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RE: GZip Content-Encoding / Reopen #404

From: <K.Morgan@iaea.org>
Date: Mon, 14 Apr 2014 15:55:44 +0000
To: <roland@zinks.de>, <ietf-http-wg@w3.org>
CC: <matthew@kerwin.net.au>
Message-ID: <0356EBBE092D394F9291DA01E8D28EC20100F4C3D4@sem002pd.sg.iaea.org>

Two more reasons against implicit C-E gzip (in addition to Matthew's lengthy list)...

#1 In HTTP/1.1 it was possible for a client to unambiguously request a resource with a single acceptable content encoding...

e.g. "Accept-Encoding: sdch, identity;q=0"

Since opting out of gzip is not allowed in HTTP/2, this is no longer possible. The best a client can do in HTTP/2 is

e.g. "Accept-Encoding: sdch, gzip;q=0.001, identity;q=0"

- which is ugly and ambiguous.

#2 The HTTP/2 charter says "Retain the semantics of HTTP/1.1" [1]. Changing the definition of the implicit value of Accept-Encoding from "Accept-Encoding: identity" to "Accept-Encoding: gzip, identity" IMO is a change in semantics.

I agree with Patrick McManus [2] and Google researchers [3] that there is a need to require compression because of crazy intermediaries (e.g. antivirus libraries) [3], but implicit C-E gzip is the wrong way to do it.  Implicit gzip on the transfer layer would have been the right way to do it, but it seems there are too many security issues surrounding compression on the transfer layer for that to be a possibility.

[1] http://datatracker.ietf.org/wg/httpbis/charter/
[2] http://lists.w3.org/Archives/Public/ietf-http-wg/2014JanMar/1283.html
[3] https://developers.google.com/speed/articles/use-compression

On Monday,14 April 2014 10:49, Roland Zinks wrote:
> +1
> Even then it might be that there are some problems, e.g. my experience in the past with an iPhone playing a mp4 file was that it didn't accepted a full response on a range request and the video was not played. See for example http://stackoverflow.com/questions/10275730/php-video-play-on-ipad.
> On 14.04.2014 04:23, Matthew Kerwin wrote:
>> Section 9.3 in the current draft (-11 and the editor's copy) says that clients MUST support gzip compression for HTTP response bodies. "Intermediaries that perform translation from HTTP/2 to HTTP/1.1 MUST decompress payloads unless the request includes an Accept-Encoding value that includes "gzip"."
>> This goes some way to resolving issue #404 -- mnot's original report: "the implication on a HTTP/1 to HTTP/2 intermediary should be spelled out" -- but there are still a lot of implications not listed in that requirement. According to my understanding of HTTP, the appropriate added requirements are:
>> For requests in the 1.1->2 direction without Accept-Encoding:gzip :
>> * strip all conditional headers (If-Modified-Since, If-Unmodified-Since, If-Match, If-None-Match, If-Range) [i]
>> * strip all Range headers [i]
>> For all responses in the 2->1.1 direction:
>> * ought to strip Accept-Ranges headers
>> For such a response with Content-Encoding:gzip that wasn't Accept-Encoding:gzip :
>> * gunzip the entity, appropriately update Content-Encoding, Content-Length [ii], and add a Warning:214
>> * maybe change :status: 200 to 203
>> * strip or update Content-MD5 [iii]
>> * strip Content-Location, ETag, Last-Modified headers from the response [i]
>> * if the request included Cache-Control:no-transform ???
>> There may be others. If so, my implementation (if I were writing one) would be lacking.
>> There's still a big hole in "Cache-Control:no-transform". We've essentially added a requirement that gateways break conformance, without giving any guidance on how to do it (let alone reasonably justifying it in the first place). We haven't even told them the appropriate way to fail.
>> In the absence of an opt-out mechanism for gzip C-E [iv], I think we should remove the support requirement. In fact I think we should remove section 9.3 altogether, but I'm not opposed to a strongly worded suggestion that everyone support gzip.
>> [i] or, in the case of a full-on transforming proxy, mint new Last-Modified (possibly even ETag) header, and be responsible for mapping the uncompressed entity on the 1.1 side to the compressed entity on the 2 side
>> [ii] since HTTP 2 is already fragmented by frames, a gateway is probably going to always respond with chunked T-E, so Content-Length should probably be stripped (if present) anyway.
>> [iii] thus explicitly acting as a non-transparent proxy according to RFC 2616, Section 13.5.2 (Non-modifiable Headers). I can't find the equivalent text in the HTTPbis drafts, but I assume there's something in there.
>> [iv] an alternative could be to use Cache-Control:no-transform as such a flag; i.e. a server MUST NOT apply gzip C-E if the request did not include Accept-Encoding:gzip and did include Cache-Control:no-transform. That seems a bit flaky to me, though, and easy to miss/get wrong.

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Received on Monday, 14 April 2014 15:56:34 UTC

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