shown in Figur e 1.6.

igur e 1.6. Using a br idge t o connect t woLANs.

n a configur at ion u sing bridges bet ween LANs, t r af fic fr om one LAN t o an ot her can be

ent t hr ough t he br idge inst ead of ont o t he backbone, pr oviding bet t er per for man ce. For

er vices such as Tel net an d FTP, t he speed diff er ence bet ween u sing a br idge an d going

hr ough a r out er ont o a heavil y used backbone can be significant .

WANs ar e an impor t an t subject , an d I l ook at t hem aga in in mor e det ail on Day 13,Managing an d Tr oubl eshoot ing TCP/IP."

Layers

uppose you h ave t o writ e a pr ogra m t ha t pr ovides net working fu nct ions t o ever ymachine on your LAN. Wr it ing a singl e soft war e packa ge t ha t accompl ishes ever y t ask

equir ed for commun icat ions bet ween diff er ent comput er s woul d be a night mar ish t ask.

Apar t fr om having t o cope wit h t he differ ent ha r dwar e ar chit ect ur es, simpl y writ ing

he code for al l t he appl icat ions you desir e woul d resul t in a pr ogr am t hat was far t oo

ar ge t o execut e or maint ain.

Dividing al l t he r equir ement s in t o simil ar -pur pose gr oups is a sensibl e appr oach, much a s

pr ogr ammer br eaks code in t o l ogica l chunks. Wit h open syst ems communica t ions, gr oups

r e quit e obvious. One group deal s wit h t he t r ansport of dat a, anot her wit h t heacka ging of messages, an ot her wit h end-user appl icat ions, an d so on. Each gr oup of

el at ed t asks is cal l ed a layer.

The l ayer s of an ar chit ect ur e are meant t o be st and-

al one, independent ent it ies. They usua l l y cannot perf orm an yobservabl e t ask wit hout int eract ing wit h ot her l ayer s, but

fr om a pr ogr amming point of view t hey a r e sel f-cont ained.


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Of cour se, some cr ossover of f un ct iona l it y is t o be expect ed, an d sever al diff er ent

ppr oach es t o t he same division of l ayer s for a n et wor k pr ot ocol wer e pr oposed. One

ha t became adopt ed as a st an dar d is t he Open Syst ems Int er connect ion Refer ence

Model (which is discussed in mor e det ail in t he next sect ion). The OSI Refer ence Model

OSI-RM) uses seven l ayer s, as shown in Figur e 1.7. The TCP/IP ar chit ect ur e is simil ar but

nvol ves onl y five l ayer s, becau se it combines some of t he OSI fu nct iona l it y in t wo

ayer s int o one. For now, t hough , consider t he seven-l ayer OSI model .

igur e 1.7. The OSI Ref er ence Model showing al l seven l ayer s.

The a ppl icat ion, pr esent at ion, an d session l ayer s ar e al l appl icat ion-orient ed in t ha t

hey are r esponsibl e for pr esent ing t he appl icat ion int erfa ce t o t he user . Al l t hr ee are

ndependent of t he l ayers bel ow t hem an d ar e t ot al l y obl ivious t o t he means by which

at a get s t o t he appl icat ion. These t hr ee l ayer s ar e cal l ed t he upper l ayers.

The l ower four l ayers deal wit h t he t r ansmission of dat a, covering t he packaging,out ing, verificat ion, an d t r an smission of each dat a gr oup. The l ower l ayer s don't

wor r y about t he t ype of dat a t hey receive or send t o t he appl icat ion, but deal simpl y

wit h t he t ask of sending it . They don't differ ent iat e bet ween t he differ ent appl icat ions

n a ny wa y.

The fol l owing sect ions expl ain each l ayer t o hel p you un der st and t he ar chit ect ur e of

he OSI-RM (an d l at er cont r ast it wit h t he ar chit ect ur e of TCP/IP).

The Application Layer

The appl icat ion l ayer is t he end-user int er fa ce t o t he OSI syst em. It is wher e t he

ppl icat ions, such as el ect r onic mail , USENET news rea der s, or dat abase displ ay modul es,

eside. The a ppl icat ion l ayer 's t ask is t o displ ay r eceived inf or mat ion a nd send t he u ser 's

ew dat a t o t he l ower l ayer s.

n dist r ibut ed appl icat ions, such as cl ient /ser ver syst ems, t he appl icat ion l ayer is wher e

he cl ient appl icat ion r esides. It commun icat es t hr ough t he l ower l ayers t o t he server .

The Presentation Layer

The pr esent at ion l ayer 's t ask is t o isol at e t he l ower l ayers fr om t he appl icat ion's dat a

or mat . It convert s t he dat a f r om t he appl icat ion int o a common for mat , oft en cal l ed


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h e canonical representation.The pr esent at ion l ayer pr ocesses mach ine-dependent dat a

r om t he appl icat ion l ayer int o a machine-independent for mat for t he l ower l ayers.

The pr esent at ion l ayer is wher e fil e for mat s an d even cha r act er f or mat s (ASCII an d

EBCDIC, for exampl e) ar e l ost . The conver sion fr om t he appl icat ion da t a f or mat t akes

l ace t hr ough a "common n et work pr ogra mming l an gua ge" (as it is cal l ed in t he OSI

Reference Model document s) t ha t has a st r uct ur ed for mat .

The pr esent at ion l ayer does t he r everse for incoming dat a. It is convert ed fr om t he

ommon for mat int o appl icat ion-specific for mat s, based on t he t ype of appl icat ion t he

machine h as inst r uct ions for . If t he dat a comes in wit hout r eformat t ing inst r uct ions,

he inf or mat ion might not be assembl ed in t he cor r ect man ner for t he u ser 's appl icat ion.

The Session Layer

The session l ayer or ganizes an d synchr onizes t he exchan ge of dat a bet ween a ppl icat ion

r ocesses. It works wit h t he a ppl icat ion l ayer t o pr ovide simpl e dat a set s cal l ed

ynchronization pointst ha t l et an appl icat ion know how t he t r an smission an d recept ion of

at a a r e pr ogr essing. In simpl ified t er ms, t he session l ayer can be t hou ght of a s a t iming

nd f l ow cont ro l l ayer .

