Question: would it be a desireable or even required property of an identifier to be identifiable as such by all? Obviously, the destination site or host would know whether one of its addresses is an identifier or a locator. But would it be important for others to be able to make this distinction? It might be good to avoid trying to map a locator to a new locator. On the other hand, this means identifiers have to come from a separate block of IPv6 address space (assuming they look like IPv6 unicast addresses). This isn't (very) compatible with an extra address negotiation solution or with interoperation with non-multihoming aware systems.