Unit 5 application layer

Unit 5 Application Layer Kritika Purohit
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Unit 5 Application Layer
Introduction-
The layers below the application layer are there to provide transport services, but they do not
do real work for users. In the application layer there is a need for support protocols, to allow
the applications to function.
World Wide Web
The World Wide Web is an architectural framework for accessing linked documents spread out over
millions of machines all over the Internet. The initial proposal for a web of linked documents came
from CERN physicist Tim Berners-Lee in 1989.
Architectural Overview-
From the users' point of view, the Web consists of a vast, worldwide collection of documents or Web
pages. Each page may contain links to other pages anywhere in the world. Users can follow a link by
clicking on it, which then takes them to the page pointed to. This process can be repeated indefinitely.
Pages are viewed with a program called a browser, of which Internet Explorer and Netscape Navigator
are two popular ones. The browser fetches the page requested, interprets the text and formatting
commands on it, and displays the page, properly formatted, on the screen [1].
Strings of text that are links to other pages, called hyperlinks, are often highlighted, by underlining,
displaying them in a special color, or both.
The Parts Of The Web Model-
Here the browser is displaying a Web page on the client machine. When the user clicks on a
line of text that is linked to a page on the abcd.com server, the browser follows the hyperlink
by sending a message to the abcd.com server asking it for the page. When the page arrives, it
is displayed. If this page contains a hyperlink to a page on the xyz.com server that is clicked
on, the browser then sends a request to that machine for the page.
Client Side-
When an item is selected, the browser follows the hyperlink and fetches the page selected.
Therefore, the embedded hyperlink needs a way to name any other page on the Web. Pages are
named using URLs (Uniform Resource Locators) [2].
The steps that occur at the client side are:
• The browser determines the URL

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• The browser asks DNS for the IP address
• DNS replies with the IP address
• The browser makes a TCP connection to port 80 on the IP address
• It sends a request asking for file
• The site server sends the file
• The TCP connection is released.
• The browser fetches and displays all the text and images in the file.
• Web pages are written in standard HTML language to make it understandable by all
browsers.
There are two possibilities: plug-ins and helper applications. A plug-in is a code module that
the browser fetches from a special directory on the disk and installs as an extension to itself.
The other way to extend a browser is to use a helper application. This is a complete program,
running as a separate process [4].
(a) A browser plug-in. (b) A helper application
Server side-
The steps to be followed by the server side are:
1. Accept a TCP connection from a client (a browser).
2. Get the name of the file requested.
3. Get the file (from disk).
4. Return the file to the client.
5. Release the TCP connection.
Processing Of Request
The processing of request on the web is as follows:
1. Resolve the name of the Web page requested.
2. Authenticate the client.
3. Perform access control on the client.
4. Perform access control on the Web page.
5. Check the cache.
6. Fetch the requested page from disk.
7. Determine the MIME type to include in the response.
8. Take care of miscellaneous odds and ends.
9. Return the reply to the client.
10. Make an entry in the server log.

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(a) Normal request-reply message sequence (b) Sequence when TCP handoff is used
Sometimes a trick, called TCP handoff, is used to get around this problem. With this trick, the
TCP end point is passed to the processing node so it can reply directly to the client.
URLs— Uniform Resource Locators-
When the Web was first created, it was immediately apparent that having one page point to
another Web page required mechanisms for naming and locating pages. In particular, three
questions had to be answered before a selected page could be displayed:-
1. What is the page called?
2. Where is the page located?
3. How can the page be accessed?
If every page were somehow assigned a unique name, there would not be any ambiguity in
identifying pages. Nevertheless, the problem would not be solved.
Consider a parallel between people and pages. In the United States, almost everyone has a
social security number, which is a unique identifier, as no two people are supposed to have the
same one. Nevertheless, if you are armed only with a social security number, there is no way
to find the owner's address, and certainly no way to tell whether you should write to the person
in English, Spanish, or Chinese. The Web has basically the same problems.
The solution chosen identifies pages in a way that solves all three problems at once. Each page
is assigned a URL (Uniform Resource Locator) that effectively serves as the page's worldwide
name.
URLs have three parts: the protocol (also known as the scheme), the DNS name of the machine
on which the page is located, and a local name uniquely indicating the specific page (usually
just a file name on the machine where it resides). As an example, the Web site for the author's
department contains several videos about the university and the city. The URL for the video
page is
http://www.hridu.in /video/index-en.html

