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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
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
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
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
Definition: Last Encounter Table
A
B
encounter at Xbetween A and B at t=10
B: loc=X, time=10C: ...
A: loc=X, time=10C: ...D: ...
X
Fixed Destination
A
Moving Destination
A
A
A
AA
A
Exponential Age Search (EASE)
time
-T
0
?
source destination
EASE: Messenger Nodes
time
-T
0
-T/2
EASE: Searching for Messenger Node
time
-T
0
-T/2
Search: who has seendest at most T/2 secs ago?
EASE: Sample RouteDef: 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
Improvement: Greedy EASE
Heterogeneous Speeds: Slow Dest
Heterogeneous Speeds: Fast Dest
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
FRESH: Last Encounter Flooding
source
destination
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