---
status: draft
last_updated: 2026-09-05
---

# alktunnels — Phase 0 Research Findings

This document captures Phase 0 (Exploration) findings and open design
questions for the `alktunnels` crate. The objective of Phase 0 per
`docs/sdd_process.md` is: *"Capture vision and guiding principles; research
options; validate approaches; converge on a recommended approach."* It is the
input to Phase 1 (Architecture), where the Architect will produce
`docs/architecture/` specs, ADRs, and open questions.

Drafted 2026-09-05, emerging from the initial setup discussion. The crate is
the sibling of alktty (`alk/tty` — terminal sessions) on the alkcall
substrate: where alktty multiplexes one service with a fixed five-stream
channel structure, alktunnels generalizes the tunnel handler shape to
arbitrary bidirectional tunnels in the `ssh -L` / `ssh -D` sense — TCP, UDP,
unix sockets, and other stream or datagram substrates.

The 2026-09-05 first revision adds the tun2proxy UDP gateway prior art
(`OQ-TN-01`, `OQ-TN-02`, `OQ-TN-07`, `OQ-TN-10`) and the transport story
clarification (TCP vs QUIC at the channels layer, invisible to this crate —
§What is already settled). This prior art was discussed with the POC agent
around the alknet-channels POC but never documented; it is captured here so
it survives into the spec.

## What is already settled

The foundation is POC-validated and ADR-pinned; this crate is not starting
from zero. It inherits:

- **The demux→Connection→handler→mux path** — validated by the
  alknet-channels POC (Target 3), now production alkcall channels. The
  tunnel payload is raw bytes inside a channels data channel; channels
  strips its 8-byte header transparently (alknet ADR-093 / alkcall
  ADR-035).
- **The two-pump handler shape** — one pump per direction, each pump MUST
  shut down the opposite sink on completion (`try_join!` alone deadlocks;
  alknet ADR-078). POC-validated with a 1 MiB backpressure test. This
  crate is the *second* two-pump consumer the ADR deferred helper
  extraction for (the first was the POC's tunnel handler; SSH
  `direct-tcpip` would be a later third).
- **The producer/consumer model** — producer registers openable channels
  via `ChannelCore::register_openable` (authorization for free via
  `AccessControl`); consumer opens tunnel channels via `ChannelClient`
  (alkcall ADR-037, ADR-043). Connection direction is independent of
  tunnel direction.
- **The backend inversion point pattern** — substrate-specific types
  (`TcpStream`, `UdpSocket`, unix sockets) confined to feature-gated
  backend modules, injected at the assembly layer, never imported from
  the shared/producer/consumer modules (alktty `TtyBackend` precedent).
- **The wasm-clean default crate** — protocol-only code compiles to
  `wasm32-unknown-unknown`; socket/platform I/O is feature-gated
  (alktty precedent).
- **The relay story** — tunnels traverse alkcall hub relays
  transparently via byte-for-byte data-channel forwarding with ID
  rewrite (alkcall ADR-042). No tunnel-specific relay work.
- **The transport story** — the underlying channels transport is TCP or
  QUIC (QUIC preferred) but that is the alkcall layer's concern, not
  this crate's: the tunnel protocol sees a `BiStream` and is
  transport-agnostic like the sibling crates. UDP tunneling (below)
  rides the same chunk stream; the stream-vs-datagram question is
  about what the tunnel protocol frames *inside* the channel, not
  about the transport.

## Prior art: the hub-owns-the-connection model (`OQ-TN-01`, `OQ-TN-03`, `OQ-TN-04`, `OQ-TN-08`)

Discussed 2026-09-05. The standing stance across the alk* crates: **the
hub owns the connection and explicitly proxies to expose resources for
others.** A produced resource is not a socket — it is an ACL-scoped
*virtual resource* on the side that can reach the real target. Applied
to tunnels, this dissolves the SSH direction model:

