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To allow the clients and servers to control the utilization of these persistent TCP connections, HTTP 1.1 :rfc:`2616` defines several new HTTP headers:
The `Connection:` header is used with the `Keep-Alive` argument by the client to indicate that it expects the underlying TCP connection to be persistent. When this header is used with the `Close` argument, it indicates that the entity that sent it will close the underlying TCP connection at the end of the HTTP response.
The `Keep-Alive:` header is used by the server to inform the client about how it agrees to use the persistent connection. A typical `Keep-Alive:` contains two parameters: the maximum number of requests that the server agrees to serve on the underlying TCP connection and the timeout (in seconds) after which the server will close an idle connection
The example below shows the operation of HTTP/1.1 over a persistent TCP connection to retrieve three URIs stored on the same server. Once the connection has been established, the client sends its first request with the `Connection: Keep-Alive` header to request a persistent connection.
The server replies with the `Connection: Keep-Alive` header and indicates that it accepts a maximum of 100 HTTP requests over this connection and that it will close the connection if it remains idle for 15 seconds.
The client sends a second request for the style sheet of the retrieved web page.
The server replies with the requested style sheet and maintains the persistent connection. Note that the server only accepts 99 remaining HTTP requests over this persistent connection.
Then the client requested the web server's icon [#ffavicon]_. This server does not contain such an icon and thus replies with a `404` HTTP status. However, the underlying TCP connection is not closed immediately.
As illustrated above, a client can send several HTTP requests over the same persistent TCP connection. However, it is important to note that all of these HTTP requests are considered to be independent by the server. Each HTTP request must be self-contained. This implies that each request must include all the header lines that are required by the server to understand the request. The independence of these requests is one of the key design choices of HTTP. As a consequence of this design choice, when a server processes a HTTP request, it does not use any other information than what is contained in the request itself. This explains why the client adds its `User-Agent:` header in all of the HTTP requests that it sends over the persistent TCP connection.
However, in practice, some servers want to provide content tuned for each user. For example, some servers can provide information in several languages. Other servers want to provide advertisements that are targeted to different types of users. To do this, servers need to maintain some information about the preferences of each user and use this information to produce content matching the user's preferences. HTTP contains several mechanisms to solve this problem. We discuss three of them below.
A first solution is to force the users to be authenticated. This was the solution used by `FTP` to control the files that each user could access. Initially, user names and passwords could be included inside URIs :rfc:`1738`. However, placing passwords in the clear in a potentially publicly visible URI is completely insecure and this usage has now been deprecated :rfc:`3986`. HTTP supports several extension headers :rfc:`2617` that can be used by a server to request the authentication of the client by providing his/her credentials. However, user names and passwords have not been popular on web servers as they force human users to remember one user name and one password per server. Remembering a password is acceptable when a user needs to access protected content, but users will not accept to remember a unique user name and password for each web sites that they visit.
A second solution to allow servers to tune that content to the needs and capabilities of the user is to rely on the different types of `Accept-*` HTTP headers. For example, the `Accept-Language:` header can be used by the client to indicate its preferred languages. Unfortunately, in practice this header is usually set based on the default language of the browser and it is difficult for a user to indicate the language it prefers by selecting options for each visited web server.
The third and widely adopted solution are HTTP cookies. HTTP cookies were initially developed as a private extension by Netscape_. They are now part of the standard :rfc:`6265`. In a nutshell, a cookie is a short string that is chosen by a server to represent a given client. Two HTTP headers are used : `Cookie:` and `Set-Cookie:`. When a server receives an HTTP request from a new client (i.e. an HTTP request that does not contain the `Cookie:` header), it generates a cookie for the client and includes it in the `Set-Cookie:` header of the returned HTTP response. The `Set-Cookie:` header contains several additional parameters including the domain names for which the cookie is valid. The client stores all received cookies on disk and every time it sends an HTTP request, it verifies whether it already knows a cookie for this domain. If so, it attaches the `Cookie:` header to the HTTP request. This is illustrated in the figure below with HTTP 1.1, but cookies also work with HTTP 1.0.
Privacy issues with HTTP cookies
The HTTP cookies introduced by Netscape_ are key for large e-commerce websites. However, they have also raised many discussions concerning their `potential misuses <http://www.nytimes.com/2001/09/04/technology/04COOK.html>`_. Consider `ad.com`, a company that delivers lots of advertisements on web sites. A web site that wishes to include `ad.com`'s advertisements next to its content will add links to `ad.com` inside its HTML pages. If `ad.com` is used by many web sites, `ad.com` could be able to track the interests of all the users that visit its client websites and use this information to provide targeted advertisements. Privacy advocates have even `sued <http://epic.org/privacy/internet/cookies/>`_ online advertisement companies to force them to comply with the privacy regulations. More recent related technologies also raise `privacy concerns <http://www.eff.org/deeplinks/2009/09/new-cookie-technologies-harder-see-and-remove-wide>`_.
Footnotes
An example of a non-retrievable URI is `urn:isbn:0-380-81593-1` which is an unique identifier for a book, through the urn scheme (see :rfc:`3187`). Of course, any URI can be made retrievable via a dedicated server or a new protocol but this one has no explicit protocol. Same thing for the scheme tag (see :rfc:`4151`), often used in Web syndication (see :rfc:`4287` about the Atom syndication format). Even when the scheme is retrievable (for instance with `http`), it is often used only as an identifier, not as a way to get a resource. See http://norman.walsh.name/2006/07/25/namesAndAddresses for a good explanation.
The list of standard URI schemes is maintained by IANA_ at http://www.iana.org/assignments/uri-schemes.html
Favorite icons are small icons that are used to represent web servers in the toolbar of Internet browsers. Microsoft added this feature in their browsers without taking into account the W3C standards. See http://www.w3.org/2005/10/howto-favicon for a discussion on how to cleanly support such favorite icons.

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This translation Propagated Empty cnp3-ebook/protocols/http
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Propagated Translated cnp3-ebook/exercises/sockets Notes de pied de page
Propagated Translated cnp3-ebook/exercises/transport Notes de pied de page
Propagated Translated cnp3-ebook/principles/naming Notes de pied de page
Propagated Translated cnp3-ebook/principles/network Notes de pied de page
Propagated Translated cnp3-ebook/principles/referencemodels Notes de pied de page
Propagated Translated cnp3-ebook/protocols/dns Notes de pied de page
Propagated Translated cnp3-ebook/principles/security Notes de pied de page
Propagated Translated cnp3-ebook/exercises/dns Notes de pied de page
Propagated Translated cnp3-ebook/exercises/email Notes de pied de page
Propagated Translated cnp3-ebook/exercises/http Notes de pied de page
Propagated Translated cnp3-ebook/exercises/tcp-2 Notes de pied de page
Propagated Translated cnp3-ebook/exercises/tcp Notes de pied de page
Propagated Translated cnp3-ebook/exercises/tls Notes de pied de page
Propagated Empty cnp3-ebook/protocols/bgp
Propagated Empty cnp3-ebook/protocols/congestion
Propagated Empty cnp3-ebook/protocols/dnssec
Propagated Empty cnp3-ebook/protocols/email
Propagated Empty cnp3-ebook/protocols/ethernet
Propagated Empty cnp3-ebook/protocols/http2

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Source string location
../../protocols/http.rst:443
String age
a year ago
Source string age
a year ago
Translation file
locale/fr/LC_MESSAGES/protocols/http.pot, string 79