A full HTTP request
message is build on these blocks:
[method] [URL]
[version] ←first section
[headers] ← middle
section
[body] ← last
section
The message is
written in ASCII text. The first section always
consists of the method, the URL, and the HTTP version (commonly it is
HTTP 1.1 which exists since 1999. Recently HTTP 2 is released but
still in beta state at time this post was created). The last
section, the body section, can contain various data
depending on the request, e. g. it can contain user log-in parameters
or, if you want to upload file, a large amount of file data. The
middle section contains at least the host HTTP header.
The header contains
information that help server process the request, e. g. content
negotiation. Let's say the client prefers to accept resources in
French, then the header entry would contain the value “fr-FR”
that would be assigned to the header “Accept-Language”. It would
look like this:
GET
http://foo.com/Articles/741.aspx HTTP/1.1
Host: foo.com
Accept-Language:
fr-FR
The specified value
in the accept header doesn't garantuee that the client will receive
the resource in the language he prefers though. For example if the
server only offers a site in English, then the client will receive a
site in English even though the client prefers French.
All of the headers
must comply the HTML specification. Other commonly used headers are
shown below:
| Header | Description |
| Referer | Here the client sends the URL of the referring page to the server. This is usually used when the user clicks on a link and you want to know from which site the link was originally clicked on. |
| User-Agent | This header contains information about the user agent (client, browser, software making the request). For example what browser, what version, etc. |
| Accept | This header tells the server what the media types the browser prefers to accept. You usually need this for content negotiation. |
| Accept- Language | This header describes the language the user agent prefers to accept. |
| If-Modified-Since | Contains the date the user agent last downloaded and also cached the resource from the server. Knowing this information the server will only send the requested resource it was modified since that date. |
The header listed
above are just a sample of possible headers. One header can contain
multiple values. An example of an HTTP request is shown below:
GET http://foo.com/
HTTP/1.1
Host: foo.com
Connection:
keep-alive
User-Agent:
Mozilla/5.0 (Windows NT 6.1; WOW64) Chrome/16.0.912.75
Safari/535.7
Accept:
text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8
Referer:
http://www.google.com/url?&q=foo
Accept-Encoding:
gzip,deflate,sdch
Accept-Language:
en-US,en;q=0.8
Accept-Charset:
ISO-8859-1,utf-8;q=0.7,*;q=0.3
The multiple values
in one header attribute describes the resources the client is willing
to accept. For example the “Accept” header indicates that it
accepts HTML, XHMTL+XML, and XML, but also everything else, i.e.
“*/*”. The key “q” says the rate of preferability (aka.
“quality value”, “relative degree of preference”). It is
always between 0.0 to 1.0. The higher the number the number the
higher the level of preferability. The default value is 1.0.
Source(s):
HTTP Succinctly by Scott Allen Syncfusion
Wikipedia