The session l ayer is invol ved in coor dina t ing commun icat ions bet ween diff er entppl icat ions, l et t ing each know t he st at us of t he ot her . An er r or in one appl icat ion

whet her on t he same mach ine or acr oss t he count r y) is ha ndl ed by t he session l ayer t o

et t he r eceiving appl icat ion know t ha t t he err or h as occur r ed. The session l ayer can

esynchr onize appl icat ions t hat ar e curr ent l y connect ed t o each ot her . This can be

ecessar y when commun icat ions ar e t empor ar il y int er r upt ed, or wh en an er r or h as

ccurr ed t hat r esul t s in l oss of dat a.

The Transport Layer

The t r an spor t l ayer , as it s na me suggest s, is designed t o pr ovide t he "t r an spar ent

r an sfer of dat a fr om a sour ce end open syst em t o a dest ina t ion en d open syst em,"

ccor ding t o t he OSI Refer ence Model . The t r an spor t l ayer est abl ishes, maint ains, an d

er mina t es commun icat ions bet ween t wo mach ines.

The t r ansport l ayer is r esponsibl e for ensur ing t hat dat a sent mat ches t he dat a

eceived. This ver ificat ion r ol e is import an t in ensur ing t hat dat a is cor r ect l y sent , wit h

r esend if an err or was det ect ed. The t r ansport l ayer man ages t he sending of dat a,


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et er mining it s or der an d it s pr ior it y.

The Network Layer

The net wor k l ayer pr ovides t he physical r out ing of t he dat a, det er mining t he pat h

et ween t he machines. The net wor k l ayer ha ndl es al l t hese r out ing issues, r el ieving

he h igher l ayer s fr om t his issue.

The net wor k l ayer examines t he net wor k t opol ogy t o det ermine t he best r out e t o send

message, as wel l as figur ing out r el ay syst ems. It is t he onl y net wor k l ayer t ha t sends

message fr om sour ce t o t ar get mach ine, man aging ot her chu nks of dat a t ha t pass

hr ough t he syst em on t heir way t o anot her machine.

The Data Link Layer

The dat a l ink l ayer, according t o t he OSI r eference paper , "pr ovides for t he cont r ol of

he physical l ayer, and det ect s an d possibl y cor r ect s err ors t ha t can occur ." In

r act ical it y, t he dat a l ink l ayer is r esponsibl e for cor r ect ing t r an smission er r ors

ndu ced dur ing t r an smission (as opposed t o er r or s in t he appl icat ion da t a it sel f, whichr e handl ed in t he t r anspor t l ayer ).

The dat a l ink l ayer is usual l y concerned wit h signal int erfer ence on t he physical

r an smission media, whet her t hr ough copper wir e, fiber opt ic cabl e, or micr owave.

nt er fer ence is common, r esul t ing f r om man y sour ces, incl uding cosmic r ays and st r ay

magn et ic int er fer ence fr om ot her sour ces.

The Physical Layer

The physical l ayer is t he l owest l ayer of t he OSI model an d deal s wit h t he "mechan ical ,

l ect r ical , fun ct ional , an d pr ocedur al means" r equir ed for t r an smission of dat a,

ccor ding t o t he OSI definit ion. This is r eal l y t he wir ing or ot her t r an smission f or m.

When t he OSI model was being devel oped, a l ot of concern deal t wit h t he l ower t woayers, becau se t hey ar e, in most cases, insepar abl e. The r eal wor l d t r eat s t he dat a l ink

ayer an d t he physical l ayer as one combined l ayer, but t he for mal OSI definit ion

t ipul at es diff er ent pur poses for each. (TCP/IP incl udes t he da t a l ink a nd physical


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ayer s as one l ayer , r ecognizing t ha t t he division is mor e academic t ha n pr act ical .)

Terminology and Notations

Bot h OSI and TCP/IP ar e r oot ed in f or mal descr ipt ions, pr esent ed as a ser ies of compl ex

ocument s t ha t define a l l aspect s of t he pr ot ocol s. To define OSI an d TCP/IP, sever al

ew t er ms wer e devel oped and int r oduced int o use; some (most l y OSI t er ms) ar e r at her

nu sual . You might find t he t erm OSI-speakused t o refer t o some of t hese ra t her

r ot esque definit ions, much as legaleser efer s t o l egal t erms.

To bet t er un derst an d t he det ail s of TCP/IP, it is necessar y t o deal wit h t hese t er ms now.

You won't see al l t hese t er ms in t his book, but you might encount er t hem when r eading

man ua l s or onl ine document at ion. Ther efore, al l t he major t erms ar e cover ed here.

Man y of t he t er ms used by bot h OSI and TCP/IP might seem

t o ha ve mul t ipl e meanings, but t her e is a definit e at t empt t o

pr ovide a singl e, consist ent definit ion f or each word.

Unfor t una t el y, t he user communit y is sl ow t o adopt newt er minol ogy, so t her e is a consider abl e amoun t of conf usion.

Packets

To t r an sfer dat a eff ect ivel y, many experiment s ha ve shown t ha t creat ing a un ifor m

hunk of dat a is bet t er t ha n sending char act ers singl y or in widel y var ying sized

r oups. Usua l l y t hese chun ks of dat a h ave some inf or mat ion ah ead of t hem (t he header)

n d somet imes an indicat or at t he end (t he trailer). These chunks of dat a a r e cal l ed

acketsin most synchr onous communica t ions syst ems.

The amount of dat a in a packet an d t he composit ion of t he hea der ca n cha nge depending

n t he commun icat ions pr ot ocol as wel l as some syst em l imit at ions, but t he concept of a

acket al ways r efers t o t he ent ir e set (incl uding header an d t r ail er) . The t ermpacketis

sed oft en in t he comput er indu st r y, somet imes when it shou l dn't be.


