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Re: Call for Consensus: Remove "reference set" from HPACK (to address #552)

From: Willy Tarreau <w@1wt.eu>
Date: Thu, 17 Jul 2014 17:45:02 +0200
To: Greg Wilkins <gregw@intalio.com>
Cc: Martin Thomson <martin.thomson@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>
Message-ID: <20140717154502.GE3440@1wt.eu>
Hi Greg,

On Wed, Jul 16, 2014 at 04:00:33PM +1000, Greg Wilkins wrote:
> On 16 July 2014 15:45, Willy Tarreau <w@1wt.eu> wrote:
> > The problem I'm seeing with this change is that you need one extra-byte to
> > send litteral headers fields that are not present in the static table.
> >
> The first 64 entries can still be sent with 1 byte.   So I think in many
> common use-cases almost all the indexes will just be 1 byte.
> > Maybe
> > on the other hand it rarely happens that they're not in the static table ?
> >
> Well the static table has few name+value fields, but the ones it does have
> are common.

I'm noticing that we need only 4 bits to send any possible static header+value
(assuming we swap accept-charset and accept-encoding), since all other values
are empty and will never be used as-is but as litterals.

So I think that based on your suggestion to split indexing of static and
dynamic, I'm suspecting we could systematically split indexes between static
and dynamic in opcodes. Indeed, some index fields are already small (6+ for
litterals and 4+ for litterals not indexed/never indexed), so having the
static first will force one extra byte for all dynamic headers. However,
stealing one bit in unused value ranges could help make that situation
better. Something along these lines seems possible (please forgive me for
errors, I'm encoding as I write) :

  11 XXXXXX => send 6+indexed header from *dynamic* table as-is  (0xC0 reserved)
  10 01XXXX => send 4+indexed header from *static* table as-is   (0x80 reserved)

  00 00XXXX => send 4+indexed *dynamic* header without indexing (unchanged)
  10 00XXXX => send 4+indexed *static* header without indexing

  10 1XXXXX => send litteral based on *dynamic* header table (0=new name or reserved)
  01 XXXXXX => send litteral based on *static* header table (0=new name)

The idea is that even when we have few bits to encode the index, we're not
forced to add an extra byte to encode the values from the other table, whatever
table we pick first as first.

I don't know if you see what I mean of if that's even more confuse.

Received on Thursday, 17 July 2014 15:45:31 UTC

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