1. Introduction to the Internet

1. Computer Networks

2. The Internet

3. Part of the Internet

4. Packet Switching

5. History of the Internet

6. Growth of the Internet

7. Communication Protocols

8. Protocols and Layering

9. TCP/IP 5-layer Reference Model

10. TCP/IP layers with some protocols

11. Data Passing Through Layers

12. Headers and Layers

13. Internet Communication Paradigms

14. Connection-Oriented Communication

15. Client-Server Model

16. Client Software

17. Server Software

18. Server Identification

19. Service Identification

20. Client-Server Interaction

21. A specific example

1.1. Computer Networks

computer networks are everywhere

they form an essential part of our infrastructure

used

o at home

o at work

o by governments

o on the move

o ...

there are many different types of networks and standards

we will concentrate on the Internet

1.2. The Internet

(the above is a very old figure showing an internet)

each ellipse represents a network connecting a number of computers

directly

an internet is a federation of computer networks, connected by routers

the Internet is the world-wide federation of packet-switched networks

running TCP/IP

important applications include email and the World Wide Web (WWW)

1.3. Part of the Internet

(the above figure is taken from the book by Kurose and Ross)

1.4. Packet Switching

early communication networks evolved from telephone systems

used physical pair of wires between two parties to form a

dedicated circuit

circuit switching was the task of deciding which circuit to use when two

parties wanted to communicate

the circuit is reserved for the two parties during communication

so it is not available to other parties

the Internet uses packet switching which is considered more efficient

packet switching

o divides data into small blocks, called packets

o allows multiple users to share a network

o includes identification of the intended recipient in each packet

o devices throughout the network each have information about how

to reach each possible destination

1.5. History of the Internet

(1957) Advanced Research Projects Agency (ARPA) established by US

Department of Defense

(1968-9) first packet-switching networks

(1972) Telnet

(1973) File Transfer Protocol (FTP); ARPANET goes international:

o University College, London (UK)

o Royal Radar Establishment (Norway)

(1974) design of TCP (Transmission Control Protocol)

(1977) email

(1982) TCP and IP (Internet Protocol) used for ARPANET

(1984) DNS (Domain Name Service) introduced

(1991) WWW released

1.6. Growth of the Internet

1.7. Communication Protocols

communication always involves at least two entities

o one that sends information and another that receives it

all entities in a network must agree on how information will be

represented and communicated

o the way that electrical signals are used to represent data

o procedures used to initiate and conduct communication

o the format of messages

all communicating parties follow the same set of rules, a set

of specifications

a specification for network communication is called a

communication protocol

1.8. Protocols and Layering

computer networks are complex systems including both hardware and

software

rather than a single, huge specification for all possible forms of

communication, designers divide the communication problem into

subparts, called layers

the interfaces between the layers are defined by protocols

layers provide for modularity, making implementation and changes

easier

the combination of layers is sometimes called a protocol stack

1.9. TCP/IP 5-layer Reference Model

physical layer corresponds to the basic network hardware

network interface, or link, layer specifies how data is divided into

packets

Internet layer specifies how packets are forwarded to

particular machines over the Internet, using the Internet Protocol (IP)

transport layer specifies how to communicate with

particular processes on machines, using the Transmission Control

Protocol (TCP) or User Datagram Protocol (UDP)

application layer specifies how applications use the Internet, and

includes protocols such as the HyperText Transfer Protocol (HTTP) and

the Domain Name System (DNS)

