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Re: HEADERS and flow control

From: James M Snell <jasnell@gmail.com>
Date: Fri, 9 May 2014 17:13:19 -0700
Message-ID: <CABP7RbdTM1qy-JbPKcqi=6cNzjvWj54PcMrNYPD=3f_E6xuE2Q@mail.gmail.com>
To: Roberto Peon <grmocg@gmail.com>
Cc: Hasan Khalil <mian.hasan.khalil@gmail.com>, ietf-http-wg@w3.org, Johnny Graettinger <jgraettinger@chromium.org>, Martin Thomson <martin.thomson@gmail.com>
And this: http://tools.ietf.org/html/draft-snell-httpbis-ext-frames-00
On May 9, 2014 5:10 PM, "James M Snell" <jasnell@gmail.com> wrote:

> Just as a reference point:
> https://datatracker.ietf.org/doc/draft-snell-httpbis-ext-frames/?include_text=1
> - James
> On May 9, 2014 5:02 PM, "Roberto Peon" <grmocg@gmail.com> wrote:
>> Assuming BLOCKED is there, it works to write it when it would all fit,
>> though it is ugly.
>> Lets assume this was being used for something RPC like (e.g. websockets+).
>> Here is an exploration in more detail.
>> The alternativesare:
>> New opcode (e.g. METADATA), flow-controlled.
>>   (a) uses HPACK format, but allowed zero state
>>   (b) HPACK context per stream, only interpreted at the eventual
>> endpoint-- proxies do not need to examine this state.
>>   (c) HPACK context per stream, SETTINGS on how much space is allowed
>> globally for these
>> Only write metadata when all of it will fit.
>>   (d) when blocked send BLOCKED frame
>>   (e) when blocked stall that stream indefinitely until enough space is
>> available
>> (f) Ignore flow control/frame doesn't get flow controlled
>> (g) Add a new, different setting which sets an upper bound on the total
>> amount of METADATA allowed to be in flight at once, and require
>> METADATA_ACK (or equivalent)
>> (h) Tell people to put each RPC onto its own stream
>> (i) Tell people to put each RPC onto its own stream, and express
>> dependency using the prioritization scheme. *Require* relative dependencies
>> to be expressed through proxies.
>> (j) Add a new flow control token (e.g. METADATA_WINDOW_UPDATE), which
>> allows for metadata to be sent. Define a setting for this (potentially).
>> As a reminder, the use-case here is something RPC and/or WebSocket like,
>> where we have streams of messages where order may matter.
>> Option (a) never deadlocks, but it allows no compression.
>> Option (b) never deadlocks, and allows compression, but requires an
>> unfortunate amount of memory of a server which wishes to interpret this
>> metadata.
>> Option (c) never deadlocks, allows compression, but requires allocations
>> of more memory at proxies and/or servers, and adds more complexity to
>> negotiations.
>> Option (d) never deadlocks, allows compression, but may stall a
>> connection for some indeterminate period of time.
>> Option (e) can deadlock, allows compression,
>> Option (f) never deadlocks but requires potentially infinite amount of
>> memory consumption in a proxy when a non-malicious actor wishing to make a
>> bunch of RPCs.
>> Option (g) never deadlocks, allows compression without increasing memory
>> footprint, but requires acknowledgement of the receipt of HEADERS/METADATA
>> from the remote side, which increases chattiness and complexity.
>> Option (h) never deadlocks, allows compression, but doesn't allow for an
>> expression of sequencing
>> Option (i) never deadlocks, allows compression, uses little memory, but
>> requires more state tracking and makes prioritization a requirement. Ick.
>> Option (j) never deadlocks, allows compression, uses little memory,
>> requires tracking one additional counter, slightly increases chattiness,
>> but arguably little since it would be tacked onto WINDOWS_UPDATE flow
>> control updates (even if not the same frame).
>> Option(f) is what would happen today. That seems problematic, and would
>> make the use of HTTP2 as the basis of an RPC system difficult .
>> Other options?
>> -=R
>> On Fri, May 9, 2014 at 3:55 PM, Johnny Graettinger <
>> jgraettinger@chromium.org> wrote:
>>> How would the interaction of stream flow control and continuations be
>>> managed? A stalled stream may also stall the connection, including control
>>> frames. Could that also lead to deadlock?
>>> On Fri, May 9, 2014 at 6:18 PM, Roberto Peon <grmocg@gmail.com> wrote:
>>>> On Fri, May 9, 2014 at 1:47 PM, Martin Thomson <
>>>> martin.thomson@gmail.com> wrote:
>>>>> /me still needs more information.
>>>>> This is a change.  The change needs greater justification than "it
>>>>> might be nice".  So far, that's all I've heard.
>>>> I have non-browser customers who are looking at HTTP2 as it stands
>>>> right now and are pointing out that this will become problematic if HEADERS
>>>> is actually used as metadata (as it was intended to be) as opposed to
>>>> simply to create streams. There is no guarantee that the metadata is going
>>>> to be small. They're mostly willing to swallow the idea that a large block
>>>> of metadata would (effectively) pause multiplexing for a short time, but
>>>> worried about the fact that this large metadata actually could account for
>>>> a fair bit of buffer, and that the product that HTTP2 (etc) would replace
>>>> did have an operational problem with the lack of this in the past.
>>>> Hmm.. in scanning the document I think we don't have the requirement in
>>>> there that HEADERS and DATA have a sequence relationship that must be
>>>> maintained. We've spoken about it numberous times, but I think we
>>>> overlooked getting that in there.
>>>>> I actually think that this is nice.  But nice doesn't cut it for me.
>>>>> Given the likelihood that header blocks after the first will be used,
>>>>> this is just another corner case.  If we use HTTP/1.1 as a guide, the
>>>>> best analogy there is to chunk extensions and trailers, i.e.,
>>>>> basically zero use.
>>>>> Is the intent to flow control PUSH_PROMISE too?
>>>> No-- PUSH_PROMISE is necessary for stream creation. Flow controlling it
>>>> might engender protocol-induced deadlock.
>>>>> (In case you haven't have noticed, I want to finish up here.)
>>>> I know. I'll point out I haven't been making trouble unless I have
>>>> people actually wanting to use the HTTP2 for stuff!
>>>> Better, the folks who are complaining about this to me are creating
>>>> implementations now, and as far as I can tell, no-one else out there is.
>>>> That implies that this change will not slow down interop.
>>>> -=R
Received on Saturday, 10 May 2014 00:13:49 UTC

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