It's a TCP stream with protobuf messages.

Each message in the stream is encapsulated with a 8-byte header, of which 4 bytes are message length and 4 bytes message type (int32_t). Available message types are defined in server/client_parser.h. Protobuf payload is defined in server/c2s.proto.

+--------------+--------------+----------------------+
| length (4B)  | type (4B)    | message (length-4B)  |
+--------------+--------------+----------------------+

The endianity is configurable in bits.h (server) and proto.ENDIAN (libclient)