During the last lecture …
(Introduction to the Internet)
We looked at the role Internet plays in today’s
computing
We reviewed some of the history and evolution of the Internet
Internet: The Enabler
Enables attractively-priced workers located
in Pakistan to provide services to overseas clients
Enables users to easily share information with others located all over
the world
Enables users to easily, inexpensively communicate with others remote
users
Enables the users to operate and run programs on computers located all
over the world
The Internet is unlike any previous human
invention. It is a world-wide resource, accessible to all
of the humankind.
Key Characteristics
Geographic Distribution
Global - reaches around the world
Robust Architecture
Adapts to damage and error
Speed
Data can
travels at near ‘c’
on copper, fiber, airwaves
Universal Access
Same functionality to everyone
Growth Rate
The fastest growing technology ever
Freedom of Speech
Promotes freedom of speech
The Digital Advantage
Is digital: can correct errors
Internet: Network of Networks
A large number of networks,
interconnected physically
Capable of communicating and sharing data with each other
From the user’s point view, Internet – a collection of interconnected
networks – looks like a single, unified
network
TCP/IP Transmission Control Protocol/Internet Protocol
TCP breaks down the message to be sent over
the Internet into packets
IP routes these packets through the Internet to get them to their
destination
When the packets reach the destination computer, TCP reassembles them
into the original message
1960's
1969
- DoD-ARPA creates an experimental network
– ARPANET – as a test-bed for emerging networking
technologies
ARPANET originally connected 4 universities & enabled scientists to
share info & resources across long
distances
1980's
1983
- The TCP/IP protocols becomes the only set
of protocols used on the ARPANET
This sets a standard for all networks, and generates the use of the term
Internet as the net of nets
1990's1993
- CERN releases WWW, developed by Tim
Berners-Lee
It uses HTTP and hypertext, revolutionizing the way info is presented &
accessed on Internet
1990's1993-1994
- Web browsers Mosaic & Netscape Navigator
are introduced
Their GUI makes WWW & Internet more appealing to the general public
Today’s Goal: Internet Services
To look at several services provided by the
Internet
–FTP
User’s
Computer
–Telnet
–Web
–eMail
–Instant messaging
–VoIP
• But first, we need to find out
about the addressing scheme used on the Internet
30.1 Internet Addressing
Regular post cannot be delivered unless we
write a destination address on the envelope
Same is true for the Internet
Regular post can be delivered at the intended address even if the given
address is not precise. That is not
the case for Internet addressing
www.zainbooks.com
203.215.177.33
IP Address
A unique identifier for a computer on a TCP/IP
network
Format: four 8-bit numbers separated by periods. Each 8-bit number can
be 0 to 255
Example:
–203.215.177.33 (IP address of the VU Web server)
Networks using TCP/IP route messages based on the IP address of the
destination
Any IP addresses (as long as they are unique) can be assigned within a
PN
However, connecting a PN to the Internet requires using unique,
registered IP addresses
Domain Names
IP addresses are fine for computers, but
difficult to recognize and remember for humans
A domain name is a meaningful, easy-to-remember ‘label’ for an IP
address
Examples:
www.zainbooks.com
216.239.33.101 www.google.com
30.2 DNS: Domain Name System
DNS is the way that Internet domain names
are located & translated into IP addresses
Maintaining a single, central table of domain name/IP address
relationships is impractical
–Billions of DNS-IP translations take place every day
–The DNS-IP tables get updated continuously
Tables of DNs & IP addresses are distributed throughout the Internet on
numerous servers
There is a DNS server at most ISPs. It converts the domain names in our
Internet requests to actual IP
addresses
In case it does not have a particular domain name in its table, it makes
a request to another DNS server on
the Internet
30.3 Internet Services
There are many, but we will look at only
the following:
FTP
Telnet
Web
eMail
Instant messaging
VoIP
FTP: File Transfer Protocol
Used to transfer files between computers on
a TCP/IP network (e.g Internet)
Simple commands allow the user to:
List, change, create folders on a remote computer
Upload and download files
DNS address IP address
Server
Address
Typical use: Transferring Web content from
the developer’s PC to the Web server
Telnet Protocol
Using Telnet, a user can remotely
log on to a computer (connected to the user’s through a TCP/IP
network, e.g. Internet) & have control over it like a local user,
including control over running various
programs
In contrast, FTP allows file operations only
Typical use: Configuring and testing of a remote Web server
The Web
The greatest, shared resource of
information created by humankind
A user may access any item on the Web through a URL, e.g.
http://www.zainbooks.com/cs/index.html
Before, going any further, let us dissect this URL
How does the web works
User launches the browser on
his/her computer
User types in the URL into the browser
The browser breaks down the URL into 3 parts :
Protocol Identifier
Server Address
Directory & File Name
Browser sends server’s name to the DNS server
Domain Name
Browser establishes a connection
with the server
http ://
www.zainbooks.com
/cs/index.html
Protocol
Identifier
Directory &
File Name
User’s
Computer
Browser
User’s
Computer
User’s
Computer
DNS
Server
Browser sends a ‘GET’ request for cs/index.html
Server sends the requested file to the browser
Browser displays index.html
email
Computer-to-computer messaging
Inexpensive, and quite quick, but not instant!
