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The HyperText Transfer Protocol
In the early days, the Internet was mainly used for remote terminal access with telnet_, email and file transfer. The default file transfer protocol, `FTP`, defined in :rfc:`959` was widely used. `FTP` clients and servers are still included in some operating systems.
Many `FTP` clients offered a user interface similar to a Unix shell and allowed clients to browse the file system on the server and to send and retrieve files. `FTP` servers can be configured in two modes :
`authenticated` : in this mode, the ftp server only accepts users with a valid user name and password. Once authenticated, they can access the files and directories according to their permissions
`anonymous` : in this mode, clients supply the `anonymous` user identifier and their email address as password. These clients are granted access to a special zone of the file system that only contains public files.
`FTP` was very popular in the 1990s and early 2000s, but today it has mostly been superseded by more recent protocols. Authenticated access to files is mainly done by using the Secure Shell (ssh_) protocol defined in :rfc:`4251` and supported by clients such as scp_ or sftp_. Nowadays, anonymous access is mainly provided by web protocols.
In the late 1980s, high energy physicists working at CERN_ had to efficiently exchange documents about their ongoing and planned experiments. `Tim Berners-Lee`_ evaluated several of the documents sharing techniques that were available at that time [B1989]_. As none of the existing solutions met CERN's requirements, they chose to develop a completely new document sharing system. This system was initially called the `mesh`. It was quickly renamed the `world wide web`. The starting point for the `world wide web` are hypertext documents. An hypertext document is a document that contains references (hyperlinks) to other documents that the reader can immediately access. Hypertext was not invented for the world wide web. The idea of hypertext documents was proposed in 1945 [Bush1945]_ and the first experiments were done during the 1960s [Nelson1965]_ [Myers1998]_ . Compared to the hypertext documents that were used in the late 1980s, the main innovation introduced by the `world wide web` was to allow hyperlinks to reference documents stored on different remote machines.
A document sharing system such as the `world wide web` is composed of three important parts.
A standardized addressing scheme that unambiguously identifies documents
A standard document format : the `HyperText Markup Language <>`_
A standardized protocol to efficiently retrieve the documents stored on a server
Open standards and open implementations
Open standards play a key role in the success of the `world wide web` as we know it today. Without open standards, the world wide web would have never reached its current size. In addition to open standards, another important factor for the success of the web was the availability of open and efficient implementations of these standards. When CERN started to work on the `web`, their objective was to build a running system that could be used by physicists. They developed open-source implementations of the `first web servers <>`_ and `web clients <>`_. These open-source implementations were powerful and could be used as is, by institutions willing to share information. They were also extended by other developers who contributed to new features. For example, the NCSA_ added support for images in their `Mosaic browser <>`_ that was eventually used to create `Netscape Communications <>`_ and the first commercial browsers and servers.
The first components of the `world wide web` are the Uniform Resource Identifiers (URI), defined in :rfc:`3986`. A URI is a character string that unambiguously identifies a resource on the world wide web. Here is a subset of the BNF for URIs ::
The first component of a URI is its `scheme`. A `scheme` can be seen as a selector, indicating the meaning of the fields after it. In practice, the scheme often identifies the application-layer protocol that must be used by the client to retrieve the document, but it is not always the case. Some schemes do not imply a protocol at all and some do not indicate a retrievable document [#furiretrieve]_. The most frequent schemes are `http` and `https`. We focus on `http` in this section. A URI scheme can be defined for almost any application layer protocol [#furilist]_. The characters `:` and `//` follow the `scheme` of any URI.
The second part of the URI is the `authority`. With retrievable URIs, this includes the DNS name or the IP address of the server where the document can be retrieved using the protocol specified via the `scheme`. This name can be preceded by some information about the user (e.g. a user name) who is requesting the information. Earlier definitions of the URI allowed the specification of a user name and a password before the `@` character (:rfc:`1738`), but this is now deprecated as placing a password inside a URI is insecure. The host name can be followed by the semicolon character and a port number. A default port number is defined for some protocols and the port number should only be included in the URI if a non-default port number is used (for other protocols, techniques like service DNS records can used).
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This translation Propagated Read only cnp3-ebook/protocols/http
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Propagated Read only cnp3-ebook/exercises/http


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locale/pot/protocols/http.pot, string 1