Introduction to the Web

Lets have a look at the Course Syllabus.

1. Using Winrar or Winzip to zip and unzip multiple files or multiple folders.
2. Navigating a file system from a console: (cd ./dir, cd .. , cd ../dir) ...
3. Programming: a. Conditionals (if..then..else) b. Loops (for, for each, while) c. Exception handling (try ... catch) d. Objects: someObject.myFunction

• The history of the Internet and World Wide Web
• Fundamental concepts that form the foundation of the Internet Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

Pr. Tim Berners-Lee (Turing Award 2016) publishes the main features of the web we know today on 1992.

• A Uniform Resource Locator (URL) to uniquely identify a resource on the WWW. example: https://www.facebook.com
• The Hypertext Transfer Protocol (HTTP) to describe how requests and responses operate
• A software program (web server software) that can respond to HTTP requests.
• Hypertext Markup Language (HTML) to publish documents.
• A program (a browser) that can make HTTP requests to URLs and that can display the HTML it receives. example: Chrome, Opera, Internet Explorer Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

Desktop Applications (1 of 2) Advantages

• They can be accessed from any Internet-enabled computer.
• They can be used with different operating systems and browser applications.
• It is easier to roll out program updates since only software on the server needs to be updated as opposed to every computer in the organization using the software.
• They have a centralized storage on the server, which means fewer security concerns about local storage (which is important for sensitive information such as healthcare data). Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

Desktop Applications (2 of 2) Disadvantages

• Requirement to have an active Internet connection
• Security concerns about sensitive private data being transmitted over the Internet.
• Concerns over the storage, licensing, and use of uploaded data.
• Problems with certain websites not having an identical appearance across all browsers.
• Restrictions on software from being installed and hardware from being accessed (like Adobe Flash on iOS).
• additional plugins might interfere with JavaScript, cookies, or advertisements. Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

https://laws.boe.gov.sa/boelaws/laws/lawdetails/b7cfae89-828e-4994-b167-adaa00e37188/1

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• Web 2.0 refers to an interactive experience where users can contribute and consume web content, thus creating a more user-driven web experience.
• For software developers, Web 2.0 also refers to a change in the paradigm. Programming logic, which previously existed only on the server, began to migrate more and more to the browser, which requires learning JavaScript Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

It is quite common for the Internet to be visually represented as a cloud Actually implemented via millions of miles of copper (

) wires

and fiber optic cables connecting millions of server computers and other networked devices!

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• The fundamental protocols that make the web possible
• How the domain name system works
• What HTTP is
• How browsers and servers work to exchange and interpret HTML Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• A protocol is a set of rules that partners use when they communicate.
• TCP/IP is an essential internet protocol!
• These protocols have been implemented in every operating system and make fast web development possible. If web developers had to keep track of packet routing, transmission details, domain resolution, checksums, and more, it would be hard to build websites. Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• The link layer is the lowest layer, responsible for both the physical transmission of data across media and establishing logical links.
• It handles issues like packet creation, transmission, reception, error detection, collisions, line sharing, and more.
• One term that is sometimes used in the Internet context is that of MAC (media access control) addresses. Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• The Internet layer (sometimes also called the IP Layer) routes packets between communication partners across networks.
• It provides best effort communication. It sends out a message to its “ ” destination but expects no reply and provides no guarantee the message will arrive intact, or at all.
• The Internet uses the Internet Protocol (IP) addresses, which are numeric codes that uniquely identify destinations on the Internet.
• Every device connected to the Internet has such an IP address. Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

The IPv4 address space was depleted (almost filled) in 2011, but the number of computers connected to the Internet continued to grow.

• Port Address Translation (PAT), allows multiple, unrelated networks to make use of the same IP address
• When you join a wireless network in a coffee shop, home, office or university, it is quite likely you are making use of PAT.
• For future growth, IPv6 will be necessary. Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• The transport layer ensures transmissions arrive in order and without error.
• First, the data is broken into packets formatted according to the Transmission Control Protocol (TCP).
  • Each data packet has a header that includes a sequence number, so the receiver can put the original message back in order
  • Each packet acknowledges its successful arrival back to the sender (ACK).
  • In the event of a lost packet (since no ACK arrived for that packet the packet will be retransmitted.
• This means you have a guarantee that messages sent will arrive and will be in order. Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• As elegant as IP addresses may be, human beings do not enjoy having to remember long strings of numbers.
• Even as far back as the days of ARPANET, researchers assigned domain names to IP addresses
• In those early days, the number of Internet hosts was small, so a list of a domains and associated IP addresses could be downloaded as needed as a hosts file.
• As the number of computers on the Internet grew, this hosts file had to be replaced with a better, more scalable, and distributed system. This system is called the Domain Name System (DNS) Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

The rightmost portion of the domain name is called the top-level domain.

