private void nextHeader() throws IOException {
		int len, chunk = 0;
		while (!eof && header.syncFrame(buf, off, maxOff) == false) {
			// buf内帧同步失败或数据不足一帧，刷新缓冲区buf
			off = header.offset();
			len = maxOff - off;
			System.arraycopy(buf, off, buf, 0, len);
			maxOff = len + instream.read(buf, len, off);
			off = 0;
			if( maxOff <= len || (chunk += BUFLEN) > 0x10000)
				eof = true;
		}
		off = header.offset();
	}