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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

Done

(To be PR'd in a later release)

• Better (non-blocking) initial connection, to allow the serving port to be left listening indefinitely.

• 2 player netplay. Not as difficult as it seems, just need an array of buffers, frames and replay.

• Merge spectator mode into net mode.

Todo

• Infinite testing, of course!
• Bugfixes: Sometimes a player joins and everybody ignores their input :(
• Bugfixes: Stalling per connection instead of per whole netplay
• Bugfixes: Transfer input device modes across netplay peers, maybe warn if nobody's actually polling a particular player.
• Password-protected serving.
• Server admin menu for kicking/demoting users etc.
• Public matchmaking server.
• Future: Auto-expanding replay buffer based on latency. Problems relate to limits, when to contract, how to determine if we don't have enough CPU time to handle it, etc.
• 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
• Spectators to a normal netplay game.
• Proto-matchmaking: Advertisement server.
• Latency reporting ala tab menu in multiplayer games.