# ADR-007: BIND in the Base Crate — the Phase Model Extended

## Status

Accepted (2026-09-20, Phase 1; POC #5 validated end to end
2026-09-15)

## Context

fast-socks5 parses `Socks5Command::TCPBind` but every handler
refuses it; no bind machinery exists. RFC 1928 BIND is the
accept-side command (the `-R` far-side listener shape): the client
asks for a listener on the producer's network; the producer replies
#1 with the bound address, waits for exactly one inbound accept, and
replies #2 with the peer address before the data plane. The use case
that makes it real (FTP active mode is the canonical vanilla reason)
plus the composed variant's hub story were found in Phase 0
(OQ-SK-08), and POC #5 validated the whole shape end to end — both
accept variants, the scope gate, the failure mapping, the vanilla
front door.

Support is purely additive: the same ALPN, same op, same params, no
framing change; RFC-refusal of BIND remains valid for old consumers
(a producer that doesn't implement BIND just keeps refusing it).
Nothing here is a one-way door.

## Decision

- **BIND ships in the base crate** (protocol layer), phase-model
  style: request → reply#1 carrying `BND.ADDR` → quiet wait (any
  control byte after reply#1 is protocol garbage; the session ends —
  exactly two exits: the accept, or the control stream's EOF/
  garbage/error) → exactly one inbound accept → reply#2 carrying the
  peer address → raw pass-through data plane (zero per-packet
  headers in-band, same as CONNECT; only the ASSOCIATE datagram
  stage is framed).
- **The accept seam is `BindAcceptFactory`** (ADR-004). Two
  provider-backed accept variants (plus the wrapper-aware in-band
  default when neither is wired), all POC #5-validated:
  1. **Local backend binds for real** (feature-gated) — the faithful
     RFC shape: a real network-B address in reply#1 (the listener's
     `local_addr`), a real peer in reply#2, vanilla front-door
     clients work. **This is the default accept variant** for the
     optional local backend.
  2. **Composed (alktunnels listen resource)** — the listener lives
     on a bind-first listen resource (`register_tunnel_bindfirst_
     openable` + `BindFirstInjections`, alktunnels 0.2.0); per-
     listener ACL for free, hub-relayable. **Vanilla-fidelity with
     no stand-ins** since the graduation: the factory no longer
     carries an expected `bind_addr` — reply#1 advertises the real
     OS-chosen `bound` (the open-op reply's `bound` field,
     alkcall 0.8.0 reply projection); reply#2 carries the real peer
     (the forwarded op's `peer` param).
  3. **Wrapper-aware only** (zeros in reply#1, sentinel reply#2) —
     retained for completeness as the in-band default when neither
     provider is wired: the handle-swap reading applied to BIND
     (the reply is RFC-compatibility surface; the data plane is the
     channel).
- **The establishment bound covers only the open-op round trip.**
  The accept wait lives in the teardown-governed pump phase — FTP-
  length BIND waits stop resolving as spurious establishment
  refusals (the bind-first graduation's stated second win;
  `poc-bind-findings.md` §Establishment-bound, superseded in shape).
- **Failure mapping is RFC-faithful end to end**: factory failure
  (pre-reply#1) → reply#1 with the mapped reply code; accept-stage
  failure (post-reply#1) → reply#2 with the mapped code, no data
  plane. No phantom listeners, no phantom data planes.

## Consequences

- The RFC 1928 command surface is complete: CONNECT + BIND +
  ASSOCIATE, all three on one op (POC #5 test 7 regression-guarded
  the other two).
- The composed accept path composes against alktunnels 0.2.0 — the
  bind-first establisher + forwarded op (ADR-008) are the substrate;
  the POC's pinned workaround (reply#1 first, the tunnel open rides
  the accept future) is retired with the graduation.
- Old consumers see no change (the refusal was always the default);
  the support is additive and gated by `socks5:bind` (ADR-006).

## References

- OQ-SK-08 (the full reasoning + the graduation's shape
  supersession), OQ-SK-01 (the accept seam's shape), ADR-004
  (the accept seam), ADR-006 (the command gate)
- POC #5 `poc-bind-findings.md` (the validation record; the
  superseded-in-shape ask ledger)
- alktunnels ADR-007/ADR-008 (the direct op; bind-first establisher
  + forwarded op), alkcall ADR-049 amendment 3 (the reply
  projection), alktunnels graduation research
  (`alktunnels/docs/research/tunnels-graduation.md`)