Transport

The bottom of the stack is the IO layer: getting raw bytes from one peer to another.

The [Link] component is the type-erased struct that will send/receive raw bytes. It holds a send queue and a receive queue of raw payloads, plus the link state (Linking, Linked, Unlinked) and some stats. Lightyear systems only ever talk to the Link; they don’t know or care how the bytes actually travel.

How the bytes travel is decided by the IO component you pair with the Link:

• UdpIo / UdpEndpoint: plain UDP sockets
• WebTransportClientIo / WebTransportEndpoint: WebTransport (QUIC)
• WebSocketClientIo / WebSocketEndpoint: WebSocket
• CrossbeamIo: in-memory channels, used for tests and host-server mode
• SteamClientIo / SteamEndpoint: Steam sockets

So a UDP client is Link + UdpIo, a WebTransport client is Link + WebTransportClientIo, and so on. Swapping transports means swapping one component.

An accepting transport component requires Endpoint, which owns the collection of per-peer Link entities through LinkOf { endpoint }. It does not imply the Server role: a P2P peer can also own an endpoint. Trigger LinkStart to open its listener.

Native accepting endpoints are available with either the transport crate’s p2p or server feature. lightyear/p2p forwards p2p to whichever optional transports you enable, so a peer can listen without enabling Lightyear’s server feature. For example, select default-features = false and features = ["std", "p2p", "udp"] to use lightyear_udp::endpoint::{UdpEndpoint, UdpEndpointPlugin} without server plugins. For WebSocket, WebTransport, and Steam, the transport’s p2p feature enables Aeronet’s accepting-side support (called server by Aeronet), not Lightyear’s authority role.

When installing an endpoint transport plugin directly, it also installs the shared endpoint lifecycle support: unlinking an endpoint unlinks and despawns its child links, and each new child inherits the endpoint’s receive conditioner. Client-only builds retain the shared Endpoint, LinkOf, and Aeronet bridge types; accepting transport components require p2p or server.

For an authoritative server, add Server alongside the endpoint component and the appropriate connection component. ServerUdpIo is the UDP shorthand that requires both UdpEndpoint and Server. Configure inherited receive conditioning with Endpoint::new(conditioner) rather than on Server; access the fan-out collection through Endpoint.