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You oft en see t he wor dpacketused as a gener ic r efer ence t o any gr oup of dat a packaged

or t r ansmission. As an appl icat ion's dat a passes t hr ough t he l ayers of t he a r chit ect ur e,

ach adds mor e inf or mat ion. The t er mpacketis f r equent l y used at each st age. Tr eat t he

ermpacketas a genera l izat ion for any dat a wit h a ddit ional infor mat ion, inst ead of t he

pecific r esul t of onl y one l ayer 's addit ion of h eader an d t r ail er. This goes against t he

ffor t s of bot h OSI an d t he TCP gover ning bodies, but it hel ps keep your san it y int act !

Subsystems

A subsystemis t he col l ect ive of a par t icul ar l ayer acr oss a n et wor k. For exampl e, if 10

mach ines ar e conn ect ed t oget her , each r un ning t he seven-l ayer OSI model , al l 10

ppl icat ion l ayers ar e t he appl icat ion subsyst em, al l 10 dat a l ink l ayers ar e t he dat a

ink subsyst em, an d so on. As you might ha ve al r eady deduced, wit h t he OSI Refer ence

Model t her e ar e seven subsyst ems.

t is ent ir el y possibl e (an d even l ikel y) t ha t al l t he individual component s in a

ubsyst em wil l not be act ive at one t ime. Using t he 10-mach ine exa mpl e aga in, onl y t hr ee

might have t he dat a l ink l ayer act ual l y act ive at any moment in t ime, but t he

umul at ive of al l t he mach ines makes up t he subsyst em.

Entities

A l ayer can have more t han one par t t o it . For exampl e, t he t r ansport l ayer can ha ve

out ines t hat ver ify checksums as wel l as r out ines t hat han dl e resending packet s t hat

idn't t r ansfer cor r ect l y. Not al l t hese rout ines ar e act ive at once, because t hey might

ot be r equir ed at an y moment . The a ct ive rout ines, t hough, ar e cal l ed ent it ies. The

word entitywas adopt ed in or der t o find a singl e t erm t ha t coul d not be confused wit h

n ot her comput er t er m such as modul e, pr ocess, or t ask.

N Notation

The not at ions N, N+1, N+2, an d so on ar e used t o ident ify a l ayer an d t he l ayers t ha t

r e rel at ed t o it . Referr ing t o Figur e 1.7, if t he t r an sport l ayer is l ayer N, t he physical

ayer is N3 an d t he pr esent at ion l ayer is N+2. Wit h OSI, N al ways ha s a val ue of 1hr ough 7 incl usive.

One r eason t his not at ion was adopt ed was t o enabl e wr it ers t o refer t o ot her l ayer s


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wit hout ha ving t o wr it e out t heir names every t ime. It al so makes fl ow char t s an d

iagr ams of int er act ions a l it t l e easier t o dr aw. The t er ms N+1 an d N1 ar e commonl y

sed in bot h OSI an d TCP for t he l ayers above and bel ow t he cur r ent l ayer,

espect ivel y, as you wil l see.

To make t hings even mor e confu sing, man y OSI st an dar ds r efer t o a l ayer by t he f ir st

et t er of it s name. This can l ead t o a r eal mess for t he casua l r eader , because "S-ent it y,"

5-ent it y," an d "l ayer 5" al l r efer t o t he session l ayer.

N-Functions

Each l ayer perf orms N-fun ct ions. The fu nct ions ar e t he differ ent t hings t he l ayer does.

Therefor e, t he funct ions of t he t r anspor t l ayer a r e t he dif ferent t asks t hat t he l ayerr ovides. For most pur poses in t h is book, fu nct ions and en t it ies mean t he same t h ing.

N-Facilities

This uses t he hiera r chical l ayer st r uct ur e t o expr ess t he idea t ha t one l ayer pr ovides a

et of fa cil it ies t o t he next higher l ayer . This is sensibl e, because t he appl icat ion l ayer

xpect s t he pr esent at ion l ayer t o pr ovide a r obust , wel l -defined set of fa cil it ies t o it . In

OSI-speak, t he (N+1)-ent it ies assume a def ined set of N-fa cil it ies fr om t he N-ent it y.

Services

The en t ir e set of N-fa cil it ies pr ovided t o t he (N+1)-ent it ies is cal l ed t he N-ser vice. In

t her wor ds, t he service is t he ent ir e set of N-fun ct ions pr ovided t o t he next higher

ayer . Ser vices might seem l ike fun ct ions, but t her e is a f or mal diff er ence bet ween t he

wo. The OSI document s go t o gr eat l engt hs t o pr ovide det ail ed descr ipt ions of ser vices,

wit h a "ser vice definit ion st an dar d" for each l ayer . This was necessar y dur ing t he

evel opment of t he OSI st an dar d so t hat t he differ ent t asks invol ved in t he

ommun icat ions pr ot ocol coul d be assigned t o differ ent l ayers, an d so t ha t t he

unct ions of each l ayer ar e bot h wel l -defined and isol at ed from ot her l ayer s.

The ser vice definit ions ar e for mal l y devel oped fr om t he bot t om l ayer (physical ) upwar d

o t he t op l ayer. The advant age of t his appr oach is t ha t t he design of t he N+1 l ayer can

e based on t he fu nct ions perf ormed in t he N l ayer, avoiding t wo funct ions t ha t


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ccompl ish t he same t ask in t wo adjacent l ayer s.

An en t ir e set of var iat ions on t he ser vice name ha s been devel oped t o appl y t hese

efinit ions, some of which ar e in r egul ar use:

An N-ser vice user is a u ser of a ser vice pr ovided by t he N l ayer t o t he n ext higher (N+1)

ayer .

An N-ser vice pr ovider is t he set of N-ent it ies t ha t ar e invol ved in pr oviding t he N l ayer

ervice.

An N-ser vice access poin t (oft en abbr evia t ed t o N-SAP) is wher e an N-ser vice is pr ovided

o a n (N+1)-ent it y by t he N-ser vice pr ovider .

N-ser vice dat a is t he packet of dat a excha nged at an N-SAP.

N-ser vice dat a un it s (N-SDUs) ar e t he individua l un it s of da t a

excha nged a t an N-SAP (so t ha t N-ser vice dat a is made u p of N-

SDUs).