- **Role follows the resource.** Whoever can reach the target is the
  producer (registers openable tunnel channels via
  `ChannelCore::register_openable`, dials/serves the target); whoever
  wants the bytes is the consumer (opens channels via `ChannelClient`).
  This holds for both SSH `-L` and `-R`: in both, the entry-point side
  opens the channel (consumer) and the target side handles the open
  (producer) — the only difference is which machine hosts the entry
  point, which is assembly-layer wiring, not protocol.
- **The "exposed port" is a virtual port.** The far side may *think* it
  exposed a port on the remote end; what actually exists is an
  ACL-scoped resource in the connection/identity registry — the same
  shape as operation registration, not a bind. Example: a self-hosted
  gitea (HTTP in a docker container, really binding a port) is tunneled
  to the hub; the hub proxies the tunnel to consumers whose ACL grants
  access to the gitea HTTP service. A SOCKS5 service is the same shape,
  further downstream.
- **The hub proxy is a producer wrapping a consumer.** The proxy is an
  assembly-layer construct. On leg 1 (gitea side ↔ hub): gitea side is
  producer, hub is consumer. On leg 2 (hub ↔ service consumer): the hub
  re-produces the resource ACL-scoped — producer wrapping its leg-1
  consumer role. Data path per hop is the same two-pump shape; the
  consumer opens a channel naming the resource, the producer's open
  handler dials the target. This means the hub *terminates and
  re-produces* rather than relaying byte-for-byte — unlike alkcall
  ADR-042's transparent data-channel relay — because ACL must apply
  per hop. (ADR-042 relays remain valid for transports that don't
  need per-hop ACL; the hub's tunnel proxy is the explicit-ACL path.)
- **`-D` (dynamic/SOCKS) composes.** A SOCKS5 server at some assembly
  layer is just a consumer that opens channels with per-connection
  dynamic targets, gated by the producer's target policy. Not a base
  crate concern (same conclusion as the original half-answer, now with
  the mechanism named).
- **No forced binding, everywhere (OQ-TN-04 resolved).** Binding is
  always assembly-layer and optional, on either side. The protocol
  never binds: a producer "produces a resource" (which may or may not
  correspond to a local bind — a docker container's port, a unix
  socket, an in-process service); a consumer "consumes it." The SSH
  mental image of "expose a port" is an illusion the assembly layer
  may create locally; the protocol only ever carries
  produce/consume bookkeeping.

**Model, one sentence:** a producer produces a resource (a stream of
basically anything, if TCP and UDP are supported); a consumer consumes
it; direction, bind, and ACL scoping are assembly-layer concerns
wrapped around that pair.

**Residue (folded into other OQs):**
- How does a consumer *discover* produced resources — resolved
  2026-09-05: the existing bidirectional ACL-filtered ops listing
  (openable channels are operations, alkcall ADR-047). See OQ-TN-08.
- Dynamic-target policy for `-D`-style opens — dissolved 2026-09-05:
  `-D` is "just tunnel a socks5 connection"; target selection lives in
  the socks5 protocol at the producing side, governed by the same
  op-level ACL. See OQ-TN-08.
- Whether the hub proxy needs anything from this crate (a
  re-produce helper? typed open-handler composition?) or composes from
  the public producer/consumer surface as-is → spec question for
  Phase 1, tracked here as part of OQ-TN-05.

## Prior art: tun2proxy UDP gateway (`OQ-TN-01`, `OQ-TN-02`, `OQ-TN-07`, `OQ-TN-10`)

`/workspace/tun2proxy/src/udpgw.rs` implements a UDP gateway over a TCP
stream — structurally the same problem this crate faces for UDP tunnels
over channels data channels. Discussed with the alknet-channels POC
agent as the example of "UDP over a stream substrate," but never
documented. Key mechanics, all of which generalize:

- **Per-datagram length framing over the stream** — the packet format is
  `LEN(u16 BE) | FLAGS(u8) | CONN_ID(u16) | [SOCKS5 address] | DATA`
  (`udpgw.rs:82-88`). Boundary preservation is re-added by the protocol,
  not by the substrate: exactly the "length-prefix each datagram inside
  the channel" half-answer in OQ-TN-02, proven in production.
- **SOCKS5 address format travels per data packet** — `ATYP`
  (0x01 IPv4 / 0x03 domain / 0x04 IPv6) + variable address + port
  (`udpgw.rs:68-76`). This is concrete prior art for OQ-TN-01's
  addressing: a scheme-tagged addressing encoding with v4/v6/domain
  coverage already standardized. Note the asymmetry with TCP tunnels:
  for UDP, the remote endpoint is per-datagram, not per-channel.
- **One stream carries many UDP flows** — `CONN_ID(u16)` multiplexes
  associations over a single gateway connection (`udpgw.rs:66`),
  with `keepalive` (0x01) and `error` (0x20) flag packets as the only
  non-data frame types (`udpgw.rs:21-26`). This is the "one channel =
  one association, per-endpoint multiplexing inside" half-answer in
  OQ-TN-02, with the refinement that the per-endpoint multiplexing key
  (`CONN_ID`) is protocol-level, allocated by the client, u16.
- **Flow lifecycle is packet-level** — `udp_timeout` idle expiry,
  `keepalive_time` heartbeats on idle connections
  (`UDPGW_KEEPALIVE_TIME = 30s`, `udpgw.rs:16`), and an MTU cap
  (`parse_udp_response` rejects `data.len() > udp_mtu`,
  `udpgw.rs:527`). Also `UDPGW_MAX_CONNECTIONS = 5` pooled gateway
  connections *above* the packet layer — a throughput choice, not a
  protocol requirement; channels gives us N channels already.
- **Implications for alktunnels:**
  - UDP boundary preservation over the chunk stream is validated
    prior art, not speculation — raises confidence in the OQ-TN-02
    half-answer considerably.
  - The frame-type set (DATA/KEEPALIVE/ERR) is a useful minimal
    vocabulary — it maps onto OQ-TN-09's establishment/error frame
    question (tun2proxy uses flag packets, alktty uses typed control
    chunks; both are self-contained frames inside the data stream).
  - Per-datagram addressing (SOCKS5-style) vs per-channel addressing
    (fixed target at open) is a real fork for the params design: TCP
    tunnels fix the target at open; UDP associations may either fix
    one endpoint at open or carry per-datagram addresses like udpgw.
    This interaction is unresolved and feeds OQ-TN-01 + OQ-TN-07.
  - NAT/keepalive concerns partially disappear on channels: the
    underlying transport (QUIC/TCP) handles connection keepalive, and
    channel liveness is the channels layer's concern. The tunnel
    protocol likely needs only flow-level idle expiry, not
    transport-level keepalive packets — TBD in the spec.