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This URL consists of three parts: the protocol (http), the DNS name of the host (www.hridu.in),
and the file name (video/index-en.html), with certain punctuation separating the pieces. The
file name is a path relative to the default Web directory at hridu.in [4].
STATIC WEB DOCUMENTS –
• The basis of the Web is transferring Web pages from server to client. In the simplest
form, Web pages are static. They are just files sitting on some server waiting to be
retrieved.
• In this context, even a video is a static web page because it is just a file.
• In this section we will look at static web page in details. In the next one, we will
examine dynamic content.
HTML—The HyperText Mark-up Language:
• HTML allows users to produce Web pages that include text, graphics, video, pointers
toother Web pages, and more.
• HTML is a mark-up language, or language for describing how documents are to be
formatted.
• Mark-up languages thus contain explicit commands for formatting. For example, in
HTML, <b> means start boldface mode, and </b> means leave boldface mode.
• Writing a browser is then straightforward: the browser simply has to understand the
markup commands [2].
• Embedding all the markup commands within each HTML file and standardizing them
makes it possible for any Web browser to read and reformat any Web page.
• While it is certainly possible to write documents like this with any plain text editor, and
many people do, it is also possible to use word processors or special HTML editors that
do most of the work.
• A Web page consists of a head and a body, each enclosed by <html> and </html> tags.
• The head is bracketed by the <head> and </head> tags and the body is bracketed by the
<body> and </body> tags. The strings inside the tags are called directives.
• Most, but not all, HTML tags have this format. That is, they use <something> to mark
the beginning of something and </something> to mark its end.
• Tags can be in either lowercase or uppercase. Thus, <head> and <HEAD> mean the
same thing, but lower case is best for compatibility.
• Some tags have (named) parameters, called attributes. For example, the <img> tag is
used for including an image inline with the text. It has two attributes, src and alt. The
first attribute gives the URL for the image [3].
• The list of special characters is given in the standard. All of them begin with an
ampersand and end with a semicolon. For example, &nbps; produces a space, è
produces e` and &eacute producese ´. Since, < >and & have special meanings, they
can be expressed only with their escape sequences, <, >, and &amp.
• The main item in the head is the title, delimited by<title> and </title> .The title itself is
not displayed on the page. Some browsers use it to label the page’s window.
• Several headings are used in each heading is generated by an <hn> tag, where n is a
digit in the range 1 to 6. Thus, <h1> is the most important heading, <h6> is the least
important one.

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• <h1> headings are large and boldface with at least one blank line above and below. In
contrast, <h2> headings are in a smaller font with less space above and below [1].
• The tags <b> and <i> are used to enter boldface and italics mode. The <p> tag starts a
paragraph. </b> the tag that exists to mark the end of a paragraph.
XML and XSL:
• XML and XSL is (eXtensible Markup Language) and (eXtensible Style Language).
• HTML, with or without forms, does not provide any structure to web pages.
• It also mixes the content with the formatting, as e-commerce and other applications
become more common, there is an increasing need for structuring pages and separating
the content from the formatting.
• The W#C has developed an enhancement to HTML to allow web pages to be structured
for automated processing.
• It defines a structure called book_list, which is a list of books. Each book has three
fields, the title, author, and year of publication.
• In this example, each of the three fields is an indivisible entity, but it is also permitted
to further subdivide the fields [2].
• The author fields could have been done as follows to give a finer-grained control over
searching and formatting.
• Example: Andrew Tanenbaum
• Each field can be subdivided into subfields and sub subfields arbitrarily deep.
• The file is a style sheet that tells how to display the page, it is design view in the xml
file.
XHTML: (The eXtended HyperText Markup Language)
HTML keeps evolving to meet new demands. Many people in the industry feel that in the
future, the majority of web-enabled device will not be PCs, but wireless, handheld PDA-type
device.