These t er ms ar e shown in Figur e 1.8. Anot her common t er m is encapsulation,which is t he

ddit ion of cont r ol infor mat ion t o a packet of dat a. The cont r ol dat a cont ains

ddressing det ail s, checksums for err or det ect ion, an d pr ot ocol cont r ol fun ct ions.

igur e 1.8. Ser vice pr ovider s and ser vice user scommunicat e t hr ough ser vice access

oint s.

Making Sense of the Jargon

t is impor t an t t o r emember t ha t al l t hese t er ms ar e used in a f or mal descr ipt ion,ecau se a formal l an guage is usua l l y t he onl y met hod t o adequat el y describe

omet hing a s compl ex as a commun icat ions pr ot ocol . It is possibl e, t hou gh, t o f it t hese

erms t oget her so t ha t t hey make a l it t l e more sense when you encount er t hem. An

xampl e shou l d hel p.

The session l ayer ha s a set of session f un ct ions. It pr ovides a set of session f acil it ies t o

he l ayer above it , t he pr esent at ion l ayer . The session l ayer is made u p of session

nt it ies. The pr esent at ion l ayer is a u ser of t he ser vices pr ovided by t he session l ayerayer 5). A pr esent at ion en t it y is a user of t he ser vices pr ovided by t he session l ayer an d

s cal l ed a pr esent at ion ser vice user .


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The session ser vice pr ovider is t he col l ect ion of session ent it ies t ha t ar e act ivel y

nvol ved in pr oviding t he pr esent at ion l ayer wit h t he session's ser vices. The point at

which t he session service is pr ovided t o t he pr esent at ion l ayer is t he session service

ccess point , wher e t he session ser vice dat a is sent . The individua l bit s of da t a in t he

ession service dat a ar e cal l ed session ser vice dat a un it s.

Confusing? Bel ieve it or not , aft er a whil e you wil l begin t o feel more comfor t abl e

wit h t hese t erms. The import an t ones t o know now ar e t ha t a l ayer pr ovides a set ofnt it ies t hr ough a service access point t o t he next higher l ayer, which is cal l ed t he

er vice u ser .The dat a is sent in ch un ks cal l ed ser vice dat a, made up of ser vice dat a

n it s.

Queues and Connections

Commun icat ion bet ween t wo par t ies (whet her over a t el ephon e, bet ween l ayer s of an

r chit ect ur e, or bet ween appl icat ions t hemsel ves) t akes pl ace in t hr ee dist inct st ages:

st abl ishment of t he conn ect ion, dat a t r an sfer , an d connect ion t ermina t ion.

Commun icat ion bet ween t wo OSI appl icat ions in t he same l ayer is t hr ough queues t o t he

ayer beneat h t hem. Each a ppl icat ion (mor e pr oper l y cal l ed a ser vice user ) ha s t wo

ueues, one for each dir ect ion t o t he ser vice pr ovider of t he l ayer beneat h (which

ont r ol s t he whol e l ayer ). In OSI-speak, t he t wo queu es pr ovide for simul t an eous (ortomic) int er act ions bet ween t wo N-ser vice act ion point s.

Dat a, cal l ed service primitives,is put int o and r et r ieved fr om t he queue by t he

ppl icat ions (ser vice user s). A ser vice pr imit ive can be a bl ock of dat a, an indicat or t ha t

omet hing is r equired or r eceived, or a st at us indicat or . As wit h most aspect s of OSI, a

exicon h as been devel oped t o descr ibe t he a ct ions in t hese queues:

A request primitiveis when one ser vice submit s a ser vice pr imit ive t o t he qu eue (t hr ough

he N-SAP) r equest ing per mission t o commun icat e wit h a not her ser vice in t he same l ayer .

An indication primitiveis what t he ser vice pr ovider in t he l ayer beneat h t he sending

ppl icat ion sends t o t he int ended receiving appl icat ion t o l et it know t ha t

ommunicat ion is desir ed.

A response primitiveis sent by t he r eceiving appl icat ion t o t he l ayer beneat h's ser vice

r ovider t o acknowl edge t he gr an t ing of commun icat ions bet ween t he t wo ser vice user s.

A confirmation primitiveis sent fr om t he ser vice pr ovider t o t he

fina l appl icat ion t o indicat e t ha t bot h appl icat ions on t he

l ayer above can now commun icat e.


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An exampl e might hel p cl ar ify t he pr ocess. Assume t ha t t wo appl icat ions in t he

r esent at ion l ayer wan t t o commun icat e wit h each ot her . They can 't do so dir ect l y

accor ding t o t he OSI model ), so t hey must go t hr ough t he l ayer bel ow t hem. These st eps

r e shown in Figur e 1.9.

igur e 1.9. Two appl icat ions communicat e t hr ough SAPs using pr imit ives.

The f ir st appl icat ion sends a r equest pr imit ive t o t he service pr ovider of t he session

ayer an d wait s. The session l ayer 's ser vice pr ovider r emoves t he r equest pr imit ive fr om

he inboun d queue fr om t he fir st appl icat ion a nd sends an indicat ion pr imit ive t o t he

econd appl ica t ion 's inbound queu e.

The second a ppl icat ion t akes t he indicat ion pr imit ive fr om it s queu e t o t he session

er vice pr ovider an d decides t o accept t he r equest for conn ect ion by sending a posit ive

esponse pr imit ive back t hr ough it s queu e t o t he session l ayer . This is r eceived by t heession l ayer ser vice pr ovider , an d a conf ir mat ion pr imit ive is sent t o t he f ir st

ppl icat ion in t he pr esent at ion l ayer . This is a pr ocess cal l ed confirmed servicebecause

he appl icat ions wait for confir mat ion t ha t commun icat ions ar e est abl ished an d rea dy.

OSI a l so pr ovides for unconfirmed service,in wh ich a r equest pr imit ive is sent t o t he ser vice

r ovider , sending t he indicat ion pr imit ive t o t he second appl icat ion. The r esponse an d

onf ir mat ion pr imit ives ar e not sent . This is a sor t of "get r eady, becau se her e it comes

whet her you want it or n ot " commun icat ion, oft en r eferr ed t o as send and pray.

