Subject: Re: [Tsv-art] Tsvart last call review of draft-ietf-httpbis-priority-10

From: Lucas Pardue <[email-masked]>
Date: Thu, 2 Dec 2021 22:43:31 +0000

Hi Bob,

Thanks for taking the time to give us a thorough review and to share the
feedback. Kazuho and I had briefly discussed your initial set of comments
and were intending to address the easy stuff, and respond to the other
things already - $dayjob interrupted that plan. Thanks for the subsequent
clarification points, we'll incorporate them into our response to the
review, which should happen soon.

Cheers
Lucas

>                                http://bobbriscoe.net/

---

From: Lucas Pardue <[email-masked]>
Date: Thu, 9 Dec 2021 15:43:28 +0000

Hi Bob,

Thanks (again) for the review.

We've been working through the editorial issues and have cut release -11 to
address the ones we agreed with. Responses to the technical issues you
presented are in-line below:

On Tue, Nov 30, 2021 at 12:22 AM Bob Briscoe via Datatracker <
[email-masked]> wrote:

> Reviewer: Bob Briscoe
> Review result: Almost Ready
>
> This document has been reviewed as part of the transport area review team's
> ongoing effort to review key IETF documents. These comments were written
> primarily for the transport area directors, but are copied to the
> document's
> authors and WG to allow them to address any issues raised and also to the
> IETF
> discussion list for information.
>
> When done at the time of IETF Last Call, the authors should consider this
> review as part of the last-call comments they receive. Please always CC
> [email-masked] if you reply to or forward this review.
>
> Version reviewed:
> Originally draft-ietf-httpbis-priority-09, but also checked -10 diff.
>
> ==Summary==
> The move to an e2e request and hop-by-hop response is a good one.
>
> I'm not sure how ready this is, until I see how the authors respond to my
> questions about the interaction model and whether the client can say "I
> dunno,
> you tell me" to the server (T#4c, T#5a and T#9a).
>
> I think all of my other points are 'just' holes in the coverage of each
> aspect
> of the protocol, but some will be quite involved to fill. There's a lot of
> vagueness still to be tied down, I'm afraid.
>
> Three sets of comments below:
> * Gaps (G#): 2
> * Technical points or suggested modifications to normative text (T#) 13
> * Editorial points (E#) 29
>
> ______________________________________________________________
> ==Gaps==
>
> G#1 Implementation status section?
>
> This review would have really benefited from an implementation status
> section.
> You will see I got suspicious that some of the sections had been written
> without the benefit of any implementation or operational experience. While
> others seemed stronger. If the implementation status had been written up, I
> wouldn't have had to guess.
>
> Here's my guesses at what has been implemented, given the waffle factor of
> the
> relevant sections ;)
>
> * e2e priority protocol protocol handlers and schedulers: most mature
>
> * intermediary priority protocol handlers and schedulers: not so mature
>
> * automated priority header generation, APIs: not so mature
>
> * priority of server push, retransmissions, probes: just ideas in theory?
>
> * investigation of deadlocks, loops, etc: early days.
>

The short answer is that HTTP prioritization is much of an optional thing.
Signals are hints to the server in charge of the multiplexed connection.
Servers have a self-interest in serving requests in a timely manner
balanced against all the other needs like resource usage, DoS avoidance and
so on. The sections that describe scheduling are some basic recommendations
that will let clients have some expectations how responses would get
prioritized should the stars align. But this is not a very specific
algorithm that all implementations will follow exactly because rarely do
stars align. The sections attempt to spell out the considerations arising
from the protocols related to this draft. We believe the editorial changes
made in -11 make it clearer where text is considerations more than
authoritative direction on what to do.

> G#2 Performance evaluation?
>
> Priorities are about improving performance. This is a stds track draft
> about a
> core IETF protocol. But there is nothing in this draft pointing to any
> studies
> that quantify how much performance is improved (or not) by the different
> aspects of the protocol. Ideally there would be a study comparing the
> HTTP/2
> priority approach with this one. Is that because the studies don't exist,
> or
> just an omission?
>

We simply overlooked citing them, which was noted in other reviews. We
added links to work from Robin Marx and Pat Meenan that motivated this
document and it's design features.

>
> ______________________________________________________________
> ==Technical Points and Modifications to Normative Statements==
>
> §2.  Motivation for Replacing RFC 7540 Priorities
>
> T#2a) Grass is always greener on the other side of the fence?
>
>    RFC 7540 priority is expressed relative to other requests on the same
>    connection.  Many requests are generated without knowledge of how
>    other requests might share a connection, which makes this difficult
>    to use reliably
>
> This says why relative values were difficult, but it doesn't say why or
> whether
> absolute values will be better. Is there lots of experience of absolute
> values
> being sufficient and easier to use? or any cases where absolute values
> might be
> insufficient? It seems obvious that at run-time you might hit the end of
> the
> number space, i.e. have to pile up objects on the same urgency value at the
> edge of the space when you really wanted some objects to have higher (or
> lower)
> urgency. There is a mention of the possibility of creating an extension
> with
> intermediate urgency values, but what does an implementation do when it
> hits
> this problem in the middle of a session? It can't quickly jump out of
> run-time,
> convene a design team to get a new scheme extension agreed then go back to
> run-time and complete the session.
>

Dependencies between the requests in a connection are a property of the
connection. This poses challenges to acting on priority signals when
passing them to components/nodes that are not part of the connection.
That's friction against some ways that HTTP commonly operates in practice.
For example, intermediaries or servers that split or coalesce requests from
different connections. The urgency and incremental parameters have been
deemed sufficient for a web browsing use case, which was our focus, without
being too fine grained and hard for developers to reason about. If other
use cases encounter limitations or problems with this scheme, I do
encourage them to bring that back to the HTTP WG so we can consider work on
extensions that address them.

>
> T#2b) Motivation for replacing 7540 included removal of dependencies?
>
> The Security Considerations says that one of the motivations for replacing
> 7540
> was that "Extensible priorities does not use dependencies, which avoids
> these
> [resource loop DoS attack] issues." Draft-09 listed this as one of the
> motivations in §2, but in draft-10 it has been removed from §2. If it is
> still
> a motivation, it ought to be listed in §2, not just in Security
> Considerations.
>
> Security Considerations seems to be in the commonly used style of just a
> list
> of pointers to other parts of the draft. So it would be consistent to say
> this
> in §2 not just in Security Considerations, which even says "Considerations
> are
> presented to implementations, describing how..." as though the details are
> elsewhere in the document.
>
> Whatever, given this seems to have been an important motivation, please
> try to
> describe this issue in a self-contained way, rather than talking obliquely
> in a
> way that requires the reader to refer to the CERT advisory (e.g. "...some
> types
> of problems...").
>

The text was written before the 7540bis activity started. We’ve been
shifting bits and pieces of RFC 7540 problems to that venue. This one has
been removed since it seems like a distraction for the topic of this
document.

>
> §2.1.  Disabling RFC 7540 Priorities
>
> T#2c) Incremental deployment
>
> Two perhaps obvious but unstated things ought to be stated:
> i) An HTTP session will always _function_ even if all priority information
> is
> ignored; it just might perform badly.
>
> ii) The semantics of the SETTINGS_NO_RFC7540_PRIORITIES setting is
> intended to
> apply to both directions (if it is?). When it says
>   "A server that receives SETTINGS_NO_RFC7540_PRIORITIES with a value of 1
> MUST
>   ignore HTTP/2 priority signals."
> I assume after "MUST ignore" it intends to add "...and MUST NOT send...".
> I assume this is stated as "server MUST ignore" rather than a protocol
> error,
> because the HTTP/2 priority signals might have come from an intermediary
> that
> doesn't understand the SETTINGS_NO_RFC7540_PRIORITIES setting.
>
> Also, it is surely a protocol error if one endpoint sets
> SETTINGS_NO_RFC7540_PRIORITIES to the opposite of the other. Or if a node
> sends
> a header after it has said it won't.
>
> If a client sets SETTINGS_NO_RFC7540_PRIORITIES to 1, but the server
> doesn't
> understand this setting, and later sends HTTP/2 priority signals (perhaps
> in
> response to an intermediary), what happens? [As I pointed out in my review
> of
> RFC7540 Priorities (when it was a draft but after IESG approval), it wasn't
> clear whether priority messages were only sent in the C-S direction, or
> also
> the reverse. I didn't receive a reply on that point and the RFC is still
> not
> clear.
> https://lists.w3.org/Archives/Public/ietf-http-wg/2015JanMar/0529.html ]
>

Francesca also mentioned this in the AD review. RFC750 seems to have
allowed server-to-client signals on the wire but didn’t specify what at all
anyone should do with them. I’m not aware of any cases of this signal being
used in the wild. I created an issue on
https://github.com/httpwg/http2-spec/issues/1000 there's further discussion
there. 7540bis deprecates stream prioritization - all that is left is the
remnant of bits on the wire that remain for wire compatibility.

SETTINGS_NO_RFC7540 is an optimization related to carriage, processing and
application of signals. Because these signals are only a hint, as long as
they have a valid wire format there is no need for protocol errors. Since
we are focused on C->S direction of signal, and the world never defined
what RFC7540 S->C signals really meant, our document doesn’t benefit from
trying to speak about them. So we define our terms of use in the Notational
Conventions and stick to them. The document consistently uses this term,

>
> §4.  Priority Parameters
>
> T#4a) Vagueness permeates what intermediaries do in this draft
>
>    Intermediaries can consume and produce priority signals in a
>    ...PRIORITY_UPDATE frame or Priority header field.
>    ...Replacing or adding a Priority header field overrides
>    any signal from a client and can affect prioritization for all
>    subsequent recipients.
>
> * Do intermediaries really both consume and produce priority signals.
> Always?
> In both directions? What does 'consume' mean (absorb and not forward, or
> read
> and forward)?
>

They can according to HTTP Semantics.

