overview of computer networks

7/24/2019 Overview of Computer Networks

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Overview of Computer NetworksNorman Matloff

Dept. of Computer Science

University of California at Davis

c

2001-200! N. Matloff

"pril 11! 200

Contents1 Significance of Networks 3

1.1 #istory . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $

1.2 %&at "re Networks Use' (or) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $

1.$ %&ic& "spects of Networks "re *mportant to +now) . . . . . . . . . . . . . . . . . . . . . $

2 An Introductory Example 3

3 Te !amous" #$errated %ut &seful '()ayer *odel +

$.1 Overview of t&e ,ayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

$.1.1 &ysical ,ayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

$.1.2 Data ,ink ,ayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . $.1.$ Network ,ayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

$.1./ ransport ,ayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

$.1. Session ,ayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

$.1. resentation ,ayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

$.1. "pplication ,ayer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

$.2 #ow t&e ,ayers *nteract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

, *ore on TC-.I- /

/.1 C* Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

1

/.1.1 C . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

/.1.2 UD . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

/.1.$ Stream 5s. Data6ram Communication . . . . . . . . . . . . . . . . . . . . . . . . . 10

/.1./ * "''resses . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

/.1. eer Communication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

/.1. 5iewin6 Current Socket Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

/.1. %&at Makes a Connection Uni7ue . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

/.2 Sample C* "pplication8 N(S . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

+ Network -rogramming 13

.1 C Socket 9:ample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1$

.1.1 Source Co'e . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1/

.1.2 %&o S&all * Say *s Callin6) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

.2 UD Socket 9:amples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

.2.1 ;asic 9:ample . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20.2.2 "'vance' Use of Sockets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21

.$ Non


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&e key to t&e computer revolution of t&e 1430s was t&e invention of t&e microprocessor in t&e 140s.

&is

6ave rise to t&e c&eap computers w&ic& &ave


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&e N*C also &as a ,"N a''ress! typically a /3-! ftp! ss an' so on.2"not&er similar term is intranet! w&ic& refers to a private collection of networks! say wit&in a particular company! w&ic&

we

'o not wis& to make accessi will


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an' it 'oes not always correspon' to specific portions of specific networks. Nevert&eless! it serves as

a useful

overview of t&e fiel'. #ere is &ow some of t&e layers relate to our sample network a


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Suppose saturnFs messa6e to olstein consists of a lar6e file transfer! say 100 me6a


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*nformation is communicate' from one layer to t&e ne:t.4 (or instance! t&ink of t&e file-transfer

e:ample

presente' earlier. &e file-transfer pro6ram! say ftp! works in t&e "pplication ,ayer. *t will call socket

service functions in t&e Session ,ayer! suc& as t&e socket to t&e 9t&ernet N*C! w&ic& will use t&e Data ,ink an' &ysical

,ayers to

put frames out onto t&e ,"N. "t any 6iven layer! a function passes t&e messa6e in a packet of


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C* is a very comple: system! t&e su is 'roppe'. UDFs virtue is t&at it is simple an' t&us

&as

very little over&ea'! compare' to C! w&ic& spen's a lot of time ne6otiatin6 an' maintainin6 a

connection


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Similarly! suppose we are


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one lon6 stream! t&e runnin6 on t&e remote mac&ine.1

(or t&e application pro6rammer! it woul'


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1$123 ! w&ic& in turn uses UD. GC is Bust w&at t&e name implies8 " pro6ram

can


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actually call a functionAwit& parametersAw&ic& will e:ecute on anot&er mac&ine. %e &ave earlier referre' to UDFs

?unrelia! t&e function open! an' on t&e former mi6&t test wps as follows8toto% wps garnacha.engr.ucdavis.edu w

1:01pm up 37 days, 17:54, 2 users, load average: 0.06, 0.00, 0.00

User tty login@ idle JCPU PCPU what

matloff ttyp1 12:58pm 1 4 1 script

matloff ttyp2 1:00pm 1 2 1 w

bslouie ttyp3 11:43am 1 42 13 tin-new

toto%

=Note t&at my name appeare' in t&e output. &is is Bust a coinci'ence! arisin6 from t&e fact t&at * &a'

telnet-e' into garnaca from toto in or'er to start up s$r.>14Kou mi6&t not &ave use' t&ese


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5 w and ps commands.

6

7 User can check load on machine without logging in

8 (or even without being able to log in).

9

10 Usage: wps remotehostname command (where "command"

11 is either w or ps).

