private void writeConnectionHeaderPreamble(OutputStream out) throws IOException {
    out.write(connectionHeaderPreamble);
    out.flush();
  }
Write the RPC header: {@code <MAGIC WORD -- 'HBas'> <ONEBYTE_VERSION> <ONEBYTE_AUTH_TYPE>}
hbase-client/src/main/java/org/apache/hadoop/hbase/ipc/BlockingRpcConnection.java