Analysis of Roaming Techniques

1. Analysis of Roaming Techniques Areg Alimian Communication Machinery Corporation aalimian@cmc.com Bernard Aboba Microsoft bernarda@microsoft.com Areg Alimian CMC, Bernard Aboba Microsoft

2. Outline • Roaming Definition & Phases • Test Configurations for roaming measurements • Contributors to handoff latency • Existing and emerging solutions for fast handoff • Conclusions Areg Alimian CMC, Bernard Aboba Microsoft

3. How do we define roaming? • Roaming latency • “The period from when the STA last receives data traffic via its old AP and when it receives data from the new AP is often referred to as the handoff latency or handoff delay”. • Triggering roaming • When the STA moves away from its current AP, the signal quality of the messages from the above AP will decrease. • At some (configurable) signal quality threshold, or after a number of failed retransmission attempts, the STA starts looking for a ‘better” AP to reassociate with, triggering a handover procedure. Areg Alimian CMC, Bernard Aboba Microsoft

4. Handoff Scenario • Latency • Contributors • 802.11 scan • 802.1X authentication • 4-way handshake • Movement detection • Address assignment • Duplicate detection • IKE renegotiation • MIP signalling • TCP adjustment period Channel 11 Channel 6 c v D AP B STA AP A c ~ 10-20 ft D ~ 100-300 ft Areg Alimian CMC, Bernard Aboba Microsoft

5. Latency Budget Areg Alimian CMC, Bernard Aboba Microsoft

6. Logical Steps/Phases in Handoff • Detection/Rate adaptation • Mobile station starts adjusting the traffic rate all the way down to the minimum for its PHY (rate fallback ). • The signal strength and the signal-to-noise ratio of the signal from a station’s current AP degrade and the station retransmits without a response. • Scanning • Mobile station initiates active scanning to probe for nearby APs. • Association/Reassociation • 802.1X (re-)authentication • STA attempts (re)authentication with the new AP. With PMK Caching/SAs the EAP authentication phase with a back-end server is not necessary. • IEEE 802.11 AKM • IP Layer Configuration • Acquiring a valid IP address • Duplicate Address Detection (DAD) • Mobile IP signaling • IKE signaling (if required) • Transport layer adjustment • TCP adjustment period Areg Alimian CMC, Bernard Aboba Microsoft

7. 802.11 Handoff Problem Space Scan + Pre-auth via Old AP Pre-Auth + Neighbor graph 4-way handshake, no 802.1X 3-way handshake, no 802.1X Association not possible DT/B Scan + Radio tuning D DTPA D DTFH D DTReassoc c DTPA Stationary Pedestrian Vehicular High Speed Station Velocity Areg Alimian CMC, Bernard Aboba Microsoft

8. Rate adaptation Rate adaptation time Packet loss during rate adaptation (Re)authentication (Re)authentication (AKM) without prior security Association states. (Re)authentication (AKM) without prior security State. (Re)authentication with IAPP. Roaming Handoff Interval Downstream loss during handover Session continuity during handover Upstream delay Scanning Passive Scanning Active Scanning Behavioral Roaming hysteresis Rate adaptation hysteresis Network Connectivity resumption Valid IP address acquisition/ IP configuration Transport adaptation Handoff Test Metrics Summary Areg Alimian CMC, Bernard Aboba Microsoft

9. Test Scenarios for Handoff Performance • Handoff Triggering Mechanisms • The power to the current AP is switched off • Decreasing the Tx power of current AP • Changing the load on the current AP • Injecting Traffic Patterns during handoff • Unidirectional upstream traffic from STA to a host on the LAN • Unidirectional downstream traffic from LAN host to STA. • Bidirectional traffic between STA and LAN host. • 2nd STA at the new AP competing for media access. Areg Alimian CMC, Bernard Aboba Microsoft

10. General Observations Based on Test Data • Handoff triggering mechanism (power off vs. Tx Power reduction) affects movement “detection” time. • Traffic pattern during roam affects overall handoff latency and packet loss during roam. • Handoff latency varies significantly based on specific equipment, especially STAs. Areg Alimian CMC, Bernard Aboba Microsoft

11. Handover Latency Summary • Detection and active scanning probe phase can be too long, therefore increasing overall roaming latency. • Rate adaptation down to 1 or 2 Mbps can take significant time and affects the throughput of other STAs if one or more STA are connected at the lower rate. • Significant delays at L3 • IP address assignment (when DHCP server is far from host) • Duplicate Address Detection (DAD) • Mobile IP signaling • Significant delays at L4 in some scenarios • Movement from high bw/low delay network to low bw/high delay network Areg Alimian CMC, Bernard Aboba Microsoft

12. The current 802.11 probe function The probe function is the IEEE 802.11 MAC active scan function And the standard specifies a scanning procedure as follows: For each channel to be scanned, • 1. STA sends a probe request with broadcast destination, SSID, and broadcast BSSID. • 2. STA starts a ProbeTimer. • 3. If medium is not busy before the ProbeTimer reaches MinChannelTime, scan the next channel, else when ProbeTimer reaches MaxChannelTime, process all received probe responses and proceed to next channel. Areg Alimian CMC, Bernard Aboba Microsoft

13. Existing Techniques for Handover Optimization • Limiting Rate adaptation range • Allowing negotiation of 1 and 2 Mbps rates is very time consuming.   • If there are one or more stations associated at lower rates, this will limit the throughput of stations associated at higher rates.  • AP Initiated Handoff • At the PHY Layer • Optimized Active Scanning • Scan most likely channels first. • Obtain channel list from the AP. • Fast Active Scanning. • Sending a probe request to a specific AP on its operation channel designating as the sole responder. Designated AP sends probe response after SIFS deferral. Areg Alimian CMC, Bernard Aboba Microsoft