1.10. TCP/IP layers with some protocols

1.11. Data Passing Through Layers

1.12. Headers and Layers

1.13. Internet Communication Paradigms

Internet supports two basic communication paradigms:

o stream paradigm

o message paradigm

stream paradigm message paradigm

connection-oriented connectionless

one-to-one communication many-to-many communication

sequence of individual bytes sequence of individual messages

arbitrary length transfer each message limited to 64 Kbytes

used by most applications used for multimedia applications

built on TCP protocol built on UDP protocol

we will focus on the stream paradigm

1.14. Connection-Oriented Communication

Internet stream service is connection-oriented

two applications must request that a connection be created

once it has been established, the connection allows the applications to

send data in either direction

finally, when they finish communicating, the applications request that

the connection be terminated

1.15. Client-Server Model

server application client application

starts first starts second

does not need to know which client will

contact it must know which server to contact

waits passively and arbitrarily long for

contact from a client

initiates contact whenever communication is

needed

communicates with a client by both sending

and receiving data

communicates with a server by both sending

and receiving data

stays running after servicing one client, and

waits for another may terminate after interacting with a server

1.16. Client Software

is an arbitrary application program that becomes a client temporarily

when remote access is needed, but also performs other computation

is invoked directly by a user, and executes only for one session

runs locally on a user's personal computer

actively initiates contact with a server

can access multiple services as needed, but usually contacts one remote

server at a time

does not require especially powerful computer hardware

1.17. Server Software

is a special-purpose, privileged program

is dedicated to providing one service that can handle multiple remote

clients at the same time

is invoked automatically when a system boots, and continues to execute

through many sessions

runs on a large, powerful computer

waits passively for contact from arbitrary remote clients

accepts contact from arbitrary clients, but offers a single service

requires powerful hardware and a sophisticated operating system (OS)

1.18. Server Identification

Internet protocols divide identification into two pieces:

o an identifier for the computer on which a server runs

o an identifier for a service on the computer

identifying a computer

o each computer on the Internet is assigned a unique 32-

bit identifier known as an Internet Protocol address (IP address)

o 4 bytes written as n1.n2.n3.n4 where each ni is a decimal number,

e.g., 18.23.0.22

o a client must specify the server's IP address

o to make server identification easy for humans, each computer is

also assigned a name, and the Domain Name System (DNS) is

used to translate a name into an address

o thus, a user specifies a name such as www.dcs.bbk.ac.uk rather

than an integer address

1.19. Service Identification

each service available in the Internet is assigned a unique 16-

bit identifier known as a protocol port number (or port number)

o examples: email -> port number 25, and the web -> port number

80

when a server begins execution

o it registers with its local OS by specifying the port number for its

service

when a client contacts a remote server to request service

o the request contains a port number

when a request arrives at a server computer

o software on the server uses the port number in the request to

determine which application on the server computer should handle

the request

1.20. Client-Server Interaction

following diagram illustrates client-server interaction

o client on the left

o server on the right

1.21. A specific example

suppose I want to retrieve a web page from www.w3.org

my browser will use DNS to find the IP address 128.30.52.37

my browser will compose a message based on HTTP asking to get the

page

HTTP will ask TCP to connect to port 80 on 128.30.52.37

TCP will ask IP to send the message to 128.30.52.37

IP will send the message to a router on the local network

this router will send the message to another router

...

the router on the local network for 128.30.52.37 will receive the

message

it will send the message to 128.30.52.37

IP will receive it and pass it up to TCP

TCP will see that it is for port 80 and will pass it to the web server

process

the web server will interpret the HTTP and send the page to my browser

2. Internet Applications

1. Representation and Transfer

2. Web Protocols

3. Some Other Application Layer Protocols

4. Uniform Resource Identifiers (URIs)

5. Uniform Resource Locators (URLs)

6. URL schemes

7. Escaping Special URI characters

8. Domain Name System (DNS)

9. DNS Design

10. Top-Level Domains

11. DNS Server Hierarchy

12. DNS Server Model

13. Name Resolution

14. URIs, URNs and URLs

15. Uniform Resource Names (URNs)

16. Internet Electronic mail

17. Sending e-mail

18. Example SMTP Session

19. Email Representation Standards

20. Multi-purpose Internet Mail Extensions (MIME)

21. MIME Headers

22. Base64 Encoding

2.1. Representation and Transfer

application-layer protocols specify two aspects of interaction

o representation

o transfer

representation:

