1. Mobility assisted routing CS 218 F2008 Ad hoc mobility generally harmful Can mobility help in routing? –Mobility induced distributed route/directory tree –Destination discovery (if coordinates not know) Last Encounter Routing FRESH: H. Dubois Ferriere et al”Age Matters: Efficient Route discovery in Mobile Ad Hoc Networks Using Encounter ages, Mobihoc June 2003. EASE: Grossglauser et al “Locating Mobile Nodes with EASE: Learning Efficient Routes from Encounter Histories Alone”, IEEE TON 2006

2. Mobility Diffusion and “last encounter” routing Imagine a roaming node “sniffs” the neighborhood and learns/stores neighbors’ IDs Roaming node carries around the info about nodes it saw before If nodes move randomly and uniformly in the field (and the network is dense), there is a trail of nodes – like pointers – tracing back to each ID The superposition of these trails is a tree – it is a routing tree (to send messages back to source); “Last encounter” routing: next hop is the node that last saw the destination

3. Last Encounter History No location service (as in geo-routing) Only information on network topology available for free at a node is local connectivity to neighboring nodes But, there is more: “history” of this local connectivity! Claim: –Collection of last encounter histories at network nodes contain enough information about current topology to efficiently route packets

4. Last Encounter Routing Can we efficiently route a packet from a source to a destination based only on LE information, in a large network with n nodes? Assumptions: –Dense encounters: O(n^2) pairs of nodes have encountered each other at least once –Time-scale separation: packet transmission (ms) << topology change (minutes, hours, days) –Memory is cheap (O(n) per node) Basic idea: –Packet carries with it: location and age of best (most recent) encounter it has seen so far –Routing: packet consults entries for its destination along the way, “zeroes in” on destination

5. Definition: Last Encounter Table A B encounter at X between A and B at t=10 B: loc=X, time=10 C:... A: loc=X, time=10 C:... D:... X

6. Fixed Destination A

7. Moving Destination A A A A A A

8. Exponential Age Search (EASE) time -T 0 ? sourcedestination

9. EASE: Messenger Nodes time -T 0 -T/2

10. EASE: Searching for Messenger Node time -T 0 -T/2 Search: who has seen dest at most T/2 secs ago?

11. EASE: Sample Route Def: anchor point of age T = pos. of dest. T sec ago EASE: -ring search nodes until new anchor point of age less than T/2 is found -go there and repeat with T=new age src dst

12. Improvement: Greedy EASE

13. Heterogeneous Speeds: Slow Dest

14. Heterogeneous Speeds: Fast Dest

15. Related Idea: Last Encounter Flooding With coordinate system –Last-encounter information: time + place –EASE/GREASE algorithms Blind, no coordinate system –Last-encounter information: time only –FRESH algorithm: flood to next anchor point –Henri Dubois-Ferrière & MG & Martin Vetterli, MOBIHOC 03

16. FRESH: Last Encounter Flooding source destination

17. Summary: Last Encounter Routing Last Encounter Routing uses position information that is diffused for free by node mobility –Last encounter history: noisy view of network topology –Packet successively refines position estimate as it moves towards destination –Mobility creates uncertainty, but also provides the means to diffuse new information No explicit location service, no transmission overhead to update state! –Only control traffic is local “hello” messages –At least for some classes of node mobility, routes are efficient! –Key ingredients: locality, homogeneity, mixing of trajectories Last Encounter Flooding (FRESH): –No coordinates Rich area for more research: –Prediction –Integration with explicit location services & routing protocols –More realistic mobility models