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Re: Out-of-order Frames

From: Sam Pullara <spullara@gmail.com>
Date: Sat, 22 Jun 2013 13:15:43 -0700
Cc: HTTP Working Group <ietf-http-wg@w3.org>
Message-Id: <6DCDC475-1017-4F3B-8ED8-4AB840596CA9@gmail.com>
To: William Chan (陈智昌) <willchan@chromium.org>
I'm all for brainstorming. The current solution works basically like this:

Chunk 1 (sent immediately):

<div id="1"></div>

Chunk 2 (calculated completed and sent later):
<script>document.getElementById("1").innerHTML = "<span>Hi William Chan!</span>";</script>
Even if the browser can do no work before receiving all the data it is still better because the bytes are sent sooner than they normally would be sent. However, I think that the browser would be able to parse complete DOM elements in the fragments to do even more work before the page has been completely rendered on the server. The contract with the browser might be that delayed frames must be complete elements like in the example. Rather than out of order frames we might have some way of specifying a named frame to be replaced later. There are a number of possible implementations.


On Jun 22, 2013, at 1:06 PM, William Chan (陈智昌) <willchan@chromium.org> wrote:

> OK, it sounds like you want to be able to send out of order byte sequences for a single resource, by sending out of order frames for the same HTTP/2 stream. This is not possible without changing HTTP semantics, and HTTP/2 is primarily only working on improving the mapping of HTTP message semantics onto a connection.
> Btw, why would you want to do this? The browser rendering engine cannot do anything with just frame 1 and frame 3 anyway. HTML parsing requires in-order parsing of the document.
> Stepping back, I still don't fully understand the problem with the existing solution that relies on script to dynamically fill in the DOM nodes as they become available. Can you clarify what you are hoping HTTP2 can do that would be better? Maybe there's a better solution here?
> On Sat, Jun 22, 2013 at 12:52 PM, Sam Pullara <spullara@gmail.com> wrote:
> Sorry, let me give a concrete example. You have an HTML page that ultimately needs to look like:
> <html>
> <body>
> <span>Hi William Chan!</span>
> </body>
> </html>
> In order for me to figure out the contents of the span I need to check an authentication database and do some kind of hard computation that might take several 100 ms. What I am proposing is that you could send 3 "frames" or "chunks" or whatever is the final form like this:
> Stream:
>  Frame 1:
>  <html>
>  <body>
>  Frame 3:
>  </body>
>  </html>
> … some time later …
>  Frame 2:
>  <span>Hi William Chan!</span>
> The client then reconstructs the final document, optionally doing some processing on the parts of the document it received before completion. I'm pretty sure the protocol doesn't allow out of order frames within a stream right now. In section 3.4 it states:
>    o  The order in which frames are sent within a stream is significant.
>       Recipients are required to process frames in the order they are
>       received.
> Sam
> On Jun 22, 2013, at 12:47 PM, William Chan (陈智昌) <willchan@chromium.org> wrote:
>> I'm having difficulty fully understanding the problem and the proposal. Can you clarify?
>> * What do you mean by "frame"? Do you mean frames in the HTTP/2 framing layer? Frames in the framing layer can appear in any order (subject to a few rules), so I don't know what out-of-order frames means. Or do you mean like video frames?
>> * If you meant video frames, then if you separate the video into multiple resources, there's no reason the resources can't be sent "out of order", since at the HTTP level there's no concept of order among resources.
>> On Sat, Jun 22, 2013 at 12:34 PM, Sam Pullara <spullara@gmail.com> wrote:
>> Commonly in dynamically generated websites there are sections of content that are static and parts that are calculated on a per request basis. The current best practice for accelerating the delivery of a page like this involves leaving identifiable DOM elements where the dynamic content would appear, flushing the entire static page, and then flushing JavaScript script nodes as calculations complete (e.g. Facebook's BigPipe and deferred rendering in mustache.java). This practice only works for HTML pages (with JavaScript enabled) and offers no acceleration for other types of content delivered over HTTP.
>> One possible solution to this problem would be to allow for out-of-order frames where the static frames are sent as quickly as the connection allows and dynamically generated frames are then sent later as they become available on the server. We would likely not want to enable this in general and would likely need to negotiate this behavior between client and server. Looking at the spec, frames might not be the right place but something on top of frames because of the size limitations.
>> Has something like this been discussed before? Would this be the right mechanism or are there better ways to do it?
>> Thanks,
>> Sam
Received on Saturday, 22 June 2013 20:16:07 UTC

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