Ad hoc On-demand Multipath Distance Vector (AOMDV) Routing

1
Ad hoc On-demand Multipath Distance Vector
(AOMDV) Routing

• Mahesh Marina Samir Das
• University of Cincinnati

2
On-demand Multipath Routing

• In each route discovery, find multiple routes
  between source and destination.
• Use alternate routes on a route failure.
• New route discovery needed only when all routes
  fail.
• Fewer number of route discoveries.
• Reduction in delay and routing overhead.

3
Main Question

• In the AODV framework, how to compute multiple
  paths between source and destination during route
  discovery?
• With minimal additional overhead.

4
Basic AODV Route Discovery
RREQ (broadcast)
A
E
D
S
B
C

• When a route is needed, source floods a route
  request for the destination.

5
Basic AODV Route Discovery
RREQ (broadcast)
Reverse Path
A
E
D
S
B
C

• Reverse path is formed when a node hears a
  non-duplicate route request.
• Each node forwards the request at most once (pure
  flooding).

6
Basic AODV Route Discovery
RREQ (broadcast)
Reverse Path
A
E
D
S
B
C

• Reverse path is formed when a node hears a
  non-duplicate route request.
• Each node forwards the request at most once (pure
  flooding).

7
Basic AODV Route Discovery
Reverse Path
A
E
D
S
B
C

• Observation Duplicate RREQ copies completely
  ignored. Therefore, potentially useful alternate
  reverse path info lost.

8
Alternate Reverse Paths A Naïve Approach
Reverse Path
A
E
D
S
B
C

• Form reverse paths using all duplicate RREQ
  copies.
• Causes routing loops.

9
Alternate Reverse Paths A Naïve Approach
Reverse Path
A
E
D
S
B
C

• Question how to form alternate loop-free
  reverse paths using some duplicate RREQ copies?
  Which some?

10
Loop Freedom

• Key idea Impose ordering among nodes in every
  path.
• Notion of upstream/downstream nodes.

i
j
d
j is downstream to i w.r.t d

• General loop-freedom rule Never form a route at
  a downstream node via an upstream node.

11
AODV Loop Freedom

• Destination sequence numbers to order routing
  events in time.
• Ordering among ltseqno, hop countgt tuples at
  different nodes on a path.
• higher seqno has precedence
• if same seqno, lower hop count has precedence
• Cant we use the same idea in AOMDV?
• No. Because hop count rule restricts us to a
  single path.
• But can use a similar idea though!

12
AOMDV Loop Freedom

• Sequence number rule Keep only routes for the
  highest dest seqno (like in AODV).
• For the same dest seqno,
• Route advertisement rule Keep multiple routes
  but always advertise only one of them to others.
  Hop count of that path is the advertised hop
  count
• Which one? Longest path at the time of first
  advertisement.
• Why? Maximize chances of forming more paths.
• Route acceptance rule Accept a route from a
  neighbor only if it has a smaller or equal
  advertised hop count. Break ties using node ids.
• No need for coordination with upstream nodes.

13
AOMDV Loop Freedom

• Sequence number rule Keep only routes for the
  highest dest seqno (like in AODV).
• For the same dest seqno,
• Route advertisement rule Keep multiple routes
  but always advertise only one of them to others.
  Hop count of that path is the advertised hop
  count
• Which one? Longest path at the time of first
  advertisement.
• Why? Maximize chances of forming more paths.
• Route acceptance rule Accept a route from a
  neighbor only if it has a smaller or equal
  advertised hop count. Break ties using node ids.
• No need for coordination with upstream nodes.

14
AOMDV Loop Freedom

• Sequence number rule Keep only routes for the
  highest dest seqno (like in AODV).
• For the same dest seqno,
• Route advertisement rule Keep multiple routes
  but always advertise only one of them to others.
  Hop count of that path is the advertised hop
  count
• Which one? Longest path at the time of first
  advertisement.
• Why? Maximize chances of forming more paths.
• Route acceptance rule Accept a route from a
  neighbor only if it has a smaller or equal
  advertised hop count. Break ties using node ids.
• No need for coordination with upstream nodes.

15
AOMDV Routing Table Entry
Dest
Seqno
Advertised hop count
Hop count 1
Next hop 1
Hop count 2
Next hop 2


16
AOMDV Loop Freedom

• Sequence number rule Keep only routes for the
  highest dest seqno (like in AODV).
• For the same dest seqno,
• Route advertisement rule Keep multiple routes
  but always advertise only one of them to others.
  Hop count of that path is the advertised hop
  count
• Which one? Longest path at the time of first
  advertisement.
• Why? Maximize chances of forming more paths.
• Route acceptance rule Accept a route from a
  neighbor only if it has a smaller or equal
  advertised hop count. Break ties using node ids.
• No need for coordination with upstream nodes.

17
Multiple Loop-free Reverse Paths
?
?
RREQ (broadcast)
A
E
D
S
?
0
B
C
?
?

• Suppose RREQ from S includes highest seqno for
  itself.

18
Multiple Loop-free Reverse Paths
?
1
RREQ (broadcast)
Reverse Path
A
E
D
S
?
0
B
C
1
?
19
Multiple Loop-free Reverse Paths
2
1
RREQ (broadcast)
Reverse Path
A
E
D
S
?
0
B
C
1
2
20
Multiple Loop-free Reverse Paths
2
1
Reverse Path
A
E
D
S
?
0
B
C
1
2
21
Multiple Loop-free Forward Paths
1
2
Forward Path
A
E
D
S
0
?
B
C
2
1

• Another modification to basic AODV route
  discovery multiple replies from destination.

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How Many Paths?

• Too many paths are not useful.
• Overhead proportional to paths.
• Diminishing utility with larger paths.
  Analytical study in Nasipuri-Das IC3N-99, ACM
  MONET J. 01
• Solution Disjoint paths.
• Automatically fewer paths.
• Paths fail independently. Better utility.
• Node or link disjoint?
• Too few node disjoint paths in dense networks
  using flooding.

23
Finding Link-disjoint Paths
Examples
How?

• Maintain last hop info in routing table.
• Ensure that next hops and last hops before
  destination are unique.
• This requires route request and replies to carry
  first hop info.

D
S
D
S
D
S
24
Performance Evaluation

• Ns-2 with CMU wireless extensions.
• 100 nodes in a 1000m x 1000m field.
• Random way point mobility model
• 25 CBR/UDP sources each sending 512 byte packets
  _at_ 2 packets/s.

25
Route discovery frequency/Average delay
26
Packet delivery fraction/Routing load
27
Summary

• AOMDV - a multipath extension to AODV
• multiple loop-free and link-disjoint paths.
• AOMDV performance relative to AODV
• more than factor of two improvement in delay.
• about 25 reduction in routing load.