During every new connection the MLAPI performs a handshake on top of the one(s) done by the transport. This is to ensure that the
NetworkConfig's match up between the Client and Server. In the
NetworkConfig you can specify to enable
ConnectionApproval. Connection approval will let you decide on a per connection basis if the connection should be allowed. Connection approval also lets you specify the player prefab to be created, allowing you to override the default behaviour on a per player basis.
ConnectionApproval is true you are also required to provide a callback where you put your approval logic inside.
connectionData parameter is any custom data of your choice that the client should send to the server. Usually, this should be some sort of ticket, room password or similar that will decide if a connection should be approved or not. The
connectionData is specified on the Client side in the
NetworkingConfig supplied when connecting.
The ConnectionData will then be passed to the server and it will decide if the client will be approved or not.
The MLAPI uses a callback system in order to allow for external validation. For example, you might have a steam authentication ticket sent as the ConnectionData (encrypted and authenticated by the MLAPI) that you want to validate against steams servers. This can take some time. If you don't call the callback method within the time specified in the
ClientConnectionBufferTimeout configuration the connection will be dropped. This time starts counting when the transport has told the MLAPI about the connection. This means that you cannot attack the MLAPI by never sending the buffer, it will still time you out.
If connection approval is enabled. Any messages sent before a connection is setup are silently ignored.
If Encryption is enabled, the connection handshake with the buffer will be encrypted AND authenticated. (AES-256 encryption and HMAC-SHA-256 authentication).
If the key exchange is not signed, a man in the middle attack can be done. If you plan on sending authentication tokens such as steam tickets. It's strongly suggested that you sign the handshake.