Explore the intricate implementation of propagation netcode in Spelunky 2 through this comprehensive 2021 GDC session. Delve into the constraints, rollback mechanics, liquid physics, and deterministic propagation techniques employed in the game. Learn about interpolation methods, latency issues, performance improvements, and the challenges of implementing input delay and multiple instances. Discover the complexities of crossplay, server expansion, and online multiplayer features. Gain insights into server analytics, invite codes, and tool problems encountered during development. Examine the ambiguity feedback system and other open challenges faced by the development team, including matchmaking. Understand the intricacies of random number generation and the importance of reliability in netcode implementation for this popular roguelike platformer.