cs101 introduction to computing lecture 7 internet services
TRANSCRIPT
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CS101 Introduction to Computing
Lecture 7Internet Services
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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
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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
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203.81.197.188
www.imt.edu.pk
IP addressDNS address
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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)
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??clientclient
serverserver
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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
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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
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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
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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
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Internet Services
There are many, but we will look at only the following:
• FTP
• Telnet
• Web
• Instant messaging
• VoIP
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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
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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
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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
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http://sst.imt.edu.pk/cs101/index.htm
ProtocolIdentifier
ServerAddress
Directory & File Name
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How does the Web work?
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User’sComputer
User launches the browser on his/her computer
Browser
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User’sComputer
User types in the URL into the browser
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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
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User’sComputer
DNSServer
Browser sends server’s name to the DNS server
Domain Name
IP Address
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User’sComputer
WebServer
Internet
Browser establishes a connection with the server
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User’sComputer
WebServer
Browser sends a ‘GET’ request for cs101/index.htm
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User’sComputer
WebServer
Server sends the requested file to the browser
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User’sComputer
Browser displays index.htm
X
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• 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
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How does an email system work?
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But first, the components:
• Email client
• SMTP server
• POP3 server
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Email Clients
• Programs used for writing, sending, receiving, and displaying eMail messages
• Examples: Outlook, Communicator, Hotmail, YahooMail
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SMTP: Simple Mail Transfer Protocol
A protocol used to send and receive email messages over a TCP/IP network
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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
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Sender’sComputer
The message is prepared using the email client
Email Client
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Sender’sComputer
SMTPServer
The email client sends it to the SMTP server
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Sender’sComputer
SMTPServer
POP3Server
If the receiver is local, it goes to the POP3 server
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Sender’sComputer
POP3Server
The receiver picks it at his/her convenience
Receiver'sComputer
SMTPServer
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Sender’sComputer
SMTPServer
SMTPServer
Internet
Otherwise, it is sent to receiver's SMTP server
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Sender’sComputer
SMTPServer
POP3Server
SMTPServer
Which forwards it to the local POP3 server
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Sender’sComputer
SMTPServer
POP3Server
SMTPServer
The receiver picks it at his/her convenience
Receiver'sComputer
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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
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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
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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
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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
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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
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How instant messaging works?
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User launches the IM client
My Computer
IM Client
Internet
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IM client finds the IM server & logs in
My Computer IM Server
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It sends communication info (IP address, etc) to the IM server
My Computer IM Server
Temporary File
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IM server finds user’s contacts & sends him/her the communication info for the ones online
My Computer IM Server
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IM server also tells the contacts that the user is online; sends his/her communication info to them
My Computer IM Server
Contact’sComputer
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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
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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
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My Computer IM Server
ContactA’s Computer
Multiple, simultaneous conversations are possible
ContactB’s Computer
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My Computer IM Server
ContactA’s Computer
When the user logs-off, his/her IM client informs the IM server
ContactB’s Computer
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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
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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
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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?
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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
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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)
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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
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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