ccnet-lec-14-socket-prog

8/6/2019 ccnet-lec-14-socket-prog

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Lecture

14Data Communication & Networks

Socket Programming

(A Developers Interface to Network)

Muhammad Yousaf


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Developer Vs. Network Designer:

Developer views the network from programming perspective

Concern about Application Programming Interfaces (API),ease of development, flexibility, etc

Network Designer views the network from actualimplementation perspective

Concerns about communication issues, problems, theirsolutions, protocols, etc


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Programmers perspective:

Why a programmer should learn about network?Demand for network applications is increasing

In Internet architecture most of the intelligence lies in theend-hosts (programmable machines)

New services on existing network can be launchedmerely by developing new applications

Knowledge of network helps programmer to built networkapplications in a better way

Developer can think about the implementation ofinnovative distributed applications, e.g. remote digitallibrary, remote shopping, entertainment, distributed OS,distributed databases, etc.


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Network Designers perspective:

Why a network designer should learn aboutprogramming?

We all demand an actually working network

We dont need a network that only lies in books & papers

Programming helps to actually implement networkingprotocols

IETFs motto: WE BELIEVE IN WORKING CODE


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Programming over network:

Socket programming provides most popularinterface to the networking stack

Although other alternatives are also available

e.g. Sun RPC (Remote Procedure Calls)

Socket APIs were introduced in 1981 with BSD-4.1Unix

Implemented as a set of library/system calls

Revolves around client/server architecture


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Socket Programming:

Sockets API provide:A transport layer service interface

Similar interfaces to TCP (stream-socket) and UDP(datagram-socket)

Can also serve as interface to IP (raw-socket)Support for both IPv4 & IPv6 is available

Linux also provides interface to MAC layer (data-link-socket)

Some other type of sockets are also available (Unix-domain-socket, routing-socket, etc.)


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Socket Programming: cont

Flexibility & Uniformity of socket interface

Socket Interface

TCP UDP

IP

Network Access


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Socket Programming: cont

Recall client/server interaction procedure:

Server:Create socket at server side

Binds with well-known Port number & IP address

Listen for the connection requests

Client:Create socket at client side

Sends a connection request

Server:Accept connection request

Client/Server:Send/Receive user Data

Either Client or Server Close the connection


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Creating Sockets:

int socket (int family, int type, int protocol);

Create a socket. Returns file descriptor or -1. Also sets errnoon failure.

family: address family / protocol family

AF_INET for IPv4

other possibilities: AF_INET6 (IPv6), AF_UNIX, AF_OSI, AF_LOCAL(Unix socket), AF_ROUTE (routing)

type: type of communication

SOCK_STREAM for TCPSOCK_DGRAM for UDP

protocol: protocol within family

typically 0 (default)


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Binding the socket:

int bind (int sockfd, struct sockaddr* serverAddr, int addrlen);Bind a socket to a local IP address and port number. Returns0 on success, -1 and sets errno on failure.

sockfd: socket file descriptor (returned from socket)serverAddr: includes IP address and port number

IP address: set by kernel if value passed is INADDR_ANY, else setby caller

port number: set by kernel if value passed is 0, else set by caller

addrlen: length of address structure

sizeof (struct sockaddr_in)


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Waiting for Connections:

int listen (int sockfd, int backlog);Put socket into passive state (wait for connections rather thaninitiate a connection). Returns 0 on success, -1 and setserrno on failure.

sockfd : socket file descriptor

backlog : bound on length of un-accept()ed connection queue(connection backlog)


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Sending connection request:

int connect (int sockfd, struct sockaddr* servaddr, int addrlen);Connect to another socket. Returns 0 on success, -1 and setserrno on failure.

sockfd : socket file descriptorservaddr : IP address and port number of server

addrlen : length of address structure


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Accepting a connection request:

int accept (int sockfd, struct sockaddr* cliaddr, int* addrlen);Accept a new connection (first one off queue of pendingconnections). Returns file descriptor or -1. Also sets errno.

sockfd : socket file descriptorcliaddr : IP address and port number of client (when returned)


addrlen is a value-result argument:

caller passes the size of address structure,

kernel returns the size of clients address (number of

bytes written)


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Sending Data:

int write (int sockfd, char* buf, size_t nbytes);Write data to a stream (TCP) or connected datagram

(UDP) socket. Returns number of bytes written or -1.Also sets errno on failure


buf: data buffer

nbytes : number of bytes to try to write

int send (int sockfd, char* buf, size_t nbytes , int flags);


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Sending Data: cont

int sendto (int sockfd, char* buf, size_t nbytes, int flags, structsockaddr* destaddr, int addrlen);

Send a datagram to another UDP socket. Returns number ofbytes written or -1. Also sets errno on failure.

