I would have expected the path to be symmetric. As it turns out, there is no reason for this to be the case. IP routing algorithms are quite capable of handling data that comes from an address that does not reside on the subnet from which it originates. I suspect that hackers are very aware of this fact, since it can act as the basis for a variety of spoofing attacks. The format is primarily left up to the Local and Foreign Agents to decide.
Security is also obviously a major concern.
There are protocols for validation of mobile devices to the Local Agent, as well as centralized control that ensures that a device will only be registered to be in one place at a time. There may well be holes in this security, but I have not spotted them as of yet. I would encourage anyone who is interested to pick up a copy of Perkins book, or at least to download the relevant RFCs. There are two points that kept coming to mind while I read the book. The first was how relevant this standard was going to be.