Clarification on Alternative Service Connection Failures

Hi,


I've got another question about HTTP Alternative Services [RFC7838]. In
section 2.4, it says the following:

> if the connection to the alternative service fails or is unresponsive,
> the client MAY fall back to using the origin or another alternative
> service

What actually counts as a failure in the connection? Is it only for
transport/network layer connection failures, or could an application
layer failure also count?

For example, I make a request to a http origin and it responds with a
valid Alt-Svc header indicating that there is a https alternative.
Connecting to the https origin succeeds and I can make my request.
However, that https alternative has been misconfigured or is otherwise
having issues and it returns a 500 Internal Server Error response for my
request. Would that be considered a failure of the connection?

In addition, if the request to another alternative or back to the origin
succeeds, would it then be wise to drop the failing alternative from my
Alt-Svc cache?

The way I've read it, I think that a 500 series response would be
considered a failure, and I would mark it as a bad alternative in my
cache and then make another request to the next highest priority
alternative (or back to the insecure origin if the application is okay
with this).


Best Regards,
Sam