Grid: Scalable AdHoc Geographic Routing

1
Grid Scalable Ad-Hoc Geographic Routing

• Li, De Couto, Morris, Kaashoek,
• Karger, and Jannotti
• http//www.pdos.lcs.mit.edu/grid

2
Motivation

• How will my smart camera communicate?
• How to organize a campus full of smart devices?
• How to design/choose
• Communication hardware.
• Automatic configuration plan.
• Scalable routing protocols.
• Deployment strategy.

3
Possible Technologies

• Campus wired ethernet w/ DHCP.
• Hard to deploy everywhere.
• Cellular modems / CDPD.
• Pervasive but slow, complex per-node setup.
• 802.11 base stations w/ DHCP.
• Hard to deploy everywhere.
• Metricom / Ricochet.
• Like cellular but faster and less pervasive.
• Ad-hoc routing.

4
Ad-Hoc Nets The Dream
Frans
Doug
Nodes forward each others packets. No
infrastructure easy to deploy fault
tolerant. Short hops are good for power and
spectrum. Can it be made to work?
5
Ad-Hoc Nets The Reality
Avg. packets transmitted per node per second
Number of nodes

• Flooding-based on-demand routing works best in
  small nets.
• Can we route without global topology knowledge?

6
Geographic Forwarding Scales Well

• Assume each node knows its geographic location.

Cs radio range
A
D
F
C
G
B
E

• A addresses a packet to Gs latitude, longitude
• C only needs to know its immediate neighbors to
  forward packets towards G.
• Geographic forwarding needs a location service!

7
Possible Designs for a Location Service

• Flood to get a nodes location (e.g. LAR, DREAM)
• excessive flooding messages
• Central static location server.
• not fault tolerant
• too much load on central server and nearby nodes
• the server might be far away for nearby nodes or
  inaccessible due to network partition.
• Every node acts as server for a few others.
• good for spreading load and tolerating failures.

8
Desirable Properties of a Distributed Location
Service

• Spread load evenly over all nodes.
• Degrade gracefully as nodes fail.
• Queries for nearby nodes stay local.
• Per-node storage and communication costs grow
  slowly as the network size grows.

9
GLSs Spatial Hierarchy
All nodes agree on the global origin of the grid
hierarchy
10
3 Servers Per Node Per Level

• s is ns successor in that square.
• (Successor is the node with least ID greater
  than n )

11
Queries Search for Destinations Successors
Each query step visit ns successor at
increasing level.
12
GLS Update (level 0)
Invariant (for all levels) For node n in a
square, ns successor in each sibling square
knows about n.
11
1
2
3
9
23
29
16
7
6
Base case Each node in a level-0 square knows
about all other nodes in the same square.
17
5
26
25
4
8
21
19
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GLS Update (level 1)
Invariant (for all levels) For node n in a
square, ns successor in each sibling square
knows about n.
9
11
1
2
3
2
11
9
6
23
29
2
16
2
23
7
6
17
5
26
25
4
8
21
19
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GLS Update (level 1)
...
Invariant (for all levels) For node n in a
square, ns successor in each sibling square
knows about n.
9
...
11
1
2
...
3
11, 2
9
6
...
23
29
2
16
...
23, 2
7
6
...
...
...
17
5
...
26
25
...
...
...
8
4
21
...
19
15
GLS Update (level 2)
...
Invariant (for all levels) For node n in a
square, ns successor in each sibling square
knows about n.
9
...
1
11
1
1
2
...
3
11, 2
9
6
...
23
29
2
16
...
23, 2
7
6
...
...
...
17
5
...
26
25
...
...
...
8
4
21
...
19
16
GLS Query
...
9
...
1
11
1
1
2
...
3
11, 2
9
6
...
23
29
2
16
...
23, 2
7
6
...
...
...
17
5
...
26
25
location table content

...
...
...
8
4
21
query from 23 for 1
...
19
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Performance Analysis

• How scalable is GLS to big mobile ad-hoc networks?

• Simulations using ns
• Mobility Model
• random way-point with speed 0-10 m/s (22 mph)
• Area of square universe grows with the number of
  nodes in the network.
• Achieve spatial reuse of the spectrum
• GLS level-0 square is 250m x 250m

18
GLS Finds Nodes in Big Mobile Networks
Biggest network simulated 600 nodes,
2900x2900m (4-level grid hierarchy)

• Failed queries are not retransmitted in this
  simulation
• Queries fail because of out-of-date information
  for destination nodes or intermediate servers.

