Communication and Networking Java Socket ... - cs.bham.ac.uk · TCP Socket Programming in Java Java.net class I Java provides a set of classes, java.net , to enable the rapid development

SSC - Communication and Networking

SSC - Communication and NetworkingJava Socket Programming (I)

Shan He

School for Computational ScienceUniversity of Birmingham

Module 06-19321: SSC


Outline

Outline of Topics

Sockets and Socket-based Communication

TCP Socket Programming in Java



Client/Server Communication

I The simplest level of network communication

I Consists of a server, client, and a network for communication

I Client: a computer running a program that makes a requestfor services

I Server: a computer running a program that offers requestedservices, e.g., processing database queries from one or moreclients

Client Server

Network (TCP/IP)

Request

Results



TCP/IP Application layer: Sockets

Transport

Application

Internet

Network Interface

FTP SMTP HTTP IMAP

SSL/TLS ACSII

TCP UDP

IP

Physical

ProtocolsTCP/IP model

Sockets



TCP/IP model: Application layer - Sockets

I The lowest level before Transport layer

I Definition: “endpoint of a two-way communication link”.

I Provides an interface for programming networkcommunication.

I Similar to performing file I/O, e.g., socket handle is treatedlike file handle.

I Independent of a programming language



Sockets

I A socket consists ofI Local socket address: Local IP address and service port numberI Remote socket address: Only for established TCP socketsI Protocol: A transport protocol, e.g., TCP or UDP.

I A socket address is the combination of an IP address (phonenumber) and service port number (extension).

I A socket API is an application programming interface (API),usually provided by the operating system.



Service ports

I Computers often communicate and provide more than onetype of service or to talk to multiple hosts/computers at atime

I Ports are used to distinguish these services

I Each service offered by a computer is identified by a portnumber

I Represented as a positive (16-bit) integer valueI Some ports have been reserved to support common services

I FTP: 21/TCPI HTTP: 80/TCP,UDPI IMAP: 143/TCP

I User-level process/services use port number value > 1024



Establish socket connection

I Step 1: The server listens to the socket of a specific portnumber for a client to make a connection request

I Step 2: the server accepts the connection if everything goeswell

I Step 3: the server gets a new socket bound to a different(usually random) port for connection with the client

I Go to Step 1

I Question: the service port in the TCP socket connectionrequest is different from the service port for the connection.Why?



Establish socket connection

Service Port

Server Client

Service Port

Server Client

Service Port

Connection request

Connection



Put them together

I Client is connected to a server at the physical addressidentified by a IP address.

I Connection binds a port number to a socket number.

I Both TCP and UDP protocols use ports to map incomingdata to a particular process running on a computer.

FTP HTTP SMTP App

Port Port Port Port

TCP or UDP

Port# Data

PacketData

Sockets

Apps



Java.net class

I Java provides a set of classes, java.net , to enable therapid development of network applications.

I Essentially provides classes, interfaces, and exceptions tosimplify the complexity involved in creating client and serverprograms

I Two key classes for creating server and client programsI ServerSocketI Socket



Socket-based client and server Java programming

I Server: ServerSocket creates a new service socket (port)

I Clinet: Socket establishes connection with service socket

I Both: exchange data using input and output streams



Java.net class

ServiceSocket

1254

Server (IP: 121.2.21.25) Client

Socket((“121.2.21.25”, 1254)

Output/write stream Input/read stream



A simple Server Program in Java (I)

Steps to create a simple server program:

I Step 1: Open the Server Socket:ServerSocket server = new ServerSocket( PORT );

I Step 2: Wait for the Client Request:Socket client = server.accept();

I Step 3: Create I/O streams for communicating to the clientDataInputStream is =

new DataInputStream(client.getInputStream());

DataOutputStream os =

new DataOutputStream(client.getOutputStream());



A simple Server Program in Java (II)

I Step 4: Perform communication with clientI Receive from client: String line = is.readLine();

I Send to client: os.writeBytes("Hello!");

I Step 5: Close the stream is.close(); os.close();

I Step 6: Close the socket: client.close();



A simple Client Program in Java

I Step 1: Open a socket.

I Step 2: Open an input stream and output stream to thesocket.

I Step 3: Read from and write to the stream according to theserver’s protocol.

I Step 4: Close the streams.

I Step 5: Close the socket.



A few explainations: PrintWriter vsDataOutputStream

I PrintWriter : “Prints formatted representations of objectsto a text-output stream.”

I DataOutputStream : “A data output stream lets anapplication write primitive Java data types to an outputstream in a portable way”

I PrintWriter inherited from Writer class, whileDataOutputStream inherited from OutputStream .

I OutputStream (therefore DataOutputStream ) is

designed for binary data, while Writer (thereforePrintWriter ) is designed for text data



A few explainations: InetAddress

I InetAddress class: “represents an Internet Protocol (IP)address.”

I getLocalHost() method: “Returns the address of thelocal host.”



A few explainations: Protocol

I A protocol is the language that the client and server haveagreed to use to communicate.

I Using Java.net class, you can build you own protocol usingsocket

I One example: KnockKnockProtocol can be found at here

http://docs.oracle.com/javase/tutorial/networking/sockets/clientServer.html



Socket programming for email

I POP3, IMAP and SMTP are all email protocols based onTCP socket communication

I Please take a look at my source code of send email using onlysocket

Communication and Networking Java Socket ... - cs.bham.ac.uk · TCP Socket Programming in Java Java.net class I Java provides a set of classes, java.net , to enable the rapid development

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