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Design FAQs, Was: Our Schedule

From: Greg Wilkins <gregw@intalio.com>
Date: Mon, 26 May 2014 21:29:45 +0200
Message-ID: <CAH_y2NFAeCfAbbU-FCfgF9iie0zu+w5TqiXe-oC4mKaq+RaGDA@mail.gmail.com>
To: HTTP Working Group <ietf-http-wg@w3.org>

thanks for the recap of the argument for small request size.    I know that
the reasoning behind every design decision cannot be put into the draft,
but it would be really good if  some way could be found that didn't leave
such knowledge only in the email archives.    Perhaps a HTTP2 FAQ would
avoid re-runs of frequent discussions.

I certain accept the reasoning behind wanting to fit many requests into a
single CWIN to avoid round trips.   I also accept that gzip is not suitable
for security reasons.

But I think there are many more FAQs needed to explain HPACK and other
aspects of HTTP/2

 + why is hpack streaming?  Its design means that common fields like the
method are likely to be emitted at the end thus requiring the whole headers
to be buffered anyway and the server must apply a max header size anyway.
 + why a single reference table? Wont this be inefficient for connections
that aggregate unrelated streams?
 + why a dynamic reference table that can be mutated by any stream?
reference table(s) that can only be mutated by stream 0 would allow other
streams to progress in parallel without serialisation between streams.
 + Does HPACK really request contiguous header frames without flow
control?  It looks like a maximum size will be applied to the initial
headers anyway, so with that known head of line blocking can be avoided.
 + I know it has been explained to me before, the END_STREAM bit that
doesn't mean the end of the stream is another FAQ that really needs to be

If the WG wants to get more feedback from a wider audience, then they are
just going to get questions like these asked again and again unless some
effort is made to pro actively explain some of the more surprising aspects
of the design.


On 26 May 2014 19:53, Mark Nottingham <mnot@mnot.net> wrote:

> 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. It’s especially persuasive when you think of the
> characteristics of an average mobile connection.
> HPACK is not as efficient as gzip, and as we’ve said many times, our goal
> is NOT extremely high compression; rather, it’s safety. If we could ignore
> the CRIME attack, we would use gzip instead, and I don’t think we’d be
> having this discussion.
> Hope this helps,
> --
> Mark Nottingham   http://www.mnot.net/

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 Monday, 26 May 2014 19:30:15 UTC

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