Re: proposed WINDOW_UPDATE text for session flow control windows
Martin Thomson <martin.thomson@gmail.com> Wed, 20 February 2013 21:44 UTC
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Date: Wed, 20 Feb 2013 13:43:00 -0800
Message-ID: <CABkgnnUH5_t4BesWz2K5gtR0k0+ve42k-EWEaObK_nteh9fOUw@mail.gmail.com>
From: Martin Thomson <martin.thomson@gmail.com>
To: "William Chan (陈智昌)" <willchan@chromium.org>
Cc: Roberto Peon <grmocg@gmail.com>, Mark Nottingham <mnot@mnot.net>, Patrick McManus <mcmanus@ducksong.com>, HTTP Working Group <ietf-http-wg@w3.org>
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Subject: Re: proposed WINDOW_UPDATE text for session flow control windows
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I believe that many people will want to turn it off and keep it off. Besides, if you send multiple huge WINDOW_UPDATE frames, your peer (who is tracking the window size) will be forced to send you a RST_STREAM when their window goes off the scale. Another alternative: Send a WINDOW_UPDATE with a value of zero. This could temporarily disable flow control on the referenced stream (or the session for stream ID 0). Similarly, sending a SETTINGS_INITIAL_WINDOW_SIZE value of zero would disable flow control for all new streams. The current doc implicitly permits a value of zero on WINDOW_UPDATE, with zero net effect. A zero on SETTINGS_INITIAL_WINDOW_SIZE would be bad, and thus might be considered "reserved" in a special kind of way. More flags is always an option, but that seems like it is a heavier, blunter instrument than this. On 20 February 2013 12:10, William Chan (陈智昌) <willchan@chromium.org> wrote: > Do we need a protocol mechanism for this? What's wrong with advertising a > huge window and sending huge WINDOW_UPDATE frames as needed? Is the goal to > allow implementations to actually not implement receiver flow control at > all, and just disable it? Considering implementations always need to support > the peer's receive window advertisements, I view the extra implementation > difficulty of supporting receive windows correctly as pretty marginal. That > said, I do guess it's possible for implementations to set it to extremely > high, and in their testing never hit those window limits, and have > everything hang because they never sent out more WINDOW_UPDATEs. I do admit > that'd be bad. > > I guess I don't feel too strongly. Just wanted to make sure there was an > agreement that we needed an extra protocol mechanism rather than just > advertising huge windows and updating as needed. > > > On Wed, Feb 20, 2013 at 12:02 PM, Roberto Peon <grmocg@gmail.com> wrote: >> >> Two proposals: >> >> 1) Assuming we need/want this on a per-stream basis: >> Use a flag value (lets say 0x2) with the delta-window-size field set to >> all bits on (i.e. 0xFFFFFF). >> all-bits-on is currently illegal (the top bit is reserved so that we might >> use negative window updates in the future, should we decide to do that), so >> this should be quite difficult to screw up. >> >> Setting this on stream '0' would disable the session-level flow control. >> >> 2) Change SETTINGS so that it also includes the flag byte. This would be >> accomplished by adding a new settings ID (10). The LSB of that settings >> value would be interpreted as the flag-byte, as if received as in scheme 1. >> Thus, a settings with id:7, value 0xFFFFFFFF and with id:10 value:0x00000002 >> would disable flow control >> >> >> >> >> On Wed, Feb 20, 2013 at 9:16 AM, Martin Thomson <martin.thomson@gmail.com> >> wrote: >>> >>> I need to know how to advertise an infinite window so that receivers >>> are able to turn flow control off. Does anyone want to propose >>> something? >>> >>> On 19 February 2013 14:54, William Chan (陈智昌) <willchan@chromium.org> >>> wrote: >>> > I've sent out the first pull request for >>> > SETTINGS_MAX_NUM_CONCURRENT_STREAMS. After that goes in, I'll rebase >>> > and >>> > re-run the HTML generator for the flow control change, and send a pull >>> > request for that. >>> > >>> > On Tue, Feb 19, 2013 at 2:48 PM, Mark Nottingham <mnot@mnot.net> wrote: >>> >> >>> >> William, can you send a pull request for your changes? >>> >> >>> >> Patrick, if you want to open an issue to remind us to revisit negative >>> >> window updates, please feel