1. Grid: Scalable Ad Hoc Wireless Networking Douglas De Couto http://pdos.lcs.mit.edu/grid

2. What is “Ad Hoc”? 802.11 “Ad hoc mode” Single-hop communications Bluetooth: master/slave All communication goes through master device We will mean multihop wireless networks without infrastructure, possibly mobile.

3. Talk Outline Motivation Research Results Geographic forwarding Grid location service (GLS) Capacity of ad hoc networks 802.11 performance Testbed Implementations In-building net Rooftop net

4. Application: Smart Devices Internet Access Point Print E-Mail Share Remote Control

5. Application: Rooftop Nets Game server School/Homework Server Internet Access

6. Application: Community Nets Cheap Incremental Automatic

7. Application: Disaster Services Disaster may have damaged phone system etc. Want to avoid N 2 plans for N services to communicate

8. Goal: Networks out of chaos AFDBECGJIH

9. Direct Contact Scales Badly AFDBECGJIH “Hello J!”

10. Solution: Multi-hop Forwarding AFDBECGJIH “A to J: Hello!”

11. Design Challenges Finding routes Cope with mobile nodes Conserving battery power Coping with malicious/faulty nodes Scaling to large networks

12. Completed Research Scalable routing: Geographic forwarding Distributed P2P location database Low-power forwarding Understanding capacity limits Avoiding malicious nodes Current research: 802.11 link selection

13. Geographic forwarding (GF) Packets addressed to  id, location  Next hop is chosen from neighbors to move packet geographically closer to destination location Per-node routing overhead constant as network size (nodes, area) grows Requires location service, which adds overhead N1 N2 N3 N4 N5 N3’s radio range N7 N6

14. A E H G B D F C J I K L Each node has a few servers that know its location. 1. Node D sends location updates to its servers (B, H, K). 2. Node J sends a query for D to one of D’s close servers. “D?” Grid Location Service (GLS) overview

15. level-0 level-1 level-2 level-3 All nodes agree on the global origin of the grid hierarchy GLS’s Spatial Hierarchy

16. 3 servers per node per level n s s s ss s s s s Node updates servers with GLS protocol sibling level-0 squares sibling level-1 squares sibling level-2 squares

17. Queries search for destination’s servers Queries search with same protocol as updates. Guaranteed to find closest location server. n s s s s s s s s s3 x s2 s1 location query path

18. Geographic forwarding is less fragile than source routing. DSR queries use too much b/w with > 300 nodes. Fraction of data packets delivered successfully Number of nodes DSR Grid GF + GLS performs well Biggest network simulated: 600 nodes, 2900x2900m (4-level grid hierarchy)

19. GLS properties Spreads load evenly over all nodes Degrades gracefully as nodes fail Queries for nearby nodes stay local Per-node storage and communication costs grow slowly as the network size grows : O(log n), n nodes More details: Li et al., Mobicom 2000

20. 802.11 Capacity Enlarge network by adding nodes, area Constant density Ideally, there is more “packet-hop” capacity, due to spatial reuse of spectrum But: more nodes producing traffic to be forwarded across network

21. 802.11 packet-hops can scale

22. Per-node capacity depends on traffic patterns “Random” traffic patterns won’t scale Per-node capacity decreases like O(1/sqrt(n)) “Local” traffic patterns scale, capacity remains constant if number of hops follows a power law distribution (e.g. GLS) More details: Li et al., Mobicom 2001

23. Implementation and Testbeds Software distributions for Linux, BSD PC, iPAQ Works with unmodified Internet software Two Grid nets deployed

24. LCS Grid Net 5 5 5 5 5 5 5 555 5 6 6 6 6 6 6 17 static nodes on 5 th /6 th floors A dozen iPaq hand-helds wired gateway

25. Roof-Top Grid Net LCS 5 4 3 1 2 6

26. A B C D E F A’s nbrs: B, 1 hop (nh: B) C, 2 hops (nh: B) D, 3 hops (nh: B) … C, 2 hops (nh: B) B’s nbrs: A, 1 hop (nh: A) C, 1 hop (nh: C) D, 2 hops (nh: C) … Distance Vector Protocol C, 1 hop (nh: C) *Nodes periodically broadcast route tables *Nodes choose route with fewest hops