## Prior art: the alktty stream-splitting pattern (`OQ-TN-09`, `OQ-TN-05`)

Discussed 2026-09-05. alktty tunnels io-like streams: the adapter
splits the `BiStream` from `accept_bi` into read/write halves
(`tokio::io::split`), wraps each half in a `ChunkReader`/`ChunkWriter`
(`wire.rs` — the 5-byte `[stream_type: u8][length: u32 be][payload]`
codec), and pumps chunks between the wire halves and the backend's
`TtyHandle` halves. Strip packets on the way in, wrap packets on the
way out; everything above the codec sees plain `AsyncRead`/`AsyncWrite`.

The proposal: that same pattern (or close) may be *the* tunnel pattern.
A tunnel is then literally "tty without the five stream types" — split
the channel's `BiStream`, wrap the halves in a codec, pump to/from
substrate halves. What differs per concern is only the codec and the
far-end handle:

| | alktty | alktunnels (TCP) | alktunnels (UDP) |
|---|---|---|---|
| wire codec | 5-byte, 5 stream types | raw pass-through or minimal framing | length-prefixed datagrams (tun2proxy-style) |
| far end | `TtyHandle` (stdin/stdout/stderr/ctrl) | `TcpStream` split halves | `UdpSocket` + flow table |
| pumps | 3 (stdout/stderr + stdin, exit future coordinates) | 2 (the ADR-078 shape) | 2 + flow expiry |

Implications:

- **OQ-TN-09 falls out naturally.** alktty's `STREAM_CTRL_IN/OUT`
  (flag frames inside the same codec) is the precedent for the
  establishment/error frame: same codec, one extra stream type
  (`TUNNEL_CTRL` or similar), typed control messages — rather than a
  separate framing layer. alktty's zero-length-sentinel EOF semantics
  carry over unchanged.
- **OQ-TN-05 (backend trait) gets a concrete shape to judge.** If
  tunnels are "split + wrap + pump," the far-end handle is just
  "(AsyncRead + AsyncWrite) halves" for stream substrates — which is
  either a very thin trait or no trait (fn returning boxed halves).
  The UDP case's flow table is the real differentiator to design
  around, not the stream case.
- **Possible convergence with OQ-TN-06 (two-pump helper).** If the
  pump loop shape is identical to alktty's modulo the number of pumps,
  a shared helper extraction (this crate's second-consumer moment,
  alknet ADR-078) should be evaluated against alktty's `pump_session`
  too — the convergence test the ADR asked for may be nearly free.
- **Wasm story intact** — the codec is pure byte manipulation; the
  split/pump plumbing is tokio-async, no substrate types in the
  protocol layer. Same as alktty.

Open: whether the tunnel codec is *literally* alktty's 5-byte format
with different stream-type constants (reuse), a stripped 5-byte
variant (1 stream type + ctrl), or raw pass-through with framing only
for UDP (the OQ-TN-01 reframe suggests TCP needs nothing beyond the
channels chunk stream). Phase 1 ADR, before the first consumer.

## Open Questions

These are the design questions Phase 0 must resolve (or explicitly defer)
before the architecture spec. They are numbered OQ-TN-01.. so they can be
referenced, tracked, and promoted into `docs/architecture/open-questions.md`
in Phase 1. Half-answers and hunches are marked as such — the point of this
document is to hold them without forcing premature decisions.

