2: Application Layer 1

Socket programming

Socket API introduced in BSD4.1 UNIX,

1981 explicitly created, used, released

by apps client/server paradigm two types of transport service via

socket API: unreliable datagram reliable, byte stream-oriented

a host-local, application-created,

OS-controlled interface (a “door”) into which

application process can both send and

receive messages to/from another

application process

socket

Goal: learn how to build client/server application that communicate using sockets

2: Application Layer 2

Socket-programming using TCP

Socket: a door between application process and end-end-transport protocol (UCP or TCP)

TCP service: reliable transfer of bytes from one process to another

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperating

system

host orserver

process

TCP withbuffers,

variables

socket

controlled byapplicationdeveloper

controlled byoperatingsystem

host orserver

internet

2: Application Layer 3

Socket programming with TCP

Client must contact server server process must first be

running server must have created socket

(door) that welcomes client’s contact

Client contacts server by: creating client-local TCP socket specifying IP address, port

number of server process When client creates socket:

client TCP establishes connection to server TCP

When contacted by client, server TCP creates new socket for server process to communicate with client allows server to talk with

multiple clients source port numbers used to

distinguish clients (more in Chap 3)

TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server

application viewpoint

2: Application Layer 4

Stream jargon

A stream is a sequence of characters that flow into or out of a process.

An input stream is attached to some input source for the process, eg, keyboard or socket.

An output stream is attached to an output source, eg, monitor or socket.

2: Application Layer 5

Socket programming with TCP

Example client-server app:1) client reads line from standard

input (inFromUser stream) , sends to server via socket (outToServer stream)

2) server reads line from socket

3) server converts line to uppercase, sends back to client

4) client reads, prints modified line from socket (inFromServer stream) ou

tToS

erve

r

to network from network

inFr

omS

erve

r

inFr

omU

ser

keyboard monitor

Process

clientSocket

inputstream

inputstream

outputstream

TCPsocket

Clientprocess

client TCP socket

2: Application Layer 6

Client/server socket interaction: TCP

wait for incomingconnection requestconnectionSocket =welcomeSocket.accept()

create socket,port=x, forincoming request:welcomeSocket =

ServerSocket()

create socket,connect to hostid, port=xclientSocket =

Socket()

closeconnectionSocket

read reply fromclientSocket

closeclientSocket

Server (running on hostid) Client

send request usingclientSocketread request from

connectionSocket

write reply toconnectionSocket

TCP connection setup

2: Application Layer 7

Example: Java client (TCP)

import java.io.*; import java.net.*; class TCPClient {

public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence;

BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in));

Socket clientSocket = new Socket("hostname", 6789);

DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream());

Createinput stream

Create client socket,

connect to server

Createoutput stream

attached to socket

2: Application Layer 8

Example: Java client (TCP), cont.

BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream()));

sentence = inFromUser.readLine();

outToServer.writeBytes(sentence + '\n');

modifiedSentence = inFromServer.readLine();

System.out.println("FROM SERVER: " + modifiedSentence);

clientSocket.close(); } }

Createinput stream

attached to socket

Send lineto server

Read linefrom server

2: Application Layer 9

Example: Java server (TCP)import java.io.*; import java.net.*;

class TCPServer {

public static void main(String argv[]) throws Exception { String clientSentence; String capitalizedSentence;

ServerSocket welcomeSocket = new ServerSocket(6789); while(true) { Socket connectionSocket = welcomeSocket.accept();

BufferedReader inFromClient = new BufferedReader(new InputStreamReader(connectionSocket.getInputStream()));

Createwelcoming socket

at port 6789

Wait, on welcomingsocket for contact

by client

Create inputstream, attached

to socket

2: Application Layer 10

Example: Java server (TCP), cont

DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();

capitalizedSentence = clientSentence.toUpperCase() + '\n';

outToClient.writeBytes(capitalizedSentence); } } }

Read in linefrom socket

Create outputstream,

attached to socket

Write out lineto socket

End of while loop,loop back and wait foranother client connection

2: Application Layer 11

Socket programming with UDP

UDP: no “connection” between client and server

no handshaking sender explicitly attaches

IP address and port of destination to each packet

server must extract IP address, port of sender from received packet

UDP: transmitted data may be received out of order, or lost

application viewpoint

UDP provides unreliable transfer of groups of bytes (“datagrams”)

between client and server

2: Application Layer 12

Client/server socket interaction: UDP

closeclientSocket

Server (running on hostid)

read reply fromclientSocket

create socket,clientSocket = DatagramSocket()

Client

Create, address (hostid, port=x,send datagram request using clientSocket

create socket,port=x, forincoming request:serverSocket = DatagramSocket()

read request fromserverSocket

write reply toserverSocketspecifying clienthost address,port number

2: Application Layer 13

Example: Java client (UDP)

sendP

ack

et

to network from network

rece

iveP

ack

et

inF

rom

Use

r

keyboard monitor

Process

clientSocket

UDPpacket

inputstream

UDPpacket

UDPsocket

Output: sends packet (TCP sent “byte stream”)

Input: receives packet (TCP received “byte stream”)

Clientprocess

client UDP socket

2: Application Layer 14

Example: Java client (UDP)

import java.io.*; import java.net.*; class UDPClient { public static void main(String args[]) throws Exception { BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); DatagramSocket clientSocket = new DatagramSocket(); InetAddress IPAddress = InetAddress.getByName("hostname"); byte[] sendData = new byte[1024]; byte[] receiveData = new byte[1024]; String sentence = inFromUser.readLine();

sendData = sentence.getBytes();

Createinput stream

Create client socket

Translate hostname to IP

address using DNS

2: Application Layer 15

Example: Java client (UDP), cont.

DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876); clientSocket.send(sendPacket); DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); clientSocket.receive(receivePacket); String modifiedSentence = new String(receivePacket.getData()); System.out.println("FROM SERVER:" + modifiedSentence); clientSocket.close(); }

}

Create datagram with data-to-send,

length, IP addr, port

Send datagramto server

Read datagramfrom server

2: Application Layer 16

Example: Java server (UDP)

import java.io.*; import java.net.*; class UDPServer { public static void main(String args[]) throws Exception { DatagramSocket serverSocket = new DatagramSocket(9876); byte[] receiveData = new byte[1024]; byte[] sendData = new byte[1024]; while(true) { DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length);

serverSocket.receive(receivePacket);

Createdatagram socket

at port 9876

Create space forreceived datagram

Receivedatagra

m

2: Application Layer 17

Example: Java server (UDP), cont

String sentence = new String(receivePacket.getData()); InetAddress IPAddress = receivePacket.getAddress(); int port = receivePacket.getPort(); String capitalizedSentence = sentence.toUpperCase();

sendData = capitalizedSentence.getBytes(); DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, port); serverSocket.send(sendPacket); } }

}

Get IP addrport #, of

sender

Write out datagramto socket

End of while loop,loop back and wait foranother datagram

Create datagramto send to client

2: Application Layer 18

UDP Sockets

int socket(int domain, int type, int protocol); int bind(int sockfd, struct sockaddr *my_addr, socklen_t addrlen); ssize_t sendto(int s, const void *buf, size_t len, int flags, const

struct sockaddr *to, socklen_t tolen); ssize_t recvfrom(int s, void *buf, size_t len, int flags, struct sock-addr

*from, socklen_t *fromlen);

2: Application Layer 19

UDP Sockets

int socket(int domain, int type, int protocol); socket() creates an endpoint for communication and returns a descriptor.

Domain:PF_INET IPv4 Internet protocols ip(7)PF_INET6 IPv6 Internet protocols

Type:SOCK_STREAM: TCP SocketsSOCK_DGRAM: UDP Sockets

Protocol:The protocol specifies a particular protocol to be used with thesocket. Normally only a single protocol exists to support a particularsocket type within a given protocol family, in which a case protocolcan be specified as 0.

2: Application Layer 20

UDP Sockets

int bind(int sockfd, struct sockaddr *my_addr, socklen_t addrlen);

Bind() gives the socket sockfd the local address my_addr. my_addr is addrlen bytes long.

Setting up the socket addr structure: struct cliAddr; cliAddr.sin_family = AF_INET; cliAddr.sin_addr.s_addr = htonl(INADDR_ANY); cliAddr.sin_port = htons(0);

2: Application Layer 21

UDP Sockets ssize_t sendto(int s, const void *buf, size_t len, int flags, const

struct sockaddr *to, socklen_t tolen);

s = Socket descriptor buf = Message to send len = Length of message flags = Normally 0, specifies the behavior of the socket to = Pointer to the sockaddr structure that contains the destination (IP

and port)• remoteAddr.sin_port• remoteAddr.sin_addr.s_addr

tolen = Length of remoteAddr socket

2: Application Layer 22

UDP Sockets

ssize_t recvfrom(int s, void *buf, size_t len, int flags, struct sock-addr *from, socklen_t *fromlen);

s = Socket descriptor buf = Pointer of buffer of which will be filled by the

message. len = The length of the buffer flags = Normally 0, specifies the behavior of the socket from = Pointer to the sockaddr structure that contains

the destination (IP and port)• remoteAddr.sin_port• remoteAddr.sin_addr.s_addr

fromlen = Length of sockaddr struct

2: Application Layer 23

UDP Client Example#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>#include <netdb.h>#include <stdio.h>#include <unistd.h>#include <string.h> /* memset() */#include <sys/time.h> /* select() */#include <stdlib.h>#include <readline/readline.h>#include <readline/history.h>

#define REMOTE_SERVER_PORT 1500 //bind to port 1500#define MAX_MSG 100

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

int sd, rc, i; struct sockaddr_in cliAddr, remoteServAddr; struct hostent *h;