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• These devices have limited memory for large browser full of heuristics that try to
somehow deal with syntactically incorrect web pages.
• There are 6 major difference in HTML and XHML:
➢ XHTML pages and browser must strictly conform to the standard. No more
shodly web pages.
➢ All tags and attributes must be in lower case, tags like <HTML> are not
valid in XHTML.
➢ Closing tags are required, even for </p>. For tags that have no natural
closing tag. Such as <br>, <hr> and <img>, a slash must proceed the
closing”>”.
Eg: - <img src=”pic01.jpg” height=”500” />
➢ Tags must be nest properly. In the past, proper nesting was not required as
long as final state achieved was correct. Tags closed in the inverse order that
they were opened.
Eg: - <center><b>Vacation Pictures</center></b>
➢ Every document must specify its document type.
Dynamic Web Documents- Dynamic web documents are created at both client and server
sides.
Server Side Generation-
The server side generation involves the following steps: [2]
• User fills in form.
• Form sent back
• Handed to CGI
• CGI queries database.
• Record found
• CGI builds page
• Page returned
• Page displayed
Client Side Generation-

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CGI, PHP, JSP, and ASP scripts solve the problem of handling forms and interactions with
databases on the server. They can all accept incoming information from forms, look up
information in one or more databases, and generate HTML pages with the results.
Usually the server side scripting is done with PHP and client side scripting is JavaScript.
Complete web pages can be generated on the fly by various scripts on the server machine. Once
they are received by the browser, they are treated as normal HTML pages and displayed [1].
Dynamic content generation is also possible on the client side. Web pages can be written in
XML and then converted to HTML according to XSL file. JavaScript programs can perform
arbitrary computations. Finally plugins and helper applications can be used to display content
in a variety of formats.
Domain Name Systems (DNS)
This is primarily used for mapping host and e-mail destinations to IP addresses but can also
be used other purposes. DNS is defined in RFCs 1034 and 1035.
Working: -
• To map a name onto an IP address, an application program calls a library procedure
called Resolver, passing it the name as a parameter.
• The resolver sends a UDP packet to a local DNS server, which then looks up the name
and returns the IP address to the resolver, which then returns it to the caller.
• Armed with the IP address, the program can then establish a TCP connection with the
destination, or send it UDP packets [4].
1. The DNS Name Space:
The Internet is divided into several hundred top level domains, where each domain covers many
hosts. Each domain is partitioned into sub domains, and these are further partitioned as so on.
All these domains can be represented by a tree, in which the leaves represent domains that have
no sub domains. A leaf domain may contain a single host, or it may represent a company and
contains thousands of hosts. Each domain is named by the path upward from it to the root [2].
The components are separated by periods (pronounced “dot”).
Eg: Sun Microsystems Engg. Department = eng.sun.com.
The top domain comes in 2 flavours: -

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• Generic: com(commercial), edu(educational instructions), mil(the U.S armed forces,
government), int (certain international organizations), net( network providers), org (non
profit organizations).
• Country: include 1 entry for every country. Domain names can be either absolute (ends
with a period e.g. eng.sum.com) or relative (doesn’t end with a period). Domain names
are case sensitive and the component names can be up to 63 characters long and full
path names must not exceed 255 characters [3].
A portion of the Internet domain name space.
Insertions of a domain into the tree can be done in 2 days:-
• Under a generic domain (Eg: - facebook.com)
• Under the domain of their country (Eg: - amazon.in)
2. Resource Records:
Every domain can have a sent of resource records associated with it. For a single host, the most
common resource record is just its IP address. When a resolver gives a domain name to DNS,
it gets both the resource records associated with that name i.e., the real function of DNS is to
map domain names into resource records. A resource record is a 5-tuple and its format is as
follows:

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3. Name Servers:
It contains the entire database and responds to all queries about it. DNS name space is divided
up into non-overlapping zones, in which each zone contains some part of the tree and also
contains name servers holding the authoritative information about that zone.
Part of the DNS name space showing the division into zones.
When a resolver has a query about a domain name, it passes the query to one of the local name
servers:
1. If the domain being sought falls under the jurisdiction of name server, it returns the
authoritative resource records (that comes from the authority that manages the record, and is
always correct) [1].
2. If the domain is remote and no information about the requested domain is available locally
the name server sends a query message to the top level name server for the domain requested.
E.g.: A resolver of flits.cs.vle.nl wants to know the IP address of the host Linda.cs.yale.edu