> * Can they really use either type of frame? Always?
>

They can, it depends on the versions of HTTP being used on the upstream or
downstream.

> * How does adding a priority header override any signal from a client? Or
> is it
> only replacing that overrides?
>
> My later comment asking for a more precise statement of the protocol's
> interaction model ought to resolve these issues as well.
>

it all depends on the model about how HTTP intermediaries convert between
versions. Frames are connection-level and headers might be e2e or
hop-by-hop.  This document shouldn’t litigate any more than it does on the
matter.

> T#4b) Really only C-S direction?
>
>     PRIORITY_UPDATE frame preserves the signal from the client, but...
>
>     ...overrides any signal from a client...
>
> Also used for S-C direction?
>
> Given this part of the draft seems to have been written solely about the
> C-S
> direction, perhaps it would be better to admit that is a good way to
> structure
> the draft with C-S first. Then add another section about S-C, and perhaps
> another about S-Int. The alternative of adding to all the definitions to
> cover
> all directions and interactions, might become incomprehensible.
>

This is different from HTTP/2 PRIORITY frames (as mentioned above). By
definition PRIORITY_UPDATE is only allowed in the C->S direction, which
eliminates the need to document the reverse direction.

>
> §4.2.  Incremental
>
> T#4c) Client doesn't always have prerequisite info to set incremental
> parameter
>
> There will surely be cases where the MIME type of the response (and
> therefore
> whether the client can render it incrementally) is not known, or cannot be
> guessed by the client when it requests a resource, or when it starts
> content
> negotiation? For instance, the client might have listed MIME types in its
> Accept list, some of which are incremental, and some not.
>
> The server can't override a client 'not incremental' message by stating
> that
> the MIME type it has served is incremental. Because, when the client says
> 'not
> incremental', that is intended to state the capability of the client, not
> the
> format of the resource.
>
> Perhaps the HTML that gave the client the hyperlink that was selected to
> get
> the resource could also include a tag giving the MIME type of the
> hyperlinked
> resource? Or perhaps the idea is that the client has to send a
> PRIORITY_UPDATE
> once it knows the MIME type (by which time it might be too late)?
>

That's a fair point. Unfortunately, there will be cases where parties lack
all of the information that could lead to perfect prioritization. Client
priority signals are only a hint. Servers can and will do whatever they
like, including serving the response in a way that does not follow the
recommendations we provide for handling the incremental parameter in
Section 10. There’s lot of additional means, outside of this specification,
that clients and servers can use to augment their understanding of
priority. There is no need to enumerate them in this document.

> §5.  The Priority HTTP Header Field
>
> T#5a) Interaction model: an example or mandatory?
>
> It would help to start by explaining (perhaps in the Intro, rather than §5)
> whether a priority message about a response can be initiated by a server or
> intermediary if there was not a priority field attached to the request
> from the
> client. I believe the draft intends this not to be possible, although this
> is
> not stated normatively anywhere, and I don't know why such a restriction
> would
> be imposed.
>
> Actually, I believe it is essential that the protocol allows the server to
> initiate priority messages, as absence of a message is currently the only
> way
> for the client to say "I have no idea, you decide". Otherwise, if the
> server is
> only allowed to follow the client, when the server knows the best order to
> serve the objects (which I believe is often the case), the client still
> has to
> request non-incremental objects in some order or other, and give them some
> priority or other. So the server doesn't know whether the client actually
> knows
> what it is doing, or whether it is just making up an ordering because it
> has
> to, even tho' it has no clue.
>
> Alternatively, could the client send a Priority header with no parameters?
> This
> would indicate that the client wants the server to prioritize, and to
> allow the
> server to tell intermediaries what to prioritize. (For more about clueless
> clients, see T#9a) "Client scheduling".)
>
> The abstract gives the only outline of the interaction model, but it's not
> clear whether this is just an example of common usage, or the only possible
> model.
>
> §5 just says the priority field can be used "when a request or response is
> issued". It goes on to state that the priority field is an e2e signal, but
> then
> in the next sentence talks about how intermediaries can combine priority
> info
> from client requests and server responses (which reflects what §8 says as
> well). So "e2e" is clearly an over-simplification. I think it's e2e in one
> direction but hop-by-hop in the other (supported by the description in the
> abstract), ie. client -> server -> intermediary/ies -> client. It's also
> possible that intermediaries are intended to (or at least allowed to) read
> but
> do not alter the messages in the C-S direction, otherwise, what would they
> 'combine' with the priority field coming from the other direction?
>
> Whatever, the interaction model(s) is never stated precisely. I've
> classified
> this as a technical point, not just editorial, because I couldn't really
> assess
> the completeness of many other technical details of the draft without
> knowing
> the answer to this fundamental question.
>

If this scheme is implemented at the server, then all requests are treated
as if they have an associated priority signal. This can be a Priority
header field (Note the serialization rules for Structured Fields Dictionary
in Section 3.2 of RFC 8941 - sending an empty header is not permitted) or a
PRIORITY_UPDATE frame. Omission of signals, or omission values in signals,
invokes default value priority parameters urgency=3, incremental=false. A
server is therefore able to determine the client’s view of the priority.

Editorial changes now in Section 10 should make it clearer that an HTTP
server’s job is to respond in a timely manner. And it always has to decide
how to use finite resources to do so. Clients can hint as some preference
but if they don’t know or don’t care, it's basically delegating the
responsibility to the server.

The purpose of the Priority header in responses is to allow origin servers
(detached from the intermediaries’ connection to the client) the ability to
also provide hints about prioritization.

The interaction model is described throughout the document, with a gist in
the intro. Duplicating details into the intro does not seem beneficial.

>
> T#5b) Normative 'cannot'?
>
>    Clients cannot interpret the
>    appearance or omission of a Priority response header as
>    acknowledgement that any prioritization has occurred.
>
> Was this intended to say 'Clents MUST NOT interpret...'?
>

Signals by design are just a hint. They can never be trusted and this
sentence highlights that fact.

> T#5c) Nothing said about caching and priority
>
> The paragraph about caching and priority just ends having talked a bit
> about
> caching but not about priority. It left me none the wiser about what a
> cache
> ought to store about priority with the response. §13.8 talks about fairness
> between multiple live connections in the presence of coalescing. But
> doesn't
> the discussion of caching and priority here need to talk about what
> must/should/may be stored about priority in a cache for later connections.
> Even
> if it's implementation dependent, wouldn't it be worth a brief discussion
> (as
> in the 2 paras below).
>
> The priority of a response is the outcome of an interaction between the
> client's original (e2e) priority combined with the server's logic about the
> resource. If only the priority outcome is stored, then when another request
> arrives at the cache from a different client, there will be no record of
> the
> original client's priority. So the  cache will not know what client
> priority
> led to the priority stored with the response. And it will not know whether
> the
> current client priority is the same or different.
>
> On the other hand, if the cache stores the original client priority with
> the
> response priority, then should it refer a request with a different (e2e)
> client
> priority to the server, then store the new pair of priorities with the
> original
> cached response? And I guess it could serve the request in parallel, rather
> than waiting for the server to tell it whether to serve the request
> urgently
> (!). This would probably scale reasonably well, given the likely small
> number
> of different client priorities. But who knows how it would scale if the
> parameter space is extended in future.
>

Answer supplied by Kazuho - As discussed in the last paragraph of section
5, CACHING defines if and how requests with different header field values
can be mapped to one response. If the capabilities provided by CACHING
(i.e. Vary) is too limited, then we should fix that as an extension to
CACHING (as have been previously proposed as draft-ietf-httpbis-key).
In practice, re Extensible Priorities, IMO, there aren't many sensible
combinations of urgency and incremental. Therefore, backend servers that
want to tune priority based on the value that the client sends can simply
send Vary: priority and call it a day.

> §9.  Client Scheduling
>
> T#9a) Client doesn't have prerequisite info about content sizes and
> dependencies
>
> Consider a web page example with a load of non-incremental objects for the
> client to request. It doesn't know their sizes, and it doesn't know which
> ones
> might contain references to further objects to request. So it requests
> A,B,C,D.
> In retrospect, it turns out that C was huge, and D referred to further
> objects
> to download. How was the client to know it should have downloaded D before
> C?
>
> To be effective, a scheduler needs to know object sizes and which objects
> will
> identify further objects to be requested (dependencies). * Size is known
> by the
> server but not by the client, at least not until the headers at the start
> of
> the object arrive. * Dependencies are known by the server, but not by the
> client until an object starts to unfold.
>
> Why is the client made to choose the priorities of the responses? It
> doesn't
> know any of this vital metadata about these objects. It can guess from file
> types that JS and HTML probably ought to come first. But it knows little
> else.
>
> So, as I already said under my question T#5a) about the interaction model,
> the
> most important capability the client must have is the ability to say "I
> dunno,
> you decide". But that's the one thing this draft doesn't allow the client
> to do
> (at least I think it doesn't? see T#5a). For a list of non-incremental
> objects,
> even if the client gives all their requests the same urgency, it can't
> send all
> the requests at the same time - it has to choose which order to send them
> in,
> even if it has no clue. This tells the server to respond in that order OR
> to
> choose a different order. But the server doesn't know whether the client
> chose
> this order deliberately or just because it didn't know any better.
>
> Alternatively, there will need to be some way for the server to tell the
> client
> what to prioritize _before_ it sends its requests (e.g. using extensions
> to the
> HTML in a base HTML document).
>

As noted in response to T#4c, we are constrained by the capabilities of
information exchange that HTTP and its uses (such as the web) allows us.
This is no different a problem than existed for RFC 7540. Only a client
knows how it wants to use resources for which it has limited knowledge. If
we use an HTML document as an example, the subresources have a dependency
chain that may or may not change while they get loaded. It’s more likely
that a client will request something with a particular priority because of
its type and usage in a given HTML document, rather than its size (even if
it knew that size). It’s going to be rare that a client doesn't have an
opinion - if the client doesn’t know, the defaults are sufficient to let it
reprioritize the request higher or lower if it finds out that need once the
response is back.

