Why Animal Crossing’s Online Play Stutters: Inside Game Sync and NAT Traversal

Game Synchronization Basics

Two common approaches are used in multiplayer games. Lockstep synchronization sends each player’s input to the other peers and reconstructs the game state from the ordered input sequence; any lost input causes severe desynchronization. State synchronization transmits the entire game state after each action, tolerating occasional lost updates while still converging on a consistent state.

High‑frequency action games (e.g., MOBA) typically use lockstep, whereas turn‑based or card games favor state sync.

Communication Architecture

Most games employ a host‑client model. One player acts as the authoritative host, receiving inputs from other clients, validating them, and broadcasting the resulting state or input sequence. The host must remain online for the session, which can overload it as player count grows, prompting the use of external dedicated servers for state synchronization.

NAT Traversal

Home routers use Network Address Translation (NAT), creating four main NAT types that affect peer‑to‑peer connectivity.

Full‑cone NAT

Once an internal address

iAddr:iPort

is mapped to an external address

eAddr:ePort

, any external host can send packets to

iAddr:iPort

via

eAddr:ePort

.

(Address‑restricted‑cone) NAT

Mapping is the same as full‑cone, but an external host can send packets only if the internal host has previously sent a packet to that external address.

Port‑restricted‑cone NAT

Similar to address‑restricted, but the external host must match both the IP address and port that the internal host previously contacted.

Symmetric NAT

Each outbound request from the same internal IP/port to a different destination receives a unique external mapping, and only the destination that received a packet can send back.

From these definitions, the following conclusions can be drawn:

Combinations involving Symmetric NAT (C‑D, D‑C, D‑D) cannot establish bidirectional communication.

Players behind Full‑cone or restricted‑cone NAT (A/B) can generally communicate with any other type.

Animal Crossing’s Online Implementation

Several quirky behaviors observed in the game stem from its networking design:

Island decorations cannot be changed while online.

When a new player joins, all existing players pause for a noticeable period.

Leaving the island triggers a synchronized save across all clients.

If a player disconnects abruptly, the island state rolls back to the last consistent snapshot.

Players cannot enter or exit the island while a dialogue window is open.

The likely implementation follows these steps:

1. Host opens a lobby and registers its external IP/port (e.g., 1.1.1.1:10001 ) with Nintendo’s matchmaking service.

2. Client discovers the host’s address, registers its own external IP/port (e.g., 2.2.2.2:10002 ), and attempts to punch through NAT using the host’s information.

3. After successful bidirectional communication, the host creates a snapshot of the entire island state and shares it with the joining client.

4. While the snapshot is being transferred, existing players wait; no new state‑changing actions are allowed.

5. Once the new player is fully loaded, gameplay resumes. Low‑frequency actions that affect persistent game data (e.g., digging, giving items) use lockstep synchronization with the host, while high‑frequency visual updates (e.g., running) are handled locally without strict ordering.

6. On normal exit, the host generates a final snapshot and each client downloads its portion to persist locally. On abnormal disconnect, the host rolls back all clients to the last saved snapshot, explaining the observed data rollback.

Common Questions

Is Nintendo’s online service poor?

The quirks arise from technical constraints rather than negligence; Nintendo balances cost and performance, and as a console publisher it does not operate a specialized distributed database service.

Why don’t game studios run their own servers?

Global server deployment is expensive and complex. Only large online‑focused publishers can justify the infrastructure; console games often rely on peer‑to‑peer or limited matchmaking services to keep costs manageable.

References

Wikipedia: Network address translation

Peer‑to‑Peer (P2P) communication across middleboxes

Overview of network synchronization techniques in online games