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Re: Fragmentation for headers: why jumbo != continuation.

From: Greg Wilkins <gregw@intalio.com>
Date: Fri, 11 Jul 2014 09:38:21 +1000
Message-ID: <CAH_y2NE=DwpMzjpOWnSqyyOhytha_dQgBcKJQrkss+qtdpp=iw@mail.gmail.com>
To: Roberto Peon <grmocg@gmail.com>
Cc: HTTP Working Group <ietf-http-wg@w3.org>
Roberto,

I agree that this is a concern, even if large headers are only 0.01% (or
whatever it is ) of the traffic.

But I don't think fragmentation on it's own is sufficient. You need
fragmentation and flow control.       To achieve this we need to stop
treating headers as a special case and forget about any deadlines by the
end of the year. I would propose that:

   - We remove HEADERS, CONTINUATION and PUSH_PROMISE from the specification
   - We retain END_SEGMENT in DATA frames
   - Streams are created by sending a stream SETTINGs frame with a PROTOCOL
   parameter!

We now have a multiplexed framing layer that is totally devoid of any
knowledge of HTTP!   The framing supports segmented data streams that are
flow controlled and of unlimited size.    We then come up with an mapping
of HTTP semantics to this framing layer:

   - HTTP streams start with a SETTINGS frame that has PROTOCOL=h2
   - Odd data segments on the stream carry header/trailers.  So a Stream
   with  1 segment is just headers. A stream with 3 segments is headers, data,
   trailers etc.

Now we have to work out how to encode the headers into those data frames.
The stateless parts of HPACK are a pretty reasonable start, using Static-H
gives a 0.66 compression factor.  However, I think there are probably other
alternatives that are less order dependent - eg sending the header set
mutations only on stream 0 and normal decoding does not mutate the table.
If we wanted to make HTTP a bit of a special case, we could go to Linear-H,
with 0.31 compression factor, but then decoding of the headers must be done
in the order they are sent - making header compression part of the framing
layer.... but I could live with a little bit of conflation for efficiency
purposes:)

With this scheme, we could even support lazy proxies that would send
HTTP/1.1 by having PROTOCOL=h1 on a stream and just sending the http/1
bytes unaltered.   Websocket could be supported the same way or it too
could have it's own segmented data mapping.

Even if we go for a less drastic way to do fragmentation of headers, I
think the process has to be the same - start with data frame semantics and
work out how to transport compressed headers.  Don't come up with a
different fragmentation/flowcontrol regime based on the content of the
frame.


cheers








On 11 July 2014 06:27, Roberto Peon <grmocg@gmail.com> wrote:

> There are two separate reasons to fragment headers
>
> 1) Dealing with headers of size > X when the max frame-size is <= X.
> 2) Reducing buffer consumption and latency.
>
> Most of the discussion thus far has focused on #1.
> I'm going to ignore it, as those discussions are occurring elsewhere, and
> in quite some depth :)
>
>
> I wanted to be sure we were also thinking about #2.
>
> Without the ability to fragment headers on the wire, one must know the
> size of the entire set of headers before any of it may be transmitted.
>
> This implies that one must encode the entire set of headers before sending
> if one will ever do transformation of the headers. Encoding the headers in
> a different HPACK context would count as a transformation, even if none of
> the headers were modified.
>
> This means that the protocol, if it did not have the ability to fragment,
> would require increased buffering and increased latency for any proxy by
> design.
>
> This is not currently true for HTTP/1-- the headers can be sent/received
> in a streaming fashion, and implementations may, at their option, choose to
> buffer in order to simplify code.
>
> -=R
>



-- 
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 Thursday, 10 July 2014 23:38:53 UTC

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