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Re: SYN_REPLY

From: 陈智昌 <willchan@chromium.org>
Date: Tue, 26 Feb 2013 20:03:25 -0800
Message-ID: <CAA4WUYiaZ6ftTePFiMkJ5y4rBd2eXjnrzk1c24-VYqAEe0ystw@mail.gmail.com>
To: Roberto Peon <grmocg@gmail.com>
Cc: Amos Jeffries <squid3@treenet.co.nz>, HTTP Working Group <ietf-http-wg@w3.org>
Probably my fault :) My understanding of the thread in respect to this
particular point
* Martin is proposing combining SYN_STREAM+SYN_REPLY+HEADERS into a single
HEADERS frame.
  - single HEADERS frame might contain priority
  - otherwise, we need a PRIORITIZE frame
* I want the initial stream frame (whether it be HEADERS or SYN_STREAM) to
contain a priority.
* Furthermore, I find it weird for subsequent frames carrying the header
name/value block to *also* carry a priority. This is what I objected to as
"tight coupling" of priority with headers name/value blocks in all
HEADERS-esque frames.
  - This somewhat implies we should use separate frames.
* I believe Amos read my statement as arguing against stream
reprioritization over its lifetime. As I've previously said on this mailing
list, I am in favor of experimenting with stream reprioritization.


On Tue, Feb 26, 2013 at 7:53 PM, Roberto Peon <grmocg@gmail.com> wrote:

> I'm about as clear as mud about what we're actually talking about now :)
>
> -=R
>
>
> On Tue, Feb 26, 2013 at 7:49 PM, William Chan (陈智昌) <willchan@chromium.org
> > wrote:
>
>> On Tue, Feb 26, 2013 at 7:37 PM, Amos Jeffries <squid3@treenet.co.nz>wrote:
>>
>>> On 27/02/2013 2:19 p.m., William Chan (陈智昌) wrote:
>>>
>>>> On Tue, Feb 26, 2013 at 4:15 PM, Martin Thomson <
>>>> martin.thomson@gmail.com <mailto:martin.thomson@gmail.**com<martin.thomson@gmail.com>>>
>>>> wrote:
>>>>
>>>>     On 25 February 2013 20:42, William Chan (陈智昌)
>>>>     <willchan@chromium.org <mailto:willchan@chromium.org>**> wrote:
>>>>     > Fully agreed it's more general. I think that unless we go all
>>>>     the way with
>>>>     > ditching SYN_STREAM too (which I disagree with), then I think
>>>>     it's a net
>>>>     > loss (primarily due to more difficulty in grokking the spec) to
>>>>     save a frame
>>>>     > type value and combine SYN_REPLY and HEADERS into one.
>>>>
>>>>     I'm interested in what you feel SYN_STREAM provides that you can't
>>>> get
>>>>     with HEADERS.
>>>>
>>>>     I don't care either way about whether the priority is in the message
>>>>     or not.  So, in the interests of saving those few bytes, that's a
>>>>     feature that could be retained (or even moved to HEADERS).
>>>>
>>>>
>>>> I'm not completely clear here on the stated proposal, so I'll just
>>>> reclarify my position here. I think that the priority should be
>>>> communicated in the same frame which starts the stream, whether that frame
>>>> be called SYN_STREAM or HEADERS. I'm not sure if it makes sense to continue
>>>> including the priority information for followup headers, that may arrive in
>>>> a HEADERS frame. I'm leaning towards saying it does not.
>>>>
>>>>
>>>>     The only other thing is the UNIDIRECTIONAL flag.  This flag is
>>>>     currently redundant: all streams sent by the client are
>>>> bidirectional,
>>>>     and all streams from the server are unidirectional without
>>>> exception.
>>>>
>>>>
>>>> I think in the normal HTTP use case, yes. But when you view HTTP/2 as a
>>>> transport layer for other protocols, then I think it might be reasonable to
>>>> have the server initiate a bidirectional stream. Currently there's no
>>>> binding for that in the web platform, but you could imagine it (register an
>>>> event handler for server initiated streams, rather than relying on hanging
>>>> GETs / client initiated WebSockets). I don't feel strongly here due to not
>>>> having a concrete use case.
>>>>
>>>>
>>>>     As I said in another mail, I'm not sure that SYN_STREAM/SYN_REPLY
>>>>     actually help with understanding the spec.  On the contrary, I think
>>>>     that they lead to false impressions about how streams start.  They
>>>>     imply negotiation, which is far from the case.
>>>>
>>>>
>>>> Intriguing. I did not read the read the earlier email and that was my
>>>> bad. I think I have a bias because it's always been called SYN_STREAM and
>>>> SYN_REPLY and that's how I conceptualize it. I'm willing to say that my
>>>> conceptions on the naming might be very biased and maybe should be
>>>> discounted.
>>>>
>>>> In summary, here's my current position:
>>>> * the first frame for a stream should include its priority (to be
>>>> clear, I don't view the PUSH_PROMISE as belonging to the promised new
>>>> stream, but to the associated stream)
>>>> * it feels weird to me for subsequent frames on the stream that include
>>>> the header name/value block to also include the priority. i don't like the
>>>> tight coupling of that.
>>>>
>>>
>>> I do like it and from earlier readings I'm not alone in that. Priority
>>> needs to be adaptable within the duration of the stream _in total_.
>>> Ignoring the idea one end adjusting priority dynamically.... client can
>>> still name its priority based on objects importance for whatever its user
>>> is doing, and server claim a higher/lower relative priority based on its
>>> own knowledge of the web site/service resource. There is no contradictions
>>> there and adjusting the priority preference after input from both ends
>>> should not be allowed to affect the traffic flow in any major way - at
>>> worst some resources may get slower response time because they initially
>>> claimed lower priority and raising it was rejected by the assigning
>>> algorithm.
>>
>>
>> Just to be clear, I am very open to reprioritization, and in fact do want
>> to experiment with it in HTTP/2. I'm just saying that I feel that it's
>> weird to couple it to whatever frame carries the header name/value block.
>> I'm trying to work through my head the implications here. I think it means
>> that *if* I want to send a follow up HEADERS frame, I'd have to remember
>> the priority of the stream, whereas today I calculate it once based off the
>> resource type and forget it. Not a huge deal, bookkeeping's easy and the
>> extra state is cheap. But it seems nice not to require it.
>>
>>
>>>
>>>
>>>  * i feel less strongly about the naming of SYN_STREAM+SYN_REPLY vs
>>>> HEADERS, after what Martin wrote. i fully admit my mental bias here.
>>>>
>>>
>>> When there are two features largely duplicating the same things bias is
>>> expected. :-)
>>>
>>> Amos
>>>
>>>
>>
>
Received on Wednesday, 27 February 2013 04:03:54 GMT

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