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Hypertext Transfer Protocol (HTTP) เป็นโปรโตคอลในระดับชั้นแอพพลิเคชัน(application-layer) ที่ใช้สำหรับการจัดส่งเอกสารประเภท hypermedia อย่างเช่นเอกสาร HTML และถูกออกแบบมาเพื่อใช้งานในการติดต่อสื่อสารระหว่าง web browser กับ web server และสามารถนำไปใช้ในจุดประสงค์อื่น ๆ ได้ด้วย ซึ่งรูปแบบการสื่อสารของโปรโตคอล HTTP เป็นไปตามโมเดลการสื่อสารแบบเครื่องลูกข่าย-แม่ข่าย(client-server model) ที่เครื่องลูกข่ายจะสร้าง connection เพื่อส่ง request ไปยังเครื่องแม่ข่าย และรอจนกว่าจะได้ response กลับมาจากเครื่องแม่ข่าย นอกจากนั้น HTTP เป็นโปรโตคอลแบบ stateless protocol เป็นลักษณะที่เครื่องแม่ข่ายไม่มีการจัดเก็บข้อมูล(state) ใด ๆ ที่เกิดขึ้นระหว่าง request เพราะข้อมูลแต่ละ request สามารถทำความเข้าใจได้ในตัวมันเอง และเนื่องจาก HTTP เป็นโปรโตคอลที่ใช้งานในระดับชั้นแอพพลิเคชัน ซึ่งอยู่ในระดับชั้นที่สูงกว่าโปรโตคอลในระดับล่างอย่าง TCP/IP ที่อยู่ในระดับ transport layer มันจึงสามารถใช้รูปแบบการติดต่อสื่อสารที่มีความน่าเชื่อถือของโปรโตคอลต่าง ๆ ในระดับชั้น transport ได้ อย่างเช่น อาจจะเลือกใช้โปรโตคอล RUDP ที่มีความน่าเชื่อถือกว่า UDP ที่มีโอกาสสูญเสียข้อมูลไปโดยไม่รู้ตัวได้
http://domaina.example) such as an HTML web page, makes a request for a resource on Domain B (http://domainb.foo), such as an image, using the img element (http://domainb.foo/image.jpg). This occurs very commonly on the web today — pages load a number of resources in a cross-site manner, including CSS stylesheets, images and scripts, and other resources.Since its original conception, as a protocol with one single method (GET) and returning only HTML pages, the HTTP protocol went through several revisions. The first documented version was HTTP/0.9 in 1991, corresponding to the original version. Very simple, it has a rudimentary search capability via the HTML {{ HTMLElement("isindex") }} element and an extension of the URL using the '?' character.
Then, in 1992, a version was published that became, with some minor changes, HTTP/1.0 (finalized in RFC 1945 in May 1996). One major improvement over the previous version was the ability to transmit files of different types, like images, videos, scripts, CSS documents, and so on, instead of only HTML files: this is achieved by using MIME types in conjunction with the Content-Type: header.
In 1995, the IETF began developing a new version of HTTP, which would become HTTP/1.1. It quickly spread into wide usage, and it was officially standardized in 1997 in RFC 2068, with minor fixes in RFC 2616 two years later.
HTTP/1.1 brought the ability to reuse established connections for subsequent requests, greatly improving the performance of the protocol by lowering the latency between them; this is especially useful with complex HTML documents that need to fetch several subsequent files, like images or style sheets. It also brought the Host: header, which allows a single server, listening on a specific port, to receive requests for several websites; this paved the way for colocating numerous websites on one single server, greatly reducing the cost of hosting.
Since then, the HTTP protocol evolved by adding new headers, defining new behaviors without the need to fundamentally change the protocol. Unknown headers are simply ignored by servers or clients.
HTTP/1.1 is currently being revised by the IETF HTTPbis Working Group.
The request method indicates the action to be performed by the server. The HTTP/1.1 standard defines seven methods and allows other methods to be added later. Over the years, a few ones have been added in standards like WebDAV. The IETF HTTPbis Working Group is currently working on an IANA registry to list them all. If a server receives a request method that it doesn't know, it must return a 501 Not implemented response; if it knows the method but is configured not to answer it, it must return a 405 Method not allowed response. Two methods are required to be supported: HEAD and GET; all others are optional.
Two specific semantics are defined in the standard and are crucial for web developers: the safety property and the idempotent property.
A safe method is a method that doesn't have any side-effects on the server. In other words, this property means that the method must be used only for retrieval of data. The safe HTTP methods defined in HTTP/1.1 are:
An idempotent method is a method such that the side-effects on the server of several identical requests with the method are the same as the side-effects of one single request.
Many more methods, such as PROPFIND or PATCH are defined in other standards-track RFCs of the IETF, like WebDAV.
The CONNECT method is defined in RFC 2817.
In HTML, different HTTP request methods can be specified in the {{ htmlattrxref("method", "form") }} attribute of the {{ HTMLElement("form") }} element, but also to the {{ htmlattrxref("formmethod", "input") }} of the {{ HTMLElement("input") }} and {{ HTMLElement("button") }} elements. But not all HTTP methods can be used with these attributes; only GET and POST method are allowed by the HTML specification. See this StackExchange answer why other HTTP request methods are not allowed by the HTML specification.
