code: public String checksum(ChecksumAlgorithm algorithm,
                             long offset, long length, String file)
          throws IOException, ServerException {
          String arguments = algorithm.toFtpCmdArgument() + " " +
              String.valueOf(offset) + " " +
              String.valueOf(length) + " " + file;
  
          Command cmd = new Command("CKSM", arguments);
          Reply reply = null;
          try {
              reply = controlChannel.execute(cmd);
              return reply.getMessage();
          } catch (UnexpectedReplyCodeException urce) {
              throw ServerException.embedUnexpectedReplyCodeException(urce);
          } catch (FTPReplyParseException rpe) {
              throw ServerException.embedFTPReplyParseException(rpe);
          }
      }
docs: Computes and returns a checksum of a file.
  transferred.
  
  @param algorithm the checksume algorithm
  @param offset the offset
  @param length the length
  @param file file to compute checksum of
  @return the computed checksum
  @exception ServerException if an error occured.