### OQ-TN-01: Target addressing format

What does the tunnel `params` on `channel/open` look like? alknet ADR-071
§ALPN table noted `alknet/tunnel` as `[0, 1]` data in/out only, but the
addressing scheme was never decided. It must cover at minimum:

- TCP dial (`host:port`)
- UDP (associate-style or endpoint-style — see OQ-TN-02)
- Unix domain sockets (path)
- Direction (who dials the target — see OQ-TN-03, resolved)
- Bind/listen vs dial semantics (see OQ-TN-04, mostly resolved)

Considerations:

- `params` is ALPN-specific JSON, interpreted by the open handler, not by
  the channels layer (alknet ADR-075 / alkcall ADR-039). alktty's
  precedent is the `NegotiateRequest` shape — a self-contained JSON
  object carried in the open op.
- The addressing string is wire-stable once a consumer exists (one-way
  door). It must be substrate-extensible without format changes (a new
  substrate should be an additive `scheme` value, not a v2 format).
- Prior art to survey: SSH forwarding models (`direct-tcpip`,
  `forwarded-tcpip`), SOCKS5 addressing (ATYP + addr + port), russh
  (`/workspace/russh` — note: russh has no UDP channel type at all, so
  there is no `direct-udp`-style prior art there; still useful for
  channel-open framing and addressing-intel generally).
- ALPN convention check (2026-09-05): the ALPN prefix swap `alknet/` →
  `alk/` happened alkcall v0.1.1, before the first published consumer.
  Everything in this crate's docs referencing `alknet/tunnel` means
  `alk/tunnel` — a small thing, but docs/ADRs must not perpetuate the
  old prefix or consumers will bake it in.

**Reframe 2026-09-05 — this is partly an XY problem.** The rich
"remote addressing" framing was chasing the wrong thing. Except in the
`-D`/dynamic case (which composes at the assembly layer), a tunnel is
either TCP or UDP, and `params` need only *identify a produced
resource* — not carry a general-purpose address. The producer owns
where the resource comes from (a local port, a docker container's
port, an in-process service, a unix socket — its problem, most likely
a local port). So `params` reduce to:

1. Which resource (a produced resource identifier — stable name or
   target address, still to decide),
2. Substrate discriminator (`tcp` / `udp` / extensible) — so UDP is
   structurally supported from day one even if the vast majority of
   use is TCP.

Rich in-band addressing (SOCKS5 ATYP, per-datagram remote addresses)
enters only through the `-D`/dynamic-target composition path
(OQ-TN-03/OQ-TN-08 residue), not through the base open-op params.

**Status:** mostly resolved 2026-09-05 (pending the resource-naming
and discovery residue). Direction of travel:
- `params` = self-contained JSON object in the open op (alktty
  `NegotiateRequest` precedent) — accepted path.
- `params` identify a produced resource + substrate discriminator;
  producer owns the backing. No URL-style general addressing.
- Residue: stable resource name vs target address as the identifier
  shape; discovery resolved 2026-09-05 — see OQ-TN-08 (the existing
  ACL-filtered ops listing IS tunnel-resource discovery, since
  openable channels are operations per alkcall ADR-047); exact JSON
  field layout (Phase 1 spec, ADR before first consumer — wire-stable
  once published).

### OQ-TN-02: Datagram substrates (UDP) — boundary preservation

Does a UDP tunnel preserve datagram boundaries end-to-end, or does the
tunnel present a byte-stream abstraction to the consumer (boundaries lost,
re-chunked arbitrarily)?

- Channels is a chunk stream with bounded buffers; the zero-length chunk
  is the EOF sentinel — datagram boundaries are *not* preserved by the
  substrate (alknet ADR-071/093; the POC only exercised TCP).
- SSH's `-D` UDP associate tunnels UDP as a stream with per-datagram
  framing re-added by the tunnel protocol (e.g. SOCKS5 UDP over TCP).
  Note (2026-09-05): russh does not support UDP channels at all — no
  `direct-udp`-style prior art exists there; the tun2proxy gateway
  (§Prior art) is the strongest framing precedent.
- iroh and quinn-proxy-poc have native datagram transports; tun2proxy has
  a full UDP-over-TCP model worth reading.
- Boundary preservation is a wire-format decision (per-datagram length
  framing inside the `BiStream`) and would need an ADR + possibly a BAST
  document (AGENTS.md convention 12). Boundary loss is cheaper but
  changes what protocols can ride the tunnel (DNS? QUIC? game traffic?).
- Datagrams also raise multiplexing questions TCP does not: one UDP
  "association" carries many remote endpoints — does one tunnel channel
  carry one endpoint or many, and how are per-endpoint replies routed?

