Recently, I had the opportunity to test this functionality out on an iPhone 5 and an iPad mini with an Aerohive WLAN. I'd like to share my results with you... and I can tell you that you won't be disappointed! How do 8.5ms roams sound?!
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| Apple iPad Fast Roaming (1) and Aerohive AP Neighbor Report (2) |
Roaming with 802.11r (Fast BSS Transition) is noticeably faster than other proprietary fast-roaming methods (OKC/PKC) and it's also faster than roaming on a Pre-Shared Key (PSK) WLAN. This is because the 4-Way Handshake exchange can be eliminated by embedding the key derivation material (ANonce, SNonce, MIC, and GTK) within the Fast Transition Information Element inside the 802.11 Authentication and Reassociation frames. There is also a Mobility Domain IE that comes into play to distinguish boundaries between different WLANs (since key material must be exchanged between APs on the backend, two separate WLANs cannot facilitate fast roaming).
Here's a look at the Fast Transition IE inside the Reassociation Response (frame 18) from the AP to the iPad:
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| 802.11r Fast Transition Information Element |
You may want to review my previous post on The Many Variations of Wi-Fi Roaming to compare the frame exchanges required with each roaming method, CWNP's whitepaper on Fast BSS Transition [PDF] and blogs (here and here) to understand the key hierarchy and exchange between the initial AP authenticator (PMK-R0) and subsequent APs (PMK-R1).
Immediately after the fast roam completes, the Apple iPad submits a Neighbor Report Request within a Management Action frame. In essence, the client is requesting a list of all the neighbors from the AP in order to build a list for future roaming events. This report can be requested on-demand by the client and can help improve roam times by reducing or eliminating the need for the client station to actively scan off-channel. This way, the client has a list of nearby APs that is always up-to-date and can quickly move to another channel where it knows another AP is waiting.
Here is a look inside the Neighbor Report sent back to the iPad from the Aerohive AP (frame 21):
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| 802.11k Neighbor Report |
There are three IEEE 802.11 amendments that come into play which are all bundled up in the Wi-Fi Alliance Voice-Enterprise certification.
Standards and Certification Recap
The core of fast roaming was drafted in the IEEE 802.11r amendment, defining "Fast BSS Transition" or just Fast Transition (FT) for short. The name is derived because every individual AP radio cell is defined as a "Basic Service Set (BSS)" in the standard, and the amendment defines a method for client stations to transition (also called roaming) very fast between AP radios. It accomplishes this by defining a Mobility Domain comprised of a set of BSSs (APs) within the same Extended Service Set (ESS, otherwise known as an SSID) which have been validated. Validated APs must coordinate with each other to exchange client station details, including pairwise master key (PMK) encryption material, and perform pre-authentication of the client prior to the roam. This speeds the client roam by eliminating the need to re-authenticate the client through 802.1X/RADIUS or having to perform the 4-Way Handshake to derive pairwise transient key (PTK) encryption key material even in the case of a simple PSK network. The 802.11r amendement was ratified in 2008.
The IEEE 802.11k amendment on "Radio Resource Measurement" defines methods for information exchange about the RF environment between APs and client stations. The goal is to enable the client stations to understand the radio environment in which they exist so that they have more information to make correct decisions about roaming and performance. Stations can take radio measurements locally, request measurement by other stations, or have measurement requested of them and return the results. One interesting aspect for fast roaming is the Neighbor Report, where a client can request an AP to measure and report the neighboring APs which are available within the same Mobility Domain, including several pieces of operational information about each neighbor such as: BSSID, channel, security policy, and capabilities for QoS, APSD (power-save), BlockAck, spectrum management, and PHY type (802.11a/b/g/n). Some other reports available with 802.11k include: channel load, noise histogram, location configuration information, link measurement, and traffic stream measurements. The 802.11k amendment was ratified in 2008 as well.
