Application Development withTCP/IP

Brian S. MitchellDrexel University

Agenda

l TCP/IP Application Development EnvironmentlClient/Server Computing with TCP/IPl Socketsl Port Numbersl The TCP/IP Application Programming Interface (API)l Example: A Distributed File Viewerl Java Socket Implementation

TCP/IP Application DevelopmentEnvironment

l TCP/IP applications are constructed using the sockets interfacel Open standard

lExtensions to support the Windows (3.1, 95, NT) environment havebeen developed by an open committee (Winsock)

Network Media (Wire)

Network Interface Card

Control Header Data Area IEEE 802.2 LLC Frame

Datagram Header Data Area IP Datagram

DatagramHeader Data Area TCP Datagram

TCP/IP Application DevelopmentInterface (Sockets)

TCP/IP Applications

TCP /IPProtocolSuite

Client/Server Model with TCP/IP

l TCP/IP provides peer to peer communication between two application processesl The peer applications can execute on the same or different machines

l Client application initiates the conversationl Server application accepts the conversationl Client applications generally request services whereas server applications

generally provide servicesl Example: CETS

Client Application

TCP/IPInterface Network

TCP/IP Interface

Server Application

Application Responsibilities

lClient Applicationsl Locate and request a service from a serverl Can run on a single (DOS, Windows 3.x) or a multi-process

(Windows 95, NT, OS2, UNIX, MVS) operating system

l Server Applicationsl Server has additional responsibilities beyond providing a service to

a clientlAuthentication - verifying the identity of the clientlAuthorization - determining if a given client is permitted to

access the service that the server supplieslData Security - guaranteeing that the data that is exchanged

between the client and the server is not revealed orcompromised

Application Responsibilities(con’t)

lPrivacy - keeping information about an individual (e.g. userID and password) from unauthorized accesslProtection - guaranteeing that network applications cannot

abuse system resourcesl Requires a multiprocessing operating system to concurrently

support more than one client.

Application Development withTCP/IP

lChecklist4IP Address4Socket4Port Number4Socket Verbs (Functions)

Sockets

l A socket is a 16-bit number that is used as an index into a socketdescriptor tablel Each entry in this table references a socket data structure.

l The socket data structure contains all of the information that isrequired to setup and exchange data over an IP conversation

l The socket model has many advantagesl Support for different communication protocols (Raw IP, UDP/IP, TCP/IP)l Makes receiving and sending data over a network synonymous with

performing read and write operations on a local file.

3780

965431569512

Socket Socket Details

Socket Data Structure

Family: PF_INETService: SOCK_STREAM (TCP)Local IP: 172.21.100.8Remote IP: 172.21.100.45Local Port: 3571Remote Port: 6358

Socket Descriptor Table

Sockets (con’t)

l When a socket is first created it does not contain enough informationto initiate a network conversation.l Special socket functions (TCP/IP API’s) must be called in order to

populate the socket data structure

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Socket Socket Details

Socket Data Structure

Family: ??????????Service: ??????????Local IP: ??????????Remote IP: ??????????Local Port: ??????????Remote Port: ??????????

Socket Descriptor Table

Port Numbers

l A port number is a 16-bit integer that represents the logical endpoint ofa conversation

l The mechanism that enables a computer to support more than one IPconversation at a time

l Port numbers can (and most likely will) be different for the sameconversation on the client and server computers

172.21.100.45

172.21.100.7

Program 1, 172.21.100.7, Port 52Program 2, 172.21.100.7 Port 87 . . .Program n, 172.21.56.2 Port 382

LAN

Program 1, 172.21.100.45 Port 97Program 2, 172.21.100.45 Port 156

Program 1, 172.21.100.45, Port 1041

172.21.56.2

Port Numbers (con’t)

l A Client application can obtain the server application port number inone of two waysl Must know the port number in advance

lProvided by the server’s documentationl Can determine the port number dynamically by sending a message to the

port mapper servicelAll services that want their port number to be found by name must

register (and deregister) with the port mapper servicelPort number is obtained by calling the getportbyname (...) API

ClientApplication

Port Mapper Service

Phone Directory Service Port 158

PortDatabase

The phone directory service islistening on port number 158

Connect 172.21.100.87:158

What port is handling the phone directory?

TCP/IP Client/ServerConversation

l TCP/IP socket programs are controlled by a series of API’s:l Socket, connect, accept, read, write, close, listen, bind, accept

Create a socket - socket( )Connect to the server - connect( )

Send data to the server - write( )Receive data from the server - read( )Terminate the conversation - close( )

socket( ) - create a socketbind( ) - bind program to a port numberlisten( ) - get ready to accept client requestsaccept( ) - wait for a client requestread( ) - Receive data from the clientwrite( ) - Send data to the clientclose( ) - Terminate the conversation

Client Program Server Program

Supporting concurrent clientswith Socket Servers

l The server creates a new process or thread to handle the clients requestl Client connection is serviced by the new socket that was returned by the accept( ) calll The original socket is freed up to handle another client request

Multiprocessing Socket Server

s1 = socket ()bind (s1, port_number)listen (s1)s2 = accept (s1)

create_process(client_handler, s2)

