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Re: Header Size? Was: Our Schedule

From: Greg Wilkins <gregw@intalio.com>
Date: Fri, 30 May 2014 10:22:45 +0200
Message-ID: <CAH_y2NGu_fJeGpM5j39KDdBpFPEqpnZVE+7LVU2_Q43p+3Mkcw@mail.gmail.com>
To: Matthew Kerwin <matthew@kerwin.net.au>
Cc: Martin J. Dürst <duerst@it.aoyama.ac.jp>, David Krauss <potswa@gmail.com>, Martin Thomson <martin.thomson@gmail.com>, "Richard Wheeldon (rwheeldo)" <rwheeldo@cisco.com>, HTTP Working Group <ietf-http-wg@w3.org>
Matthew,

firstly I'm sure there are forms of header compression that can have a
shared state table that are not so highly order dependent.  The problem
with hpack is that every field in every frame of stream can mutate the
shared state table.  This gives us a really hard serialisation problem, so
that setting the table size to zero does not help, as you still have to
prevent interleaving so you can decode in order and watch for an increase
in the table size.

I think we could get a lot of benefit from a compression scheme that uses
header frames transmitted on channel 0 to set the shared state.   All the
user-agent guff could then be sent once and only once and all the stream
header decoding would then be read only (and thus could happen in any
order).    If you wanted to put cookies into the shared table, then there
are still some ordering issues, but not as hard as the current ones and
with lots of potential solutions (eg table versions or multiple table ids
etc.).

To Martins idea,  that would help in some respects if the subsequent frames
are excluded from hpack.  This would let us allow interleaving.  However,
it does not prevent the server from needing to hold onto large header
tables during request handling.  So we still should include headers in the
flow control, so the receiver can say "stop already!" when large headers
are being sent.

cheers



On 30 May 2014 09:34, Matthew Kerwin <matthew@kerwin.net.au> wrote:

> On 30 May 2014 16:51, "Martin J. Dürst" <duerst@it.aoyama.ac.jp> wrote:
>
>> This is just a thought:
>>
>> Would it be possible to allow arbitrarily large amounts of header data
>> (either via continuations or via multiple header frames), but to limit
>> compression to a single header frame.
>>
>> While in general, there is a stronger need to compress larger stuff, such
>> a solution could come with various benefits:
>> - Simplified compression (less/no state)
>> - Keep the main benefit (quick start)
>> - Penalty against large amounts of header data
>>   (because that's not the way to do things anyway)
>>
>> Regards,   Martin.
>>
>>
> If you send SETTINGS_HEADER_TABLE_SIZE=0 and a HEADERS with [0x30,0x20] in
> the first block fragment you effectively disable the context, and are left
> with only Huffman coding (which has a per-frame context).
>
> As Roberto reminded me yesterday, the thing about a header block is that
> when it ends, you get everything else in the reference set (carried over
> from the previous header block). The biggest gain in HPACK compression
> comes from not actually sending identical headers again and again, which
> means not only sharing context between multiple frames, but between frames
> from multiple streams. I don't know if, in practice, any per-frame
> compression scheme would come close to HPACK's connection-based delta
> compression, and that would be a big hit to the protocol's appeal.
>
>
> --
>   Matthew Kerwin
>   http://matthew.kerwin.net.au/
>



-- 
Greg Wilkins <gregw@intalio.com>
http://eclipse.org/jetty HTTP, SPDY, Websocket server and client that scales
http://www.webtide.com  advice and support for jetty and cometd.
Received on Friday, 30 May 2014 08:23:15 UTC

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