Let's say I have 2 connecting flights, first operated by airline A, second operated by airline B, on a ticket issued by airline C, booked on a booking site D. Who is responsible for rebooking and getting me to the final destination if the first flight is delayed and I miss the connection? A, B, C or D? If everyone claims it's not him, the other guys should do that, what document can I hold in front of one of them and say "No, you are responsible for rebooking, it's clearly written here"?
For the sake of argument let's assume I am at the connecting airport, airline A was a small regional with one flight per day and no presence (no ticketing desk, no representative) in the connecting airport and the online agency is already closed for that day.
I have read similar questions here, but none of the answers provided any sources for their opinion. If you are convinced A, B, C or D is responsible, could you please provide the basis for your claims, what documents / laws / conventions outline this responsibility in your opinion?