alkhttp/docs/architecture/decisions/010-alpn-router-and-endpoint.md

ADR-010: ALPN Router and Endpoint

Ported from alknet ADR-010 (ALPN Router and Endpoint); re-targeted to alkhttp.

Status

Accepted

Context

ADR-001 establishes ALPN-based protocol dispatch: a single endpoint accepts connections, and the ALPN negotiated during the TLS handshake routes each connection to the correct ProtocolHandler. ADR-002 defines the ProtocolHandler trait. ADR-006 establishes one ALPN per connection. ADR-007 defines Connection and BiStream.

The question is: how does the endpoint work? What accepts connections, negotiates ALPN, and hands connections to handlers? This is the central runtime piece of the shared core — every handler depends on it. The endpoint itself is an alkcall/alknet-side concern; this ADR is ported because alkhttp's HttpAdapter is a consumer of its dispatch model (registration, stealth mode, static ALPN registration), not because alkhttp owns an endpoint.

Multiple connectivity modes, not multiple transports

The reference implementation supports three connectivity modes that serve fundamentally different deployment contexts:

1. QUIC+TLS (public) — The node has a public IP and open ports. TLS provides protocol routing via ALPN negotiation. The TLS certificate is the node's network-facing identity — it's what clients verify when connecting to alknet.example.com:4433. This is the mode for replicators, VPS hosts, service providers. SSH key auth still handles authentication — the TLS cert is not the auth identity, it's the network identity.

2. iroh P2P (NAT traversal) — The node has no public IP or open ports. iroh's relay handles NAT traversal and connection brokering. Node identity comes from iroh's NodeId (Ed25519 key pair). The relay is a signaling service, not a proxy — it helps peers establish direct QUIC connections. This is the mode for home servers, IoT devices, anything behind NAT.

3. TCP (local/dev) — Bare SSH over TCP. Port 22. No TLS, no ALPN, no certs. SSH key exchange handles both identity and authentication. This is the mode for local network access and development.

These are not interchangeable "transports" to be abstracted behind a trait. They are different ways a node can be reached, each with different identity and authentication implications:

Mode	Identity source	Auth mechanism	Requires public IP	Use case
QUIC+TLS	TLS cert (network) + SSH key (auth)	SSH key, API key	Yes	VPS, replicators
iroh P2P	NodeId (Ed25519)	NodeId, SSH key	No	Home servers, NAT
TCP	SSH host key	SSH key	Yes (local)	Dev, LAN

What the old "stealth mode" actually was

The reference implementation's "stealth mode" is SSH-over-TLS on port 443. The TLS cert is NOT the node's identity — it's camouflage. The purpose is to make port 443 look like a web server to port scanners and DPI systems. Non-SSH traffic gets a fake nginx 404. SSH auth still happens via SSH key exchange inside the TLS tunnel.

In the ALPN model, this concept maps to: the endpoint speaks TLS with ALPN, and the HTTP handler can serve a decoy website on h2/http/1.1 while real services use alk/ssh, alk/call, etc. (ALPN prefix per the alkcall crate's ADR-004). The ALPN router does the "stealth" job — unknown ALPNs get the HTTP handler, which can serve whatever fronting content is desired. No byte-peeking needed.

iroh produces QUIC connections with ALPN

iroh's Endpoint::accept() produces incoming QUIC connections with ALPN negotiation (step 4 of iroh's connection establishment). The iroh::Endpoint supports set_alpns() to configure which ALPNs the endpoint advertises — the same mechanism iroh's own Router uses internally.

This means the iroh integration is not a separate dispatch path. It uses the same ALPN dispatch as the quinn path. The iroh::Endpoint accepts connections, negotiates ALPN, and our HandlerRegistry dispatches to the right handler — exactly like iroh's own Router does with its ProtocolMap.

We do NOT wrap iroh's Router. We use iroh::Endpoint directly and run our own accept loop, because:

• Our HandlerRegistry is shared between quinn and iroh connection sources
• Our AuthContext construction differs per connection source
• Our shutdown and error handling patterns are our own

The relationship is: iroh's Router is a reference implementation of the pattern we're building. Our endpoint generalizes it to support multiple connection sources with the same dispatch.

