alknet/docs/architecture/transport.md

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status	last_updated
reviewed	2026-06-02

Transport Layer

What

The transport layer produces a duplex byte stream (AsyncRead + AsyncWrite + Unpin + Send) that the SSH layer consumes via russh::client::connect_stream() or russh::server::run_stream(). The SSH layer is completely unaware of what transport it runs over.

Why

Pluggable transports are the core architectural insight. They enable:

• Simple deployment: TCP on port 22 for basic use
• Censorship resistance: TLS on port 443 looks like HTTPS
• NAT traversal: iroh QUIC allows connections without public IPs
• Composability: transports can be layered (iroh through SOCKS5 through SSH through TLS)

Without this abstraction, each transport mode would need its own SSH connection logic. With it, there's one SSH implementation and N transport implementations.

Architecture

Transport Trait

// The core abstraction. Each transport produces ONE duplex stream.
// The SSH session runs over this stream for its entire lifetime.

#[async_trait]
pub trait Transport: Send + Sync + 'static {
    type Stream: AsyncRead + AsyncWrite + Unpin + Send + 'static;

    /// Connect to the remote endpoint and return a duplex stream.
    /// For client-side transports.
    async fn connect(&self) -> Result<Self::Stream>;

    /// Return a human-readable description of this transport for logging.
    fn describe(&self) -> String;
}

Server-Side Transport Acceptor

#[async_trait]
pub trait TransportAcceptor: Send + Sync + 'static {
    type Stream: AsyncRead + AsyncWrite + Unpin + Send + 'static;

    /// Accept an incoming connection and return a duplex stream.
    async fn accept(&self) -> Result<(Self::Stream, TransportInfo)>;
}

/// Metadata about the incoming connection.
pub struct TransportInfo {
    pub remote_addr: Option<SocketAddr>,
    pub transport_kind: TransportKind,
}

pub enum TransportKind {
    Tcp,
    Tls { server_name: Option<String> },
    Iroh { endpoint_id: String },
}

Transport Implementations

Transport	Client	Server	Stream Type
TcpTransport	TcpStream::connect(addr)	TcpListener::accept()	TcpStream
TlsTransport	TlsStream<TcpStream> (client TLS)	TlsStream<TcpStream> (server TLS)	tokio_rustls::client::TlsStream<TcpStream>
IrohTransport	endpoint.connect(peer, alpn) then conn.open_bi() then join(recv, send)	endpoint.accept() then conn.accept_bi() then join(recv, send)	tokio::io::Join<RecvStream, SendStream>

Iroh Stream Join

Since QUIC splits streams into separate RecvStream (implements AsyncRead) and SendStream (implements AsyncWrite), while russh expects a single duplex stream, they are combined using tokio::io::join(recv_stream, send_stream) which produces a Join<RecvStream, SendStream> implementing both traits.

See ADR-003 for the decision to use tokio::io::join over a custom wrapper.

iroh Relay Configuration

By default, iroh transport uses n0's free relay servers (https://relay.iroh.network/). This provides zero-config NAT traversal for testing and development. For production deployments, users override with --iroh-relay <url> to point to a self-hosted relay.

The relay URL is passed to iroh's Endpoint::builder() configuration. Self-hosted relay setup is documented in the project wiki.

See ADR-009 for the decision to default to n0's relay with override.

Transport Chaining

Transports can be nested. The CLI supports --transport iroh --proxy socks5://... natively (ADR-010):

wraith connect --transport iroh --proxy socks5://127.0.0.1:1080

This routes iroh's outbound TCP connections through the specified SOCKS5 proxy. The iroh transport supports SOCKS5 and HTTP proxy configuration for its outbound connections — the proxy URL is applied during transport initialization.

For other combinations:

• TCP + TLS is already implicit (TLS wraps TCP in TlsTransport)
• TLS + SOCKS5 proxy is also supported via --proxy with --transport tls

Note: --proxy has different semantics on the client vs the server (ADR-019):

• Client: --proxy routes the transport connection through the proxy (e.g., iroh endpoint → SOCKS5 → iroh relay)
• Server: --proxy routes outbound target connections through the proxy (e.g., SSH channel request → SOCKS5 → target host)

Connection Lifecycle

Client                                          Server
  │                                               │
  │  transport.connect()                          │  transport_acceptor.accept()
  │  ─────────────────────────────────────────────▶│
  │        (duplex byte stream established)        │
  │                                               │
  │  russh::client::connect_stream(config,        │  russh::server::run_stream(config,
  │      stream, handler)                          │      stream, handler)
  │                                               │
  │  ═══════ SSH session over stream ═════════════ │
  │  ═════════════════════════════════════════════ │
  │                                               │
  │  channel_open_direct_tcpip(host, port, ...)  │
  │  ─────────────────────────────────────────────▶│
  │                                               │
  │  ┌─────── TCP proxy ──────────────────┐       │
  │  │  SSH channel ←→ TcpStream::connect │       │
  │  └────────────────────────────────────┘       │

Constraints

• SSH sees only the stream. It never opens its own TCP connections. (ADR-004)
• Each transport produces exactly one stream per SSH session. Multiple sessions need multiple connect() calls.
• The iroh transport reuses a single Endpoint across multiple sessions (one QUIC connection per peer, multiple open_bi() streams). The endpoint is created once and shared.
• TLS transport requires certificate configuration on the server side. The client can accept any certificate (self-signed) or verify against a CA. Server-side ACME is supported (ADR-008).

Open Questions

None — all resolved.

Design Decisions

ADR	Decision	Summary
001	Pluggable transport	Transport trait produces stream, SSH consumes it
003	iroh stream join	tokio::io::join combines QUIC halves
004	SSH over transport	SSH never touches TCP/iroh/TLS directly
008	ACME/Let's Encrypt	Auto-provision TLS certs, domain and IP paths
009	Default iroh relay	n0 relay by default, --iroh-relay override
010	Transport chaining	--proxy works with all transports natively
019	Proxy dual semantics	--proxy routes transport on client, data on server