But really this all comes down to making all the actors aware of the
challenges and stating that priority signals are just hints in the decision
making. If a client finds that the server is making scheduling choices when
using defaults, then it is empowered to provide better signals.

The general problem is not solvable so I do not believe there is anything
more we can add to the document.

>
> §10.  Server Scheduling
>
> T#10a) Server push priority sounds like waffle
> The discussion of priority for server push seems to say "This might not
> work".
> If this conclusion is based on operational experience it ought to say so.
> And
> if it's not, it ought to say that it's just conjecture.
>

The topic of server push is not helped by the fact that it’s deployment
story, outside this I-D, is one of disappointment. But we’re stuck with
that feature in HTTP/2 and HTTP/3 and a server that chooses to use it while
implementing this scheme has to make some choices. There's no case where
push will fail but there are cases that could cause it to perform badly.
The text in paragraphs 10 and 11 provide considerations that a server that
does implement server push will have to make (because things are _always_
contending for resource). That said, I don’t think we need operational
experience to conclude that if you push things incorrectly, you could
really hurt what the client is trying to achieve.

> §12.  Retransmission Scheduling
>
> T#12a) Sounds like more waffle
> Similarly, if retransmission scheduling and/or probe scheduling has limited
> operational experience or limited usefulness, it would be better to say so,
> rather than trying to sound authoritative without really saying anything.
>

Again, this is something a server has to do anyway and we want to present
the considerations at play. Our editorial change makes it sound less
authoritative by nixing the sentence containing “...its effectiveness can
be further enhanced…”.

>
> §13. Fairness
>
> T#13a) Please define fair. Seriously.
>
> A prerequisite question is, "What parameters does a server scheduler
> manipulate?" The text implies the server can only control the order in
> which it
> starts a response to each request, and whether responses start while other
> responses are in progress or wait for their completion. If so, I'm not sure
> what fairness could mean.
>
> Presumably a server can also determine the relative rate at which it sends
> different streams. And it could stall a stream to allow another absolute
> priority. In this context, fairness might mean instantaneously equal
> rates. But
> that's not fair if the objects are of different sizes.
>
> So we genuinely do need to see a definition of what fairness means here.
>

Good point, we addressed this in issue 1819 -
https://github.com/httpwg/http-extensions/issues/1819

>
> T#13b) Why not make scheduling decisions across different clients?
>
>    As a general guideline, a server SHOULD NOT use priority information
>    for making scheduling decisions across multiple connections, unless
>    it knows that those connections originate from the same client.
>
> Why does the IETF have anything to say about this? It's surely an operator
> policy decision.
>

We disagree a bit with this but we discused some more on issue 1820 -
https://github.com/httpwg/http-extensions/issues/1820

> ______________________________________________________________
> ==Editorial Comments==
>
> General (all sections).
>
> E#0a) Are precedence and priority interchangeable with urgency?
>
> It's called a priority field, and the parameter is called urgency, but
> sometimes the term priority or precedence is used to describe urgency.
> Please
> go through the draft using priority or urgency consistently, and remove
> precedence unless it's there for good reason.
>
> §1. Introduction
>
> E#1a) The "For example" in para 2 is actually the general point not the
> example, whereas the example is in the previous para.
>
> E#1b) The last para "   The prioritization scheme and priority signals
> defined
> herein can act as a substitute for RFC 7540 stream priority." would fit
> better
> 3 paras earlier, just after "HTTP/2 [HTTP2] has consequently deprecated
> the use
> of these stream priority signals."
>
> E#1c) It would help to state what the interaction model is for the priority
> field. I believe it's not as simple as just "e2e" (see technical comment
> earlier about §5). Although I've suggested that normative text about this
> ought
> to be in the §5, it needs to be stated early on (probably in the intro, not
> just the abstract), because the reader needs it to understand §§2 & 4.
>
> §1.1.  Notational Conventions
>
> E#1d) Was HTTP/2 priority only from C-S?
>
>    The term HTTP/2 priority signal is used to describe the priority
>    information sent from clients to servers in HTTP/2 frames;
>
> Neither RFC7540 nor the http2bis draft says that an HTTP/2 priority signal
> cannot be sent by a server. It may be that this was the unstated intention
> but,
> if it wasn't, the above sentence is incorrect.
>
> §2.  Motivation for Replacing RFC 7540 Priorities
>
> E#2a) Not just absence
>
> CURRENT:
>
>    compatibility (see Section 5.3.2 of [HTTP2]), which means that they
>    might still be used in the absence of alternative signaling, such as
>    the scheme this document describes.
>
> PROPOSED:
>
>    compatibility (see Section 5.3.2 of [HTTP2]), because they
>    might still be used by other nodes.
>
> REASONING:
> 7540 priority fields are not only used in the absence of alternative
> signalling. They are used by the client in parallel to extensible
> priorities
> before it receives the server's SETTINGS frame.
>
> 2.1.1. Advice when Using Extensible Priorities as the Alternative
>
> E#2b)
> CURRENT:
>
>    might be useful to nodes behind the server that the client is
>    directly connected to.
>
> PROPOSED:
>
>    might be useful to a server behind the directly connected node.
>
> REASONING:
> Removes ambiguity - I initially read this as "nodes (behind the server)
> that"
> rather than "...the server that".
>
> §3.  Applicability of the Extensible Priority Scheme
>
> E#3a)
>
>    The priority scheme defined by this document considers only the
>    prioritization of HTTP messages and tunnels, see Section 9,
>    Section 10, and Section 11.
>
> Is this sentence meant to be as mysterious as it sounds? If this document
> only
> considers messages and tunnels, what else doesn't it consider? Is 'HTTP
> messages' deliberately used instead of 'HTTP responses'? If it is wider
> than
> just responses, then all the places in the draft where it says it is about
> http
> responses need to be generalized (e.g. §4.1, §4.2, §8, §10 §14). But will
> this
> make the draft so abstract that it becomes incomprehensible? Perhaps
> better to
> explain here that it is applicable to a few other odd messages and
> tunnels, but
> it is primarily about responses, so where the draft talks about responses,
> it
> is not intended to preclude the other less-common cases.
>
> It's not clear what the references to §§9,10,11 are there for. Are they
> further
> info about tunnels (only §10 has one sentence on tunnels)? Or are they
> meant to
> list all the sections about scheduling things? if so, why not also §12 on
> scheduling retransmissions?
>
> E#3b)
> CURRENT:
>
>    they can also define how this priority scheme
>    can be applied.
>
> PROPOSED:
>
>    they can also define how the present priority scheme
>    can be extended to support the new extension.
>
> RATIONALE:
> I think this is what was intended?
>
> §4.  Priority Parameters
>
> E#4a) First use of reprioritize
> Ought to explain long-hand why reprioritization might be needed here,
> given it
> hasn't been mentioned yet.
>
> E#4b)
>
>    Note that handling of omitted parameters is different
>    when processing an HTTP response
>
> When what processes an HTTP response? The client? An intermediary? Both?
>
> §4.1. Urgency
> E#4c)
>
>    integer between 0 and 7, in
>    descending order of priority
>
>    The smaller
>    the value, the higher the precedence.
>
> Suggest the latter is moved up 2 paras.
> (BTW, what possessed anyone to define this in the most illogical way
> possible?
> So that the highest numerical urgency means the lowest urgency?)
>
> E#4d) Priority = Precedence = Urgency?
> This section interchanges the words 'priority', 'precedence' and
> 'urgency'. Why
> not just use urgency throughout? Otherwise you have to define that
> priority and
> precedence mean exactly the same as urgency.
>
> §4.3.  Defining New Parameters
>
> E#4e) Not new parameters for everything
>
> Suggested replacement section heading: "Defining New Priority Parameters"
>
> s/   When attempting to define new parameters,/
>  /   When attempting to define new priority parameters,/
>
> §4.3.1.  Registration
>
> E#4f)
>
> s/in Structured Fields Dictionary/
>  /in the Structured Fields Dictionary
>
> §5.  The Priority HTTP Header Field
>
> E#5a)
>
> s/carries priority parameters Section 4./
>  /carries priority parameters (Section 4)./
>
> E#5b)
>
> s/As is the ordinary case for HTTP caching [CACHING], a response with a
>   Priority header field might be cached /
>  /A response with a Priority header field might be cached [CACHING]/
>
> §7.  The PRIORITY_UPDATE Frame
> E#7a)
>
> s/which can can be bound by/
>  /which can can be bounded by/
>
> (Oxford dictionary example '‘the ground was bounded by a main road on one
> side
> and a meadow on the other’')
>
> §8.  Merging Client- and Server-Driven Parameters
>
> E#8a)
> First para: The 2nd example isn't a particularly good example of 'server
> knows
> best'; it even admits that it's the visual-ness of the client that
> determines
> the priority.
>
> #8b) What is 'the logic being defined' meant to mean?
>
> s/This is different from the logic being defined for the request header
> field,/
>  /This is different from the approach for the request header field,/
>
> Perhaps?
>
> §10.  Server Scheduling
>
> E#10a) No guidance is provided,... except for a page and a half
>
>    No guidance is provided about how this can or
>    should be done.  ...
>    For these reasons, ... this document only provides some basic
>    recommendations for implementations.
>
> (Contradictory.)
> It seems like the rather over-negative caveats in the first 3 paras need
> to be
> revisited now that the page and a half of recommendations (some normative)
> has
> been added after them.
>
> §10.1.  Intermediaries with Multiple Backend Connections
>
> E#10b)
> s/inflight/in flight/
>
> §11.  Scheduling and the CONNECT Method
>
> #E11a)
>
>    A
>    client that issues multiple CONNECT requests can set the incremental
>    parameter to true, servers that implement the recommendation in
>    Section 10 will schedule these fairly.
>
> s/...true, servers.../
>  /...true. Servers.../
>
> There are 3 recommendations in §10. Which one?
>
> Given §10 starts with a load of caveats about how hard this stuff is to get
> right, is it appropriate to assert with such certainty that scheduling
> will be
> fair?
>
> §12.  Retransmission Scheduling
>
> E#12a)
>
> s/Section 6.2.4 of [QUIC-RECOVERY], also highlights/
>  /Section 6.2.4 of [QUIC-RECOVERY] also highlights/
>
> §13.1.  Coalescing Intermediaries
>
> E#13a)
>
>    It is sometimes beneficial for the server running behind an
>    intermediary to obey to the value of the Priority header field.
>
> This seems an odd sentence to have in this draft. I think it is meant to
> be in
> the context of the previous sentence about coalesced requests with
> priorities
> set by different clients. Needs rewriting, I think.
>
> s/obey to/
>  /obey/
>
> s/the Priority header field/
>  /each Priority header field/
>
> s/as another signal in its prioritization decisions./
>  /as another input in its prioritization decisions./
>
> §14.  Why use an End-to-End Header Field?
>
> E#14a)
>
> s|Contrary to the prioritization scheme of HTTP/2|
>  |In contrast to the prioritization scheme of HTTP/2|
>
> Contrary in this form has an implication that HTTP/2 was wrong. That might
> be
> intended. But 'in contrast to' has less of a 'know-it-all' feel.
>
> E#14b) Answering a different question
>
>    rather
>    than how relatively urgent each response is to others.
>
> This seems to be a (weak) rationale for relative rather than absolute
> priorities, whereas the section heading promises the rationale for an e2e
> header. I suggest the whole first sentence of this 2nd para is deleted,
> because
> the next sentence gives a sufficient rationale.
>
> E#14c) Answering another different question
>
>    It should also be noted that the use of a header field carrying a
>    textual value makes the prioritization scheme extensible; see the
>    discussion below.
>
> This is also not a rationale for an e2e header. Perhaps it belongs in the
> Intro?
>
> Alternatively, the section title could be changed to "Rationale for
> Priority
> Protocol Design" or something. Then it could give rationale for absolute
> priority values and textual values as well as e2e header fields.
>
> E14d) see what "discussion below"?
>
> Perhaps this refers to the sentence in Security Considerations that refers
> to
> [STRUCTURED FIELDS]? If so, it's hardly a "discussion".
>
> §16.  IANA Considerations
>
> E#16a)
>
>    populate it with the
>    types defined in Section 4; see Section 4.3.1 for its associated
>    procedures.
>
> I suspect IANA will prefer the exact text they should use to be written
> here.
>

