+/**\r
+ Start the MTFTP session to download.\r
+\r
+ It will first initialize some of the internal states then build and send a RRQ\r
+ reqeuest packet, at last, it will start receive for the downloading.\r
+\r
+ @param Instance The Mtftp session\r
+ @param Operation The MTFTP opcode, it may be a EFI_MTFTP4_OPCODE_RRQ\r
+ or EFI_MTFTP4_OPCODE_DIR.\r
+\r
+ @retval EFI_SUCCESS The mtftp download session is started.\r
+ @retval Others Failed to start downloading.\r
+\r
+**/\r