How Multiplayer Games Use UDP for Movement Sync

Online multiplayer games rely on the User Datagram Protocol (UDP) to handle fast-paced player movement synchronization where low latency is critical. Unlike protocols that guarantee packet delivery, UDP prioritizes speed and immediate data throughput, allowing game clients and servers to exchange position data continuously with minimal delay. To compensate for UDP’s lack of built-in reliability, game developers implement techniques such as client-side prediction, server reconciliation, interpolation, and custom packet ordering to ensure smooth, accurate, and cheat-resistant movement.

Why Games Choose UDP Over TCP

The Transmission Control Protocol (TCP) guarantees that every packet arrives intact and in order. If a packet is lost, TCP halts further data processing until that packet is retransmitted—a phenomenon known as head-of-line blocking. In fast-paced multiplayer games, a delayed position update from 100 milliseconds ago is useless because newer data has already superseded it.

UDP eliminates connection overhead and retransmission delays. It allows the game to stream position, velocity, and orientation updates as fast as possible. If a packet drops, the game simply uses the next incoming packet, keeping the gameplay fluid and responsive.

Client-Side Prediction

To prevent the local player from feeling input lag, games do not wait for the server to confirm movement.

1. Local Execution: When a player presses a movement key, the client immediately updates the local character’s position on the screen.
2. Sending Inputs: Simultaneously, the client sends a UDP packet containing the inputs (e.g., movement direction, jump state, sequence number, and timestamp) to the server.
3. Instant Feedback: The player experiences zero latency in their own controls, even with high network ping.

Authoritative Server Reconciliation

Because the server is the ultimate authority to prevent cheating and maintain game state consistency, it validates all player actions.

• Server Simulation: The server receives the client’s inputs, simulates the movement based on the game’s physics rules, and calculates the true position.
• State Broadcast: The server broadcasts the authoritative game state back to the client via UDP.
• Correction (Reconciliation): When the client receives the server’s update, it compares the server position with its own historical record for that specific tick. If a discrepancy exists (due to collisions, external forces, or packet loss), the client snaps or blends the character to the server’s position and replays any inputs that occurred after that tick.

Entity Interpolation and Extrapolation

While client-side prediction handles the local player, rendering other players smoothly requires additional synchronization techniques:

• Interpolation (Snapshot Buffering): Instead of rendering other players at the exact moment a UDP packet arrives, the client buffers a few snapshots (typically 50–100ms) and interpolates between the two most recent confirmed positions. This creates smooth, continuous visual motion despite jitter in UDP delivery.
• Extrapolation (Dead Reckoning): If network packets are dropped or delayed, the client projects a remote player’s future position based on their last known velocity, acceleration, and direction until a new authoritative packet arrives.

Managing UDP’s Unreliability

Because raw UDP does not manage packet order or loss, game engines build lightweight reliability layers on top of it:

• Sequence Numbers: Each packet includes an incrementing sequence ID. The receiver discards any packet that arrives with a sequence number older than the most recently processed update.
• Delta Compression: To conserve bandwidth, the server sends only the differences (deltas) between the current state and the last acknowledged state of the world.
• Redundant Data: Critical movement data (such as the last few inputs) is often bundled redundantly across consecutive UDP packets so that a single dropped packet does not interrupt game logic.