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Re: Header Compression Clarifications

From: Roberto Peon <grmocg@gmail.com>
Date: Thu, 4 Jul 2013 11:07:28 -0700
Message-ID: <CAP+FsNe1qDFRSv1BW+visFRyH9q_0tu4SaRSzJF8BzzQZCNG2w@mail.gmail.com>
To: RUELLAN Herve <Herve.Ruellan@crf.canon.fr>
Cc: James M Snell <jasnell@gmail.com>, "ietf-http-wg@w3.org" <ietf-http-wg@w3.org>
The approach of add-then-enforce guarantee has a glaring and huge problem--
 it cannot guarantee the amount of memory I'll use.
I'd much rather deal with some small complexity (demonstrably not big) here
and have that guarantee.
-=R


On Thu, Jul 4, 2013 at 6:09 AM, RUELLAN Herve <Herve.Ruellan@crf.canon.fr>wrote:

> Trying to catch on the discussion, I see three proposals for solving the
> problem of substitution and eviction.
>
> 1. Size adjustment BEFORE doing the substitution.
> This has several edge case problems as James showed and would lead to
> complex implementations.
>
> 2. Size adjustment AFTER doing the substitution.
> The problem is that the new entry may be dropped just after adding it.
>
> 3. Size adjustment BEFORE doing the substitution with pinning of the
> replaced entry (Roberto's proposal).
>
>
> I think that 2 and 3 are in fact close together.
> With 2, a bad encoder could just have its new entry dropped from the table
> just after adding it. However a good encoder could use an algorithm like
> the one propose in 3 to find the right entry to replace and prevent
> dropping the new entry.
> On the decoder side, 2 is simpler.
>
> Taking James' example for what a "good" encoder should do.
> With existing table (max size 15)
>   0: FOO = 123
>   1: BAR = 321
>   2: BA = Z
>
> The encoder wants to substitute: TESTING = FOO at Index #0 (because entry
> #0 is old or whatever).
> It detects that index #0 and #1 need to be dropped, and therefore adjust
> the substitution to replace Index #2.
> It then can apply the substitution, and after that adjust the size.
>
>
> So my preference is for the second solution. True, it can lead to
> under-optimal usage of the header table. But I'm not in favor of making all
> implementations more complex to help optimize bad implementations.
> We should however warn implementers of this problem.
>
> Hervé.
>
>
> > -----Original Message-----
> > From: James M Snell [mailto:jasnell@gmail.com]
> > Sent: mercredi 3 juillet 2013 02:36
> > To: Roberto Peon
> > Cc: ietf-http-wg@w3.org
> > Subject: Re: Header Compression Clarifications
> >
> > Yes, I was simplifying :-) I think that rule should work..
> > particularly in that it allows me to avoid having to check for as many
> of these
> > weird edge cases.
> >
> > On Tue, Jul 2, 2013 at 5:27 PM, Roberto Peon <grmocg@gmail.com> wrote:
> > > Correct (assuming the overhead per item was assumed to be zero, which
> > > isn't the case, but is good in example-land :) )
> > >
> > > -=R
> > >
> > >
> > > On Tue, Jul 2, 2013 at 5:18 PM, James M Snell <jasnell@gmail.com>
> wrote:
> > >>
> > >> So to make sure I have it right... Given the two examples I gave...
> > >>
> > >>   Header Table, Max size = 15
> > >>
> > >>   1  A = B
> > >>   2  C = D
> > >>   3  E = F
> > >>   4  G = H
> > >>   5  I = J
> > >>
> > >>   Substitute #5 with FOOBARBAZ = 123456
> > >>
> > >> The result would be a Header table with one item "FOOBARBAZ = 123456"
> > >>
> > >> And...
> > >>
> > >>   Header Table, Max size = 20
> > >>
> > >>   1  A = B
> > >>   2  C = D
> > >>   3  E = F
> > >>   4  G = H
> > >>   5  I = J
> > >>   6  K = L
> > >>   7  M = N
> > >>
> > >>   Substitute #3 with FOOBARBAZ = 123456
> > >>
> > >> The result would be a Header table with three items...
> > >>
> > >>   FOOBARBAZ = 123456
> > >>   K = L
> > >>   M = N
> > >>
> > >> Is that correct?
> > >>
> > >> On Tue, Jul 2, 2013 at 5:07 PM, Roberto Peon <grmocg@gmail.com>
> > wrote:
> > >> > The biggest reason that I don't like this is that it requires the
> > >> > encoder keep more state.
> > >> > I prefer to make this simple by having an easy-to-follow rule for
> > >> > when it the slot it would have replaced would have been evicted
> > >> > (once all predecessors to that slot have been evicted, then
