CS 640: Comput er Net working Ashut osh Shukla Lect ure 3 Net work Programming Topics • Client -server model • Socket s int erf ace • Socket primit ives • Example code f or echoclient and echoserver • Debugging Wit h GDB • Programming Assignment 1 (MNS) Client / sever model • Client asks ( request ) –server provides ( response ) • Typically: single server - mult iple client s • The server does not need t o know anyt hing about t he client – even t hat it exist s • The client should always know somet hing about t he server – at least where it is locat ed 1. Client sends request Client Server Resource process process 4. Client 2. Server 3. Server sends response handles handles response request Note: clients and servers are processes running on hosts (can be the same or dif f erent hosts). Page 1
Socket s as means f or int er-process communicat ion (I PC) application layer application layer Internet Client Process Server Process Socket Socket transport layer (TCP/UDP) transport layer (TCP/UDP) OS network OS network Internet network layer (IP) network layer (IP) stack stack link layer (e.g. ethernet) link layer (e.g. ethernet) Internet physical layer physical layer The int erf ace t hat t he OS provides t o it s net working subsyst em I nt ernet Connect ions (TCP/ I P) • Address t he machine on t he net work – By I P address • Address t he process – By t he “port ”-number • The pair of I P-address + port – makes up a “ socket -address” Client socket address Server socket address 128. 2. 194. 242:3479 208. 216. 181. 15:80 Server Client (port 80) Connection socket pair (128. 2. 194. 242:3479, 208. 216. 181. 15:80) Client host address Server host address 128. 2. 194. 242 208. 216. 181. 15 Note: 3479 is an Note: 80 is a well- known port associated with Web servers ephemeral port allocated by the kernel Client s • Examples of client programs – Web browsers, ftp , telnet , ssh • How does a client f ind t he server? – The I P address in t he server socket address ident if ies t he host – The (well-known) port in t he server socket address ident if ies t he service, and t hus implicit ly ident if ies t he server process t hat perf orms t hat service. – Examples of well known port s • Port 7: Echo server • Port 23: Telnet server • Port 25: Mail server • Port 80: Web server Page 2
Using Port s t o I dent if y Services Server host 128. 2. 194. 242 Client host Service request f or Web server 128. 2. 194. 242:80 (port 80) (i. e. , the Web server) Kernel Client Echo server (port 7) Service request f or Web server 128. 2. 194. 242:7 (port 80) (i. e. , the echo server) Kernel Client Echo server (port 7) Servers • Servers are long-running processes (daemons). – Creat ed at boot -t ime (t ypically) by t he init process (process 1) – Run cont inuously unt il t he machine is t urned of f . • Each server wait s f or request s t o arrive on a well-known port associat ed wit h a part icular service. – Port 7: echo server See / etc/ services f or a comprehensive list of the – Port 23: t elnet server services available on a – Port 25: mail server Linux machine. – Port 80: HTTP server • Ot her applicat ions should choose bet ween 1024 and 65535 Socket s • What is a socket ? – To t he kernel, a socket is an endpoint of communicat ion. – To an applicat ion, a socket is a f ile descript or t hat let s t he applicat ion read/ writ e f rom/ t o t he net work. • Remember: All Unix I / O devices, including net works, are modeled as f iles. • Client s and servers communicat e wit h each by reading f rom and writ ing t o socket descript ors. • The main dist inct ion bet ween regular f ile I / O and socket I / O is how t he applicat ion “opens” t he socket descript ors. Page 3
Socket Programming Cliches • Net work Byt e Ordering – Net work is big-endian, host may be big- or lit t le-endian – Funct ions work on 16-bit (short ) and 32-bit (long) values – ht ons() / ht onl() : convert host byt e order t o net work byt e order – nt ohs() / nt ohl(): convert net work byt e order t o host byt e order – Use t hese t o convert net work addresses, port s, … struct sockaddr_in serveraddr; / * f ill in serveraddr with an address */ … / * Connect takes (struct sockaddr * ) as its second argument * / connect(clientf d, (struct sockaddr * ) &serveraddr, sizeof (serveraddr)); … • St ruct ure Cast s – You will see a lot of ‘st ruct ure cast s’ Socket primit ives • SOCKET : int socket(int domain , int type , int protocol) ; – domain := AF_I NET (I Pv4 prot ocol) – t ype := (SOCK_DGRAM or SOCK_STREAM ) – prot ocol := 0 (I PPROTO_UDP or I PPROTO_TCP) – ret urned : socket descript or ( sockf d ), -1 is an error • BI ND : int bind(int sockf d , struct sockaddr * my_addr , int addrlen ); – sockf d - socket descript or (ret urned f rom socket ()) – my_addr: socket address, st ruct sockaddr_in is used – addrlen := sizeof (st ruct sockaddr) struct sockaddr_in { unsigned short sin_family; /* address family (always AF_INET) */ unsigned short sin_port; /* port num in network byte order */ struct in_addr sin_addr; /* IP addr in network byte order */ unsigned char sin_zero[8]; /* pad to sizeof(struct sockaddr) */ }; • LI STEN : int listen(int sockf d , int backlog ); – backlog : how many connect ions we want t o queue • ACCEPT : int accept(int sockf d , void * addr , int * addrlen ); – addr : here t he socket -address of t he caller will be writ t en – ret urned: a new socket descript or (f or t he t emporal socket ) • CONNECT : int connect(int sockf d , struct sockaddr * serv_addr , int addrlen ); / / used by TCP client – paramet ers are same as f or bind() • SEND : int send(int sockf d , const void * msg , int len , int f lags ); – msg : message you want t o send – len: lengt h of t he message – f lags : = 0 – ret urned: t he number of byt es act ually sent • RECEI VE : int recv(int sockf d , void * buf , int len , unsigned int f lags ); – buf : buf f er t o receive t he message – len: lengt h of t he buf f er (“don’t give me more!”) – f lags := 0 – ret urned: t he number of byt es received Page 4
• SEND (DGRAM-st yle): int sendto(int sockf d , const void * msg , int len , int f lags, const struct sockaddr * to , int tolen ); – msg : message you want t o send – len: lengt h of t he message – f lags : = 0 – t o: socket address of t he remot e process – t olen : = sizeof (st ruct sockaddr) – ret urned: t he number of byt es act ually sent • RECEI VE (DGRAM-st yle): int recvfrom(int sockf d , void * buf , int len , unsigned int f lags, struct sockaddr * from , int * f romlen ); – buf : buf f er t o receive t he message – len: lengt h of t he buf f er (“don’t give me more!”) – f rom : socket address of t he process t hat sent t he dat a – f romlen := sizeof (st ruct sockaddr) – f lags := 0 – ret urned: t he number of byt es received • CLOSE : close ( socketf d ); Client +server: connect ionless CREATE BIND SEND RECEIVE SEND CLOSE Client +server: connect ion-orient ed BIND SOCKET LISTEN CONNECT ACCEPT TCP three-way handshake SEND RECEIVE SEND RECEIVE CLOSE Concurrent server Page 5
EchoClient .c – # include’s # include < st dio.h> / * f or print f () and f print f () */ # include < sys/ socket .h> / * f or socket (), connect (), sendt o(), and recvf rom() */ # include < arpa/ inet .h> / * f or sockaddr_in and inet _addr() */ # include < st dlib.h> / * f or at oi() and exit () */ # include < st ring.h> / * f or memset () */ # include < unist d.h> / * f or close() */ # def ine ECHOMAX 255 / * Longest st ring t o echo */ EchoClient .c -variable declarat ions int main(int argc, char * argv[]) { int sock; / * Socket descript or * / st ruct sockaddr_in echoServAddr; / * Echo server address */ st ruct sockaddr_in f romAddr; / * Source address of echo */ unsigned short echoServPort =7; / * Echo server port */ unsigned int f romSize; / * address size f or recvf rom() */ char * servI P=“172.24.23.4”; / * I P address of server */ char * echoSt ring=“I hope t his works”; / * St ring t o send t o echo server */ char echoBuf f er[ECHOMAX+1]; / * Buf f er f or receiving echoed st ring * / int echoSt ringLen; / * Lengt h of st ring t o echo */ int respSt ringLen; / * Lengt h of received response */ EchoClient .c - creat ing t he socket and sending / * Creat e a dat agram/ UDP socket */ sock = socket (AF_I NET, SOCK_DGRAM, 0); / * Const ruct t he server address st ruct ure */ memset (&echoServAddr, 0, sizeof (echoServAddr)); / * Zero out st ruct ure */ echoServAddr.sin_f amily = AF_I NET; / * I nt ernet addr f amily * / echoServAddr.sin_addr.s_addr = ht onl(servI P); / * Server I P address * / echoServAddr.sin_port = ht ons(echoServPort ); / * Server port */ / * Send t he st ring t o t he server * / sendt o(sock, echoSt ring, echoSt ringLen, 0, (st ruct sockaddr *) &echoServAddr, sizeof (echoServAddr); / * Recv a response */ Page 6
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