alkcall/docs/architecture/channels-adapter.md

status, last_updated

status	last_updated
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channels-adapter.md — ChannelsAdapter and ChannelManager

The two internal components of the channels crate: the read/demux half (ChannelsAdapter) and the reassemble/allocate half (ChannelManager). ADR-039 is the decision; this doc specifies the contracts and the demux/mux invariants. The channels layer has no stream_type concept (ADR-035) — the demux routes by channel_id only, and the reassembly buffer is one per channel (not per (channel_id, stream_type)).

The split

Component	Role	What it knows
ChannelsAdapter	ProtocolHandler on alknet/channels; reads 8-byte chunk headers off every bidi stream the transport yields and routes to ChannelManager. Substrate-agnostic (ADR-034 §substrate modes, as amended by ADR-035).	The transport stream(s); the ChannelManager handle. ALPN-blind.
ChannelManager	Shared state; holds channel_id → ChannelState, HandlerRegistry. Constructs ChannelBidiStreamSource per channel. What channel/open closes over (in channels-call).	The channel map; the handler registry for ALPN lookup. ALPN-blind (looks up ALPNs, doesn't parse their protocols).

The split mirrors the TTY crate's ChunkReader/ChunkWriter + adapter pattern, generalized to N channels: the adapter drives N channels, and channel 0 is special only in that it's pre-allocated (by channels-call).

ChannelsAdapter::handle (substrate-agnostic)

#[async_trait]
impl ProtocolHandler for ChannelsAdapter {
    fn alpn(&self) -> &'static [u8] { b"alknet/channels" }

    async fn handle(&self, connection: Connection, auth: &AuthContext)
        -> Result<(), HandlerError>
    {
        // 1. Channel 0 is pre-negotiated (ADR-036). The first bidi stream
        //    the transport yields is channel 0. The consumer (channels-call)
        //    installs the CallAdapter on it.
        let bidi = connection.accept_bi().await?;
        self.manager.preinstall_channel_0(bidi, auth).await?;

        // 2. Accept remaining bidi streams and read 8-byte headers off each.
        //    On an in-line transport, accept_bi() yields once and the header
        //    demuxes N channels from that stream. On QUIC native, accept_bi()
        //    yields repeatedly — each stream carries one logical channel.
        //    Same code path, same wire format (ADR-034 §substrate modes,
        //    as amended by ADR-035).
        self.manager.run_demux_loop(connection).await
    }
}

The preinstall_channel_0 step (provided by channels-call, ADR-044) constructs the reassembly buffer for channel_id = 0, wraps it as a Connection via Connection::from_source with a ChannelBidiStreamSource (ADR-038, as amended by ADR-035 — accept_bi yields a BiStream), and hands that Connection to the CallAdapter. The ChannelsAdapter in channels-core exposes the hook; channels-call provides the implementation.

run_demux_loop continues accepting bidi streams from the transport. For each stream, it reads 8-byte headers and routes payloads to the matching channel_id's reassembly buffer. On an in-line transport, there is only one stream (channel 0 rides inside it via the header); the header demuxes all channels. On QUIC, each subsequent stream is a new channel; the header's channel_id correlates it. The loop is the same; only the transport's stream count differs.

ChannelManager

// In alknet-channels-core:
pub struct ChannelManager {
    channels: Mutex<HashMap<u32, ChannelState>>,
    handlers: Arc<HandlerRegistry>,
    // Note: no call_ops field — the call-protocol coupling lives in
    // channels-call (ADR-044). The ChannelManager is ALPN-blind and
    // call-protocol-blind.
    next_id: AtomicU32,       // monotonic; wraps at u32::MAX
    buffer_cap: usize,        // default 1 MiB (ADR-040)
    max_channels: usize,      // default 256 (ADR-040) — per-connection
                              // memory bound, NOT a DoS defense. The
                              // per-identity DoS defense is the
                              // ChannelLifecyclePolicy consulted by the
                              // channel/open handler in channels-call
                              // (ADR-041). The auth-blindness that forces
                              // the cap out of this struct is ADR-039's
                              // "no auth state" rule.
}

struct ChannelState {
    alpn: String,
    /// One reassembly buffer per channel (not per (channel_id, stream_type) —
    /// the channels layer has no stream_type concept per ADR-035). Yields
    /// a BiStream to the handler.
    reassembly: ReassemblyBuffer,
    handler_task: JoinHandle<()>,
}

ChannelManager is Clone (cheap — Arc internally) so the ChannelsAdapter, the channel/open operation handler, and relay logic can all hold a handle.

