I think this is one of the most popular interview questions for Software Engineering Positions. Recently it has been said this has been asked in one of the Tiktok Interview.
To answer the question “What happens when you type https://www.google.com in the browser?” in a detailed and structured manner for an interview, break the process into key steps that span DNS lookup, TCP/SSL handshake, request processing, and rendering the response. Here’s a comprehensive explanation:
URL Parsing
When you type the URL https://www.google.com and press enter:
- Scheme: The browser identifies the scheme as https, meaning it will use HTTP over TLS (encrypted communication).
- Host: The browser identifies www.google.com as the domain.
- Path: By default, the path is /, which refers to the homepage since no specific path is provided.
DNS Lookup
The browser needs to convert the domain name www.google.com into an IP address. This happens in several steps:
- Browser Cache: The browser first checks its own cache to see if it already has the IP address for www.google.com.
- OS Cache: If not found, the browser asks the operating system to check its cache.
- Router Cache: If the OS doesn’t know the IP, the request is forwarded to the router, which checks its own cache.
- ISP DNS Server: If still not found, the router queries the ISP’s DNS server.
- Recursive DNS Lookup: If the ISP doesn’t have the IP cached, it queries the DNS hierarchy (root, TLD, and authoritative DNS servers). Eventually, the IP address of www.google.com is resolved (e.g., 142.250.72.196).
Establishing a TCP Connection
Once the IP address is known, the browser must establish a connection to the Google server. This is done using:
TCP 3-Way Handshake:
- SYN: The client (browser) sends a SYN (synchronize) packet to the server to initiate a connection.
- SYN-ACK: The server responds with a SYN-ACK (synchronize-acknowledge) packet.
- ACK: The client sends an ACK packet, and the connection is established.
SSL/TLS Handshake (for HTTPS)
Since the URL uses HTTPS, a secure connection is established using SSL/TLS:
- The browser and server negotiate a cryptographic protocol (TLS version) and exchange encryption keys.
- The server sends its SSL certificate, which the browser verifies to ensure the server’s identity.
- A session key is generated to encrypt subsequent communication.
HTTP Request
After the secure connection is established, the browser sends an HTTP GET request to the server:
- Method: GET
- Headers: Includes details like browser type, cookies, and cache information.
- Host: www.google.com
- Path: /
Server Processing
Google’s servers, behind load balancers, receive the request:
The request might be routed through several reverse proxies and load balancers, often spread across multiple data centers for availability and performance.
Google’s web server processes the request, checks for the requested resource (Google’s homepage), and prepares a response.
HTTP Response
The server responds with an HTTP 200 OK status and sends the necessary HTML, CSS, JavaScript, and other resources to the browser.
The response includes headers (e.g., Content-Type, Cache-Control) and the response body (HTML content of Google’s homepage).
Rendering in the Browser
The browser now has the HTML and begins rendering the page:
- HTML Parsing: The browser parses the HTML to construct the DOM (Document Object Model).
- CSS Parsing: Any linked or embedded CSS is downloaded and applied to the DOM to style the elements.
- JavaScript Execution: Any JavaScript is downloaded and executed. JavaScript might further manipulate the DOM or make additional network requests (AJAX) to update the page dynamically.
- Rendering: The browser’s rendering engine paints the parsed and styled content to the screen, creating the visible webpage.
Additional Resource Requests
As the browser parses the HTML, it identifies additional resources (images, stylesheets, JavaScript files) that are needed:
These resources are fetched via additional HTTP/HTTPS requests. This process repeats with multiple parallel connections to download and render assets quickly.
Browser Caching & Optimization
The browser will cache certain resources (images, scripts, styles) based on cache headers (Cache-Control, ETag).
Modern browsers use optimizations like HTTP/2 multiplexing to download multiple resources over a single TCP connection and reduce latency.
Final Page Display
Once all resources are downloaded, parsed, and rendered, the user can interact with the fully loaded webpage. Further user actions (clicks, typing, etc.) may trigger more network requests (e.g., submitting a form, AJAX updates).
Bonus Points to Mention
- CDN (Content Delivery Network): Google uses a CDN to serve content from geographically close servers, reducing latency and improving load times.
- Security Features: HSTS (HTTP Strict Transport Security) ensures all requests to Google are made over HTTPS. Google’s certificate pinning ensures that the server’s SSL certificates are not tampered with.
- Service Workers: If enabled, a service worker may intercept requests to provide cached responses or enable offline functionality.
This detailed breakdown covers everything from the initial user input to the final rendered page in the browser, touching on key topics like DNS, TCP/IP, TLS, HTTP, and browser rendering, which is ideal for a system design or software engineering interview.
–EOF (The Ultimate Computing & Technology Blog) —
loading...
Last Post: Crypto is Just the Tip of the Blockchain Iceberg
Next Post: The Memory Manager in C/C++ (Heap vs Stack)