En HTTP, la négociation de contenu est le mécanisme utilisé pour fournir différentes représentations d'une ressource à la même URI, afin que l'agent utilisateur puisse spécifier celle qui convient le mieux à l'utilisateur (par exemple, la langue d'un document, le format d'image, ou l'encodage du contenu).
Un document spécifique s'appelle une ressource. Lorsqu'un client veut y accéder, il le demande en utilisant son URL. Le serveur utilise cette URL pour choisir une des différentes versions qu'il peut fournir - chaque version étant appelée une représentation - et renvoie cette représentation spécifique au client. La ressource globale, ainsi que chacune de ses représentations, ont une URL spécifique. La façon dont une représentation spécifique est choisie est déterminée par la négociation de contenu et il existe plusieurs façons de négocier entre le client et le serveur.
La sélection de la représentation la mieux adaptée se fait par l'un des deux mécanismes suivants:
Au fil des ans, d'autres propositions de négociation de contenu, comme la négociation transparente du contenu et l'en-tête Alternates, ont été proposées. Elles n'ont pas réussi à emporter l'adhésion et ont été abandonnées.
Dans la négociation de contenu gérée par le serveur, ou négociation proactive de contenu, le navigateur (ou tout autre type de client) envoie plusieurs en-têtes HTTP avec l'URL décrivant les choix préférés de l'utilisateur. Le serveur les utilise comme indications et un algorithme interne choisit le meilleur contenu à servir au client. L'algorithme est spécifique au serveur et n'est pas défini dans la norme. Voir, par exemple, l'algorithme de négociation d'Apache 2.2.
La norme HTTP/1.1 définit la liste des en-têtes standard qui initient la négociation pilotée par le serveur ({{HTTPHeader("Accept")}}, {{HTTPHeader("Accept-Charset")}}, {{HTTPHeader("Accept-Encoding")}}, {{HTTPHeader("Accept-Language")}}). Bien qu'à proprement parler {{HTTPHeader("User-Agent")}} ne figure pas dans la liste, il est aussi parfois utilisé pour envoyer une représentation spécifique de la ressource demandée, bien que cela ne soit pas considéré comme une bonne pratique. Le serveur utilise l'en-tête {{HTTPHeader("Vary")}} pour indiquer quels en-têtes il a effectivement utilisés pour la négociation de contenu (ou plus précisément les en-têtes de réponse associés), pour que les caches puissent fonctionner de manière optimale.
En outre, il existe une proposition expérimentale visant à ajouter d'autres en-têtes à la liste des en-têtes disponibles, appelés indications (hints) du client. Ces hints indiquent sur quel type de périphérique l'agent utilisateur fonctionne (par exemple, s'il s'agit d'un ordinateur de bureau ou d'un périphérique mobile).
Même si la négociation de contenu gérée par le serveur est le moyen le plus courant de s'entendre sur une représentation spécifique d'une ressource, elle présente plusieurs inconvénients:
Accept headerThe {{HTTPHeader("Accept")}} header lists the MIME types of media resources that the agent is willing to process. It is comma-separated lists of MIME types, each combined with a quality factor, a parameter indicating the relative degree of preference between the different MIME types.
The {{HTTPHeader("Accept")}} header is defined by the browser, or any other user-agent, and can vary according to the context, like fetching an HTML page or an image, a video, or a script: It is different when fetching a document entered in the address bar or an element linked via an {{ HTMLElement("img") }}, {{ HTMLElement("video") }} or {{ HTMLElement("audio") }} element. Browsers are free to use the value of the header that they think is the most adequate; an exhaustive list of default values for common browsers is available.
Accept-CH header {{experimental_inline}}Note : This is part of an experimental technology called Client Hints. Initial support is in Chrome 46 or later. The Device-Memory value is in Chrome 61 or later.
The experimental {{HTTPHeader("Accept-CH")}} lists configuration data that can be used by the server to select an appropriate response. Valid values are:
| Value | Meaning |
|---|---|
Device-Memory |
Indicates the approximate amount of device RAM. This value is an approximation given by rounding to the nearest power of 2 and dividing that number by 1024. For example, 512 megabytes will be reported as 0.5. |
DPR |
Indicates the client's device pixel ratio. |
Viewport-Width |
Indicates the layout viewport width in CSS pixels. |
Width |
Indicates the resource width in physical pixels (in other words the intrinsic size of an image). |
Accept-Charset headerThe {{HTTPHeader("Accept-Charset")}} header indicates to the server what kinds of character encodings are understood by the user-agent. Traditionally, it was set to a different value for each locale for the browser, like ISO-8859-1,utf-8;q=0.7,*;q=0.7 for a Western European locale.
