The posters interpretation of why Envoy is using downstream on listener side and upstream on backend side is wrong. There's a formal definition in RFC 2616:
upstream/downstream
Upstream and downstream describe the flow of a message: all messages flow from upstream to downstream.
Now HTTP is obviously bi-directional but since for most reverse proxy usecases responses far outweigh the requests upstream means "the backend"
> Umm, no. Fix the Envoy proxy terminology so that it isn't obviously wrong.