cs101 introduction to computing lecture 7 internet services

CS101 Introduction to Computing

Lecture 7Internet Services

Today’s Goal: Internet Services

• To look at several services provided by the Internet

– FTP– Telnet– Web– eMail– Instant messaging– VoIP

But first, we need to find out about the addressing scheme used on the Internet

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

203.81.197.188

www.imt.edu.pk

IP addressDNS address

IP Address (1)

• 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:

– 203.81.197.188 (IP address of the IMT Web server)

??clientclient

serverserver

IP Address (2)

• 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:203.81.197.188 www.imt.edu.pk216.239.33.101 www.google.com

DNS: Domain Name System (1)

• 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

DNS: Domain Name System (2)

• 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

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

• 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://sst.imt.edu.pk/cs101/index.htm

• Before, going any further, let us dissect this URL

http://sst.imt.edu.pk/cs101/index.htm

ProtocolIdentifier

ServerAddress

Directory & File Name

How does the Web work?

User’sComputer

User launches the browser on his/her computer

Browser

User’sComputer

User types in the URL into the browser

User’sComputer

The browser breaks down the URL

http://sst.imt.edu.pk/cs101/index.htm

httpProtocolIdentifier

sst.imt.edu.pkServer’s Name

cs101/index.htmDirectory &File Name

User’sComputer

DNSServer

Browser sends server’s name to the DNS server

Domain Name

IP Address

User’sComputer

WebServer

Internet

Browser establishes a connection with the server

User’sComputer

WebServer

Browser sends a ‘GET’ request for cs101/index.htm

User’sComputer

WebServer

Server sends the requested file to the browser

User’sComputer

Browser displays index.htm

X

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

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

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

Sender’sComputer

The message is prepared using the email client

Email Client

Sender’sComputer

SMTPServer

The email client sends it to the SMTP server

Sender’sComputer

SMTPServer

POP3Server

If the receiver is local, it goes to the POP3 server

Sender’sComputer

POP3Server

The receiver picks it at his/her convenience

Receiver'sComputer

SMTPServer

Sender’sComputer

SMTPServer

SMTPServer

Internet

Otherwise, it is sent to receiver's SMTP server

Sender’sComputer

SMTPServer

POP3Server

SMTPServer

Which forwards it to the local POP3 server

Sender’sComputer

SMTPServer

POP3Server

SMTPServer

The receiver picks it at his/her convenience

Receiver'sComputer

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

Using Instant Messaging (1)

• 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

Using Instant Messaging (2)

• 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

Using Instant Messaging (3)

• 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

How instant messaging works?

User launches the IM client

My Computer

IM Client

Internet

IM client finds the IM server & logs in

My Computer IM Server

It sends communication info (IP address, etc) to the IM server

My Computer IM Server

Temporary File

IM server finds user’s contacts & sends him/her the communication info for the ones online

My Computer IM Server

IM server also tells the contacts that the user is online; sends his/her communication info to them

My Computer IM Server

Contact’sComputer

My Computer IM Server

Contact’sComputer

Now the user’s & the contact’s IM clients are ready to communicate directly (P2P)

The IM server doesn’t play any part in this P2P communication

My Computer IM Server

ContactA’s Computer

As new contact’s come online, IM server informs them about the user being online & vice versa

ContactB’s Computer

My Computer IM Server

ContactA’s Computer

Multiple, simultaneous conversations are possible

ContactB’s Computer

My Computer IM Server

ContactA’s Computer

When the user logs-off, his/her IM client informs the IM server

ContactB’s Computer

My Computer IM Server

ContactA’s Computer

IM server erases the temporary file and informs the user’s contact’s about his/her ‘offline’ status

ContactB’s Computer

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

VoIP: Voice over IP

• Voice delivered from one device to another using the Internet Protocol

• Voice is first converted into a digital form, is broken down into packets, and then transmitted over a TCP/IP network (e.g. Internet)

• Four modes:– C2C– C2T– T2C– T2T (with a TCP/IP net somewhere in between)

Pro

Much cheaper than traditional phone service

Con

Noticeably poor quality of voice as compared with land-line phone service, but not much

worse than cell phone service

Today’s Goal: Internet Services

• We looked at several services provided by the Internet– FTP– Telnet– Web– eMail– Instant messaging– VoIP

• We also found out about the addressing scheme used on the Internet