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Re: per stream (initial) flow window size

From: Mark Nottingham <mnot@mnot.net>
Date: Wed, 9 Oct 2013 16:41:02 -0700
Cc: "ietf-http-wg@w3.org Group" <ietf-http-wg@w3.org>
Message-Id: <B230BAED-6140-408B-AC3D-C887639D843F@mnot.net>
To: Ilya Grigorik <ilya@igvita.com>
Hi Ilya,

I've created an issue for this:
  https://github.com/http2/http2-spec/issues/261

Not sure if we'll get to discussing it at the meeting this week.

Cheers,


On 08/10/2013, at 4:14 PM, Ilya Grigorik <ilya@igvita.com> wrote:

> SETTINGS allows us to set a default starting window for all streams + update this default later. However, I think there are compelling cases where it would be beneficial to allow different streams to start with a different initial window size (similar to optional priority in headers frame). Given that this would effectively be a new feature... let me try to motivate why the extra complexity is worth it based on some of the recent work I've been involved with: 
> 
> We want to make browser pre{fetch,rendering} smarter [1,2]. Meaning, we want to be able to discover critical page resources as early as possible and kick off fetches for those assets also. However, since some of these assets are large (and there can be many of them), we need to control how much data we fetch (e.g. if we are too aggressive we may slow down the host page, and/or incur high overhead for the user). 
> 
> A concrete example of this is the work we've been experimenting with in PageSpeed [3], where we are aggressively inlining CSS/JS and even doing "page splits": the rewriter effectively creates two pages, the first is the "prefetch friendly" version which fits in < 15KB (one RTT), and second is the diff that fills in the rest once the navigation is triggered via an XHR -- if that sounds crazy, then that's not too far from the truth. That said, we have a working prototype, and it's showing impressive results.. For example, we can take a mobile wikipedia page, which currently takes 9s (!!!) to first render (on a 3G profile), down to 1.6s [3] by using the combination of prefetch + page split strategies.
> 
> What does flow control have to do this with all this? When issuing a prefetch, we would like to be able to open a request with a lower initial window (e.g. 15KB), and fetch just the head of the document - in most cases, this will allow us to discover the critical resources, initiate requests for them, etc. Then, if the navigation is triggered, we would just increment the window and "resume" the stream to get the rest of the page... This eliminates the need for server rewriting / page splitting, which has a lot of gnarly edge conditions. Handling this at the transport layer would make it much, much simpler and more powerful.
> 
> (Yes, we could lower the initial window for all streams, but that's counter productive in majority of cases... we'd just end up sending a lot more WINDOW_UPDATE frames for non-prefetch request.)
> 
> Further, the ability to set a custom window size also allows us to adjust this logic based on type of asset, prior knowledge of the site, or other signals. For example, the other use case is progressive rendering of images - i.e. using flow control to fetch image layers and having control over where and when to stop. Concretely, we could fetch first X KB, which may provide a reasonable low-res preview of the asset, and then continue fetching subsequent layers until some condition is met (end of file, or max resolution of device is reached -- if you're a 2x device, don't fetch the 3x layers). I'm intentionally skipping over the image container discussions here, since that's a separate conversation.. But I'll note that many sites are already trying to provide this sort of experience, except through rather poor implementation: inline low-res asset, render that, then initiate a separate XHR to fetch image and replace with high-res version. Needless to say, we can do much better, and there is strong interest in providing this sort of functionality natively...
> 
> --
> 
> In short, there are interesting cases where the initiator of the stream may want to control the initial window size: to decrease it, and perhaps even to increase it in some cases. There are multiple ways to achieve this, but one plausible strategy would be to allow an optional SETTINGS_INITIAL_WINDOW_SIZE payload in the headers frame, not unlike the optional priority field -- minimal overhead, no races between initiating the request and WINDOW_UPDATE, etc. 
> 
> Thoughts?
> 
> Ilya
> 
> [1] https://docs.google.com/document/d/1wck0tFTiibKzZDuBeyK0lrKTKsZ5pqfvcR__1r2m834/edit
> [2] http://lists.w3.org/Archives/Public/public-web-perf/2013Aug/0010.html
> [3] https://developers.google.com/speed/pagespeed/optimization 
> [4] http://www.webpagetest.org/result/130715_PZ_08063384bd76cd2206a1b39e8678e438/3/details/

--
Mark Nottingham   http://www.mnot.net/
Received on Wednesday, 9 October 2013 23:41:28 UTC

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