Key design questions

1. How many endpoints can a node have? A node may need to listen on quinn (public QUIC+TLS) AND iroh (P2P relay) simultaneously. These are not alternatives — they're complementary connectivity modes.
2. Handler registration: Static (at startup) or dynamic (at runtime)?
3. Connection lifecycle: Who owns the endpoints? How does graceful shutdown work?
4. Error handling: What happens when a handler panics? When ALPN negotiation fails?

Decision

A node can have multiple endpoints

The endpoint type manages one or more QUIC connection sources. Each source produces connections that feed into the same HandlerRegistry:

pub struct Endpoint {
    // One or more QUIC connection sources
    quinn: Option<quinn::Endpoint>,       // Public QUIC+TLS
    iroh: Option<iroh::Endpoint>,         // P2P relay-assisted

    handlers: Arc<HandlerRegistry>,
    dynamic: Arc<ArcSwap<DynamicConfig>>,
    identity_provider: Arc<dyn IdentityProvider>,
    shutdown: watch::Receiver<bool>,
}

A node that has a public IP runs with quinn: Some(...) — it listens on a public address with TLS+ALPN. A node behind NAT runs with iroh: Some(...) — it connects to a relay and accepts P2P connections. A node that has both runs with both — it's reachable via either path, and both feed into the same ALPN router.

TCP mode is not an endpoint concern. TCP mode in the reference implementation is SSH over raw TCP on port 22. This is not QUIC and doesn't have ALPN. In the new model, TCP access to SSH is handled by the SSH handler directly — it can listen on a TCP socket independently of the ALPN endpoint. This is a handler-specific concern, not a core endpoint concern.

HandlerRegistry maps ALPN strings to ProtocolHandler instances

pub struct HandlerRegistry {
    handlers: HashMap<&'static [u8], Arc<dyn ProtocolHandler>>,
}

Registration is static at startup (OQ-04). The CLI binary constructs a HandlerRegistry, inserts handlers, and passes it to Endpoint::new().

The ALPN strings for the quinn endpoint's TLS ServerConfig are derived from the registry's keys. The iroh endpoint's ALPN strings are also derived from the registry — both endpoints advertise the same set of ALPNs.

Accept loop: accept from all sources, dispatch by ALPN

The endpoint runs accept loops for each active connection source. All loops dispatch through the same HandlerRegistry:

// Quinn accept loop (if configured)
loop {
    incoming = quinn_endpoint.accept().await
    connection = incoming.await  // TLS handshake + ALPN negotiation
    dispatch(connection)
}

// iroh accept loop (if configured)
loop {
    incoming = iroh_endpoint.accept().await
    connection = incoming.await  // iroh QUIC connection + ALPN
    dispatch(connection)
}

fn dispatch(connection) {
    alpn = connection.alpn()
    handler = registry.get(alpn)
    match handler {
        Some(h) => {
            auth = AuthContext::from_connection(&connection)
            conn = Connection::from_quinn(connection) // or from_iroh
            tokio::spawn(h.handle(conn, &auth))
        }
        None => connection.close()
    }
}

Both accept loops are tokio::select!-ed against the shutdown signal.

TLS certificate and the distinction between network identity and auth identity

For the quinn endpoint, the TLS cert serves as network-facing identity — it's what clients verify when connecting to a domain name. It is NOT the node's authentication identity. Authentication is handled by handlers (SSH key exchange, API tokens, etc.).

This is the same model as the reference implementation's TLS mode: the cert makes the port look legitimate and encrypts traffic, but SSH key exchange handles the actual authentication. The ALPN model extends this: the cert + ALPN routing is the network layer, handler-specific auth is the application layer.

For the iroh endpoint, the NodeId serves as network identity. No TLS cert is needed — iroh's QUIC uses the NodeId for connection verification.

RFC 7250: Raw Public Keys in TLS

iroh uses RFC 7250 raw public keys instead of X.509 certificates for TLS. The implementation is strikingly simple (see iroh/iroh/src/tls/resolver.rs): take an Ed25519 key, wrap its SPKI public key as a CertificateDer, and tell rustls only_raw_public_keys() -> true. No X.509, no CAs, no domain names, no cert renewal.

rustls already supports RFC 7250. This means the quinn endpoint can also use raw Ed25519 public keys instead of X.509 certs. The implications:

1. No domain required. A node without a domain name can use raw public keys for the quinn path — the same key-based identity model as iroh, but with direct QUIC over UDP instead of relay-assisted connections.
2. Key = identity. The Ed25519 public key IS the node's identity. No CA trust chain, no cert expiry, no renewal. The key is derived from alkvault or generated at startup.
3. X.509 is optional. Domain-facing identity (for replicators, public services) uses X.509 certs. Key-based identity (for personal nodes, P2P) uses raw public keys. Both work with the same quinn endpoint.
4. Browser compatibility. Browsers don't support RFC 7250 — they require X.509. For browser/WebTransport clients, X.509 certs are needed. For alknet-native clients, raw public keys work fine.

