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alknet/docs/architecture/interface.md

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draft 2026-06-09

Interface (Layer 2)

What

The Interface layer sits between Transport (Layer 1) and Protocol (Layer 3). Interfaces consume byte streams from Transports or manage their own transports, and produce call protocol sessions or handle discrete requests. SSH is an interface, not a transport — it wraps a byte stream in session semantics. Raw framing (4-byte length prefix + JSON EventEnvelope) is another interface. HTTP and DNS are message-based interfaces that handle individual request/response pairs without persistent sessions.

Why

In the original architecture, SSH was deeply embedded in ServerHandler. This tangling of transport, interface, and protocol made it impossible to:

  • Run the call protocol over DNS queries without wrapping SSH inside DNS
  • Use raw framing for local service mesh (no SSH overhead)
  • Support WebTransport direct call protocol for browsers
  • Separate auth mechanics from channel management
  • Accept HTTP requests and map them to call protocol operations

The three-layer model (ADR-026) cleanly separates these concerns. Transport produces bytes. Interface parses bytes into sessions or handles requests. Protocol carries semantics. A connection is always a (Transport, Interface) pair for stream-based interfaces, or a standalone message-based interface.

Phase 2 research identified that HTTP and DNS don't fit the persistent session model — they're stateless per-request. This led to the StreamInterface / MessageInterface split (ADR-035), which gives each interface category its own trait with the right lifecycle and ownership model.

Architecture

Three-Layer Model

Layer 3: Protocol    (Call protocol, Operations, OperationEnv)
Layer 2: Interface   (StreamInterface: SSH, raw framing | MessageInterface: HTTP, DNS)
Layer 1: Transport   (TCP, TLS, iroh, WebTransport)
  • Layer 1: Transport — produces byte streams (`AsyncRead + AsyncWrite + Unpin
    • Send`). Unchanged per ADR-001. DNS is NOT a transport.
  • Layer 2: Interface — two categories:
    • StreamInterface: consumes a TransportStream and produces a long-lived session that yields InterfaceEvent frames.
    • MessageInterface: handles individual InterfaceRequest → InterfaceResponse pairs. Manages its own transport.
  • Layer 3: Protocol — carries semantics. Call protocol events, operation registry, service calls. Agnostic to both Transport and Interface below it.

StreamInterface Trait

#[async_trait]
pub trait StreamInterface: Send + Sync + 'static {
    type Session: InterfaceSession;

    async fn accept(
        &self,
        stream: Box<dyn TransportStream>,
        config: &InterfaceConfig,
    ) -> Result<Self::Session>;
}

The session produced by a StreamInterface is consumed by the call protocol handler. Different stream interfaces produce different session types, but the call protocol handler receives InterfaceEvent frames from any stream interface.

MessageInterface Trait

#[async_trait]
pub trait MessageInterface: Send + Sync + 'static {
    async fn handle_request(&self, request: InterfaceRequest) -> Result<InterfaceResponse>;
}

Message-based interfaces handle individual requests without persistent sessions. They manage their own transport (HTTP server, DNS server) and normalize requests into InterfaceRequest / InterfaceResponse.

InterfaceRequest / InterfaceResponse

pub struct InterfaceRequest {
    pub operation_path: String,         // e.g., "/head/auth/verify"
    pub input: Value,                   // JSON input payload
    pub auth_token: Option<AuthToken>,  // Extracted from wire format
    pub metadata: HashMap<String, String>,
}

pub struct InterfaceResponse {
    pub result: Result<Value, CallError>,
    pub status: u16,                    // HTTP status, DNS result code, etc.
    pub headers: HashMap<String, String>,
}

The call protocol handler processes InterfaceRequest the same way it processes InterfaceEvent frames — both resolve to operation invocations through OperationEnv. The difference is framing: stream interfaces produce InterfaceEvent frames from a continuous byte stream, message interfaces construct InterfaceRequest from their wire format.

InterfaceSession

Every stream interface session implements InterfaceSession:

pub struct InterfaceEvent {
    pub envelope: EventEnvelope,
    pub identity: Option<Identity>,
}

#[async_trait]
pub trait InterfaceSession: Send {
    async fn recv(&mut self) -> Option<InterfaceEvent>;
    async fn send(&mut self, envelope: EventEnvelope) -> Result<()>;
}

InterfaceEvent carries an EventEnvelope and the authenticated Identity. The call protocol handler (Layer 3) receives InterfaceEvent frames and processes them uniformly, regardless of whether they arrived over SSH or raw framing.

SshInterface (StreamInterface)

Wraps the existing ServerHandler logic. This is the most complex stream interface because SSH provides channel multiplexing, auth negotiation, and proxy management within a single session.