Thanks for these, they were by and large great suggestions. See issue 1802
for how we tracked our response
https://github.com/httpwg/http-extensions/issues/1802

Cheers,
Lucas

---

From: Lucas Pardue <[email-masked]>
Date: Tue, 21 Dec 2021 18:16:06 +0000

Hey Bob,

Responses inline (tagged [LP] because my mail client decided to be weird
today)

On Tue, Dec 21, 2021 at 12:16 PM Bob Briscoe <[email-masked]> wrote:

> Lucas,
>
> Thanks. Responses tagged [BB] inline where necessary. No comment =
> agreed/understood.
>
> Although I wrote this on 10 Dec, I held back from sending in order to
> think about fairness, but then forgot to finish it. I hope the extra time
> stewing has been worth it. I haven't gone back to all the issue trackers
> since then, except the one on fairness.
>
> Altho' you ack'd receipt of my updated review, I notice you've responded
> to the original incorrect one.
> So I've added back in my corrections and clarifications, tagged
> [BB-follow-up], because they toned down some of my snottiness, which I was
> trying to retract.
>
>
> On 09/12/2021 15:43, Lucas Pardue wrote:
>
> Hi Bob,
>
> We've been working through the editorial issues and have cut release -11
> to address the ones we agreed with. Responses to the technical issues you
>> Here's my guesses at what has been implemented, given the waffle factor
>> of the
>> relevant sections ;)
>>
>> [BB-follow-up]: Sorry, 'waffle factor' was a bit derogatory. I meant that
>> the sections that I have identified below as 'less mature' tended not to
>> say anything significant, while apparently looking as if they were saying
>> something significant.
>>
>>
> [BB] I think you might have missed my point. I meant a section telling us
> what degree of implementation there has been of each aspect (to be removed
> before publication as an RFC). I should have referenced BCP 205 [RFC7942].
>
> Reason: Unless readers put in effort to track what everyone else is doing,
> they have no idea whether the various aspects of this draft are based on
> arm-waving or real operational experience. For anyone who is implementing
> themselves, that colours how much weight they should give to the advice
> relative to their own experience.
>

[LP] The nature of this spec means that the majority of the work - server
scheduling - is delegated solely to server implementation detail. The
considerations presented hold whether people choose to take them into
account or not. How large an effect the considerations have on any single
implementation depends on the nature of the implementation, the deployment
environment, the runtime conditions under which any single connection is
operating, etc.

Given where we are at in the document cycle, adding such a section would
just delay progress and I'm not convinced it will improve this document for
the target audience.

>> [BB-follow-up]: I was wrong to say there is nothing pointing to any
>> studies, just that the studies pointed to haven't been cited. I should have
>> just asked for references to be provided at the end of §2, after:
>>
>>     "Multiple experiments from independent research..."
>>
>>
>> The Security Considerations says that one of the motivations for
>> replacing 7540
>> in §2 not just in Security Considerations, which even says
>> "Considerations are
>> describe this issue in a self-contained way, rather than talking
>> obliquely in a
> [BB] I notice it's now removed from Sec Consids as well.
>
> That's fine, but BTW, it wasn't a distraction for me. It seemed to be one
> of the stronger reasons for moving away from dependencies, because (in many
> cases) I can't understand how priorities can be implemented without knowing
> the dependencies (see T#9a).
>

[LP] Ack. We think the text in the latest version now pays enough detail to
RFC 7540 without treading on 7540bis toes.

>> i) An HTTP session will always _function_ even if all priority
>> information is
> [BB] I think you've overlooked this point.
>
> In your very first response above - about implementation status - you say
> "The short answer is that HTTP prioritization is much of an optional
> thing." The point I was making here is that I don't believe the draft ever
> actually explicitly says the obvious - that HTTP will function without any
> priorities at all. And even if the lack of MUSTs ultimately implies
> everything is optional, it would be worth making that clear from the outset
> in the Introduction.
>
> BTW, when I asked for the refs to evaluations, I was trying to establish
> whether the gain from prioritization is turning out to be worth the pain of
> implementing it and driving it at run-time.
>

[LP] HTTP/2 and HTTP/3 (and QUIC) are the protocols that define
mulitplexing and concurrency and they contain comments roughly equating to
prioritization is good, no prioritization is bad. Neither 7540bis, RFC 9000
or draft-ietf-quic-http-34 define prioritiy signals.

Can you elaborate on what problem is solved by adding the statement you
suggest to this document?

>>   "A server that receives SETTINGS_NO_RFC7540_PRIORITIES with a value of
>> 1 MUST
> [BB] I don't think you've addressed this point below or in the linked
> issue you've opened. Altho there is no appetite in the WG for defining what
> 7540 should have said, surely it would be worth closing off any uncertainty
> for the future, given all the opinions in response to this point were
> purely based on what they /think/ has not been implemented. It would do no
> harm to just add that a 7540bis server in receipt of
> SETTINGS_NO_RFC7540_PRIORITIES =1 MUST NOT send HTTP/2 priority messages
> (as well as ignoring incoming).
>

[LP] Other documents talk in terms of servers operating on client signals.
They don't mention server->client PRIORITY frames. So in effect, the signal
is always ignored anyway. Trying to talk to that direction of flow of the
signal here seems like it would add complication to the document and I
think a distraction.

If there is some use of server->client signals, then the HTTP WG is not
aware of it. If the people that use server->client signals feel like they
need to disable them, then then I think they are best placed to document
the concerns for that.

