The connection-mode (passive or active) of data-channels.
When the connection-mode is active, the server will initiate connections to the client, meaning that the client must open a socket and wait for the server to connect to it. This often causes problems if the client is behind a firewall.
When the connection-mode is passive, the client will initiates connections to the server, meaning that the client will connect to a particular socket on the server. This is generally used if the client is behind a firewall.