Re: Header compression question: duplicate header entry and current index on computing working set

On Thu, Jul 18, 2013 at 1:36 AM, Martin Thomson <martin.thomson@gmail.com>wrote:

> On 17 July 2013 08:56, Tatsuhiro Tsujikawa <tatsuhiro.t@gmail.com> wrote:
> > In 3.4, to compute working set from reference set of headers, the index
> of
> > entry in header table is required.
> > The question is, when the duplicate entries are in the header table,
> which
> > index is used as the index of working set?
>
> If, for some strange reason, a compressor created multiple identical
> entries in the table, the decompressor is required to respect that
> choice, even if it is likely to be a bug.  This prevents the
> decompressor and compressor from getting out of sync.
>
> The compressor can then reference any of the entries when using an index.
>

If the choice is arbitrary, then the compressor and decompressor may choose
different index and
can get out of sync.

For example,
Current header table:
|0|name1|value1|
|1|name1|value1|

If name1/value1 is in reference set, compressor chooses index 0, and
decompressor chooses index 1.
compressor wants to remove name1/value1, so reference index 0.
In decompressor side, however, seeing index header representation with
index 0 and it is not in its reference set
(because name1/value1 is index 1), retrieve index 0 from header table and
add it to working set.
Maybe I misunderstand the draft.

If multiple identical entries are considered as a bug, then it would be
better to
prohibit it in the spec and we are happy to not to consider these things.

Best regards,

Tatsuhiro Tsujikawa