Fast Roaming with Multiple Concurrent Associations

1. Multiple Concurrent Associations as a Means of Doing Fast Roaming Bob Beach Symbol Technologies Bob Beach

2. What makes roaming slow? • The association handshake is simple and fast • Two packet exchange and we are associated • It’s the stuff after association that is painful and takes time • Authentication and key negotiation/security context • QoS negotiation • If we could just do the association handshake, then roaming, at least the active transitioning part, would go quickly Bob Beach

3. What Does “Being Associated” Mean? • Defines the BSS the STA addresses packets to and receives packets from • Enables the AP to forwards packets from/to a station • DS to RF, RF to DS, RF to RF • AP assigns AID • Used for polling, etc. • May trigger authentication process • Strictly speaking, association is independent of authentication and QoS negotiation Bob Beach

4. Why cannot a STA be associated with multiple APs concurrently? • The most obvious problem • Multiple APs will forward packets for a given STA • Wastes bandwidth • Actually the APs will not forward the packets since most wired infrastructures will prevent it • Ethernet switches will forward packets only via one port to one AP • If one tells the infrastructure, the AP will not receive any traffic for the MU and hence the AP will have nothing to send • A few other problems • AID usage/TIM fields • 2K STAs per AP are sufficient • Memory space within AP • Its cheap • QoS reserved bandwidth • Could be released/timed out Bob Beach

5. How might multiple associations work? • Several possible approaches, this is just one • Associate and authenticate just like one does now for all APs of interest • Maintain key, sequence number, etc.. Information for all of them • “Cost” is mostly memory space on STA • When one wants to roam send a packet to infrastructure via new AP • An empty data packet with my Source Address • Wired switches will reconfigure themselves accordingly to indicate my new location • This packet is secure since it is encrypted using keys of new AP • AP responds with a packet to indicate mode change • 802.11 protocol changes • APs do not tell one another of roaming nor do they delete STA from internal data base • Cost is mostly memory on AP Bob Beach

6. From the AP’s perspective • The STA is just “quiet” with no packets moving either direction • There is precedence for such modes in the PSP mode • STA goes away for potentially long period of time (minutes/hours) • STA can pick up again at any point in time • One might define a new mode “suspend” • Similar to PSP mode • Frees any QoS reservations Bob Beach

7. Conclusions • “Fast roaming”, at least the active transition part, may be accomplished with relatively simple mechanisms • We need to challenge to some of the basic assumptions of 802.11 regarding what an association involves Bob Beach