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glm-5.3-flash bd7d1ad8ec docs: Phase 1 architecture spec — ADRs 001..006, spec docs, OQ promotion 
The Phase 0 OQ ledger (OQ-TN-01..10) promoted into
docs/architecture/open-questions.md (now OQ-TN-01..14, with four
new Phase 1 residues). Six ADRs and four spec docs, all Draft, all
decision-shaped per the SDD process (ADRs carry the WHY; specs carry
the WHAT and reference by number):

ADRs:
- 001 open-params-layout: params = {resource, substrate} — the
  producer's stable name, not an address; wire-stable from the first
  consumer; unknown substrates fail loudly at schema time
- 002 alpn-strategy: single alk/tunnel ALPN (option A); substrate in
  params selects the framing; prior art (SSH/SOCKS5/udpgw) gives no
  reason for the split
- 003 codec-and-udp-framing: raw pass-through (stream) / mandatory
  [len: u16 BE] (UDP) — F-2 mandate recorded (the codec is mandatory
  for correctness, not cosmetics); len=0 = legal empty datagram;
  OQ-TN-13 resolved fail-loud (truncation)
- 004 no-backend-trait: halves functions at the assembly layer;
  listen is an establisher shape; hub re-produce deferred(OQ-TN-12);
  trait re-evaluated at the alknet ADR-078 convergence threshold
- 005 consumer-session-owns-teardown: TunnelSession with
  open/adopt/stream_halves/take_halves/pump_against/close/join/Drop —
  the W3 adopter gap closes structurally; Drop is the best-effort
  fallback
- 006 access-control-posture: the open gate is the boundary;
  TUNNEL_OPEN_SCOPE = tunnel:open (stable once published); op-level
  ACL; ownership seam; no allowlists in v1; identity = 0.7.0's
  precedence chain

Spec docs:
- overview.md: purpose, resource model, deps (alkcall 0.7.0,
  wasm-clean default, local feature), module map
- wire.md: the open op (params/reply/typed errors), the data plane
  by substrate, sentinels + half-close, byte diagrams
- producer.md: spec, establisher (dial + listen shapes), pump
  handler (pump_bidi inline, R-02), registration API, ACL posture
- consumer.md: TunnelSession (forward open + reverse adopt
  construction), data plane, teardown API (close/join/Drop incl.
  pump-less join semantics)
- bast.md: the BAST doc for the UDP codec (convention 12's trigger
  fired — the framing IS binary beyond pass-through)
- open-questions.md: OQ-TN-01..14; 01..10 promoted (faithful to the
  phase-0 ledger's final states), 11 partially resolved (collision
  domain = per-producer registry per ADR-001; lifecycle open),
  12 deferred(scope), 13 resolved fail-loud (ADR-003), 14 open

Verified by an architecture-reviewer pass (2 criticals, 6 majors,
10 minors — all fixed: ADR-001/003-vs-OQ decision-state contradictions
resolved; the Layer-2 mislabel corrected to the ADR-047 §4 per-session
fork; alktty/alknet ADR misattributions fixed; codec placement pinned
to the establisher (pump stays substrate-agnostic); reverse-path
construction named (TunnelSession::adopt + pump_against); security
posture promoted to ADR-006; BAST doc written; F-2 rationale deduped;
README tables completed; channel_id > 0; MTU wording fixed; ADR-047 §5
attributions corrected; impacts lines added to unresolved OQs; review/
ledger pointer paths added to README references).

Verification: doc set internal cross-refs all resolve; cargo test,
fmt --check, doc --no-deps clean
2026-09-07 18:36:44 +00:00

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---
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)
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