I have an upcoming trip IAD-YYZ-FCO that I booked on United.com, with both flights as UA codeshares operated by AC. Turns out the IAD-YYZ leg is often delayed and sometimes cancelled. If IAD-YYZ is delayed long enough that I miss the YYZ-FCO flight, do I work with UA or AC to rebook? Does that depend on whether I know I'm going to misconnect before or after I depart from IAD? Any advice folks might have for handling this situation? TIA!