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Step 1: Resolver sends a query containing domain name sought the type and the class to local
name server, cs.vu.nl.
Step 2: Suppose local name server knows nothing about it, it asks few others nearby name
servers. If none of them know, it sends a UDP packet to the server for edu-server.net.
Step 3: This server knows nothing about Linda.cs.yale.edu or cs.yale.edu and so it forwards
the request to the name server for yale.edu.
Step 4: This one forwards the request to cs.yale.edu which must have authoritative resource
records.
Step 5 to 8: The resource record requested works its way back in steps 5-8 This query method
is known as Recursive Query
3. When a query cannot be satisfied locally, the query fails but the name of the next server
along the line to try is returned.
Electronic Mail (E-Mail)
Architecture and Services-
E-mail systems consist of two subsystems. They are:-
1. User Agents, which allow people to read and send e-mail
2. Message Transfer Agents, which move messages from source to destination
E-mail systems support 5 basic functions: -
1. Composition: It refers to the process of creating messages and answers. Any text editor
is used for body of the message. While the system itself can provide assistance with
addressing and numerous header fields attached to each message.
2. Reporting: It has to do with telling the originator what happened to the message that
is, whether it was delivered, rejected (or) lost.
3. Transfer: It refers to moving messages from originator to the recipient.
4. Displaying: Incoming messages are to be displayed so that people can read their email.
5. Disposition: It concerns what the recipient dose with the message after receiving it.
Possibilities include throwing it away before reading (or) after reading, saving it and so
on.
Most systems allow users to create mailboxes to store incoming e-mail. Commands are needed
to create and destroy mailboxes, inspect the contents of mailboxes, insert and delete messages
from mailboxes, and so on.

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Envelopes and messages. (a) Paper mail. (b) Electronic mail.
(1) The User Agent-
A user agent is normally a program (sometimes called a mail reader) that accepts a variety of
commands for composing, receiving, and replying to messages, as well as for manipulating
mailboxes.
Sending Mail-
To send an e-mail message, a user must provide the message, the destination address, and
possibly some other parameters. The message can be produced with a free-standing text editor,
a word processing program, or possibly with a specialized text editor built into the user agent.
The destination address must be in a format that the user agent can deal with. Many user agents
expect addresses of the form user@dns-address [4].
Reading Mail-
When a user agent is started up, it looks at the user's mailbox for incoming e-mail before
displaying anything on the screen. Then it may announce the number of messages in the
mailbox or display a one-line summary of each one and wait for a command.
(2) Message Format-
RFC 822
Messages consist of a primitive envelope (described in RFC 821), some number of header
fields, a blank line, and then the message body. Each header field (logically) consists of a single
line of ASCII text containing the field name, a colon, and, for most fields, a value.

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RFC 822 header fields related to message transport
MIME — The Multipurpose Internet Mail Extensions
RFC 822 specified the headers but left the content entirely up to the users. Nowadays, on the
worldwide Internet, this approach is no longer adequate. The problems include sending and
receiving
1. Messages in languages with accents (e.g., French and German).
2. Messages in non-Latin alphabets (e.g., Hebrew and Russian).
3. Messages in languages without alphabets (e.g., Chinese and Japanese).
4. Messages not containing text at all (e.g., audio or images).
A solution was proposed in RFC 1341 called MIME (Multipurpose Internet Mail Extensions)
The basic idea of MIME is to continue to use the RFC 822 format, but to add structure to the
message body and define encoding rules for non-ASCII messages. By not deviating from RFC
822, MIME messages can be sent using the existing mail programs and protocols. All that must
be changed are the sending and receiving programs, which users can do for themselves [3].
RFC 822 headers added by MIME
Message Transfer-
The message transfer system is concerned with relaying messages from the originator to the
recipient. The simplest way to do this is to establish a transport connection from the source
machine to the destination machine and then just transfer the message.
SMTP—The Simple Mail Transfer Protocol
SMTP is a simple ASCII protocol. After establishing the TCP connection to port 25, the
sending machine, operating as the client, waits for the receiving machine, operating as the
server, to talk first. The server starts by sending a line of text giving its identity and telling
whether it is prepared to receive mail. If it is not, the client releases the connection and tries
again later.
Even though the SMTP protocol is completely well defined, a few problems can still arise.

