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

I thought that section refers only to eviction when appending/replacing
(which is definitely lowest index first) as opposed to eviction when
changing the max size (which isn't really treated in the spec).


On Wed, Sep 18, 2013 at 3:58 PM, Roberto Peon <grmocg@gmail.com> wrote:

> 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****
>>
>>    ** **
>>
>>
>