Network Protocols in Java - University of Readingsis06gd/res/SE2JA11-Java...Network Protocols in...

BSc Computer Science - “Java” (SE2JA11)

Network Protocols in Java

Dr. Giuseppe Di Fatta Associate Professor of Computer Science

Web: http://www.personal.reading.ac.uk/~sis06gd/ Email: [email protected]

Director of the MSc Advanced Computer Science http://www.reading.ac.uk/sse/pg-taught/sse-mscadvancedcomputerscience.aspx

School of Systems Engineering

These lecture slides are available at: http://www.personal.reading.ac.uk/~sis06gd/resources.html

Java, Dr. Giuseppe Di Fatta, 2007-2013 2

Outline

Introduction to Networking • Network protocols

• Client-server network applications

Java Socket programming using TCP and UDP

Simple Mail Transfer Protocol (SMTP)

Code examples: • How to send an email in Java

• How to implement a simple FTP client in Java


What’s a protocol?

human protocols: • “what’s the time?” • “I have a question” • introductions

… specific msgs sent … specific actions taken

when msgs received, or other events

network protocols: • machines rather than

humans • all communication

activity in Internet governed by protocols

A protocol defines: • format and order of messages

sent and received among network entities

• actions taken on message transmission, receipt


What’s a protocol?

a human protocol and a computer network protocol:

Hi

Hi Got the time? 2:00

TCP connection req

TCP connection response Get http://www.google.com/

<file> time


Internet protocol stack

• application: supporting network applications – FTP, SMTP, STTP

• transport: host-host data transfer – TCP, UDP

• network: routing of datagrams from source to destination – IP, routing protocols

• link: data transfer between neighboring network elements – PPP, Ethernet

• physical: bits “on the wire”

application

transport

network

link

physical


Network applications: some jargon

Process: program running within a host.

• within same host, two processes communicate using interprocess communication (defined by OS).

• processes running in different hosts communicate with an application-layer protocol

user agent: interfaces with user “above” and network “below”.

• implements user interface & application-level protocol – Web: browser – E-mail: mail reader – streaming audio/video:

media player


Applications and application-layer protocols

Application: communicating, distributed processes – e.g., e-mail, Web, P2P file

sharing, instant messaging – running in end systems (hosts) – exchange messages to

implement application Application-layer protocols

– one “piece” of an app – define messages exchanged by

apps and actions taken – use communication services

provided by lower layer protocols (TCP, UDP)

application transport network data link physical




Application-Layer Protocol

A protocol defines: • Types of messages

exchanged, e.g., request & response messages

• Syntax of message types: what fields in messages & how fields are delineated

• Semantics of the fields, i.e., meaning of information in fields

• Rules for when and how processes send & respond to messages

Public-domain protocols: • standardised by IETF • allow for interoperability • e.g., HTTP, SMTP, FTP

Example of P2P protocols: • BitTorrent

Example of some proprietary

P2P protocols: • KaZaA • Skype


ARPANET-INTERNET Transition

• The Internet was born on January 1st, 1983: when ARPANET changed from NCP to the TCP/IP protocol suite. – Flag day: a change which requires a complete restart or conversion of a relevant

software component or data. The change is large and expensive; reversing the change is difficult or impossible.

– “The goal is to make a complete switch over from the NCP to IP/TCP by 1 January 1983.” (Jon Postel, NCP/TCP Transition Plan, RFC 801)

Bob Kahn and Vint Cerf - fathers of the Internet: inventors of the TCP/IP protocol suite


IETF

The Internet Engineering Task Force (IETF) is a loosely self-organized group of people who contribute to the engineering and evolution of Internet technologies. The IETF in no way "runs the Internet“.

IETF quotes (believes) Robert Kahn (interview, 2002):

People don't think of the Internet as a logical architecture. They think of it as what AOL does. The Internet is an architectural philosophy, rather than a technology.

David Clark (1992-07-16, 24th IETF meeting):

We reject: kings, presidents and voting. We believe in: rough consensus and running code.

Jon Postel (RFC 793 - September 1981) - Robustness Principle:

TCP implementations will follow a general principle of robustness: be conservative in what you do, be liberal in what you accept from others.

Jon Postel (1943-1998): see RFC 2468, October 1998


Client-Server Paradigm

Typical network app has two pieces: client and server



Client: • initiates contact with server

(“speaks first”) • typically requests service from

server, • Web: client implemented in

browser; e-mail: in mail reader

request

reply

Server: • provides requested service to client • e.g., Web server sends requested Web

page, mail server delivers e-mail


Addressing Hosts and Processes

• For a process to receive messages, it must have an identifier

• Every host has a unique 32-bit IP address

• Q: does the IP address of the host on which the process runs suffice for identifying the process?

