private void doSecurityPreambleResponse() throws IOException {
    if (rpcServer.isSecurityEnabled) {
      SecurityPreamableResponse resp = SecurityPreamableResponse.newBuilder()
        .setServerPrincipal(rpcServer.serverPrincipal).build();
      doPreambleResponse(resp);
    } else {
      // security is not enabled, do not need a principal when connecting, throw a special exception
      // to let client know it should just use simple authentication
      doRespond(getErrorResponse("security is not enabled", new SecurityNotEnabledException()));
    }
  }
Has the request header and the request param and optionally encoded data buffer all in this one
array.
<p/>
Will be overridden in tests.
hbase-server/src/main/java/org/apache/hadoop/hbase/ipc/ServerRpcConnection.java