12 */

13

14

15 /* these are needed for socket calls */

16 #include

17 #include

18 #include

19 #include

20 #include

21

22

23 #define WPSPORT 2000 /* server port number */

24 #define BUFSIZE 1000

2526

27 main(argc,argv)

28 int argc; char **argv;

29

30 { int SD,MsgSize;

31 struct sockaddr_in Addr;

32 struct hostent *HostPtr,*gethostbyname();

33 char Buf[BUFSIZE];

34

35 /* open a socket */

36 SD = socket(AF_INET,SOCK_STREAM,0);

40

41 /* set up for an Internet connection to the host whose

42 name was specified by the user on the command line */43 Addr.sin_family = AF_INET;

44 Addr.sin_port = WPSPORT;

45 /* get IP address of host */

46 HostPtr = gethostbyname(argv[1]);

47 memcpy(&Addr.sin_addr.s_addr,

48 HostPtr->h_addr_list[0],HostPtr->h_length);

49

50 /* OK, now connect */

51 connect(SD,&Addr,sizeof(Addr));

55

56 /* send user command */

57 write(SD,argv[2],strlen(argv[2]));

58

59 /* display response */

60 MsgSize = read(SD,Buf,BUFSIZE);

1/61 write(1,Buf,MsgSize);

62 }

63

1 /* WPsServer.c */

2

3 /* A server for remote versions of the w and ps

4 commands.

5

6 User can check load on machine without logging

7 in (or even without being able to log in).

8 */9

10


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11 /* these are needed for socket calls */

12 #include

13 #include

14 #include

15 #include

16 #include

17

18

19 /* this is needed for the disk read */

20 #include

21

22

23 #define WPSPORT 2000 /* server port number */

24 #define BUFSIZE 1000

25

26

27 int ClntDescriptor, /* socket descriptor to client */

28 SrvrDescriptor; /* socket descriptor for server */

29

30

31 char InBuf[BUFSIZE], /* messages from client */32 OutBuf[BUFSIZE]; /* messages to client */

33

34

35 Write()

36

37 { int FD,NB;

38

39 FD = open("tmp.client",O_RDONLY);

40 NB = read(FD,OutBuf,BUFSIZE);

41 write(ClntDescriptor,OutBuf,NB);

42 unlink("tmp.client");

43 }

44

4546 Respond()

47

48 { memset(OutBuf,0,sizeof(OutBuf)); /* clear buffer */

49 if (!strcmp(InBuf,"w"))

50 system("w > tmp.client");

51 else if (!strcmp(InBuf,"ps"))

52 system("ps -ax > tmp.client");

53 else

54 system("echo invalid command > tmp.client");

155 Write();

56 }

57

58

59 main(argc,argv)

60 int argc; char **argv;

61

63 { struct sockaddr_in BindInfo;

66

67 /* create an Internet TCP socket */

68 SrvrDescriptor = socket(AF_INET,SOCK_STREAM,0);

73

74 /* bind it to port 2000 (> 1023, to avoid the

75 "well-known ports"), allowing connections from

76 any NIC */

77 BindInfo.sin_family = AF_INET;

78 BindInfo.sin_port = WPSPORT;79 BindInfo.sin_addr.s_addr = INADDR_ANY;

80 bind(SrvrDescriptor,&BindInfo,sizeof(BindInfo));


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86

87 /* OK, set queue length for client calls */

88 listen(SrvrDescriptor,5);

91

93 while (1) {

94 /* wait for a call */

95 ClntDescriptor = accept(SrvrDescriptor,0,0);

106 /* read client command */

107 memset(InBuf,0,sizeof(InBuf));

108 read(ClntDescriptor,InBuf,sizeof(InBuf));

113 /* process the command */

114 Respond();

115 }

116 }

117

Note t&e Pinclu'e files. "lso! &ere are some ot&er notes on pro6rammin68

On Solaris mac&ines! you may nee' to a'' (lsocket an' possi! Bust

in case t&ere are still some packets to come in. &us a 6iven port will not


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%e are


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send

Now letFs take a look at t&e server co'e.

Line 6#:

#ere t&e server creates a socket.

Lines -#5:

&e system function 5ind


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Normally we 'o not nee' to know w&ic& ep&emeral port &as actually su


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(ollowin6 are a pair of pro6rams w&ic& communicate usin6 UD sockets81/* BasicCln.c */2

$/* introductory UDP example (client), with client sending a one-line

/message to server at port 4000 */

#include

#include 3#include

4#include

10#include 11

12main()1$

1/{ struct hostent *h_name;

1int sockfd;

1char buf[10];

1struct sockaddr_in your;13

14your.sin_family = AF_INET;

20your.sin_port = htons(4000);

21h_name = gethostbyname("sgi8.cs.ucdavis.edu");

22your.sin_addr.s_addr = *(u_long *) h_name->h_addr_list[0];2$

2/if ((sockfd = socket(AF_INET,SOCK_DGRAM,0)) < 0) {

2printf("socket error\n");

2exit(1);

2}23

24strcpy(buf,"OK");$0

$1if (sendto(sockfd,buf,strlen(buf),0,&your,sizeof(your)) < strlen(buf)) {

$2printf("send error\n");

$$exit(1);

