I understand that TCP and UDP are multiplexed protocols, and the multiplexing key in them is the (sender.ip, sender.port, receiver.ip, receiver.port) tuple. I understand the reasoning behind sender.ip, sender.port and receiver.port but can't really see how receiver.ip helps with identifying the corresponding socket on the receiver side.
I am very new to the whole idea but on the other hand, it appears to me that by removing receiver.ip from the tuple, a whole new range of Internet censorship bypassing techniques could arise. Fragmenting IP diagrams into diagrams with various IP addresses of the same receiver could at least in theory help Internet freedom.
So my question is what is the reasoning behind having receiver.ip in TCP and UDP multiplexing key tuple? What would break if we just went with (sender.ip, sender.port, receiver.port)? I apologize in advance if this is a stupid question.