---
status: draft
last_updated: 2026-09-07
---

# alktunnels — Architecture

Arbitrary bidirectional tunnels over alkcall channels: the
`alk/tunnel`-family producer/consumer protocol crate. This directory
holds the architecture spec docs and the ADRs. It is the sibling of
alktty's architecture (`/workspace/@alkdev/alktty/docs/architecture/`)
— same substrate (alkcall channels), same shape (a producer
open-handler + a typed consumer session), different service (tunnels:
raw byte/datagram transport, not terminal sessions).

## Documents

| Document | Status | Description |
|----------|--------|-------------|
| [overview.md](overview.md) | draft | Crate purpose, the resource model in brief, dependencies, ALPN, feature gates, module map |
| [wire.md](wire.md) | draft | The wire format: open-op params JSON (ADR-001), the data-plane codec (raw pass-through for stream substrates; mandatory `[len: u16 BE]` framing for UDP — ADR-003), sentinels, half-close semantics |
| [producer.md](producer.md) | draft | The producer half: the `channels/tunnel/sub` open op, the establisher (dial + listen variants), the pump handler, registration, scope gating |
| [consumer.md](consumer.md) | draft | The consumer half: `TunnelSession` (stream + datagram variants), the reverse-flow initiator, teardown ownership (ADR-005) |
| [bast.md](bast.md) | draft | The BAST (Binary Abstract Syntax Tree) document for the data-plane's binary framing (the UDP `[len: u16 BE]` codec — ADR-003); the stream pass-through has no binary framing to describe |
| [open-questions.md](open-questions.md) | draft | Centralized OQ tracker (OQ-TN-01..14; the Phase 0 ledger promoted) |

## Applicable ADRs

alktunnels-native ADRs (001..005). Upstream ADRs (alkcall/alknet) are
referenced by their own numbers and are NOT ported — the alkcall
originals at `/workspace/@alkdev/alkcall/docs/architecture/decisions/`
remain authoritative for the shared surfaces (establishment, channels
wire format, `pump_bidi`, identity seam).

| ADR | Title | Status |
|-----|-------|--------|
| [001](decisions/001-open-params-layout.md) | Open-Op Params Layout: Resource + Substrate | Accepted |
| [002](decisions/002-alpn-strategy.md) | Single `alk/tunnel` ALPN (Option A) | Accepted |
| [003](decisions/003-codec-and-udp-framing.md) | Data-Plane Codec: Raw Pass-Through (Stream) / Mandatory Length Framing (UDP) | Accepted |
| [004](decisions/004-no-backend-trait.md) | No TunnelBackend Trait — Halves Functions at the Assembly Layer | Accepted |
| [005](decisions/005-consumer-session-owns-teardown.md) | The Consumer Session Type Owns Teardown | Accepted |
| [006](decisions/006-access-control-posture.md) | Access-Control Posture — the Open Gate Is the Security Boundary | Accepted |

## Key Design Principles

1. **A tunnel is a resource, not an address.** `params` identify a
   produced resource + substrate discriminator; the producer owns the
   backing (local port, docker container, in-process service, unix
   socket). No URL-style general addressing in the wire — rich
   addressing enters only through `-D`/dynamic composition, which
   speaks its protocol inside the tunnel payload. See
   [ADR-001](decisions/001-open-params-layout.md) and
   [wire.md](wire.md).

2. **Role follows the resource; connection direction is independent.**
   Whoever can reach the target is the producer (registers openable
   channels); whoever wants the bytes is the consumer. `-L` and `-R`
   are the same open op with the entry point on different machines —
   no protocol-level direction (the hub-owns-the-connection model,
   OQ-TN-03). Validated by the reverse-flow POC: the worker serves on
   the connect side; the hub initiates per accept. See
   [producer.md](producer.md) and [consumer.md](consumer.md).

3. **Raw pass-through for stream substrates; mandatory length framing
   for UDP.** A tunnel has one data stream per direction — no
   sub-demux key, no tunnel-level header on stream substrates (0 B
   tunnel overhead; the channels 8-byte header is the only cost).
   UDP rides `[len: u16 BE]` framing — mandatory for correctness
   (F-2; the rationale and the executable pin live in ADR-003). See
   [ADR-003](decisions/003-codec-and-udp-framing.md) and
   [wire.md](wire.md).

