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Tutorial 8

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Socket programming What is socket ? Sockets architecture Types of Sockets The Socket system calls Data Transfer Service functions for sockets

programming Examples for Client - Server

communication STREAM Communication (TCP) DATAGRAM Communication

(UDP) Port reservation Example

Contents

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What is Sockets ?

First appeared in 4.1 BSD UNIX 1982.

Sockets: is an abstraction used as communication

endpoints. is implemented as file descriptor with

some state information stored in the socket library.

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The Client Server model

clie n t 1ip = 132.68.44 .73

clie n t 2ip = 132.68.44 .79

s e rv e rip = 132.68.8 .1

telnet port 24

f tpport 3328

port 2353

port 20

request

reply

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Example:UDP Echo

The simplest form of client-server interaction uses unreliable datagram delivery to convey messages from client to server and back.

client server

• Waits for a datagram to arrive the echo port• Reserve the source and destination addresses (IP addresses and ports)• Return the datagram to its original sender

Request sent to a well-known port

• The program become a UDP echo client when it allocates an unused UDP protocol port•sends a UDP message to the UDP echo server•Waits for the replay

Response sent to the client’s port

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Example: UDP Echo (Cont.)

The example illustrates two important points that are generally true about client-server interaction: The difference between the lifetime of

servers and clients. The use of reserved and non-reserved port

identifiers.

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Sockets architecture

Kernel

User

Application

TCP/UDP layer

IP (network) layer

Socket API

MAC layer

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Type of Sockets

Socket has an associated type that determine the semantic of the communication : SOCK_STREAM: connection

oriented byte stream - TCP. SOCK_DGRAM: unreliable,

connectionless communication - UDP

and more

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Protocol family for sockets functions

The family specifies the address family: group of protocols with the same address format

Example of address family constants: AF_INET - IPv4 protocols AF_INET6 - IPv6 protocols AF_LOCAL (AF_UNIX) - Unix

domain protocols

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Data Structures

struct sockaddr_in

{

short int sin_family; /* we use AF_INET */

unsigned short int sin_port; /* port number */

struct in_addr sin_addr; /* comp. address */

unsigned char sin_zero[8]; /* filled with 0s */

};

struct in_addr

{

u_long s_addr; /* unsigned long */

};

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The Socket system calls The Socket system calls: int socket(int family, int type, int protocol)

allocate a socket

return: a socket descriptor or -1

family: communication domain

type: type of socket- SOCK_STREAM, SOCK_DGRAM, SOCK_RAW

protocol: particular protocol to use - 0

int bind(int sd, struct sockaddr *addr, size_t addrlen)

Bind a name to a socket

return: 0 on success or -1

sd: socket descriptor (from the socket() system call)

addr: pointer to the socket address

addrlen: socket address length

int connect(int sd, const struct sockaddr *addr,

size_t addrlen)

Try to connect to a socket (done by the client)

return: 0 on success or -1

sd: socket descriptor (from the socket() system call)

addr: pointer to the socket address

addrlen: socket address length

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The Socket system calls (cont) int listen(int sd, int backlog)

a queue for incoming connections is activated using

listen()

return: 0 on success or -1

sd: socket descriptor (from the socket() system call)

backlog : Maximum queue length of processes waiting for connection.

int accept(int sd, struct sockaddr *addr, size_t

*addrlen);

Wait for a connection request (done by the server)

return: new socket descriptor or -1 in case of failure

sd: socket descriptor (from the socket() system call)

addr: pointer to the socket address

addrlen: socket address length. int close(int sd)

Close the socket or the connection with the remote host.

sd: socket descriptor (from the socket() system call)

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Data Transfer - send int send(int sd, char *buf, int buf_len)

transmit a message to another socket. send() can be

used only when the socket is in a connected state.

return: the number of bytes that has been delivered or -1.

sd: socket descriptor (from the socket() system call)

buf: the message you send.

buf_len: the data length of buf

int sendto(int sd, char *buf, int buf_len,

const struct sockaddr *to_addr, size_t to_len)

used to transmitting data to remote host

return: the number of bytes that has been delivered or -1.

sd: socket descriptor (from the socket() system call)

buf: a pointer to the data location

buf_len: the data length

to_addr: the socket address of the destination.

to_len: the “to_addr” structure length.

