When a node receives a bundle, it's often helpful to know which node sent that bundle. (We need this for BABs, for example, and there are other potential uses.) That information could be provided in a previous-hop extension block, and if we did a 5050bis it could even be provided in the primary block; in either case, though, how would we know that information was trustworthy? We could use something in BSP to authenticate it, but maybe not all nodes will have BSP.
Currently in ION we figure this out by looking at the sender's convergence-layer endpoint ID: LTP engine number, or TCP or UDP socket address. For LTP, no problem because LTP engine numbers are the same as BP node numbers by convention; of course, that breaks down if somebody decides not to follow the convention. For TCP and UDP it's trickier: we have to walk through the routing Plans, looking for a Plan that maps a node number to an outduct with matching duct name, and that's not guaranteed to work because at most we can identify the node's host address -- if there are multiple nodes on the host, we can't tell which one sent the bundle.
Maybe an easier, more reliable, and safer mechanism would be simply to manage a lookup table that directly and explicitly associates nodes with the convergence layer endpoint IDs they use for bundle transmission. The best mechanism for populating that table isn't yet obvious, but maybe it's not an insuperable problem.