Network Programming using sockets

TCP/IP layers

Layers Application

Message

Messages (UDP) or Streams (TCP) Transport UDP or TCP packets Internet IP datagrams Network interface Network-specific frames Underlying network

Network Programming

The programmer's conceptual view of a TCP/IP Internet

Application TCP IP

Application UDP

Network Programming

Socket programming
Goal: learn how to build client/server application that communicate using sockets Socket API
r introduced in BSD4.1 UNIX,

socket
a host-local, applicationcreated/owned, OS-controlled interface (a door) into which application process can both send and receive messages to/from another (remote or local) application process

1981 r explicitly created, used, released by apps r client/server paradigm r two types of transport service via socket API: m unreliable datagram m reliable, byte streamoriented

Network Programming

Sockets and ports

socket

any port

agreed port socket

message client other ports Internet address = 138.37.94.248 Internet address = 138.37.88.249 server

Network Programming

Berkeley Sockets (1)

r Socket primitives for TCP/IP.
Primitive Socket Bind Listen Accept Connect Send Receive Close Meaning Create a new communication endpoint Attach a local address to a socket Announce willingness to accept connections Block caller until a connection request arrives Actively attempt to establish a connection Send some data over the connection Receive some data over the connection Release the connection

Network Programming

Socket programming with TCP

Client must contact server
r server process must first r When client creates socket:

be running r server must have created socket (door) that welcomes clients contact Client contacts server by: r creating client-local TCP socket r specifying IP address, port number of server process

client TCP establishes connection to server TCP

r When contacted by client,

server TCP creates new socket for server process to communicate with client m allows server to talk with multiple clients

application viewpoint TCP provides reliable, in-order transfer of bytes (pipe) between client and server
Network Programming 7

Socket programming with TCP

Example client-server app:

r client reads line from

standard input (inFromUser stream) , sends to server via socket (outToServer stream) r server reads line from socket r server converts line to uppercase, sends back to client r client reads, prints modified line from socket (inFromServer stream)

Input stream: sequence of bytes into process Output stream: sequence of bytes out of process
iinFromServer
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inFromUser

client socket
Network Programming

outToServer

Client/server socket interaction: TCP

Server

(running on hostid)

Client

create socket, port=x, for incoming request: welcomeSocket = ServerSocket () wait for incoming connection request connection connectionSocket = welcomeSocket .accept()

TCP

setup

create socket, connect to hostid, port=x clientSocket = Socket() send request using clientSocket

read request from connectionSocket write reply to connectionSocket close connectionSocket

read reply from clientSocket close clientSocket Network Programming 9

Berkeley Sockets (2)

r Connection-oriented communication pattern using sockets.

Network Programming

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Sockets used for streams

Requesting a connection

Listening and accepting a connection

s = socket(AF_INET, SOCK_STREAM,0)

s = socket(AF_INET, SOCK_STREAM,0) bind(s, ServerAddress); listen(s,5); sNew = accept(s, ClientAddress);

connect(s, ServerAddress)

write(s, "message", length)

n = read(sNew, buffer, amount)

ServerAddress and ClientAddress are socket addresses

Network Programming

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Example: Java client (TCP)

import java.io.*; import java.net.*; class TCPClient { public static void main(String argv[]) throws Exception { String sentence; String modifiedSentence; Create input stream Create client socket, connect to server Create output stream attached to socket BufferedReader inFromUser = new BufferedReader(new InputStreamReader(System.in)); Socket clientSocket = new Socket("hostname", 6789); DataOutputStream outToServer = new DataOutputStream(clientSocket.getOutputStream ());

Network Programming

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Example: Java client (TCP), cont.

Create input stream attached to socket

BufferedReader inFromServer = new BufferedReader(new InputStreamReader(clientSocket.getInputStream ())); sentence = inFromUser.readLine();

Send line to server Read line from server

outToServer.writeBytes(sentence + '\n'); modifiedSentence = inFromServer.readLine(); System.out.println("FROM SERVER: " + modifiedSentence); clientSocket.close(); } }

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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()));

Create welcoming socket at port 6789 Wait, on welcoming socket for contact by client Create input stream, attached to socket

Network Programming

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Example: Java server (TCP), cont

Create output stream, attached to socket Read in line from socket Write out line to socket } }

DataOutputStream outToClient = new DataOutputStream(connectionSocket.getOutputStream()); clientSentence = inFromClient.readLine(); capitalizedSentence = clientSentence.toUpperCase() + '\n'; outToClient.writeBytes(capitalizedSentence); } End of while loop, loop back and wait for another client connection

Network Programming

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Socket programming with UDP

UDP: no connection between client and server

r no handshaking r sender explicitly attaches

IP address and port of destination r server must extract IP address, port of sender from received datagram

application viewpoint UDP provides unreliable transfer of groups of bytes (datagrams) between client and server

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

Network Programming

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Client/server socket interaction: UDP

Server

(running on hostid)

Client
create socket, clientSocket = DatagramSocket ()

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

read request from serverSocket write reply to serverSocket specifying client host address, port umber

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

read reply from clientSocket close clientSocket

Network Programming

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Sockets used for datagrams

Sending a message
s = socket(AF_INET, SOCK_DGRAM, 0)

Receiving a message
s = socket(AF_INET, SOCK_DGRAM, 0) bind(s, ServerAddress) amount = recvfrom(s, buffer, from)

bind(s, ClientAddress) sendto(s, "message", ServerAddress)

ServerAddress and ClientAddress are socket addresses

Network Programming

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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();
Network Programming 19

Create input stream Create client socket Translate hostname to IP address using DNS

Example: Java client (UDP), cont.

Create datagram with data-to-send, length, IP addr, port Send datagram to server Read datagram from server

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(); }

}
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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) {

Create datagram socket at port 9876

Create space for received datagram Receive datagram

DatagramPacket receivePacket = new DatagramPacket(receiveData, receiveData.length); serverSocket.receive(receivePacket);

Network Programming 21

Example: Java server (UDP), cont

String sentence = new String(receivePacket.getData());

Get IP addr port #, of sender

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

Create datagram to send to client Write out datagram to socket

} }

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

End of while loop, loop back and wait for another datagram
Network Programming 22

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