code: protected void writeChallenge(final HttpResponseMessage httpResponse,
                                    final NextFilter nextFilter,
                                    final IoSession session,
                                    final String httpChallengeScheme) {
          if (logger != null && logger.isTraceEnabled()) {
              logger.trace("Sending HTTP challenge for auth scheme {}",
                      httpChallengeScheme);
          }
          nextFilter.filterWrite(session, new DefaultWriteRequestEx(httpResponse,
                                                                    new DefaultWriteFutureEx(session)));
      }
docs: Write a challenge response (http status code 401) from the server to the client.
  The challenge is encoded in the "WWW-Authenticate" header of the provided httpResponse.
  
  @param httpResponse    the response contaiing the challenge and a 40 status code
  @param nextFilter      the next filter to write the response with
  @param session         the client session
  @param httpChallengeScheme the challenge scheme for this session