computer network

1. The Application Layer
10/29/2023 Dr.T.Thendral - CN

2. DNS—The Domain Name System
• IP addresses: These addresses are hard for
people to remember
• To solve these problems, DNS (the Domain
Name System) was invented.
• DNS is the phonebook of the Internet
• DNS translates domain names to IP addresses
so browsers can load Internet resources.
10/29/2023 Dr.T.Thendral - CN

3. The DNS Name Space
• A portion of the Internet domain name space
10/29/2023 Dr.T.Thendral - CN

4. Reference
10/29/2023 Dr.T.Thendral - CN

5. EXAMPLES
• 1. cs.yale.edu (Yale University, in the United
States)
• 2. cs.vu.nl (Vrije Universiteit, in The
Netherlands)
• 3. cs.keio.ac.jp (Keio University, in Japan)
10/29/2023 Dr.T.Thendral - CN

6. Resource Records
• Every domain, whether it is a single host or a
top-level domain, can have a set of resource
records associated with it
• For a single host, the most common resource
record is just its IP address
• A resource record is a five-tuple
• The format we will use is as follows:
Domain_name Time_to_live Class Type Value
10/29/2023 Dr.T.Thendral - CN

7. • The Domain_name tells the domain to which
this record applies
• The Time_to_live field gives an indication of
how stable the record is
• The third field of every resource record is the
Class
• For Internet information, it is always IN.
10/29/2023 Dr.T.Thendral - CN

8. The Principal DNS Resource Record Types
For Internet Protocol version 4 (IPv4)
10/29/2023 Dr.T.Thendral - CN
IPv4 an address consists of 32 bits

9. 10/29/2023 Dr.T.Thendral - CN

10. Name Servers
• DNS name server is a server that stores the
DNS records
• To avoid the problems associated with having
only a single source of information, the DNS
name space is divided into nonoverlapping
zones
10/29/2023 Dr.T.Thendral - CN

11. 10/29/2023 Dr.T.Thendral - CN

12. 10/29/2023 Dr.T.Thendral - CN

13. Electronic Mail
• Electronic mail, or e-mail, as it is known to its many
fans, has been around for over two decades
• Before 1990, it was mostly used in academia
• E-mail, like most other forms of communication, has
its own conventions and styles
• In particular, it is very informal and has a low
threshold of use
• E-mail is full of jargon such as BTW (By The Way),
ROTFL (Rolling On The Floor Laughing), and IMHO (In
My Humble Opinion)
10/29/2023 Dr.T.Thendral - CN

14. Some smileys
• Many people also use little ASCII symbols
called smileys or emoticons in their e-mail
10/29/2023 Dr.T.Thendral - CN

15. Some of the complaints were as follows:
1. Sending a message to a group of people was inconvenient. Managers often
need this facility to send memos to all their subordinates.
2. Messages had no internal structure, making computer processing difficult
For example, if a forwarded message was included in the body of another
message, extracting the forwarded part from the received message was
difficult.
3. The originator (sender) never knew if a message arrived or not.
4. If someone was planning to be away on business for several weeks and
wanted all incoming e-mail to be handled by his secretary, this was not easy to
arrange.
5. The user interface was poorly integrated with the transmission system
requiring users first to edit a file, then leave the editor and invoke the file
transfer program.
6. It was not possible to create and send messages containing a mixture of
text, drawings, facsimile, and voice
10/29/2023 Dr.T.Thendral - CN

16. Architecture and Services
• They normally consist of two subsystems:
• The user agents, which allow people to read
and send e-mail, and the message transfer
agents, which move the messages from the
source to the destination
• Typically, e-mail systems support five basic
functions
• Composition, Transfer, Reporting, Displaying,
Disposition
10/29/2023 Dr.T.Thendral - CN

17. • 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.
• This gives rise to the idea of a mailing list,
which is a list of e-mail addresses
• A key idea in e-mail systems is the distinction
between the envelope and its contents
• The message inside the envelope consists of
two parts: the header and the body
10/29/2023 Dr.T.Thendral - CN

18. The User Agent
• Sending E-mail, Reading E-mail
• An example display of the contents of a
mailbox
10/29/2023 Dr.T.Thendral - CN

19. Message Formats
10/29/2023 Dr.T.Thendral - CN
RFC 822 header fields related to message transport

20. 10/29/2023 Dr.T.Thendral - CN

21. MIME—The Multipurpose Internet Mail
Extensions
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)
10/29/2023 Dr.T.Thendral - CN

22. 10/29/2023 Dr.T.Thendral - CN

23. 10/29/2023 Dr.T.Thendral - CN

24. Message Transfer
• SMTP—The Simple Mail Transfer Protocol
• (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
(internet service provider)
10/29/2023 Dr.T.Thendral - CN

25. Network Security
• Most security problems are intentionally
caused by malicious people trying to gain
some benefit, get attention, or to harm
someone
• Some people who cause security problems
and why
10/29/2023 Dr.T.Thendral - CN

26. • Network security problems can be divided
roughly into four areas:
• secrecy, authentication, nonrepudiation, and
integrity control
10/29/2023 Dr.T.Thendral - CN

27. Cryptography
• Cryptography comes from the Greek words for
''secret writing.'‘
• Professionals make a distinction between
ciphers and codes
• A cipher is a character-for-character or bit-
for-bit transformation, without regard to the
linguistic structure of the message
• In contrast, a code replaces one word with
another word or symbol
10/29/2023 Dr.T.Thendral - CN

28. Introduction to Cryptography
10/29/2023 Dr.T.Thendral - CN

29. Substitution Ciphers
• In a substitution cipher each letter or group of
letters is replaced by another letter or group
of letters to disguise it
• One of the oldest known ciphers is the Caesar
cipher, attributed to Julius Caesar.
• In this method, a becomes D, b becomes E, c
becomes F, ... , and z becomes C
10/29/2023 Dr.T.Thendral - CN

30. • The next improvement is to have each of the symbols
in the plaintext, say, the 26 letters for simplicity, map
onto some other letter.
• For example,
• plaintext: a b c d e f g h i j k l m n o p q r s t u v w x y z
• ciphertext: Q W E R T Y U I O P A S D F G H J K L Z X C
V B N M
• The general system of symbol-for-symbol
substitution is called a monoalphabetic substitution
• The plaintext attack would be transformed into the
ciphertext QZZQEA
10/29/2023 Dr.T.Thendral - CN

31. Transposition Ciphers
• Transposition ciphers, in contrast, reorder the
letters but do not disguise them
• In this example, MEGABUCK is the key
10/29/2023 Dr.T.Thendral - CN

32. One-Time Pads
• First choose a random bit string as the key
• Then convert the plaintext into a bit string, for
example by using its ASCII representation
• With a one-time pad, the encryption
algorithm is simply the XOR operation
• The resulting ciphertext cannot be broken,
This method, known as the one-time pad
10/29/2023 Dr.T.Thendral - CN

33. 10/29/2023 Dr.T.Thendral - CN

34. Two Fundamental Cryptographic
Principles
• Redundancy - The first principle is that all
encrypted messages must contain some
redundancy, that is, information not needed
to understand the message
• Cryptographic principle 1: Messages must
contain some redundancy
• EX: CRC, Hamming or Reed Solomon code is a
more efficient way to do error detection and
correction
10/29/2023 Dr.T.Thendral - CN

35. Freshness
• Cryptographic principle 2: Some method is
needed to foil replay attacks
• The second cryptographic principle is that
some measures must be taken to ensure that
each message received can be verified as
being fresh, that is, sent very recently
10/29/2023 Dr.T.Thendral - CN