The document discusses the Domain Name System (DNS) which translates domain names to IP addresses, allowing browsers to load internet resources. It describes DNS name space and provides examples of domain names. It also discusses DNS resource records which contain information about a domain, including the domain name, time to live, class, type and value. The principal DNS record types for IPv4 are described, including A, MX, and NS records. The document also discusses DNS name servers which store DNS records and avoid problems with a single information source by dividing the DNS name space into zones.

1. The Application Layer
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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.
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3. The DNS Name Space
• A portion of the Internet domain name space
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4. Reference
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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)
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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
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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.
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8. The Principal DNS Resource Record Types
For Internet Protocol version 4 (IPv4)
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IPv4 an address consists of 32 bits

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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
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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)
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14. Some smileys
• Many people also use little ASCII symbols
called smileys or emoticons in their e-mail
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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
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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
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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
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18. The User Agent
• Sending E-mail, Reading E-mail
• An example display of the contents of a
mailbox
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19. Message Formats
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RFC 822 header fields related to message transport

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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)
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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)
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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
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26. • Network security problems can be divided
roughly into four areas:
• secrecy, authentication, nonrepudiation, and
integrity control
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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
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28. Introduction to Cryptography
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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
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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
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31. Transposition Ciphers
• Transposition ciphers, in contrast, reorder the
letters but do not disguise them
• In this example, MEGABUCK is the key
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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
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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
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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
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