What ever happened to Tandem and NonStop OS 2017

You have to care in a variety of ways if you intend to operate a internetworking protocol, or even a protocol that will ever operate beyond its original design space. If you only have a couple endpoints and everybody knows about each other, then the need is less apparent or even unimportant- but that is a degenerate case of internetworking. If you are protocol X, then perhaps you don't care where a packet comes from or through which interface it arrived, but that is because of the idiosyncracies of protocol X.

ISTM your issue above is confusing layers. If you are Protocol X, you may indeed not care about comm wires, but you probably will need to know the sender of the packet. It is important that the protocol engineers not get hung up on the buttociation of wires and addresses despite the temptation to do so- it is sufficient that the (presumably) network layer beneath provides a consistent view of addresses such that a Protocol X receiver can get response data units back to Protocol X on the sender. If Protocol X is trying to own or identify or influence the selection of comm wires, then there is likely a design problem.

Gregm