- From: James M Snell <jasnell@gmail.com>
- Date: Wed, 1 May 2013 10:26:26 -0700
- To: William Chan (陈智昌) <willchan@chromium.org>
- Cc: Roberto Peon <grmocg@gmail.com>, HTTP Working Group <ietf-http-wg@w3.org>, Martin Thomson <martin.thomson@gmail.com>
Why not just bring the UNIDIRECTIONAL flag back as a PUSH_PROMISE frame-specific flag? If a PUSH_PROMISE frame has the unidirectional flag set, the stream is automatically half-closed in the return direction. If the flag is unset, the promised stream remains half-open until the client half-closes or a rst_stream is sent. On Mon, Apr 29, 2013 at 2:44 PM, William Chan (陈智昌) <willchan@chromium.org> wrote: > Remember we originally *had* a flag for UNIDIRECTIONAL, which we removed > because it was redundant in the traditional HTTP use cases. > > > On Mon, Apr 29, 2013 at 6:39 PM, Roberto Peon <grmocg@gmail.com> wrote: >> >> At worst, we burn a flag which states it is half-closed or unidirectional, >> or provide some other information which identifies the IANA port number for >> the overlayed protocol or something. >> Anyway, *shrug*. >> -=R >> >> >> On Mon, Apr 29, 2013 at 2:32 PM, William Chan (陈智昌) >> <willchan@chromium.org> wrote: >>> >>> On Mon, Apr 29, 2013 at 6:17 PM, James M Snell <jasnell@gmail.com> wrote: >>>> >>>> +1 on this. I like this approach. >>>> >>>> On Apr 29, 2013 2:15 PM, "Roberto Peon" <grmocg@gmail.com> wrote: >>>>> >>>>> I had thought to provide no explicit limit for PUSH_PROMISE, just as >>>>> there is no limit to the size of a webpage, or the number of links upon it. >>>>> The memory requirements for PUSH are similar or the same (push should >>>>> consume a single additional bit of overhead per url, when one considers that >>>>> the URL should be parsed, enqueued, etc.). >>>>> If the browser isn't done efficiently, or, the server is for some >>>>> unknown reason being stupid and attempting to DoS the browser with many >>>>> resources that it will never use, then the client sends RST_STREAM for the >>>>> ones it doesn't want, and makes a request on its own. all tidy. >>> >>> >>> I don't feel too strongly here. I do feel like this is more of an edge >>> case, possibly important for forward proxies (or reverse proxies speaking to >>> backends over a multiplexed channel like HTTP/2). It doesn't really matter >>> for my browser, so unless servers chime in and say they'd prefer a limit, >>> I'm fine with this. >>> >>>>> >>>>> As for PUSH'd streams, the easiest solution is likely to assume that >>>>> the stream starts out in a half-closed state. >>> >>> >>> I looked into our earlier email threads and indeed this is what we agreed >>> on (http://lists.w3.org/Archives/Public/ietf-http-wg/2013JanMar/1106.html). >>> I voiced some mild objection since if you view the HTTP/2 framing layer as a >>> transport for another application protocol, then bidirectional server >>> initiated streams might be nice. But in absence of any such protocol, this >>> is a nice simplification. >>> >>>>> >>>>> -=R >>>>> >>>>> >>>>> On Mon, Apr 29, 2013 at 12:33 PM, William Chan (陈智昌) >>>>> <willchan@chromium.org> wrote: >>>>>> >>>>>> On Mon, Apr 29, 2013 at 3:46 PM, James M Snell <jasnell@gmail.com> >>>>>> wrote: >>>>>>> >>>>>>> >>>>>>> On Apr 29, 2013 11:36 AM, "William Chan (陈智昌)" >>>>>>> <willchan@chromium.org> wrote: >>>>>>> > >>>>>>> [snip] >>>>>>> >>>>>>> > >>>>>>> > >>>>>>> > Oops, forgot about that. See, the issue with that is now we've made >>>>>>> > PUSH_PROMISE as potentially expensive as a HEADERS frame, since it does more >>>>>>> > than just simple stream id allocation. I guess it's not really a huge issue, >>>>>>> > since if it's used correctly (in the matter you described), then it >>>>>>> > shouldn't be too expensive. If clients attempt to abuse it, then servers >>>>>>> > should probably treat it in a similar manner as they treat people trying to >>>>>>> > abuse header compression in all other frames with the header block, and kill >>>>>>> > the connection