o syntax of data items exchanged

o specific form during transfer

o translation of integers, characters and files between computers

transfer:

o interaction between client and server

o message syntax and semantics

o valid and invalid exchange

o error handling

o termination of interaction

2.2. Web Protocols

World Wide Web (WWW) is one of the most widely used services on

the Internet

major WWW standards are

o HyperText Markup Language (HTML): representation standard

specifying contents and layout of a web page

o Uniform Resource Identifier (URI): representation standard

specifying format and meaning of web page identifiers

o HyperText Transfer Protocol (HTTP): transfer protocol specifying

how a browser interacts with a web server

2.3. Some Other Application Layer Protocols

telnet (for remote login)

o defined in RFC 318 (1972)

ftp (file transfer protocol)

o defined in RFC 454 (1973)

email protocols

o SMTP (Simple Mail Transfer Protocol)

o POP3 (Post Office Protocol version 3)

o IMAP4 (Internet Mail Access Protocol)

DNS (Domain Name System)

o defined in RFC 1034 and RFC 1035 (1987)

RTP (Real-time Transfer Protocol) for audio and video

o defined in RFC 3550 (2003)

2.4. Uniform Resource Identifiers (URIs)

a Uniform Resource Identifier (URI) is a unique identifier for identifying

a resource on the internet

basic syntax is:

scheme ":" scheme-specific-part

where

o scheme identifies a naming scheme, e.g., http

o scheme-specific-part identifies resource in some way specific to

the scheme

o most commonly used URIs are Uniform Resource

Locators (URLs)

2.5. Uniform Resource Locators (URLs)

scheme examples include

o ftp, http, https, mailto, telnet

in the following syntax [ ... ] denotes optional

everything else not in quotes denotes a string to be supplied

scheme specific part has syntax "//" [ user [ ":" password ] "@" ] host [ ":" port ] [ "/" url-

path ] [ "?" query-string ] [ "#" anchor ]

where

o user and password are not often used

o host is a fully qualified domain name or IP address

o port is optional (usually a default)

o url-path is the path to the resource, specific to scheme

o query-string includes parameters associated with the request

(usually form fields)

o anchor is a reference to a part of a resource (a fragment identifier)

e.g. in http://vili.dcs.bbk.ac.uk/dept/staffperson05.asp?name=ptw

o http is the scheme

o vili.dcs.bbk.ac.uk is the host

o dept/staffperson05.asp is the url-path

o name=ptw is the query string

2.6. URL schemes

http o user name and password not applicable

o default port number is 80 https

o HTTP over Secure Sockets Layer (SSL)

o default port number is 443 ftp

o user name and password can be given

o if not, anonymous ftp used

o default port number is 21 telnet

o host is mandatory

o default port number is 23 mailto

o no need for url-path to be specified

o program should prompt user for message, then send using SMTP

2.7. Escaping Special URI characters

the space character is not allowed in URIs

/, #, ? have special meanings in URIs

also + is used to separate parameters in a query string

so if we need any of these as an ordinary character in a URI, we use

the escaped version

the escaped version is the character % followed by the ASCII

hexadecimal value of the character

now % has a special meaning too

the escaped versions are as follows:

symbol escaped version

% %25

/ %2F

# %23

? %3F

space %20

+ %2B

2.8. Domain Name System (DNS)

provides a service mapping (human-readable) DNS names to IP

addresses

browsers, mail software and most other Internet applications use DNS

two advantages:

o easier to remember www.w3.org than 128.30.52.37

o higher level of abstraction allows simpler reorganisation

names are organised hierarchically:

o most significant part of the name on the right (specified by DNS)

o left-most segment of a name is the name of an individual

computer

DNS is essentially

o a distributed database implemented as a hierarchy of DNS servers

o an application-layer protocol allowings hosts to query the

database

2.9. DNS Design

why is DNS distributed?