TLDs limited to two broad categories, plus a third reserved for other use.

1. Generic top-level domain (gTLD) 2. Country code top-level domain (ccTLD) 3. .arpa (used for reverse DNS lookups)

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Generic top-level domains (gTLD) include the famous .com and .org, There are 3 subtypes of gTLD.

• Unrestricted. TLDs include .com, .net, .org, and .info.
• Sponsored. TLDs including .gov, .mil, .edu, and others
• New. Starting in June 2012, ICANN invited companies to launch new TLDs in order to provide more choice. Since then over 1000 new TLD have been created including .art, .cash, .cool, .jobs, .tax and so on Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

Country code top-level domain (ccTLD) are under the control of the countries which they represent, which is why each is administered differently.

• In the United Kingdom, for example, businesses must register subdomains to co.uk rather than second-level domains directly whereas in Canada, .ca domains can be obtained by any person, company, or organization living or doing business in Canada.
• Internationalized top-level domain name (IDN) allows domains to use non-ascii characters and has been deployed since 2009. There are over 9 million IDN domains Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• Q: How then are domain names assigned?
• A: Special organizations or companies called domain name registrars manage the registration of domain names. These domain name registrars are given permission to do so by the appropriate generic top-level domain (gTLD) registry and/or a country code top-level domain (ccTLD) registry.
• The non-profit Internet Corporation for Assigned Names and Numbers (ICANN) oversees the management of top-level domains, accredits — registrars, and coordinates other aspects of DNS.
• Examples: domain.com, godaddy, hostgator, bluehost ... Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• A port is a type of software connection point used by the underlying TCP/IP protocol and the connecting computer.
• Although the port attribute is not commonly used in production sites, it can be used to route requests to a test server, to perform a stress test, or even to circumvent Internet filters.
• If no port is specified, the protocol determines which port to use. For instance, port 80 is the default port for web-related HTTP requests.
• Syntax is to add a colon after the domain, then specify an integer port number. http://funwebdev.com:8080/ would connect on port 8080 Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• The path is an important concept to anyone who has ever used a computer file system.
• The root of a web server corresponds to a folder somewhere on that server.On many Linux servers that path is /var/www/html/ or something similar (for Windows it is often /inetpub/wwwroot/).
• The path is optional. However, when requesting a folder or the top-level page of a domain, the web server will decide which file to send you. Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

Headers are sent in the request from the client and received in the response from the server. Headers are one of the most powerful aspects of HTTP.

• Request headers include data about the client machine
  • Host, User-Agent, Cache settings and more
• Response headers have information about the server answering the request and the data being sent
  • Server, Last Modified, Content Type, Encoding, Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• The most common requests are the GET and POST request, along with the HEAD request
• Other HTTP verbs such as CONNECT, TRACE, and OPTIONS are less commonly used and are not covered. Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• Response codes are integer values returned by the server as part of the response header.
• These codes describe the state of the request, including whether it was successful, had errors, requires permission, and more.
• The codes use the first digit to indicate the category of response.
  • 2## codes are for successful responses,
  • 3## are for redirection-related responses,
  • 4## codes are client errors.
  • 5## codes are server errors. Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• Seeing a single web page is facilitated by the browser, which
  • requests the initial HTML page, then
  • parses the returned HTML to find all the resources referenced from within it (like images, style sheets, and scripts).
• Only when all the files have been retrieved is the page fully loaded for the user
• The algorithms within browsers to download, parse, layout, fetch assets, and create the final interactive page for the user are commonly referred to collectively as the rendering of the page Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

• A web server is, at a fundamental level, nothing more than a process that responds to HTTP requests.
• Real-world websites typically have many web servers configured together in web farms.
• Regardless of the physical characteristics of the server, one must choose an application stack to run a website. This application stack will include
  • an operating system,
  • web server software,
  • a database,
  • a backend programming language and runtime,
  • a set of languages/framewors/libraries for the front-end. Copyright © 2021, 2018, 2015 Pearson Education, Inc. All Rights Reserved

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