Proactive Key Caching (PKC) or Opportunistic Key Caching (OKC) - In the 802.11i specification there is a suggestion towards WPA2 Pairwise Master Key ID (PMKID) caching. PKC/OKC is one interpretation of this suggestion. The station remembers the PMKID from its initial BSSID with which it associated and authenticated. When the client roams to another BSSID it uses the original PMKID and hashes it with the new BSSID and comes up with new keys. It needs a centralised controller to distribute the PMKID out to the AP's to make it all work.