a simpler design would have been to have one DNS server storing all the

mappings

problems with this centralised design include:

o it is a single point of failure

o the need to handle huge volumes of queries

o a single server cannot be "close" to all clients

o it would also have to handle all updates for new hosts

2.10. Top-Level Domains

right-most domains of the hierarchy are top-level domains:

o either country-code top-level domain (ccTLD)

o or generic top-level domain (gTLD)

ccTLD represented by two-letter country-codes from ISO 3166,

e.g., uk, fr, de, ch

gTLD given in Internet informational RFC 1591:

o edu: educational institutions

o com: commercial entities, i.e., companies

o net: network providers

o org: organisations, e.g. NGOs

o gov: government agencies

o mil: US military

o int: organisations established by international treaties

2.11. DNS Server Hierarchy

the following shows a portion of the hierarchy of DNS servers

there are 13 root DNS servers (each is actually a cluster of replicated

servers)

o these return IP addresses of top-level domain servers

top-level domain servers are responsible for top-level domains

o they return IP addresses of authoritative servers for organisations

each organisation must provide an authoritative DNS server for its

publically accessible hosts

2.12. DNS Server Model

each organization is free to choose how to organise its servers

o a small organisation might use an ISP to run a DNS server

o a larger organisation might place all names on a single server

o a large organisation might divide its names among several servers

DNS allows each organization to assign names to computers or

to change those names without informing a central authority

each DNS server contains information linking it to other DNS servers up

and down the hierarchy

a given server can be replicated

replication is useful for heavily used servers, such as root servers that

provide information about top-level domains

DNS servers employ caching in order to improve performance and

reduce load

2.13. Name Resolution

translation of a domain name into an address is called name resolution

the name is said to be resolved to an address

software to perform the translation is known as a name resolver (or

simply resolver)

DNS server is used by a browser to map DNS name to IP address:

2.14. URIs, URNs and URLs

a Uniform Resource Identifier (URI) is either

o a Uniform Resource Name (URN), or

o a Uniform Resource Locator (URL)

URN names a resource, while URL gives its address

URN vs URL analogous to DNS name vs IP address

2.15. Uniform Resource Names (URNs)

overcome disadvantages of using URLs, namely:

o dependence on host names

o dependence on file structure on host

o ease with which URL can be invalidated

no syntactic difference between URN and URL

URNs not yet supported by browsers

syntax for URNs:

"urn:" <NID> ":" <NSS>

where

o scheme is urn

o scheme specific part is <NID> ":" <NSS>

o <NID> is Namespace IDentifier, e.g., isbn

o <NSS> is Namespace Specific String

2.16. Internet Electronic mail

e-mail client responsible for

o retrieving mail from server (POP3, IMAP4)

o sending mail to server (SMTP)

e-mail server responsible for

o collecting mail from client (SMTP)

o distributing mail to client (POP3, IMAP4)

o relaying mail between e-mail servers (SMTP)

2.17. Sending e-mail

SMTP (Simple Mail Transfer Protocol)

defined in RFC 821 and 822 (1982), superceded by RFC 2822 (2001)

use mailto: prefix in URI in browser

uses TCP port 25

address of recipient is of the form name@dept.inst.ac.uk

uses DNS (Domain Name System) to map domain name to IP address

2.18. Example SMTP Session

mail message is transferred from user John_Q_Smith on

computer example.edu to two users on computer somewhere.com

2.19. Email Representation Standards

two important standards exist

o RFC (Request For Comments) 2822 mail message format

o Multi-purpose Internet Mail Extensions (MIME)

