Sockets A popular API for client-server interaction

Complete view of a socket Application Presentation Session Transport Network Data Link Physical Application Presentation Session Transport Network Data Link Physical Applications talk to each other via APIs like sockets. Sockets provide the interface to the other layers.

Programmer View of a socket Application socket The networking aspects of a socket are transparent Similar to writing to and reading from a file /7055 IP PORT /1234 IP PORT server client

Socket API Three popular variations –berkley - older outdated unix API –TLI - newer unix version –winsock - microsoft All very similar Most examples in c but available in other languages

General Approach SERVER CLIENT Sets up a connection to wait for client (port) Asks for connection to server port Responds & sets up a different connection for the dialog Client asks for service Server responds time

SERVER CLIENT Sets up a connection to wait for client (port) Asks for connection to server port Responds & sets up a different connection for the dialog Client asks for service Server responds time /* get an internet domain socket */ if ((sd = socket(AF_INET, SOCK_STREAM, 0)) == -1) { /* bind the socket to the port number */ if (bind(sd, (struct sockaddr *) &sin, sizeof(sin)) == -1) { /* show that we are willing to listen */ if (listen(sd, 5) == -1) { /* grab an Internet domain socket */ if ((sd = socket(AF_INET, SOCK_STREAM, 0)) == -1) { /* connect to PORT on HOST */ if (connect(sd,(struct sockaddr *) &pin, sizeof(pin)) == -1) { /* wait for a client to talk to us */ if ((sd_current = accept(sd, (struct sockaddr *) &pin, &addrlen)) == -1) { /* send a message to the server PORT on machine HOST */ if (send(sd, argv[1], strlen(argv[1]), 0) == -1) { /* wait for a message to come back from the server */ if (recv(sd, dir, DIRSIZE, 0) == -1) { /* get a message from the client */ if (recv(sd_current, dir, sizeof(dir), 0) == -1) { /* get the directory contents */ read_dir(dir); /* acknowledge the message, reply w/ the file names */ if (send(sd_current, dir, strlen(dir), 0) == -1) {

#define PORT 0x1234 #define DIRSIZE 8192 main() { char dir[DIRSIZE]; int sd, sd_current, cc, fromlen, tolen; int addrlen; struct sockaddr_in sin; struct sockaddr_in pin; /* get an internet domain socket */ if ((sd = socket(AF_INET, SOCK_STREAM, 0)) == -1) { perror("socket"); exit(1); } /* complete the socket structure */ memset(&sin, 0, sizeof(sin)); sin.sin_family = AF_INET; sin.sin_addr.s_addr = INADDR_ANY; sin.sin_port = htons(PORT); /* bind the socket to the port number */ if (bind(sd, (struct sockaddr *) &sin, sizeof(sin)) == -1) { perror("bind"); exit(1); } /* show that we are willing to listen */ if (listen(sd, 5) == -1) { perror("listen"); exit(1); } /* wait for a client to talk to us */ if ((sd_current = accept(sd, (struct sockaddr *) &pin, &addrlen)) == -1) { perror("accept"); exit(1); } /* get a message from the client */ if (recv(sd_current, dir, sizeof(dir), 0) == -1) { perror("recv"); exit(1); } /* get the directory contents */ read_dir(dir); /* acknowledge the message, reply w/ the file names */ if (send(sd_current, dir, strlen(dir), 0) == -1) { perror("send"); exit(1); } /* close up both sockets */ close(sd_current); close(sd); /* give client a chance to properly shutdown */ sleep(1); } SERVER

