- From: James M Snell <jasnell@gmail.com>
- Date: Fri, 28 Jun 2013 22:25:44 -0700
- To: Roberto Peon <grmocg@gmail.com>
- Cc: Martin Thomson <martin.thomson@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>
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 05:26:31 UTC