Re: Stream State and PRIORITY Frames

Tom Bergan <> Fri, 20 January 2017 18:32 UTC

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From: Tom Bergan <>
Date: Fri, 20 Jan 2017 10:28:59 -0800
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To: Scott Mitchell <>
Cc: laike9m <>, Martin Thomson <>, Tatsuhiro Tsujikawa <>, HTTP Working Group <>
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Subject: Re: Stream State and PRIORITY Frames
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IIUC, PUSH_PROMISEs don't count against the stream limit due to a
(hypothetical) scenario like the following: A server currently has 199
streams open on a connection, with limit of 200 per connection, and the
client makes another request. Now the server has 200 streams open. The
server wants to push a resource. Since the PUSH_PROMISE does not count
against the stream limit, the server is allowed to send a PUSH_PROMISE,
which notifies the client that the resource will be pushed (the client does
not need to request it).

I submit that this is a nearly-pointless optimization. Broadly speaking,
there are two reasons a server might want to push a resource:

1. The server speculates that the client will need that resource shortly.
If the pushed resource is not more important than basically all of the 200
streams currently in flight, there is little reason to push that resource.
This is a speculative push, meaning the goal is to reduce latency, but we
can't reduce latency if the push will be queued behind 200 other streams
anyway -- we might as well wait for the client to make the request.

2. The server is participating in a notification protocol where the push
used to notify the client of some event. In this case, the PUSH_PROMISE
does not need to be sent immediately because the server is not speculating
about a future client request. It would be fine to queue the PUSH_PROMISE
in the server until one of the 200 concurrent streams is closed.

Therefore, I propose the following convention: servers should count
PUSH_PROMISEs towards the concurrent stream limit. This doesn't
necessarily help clients, since a client cannot rely on every server to
implement this behavior, but overall it seems like a much simpler
situation. FWIW, this is what I have implemented in Go's HTTP/2 server:

On Fri, Jan 20, 2017 at 9:01 AM, Scott Mitchell <>

> On Fri, Jan 20, 2017 at 5:17 AM, laike9m <> wrote:
>> If you don’t include RESERVED streams in the count for
>> SETTINGS_MAX_CONCURRENT_STREAMS then how do you limit the amount of
>> RESERVED streams, and how does your peer know about this limit? I have
>> imposed an implementation specific metric in the past, but this seems less
>> preferable than relying on something in the RFC that the peer is aware of.
>> Either way having infinite of something doesn’t work in practice.
>> As Martin has explained, H2 doesn’t limit the amount of RESERVED streams,
>> based on the notion that HEADERS are close to free.
> "HEADERS are free" sounds like an over simplification which becomes more
> apparent as the concurrency of streams and connections grows. There are
> other provisions in the RFC to limit the amount of state consumed by
> In addition to headers this may require additional state to be allocated
> for stream management. The specification also has mechanisms to limit state
> consumed by streams.
> It’s true that one trying to send infinite number of PUSH_PROMISEs to
>> client can cause problems, but 1. This only happens if the server is
>> malicious, and if it’s malicious,having a limit in the RFC won’t prevent
>> anything, and 2. Not counting PUSH_PROMISEs is a tradeoff for fast delivery
>> of PUSH_PROMISEs, which stops client from sending more requests. I guess
>> this is what Martin meant by “If you limit server push by applying a stream
>> limit, then you prevent it from being used in time for the client to use
>> it.”
> (Forgot to reply to all :P)
> Malicious actors is a concern and must be dealt with. However there may be
> proxy-like systems with large amounts of concurrency or other memory
> constraint systems that already control their resources but the peer's only
> mechanism to know about these limits is to try-then-fail. Assuming clients
> impose some limit to their state (infinite state isn't practical) then the
> problem of "the client won't accept this push" exists if the server knows
> about it before hand or not. Knowing about it before hand gives the server
> the ability to potentially prioritize which resources it wants to push or
> make other more informed decisions.
>> On Thu, Jan 19, 2017 at 9:21 AM, Scott Mitchell <
>> > wrote:
>>> On Tue, Jan 17, 2017 at 2:43 PM, Scott Mitchell <
>>>> wrote:
>>>> From my perspective I would like to see two clarifications:
>>>> 1. It is clear to me that PRIORITY doesn't impact state.
>>> Just to clarify ... it is clear that a PRIORITY frame doesn't impact the
>>> state of the stream  it is carrying priority information for. The impacts
>>> PRIORITY frames have on other streams is not clear due to the wording in
>>> section 5.1.1.
>>>> However Section 5.1.1 states "first use of a new stream identifier"
>>>> which makes no reference to stream state. If stream state is
>>>> important/implied here better to be specific about it. I don't think the
>>>> one-off example below this text is sufficient to convey the intended
>>>> implications of this statement.
>>>> 2. Section 5.1.2 states "Streams in either of the 'reserved' states do
>>>> not count toward the stream limit." which seems to conflict with section
>>>> 8.2.2 "A client can use the SETTINGS_MAX_CONCURRENT_STREAMS setting to
>>>> limit the number of responses that can be concurrently pushed by a
>>>> server.". These two statements appear to contradict each other. Since
>>>> SETTINGS_MAX_CONCURRENT_STREAMS is really the only mechanism to limit
>>>> resources due to server push I'm assuming section 5.1.2 is overly
>>>> restrictive.
>>>> On Tue, Jan 17, 2017 at 2:27 PM, Martin Thomson <
>>>>> wrote:
>>>>> On 18 January 2017 at 01:37, Tatsuhiro Tsujikawa <
>>>>>> wrote:
>>>>> > If my understanding is correct, this only refers to the new stream
>>>>> ID used
>>>>> > by HEADERS, and PUSH_PROMISE frames which open or reserve streams.
>>>>> The
>>>>> > example text following that statement uses HEADERS which opens new
>>>>> stream.
>>>>> > PRIORITY frame does not change stream state, and there is no reason
>>>>> to close
>>>>> > all unused streams lower than bearing stream ID.  That said, I agree
>>>>> that
>>>>> > this is not crystal clear in the document.  In practice, this is
>>>>> probably
>>>>> > rather rare case.
>>>>> This is, I think, the expectation.
>>>>> I think that we probably want to clarify the point by explicitly
>>>>> saying that PRIORITY doesn't affect stream states.  We say that it can
>>>>> be sent in any state, but we don't also mention that important point.
>>>>> Do people here agree that an erratum on this point is appropriate
>>>>> here?