> [BB] This was a question really. The subsequent points depend on the
> answer.
>
> If the answer is that HTTP/2 priority signals could never arrive at the
> server from an intermediary, and if a client says it won't send a certain
> header, but subsequently the server receives one anyway, surely that is a
> strong case for protocol error.
>

>> SETTINGS_NO_RFC7540_PRIORITIES to the opposite of the other.
>
>
> [BB] I don't think you've addressed this point either.
> What is meant to happen if
> * an HTTPbis client sends SETTINGS_NO_RFC7540_PRIORITIES = 1
> * the HTTPbis server sends SETTINGS_NO_RFC7540_PRIORITIES = 0
> ?
>
> This means the client is "not using" HTTP/2 priority signals and the
> server says I "won't ignore" HTTP/2 priority signals I receive. Altho' that
> would lead to no protocol problems, doesn't it imply that the server is
> somehow broken? I agree that the old HTTP/2 priority signals are hints, but
> this SETTINGS message is not a hint. It's a configuration setting
> introduced in HTTPbis. So isn't it worth defining it fully?
>
>
> Or if a node sends
> [BB] Again, this relates to the new SETTINGS message, not the fact that
> 7540 priority messages are hints. Surely errors like this need to be caught
> (they could e.g. be due to a load balancer switching to an endpoint with a
> different config).
>

[LP] Responding to the three points as one:

Given the current constrains and how loosely coupled things, I'm not sure
what problem is solved by a protocol error.

H2 says - "Settings are not negotiated; they describe characteristics of
the sending peer, which are used by the receiving peer.". And that governs
a lot of what this section can do.

The initial value of SETTINGS_NO_RFC7540_PRIORITIES is 0. A client that
understands the setting will assume 0 when SETTINGS arrive and
SETTINGS_NO_RFC7540_PRIORITIES is not present. So the client=1, server=0
case exists already implicitly.

One likely deployment model for the Extensible priorities scheme is for
HTTP applications to add the priority header on request messages, in
ignorance of whatever the HTTP stack underneath supports or is doing. That
lets them write version-independent logic. Being too strict with settings
rules risks adding hard-to-diagnose errors for such applications.

> response to an intermediary), what happens?
>
[As I pointed out in my review of
> [BB-follow-up]: That wasn't intended to sound like sour grapes. In the
> opening sentence of my review of http/2 I had said I was aware that my
> (long) review had arrived after http/2 had been approved by the IESG. My
> review was unsolicited. And I guess the last thing anyone wants to do when
> their draft has just been approved is answer a load of points about
> something they are not going to change. But I guess it should have been
> handled eventually, if only for errata.
>
>

[BB] Above, I have highlighted again those of my points that were about how
> the SETTINGS message itself (not the 7540 hints) is not fully tied down.
>

[BB] Then this becomes just an editorial point, that it would be useful to
say explicitly at the start of §4.

CURRENT:

   PRIORITY_UPDATE frames (Section 7.1 and Section 7.2) are used by
   clients...

PROPOSED

   PRIORITY_UPDATE frames (Section 7.1 and Section 7.2) are sent by
   clients (not by servers)...

There seems to be a pattern developing (RFC7540 and now here) where the
people who write the spec know what the interaction model is and they
assume everyone else does, which sometimes becomes revealed implicitly as
the reader gets into the details, rather like a game of Cluedo. It would be
really, really helpful if you just stated it early on.

The useful change is to clarify it's the client that sends the frames. I
made https://github.com/httpwg/http-extensions/pull/1866 to do that.
Stating client (not server), server (not client) is not a style that I
think would improve the readability of this draft.

> [BB-follow-up]: I intended to add that I suspect in common cases the
> client's guess of what MIME type is likely to served will be sufficient and
> correct. This comment was meant to be about the remainder of cases.
>
>
>

[BB] Strictly then, the incremental parameter is a property of each MIME
type in the Accept list, rather than a property of the request as a whole.
But I guess it is a reasonable assumption that most of the commonly used
selections of alternative MIME types will all share the same incremental
property.

[LP] Ack. I think what we have is ample enough.

> [BB-follow-up]: When I said "which I believe is often the case" I didn't
> intend to mean the majority of cases. I meant "not an insignificant number".
> I'm sure the client can have a crack at how to set the priorities in
> common cases. Which can then be modified by the server if necessary.
>
> The point I was trying to highlight was that the client has no way to tell
> the server how to distinguish between "I'm certain about these priorities"
> and "I'm not really sure, but the protocol requires me to look as if I'm
> sure, even if I just leave everything as default priority."
> I think I explained it better under T#9a.
>
>

[BB] This is a major problem for the reader. All the way through, I'm
> having to build a set of possibilities of what might be meant, then
> gradually remove them one at a time, which is why I likened it to a game of
> Cluedo.
>
> An interaction model is never about 'details'. I'm not asking for details.
> I'm asking that, if the model described in the abstract is the only model,
> it says so. And if it's only an example, it says so. It is a common mistake
> for the authors of a protocol to omit description of the big picture,
> probably because there has never been any other possibility of a different
> big picture in /their/ minds.
>

[LP] In that case, I don't really follow what you're asking for. We note
the actors and the messages between them. Just adding a sentence to Section
1 of paragraph 6 that says "the only thing we've defined applies to the
only this we've defined" seems like waffle to me. Maybe you have a better
concrete suggestion?

Answer supplied by Kazuho - As discussed in the last paragraph of section
5, CACHING defines if and how requests with different header field values
can be mapped to one response. If the capabilities provided by CACHING
(i.e. Vary) is too limited, then we should fix that as an extension to
CACHING (as have been previously proposed as draft-ietf-httpbis-key). In
practice, re Extensible Priorities, IMO, there aren't many sensible
combinations of urgency and incremental. Therefore, backend servers that
want to tune priority based on the value that the client sends can simply
send Vary: priority and call it a day.

[BB] I think my point has been missed. I'll try an example:
> Client A requests
>     priority u=4
> Server responds
>     priority u=2,
> which gets cached.
> Client B requests same object
>     priority u=4.
> Client C requests same object
>     priority u=0
>
> If requests B & C were forwarded to the origin, it would respond with
>     priority u=2    # for B
>     priority u=0    # for C
>
> However, even though the cached object has the same priority header that
> the origin server would give to Client B's request, it's different to that
> cached. And the cache cannot tell that B's request would match, but C's
> wouldn't.
>
> Vary doesn't help here, does it? At least not without storing the client
> request priority as well as the server response.
>

[LP] I'll let Kazuho respond to this one.

> identify further objects to be requested (dependencies).
> * Size is known by the server but not by the client, at least not until
> the headers
> at the start of the object arrive.
> * Dependencies are known by the server, but not by the client until an
> object
> starts to unfold.
>

As noted in response to T#4c, we are constrained by the capabilities of
information exchange that HTTP and its uses (such as the web) allows us.
This is no different a problem than existed for RFC 7540. Only a client
knows how it wants to use resources for which it has limited knowledge.

[BB] it's not the case that only a client knows how to use resources...
> That's the whole problem.
> The position is that B (the server) has to decide how to prioritize, but B
> knows info that A needs in order to tell B what to prioritize. And there's
> limited time for them to chat about it before prioritization becomes moot.
>
> So either B tells A the missing info, so A can tell B what to prioritize.
> Or A tells B its various strategies that depend on the info B holds. The
> latter is complex, the former would make more sense, but only if the
> protocol exchange had started earlier (!)
>
> I think the new priority approach is deliberately not fully solving this
> problem (unlike http/2 priorities), on the basis that httpbis can still add
> benefit for the current typical way web objects are structured. If you
> agree, I this is true, the spec ought to say it (nothing to be ashamed
> about).
>

[LP] RFC 7540 didn't have a solution to this. The information available to
clients is independent of the scheme used to signal a priority hint from
client to server.

In the "I dunno" case for RFC 7540, the defaults are invoked (non-exclusive
dependent on stream 0x0, weight 16) [1]. So all requests had an implicit or
explicit priority; an explicit prioritization to stream 0x0 with weight 16
is indistinguishable from an implicit priority.

I'm still unclear what you're asking for. Is it some short editorial text
to discuss client considerations for choosing the value of u or i
parameters?

If we use an HTML document as an example, the subresources have a
dependency chain that may or may not change while they get loaded. It’s
more likely that a client will request something with a particular priority
because of its type and usage in a given HTML document, rather than its
size (even if it knew that size). It’s going to be rare that a client
doesn't have an opinion - if the client doesn’t know, the defaults are
sufficient to let it reprioritize the request higher or lower if it finds
out that need once the response is back.

[BB] Apart from the fundamental problem of A&B having info in the wrong
> place (see above)... I believe the missing piece is an ability for the
> client to be able to distinguish between "I'm certain" and "I really don't
> know". As you explained earlier, "all requests are treated as if they have
> an associated priority signal", so the client has no way to express "I
> really don't know".
>
> I've said that multiple times now, and it seems to be falling on stony
> ground,  so I'll stop. Maybe I'm wrong, but I've said as strongly as I can
> that I think this will be a problem.
>

I think we can agree to disagree on this one. If it does really turn into a
problem, we can convert that into an opportunity to define a new parameter
for this scheme to address it. For example, imagine a new "confidence"
parameter between 0 and 100 that expresses the confidence of the priority
signal it was sent with.,

> [BB-follow-up]: When I said "which I believe is often the case" I didn't
> intend to mean the majority of cases. I meant "not an insignificant number".
> I'm sure the client can have a crack at how to set the priorities in
> common cases. Which can then be modified by the server if necessary.
>
> The point I was trying to highlight was that the client has no way to tell
> the server how to distinguish between "I'm certain about these priorities"
> and "I'm not really sure, but the protocol requires me to look as if I'm
> sure, even if I just leave everything as default priority."
> I think I explained it better under T#9a.
>
>

[BB] This doesn't answer my question. The text is essentially blustering.
> Please just say that there is little operational experience to go on, if
> there isn't. Rather than trying to say something that sounds wise,... and
> failing.

[LP] I disagree. Server push is the initiation of a response based on a
request that a client never made. The server has to pick something. And we
know that what it picks will interplay with non-pushed resources. We spell
out the problem that if you don't consider these things, problems could
happen. We don't need operational experience to reason that guessing wrong
is suboptimal when picking scheduling using this scheme.

