D006: Networking — Pluggable via Trait
Revision note (2026-02-22): Revised to clarify product-vs-architecture scope. IC ships one default/recommended multiplayer netcode for normal play, but the NetworkModel abstraction remains a hard requirement so the project can (a) support deferred compatibility/bridge experiments (M7+/M11) with other engines or legacy games where a different network/protocol adapter is needed, and (b) replace the default netcode under a separately approved deferred milestone if a serious flaw or better architecture is discovered.
Decision: Abstract all networking behind a NetworkModel trait. Game loop is generic over it.
Rationale:
- Sim never touches networking concerns (clean boundary)
- Full testability (run sim with
LocalNetwork) - Community can contribute netcode without understanding game logic
- Enables deferred non-default models under explicit decision/overlay placement (rollback, client-server, cross-engine adapters)
- Enables bridge/proxy adapters for cross-version/community interoperability experiments without touching
ic-sim - De-risks deferred netcode replacement (better default / serious flaw response) behind a stable game-loop boundary
- Selection is a deployment/profile/compatibility policy by default, not a generic “choose any netcode” player-facing lobby toggle
Key invariant: ic-sim has zero imports from ic-net. They only share ic-protocol.
Cross-engine validation: Godot’s MultiplayerAPI trait follows the same pattern — an abstract multiplayer interface with a default SceneMultiplayer implementation and a null OfflineMultiplayerPeer for single-player/testing (which validates IC’s LocalNetwork concept). O3DE’s separate AzNetworking (transport layer: TCP, UDP, serialization) and Multiplayer Gem (game-level replication, authority, entity migration) validates IC’s ic-net / ic-protocol separation. Both engines prove that trait-abstracted networking with a null/offline implementation is the industry-standard pattern for testable game networking. See research/godot-o3de-engine-analysis.md.