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The big picture

For implementation details see files located at lib/network directory.

VCMI uses connection using TCP to communicate with server, even in single-player games. However, even though TCP is stream-based protocol, VCMI uses atomic messages for communication. Each message is a serialized stream of bytes, preceded by 4-byte message size:

int32_t messageSize;
byte messagePayload[messageSize];

Networking can be used by: - game client (vcmiclient / VCMI_Client.exe). Actual application that player interacts with directly using UI. - match server (vcmiserver / VCMI_Server.exe / part of game client). This app controls game logic and coordinates multiplayer games. - lobby server (vcmilobby). This app provides access to global lobby through which players can play game over Internet.

Following connections can be established during game lifetime:

  • game client -> match server: This is main connection for use during gameplay, created once player requests to move from main menu to pregame / match lobby (e.g. after pressing New Game / Load Game)
  • game client -> lobby server: This connection is used to access global lobby, for multiplayer over Internet. Created when player logs into a lobby (Multiplayer -> Connect to global service)
  • match server -> lobby server: This connection is established when player creates new multiplayer room via lobby. It is used by lobby server to send commands to match server

Gameplay communication

For gameplay, VCMI serializes data into a binary stream. See Serialization for more information.

Global lobby communication

For implementation details see: - game client: client/globalLobby/GlobalLobbyClient.h - match server:server/GlobalLobbyProcessor.h - lobby server: `client/globalLobby/GlobalLobbyClient.h

In case of global lobby, message payload uses plaintext json format - utf-8 encoded string:

int32_t messageSize;
char jsonString[messageSize];

Every message must be a struct (json object) that contains "type" field. Unlike rest of VCMI codebase, this message is validated as strict json, without any extensions, such as comments.

Communication flow

Notes: - invalid message, such as corrupted json format or failure to validate message will result in no reply from server - in addition to specified messages, match server will send operationFailed message on failure to apply player request

New Account Creation

  • client -> lobby: clientRegister
  • lobby -> client: accountCreated


  • client -> lobby: clientLogin
  • lobby -> client: loginSuccess
  • lobby -> client: chatHistory
  • lobby -> client: activeAccounts
  • lobby -> client: activeGameRooms

Chat Message

  • client -> lobby: sendChatMessage
  • lobby -> every client: chatMessage

New Game Room

  • client starts match server instance
  • match -> lobby: serverLogin
  • lobby -> match: loginSuccess
  • match accepts connection from client
  • client -> lobby: activateGameRoom
  • lobby -> client: joinRoomSuccess
  • lobby -> every client: activeAccounts
  • lobby -> every client: activeGameRooms

Joining a game room

See #Proxy mode

Leaving a game room

  • client closes connection to match server
  • match -> lobby: leaveGameRoom

Sending an invite:

  • client -> lobby: sendInvite
  • lobby -> target client: inviteReceived

Note: there is no dedicated procedure to accept an invite. Instead, invited player will use same flow as when joining public game room


  • client closes connection
  • lobby -> every client: activeAccounts

Proxy mode

In order to connect players located behind NAT, VCMI lobby can operate in "proxy" mode. In this mode, connection will be act as proxy and will transmit gameplay data from client to a match server, without any data processing on lobby server.

Currently, process to establish connection using proxy mode is: - Player attempt to join open game room using joinGameRoom message - Lobby server validates requests and on success - notifies match server about new player in lobby using control connection - Match server receives request, establishes new connection to game lobby, sends serverProxyLogin message to lobby server and immediately transfers this connection to VCMIServer class to use as connection for gameplay communication - Lobby server accepts new connection and then sends reply to client using joinRoomSuccess message. - Game client receives message and establishes own side of proxy connection - connects to lobby, sends clientProxyLogin message and transfers to ServerHandler class to use as connection for gameplay communication - Lobby server accepts new connection and moves it into a proxy mode - all packages that will be received by one side of this connection will be re-sent to another side without any processing.

Once the game is over (or if one side disconnects) lobby server will close another side of the connection and erase proxy connection