[BB] I've read the issue, but I think it reveals unstated assumptions about
> what fair means. To understand what I mean by "equal rates are not fair if
> the objects are of different sizes", please see the description around Fig
> 1 in this discussion paper:
> https://bobbriscoe.net/projects/latency/per-flow_tr.pdf#page.3
>
> (The context is transport layer, not app-layer, but the arguments are the
> same. In the next response, I try to pull together app-layer and transport
> layer fairness).
>

[BB] I've read 1820, and the new text in draft-11.

   Typically, HTTP implementations depend on the underlying transport to
   maintain fairness between connections competing for bandwidth.

Interesting. But you (the apps area) don't appear to know your own
> strength...
>
> When the endpoints set the non-incremental parameter on streams A,B,C,D,E
> , it is equivalent to giving B-E zero rate until A completes, then C-E zero
> rate until B completes, etc.
>
> So, I think most transport area people would say the opposite of the
> above; the transport layer handles relative rates of connections, but only
> in the absence of more conscious bandwidth scheduling driven from the app
> layer (or intervention by the network operator, which is the other
> stakeholders that might apply bandwidth policy, but it has very weak
> visibility and knowledge of the information it is handling).
>
> The transport area offers no meaningful guidance on app-layer fairness.
> For instance, the app-layer decides:
> a) which streams to hold back from being allowed to send (as just
> described above);
> b) which streams are collected together within one 5-tuple flow;
> c) how much data to send into each socket.
> So the app-layer determines the units of fairness that the transport layer
> has to work with. The transport area's only contribution has been
> rate-equality between flow IDs competing in a common bottleneck. So I
> suggest you consider transport layer 'fairness' as just an arbitrary
> default policy that is merely the easiest way to prevent starvation. But
> always remember that the app-layer can override the transport layer and
> impose starvation (e.g. with the non-incremental parameter, as described
> above).
>
> In the transport area, there have been some recent advance in
> understanding, allowing that rate equality between flow IDs only becomes
> more necessary as congestion rises. The corollary being that, when
> congestion is low, more inequality is OK. IOW rate-equality is most useful
> as a famine response.
>
>    When
>    HTTP requests are forwarded through intermediaries, progress made by
>    each connection originating from end clients can become different
>    over time, depending on how intermediaries coalesce or split requests
>    into backend connections.  This unfairness can expand if priority
>    signals are used.  Section 13.1 <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-priority-11#section-13.1> and Section 13.2 <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-priority-11#section-13.2> discuss mitigations
>    against this expansion of unfairness.
>
>
> There seems to be an (incorrect) implication that different progress
> between end-clients is unfair. However, given end-clients have different
> available bandwidths in the general case, it will be 'fair' that their
> rates are unequal. For instance:
> * Client A has a 100Mb/s downstream access bottleneck, but it is currently
> shared by 4 other applications
> * Client B is using a 120Mb/s downstream bottleneck exclusively for itself.
> It would be wrong here for an http intermediary to allocate equal
> bandwidth between two http applications run by these two clients, which
> would merely serve to take bandwidth from the other 4 applications, and
> waste some of B's capacity.
>
> This is the sense in which it's best to allow the transport layer to share
> out the bandwidth (which includes some ideas that never took off, like
> bandwidth brokers between applications). But the app layer is still
> controlling the bigger levers of bandwidth allocation.

[LP] Thanks. There's some good knowledge here that I'll defer to Kazuho to
respond on better than I. I'm open to nailing down definitions better if it
will help.

FWIW we give recommendations for servers to avoid starving on situations
like 5 non-incremental requests. Really, servers want to keep transport
connection bandwidth capacity as full as possible. The available capacity
is a product of various factors - HTTP/2 flow control can add a whole
dimension of implementer fun. If we imagine a scenario where the client
makes 5 non-incremental requests where the contents are written to 5
different disk drives; if response A gets flow-control blocked by the
client because the disk is full, then the server can switch to B if it
likes.

Cheers
Lucas

Bob

[1] - https://datatracker.ietf.org/doc/html/rfc7540#section-5.3.5

---

From: Lucas Pardue <[email-masked]>
Date: Fri, 24 Dec 2021 01:37:13 +0000

Hey Bob,

Responses in line, I used LP2 just to aid navigation

On Thu, Dec 23, 2021 at 4:29 PM Bob Briscoe <[email-masked]> wrote:

> Lucas,
>
> I'm trying to close down as many of my points as I can. But still hanging
> on with some... see [BB2]...
> (BTW, I'm not subscribed to any w3.org lists, so it's blocking my
> responses. I think that's OK, 'cos so far you've not snipped any quoting.)
>
> On 21/12/2021 18:16, Lucas Pardue wrote:
>
> Hey Bob,
>
>>> authors and WG to allow them to address any issues raised and also to
>>> the IETF
>>> [BB-follow-up]: Sorry, 'waffle factor' was a bit derogatory. I meant
>>> that the sections that I have identified below as 'less mature' tended not
>>> to say anything significant, while apparently looking as if they were
>>> saying something significant.
>>>
>>>
>> The short answer is that HTTP prioritization is much of an optional
>> thing. Signals are hints to the server in charge of the multiplexed
>> connection. Servers have a self-interest in serving requests in a timely
>> manner balanced against all the other needs like resource usage, DoS
>> avoidance and so on. The sections that describe scheduling are some basic
>> recommendations that will let clients have some expectations how responses
>> would get prioritized should the stars align. But this is not a very
>> specific algorithm that all implementations will follow exactly because
>> rarely do stars align. The sections attempt to spell out the considerations
>> arising from the protocols related to this draft. We believe the editorial
>> changes made in -11 make it clearer where text is considerations more than
>> Reason: Unless readers put in effort to track what everyone else is
>> doing, they have no idea whether the various aspects of this draft are
>> based on arm-waving or real operational experience. For anyone who is
>> implementing themselves, that colours how much weight they should give to
>> the advice relative to their own experience.
>>
>
> [BB2] An implementation status section is most useful for reviewers;
> indeed it's recommended that it's deleted before publication. So I accept
> it would be a pain to add it just for the last few final reviews from the
> wider Internet community. But bear in mind that you have to get through the
> IESG as well.
>
> Whatever, hopefully you might consider one for future drafts.
>

[LP2] I agree a record of interop and implementation status is useful for
several purposes. I'll bear communication of status to reviewers in mind
for future drafts.

FWIW In the QUIC WG we have typically tried to record some results during
hackathon events or Martin Seeman's hosted interop runner [1]. In the HTTP
WG things are a bit different. Surveying implementation status of various
HTTP features can be tricky, there's a few things like mnot's cache tests
[2] or h2spec.

>>> priority approach with this one. Is that because the studies don't
>>> exist, or
>>> just an omission?
>>>
> [BB] I've now read the two references you added. I think only [MARX] fits
> after "Multiple experiments from independent research". [MEENAN] is more of
> an explanation of the problem (not the problem with 7540, but the problem
> of not having a prioritization protocol at all). And it is not really
> intended to be scientific research - the only quantified improvement is "We
> have seen 50% faster on multiple occasions" - there are no specific
> experiments or results. If you want to keep it, it could be added at the
> end of the first para of the Intro, perhaps. But you might find the RFC
> Editor cuts it out, because it is not an archival reference (it's a blog).
>
>
>>
>>>
>>> of pointers to other parts of the draft. So it would be consistent to
>>> say this
>>> way that requires the reader to refer to the CERT advisory (e.g.
>>> "...some types
>>> of problems...").
>>>
>>
> [LP] Ack. We think the text in the latest version now pays enough detail
> to RFC 7540 without treading on 7540bis toes.
>
>
>>
>>
>>
>>>
> [BB2] An initial implementation can discover that they can defer reading
> the spec until later; without having to read it to find out they didn't
> have to.
>

[LP2] Priority signaling and prioritization of concurrent resources are
different things. A server implementer has to think about prioritization
independent of RFC 7540 priority signals or Extensible priority signals.
The introduction states what clients and servers can do, especially that
servers can do whatever they like. The only thing I can think to suggest
for this point is modifiying Section 1 paragraph 3 to say something like
"HTTP/2 and HTTP/3 servers can schedule transmission of concurrent response
data by any means they decide. Servers can ignoring client priority signals
and still successfully serve HTTP. However, servers that operate in
ignorance of clients ...<reamining text>"

If that sounds reasonable I'll open a PR. But if you're looking for some
other type of statement that labours the point about HTTP working without
signals, then I don't think we need it here because its covered elsewhere.

