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506 Variant Also Negotiates

Written by Evert Pot / Original link on Aug. 13, 2019

In 1998 RFC2295 was published. It’s experimental, and meant to introduce a new way to do content negotiation in HTTP. As far as I personally know, I don’t think it got a lot of traction.

Traditionally, when a HTTP client wants to do content-negotation, they will send one or more accept headers:

GET / HTTP/1.1
Accept: text/html; image/png; text/*; q=0.9
Accept-Language: en-CA; en
Accept-Charset: UTF-8
Accept-Encoding: gzip, brotli

RFC2295 intended to introduce a new way to do this, with a lot more flexibility and features. The RFC talks about selecting specific variants not just based on mimetype, but also HTML features a browser supports, color capabilities, screen resolution, speed preference, paper size for printers and even selecting content for specific devices like VR goggles and PDA’s.

An interesting feature is that it can also return a list of urls for specific variations, changing the HTTP model a bit by giving every representation and variant their own url, and returning all this in with a 300 Multiple Choices response.

An example of such a response (from the RFC):

HTTP/1.1 300 Multiple Choices
Date: Tue, 11 Jun 1996 20:02:21 GMT
TCN: list
Alternates: {"paper.1" 0.9 {type text/html} {language en}},
{"paper.2" 0.7 {type text/html} {language fr}},
{"paper.3" 1.0 {type application/postscript}
{language en}}
Vary: negotiate, accept, accept-language
ETag: "blah;1234"
Cache-control: max-age=86400
Content-Type: text/html
Content-Length: 227

<h2>Multiple Choices:</h2>
<ul>
  <li><a href=paper.1>HTML, English version</a>
  <li><a href=paper.2>HTML, French version</a>
  <li><a href=paper.3>Postscript, English version</a>
</ul>

The RFC introduces a new error code: 506 Variant Also Negotiates. To the best of my understanding, this error returned when a server is misconfigured and a ‘negotiating resource’ is pointing to another resource that doesn’t serve a representation, but instead also tries to negotiate.

I can imagine that a negotiating resource could for example point to itself, or sets up something like a redirection look. I think 506 is a specific error that a server could return for this case.

Should you use this?

I often include a section that answers whether you should use this status code. In this case, I think it’s better to dive into whether you should support the negotiation feature.

The issue with the feature is that it never left the experimental phase, and as far as I know got very little adoption. It was defined before HTTP/1.1 was finalized, and for all intents and purposes I think it can be considered dead.

However, it solves a couple of really interesting problems that aren’t solved well in the content-negotation system that we have today.

For example, it allows content-negotation of arbitrary features and it provides a way to canonicalize specific representations.

Right now when a proxy needs to create a ‘key’ for storing a representation, it can only really do so based on the Vary header, and it treats headers appearing in this list as opaque strings.

This means that any variation of an Accept header results in a new representation in cache, even if there’s only for example 2 supported content-types.

So my conclusion here is that I think it’s an interesting enough specification that it might warrant a look, with the following caveats:

  1. The spec is poorly supported, so it only really makes sense if you can add support to both server and client.
  2. Make sure that a fallback exists for clients that don’t support this feature. This should not be terribly difficult.

References

evertpot

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