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Networking review
Chapter 1, Chapter 2, Chapter 3
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Theory chapters
Why Networked Java?
Basic Network Concepts
Basic Web Concepts
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Networking tasks
Java Network Programming will help you to take advantage of Java's network class library
to quickly and easily write programs that accomplish many common networking
tasks. Some of these include :
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Browsing the Web with HTTP
Parsing and rendering HTML
Sending email with SMTP
Receiving email with POP and IMAP
Writing multithreaded servers
Installing new protocol and content handlers into browsers
Encrypting communications for confidentiality, authentication, and guaranteed message
integrity
Designing GUI clients for network services
Posting data to server-side programs
Looking up hosts using DNS
Downloading files with anonymous FTP
Connecting sockets for low-level network communication
Distributing applications across multiple systems with Remote Method Invocation
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Why Networked Java ?
Java was the first programming language designed from the ground up
with networking in mind and it makes writing network programs easy.
This course shows you many of complete programs that take advantage
of the Internet.
Some are simple textbook examples, while others are completely
functional applications. One thing you'll notice in the fully functional
applications is just how little code is devoted to networking.
The part of the program that deals with the network is almost always
the shortest and simplest.
In brief, it is easy for Java applications to send and receive data across the
Internet
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What Can a Network Program Do ?
 Retrieve Data
Network client retrieves data from a server. Network clients written in Java can
speak standard protocols like HTTP, FTP, or SMTP to communicate with
existing servers written in a variety of languages. More importantly, programs
can speak custom protocols designed for specific purposes.
Send Data
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Web browsers are optimized for retrieving data: they send only limited amounts
of data back to the server. Java programs have no such limitations.
(File storage and Massively parallel computing)
Peer-to-Peer Interaction
(Games, Real-time chat, Collaborative work, Video conference, File
sharing, e.g. BitTorrent)
 Servers (file servers, print servers, email servers, directory servers, domain name
servers, FTP servers, TFTP servers or custom servers like online games server )
 Searching the Web (Google search, spiders, agents)
 Electronic Commerce (Java applets)
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Basic Network Concepts
A network is a collection of computers and other devices that can
send data to and receive data from each other.
Each machine on a network is called a node. Most nodes are
computers, printers, routers, bridges, gateways, etc. The word host
to refer to a node that is a general-purpose computer.
Every network node has an address.
All modern computer networks are packet-switched networks:
data traveling on the network is broken into chunks called
packets and each packet is handled separately. Each packet
contains information about who sent it and where it's going.
A protocol is a precise set of rules defining how computers
communicate: the format of addresses, how data is split into
packets, and so on.
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The Layers of a Network
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The Internet Layer
The Internet Protocol (IP) is the most widely used
network layer protocol in the world and the only
network layer protocol Java understands.
Data is sent across the internet layer in packets called
datagrams. Each IP datagram contains a header between
20 and 60 bytes long and a payload that contains up to
65,515 bytes of data. (In practice, most IP datagrams are
much smaller, ranging from a few dozen bytes to a little
more than eight kilobytes.)
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Transport Layer
The transport layer is responsible for ensuring that packets are
received in the order they were sent and making sure that no
data is lost or corrupted. If a packet is lost, the transport layer
can ask the sender to retransmit the packet.
Transmission Control Protocol (TCP), is a high-overhead
protocol that allows for retransmission of lost or corrupted
data and delivery of bytes in the order they were sent.
User Datagram Protocol (UDP), allows the receiver to detect
corrupted packets but does not guarantee that packets are
delivered in the correct order (or at all). However, UDP is
often much faster than TCP.
TCP is called a reliable protocol; UDP is an unreliable protocol.
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The Application Layer
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The layer that delivers data to the user is called the
application layer.
HTTP (for the World Wide Web) makes sure that your
web browser knows to display a graphic image as a
picture, not a long stream of numbers.
SMTP, POP, and IMAP for email;
FTP, FSP, and TFTP for file transfer;
NFS for file access; NNTP for news transfer;
Gnutella, FastTrack, BitTorrent, and Freenet for file
sharing; and many, many more.
In addition, your programs can define their own
application layer protocols as necessary.
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Internet addresses
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Every host on the Internet is identified by a unique, four-byte
Internet Protocol (IP) address.
This is written in dotted quad format like 199.1.32.90 where
each byte is an unsigned integer between 0 and 255.