Type-name convention: ChannelManager, ChannelsAdapter, ChannelBidiStreamSource, and ChannelClient are the public API surface (contract). ReassemblyBuffer, Demux, MuxHandle/MuxRunner, MpscSendStream/MpscRecvStream, and ChannelOperations are illustrative internal type names — the channels crate's implementation may name them differently. The contracts are the invariants (REQ-CH-01..04, 06) and the public API; the internal names are not contractual.

ChannelManager is ALPN-blind and auth-blind

The ChannelManager deliberately does not hold:

• No ProtocolHandler implementations. It holds a HandlerRegistry reference for ALPN lookup, but it doesn't be a handler. Handlers live in their crates and register on the same registry.
• No ALPN-specific parsing. It does not parse NegotiateRequest JSON, SSH frames, or tunnel target strings. It hands params JSON to the handler and gets back a handler task. The channels layer carries the handler's framing transparently in the payload — it does not interpret the payload bytes.
• No auth state. Auth lives in the OperationContext that the call protocol passes to channel/open. The ChannelManager doesn't check scopes or ownership — that's AccessControl::check in OperationRegistry::invoke, run before the channel/open handler.
• No transport coupling. It talks to the transport only through the ChannelsAdapter's read loop and the per-channel write pumps, both of which use AsyncRead + AsyncWrite.
• No stream_type concept. Per ADR-035, the channels layer routes by channel_id only. There is one reassembly buffer per channel (yielding a BiStream), not one per (channel_id, stream_type). The handler owns its sub-stream multiplexing on the BiStream it receives.

This is what makes the channels layer WASM-compatible and transport-agnostic — the ChannelManager is pure byte routing with no platform or protocol dependencies.

The channel/open handler

The channel/open (and channel/close, channel/control, channel/resources/subscribe) operations are registered on the call protocol's OperationRegistry at registration time. The ChannelOperations constructor takes a ChannelLifecyclePolicy (ADR-041) — the default is PerIdentityChannelPolicy::new(256) (a real per-identity cap, not NoOp):

let policy = Arc::new(PerIdentityChannelPolicy::new(256));
let channel_ops = ChannelOperations::new(manager.clone(), policy);
channel_ops.register_on(&mut call_registry)?;

The same Arc<PerIdentityChannelPolicy> is shared across every channels connection this peer accepts — that is what makes the cap per-identity, not per-connection. A hub constructs one policy and shares it across all worker and browser legs; a worker accepting direct channels constructs one policy and shares it across whatever connections it accepts. See ADR-041 for the policy trait and the default/opt-out variants.

The channel/open handler (ADR-037):

1. ACL is already checked by OperationRegistry::invoke before this handler runs.
2. Looks up the ALPN in HandlerRegistry → channel:unknown_alpn if missing.
3. Per-identity cap check (ADR-041): policy.check_open(&op_ctx.identity)? — deny with channel:too_many_channels if the identity is over its cap. The identity is the direct caller (the peer on this channels connection); forwarded_for is metadata and is NOT consulted (ADR-026). For the hub-relay path, the spoke sees the hub as the direct caller — the hub's quota on the spoke reflects the aggregate of all relayed channels (ADR-041 §5).
4. Allocates the channel_id via next_id.fetch_add(1, Relaxed) (DP-1: server-assigned). The per-connection max_channels (ADR-040) is checked here too — the per-connection memory bound; if hit, the same channel:too_many_channels error is returned (which cap fired first is an implementation detail — ADR-041 §4).
5. Constructs the ChannelBidiStreamSource (ADR-038, as amended by ADR-035) — one reassembly buffer, yielding a BiStream.
6. Spawns the handler task — tokio::spawn(handler.handle(conn, &auth)). Identical to what TtyAdapter::handle does today, but on a channels-backed Connection.
7. Records the ChannelState.
8. Returns the channel_id.

The channel/close handler (ADR-037) gains a symmetric policy.on_close(&op_ctx.identity) call after the drain completes (the same point ADR-040 marks the channel_id as eligible for reuse) — decrementing the per-identity count.

Demux invariants (REQ-CH-02, 04)

REQ-CH-02: transport close → all channel senders drop → all handlers see EOF

On transport EOF, run_demux_loop clears the channels map, dropping all ReassemblyBuffer senders. Every handler's reassembled BiStream sees EOF even without an explicit zero-length sentinel on the wire. Without this, read_to_end / tokio::io::copy in handlers hangs forever waiting for a sender that never drops. This is a teardown invariant of the ChannelsAdapter::handle contract.

REQ-CH-04: lenient unknown-channel_id handling

A chunk with an unallocated channel_id is dropped with a debug log and an error counter (exposed via Demux::stats()), and the demux continues. This matches SSH's behavior and survives transient mis-ordering during teardown. Validated by the POC (demux_unknown_channel_drops_lenient).