2: Application Layer 24

/* get server IP address (no check if input is IP address or DNS name */ h = gethostbyname(argv[1]); if(h==NULL) { printf("%s: unknown host '%s' \n", argv[0], argv[1]); exit(1); }

printf("%s: sending data to '%s' (IP : %s) \n", argv[0], h->h_name, inet_ntoa(*(struct in_addr *)h->h_addr_list[0]));

remoteServAddr.sin_family = h->h_addrtype; memcpy((char *) &remoteServAddr.sin_addr.s_addr,

h->h_addr_list[0], h->h_length); remoteServAddr.sin_port = htons(REMOTE_SERVER_PORT);

/* socket creation */ sd = socket(AF_INET,SOCK_DGRAM,0); if(sd<0) { printf("%s: cannot open socket \n",argv[0]); exit(1); }

/* bind any port */ cliAddr.sin_family = AF_INET; /* pick any available network interface */ cliAddr.sin_addr.s_addr = htonl(INADDR_ANY); /* bind to any local port */ cliAddr.sin_port = htons(0);

2: Application Layer 25

rc = bind(sd, (struct sockaddr *) &cliAddr, sizeof(cliAddr)); if(rc<0) { printf("%s: cannot bind port\n", argv[0]); exit(1); }

char* line; int sent; /* send data */ while(1){

/* Prompt user for string and send to the server */ line = readline("Enter text: "); sent = sendto(sd, line, strlen(line)+1, 0,

(struct sockaddr *) &remoteServAddr, sizeof(remoteServAddr));

if(sent<0) { printf("%s: cannot send data\n",line); close(sd); exit(1); }

2: Application Layer 26

char msg[MAX_MSG]; /* receive response from server */ int remoteServSize = sizeof(remoteServAddr); rc = recvfrom(sd,msg,MAX_MSG, 0, (struct sockaddr*) &remoteServAddr,

&remoteServSize);

if(rc<0){ printf("Error occurred receiving from server\n"); exit(1); } printf("%s\n", msg); } return 1;

}

2: Application Layer 27

UDP Server Example#include <sys/types.h>#include <sys/socket.h>#include <netinet/in.h>#include <arpa/inet.h>#include <netdb.h>#include <stdio.h>#include <unistd.h> /* close() */#include <string.h> /* memset() */#include <stdlib.h>#include <string.h>

#define LOCAL_SERVER_PORT 1500 /* Listen on port 1500 */#define MAX_MSG 100#define MAX_RESPONSE 1024

int main(int argc, char *argv[]) { int sd, rc, n, cliLen; struct sockaddr_in cliAddr, servAddr; char msg[MAX_MSG];

/* socket creation */ sd=socket(AF_INET, SOCK_DGRAM, 0); if(sd<0) { printf("%s: cannot open socket \n",argv[0]); exit(1); }

2: Application Layer 28

/* bind local server port */ servAddr.sin_family = AF_INET; /* Accept a connection from interface */ servAddr.sin_addr.s_addr = htonl(INADDR_ANY); /* Listen on Port 1500 */ servAddr.sin_port = htons(LOCAL_SERVER_PORT); rc = bind (sd, (struct sockaddr *) &servAddr,sizeof(servAddr)); if(rc<0) { printf("%s: cannot bind port number %d \n",

argv[0], LOCAL_SERVER_PORT); exit(1); }

printf("%s: waiting for data on port UDP %u\n", argv[0],LOCAL_SERVER_PORT);

/* server infinite loop */ while(1) { /* init buffer */ memset(msg,0x0,MAX_MSG);

/* receive message */ cliLen = sizeof(cliAddr); n = recvfrom(sd, msg, MAX_MSG, 0,

(struct sockaddr *) &cliAddr, &cliLen);

2: Application Layer 29

if(n<0) { printf("%s: cannot receive data \n",argv[0]); continue; } /* print received message */ printf("%s: from %s:UDP%u : %s \n",

argv[0],inet_ntoa(cliAddr.sin_addr), ntohs(cliAddr.sin_port),msg);

char response[MAX_RESPONSE]; char* responseMsg = "Received your message: "; int len = strlen(responseMsg) + 1; strcpy(response,responseMsg); strncat(response,msg,MAX_RESPONSE-len);

n = sendto(sd, response, strlen(response)+1,0,(struct sockaddr*) &cliAddr, sizeof(cliAddr)); if(n<0){ printf("Error sending to client\n"); exit(1); }

}/* end of server infinite loop */

return 0;

}

2: Application Layer 30

GDB Basics

How to setup the environment to dump core files: csil% ulimit –c unlimited

How to open a core file csil% gdb –q ./executable corename (the parameter –q is optional. The will prevent GDB

from printing out extra garbage when it initially loads) What to do when a core file is loaded up

(gdb) bt (backtrace) • Will show you the stack when the program crashed

More useful to run your program with GDB. csil% gdb –q ./executable

• Set breakpoints: (gdb) b <line number> OR b <function name>• Run the program: (gdb) r <commandline params>

– The program will break when it reaches a breakpoint– Print value of a variable: (gdb) print <variable name>– Step through a function: (gdb) s– Skip to the next instruction: (gdb) n– Continue until the program exits or another breakpoint is reached:

(gdb) c