> https://github.com/httpwg/http2-spec/issues/1000 there's further
> discussion there. 7540bis deprecates stream prioritization - all that is
> left is the remnant of bits on the wire that remain for wire compatibility.
>
> [BB] Above, I have highlighted again those of my points that were about
>> how the SETTINGS message itself (not the 7540 hints) is not fully tied down.
>>
>
> [BB2] I'll stop harping on about SETTINGS_NO_RFC7540 now. I clearly don't
> understand the subtleties of http incremental deployment.
>
>
>
>
>>
>> §4.  Priority Parameters
>>
> [BB2] Thanks for the change you have made.
>
> You say "By definition PRIORITY_UPDATE is only allowed in the C->S
> direction". By what definition? It is not in the draft. Here are two places
> where you might think it is defined:
>
> §4 In order to reprioritize a request (Section 6 <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-priority#section-6>), HTTP-
>    version-specific PRIORITY_UPDATE frames (Section 7.1 <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-priority#section-7.1> and Section 7.2 <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-priority#section-7.2>)
>    are used by clients to transmit the same information on a single hop.
>
> §7 PRIORITY_UPDATE frames are sent by clients on the control stream,
>    allowing them to be sent independent from the stream that carries the
>    response.
>
>
> When I read the §4 sentence, I thought it might mean that servers could
> respond with a PRIORITY_UPDATE to their original priority message, but that
> it only mentions the client's update because it is newly defined for a
> single hop, while the server's was already hop-by-hop.
>
> The §7 sentence merely says which stream a PRIORITY_UPDATE should be on if
> it's in the C->S direction. Indeed, it could be read as if it also implies
> that responses will carry a PRIORITY_UPDATE frame on the relevant stream
> itself, not on the control stream. Given this protocol is extensible, why
> shouldn't a server be able to update a previous priority message? Will
> intermediaries ignore such a message? Must they?
>
> In this draft, it has not been considered necessary to explicitly state
> the interaction model. So valid interactions only appear implicitly in
> sentences that are primarily there to say something else. This is an
> extensible protocol, so the designers of extensions that want their new
> message to be consumed by a particular type of node are entitled to think
> that they can define it, unless this draft specifically precludes it.
>
> So, I beg to differ about negatives - they remove ambiguity. Absence of
> any statements about a frame in the S->C direction does not define that it
> is not used in that direction. And anyway, to find an absence the reader
> has to carefully check the whole draft (like Cluedo). This is the
> antithesis of improved readability.
>

[LP2] Section 7.1 and 7.2 both include the statement that only you're
looking for. 7.2 is the stronger of these, we can update 7.1 to match it (I
think that was an editorial oversight, fixed by
https://github.com/httpwg/http-extensions/pull/1869).

>From section 7.2 para 8

  PRIORITY_UPDATE frames of either type are only sent by clients. If a
client receives a PRIORITY_UPDATE frame, this MUST be treated as a
connection error of type H3_FRAME_UNEXPECTED.

So by this definition, there is no way for a server to successfully send a
PRIORITY_UPDATE frame. We could probably hammer this home more upfront in
section 4 with an editorial tweak but it doesn't change the current design
of the document.

My comment about readability is in response to understanding whether you
want X (not Y) on every instance. That seems overly wordy to me. If the
suggestion was more geared around surfacing the constraint succinctly front
and center, I'm more willing to consider it. I'll note even documents like
draft-ietf-httpbis-semantics [4] don't seem to be trying to tie themselves
in knots about stating only clients (not servers) send requests and only
servers (not clients) send responses.

>> client. I believe the draft intends this not to be possible, although
>> this is
>> The point I was trying to highlight was that the client has no way to
>> tell the server how to distinguish between "I'm certain about these
>> priorities" and "I'm not really sure, but the protocol requires me to look
>> as if I'm sure, even if I just leave everything as default priority."
>> Alternatively, could the client send a Priority header with no
>> parameters? This
>> issued". It goes on to state that the priority field is an e2e signal,
>> but then
>> possible that intermediaries are intended to (or at least allowed to)
>> read but
>> An interaction model is never about 'details'. I'm not asking for
>> details. I'm asking that, if the model described in the abstract is the
>> only model, it says so. And if it's only an example, it says so. It is a
>> common mistake for the authors of a protocol to omit description of the big
>> picture, probably because there has never been any other possibility of a
>> different big picture in /their/ minds.
>>
>
> [BB2] I would say the most relevant place to do this would be to limit
> extensibility. Otherwise the interaction model could be read as just the
> one needed for the parameters defined so far. So, straight after the last
> sentence of §3:
>
>    Such extensions can
>    define themselves how this priority scheme is to be applied.
>
> Add:
>
>    However, they have to {MUST?} keep to the same interaction model where
>    the client sends an end-to-end Priority field to the origin server
>    and the server sends hints in the Priority field to downstream
>    intermediaries, or the client sends hop-by-hop PRIORITY_UPDATE frames
>    to intermediaries.
>
>
> BTW, I notice the sentence below from §4 says that the Priority field is
> e2e in both directions, so I think it will need to be corrected to say
> hop-by-hop in the S->C direction.
>
>    The Priority HTTP header field (Section 5 <https://datatracker.ietf.org/doc/html/draft-ietf-httpbis-priority#section-5>) is an end-to-end way to
>    transmit this set of priority parameters when a request or a response
>    is issued.
>
> [LP2] I hope the response to the prior point transfers a bit here. I don't
see how extensions can change the interaction model we define unless they
wish to extend it. In which case why would we have anything to say here
about what they extend it to do?

>> must/should/may be stored about priority in a cache for later
>> connections. Even
>> led to the priority stored with the response. And it will not know
>> whether the
>> to download. How was the client to know it should have downloaded D
>> before C?
>>
>> So, as I already said under my question T#5a) about the interaction
>> model, the
>> The position is that B (the server) has to decide how to prioritize, but
>> B knows info that A needs in order to tell B what to prioritize. And
>> there's limited time for them to chat about it before prioritization
>> becomes moot.
>>
> In the "I dunno" case for RFC 7540, the defaults are invoked
> (non-exclusive dependent on stream 0x0, weight 16) [1]. So all requests had
> an implicit or explicit priority; an explicit prioritization to stream 0x0
> with weight 16 is indistinguishable from an implicit priority.
>
> [BB2] Here I was reacting to your statement that "Only a client knows how
> it wants to use resources for which it has limited knowledge. "
> Last night I noticed that [MEEHAN] starts out by saying the opposite:
>     "Historically the browser has been in control of deciding how and when
> web content is loaded. Today we are introducing a radical change to that
> model ... that puts control into the hands of the site owner directly. "
>
> A control point cannot exercise its control well if it is lacking
> necessary information. That's what a protocol is for. Here, I was trying to
> analyse what information was lacking where and when. But, I think I had
> better just step back from trying to improve the protocol at this stage.
>

[LP2] The challenges are innate to HTTP. There are some means to give the
client better data, some exist in HTTP and others exist in HTML or other
things that are outside the remit of the IETF. This is why we acknowledge
servers might look beyond just the priority signal, to other client headers
or anything else that might fill in some gaps. The hardest thing is that
client and server are flying close to the latency edge and that
reprioritization can be too slow to make it worthwhile after the client
learns what the server knows.

I can imagine some new Extensible Priority parameters that might help to
improve the situation. For example, a client could present multiple sets of
parameters and instruct the server to select on based some variable
condition (if content-length < N use set 1, else use set 2). However, this
is a new speculative capability that so far no one has mentioned or
screamed for.

> [BB2] Nonetheless, I just noticed a new editorial point, in §9 on client
> scheduling. The text made me think §9 is about what priority values a
> client puts in its requests. But perhaps it meant:
> CURRENT:
>
>    A client MAY use priority values to make local processing or
>    scheduling choices about the requests it initiates.
>
> Was this intended to say?:
>
>    A client MAY use priority values to make local processing or
>    scheduling choices about the responses to the requests it initiated.
>
>
This text is quite old. I think (personally) it probably actually is meant
to say the values MAY apply to both requests and responses. Worth further
thought so I created https://github.com/httpwg/http-extensions/issues/1868

> I think we can agree to disagree on this one. If it does really turn into
> a problem, we can convert that into an opportunity to define a new
> parameter for this scheme to address it. For example, imagine a new
> "confidence" parameter between 0 and 100 that expresses the confidence of
> the priority signal it was sent with.,
>
>
> [BB2] Yes. Again, I'll step back from trying to improve the protocol at
> this stage.
>
>
>
>
>>
>> §10.  Server Scheduling
>>
>> The point I was trying to highlight was that the client has no way to
>> tell the server how to distinguish between "I'm certain about these
>> priorities" and "I'm not really sure, but the protocol requires me to look
>> as if I'm sure, even if I just leave everything as default priority."
> [BB2] OK, I should have said that the draft needs to say that any
> /solution/ would be conjecture due to lack of operational experience
> (you're right, the problem can be predicted without ops experience).
>
> If there were operational experience, the draft might be able to give some
> advice on what is the least harmful thing to do. But, as it stands, these
> paras end in mid-air, with no explanation for why no advice is given.
>

[LP2] Thanks for the clarification. That's a slightly different sort of
problem. In these cases the answer is implementation driven and we know
that sever implementations do different things. I think the document is
consistent in its treatment. Stating something like "these considerations
mean we have no specific advice" doesn't seem much help.

> [BB] I've read the issue, but I think it reveals unstated assumptions
>> about what fair means. To understand what I mean by "equal rates are not
>> fair if the objects are of different sizes", please see the description
>> around Fig 1 in this discussion paper:
>> So the app-layer determines the units of fairness that the transport
>> layer has to work with. The transport area's only contribution has been
>> * Client A has a 100Mb/s downstream access bottleneck, but it is
>> currently shared by 4 other applications
>> This is the sense in which it's best to allow the transport layer to
>> share out the bandwidth (which includes some ideas that never took off,
>> like bandwidth brokers between applications). But the app layer is still
> [BB2] OK, I'll watch for the responses deferred to Kazuho on two points.
> I'll probably be going off line until the New Year now.
>

Me too, enjoy!

