Network Protocol

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This article describes the network protocol used by Mario Kart Wii.


The following terms are used in this article.

Game Features
Term Description
ARP Address Resolution Protocol, a network routing protocol.
cfc The console friend code. It is displayed as 16 digits in the format "XXXX-XXXX-XXXX-XXXX".
Client A Wii system that takes part in the race.
fc, fc8 The friend code of the first user of a client. Sometimes it is extended by ".1" or ".2" to identify the user of the client. A friend code is displayed as 12 digits using format "XXXX-XXXX-XXXX".
GPCM The client server.
GPSP The server that retrieves friends.
Guest All non-host clients.
Home client The observed client where the network traffic comes from.
Host The client who has opened the room.
MASTER The master matchmaking server.
Mii The Mii avatar of a user.
MS The matchmaking retrieving server.
N-server One of Nintendo's servers.
NAS, NASWII The login server.
NATNEG The server that enables client to client connections behind firewalls. It stands for Network Address Translation NEGotiation.
Packet A packet is a technical network packet. Mario Kart Wii uses ARP, TCP and UDP (including name resolution) packets.
Player A player that takes part of the race, with a maximum of 12. Players are all users of all clients.
Player ID An alternative name for Profile ID.
Profile ID, pid, fc4 The Profile ID is a short version of the friend code with only the lowest 4 bytes (32 bit). It is used in the network protocols for identification.
TCP Transmission Control Protocol, a network protocol.
Record A packet can be split into logical records. Each record has its own meaning and its own data structure.
UPD User Datagram Protocol, a network protocol.
User A user of a client. Each client supports 1 or 2 users. The users of the client are numbered 1 and 2.


The Mario Kart Wii traffic starts with some name resolutions. Then, it will use TCP connection to the servers and UDP packets for the peer-to-peer data transfer. First it connects the N-server (Wiimmfi server). In a room or while racing, all consoles communicate directly to each other. In a race, only some active records are send to an N-server.

Connecting to Servers

When connecting to the Nintendo Wi-Fi Connection, the console starts with DNS queries to locate the IP addresses of the following servers:


The Wiimmfi equivalents are the same servers with "" replaced with "". Then it starts the communication with the servers and the first packet goes to More details about the servers can be found here.

Point to Point Communication

The Wii uses a frequency of 59.94Hz (NTSC) as time base. All clients send a status packet to one of the other clients every second pulse (around 1/30 seconds). For example, when there are three clients A, B and C, then A will send packets to B and C in turn every 1/30 seconds. This means that the individual status update is only done every 1/15 seconds. This update time is reduced to 11/30 seconds when there are 12 clients in a race. This is the main reason for lag when racing with many players. The average packet length is about 220 bytes, so the total traffic is about 8 KB/s in both directions, independent of the number of clients.

Main records

Each UDP packet can be split into records, which are logical units. Sometimes, a packet is prefixed by a SLOT record to reduce a client's needed upload bandwidth. The CHECKSUM algorithm is described here. RACE records are sent from every client to all other clients during a race including the race preparation. The following table shows the packet structure and the different records.

Name Size Description
HEADER 0x10 The packet header.
RACEHEADER_1 0x28 Race header.
RACEHEADER_2 0x28 Another race header.
ROOM 0x04 Room records are only sent in room events. The first byte decides the sub type:
  • 0: Unknown.
  • 1: Event start sent by the host (GP, Team, Balloon or Coin).
  • 2: Unknown.
  • 3: Unknown.
  • 4: Send room message (online chat).
SELECT 0x38 This record is sent while selecting character and track. It is always placed before a USER record. One byte decides the phase.
  • 0: Race preparation.
  • 1: Character selection.
  • 2: Track selection.
RACEDATA 0x40 Actual race data. May exist twice if there are two players on one console.
USER 0xC0 User information including a friend code and 2 Miis for both users of the client.
ITEM 0x08 Record indicating the item status.
EVENT ≥ 0x18 Record indicating the event status.

Additional Records

The following table shows additional records.

Name Record ID Length Description
SLOT 46fc.570x 0x08 The following message is related to the client slot "X". This is mainly used in combination with ANNOUNCE and USER to tie information to specific client slots. If a client has bandwidth problems, it can use another client as a proxy to reduce its own traffic during the race. Such indirect packets are also prefixed by a SLOT record.
STATUS bb49.cc4d 0x14+N A status record with five different extensions. Byte 0x08 describes the status type. One task is to select and advice client slots to clients.
HANDSHAKE fdfc 0x14+N NATNEG records. Only CONNECT_PING is sent peer-to-peer, all other records via the server.
QUIT fefe.68 0x03 This record is sent when a client quits a room or a race. It is not prefixed by a CHECKSUM record.


This section describes what peer-to-peer data is transmitted by the clients during the different stages of a match.


When a player wants to enter a friend room, it sends a "GMCP90" type 1 message to the MASTER server. This asks the server for permission to enter the room, which will either be answered with type 2 (access granted) or type 3 (denied, room full). When the host allows the new guest to enter, they start doing NATNEG requests to open a peer-to-peer-connection.

After NATNEG is done, the players start HANDSHAKE with a few FEXX status messages. After exchanging these messages, they send each other an empty race packet (ANNOUNCE) to indicate the successful connection.

After that, the new player appears in the room and starts doing NATNEG with all other players. Because NATNEG is very slow and does not run in threads, only one player can join a room at the same time. This is the reason why starting a match takes a long time when the host starts a match as soon as everyone is in the room. All the peer-to-peer connections have not been set up yet, which are about 60 connections in a full room.


As soon as the client has found a room via MS, it sends an "asking for entrance" status message. When the room host is in the friend list, GPCM90 messages are used. If the host is not in the friend list, STATUS messages are being proxied by MS and/or MASTER.

Then, same as in the room, NATNEG and a few status messages are done. The Wii then sends an empty record header (ANNOUNCE) to the host and starts sending packets containing a user record.

During the viewing phase, race packets with only RACEHEADER_1 and EVENT are sent.

Start of Event

At event begin, the host sends a ROOM message to indicate the start of an event. The players select their characters and vehicle, and the track selection starts. The track selection has three phases.

  1. SELECT/PREPARE - The waiting screen until all consoles are ready.
  2. SELECT/WAITING - The track selection screen and waiting until everyones has voted for a track.
  3. SELECT/LOTTERY - The roulette that determines the track. This is already determined before it starts.


Before the countdown, the host sends a "ready?" request to all clients. Until all clients have answered this, the loading icon appears in the bottom right corner of the screen. When the clients are syncronized and all ready for the event, the countdown and the race begins.


As soon as a player finishes, he sends his final time to all the other players. Then the other players have 30 seconds left to complete the race, given that the track-specific minimum time race length has been completed. When the race is finished by either everyone crossing the finish line, 5 minutes being over, or the 30 seconds being reached, the "finished" phase is activated. The rankings will show and the next race will begin.