**Status:** open — survey mostly resolved by tun2proxy prior art
(§Prior art): per-datagram length framing over the stream is
production-proven (`LEN | FLAGS | CONN_ID | [addr] | DATA`), one
stream carries many UDP flows via a protocol-level `CONN_ID`, and
flow lifecycle (idle timeout + keepalive) is packet-level. Remaining:
whether alktunnels fixes the UDP endpoint at open (per-channel, TCP-
like) or carries per-datagram addresses (udpgw-like), and whether a
u16 conn-id vocabulary is right for channels (vs the channel ID
itself doing the demux and one channel per UDP flow). Note (2026-09-05,
hub model §Prior art): if UDP resources are produced like any other
resource, endpoint-at-open aligns naturally with resource naming
(OQ-TN-01); per-datagram addressing matches the `-D`/dynamic-target
composition path instead. A targeted POC (OQ-TN-10 #1) is likely still
+EV for the chosen shape.

### OQ-TN-03: Direction semantics (`-L` / `-R` / dynamic)

**Status: resolved 2026-09-05** by the hub-owns-the-connection model
(§Prior art: the hub-owns-the-connection model). There is no
protocol-level direction: role follows the resource. Whoever can reach
the target is the producer (registers openable channels); whoever wants
the bytes is the consumer (opens channels). SSH `-L` and `-R` are the
same producer/consumer pair with the entry point on different machines —
assembly-layer wiring, not protocol. The "exposed port" is a virtual,
ACL-scoped resource on the producing side; the hub proxy is a producer
wrapping a consumer. `-D`/SOCKS composes as a consumer opening channels
with per-connection dynamic targets, gated by the producer's target
policy (OQ-TN-08) — not a base-crate concern. Original question retained
below for context.

Considerations (original):

SSH has three forwarding flavors; the crate must model them without
"server/client" framing:

- `-L` (local forward): consumer dials a local port; producer dials the
  target. Channels flows consumer→producer; target dial happens on the
  producer side. This is the POC's shape.
- `-R` (remote forward): producer (or a third party) listens; the
  *consumer's* side dials or accepts incoming connections and asks the
  other side to carry them. Channels flows producer→consumer.
- `-D` (dynamic/SOCKS): one side runs a SOCKS5 server; the target is
  chosen per-connection by the client. Addressing arrives per-channel,
  not per-tunnel-registration.

Both sides can be producer and consumer simultaneously (alkcall ADR-022/037
direction semantics), so the model must not bake direction into the
connection. The open questions:

- Is direction a field in `params`, or two distinct open-handler shapes /
  ALPNs?
- How does `-R` register availability (the side that will carry traffic
  advertises listen targets)? Does it interact with `channel/open` at all,
  or is it a call-level operation ("please open a tunnel channel to me
  when a local accept happens")?
- Dynamic (-D) may not be a tunnel concern at all — it may compose as
  "SOCKS5 server implemented over alktunnels dial primitives" in a
  separate crate. Keep or cut for v1?

**Status:** open — needs architecture decision. Half-answer (hunch): `-L`
is the channel/open handler; `-R` needs a small advertisement/lifecycle
surface; `-D` composes on top and is out of scope for the base crate.

### OQ-TN-04: No forced local binding

A tunnel must not require the producer (or consumer) to bind a local port.
The POC's shape dialed a target from the handler; binding is optional and
belongs to the caller (assembly layer), not the protocol crate. The API
surface must support:

- Dial flows with no local bind (POC shape) — covered.
- Listen flows where the binding happens on one side only.
- Unbound/abstract flows (e.g. unix socketpair-style, stdio bridges,
  in-process pipes) where neither side binds.

The protocol layer must express "carry bytes between this target and this
channel" without assuming either endpoint is a bound socket. Substrate
modules (behind feature flags) own actual `bind()` calls; the protocol
owns bookkeeping only.

**Status:** open — mostly resolved (2026-09-05, see §Prior art: the
hub-owns-the-connection model): binding is always assembly-layer and
optional, on either side; the protocol never binds. What remains is the
concrete API surface — who calls what to start a tunnel in each mode
(produce-with-dial, produce-without-dial/accept-style, consume). This is
now a spec-shape task, not a research question.

