Re: Clarification of dynamic table size change

Hervé Ruellan <herve.ruellan@crf.canon.fr> Thu, 22 October 2015 16:22 UTC

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From: =?UTF-8?Q?Herv=c3=a9_Ruellan?= <herve.ruellan@crf.canon.fr>
To: Cory Benfield <cory@lukasa.co.uk>
CC: "ietf-http-wg@w3.org" <ietf-http-wg@w3.org>, Tatsuhiro Tsujikawa <tatsuhiro.t@gmail.com>
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Date: Thu, 22 Oct 2015 18:18:17 +0200
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Subject: Re: Clarification of dynamic table size change
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I agree that the wording is ambiguous here.

However, my reading is the same a Cory's: you don't have to send a dynamic table update if the *actual* value is not changed.

Hervé

On 19/10/15 19:15, Cory Benfield wrote:
>
> > On 19 Oct 2015, at 17:26, Tatsuhiro Tsujikawa <tatsuhiro.t@gmail.com
> > <mailto:tatsuhiro.t@gmail.com>> wrote:
> > ​Could be.  But I don't think we mixed HTTP/2 layer and HPACK layer.
> > More specifically, we input the max header table size to HPACK, and
> > see what it react to it in this case.  It is specific to HPACK
> > implementation behaviour.
> >
> > In general, we choose simpler path in HTTP/2​, I mean single execution
> > path rather than "do this if we have x otherwise do that".  For
> > example, we require client preface even if we negotiated HTTP/2 over ALPN.
> > We send HTTP2-Settings in HTTP Upgrade request, but still we need to
> > send SETTINGS frame in client preface.
> > Taking into account of this split, it is more natural to always send
> > header table size update as acknowledgement for
> > SETTINGS_HEADER_TABLE_SIZE.  We can do more strict validation about
> > the peer; it might forget to implement it anyway.
>
> I don’t know that I buy that. The acknowledgement for
> SETTINGS_HEADER_TABLE_SIZE is a SETTINGS frame with the ACK flag set.
> Why is further acknowledgement required? As far as I can see it, there
> are two (slightly different) values here:
>
> - SETTINGS_HEADER_TABLE_SIZE is the *maximum* value the encoder may use.
> - Whatever is sent by the HPACK encoder in a dynamic table size update
> is the *actual* value being used.
>
> You may be able to change the first without affecting the second (e.g.
> if you raise or leave unchanged the value of
> SETTINGS_HEADER_TABLE_SIZE). In the case that the second is not
> affected, it seems like unnecessary noise for the encoder to be forced
> to emit a dynamic table size update that has no change.
>
> I don’t strongly object to adding an erratum to RFC 7541 that requires
> that a dynamic table update be emitted for any change to
> SETTINGS_HEADER_TABLE_SIZE, even if that change does not actually affect
> the size of the table the encoder will use, but my current reading of
> the specification does not require that such an update be emitted.
>
> Cory