Chrome should fall back to a direct connection when it can't connect to the data reduction proxy, but doesn't fall back if it succeeds in connecting but cannot use the proxy.
For example, a misconfigured dns mapping could point Chrome to the wrong IP address, a connection could be established, but the server at that address might not operate as a data reduction proxy. See https://bugs.chromium.org/p/chromium/issues/detail?id=756645.
Chrome should verify that the connection is actually to the data reduction proxy and that the connection can served resources. If not, Chrome should fall back to a direct connection immediately.
Note that the verification mechanism should be robust to failures of server that serves a verification URL.
Comment 1 by bengr@chromium.org
, Aug 29 2017