Cheers
Lucas

[1] https://interop.seemann.io/
[2] https://cache-tests.fyi/
[3] https://github.com/summerwind/h2spec
[4]
https://httpwg.org/http-core/draft-ietf-httpbis-semantics-latest.html#rfc.section.1.3.p.2

---

From: Lucas Pardue <[email-masked]>
Date: Fri, 24 Dec 2021 14:08:56 +0000

Hey,

Responses as LP3 from me :-)

On Fri, Dec 24, 2021 at 9:58 AM Bob Briscoe <[email-masked]> wrote:

> Lucas, see [BB3]
>
> On 24/12/2021 01:37, Lucas Pardue wrote:
>
> Hey Bob,
>
>>> [BB] I think you might have missed my point. I meant a section telling
>>> us what degree of implementation there has been of each aspect (to be
>>> removed before publication as an RFC). I should have referenced BCP 205
>>> [RFC7942].
>>>
> [BB3] Sry, I should have tagged the above as [BB2], which is probably why
> you missed it.
>

[LP3] The experiment is in replacing wholesale the RFC 7540 tree-based
prioritization with the scheme diagramed mid-way down the page. This
continues to operate and meets the criteria of the sentence fragment
"simpler schemes can reach at least equivalent performance". I accept that
this is not of the rigor of a scientific paper and I can remove it if it's
a weak citation or distraction. The important link is that the work and the
scheme were part of the original motivation for Extensible Priorities work
but we capture that in the acknowledgements already.

>>>> insufficient? It seems obvious that at run-time you might hit the end
>>>> of the
>>>> edge of the space when you really wanted some objects to have higher
>>>> (or lower)
>>>> motivations in §2, but in draft-10 it has been removed from §2. If it
>>>> is still
>>>> Security Considerations seems to be in the commonly used style of just
>>>> a list
>>> That's fine, but BTW, it wasn't a distraction for me. It seemed to be
>>> one of the stronger reasons for moving away from dependencies, because (in
>>> many cases) I can't understand how priorities can be implemented without
>>> knowing the dependencies (see T#9a).
>>>
>>
>>> In your very first response above - about implementation status - you
>>> say "The short answer is that HTTP prioritization is much of an optional
> [BB3] It doesn't also say that clients don't need to send priority
> signals, but it's better than it was. Esp. if you can s/servers/servers or
> intermediaries/ in both the new sentences. Thank you.
> Also s/can ignoring/can ignore/
>

[LP3] Thanks, see https://github.com/httpwg/http-extensions/pull/1871

>>>>   "A server that receives SETTINGS_NO_RFC7540_PRIORITIES with a value
>>>> of 1 MUST
>> [LP] Other documents talk in terms of servers operating on client
>> signals. They don't mention server->client PRIORITY frames. So in effect,
>> the signal is always ignored anyway. Trying to talk to that direction of
>> flow of the signal here seems like it would add complication to the
>> document and I think a distraction.
>>
>>>> because the HTTP/2 priority signals might have come from an
>>>> intermediary that
>>> understand this setting, and later sends HTTP/2 priority signals
>>> (perhaps in
>> SETTINGS_NO_RFC7540 is an optimization related to carriage, processing
>> and application of signals. Because these signals are only a hint, as long
>> as they have a valid wire format there is no need for protocol errors.
>> Since we are focused on C->S direction of signal, and the world never
>> defined what RFC7540 S->C signals really meant, our document doesn’t
>> benefit from trying to speak about them. So we define our terms of use in
>> the Notational Conventions and stick to them. The document consistently
>> uses this term,
>>
>>
>> They can, it depends on the versions of HTTP being used on the upstream
>> or downstream.
>>
>>
>>> * How does adding a priority header override any signal from a client?
>>> Or is it
>> [BB] Then this becomes just an editorial point, that it would be useful
>> to say explicitly at the start of §4.
>>
>> CURRENT:
>>
>> The §7 sentence merely says which stream a PRIORITY_UPDATE should be on
>> if it's in the C->S direction. Indeed, it could be read as if it also
>> implies that responses will carry a PRIORITY_UPDATE frame on the relevant
>> stream itself, not on the control stream. Given this protocol is
>> extensible, why shouldn't a server be able to update a previous priority
>> message? Will intermediaries ignore such a message? Must they?
>>
> [BB3] The recent changes have fixed and will fix this for PRIORITY_UPDATE.
> The 'X (not Y)' formulation was only a suggestion of how you might express
> this. Thanks.
>

[LP3] Cool.

>>> the MIME type it has served is incremental. Because, when the client
>>> says 'not
>>> incremental', that is intended to state the capability of the client,
>>> not the
>>> format of the resource.
>>>
>> That's a fair point. Unfortunately, there will be cases where parties
>> lack all of the information that could lead to perfect prioritization.
>> Client priority signals are only a hint. Servers can and will do whatever
>> they like, including serving the response in a way that does not follow the
>>> initiate priority messages, as absence of a message is currently the
>>> only way
>>> priority or other. So the server doesn't know whether the client
>>> actually knows
>> If this scheme is implemented at the server, then all requests are
>> treated as if they have an associated priority signal. This can be a
>> Priority header field (Note the serialization rules for Structured Fields
>> Dictionary in Section 3.2 of RFC 8941 - sending an empty header is not
>> permitted) or a PRIORITY_UPDATE frame. Omission of signals, or omission
>> values in signals, invokes default value priority parameters urgency=3,
>> incremental=false. A server is therefore able to determine the client’s
>> view of the priority.
>>
>> The purpose of the Priority header in responses is to allow origin
>> servers (detached from the intermediaries’ connection to the client) the
>> ability to also provide hints about prioritization.
>>
>> The interaction model is described throughout the document, with a gist
>> in the intro. Duplicating details into the intro does not seem beneficial.
>>
>>
> [BB3] I think your response below overlooked the 'BTW' editorial point
> above.
>
> [BB3] You say "the interaction model we define". It is now defined for
> PRIORITY_UPDATE. But, still labouring my point, for the Priority header,
> the interaction model is not /defined/. Some interactions are mentioned.
> But the the set of allowed interactions is not defined normatively.
>
> If the interaction model is defined well, it can be narrowed but not
> widened by extending a parameter. For instance, if an intermediary is not
> meant to look at an end-to-end Priority field in one direction, no-one can
> add an extension parameter within a Priority field in that direction that
> is intended for an intermediary.
>
> But if the interaction model is unclear (as it still is for the Priority
> header), you get the sort of discussions like we had about the SETTINGS as
> to whether some implementations might have interpreted a vague interaction
> model (RFC7540 in this case) in a different way to how it was intended.
>
> This is why an interaction model is primarily about which interactions are
> /not/ supported.
>

[LP3] Responding to both of these points. I'm not sure I understand your
interpretation of the header treatment here. The header is end-to-end for
both request and response. HTTP semantics defines the interaction model for
messages. HTTP allows intermediaries to do a lot of things with the request
and response messages that they forward. I don't see the need to litigate
HTTP semantics in this draft.

The whole point of this statement you quote is to handle applicability to
HTTP/2 or HTTP/3 extensions that allow new data carriage mechanisms that
don't follow existing HTTP semantics. For example,
draft-ietf-masque-h3-datagram [1] defines how HTTP/3 endpoints can use QUIC
datagrams to send unreliable messages. Unreliable messages have no HTTP
semantic, so we don't have the means to predict how a priority signal would
be sent or the means to tell a server how it might prioritize datagram data
against response data. Defining such things can be done by the extension if
they care to do it - for example by binding datagram flows to requests and
stating how Extensible priorities does or does not apply. But we can't
predict the future, which is why this draft shouldn't be overly restrictive
for future uses.

>>> if it's implementation dependent, wouldn't it be worth a brief
>>> discussion (as
>>> in the 2 paras below).
>>>
>>> client to request. It doesn't know their sizes, and it doesn't know
>>> which ones
>>> To be effective, a scheduler needs to know object sizes and which
>>> objects will
>>> you decide". But that's the one thing this draft doesn't allow the
>>> client to do
>>> the requests at the same time - it has to choose which order to send
>>> them in,
>>> even if it has no clue. This tells the server to respond in that order
>>> OR to
>>> So either B tells A the missing info, so A can tell B what to
>>> prioritize. Or A tells B its various strategies that depend on the info B
>>> holds. The latter is complex, the former would make more sense, but only if
>>> the protocol exchange had started earlier (!)
>>>
>> [LP] RFC 7540 didn't have a solution to this. The information available
>> to clients is independent of the scheme used to signal a priority hint from
>> client to server.
>>
>>     "Historically the browser has been in control of deciding how and
>> when web content is loaded. Today we are introducing a radical change to
>> that model ... that puts control into the hands of the site owner directly.
>> "
>>
> [BB3] I've responded there.
>

[LP3]  Cool

>>> If this conclusion is based on operational experience it ought to say
>>> so. And
>> If there were operational experience, the draft might be able to give
>> some advice on what is the least harmful thing to do. But, as it stands,
>> these paras end in mid-air, with no explanation for why no advice is given.
>>
>
> [BB3] I asked to say sthg like "without more implementation experience no
> useful advice can be given." but this isn't important enough for me to keep
> arguing about.
>

[LP3]  In the server push section we already say " There is no general
guidance on the best way to apply these.". Even with implementation
experience, I doubt useful advice can be given. Therefore, making a
statement that implementation experience would produce useful advice is
highly speculative and next to useless IMO. I think its best for us to move
on from this point.

>>> (The context is transport layer, not app-layer, but the arguments are
>>> the same. In the next response, I try to pull together app-layer and
>>> transport layer fairness).
>>>
>>
>>
>>> When the endpoints set the non-incremental parameter on streams
>>> A,B,C,D,E , it is equivalent to giving B-E zero rate until A completes,
>>> then C-E zero rate until B completes, etc.
>>>
>> [LP] Thanks. There's some good knowledge here that I'll defer to Kazuho
>> to respond on better than I. I'm open to nailing down definitions better if
>> it will help.
>>
> --
> ________________________________________________________________
> Bob Briscoe                               http://bobbriscoe.net/
>
>