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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.

Todo

• Infinite testing, of course!
• Prevent changing pad devices while netplay peers are connected.
• Server admin menu for kicking/demoting users etc.
• Netplay input support for devices other than the simple retropad.
• Other netplay modes: Master/slave mode, in which the client only acts on its own input when the server says so; and streaming mode, which is master/slave mode but with video/audio streaming instead of the actual core.
• 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. Certainly necessary for e.g. keyboards.

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.
• Remove conventional stalling. So long as all parties catch up, stalling helps no one. The server now demands stalling when one player is ahead and another is behind.
• If netplay host is unset, pop up a dialog to request it when the netplay "connect to host" button is pressed.
• Optional input latency to counter network latency.
• Public matchmaking server (not written by me).
• Netplay-based recording/replay.

Suggestions

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