sockfd: socket file descriptor

buf: data buffer

nbytes : number of bytes to try to read

flags : typically use 0

destaddr : IP address and port number of destination socketaddrlen : length of address structure


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Receiving Data:

int read (int sockfd, char* buf, size_t nbytes);

Read data from a stream (TCP) or connected datagram

(UDP) socket. Returns number of bytes read or -1. Alsosets errno on failure. Returns 0 if socket closed.


buf: data buffer


int recv (int sockfd, char* buf, size_t nbytes , int flags);


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Receiving Data: cont

int recvfrom (int sockfd, char* buf, size_t nbytes, int flags,struct sockaddr* srcaddr, int* addrlen);

Read a datagram from a UDP socket. Returns number of bytesread (0 is valid) or -1. Also sets errno on failure.

sockfd :socket file descriptor

buf: data buffer


flags : typically use 0

srcaddr : IP address and port number of sending socket(returned from call)



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Closing a connection:

int close (int sockfd);Closes a socket and deletes descriptor from system

tables. Returns 0 on success, -1 and sets errno onfailure.


All data sent before close are delivered to other sideAfter close() , sockfd is not valid for reading orwriting.


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Example code:

Following a sample code for client and serverCode is written in simple C-Language

System used: Linux

Although server and client are doing very simple

work, but it will give an idea about networkprogramming

Code can be compiled/run over command promptterminal1# gcco server server.c

./serverterminal2# gcco client client.c

./client serverAddress


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server.c: (include, define)

#include

#include

#include

#include

#include #include

#include

#include

#define PORT 1233 /* my well-known port */

#define BACKLOG 10/* length of pending connections queue */


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server.c: (main, socket)

main()

{

int sockfd, new_fd;/*listen on sockfd, new connection on new_fd */

struct sockaddr_in my_addr; /* servers address */

struct sockaddr_in their_addr; /* client addr */int sin_size; /* address size */

if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1) {

perror("socket");exit(1);

}


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server.c: (bind)

my_addr.sin_family = AF_INET; /* host byte order */

my_addr.sin_port = htons(MYPORT); /* convert to short,network byte order */

my_addr.sin_addr.s_addr = htonl(INADDR_ANY);/* automatically fill with my IP */

bzero(&(my_addr.sin_zero), 8); /* zero the rest of the struct */

if(bind(sockfd, (struct sockaddr *)&my_addr, sizeof(struct

sockaddr)) == -1) {perror("bind");

exit(1);

}


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server.c: (listen, accept)

if(listen(sockfd, BACKLOG) == -1) {

perror("listen"); exit(1);

}

while(1) { /* main accept() loop ; always true*/

sin_size = sizeof(struct sockaddr_in);

if((new_fd = accept(sockfd, (struct sockaddr*)&their_addr, &sin_size)) == -1) {

perror("accept"); continue;

}

printf("server: got connection from %s\n",inet_ntoa(their_addr.sin_addr));


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server.c: (send, close)

if(!fork()) { /* this is the child process */

close(sock_fd); /* child doesn't need it */

if(send(new_fd, "Hello, world!\n", 14, 0) == -1)

perror("send");

close(new_fd); exit(0);

}

close(new_fd); /* parent doesn't need this */

while(waitpid(-1,NULL,WNOHANG) > 0);/* clean up allchild processes */

}/* outer while loop ends here */

} /* main function ends here */


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client.c: (include, define)

#include

#include

#include

#include

#include

#include

#include

#include

#define PORT 1233 /* servers well-known port */

#define MAXDATASIZE 100/* max number of bytes we canget at once */


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client.c: (main)

int main (int argc, char* argv[])

{

int sockfd, numbytes;

char buf[MAXDATASIZE];

struct hostent* he;

struct sockaddr_in their_addr;/* connectors address information */if(argc != 2) {

fprintf (stderr, usage: client hostname\n); exit(1);

}

if((he = gethostbyname (argv[1])) == NULL) {/* get the host info */

perror (gethostbyname); exit (1);

}


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client.c: (socket, connect)

if((sockfd = socket(AF_INET, SOCK_STREAM, 0)) == -1){

perror (socket); exit (1);

}

their_addr.sin_family = AF_INET;/* interpreted by host */

their_addr.sin_port = htons (PORT);

their_addr.sin_addr = *((struct in_addr*)he->h_addr);

bzero (&(their_addr.sin_zero), 8);/* zero the rest of struct */

if(connect (sockfd, (struct sockaddr*)&their_addr, sizeof(struct sockaddr)) == -1) {

perror (connect); exit (1);

}


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client.c: (receive, close)

if((numbytes = recv (sockfd, buf, MAXDATASIZE, 0))== -1) {

perror (recv);

exit (1);

}

buf[numbytes] = \0;

printf (Received: %s, buf);

close (sockfd);

return 0;

}/* main function ends */


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ccnet-lec-14-socket-prog