GregOrigin - Rollback Core Pro: Accessible Netcode Framework for Fighting & Platform Games

[andgregori (Andras Gregori)]

Valhalla Rumble: one of the sample multiplayer games shipped with 0.7.0:

No more input lag & latency in Unreal Engine: innovative rollback netcode framework (UE5.5 - 5.8)

[andgregori (Andras Gregori)]

A video tutorial is now available:

No more input lag & latency in Unreal Engine: innovative rollback netcode framework (UE5.5 - 5.8)

[andgregori (Andras Gregori)]

[0.8.0] update, 2026-10-10

Legend: demos · fix · act on this in your own project · behaviour change · covered by tests · soak harness · docs · new

Animation montages now roll back, an offline match can rewind, and components can own rollback
state. A second sample game, Rewind Arena, shows all three in 3D, and shows two peers playing
over the UDP transport after finding each other through an OnlineSubsystem Null LAN session.
Supported engines are unchanged (5.5, 5.6, 5.7, 5.8; 5.8 primary) and so is the wire protocol
(v6): a 0.8.0 peer plays a 0.7.0 peer as long as both run the same game.

Added — runtime

• URollbackAnimationComponent — montages as rollback state. Each character’s montage
  timeline (which montage, from which simulation frame, at which integer play rate, from which
  section and offset, blending from what) lives in AnimState, which the component contributes to
  the actor’s snapshot. A rollback, a rewind or a determinism audit restores the pose exactly —
  mid-swing, mid-blend — because the pose shown is evaluated from the state for the frame being
  presented (URollbackAnimInstance, Native drive mode), not advanced by an animation clock.

  • Simulation API, all taking the frame being simulated: PlayMontage / PlayMontageAsset,
    JumpToSection, StopMontage, DelayTimeline (hit-stop that also holds the windows and root
    motion), IsInNotifyWindow, WasNotifyFired, GetSectionAt, GetMontageFrame,
    GetRootMotionDelta (actor space; RootMotionAxes handles the UE mannequin’s −90° mesh).
  • MontageSync drive mode keeps your own Anim Blueprint: the component holds the montage paused
    and sets its position every frame.
  • Corrections you can see: AnimationCorrectionCount and OnAnimationCorrected(Frame, Before, After) report every rollback that changed what a character was playing.
  • PresentationFrameOverride scrubs the pose to any frame the timeline covers.

  RollbackCorePro.Animation.*.

• URollbackMontageData, URollbackNotifyWindow, Bake Rollback Montage Data. Gameplay never
  asks the animation system anything. Each montage carries a baked per-frame table (animation
  metadata, so it cooks with the asset): length, sections and links, notify windows, blend times and
  cumulative root translation at the project’s fixed tick rate. Rollback Window notify states
  (URollbackNotifyWindow) let designers place hit / cancel / invulnerability windows on the
  animation. Bake from the Content Browser (right-click montages) or
  URollbackMontageBakeLibrary::BakeMontages; IsMontageBakeCurrent reports a stale bake. Montages
  without data are baked at first use, with a warning, when bAllowRuntimeBake is on.
  RollbackCoreProEditor.BakeLibrary.RejectsUnusableMontages, RollbackCorePro.Animation.BakedMontageLookups.

• IRollbackStateContributor — state that lives on a component. A component that implements it
  names its own properties; the actor’s URollbackStateComponent saves, restores, checksums and diffs
  them after the actor’s own (ordered by component name, then property), names them
  Component.Property in desync reports, and calls OnRollbackStateRestored(Frame) after a load.
  New on the state component: GetTrackedPropertyNames() and RefreshTrackedProperties().
  RollbackCorePro.State.*Contributor*.

• URollbackManager::RewindToFrame(Frame) — offline rewind without replay. Every entity is
  restored to the frame, which becomes the present; the next AdvanceFrame() continues on a new
  branch, and each entity’s history after the frame is discarded
  (IRollbackEntity::DiscardRollbackHistoryAfter, a new virtual with an empty default). Refused while
  a peer is connected, during a rollback, and for a frame without a complete snapshot. Plus
  CanRewindToFrame, GetOldestRewindFrame and RewindCount (rewinds are not rollbacks).
  RollbackCorePro.Rewind.*.

• URollbackManager::OnRollbackBegin / OnRollbackEnd (native multicast, (RollbackFrame, CurrentFrame)): fired by every accepted rollback before state is restored and after the replay
  caught up, never by a rewind. Presentation that wants to know what a rollback changed records its
  view on Begin and compares on End — the animation component detects corrections this way.

