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Re: Tsvart last call review of draft-ietf-httpbis-priority-10

From: Lucas Pardue <lucaspardue.24.7@gmail.com>
Date: Thu, 9 Dec 2021 15:43:28 +0000
Message-ID: <CALGR9oZJKr_guJfP_QxhTcAjXCGDT+a-gJHjFfksFisc-ZAxnA@mail.gmail.com>
To: Bob Briscoe <ietf@bobbriscoe.net>
Cc: tsv-art@ietf.org, draft-ietf-httpbis-priority.all@ietf.org, HTTP Working Group <ietf-http-wg@w3.org>, last-call@ietf.org
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 <
noreply@ietf.org> 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
> tsv-art@ietf.org 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
Received on Thursday, 9 December 2021 15:43:59 UTC

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