glm-5.2
a3cb44968e
Prune the channels spec to reflect the stream-unification resolution (docs/research/stream-unification/findings.md): the channels wire format goes from 9 bytes to 8 bytes, the channels layer no longer carries a stream_type concept, into_sub_streams() is removed, and TTY always uses its 5-byte format (carried transparently in the channels payload). ADR-093 is the umbrella decision (the channels-layer consequence of ADR-092's BiStream handler leaf): every channel is a BiStream, the handler owns its sub-stream multiplexing, the channels layer routes by channel_id only. Amends ADR-071 (8-byte header, no stream_type), ADR-074 (into_sub_streams removed, accept_bi yields BiStream), reverses ADR-077 (TTY always 5-byte), and the channels-facing clauses of ADR-072/073/075/076/080/081. Adds ADR-092 forward-reference note (into_sub_streams preservation subsequently reversed by ADR-093) and the missing ADR-092 cross-reference on ADR-070. Adds OQ-68 (add/strip API shape — built-in vs utility; the contract is decided in ADR-093, the function surface is open; two-way door, low priority, decision-ready when the channels crate's implementation begins). Rewrites the 7 channels spec docs (README, overview, channels-wire, channels-connection, channels-adapter, channel-operations, channel-client) to describe the post-amendment shape as current, with the 8-byte header, the add/strip composition, single accept_bi accessor, BiStream per channel, and TTY-always-5-byte. Touch-up cross-references in hub README, client README, ADR-085, and the OQ-45/47/65 question files (TTY-internal stream_type 3 → STREAM_CTRL_IN; channels 9-byte → 8-byte).
156 lines
7.7 KiB
Markdown
156 lines
7.7 KiB
Markdown
# ADR-072: Channel 0 Is Pre-Negotiated `alknet/call`
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## Status
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Accepted (amended 2026-07-18 by ADR-093 — channel 0's `stream_types`
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field is removed; the channels layer has no `stream_type` concept; the
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call protocol's `EventEnvelope` framing is the channels payload, carried
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transparently — see "Amendment (ADR-093, 2026-07-18)" below)
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## Amendment (ADR-093, 2026-07-18)
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Channel 0's `stream_types` field (the `[0, 1]` active set) is **removed**.
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The channels layer has no `stream_type` concept (ADR-093) — it carries the
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call protocol's `EventEnvelope` framing (ADR-064) transparently in the
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8-byte header's payload. The call protocol's bidirectionality (client
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writes requests, server writes responses) is a call-protocol concern,
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not a channels-layer concern; the channels layer routes by `channel_id`
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only and yields a `BiStream` to the `CallAdapter`. The `CallAdapter`'s
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`accept_bi()` returns one `BiStream` (per ADR-092); the call protocol
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reads/writes `EventEnvelope` frames on it, exactly as on a top-level
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`alknet/call` connection.
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The body below describes the **original** (with `stream_types`) shape;
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the amendment above is the operative decision. See ADR-093 for the
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resolution rationale and the cross-ADR impacts.
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## Context
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A channels connection carries N logical channels. One of them must carry the
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call protocol — the JSON-RPC layer that orchestrates channel lifecycle
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(`channel/open`, `channel/close`, `channel/control`, `channel/resources`).
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The question is how channel 0 relates to the call protocol: is it a special
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"control plane" with its own framing, or is it just `alknet/call` pre-
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negotiated?
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The phase-0 research (`docs/research/alknet-channels/phase-0-findings.md`
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§DP-2) recommends channel 0 is `alknet/call` pre-negotiated — no special
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framing, no separate control-plane wire format. The call protocol runs on
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channel 0 exactly as it runs on a top-level `alknet/call` QUIC connection.
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This matters because the alternative (a special control plane) would mean
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the channels layer has its own JSON protocol for channel lifecycle, parallel
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to and duplicating the call protocol's `OperationRegistry`, `AccessControl`,
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`OperationContext`, and `forwarded_for` machinery. That duplication is the
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"re-implement every protocol's framing per transport" problem the hub
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motivation (§Hub Motivation) identifies as the thing channels exists to
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collapse.
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## Decision
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**Channel 0 is `alknet/call`, pre-negotiated.** Both sides of a channels
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connection know that `channel_id = 0` is routed to the `CallAdapter` without
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an explicit `channel/open` exchange. The `CallAdapter` receives a
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`Connection` backed by channel-0 chunk reassembly and dispatches operations
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exactly as it does on a top-level `alknet/call` connection.
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### What this means concretely
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1. **Channel 0 uses the same 9-byte chunk format as every other channel**
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(ADR-071). Its chunks have `channel_id = 0` in the header. No special
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first-byte trick, no separate framing.
