SWAN Fast Secure Roaming (CCKM)

When you deploy EAP/802.1x as the security mechanism, you need to address performance aspects when a user roams from an AP to another AP (whether Layer 2 or Layer 3 roam). As discussed in previous chapters, during an 802.11 reassociation process, you must reauthenticate the WLAN user to avoid man-in-the-middle (MitM) attacks. A full EAP/802.1x authentication is likely to increase the roaming delay between the APs. In the case of a remote branch office, if the RADIUS authentication is to take place over a WAN link (such as a RADIUS infrastructure located at the headquarters [HQ]), this will further increase the roaming delay.

The roaming delay during EAP/802.1x reassociation might impact some applications, such as voice over IP (VoIP). If the roaming delay is to exceed 150 ms, the user is likely to notice jitter (that is, decreased voice quality). Therefore, you must expedite between APs roaming for devices enabled with latency/jitter-sensitive applications such as VoIP. The SWAN fast secure roaming feature expedites roaming (that is, 802.11 reassociation) for EAP/802.1x users using Cisco Centralized Key Management (CCKM). Essentially, CCKM is a centralized key management functionality that removes the need for full 802.1x reauthentication and 4-way handshake (as defined by 802.11i/WPA specifications to refresh the unicast keys) during the 802.11 roam process. Refer to Chapter 8, "WLAN Encryption and Data Integrity Protocols," for more discussion on 802.11i and WPA, including unicast key derivation via 4-way handshake.

As of this writing (summer of 2004), fast secure roaming is supported on Cisco and CCX clients for LEAP and EAP-FAST authentication. However, fast secure roaming is independent of the EAP authentication type and can be supported on any EAP type.

Figure 9-7 illustrates fast secure roaming in large campus network and remote branch office scenarios. As shown in the figure, switch-based WDS is used for fast secure roaming in a large campus environment (for scalability reasons). In the remote branch office deployment scenario, you can use an AP or branch office router (and possibly a low-end Catalyst switch) as the WDS. When you use a localized WDS in the remote branch office scenario, the WDS can expedite roaming for CCKM users (for example, LEAP with CCKM and WPA-TKIP enabled VoIP devices) without having to reauthenticate the users to the RADIUS server. As noted previously, this provides a considerable amount of improvement in roaming performance in a remote branch office scenario and accommodates delay/jitter-sensitive applications such as VoIP over WLAN.

During the initial EAP/802.1x authentication process of a WLAN client, credentials are cached on the WDS server. When the client roams, the cached credentials to expedite the roaming process. Thus, SWAN enables infrastructure-based, scalable, and centrally manageable fast secure roaming for EAP/802.1x users. Figure 9-8 illustrates the initial EAP/802.1x authentication process in a SWAN-enabled network for a WLAN client. As shown in the figure, the WLAN client discovers the fast secure roaming?enabled AP during the initial discovery process using WPA Information Element (IE) and associates to the AP (Step 1). During the initial association process, the WLAN client needs to negotiate the ciphers such as Cisco Temporal Key Integrity Protocol (Cisco TKIP, also known as CKIP) or WPA-TKIP (Wi-Fi Protected Access TKIP). This process is similar to initial WPA association.

As shown in Figure 9-8, when the initial 802.11 association is successful (Step 2), the WLAN client authenticates with the WDS client AP, which relays the authentication request using the WLCCP protocol to the WDS server (Step 3). The WDS server communicates with the RADIUS server to authenticate the WLAN client. Thus, the WDS client AP does not communicate directly with the RADIUS server. At the end of initial Fast Secure Roaming authentication, both the WLAN client and the RADIUS server derive the Network Session Key (NSK). The RADIUS server securely communicates the derived NSK to the WDS server (Step 4). Subsequently, the WLAN client and the WDS server initiate a 4-way handshake through the WDS client AP where nonces (that is, random numbers) are exchanged to derive the Base Transient Key (BTK) and the Key Refresh Key (KRK), as shown in the figure (Step 5). For more discussion on the 802.11i (and WPA) 4-way handshake and key-derivation process, refer to Chapter 8.

As shown in Figure 9-9, the reassociation process is expedited using the WDS server, and a total of three messages are required between the WLAN client and the new AP. Note that in the case of a non-CCKM EAP/802.1x client, a full EAP/802.1x reauthentication is required. In the case of a non-CCKM WPA client, a full EAP/802.1x reauthentication, along with 4-way handshake, is required to reassociate with the new AP. Thus, there is a significant performance improvement in using the SWAN-enabled fast secure roaming implementation for latency-sensitive applications such as VoIP.