The internet and its uses (3)
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1.
Explain the roles of DNS, HTTP, and TCP in the process of retrieving a web page. Provide a brief description of each protocol and how it contributes to the overall process.
Here's a breakdown of the roles:
DNS (Domain Name System): DNS acts like a phonebook for the internet. It translates human-readable domain names (like www.example.com) into numerical IP addresses (like 93.184.216.34) that computers use to locate each other. Without DNS, we would have to remember long strings of numbers to access websites.
HTTP (Hypertext Transfer Protocol): HTTP is the protocol used for transferring data between a web browser and a web server. It defines how the browser requests resources (like HTML, CSS, and JavaScript) and how the server responds. HTTP uses requests and responses to exchange information. It specifies the format of the data being transferred and the actions to be performed.
TCP (Transmission Control Protocol): TCP is a reliable, connection-oriented protocol that ensures data is transmitted accurately and in the correct order. It breaks data into packets, sends them across the network, and reassembles them at the destination. TCP provides error detection and correction, guaranteeing that the data arrives completely and without errors. It establishes a connection before data transfer and terminates it afterwards.
In summary, DNS allows the browser to find the server, HTTP allows the browser to request and receive the web page data, and TCP ensures that the data is transmitted reliably.
2.
Describe, in detail, the process a user's device follows from the moment they enter a URL into a web browser until the web page is fully displayed on the screen. Your answer should include the roles of DNS, HTTP, and the browser's rendering engine.
When a user enters a URL (e.g., www.example.com) into a web browser, the following steps occur:
- URL Parsing: The browser first parses the URL to identify its components, including the protocol (e.g., HTTP), the domain name (e.g., example.com), and the path (e.g., /index.html).
- DNS Lookup: The browser uses the Domain Name System (DNS) to translate the domain name into an IP address. It first checks its cache, then queries a DNS server. The DNS server recursively queries other DNS servers until it finds the IP address associated with the domain name. This IP address is a numerical address that identifies the server hosting the website.
- TCP Connection: The browser establishes a Transmission Control Protocol (TCP) connection with the web server at the IP address obtained from the DNS lookup. TCP ensures reliable data transmission. A three-way handshake is used to establish the connection.
- HTTP Request: The browser sends an HTTP request to the web server. This request specifies the desired resource (e.g., /index.html) and the HTTP method (e.g., GET). The request includes information about the browser (User-Agent) and any cookies.
- Server Response: The web server receives the HTTP request and processes it. It then sends an HTTP response back to the browser. This response includes the requested resource (e.g., the HTML code of index.html), along with HTTP headers that provide information about the response (e.g., content type).
- Rendering Engine: The browser's rendering engine (e.g., Blink in Chrome, Gecko in Firefox) receives the HTTP response. It parses the HTML code, builds the Document Object Model (DOM), and then creates the Composite View Tree (CVT). The CVT represents the visual structure of the web page.
- Layout and Painting: The rendering engine calculates the layout of the page based on the CSS rules and then paints the page on the screen. This involves calculating the position and size of each element and drawing it on the screen.
3.
Question 2
Explain the process of an HTTP request and the corresponding HTTP response. Include details of the headers involved in each stage.
An HTTP request-response cycle works as follows:
- Request Initiation: A client (e.g., a web browser) initiates a request by typing a URL or clicking a link.
- Request Transmission: The browser constructs an HTTP request message. This message includes:
- Method: Indicates the action to be performed (e.g., GET, POST, PUT, DELETE). GET is used to retrieve data, POST is used to send data to the server.
- URL: Specifies the resource being requested.
- Headers: Provide additional information about the request, such as the browser type (User-Agent), accepted content types (Accept), and cookies (Cookie).
- Body (optional): Contains data being sent to the server (e.g., form data in a POST request).
- Server Processing: The web server receives the request and processes it. This may involve retrieving data from a database, performing calculations, or generating a response.
- Response Generation: The server constructs an HTTP response message. This message includes:
- Status Code: A three-digit code indicating the outcome of the request (e.g., 200 OK, 404 Not Found, 500 Internal Server Error).
- Headers: Provide additional information about the response, such as the content type (Content-Type), the server software (Server), and caching instructions (Cache-Control).
- Body: Contains the requested data (e.g., HTML code for a web page, an image, or JSON data).
- Response Transmission: The server sends the HTTP response back to the client.
- Response Rendering: The browser receives the response and renders the content for the user.