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

From: Rob Trace <Rob.Trace@microsoft.com>
Date: Fri, 11 Jul 2014 00:44:06 +0000
To: Greg Wilkins <gregw@intalio.com>, Roberto Peon <grmocg@gmail.com>
CC: HTTP Working Group <ietf-http-wg@w3.org>
Message-ID: <a4ff55d2e509417a831d6d52136dbc95@BL2PR03MB372.namprd03.prod.outlook.com>
I am very much -1 to starting over.  I would prefer to keep the threads to the issues at hand, there is already too much traffic to read…

Thanks!!

-Rob

From: Greg Wilkins [mailto:gregw@intalio.com]
Sent: Thursday, July 10, 2014 4:38 PM
To: Roberto Peon
Cc: HTTP Working Group
Subject: Re: Fragmentation for headers: why jumbo != continuation.


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<mailto: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<mailto: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 Friday, 11 July 2014 00:44:54 UTC

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