fb103ac5ae
Every registered string now lives in one table (codecs/registry.rs) with its kind, status, media class, registering SPEC section, edition and registration date. `wai registry export` writes registry/capabilities.json, its JSON Schema, the generated table in REGISTRY.md and vocab v2; --check fails on drift. vocab v1 stays stable. A candidate capability (a registered string whose payload format no revision has pinned) must not be emitted. The C packers, the browser writers and `wai wrap` refuse one, and `codecs::is_dispatchable` is the single dispatch and selection predicate (SPEC §4): candidates, companion-only capabilities, records and derivations are not dispatchable, and an unregistered string is decoded only where the sink advertises it. The codec glossary is linked to the registry through a reviewed table and held to its schema by a library test; `org` becomes `body`, from a closed allowlist. The SPEC gains §5 sections on kinds and candidates, a §8 edition rule, corrected citations and rewritten appendices. The quantum-ops vectors are regenerated with neutral device ids. CI declares the feature-gated test targets with required-features, checks the registry export, builds wai-web for wasm32, and runs two public-text checks (comparative prose, and private name patterns) with a self-test. The specification, extensions, research notes, READMEs, glossary and source comments are rewritten to name no company or product and to make no comparisons. Co-Authored-By: Claude Opus 5.5 <[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, which has only
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"supported", gains a range by adopting it.
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