Is there a de facto or established reason why multipart HTTP responses aren't generally supported in browsers?

Question

The HTTP protocol has supported multipart responses for a long time. I've used them before for APIs with appropriately equipped consumers, but it doesn't appear browser support for them is very good, nor has it improved in the last half-decade. I've had difficulty finding much information on why this might be. I'd love to be able to cut down on HTTP requests by sending all of the assets I know a webapp will need on the initial request, especially for apps that employ client-side frameworks like Backbone.js.

Are there any white-papers, trade articles, failed experiments, or other evidence on why neither browser-makers or web-performance evangelists are paying this long-time HTTP construct any attention?

To be utterly clear, I'm not looking for an opinion, but veritable evidence indicating why this might be. For example, if Mozilla published something about this a few years ago, or there is a closed ticket in the Firefox bug tracker where a lead developer comments about why they won't implement this.

Answer 1

Actually older versions of IE would process multipart application/octet-stream responses and save all the files during a download operation, but this was recently removed (as of IE7, I think) and was specific to downloading only.

I doubt you're going to find the "evidence" you're looking for, because I don't think what you have proposed is in keeping with the "spirit" of the HTTP specification. I will try to explain what I mean by that. The basic paradigm of HTTP is a client-driven request and the server's response to that request. But you seem to be proposing that the server would return arbitrary files with the assumption that the client would know what to do with them.

However, if you were to propose that the client first explicitly requests multiple files, then I would say you could be on to something. The HTTP 1.1 specification does allow the Accept client-request header to indicate multipart support, so this would appear to be how the HTTP designers envisioned this working. Unfortunately the spec is silent about how the client should identify the files it expects to receive, and this is understandable if you look at HTTP in a vacuum, as it is defined, rather than through the lens of browsers and websites. That is an implementation detail that is left up to the client and server to settle. It is a concern which applies to a different layer -- what the content is and how it is consumed, rather than how to request it and transport it.

It is easy to imagine various solutions, of course, but without a standard to refer to, it wouldn't seem to warrant the effort on the part of the browser developers. I could imagine someone like Microsoft (with control over a widely-adopted server and browser) implementing this, but they'd be inventing a spec and people would complain. Apparently we've decided it's better to wait 10 years for the W3C to agree on something...

Answer 2

Directly from W3 org itself (at http://www.w3.org/Protocols/rfc2616/rfc2616-sec3.html#sec3.7.2):

3.7.2 Multipart Types

MIME provides for a number of "multipart" types -- encapsulations of one or more entities within a single message-body. All multipart types share a common syntax, as defined in section 5.1.1 of RFC 2046

[40], and MUST include a boundary parameter as part of the media type value. The message body is itself a protocol element and MUST therefore use only CRLF to represent line breaks between body-parts. Unlike in RFC 2046, the epilogue of any multipart message MUST be empty; HTTP applications MUST NOT transmit the epilogue (even if the original multipart contains an epilogue). These restrictions exist in order to preserve the self-delimiting nature of a multipart message- body, wherein the "end" of the message-body is indicated by the ending multipart boundary.

In general, HTTP treats a multipart message-body no differently than any other media type: strictly as payload. The one exception is the "multipart/byteranges" type (appendix 19.2) when it appears in a 206 (Partial Content) response, which will be interpreted by some HTTP caching mechanisms as described in sections 13.5.4 and 14.16. In all other cases, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. The MIME header fields within each body-part of a multipart message- body do not have any significance to HTTP beyond that defined by their MIME semantics.

In general, an HTTP user agent SHOULD follow the same or similar behavior as a MIME user agent would upon receipt of a multipart type. If an application receives an unrecognized multipart subtype, the application MUST treat it as being equivalent to "multipart/mixed".

  Note: The "multipart/form-data" type has been specifically defined
  for carrying form data suitable for processing via the POST
  request method, as described in RFC 1867 [15].