When t wo ser vice user s ar e using confir med ser vice t o commun icat e, t hey a r e consider ed

onnect ed. Two appl icat ions ar e t al king t o each ot her , awar e of what t he ot her is

oing wit h t he ser vice dat a. OSI r efer s t o t he est abl ishment an d maint ena nce of state

nformationbet ween t he t wo, or t he fact t hat each knows when t he ot her is sending or

eceiving. OSI cal l s t his connection-orientedor connection-modecommunica t ions.

Connectionlesscommun icat ion is when ser vice dat a is sent independen t l y, as wit h

n conf ir med ser vice. The ser vice dat a is sel f-cont ained, possessing ever yt hing a r eceivinger vice user needs t o know. These ser vice dat a packet s ar e oft en cal l ed datagrams.The

ppl icat ion t ha t sends t he dat agr am ha s no idea who receives t he dat agr am an d how it is

a ndl ed, and t he r eceiving service users have no idea who sent it (ot her t ha n

nfor mat ion t ha t might be cont ained wit hin t he dat agr am it sel f). OSI cal l s t his

onnectionless-mode.

OSI (an d TCP/IP) use bot h con nect ed an d connect ionl ess syst ems bet ween l ayer s of t heir

r chit ect ur e. Each ha s it s benefit s an d ideal impl ement at ions. Al l t hese commun icat ions

r e bet ween a ppl icat ions (ser vice user s) in each l ayer , using t he l ayer beneat h t oommun icat e. Ther e a r e man y ser vice user s, and t his pr ocess is going on al l t he t ime. It 's

uit e amaz ing when you t hink about it .


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Standards

eopl e don't quest ion t he n eed for r ul es in a boar d game. If you didn't ha ve ru l es, each

l ayer coul d be happil y pl aying as it suit s t hem, r egar dl ess of whet her t heir pl ay wasonsist ent wit h t hat of ot her pl ayer s. The exist ence of ru l es ensur es t hat each pl ayer

l ays t he game in t he same way, which might not be as much fu n a s a fr ee-for -al l .

However, when a f ight over a pl ayer 's act ions ar ises, t he writ t en r ul es cl ear l y indicat e

who is r ight . The r ul es ar e a set of st an dar ds by which a game is pl ayed.

t an dar ds pr event a sit ua t ion a r ising wher e t wo seemingl y compat ibl e syst ems r eal l y

r e not . For exampl e, 10 year s ago wh en CP/M was t he domina nt oper at ing syst em, t he

.25-inch fl oppy was used by most syst ems. But t he f l oppy f r om a Kaypr o II coul dn 't be

ead by an Osbour ne I because t he t r acks wer e l aid out in a differ ent man ner . A ut il it yr ogr am coul d conver t bet ween t he t wo, but t hat ext r a st ep was a major ann oyance for

machine user s.

When t he IBM PC became t he pl at for m of ch oice, t he 5.25-inch for mat used by t he IBM

C was adopt ed by ot her companies t o ensur e disk compat ibil it y. The IBM for mat became

de fa ct o st an dar d, one a dopt ed because of mar ket pr essur es an d cust omer deman d.

Setting Standards

Cr eat ing a st an dar d in t oday's wor l d is not a simpl e mat t er . Severa l organ iza t ions ar e

edicat ed t o devel oping t he st an dar ds in a compl et e, un ambiguou s man ner . The most

mport an t of t hese is t he Int ern at ional Or ganiza t ion for St an dar diza t ion, or ISO (oft en

al l ed t he Int ernat ional St andardizat ion Or ganizat ion t o f it t heir acronym, al t hough

his is incor r ect ). ISO consist s of st an dar ds or gan iza t ions fr om man y count r ies who t r yo agr ee on int ern at ional crit er ion. The American Nat ional St an dar ds Inst it ut e (ANSI),

Br it ish St an dar ds Inst it ut e (BSI), Deut sches Inst it ut fu r Nor mun g (DIN), an d Associat ion

r ancaise du Nor mal iza t ion (AFNOR) ar e a l l member gr oups. The ISO devel oped t he Open

yst ems Int er connect ion (OSI) st an dar d t ha t is discussed t hr oughou t t his book.

Each nat ion's st andar ds organizat ion can creat e a st andar d for t hat count r y, of

our se. The goal of ISO, however , is t o agr ee on worl dwide st an dar ds. Ot her wise,

ncompat ibil it ies coul d exist t ha t woul dn't al l ow one coun t r y's syst em t o be used in

n ot her . (An exa mpl e of t his is wit h t el evision signa l s: t he US r el ies on NTSC, wher easEur ope uses PALsyst ems t ha t ar e incompat ibl e wit h ea ch ot her .)

Cur iousl y, t he l an guage used for most int er nat ional st an dar ds is Engl ish, even t hough


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he major it y of par t icipan t s in a st an dar ds commit t ee ar e not fr om Engl ish-speaking

ount r ies. This can cau se quit e a bit of conf usion, especial l y because most st an dar ds ar e

wor ded awkwar dl y t o begin wit h.

The r eason most st an dar ds invol ve awkwar d l an gua ge is t ha t t o descr ibe somet hing

n ambiguousl y can be ver y diff icul t , somet imes necessit at ing t he cr eat ion of new t er ms

hat t he st andar d defines. Not onl y must t he concept s be cl ear l y defined, but t he

bsol ut e beha vior is necessar y t oo. Wit h most t hings t ha t st an dar ds appl y t o, t his meanssing nu mber s and physical t er ms t o pr ovide a concr et e definit ion. Defining a 2x4 piece of

umber necessit at es t he u se of a measur ement of some sor t , an d simil ar l y defining

omput er t er ms r equir es mat hemat ics.

impl y def in ing a met hod of communica t ions, such as TCP/IP, woul d be fa ir l y

t r aight for war d if it weren 't for t he compl icat ion of defining it for open syst ems. The

se of an open syst em adds an ot her diff icul t y because al l aspect s of t he st an dar d must

e mach ine-independen t . Imagine t r ying t o define a 2x4 wit hou t using a measur ement your e famil iar wit h, such as inch es, or if inches ar e adopt ed, it woul d be diff icul t t o define

nches in an u nambiguous way (which indeed is wha t ha ppens, because most unit s of

engt h ar e defined wit h r espect t o t he wavel engt h of a par t icul ar kind of coherent

ight ).

