This is a compatibility layer for translating V3 commands into V2/V1 format, currently for internal use.
Starting from V3, the protocol uses JSON to carry messages between the server and the client, and command arguments are now recorded using a Map.
These designs make V3 incompatible with V2/V1, so we need a translation layer to translate V3 commands into the V2/V1 format.
So this is here.