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RE: Header Size? Was: Our Schedule

From: Richard Wheeldon (rwheeldo) <rwheeldo@cisco.com>
Date: Fri, 30 May 2014 16:06:06 +0000
To: Michael Sweet <msweet@apple.com>, Greg Wilkins <gregw@intalio.com>
CC: Matthew Kerwin <matthew@kerwin.net.au>, "Martin J. Dürst" <duerst@it.aoyama.ac.jp>, "David Krauss" <potswa@gmail.com>, Martin Thomson <martin.thomson@gmail.com>, "HTTP Working Group" <ietf-http-wg@w3.org>
Message-ID: <0566CA5E9B906D40B6737DD47DA9FB8F1AFCF12F@xmb-rcd-x04.cisco.com>
Speaking as a proxy developer, I like the idea of putting common header stuff onto frame 0. Common identity-specific stuff (user-agent) can easily be shared. We already do similar things -(re-using state from earlier requests on a keep alive connection. It's a big win over re-sending,


From: Michael Sweet [mailto:msweet@apple.com]
Sent: 30 May 2014 09:14
To: Greg Wilkins
Cc: Matthew Kerwin; "Martin J. Dürst"; David Krauss; Martin Thomson; Richard Wheeldon (rwheeldo); HTTP Working Group
Subject: Re: Header Size? Was: Our Schedule


I don't see shared state like that working for proxies.

On May 30, 2014, at 4:22 AM, Greg Wilkins <gregw@intalio.com<mailto:gregw@intalio.com>> wrote:

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.

On 30 May 2014 09:34, Matthew Kerwin <matthew@kerwin.net.au<mailto:matthew@kerwin.net.au>> wrote:
On 30 May 2014 16:51, "Martin J. Dürst" <duerst@it.aoyama.ac.jp<mailto: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

Greg Wilkins <gregw@intalio.com<mailto:gregw@intalio.com>>
http://eclipse.org/jetty HTTP, SPDY, Websocket server and client that scales
http://www.webtide.com<http://www.webtide.com/>  advice and support for jetty and cometd.

Michael Sweet, Senior Printing System Engineer, PWG Chair
Received on Friday, 30 May 2014 16:06:44 UTC

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