HTTP 101.

Hypertext Transfer Protocol

• It is a request-response protocol between a client and server.

HTTP Messages

HTTP-message = <request> | <response> ; HTTP/1.1 messages  

HTTP Requests and Responses

# HTTP requests and responses use the generic format 
# consists of the following four items.

* Start-line   = Request-Line | Status-Line
* Zero or more header fields followed by CRLF (return)
* An empty line (indicating the end of the header fields
* Optionally a message-body


# Example
Request-Line = "GET /hello.htm HTTP/1.1"
Status-Line  = "HTTP/1.1 200 OK"
  

HTTP Request Methods

• GET

/test/demo_form.asp?name1=value1&name2=value2

• GET requests can be cached 
• GET requests remain in the browser history 
• GET requests can be bookmarked 
• GET requests should never be used when dealing with sensitive data 
• GET requests have length restrictions 
• GET requests should be used only to retrieve data
  
• POST

POST /test/demo_form.asp HTTP/1.1
Host: w3schools.com
name1=value1&name2=value2

• POST requests are never cached 
• POST requests do not remain in the browser history 
• POST requests cannot be bookmarked 
• POST requests have no restrictions on data length
  
• HEAD
• PUT
• DELETE
• OPTIONS
• CONNECT

HTTP Message Header Fields

message-header = field-name ":" [ field-value ]

• General-header
• Request-header
• Response-header
• Entity-header

HTTP Message Body Fields (Optional)

• Content-Type and Content-Length headers lines specify the nature of the body associated.

HTTP Response/Status Line

* HTTP/1.0 200 OK
* HTTP/1.0 404 Not Found

* The status code is a three-digit integer
   - 1xx indicates an informational message only
   - 2xx indicates success of some kind
   - 3xx redirects the client to another URL
   - 4xx indicates an error on the client's part
   - 5xx indicates an error on the server's part