@Override
    HttpRoute determineRoute(final HttpHost httpHost, final HttpRequest request, final HttpClientContext clientContext) throws HttpException {
        final HttpRoute route = routePlanner.determineRoute(httpHost, request, clientContext);
        if (route.isTunnelled()) {
            throw new HttpException("HTTP/2 tunneling not supported");
        }
        return route;
    }
Internal implementation of HTTP/2 only {@link CloseableHttpAsyncClient}.
<p>
Concurrent message exchanges with the same connection route executed by
this client will get automatically multiplexed over a single physical HTTP/2
connection.
</p>

@since 5.0
httpclient5/src/main/java/org/apache/hc/client5/http/impl/async/InternalH2AsyncClient.java