Računarske mreže. Čas 1. Ivana Tanasijević Matematički fakultet, Beograd
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1 Računarske mreže Čas 1 Ivana Tanasijević ivana@matf.bg.ac.rs Matematički fakultet, Beograd 1
2 Osnovni pojmovi 2
3 Mreža Za sistem na kome korisnik radi često se kaže da je lokalni dok su ostali sistemi udaljeni (eng. remote). Sistem koji vrši neku uslugu za drugi sistem naziva se server, dok je sistem za koji se usluga vrši klijent. Mreže mogu biti podeljene na podmreže. Računar preko koga podmreža komunicira sa ostatkom mreže naziva se gateway. Računari u mreži (eng. host) imaju svoja imena i IP adrese. 3
4 IP adresa IP adresa je jedinstven identifikator računara u mreži. Sastoji se od 4 bajta. Obično se zapisuje bajt po bajt: Adresa se deli na dva dela Mrežni deo bitovi manje težine identifikuju lokalnu mrežu Host deo bitovi manje težine identifikuju računare na mreži Veličina mrežnog dela se odredjuje mrežnom maskom. Mrežni deo odgovara bitovima maske najveće težine jednakim 1. 4
5 IP adresa Smatra se da su dva računara u istoj mreži ako njihove adrese u bitskoj konjunkciji sa mrežnom maskom daju iste rezultate. Ovakvi računari mogu da komuniciraju direktno, dok su u suprotnom potrebne informacije o putanji (rutiranju) izmedju njih (eng. Routing) Primer: IP adresa , maska , mrežna adresa
6 IP adresa Adresa sa host delom koji se sastoji od svih jedinica se naziva broadcast adresom i koristi se za slanje poruka svim računarima u mreži odjednom. IPv4 adresiranje 32 bitna adresa, ukupno adresa. IPv6 adresiranje 128 bitna adresa, 3.4x10^38 adresa, 2.29x1020 adresa na svaki cm kubni zemlje. 6
7 MAC adresa Na nižem nivou, mrežni hardver takodje koristi adrese. One se zovu MAC (media access control) adrese i predstavljene su sa 6 heksadekadnih cifara (00:50:BF:19:7E:99). MAC adrese su korisne samo za lokalne mreže. 7
8 DNS serveri IP adrese nisu zgodne za korišćenje od strane ljudi, pa se umesto njih koriste jasnija imena, kao: a prevođenje imena u IP adrese se vrši pomoću tzv. DNS (Domain Name Server). DNS koristi serije servera, od kojih je svaki odgovoran za neki deo Interneta. Lokalni DNS serveri, koji znaju za lokalne računare i kako da izvrše ispitivanje drugih DNS servera su najvažniji sa perspektive osnovne konfiguracije mreže. Neophodno je proslediti im IP adresu za bar jedan DNS server. 8
9 Razrešavanje imena U slučaju manjih mreža korišćenje DNS servera nije neophodno. Linux koristi /etc/hosts fajl za zapis informacija o imenima. Unosi su oblika: IP_adresa ime alijasi 9
10 DHCP server IP adrese mogu biti statički dodeljene računarima i u tom slučaju se ne menjaju, a mogu se i dinamički dodeljivati i tad su podložne promenama. Dinamičke adrese dodeljuje tzv. DHCP server (Dynamic Host Configuration Protocol). Ova dodela se vrši na određeno vreme tipično nekoliko sati. Posle određenog vremena klijent pokušava da obnovi adresu. Ovaj pokušaj nekad mora biti ponovljen. 10
11 Mrežni interfejs U Unix terminologiji mrežni interfejs je logički entitet koji podrazumeva mrežni adapter i njegovu konfiguraciju na nivou operativnog sistema. Posebna vrsta mrežnog interfejsa na Unix sistemima je loopback interfejs koji nema fizički adapter, već pakete koji se preko njega šalju vraća nazad na isti sistem, ali kao da su došli sa udaljenog računara. Njegova IP adresa je: , a ime localhost. 11
12 Mrežno programiranje 12
13 Neki sistemski pozivi Podsećanje funkcija iz standardne biblioteke stdio.h open, close, dup, dup2, fork write, read Primeri: cp, cp2, cp3, dup2, fork 13
