alknet/tasks/architecture/spec-interface.md

id, name, status, depends_on, scope, risk, impact, level

id	name	status	depends_on	scope	risk	impact	level
architecture/spec-interface	Create interface.md architecture spec (Layer 2)	completed	architecture/adr-026-transport-interface-separation architecture/adr-033-operationenv-irpc-call-protocol	moderate	high	project	implementation

Description

Create docs/architecture/interface.md — the new Layer 2 spec defining the Interface abstraction. This is the most architecturally significant new spec document.

Currently, SSH is deeply embedded in the server handler. The Interface trait extracts it into a pluggable Layer 2 component:

#[async_trait]
pub trait Interface: Send + Sync + 'static {
    type Session;
    async fn accept(stream: TransportStream, config: &InterfaceConfig) -> Result<Self::Session>;
    // The session produces call protocol events and handles responses
}

The spec must define:

1. The Interface trait — consumes a Transport::Stream and produces call protocol sessions
2. SshInterface — wraps existing russh handler, produces SSH channels + control channel
3. RawFramingInterface — reads length-prefixed JSON EventEnvelope frames directly, no SSH wrapping
4. Valid (Transport, Interface) pairs — enumerated per ADR-026
5. How the call protocol is interface-agnostic — it receives EventEnvelope frames from any interface

The hard part: The existing ServerHandler owns auth, channel management, and proxy logic. The spec must cleanly define what moves to Layer 3 (call protocol), what stays in the interface, and what's shared. The split needs to be clean — this is explicitly flagged as needing careful design review in the integration plan.

Depends on ADR-033 because: The Interface trait must produce call protocol events, and OperationEnv defines how those events are dispatched. The Interface needs to produce the right shape of events for the OperationEnv to consume.

Acceptance Criteria

• docs/architecture/interface.md exists with YAML frontmatter (status: draft)
• Follows spec format: What, Why, Architecture, Constraints, Open Questions, Design Decisions
• Defines the three-layer model (reference ADR-026): Transport (Layer 1), Interface (Layer 2), Protocol (Layer 3)
• Defines Interface trait with signature, bounds, and lifecycle
• Defines SshInterface — what it wraps from existing ServerHandler (auth delegation, channel open, proxy)
• Defines RawFramingInterface — 4-byte BE length prefix + JSON EventEnvelope, no SSH
• Enumerates valid (Transport, Interface) pairs per ADR-026 table
• Defines what InterfaceConfig contains (different per interface type)
• Clearly separates what moves to call protocol (Layer 3) vs what stays in the interface (Layer 2)
• Shows how auth_publickey() maps to SshInterface (not RawFramingInterface which uses token auth)
• Shows how channel_open_direct_tcpip() proxy logic relates to Layer 3 forwarding policy
• DNS control channel explicitly defined as (DNS transport, raw framing interface) — NOT SSH inside DNS
• Call protocol is interface-agnostic: receives EventEnvelope from any interface
• References ADR-026, ADR-033
• docs/architecture/README.md updated to include interface.md
• Flags any open design questions (e.g., exactly how auth maps across interfaces)

References

• docs/research/integration-plan.md — Phase 1.8, three-layer model, valid (Transport, Interface) pairs
• docs/research/core.md — DNS transport, interface concept
• docs/architecture/decisions/026-transport-interface-separation.md
• docs/architecture/decisions/033-operationenv-irpc-call-protocol.md
• docs/architecture/server.md — current ServerHandler (source of SshInterface logic)

Notes

To be filled by implementation agent

Summary

To be filled on completion