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Re: [#153] PUSH_PROMISE headers

From: Roberto Peon <grmocg@gmail.com>
Date: Fri, 28 Jun 2013 22:36:10 -0700
Message-ID: <CAP+FsNcEf6s5s7Jk=NLKdrdU8fV1AsSJ4u-8CZNT8P7YXvxkag@mail.gmail.com>
To: James M Snell <jasnell@gmail.com>
Cc: Martin Thomson <martin.thomson@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>
... because by then you've opened up a stream., and you're back into
problematic territorry.
PUSH_PROMISE exists because we need to indicate to the browser all of the
information it needs to make a determination about whether or not it wants
the stream (and to short circuit the inlining/push mechanism when it
already has what it needs!)
-=R


On Fri, Jun 28, 2013 at 10:34 PM, Roberto Peon <grmocg@gmail.com> wrote:

> Any content negotiation would be an appropriate example. :)
>
> You don't want to have to wait for the HEADERS frame to indicate to the
> client which resource it might already have (it should have the opportunity
> to RST_STREAM if it has it in cache, for instance).
> -=R
>
>
> On Fri, Jun 28, 2013 at 10:25 PM, James M Snell <jasnell@gmail.com> wrote:
>
>> Have an example handy?
>>
>> Here's an example that shows that response headers in the PUSH_PROMISE
>> would not be necessary... Let's say I send a PUSH_PROMISE with the
>> following bits of info...
>>
>> PUSH_PROMISE
>>   :path = /images/f.jpg
>>   :method = GET
>>   :host = example.org
>>   :scheme = http
>>   accept = image/jpeg
>>   if-match: "my-etag1"
>>   cache-control: max-age=1000
>>
>> These headers are giving me everything I would need to determine if
>> there is a matching resource in my local cache. I have the method, I
>> have the etag, I have the cache-control parameters, accept... There's
>> no need for response headers at this point.
>>
>> Later, once I start accepting the frames for the pushed content, I
>> would get something like...
>>
>> HEADERS
>>   :status = 200
>>   content-type: image/jpeg
>>   content-length: 123
>>   etag: "my-etag1"
>>   vary: accept
>>   cache-control: public
>>
>> On the off chance that the PUSH_PROMISE doesn't give me what I need,
>> the follow on HEADERS frame will give me the rest.
>>
>>
>>
>> On Fri, Jun 28, 2013 at 9:55 PM, Roberto Peon <grmocg@gmail.com> wrote:
>> > Depending on how the request might have been been constructed, response
>> > headers may be necessary to identify the resource in the cache, as
>> compared
>> > to the resource specified in the HTML (I'm thinking about vary: stuff).
>> >
>> > -=R
>> >
>> >
>> > On Fri, Jun 28, 2013 at 9:44 PM, James M Snell <jasnell@gmail.com>
>> wrote:
>> >>
>> >> Let's take a step back and consider what a pushed stream is...
>> >>
>> >> A pushed stream is essentially an "Implied GET". This means that a
>> >> server is going to assume that the client was going to send a GET for
>> >> the pushed resource. This also means that the server has to make some
>> >> assumptions about the make up of that implied GET.
>> >>
>> >> Now, consider how HTTP caching works. When a cache receives a request
>> >> for a resource, how does it determine whether or not it has a
>> >> representation of the resource already available? Does it look at the
>> >> request headers or the response headers? Obviously, it looks at the
>> >> request headers. It uses the response headers when populating the
>> >> cache.
>> >>
>> >> So, if we look at the pushed resource sent by the server, what we need
>> >> is for A) the server to first let us know about the implied GET
>> >> request.. which means pushing down a set of request headers then B)
>> >> the server to send the actual resource, which means pushing down the
>> >> response headers.
>> >>
>> >> Already in our design for pushed resources, we have the server sending
>> >> a PUSH_PROMISE frame that contains a header block, followed by a
>> >> HEADERS frame that also contains a headers block. It stands to reason
>> >> that the PUSH_PROMISE frame would contain the set of request headers