> > >> > elements following the element-to-be-replaced are removed, leaving
> > >> > the new element at the head of the list).
> > >> >
> > >> > The pseudo code for this is:
> > >> >
> > >> > if not replacement_idx or new_element_size > max_table_size:
> > >> >   PROTOCOL_ERROR()
> > >> > if max_table_size ==new_element_size:
> > >> >   table.clear()
> > >> >   table[0] = new_element
> > >> >   return
> > >> >
> > >> > # above is boilerplate true for any algorithm
> > >> >
> > >> > table[replacement_idx].clear()
> > >> > table[replacement_idx].pin()
> > >> > first_non_pinned = 0
> > >> > while new_element_size + table_byte_size() > max_table_size:
> > >> >     if table[first_non_pinned].pinned():
> > >> >       ++first_non_pinned
> > >> >        continue
> > >> >       table[first_non_pinned].pop()
> > >> >
> > >> > This adds some small complexity here, but it makes encoding
> > >> > significantly easier (you can have a naive encoder which leaps
> > >> > without looking, which is far less complicated than having to look
> > >> > before leap, and may still prove reasonable in terms of compressor
> > >> > efficiency).
> > >> >
> > >> > I admit that I'm attracted to your idea. I just am afraid of what
> > >> > it makes the encoder look like :) -=R
> > >> >
> > >> >
> > >> > On Tue, Jul 2, 2013 at 4:37 PM, James M Snell <jasnell@gmail.com>
> > wrote:
> > >> >>
> > >> >> On Tue, Jul 2, 2013 at 4:00 PM, Roberto Peon <grmocg@gmail.com>
> > wrote:
> > >> >> [snip]
> > >> >> >
> > >> >> > So, an example:
> > >> >> > Imagine that you're replacing entry #10 with something 10
> > >> >> > characters long.
> > >> >> > The previous entry in that slot was 5 characters long, and the
> > >> >> > table was already at max size.
> > >> >> > This implies that you need to get rid of 5 characters before
> > >> >> > replacing.
> > >> >> > Assuming that items 1 and 2 are the oldest items and item 1 is 3
> > >> >> > chars, and item 2 is 3 chars, you need to pop two.
> > >> >> >
> > >> >> > You now stick the 10 characters into what was formerly entry #10.
> > >> >> >[snip]
> > >> >>
> > >> >> That's problematic too. Let's go back to my example:
> > >> >>
> > >> >> Header Table, Max size = 15
> > >> >>
> > >> >> 1  A = B
> > >> >> 2  C = D
> > >> >> 3  E = F
> > >> >> 4  G = H
> > >> >> 5  I = J
> > >> >>
> > >> >> Substitute #5 with FOOBARBAZ = 123456
> > >> >>
> > >> >> Obviously, we end up popping all five entries, saying "stick the
> > >> >> new characters into what was formerly entry #5" does not make any
> > >> >> sense because the thing that was "formerly entry #5" no longer
> exists.
> > >> >>
> > >> >> Now a variation on the same problem:
> > >> >>
> > >> >> Header Table, Max size = 20
> > >> >>
> > >> >> 1  A = B
> > >> >> 2  C = D
> > >> >> 3  E = F
> > >> >> 4  G = H
> > >> >> 5  I = J
> > >> >> 6  K = L
> > >> >> 7  M = N
> > >> >>
> > >> >> Substitute #3 with FOOBARBAZ = 123456
> > >> >>
> > >> >> We begin popping things off to make room before doing the
> > >> >> substitution... 4 entries are removed, including the item being
> > >> >> replaced... leaving
> > >> >>
> > >> >> 1  I = J
> > >> >> 2  K = L
> > >> >> 3  M = N
> > >> >>
> > >> >> What exactly do we replace? Are we replacing "M = N" (the current
> > #3)?
> > >> >> If so, how does that sync up with the "thing that was formerly
> > >> >> entry #3" idea?
> > >> >>
> > >> >> I think the only reliable approach is to substitute AFTER freeing
> > >> >> up space, substitute into whatever is in the index position after
> > >> >> freeing up space, and if nothing is in that space, return an
> > >> >> error. This means that the sender has to be careful to avoid
> > >> >> getting into this state in the first place, which means very
> > >> >> careful control over when and how substitution is being used.
> > >> >> Given the current eviction strategy, that would be the most
> > >> >> reliable approach I think. So in the two examples above, the first
> > >> >> case returns an error and the second case results in "M = N" being
> > replaced.
> > >> >
> > >> >
> > >
> > >
>
>
Received on Thursday, 4 July 2013 18:07:56 UTC

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