HTTP

HTTP messages consist of requests from client to server and responses from server.

CRLF (Carriage Return/move to start, Line Feed/newline)
the request message includes method applied to the resource, identifier of the resource, and protocol version
request-line begins with method token, followed by Request-URI, protocol version, ends with CRLF
- Methods include OPTIONS GET HEAD POST PUT DELETE TRACE CONNECT
  - 405 (Method Not Allowed) known and disallowed, 501 (Not Implemented) unknown or not implemented by server
- The Request-URI is a uniform resource identifier (which resource)
  - Request-URI = "*" | absoluteURI | abs_path | authority
  - the absoluteURI form is required when the request is made to a proxy, GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1
  - authority form is only used by the CONNECT method
  - common request-URI is used to identify a resource on an origin server or gateway
    - absolute path of the URI is transmitted as the Request-URI
    - Network location of the URI (authority) must be transmitted in a Host header field
    - transparent proxies must not rewrite the abs_path part of the received Request-URI when forwarding
```
 GET /pub/WWW/TheProject.html HTTP/1.1
 Host: www.w3.org
```

Part of requests and responses, contains fields defining the operating parameters of an HTTP transaction

Accept: text/plain
Accept-Charset: utf-8
Host: en.wikipedia.org domain name
User-Agent: string
Cache-Control: comma delimited list of response directives:
- private || public, whether or not it should be cached
- no-cache, cache should revalidate this resource every time
expires
etag, response header, critical for caching and used for conditional requests (invoking 304s)
vary
pragma: no-cache
Location header is returned to ask a web browser to load a different web page (URL redirection) or sent with HTTP status codes 3xx

301 is a permanent redirect, when content is moved forever, browser should always request the new URL instead
302 is a temporary redirect, tell the browser that the content is temporarily located at a different location, but they should continue to attempt the original URL on later requests
304 not modified, the conditional GET request would have received 200 OK, but the condition was false.
- minimize the amount of information transfer to the client
  
  4xx Client error
416 request range not satisfiable

HTTP/2

2015 revision of the protocol, provided improvements to network efficiency and enabled real-time communication through long-lived connections