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2. **The `CallAdapter` is unchanged.** It receives a `Connection`, calls
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`accept_bi()`, gets one bidi stream (the channel-0 reassembled stream),
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and runs its dispatch loop. `EventEnvelope` frames ride on `stream_type =
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0` of channel 0. The `CallAdapter` does not know it is inside a channels
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connection.
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3. **Channel lifecycle operations are call operations.** `channel/open`,
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`channel/close`, `channel/control`, `channel/resources` are registered on
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the call protocol's `OperationRegistry` at assembly time (ADR-073). They
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are dispatched through the existing `OperationContext` (identity, scopes,
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capabilities, ownership, `forwarded_for`), gated by the existing
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`AccessControl::check`. No new auth machinery, no new framing, no
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protocol version bump.
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4. **Channel 0 is allocated at `ChannelsAdapter::handle` entry.** The
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`ChannelsAdapter` constructs channel 0's reassembly buffers, wraps them
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as a `Connection` (via `Connection::from_source` with a
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`ChannelBidiStreamSource` — ADR-070/074), and hands that `Connection` to
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the `CallAdapter` — exactly as if `alknet/call` had been the top-level
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ALPN. The `CallAdapter` is looked up in the same `HandlerRegistry` as
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every other ALPN.
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### Channel 0's stream_type usage
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| stream_type | direction | purpose |
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|-------------|-----------|---------|
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| 0 | write (client→server) | `EventEnvelope` frames from the client (call.requested, call.aborted) |
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| 1 | read (server→client) | `EventEnvelope` frames from the server (call.responded, call.completed, call.error) |
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Channel 0 uses stream_types [0, 1] — the call protocol is bidirectional via
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two unidirectional halves, the same way every channel type works
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(ADR-071 §stream_type decomposition). The call protocol's `(SendStream,
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RecvStream)` pair maps directly: `SendStream` backed by stream_type 0,
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`RecvStream` backed by stream_type 1. Both sides write to their write half
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and read from their read half — no shared stream_type both sides write to.
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The call protocol is JSON-only and single-stream by design (ADR-064).
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stream_types 2-255 on channel 0 are reserved for future call-protocol
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sub-streams.
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## Consequences
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**Positive:**
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- No control-plane duplication. The channels layer reuses the call protocol's
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`OperationRegistry`, `AccessControl`, `OperationContext`, `forwarded_for`,
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and `StreamingHandler` (ADR-049) machinery verbatim. Channel lifecycle is
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just another class of call operations.
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- The `CallAdapter` is transport-agnostic by construction — it works
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identically whether the `Connection` is a top-level QUIC stream or a
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channels-reassembled channel-0 stream. This is the "streams are streams"
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insight made concrete.
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- `channel/resources/subscribe` (ADR-073) is a `Subscription` operation on
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channel 0, using the already-implemented `StreamingHandler` /
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`invoke_streaming` path (ADR-049). The resource registry is a live view,
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not a polled snapshot.
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- Auth is inherited: `channel/open` goes through `AccessControl::check`
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exactly like any other call operation. The channels layer does not re-
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implement auth.
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**Negative:**
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- Channel 0 is a single point of orchestration. If channel 0's `CallAdapter`
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hangs, no new channels can be opened. This is the same property as the call
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protocol today (one dispatch loop per connection) and is not a new
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vulnerability.
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- The call protocol's JSON-only nature means channel lifecycle operations
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are JSON. For high-frequency control (e.g., per-keystroke resize), this is
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more overhead than a binary control frame. The division (ADR-073 §DP-4)
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handles this: `stream_type 3` on the data channel for data-ordered control,
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call operations for lifecycle and infrequent control.
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## Door type
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**One-way.** Channel 0's role as `alknet/call` pre-negotiated is a wire-
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format and protocol-structure commitment. Changing it after deployments
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exist (e.g., to a special control plane) requires a version migration and
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re-architecting the channel lifecycle operations. The reservation of
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`stream_type` 1-255 on channel 0 is a two-way-door detail (they're currently
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unused; assigning them is additive).
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## References
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- ADR-071: channels wire format (the 8-byte chunk header channel 0 uses,
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as amended by ADR-093)
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- ADR-093: channels pure channel multiplexing (amends this ADR —
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channel 0's `stream_types` field removed; the call protocol's framing
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is the channels payload, carried transparently)
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- ADR-073: channel lifecycle operations (registered on channel 0's
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`OperationRegistry`)
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- ADR-064: irpc never integrated — hand-rolled EventEnvelope framing (the
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call protocol channel 0 carries)
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- ADR-049: StreamingHandler for subscriptions (the machinery
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`channel/resources/subscribe` uses)
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- ADR-070: BidiStreamSource trait (the `Connection` extension point)
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- `docs/research/alknet-channels/phase-0-findings.md` §DP-2, §Channel 0 |