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Re: Our Schedule

From: Mark Nottingham <mnot@mnot.net>
Date: Tue, 27 May 2014 03:53:21 +1000
Cc: Patrick McManus <pmcmanus@mozilla.com>, James M Snell <jasnell@gmail.com>, Cory Benfield <cory@lukasa.co.uk>, Greg Wilkins <gregw@intalio.com>, HTTP Working Group <ietf-http-wg@w3.org>
Message-Id: <66150082-2431-47FA-893C-4DB1DB5A47D9@mnot.net>
To: Michael Sweet <msweet@apple.com>
Michael,

On 27 May 2014, at 2:35 am, Michael Sweet <msweet@apple.com> wrote:

> Patrick,
> 
> On May 26, 2014, at 10:45 AM, Patrick McManus <pmcmanus@mozilla.com> wrote:
>> ...
>> I disagree. the fundamental value of http/2 lies in mux and priority, and to enable both of those you need to be able to achieve a high level of parallelism. Due to CWND complications the only way to do that on the request path has been shown to be with a compression scheme. gzip accomplished that but had a security problem - thus hpack. Other schemes are plausible, and ones such as james's were considered, but some mechanism is required.
> 
> I see several key problems with the current HPACK:
> 
> 1. The compression state is hard to manage, particularly for proxies.
> 2. HEADER frames hold up the show (issue #481)
> 3. There is no way to negotiate a connection without Huffman compression of headers (issue #485).
> 
> *If* we can come up with a header compression scheme that does not suffer from these problems, it might be worth the added complexity in order to avoid TCP congestion window issues.  But given that we are already facing 3.5 RTTs worth of latency just to negotiate a TLS connection I'm not convinced that compressing the request headers will yield a user-visible improvement in the speed of their web browsing experience.

The previous discussion that Patrick was referring to has a lot of background.

In a nutshell, he made an argument for header compression a while back (I can dig up the references if you like), where he basically showed that for a very vanilla page load, merely getting the requests out onto the wire (NOT getting any responses) would take something like 8-11 RTs, just because of the interaction between request header sizes and congestion windows. This assumes that the page has 80 assets (the average is not over 100, according to the Web archive), and request headers are around 1400 bytes (again, not uncommon).

In contrast, with compressed headers (his experiment was with gzip), you can serialise all of those requests into one RTT, perhaps even a single packet.

This is a very persuasive argument when our focus is on reducing end-user perceived latency. Its especially persuasive when you think of the characteristics of an average mobile connection.

HPACK is not as efficient as gzip, and as weve said many times, our goal is NOT extremely high compression; rather, its safety. If we could ignore the CRIME attack, we would use gzip instead, and I dont think wed be having this discussion.

Hope this helps,

--
Mark Nottingham   http://www.mnot.net/
Received on Monday, 26 May 2014 17:53:55 UTC

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