Breaking the Ankh - Deterministic Propagation Netcode in 'Spelunky 2'

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.

Introduction
What is Spelunky
Constraints
How rollback works
Liquid physics
Deterministic propagation
Interpolation
Day 1 rollback
Comparison with Simple Interpolation
Latency Issues
Performance Improvements
Input Delay
Multiple Instances
Invite Criteria
Ghost Bug
Random Number Generator
Server Expansion
Reliability
Crossplay
The Promise Day
Online Multiplayer
Server Analytics
Invite Codes
Tool Problems
Ambiguity Feedback
Other Open Challenges
Matchmaking