4. **The two-pump shape is `alkcall::channels::pump_bidi` — never
   hand-rolled.** The producer's pump handler and the consumer-side
   pump both consume the upstream helper (ADR-050); each pump shuts
   down the opposite sink on completion (half-close semantics fall
   out). The `OpenHandler`'s returned `JoinHandle` tracks the
   data-plane lifetime (early return = teardown-at-birth, R-02). See
   [producer.md](producer.md).

5. **No backend trait.** Stream substrates need "produce boxed halves
   for a resource" — a function, not a trait (the UDP POC's OQ-TN-05
   finding). The inversion point is the assembly layer wiring
   substrate halves into the establisher/pump; substrate types stay
   in feature-gated backend modules. The listen variant (producer
   accepts instead of dialing) is an establisher shape, not a trait.
   See [ADR-004](decisions/004-no-backend-trait.md).

6. **The consumer session type owns teardown.** One session = one
   channel = one tunnel (the channel ID is the flow key). The adopted
   side has no wrapper awaiting its pump (reverse POC W3) — the
   session must hold the pump handle and reap the channel on
   completion/close. See [ADR-005](decisions/005-consumer-session-owns-teardown.md)
   and [consumer.md](consumer.md).

7. **Backpressure and limits are inherited, not redefined.** Bounded
   per-channel buffers, the 256-channel cap, monotonic IDs, and the
   zero-length-sentinel EOF convention are alkcall channels
   invariants; the tunnel crate consumes them and does not build a
   second demux/mux (AGENTS.md convention 10).

8. **Security posture: the open gate is the boundary.** Scope-gated
   opens (`TUNNEL_OPEN_SCOPE = "tunnel:open"`, stable once published
   — ADR-006), alkcall's op-level ACL via `register_openable`; caller
   identity resolves in the 0.7.0 precedence order (token >
   `ServingConfig.identity` > transport); the establisher/pump
   handler see the per-call opener identity. No target allowlists, no
   tunnel-specific policy machinery in v1. See
   [ADR-006](decisions/006-access-control-posture.md) and
   [producer.md](producer.md).

9. **Wasm-clean default crate.** The default crate (protocol only)
   compiles to `wasm32-unknown-unknown`; socket/platform I/O lives in
   feature-gated backend modules (AGENTS.md conventions 4/14), same
   as alktty.

## Relevant Open Questions

All open questions are tracked in
[open-questions.md](open-questions.md). The Phase 0 OQ ledger
(OQ-TN-01..10, `docs/research/phase-0-findings.md`) is promoted there;
the questions affecting the spec set are summarized in each doc's Open
Questions section.

## References

- Phase 0 research: `docs/research/phase-0-findings.md` (OQ ledger),
  `docs/research/poc-summary.md` (UDP POC),
  `docs/research/reverse-poc-summary.md` (reverse-flow POC),
  `docs/research/ssh-socks5-survey.md` (SSH/SOCKS5/udpgw prior art)
- Upstream: alkcall ADR-047 (openable ALPNs are operations), ADR-049
  (establishment phase + amendment 2 plan payload), ADR-050
  (`pump_bidi`), ADR-022 §2 (both-sides serving), ADR-047 §5
  (connection-owner allocation), ledger CF-005/CF-006 (identity seam);
  alknet ADR-078 (two-pump contract), ADR-071/093 (channels wire
  format); alkcall ADR-042 (hub relay)
- Upstream reviews/ledger (the spec cites these IDs):
  `/workspace/@alkdev/alkcall/docs/reviews/007-establishment-follow-ups-review.md`
  (R-01 = the `Establishment` plan payload, R-02 = the `OpenHandler`
  lifetime contract, R-03 = `pump_bidi`) and
  `/workspace/@alkdev/alkcall/docs/reviews/consumer-findings-ledger.md`
  (CF-005/CF-006 = the identity seam, resolved in 0.7.0)
- Sibling precedent: alktty (`/workspace/@alkdev/alktty`) — the
  producer/consumer-on-channels shape, wasm-clean default,
  feature-gated local backend, BAST convention
- POC code: `/workspace/alktunnels-udp-poc` (forward),
  `/workspace/alktunnels-reverse-poc` (reverse)