wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

48-10 Industrial Communication Systems
Loss of connection
Roaming-trigger/
search phase
Open systems
authentication
Association
Robust security
network association
Client
(mobile station)
Old AP New AP
Secure association and data exchange
Probe request Ch 1/probe response Ch 1
t Search
Probe request Ch n/probe response Ch n
802.11 Authentication request/response
t Auth
t Asso
(Re) Association request/response
Secure authentication
Key negotiation (4-Way-handshake)
Secure association and data exchange
Authentication
server (e.g., radius)
t
FIGURE 48.4
Handover procedure in 802.11 WLANs.
open authentication phase, the association phase, and the robust security network association (RSNA)
phase as shown in Figure 48.4. First, the connection loss will be detected and the search phase starts by
means of scanning all channels for available APs. Whenever a suitable new AP is found, the client starts
the authentication frame exchange followed by the association. Finally, RSNA is established and data
can be transmitted again using a secure connection.
In order to minimize the overall handover duration, either the search phase or RSNA can be optimized.
RSNA depends mainly on the AP side, whereas the search phase is determined by the client side. In [PCK07],
a good survey of recent works in the area of fast handover and currently achievable times is provided.
48.7.1 Mechanisms on the AP Side
The mechanisms on the AP side mainly address a reduction of RSNA. In order to reduce time-consuming
authentication against an authentication server, the IEEE 802.11i [IEE04i] standard amendment specifies
a preauthentication mechanism and allows a caching of the corresponding pair-wise master keys
(PMKs). Preauthentication provides an option for the client to perform a full IEEE 802.1X authentication
with other APs in range, while it is still associated to its current AP. The PMK caching feature allows
both AP and the client to store the results of a first full 802.1X authentication. In other words, whenever
the client roams back to an AP it had been previously associated to, only a four-way handshake is necessary
to establish the temporal keys for encryption.
A further improvement is achieved by methods specified in the 802.11r standard amendment [IEE08r]
for a fast BSS transition. Once a station joins the wireless network, a full 802.1X authentication is done
with the result of a generated PMK that is then distributed to all APs within the same mobility domain
(MD). Hence, when a station decides to handover to a new AP, PMK can be assumed to be already present.
However, the important question of a feasible key distribution has been left open by the amendment. After
this the four-way handshake and the resource reservation for QoS are embedded into the mandatory
reassociation procedure. To sum up, three enhancements can be identified in the 802.11r amendment.
© 2011 by Taylor and Francis Group, LLC