802.11r: Wireless LAN Fast Roaming

For Wi-fi to serve as a foundation for mobile applications and voice, networks must provide secure mobility. And to achieve that, mobile devices need robust authentication and encryption, fast roaming and QoS (quality of service).

Enterprise IT pros should pay attention to the IEEE's 802.11r fast-roaming task group, whose standard is likely to make its way to market by late 2007 or early 2008, with broad adoption by enterprise solution providers in 2008.

The arrival of 11r is timely. Voice over Wi-Fi is expected to generate increased interest as more dual-mode Wi-Fi/cellular solutions are delivered this year. Many enterprises have moved from VPNs and captive portals to WLAN security architectures built around 802.11i/WPA2. Vendors have been offering proprietary secure-mobility solutions for years, and many of the underlying foundations for 802.11r have been tested on real networks. Given the need to integrate with 802.11i security and 802.11e QoS standards, developing a workable standard for fast roaming is challenging. Not surprisingly, 11r defines a complex architecture, though one that is not expected to require forklift upgrades of existing hardware. But getting all the software right will be difficult.

Security Is Easy, Mobility Is Hard

The expected deployment of time-sensitive applications like Vo-Fi on enterprise networks is driving 11r. Most Vo-Fi vendors have advocated virtual WLANs dedicated to voice, with WEP encryption often serving as the shaky foundation for privacy and a variety of techniques, largely proprietary, prioritizing voice traffic.

That's yesterday. Tomorrow's Vo-Fi installations will use advanced authentication and dynamic encryption-key techniques made possible by 802.11i and QoS capabilities defined by 802.11e. Unfortunately, upgrading to these standards while still providing fast roaming between APs may be tricky. Full authentication using 802.11i, for example, can create delays of several hundred milliseconds during roaming. The new Fast BSS Transition defined by 802.11r eliminates much of the handshaking overhead.

802.11r provides a no-compromise solution for secure wireless voice, providing fast-roaming transitions of about 50 ms while preserving a device's security and QoS context. It effectively "mobilizes" 802.11i's security services and 802.11e's QoS functions. The increasing number of enterprises that have begun or completed a migration to 802.11i and WPA2 will be poised to take advantage of 802.11r; those with legacy setups will have more to do. The IEEE's 802.11r standards initiative was established in 2004 to address 11i's limitations. The IEEE approved 11i while creating the 11r task group to address secure fast roaming. Many vendors--including Aruba, Broadcom, Cisco, Intel, Motorola, Nokia, SpectraLink and Texas Instruments--have been active in the 11r process.

The Wi-Fi Alliance is studying fast hand-off interoperability testing in its Enterprise Voice over Wi-Fi task group with a goal of releasing a certification plan in early 2008.

Fast Hand-off The current 802.11i authentication process is notoriously slow. Although 11i included optional mechanisms such as pairwise master key caching and pre-authentication to minimize roaming times, these haven't been broadly implemented by vendors. In pure 11i, once a client has decided it needs to roam to a new AP, it must exchange association messages with the AP. After a user's login credentials have been authenticated, a master session key is derived. 802.11r ensures that the authentication processes and encryption keys are established before a roam takes place.

To speed up roaming, 802.11r introduces "fast hand-off." Authentication occurs only once, when a client enters the mobility domain. Subsequent roams within a mobility domain use cryptographic material derived from the initial authentication, decreasing roam times and reducing load on back-end authentication servers.

To securely cache and distribute encryption keys, 802.11r includes a new key-management hierarchy. In this multilevel setup, the highest-level key holder (a WLAN controller, for instance) has access to the original cryptographic material and is responsible for deriving keys for lower-level key holders (APs). 802.11r's key-derivation algorithms are based on a one-way hash function ensuring that a compromised lower-level key cannot be used to decipher the original master key.

802.11r also tackles QoS. Even if a Wi-Fi device establishes QoS-based resource reservation when it connects to the network, when transitioning to a new AP, QoS is not preserved automatically. An optional mechanism in 11r lets a client request QoS resources on a target AP before choosing to roam. Most vendors we spoke to said they would support 11r. Aruba, Cisco and Meru have developed proprietary methods to deal with the shortcomings addressed by 11r. How fast they will migrate to a standards-based approach is uncertain, as is the pace at which client devices will be upgraded. To the degree that Cisco includes 11r support as part of its CCX certification program, the company will play a key role in promoting adoption of this standard.

Dave Molta is NWC's editor at large. he is also assistant dean for technology at the school of information studies and director of the center for emerging network technologies at Syracuse University. write to him at [email protected].