IP mobility support typically implies that packets incur lengthened routing paths by virtue of them being sent through a stationary home agent. However, "route optimization" in the Mobile IPv6 protocol enables normal and direct routing between a mobile node and its correspondent node. To securely allow this feature between initially unacquainted parties, the so-called return-routability procedure was built into Mobile IPv6. Recently, a number of improvements or optional alternatives have been suggested to the standard procedure. This document summarizes the goals for enhancements to route optimization, discusses the security threats that such enhancements must consider, categorizes the techniques that one can use for optimization, highlights the key ideas of various recent proposals, evaluates the performance gain that such proposals can yield, and compares these to ongoing optimization work in other parts of the network stack. Finally, the paper identifies needs for additional research.