14. Existing Techniques for Handover Optimization - 2 • Providing “Candidate Lists” to roaming STA • Roaming Station can request a candidate list from the AP to obtain relevant information about neighborhood STAs. • A “Site Report” is not necessarily the same as a “candidate list” • Difference: The list of all neighbors vs. the list of authorized, functional neighbors • Optimized IP Layer configuration • Significant delays in Layer 3 due to Duplicate Address Detection (DAD) and IP address assignment • IPv4: significant delay in DHCP where the DHCP server is far away from the host. • IPv6: delays due to movement detection constants • DNA reduces IP address assignment delays for intra-subnet roaming, provided there are reliable “hints” from L2 • Optimistic DAD (IPv6 only) reduces DAD delays Areg Alimian CMC, Bernard Aboba Microsoft

15. Detection of Network Attachment (DNA) • The time required to detect movement (or lack of movement) between subnets, and to obtain (or continue to use) a valid IP address may be significant as a fraction of the total delay in moving between points of attachment. As a result, optimizing detection of network attachment is important for mobile hosts. • Detection of Network Attachment follows the principles below: • Treatment of Link-Up indications from the Link Layer • Link-Local addressing as a mechanism of last resort • Utilization of hints from the Link Layer on current Subnet • Performing reachability test instead address acquisition where a valid IP address exists on the “most likely” point of attachment • Sending a DHCPDISCOVER instead of a DHCPREQUEST if the subnet is likely to have changed. Areg Alimian CMC, Bernard Aboba Microsoft

16. Issues with DNA • Today, there are no reliable “hints” of subnet attachment • SSID is not a reliable “hint” of subnet attachment • “Default” SSIDs are common; can disambiguate w/BSSID • STA may change prefix within same SSID • STA may keep same prefix when changing SSIDs (less likely) • DNA will not optimize the IP configuration phase significantly without reliable link layer hints Areg Alimian CMC, Bernard Aboba Microsoft

17. Factors Affecting STA Roam Decision • Factors that may affect the quality of the connection between the AP and the STA include: - Received Signal Power - Retransmissions • Factors that affect which AP, currently, would be the best choice for a STA to (re)associate with to maintain the upper layer connection include the above considerations plus: • Loading/Load Balancing Considerations • Capability matching • SNR • Received Signal Strength • Security • SSID Areg Alimian CMC, Bernard Aboba Microsoft

18. Using Candidate List Reports • A “candidate list report” contains information on APs that are valid handoff candidates for a STA • Valid = not a rogue, connected to the DS, forwarding frames, etc. • In response to a “candidate list request”, AP in response will send • Candidate list report for the ESS specified. • If the SSID IE is not present it will send a Candidate List Report for the SSID for the current ESS. • If the AP has no information on the ESS of which the SSID has been requested it will send a Candidate List Response with a length of zero. Areg Alimian CMC, Bernard Aboba Microsoft

19. Issues with the “Site Report” • “Site report” may or may not be equivalent to a “candidate list report” • Is purpose of “site report” to obtain a list of all APs, or just valid roaming candidates? • Site Report Response uses mgmt action frames which are not secured in the current specification. • Even if the STA has the BSSID of the AP to pre-authenticate to, it needs to be within the AP’s coverage area to reassociate. • The site report may not narrow the roaming candidates • “Site Report” may contain unsuitable roaming candidates • SNR is necessary to choose between roaming candidates • Using a “site report” as a “candidate list report” may cause the station to pre-authenticate to more APs, increasing load. Areg Alimian CMC, Bernard Aboba Microsoft

20. Alternative Approaches • Obtain neighbor information only after completion of authenticated key management (AKM) • Neighbor information obtained only from authenticated APs • “Candidate list” exchange is authenticated via a unicast key, not a group key • Semantics provide a “candidate list” not a “site report” Areg Alimian CMC, Bernard Aboba Microsoft

21. Handoff – Alternative Approach • AP-Initiated handoff • WLAN switch approach • PMKs made available to “dumb APs” by WLAN switch • IAPP approach • PMKs propagated between APs • PHY layer approach • Same SSID, same BSSID, same channel. • STA does not know that it is roaming. • Result is very small handoff latency. • Realities • This approach is now ubiquitous (but non-interoperable). • Standardizing AP-initiated handoff is not a worthwhile activity • Probably more profitable to focus on other issues Areg Alimian CMC, Bernard Aboba Microsoft

22. Related Work • Papers on this topic include: • http://www.ieee802.org/11/Documents/DocumentHolder/3-417.zip • http://www.ieee802.org/11/Documents/DocumentHolder/3-416.zip • http://www.it.kth.se/~vatn/research/handover-perf.pdf • http://www.drizzle.com/~aboba/IEEE/692.zip • http://www.cs.umd.edu/~waa/pubs/handoff-lat-acm.pdf • http://www.it.kth.se/~hvelayos/papers/TRITA-IMIT-LCN%20R%2003-02%20Handover%20in%20IEEE%20802.pdf • http://www.cs.cmu.edu/~glennj/scp/FixingAPSelection.html Areg Alimian CMC, Bernard Aboba Microsoft

23. Conclusions • Biggest challenges occur prior to authentication • Detection algorithms (when to roam) • Rate adaptation algorithms • Scanning latency (particularly for 802.11a/b/g devices) • Potential solutions are available • Channel maps • Roaming Candidate lists • Active scan optimizations • Rate adaptation limits • DNA • Optimistic DAD • Key management techniques not a high priority • TGi pre-authentication, PMK caching enables working systems today • Fitting within 50ms VOIP budget is possible…. • And involves hard implementation work, not rocket science. Areg Alimian CMC, Bernard Aboba Microsoft

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