The IEEE 802.11v amendment on "Wireless Network Management (WNM)" defines methods for stations to exchange information for the purpose of improving overall performance of the wireless network. Where 802.11k is concerned only with the radio environment, 802.11v expands it to include broader operational data surrounding existing network conditions allowing stations to be more cognizant of the topology and state of the network. There are a multitude of WNM services, the most interesting (for me, at least) is the BSS Transition Management capability, whereby an AP can request a client to roam to another AP for better performance or capacity. Some other services include: co-located interference, diagnostic reporting, directed multicast services, location services, multiple BSSID capability, proxy ARP, QoS traffic capability, and traffic filtering service, to name only a few. The 802.11v amendment was ratified in 2011.
Each of these amendments define numerous capabilities, of which I will only scratch the surface in this post to highlight a few. If you are interested in learning more about the services defined in each of the amendments, visit the IEEE Get Program website to download the 802.11-2012 standard, or search the web for PDF versions of each amendment.
Aerohive WLAN Configuration
Prior to being able to test and execute a fast transition (FT) roam, you need to configure the WLAN infrastructure to support the 11r/k/v features. In Aerohive HiveManager, navigate into the Configuration section and edit the SSID on which FT roaming should be supported. In the Advanced section of the SSID configuration you will see two sections, one for WMM and one for Voice Enterprise.
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| Aerohive Voice-Enterprise Configuration (IEEE 802.11r, k, v) |
Upon checking the first check box for Voice Enterprise, you will be presented with the following notice, informing you that Voice-Enterprise requires 802.11rkv and WMM AC-Voice which will all be enabled automatically.
You may have also noticed the note which states 802.11r requires WPA2 key management. This is because 802.11r advertises FT support in-part through the Authentication and Key Management (AKM) suites in the Robust Security Network (RSN) Information Element, which was included in the 802.11i amendment and WPA2 certification program. Pre-standard WPA did not include the RSN IE and therefore cannot support fast transition. So make sure you're using WPA2 (with either 802.1X or PSK) on the SSID as well.
Save and upload this configuration to at-least two APs, which will then begin including the Mobility Domain IE, Fast Transition IE, and Radio Management capabilities in beacons and probe responses to advertise these capabilities to clients.
Note - no explicit configuration is required to enable Voice-Enterprise on Apple iOS devices. Simply run iOS 6.01 or later and join a Voice-Enterprise enabled WLAN.
Client Limitations
In addition, the RSN IE which advertises encryption ciphers and authentication and key management (AKM) methods in-use on the WLAN to clients now includes a new AKM type to advertise Fast Transition key management. Some existing client drivers have issues parsing the RSN IE with additional AKM and will fail to association to the WLAN - in fact, they won't even try. Until client drivers are updated by manufacturers to support this addition AKM type, they will be unable to join any SSID that has Voice-Enterprise (specifically 802.11r) enabled.
Therefore, it is recommended to create a separate SSID specifically for Fast Transition capable clients and migrate them over to the new SSID.
Final Thoughts
Wi-Fi roaming performance has been a painful sore spot on the industry for many years. Problems were initially obscured through the use of open or WEP encrypted networks where roaming was relatively fast due to the simple security models implemented. However, as security improved with 802.11i and WPA2-Enterprise, roaming performance became a glaring issue, often taking >500ms or worse! This impacted the usability of real-time applications on an enterprise WLAN, forcing many network administrators to rely on less-secure PSK security methods.
Some vendors responded with proprietary fast roaming methods such as CCKM, OKC, and PKC. However, this served to fragment the industry and support for these methods were spotty at best. The IEEE thankfully stepped in and ratified the 802.11r amendment in 2008, yet it has taken nearly 5 years since then for enough momentum to build to finally implement standards-based testing and certification of fast roaming through the Wi-Fi Alliance Voice-Enterprise certification program.
However, now that standards-based fast roaming is here, IT IS GLORIOUS! I applaud Apple for being an advocate for fast roaming and implementing it into their iOS platform, likely because their devices get blamed for poor performance all the time. I encourage other mobile device manufacturers to follow suit, especially if their devices are used with real-time voice or video applications.
Cheers,
Andrew
