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Re: JavaScript header compressor/decompressor updated to HPACK-03

From: Roberto Peon <grmocg@gmail.com>
Date: Wed, 18 Sep 2013 15:58:07 -0700
Message-ID: <CAP+FsNdYEUi7OXZkDHfLto+v7xK8wXDXLrCCKR9YpbDm4J0zXA@mail.gmail.com>
To: Mike Bishop <Michael.Bishop@microsoft.com>
Cc: Fred Akalin <akalin@google.com>, HTTP Working Group <ietf-http-wg@w3.org>
Well, it is the way Herve and I designed it. The ordering of the header
table is not necessarily in index order. :/

I'd be perfectly happy to not have substitution, and not have the potential
confusion, but I'm fairly sure Herve would object :)

-=R


On Wed, Sep 18, 2013 at 2:54 PM, Mike Bishop
<Michael.Bishop@microsoft.com>wrote:

>  In HPACK-03, section 3.2.4 says that “To achieve this [size within
> bounds], repeatedly, the first entry of the header table is removed, until
> enough space is available for the modification.”  I don’t see any
> references in the spec to “oldest” or entry age.****
>
> ** **
>
> Or are we talking about different things?****
>
> ** **
>
> *From:* Roberto Peon [mailto:grmocg@gmail.com]
> *Sent:* Wednesday, September 18, 2013 2:34 PM
> *To:* Fred Akalin
> *Cc:* HTTP Working Group
> *Subject:* Re: JavaScript header compressor/decompressor updated to
> HPACK-03****
>
> ** **
>
> If you added an item A at index 0, then B at 1, then substituted C in at
> 0, then B is the oldest.****
>
> -=R****
>
> On Sep 18, 2013 2:28 PM, "Fred Akalin" <akalin@google.com> wrote:****
>
>  Just to clarify, is the oldest item the one with index 0, or some other
> definition of oldest?****
>
> ** **
>
> On Fri, Sep 13, 2013 at 3:16 PM, Roberto Peon <grmocg@gmail.com> wrote:***
> *
>
> Awesome!****
>
> The expiry mechanism should always be oldest item first.****
>
> -=R****
>
> On Sep 13, 2013 2:53 PM, "Fred Akalin" <akalin@google.com> wrote:****
>
>  Hey all,****
>
> ** **
>
> I updated
> http://akalin-chromium.github.io/httpbis-header-compression/compressor_test.html to
> implement the HPACK-03 draft. In particular, I tried to make it a complete
> an implementation as possible, and I added copious comments and references
> to the spec to make it easy to validate and understand.****
>
> ** **
>
> The only thing I didn't implement is UTF-8 validation for header values.
> Hopefully, the need for that will go away.****
>
> ** **
>
> Some thoughts:****
>
> ** **
>
> - There aren't any tests. I wanted to see how correct I can make the
> implementation without them (which will be measured when the compliance
> suite comes out). I'm sure there are bugs.****
>
> ** **
>
> - I didn't try very hard to make the encoder smart, but I did try to make
> it exercise all the opcodes.****
>
> ** **
>
> - I found it quite helpful that the encoding context was precisely defined
> (as a header table plus the reference set). However, I ultimately found it
> better to encode the reference set as part of the header table (by having a
> bit per entry) instead of having a separate data structure, since it
> eliminates a bunch of logic to keep the indices in the two in sync. This
> may have been obvious to some people, but not to me. I wonder if it's in
> the scope of the spec to suggest this.****
>
> ** **
>
> - I also found it helpful to have a 'touch' flag per entry since
> encoding/decoding requires processing of the untouched subset of the
> reference set.****
>
> ** **
>
> - For encoding I also needed to keep track of the number of touches
> (representing the number of times the entry would be explicitly emitted),
> and I needed to make a distinction between no touches and 0 touches
> (representing an implicit emission). This is to support duplicate headers,
> which was tricky to get right.****
>
> ** **
>
> - It would be nice to have explicit bounds for encoded integers, string
> lengths, header lengths, etc. I didn't try to make the encoder/decoder
> streaming, since that would complicate the implementation, but it seems
> difficult to guarantee memory bounds without the above explicit bounds.***
> *
>
> ** **
>
> - It would be nice to clarify the behavior when the max header table size
> is reduced. I just implemented popping from the front until the new bound
> is satisfied.****
>
> ** **
>
> - I didn't find the need to encode index vs. index + 1 too confusing this
> time around. I feel like making the header table start at 1 would simply
> move the off-by-one bugs someplace else. I don't feel too strongly about
> this, though.****
>
> ** **
>
> Comments, pull requests, etc. welcome!****
>
> ** **
>
> -- Fred****
>
>    ** **
>
>
Received on Wednesday, 18 September 2013 22:58:34 UTC

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