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