I have a Dell (UP2716D) monitor connected to a mid-2014 13" rMBP via Mini DisplayPort. Only when I connect the USB upstream will the USB ports on Dell work, where my speakers are connected by USB to monitor. Is my monitor/cable faulty or is this how mDP supposed to work -- not transmit USB ports from monitor to laptop? I am just curious because I always connect both USB upstream and mDP to laptop and could not find info about this online.