Unique identifier for this connection that is assigned by the transport layer.
On a server, this Id is unique for every connection on the server. On a client this Id is local to the client, it is not the same as the Id on the server for this connection.
Transport layers connections begin at one. So on a client with a single connection to a server, the connectionId of that connection will be one. In NetworkServer, the connectionId of the local connection is zero.
Clients do not know their connectionId on the server, and do not know the connectionId of other clients on the server.