free. >>> >> >>> >> Cheers, >>> >> >>> >> >>> >> On 16/02/2013, at 11:10 AM, William Chan (陈智昌) <willchan@chromium.org> >>> >> wrote: >>> >> >>> >> > Thanks for the thoughts here. I will need to investigate on our end >>> >> > how >>> >> > much RAM we see get consumed here and if this would bring practical >>> >> > wins. If >>> >> > you feel strongly or anyone else supports this, let's add protocol >>> >> > support. >>> >> > Otherwise, out of inclination for fewer features and also a mild >>> >> > fondness >>> >> > for being able to be stricter in the protocol (enforcing >>> >> > WINDOW_UPDATE >>> >> > compliance). I don't feel strongly and I'm happy to revisit later >>> >> > on. This >>> >> > part is easy to change if desired. >>> >> > >>> >> > >>> >> > On Tue, Feb 12, 2013 at 6:54 AM, Patrick McManus >>> >> > <mcmanus@ducksong.com> >>> >> > wrote: >>> >> > >>> >> > >>> >> > >>> >> > On Sun, Feb 10, 2013 at 2:14 PM, William Chan (陈智昌) >>> >> > <willchan@chromium.org> wrote: >>> >> > Do servers often have a need to immediately revoke buffer size >>> >> > promises? >>> >> > In absence of negative window updates, I would think servers would >>> >> > just stop >>> >> > sending WINDOW_UPDATEs. Is that mechanism insufficient? >>> >> > >>> >> > >>> >> > s/servers/receivers >>> >> > >>> >> > In this case I was thinking about firefox. In general we don't have >>> >> > a >>> >> > ram budget for transactions in the way a server does, so the >>> >> > reasonable >>> >> > thing to do in the general case is to set flow control to a very >>> >> > high value >>> >> > to ensure it isn't a choke point, right? However, RAM does have a >>> >> > way of >>> >> > suddenly appearing to be low and we get notifications of that. Lots >>> >> > of times >>> >> > this is due to other unrelated system activity - this is especially >>> >> > true on >>> >> > mobile. Currently we do a handful of things in reaction to that >>> >> > (dumping >>> >> > decoded image caches for example). Another reasonable reaction to >>> >> > that is to >>> >> > squelch some active streams and shrink their associated buffers.. >>> >> > this is >>> >> > the context I was thinking about. >>> >> > >>> >> > waiting for a very large window to drain via lack-of-updates could >>> >> > take >>> >> > an extremely long time. >>> >> > >>> >> > >>> >> > All in all, I don't feel very strongly on this. I'd rather hear from >>> >> > more proxy/server vendors that they want this, rather than adding it >>> >> > in just >>> >> > because it might be useful. Or are you suggesting that Firefox would >>> >> > like to >>> >> > use this? >>> >> > >>> >> > >>> >> > >>> >> > >>> >> >>> >> -- >>> >> Mark Nottingham http://www.mnot.net/ >>> >> >>> >> >>> >> >>> > >>> >> >
- proposed WINDOW_UPDATE text for session flow cont… William Chan (陈智昌)
- Re: proposed WINDOW_UPDATE text for session flow … Patrick McManus
- Re: proposed WINDOW_UPDATE text for session flow … William Chan (陈智昌)
- RE: proposed WINDOW_UPDATE text for session flow … RUELLAN Herve
- RE: proposed WINDOW_UPDATE text for session flow … Roberto Peon
- Re: proposed WINDOW_UPDATE text for session flow … Patrick McManus
- Re: proposed WINDOW_UPDATE text for session flow … William Chan (陈智昌)
- Re: proposed WINDOW_UPDATE text for session flow … William Chan (陈智昌)
- Re: proposed WINDOW_UPDATE text for session flow … Mark Nottingham
- Re: proposed WINDOW_UPDATE text for session flow … William Chan (陈智昌)
- Re: proposed WINDOW_UPDATE text for session flow … Martin Thomson
- Re: proposed WINDOW_UPDATE text for session flow … Roberto Peon
- Re: proposed WINDOW_UPDATE text for session flow … William Chan (陈智昌)
- Re: proposed WINDOW_UPDATE text for session flow … Martin Thomson
- Re: proposed WINDOW_UPDATE text for session flow … Roberto Peon
- Re: proposed WINDOW_UPDATE text for session flow … Martin Thomson
- Re: proposed WINDOW_UPDATE text for session flow … Roberto Peon
- Re: proposed WINDOW_UPDATE text for session flow … Martin Thomson
- Re: proposed WINDOW_UPDATE text for session flow … Mark Nottingham
- Re: proposed WINDOW_UPDATE text for session flow … Martin Thomson
- Re: proposed WINDOW_UPDATE text for session flow … Roberto Peon
- Re: proposed WINDOW_UPDATE text for session flow … William Chan (陈智昌)
- Re: proposed WINDOW_UPDATE text for session flow … Martin Thomson