This Article will describe the input / output (Request / Response) message formats.
Key Concept: HTTP requests are the means by which HTTP clients ask servers to take a particular type of action, such as sending a file or processing user input. Each request message begins with a request line, which contains three critical pieces of information: the method (type of action) the client is requesting; the URI of the resource upon which the client wishes the action to be performed, and the version of HTTP that the client is using. After the request line come a set of message headers related to the request, followed by a blank line and then optionally, the message body of the request.
The client initiates an HTTP session by opening a TCP connection to the HTTP server with which it wishes to communicate. It then sends request messages to the server, each of which specifies a particular type of action that the user of the HTTP client would like the server to take. Requests can be generated either by specific user action (such as clicking a hyperlink in a Web browser) or indirectly as a result of a prior action (such as a reference to an inline image in an HTML document leading to a request for that image.)
HTTP requests use a message format that is based on the generic message format described in the preceding topic, but specific to the needs of requests. The structure of this format is as follows:
This diagram shows the structural elements of an HTTP request and an example of the sorts of headers a request might contain. Like most HTTP requests, this one carries no entity, so there are no entity headers and the message body is empty.
The generic start line that begins all HTTP messages is called a request line in request messages. It has a three-fold purpose: to indicate the command or action that the client wants performed; to specify a resource upon which the action should be taken; and to indicate to the server what version of HTTP the client is using. The formal syntax for the request line is:
The method is simply the type of action that the client wants the server to take; it is always specified in upper case letters. There are eight standard methods defined in HTTP/1.1, of which three are widely used: GET, HEAD and POST. They are called “methods” rather than “commands” because the HTTP standard uses terminology from object-oriented programming. I explain this and also describe the methods themselves in the topic describing HTTP methods.
The request URI is the uniform resource identifier of the resource to which the request applies. While URIs can theoretically refer to either uniform resource locators (URLs) or uniform resource names (URNs), at the present time a URI is almost always an HTTP URL that follows the standard syntax rules of Web URLs.
Interestingly, the exact form of the URL used in the HTTP request line usually differs from that used in HTML documents or entered by users. This is because some of the information in a full URL is used to control HTTP itself. It is needed as part of the communication between the user and the HTTP client, but not in the request from the client to the server. The standard method of specifying a resource in a request is to include the path and file name in the request line (as well as any optional query information) while specifying the host in the special Host header that must be used in HTTP/1.1 requests.
For example, suppose the user enters a URL such as this:
We obviously don't need to send the “http:” to the server. The client would take the remaining information and split it so the URI was specified as “/chatware/chatroom.php” and the Host line would contain “www.myfavoritewebsite.com:8080”. Thus, the start of the request would look like this:
GET /chatware/chatroom.php HTTP/1.1
The exception to this rule is when a request is being made to a proxy server. In that case, the request is made using the full URL in its original form, so that it can be processed by the proxy just as the original client did. The request would be:
GET http://www.myfavoritewebsite.com:8080/chatware/chatroom.php HTTP/1.1
Finally, there is one special case where a single asterisk can be used instead of a real URL. This is for the OPTIONS method, which does not require the specification of a resource. (Nominally, the asterisk means the method refers to the server itself.)
The HTTP-VERSION element tells the server what version the client is using so the server knows how to interpret the request, and what to send and not to send the client in its response. For example, a server receiving a request from a client using versions 0.9 or 1.0 will assume that a transitory connection is being used rather than a persistent one, and will avoid using version 1.1 headers in its reply. The version token is sent in upper case as “HTTP/0.9”, “HTTP/1.0” or “HTTP/1.1”—just the way I've been doing throughout my discussion of the protocol.
After the request line come any of the headers that the client wants to include in the message; it is in these headers that details are provided to the server about the request. The headers all use the same structure, but are organized into categories based on the functions they serve, and whether they are specific to one kind of message or not
General Headers: General headers refer mainly to the message itself, as opposed to its contents, and are used to control its processing or provide the recipient with extra information. They are not particular to either request or response messages, so they can appear in either. They are likewise not specifically relevant to any entity the message may be carrying.
* Request Headers: These headers convey to the server more details about the nature of the client's request, and give the client more control over how the request is handled. For example, special request headers can be used by the client to specify a conditional request—one that is only filled if certain criteria are met. Others can tell the server which formats or encodings the client is able to process in a response message.
* Entity Headers: These are headers that describe the entity contained in the body of the request, if any.
Request headers are obviously used only in request messages, but both general headers and entity headers can appear in either a request or a response message. Since there are so many headers and most are not particular to one message type, they are described in detail in their own complete section.
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