14 Klijent i server Koncept na kome se bazira pisanje programa je klijent server koncept Primer: klijent kontaktira server da dobije informaciju o trenutnom vremenu. Moramo precizirati od kog procesa na serverskom računaru mi želimo informaciju. Postoje portovi (fajl deskriptori) na kojima možemo da komuniciramo sa programima na serveru. Npr. ako hoćemo poštu da prenesemo kod nas obraćamo se pop3 servisu. 14
15 Klijent i server Port za webserver je 80, za pop3 je 110, po dogovoru. Za naše programe ćemo koristiti portove >1000. Daytime primer 15
16 Protokoli Protokoli imaju 4 sloja Aplication layer dogovor izmedju klijenta i servera Transport layer često je TCP ili UDP seckanje poruke Network layer odnosi se na IP adrese Physical layer kog tipa je mreža, da li je bežična i slično. TCP (Transmission Control Protocol) ako se radi seckanje poruke i provera da li je svaki komad stigao. Korisnom sadržaju poruke moraju biti pridruženi brojači, odakle dokle je poslato, primljeno, potvrda prijema. UDP (User datagram Protocol) ako se ne radi seckanje i ne proverava se da li su poslati paketi i stigli. Ima smisla kod slanja video audio zapisa. 16
17 Adresna struktura Može da se prenosi se u dva pravca od kernela ka procesu i obrnuto. Kao argument funkcija se uglavnom prenosi pokazivač na ovu strukturu. Svaki protokol definiše svoju strukturu, a ime svake počinje sa sockaddr_(in, un) POSIX standard zahteva postojanje tri člana: sin_family, sin_addr i sin_port. - sin_family se odnosi na familiju IP adresa - sin_addr sadrži adresu sa kojom želimo da komuniciramo. Mora da bude network order. - sin_port vrednost takođe mora da bude network order Ove strukture se koriste samo na datom hostu. Funkcije za rad sa soketima su definisane tako da rade sa generičkim pokazivačima na ove strukture.
18 Funkcije za organizaciju bajtova Postoje dva načina da se smeste bajtovi u memoriju Little-endian: bajt najmanje težine na manju adresu Big-endian: bajt najveće težine na manju adresu Redosled na datoj mašini se zove host byte order, dok je dogovoreni redosled network byte order. Funkcije kojima se menja redosled su: htons, htonl, ntohs, ntohl. n-network, h-host, s-short, l-long
19 Funkcije za rad sa bajtovima Funkcije koje počinju sa str uglavnom rade sa stringovima, koji su terminirani nulom. Nama je potrebno da radimo sa bajtovima, neznajući koji je sadržaj. Za te svrhe koristimo bzero, bcopy, bcmp, memcpy.
20 Konverzija adresa Funkcije inet_aton, inet_addr i inet_ntoa, rade konverziju između IPv4 adrese predstavljene stringom i njene binarne vrednosti. Novije funkcije inet_pton i inet_ntop rade i sa IPv4 i sa IPv6 adresama (n-numeric, p-presentation).
21 Primer: Popunjavanje adresne strukture struct sockaddr_in address; /* Promenljiva za adresu. */ /* Server formira adresu na kojoj prihvata poruke od klijenata. */ bzero (&address, sizeof (struct sockaddr_in)); address.sin_family = AF_INET; address.sin_port = htons (PORT); address.sin_addr.s_addr = htonl (INADDR_ANY); /* Klijent formira adresu servera. */ bzero (&address, sizeof (struct sockaddr_in)); address.sin_family = AF_INET; address.sin_port = htons (PORT); if (inet_pton (AF_INET, argv[1], &address.sin_addr) <= 0) error_fatal ("%s inet_pton() error: %s\n", argv[0], strerror (errno));
22 Kreiranje soketa Komunikacija preko mreže se izvodi preko posebnih fajlova tipa soketa. Funkcija socket kreira soket. Ima tri argumenta: - familija protokola - može da bude jedna od konstanti PF_INET, PF_INET6, PF_LOCAL, itd. - tip soketa - SOCK_STREAM, SOCK_DGRAM, SOCK_ROW. - dodatne opcije - imaju smisla kod row soketa, u ostalim slučajevima se postavljaju na 0. PF_INET se odnosi na familiju protokola, dok se AF_INET odnosi na familiju adresa. Zamisao je bila da jedna familija protokola može da podrži više familija adresa, ali to nije zaživelo. PF_INET se koristi kod socket funkcije, dok se AF_INET koristi u adresnoj strukturi.
