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

From: Roberto Peon <grmocg@gmail.com>
Date: Wed, 18 Sep 2013 14:33:34 -0700
Message-ID: <CAP+FsNfVGPjJF_2BdrOW+xjKXPQNQTxaJZn_5iFppJys_9s8tg@mail.gmail.com>
To: Fred Akalin <akalin@google.com>
Cc: HTTP Working Group <ietf-http-wg@w3.org>
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 21:34:01 UTC

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