>> >> that the server is assuming for the implied GET. These are delivered
>> >> to the client, which uses those to determine whether or not a cached
>> >> representation of the resource is already available (just as any cache
>> >> would do using the request headers). The server would then send it's
>> >> response headers in a HEADERS frame, just as it would any response to
>> >> any other kind of GET.
>> >>
>> >> Two examples to show how this naturally fits... First, let's look at a
>> >> normal GET request sent by the client to the server...
>> >>
>> >> Client                 Server
>> >> ------                 ------
>> >>   |                        |
>> >>   | ---------------------> |
>> >>   |   HEADERS              |
>> >>   |     GET                |
>> >>   |     /images/f.jpg      |
>> >>   |     If-Match: etag1    |
>> >>   |     Accept: image/jpeg |
>> >>   |                        |
>> >>   | <--------------------- |
>> >>   |   HEADERS              |
>> >>   |     200                |
>> >>   |     Content-Type:      |
>> >>   |       image/jpeg       |
>> >>   |     Content-Length:    |
>> >>   |       123              |
>> >>   |                        |
>> >>   | <--------------------- |
>> >>   |   DATA....DATA....     |
>> >>   |                        |
>> >>
>> >> Now consider the same resource being pushed by the server using
>> >> PUSH_PROMISE...
>> >>
>> >> Client                 Server
>> >> ------                 ------
>> >>   |                        |
>> >>   | <--------------------- |
>> >>   |   PUSH_PROMISE         |
>> >>   |     GET                |
>> >>   |     /images/f.jpg      |
>> >>   |     If-Match: etag1    |
>> >>   |     Accept: image/jpeg |
>> >>   |                        |
>> >>   | <--------------------- |
>> >>   |   HEADERS              |
>> >>   |     200                |
>> >>   |     Content-Type:      |
>> >>   |       image/jpeg       |
>> >>   |     Content-Length:    |
>> >>   |       123              |
>> >>   |                        |
>> >>   | <--------------------- |
>> >>   |   DATA....DATA....     |
>> >>   |                        |
>> >>
>> >>
>> >> Note that the only difference here is the direction and type of the
>> >> first frame. Everything else is identical. The PUSH_PROMISE contains
>> >> everything the client needs to determine whether or not it already has
>> >> the resource in it's local cache (request URI, etag, content-type...).
>> >>
>> >> There's no need to get any more complicated than this. We already
>> >> require two distinct header blocks for every request. We already send
>> >> two distinct header blocks for each pushed stream. We already indicate
>> >> that a pushed stream is an implied GET. To make it work, we simply
>> >> state that the PUSH_PROMISE contains the Request headers that the
>> >> server has assumed for the implied GET request, while the HEADERS
>> >> frame sent later contains the Response headers. If the request headers
>> >> in the PUSH_PROMISE end up not being adequate enough to properly
>> >> determine if the resource is already cached, then we treat it as just
>> >> another cache miss.
>> >>
>> >> On Fri, Jun 28, 2013 at 5:21 PM, Martin Thomson
>> >> <martin.thomson@gmail.com> wrote:
>> >> > https://github.com/http2/http2-spec/issues/153
>> >> >
>> >> > The current text describes PUSH_PROMISE as having a few request
>> >> > headers, plus some response headers, but it's quite vague.
>> >> >
>> >> > I think that if this is going to be properly workable across a wide
>> >> > range of uses with lots of different headers, PUSH_PROMISE needs to
>> >> > include two sets of headers: the ones that it overrides from the
>> >> > associated request (:path being foremost of those) and the ones that
>> >> > it provides as a "preview" of the response (e.g., ETag might allow
>> >> > caches to determine if they were interested in the rest of the
>> >> > response).
>> >> >
>> >>
>> >
>>
>
>
Received on Saturday, 29 June 2013 05:36:37 UTC

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