The most popular service on the Internet, even more than surfing, but
soon to be overtaken by instant
messaging
Billions are sent every day
30.3 How does an eMail system work?But first, the
components:
eMail client
SMTP server
POP3 server
eMail Clients
Programs used for writing,
sending, receiving, and displaying eMail messages
Examples: Outlook, Communicator, Hotmail, YahooMail
SMTP: Simple Mail Transfer Protocol
A protocol used to send and
receive eMail messages over a TCP/IP network
User’s
Computer
Web
Server
Internet
User’s
Computer
Web
Server
User’s
Computer
POP3: Post Office Protocol
A protocol used for receiving
eMail messages
A POP3 server maintains text files (one file per user account)
containing all messages received by a user
eMail client interacts with the POP3 server for discovering and
downloading new eMail messages
The message is prepared using the eMail client
The eMail client sends it to the SMTP server
If the receiver is local, it goes to the POP3 server
Sender’s
Computer eMail Client
Sender’s
Computer
SMTP
Server
Sender’s
Computer
SMTP
POP3
The receiver picks it at his/her convenience
Otherwise, it is sent to receiver's SMTP server
Which forwards it to the local POP3 server
Receiver's
Computer
Sender’s
Computer
POP3
Server
SMTP
Server
SMTP
Server
SMTP
Server
Sender’s
Computer
Internet
Sender’s
Computer
SMTP
Server
POP3
Server
SMTP
Server
The receiver picks it at his/her convenience
The Trouble with email
Slow response times
No way of knowing if the person we are sending eMail to is there to read
it
The process of having a conversation through eMail by exchanging several
short messages is too
cumbersome
Instant messaging (IM) solves these problems
Instant Messaging
•
The IM services available on the Internet
(e.g. ICQ, AIM, MSN Messenger, Yahoo! Messenger)
allow us to maintain a list of people (contacts) that we interact with
regularly
•
We can send an instant messages to any of
the contacts in our list as long as that contact is online
30.4 Using Instant Messaging
Whenever a contact in our list
comes online, the IM client informs us through an alert message and by
playing a sound
To send an instant message to a contact, just click on the contact in
the IM client, and start typing the
message
The selected contact will receive that message almost immediately after
you press ‘Enter’
When the contact’s IM client receives the message, it alerts the contact
with a blinking message and by
playing a sound
That contact then can type a response to the received message, and send
it instantly
Several such conversations can be carried out simultaneously, each
occupying a separate IM windows
Sender’s
Computer
SMTP
Serve
r
POP3
Serve
r
SMTP
Serve
r
Receiver'
s
Computer
How instant messaging
works?
User launches the IM
client
IM client finds the IM server & logs in
It sends communication info (IP address, etc) to the IM server
IM server finds user’s contacts & sends him/her the communication info
for the ones online
My Computer
IM Client
Internet
My Computer
IM
Server
My
Computer
IM
Server
Temporary File
My
Computer IM Server
My Computer
IM server also tells the contacts that the user is online; sends
his/her communication info to them
Now the user’s & the contact’s IM clients are ready to communicate
directly (P2P)
My Computer
IM Server
Contact’s
Computer
IM Server
Contact’s
Computer
The IM server doesn’t play any
part in this P2P communication
As new contact’s
come online, IM server informs them about the user being online & vice
versa
Multiple, simultaneous conversations are possible
My
Computer
IM Server
Contact
A’s Computer
Contact
B’s
Computer
My Computer
IM
Server
Contact
A’s Computer
Contact
B’s Computer
When the user logs-off, his/her IM client informs the IM server
IM server erases the temporary file and informs the user’s contact’s
about his/her ‘offline’ status
Key Point
Once the IM server provides the
communication info to the user and his/her contact’s IM client, the two
are able to communicate with each other without the IM server’s
assistance
This server-less connection is termed as a P2P connection
Question
Why do we require the server in
the first place?
Why doesn’t my IM client look for the user’s contact’s IM client without
the IM server’s help?
Answer
Many users (including almost all
home users) do not have permanent IP addresses. They are assigned
temporary IP addresses by their ISP each time they connect to the
Internet
The server-based IM scheme removes the need of having permanent IP
numbers
It also gives IM users true mobility, allowing them the use of IM from
any Internet-connected computer
My Computer IM
Server
Contact
A’s Computer
Contact
B’s Computer
My Computer
IM Server
Contact
A’s Computer
Contact
B’s
Computer
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