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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