CLIENT #define PORT 0x1234 /* REPLACE with your server machine name*/ #define HOST "america3" #define DIRSIZE 8192 main(argc, argv) int argc; char **argv; { char hostname[100]; char dir[DIRSIZE]; intsd; struct sockaddr_in sin; struct sockaddr_in pin; struct hostent *hp; strcpy(hostname,HOST); if (argc>2) { strcpy(hostname,argv[2]); } /* go find out about the desired host machine */ if ((hp = gethostbyname(hostname)) == 0) { perror("gethostbyname"); exit(1); } /* fill in the socket structure with host information */ memset(&pin, 0, sizeof(pin)); pin.sin_family = AF_INET; pin.sin_addr.s_addr = ((struct in_addr *)(hp->h_addr))->s_addr; pin.sin_port = htons(PORT); /* grab an Internet domain socket */ if ((sd = socket(AF_INET, SOCK_STREAM, 0)) == -1) { perror("socket"); exit(1); } /* connect to PORT on HOST */ if (connect(sd,(struct sockaddr *) &pin, sizeof(pin)) == -1) { perror("connect"); exit(1); } /* send a message to the server PORT on machine HOST */ if (send(sd, argv[1], strlen(argv[1]), 0) == -1) { perror("send"); exit(1); } /* wait for a message to come back from the server */ if (recv(sd, dir, DIRSIZE, 0) == -1) { perror("recv"); exit(1); } /* spew-out the results and bail out of here! */ printf("%s\n", dir); close(sd); }

Disconnect between Sending and Receiving CLIENTSERVER char one[]=“one”; char three[]=“3”; if (send(sd, one, 3, 0) == -1) {.. if (send(sd, three, 1, 0) == -1) {.. one 3 OS 3one (3eno) 3one Char recvbuf[100] if (recv(sd_current, recvbuf, 100, 0)... 3one A question of how the two synchronize their respective activities!

Not what you expect! CLIENTSERVER char one[]=“one”; char three[]=“3”; if (send(sd, one, 3, 0) == -1) {.. if (send(sd, three, 1, 0) == -1) {.. if (send(sd, one, 3, 0) == -1) {.. if (send(sd, three, 1, 0) == -1) {.. Char recvbuf[100] if (recv(sd_current, recvbuf, 100, 0) … if (recv(sd_current, recvbuf, 100, 0)... NOT a recv for every send! (but not as bad as you think)

Something like this CLIENTSERVER char one[]=“one#”; char three[]=“3#”; if (send(sd, one, 4, 0) == -1) {.. if (send(sd, three, 2, 0) == -1) {.. if (send(sd, one, 4, 0) == -1) {.. if (send(sd, three, 2, 0) == -1) {.. Char recvbuf[100] bool notdone=true; while (notdone) { if (recv(sd_current, recvbuf, 100, 0) … // iterate char by char if (recvbuf[i] == “#”) notdone = false; // only read when you exhaust all // of the recvbuff ……... Actually like if (i==bufflen-1) if (recv…. else i++;

Framing! Message1 Message2 Message3 Should be sent as Message1# Message2# Message3# And will be received as Message1#Message2#Message3# Sometimes the application is less demanding! When the input is predictable in both size and nature of data (like 230 programs)

C issues (Main) routine / parameter declaration –main(argc, argv) –int argc; char **argv; –{ Parameters by reference –int addone(a) – int *a; – {…} –called by x=7; addone (&x) I/O –scanf (“%d %d”,%x,%y) –printf (“Value of x is %d and y is %d”,&x,&y);

Adapting the example to your problem Setup and disconnect is very similar Interaction is not as simple as the example read/write are basically executed in your main loop.

Using C++ No standard library A library available online from Rob Tougher.. Linux Gazette See – sockets.html

How about java? Good support in java Will not be covered (at least now) You’re encouraged to investigate on your own as CS611 will use java extensively

Fundamental difference with your programming experience Your programs have predictable input output sequences. Typically defined by order of data in a file or which button was clicked. Can NOT control order in which peer queries or responds. Requires an event-driven paradigm

Event-driven apps While (NOT A TERMINATE EVENT) get next event if a reasonable event occurred process it else error or terminate Consider deadlocks: A waits for B to send a message X B waits for A to send message Y neither does -> deadlock! Like windows programs! Will address this more later when we cover protocols.

Problems with socket apps Timing problems are new for you Apps typically will deadlock Since input is less predictable, harder to debug. Will need to send/get info through socket and echo to a standard output device whatever is sent/read to aid in debugging

See socket code examples Check out the lab activity on the Course Resources part of the web page

Non-socket APIs Other than the three types of sockets, one will typically find RPC in use. RPC stands for Remote Procedure Call RPC effectively extracts a function from your code and runs it on another machine X = sqrt(y) Calc sqrt(y) return results y x

Additional considerations Setting up and launching a server on your OS How many simultaneous servers can run How to handle queues of clients waiting for service These and many others are addressed in CS611