alknet/docs/architecture/crates/channels

status, last_updated

status	last_updated
draft	2026-07-12

alknet-channels

A multiplexing proxy: a ProtocolHandler on alknet/channels that decomposes a single bidirectional transport stream into N logical channels, each carrying a different ALPN. Channel 0 is pre-negotiated as alknet/call (ADR-072); every other channel is opened dynamically via call operations on channel 0 and routed through the same HandlerRegistry as top-level connections. The channels layer is a re-framing proxy — it converts between "one transport stream carrying N channels" (the wire) and "N independent AsyncRead + AsyncWrite handles" (what handlers see) — and it does no protocol work itself.

Documents

Document	Status	Description
overview.md	draft	Crate purpose, the multiplexing collapse, dependencies, ALPN, transport agnosticism, WASM, relationship to existing crates
channels-wire.md	draft	The 9-byte chunk format ([channel_id:u32 be][stream_type:u8][length:u32 be][payload]), stream types, sentinels, framing disambiguation, wire-level invariants (REQ-CH-01..05)
channels-connection.md	draft	ChannelBidiStreamSource (implements BidiStreamSource — ADR-070/074), into_sub_streams() typed destructure, recursive composition
channels-adapter.md	draft	ChannelsAdapter (ProtocolHandler on alknet/channels), ChannelManager, demux/mux contracts (REQ-CH-01..04), the two-pump pattern (ADR-078)
channel-operations.md	draft	channel/open, channel/close, channel/control, channel/resources/subscribe — call-protocol operations on channel 0, ACL flow, direction semantics, the hub relay contract (ADR-079)
channel-client.md	draft	ChannelClient — the client side of a channels connection; transport-agnostic from_connection primary, connect_quic convenience; bidirectionality preserved

Applicable ADRs

ADR	Title	Relevance
071	channels Wire Format — 9-Byte Chunk Header	The chunk format; unidirectional stream_types in groups of 3; substrate-agnostic; one-way door
072	Channel 0 Is Pre-Negotiated alknet/call	Channel 0 = call protocol, stream_types [0,1]; no special control plane
073	Channel Lifecycle Operations on the Call Protocol	channel/open/close/control/resources/subscribe; direction semantics; subscribe not poll
074	ChannelConnection — BidiStreamSource over Chunk Reassembly	Per-channel BidiStreamSource impl; into_sub_streams() with SubStreamHandle enum (Send/Recv)
075	ChannelsAdapter and ChannelManager	Substrate-agnostic demux loop; REQ-CH-01..04 contracts
076	Backpressure, Channel Limits, and ID Reuse	Bounded-buffer (1 MiB default), 256-channel cap, monotonic IDs with wrap
077	TTY Inside Channels — Sub-Streams, Not Wire Format	TTY's two modes (direct vs channels); 5 sub-streams; control bidirectional via 3/4; amends ADR-052 scope
078	Two-Pump Shutdown-on-Completion Pattern	The two-pump deadlock contract; handler-level, not channels-layer
079	Hub Relay — Translate, Not Transparently Forward	The hub translates channel 0, byte-forwards data channels with ID rewrite
080	ChannelClient — the Client Side of a Channels Connection	ChannelClient, transport-agnostic from_connection primary + connect_quic convenience; AlknetClient dial-seam deferred (OQ-55)
081	channels Sub-Crate Decomposition	channels-core (pure multiplexer) / channels-call (call coupling + ChannelClient); hub and worker are consumers, not sub-crates
070	BidiStreamSource Trait	The Connection extension point ChannelBidiStreamSource implements
065	Connection::from_stream	The transport-agnostic Connection the channels layer rides on
052	alknet-tty Wire Format	The 5-byte format the 9-byte format generalizes (amended by ADR-077 — scoped to direct TTY)
049	StreamingHandler for Subscriptions	The machinery channel/resources/subscribe uses
032	Forwarded-For Identity	The auth chain for hub-relayed channel opens
003	Crate Decomposition	alknet-channels depends on alknet-core only; no handler-depends-on-handler

