Why the ProxySG sends a header "X-Bluecoat-Via:"
You want to know why the proxy sends a header "X-Bluecoat-Via:"
The purpose of the ProxySG sending the X-Bluecoat-Via header is to detect loops in the network. If the proxy receives a request with the exact same X-Bluecoat-Via header that it generates on the server-side then it knows that the request was generated from itself and it will not propagate the request on. Each proxy generates a different X-Bluecoat-Via header.