Re: Reference set in HPACK

You're basing conclusions on today's data, instead of looking forward as to
what might happen when the set of headers sent adapts to the compression
method, making it significantly more likely for items in the reference set
to be emitted.

You may want to look at how many of those entries would be regularized if
HPACK was in use and servers/clients intended on sending headers that were
similar.
-=R


On Tue, Jul 1, 2014 at 10:30 PM, Kazu Yamamoto <kazu@iij.ad.jp> wrote:

> Hi,
>
> As you may remember, I implemented several HPACK *encoding* algorithms
> and calculated compression ratio. I tried it again based on HPACK
> 08. I have 8 algorithms.
>
> - Naive    -- No compression
> - Naive-H  -- Using Huffman only
> - Static   -- Using static table only
> - Static-H -- Using static table and Huffman
> - Linear   -- Using header table
> - Linear-H -- Using header table and Huffman
> - Diff     -- Using header table and reference set
> - Diff-H   -- Using header table, reference set and Huffman
>
> The implementations above pass all test cases in
> https://github.com/http2jp/hpack-test-case/.  Using this test cases as
> input, I calculated compression ratio again. The ratio is calculated
> by dividing the number of bytes after compression by that before
> compression.
>
> Here is results:
>
> Naive     1.10
> Naive-H   0.86
> Static    0.84
> Static-H  0.66
> Linear    0.39
> Linear-H  0.31
> Diff      0.39
> Diff-H    0.31
>
> Linear-H and Diff-H results in almost the same. To my calculation,
> Diff-H is only 1.6 byte shorter than Linear-H in average. This means
> that reference set does NOT much contribute to compress headers
> although it is very difficult to implement.
>
> I have NOT seen any header examples for which reference set work
> effectively so far.
>
> So, if the authors of HPACK want to retain reference set, I would like
> to see evidence that there are some cases in which reference set
> contributes the compression ratio. HPACK 08 says "Updated Huffman
> table, using data set provided by Google". So, I guess that the
> authors can calculate the compression ratio based on this data.
>
> If there is not such an evidence, I would like to strongly recommend
> to remove reference set from HPACK. This makes HPACK much simpler, so
> implementations gets bug less and inter-operability is improved. Plus,
> the order of headers is reserved always.
>
> Regards,
>
> --Kazu
>
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