There are about four billion unique IP addresses (2³²).
When data is transmitted across the network, the packet’s
header includes the address of the machine for which the
packet is intended (the destination address) and the address
of the machine that sent the packet (the source address).
Routers along the way choose the best route to send the
packet along by inspecting the destination address. The source
address is included so the recipient will know who to reply to.
Domain Name System (DNS)
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Numeric addresses are mapped to names like
www.uop.edo.jo by DNS.
Each site runs domain name server software that
translates names to IP addresses and vice versa
DNS is a distributed system
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The InetAddress Class
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The java.net.InetAddress class represents an IP
address.
It converts numeric addresses to host names and host
names to numeric addresses.
It is used by other network classes like Socket and
ServerSocket to identify hosts
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Ports
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In general a host has only one Internet address
This address is subdivided into 65,536 ports.
Ports are logical abstractions that allow one host to
communicate simultaneously with many other hosts
Many services run on well-known ports. For example,
http tends to run on port 80
Ports numbers are used by transport layer
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The Client/Server Model
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IETF RFCs
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Requests For Comment
Document how much of the Internet works
Various status levels from obsolete to required to
informational
TCP/IP, telnet, SMTP, MIME, HTTP, and more
http://www.faqs.org/rfc/
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W3C (World Wide Web Consortium)
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IETF is a relatively informal, democratic body open to
participation by any interested party.
W3C is a vendor organization, controlled by dues-paying
member corporations, that explicitly excludes
participation by individuals.
For the most part, the W3C tries to define standards in
advance of implementation.
W3C standards include HTTP, HTML, and XML.
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Basic Web Concepts
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Many of your programs will be applets on web pages,
servlets running on the server, or web services that need
to talk to other web servers and clients.
Therefore, it’s important to have a solid understanding of
the interaction between web servers and web browsers.
The Hypertext Transfer Protocol (HTTP) is a standard
that defines how a web client talks to a server and how
data is transferred from the server back to the client.
The most common format for data transferred over the
Web is the Hypertext Markup Language (HTML).
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URL (I)
HTML is a “hypertext markup language” because it
includes a way to specify links to other documents
identified by URLs.
 A URL, short for "Uniform Resource Locator", is a way to
clearly identify the location of a resource on the Internet.
 The syntax of a URL is:
protocol://username@hostname:port/path/filename?query#frag
ment
 Protocol part can be file, ftp, http, https …
 Hostname part of a URL is the name of the server that
provides the resource you want, such as www.oreilly.com
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URL (II)
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The username is an optional name for the server. The port
number is also optional.
The path points to a particular directory on the specified
server.
The filename points to a particular file in the directory
specified by the path.
The query string provides additional arguments for the server.
It’s commonly used only in http URLs, where it contains form
data for input to programs running on the server.
The fragment references a particular part of the remote
resource.
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URL (III)
A named anchor is created in an HTML document with a
tag, like this:
<A NAME="xtocid1902914">Comments</A>
 This tag identifies a particular point in a document. To
refer to this point, a URL includes not only the
document’s filename but the named anchor separated
from the rest of the URL by a #:
http://www.cafeaulait.org/javafaq.html#xtocid1902914
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Example URLs
http://java.sun.com/
file:///Macintosh%20HD/Java/Docs/JDK%201.1.1%20docs/api/ja
va.net.InetAddress.html#_top_
http://www.macintouch.com:80/newsrecent.shtml
ftp://ftp.info.apple.com/pub/
mailto:[email protected]
telnet://utopia.poly.edu
ftp://mp3:[email protected]:21000/c%3a/stuff/mp3/
http://[email protected]/
http://metalab.unc.edu/nywc/comps.phtml?category=Choral+Wo
rks
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Normal web surfing uses these two steps:
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The browser requests a page
The server sends the page
Data flows primarily from the server to the client.
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Forms
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There are times when the server needs to get data from
the client rather than the other way around. The common
way to do this is with a form like this one:
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CGI
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The user types data into a form and hits the submit button.
The browser sends the data to the server using the
Common Gateway Interface, CGI.
CGI uses the HTTP protocol to transmit the data, either as
part of the query string or as separate data following the
MIME (Multipurpose Internet Mail Extensions)header.
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GET and POST
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When the data is sent as a query string included with the
file request, this is called CGI GET.