RFC 2822 format comprises

o a header section

o a blank line

o and a body

header lines each have the form keyword: information

where keywords include From, To, Subject, Cc

the mail message (including headers) makes up the DATA as sent by

SMTP

2.20. Multi-purpose Internet Mail Extensions (MIME)

SMTP uses 7-bit ASCII format

inadequate for non-English and non-textual data

MIME defined in RFCs 2045, 2046, 2047, 2048, 2049; allows

o non-ASCII message bodies

o extensible set of different formats for non-textual bodies

o multi-part message bodies

o non-ASCII textual header information

2.21. MIME Headers

MIME headers include: o MIME-Version o Content-Type: specifies a type and subtype

o Content-Transfer-Encoding: specifies auxiliary encoding for

transfer

contents of the Content-Type header is the MIME type

examples of MIME types are text/html, image/gif and multipart/mixed

example of Content-Transfer-Encoding is base64:

o preferred encoding for 8-bit binary data

o each group of 3 bytes (24 bits) is encoded as 4 ASCII characters

2.22. Base64 Encoding

0x00 0x10 0x20 0x30

0 A Q g w

1 B R h x

2 C S i y

3 D T j z

4 E U k 0

5 F V l 1

6 G W m 2

7 H X n 3

8 I Y o 4

9 J Z p 5

A K a q 6

B L b r 7

C M c s 8

D N d t 9

E O e u +

F P f v /

values in top row and leftmost column are hexadecimal numbers

range of values is 0x00 to 0x3F (111111)

encode 01011010, 10001010, 00011101, e.g., by

1. splitting into 4 6-bit values: 010110, 101000, 101000, 011101

2. converting to hex: 0x16, 0x28, 0x28, 0x1D

3. use table to encode: W, o, o, d

QUIZ

1. What does WWW stand for?

World Wacky Web

Wide World Wumpus

World Wide Web

Wide World of Why?

2. Which one of the following is a search engine?

Netscape

Java

Altavista

Internet

3. What is the URL of the search engine in question 2?

http://www.yahoo.com

http://www.altavista.net

http://www.yahooligans.org

http ://www.altavista.com

4. What does URL stand for?

united route link

uniform resource locator

unknown redirection link

up real late

5. What is the name of the language you use to write a web page?

HTTP

FTP

URL

HTML

6. What do you do if you accidentally end up at an inappropriate web site?

Hit "Back" button, and continue surfing

Yell out "Hey look at this everyone!!"

Hit "Back" button immediately, and raise your hand to tell

the teacher or tell your parent

Leave it on your screen until the teacher finds out

7. What are the consequences if download files from the Internet to your computer?

You can play lot of games

Or listen to lot of music

Your computer will get full up of junk

You might download a virus and put your computer and

other at risk, always check if the download site is trustworthy

8. Which of the following terms is a "browser"?

Netscape

World Wide Web

Launcher

E-mail

9. All web addresses start with which of the following?

htp

http://

http:/

WWW

10. A word that looks underlined on a web page is usually what?

an important word

the web address

a "link" to another web page

a mistake

11. What is the World Wide Web?

(A) a computer game

(B) a software program

(C) the part of the Internet that enables

information-sharing via interconnected

pages

(D) another name for the Internet

12. Which is the best search tool for finding Web sites that have been handpicked and

recommended by someone else?

(A) subject directories

(B) search engines

(C) meta-search engines

(D) discussion groups

13. The Internet was originally developed by

whom?

(A) computer hackers

(B) a corporation

(C) the U.S. Department of Defense

(D) the University of Michigan

14. Which description does NOT apply to the

Internet?

(A) an interconnected system of networks

that allows for communication through e-mail, LISTSERVS, and the World Wide

Web

(B) a public network neither owned nor run

by any one group or individual

(C) a vast network that connects millions of

computers around the world

(D) a catalog of information organized and

fact-checked by a governing body

15. Which one of the following is a search engine?

(A) Macromedia Flash

(B) Google

(C) Netscape

(D) Librarians’ Index to the Internet

16. Which of the following is a TRUE statement?

(A) You are free to copy information you

find on the Web and include it in your

research report.

(B) You do not have to cite the Web sources

you use in your research report.

(C) You should never consult Web sources

when you are doing a research report.

(D) Just like print sources, Web sources

must be cited in your research report. You are not free to plagiarize

information you find on the Web.

