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"
Comments
Submitted, because I just found out that even after 28 years, the definition of "upstream" and "downstream" is still heavily contested.
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:
Now HTTP is obviously bi-directional but since for most reverse proxy usecases responses far outweigh the requests upstream means "the backend"