This reframes the connectivity model. The quinn and iroh paths are not distinguished by their identity model (both can use Ed25519 keys). They're distinguished by how the connection is established:

Path	Connection establishment	Identity model (v1)	Identity model (future)
quinn	Direct UDP, public IP	X.509 (domain)	X.509 or RFC 7250 raw key
iroh	Relay-assisted P2P	RFC 7250 raw key (NodeId)	Same

Error taxonomy

EndpointError is removed per ADR-083 (Amendment 2026-07-15 + the EndpointError-removal amendment; see the alkcall crate docs). BindFailed is vestigial (the endpoint takes pre-bound transports); TlsConfig is removed (the endpoint takes no TLS config); HandlerNotFound is swallowed by dispatch (close + log, not an error). shutdown() is infallible. The sketch below is the historical shape; it does not survive into the endpoint crate. HandlerError is unchanged.

// HISTORICAL — removed per ADR-083. See the note above.
pub enum EndpointError {
    BindFailed(io::Error),
    TlsConfig(io::Error),
    HandlerNotFound(Vec<u8>),  // ALPN string with no registered handler
}

pub enum HandlerError {
    ConnectionClosed,
    StreamError(io::Error),
    AuthRequired,
    Internal(Box<dyn std::error::Error + Send + Sync>),
}

• EndpointError: removed (ADR-083; see the alkcall crate docs). The endpoint takes pre-built transports and swallows no-handler matches; shutdown() is infallible.
• HandlerError: Problems within a handler's handle() method. Non-fatal — the connection is closed, but the endpoint keeps running.

Consequences

Positive:

• A node can be reachable via multiple paths simultaneously (public QUIC+TLS, iroh P2P)
• ALPN router is transport-agnostic — dispatches by ALPN string regardless of connection source
• Adding a handler is registering an ALPN string — no endpoint code changes
• Handler panics are isolated — one bad handler can't take down the endpoint
• "Stealth mode" maps naturally to the HTTP handler serving decoy content on h2/http/1.1 — in alkhttp this is the HttpAdapter's decoy surface
• Both iroh and quinn produce QUIC connections — same Connection type works for both

Negative:

• The core crate depends on both quinn and iroh (mitigated: both are feature-gated; a node that only needs one doesn't compile the other)
• The endpoint is more complex than a single quinn listener — it manages multiple accept loops
• TLS identity provisioning has two distinct use cases: RFC 7250 raw keys (default for P2P/key-based identity) and X.509 certs (for domain-hosted services and browsers). ACME auto-provisioning and RawKey decoupling from the iroh feature are designed in ADR-027 (see the alkcall crate docs). See OQ-12.
• No runtime handler registration without regenerating the TLS config (mitigated: two-way door, start static, add ArcSwap later if needed)

References

• ADR-001: ALPN-based protocol dispatch
• ADR-002: ProtocolHandler trait
• ADR-006: ALPN string convention and connection model (see the alkcall crate docs)
• ADR-007: BiStream type definition — Connection, SendStream, RecvStream (see the alkcall crate docs)
• ADR-009: One-way door decision framework (alknet mono-repo)
• OQ-04: Dynamic handler registration (two-way door, start static)
• OQ-05: Multi-transport endpoint (now: multi-connectivity endpoint)
• iroh Router pattern (alknet mono-repo): docs/research/references/iroh/
• Reference implementation (alknet mono-repo): alknet-main/crates/alknet-core/src/server/serve.rs
• Reference stealth mode (alknet mono-repo): alknet-main/crates/alknet-core/src/server/stealth.rs
• Reference iroh transport (alknet mono-repo): alknet-main/crates/alknet-core/src/transport/iroh_transport.rs

Amendments

Amendment 1 (2026-07-09): TCP+TLS can dispatch through the ALPN router via from_stream

This ADR's Decision section states: "TCP mode is not an endpoint concern." The rationale was that bare TCP (SSH over port 22) does not use QUIC or ALPN, so TCP access is handled by individual handlers listening on a TCP socket independently — a handler-specific concern, not a core endpoint concern.

