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This is a fork of RetroArch intended to fix and improve its Netplay. The crucial fixes have all been merged upstream. See also my info on which cores are tested.

Done and merged upstream

• Removed the "here there be dragons" sign that prevented anyone else from looking at this code.
• Added a README that lightly documents how RetroArch's Netplay works.
• Netplay ring buffer hardened to prevent overruns, which previously caused insane desyncs.
• Ripped out UDP. Everything is done over TCP now. The previous implementation demanded reliability and in-order guarantees anyway, so UDP was just silly. Note that the protocol has also changed in more fundamental ways, so it's not really possible to make a "UDP or TCP" option except by having two incompatible implementations of Netplay.
• Removed all limits on the size of the replay buffer. You want to allow 10 seconds of drift? 10 seconds of drift it is!
• Made stalling work without blocking the UI thread.
• General bugfixes to make Netplay bulletproof.
• Remote pausing.
• Stalling without rerunning frames. Netplay is now one of the many reasons the frontend may pause.
• Support for loading state over Netplay.
• Frame CRCing as a last resort for sync. Configurable by netplay_check_frames or the --check-frames option.
• Deterministic player flipping.
• Fixed spectator mode. In addition, it's now far more similar to net (normal) mode than it was before.
• Netplay menu and support for launching Netplay on an existing session.
• Savestate "quirks" API. Allows cores to describe the quirkiness of their serialization code so that frontend features can adapt. Particularly useful since many cores have session-specific savestates that cannot be transferred over the network.
• Update all quirky cores to identify their quirkiness.
• Compression of savestates when sent over the network.
• IPv6 fixes
• NAT traversal (UPnP)
• LAN scanning for netplay hosts
• Better (non-blocking) initial connection, to allow the serving port to be left listening indefinitely.

• 2 player netplay.

• Merge spectator mode into net mode.
• Password-protected serving.
• New alternative to delay_frames:
  • Keep running (window-based) average of frame runtimes to guess how many frames per second you're capable of rewinding.
  • Always buffer for a max of 60 frames (i.e., what was once delay_frames is now fixed at 60).
  • Stall if you're ahead by three real frames worth of replay frames (i.e., if it would be a significant stall to rewind for new data)
  • Let catch-up detection keep things generally in sync.

Todo

• Infinite testing, of course!
• Bugfixes:
  • Prevent changing pad devices while netplay peers are connected.
• If netplay host is unset, pop up a dialog to request it when the netplay "connect to host" button is pressed.
• Server admin menu for kicking/demoting users etc.
• Public matchmaking server.
• Super-future: Netplay input support for devices other than the simple retropad.
• Maybe: Rewindless mode. This is strictly-wrong Netplay, but with frame CRCing, can work fine. If one disabled both rewinds and frame CRCing, one would be left with ZSNES-style netplay (no attempt at sync)
• Maybe: Support for input grappling (two players controlling the same input device). Useful for e.g. handhelds.

Suggestions

• Chat
• Proto-matchmaking: Advertisement server.
• Latency reporting ala tab menu in multiplayer games.
• Faster netplay with delay_frames=0 by introducing input latency.