Relevant Open Questions

OQ	Title	Status	Relevance
OQ-55	AlknetClient / Client Establishment Extraction	deferred(scope)	ChannelClient's API is decided (ADR-080): transport-agnostic from_connection primary, connect_quic convenience. AlknetClient core extraction stays deferred — blocked on a second transport's dial (the shared dial+TLS seam), not a second client
OQ-56	Full channel-level flow-control windowing	deferred(scope)	Bounded-buffer is decided (ADR-076); full windowing is an extension blocked on "a real deployment observes HOL blocking on a saturated channel where bounded buffer is insufficient"
OQ-57	Two-pump helper extraction to alknet-core	deferred(scope)	The shutdown-on-completion contract is decided (ADR-078); the helper extraction is blocked on a second two-pump handler existing (shape convergence)

Key Design Principles

1. Streams are streams. A TTY session, an SSH channel, a forwarded TCP connection, a QUIC bidi stream — they're all AsyncRead + AsyncWrite handles. The differences are only in how they're opened (negotiation via channel/open on channel 0) and what multiplexing layer carries them (the 9-byte chunk format). Once normalized, every channel is an ALPN routed through the same HandlerRegistry. See overview.md and ADR-071.

2. Channel 0 is alknet/call pre-negotiated, not a special control plane. The call protocol runs on channel 0 exactly as on a top-level alknet/call connection. Channel lifecycle operations (channel/open, channel/close, channel/control, channel/resources/subscribe) are call operations on channel 0's OperationRegistry, gated by the existing AccessControl::check. No new auth machinery, no new framing. See ADR-072, ADR-073.

3. The channels layer is a re-framing proxy, not a protocol engine. It converts between "one transport stream carrying N channels" (the wire) and "N independent AsyncRead + AsyncWrite handles" (what handlers see). It does no ALPN-specific parsing, no auth, no transport coupling. This makes it WASM-compatible and transport-agnostic by construction. See channels-adapter.md and ADR-075.

4. channel/resources/subscribe is a Subscription, not a polled Query. The call protocol has StreamingHandler / invoke_streaming (ADR-049, implemented and tested). The first consumer (the hub aggregating worker resources) needs live updates. Polling would be built and immediately reworked. See ADR-073.

5. Bidirectional open. Either side can open a channel to the other, just like the call protocol's operation overlay. The direction field on channel/open pins who is the ALPN-server vs ALPN-client. See ADR-073 §Direction semantics.

6. Wire-level invariants are contracts, not implementation details. The POC surfaced five invariants (REQ-CH-01..04, plus REQ-CH-06 for close ordering) that hang channels silently if underspecified: shutdown emits a zero-length sentinel; transport close drops all senders; the mux supports dynamic registration; unknown channel_id is lenient-dropped; bounded-buffer backpressure doesn't deadlock; data chunks flush before channel/close. See channels-wire.md and channels-adapter.md.

7. The hub translates, not transparently forwards. The hub terminates channel 0 on both legs, runs AccessControl::check, and re-issues channel/open on the spoke leg with forwarded_for (ADR-032). Data channels are byte-forwarded with channel_id rewrite. This preserves the auth model. See ADR-079.

References

• docs/research/alknet-channels/phase-0-findings.md — Phase 0 research (vision, hub motivation, wire format, negotiation, internals, DPs, OQs)
• docs/research/alknet-channels/poc-summary.md — the de-risk POC (28 tests, three validated targets, REQ-CH-01..06 wire-level invariants surfaced; REQ-CH-07 is a cosmetic clippy item, not a wire invariant)
• docs/research/alknet-channels/poc-plan.md — the POC plan
• /workspace/alknet-channels-poc/ — the POC codebase
• docs/research/alknet-tty/phase-0-findings.md — the TTY crate's chunk format (the seed of the channels generalization)
• docs/research/alknet-ssh/phase-0-findings.md — SSH's channel multiplexer (the prior art for N-channel multiplexing)
• docs/architecture/crates/hub/README.md — the hub crate (the primary consumer; the relay implementation's home)