# ADR-0006 — Ingress posture: WebRTC-first; SIP endpoint deferred

- **Status:** Accepted
- **Date:** 2026-06
- **Related:** [ADR-0003](0003-sip-rust-native-trunk.md) (trunk SIP), [ADR-0002](0002-north-star-and-fused-core.md) (fused vertical)

## Context

The fused per-call vertical ([ADR-0002](0002-north-star-and-fused-core.md)) needs an answer for
**human-participant ingress** — agents and supervisors joining/taking calls. The thin-slice first
proof starts at WebRTC loopback (vision-revision §10). The question: which protocols carry humans
into calls, and is inbound **endpoint** SIP (desk/soft phones) in scope?

## Decision

### 1. WebRTC is the first-party human-participant ingress

Agents and supervisors use the browser as the softphone: SSO in, one peer-connection,
DTLS-SRTP + ICE handled by the browser, **zero device-provisioning tail**. This is also the modern
hosted UX. Escalation (capability rung 2) is a UX gesture: an agent clicks *take this call* in the
dashboard → a WebRTC leg joins → audiohook/barge handoff. Writes itself onto the audiohook primitive
(PORT_PLAN) and shares step-1 infrastructure (WebRTC ships first regardless).

### 2. SIP trunk client = core (per ADR-0003)

Bounded: a few documented, IP-allowlisted, cooperative carriers.

### 3. Inbound SIP *endpoint* registrar = deferred / community edge, NOT core

`REGISTER` from Polycoms/Yealinks/Zoiper, BLF/MWI subscribe-notify, RFC2833-vs-SIP-INFO DTMF,
per-device NAT keepalives and provisioning quirks are **deferred out of the core**:

- **The real reason** is not "SIP is hard" — it's that an inbound endpoint server **re-imports the
  unbounded device-interop tail** [ADR-0003](0003-sip-rust-native-trunk.md) deliberately closed.
  ADR-0003's "tail collapses" argument survives only because rutster talks to *documented carriers*,
  not *thousands of desk-phone UAs*. Opening an endpoint server reopens that seam.
- In an SSO/browser-agent contact center the desk-phone demographic has collapsed to niche ops
  floors already served by dated OSS (FreePBX/Vicidial). Wrong place to spend a solo multi-year
  build — a pointlessly-and-diverting distraction in a modern hosted-UX/SSO world.
- **Don't burn the bridge:** if an operator genuinely needs desk phones, the right home is an
  **optional, out-of-tree SBC-style adapter** (the Kamailio-flavored thing ADR-0003 chose not to put
  in the core) fronting the public API or B2B-SIP'ing into rutster — community-owned, **outside the
  trust boundary** so its tail can't compromise the wedge.

### 4. Core interop perimeter is deliberately bounded

**SIP trunk client + WebRTC ingress.** Nothing else inbound.

## Security seam — tap ≠ ingress

Two distinct extension points with **opposite** security postures; never unified into one model:

- **Tap** (agent brain): **egress**, core-as-**client**, **no inbound tap port** on the core. The
  boundary dials out to the brain. (See ARCHITECTURE.md, "Agent tap.")
- **Ingress** (humans): **inbound**, core-as-**server**, with SSO + RBAC + per-tenant scoping.
  Legitimately a server surface — that's fine, it's auth'd; the "core-as-client" move was
  specifically about the egress brain tap, where *egress* is the dangerous direction.

## Consequences

- **Positive:** first-call via WebRTC ships before any SIP; escalation is a UX click not a new
  protocol; the core's interop surface stays bounded; no device-provisioning tail in the trust
  boundary.
- **Negative:** desk-phone-only operators are not served by the core — accepted, the community
  adapter path exists.
