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Re: Header Compression Clarifications

From: James M Snell <jasnell@gmail.com>
Date: Tue, 2 Jul 2013 16:37:44 -0700
Message-ID: <CABP7RbfwnBZLr+UH0js+34-LC=-ivw_sh+rdN4r4VzJz6HpYWA@mail.gmail.com>
To: Roberto Peon <grmocg@gmail.com>
Cc: "ietf-http-wg@w3.org" <ietf-http-wg@w3.org>
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 Tuesday, 2 July 2013 23:38:31 UTC

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