19
GLS Protocol Overhead Grows Slowly
Avg. packets transmitted per node per second
Number of nodes

• Protocol packets include GLS update, GLS
  query/reply

20
Fraction of Data Packets Delivered

• Geographic forwarding is less fragile than
  source routing.
• DSR queries use too much b/w with gt 300 nodes.

21
Grid Status

• Deployed 16 nodes with preliminary software.
• Mostly stationary nodes
• Linux, Click, 802.11 radios.
• Aiming for campus-wide deployment.
• Currently adding wireless handhelds (iPaq)
• Will add location sensors

22
Real-world Challenges

• Smart protocols to save battery lifetime.
• Geography holes, congestion, position
  estimation.
• Node density adaptivity variable transmit power.
• Model for system capacity.
• Appropriate applications.
• Privacy, security, anonymity.
• Internet access.
• Relays and wires for complete campus coverage.

23
Practical Problems

• Location sensors add cost, size, and power
  requirements
• Connectivity holes
• The world is not randomly uniform!
• Radio ranges vary
• Power not all devices can expend energy
  forwarding for others
• Capacity does network capacity grow with the
  number of nodes?

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Only Some Nodes Know Position

• E.g. GPS doesnt work well inside
• Previous work assumed every node knew position
• To choose next forwarding hop
• To address packet to destination
• To run GLS

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Solution Location Proxies

• Nodes that know their location can act as
  location proxies.
• Location proxies can communicate with each other
  using geographic forwarding and local routing
  protocol (DV distance vector).
• Nodes without location select proxies, and
  communicate through them using only local DV.
• Proxies are not special besides knowing locations.

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Location Proxies (2)
P location proxies N nodes without location
N3
P
P2
P1
P
N
N2
P
N
N
N1
N
local DV protocol
Geographic forwarding local DV protocol
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Choosing Location Proxies

• Location ignorant nodes always remember and
  advertise any routes to location equipped nodes
• Routes to nodes with location will propagate to
  all location ignorant nodes
• Location ignorant nodes choose closest (in hops)
  location equipped node as proxy
• Advertise proxys location to location service as
  own
• Forward all data via proxy

28
Proxy Example (finding proxies)
P position equipped N position ignorant
Advertisements
P
N4
N3
N2
N1
Routes
P, 4
P, 3
P, 2
P, 1





29
Receiving Packets Using Proxies

• Proxy P will receive packets for N via geographic
  forwarding.
• P uses local routing to send packets to N.
• P will learn the route to N.

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How Proxies Learn Routes

• Location ignorant node N sets propagate_hops
  field in its own route advertisements to hop
  count of proxy P.
• When receiving a route entry with propagate_hops
  field set, decrement field and advertise the
  entry.

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How Proxies Learn Routes
N1s route should be propagated for 4 hops to
reach proxy P.
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Proxy Example (complete)
P position equipped N position ignorant
Advertisements
P
N4
N3
N2
N1
Routes
N1, 4
P, 4
P, 3
P, 2
P, 1
N1, 1
N1, 2
N1, 3





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Proxy Results scenario

• 10 know location
• Nodes with location placed along top edge

0
10
Knows loc
Ignorant
100
34
Location Proxies are Effective
35
What is a hole?

• There could be no geographically closer next hop

Cs radio range
B
A
F
G
C
?
D
E
Next hop D is backwards!
36
Intermediate Node Forwarding

• Idea If geographic route fails, try another
  route!
• Select an intermediate location L.
• Forward packets to destination via L
• Use geographic forwarding from source to L, and
  from L to destination
• L can be backwards, or to the side, avoiding
  holes.

37
Implementing INF

• Select L randomly from circle c about midpoint m
• If routes continue to fail, increase radius of
  circle c, e.g. double the radius, and choose new
  L.

38
Intermediate Node ForwardingExample
INF through L1 will fail (A-B-C-?)
B
A
G
L1
C
F
L2
INF through L2 will succeed (A-B-C-D-E-F-G)
D
E
39
Summary

• GLS geographic forwarding is scalable
• Practical challenges
• No location info
• Geographic connectivity holes
• Power
• Capacity
• Basic system is functioning, we are addressing
  the challenges.