Re: [Editorial Errata Reported] RFC7540 (4871)

[laike9m <[e-mail]>]

Since it’s me that brought this question
<https://github.com/http2/http2-spec/issues/758> to Martin, I also got some
thoughts on it(from a reader's perspective):

Basically the question is not about the example being wrong, it’s about the
above description being contradictory with the example.

…Resources are shared between streams with the same parent stream, which
means that if a stream *in that set* closes or becomes blocked, any spare
capacity allocated to a stream is distributed to the immediate *neighbors*
of the stream. *However, if the common dependency is removed from the tree,
those streams share resources with streams at the next highest level.*

So, let’s assume the example is correct, then there are three problems with
this paragraph.

   1. *“in that set”*: “set” here apparently means “streams with the same
   parent stream”, so emphasizing “in that set” makes readers feel the
   resource reallocation process is between those streams. Better remove it.
   2. *“neighbors”*: it’s true that in graph theory “neighbors” means nodes
   that directly connect to a specific node(like C, D to A), but since we’re
   talking about a tree, if we could use more precise words like “dependent
   streams” or “child streams”, then there would be no controversy. I searched
   the whole RFC, “neighbor” is only used once, which does cause some
   confusion.
   3. *“However, if the common dependency is removed from the tree, those
   streams share resources with streams at the next highest level.”*: this
   whole sentence, in such case, is not even necessary.

So the modified version would be something like this:

*Resources are shared between streams with the same parent stream, which
means that if a stream closes or becomes blocked, any spare capacity
allocated to the stream is distributed to the dependent streams of it.*

But yeah, like Cory said, Martin definitely proposed the erratum for a
reason, and it's still possible the erratum is what the RFC writers really
meant.
​

On Wed, Nov 30, 2016 at 10:54 PM, Stefan Eissing <
[e-mail]> wrote:

> A stream is blocked only by flow-control, as I read it. The errata would
> suggest that the flow control of the parent would effectively rule those of
> its descendants? That does not make much sense to me.
>
> This could easily become very messy. If flow control of dependant nodes
> become entangled with their ancestors, then we basically have nested flow
> control windows? Please. no.
>
> -Stefan
>
> > Am 30.11.2016 um 14:49 schrieb Tatsuhiro Tsujikawa <
> [e-mail]>:
> >
> >
> >
> > On Wed, Nov 30, 2016 at 10:29 PM, Kazuho Oku <[e-mail]>
> wrote:
> > 2016-11-30 22:05 GMT+09:00 Cory Benfield <[e-mail]>:
> > >
> > >
> > > On 30 Nov 2016, at 13:04, Kazuho Oku <[e-mail]> wrote:
> > >
> > > My understanding is that you do not need to distinguish between
> completed, idle and blocked states.
> > >
> > > For a stream under either of the three states, the weight associated
> to the stream is distributed to the dependents.
> > >
> > > That is what nghttp2 does and H2O does (except for the fact that it
> does not remember closed streams), and I this behavior is what is suggested
> by the spec.
> > >
> > >
> > > My understanding of what Martin is suggesting is that that isn’t true:
> blocked streams do not distribute their weight to their dependants.
> >
> >
> > Thank you for pointing that out.
> >
> > My understanding is that there is no special casing for blocked
> > streams. Section 5.3.1 handles all the states we are discussing
> > equally, quote:
> >
> >    Inside the dependency tree, a dependent stream SHOULD only be
> >    allocated resources if either all of the streams that it depends on
> >    (the chain of parent streams up to 0x0) are closed or it is not
> >    possible to make progress on them.
> >
> > I also do not see why it would be beneficial to treat them differently.
> >
> >
> > ​I agree with Kazuho.  I think RFC 7540 is written based on the idea
> that dependent stream ca​n receive resource if the streams between it and
> root are all either in closed, idle or blocked.
> >
> > Actually, from nghttp2 commit log, I made a commit which implemented the
> proposed  text.  But we later reverted it, since there is no text in the
> draft at that time to mandate that behaviour.
> >
> > Best regards,
> > Tatsuhiro Tsujikawa
> >
> >
> >
> > >
> > > However, that’s also what the Python Priority implementation does.
> > >
> > > Cory
> > >
> >
> >
> >
> > --
> > Kazuho Oku
> >
> >
>
>
>