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• One problem relates to message length. Some older implementations cannot handle
messages exceeding 64 KB.
• Another problem relates to timeouts. If the client and server have different timeouts,
one of them may give up while the other is still busy, unexpectedly terminating the
connection.
• Finally, in rare situations, infinite maelstroms can be triggered.
For example, if host 1 holds mailing list A and host 2 holds mailing list B and each list contains
an entry for the other one, then a message sent to either list could generate a never-ending
amount of e-mail traffic unless somebody checks for it.
Final Delivery-
With the advent of people who access the Internet by calling their ISP over a modem, it breaks
down. One solution is to have a message transfer agent on an ISP machine accept e-mail for its
customers and store it in their mailboxes on an ISP machine. Since this agent can be on-line all
the time, e-mail can be sent to it 24 hours a day[3].
POP3-
(a) Sending and reading mail when the receiver has a permanent Internet connection
and the user agent
runs on the same machine as the message transfer agent.
(b) Reading e-mail when the receiver has a dial-up
connection to an ISP
POP3 begins when the user starts the mail reader. The mail reader calls up the ISP (unless
there is already a connection) and establishes a TCP connection with the message transfer
agent at port 110. Once the connection has been established, the POP3 protocol goes through
three states in sequence:
1. Authorization.
2. Transactions.
3. Update.
• The authorization state deals with having the user log in.

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• The transaction state deals with the user collecting the e-mails and marking them for
deletion from the mailbox.
• The update state causes the e-mails to be deleted.
IMAP (Internet Message Access Protocol)-
POP3 normally downloads all stored messages at each contact, the result is that the user's e-
mail quickly gets spread over multiple machines, more or less at random; some of them not
even the user's.
This disadvantage gave rise to an alternative final delivery protocol, IMAP (Internet Message
Access Protocol).
IMAP assumes that all the e-mail will remain on the server indefinitely in multiple mailboxes.
IMAP provides extensive mechanisms for reading messages or even parts of messages, a
feature useful when using a slow modem to read the text part of a multipart message with large
audio and video attachments.
File Transfer Protocol
File Transfer Protocol (FTP) is an application layer protocol which moves files between local
and remote file systems. It runs on the top of TCP, like HTTP. To transfer a file, 2 TCP
connections are used by FTP in parallel: control connection and data connection.
Control Connection-For sending control information like user identification, password,
commands to change the remote directory, commands to retrieve and store files etc., FTP
makes use of control connection. Control connection is initiated on port number 21.
Data Connection-For sending the actual file, FTP makes use of data connection. Data
connection is initiated on port number 20.
FTP sends the control information out-of-band as it uses a separate control connection. Some
protocols send their request and response header lines and the data in the same TCP connection.
For this reason, they are said to send their control information in-band. HTTP and SMTP are
such examples.
FTP Session-
When a FTP session is started between a client and a server, the client initiates a control TCP
connection with the server side. The client sends the control information over this. When the
server receives this, it initiates a data connection to the client side. Only one file can be sent
over one data connection. But the control connection remains active throughout the user
session. As we know HTTP is stateless i.e. it does not have to keep track of any user state. But
FTP needs to maintain a state about its user throughout the session.
FTP sessions work in passive or active modes. In active mode, after a client initiates a session
via a command channel request, the server initiates a data connection back to the client and
begins transferring data. In passive mode, the server instead uses the command channel to send
the client the information it needs to open a data channel. Because passive mode has the client
initiating all connections, it works well across firewalls and Network Address Translation
(NAT) gateways.

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Data Structures : FTP allows three types of data structures :
1. File Structure – In file-structure there is no internal structure and the file is
considered to be a continuous sequence of data bytes.
2. Record Structure – In record-structure the file is made up of sequential records.
3. Page Structure – In page-structure the file is made up of independent indexed
pages.
FTP Commands – Some of the FTP commands are :
• USER – This command sends the user identification to the server.
• PASS – This command sends the user password to the server.
• CWD – This command allows the user to work with a different directory or dataset for
file storage or retrieval without altering his login or accounting information.
• RMD – This command causes the directory specified in the path-name to be removed
as a directory.
• MKD – This command causes the directory specified in the path name to be created as
a directory.
• PWD – This command causes the name of the current working directory to be
returned in the reply.
• RETR – This command causes the remote host to initiate a data connection and to
send the requested file over the data connection.
• STOR – This command causes to store a file into the current directory of the remote
host.
• LIST – Sends a request to display the list of all the files present in the directory.
• ABOR – This command tells the server to abort the previous FTP service command
and any associated transfer of data.
• QUIT – This command terminates a USER and if file transfer is not in progress, the
server closes the control connection.
FTP Replies – Some of the FTP replies are:
• 200 Command okay.
• 530 Not logged in.
• 331 User name okay, need password.

