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内容协商

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在 HTTP 协议中,内容协商是这样一种机制,通过为同一URI指向的资源提供不同的展现形式,可以使用户代理指定哪种展现形式(例如,文档使用的自然语言,图片的格式,或者内容编码形式)能够更好地适配用户的需求。

内容协商的基本原则

一份特定的文档称为一项资源。当客户端获取资源的时候,会向其对应的URL发送请求。服务器使用这个URL来选择其中的一个变体——每一个变体称为一个表示——然后将这个选定的表示返回给客户端。整个资源,连同它的各个表示,共用一个特定的URL。当一项资源被访问的时候,如何选择一个特定的表示是通过内容协商机制来决定的。客户端和服务器端之间存在多种协商方式。

最佳表示的选取通过两种机制实现:

  • 客户端设置特定的 HTTP 首部 (又称为服务器驱动协商或者主动协商);这是进行内容协商的标准方式;
  • 服务器返回 300 (Multiple Choices) 或者 406 (Not Acceptable) HTTP 状态码 (又称为代理驱动协商或者被动协商);这种方式一般用作备选方案。

随着时间的推移,也有其他一些内容协商的提案被提出来,比如透明内容协商以及 Alternates 首部。但是它们都没有获得人们的认可而逐渐被遗弃。

Server-driven content negotiation

In server-driven content negotiation, or proactive content negotiation, the browser (or any other kind of user-agent) sends several HTTP headers along with the URL. These headers describe the preferred choice of the user. The server uses them as hints and an internal algorithm chooses the best content to serve to the client. The algorithm is server-specific and not defined in the standard. See, for example, the Apache 2.2 negotiation algorithm.

The HTTP/1.1 standard defines list of the standard headers that start server-driven negotiation (Accept, Accept-Charset, Accept-Encoding, Accept-Language). Though strictly speaking User-Agent is not in this list, it is sometimes also used to send a specific representation of the requested resource, though this is not considered as a good practice. The server uses the Vary header to indicate which headers it actually used for content negotiation (or more precisely the associated response headers), so that caches can work optimally.

In addition to these, there is an experimental proposal to add more headers to the list of available headers, called client hints. Client hints advertise what kind of device the user agent runs on (for example,  if it is a desktop computer or a mobile device).

Even if server-driven content negotiation is the most common way to agree on a specific representation of a resource, it has several drawbacks:

  • The server doesn't have total knowledge of the browser. Even with the Client Hints extension, it has not a complete knowledge of the capabilities of the browser. Unlike reactive content negotiation where the client makes the choice, the server choice is always somewhat arbitrary.
  • The information by the client is quite verbose (HTTP/2 header compression mitigates this problem) and a privacy risk (HTTP fingerprinting)
  • As several representations of a given resource are sent, shared caches are less efficient and server implementations are more complex.

The Accept header

The 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 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 <img>, <video> or <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.

The Accept-CH header 

This is part of an experimental technology called Client Hints and currently implemented only in Chrome 46 and later

The experimental Accept-CH lists configuration data that can be used by the server to select an appropriate response. Valid values are:

Value Meaning
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).

The Accept-Charset header

The 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, most browsers omit the Accept-Charset header: Internet Explorer 8, Safari 5, Opera 11 and Firefox 10 have abandoned this header.

The Accept-Encoding header

The 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.

The Accept-Language header

The 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:

  • They should always provide a way to overcome the server-chosen language, e.g., by providing a language menu on the site. Most user-agents provide a default value for the 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.
  • Once a user has overridden the server-chosen language, a site should no longer use language detection and should stick with the explicitly-chosen language. In other words, only entry pages of a site should select the proper language using this header.

The User-Agent header

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 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 ';'.

The Vary response header

In opposition to the previous Accept-* headers which are sent by the client, the 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 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 agent-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.

Agent-driven negotiation

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.

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