HTTP: protocol that allows browsers to retrieve resources from the server

The browser initiates the HTTP request process

1. Build the request

Build the request line information GET /index.html HTTP1.1 and then prepare to initiate the request

2. Find the cache (before the actual request is made)

Browser cache: saves a copy of the resource locally for direct use by the next request. If there is a copy -> intercept request -> Return copy -> end the request

  • Reduce server pressure and improve performance
  • For web sites, caching enables fast loading of resources

**3.** Prepare IP addresses and ports

Domain Name System (DNS) : mapping IP addresses and Domain names

  • The browser requests DNS to return the corresponding IP address (DNS data caching can reduce a network request)
  • Port number: URL not specified, default port 80

4. Wait **** to wait for the TCP queue

  • Http1.1 A maximum of six TCP connections can be established in the same domain name. A TCP connection can process only one request at a time
  • Http2 can request resources in parallel, maintaining one TCP connection per domain name

5. Establish the ****TCP connection

6. Send ****HTTP request: Sends a request line to the server

The server side handles the HTTP request flow

1. Return the request

2. Disconnect the device

Connection: keep-alive Enables the browser to continue sending TCP requests through the same TCP Connection. This saves the time for establishing a Connection for the next request and speeds up resource loading

3. The redirection

conclusion

  • Why do some sites open faster the second time

  • DNS cache

  • Page resource cache

  • The server returns an HTTP response header. The browser sets whether to Cache the resource and the Cache expiration time by cache-control: max-age =2000

  • If the cache expires, the browser continues to make requests and puts if-none-match in the HTTP request header :” 4f80F – 13C-3a1XB12a “The server uses if-none-match to judge whether the requested resources have been updated

  • Priority: Cache-Control > Expires Etag > last-Modified

  • Cache-control: max-age: Cache duration (s) S-maxAge: Cache duration for the proxy server Public: can be cached by any Cache private: can only be cached by an individual user, but cannot be cached by the proxy server no-cache: Will be cached, but each time a request is made, the cache evaluates the validity of the cache response to the serverCopy the code

  • How is the login status maintained