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A simple HTML page
Over the years, various extensions to HTML have been proposed and implemented. These include the specification of style sheets that adjust the layout of the document and the possibility of adding or referencing javascript code. Additional details about the various extensions to HTML may be found in the `official specifications <http://www.w3.org/MarkUp/>`_ maintained by W3C_.
The third component of the `world wide web` is the HyperText Transfer Protocol (HTTP). HTTP is a text-based protocol like SMTP. The client sends a request and the server returns a response. HTTP runs above the bytestream service and HTTP servers listen by default on port `80`. The design of HTTP has largely been inspired by the Internet email protocols. Each HTTP request contains three parts :
a `method`, that indicates the type of request, a URI, and the version of the HTTP protocol used by the client
a `header`, that is used by the client to specify optional parameters for the request. An empty line is used to mark the end of the header
an optional MIME document attached to the request
The response sent by the server also contains three parts :
a `status line` , that indicates whether the request was successful or not
a `header`, that contains additional information about the response. The response header ends with an empty line.
a MIME document
Several types of method can be used in HTTP requests. The three most important ones are :
the `GET` method is the most popular one. It is used to retrieve a document from a server. The `GET` method is encoded as `GET` followed by the path of the URI of the requested document and the version of HTTP used by the client. For example, to retrieve the http://www.w3.org/MarkUp/ URI, a client must open a TCP connection on port `80` with host `www.w3.org` and send a HTTP request containing the following line:
the `HEAD` method is a variant of the `GET` method that allows the retrieval of the header lines for a given URI without retrieving the entire document. It can be used by a client to verify if a document exists, for instance.
the `POST` method can be used by a client to send a document to a server. The document is attached to the HTTP request as a MIME document.
HTTP clients and servers can include different HTTP headers in HTTP requests and responses. Each HTTP header is encoded as a single ASCII-line terminated by `CR` and `LF`. Several of these headers are briefly described below. A detailed discussion of the standard headers may be found in :rfc:`1945`. The MIME headers can appear in both HTTP requests and HTTP responses.
the `Content-Length:` header is the :term:`MIME` header that indicates the length of the MIME document in bytes.
the `Content-Type:` header is the :term:`MIME` header that indicates the type of the attached MIME document. HTML pages use the `text/html` type.
the `Content-Encoding:` header indicates how the :term:`MIME document` has been encoded. For example, this header would be set to `x-gzip` for a document compressed using the gzip_ software.
:rfc:`1945` and :rfc:`2616` define headers that are specific to HTTP responses. These server headers include:
the `Server:` header indicates the version of the web server that has generated the HTTP response. Some servers provide information about their software release and optional modules that they use. For security reasons, some system administrators disable these headers to avoid revealing too much information about their server to potential attackers.
the `Date:` header indicates when the HTTP response has been produced by the server.
the `Last-Modified:` header indicates the date and time of the last modification of the document attached to the HTTP response.
Similarly, the following header lines can only appear inside HTTP requests sent by a client:
the `User-Agent:` header provides information about the client that has generated the HTTP request. Some servers analyze this header line and return different headers and sometimes different documents for different user agents.
the `If-Modified-Since:` header is followed by a date. It enables clients to cache in memory or on disk recent or most frequently used documents. When a client needs to request a URI from a server, it first checks whether the document is already in its cache. If it is, the client sends an HTTP request with the `If-Modified-Since:` header indicating the date of the cached document. The server will only return the document attached to the HTTP response if it is newer than the version stored in the client's cache.
the `Referrer:` header is followed by a URI. It indicates the URI of the document that the client visited before sending this HTTP request. Thanks to this header, the server can know the URI of the document containing the hyperlink followed by the client, if any. This information is very useful to measure the impact of advertisements containing hyperlinks placed on websites.
the `Host:` header contains the fully qualified domain name of the URI being requested.
The importance of the `Host:` header line
The first version of HTTP did not include the `Host:` header line. This was a severe limitation for web hosting companies. For example consider a web hosting company that wants to serve both `web.example.com` and `www.example.net` on the same physical server. Both web sites contain a `/index.html` document. When a client sends a request for either `http://web.example.com/index.html` or `http://www.example.net/index.html`, the HTTP 1.0 request contains the following line :
By parsing this line, a server cannot determine which `index.html` file is requested. Thanks to the `Host:` header line, the server knows whether the request is for `http://web.example.com/index.html` or `http://www.dummy.net/index.html`. Without the `Host:` header, this is impossible. The `Host:` header line allowed web hosting companies to develop their business by supporting a large number of independent web servers on the same physical server.
The status line of the HTTP response begins with the version of HTTP used by the server (usually `HTTP/1.0` defined in :rfc:`1945` or `HTTP/1.1` defined in :rfc:`2616`) followed by a three digit status code and additional information in English. HTTP status codes have a similar structure as the reply codes used by SMTP:

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read-only
Source string location
../../protocols/http.rst:158
String age
3 years ago
Source string age
3 years ago
Translation file
locale/pot/protocols/http.pot, string 38