What stays in SshInterface (Layer 2):

  • SSH handshake and session management
  • Auth delegation to IdentityProvider (via auth_publickey() callback)
  • Channel multiplexing (multiple channels per session)
  • alknet-control:0 channel routing to call protocol

What moves to Layer 3 (call protocol handler):

  • Operation registry and dispatch
  • Forwarding policy checks (per ADR-031)
  • Operation context construction (Identity, scopes)

What moves to per-connection state:

  • Port forwarding proxy logic

Current implementation note: SshSession::recv() and SshSession::send() are stubs. The bridge from SSH channels to InterfaceEvent frames is scheduled for Phase 2 implementation (see integration-plan.md Phase 2.1).

RawFramingInterface (StreamInterface)

Reads 4-byte big-endian length prefix + JSON EventEnvelope frames directly from the transport stream. No SSH wrapping. No channel multiplexing — the entire stream is a single call protocol channel.

pub struct RawFramingInterface;

impl StreamInterface for RawFramingInterface {
    type Session = RawFramingSession;
    // Reads length-prefixed EventEnvelope frames from the stream
}

Used for:

  • Local service mesh (TCP + raw framing, no SSH overhead)
  • Secure mesh (TLS + raw framing)
  • WebTransport direct call protocol (future: WebTransport + raw framing)

Auth for raw framing: AuthToken in frame header, resolved via IdentityProvider::resolve_from_token().

Current implementation note: RawFramingInterface::accept() returns an error. Frame reading/writing is scheduled for Phase 2 implementation (see integration-plan.md Phase 2.2).

HttpInterface (MessageInterface)

Accepts standard HTTP requests and maps them to call protocol operations:

POST /v1/{namespace}/{op}     → registry.invoke(namespace, op, input)  (mutation)
GET  /v1/{namespace}/{op}     → registry.invoke(namespace, op, input)  (query)
GET  /v1/{namespace}/{op} SSE → registry.subscribe(namespace, op, input) (subscription)
GET  /v1/schema               → registry.list_operations()

Auth: Authorization: Bearer <token> header, resolved via IdentityProvider::resolve_from_token(). Both AuthTokens and API keys are accepted.

The HTTP interface runs inside the existing stealth mode byte-peek architecture: after a TLS handshake, the server peeks at the first bytes. If they're SSH-2.0-, the stream goes to SshInterface. Otherwise, the stream goes to the axum HTTP router.

Phase 2 scope: Auth middleware, stealth handoff, and default 404 handler only. Specific operation routes and path conventions are Phase 5+. The ListenerConfig::Http variant spawns an axum router that reaches auth context; routing inside axum is a later concern.

DnsInterface (MessageInterface)

A DNS server that encodes/decodes EventEnvelope frames as DNS query/response pairs. AuthToken is embedded in DNS query labels. Resolution via IdentityProvider::resolve_from_token().

This is a MessageInterface — it manages its own transport (UDP/TCP port 53) and handles individual DNS queries as request/response pairs. DNS is NOT a transport.

Phase: DNS interface implementation is Phase 5+. The ListenerConfig::Dns variant and DnsInterface stub are defined now; implementation is deferred.

Stream-Based Interface Pairs

Transport StreamInterface Credential Presentation Use case
TLS SshInterface SSH key handshake Standard alknet tunnel
TCP SshInterface SSH key handshake Plain SSH tunnel
iroh SshInterface SSH key handshake P2P SSH tunnel
TCP RawFramingInterface AuthToken in frame header Local service mesh
TLS RawFramingInterface AuthToken in frame header Secure mesh
WebTransport RawFramingInterface AuthToken in CONNECT request Browser call protocol (future)

Message-Based Interface Pairs

MessageInterface Credential Presentation Owns transport? Use case
HttpInterface Authorization: Bearer header Yes (axum) REST API, dashboard, integrations
DnsInterface AuthToken in query labels Yes (DNS server) Censorship-resistant control channel
WebSocketInterface AuthToken in handshake Yes (WS server) Browser persistent connection (future)

Message-based interfaces manage their own transport. They don't need a Transport from Layer 1 — they ARE the transport+interface combined.

ListenerConfig

The server's accept loop configuration covers both stream and message interfaces:

pub enum ListenerConfig {
    Stream {
        transport: TransportKind,
        interface: StreamInterfaceKind,
    },
    Http {
        bind_addr: SocketAddr,
        tls: bool,
        stealth: bool,    // byte-peek protocol detection on shared port
    },
    Dns {
        bind_addr: SocketAddr,
        tls: bool,
    },
}

pub enum StreamInterfaceKind {
    Ssh,
    RawFraming,
}

pub enum TransportKind {
    Tcp,
    Tls { server_name: Option<String> },
    Iroh { endpoint_id: String },
    WebTransport,  // Phase 5+: tag only, no acceptor yet
}

Note: TransportKind::Dns does NOT exist. DNS is a MessageInterface, not a transport. The ListenerConfig::Dns variant handles DNS listener configuration directly.

