1. 1 Architecture and Techniques for Diagnosing Faults in IEEE 802.11 Infrastructure Networks Atul Adya, Victor Bahl, Ranveer Chandra, Lili Qiu Microsoft Research

2. 2 Wireless Network Woes How many times have you heard users say: –“My machine says: wireless connection unavailable” –“Why can’t my machine authenticate?” –“My performance on wireless really sucks” IT Dept: Several hundred complaints per month You may have heard network admins say: –“I wonder if some one has sneakily installed an unauthorized access point” –“Do we have complete coverage in all the buildings?”

3. 3 Enterprise Wireless Problems Main problems observed by IT department: –Connectivity: RF Holes –Authentication: 802.1x protocol issues –Performance: Unexplained delays –Security: Rogue APs

4. 4 Existing Products Provide management/diagnostic functions –E.g., AirWave, CA’s NSM, Air Defense, Air Magnet Insufficient functionality: –No support for disconnected clients –Weak root-cause analysis (raw data, mostly) –Diagnosis only from the AP perspective –Sometimes need expensive sensor deployment

5. 5 Our Contributions Flexible client-based framework for detection and diagnosis of wireless faults Client Conduit: communication for disconnected clients via nearby connected clients Diagnostic mechanisms –Approximate location of disconnected clients –Rogue AP detection –Performance problem analysis

6. 6 Talk Outline Diagnostics architecture and implementation Client Conduit: diagnosing disconnected clients Diagnostic mechanisms –Locating disconnected clients –Detecting unauthorized APs –Analyzing performance problems Summary and Future Work

7. 7 Assumptions Can install diagnostic software on clients –APs are typically closed platforms –Can provide improved diagnosis with modified APs Nearby clients available for fault diagnosis –At least 13 active clients on our floor (approx. 2500 sq. feet) Network admins maintain AP Location Database

8. 8 Diagnostic AP Module (DAP) Client-Centric Architecture RADIUSKerberos Legacy AP Disconnected Client Client Conduit Authentication/User Info Diagnostic Client Module (DC) Diagnostic Server (DS)

9. 9 Diagnostic Architecture Properties Exploits client-view of network (not just APs) Supports proactive and reactive mechanisms Scalable Secure

10. 10 Client Implementation Prototype system on Windows Native WiFi: Extensibility framework for 802.11 [Microsoft Networking 2003] Daemon: most of functionality and main control flow IM driver: limited changes –Packet capture & monitoring

11. 11 Talk Outline Diagnostics architecture and implementation Client Conduit: diagnosing disconnected clients Diagnostic mechanisms –Locating disconnected clients –Detecting unauthorized APs –Analyzing performance problems Summary and Future Work

12. 12 Cause of Disconnection Lack of coverage –In an RF Hole –Just outside AP range Authentication issues, e.g., stale certificates Protocol problems, e.g., no DHCP address Can we communicate via nearby connected clients?

13. 13 Communication via Nearby Clients Possible (unsatisfactory) solutions: Multiple radios: extra radio for diagnostics MultiNet [InfoCom04]: Multiplex “Happy” between Infrastructure/Adhoc modes Penalizing normal case behavior for rare scenario Connected Client “Happy” (Infrastructure) Disconnected Client “Grumpy” Access Point Cannot be on 2 networks. Packet dropped! SOS Adhoc Mode

14. 14 Stops beaconing Our Solution: Client Conduit Connected Client “Happy” Disconnected Client “Grumpy” Access Point Disconnected station detected Becomes an Access Point (Starts beaconing) SOS (Beacon) SOS Ack (Probe Req) Ad hoc network via MultiNet Help disconnected wireless clients with: Online diagnosis Certificate bootstrapping Disconnected Client “Not-so-Grumpy”

15. 15 Client Conduit Features Incurs no extra overhead for connected clients –Use existing 802.11 messages: beacons & probes Works with legacy APs Includes security mechanisms to avoid abuses

16. 16 Time for “Grumpy” to get connected < 7 seconds –Reduced time can enable transparent recovery Bandwidth available for diagnosis > 400 Kbps (when “Happy” donates only 20% of time) Client Conduit Performance

17. 17 Talk Outline Diagnostics architecture and implementation Client Conduit: diagnosing disconnected clients Diagnostic mechanisms –Locating disconnected clients –Detecting unauthorized APs –Analyzing performance problems Summary and Future Work

18. 18 Locating Disconnected Clients Goal: Approximately locate to determine RF Holes Solution: Use nearby connected clients “Grumpy” starts beaconing Nearby clients report signal strength to server Diagnostic server uses RADAR [ InfoCom00 ] twice –Locates connected clients –Locates “Grumpy” with clients as “anchor points” Location error: 10 – 15 meters

19. 19 Talk Outline Diagnostics architecture and implementation Client Conduit: diagnosing disconnected clients Diagnostic mechanisms –Locating disconnected clients –Detecting unauthorized APs –Analyzing performance problems Summary and Future Work

20. 20 Rogue AP Problems Why problematic? Allow network access to unauthorized users Hurt performance: interfere with existing APs Detection goals: Common case: mistakes by employees Detect unauthorized IEEE 802.11 APs –Not considering non-compliant APs Solution: Use clients for monitoring nearby APs

21. 21 Rogue AP Detection Clients monitor nearby APs. Send to server: –MAC address, Channel, SSID, RSSI (for location) Server checks 4-tuple in AP Location Database Obtaining AP Information at clients: –Same/overlapping channel as client: from Beacons –AP on non-overlapping channel: Active Scan periodically AP information from Probe Response

22. 22 Rogue AP Detection Overheads Bandwidth usage < 0.2 Kbps per client Can active scans be performed without disruption? –Sufficient idleness available (2½ – 3 min.) –Simple threshold-based prediction: Active scan completed in idle period for 95% cases

23. 23 Talk Outline Diagnostics architecture and implementation Client Conduit: diagnosing disconnected clients Diagnostic mechanisms –Locating disconnected clients –Detecting unauthorized APs –Analyzing performance problems Summary and Future Work

24. 24 Summary Diagnostics critical for 802.11 deployments Client-centric architecture Client Conduit Diagnosis using nearby clients –Locate disconnected clients –Detect rogue APs –Analyze performance problems Prototype in Windows using Native WiFi –Mechanisms are effective with low overheads

25. 25 Future Work Detecting Rogue Ad Hoc networks 802.1x protocol analyzer Detailed wireless delay analyzer Automated recovery after fault diagnosis