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Data Transfer - receive recv(int sd, char *buf, int bub_len)

Obtain data from the socket. Used only when the socket is in a connected state.

return: the number of bytes at the obtain input or -1.

sd: socket descriptor (from the socket() system call)

buf: the buffer address (in memory) for holding the received data.

buf_len: the buffer length

int recvfrom(int sd, char *buf, int bub_len, struct sockaddr *from_addr, int from_len)

Obtain data from the socket.

return: the number of bytes at the obtain input or -1.

sd: socket descriptor (from the socket() system call)

buf: the buffer address (in memory) for holding the received data.

buf_len: the buffer length

from_addr: the socket address of the transmitter

from_len: the “from_addr” structure length

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Service functions for sockets programming

Byte ordering short htons(short) - convert 16-bit value from host to

network long htonl(long) - convert 32-bit value from host to

network short ntohs(short) - convert 16-bit value from network to

host long ntohl(long) - convert 32-bit value from network to

host

Deal with IP address ina.sin_addr.s_addr = inet_addr("132.68.1.8") returns the address in Network Byte Order already.

DNS int gethostname(char* name, int namelen)

return local host name struct hostent *gethostbyname(char* name)

use to get computer address by the name. struct hostent *gethostbyaddr(char *addrp, int len, int

type)

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From Beej's Guide to Network Programming

socket()--Get the File Descriptor! bind()--What port am I on? connect()--Hey, you! listen()--Will somebody please call

me? accept()--"Thank you for calling port

3490." send() and recv()--Talk to me, baby! sendto() and recvfrom()--Talk to me,

DGRAM-style close() and shutdown()--Get outta my

face!

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Stream SocketsTCP Client TCP Server

sd = socket(…)

bind(sd,port)

connect(sd,dest)

send(sd,…)

recv(sd,…)

close(sd)

sd = socket(…)

bind(sd,port)

listen(sd)

new_sd = accept(sd)

recv(new_sd,…)

send(new_sd,…)

close(new_sd,...)

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Datagram Sockets

UDP Client UDP Server

sd = socket(…)

sendto(sd,…)

recvfrom(sd,…)

close(sd)

sd = socket(…)

bind(sd,port)

recvfrom(sd,…)

sendto(sd,…)

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Port reservation

Ports 0 through 1023 are reserved.

Other ports are used by system for assignment to clients.

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Example (UDP Client)// This program sends UDP packets to the given address #include <sys/types.h>#include <sys/socket.h> #include <netinet/in.h>#include <arpa/inet.h>#include <stdio.h>

#define SERVER_ADDR "127.0.0.1"#define SERVER_PORT 5555

void error(char *str){ printf("\n%s", str); exit(0);}

int main(int argc, char *argv[]){ char message[100], message2[10]; int sockfd, res; struct sockaddr_in client_addr, server_addr; int i, mesNum;

printf("\nClient is running...");

if (argc < 2) error("\nYou should supply parameter: the number of

messages to send");

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Example (UDP Client)

// Opening socket if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) error("Could not open socket");

// Sending a message to the server bzero((char*) &server_addr, sizeof(server_addr)); server_addr.sin_family = AF_INET; server_addr.sin_addr.s_addr = inet_addr(SERVER_ADDR); server_addr.sin_port = htons(SERVER_PORT);

mesNum = atoi(argv[1]); if (mesNum == 0) error("\nIllegal parameter");

for (i=0; i<mesNum; i++) { strcpy(message, "Test message: "); sprintf(message2, "%d", i+mesNum); strcat(message, message2);

res = sendto(sockfd, message, strlen(message)+1, 0, (struct sockaddr*)&server_addr, sizeof(server_addr));

printf("\nClient sent %d bytes", res); }}

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Example (UDP Server)

// This program receives UDP packets#include <sys/types.h>#include <sys/socket.h> #include <netinet/in.h>#include <arpa/inet.h>#include <stdio.h>

#define SERVER_PORT 5555#define MAX_MESSAGE_SIZE 100

void error(char *str){ printf("\n%s", str); exit(0);}

int main(){ char message[MAX_MESSAGE_SIZE]; int sockfd, res; struct sockaddr_in client_addr, server_addr; int addr_len; printf("\nServer is running..."); fflush(stdout);

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Example (UDP Server)// Opening socket if ((sockfd = socket(AF_INET, SOCK_DGRAM, 0)) < 0) error("Could not open socket");

// Bind local ip and process addresses bzero((char*) &server_addr, sizeof(server_addr)); server_addr.sin_family = AF_INET; server_addr.sin_addr.s_addr = htonl(INADDR_ANY); server_addr.sin_port = htons(SERVER_PORT);

if (bind(sockfd, (struct sockaddr*)&server_addr, sizeof(server_addr)) < 0)

error("Could not bind to the socket");

while (1) { // Receiving a message from the client addr_len = sizeof(client_addr); res = recvfrom(sockfd, message, MAX_MESSAGE_SIZE,

0, (struct sockaddr*)&client_addr, &addr_len);

printf("\nServer received %d bytes", res); printf("\n%s", message); fflush(stdout); }}