====== HTTP ======

===== Intro =====

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 QUIC (Quick UDP). The main idea that we eliminate two handshaks that were needed in HTTPs (TCP + TLS).

In QUIC TLS handshake is absorbed into the initial setup handshake. HTTP/3 does not run TLS "on top" of QUIC. Instead, **TLS 1.3** is built into the QUIC protocol itself.

QUIC mitigates HOL (Head of Line) problem by treating each stream within a single connection independently. If one particular frame is lost, QUIC will handle it separately without blocking the rest of the streams.

===== HTTP Request Message =====

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

Request line has three fields:
  - Method
  - URL
  - HTTP version

==== HTTP Response Message ====

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:
  - Protocol version
  - Status code
  - Corresponding status message

==== Cookie ====

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