27. Implementation Click modular software router (userlevel) Portable: userlevel or kernel Rich APIs, e.g. Vector, HashMap, etc. Any 802.11 card with std. “ad hoc mode” Aironet 340/350 cards on Linux/BSD Lucent-based cards on Linux Best performance with driver support for signal statistics (minor patches)

28. Grid Protocol All packets have Grid header Own Ethernet type code (not IP packets) Transmitter information: ID, location Control packets Route advertisements (broadcasts) Location queries and replies Data packets Encapsulated IP Link information is included

29. Packet Handling Kernel Userlevel eth0 Grid routing process demux IP Stack Route lookup Route table Control packets (broadcast) Encapsulated data packets Grid packets (via pcap) IP packets (via tun/tap) Add/remove encapsulation Applications

30. Packet Handling: Control Kernel Userlevel eth0 Grid routing process demux IP Stack Route lookup Route table Control packets (broadcast) Encapsulated data packets Grid packets (via pcap) IP packets (via tun/tap) Add/remove encapsulation Applications

31. Packet Handling: Data Kernel Userlevel eth0 Grid routing process demux IP Stack Route lookup Route table Control packets (broadcast) Encapsulated data packets Grid packets (via pcap) IP packets (via tun/tap) Add/remove encapsulation Applications

32. Does Grid Find Useful Paths? AFDBECGJIH

33. Mistake: Shortest-Path Routes AFDBECGJIH A’s max range

34. Link Quality Isn’t Bi-modal

35. Obstacles to Better Routing Use low-loss paths, but… Loss rate masked by 802.11 re-sends Changes quickly with time, motion What’s the best metric to minimize? Expected total packet transmissions Fight strong bias towards shortest paths

36. How to choose links? Signal strength?

37. Should we use “quality”? Aironet “quality”

38. Current Approach: Measure loss rates Receiver measures loss rate of sender Receiver ping-pongs loss rate to sender Meaured with broadcast But: each node broadcasts every ~1.3s What period to measure over? How to smooth? Trying exponentially time-weighted avg.

39. Installing Grid ipkg install grid Follow prompts, be sure to set IPADDR Is it working?

40. Grid Summary Grid routing protocols are Self-configuring Easy to deploy Scalable Software etc. at: http://www.pdos.lcs.mit.edu/grid

41. References GLS: Li et al., “A Scalable Location Service for Geographic Ad Hoc Routing”. Proc. ACM MobiCom, August 2000. pp. 120--130 Capacity: Li et al., “Capacity of Ad Hoc Wireless Networks”. Proc. ACM MobiCom, July 2001. pp. 61--69 Link quality: De Couto et al., “Effects of Loss Rate on Ad Hoc Wireless Routing”. MIT LCS TR #836

42. End Of Talk Demo…

43. Links Aren’t Symmetric

44. Application: Smart Devices Internet Access Point Print E-Mail Share Remote Control

45. Application: Rooftop Nets Game server School/Homework Server Internet Access

46. Application: Disaster Services Disaster may have damaged phone system &c Want to avoid N 2 plans for N services to communicate

47. Direct Contact Scales Badly AFDBECGJIH “Hello J!”

48. Design Challenges Cope with mobile nodes Finding routes Conserving battery power Coping with malicious/faulty nodes Scaling to large networks

49. Topology Distribution Scales Badly 1. “C can reach A and B.” ABCDF 3. Data from F to B. 2. “D can reach A, B, and C.” G

50. Geographic Forwarding Scales Well Longitude Latitude AFDBECG “Send towards lat G / lon G.”

51. Location Database Longitude Latitude AFDBECG DB 1. “G is at lat G / lon G” 2. “Where is G?”

52. Distributed Location Database Each node is DB for a few other nodes How to find a node’s location server(s)? Every node has an unchanging ID hash(ID) maps ID to position in unit square

53. G’s Location Server is a Point G hash(G) = 0.1,0.9 x (0,0) H I

54. Spatial Grid Hierarchy All nodes agree on the global origin of the Grid hierarchy

55. Multiple Servers per Node G c ba

56. Lookups Expand in Scope G c ba A ?

57. Grid Protocol Overhead Grows Slowly Protocol packets include: Grid update, Grid query/reply. Number of nodes Protocol Overhead (packets per second)