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Re: Removing dropped entry from ref set

From: Roberto Peon <grmocg@gmail.com>
Date: Mon, 23 Sep 2013 19:27:34 -0700
Message-ID: <CAP+FsNfgHKOO5Q58+qeYBkS4RFB89mqGe_62h2xnymKOuYKVwA@mail.gmail.com>
To: Mike Bishop <Michael.Bishop@microsoft.com>
Cc: Fred Akalin <akalin@google.com>, HTTP Working Group <ietf-http-wg@w3.org>
The appropriate action is to remove the item, and then you are done. Tests
I did in the past indicated that inokicit expiry was better than explicit
expiry both in terms of on the wire size and in terms of reducing error
cases.

The compressor and decompressor must always make the same decision, so the
compressor needs to ensure that it emits what is necessary to encode the
correct data. This may well involve encoding data that used to be present
or was present when the first key/value was encoded but was dropped before
the encoder realized it could have used it.

This is one of the places where a better implementation of an encoder will
have a benefit-- it can construct a better ordering (which may toggle the
entry twice) in order to yield better compression.

-=R
On Sep 23, 2013 7:16 PM, "Mike Bishop" <Michael.Bishop@microsoft.com> wrote:

>  Yes, thatís a reasonable interpretation.  While the spec could be
> clearer, I think thatís the correct reading.  That still leaves open the
> question about correct behavior, though.  You said ďYou'll need to take the
> appropriate action when an entry in the reference set gets dropped, which
> is most likely explicitly emitting it before it gets dropped.Ē  Is this
> appropriate action on the part of the compressor, or the decompressor?
>
>  If the compressor, what action is that -- does it need to notice that an
> entry it cares about is going to be dropped and immediately encode two
> indexed references to it, forcing it to be emitted before itís dropped?  Or
> is it an appropriate action on the decompressorís part, which means the
> compressor needs to ensure that nothing in the reference set it wishes to
> remove falls off the list before it gets removed?
>
>  Sent from Windows Mail
>
>   *From:* Fred Akalin <akalin@google.com>
> *Sent:* Monday, September 23, 2013 1:51 PM
> *To:* Mike Bishop <Michael.Bishop@microsoft.com>
> *Cc:* ietf-http-wg@w3.org
>
>   On Mon, Sep 23, 2013 at 1:37 PM, Mike Bishop <
> Michael.Bishop@microsoft.com> wrote:
>
>>  Found a corner case that Iím not sure HPACK handles, or if it does Iím
>> not finding it and we might want to clarifyÖ.
>>
>>  Sequence of events:
>>
>>    - Header X is present in reference set and header table.
>>    - Some other addition to the header table causes header X to be
>>    dropped.  (This could be a header that causes X to be dropped, or a
>>    substitution operation that replaces X.)
>>
>>
>>    - Now the compressor wishes to remove X from the reference set.  How
>>    does it do so?
>>
>>
>>  Per 3.2.1, the only operation that has the effect of removing an entry
>> from the reference set is an indexed representation of a header already in
>> the reference set.  But you canít send the indexed representation of a
>> header no longer in the header table.
>>
>
>  >From 3.1.3: "A reference set is defined as an unordered set of
> references to entries of the header table." I interpret this to mean that
> when an entry is removed from the header table, it's removed from the
> reference set also.
>
>
>>   To address it, we could say:
>>
>>    -  *Replacing X with another entry in the header table also replaces
>>    X in the reference set.*  This makes perfect sense, but needs to be
>>    stated.
>>    -  *Dropping X from the header table also drops it from the reference
>>    set.*  This is more problematic, because the compressor canít just
>>    pass over headers it wants to communicate which are already in the
>>    reference set.  Instead, it needs to leave them until the end, then check
>>    if theyíre all still in the reference set.  If not, it needs to take
>>    another action to emit one or more, then check that all the remaining are
>>    still in the reference setÖ  This loops until the entire remaining set is
>>    still present.
>>
>>
>  You'll need to take the appropriate action when an entry in the
> reference set gets dropped, which is most likely explicitly emitting it
> before it gets dropped. I ran into this same problem, which Tatsuhiro
> Tsujikawa pointed out: see
> https://github.com/akalin-chromium/httpbis-header-compression/issues/1 .
>
Received on Tuesday, 24 September 2013 02:28:01 UTC

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