HTTP messages consist of requests from client to server and responses from server to client.
HTTP-message = Simple-Request ; HTTP/0.9 messages | Simple-Response | Full-Request ; HTTP/1.0 messages | Full-Response
Full-Request and Full-Response use the generic message format of RFC 822  for transferring entities. Both messages may include optional header fields (also known as "headers") and an entity body. The entity body is separated from the headers by a null line (i.e., a line with nothing preceding the CRLF).
Full-Request = Request-Line ; Section 5.1 *( General-Header ; Section 4.3 | Request-Header ; Section 5.2 | Entity-Header ) ; Section 7.1 CRLF [ Entity-Body ] ; Section 7.2 Full-Response = Status-Line ; Section 6.1 *( General-Header ; Section 4.3 | Response-Header ; Section 6.2 | Entity-Header ) ; Section 7.1 CRLF [ Entity-Body ] ; Section 7.2
Simple-Request and Simple-Response do not allow the use of any header information and are limited to a single request method (GET).
Simple-Request = "GET" SP Request-URI CRLF Simple-Response = [ Entity-Body ]
Use of the Simple-Request format is discouraged because it prevents the server from identifying the media type of the returned entity.