Comput er s commun icat e t hr ough bit s of dat a, but t hose bit s can r epr esent cha r act er s,

u mber s, or somet hing el se. Number s coul d be int eger s, fr act ions, or oct al

epr esent at ions. Again, you must define t he u nit s. You can see t ha t t he compl icat ions

mount , one on t op of t he ot her .

To hel p define a st an dar d, an abst r act appr oach is usual l y used. In t he case of OSI, t he

meaning (cal l ed t he seman t ics) of t he dat a t r an sfer r ed (t he abst r act synt ax) is fir st

eal t wit h, and t he exact r epr esent at ion of t he dat a in t he machine (t he concr et e

ynt ax) an d t he means by which it is t r ansferr ed (t r an sfer synt ax) ar e handl ed

epar at el y. The separ at ion of t he abst r act l et s t he dat a be r epr esent ed as an ent it y,

wit hout concer n for what it r eal l y means. It 's a l it t l e l ike t r eat ing your car as a unit

nst ead of a n en gine, t r an smission, st eering wheel , an d so on. The a bst r act ion of t he

et ail s t o a simpl er whol e makes it easier t o convey infor mat ion. ("My car is br oken" isbst r act , whereas "t he power st eer ing fl uid has al l l eaked out " is concr et e.)

To descr ibe syst ems abst r act l y, it is necessar y t o have a l an gua ge t ha t meet s t he

ur pose. Most st andar ds bodies ha ve devel oped such a syst em. The most commonl y u sed is

SO's Abst r act Synt ax Not at ion One, fr equent l y shor t ened t o ASN.1. It is suit ed

special l y for descr ibing open syst ems net wor king. Thu s, it 's not sur pr ising t o find it used

xt ensivel y in t he OSI and TCP descr ipt ions. Indeed, ASN.1 was devel oped concu r r ent l y

wit h t he OSI st an dar ds when it became necessar y t o descr ibe upper -l ayer fu nct ions.

The pr imar y concept of ASN.1 is t ha t al l t ypes of da t a, r egar dl ess of t ype, size, or igin, or

ur pose, can be r epr esent ed by an object t ha t is independent of t he ha r dwar e, oper at ing


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yst em soft war e, or appl icat ion. The ASN.1 syst em defines t he con t ent s of a dat agr am

r ot ocol headert he chun k of infor mat ion at t he beginn ing of an object t hat descr ibes

he cont ent s t o t he syst em. (Header s ar e discussed in mor e det ail in t he sect ion t it l ed

Pr ot ocol Headers" l at er in t his chapt er.)

ar t of ASN.1 descr ibes t he l an guage u sed t o descr ibe object s an d dat a t ypes (such as a

at a descr ipt ion l an gua ge in dat abase t er minol ogy). Anot her par t defines t he basic

ncoding r ul es t ha t deal wit h moving t he da t a object s bet ween syst ems. ASN.1 definesat a t ypes t ha t ar e used in t he const r uct ion of dat a packet s (dat agr ams). It pr ovides for

ot h st r uct ur ed an d unst r uct ur ed dat a t ypes, wit h a l ist of 28 support ed t ypes.

Don't be t oo worr ied about l ear ning ASN.1 in t his book. I

r efer t o it in passing in on l y a coupl e of pl aces. It is usefu l ,t hough, t o know t hat t he l anguage is pr ovided for t he formal

definit ion of al l t he a spect s of TCP/IP.

Internet Standards

When t he Defen se Advanced Resear ch Pr oject s Agency (DARPA) was est abl ished in 1980,

gr oup was for med t o devel op a set of st andar ds for t he Int ern et . The group, cal l ed t he

nt ern et Configur at ion Cont r ol Boar d (ICCB) was r eor ganized int o t he Int ern et

Act ivit ies Boar d (IAB) in 1983, whose t ask was t o design , engineer , and manage t he

nternet .

n 1986, t he IAB t ur ned over t he t ask of devel oping t he Int er net st an dar ds t o t hent er net Engineer ing Task For ce (IETF), an d t he l ong-t er m r esear ch was assigned t o t he

nt er net Resear ch Task For ce (IRTF). The IAB r et ained fina l au t hor iza t ion over

n yt hing pr oposed by t he t wo t ask for ces.

The l ast st ep in t his saga wa s t he for mat ion of t he Int er net Societ y in 1992, when t he

AB was r ena med t he Int er net Ar chit ect ur e Boar d. This gr oup is st il l r esponsibl e for

xist ing and fu t ur e st andar ds, r eport ing t o t he board of t he Int ernet Societ y.

Aft er a l l t ha t , what ha ppened dur ing t he shu ffl ing? Al most fr om t he beginning, t hent ernet was defined as "a l oosel y or ganized int erna t ional col l aborat ion of

u t onomous, int er conn ect ed net wor ks," which suppor t ed host -t o-host commun icat ions

t hr ough vol un t ar y adherence t o open pr ot ocol s an d pr ocedur es" defined in a t echnical


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aper cal l ed t he Int er net St an dar ds, RFC 1310,2. Tha t definit ion is st il l used t oday.

The IETF cont inu es t o work on r efining t he st an dar ds used for commun icat ions over t he

nt er net t hr ough a nu mber of wor king gr oups, each on e dedicat ed t o a specific aspect of

he overal l Int ernet pr ot ocol suit e. There a r e working groups dedicat ed t o net wor k

man agement , secur it y, user ser vices, rou t ing, and man y mor e t hings. It is int er est ing t ha t

he IETF's gr oups ar e considera bl y mor e fl exibl e an d efficient t ha n t hose of, say, t he

SO, whose wor king gr oups can t ake year s t o agr ee on a st an dar d. In man y cases, t heETF's gr oups can f or m, cr ea t e a r ecommendat ion , and disband wit h in a year or so. This

el ps cont inuousl y r ef ine t he Int ernet st andar ds t o ref l ect changing har dwar e and

oft war e capabil it ies.