### OQ-TN-05: Backend inversion point — is there a `TunnelBackend` trait?

alktty has `TtyBackend` because backends (local PTY, docker, SSH) produce
handles and the adapter pumps them. For tunnels, the producer side's
substrate action is narrower — dial a target, or accept on a listener —
so the question:

- Is a `TunnelBackend`-style trait needed at all, or is the two-pump
  handler + feature-gated substrate modules (dial/listen helpers) the
  whole story, with the assembly layer wiring substrate streams directly?
- If a trait: what is the handle type? A tunnel "handle" is just an
  `AsyncRead + AsyncWrite` stream (or a datagram endpoint) — much thinner
  than `TtyHandle`'s stdin/stdout/stderr/exit-code quadruple. The trait
  may collapse to "produce a boxed stream for this target" plus a
  listener variant.
- Backpressure/limits come from channels (AGENTS.md convention 10); the
  backend trait must not add a second layer of them.
- New sub-question from the hub model (2026-09-05, §Prior art): the
  hub's tunnel proxy *re-produces* a resource it consumes (producer
  wrapping a consumer). Does the proxy need a composition helper from
  this crate, or does it assemble from the public producer/consumer
  surface as-is? If a helper is warranted, it may share shape with the
  substrate dial/listen trait — which would argue for the trait.

**Status:** open — needs a survey of what backends would actually
implement (local TCP? docker exec? ssh -w?) before deciding trait vs
no-trait. Half-answer (hunch): a thin trait (or just a fn alias) for
"obtain a bidirectional substrate stream for a target," possibly no
trait at all if the only meaningful backends are local sockets — decide
after surveying candidate backends.

### OQ-TN-06: The two-pump helper — extract now?

alknet ADR-078 deferred helper extraction until a second two-pump consumer
exists ("a genuine deferral... the contract is decided (shutdown-on-
completion), only the extraction is deferred"). This crate is that second
consumer (POC tunnel was the first; SSH `direct-tcpip` would be a third).

- Does the helper live here (as a pub utility other handler crates can
  use), or upstream in alkcall (which already owns `core` types)?
- Shape: `pump_bidi(recv, send) -> (Future, Future)` returning both
  pumps with the shutdown-on-completion wired in? Or a
  `join_two_pumps(a, b)` combinator?
- alknet ADR-057 (two-pump helper extraction OQ) noted the helper from
  one consumer would bake in a wrong shape; with two consumers the shapes
  should be compared before extraction.

**Status:** open — decide when the first real tunnel handler is written;
not a blocker for the spec. Half-answer: the helper probably belongs
upstream (alkcall, near the channels-adapter handler-integration
conventions) but only if the two shapes genuinely converge.

### OQ-TN-07: ALPN strategy

This crate owns the `alk/tunnel`-family ALPN(s). alkcall ADR-004: one
ALPN per protocol; `alk/` prefix. If stream (TCP/unix) and datagram (UDP)
tunnels get distinct ALPNs, the split must be decided before the first
consumer — ALPN strings are wire-stable once published.

- Option A: single `alk/tunnel` ALPN; substrate is a `params` field
  (and datagram framing, if any, is self-describing inside the channel).
- Option B: `alk/tunnel` (stream) + `alk/tunnel-dgram` (datagram), so
  the wire framing differs per ALPN cleanly.
- Channels' `params` is ALPN-specific, and the open-handler registry
  dispatches per ALPN — both options are cheap mechanically; the cost is
  consumer-side API bifurcation (two session types vs one with a
  substrate enum).

**Status:** open — needs the OQ-TN-02 outcome first (if datagrams need
different framing, option B gets stronger). Note from the tun2proxy
prior art (§Prior art): udpgw runs its packet framing over a plain TCP
stream — one framing covers both the stream and datagram cases there.
If alktunnels follows the same shape (datagram framing self-describing
inside the channel), option A (single `alk/tunnel` ALPN) stays viable
even with UDP support; option B remains cleaner if the datagram
channel needs structurally different framing from the first chunk on.

### OQ-TN-08: Access control and ownership scope

Tunnels reach local networks — the open gate is the security boundary.
Shape follows alktty: `TUNNEL_OPEN_SCOPE` scope-gate, and the channels
path gets `AccessControl` wiring for free via
`ChannelCore::register_openable`. Open sub-questions:

- Should ownership (`OwnershipProvider.owns(...)`) be consulted for
  tunnel targets, and what is the resource identity of a tunnel target
  (a `host:port`? a registered tunnel name?), given targets may be
  arbitrary strings and wildcard targets (`0.0.0.0/0`-style egress) may
  be intentionally allowed for some identities?
- Is there a target-allowlist concept (per-identity reachable target
  sets), and does it live in `AccessControl` or in the open handler's
  params validation?