With UTF-8 now being well-supported, being the preferred way of encoding characters, and to guarantee better privacy through less configuration-based entropy, browsers omit the Accept-Charset header: Internet Explorer 8, Safari 5, Opera 11, Firefox 10 and Chrome 27 have abandoned this header.
Accept-CH-Lifetime headerNote : This is part of an experimental technology called Client Hints and is only available in Chrome 61 or later.
The {{HTTPHeader("Accept-CH-Lifetime")}} header is used with the Device-Memory value of the Accept-CH header and indicates the amount of time the device should opt-in to sharing the amount of device memory with the server. The value is given in miliseconds and it's use is optional.
Accept-Encoding headerThe {{HTTPHeader("Accept-Encoding")}} header defines the acceptable content-encoding (supported compressions). The value is a q-factor list (e.g.: br, gzip;q=0.8) that indicates the priority of the encoding values. The default value identity is at the lowest priority (unless otherwise declared).
Compressing HTTP messages is one of the most important ways to improve the performance of a Web site, it shrinks the size of the data transmitted and makes better use of the available bandwidth; browsers always send this header and the server should be configured to abide to it and to use compression.
Accept-Language headerThe {{HTTPHeader("Accept-Language")}} header is used to indicate the language preference of the user. It is a list of values with quality factors (like: "de, en;q=0.7"). A default value is often set according the language of the graphical interface of the user agent, but most browsers allow to set different language preferences.
Due to the configuration-based entropy increase, a modified value can be used to fingerprint the user, it is not recommended to change it and a Web site cannot trust this value to reflect the actual wish of the user. Site designers must not be over-zealous by using language detection via this header as it can lead to a poor user experience:
Accept-Language header, adapted to the user interface language and end users often do not modify it, either by not knowing how, or by not being able to do it, as in an Internet café for instance.User-Agent headerNote : Though there are legitimate uses of this header for selecting content, it is considered bad practice to rely on it to define what features are supported by the user agent.
The {{HTTPHeader("User-Agent")}} header identifies the browser sending the request. This string may contain a space-separated list of product tokens and comments.
A product token is a name followed by a '/' and a version number, like Firefox/4.0.1. There may be as many of them as the user-agent wants. A comment is a free string delimited by parentheses. Obviously parentheses cannot be used in that string. The inner format of a comment is not defined by the standard, though several browser put several tokens in it, separated by ';'.
Vary response headerIn opposition to the previous Accept-* headers which are sent by the client, the {{HTTPHeader("Vary")}} HTTP header is sent by the web server in its response. It indicates the list of headers used by the server during the server-driven content negotiation phase. The header is needed in order to inform the cache of the decision criteria so that it can reproduce it, allowing the cache to be functional while preventing serving erroneous content to the user.
The special value of '*' means that the server-driven content negotiation also uses information not conveyed in a header to choose the appropriate content.
The Vary header was added in the version 1.1 of HTTP and is necessary in order to allow caches to work appropriately. A cache, in order to work with server-driven content negotiation, needs to know which criteria was used by the server to select the transmitted content. That way, the cache can replay the algorithm and will be able to serve acceptable content directly, without more request to the server. Obviously, the wildcard '*' prevents caching from occurring, as the cache cannot know what element is behind it.
Server-driven negotiation suffers from a few downsides: it doesn't scale well. There is one header per feature used in the negotiation. If you want to use screen size, resolution or other dimensions, a new HTTP header must be created. Sending of the headers must be done on every request. This is not too problematic with few headers, but with the eventual multiplications of them, the message size would lead to a decrease in performance. The more precise headers are sent, the more entropy is sent, allowing for more HTTP fingerprinting and corresponding privacy concern.
From the beginnings of HTTP, the protocol allowed another negotiation type: agent-driven negotiation or reactive negotiation. In this negotiation, when facing an ambiguous request, the server sends back a page containing links to the available alternative resources. The user is presented the resources and choose the one to use.
Unfortunately, the HTTP standard does not specify the format of the page allowing to choose between the available resource, which prevents to easily automatize the process. Besides falling back to the server-driven negotiation, this method is almost always used in conjunction with scripting, especially with JavaScript redirection: after having checked for the negotiation criteria, the script performs the redirection. A second problem is that one more request is needed in order to fetch the real resource, slowing the availability of the resource to the user.