Cr eat ing a n ew Int er net st an dar d (which ha ppened wit h TCP/IP) fol l ows a wel l -defined

r ocess, shown schemat ica l l y in Figur e 1.10. It begins wit h a r equest for comment (RFC).

This is usual l y a document cont a in ing a specific pr oposa l , somet imes new a nd somet imes a

modificat ion of a n exist ing st an dar d. RFCs ar e widel y dist r ibut ed, bot h on t he n et worksel f a nd t o int er est ed par t ies as pr int ed document s. Impor t an t RFCs an d inst r uct ions

or r et r ieving t hem ar e incl uded in t he appendixes at t he end of t his book.

igur e 1.10. The pr ocess for adopt ing a newInt e r ne t st a nd ar d .

The RFC is usual l y discussed for a whil e on t he net work it sel f, wher e anyone can

xpr ess t heir opinion, as wel l as in for mal IETF wor king gr oup meet ings. Aft er a suit abl e

moun t of r evision a nd con t inu ed discussion, an Internet draftis cr eat ed and dist r ibut ed.

This dr af t is cl ose t o fina l for m, pr oviding a consol idat ion of a l l t he comment s t he RFCener at ed.

The next st ep is usua l l y a proposed standard,which r emains as such for at l east six

mont hs. Dur ing t his t ime, t he Int ern et Societ y r equir es at l east t wo independent an d

nt er oper abl e impl ement at ions t o be writ t en a nd t est ed. Any pr obl ems ar ising fr om t he

ct ua l t est s can t hen be addressed. (In pr act ice, it is usual for man y impl ement at ions t o

e wr it t en and given a t hor ough t est ing.)

Aft er t ha t t est ing an d r efinement pr ocess is compl et ed, a draft standardis wr it t en, which

emains for at l east fou r mont hs, dur ing which t ime man y mor e impl ement at ions ar e

evel oped and t est ed. The l ast st epaf t er man y mont hsis t he adopt ion of t he

t an dar d, at which point it is impl ement ed by al l sit es t ha t r equir e it .

Protocols

Dipl omat s fol l ow ru l es when t hey conduct business bet ween n at ions, which you see


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eferr ed t o in t he media a s pr ot ocol . Dipl omat ic pr ot ocol r equir es t ha t you don't insul t

our host s an d t hat you do respect l ocal cust oms (even if t ha t means you ha ve t o eat

ome u nappet izing dinn er s!). Most embassies an d commissions ha ve special ist s in pr ot ocol ,

whose fu nct ion is t o ensur e t ha t ever yt hing pr oceeds smoot hl y when commun icat ions

r e t aking pl ace. The pr ot ocol is a set of r ul es t ha t must be fol l owed in or der t o "pl ay

he game," as car eer dipl omat s ar e fond of saying. Wit hout t he pr ot ocol s, one side of t he

onver sat ion might not r eal l y under st and what t he ot her is saying.

imil ar l y, comput er pr ot ocol s define t he man ner in which commun icat ions t ake pl ace. If

ne comput er is sending infor mat ion t o anot her an d t hey bot h fol l ow t he pr ot ocol

r oper l y, t he message get s t hr ough, r egar dl ess of what t ypes of mach ines t hey ar e and

wha t oper at ing syst ems t hey r un (t he basis for open syst ems). As l ong a s t he mach ines

a ve soft war e t ha t can man age t he pr ot ocol , commun icat ions ar e possibl e. Essent ial l y,

comput er pr ot ocol is a set of r ul es t ha t coordina t es t he exchan ge of infor mat ion.

r ot ocol s ha ve devel oped fr om ver y simpl e pr ocesses ("I'l l send you on e cha r act er , youend it back, an d I'l l make sur e t he t wo mat ch") t o el abor at e, compl ex mechan isms t ha t

over al l possibl e pr obl ems an d t r an sfer condit ions. A t ask such as sending a message

r om one coast t o anot her can be ver y compl ex when you consider t he man ner in which

moves. A singl e pr ot ocol t o cover a l l aspect s of t he t r an sfer woul d be t oo l ar ge,

n wiel dy, an d overl y special ized. Ther efor e, sever al pr ot ocol s ha ve been devel oped,

ach ha ndl ing a specific t ask.

Combining sever al pr ot ocol s, each wit h t heir own dedicat ed pur poses, woul d be a

ight mar e if t he int er act ions bet ween t he pr ot ocol s wer e not cl ear l y defined. Theoncept of a l ayered st r uct ur e was devel oped t o hel p keep each pr ot ocol in i t s pl ace

n d t o define t he man ner of int er act ion bet ween each pr ot ocol (essent ial l y, a pr ot ocol

or commun icat ions bet ween pr ot ocol s!).

As you saw ea r l ier , t he ISO ha s devel oped a l ayer ed pr ot ocol syst em cal l ed OSI. OSI

efines a pr ot ocol as "a set of r ul es an d for mat s (seman t ic an d synt act ic), which

et er mines t he commun icat ion behavior of N-ent it ies in t he per for man ce of N-fu nct ions."

You might r emember t ha t Nr epr esent s a l ayer , an d an en t it y is a ser vice component of a

ayer .

When mach ines commun icat e, t he r ul es ar e for mal l y defined a nd a ccoun t for possibl e

nt err upt ions or f au l t s in t he fl ow of infor mat ion, especial l y when t he fl ow is

onnect ionl ess (no f or mal connect ion bet ween t he t wo mach ines exist s). In such a

yst em, t he abil it y t o pr oper l y rou t e and verify each packet of dat a (dat agr am) is

it al l y import an t . As discussed ear l ier, t he dat a sent bet ween l ayers is cal l ed a ser vice

at a u nit (SDU), so OSI defines t he a na l ogous dat a bet ween t wo machines as a pr ot ocol

at a u nit (PDU).

The fl ow of infor mat ion is cont r ol l ed by a set of act ions t ha t define t he st at e machine

or t he pr ot ocol . OSI defines t hese act ions as pr ot ocol cont r ol inf or mat ion (PCI).