$/}

$}

1/* BasicSrv.c */2

$/* introductory UDP example (server), with client sending a one-line

/message to server at port 4000 */

#include

#include

3#include

204#include

10#include 11

12main()1$

1/{ struct hostent *h_name;

1int sockfd;

1char buf[10];

1struct sockaddr_in mine;

13/* these two are needed as placeholders but are not used here: */

14struct sockaddr_in rcvaddr;

20int addlen;21

22mine.sin_family = AF_INET;

2$mine.sin_port = htons(4000);

2/mine.sin_addr.s_addr = INADDR_ANY;2

2if ((sockfd = socket(AF_INET,SOCK_DGRAM,0)) < 0) {

2printf("socket error\n");

23exit(1);

24}

$0$1if (bind(sockfd,(struct sockaddr *)&mine,sizeof(mine)) < 0) {

$2close(sockfd);


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$$printf("bind error\n");

$/exit(1);

$}$

$recvfrom(sockfd,buf,sizeof(buf),0,&rcvaddr,&addlen);

$3printf("%s\n",buf);

$4}

%e &ave use' t&e tons! it 'oes a call to

sendto81/* BCastCln.c */2

$/* introductory UDP example (client), with client sending a one-line

/message to all servers on the given network (see below), at port 4000

the server is still BasicSrv.c, no change */

3#include

4#include

10#include

11#include

12#include 1$

1/main()1

1{ struct hostent *h_name;

1int sockfd;

13char buf[10];

14struct sockaddr_in your;20

21const int turn_option_on = 1;22

2$your.sin_family = AF_INET;

2/your.sin_port = htons(4000);

2/* UCD CS and Engrg. machines are on the network 128.120.*.*,

2i.e. hex IP addresses 0x8078zzzz, so the broadcast address


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2replaces zzzz by ffff (after this program was written,

23the addresses became 169.237.*.*; in general, run

24/sbin/ifconfig to determine broadcast address */

$0your.sin_addr.s_addr = 0x8078ffff;$1

$2if ((sockfd = socket(AF_INET,SOCK_DGRAM,0)) < 0) {

$$printf("socket error\n");

$/exit(1);$}$

$setsockopt(sockfd,SOL_SOCKET,SO_BROADCAST,&turn_option_on,

$3sizeof(turn_option_on));$4

/0strcpy(buf,"OK");/1

/2if (sendto(sockfd,buf,strlen(buf),0,&your,sizeof(your)) < strlen(buf)) {

/$printf("send error\n");

//exit(1);

/}

/}

"not&er e:ample of t&e a'vance' use of sockets is raw sockets. #ere one can


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telnet pc8.cs.ucdavis.edu 1088

*f we 6et a response &ere

*n 6eneral! 'e


22/26

02 I-%yte B@ &e first four .

03 Eternet-ream5le@ Start-of-frame in'icator! a special /-


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2

So! t&e effect of t&e write fiel'. "fter creatin6 t&is packet! C at onda will pass it to * in t&e Network ,ayer

of

onda! alon6 wit& t&e information t&at t&e 'estination will

&e Data ,ink ,ayer will t&en create an 9t&ernet frame. *t will put 0:/12$/34a< for t&eDestination

9t&ernet *D! an' 0:12$/34a< for t&e Source 9t&ernet *D. *t will fill in 0:0300 for t&e ype fiel'.

"n'

it will put in t&e entire * packet it receive' from t&e Network ,ayer in for t&e current Data fiel'.

(inally!

t&e Data ,ink ,ayer will pass t&is 9t&ernet frame to t&e 9t&ernet 'evice 'river on onda.

&e 9t&ernet 'evice 'river on onda will t&en put t&e frame on network ;. "ll N*Cs on t&at network

will

see it! inclu'in6 t&e N*C 0:/12$/34a< on;eep.

&at N*C will say! ?O&! t&is frame is for meI@ Note t&at t&e N*C will not notice t&at t&e ultimate

'estination

of t&e frame is $enus all t&e N*C cares a


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own. "ll t&e N*C will 'o is pass t&is frame up to t&e ne:t layer at ;eep!2 w&ic& will

&e frame will t&en 6o up t&e protocol stack at $enus like it 'i' at;eep! . Suppose you are lma5ccom

an'

are sen'in6 e-mail to u$wxy:org! an' your messa6e is 6oin6 to


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1HELO abc.com

2MAIL FROM: [email protected]

$RCPT TO: [email protected]

/DATA

Hi, how have you been?

.

QUIT

=&e en' of t&e e-mail messa6e itself is in'icate' &e mac&ine woul' respon'


26/26

%in'ows networks use MicrosoftFs own name lookup system! %*NS.> ;riefly! your OS will &ave one

or

more DNS mac&ines w&ic& it can 7uery to convert t&e ?9n6lis&@ name! say wwwyaoocom to an *

num

overview of computer networks

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