glm-5.2
fd83fc1685
Three simplifications to the channels spec, all flowing from the review: 1. Substrate simplification (ADR-071 revised): the 9-byte chunk header is used in ALL substrates — in-line (TCP+TLS, WebTransport), native (QUIC bidi streams), and multi-connection. The ChannelsAdapter reads headers off every bidi stream it accepts; the transport's native multiplexing is a performance optimization (independent flow-control windows), not a protocol change. One wire format, one code path, one handler experience. The channel_id in the header is the correlation key across substrates. 2. Stream_type decomposition (ADR-071 revised): every stream_type is unidirectional. Bidirectionality is two stream_types (write + read), not one 'bidirectional' stream_type. Grouped in threes: 0/1/2 = data write/read/err, 3/4/5 = control write/read/err, % 3 formula. This resolves the TTY control channel's 'not actually bidirectional' flaw — control is now 3 (write, client→server) + 4 (read, server→client), each with its own reassembly buffer and EOF. Channel 0 uses [0,1] (call frames bidirectional via 0=in, 1=out). TTY uses [0,1,2,3,4]. ADR-072, 073, 074, 077 updated for the new stream_type assignments. 3. Sub-crate decomposition (ADR-081 new): channels-core (pure multiplexer — wire format, demux/mux, ChannelBidiStreamSource, ChannelManager; depends on alknet-core only, no call dependency, ALPN-blind) / channels-call (channel 0 pre-negotiation + lifecycle op registrations; depends on channels-core + alknet-call) / channels-hub (relay) / channels-worker (ChannelClient). Isolates the call-protocol coupling from the pure multiplexer so the dependency graph is honest. ADR-077 (TTY inside channels) updated: TTY now uses 5 sub-streams [0,1,2,3,4] with control properly bidirectional via 3/4; amends ADR-052's stream_type assignments for direct mode too (direct alknet/tty now uses 0-4, not 0-3). Spec docs updated: channels-wire, channels-connection, channels-adapter, channel-operations, overview, README.
132 lines
6.3 KiB
Markdown
132 lines
6.3 KiB
Markdown
# ADR-072: Channel 0 Is Pre-Negotiated `alknet/call`
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## Status
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Accepted
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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 9-byte chunk header channel 0 uses)
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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 |