Mid-stream I/O errors (e.g. Connection reset by peer) are never propagated out of transport sockets, and connections terminated because of those errors are closed exactly the same way as in case of a clean shutdown, which results in silent truncation, and the information about that is not available in neither %CONNECTION_TERMINATION_DETAILS%, %UPSTREAM_TRANSPORT_FAILURE_REASON%, nor %RESPONSE_FLAGS%.

While some L7 network filters can detect premature connection termination because of truncated and/or missing frames, L4 network filters (e.g. TCP proxy) are completely lost here.

cc @alyssawilk @antoniovicente @ggreenway @kyessenov