Fixed

• Struct members without SaveGame were silently not rolled back. Snapshots serialize with
  the SaveGame flag, and the engine then skips any member of a tracked struct that is not
  UPROPERTY(SaveGame) — on save and on restore, so no checksum ever disagreed. The state component
  now logs a warning naming each such member when it builds its schema. Natively serialized structs
  (FVector, FQuat, …) are written whole and are unaffected. If the warning names one of your
  members, mark it SaveGame (or confirm it is meant to stay out).
  RollbackCorePro.State.StructMembersWithoutSaveGameAreReported.

• An empty UDP datagram stopped the transport reading for the rest of that pump. PumpSocket
  treated a zero-length datagram like “nothing to read” and stopped, so every packet queued behind
  it waited for the next pump — and the empty datagram itself was never counted. It is now counted,
  rejected and skipped. Found by the new malformed-packet test; RecvFrom returns false for “no
  data” on both the BSD and EOS socket subsystems, so a successful zero-byte read is always a real
  empty packet. RollbackCorePro.Network.MalformedPacketsAreRejected — 53 hostile datagrams
  (empty, truncated, oversized and negative counts, NaN axes, negative frames, unknown types and
  flags, zero nonce, foreign magic, seeded random bytes): every one rejected exactly once in a single
  pump, no peer created, no input buffered, and a real peer still connects afterwards.

Added — Rewind Arena, a 3D sample game

• Rewind Arena (RollbackArenaGame, a non-Shipping DeveloperTool module; content in
  /RollbackCorePro/ArenaGame). Open Map_RewindArena and press Play, or run Rollback.PlayArena.
  Two UE5 mannequins, Ember and Frost, fight on a rune-lit platform at sunset, animated only by
  montages that roll back.

  • Modes: Versus CPU (three difficulties), Local Versus, Training with hold-to-rewind (R /
    left trigger runs fighters, montages and clock backwards; let go to play on), Netplay Lab,
    Online, Showcase, Options, About.
  • Fighting from the montage: a Jab → Cross → Hook chain that may continue only inside its
    RB_Cancel windows, a Roundhouse, a meter-powered Rush carried by baked root motion, block with
    chip, sidesteps invulnerable inside RB_Invuln, knockdowns, hit-stop via DelayTimeline. Every
    hit window is a notify state on the animation.
  • Rollback Lens: a ghost of every mispredicted pose, drawn from the correction’s Before
    timeline, and a panel with rollbacks, resimulation depth, montage corrections, ghosts drawn,
    replayed effects skipped, mispredicted inputs or desyncs, and the link.
  • Online, two peers: Host LAN Match advertises a Null OSS session through the plugin’s
    HostRollbackSession node; Find LAN Matches lists sessions (FindRollbackSessions); joining
    one (JoinRollbackSession) handshakes the UDP transport and both machines start the fight — host
    seat 0, joiner seat 1. Direct IP is the fallback. Match rules are rollback state, ticked after
    both fighters, so trades and rounds replay together; rematch needs both players. If the rival
    disconnects, the match stops cleanly and says so instead of stalling.
  • Presentation reads the simulation and never writes it: an orbit / fight camera, impact
    sparks and rings deduplicated by FRollbackCosmeticEventFilter, a rewind grade, a glyph halo
    above the arena that spins backwards while rewinding, flickering braziers, CC0 audio reused from
    Valhalla Rumble. HUD and menus are UMG widgets painted in NativePaint, so Movie Render Queue
    records them.
  • Content (~47 MB): the UE5 mannequins from Unreal Engine’s templates (Epic Games), 19 original
    animations and 17 baked montages authored in Blender by script, an original procedural arena, all
    authored in UE 5.5 (Tools/ArenaArt/Author-ArenaAssets.ps1) so it loads on every supported
    engine.

  20 tests under RollbackArenaGame.*: Combat, Assets, Animation, Fighter,
  Rewind, Online, Flow, Presentation.

• Demo/Run-ArenaP2PDemo.ps1 — host and joiner side by side on one machine (-Auto: the AI
  plays both seats; -Packaged: a packaged build).

• Tools/Verify-ArenaOnline.ps1 — the same two processes headless, CI-safe: fails unless the
  joiner finds the session through Null, both peers start and keep simulating, neither reports a
  desync and at least one rolled back. First run on UE 5.8: 4,560 frames per peer, 2,223 rollbacks,
  483 montage corrections, 0 desyncs.

