code: public void extendedGet(String remoteFileName,
                              long offset,
                              long size,
                              DataSink sink,
                              MarkerListener mListener)
          throws IOException,
                 ClientException,
                 ServerException {
  
          // servers support GridFTP?
          checkGridFTPSupport();
          // all parameters set correctly (or still unset)?
          checkTransferParamsGet();
  
          gLocalServer.store(sink);
  
          controlChannel.write(new Command("ERET",
                                           "P " + offset + " " + size
                                           + " " + remoteFileName));
  
          transferRunSingleThread(localServer.getControlChannel(),
                                  mListener);
      }
docs: Performs extended retrieve (partial retrieve mode).
  
  @param remoteFileName file to retrieve
  @param offset the staring offset in the remote file
  @param size number of bytes of remote file to transmit
  @param sink data sink to store the file
  @param mListener marker listener