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Breaking Data Apart

t is necessar y t o in t r oduce a f ew mor e t er ms commonl y used in OSI and TCP/IP, but

uckil y t hey ar e readil y un der st ood becau se of t heir r eal -wor l d conn ot at ions. These

er ms ar e necessar y because dat a doesn't usual l y exist in man ageabl e chun ks. The dat a

might ha ve t o be br oken down int o smal l er sect ions, or sever al smal l sect ions can be

ombined int o a l ar ge sect ion for mor e efficient t r an sfer . The basic t er ms ar e as fol l ows:

egmentationis t he pr ocess of br eaking an N-ser vice dat a un it (N-SDU) int o sever al N-

r ot ocol dat a un it s (N-PDUs).

eassemblyis t he pr ocess of combining sever a l N-PDUs in t o an N-SDU (t he r ever se ofegment at ion).

lockingis t he combina t ion of sever al SDUs (which might be fr om diff er ent ser vices) int o

l ar ger PDU wit hin t he l ayer in which t he SDUs originat ed.

Unblockingis t he br eaking up of a PDU int o sever al SDUs in t he same l ayer .

Concatenationis t he pr ocess of on e l ayer combining sever al N-PDUs fr om t he n ext higher

ayer int o one SDU (l ike bl ocking except occur r ing acr oss a l ayer boun dar y).

Separationis t he rever se of concat enat ion, so t hat a l ayer

br eaks a singl e SDU int o severa l PDUs for t he n ext l ayer

higher (l ike unbl ocking except acr oss a l ayer bounda r y).

These six pr ocesses ar e shown in Figur e 1.11.

igur e 1.11. Segment at ion, r ea ssembl y, bl ock ing,unbl ocking, concat enat ion, andepar a t ion.

ina l l y, her e is one l ast set of definit ions t ha t deal wit h connect ions:

Multiplexingis when sever al connect ions ar e suppor t ed by a singl e conn ect ion in t he next

ower l ayer (so t hr ee pr esent at ion service connect ions coul d be mul t ipl exed int o a

ingl e session con nect ion).

Demultiplexingis t he r everse of mul t ipl exing, in which one conn ect ion is spl it int o severa l

onnect ions for t he l ayer a bove it .


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plittingis when a singl e connect ion is suppor t ed by sever al connect ions in t he l ayer

el ow (so t he dat a l ink l ayer might have t hr ee connect ions t o support one net wor k

ayer conn ect ion).

Recombiningis t he r ever se of spl it t ing, so t hat severa l

connect ions ar e combined int o a singl e one for t he l ayer above.

Mul t ipl exing a nd spl it t ing (and t heir r ever ses, demul t ipl exing a nd r ecombining) ar e

iffer ent in t he man ner in which t he l ines ar e spl it . Wit h mul t ipl exing, sever al

onnect ions combine int o one in t he l ayer bel ow. Wit h spl it t ing, however, one

onnect ion can be spl it int o sever al in t he l ayer bel ow. As you might expect , each h as

s impor t ance wit h in TCP an d OSI.

Protocol Headers

r ot ocol cont r ol infor mat ion is infor mat ion about t he dat agr am t o which it is

t t ached. This infor mat ion is usua l l y assembl ed int o a bl ock t ha t is at t ached t o t he

r ont of t he dat a it accompan ies an d is cal l ed a headeror protocol header.Pr ot ocol

eaders ar e used for t r an sfer r ing infor mat ion bet ween l ayers as wel l as bet ween

mach ines. As ment ioned ear l ier , t he pr ot ocol hea ders ar e devel oped accor ding t o r ul es

a id down in t he ISO's ASN.1 document set .

When a pr ot ocol header is passed t o t he l ayer beneat h, t he dat agr am incl uding t he

ayer 's header is t r eat ed as t he ent ir e dat agra m for t hat r eceiving l ayer , which adds it s

wn pr ot ocol header t o t he fr ont . Thus, if a dat agra m st ar t ed at t he appl icat ion l ayer ,

y t he t ime it r eached t he physical l ayer, it woul d have seven set s of pr ot ocol headers

n it . These l ayer pr ot ocol headers ar e used when moving back up t he l ayer st r uct ur e;

hey ar e st r ipped off as t he dat agr am moves up. An il l ust r at ion of t his is shown in Figur e

.12.

igu r e 1.12. Adding each l ayer ' s pr o t oco l header t o u se r d at a .

t is easier t o t hink of t his pr ocess as l ayer s on an onion. The inside is t he dat a t ha t is t o

e sent . As it passes t hr ough ea ch l ayer of t he OSI model , an ot her l ayer of onion skin is

dded. When it is finished moving t hr ough t he l ayer s, severa l pr ot ocol hea ders ar e

ncl osing t he dat a. When t he dat agr am is passed back up t he l ayer s (pr obabl y on

not her machine), each l ayer peel s off t he pr ot ocol header t hat cor r esponds t o t he

ayer . When it r eaches t he dest inat ion l ayer , onl y t he dat a is l ef t .

This pr ocess makes sense, becau se each l ayer of t he OSI model r equir es diff er ent

nfor mat ion fr om t he dat agr am. By using a dedicat ed pr ot ocol header for each l ayer of

he dat agr am, it is a r el at ivel y simpl e t ask t o r emove t he pr ot ocol header, decode it s


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nst r uct ions, an d pass t he r est of t he message on. The al t ern at ive woul d be t o have a

ingl e l ar ge header t hat cont ained al l t he informat ion, but t his woul d t ake l onger t o

r ocess. The exact cont ent s of t he pr ot ocol header a r e not import an t r ight now, but I

xamine t hem l at er when l ooking at t he TCP pr ot ocol .

As usua l , OSI ha s a for mal descript ion for al l t his, which st at es t ha t t he N-user dat a t o

e t r an sfer r ed is pr epended wit h N-pr ot ocol cont r ol inf or mat ion (N-PCI) t o for m an N-

r ot ocol da t a unit (N-PDU). The N-PDUs ar e passed a cr oss an N-ser vice a ccess poin t (N-AP) as one of a set of ser vice par amet er s compr ising an N-ser vice da t a unit (N-SDU). The

er vice par amet er s compr ising t he N-SDU ar e ca l l ed N-ser vice user da t a (N-SUD), which

s pr epended t o t he (N1)PCI t o for m anot her (N1)PDU.

or every s

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