HTTP can run in two modes:

• Non-Persistent - each response/request is sent over separate TCP connection
• Persistent - re-using same TCP connection for all request/responses

There are two HTTP message types:

• Request
• Response

Both have a start line, headers, an empty line and optional body.

Version - the application doesn't usually care which protocol version is being used. The web-server translates the protocol details into standard request before passing it to the application.

The version is negotiated during TLS handshake. The browser advertises the application protocols it supports using TLS extension called ALPN.

There is no TLS handshake for HTTP, so the browser simply starts with HTTP/1.1.

HTTP/3
HTTP/3 runs on top of the QUICK (Quick UDP). The main idea that we eliminate two handshaks that were needed in HTTPs (TCP + TLS).

First line in the Request Message is called a request line. The subsequent lines are called headers line.

Request line has three fields:

1. Method
2. URL
3. HTTP version

First line in Response Message is called a status line. Then we have six header lines and then entity body.

Status line has three fields:

1. Protocol version
2. Status code
3. Corresponding status message

HTTP is a Stateless protocol. Cookie is the primary mechanism for maintaining state. Cookies allows a web server to know:

• Whether you've visited before
• Whether you're logged in
• What's in your shopping cart
• What language you selected