5 Request
A request message from a client to a server includes, within the first line of that message, the method to be applied to the resource, the identifier of the resource, and the protocol version in use.

          Request       = Request-Line           ; Section 5.1
                          *(( general-header     ; Section 4.5
                        | request-header         ; Section 5.3
                        | entity-header ) CRLF)  ; Section 7.1
                          CRLF
                          [ message-body ]       ; Section 4.3

5.1 Request-Line
The Request-Line begins with a method token, followed by the Request-URI and the protocol version, and ending with CRLF. The elements are separated by SP characters. No CR or LF is allowed except in the final CRLF sequence.

          Request-Line   = Method SP Request-URI SP HTTP-Version CRLF

5.1.1 Method
The Method token indicates the method to be performed on the resource identified by the Request-URI. The method is case-sensitive.

         Method         = "OPTIONS"                ; Section 9.2
                        | "GET"                    ; Section 9.3
                        | "HEAD"                   ; Section 9.4
                        | "POST"                   ; Section 9.5
                        | "PUT"                    ; Section 9.6
                        | "DELETE"                 ; Section 9.7
                        | "TRACE"                  ; Section 9.8
                        | "CONNECT"                ; Section 9.9
                        | extension-method
         extension-method = token

The list of methods allowed by a resource can be specified in an Allow header field (section 14.7). The return code of the response always notifies the client whether a method is currently allowed on a resource, since the set of allowed methods can change dynamically. An origin server SHOULD return the status code 405 (Method Not Allowed) if the method is known by the origin server but not allowed for the requested resource, and 501 (Not Implemented) if the method is unrecognized or not implemented by the origin server. The methods GET and HEAD MUST be supported by all general-purpose servers. All other methods are OPTIONAL; however, if the above methods are implemented, they MUST be implemented with the same semantics as those specified in section 9.5.1.2 Request-URI The Request-URI is a Uniform Resource Identifier (section 3.2) and identifies the resource upon which to apply the request.

         Request-URI    = "*" | absoluteURI | abs_path | authority

The four options for Request-URI are dependent on the nature of the request. The asterisk "*" means that the request does not apply to a particular resource, but to the server itself, and is only allowed when the method used does not necessarily apply to a resource. One example would be

         OPTIONS * HTTP/1.1

The absoluteURI form is REQUIRED when the request is being made to a proxy. The proxy is requested to forward the request or service it from a valid cache, and return the response. Note that the proxy MAY forward the request on to another proxy or directly to the server specified by the absoluteURI. In order to avoid request loops, a proxy MUST be able to recognize all of its server names, including any aliases, local variations, and the numeric IP address. An example Request-Line would be:

         GET http://www.w3.org/pub/WWW/TheProject.html HTTP/1.1

To allow for transition to absoluteURIs in all requests in future versions of HTTP, all HTTP/1.1 servers MUST accept the absoluteURI form in requests, even though HTTP/1.1 clients will only generate them in requests to proxies. The authority form is only used by the CONNECT method (section 9.9). The most common form of Request-URI is that used to identify a resource on an origin server or gateway. In this case the absolute path of the URI MUST be transmitted (see section 3.2.1, abs_path) as the Request-URI, and the network location of the URI (authority) MUST be transmitted in a Host header field. For example, a client wishing to retrieve the resource above directly from the origin server would create a TCP connection to port 80 of the host "www.w3.org" and send the lines:

         GET /pub/WWW/TheProject.html HTTP/1.1
         Host: www.w3.org

followed by the remainder of the Request. Note that the absolute path cannot be empty; if none is present in the original URI, it MUST be given as "/" (the server root). The Request-URI is transmitted in the format specified in section 3.2.1. If the Request-URI is encoded using the "% HEX HEX" encoding [42], the origin server MUST decode the Request-URI in order to properly interpret the request. Servers SHOULD respond to invalid Request-URIs with an appropriate status code. A transparent proxy MUST NOT rewrite the "abs_path" part of the received Request-URI when forwarding it to the next inbound server, except as noted above to replace a null abs_path with "/".

Note: The "no rewrite" rule prevents the proxy from changing the meaning of the request when the origin server is improperly using a non-reserved URI character for a reserved purpose. Implementors should be aware that some pre-HTTP/1.1 proxies have been known to rewrite the Request-URI.

5.2 The Resource Identified by a Request
The exact resource identified by an Internet request is determined by examining both the Request-URI and the Host header field. An origin server that does not allow resources to differ by the requested host MAY ignore the Host header field value when determining the resource identified by an HTTP/1.1 request. (But see section 19.6.1.1 for other requirements on Host support in HTTP/1.1.) An origin server that does differentiate resources based on the host requested (sometimes referred to as virtual hosts or vanity host names) MUST use the following rules for determining the requested resource on an HTTP/1.1 request:

1. If Request-URI is an absoluteURI, the host is part of the Request-URI. Any Host header field value in the request MUST be ignored.
2. If the Request-URI is not an absoluteURI, and the request includes a Host header field, the host is determined by the Host header field value.
3. If the host as determined by rule 1 or 2 is not a valid host on the server, the response MUST be a 400(Bad Request) error message.

Recipients of an HTTP/1.0 request that lacks a Host header field MAY attempt to use heuristics (e.g., examination of the URI path for something unique to a particular host) in order to determine what exact resource is being requested.

5.3 Request Header Fields
The request-header fields allow the client to pass additional information about the request, and about the client itself, to the server. These fields act as request modifiers, with semantics equivalent to the parameters on a programming language method invocation.

         request-header = Accept                   ; Section 14.1
                        | Accept-Charset           ; Section 14.2
                        | Accept-Encoding          ; Section 14.3
                        | Accept-Language          ; Section 14.4
                        | Authorization            ; Section 14.8
                        | Expect                   ; Section 14.20
                        | From                     ; Section 14.22
                        | Host                     ; Section 14.23
                        | If-Match                 ; Section 14.24
                        | If-Modified-Since        ; Section 14.25
                        | If-None-Match            ; Section 14.26
                        | If-Range                 ; Section 14.27
                        | If-Unmodified-Since      ; Section 14.28
                        | Max-Forwards             ; Section 14.31
                        | Proxy-Authorization      ; Section 14.34
                        | Range                    ; Section 14.35
                        | Referer                  ; Section 14.36
                        | TE                       ; Section 14.39
                        | User-Agent               ; Section 14.43

Request-header field names can be extended reliably only in combination with a change in the protocol version. However, new or experimental header fields MAY be given the semantics of request- header fields if all parties in the communication recognize them to be request-header fields. Unrecognized header fields are treated as entity-header fields.