17. What is a URL?

(A) a computer software program

(B) a type of UFO

(C) the address of a document or "page" on

the World Wide Web

(D) an acronym for Unlimited Resources for

Learning

18. What are the three main search expressions,

or operators, recognized by Boolean logic?

(A) FROM, TO, WHOM

(B) AND, OR, NOT

(C) SEARCH, KEYWORD, TEXT

(D) AND, OR, BUT

19. Which of the following is a true statement about the Internet and the library?

(A) They both have an expert librarian or

specialist to answer your questions.

(B) They both provide up-to-the-minute

news and information.

(C) They both close after hours.

(D) They both provide access to

newspapers, magazines, and journals.

20. http://www.classzone.com is an example of

what?

(A) a URL

(B) an access code

(C) a directory

(D) a server

21. HTML is used to

• Plot complicated graphs • Solve equations

• Author webpages • Translate one language into another

22. The "http" you type at the beginning of any site's address stands for

• HTML Transfer Technology Process

• Hyperspace Techniques and Technology Progress

• Hyper Text Transfer Protocol • Hyperspace Terms and Technology Protocol

23. "www" stands for • World Wide Wait

• World Wide Web • World Wide War

• World Wide Wares

24. ISP stands for

• Integrated Service Provider • Internet Security Protocol

• Internet Survey Period

• Internet Service Provider

25. Google (www.google.com) is a

• Number in Math • Chat service on the web

• Directory of images • Search Engine

26. Internet Explorer is a • Web Browser

• News Reader • Graphing Package

• Any person browsing the net

27. Modem stands for

• Memory Demagnetization • Monetary Devaluation Exchange Mechanism

• Modulator Demodulater • Monetary Demarkation

28. On which of the following sites can you set up your email account:

• www.gre.org

• www.hotmail.com

• www.linux.org • www.syvum.com

29. The speed of your net access is defined in terms of • MHz

• Megabytes • RAM

• Kbps

30. AOL stands for

• America Over LAN • America Online

• Arranged Outer Line

• Audio Over LAN

31. Which of the following is not a method of accessing the

web? • ISDN

• DSL • Modem

• CPU

32. Yahoo (www.yahoo.com) is a

• Super Computer • Organization that allocates web addresses

• Website for Consumers

• Portal

33. What is the name given to the temporary storage area that a web browser uses to store pages and graphics that it has

recently opened? • Niche

• Cellar • Cache

• Webspace

34. A computer on the Internet that hosts data, that can be

accessed by web browsers using HTTP is known as: • Web Server

• Web Rack • Web Space

• Web Computer

35. Linux is

• A Web Browser • An Operating System

• A Web Server • An non profit organization

36. Microsoft Windows is

• A Web Browser • A Web Server

• An Operating System • A Spreadsheet Package

37. A domain name ending with "org" is

• A commercial website • An organization

• A network site • A site which has very high traffic

38. At which of the following sites would you most probably

buy books? • www.hotmail.com

• www.amazon.com • www.sun.com

• www.msn.com

39. What can you do with the Internet?

Exchange information with friends and colleagues Access pictures, sounds, video clips and other media

elements Find diverse perspective on issues from a global audience

Post and respond to inquiries on a variety of subjects

40. The Internet was developed in the... early 1990s

late 1980s early 1970s

late 1960s

41. According to CNN, how much did Internet traffic increase

between 1994 and 1996? Two times

Five times Ten times

Twenty-five times

42. USENET is... A set of tools reserved exclusively for Internet

administrators Short for United States Electronic Network

A bulletin board system that allows for posting and responding to messages on the Internet

A precursor to the Internet that is now obsolete

43. True or false: The Internet is managed by the U.S. government

True False

44. What is a spider? A computer virus

A program that catalogs Web sites A hacker who breaks into corporate computer systems

An application for viewing Web sites

45. What is not always necessary for accessing the Web? A Web browser

A connection to an Internet Access Provider A computer

A modem