nghttpx server push is initiated by looking for Link header field from
backend server response.  Currently we only enable server push for
HTTP/1 backend and without HTTP/2 proxy mode.  The URIs which have
rel=preload are eligible to resource to be pushed.

We don't rewrite them if -s or -p is used

We have now Downstream retry count to be limited to 5 times.  At 6th
failure, we send 503 message to client.

Kudos to h2spec to find this details