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• 225 Data connection open; no transfer in progress.
• 221 Service closing control connection.
• 551 Requested action aborted: page type unknown.
• 502 Command not implemented.
• 503 Bad sequence of commands.
• 504 Command not implemented for that parameter.
Anonymous FTP-
Anonymous FTP is enabled on some sites whose files are available for public access. A user
can access these files without having any username or password. Instead, username is set to
anonymous and password to guest by default. Here, the user access is very limited. For
example, the user can be allowed to copy the files but not to navigate through directories.
Network Security
• The requirements of information security within an organization have undergone two
major changes in the last decades.
• The generic name for the collection of tools designed to protect data and to prevent
hackers is computer security.
• Another important thing is that affected security is the introduction of distributed
systems and the use of networks and communication facilities for carrying data between
terminal user and computer and between computer and compiler.
• Network security measures are needed to protect data during their transmission and to
guarantee that data transmissions are authentic [1].
Security Attacks-Any action that compromises the security of information owned by an
organization. Two types of security attacks are
1. Passive Attack-Just listens the message.
2. Active Attack-Alter message.
Security Mechanisms-A process that designed to detect, prevent, recover from security attack.
Security Service-Use of one or more security mechanisms to provide service.
Important Features of Security-
1. Confidentiality-It is the protection of transmitted data from passive attacks.
2. Authentication-The recipient that the message is from source that it claims to be from.
3. Non-repudiation -It prevent either sender or receiver.
• When a message is sent, the receiver can prove that the alleged sender in fact sent the
message similarly
• when a message is received, the sender can prove that alleged receiver in fact received
the message.
Security requirements and attacks-
Computer and network security address three requirements: -

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Secrecy-Secrecy requires that the information in a computer system only be accessible for
reading by authorized parties. This type of access includes printing, displaying and other forms
of disclosure, including simply revealing the existence of an object [1].
Integrity-Integrity requires that computer system can be modified only by authorized parties.
Modification includes writing, changing status, deleting and creating.
Availability-
• Availability requires that computer systems are available to authorized parties.
• The types of attacks on the security of a computer system or network can be viewed by
the function of the computer system.
In general, there is a flow of information from a source to destination. This is called normal
flow-
The following are the four categories of attack: -
• Interruption- An asset of the system is destroyed, or it becomes unavailable or
unusable. This is an attack on availability. Examples: destruction of a piece of hardware
such as hard disk, the cutting of a communication line, or the disabling of the file
management system.
• Interception- An authorized party gains access to an asset. This is an attack on
confidentiality. The authorized party could be a person or program or a computer. E.g.:
wiretapping to capture data in a network and the illicit copying of files or programs.
• Modification- An authorized party gains access to an asset. This is an attack on
confidentiality. This is an attack on integrity. E.g: changing the values in a data file,
altering a program so that it performs differently and modifying the content of messages
being transmitted in a network.
• Fabrication- An authorized party inserts counterfeit objects into the system. This is an
attack on authenticity. Examples include the insertion of spurious messages in a
network or the addition of records to a file.
Passive Attacks- Passive attacks means the eavesdropping on or monitoring of, transmissions.
The goal of the component is to obtain information that is being transmitted. Two types of
attacks are involved here.
• Release of message contents-In this, a telephone conversation, an e-mail message, a
transferred file may contain sensitive or confidential information. This helps to prevent
the opponent from learning the content of these transmissions.
• Traffic analysis- In this encryption is used for masking the contents which helps to
observe the pattern of the messages. The opponent could determine the location and