When the form data is sent as data attached to the
request following the MIME header, this is called CGI
POST
Other request types:
HEAD retrieves only the header for the file, not the
actual data.
PUT uploads a resource to the server,
and DELETE removes a resource from the server.
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HTTP
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Web browsers communicate with web servers through a
standard protocol known as HTTP, an acronym for
HyperText Transfer Protocol.
This protocol defines
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how a browser requests a file from a web server
how a browser sends additional data along with the request (e.g.
the data formats it can accept),
how the server sends data back to the client
response codes
A Typical HTTP Connection
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Client opens a socket to port 80 on the server.
Client sends a GET request including the name and path of
the file it wants and the version of the HTTP protocol it
supports.
The client sends a MIME header.
The client sends a blank line.
The server sends a MIME header
The server sends the data in the file.
The server closes the connection.
What the client sends to the server
GET /index.html HTTP/1.0 (operation to return resource)
Accept: text/html, text/plain, image/gif, image/jpeg (data
accepted by client)
User-Agent: Lynx/2.4 libwww/2.1.4 (browser type)
Host: www.cafeaulait.org (server name)
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MIME
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MIME is an acronym for "Multipurpose Internet Mail
Extensions".
an Internet standard defined in RFCs 2045 through 2049
originally intended for use with email messages, but has
been adopted for use in HTTP to describe a file’s
contents so that client software can tell the difference
between different kinds of data.
For example, HTML’s content type is text/html; the type is
text, and the subtype is html. The content type for a GIF
image is image/gif; the type is image, and the subtype is gif.
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Server Response
When a web server responds to a web browser it sends a response message
and a MIME header along with the response that looks something like this:
HTTP/1.1 200 OK
Date: Mon, 15 Sep 2003 21:06:50 GMT
Server: Apache/2.0.40 (Red Hat Linux)
Last-Modified: Tue, 15 Apr 2003 17:28:57 GMT
Connection: close
Content-Type: text/html; charset=ISO-8859-1
Content-length: 107
<html>
<head>
<title>
A Sample HTML file
</title>
</head>
<body>
The rest of the document goes here
</body>
</html>
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Server-Side Programs
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Server generates web pages dynamically to meet user
requests, instead of sending static files form hard drive.
Such server-side programs are often written using
servlets or Java Server Pages (JSP). They can also be
written with other languages, such as C and Perl, or other
frameworks, such as ASP and PHP.
The concern in this book is not so much with how these
programs are written as with how your programs
communicate with them. One advantage to HTTP is that
it really doesn’t matter how the other side of the
connection is written, as long as it speaks the same basic
HTTP protocol.
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Server-Side Programs
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The simplest server-side programs run without any input from the user. From the viewpoint
of the client, these programs are accessed like any other web page.
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A slightly more complex server-side program processes user input from HTML forms.
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Example 3-1. A simple form with input fields for a name and an email address
<HTML>
<HEAD>
<TITLE>Sample Form</TITLE>
</HEAD>
<BODY>
<FORM METHOD=GET
ACTION="/cgi/reg.pl">
<PRE>
Please enter your name: <INPUT
NAME="username" SIZE=40>
Please enter your email address: <INPUT
NAME="email" SIZE=40>
</PRE>
<INPUT TYPE="SUBMIT">
</FORM>
</BODY>
</HTML>
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Server-Side Programs
The web browser reads the data the user types and encodes it in a simple fashion.
The name of each field is separated from its value by the equals sign (=). Different
fields are separated from each other by an ampersand (&).
For example, the data from the form in Figure 3-3 is encoded as:
username=Elliotte+Harold&email=elharo%40macfaq.com
This is called the query string.
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Server-Side Programs
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There are two methods by which the query string can be sent to the
server: GET and POST. If the form specifies the GET method, the browser
attaches the query string to the URL it sends to the server. Forms that
specify POST send the query string on an output stream.
GET /cgi/reg.pl?username=Elliotte+Harold&email=elharo%40macfaq.com
HTTP/1.0
POST /cgi-bin/register.pl HTTP 1.0
Content-type: application/x-www-form-urlencoded
Content-length: 65
username=Elliotte+Harold&email=elharo%40metalab.unc.edu
 GET is intended for noncommital actions, like browsing a static web page.
POST is intended for actions that commit to something.
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