cs101 introduction to computing lecture 7 internet services
TRANSCRIPT
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
• 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
• 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