That rationale holds for bare TCP (no TLS, no ALPN). But ADR-065 (see the alkcall crate docs) adds Connection::from_stream / from_bidi, which construct a Connection from any AsyncRead + AsyncWrite pair — including a TlsStream<TcpStream>. A TCP+TLS accept loop can now call Connection::from_bidi(tls_stream, alpn, remote_addr) and dispatch through the same HandlerRegistry as QUIC connections, by the ALPN negotiated in the TLS handshake. This is not a parallel listener bypassing the core — it's the same ALPN dispatch, over a non-QUIC transport.

Revised reading of "TCP is not an endpoint concern": the endpoint struct (quinn + iroh) remains QUIC-only — the endpoint does not own a TCP+TLS accept loop. But a TCP+TLS accept loop can be constructed outside the endpoint (by the assembly layer or a handler) and feed connections into the same HandlerRegistry the endpoint uses. The endpoint is one accept-loop source; a TCP+TLS loop is another source that shares the registry. The "not an endpoint concern" framing is preserved at the struct level (no tcp: Option<TcpListener> on the endpoint); the "TCP can't participate in ALPN dispatch" framing is reversed — from_stream is the primitive that lets TCP+TLS participate without changing the endpoint design.

The unblocked follow-ups (not part of ADR-065, but enabled by it):

• Standard HTTP over TCP+TLS (api.alk.dev's requirement): a TLS accept loop wraps each TlsStream<TcpStream> as a Connection via from_bidi and dispatches to HttpAdapter by the negotiated ALPN (h2/http/1.1). HttpAdapter::handle runs hyper over a bidirectional stream (BiStream) yielded by Connection::accept_bi() — unchanged from the QUIC path. No handler code changes.
• SSH channel dispatch: an SSH handler wraps each russh channel as a Connection via from_stream and dispatches by channel-type (treated as the ALPN string) through HandlerRegistry. One SSH connection carries heterogeneous channels — a multiplexing power QUIC's per-connection ALPN doesn't provide natively.
• WebTransport stream dispatch (parked per ADR-044, unblocked structurally; an alknet-side concern — out of scope in alkhttp, ADR-069): the WT handler wraps each WT stream via from_stream.

The iroh 0.35 → 1.0.2 migration (commit acd049e, 2026-07-09) is a related cleanup: it bumps the iroh dep to 1.0, unblocking alknet-blobs (which pulls iroh 1.0 transitively). It is not an architectural change — 6 API surface edits in endpoint.rs / types.rs (the Endpoint::builder preset, SecretKey::from_bytes/generate signatures, Connection::remote_id/alpn return types). No ADR needed; the endpoint design is unchanged.

Amendment 2 (2026-07-14): TCP+TLS is a first-class owned transport (supersedes Amendment 1's struct-level exclusion)

Amendment 1 preserved the "not an endpoint struct concern" framing at the struct level — no tcp: Option<TcpListener> field on the endpoint. The rationale was that the endpoint built transports internally (quinn, iroh), and TCP+TLS couldn't fit that construction shape, so it was a sibling loop outside the struct.

ADR-083 (see the alkcall crate docs) removes that rationale: the endpoint no longer builds transports at all — it runs accept loops on whatever it's given via builder methods. TCP+TLS is a listener transport, same shape as quinn and iroh (accept → extract ALPN + fingerprint → Connection::from_bidi → dispatch). The endpoint now owns it via with_tcp_tls(listener, acceptor) (behind a tcp feature), runs its accept loop inside run(), and stops it on shutdown(). The struct gains a tcp_tls: Option<TcpTlsListener> field.

Amendment 1's dispatch contribution survives — the public dispatch method and Connection::from_bidi are what make TCP+TLS dispatch work. Amendment 1's struct-level exclusion (no tcp field, sibling loop outside) is superseded: TCP+TLS is now a first-class owned transport. The dispatch method stays public, but for genuinely external shapes (SSH channels, future WebTransport streams) — connection-internal multiplexing, not listener transports.

This also means shutdown is single-owner: the endpoint owns all its accept loops (quinn, iroh, TCP+TLS); one shutdown() stops them all. The multi-owner shutdown coordination problem (OQ-61) does not arise.