Credential Presentation Across Interfaces

Every interface resolves to the same Identity through IdentityProvider:

SSH fingerprint       → IdentityProvider::resolve_from_fingerprint → Identity
AuthToken (Bearer)   → IdentityProvider::resolve_from_token       → Identity
API key (Bearer)     → IdentityProvider::resolve_from_token       → Identity
DNS embedded token    → IdentityProvider::resolve_from_token       → Identity

The credential presentation differs per (Transport, Interface) pair, but the resolution result is always an Identity. See definitions.md for the full table and terminology rules.

Server Accept Loop

With both stream and message interfaces, the accept loop becomes:

for listener in listeners {
    match listener {
        ListenerConfig::Stream { transport, interface } => {
            // Spawn accept loop: transport.accept() → interface.accept(stream)
        }
        ListenerConfig::Http { bind_addr, tls, stealth } => {
            // Spawn axum HTTP server on bind_addr
            // If stealth: byte-peek after TLS, route SSH vs HTTP
        }
        ListenerConfig::Dns { bind_addr, tls } => {
            // Spawn DNS server on bind_addr
        }
    }
}

Constraints

  • StreamInterface and MessageInterface are independent traits with different signatures, lifecycles, and transport ownership. No common super-trait (ADR-035).
  • SshInterface is the most invasive refactoring. The existing SshHandler owns auth, channel management, and proxy logic — extracting these cleanly requires careful design (integration-plan Phase 1.8, completed in Phase 1).
  • DNS interface implementation is Phase 5 work. DnsInterface is defined as a MessageInterface stub; implementation is deferred.
  • HTTP interface Phase 2 scope is limited to auth middleware and stealth handoff. Specific operation routes are Phase 5+.
  • WebTransport is Phase 5 work. TransportKind::WebTransport and StreamInterfaceKind::WebTransport are tags only for now.
  • TransportKind::Dns does not exist. DNS is a MessageInterface, not a transport. This was TransportKind enum pollution from an earlier design.
  • The Interface trait (singular) in the current codebase needs to be renamed to StreamInterface. This is a rename, not a semantic change.

Open Questions

  • OQ-IF-02: Should SshInterface own the ForwardingPolicy check for channel_open_direct_tcpip, or should that move to Layer 3? Resolved: ForwardingPolicy is Layer 3, but channel open/close lifecycle is Layer 2. SshInterface reports channel requests to Layer 3; Layer 3 applies policy.

  • OQ-P2-01: Should MessageInterface and StreamInterface share a common trait? Recommendation: No. Independent traits with different signatures, lifecycles, and transport ownership. A common super-trait adds complexity without clear benefit. (See ADR-035.)

  • OQ-P2-02: Should the HTTP interface share a port with the SSH listener? Recommendation: Start with separate ports. ALPN multiplexing on port 443 is a future optimization that doesn't change the interface abstraction. Stealth mode byte-peek already handles shared-port detection for the common case.

Design Decisions

ADR Decision Summary
026 Three-layer model SSH is Layer 2, not Layer 1
035 StreamInterface / MessageInterface Two trait categories at Layer 2
033 OperationEnv Protocol is interface-agnostic
029 Identity as core type Auth resolution across interfaces
031 Forwarding policy Layer 3 policy applied to Layer 2 channel requests

Phase 2 Implementation Notes

  • Interface trait renamed to StreamInterface throughout alknet-core (ADR-035 implemented)
  • MessageInterface trait added with handle_request(InterfaceRequest) -> Result<InterfaceResponse> (ADR-035 implemented)
  • InterfaceRequest and InterfaceResponse types implemented
  • HttpInterface and DnsInterface stub structs added (Phase 5 for full implementation)
  • InterfaceConfig split into StreamInterfaceConfig and MessageInterfaceConfig
  • StreamInterfaceKind and MessageInterfaceKind enums added
  • ListenerConfig restructured from flat struct to enum with Stream, Http, Dns variants
  • TransportKind::Dns removed from the enum (DNS is a MessageInterface, not a transport)
  • TransportKind::WebTransport updated from { host: String } to { server_name: Option<String> }
  • RawFramingInterface fully implemented with first-frame auth
  • SshSession::recv()/send() bridge to call protocol via alknet-control:0 channel implemented, using ControlChannelBridge with mpsc channels

References

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