**Status: mostly resolved 2026-09-05** — this OQ pointed at a general
conceptual tangle ("the host owns the resource and proxies on top of
that" — how does that manifest for tunnels?), and the resolution is
the same posture alktty already uses:

- **Assume the resource is owned by the other side of the connection.**
  At these protocol-crate levels (alktunnels, like alktty), the
  protocol works under the assumption that a produced resource belongs
  to the far side — so the ACL story is exactly alkcall's existing
  op-level ACL. No new policy layer, no target allowlists, no
  tunnel-specific ownership machinery. The actual proxy/overlay
  mechanism (hub workers connecting in and exposing tunnels, the hub
  providing an overlay to those resources based on the other side's
  ACL) is a *downstream* (assembly-layer) concern.
- **Discovery is the existing bidirectional ops listing.** alkcall
  already has an underlying bidirectional discovery mechanism — each
  side can obtain the list of ops available to it (ACL-filtered). Since
  openable channels *are* operations (alkcall ADR-047), that listing
  *is* tunnel-resource discovery: a consumer asks "what ops are
  available to me" and produced tunnel resources appear there, scoped
  by identity. This is exactly how a consumer learns a socks5 tunnel,
  a postgres TCP tunnel, or a redis tunnel is an available resource.
  Examples of the pattern: services typically served over a VPN or
  SSH tunnels (postgres, redis, gitea HTTP) — workers connect to a
  hub, expose those tunnels, and the hub proxies them per-ACL.
- **`-D` simplifies to "just tunnel a socks5 connection."** The socks5
  server lives on the producing side; the consumer opens an ordinary
  tunnel channel to that resource and speaks socks5 inside it. Target
  selection happens in the socks5 protocol at the far side — *not* in
  tunnel params — so the previously-tracked "dynamic-target policy
  hook for `-D`-style opens" residue dissolves: whatever ACL governs
  the socks5 resource governs everything reachable through it, plus
  whatever policy the socks5 implementation itself applies downstream.

Residue for Phase 1 (spec-shape, not research): confirm the
ops-listing surface carries enough per-resource metadata (substrate
type, resource name/description) for a consumer UI to distinguish
produced tunnels — or whether `params`-describing metadata rides
alongside the operation spec. That is an alkcall ADR-047 interaction,
not a new mechanism.

**Upstream posture (2026-09-05):** we own the upstream. Both prior
downstream crates (alktty, alkhttp — `/workspace/@alkdev/alkhttp`)
required fixes/additions to alkcall, and the rule is to make those
asks *early*: there are only three downstream dependents right now,
and each issue resolved upstream makes the next dependent cheaper. So
if the ops listing needs per-resource metadata (or anything else in
the ADR-047 interaction), the expectation is a small upstream alkcall
change in Phase 0/1 — not a workaround in this crate.

### OQ-TN-09: Lifecycle, teardown, and error reporting

The two-pump shape gives byte-level teardown for free (EOF sentinels;
channels drops per-channel senders on transport EOF — alknet ADR-078,
POC issue #6). What's missing is the error/level above bytes:

- How does a failed target dial reach the consumer (e.g. "connection
  refused to 10.0.0.5:80")? Is there a structured error frame in the
  channel before close, a `channel/close` with reason, or call-level
  error on the open op?
- Is there a "tunnel established/failed" ack before byte pumping starts
  (alktty has the negotiation frame; the POC's tunnel handler had
  nothing — it dialed and pumped)?
- Half-open semantics: one direction EOFs, the other keeps pumping
  (standard two-pump behavior) — is that always desired, or does the
  consumer need a "close both" control?

**Status: direction set 2026-09-05** — the original half-answer is
accepted: a self-contained control frame (alktty ADR-006 shape)
carrying an establishment result/error, sent before any data chunk;
dial errors are tunnel-closing (the whole channel dies), whereas
byte-level EOFs stay per-direction. Beyond that, the frame vocabulary
should follow the alktty stream-splitting pattern (§Prior art: the
alktty stream-splitting pattern) rather than invent a parallel
mechanism. Concrete frame set lands as a Phase 1 ADR (wire-stable
before the first consumer).

### OQ-TN-10: POC scope for what remains unvalidated

The alknet-channels POC validated TCP only. Candidate targeted POCs
Phase 0 may need (in rough priority order, per the SDD process's
"validate promising approaches"):

1. **UDP tunnel POC** — boundary-preserving length framing over a
   channels channel, per-endpoint multiplexing inside one association,
   backpressure behavior. Partially derisked by the tun2proxy prior
   art (§Prior art) — the POC now mainly validates *channels-layer*
   fit: chunk-size vs datagram-size interaction, MTU cap against the
   channels bounded buffers, idle-expiry mapping, and the chosen
   endpoint-addressing shape. Derisks OQ-TN-02 (and OQ-TN-07's option B).
2. **Reverse-flow POC** — `-R`-style: the accept side listens, the far
   side carries. Derisks OQ-TN-03's advertisement/lifecycle shape.
3. **Unix socket + stdio bridge POC** — cheap; validates "substrate
   agnostic" beyond IP substrates.
4. **Two-pump helper extraction spike** — OQ-TN-06, only after 1–3.

