Interview question: What happens when you enter a URL in the browser and press Enter?

The overall flow chart is as follows:

1. URL parsing

The browser first parses the URL to find out the protocol, domain name, port, resource path, parameters, etc.

2. DNS domain name resolution

Generally speaking, domain names are easier to remember than IP addresses, so we are more accustomed to entering domain names rather than IP addresses in the browser. The computer identifier recognized by computer network communications is the IP address, so we first need to convert a domain name into a corresponding IP address. This is what the DNS protocol does.

DNS is like the address book in our mobile phone. The address book contains the other party's name (similar to the domain name), but what is actually needed when making a call is the phone number (similar to the IP address). The address book is used to convert a name into the corresponding phone number.

3. Establishing a TCP connection

Generally, when you enter a URL in a browser, the application layer protocol is HTTP/HTTPS, which requires reliable service, and the transport layer protocol used is TCP.

After domain name resolution, the browser obtains the server's IP address and initiates a TCP connection to the server, at which point a three-way handshake occurs.

4. Send HTTP request

When the browser establishes a connection with the server, data communication can begin. The browser sends an HTTP request message to the server, which includes a request line, a request header, a blank request line, and a request body. The request line specifies the method, resource path, and HTTP version. The resource path specifies the location of the resource to be operated on the server, and the method specifies what operation to perform on the resource.

When you enter the URL from the browser, the resource path has been parsed in the first step, and the method is GET, indicating that you want to get the resource, which is equivalent to the query in the addition, deletion, modification and query.

5. The server processes the request and responds

After receiving the request message sent by the browser, the server will process the request message accordingly and return a response message to the browser. For example, if the request message wants to get (GET) the file index.html, the server will find the index.html file and send it to the browser as the response body in the response message.

The response message includes a response line, a response header, a response blank line, and a response body. The response line specifies the HTTP version, status code, and explanation of the status code, such as HTTP/1.1 200 OK, where 200 is the response code, indicating that the request was processed normally, which means success.

6. Browser parses and renders the page

After receiving the response message from the server, the browser obtains the corresponding resources from the response body, such as HTML files, pictures, videos, etc., renders them, and then presents the results to the user.

7. Disconnect TCP connection

When the data completes the process from request to return, you can choose whether to disconnect the TCP connection based on the Keep-Alive attribute of the Connection in the request/response header. If data communication is no longer required, the connection can be closed, and four handshakes will occur.

Notice:

In order to improve performance, the browser will actually query whether there is a cache after URL parsing. If the cache hits, the cached resource will be returned directly.

If it is the HTTPS protocol, after establishing the TCP connection, an SSL/TLS handshake process is required to negotiate a session key for message encryption and improved security.

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