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Re: Negotiating compression

From: Michael Sweet <msweet@apple.com>
Date: Thu, 29 May 2014 11:00:32 -0400
Cc: Jason Greene <jason.greene@redhat.com>, Willy Tarreau <w@1wt.eu>, Martin Thomson <martin.thomson@gmail.com>, Simone Bordet <simone.bordet@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>
Message-id: <125168CA-1243-4913-9D67-ED1EF1D4FBB4@apple.com>
To: David Krauss <potswa@gmail.com>
David,

On May 29, 2014, at 10:27 AM, David Krauss <potswa@gmail.com> wrote:
> On 2014Ė05Ė29, at 9:51 PM, Jason Greene <jason.greene@redhat.com> wrote:
> 
>> 
>>> On May 29, 2014, at 1:40 AM, Willy Tarreau <w@1wt.eu> wrote:
>>> 
>>> These ones could be advertised in the ALPN name (h2 = failsafe, h2h =
>>> hpack version for example) so that we don't need an extra round trip
>>> to know what is supported.
>> 
>> You mean disable/disallow the Huffman encoding bit in HPACK right? HPACK with a size 0 table is easy for embedded devices (and everyone else), offers decent reduction in header sizes, and runs no risk of a CRIME style attack.
> 
> Iíve just started debugging an 8-bit friendly Huffman decoder. It looks like under a kilobyte of code & tables, and on the order of 100 instructions executed per symbol. (Could be less than a hundred, could be more. Havenít profiled yet.)
> 
> Huffman is just a drop in the bucket. Iíll share the code when itís working.

I do appreciate your efforts.  But 100 instructions per byte is at least one order of magnitude more instructions than a literal copy of those bytes requires...  For 16k worth of header data that translates into 1.6 million instructions executed just for one header frame - fairly significant for an embedded processor...

For point of reference, proxying Richard's ~6B requests would require on the order of 10^15 instructions to be executed to decode all of the headers, and then perhaps another 10^15 instructions to re-encode them (depends on how it stores its header tables).  Perhaps not unreasonable for fast proxy hardware over a period of a day, but is it necessary?

_________________________________________________________
Michael Sweet, Senior Printing System Engineer, PWG Chair


Received on Thursday, 29 May 2014 15:01:11 UTC

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