CS101 Introduction to Computing Lecture 7 Internet Services

Today’s Goal: Internet Services To look at several services provided by the Internet –FTP –Telnet –Web – –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

IP address DNS 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: – (IP address of the IMT Web server)

? client server

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: 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. Before, going any further, let us dissect this URL

Protocol Identifier Server Address Directory & File Name

How does the Web work?

User’s Computer User launches the browser on his/her computer Browser

User’s Computer User types in the URL into the browser

User’s Computer The browser breaks down the URL http Protocol Identifier sst.imt.edu.pk Server’s Name cs101/index.htm Directory & File Name

User’s Computer DNS Server Browser sends server’s name to the DNS server Domain Name IP Address

User’s Computer Web Server Internet Browser establishes a connection with the server

User’s Computer Web Server Browser sends a ‘GET’ request for cs101/index.htm

User’s Computer Web Server Server sends the requested file to the browser

User’s Computer 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 system work?

But first, the components: client SMTP server POP3 server

Clients Programs used for writing, sending, receiving, and displaying messages Examples: Outlook, Communicator, Hotmail, YahooMail

SMTP: Simple Mail Transfer Protocol A protocol used to send and receive messages over a TCP/IP network

POP3: Post Office Protocol A protocol used for receiving messages A POP3 server maintains text files (one file per user account) containing all messages received by a user client interacts with the POP3 server for discovering and downloading new messages

Sender’s Computer The message is prepared using the client Client

Sender’s Computer SMTP Server The client sends it to the SMTP server

Sender’s Computer SMTP Server POP3 Server If the receiver is local, it goes to the POP3 server

Sender’s Computer POP3 Server The receiver picks it at his/her convenience Receiver's Computer SMTP Server

Sender’s Computer SMTP Server SMTP Server Internet Otherwise, it is sent to receiver's SMTP server

Sender’s Computer SMTP Server POP3 Server SMTP Server Which forwards it to the local POP3 server

Sender’s Computer SMTP Server POP3 Server SMTP Server The receiver picks it at his/her convenience Receiver's Computer

The Trouble with Slow response times No way of knowing if the person we are sending to is there to read it The process of having a conversation through 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 ComputerIM Server

It sends communication info (IP address, etc) to the IM server My ComputerIM Server Temporary File

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

IM server also tells the contacts that the user is online; sends his/her communication info to them My ComputerIM Server Contact’s Computer

My ComputerIM Server Contact’s Computer 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 ComputerIM Server Contact A’s Computer As new contact’s come online, IM server informs them about the user being online & vice versa Contact B’s Computer

My ComputerIM Server Contact A’s Computer Multiple, simultaneous conversations are possible Contact B’s Computer

My ComputerIM Server Contact A’s Computer When the user logs-off, his/her IM client informs the IM server Contact B’s Computer

My ComputerIM Server Contact A’s Computer IM server erases the temporary file and informs the user’s contact’s about his/her ‘offline’ status Contact B’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 – –Instant messaging –VoIP We also found out about the addressing scheme used on the Internet