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identity of communicating hosts and could observe the frequency and length of
messages being exchanged.
Passive attacks are difficult to detect because they do not involve any alteration of data.
Active attacks- In this, the attacks involve some modification of the data stream or creation of
false stream. This is divided into four categories.
1. Masquerade-A masquerade takes place when one entity pretends to be a different
entity. A masquerade attack usually includes one of the other forms of active attacks.
2. Replay-Replay involves the passive capture of a data unit and its subsequent
retransmission to produce an unauthorized effect.
3. Modification of Message-This means that some portion of a legitimate message is
altered or that messages are delayed or reordered to produce an unauthorized effect.
4. Denial of service- The denial of service prevents or inhibits the normal size or
management of communication facilities.
Active attacks present the opposite characteristics of passive attacks. It is quite difficult to
prevent active attacks absolutely and would always require physical protections of all
communications facilities and paths.
P2P File Sharing
In Computer Networking, P2P is a file sharing technology, allowing the users to access mainly
the multimedia files like videos, music, e-books, games etc. The individual users in this
network are referred to as peers. The peers request for the files from other peers by establishing
TCP or UDP connections.
Working-
A peer-to-peer network allows computer hardware and software to communicate without the
need for a server. Unlike client-server architecture, there is no central server for processing
requests in a P2P architecture. The peers directly interact with one another without the
requirement of a central server.
Now, when one peer makes a request, it is possible that multiple peers have the copy of that
requested object. Now the problem is how to get the IP addresses of all those peers. This is
decided by the underlying architecture supported by the P2P systems. By means of one of these
methods, the client peer can get to know about all the peers which have the requested object/file
and the file transfer takes place directly between these two peers.
1. Centralized Directory-
• It is somewhat like client server architecture in the sense that it maintains a huge central
server to provide directory service.
• All the peers inform this central server of their IP address and the files they are making
available for sharing.

19 | P a g e
• The server queries the peers at regular intervals to make sure if the peers are still
connected or not.
• So basically, this server maintains a huge database regarding which file is present at
which IP addresses.
Working-
• Now whenever a requesting peer comes in, it sends its query to the server.
• Since the server has all the information of its peers, so it returns the IP addresses of all
the peers having the requested file to the peer.
• Now the file transfer takes place between these two peers.
The first system which made use of this method was Napster, for the purpose of Mp3
distribution.
P2P paradigm with centralized directory
The major problem with such an architecture is that there is a single point of failure. If the
server crashes, the whole P2P network crashes. Also, since all the processing is to be done by
a single server, so a huge amount of database has to be maintained and regularly updated [2].
2. Query Flooding-
• Unlike the centralized approach, this method makes use of distributed systems.
• In this, the peers are supposed to be connected into an overlay network. It means if a
connection/path exists from one peer to other, it is a part of this overlay network.
• In this overlay network, peers are called as nodes and the connection between peers is
called an edge between the nodes, thus resulting in a graph-like structure.
Working
• Now when one peer requests for some file, this request is sent to all its neighboring nodes
i.e. to all nodes which are connected to this node. If those nodes don’t have the required
file, they pass on the query to their neighbors and so on. This is called as query flooding.
• When the peer with requested file is found (referred to as query hit), the query flooding
stops and it sends back the file name and file size to the client, thus following the reverse
path.

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• If there are multiple query hits, the client selects from one of these peers.
Gnutella was the first decentralized peer to peer network
This method also has some disadvantages like, the query must be sent to all the neighbouring
peers unless a match is found. This increases traffic in the network.
3. Exploiting Heterogeneity-
• This P2P architecture makes use of both the above discussed systems.
• It resembles a distributed system like Gnutella because there is no central server for
query processing.
• But unlike Gnutella, it does not treat all its peers equally. The peers with higher
bandwidth and network connectivity are at a higher priority and are called as group
leaders/super nodes. The rest of the peers are assigned to these super nodes.
• These super nodes are interconnected and the peers under these super nodes inform
their respective leaders about their connectivity, IP address and the files available for
sharing.
KaZaA technology is such an example which makes use of Napster and Gnutella both.
Thus, the individual group leaders along with their child peers form a Napster-like
structure. These group leaders then interconnect among themselves to resemble a Gnutella-
like structure.
Working-
• This structure can process the queries in two ways.
• The first one is that the super nodes could contact other super nodes and merge their
databases with its own database. Thus, this super node now has information of many
peers.
• Another approach is that when a query comes in, it is forwarded to the neighboring super
nodes until a match is found, just like in Gnutella. Thus, query flooding exists but with
limited scope as each super node has many child peers. Hence, such a system exploits the
heterogeneity of the peers by designating some of them as group leaders/super nodes and
others as their child peers [2].

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References-
(1) Computer-Networks by Andrew S. Tanenbaum 5th Edition
(2) Computer Networking by Kurose and Ross 6th Edition
(3) Data Communication and Computer Networks by Sanjay Sharma 1st Edition
(4) Data Communication by Forouzan 4th Edition

Unit 5 application layer

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Unit 5 application layer