23 Primer: Kreiranje soketa int server; /* Fajl deskriptor za soket. */ /* Kreira se socket za komunikaciju sa klijentima ili sa serverom. */ if ((server = socket (PF_INET, SOCK_DGRAM, 0)) < 0) error_fatal ("%s socket() error: %s\n", argv[0], strerror (errno));
24 Privezivanje soketa Funkcija bind pridružuje lokalnu adresu soketu. Kod internet protokola lokalna adresa je kombinacija IP adrese i broja porta. Kako su serveri poznati upravo po portu na kome se nalaze, potrebno je da sami na ovaj način odrede port na kome će komunucirati, umesto da to uradi kernel. Pa tako nju poziva server da bi pridružio željeni port i adresu (ili jedno od njih ili oba ili nijedno) datom soketu. Ako se ne pozove ova funkcija, kernel sam pridruži neki efemeralni port soketu pri prvoj upotrebi soketa, na primer, kada se pozove connect ili listen funkcija. Adresa za koju se privezuje soket mora pripadati interfejsu datog hosta.
25 Privezivanje soketa Mogu se koristiti i džoker konstante. Za IP adrese je to INADDR_ANY, dok je za portove 0. Da bismo saznali koji efemeralni port nam je pridružio kernel koristimo funkciju getsockname. Primer kada se ne koriste džokeri: postoje organizacije u okviru jedne mreže sa prvim delom IP adrese istim. Primer kada se koriste: hoćemo da prihvatimo klijente koji se konektuju na svaku od ovih specifičnih adresa. Najčešća greška koju vraća bind je EADDRINUSE. Da bismo to izbegli, koristimo funkciju setsockopt i opciju SO_REUSEADDR.
26 Primer: Privezivanje soketa i postavljanje opcija /* Vezuje se socket za datu adresu i port na serveru. */ if (bind (server, (struct sockaddr *) &address, sizeof (struct sockaddr_in)) < 0) error_fatal ("%s bind() error: %s\n", program, strerror (errno)); /* Postavlja se odgovarajuca opcija socket-a. */ int on = 1; if (setsockopt(server, SOL_SOCKET, SO_REUSEADDR, &on, sizeof(int)) < 0) rror_fatal("%s setsockopt() error: %s\n", program, strerror(errno));
27 Slanje i prihvatanje paketa Može se izvesti funkcijama sendto i recvfrom. Obe imaju po 6 argumenata - prva tri su jednaka argumentima funkcije write i read, - četvrti je flag kojim se uključuju specifične opcije prenosa, - peti i šesti kod funkcije sendto su pokazivač na adresnu strukturu popunjenu informacijama o lokaciji kojoj se šalju podaci, kao i veličina te adresne strukture, - peti i šesti kod funkcije recvfrom su takodje pokazivač na adresnu strukturu i pokazivač na veličinu te adresne strukture, koji će biti popunjeni adresom sa koje je stigao paket. Funkcije vraćaju broj poslatih, odnosno primljenih bajtova.
28 Slanje i prihvatanje paketa U redu je da se pošalje paket veličine 0. Na taj način server može da dobije informaciju o tome ko je poslao paket, pa i da odgovori klijentu Takođe, u redu je i da recvfrom vrati 0. Kako ne postoji nešto kao što je konekcija (kao kod TCP), ovo ne znači raskid konekcije. Na ovaj način server može da dobije infomaciju adresi klijenta i da mu odgovori Ako se u funkciji recvform postavi za adresu klijenta NULL pokazivač, tada se i za dužinu te adrese takođe mora postaviti NULL pokazivač. Ovo se koristi kada server nije zainteresovan u to ko šalje pakete.
29 Primer: Slanje i prihvatanje paketa char bufer[buffer_size]; /* Bafer za razmenu poruka. */ int count; /* Duzina poruke. */ struct sockaddr_in address; /* Adresa klijenta/servera. */ socklen_t length; /* Duzina gornje adrese. */ /* Prosledjuje se poruka serveru. */ count = strlen (bufer); if (sendto (server, bufer, count, 0, address, length)!= count) error_fatal ("%s sendto() error: %s\n", program, strerror (errno)); /* Ucitava poruka od klijenta. */ length = sizeof (address); if ((count = recvfrom (server, bufer, BUFFER_SIZE, 0, (struct sockaddr *) &address, &length)) < 0) error_fatal ("%s recvfrom() error: %s\n", program, strerror (errno));
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