code: public void getPayload(byte[] buff, int offset) {
          buffer.position(FIXED_HEADER_SIZE);
          buffer.get(buff, offset, buffer.limit() - FIXED_HEADER_SIZE);
      }
docs: Reads the data transported by RTP in a packet, for example
  audio samples or compressed video data.
  
  @param buff the buffer used for reading
  @param offset the initial offset inside buffer.