Network protocols and Java programming

School of Systems Engineering
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: G.DiFatta@reading.ac.uk

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

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

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

Java, Dr. Giuseppe Di Fatta, 2007-2013

2

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


3

What’s a protocol?
a human protocol and a computer network protocol:

Hi

TCP connection
req

Hi

TCP connection
response

Got the
time?

Get http://www.google.com/

2:00

<file>
time


4

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

application

transport
network

link
physical

• physical: bits “on the wire”


5

Network applications: some jargon
Process: program running
user agent: interfaces with
within a host.
user “above” and network
• within same host, two
“below”.
processes communicate
• implements user interface
using interprocess
& application-level
communication (defined by
protocol
OS).
– Web: browser
• processes running in
– E-mail: mail reader
different hosts
– streaming audio/video:
communicate with an
media player
application-layer protocol


6

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
transport
network
data link
physical

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
transport
network
data link
physical

7

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


8

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


9

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

10

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

application
transport
network
data link
physical

request

Server:
• provides requested service to client
• e.g., Web server sends requested Web
page, mail server delivers e-mail

reply
application
transport
network
data link
physical

11

Addressing Hosts and Processes
• For a process to receive
messages, it must have an
identifier
• Every host has a unique 32bit 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


12

IP Addresses
Current world population:
7 billion

IP v4 addresses: 232 = 4 billion
IP v6 addresses: 2128

13

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

host or
server

host or
server

process

controlled by
app developer

process
socket

socket
TCP with
buffers,
variables

Internet

TCP with
buffers,
variables

controlled
by OS

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

14

Internet transport protocols services
TCP service:

UDP 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

• 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?


15

Internet Applications

Application
e-mail
remote terminal access
Web
file transfer
streaming multimedia

Internet telephony

Application
layer protocol

Underlying
transport protocol

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

TCP
TCP
TCP
TCP
TCP or UDP

proprietary

typically UDP


16

Socket programming
Goal: learn how to build client/server application that
communicate using sockets.
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)

socket
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.


17

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
controlled by
application
developer
controlled by
operating
system

process

process
socket
TCP with
buffers,
variables

host or
server

internet

socket
TCP with
buffers,
variables

controlled by
application
developer
controlled by
operating
system

host or
server

18

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
application viewpoint

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


19

Stream jargon
stream of bytes

file

stream of bytes

stream of bytes

Process

• 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.

Input streams

Internet

Output streams
stream of bytes

file

stream of bytes

stream of bytes


Internet
20

Socket programming with TCP

Process
Client
Process

output
stream

monitor

inFromUser

input
stream

outToServer

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)

keyboard

inFromServer

Example client-server app:

input
stream

client TCP
clientSocket
socket
to network


TCP
socket

from network

21

Client/server socket interaction: TCP
2

4

Server

Client
TCP connection

3
-1

1

create socket for
incoming request:
welcomeSocket =
ServerSocket()

TCP connection setup

wait for incoming
connection request
connectionSocket =
welcomeSocket.accept()
read request from
connectionSocket
write reply to
connectionSocket
close
connectionSocket

2

3

create socket to server

0

clientSocket =
Socket()
reliable fullduplex channel

1

send request using
clientSocket

read reply from
4 clientSocket
close
clientSocket

22

Example: Java TCP client (1)
public static void main(String argv[])
{
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());

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

23

Example: Java TCP client (2)
Create
input stream
attached to socket

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

sentence = inFromUser.readLine();
Send line
to server

outToServer.writeBytes(sentence + 'n');

Read line
from server

modifiedSentence = inFromServer.readLine();

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


24

Example: Java TCP server (1)

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

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()));


25

Example: Java TCP server (2)
Create output
stream, attached
to socket

DataOutputStream outToClient =
new DataOutputStream(connectionSocket.getOutputStream());

Read in line
from socket

clientSentence = inFromClient.readLine();
capitalizedSentence = clientSentence.toUpperCase() + 'n';

Write out line
to socket

outToClient.writeBytes(capitalizedSentence);

}
}
End of while loop,
loop back and wait for
another client connection


26

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

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


27

Client/server socket interaction: UDP
Server (running on hostid)
create socket,
port=x, for
incoming request:
serverSocket =
DatagramSocket()

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

Client
create socket,
clientSocket =
DatagramSocket()
Create, address (hostid, port=x,
send datagram request
using clientSocket

read reply from
clientSocket
close
clientSocket


28

Example: Java client (UDP)

input
stream

Client
process

monitor

inFromUser

keyboard

Process

Input: receives

packet (TCP
received “byte
stream”)

UDP
packet

receivePacket

packet (TCP sent
“byte stream”)

sendPacket

Output: sends

client UDP
clientSocket
socket
to network

UDP
packet

UDP
socket

from network


29

Example: Java UDP client (1)

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

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();


30

Example: Java UDP client (2)
Create datagram
with data-to-send,
length, IP addr, port

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

Send datagram
to server

clientSocket.send(sendPacket);

Read datagram
from server

clientSocket.receive(receivePacket);

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

String modifiedSentence =
new String(receivePacket.getData());
System.out.println("FROM SERVER:" + modifiedSentence);
}

31

Example: Java UDP server (1)

Create
datagram socket
at port 9876

public static void main(String args[])
{
DatagramSocket serverSocket = new DatagramSocket(9876);
byte[] receiveData = new byte[1024];
byte[] sendData = new byte[1024];
while(true)
{

Create space for
received datagram
Receive
datagram

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


32

Example: Java UDP server (2)
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

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

Write out
datagram
to socket

serverSocket.send(sendPacket);
}
}

End of while loop,
loop back and wait for
another datagram

33

Electronic Mail
outgoing
message queue

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
agent

user mailbox

mail
server

SMTP
SMTP
mail
server

SMTP

user
agent
mail
server

user
agent

user
agent

user
agent

user
agent


34

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

SMTP
mail
server

user
agent

SMTP
mail
server

SMTP
mail
server


IMAP
or
POP3

user
agent
35

SMTP [RFC 2821]
http://tools.ietf.org/html/rfc2821
http://tools.ietf.org/html/rfc821

• 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

36

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)

user
agent

SMTP

mail
server

telnet+SMTP
TCP
IP
Ethernet

telnet
connection
> 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: <myUserID@reading.ac.uk>
250 OK
rcpt to: <toSomeone@reading.ac.uk>
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
>

– …

37

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

header

blank
line

body

– the message-body

• Termination sequence
– “n.n”

38

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;
}


39

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 = "G.DiFatta@reading.ac.uk";
String dstAddress = "G.DiFatta@reading.ac.uk";
String command, answer;
Socket clientSocket = new Socket(mailServer, smtp_port);
command = "helo myself.uk";
answer = sendCommand(clientSocket, command);
command = "mail from: <" + myEmailAddress + ">";
command = "rcpt to: <" + dstAddress + ">";
command = "data";
//send the content of the email:
StringBuffer data = new StringBuffer();
data.append("blabla...n");
data.append("blabla...");
command = data.toString() + "n.n";
command = "quit";

}


40

Java Example: a simple FTP client
String ftp_url="ftp://anonymous:na@ftp.fu-berlin.de/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();
}


41

Conclusions
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 

 Network programming in Java is easy and fun.
These lecture slides are available at:
http://www.personal.reading.ac.uk/~sis06gd/resources.html

42

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