• Console commands Rollback.PlayArena, ArenaVsAI, ArenaLocal2P, ArenaTraining, ArenaLab,
  ArenaShowcase ([ember|frost] [auto]), ArenaRewind, ArenaNetHost, ArenaNetJoin,
  ArenaDirectHost, ArenaDirectJoin, ArenaMenu.

Changed

• The Setup Wizard’s Package The Sample packages both samples: it adds
  /RollbackCorePro/VikingGame and /RollbackCorePro/ArenaGame to Directories To Always Cook and
  both maps to Maps To Cook. A project that packaged only the Viking sample with 0.7 is offered the
  button again. RollbackCoreProEditor.Wizard.PackageTheSampleCooksContentAndMap.
• RollbackCorePro.uplugin lists the RollbackArenaGame module (DeveloperTool, Win64 / Linux /
  Android).

Soak harness: §12.13 and §12.14 closed

• §12.14, the soak’s intermittent permanent divergence, was the harness. It set the local
  pawn’s input once per loop iteration, so when one Tick() caught up several frames the same input
  was scheduled for all of them while the send loop put the correct per-frame inputs on the wire. This
  machine never produced a multi-frame Tick on its own (0 in 8 full soak runs, with and without the
  harness’s repair), so the soak now forces them: RCPSoak.Hitch.CatchUpAdvancesStayInSync
  stalls peer A’s process for 3 frames every 90 and requires zero divergence across 13 catch-up
  advances; RCPSoak.Hitch.ControlWithoutRepairDiverges runs the same hitches with the repair off and
  requires the worlds to diverge (first desync 13 frames after the first catch-up; 61 and 73
  steady-state desyncs). An integration that sends what its state component scheduled
  (GetInputForFrame(Frame), as both samples do) cannot diverge this way.
• §12.13, BurstLoss’s window: bursts are now scheduled (the link goes dark both ways for 13
  frames at 6 s and 13 s) instead of Poisson-distributed, and the window of 32 carries its
  derivation (13 + 2 round trips + input delay = 25, + jitter margin). Six standalone runs: peak
  confirmation lag 20–22 every time (the random model ranged 26–46), deepest rollback 30.
• Tools/Soak-Pacing.ps1 expects 9 tests.

Verification

• 207 automation tests pass on each of UE 5.5.4, 5.6.1, 5.7.4 and 5.8.3 (clean build per engine,
  counted from the engine’s JSON report): 106 RollbackCorePro.* (30 new), 4 RollbackCoreProEditor.*,
  77 RollbackVikingGame.*, 20 RollbackArenaGame.*. Tools/Run-AutomationTests.ps1 now includes
  RollbackArenaGame by default and fails below 200 tests.
• RunUAT BuildPlugin -StrictIncludes (no PCH, no unity): UE 5.8 and 5.7 for Win64 and Android
  (NDK Clang, arm64 + x64), 5.6 for Win64 and Android, 5.5 for Win64 (no Android support installed
  for 5.5). The strict build found one include a unity build had hidden, in a test file; fixed.
• Linux game target on UE 5.8 with Clang 20.1.8: every module, 0 warnings.
• Pacing soak 9/9 on UE 5.8; BurstLoss 6/6 more on its own.
• Tools/Verify-Matchmaking.ps1 and Tools/Verify-ArenaOnline.ps1 (two processes each) pass.
• Every shipped asset loads on UE 5.5 (Check-AssetEngineCompat.ps1: 276 assets ≤ package version
  1013).
• Looked at, not just logged: Rewind Arena’s title, menus, online page, Versus / Showcase fights,
  the Netplay Lab with ghosts, Training’s rewind on UE 5.8; the Showcase and Netplay Lab on UE 5.5; and
  the two-window online demo with both windows on the same frame and 0 desyncs. Looking found three
  things no test had: impact rings three times too large, fighters lit by their own rim term at
  dusk exposure, and HUD / ghost colours that followed the seat instead of the fighter (now pinned by
  RollbackArenaGame.Flow.MenuStartsTheChosenModeAndFighter).

Docs

• New Docs/arena-demo.md. Manual: new sections Animation Rollback and Rewind (later sections
  renumbered), component state and the SaveGame-member rule under State Capture, API and console
  tables. QuickStart: State that lives on a component, Montages that roll back, Rewind
  (offline). FAQ: montages, component state, rewind. credits.md covers Rewind Arena.

[andgregori (Andras Gregori)]

A tutorial is now available for the 0.8.0 update specifically:

No more input lag & latency in Unreal Engine: innovative rollback netcode framework (UE5.5 - 5.8)