HTTP uses the terms inbound and outbound to describe transactional direction. Messages travel inbound to the origin server, and when their work is done, they travel outbound back to the user agent.
HTTP messages flow like rivers. All messages flow downstream, regardless of whether they are request messages or response messages. The sender of any message is upstream of the receiver. In following figure, proxy 1 is upstream of proxy 3 for the request but downstream of proxy 3 for the response.