Re: Transfer-codings, mandatory content-coding support and intermediaries

FWIW, *if* the goal is to provide semantic compatibility between 1.1 and 2.0, and if we envision 2.0 -> 1.1 proxies being used, then I don't think that a gzip bit on each DATA frame is the right approach.

A 2.0 proxy talking to a 1.1 server can fairly easily convert a 2.0 request using a gzip bit into a 1.1 request using T-E: gzip (just concatenate the gzip streams from each DATA frame), but the reverse will require the proxy to decompress the T-E: gzip stream from the 1.1 server and then either provide an uncompressed response to the 2.0 client or re-compress each DATA frame.


On Apr 18, 2014, at 4:50 PM, K.Morgan@iaea.org wrote:

> Despite my comments below, if all we can agree on for this implementation draft is a single gzip bit ("AT RISK"), I think that's better than nothing so we can have some more experience before the discussion in NYC. - As long as something like Matthew's proposal is still on the table (for /2 or /3).
> 
> 
> 
> See my comments in-line below...
> 
> 
> 
> On 18 April 2014 18:38 Martin Thomson wrote:
>> ... we could reasonably mandate the use of gzip on a segment by
>> segment basis, which should help with the security concerns (and
>> provide a better excuse for including END_SEGMENT).  Strategic
>> segmentation could be used to separate content.
> 
> The beauty of Matthew's proposal is the independent compression context for each frame. It considerably reduces the implementation complexity with a relatively negligible loss in compression factor (see [1] for our test results).
> 
> Consider a client managing 100 simultaneous streams. Depending on the interleaving, the client could potentially need to keep around 100 separate decompression contexts. Doing decompression frame-by-frame allows for the possibility of a simple implementation that uses a single context to decompress each compressed frame as it arrives.
> 
> If we go with Matthew's proposal (or similar), I still think the security concerns should be managed with language similar to the following...
> 
>>> Intermediaries MUST NOT add transfer coding(s).  Intermediaries MAY remove transfer coding(s) if necessary. Intermediaries MAY remove and re-apply transfer coding(s), but the transfer coding(s) MUST be re-applied in the same order as applied by the origin.  For example, an intermediary might elect to remove a gzip transfer coding for local storage in its cache in order to more easily extract a range of the content. In this case, when serving a range, the intermediary can choose to send the range with no transfer coding or with gzip transfer coding, but not with any other transfer coding(s).
> 
> 
> 
> There's no way to let intermediaries add compression without some complicated scheme that requires the origin to mark every frame as sensitive or not.
> 
> 
> 
> I am adamant that intermediaries be allowed to remove compression at the transfer layer or we'll have problems transforming 2 -> 1.x.
> 
> I also think intermediaries should be allowed to remove compression and then re-apply it, if desired, for exactly the example case mentioned above which was brought up by Amos (see [2]).
> 
>> Matthew's proposal ... could (and should) be significantly reduced in complexity. ...
>> No alternative compression schemes or negotiation - those lead to interoperability failures.
> 
> To be clear, getting rid of negotation means you don't want Matthew's proposal, you just want a single compression bit.
> 
> 
> 
> I agree it adds complexity, but on the other hand it would be a shame to lock in to gzip considering there are already compression schemes better suited to dynamic compression (i.e. better CPU performance) e.g. LZ4 is ~10-20 faster than gzip (see [3]).
> 
> [1] http://lists.w3.org/Archives/Public/ietf-http-wg/2014AprJun/0156.html
> 
> [2] http://lists.w3.org/Archives/Public/ietf-http-wg/2014AprJun/0115.html
> 
> [3] http://code.google.com/p/lz4/
> 
> 
>> On 18 April 2014 01:03,  <K.Morgan@iaea.org> wrote:
>>> On 18 April 2014 08:52 mnot@mnot.net wrote:
>>>> At this point, it looks like the folks who have asked for TE support in HTTP/2
>>>> are willing to wait on incorporating such a feature into HTTP/2.
>>> 
>>> We're not willing to wait.
>>> 
>>>> the status quo seems like a good path forward ...
>>>> we should discuss this in more depth in NYC.
>>> 
>>> If we're going to discuss this in NYC, then it sounds like a really good reason to include it in the current
> implementation draft - even if it's marked "AT RISK" - so we have experience upon which we can base our discussion.
>>> 
>>> The primary concern surrounding transfer codings (whatever the variety) seems to be security vulnerabilities related to
> intermediaries applying compression without proper context.
>>> 
>>> To appease those concerns, I suggest the following language (or similar language applied to Matthew's frame-level
> transfer coding proposal or the single-bit gzip proposal - we also support Matthew's proposal)...
>>> 
>>> 9.4 Transfer Codings
>>> 
>>> HTTP/2 retains the semantics of Transfer-Encoding with the following additional restrictions and requirements.
>>> 
>>> Servers MUST NOT use the "chunked" transfer coding.
>>> 
>>> Intermediaries MUST NOT add transfer coding(s).  Intermediaries MAY remove transfer coding(s) if necessary.
> Intermediaries MAY remove and re-apply transfer coding(s), but the transfer coding(s) MUST be re-applied in the same order
> as applied by the origin.  For example, an intermediary might elect to remove a gzip transfer coding for local storage in
> its cache in order to more easily extract a range of the content. In this case, when serving a range, the intermediary can
> choose to send the range with no transfer coding or with gzip transfer coding, but not with any other transfer coding(s).
>>> 
>>> 
>>> 
>>> Others have mentioned it is "naiive" to think that intermediaries will keep these rules. Agreed. But it is eqaully
> "naiive" to think intermediares won't do the same thing with C-E. The only solution, IMO, to combat both problems would be
> signed headers (signed by the origin) - a benefit in favor of TE/Transfer-Encoding over frame-level codings, but we would
> be happy with either approach.
>>> 
>>>> However, discussion has also uncovered some problematic aspects of requiring clients to
>>>> support GZIP content-encoding in HTTP/2.
>>>> ...
>>>> That's pretty clearly a major reduction in functionality, and effectively removes
>>>> what we used to call semantic transparency from all HTTP1.x->HTTP2 proxies.
>>> 
>>> +1
>>> 
>>> There were no objections raised by anybody when Matthew, Roland and I brought up these issues, so there doesn't seem to
> be a good reason to keep it for this implementation draft.
>>> 
>>> I still think there needs to be a way to allow servers to compress as much as possible (as brought up by Patrick McManus
> [1] and others [2]).  Implicit gzip on the transfer layer is the right way to accomplish that and retain "semantic
> transparency" because a HTTP1.x -> HTTP2 intermediary can remove transfer codings without consequence.
>>> 
>>> [1] http://lists.w3.org/Archives/Public/ietf-http-wg/2014JanMar/1283.html
>>> [2] https://developers.google.com/speed/articles/use-compression
>>> 
> 
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_________________________________________________________
Michael Sweet, Senior Printing System Engineer, PWG Chair

Received on Friday, 18 April 2014 21:50:22 UTC