Mux invariants (REQ-CH-03)

REQ-CH-03: dynamic registration (handle/runner split)

The mux frames per-channel bytes back onto the transport. The POC surfaced that Mux::run(self, transport) (consume, run pre-registered pumps) does not compose with the dynamic channel/open model — channels are opened after the run loop starts.

The mux is split into:

• MuxHandle — clone-able, register(channel_id) -> Sender<Bytes> callable at any time after the runner starts.
• MuxRunner — owns the transport, select!s on new-pump registrations and per-channel write pumps.

The runner's select! loop exits when all MuxHandle clones drop (the new_pumps sender closes) — the natural shutdown signal. This matches the dynamic channel/open model.

The two-pump pattern (ADR-078 — documented here for handler authors)

Handlers with a two-pump shape (two tokio::io::copy pumps, one per direction — tunnel, SSH direct-tcpip) MUST shut down the opposite sink when one pump completes. tokio::try_join! alone deadlocks: each pump waits for the other's EOF, which only comes after the opposite pump shuts down its sink.

let c2t = async {
    tokio::io::copy(&mut recv, &mut tcp_write).await?;
    tcp_write.shutdown().await.ok();  // shut down the peer's sink
    Ok::<_, std::io::Error>(())
};
let t2c = async {
    tokio::io::copy(&mut tcp_read, &mut send).await?;
    send.shutdown().await.ok();  // shut down the peer's sink (emits sentinel — REQ-CH-01)
    Ok::<_, std::io::Error>(())
};
tokio::try_join!(c2t, t2c)?;

The three-pump pattern (TTY's pump_session, coordinating via the exit_code future) does not have this deadlock — the exit_code future is the third signal. The two-pump pattern is documented in ADR-078; the shutdown-on-completion contract is a handler-level concern, not a channels-layer one.

The hub relay interface

The hub relay (ADR-042) uses the ChannelManager's interface to bridge two channels connections:

// For channel_id=7 on browser side, channel_id=12 on spoke side:
tokio::spawn(async move {
    let mut b_bidi = browser_mgr.open_channel_stream(7).await;
    let mut s_bidi = spoke_mgr.open_channel_stream(12).await;
    tokio::join!(
        pump(&mut b_bidi, &mut s_bidi),  // browser → spoke (with channel_id rewrite)
        pump(&mut s_bidi, &mut b_bidi),  // spoke → browser (with channel_id rewrite)
    );
});

The relay reads opaque bytes off one ChannelManager's reassembled BiStream and writes them onto the other's write-half, which re-chunks them with the other leg's channel_id (a 4-byte rewrite within the 8-byte header). The relay does not parse the payload — it doesn't know if the bytes are TTY chunks, SSH frames, or tunnel data. The hub translates channel/open on channel 0 (re-issues on the spoke leg with forwarded_for); data channels are byte-forwarded with channel_id rewrite. See ADR-042 for the full relay contract.

Design Decisions

All design decisions are documented as ADRs in decisions/.

ADR	Decision	Summary
075	ChannelsAdapter and ChannelManager	The split; the contracts
093	channels Pure Channel Multiplexing	The umbrella decision: 8-byte header, no stream_type, one reassembly buffer per channel
076	Backpressure, Limits, ID Reuse	Bounded-buffer, 256-channel per-connection memory bound, monotonic IDs (DoS defense reframed by ADR-041)
094	Per-Identity Channel Cap	256 per PeerId, enforced via ChannelLifecyclePolicy in channels-call; per-connection max_channels reframed as a memory bound
078	Two-Pump Pattern	Shutdown-on-completion contract
079	Hub Relay	Translate channel 0, byte-forward data channels

References

• ADR-039: ChannelsAdapter and ChannelManager (the decision)
• ADR-035: channels pure channel multiplexing (the umbrella decision that amends ADR-034/074/077)
• ADR-036: channel 0 pre-negotiated (the preinstall_channel_0 step)
• ADR-037: channel lifecycle operations (the ops registered on call_ops)
• ADR-038: ChannelBidiStreamSource (what the manager constructs per channel, as amended by ADR-035)
• ADR-040: backpressure and limits (buffer_cap, max_channels — the per-connection memory bound)
• ADR-041: per-identity channel cap (the ChannelLifecyclePolicy consulted by the channel/open handler; the relay consequence for hub-relayed channels)
• docs/research/alknet-channels/poc-summary.md §Issues Surfaced #4-#7 (REQ-CH-01..04, the two-pump deadlock)
• docs/research/stream-unification/findings.md — the research that surfaced the pure-multiplexing resolution