Network Protocol
This page describes the network protocol used by Mario Kart Wii.
Introduction
The analysis of the data is done with the tool mkw-ana. So the naming of mkw-ana for objects (records, stages, ...) is used here. Because of the very early phase of the analysis, names and structures may completely change.
You can discuss this topic at Wii-Homebrew.com (English and German).
Terms and definition
General description
The Mario Kart Wii traffic starts with some name resolutions. Then it will use TCP connection to the servers and UDP packets for the p2p data transfer.
First it connects the Wiimmfi server (N-server). In a room or while racing, all Wiis communicate directly to each other. In a race, only some alive records are send to a N-server.
Connecting the Wiimmfi-Server(s)
When entering the Mario Kart WFC, the Wii starts with DNS queries to find out the IP addresses of the following servers:
- gpcm.gs.nintendowifi.net
- gpsp.gs.nintendowifi.net
- gamestats.gs.nintendowifi.net
- gamestats2.gs.nintendowifi.net
- mariokartwii.available.gs.nintendowifi.net
- mariokartwii.natneg1.gs.nintendowifi.net
- mariokartwii.natneg2.gs.nintendowifi.net
- mariokartwii.natneg3.gs.nintendowifi.net
- mariokartwii.master.gs.nintendowifi.net
- mariokartwii.gamestats.gs.nintendowifi.net
- mariokartwii.gamestats2.gs.nintendowifi.net
- mariokartwii.ms19.gs.nintendowifi.net
- naswii.nintendowifi.net
("nintendowifi.net" replaced with "wiimmfi.de" for the Wiimmfi server)
Then it starts the communication with the servers and the first packet goes to mariokartwii.available.gs.nintendowifi.net.
→ More Details about the servers
Point to point communication
The Wii uses a frequency of 59.94Hz (based on NTSC, about 1/60s) as time base. All clients send a status packet to one of the other clients every second pulse (~1/30s). When there are for example 3 clients A, B and C, then A sends first a packet to B and then, 1/30s later, a packet to C, and again 1/30s later, the next packet to B. This means, that the individual status update is only done every 1/15s. This update time is reduced to 11/30s (~1/3s), when there are 12 clients in a race. And that's the reason for the many lagging effects when playing 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 ("RACE" records)
Each UDP packet can be split in records. Records are logical units.
A race packet consists of records. Sometimes, a packet is prefixed by a SLOT-record to reduce a client's needed upload bandwidth.
After SLOT (if it exists), the main race header (16 byte) follows:
typedef struct header_t
{
/*00*/ u32 checksum_start // always 0x0000 0000 (start of checksum part)
/*04*/ u32 checksum // Checksum record
/*08*/ u8 header_len; // always 0x10 (length of this header)
/*09*/ u8 raceheader1_len; // length of the first race header. 0x28 or 0x00
/*0a*/ u8 raceheader2_len; // length of the 2nd race header. 0x28 or 0x00
/*0b*/ u8 room_select_len; // length of the ROOM or SELECT record. 0x04 (for room), 0x38 (for select), or 0x00
/*0c*/ u8 racedata_len; // length of RACEDATA. 0x40 (for 1 player), 0x80 (for 2 players) or 0x00
/*0d*/ u8 user_len; // length of USER. 0xc0 or 0x00
/*0e*/ u8 item_len; // length of ITEM. 0x08 (for 1 player), 0x10 (for 2 players) or 0x00
/*0f*/ u8 event_len; // length of EVENT. 0x00, or >= 0x18
/*10*/
}
__attribute__ ((packed)) header_t;
(The CHECKSUM algorithm is described here: CHECKSUM)
RACE records are send 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, including:
- A name used by mkw-ana.
- Data length
- Description.
| Name | Length | Description |
|---|---|---|
| HEADER | 0x10 | The packet header described above |
| RACEHEADER_1 | 0x28 | Race header |
| RACEHEADER_2 | 0x28 | Another race header |
| ROOM | 0x04 | The first byte decides the sub type:
Room records are only sent in room events. |
| SELECT | 0x38 | This record is send while selecting driver and track. It is always placed before an USER record. One byte decides the phase:
|
| RACEDATA | 0x40 | Actual race data. May exist twice if there are two players on one console. |
| USER | 0xc0 | A user information including a FC 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:
- A name used by mkw-ana.
- Value of the first 2-4 bytes (Record ID, big endian, hex).
- Length of the record.
- Description.
| Name | Record ID | Length | Description |
|---|---|---|---|
| SLOT | 46fc.570x | 0x08 | The following message is related to client slot x (default: for the sender). 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 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 5 different extensions. Byte 0x08 describes the status type. One task is to select and advice client slots to clients. |
| NATNEG | fdfc | 0x14+N | NATNEG records. Only "CONNECT_PING" is send peer-to-peer, all other records via server: Natneg. |
| QUIT | fefe.68 | 0x03 | This Record is sent when a client quits a rooms or a race. This record isn't prefixed by a CHECKSUM-Record. |
Stages
Here will be described what data is transmitted by the clients during the different stages of the game. Only peer-to-peer data will be mentioned here, not anything server-related.
Room
When a player wants to enter a friend room, he sends a "GMCP90"-Message type 1 (Ask for entrance, see server documentation for more details) via MASTER, this 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 to open a peer-to-peer-connection.
After NATNEG is done, the players start handshaking with a few FEXX-Status-Messages. After exchanging status messages, they send eachother 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 the other players. Because NATNEG is very slow and does not run in threads (only one-by-one), only one player can join a room at the same time (others get 86420 after a timeout). 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 aren't set up yet - which are about 60 connections in a full room.
Global match
As soon as the client has found a room via MS, it sends a "Asking for entrance"-status message (same as in the room). When the room host is in the friend list, GPCM90-Messages are used. If the host is not in the friend list (more likely), STATUS-messages are being proxied by MS/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 an USER record.
During the VIEW phase (you are seeing the other players racing), race packets with only RACEHEADER_1 and EVENT are sent.
Event begin
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:
Phase 1: SEL/PREPARE: You'll see the string "Preparing race ..." and wait until everyone is ready.
Phase 2: SEL/WAITING: You'll see the track selection screen and can choose a track to drive, or you are waiting for others to complete their track choice. At this point, the engine class is already determined.
Phase 3: SEL/LOTTERY: Everyone has chosen a track and the lottery begins. As soon as the lottery begins, the winning user and track is already know to all clients.
Countdown
Before countdown, the host sends a request "Ready for event?" to all clients. Until all clients have answered "ready!", you'll see the loading icon in the bottom right corner of the screen.
If someone has a broken image or is in a CT room without having CTs, you'll all have to wait until he turns off his wii, as his console won't answer with "ready".
When the clients are syncronized and all ready for the event, the countdown and the race begins.
Finished
As soon as a player finishes, he sends his final time to all the other players. Now, the other players have 30 seconds left to complete the race (but a race has a minimum time of 2 minutes and a maximum time of 5 minutes). When the race is finished (either by everyone crossing the finish line, 5 minutes being over or the 30-seconds being reached, the phase "All finished" is activated. You'll see the rankings and the next race will begin.
Links
- Wiimms mkw-ana (tool)
- Wiimmfi-Project (Project to create own MKWii servers)