In client-server protocols, like HTTP, sessions consist of three phases:
As of HTTP/1.1, the connection is no longer closed after completing the third phase, and the client is now granted a further request: this means the second and third phases can now be performed any number of times.
In client-server protocols, it is the client which establishes the connection. Opening a connection in HTTP means initiating a connection in the underlying transport layer, usually this is TCP.
With TCP the default port, for an HTTP server on a computer, is port 80. Other ports can also be used, like 8000 or 8080. The URL of a page to fetch contains both the domain name, and the port number, though the latter can be omitted if it is 80. See Identifying resources on the Web for more details.
FetchAPIs, using the HTML WebSockets API, or similar protocols.
Once the connection is established, the user-agent can send the request (a user-agent is typically a web browser, but can be anything else, a crawler, for example). A client request consists of text directives, separated by CRLF (carriage return, followed by line feed), divided into three blocks:
Fetching the root page of developer.mozilla.org, i.e. http://developer.mozilla.org/, and telling the server that the user-agent would prefer the page in French, if possible:
GET / HTTP/1.1 Host: developer.mozilla.org Accept-Language: fr
Observe that final empty line, this separates the data block from the header block. As there is no
Content-Length provided in an HTTP header, this data block is presented empty, marking the end of the headers, allowing the server to process the request the moment it receives this empty line.
For example, sending the result of a form:
POST /contact_form.php HTTP/1.1 Host: developer.mozilla.org Content-Length: 64 Content-Type: application/x-www-form-urlencoded name=Joe%20User&request=Send%20me%20one%20of%20your%20catalogue
HTTP defines a set of request methods indicating the desired action to be performed upon a resource. Although they can also be nouns, these requests methods are sometimes referred as HTTP verbs. The most common requests are
GETmethod requests a data representation of the specified resource. Requests using
GETshould only retrieve data.
POSTmethod sends data to a server so it may change its state. This is the method often used for HTML Forms.
After the connected agent has sent its request, the web server processes it, and ultimately returns a response. Similar to a client request, a server response is formed of text directives, separated by CRLF, though divided into three blocks:
Successful web page response:
HTTP/1.1 200 OK Date: Sat, 09 Oct 2010 14:28:02 GMT Server: Apache Last-Modified: Tue, 01 Dec 2009 20:18:22 GMT ETag: "51142bc1-7449-479b075b2891b" Accept-Ranges: bytes Content-Length: 29769 Content-Type: text/html <!DOCTYPE html... (here comes the 29769 bytes of the requested web page)
Notification that the requested resource has permanently moved:
HTTP/1.1 301 Moved Permanently Server: Apache/2.2.3 (Red Hat) Content-Type: text/html; charset=iso-8859-1 Date: Sat, 09 Oct 2010 14:30:24 GMT Location: https://developer.mozilla.org/ (this is the new link to the resource; it is expected that the user-agent will fetch it) Keep-Alive: timeout=15, max=98 Accept-Ranges: bytes Via: Moz-Cache-zlb05 Connection: Keep-Alive X-Cache-Info: caching X-Cache-Info: caching Content-Length: 325 (the content contains a default page to display if the user-agent is not able to follow the link) <!DOCTYPE HTML PUBLIC "-//IETF//DTD HTML 2.0//EN"> <html><head> <title>301 Moved Permanently</title> </head><body> <h1>Moved Permanently</h1> <p>The document has moved <a href="https://developer.mozilla.org/">here</a>.</p> <hr> <address>Apache/2.2.3 (Red Hat) Server at developer.mozilla.org Port 80</address> </body></html>
Notification that the requested resource doesn't exist:
HTTP/1.1 404 Not Found Date: Sat, 09 Oct 2010 14:33:02 GMT Server: Apache Last-Modified: Tue, 01 May 2007 14:24:39 GMT ETag: "499fd34e-29ec-42f695ca96761;48fe7523cfcc1" Accept-Ranges: bytes Content-Length: 10732 Content-Type: text/html <!DOCTYPE html... (contains a site-customized page helping the user to find the missing resource)
HTTP response status codes indicate if a specific HTTP request has been successfully completed. Responses are grouped into five classes: informational responses, successful responses, redirects, client errors, and servers errors.
200: OK. The request has succeeded.
301: Moved Permanently. This response code means that the URI of requested resource has been changed.
404: Not Found. The server cannot find the requested resource.
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