• Answer: No, many processes can be running on same host

• Identifier includes both the IP address and port numbers associated with the process on the host.

• Example port numbers: – HTTP server: 80 – Mail server: 25


IP Addresses

Current world population: 7 billion

IP v4 addresses: 232 = 4 billion

IP v6 addresses: 2128


Processes communicating across network

• process sends/receives messages to/from its socket

• socket analogous to door – sending process shoves

message out door – sending process assumes

transport infrastructure on other side of door which brings message to socket at receiving process

process

TCP with buffers, variables

socket

host or server

process


socket

host or server

Internet

controlled by OS

controlled by app developer

• API: (1) choice of transport protocol; (2) ability to fix a few parameters


Internet transport protocols services

TCP service: • connection-oriented: setup

required between client and server processes

• reliable transport between sending and receiving process

• flow control: sender won’t overwhelm receiver

• congestion control: throttle sender when network overloaded

• does not providing: timing, minimum bandwidth guarantees

UDP service: • unreliable data transfer

between sending and receiving process

• does not provide: connection setup, reliability, flow control, congestion control, timing, or bandwidth guarantee

Q: why bother? Why is there a UDP?


Internet Applications

Application

e-mail remote terminal access

Web file transfer

streaming multimedia

Internet telephony

Application layer protocol SMTP [RFC 2821] Telnet [RFC 854] HTTP [RFC 2616] FTP [RFC 959] proprietary proprietary

Underlying transport protocol TCP TCP TCP TCP TCP or UDP typically UDP


Socket programming

Socket API • introduced in BSD4.1 UNIX, 1981 • explicitly created, used, released by

applications • client/server paradigm • two types of transport service via socket

API: – unreliable datagram (UDP) – reliable, byte stream-oriented (TCP)

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.


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


socket

controlled by application developer

controlled by operating

system

host or server

process


socket

controlled by application developer

controlled by operating system

host or server

internet


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

TCP provides reliable, in-order transfer of bytes (“pipe”) between client and server

application viewpoint


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.

stream of bytes file

stream of bytes

stream of bytes

stream of bytes

Input streams

file

stream of bytes

stream of bytes

Output streams

Internet

Internet

Process


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

inF

rom

Ser

ver

inF

rom

Use

r

keyboard monitor

Process

clientSocket

inputstream

inputstream

outputstream

TCPsocket

Client Process

client TCP socket


Client/server socket interaction: TCP

wait for incoming connection request connectionSocket = welcomeSocket.accept()

create socket for incoming request: welcomeSocket =

ServerSocket()

create socket to server clientSocket =

Socket()

close connectionSocket

read reply from clientSocket

close clientSocket

send request using clientSocket read request from

connectionSocket

write reply to connectionSocket

TCP connection setup

reliable full-duplex channel

Server Client TCP connection

3

4

1

2

1

-1

0

2

3 4


Example: Java TCP client (1)

public static void main(String argv[]) { 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

Note: to run this example you still need to add some code to manage exceptions.


Example: Java TCP client (2)

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



Example: Java TCP server (1)

public static void main(String argv[]) { 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



Example: Java TCP server (2)

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



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


Client/server socket interaction: UDP

close clientSocket

Server (running on hostid)

read reply from clientSocket

create socket,

clientSocket = DatagramSocket()

Client

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

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

read request from serverSocket

write reply to serverSocket specifying client host address, port number


Example: Java client (UDP)

send

Pack

et

to network from network

rece

iveP

acke

t

inFr

omU

ser

keyboard monitor

Process

clientSocket

UDPpacket

inputstream

UDPpacket

UDPsocket

Output: sends packet (TCP sent “byte stream”)

Input: receives packet (TCP received “byte stream”)

Client process

client UDP socket


Example: Java UDP client (1)

public static void main(String args[]) { 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();

Create input stream

Create client socket

Translate hostname to IP

address using DNS



Example: Java UDP client (2)

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



Example: Java UDP server (1)

public static void main(String args[]) { 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



Example: Java UDP server (2) 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



Electronic Mail

Three major components: • user agents • mail servers • Simple Mail Transfer Protocol

(SMTP)

User Agent • a.k.a. “mail reader” • composing, editing, reading mail

messages • e.g., Eudora, Outlook, Elm,

Mozilla Thunderbird • outgoing, incoming messages

stored on server

user mailbox

outgoing message queue

mail server

user agent

user agent

user agent

mail server

user agent

user agent

mail server

user agent

SMTP

SMTP

SMTP


Mail Servers

Mail Servers • mailbox contains incoming

messages for user • message queue of outgoing

(to be sent) mail messages SMTP protocol between mail

servers to send email messages – client: sending mail server – “server”: receiving mail

server

mail server

mail server

user agent

mail server

user agent

SMTP

SMTP

SMTP

IMAP or

POP3


SMTP [RFC 2821]

• uses TCP to reliably transfer email message from client to server: TCP port 25

• direct transfer: sending server (or user client) to receiving server • 3 phases of transfer

– handshaking (greeting) – transfer of messages – closure

• command/response interaction – commands: ASCII text – response: status code and phrase

• messages must be in 7-bit ASCII

http://tools.ietf.org/html/rfc2821 http://tools.ietf.org/html/rfc821


Try SMTP yourself

We can send email without using an email client (thunderbird, outlook, etc.)

