Re: [hybi] permessage-deflate parameter negotiation

Takeshi Yoshino <tyoshino@google.com> Thu, 13 June 2013 15:31 UTC

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From: Takeshi Yoshino <tyoshino@google.com>
Date: Fri, 14 Jun 2013 00:31:01 +0900
Message-ID: <CAH9hSJZwMFUWTyk6jjmAN2oMxveVhY=SMs8NLCUHy47Uc+Mj1Q@mail.gmail.com>
To: Tobias Oberstein <tobias.oberstein@tavendo.de>
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Cc: "hybi@ietf.org" <hybi@ietf.org>
Subject: Re: [hybi] permessage-deflate parameter negotiation
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On Thu, Jun 13, 2013 at 2:20 AM, Tobias Oberstein <
tobias.oberstein@tavendo.de> wrote:

> After implementing permessage-deflate client/server and with all features,
> I think the initial
> parameter negotiation is by far the most complex part of the extension.
>
> I still have some questions. To keep it simple, here are a couple of
> examples:
>
> 1)
> client offers:
> permessage-deflate; c2s_max_window_bits
>
> server response:
> permessage-deflate; s2c_no_context_takeover
>

This is fine. The client may reset context if it wants.

Sorry about Chrome's bug. It rejects this now.


>
> 2)
> client offers:
> permessage-deflate; c2s_max_window_bits
>
> server response:
> permessage-deflate; s2c_max_window_bits=10
>

Fine. The client should be able to accept data compressed using any wbits
value.


>
> 3)
> client offers:
> permessage-deflate; c2s_max_window_bits ; s2c_max_window_bits=10
>
> server response:
> permessage-deflate; s2c_max_window_bits=8
>
>
Fine, too.


> 4)
> client offers:
> permessage-deflate; c2s_max_window_bits; s2c_max_window_bits=10
>
> server response:
> permessage-deflate; s2c_max_window_bits=12
>
>
Bad. Maximum wbits the client can afford is 10.


> ===
>
> Chrome Canary fails 1 + 2 (bails out "Received an unexpected
> permessage-deflate extension parameter").
>
> Chrome offers a fixed "permessage-deflate; c2s_max_window_bits", so I
> could not test 3 and 4.
>
> The current draft (in my reading) allows 1 + 2, but not 3, and not 4.
>
>
I'll fix it to allow 3.


> I think 1-3 should be successful, and 4 failed by client.
>
> As in my other mail, I also think a client should explicitly announce
> support for "c2s_no_context_takeover":
>
> permessage-deflate; c2s_max_window_bits; c2s_no_context_takeover
>
> /Tobias
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