alktunnels/docs/architecture/decisions/001-open-params-layout.md

ADR-001: Open-Op Params Layout — Resource + Substrate

Status

Accepted (2026-09-07)

Context

The tunnel open op's params (the input of the channels/tunnel/sub operation, alkcall ADR-047) is the wire-stable carrier of "what to open." alknet ADR-071 §ALPN table noted alknet/tunnel as [0, 1] data in/out only; the addressing scheme was never decided there. Phase 0 resolved the framing of the problem (the OQ-TN-01 reframe, 2026-09-05): the rich "remote addressing" framing was an XY problem — 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.

Constraints that shaped the decision:

• params is ALPN-specific JSON, interpreted by the open op's establisher/handler — never by the channels layer (alkcall ADR-039). The layout is a one-way door once a consumer exists (wire-stable once published; additive changes only).
• The identifier must be substrate-extensible without format changes (a new substrate is an additive substrate value, not a v2 format). SSH's direct-streamlocal (same open-op shape, degenerate address slots, new type string) and SOCKS5's ATYP (scheme-tagged addresses) are the prior-art anchors; both support the "one identifier + one discriminator" reduction.
• The producer owns the backing: the consumer never learns an address. A host:port in params would leak the producer's topology into the wire and pin the wire to address-shaped targets.
• alkcall's registry schema-validates the open op's input (input_schema runs before the establisher — review 007 Part B), so the layout must be schema-describable (no per-substrate polymorphic payloads in v1).
• Both POCs used exactly this shape (TunnelParams {resource, substrate}); 17 + 16 tests rode it end-to-end.

Decision

params for the channels/tunnel/sub open op is a self-contained JSON object:

{
  "resource": "postgres-primary",
  "substrate": "tcp"
}

• resource (string, required) — the produced resource identifier: the producer's stable name for the tunnel target. NOT an address; the producer's registry (an assembly-side construct, OQ-TN-11) maps the name to its backing (a local port, a docker container's port, an in-process service, a unix socket path).
• substrate (string, required) — the extensible discriminator: "tcp" | "udp" | "unix" in v1. A new substrate is a new value, not a format change — the same additive property SSH gets from channel-type strings and SOCKS5 gets from ATYP values. The discriminator also selects the data-plane framing (ADR-003): the establisher and both pumps must agree on it, so it rides the open op rather than being inferred.
• The open op's input_schema pins both fields as required strings; substrate is enum-validated against the v1 set. Unknown substrate values are a schema failure (typed open error) — a consumer speaking a newer substrate to an older producer fails loudly, not silently.
• Substrate-specific detail (the path of a unix socket, the UDP associate semantics) is owned by the producer's registry, not the wire. The resource name is opaque to the protocol.
• The open-failure error path rides alkcall ADR-049 (typed channel:open_failed with details.reason ∈ dial_failed / unknown_resource / resource_shortage / handler_error / timeout): an unknown resource name is unknown_resource — no params-level error field, no establishment frame (the pre-ADR-049 OQ-TN-09 hunch is superseded).

Rich in-band addressing (SOCKS5 ATYP, per-datagram remote addresses) enters only through the -D/dynamic-target composition path — it is carried INSIDE the tunnel payload by whatever protocol the tunnel carries (socks5, udpgw-style framing), never in base params.

Consequences

• Wire-stable from the first consumer. Both fields are required; renames are a breaking wire change (do not). Adding an optional field is additive-safe; adding a substrate value is consumer-progressive (an older producer rejects with invalid_input-class schema error — the SSH "unknown channel type" posture, loud not silent).
• The BAST question is moot for params. Params is JSON inside the channels open op (which is already a JSON envelope); the data plane's binary framing (ADR-003) carries its BAST doc at docs/architecture/bast.md. Per AGENTS.md convention 12, the hand-rolled codec remains the runtime implementation with the BAST doc as the contract.
• Producers own a naming discipline. Resource ids are producer-scoped names (OQ-TN-11: collision domain = the producer's registry). Consumers learn available resources through the ACL-filtered ops listing (OQ-TN-08) — the spec's OperationSpec.description SHOULD carry a human-readable hint (e.g. "postgres on the worker's tailnet") since it round-trips through discovery.
• No per-connection dynamic targets in base params. -D/SOCKS composes by tunneling a socks5 connection (OQ-TN-08); a future multi-endpoint UDP gateway resource would be self-describing framing inside the channel payload, invisible to params.

References

• Phase 0: OQ-TN-01 (the reframe + survey input), docs/research/ssh-socks5-survey.md §OQ-TN-01 residue
• alkcall ADR-047 (openable ALPNs are operations; input_schema), ADR-049 (typed establishment errors)
• Forward POC: docs/research/poc-summary.md (the shape riding 17 tests); reverse POC: docs/research/reverse-poc-summary.md
• OQ-TN-01 (promoted), OQ-TN-11 (naming residue)