We can talk SMTP directly to

the server: telnet servername 25 see “220” reply from server Commands:

– HELO – MAIL FROM – RCPT TO – DATA – QUIT

Replies: – 220 mailserver time – 250 ok – 500 unrecognized command – 501 Syntactically invalid HELO

argument(s) – …

> telnet smtp.reading.ac.uk 25 220 vimp1.rdg.ac.uk ESMTP Exim Wed, 28 Feb 2007 16:40 helo whatsoever.blabla.org 250 vimp1.rdg.ac.uk Hello imogen.rdg.ac.uk mail from: <[email protected]> 250 OK rcpt to: <[email protected]> 250 Accepted data 354 Enter message, ending with "." on a line by itself Hi there! bye. . 250 OK id=1HMRrh-0007OW-BX quit 221 vimp1.rdg.ac.uk closing connection >

user agent

mail server SMTP

telnet connection

Ethernet IP TCP

telnet+SMTP


SMTP: final words • SMTP uses persistent connections • SMTP requires message (header & body) to be in 7-bit ASCII • SMTP server uses CRLF.CRLF to determine end of message

SMTP: protocol for exchanging email msgs

RFC 822: standard for text message format:

• header lines, e.g., – to, from, subject different from SMTP commands!

• body – the message-body

• Termination sequence – “\n.\n”

header

body

blank line


Java Code: send an email (1/2) private static void pause(int seconds) { long msec = 1000 * seconds; try { Thread.sleep(msec); } catch (InterruptedException e) { } } public static String sendCommand(Socket theSocket, String cmd) { DataOutputStream outToServer = new DataOutputStream(theSocket.getOutputStream()); BufferedReader inFromServer = new BufferedReader(new

InputStreamReader(theSocket.getInputStream())); pause(1); //needed for some MS servers! System.out.println(“SENT TO SERVER: " + cmd); outToServer.writeBytes(cmd + '\n'); String res = inFromServer.readLine(); System.out.println(“REPLY FROM SERVER: " + res); System.out.println(); return res; }



Java Code: send an email (2/2) public static void main(String args[]) { String mailServer = "smtp.reading.ac.uk"; int smtp_port = 25; String myEmailAddress = "[email protected]"; String dstAddress = "[email protected]"; String command, answer; Socket clientSocket = new Socket(mailServer, smtp_port); command = "helo myself.uk"; answer = sendCommand(clientSocket, command); command = "mail from: <" + myEmailAddress + ">"; answer = sendCommand(clientSocket, command); command = "rcpt to: <" + dstAddress + ">"; answer = sendCommand(clientSocket, command); command = "data"; answer = sendCommand(clientSocket, command); //send the content of the email: StringBuffer data = new StringBuffer(); data.append("blabla...\n"); data.append("blabla..."); command = data.toString() + "\n.\n"; answer = sendCommand(clientSocket, command); command = "quit"; answer = sendCommand(clientSocket, command); clientSocket.close(); } Note: to run this example you still need to add some code to manage exceptions.


Java Example: a simple FTP client String ftp_url="ftp://anonymous:[email protected]/pub/misc/movies/database/"; String type = ";type=i"; String file1 = "filesizes"; try{ String filename=file1; URL url = new URL(ftp_url+filename+type); URLConnection urlc = url.openConnection(); InputStream is = urlc.getInputStream(); //For text files use BufferedReader and InputStreamReader //For binary files use BufferedInputStream BufferedReader bin = new BufferedReader(new InputStreamReader(is)); String line; System.out.println("--------- "+filename+" -----------"); while((line = bin.readLine()) != null){ System.out.println(line); } System.out.println("--------------------"); bin.close(); } catch (MalformedURLException e1) { System.out.println(e1); e1.printStackTrace(); } catch (IOException e1) { System.out.println(e1); e1.printStackTrace(); }

Conclusions

Network programming in Java is easy and fun.


Java is: Object-Oriented Programming (OOP) language Strongly typed, data declarations, statements A standard set of Application Programming Interfaces (API) Main features: Garbage collection Threads Exceptions Network oriented

These lecture slides are available at: http://www.personal.reading.ac.uk/~sis06gd/resources.html

Network Protocols in Java - University of Readingsis06gd/res/SE2JA11-Java...Network Protocols in...

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