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Two seams the tree could not express, both surfaced by the landscape survey.
CAPABILITY CONSTRAINTS. WAI dispatch treats support as binary — a sink either
claims a capability or does not. Real sinks support a codec TO A LEVEL: up to a
resolution, a frame rate, a bit depth, a profile. "This capability, but only to
4K60 10-bit, this profile" is the normal case and was unexpressible, so partial
support failed at DECODE time rather than SELECTION time, and rendition selection
had nothing to evaluate and deferred to a deployer policy the spec never states.
Ingests the constraint-set grammar and register pattern from AMWA's ratified
Receiver/Sender Capabilities work (BCP-004-01 / -02, v1.0.0) — ORed sets of
enum/min/max constraints over register-named parameters. Grammar and pattern
only: not the transport, not the discovery model, and explicitly NOT the
pre-flight negotiation framing, because capability-plus-budget binding before
execution is already owned by JCP and two mechanisms for one job is worse than
none.
The honest limit is stated in §3 rather than hidden: the ratified registers are
broadcast-essence-shaped and cover only WAI's conventional media families. There
are no entries for neural, quantum, world or haptic, and WAI must author those
itself — until it does, those capabilities remain unconstrained, which is the
true state rather than a pretended one.
UNIDIRECTIONAL DELIVERY. Three assumptions quietly presume the sink can talk
back: jwp-receipts assumes relay-chain linkage, targeting presumes the source
learns something about the sink, and pre-emption has no signer class because
nothing in WAI has needed to assert authority over a sink before. Terrestrial
broadcast, satellite and multicast object carriage break all three, and none is
repaired by defining a bearer.
* Sink-evaluated predicates (§2): the object carries a condition the SINK
evaluates against local state the source never learns, under a
predicate-equivalence contract in the shape of state-feeds §3. The
specifiable property is determinism and auditability, NOT enforcement —
every sink receives every byte and can lie about its own state, and this
text says so. The real claim is narrow and worth having: the source learns
nothing, which is the dual of source-side targeting.
* Authority and pre-emption (§3): pre-emption is the highest-authority
instruction a media object can carry — it displaces what a person chose to
watch — and today an object either lacks the field or has one anyone may
set. Requires a sink-held trust anchor provisioned out of band; a trust
anchor delivered over the bearer it authorises is not one. A valid signature
establishes who signed, never that they were entitled to.
* Detached receipts (§4): the deliverable is an honest profile, not a proof.
Such receipts attest WHAT WAS TRANSMITTED, not what was received or
rendered, and MUST declare the bearer unidirectional. No receipt can prove
rendering over a bearer with no return path; what this prevents is the
quieter error of a receipt saying "delivered" because a transmitter
transmitted, on a link that cannot tell it whether anyone was listening.
Generalised from deployed national early-warning systems, which are where a sink
evaluating a condition the source never learns has had to work at scale.
Ingest queue items 10 and 11 (#80 §4.2, §4.3).
Co-Authored-By: Claude Opus 5 <[email protected]>
84 lines
4.3 KiB
Markdown
84 lines
4.3 KiB
Markdown
# WAI Extension: Capability Constraints
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> Status: Draft. Makes capability dispatch (WAI SPEC.md §4) a **precondition
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> check** rather than a binary claim, by ingesting the constraint-set grammar and
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> register pattern the broadcast-IP world already ratified. No transport, no
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> registry service, no negotiation protocol — it evaluates values WAI already
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> carries.
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> Keywords **MUST**, **MUST NOT**, **SHOULD**, **MAY** are RFC 2119/8174.
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## 1. The problem: support is not binary
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WAI dispatch treats a capability as present or absent: a sink either claims
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`cap` or does not. Real sinks are not like that. A decoder supports a codec *to
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a level* — up to a resolution, a frame rate, a bit depth, a profile. "This
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capability, but only to 4K60 10-bit, this profile" is the normal case and is
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currently unexpressible.
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The consequence is that partial support fails at **decode time** rather than at
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**selection time**. A sink advertises the capability, receives an object it
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cannot actually handle, and fails after committing to the bytes. Worse, WAI's
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rendition selection has nothing to evaluate, so it defers to a deployer policy
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the specification never states.
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## 2. Constraint sets
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A **constraint set** is an ORed set of constraints over named parameters, each
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constraint being an enumeration, a minimum, a maximum, or a combination. A sink
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satisfies a capability when it advertises `cap` **and** the object's declared
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parameters satisfy at least one of the registered constraint sets for `cap`.
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This grammar is deliberately not invented here. It is the shape AMWA's Receiver
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and Sender Capabilities specifications (BCP-004-01 and BCP-004-02, both v1.0.0)
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use, with parameters drawn from a public register rather than a bespoke schema
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per implementation. WAI ingests the **grammar and the register pattern**, not the
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transport or the discovery model those specifications sit in.
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- A capability registration **MAY** publish one or more constraint sets. A
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capability with none is unconstrained, which is the current behaviour and
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remains valid.
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- A sink **MUST** evaluate constraint sets **before** dispatch, not during it.
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The point is to move the failure earlier; a check performed at decode has
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bought nothing.
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- A sink **MUST NOT** advertise a capability whose constraint sets it cannot
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evaluate. Advertising a capability while ignoring its constraints is the
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binary behaviour this extension exists to replace.
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- Where an object's declared parameters satisfy no set, the sink **MUST** treat
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the capability as unsupported for that object — not as a decode error, and not
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as a reason to attempt it anyway.
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## 3. Parameters and the register
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Parameters are named in a register, so two implementations constrain the same
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property under the same name. WAI adopts the pattern; it does not adopt the
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existing register wholesale, for a reason worth stating plainly:
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**The ratified registers are broadcast-essence-shaped.** They cover media type,
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frame rate, colour sampling, bit depth, codec profile — which serves WAI's
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conventional media capabilities and nothing else. There are no entries for the
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neural, quantum, world or haptic families. Those register entries **WAI must
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author itself**, and until it does, those capabilities remain unconstrained under
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§2, which is the honest state rather than a pretended one.
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- A parameter name **MUST** resolve in a register. An implementation-local name
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is not a constraint anyone else can evaluate.
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- WAI-authored parameters **MUST** be namespaced so they cannot collide with
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entries authored elsewhere.
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## 4. What this is not
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- It is **not** a negotiation protocol. There is no pre-flight exchange, no
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controller, and no registry service. Capability-plus-budget binding before
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execution is already owned by JCP, and duplicating it here would put two
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mechanisms in the tree for one job.
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- It is **not** a transport concern. Constraint evaluation reads values the
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object already declares.
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- It is **not** an energy mechanism. A constraint says what a sink can decode,
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never what it should spend.
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## 5. Framing
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The grammar is thin — close to a JSON-Schema subset — and that is fine. The value
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is not expressiveness; it is that a ratified, register-backed way of saying
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"supported, to here" already exists, and WAI's dispatch is strictly worse for
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having only "supported".
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