// Create a socket// Bind the socket to a port// Place the socket into a listening mode// Wait for a client connection

accept( ) returns a new socket tohandle the client connection

// Create a new process to service the// client request

client_handler (client_socket)read(client_socket)write(client_socket)close(client_socket)

Example: A Distributed FileViewer Service

l The distributed file service will consist of the following componentsl Client Program

lConnect to the remote server programlRequest a file by namelReceive an image of the actual file from the server programlDisplay the file image for the user to view

l Server ProgramlWait for a client requestlRead a file from the local hard disk based on the client requestlSend an image of the file to the remote client

Client Program

ServerProgram

HardDisk

Get me file ‘c:\test.txt’

Image of the ‘c:\test.txt’ file

Example: A Distributed FileServer (con’t)

l Read the file ‘c:\test.txt’ on the server givenl Client IP = 172.21.100.7l Server IP = 172.21.100.45l File read service port number = 1031

ClientProgram TCP/IP Conversation

Server ProgramPort 1031

171.21.100.45

Hard Disk

Clientsocket ( )connect (172.21.100.45,1031)write (‘c:\test.txt’)read (data-area)close ( )display_data(data_area)

Serversocket ()bind (1031)listen ( )accept ( )read (file_name)read_file_from_disk (file_name, data_area)write (data_area)close ()

171.21.100.7

Sockets in Java

l The java.net package provides several classes that enablesocket-based communication in Java

l The Java socket classes are:l java.net.Socketl java.net.ServerSocketl java.net.DatagramSocketl java.net.SocketImpl

l See the Java documentation for details on these classes

Parameters in Java ClassConstructors

l String host: String host can be specified either as machinename or IP address. For queen.mcs.drexel.edu, machinename is ”queen.mcs.drexel.edu", IP address is"129.25.6.176". In unix, if you know the machine name,then you can use "nslookup" to identify the IP address ofthe machine if the machine is connected to the network.

l int port: integer port numberl boolean stream: It specify whether communication

through the socket based on TCP protocol, or UDPdatagram protocol. The defaulat protocol is TCP.

Parameters in Java ClassConstructors (con’t)

l InetAddress address: InetAddress is the class thatrepresents an Internet Protocol(IP) address. It does nothave a constructor, but instead it has several methodswhich return the InetAddress objects. MethodgetLocalHost() returns an InetAddress for the local host.GetAllByName() returns an array of InetAddress thatrepresents all of the available addresses for a host specifiedby name; getByName() returns the InetAddress of a hostspecified by name. The InetAddress class definition isshown on the next slide

The InetAddress Class

public final class InetAddress extends Object { // Methods public boolean equals(Object obj); public byte[] getAddress(); public static InetAddress[] getAllByName(String host); public static InetAddress getByName(String host); public String getHostName(); public static InetAddress getLocalHost(); public int hashCode(); public String toString();}

Implementing A Server

lCreate a socket, use the ServerSocket classl ServerSocket s = new ServerSocket(4444)l Socket listens to port 4444

lAccept a connection request by a clientl Socket client_socket=s.accept();l Creates a socket that communicates with the client

lOpen the input and output stream to/from the clientlDataInputStream in = new

DataInputStream(client_socket.getInputStream());l PrintStream out = new

PrintStream(client_socket.getOutputStream());

Implementing a Server

lClose the socketsl s.close();l client_socket.close();l in.close();l out.close();

lClosing the sockets and streams terminates the sessionbetween the server process and the client process

Implementing a Client

lCreate a socketl Specify server host name or IP address and port numberl Socket c= new Socket(”queen.mcs.drexel.edu", 4444)l If you only have a single machine you can use the “127.0.0.1”

local loopback address

lOpen the input and output stream on the socketlDataInputStream in = new DataInputStream(c.getInputStream());

// get input streaml PrintStream out = new PrintStream(c.getOutputStream());

// get output stream

Implementing the Client

lClose the socketsl in.close(); // close the input streaml out.close(); // close the output streaml c.close(); // close the client socket

Sample Server

class EchoServer {

public static void main(String[] args) {

try { ServerSocket s = new ServerSocket(4444); Socket client_socket=s.accept(); DataInputStream in = new DataInputStream

(client_socket.getInputStream()); PrintStream out = new PrintStream

(client_socket.getOutputStream()); ...

Sample Server

…out.println("Hello! Enter BYE to exit. \r"); boolean done = false;

while (!done) { String str = in.readLine(); if (str == null)

done = true; else {

out.println("Echo: " + str + "\r"); if (str.trim().equals("BYE"))

done = true; }

}...

Sample Server

client_socket.close(); } catch (Exception e) {

System.out.println(e); }

}} //end of class EchoServer

Sample Client

class EchoClient { public static void main(String[] args) {

try { Socket c = new Socket(”queen.mcs.drexel.edu", 4444); DataInputStream in = new DataInputStream(c.getInputStream()); bolean more = true; while (more) {

String str = in.readLine(); if (str == null)

more = false;else

System.out.println(str); }...

Sample Client

} catch (IOException e) {

System.out.println("Error" + e); }

} //end of class EchoClient