accordingly. >>>>>>> > >>>>>>> >>>>>>> Not just "potentially" as expensive.. As soon as we get a push >>>>>>> promise we need to allocate state and hold onto it for an indefinite period >>>>>>> of time. We do not yet know exactly when that compression context can be let >>>>>>> go because it has not yet been bound to stream state. Do push streams all >>>>>>> share the same compression state? Do those share the same compression state >>>>>>> as the originating stream? The answers might be obvious but they haven't yet >>>>>>> been written down. >>>>>> >>>>>> >>>>>> I guess I don't see per-stream state as being that expensive. >>>>>> Compression contexts are a fixed state on a per-connection basis, meaning >>>>>> that additional streams don't add to that state. The main cost, as I see it, >>>>>> is the decompressed headers. I said potentially since that basically only >>>>>> means the URL (unless there are other headers important for caching due to >>>>>> Vary), and additional headers can come in the HEADERS frame. Also, >>>>>> PUSH_PROMISE doesn't require allocating other state, like backend/DB >>>>>> connections, if you only want to be able to handle (#MAX_CONCURRENT_STREAMs) >>>>>> of those backend connections in parallel. >>>>>> >>>>>> If they're not specified, then we should specify it, but I've always >>>>>> understood the header compression contexts to be directional and apply to >>>>>> all frames sending headers in a direction. Therefore there should be two >>>>>> compression contexts in a connection, one for header blocks being sent and >>>>>> one for header blocks being received. If this is controversial, let's fork a >>>>>> thread and discuss it. >>>>>> >>>>>>> >>>>>>> >> >>>>>>> >> >>>>>>> >> > As far as the potential problem above, the root problem is that >>>>>>> >> > when you >>>>>>> >> > have limits you can have hangs. We see this all the time today >>>>>>> >> > with browsers >>>>>>> >> > (it's only reason people do domain sharding so they can bypass >>>>>>> >> > limits). I'm >>>>>>> >> > not sure I see the value of introducing the new proposed limits. >>>>>>> >> > They don't >>>>>>> >> > solve the hangs, and I don't think the granularity addresses any >>>>>>> >> > of the >>>>>>> >> > costs in a finer grained manner. I'd like to hear clarification >>>>>>> >> > on what >>>>>>> >> > costs the new proposed limits will address. >>>>>>> >> >>>>>>> >> I don't believe that the proposal improves the situation enough >>>>>>> >> (or at >>>>>>> >> all) to justify the additional complexity. That's something that >>>>>>> >> you >>>>>>> >> need to assess for yourself. This proposal provides more granular >>>>>>> >> control, but it doesn't address the core problem, which is that >>>>>>> >> you >>>>>>> >> and I can only observe each other actions after some delay, which >>>>>>> >> means that we can't coordinate those actions perfectly. Nor can >>>>>>> >> be >>>>>>> >> build a perfect model of the other upon which to observe and act >>>>>>> >> upon. >>>>>>> >> The usual protocol issue. >>>>>>> > >>>>>>> > >>>>>>> > OK then. My proposal is to add a new limit for PUSH_PROMISE frames >>>>>>> > though, separately from the MAX_CONCURRENT_STREAMS limit, since PUSH_PROMISE >>>>>>> > exists as a promise to create a stream, explicitly so we don't have to count >>>>>>> > it toward the existing MAX_CONCURRENT_STREAMS limit (I searched the spec and >>>>>>> > this seems to be inadequately specced). Roberto and I discussed that before >>>>>>> > and may have written an email somewhere in spdy-dev@, but I don't think >>>>>>> > we've ever raised it here. >>>>>>> > >>>>>>> >>>>>>> Well, there is an issue tracking it in the github repo now, at >>>>>>> least. As currently defined in the spec, it definitely needs to be >>>>>>> addressed. >>>>>> >>>>>> Great. You guys are way better than I am about tracking all known >>>>>> issues. I just have it mapped fuzzily in my head :) >>>>> >>>>> >>> >> >
Received on Wednesday, 1 May 2013 17:27:13 UTC