When answering a client request, the server sends back a three-digit number indicating whether the request was successfully processed. These codes can be grouped in five categories:
1xx) are provisional responses. Most of the time neither the end user, nor the web developer or webmaster should have to bother with these. The most common is the 100 Continue response, indicating that the client should continue to send its request.
2xx) are for successfully processed requests. The 200 OK response is by far the most common of these responses, but the 206 Partial Content is also often seen when fetching a file or some media data like video or audio.3xx) indicate that the resource that the client requested has moved and the server is not able to serve it directly. Most of these responses contain some location information telling where to find the requested resource; user-agents often then retrieve it without further user interaction. The most common responses of this type are 301 Moved Permanently, indicating that the URI given is no longer valid and has been moved to another place, and 302 Found which indicates that the resource has been temporarily moved to another place.
4xx) indicate that the request sent by the client is either invalid, incomplete, or doesn't have enough rights to be performed. The most common such response is 404 Not Found which is sent back when the URI requested doesn't exist, but a few others are often presented to the end user, like 400 Bad Request sent when the request isn't a valid HTTP request (as this shouldn't happen but may indicate a bug into the user agent or, less likely, the server) or 403 Forbidden, sent when the client request a resource that does exist but isn't allowed to be transmitted (like a directory content).5xx) indicate that the server had a problem handling the valid client request. The two most common such responses are 500 Internal Server Error, a generic error code indicating a bug in the server or 503 Service Unavailable indicating that the server cannot process the request due to a temporary problem, like a disabled service for maintenance purposes or the non-availability of a database.A web developer shouldn't encounter many other response codes, but people building requests using the XMLHTTPRequest function may hit less usual response codes.
Starting in Gecko 9.0 {{ geckoRelease("9.0") }}, redirections (such as 301 and 307) that specify a javascript: URI are no longer performed. Instead, a bad connection error is presented. This helps avoid cross-site scripting attacks. See {{ bug(255119) }} if you want more details.
HTTP headers allow the client and the server to pass additional information with the request or the response. A request header consists of its case-insensitive name followed by a colon ':', then by its value (without CRLF in it). Leading white space before the value is ignored.
Headers are grouped according the context in which they may appear:
Headers can also be grouped according to how caching and non-caching proxies handle them:
In order to learn about the specific semantic of each header, see its entry in the comprehensive list of HTTP headers.
Among the numerous HTTP request headers, several are especially useful when set correctly. If you are building your own requests, by using XMLHTTPRequest or when writing an extension and sending custom HTTP requests via XPCOM, then it is important to ensure the presence of headers that are often set by browsers based on the preferences of the user.
XMLHTTPRequest:, it is a good idea to use the {{ httpheader("If-Modified-Since") }} header (and other similar ones) in order to fetch the new content only if it has changed. This approach lowers the burden on the network.The configuration of a web server is a critical part to ensure good performance and optimal security of a web site. Among the numerous HTTP response headers, several are of specific importance and should be configured on the server
Several cross-site scripting (XSS) attacks take advantage of the ability to put third-party content inside an {{ HTMLElement("frame") }} or {{ HTMLElement("iframe") }}. In order to mitigate that risk, modern browsers have introduced the CSP frame-ancestors directive. By setting it with the value 'none', it prevents the browser from displaying this resource inside of a frame. Using it on critical resources (like those containing a formularies or critical information) will reduce the risk caused by XSS attacks. Note that this specific HTTP response header is not the only way to mitigate XSS risks; other techniques, like setting some Content Security Policies, may be helpful too.
Minimizing the amount of data transferred accelerates the display of a web page. Though most techniques, like CSS Sprites, should be applied on the site itself, compression of data must be set at the web server level. If set, resources requested by the client with an {{ httpheader("Accept-Encoding") }} request header are compressed using the appropriate method and sent back with a {{ httpheader("Content-Encoding") }} response header. Setting these in Apache 2 servers is done by using the mod_deflate module.
HTTP Caching is a technique that prevents the same resource from being fetched several times if it hasn't change. Configuring the server with the correct response headers allows the user-agent to adequately cache the data. In order to do that, be sure that:
ExpiresDefault "access plus 1 month".The MIME type is the mechanism to tell the client the kind of document transmitted: the extension of a file name has no meaning on the web. It is therefore important that the server is correctly set up so that the correct MIME type is transmitted with each document: user-agents often use this MIME-type to determine what default action to do when a resource is fetched.
AddType type directive like AddType image/jpeg jpg.application/octet-stream MIME type; for security reasons, most browsers, like Firefox, do not allow setting a custom default action for such resources and force the user to store it to disk in order to use it. Some common cases of often incorrectly configured servers happens for the following file types:
Rar-encoded files. The ideal would be to be able to set the real type of the encoded files; this often is not possible (as it may not be known to the server and these files may contains several resource of different types). In that case, configure the server to send the application/x-rar-compressed MIME type or some users won't be able to define a useful default action for them.
Audio and video files. Only resources with the proper MIME Type will be recognized and played, using a {{ HTMLElement("video") }} or {{ HTMLElement("audio") }} elements. Be sure to use the correct MIME type for audio and video resources.
Proprietary file types. Pay special attention when serving a proprietary file type. Be sure not to forget to add an x-prefixed type for it; otherwise, special handling won't be possible. This is especially true with resources using the Keyhole Markup Language, which should be served as application/vnd.google-earth.kml+xml for an optimal user experience.
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