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2: Application Layer

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Chapter 2�Application Layer:

Socket Programming

Jaringan Komputer

Departemen Informatika

Universitas Diponegoro

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Network Applications

Protocol standard defined 🡪 RFC

RFC 959 🡪 FTP

Proprietary

single developer 🡪 client-server
Not to use one of the well-known port numbers defined in the RFC.

T C P or UDP ?

T C P 🡪 connection oriented, reliable
UDP 🡪 connectionless, unreliable

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Example of �Client Server Communication

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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:

UDP
TCP

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An 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

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Socket programming basics

Server must be running before client can send anything to it.
Server must have a socket (door) through which it receives and sends segments
Similarly client needs a socket

Socket is locally identified with a port number

Analogous to the apt # in a building

Client needs to know server IP address and socket port number.

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Chapter 2: Application layer

2.1 Principles of network applications
2.2 Web and HTTP
2.3 FTP (skip)
2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS

2.6 P2P applications (skip)
2.7 Socket programming with TCP
2.8 Socket programming with UDP

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Socket-programming using TCP

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

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process

TCP with

buffers,

variables

socket

controlled by

application

developer

controlled by

operating

system

host or

server

process

TCP with

buffers,

variables

socket

controlled by

application

developer

controlled by

operating

system

host or

server

internet

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TCP: Client Socket, Welcoming Socket, and Connection Socket

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1

2

3

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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)

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TCP provides reliable, in-order

transfer of bytes (“pipe”)

between client and server

application viewpoint

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Client/server socket interaction: TCP

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wait for incoming

connection request

connectionSocket =

welcomeSocket.accept()

create socket,

port=x, for

incoming request:

welcomeSocket =

ServerSocket()

create socket,

connect to hostid, port=x

clientSocket =

Socket()

close

connectionSocket

read reply from

clientSocket

close

clientSocket

Server (running on hostid)

Client

send request using

clientSocket

read request from

connectionSocket

write reply to

connectionSocket

TCP

connection setup

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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, e.g., keyboard or socket.
An output stream is attached to an output source, e.g., monitor or socket.
The DataOutputStream class has some methods to convert (and send out) data into any primitif data type, depend on our need; Such as byte, integer, char, etc

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Client

process

client TCP socket

Stream jargon

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Socket programming with TCP

Example client-server app scenario:

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,

4) Server sends back to client

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

Let’s Code the Client and Server Programs…

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client TCP socket

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Example: �Java client (TCP)

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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());

Create

input stream

Create

client socket,

connect to server

Create

output stream

attached to socket

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Example: �Java client (TCP), �cont.

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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();

}

Create

input stream

attached to socket

Send line

to server

Read line

from server

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Example: �Java server (TCP)

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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

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Example: �Java server (TCP), �cont

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DataOutputStream outToClient =

new DataOutputStream(connectionSocket.getOutputStream());

clientSentence = inFromClient.readLine();

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

outToClient.writeBytes(capitalizedSentence);

}

Read in line

from socket

Create output

stream, attached

to socket

Write out line

to socket

End of while loop,

loop back and wait for

another client connection

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TCP observations & questions

Server has two types of sockets:

ServerSocket and Socket

When client knocks on serverSocket’s “door,” server creates connectionSocket and completes TCP conx.
Dest IP and port are not explicitly attached to segment.
Can multiple clients use the server?

�

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Exercise: Java client (TCP)

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import java.io.*;

import java.net.*;

class TCPClient {

public static void main(String argv[]) throws Exception

{

String sentence;

String modifiedSentence;

// 1. .......................................................................................

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

// 2. .......................................................................................

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

// 3. .......................................................................................

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

// 4. .......................................................................................

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

// 5. .......................................................................................

sentence = inFromUser.readLine();

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

// 6. .......................................................................................

modifiedSentence = inFromServer.readLine();

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

// 7. .......................................................................................

clientSocket.close();

}

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Java client (TCP)

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import java.io.*;

import java.net.*;

class TCPClient {

public static void main(String argv[]) throws Exception

{

String sentence;

String modifiedSentence;

// 1. Membuat objek yang menerima aliran karakter dari perangkat input user (keyboard)

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

// 2. Membuat objek socket pd klien dg mencoba melakukan koneksi ke mesin hostname pd port 6789

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

// 3. Membuat objek untuk mengirimkan aliran data melalui socket

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

// 4. Membuat objek untuk menerima aliran karakter dari socket

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

// 5. Membaca baris karakter dari keyboard, lalu mengirim baris karakter tsb ke socket dlm aliran Byte

sentence = inFromUser.readLine();

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

// 6. membaca baris karakter yg datang dari socket, lalu mengirim ke perangkat output user (monitor)

modifiedSentence = inFromServer.readLine();

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

// 7. Menutup socket klien

clientSocket.close();

}

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Chapter 2: Application layer

2.1 Principles of network applications
2.2 Web and HTTP
2.3 FTP (skip)
2.4 Electronic Mail

SMTP, POP3, IMAP

2.5 DNS

2.6 P2P applications (skip)
2.7 Socket programming with TCP
2.8 Socket programming with UDP

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Socket programming with UDP

UDP: no “connection” between client and server

no handshaking
sender explicitly attaches IP address and port of destination to each segment
OS attaches IP address and port of sending socket to each segment
Server can extract IP address, port of sender from received segment

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application viewpoint

UDP provides unreliable transfer

of groups of bytes (“datagrams”)

between client and server

Note: the official terminology

for a UDP packet is “datagram”. �In this class, we instead use “UDP �segment”.

