When you type a URL into your browser and press “Enter,” a complex sequence of actions occurs to fetch and display the requested web page. Here’s a detailed step-by-step explanation:
1. URL Parsing
The browser interprets the URL to understand:
• Protocol: e.g., HTTP or HTTPS.
• Domain Name: e.g., example.com.
• Path: e.g., /about.
• Query Parameters: e.g., ?key=value.
2. DNS Resolution
• The browser translates the domain name (e.g., example.com) into an IP address.
• This is done by querying the DNS (Domain Name System):
1) Check local DNS cache (browser, operating system).
2) If not found, query the configured DNS server.
3) The DNS server resolves the IP address and returns it to the browser.
Example: example.com → 93.184.216.34
3. Establish a TCP Connection
The browser establishes a TCP (Transmission Control Protocol) connection with the web server at the resolved IP address:
1) Three-Way Handshake:
• The browser sends a SYN packet to the server.
• The server responds with a SYN-ACK packet.
• The browser replies with an ACK packet, completing the handshake.
4. Secure the Connection (if HTTPS)
• For HTTPS, the browser initiates an SSL/TLS handshake to secure the connection:
1) The server presents its SSL certificate.
2) The browser verifies the certificate (ensuring the connection is secure and trustworthy).
3) Encryption keys are exchanged, and the connection becomes secure.
5. Sending the HTTP Request
The browser sends an HTTP or HTTPS request to the server. The request includes:
• Method: e.g., GET (fetch resource), POST (submit data).
• Headers: Metadata about the request (e.g., browser type, cookies).
• Payload (if applicable): Data sent with requests like POST.
Example Request:
GET /about HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.06. Server Processing
• The server receives the request and processes it:
1) Match the request to a resource (e.g., HTML file, API endpoint).
2) Run backend logic, if necessary (e.g., database queries, authentication).
3) Generate an HTTP response with:
• Status Code: e.g., 200 (OK), 404 (Not Found), 500 (Server Error).
• Headers: Metadata (e.g., content type, cache rules).
• Body: The actual content (HTML, JSON, image, etc.).
Example Response:
HTTP/1.1 200 OK
Content-Type: text/html
Content-Length: 34217. Browser Receives the Response
The browser receives the server’s response and begins processing:
• If the response is compressed (e.g., gzip), the browser decompresses it.
8. Rendering the Web Page
1) HTML Parsing:
• The browser parses the HTML and constructs the DOM (Document Object Model) tree.
• It identifies additional resources (e.g., CSS, JavaScript, images) and requests them.
2) CSS Parsing:
• The browser parses CSS to style the DOM elements.
3) JavaScript Execution:
• JavaScript files are downloaded and executed to add interactivity or modify the DOM dynamically.
4) Layout and Painting:
• The browser calculates the layout of elements and renders them onto the screen.
9. Additional Requests
• As specified in the HTML, the browser sends additional requests for resources like images, CSS, and JavaScript.
• These resources are fetched, cached, and rendered.
10. Caching
• The browser may store static resources (e.g., CSS, images) locally based on caching rules from the server.
• This speeds up future visits to the same site.
11. User Interaction
• Once the page is loaded, the browser remains ready to handle user interactions, like clicks and form submissions.
Error Handling
If something goes wrong:
• DNS Failure: If the domain cannot be resolved, a “DNS not found” error is displayed.
• Server Down: If the server does not respond, a “Connection Timeout” error occurs.
• 404 Error: If the requested resource is missing.
This process involves multiple layers of protocols (DNS, TCP/IP, HTTP/HTTPS) and components (browser, server, DNS resolver) working seamlessly to deliver a web page in seconds.