POC placement conventions (2026-09-05): a POC that needs code from
this repo runs in a worktree/branch (`.worktrees/research/<task-id>/`
per the SDD process); a POC that is self-contained runs as a
standalone crate in the global workspace (the
`/workspace/alknet-channels-poc` precedent) with its findings written
into `docs/research/` here. Both are valid; pick per the POC's
dependency footprint. Findings always land in `docs/research/`
regardless of where the code lives.

**Status:** open — pick 1 (and probably 2) after the research pass;
3 is cheap enough to fold into whichever POC runs first.

## Survey / prior-art list

Candidate reading for the research specialist (to be expanded):

- SSH channel/forwarding model: RFC 4254 §7 (direct-tcpip /
  forwarded-tcpip), OpenSSH `-L`/`-R`/`-D` semantics, russh's
  `ChannelOpen` framing (russh is already in `/workspace/russh`;
  confirmed 2026-09-05: no UDP channel support at all — no UDP prior
  art there, but useful for open-op framing intel).
- SOCKS5 (RFC 1928): addressing (ATYP), UDP ASSOCIATE framing,
  per-endpoint multiplexing — the closest standardized "arbitrary
  tunnel + UDP" model. Relevant to the `-D` composition path
  (OQ-TN-03 residue), not the base open-op params (OQ-TN-01 reframe).
- tun2proxy (`/workspace/tun2proxy`, `src/udpgw.rs`): UDP gateway over
  TCP — per-datagram length framing, SOCKS5 per-datagram addressing,
  CONN_ID flow multiplexing, keepalive/ERR flag packets, MTU cap,
  idle expiry. Analyzed in §Prior art. Its `socks.rs` /
  `proxy_handler.rs` are also relevant for the `-D` (dynamic/SOCKS)
  composition question (OQ-TN-03).
- quinn-proxy-poc (`/workspace/quinn-proxy-poc`) and iroh
  (`/workspace/iroh`): datagram-native transports; how they model
  per-endpoint flows.
- alknet docs: ADR-071 §ALPN table (`alknet/tunnel` row), ADR-078,
  `docs/architecture/crates/channels/channel-operations.md` (`params`
  for `alknet/tunnel` is "the target resource"), and the hub-relay
  interaction (ADR-042/079).
- alktty: `NegotiateRequest` shape (self-contained negotiation
  precedent), `TtyBackend` inversion point, `TTY_OPEN_SCOPE` access
  gate.

## Convergence checklist (what Phase 0 must produce)

- [ ] Survey notes: SSH/SOCKS5 addressing + UDP framing
      (OQ-TN-01, OQ-TN-02) — tun2proxy UDP gateway done (§Prior art);
      SSH/SOCKS5 survey delegated to research specialist
- [x] Reframe landed (OQ-TN-01) — params = self-contained JSON open-op
      object identifying a produced resource + substrate
      discriminator (`tcp`/`udp`/extensible); producer owns the
      backing; no URL-style general addressing. Residue: resource
      naming shape, exact JSON layout (Phase 1 ADR); discovery
      resolved (OQ-TN-08)
- [x] Direction model resolved (OQ-TN-03) — hub-owns-the-connection
      model: role follows the resource, no protocol-level direction;
      `-D` composes at the assembly layer
- [x] No-forced-binding requirement encoded (OQ-TN-04) — binding is
      always assembly-layer and optional; remaining work is the
      concrete produce/consume API surface sketch (spec task)
- [x] Access control + discovery resolved (OQ-TN-08) — resource
      ownership assumed on the far side; alkcall's existing ACL
      applies as-is; ops listing (ADR-047) is tunnel-resource
      discovery; `-D` = "tunnel a socks5 connection" (target selection
      in the socks5 protocol, not params). Residue: per-resource
      metadata in the ops listing — small upstream alkcall ask,
      early per the upstream posture (alktty/alkhttp precedent)
- [x] Lifecycle/error direction set (OQ-TN-09) — establishment/error
      control frame accepted in principle (dial errors are
      channel-closing); frame vocabulary follows the alktty
      stream-splitting pattern; concrete set = Phase 1 ADR
- [ ] Codec decision (new, from the alktty pattern prior art) — reuse
      5-byte format vs stripped variant vs raw pass-through + UDP
      framing; feeds OQ-TN-07 (ALPN) and the OQ-TN-05 trait shape
- [ ] Decision input: backend trait vs no-trait (OQ-TN-05), now
      including the hub re-produce composition question
- [ ] Targeted POC(s) run + summary (OQ-TN-10) — UDP first, reverse
      flow second
- [ ] Open questions promoted to Phase 1
      `docs/architecture/open-questions.md` with statuses