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Running example

Client:

User types line of text
Client program sends line to server

Server:

Server receives line of text
Capitalizes all the letters
Sends modified line to client

Client:

Receives line of text
Displays

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Client/server socket interaction: UDP

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Server (running on hostid)

close

clientSocket

read datagram from

clientSocket

create socket,

clientSocket =

DatagramSocket()

Client

Create datagram with server IP and

port=x; send datagram via� clientSocket

create socket,

port= x.

serverSocket =

DatagramSocket()

read datagram from

serverSocket

write reply to

serverSocket

specifying �client address,

port number

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Example: Java client (UDP)

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Output: sends packet (recall

that TCP sent “byte stream”)

Input: receives packet (recall thatTCP received “byte stream”)

Client

process

client UDP socket

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Example: �Java client (UDP)

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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]; // 1 Kbyte

byte[] receiveData = new byte[1024];

String sentence = inFromUser.readLine();

sendData = sentence.getBytes(); // get bytes from sentence

Create

input stream

Create

client socket

Translate

hostname to IP

address using DNS

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Example: �Java client (UDP), �cont.

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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 datagram

to server

Read datagram

from server

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Example: �Java server (UDP)

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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);

Create

datagram socket

at port 9876

Create space for

received datagram

Receive

datagram

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Example: �Java server (UDP), �cont

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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 addr

port #, of

sender

Write out

datagram

to socket

End of while loop,

loop back and wait for

another datagram

Create datagram

to send to client

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UDP observations & questions

Both client server use DatagramSocket
Dest IP and port are explicitly attached to segment.
What would happen if change both clientSocket and serverSocket to “mySocket”?
Can the client send a segment to server without knowing the server’s IP address and/or port number?
Can multiple clients use the server?

�

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Exercise: Java client (UDP)

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class UDPClient {

public static void main(String args[]) throws Exception

{

// 1. .......................................................................................

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

// 2. .......................................................................................

DatagramSocket clientSocket = new DatagramSocket();

// 3. .......................................................................................

InetAddress IPAddress = InetAddress.getByName("hostname");

byte[] sendData = new byte[1024];

byte[] receiveData = new byte[1024];

// 4. .......................................................................................

String sentence = inFromUser.readLine();

sendData = sentence.getBytes();

// 5. .......................................................................................

DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);

clientSocket.send(sendPacket);

// 6. .......................................................................................

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

clientSocket.receive(receivePacket);

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

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

// 7. .......................................................................................

clientSocket.close();

}

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Exercise: Java client (UDP)

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class UDPClient {

public static void main(String args[]) throws Exception

{

// 1. Membuat objek untuk menerima input dari user

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

// 2. Membuat objek socket klien UDP

DatagramSocket clientSocket = new DatagramSocket();

// 3. Mencatat IP server, menyiapkan ukuran byte data untuk kirim/ terima

InetAddress IPAddress = InetAddress.getByName("hostname");

byte[] sendData = new byte[1024];

byte[] receiveData = new byte[1024];

// 4. Membaca baris karakter dari keyboard, lalu mengubah data dalam Byte

String sentence = inFromUser.readLine();

sendData = sentence.getBytes();

// 5. Membuat objek paket (kirim) lalu mengirimkannya melalui socket

DatagramPacket sendPacket = new DatagramPacket(sendData, sendData.length, IPAddress, 9876);

clientSocket.send(sendPacket);

// 6. Membuat objek paket (terima) lalu menerima paket dari socket, dan menampilkan pada monitor

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

clientSocket.receive(receivePacket);

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

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

// 7. menutup koneksi socket

clientSocket.close();

}

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Appendix

Java Network Programming
Java I/O byte stream
Java I/O char stream
High level class in stream chaining

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Java Network Programming

Java.io

InputStream, OutputStream 🡪 byte
Reader, Writer 🡪 text

Java.net

TCP Programming

Java.net.Socket 🡪 client, server
Java.net.ServerSocket 🡪 server

UDP Programming

Java.net.DatagramSocket 🡪 client, server
Java.net.DatagramPacket 🡪 create packet

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Java I/O�byte streams

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Java I/O�char streams

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High Level Class�In Stream Chaining