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

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Ad hoc On-demand Multipath Distance Vector (AOMDV) Routing. Mahesh Marina Samir Das University of Cincinnati. On-demand Multipath Routing. In each route discovery, find multiple routes between source and destination. Use alternate routes on a route failure.

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

Mahesh Marina Samir Das

University of Cincinnati

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.
Main Question
• In the AODV framework, how to compute multiple paths between source and destination during route discovery?
Basic AODV Route Discovery

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

A

E

D

S

B

C

Basic AODV Route Discovery

Reverse Path

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

A

E

D

S

B

C

Basic AODV Route Discovery

Reverse Path

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

A

E

D

S

B

C

Basic AODV Route Discovery

Reverse Path

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

A

E

D

S

B

C

Alternate Reverse Paths: A Naïve Approach

Reverse Path

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

A

E

D

S

B

C

Alternate Reverse Paths: A Naïve Approach

Reverse Path

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

A

E

D

S

B

C

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.
AODV Loop Freedom
• Destination sequence numbers to order routing events in time.
• Ordering among <seqno, hop count> tuples at different nodes on a path.
• higher seqno has precedence
• if same seqno, lower hop count has precedence
• Can’t 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!
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”

• 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.
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”

• 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.
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”

• 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.
AOMDV Routing Table Entry

Dest

Seqno

Hop count 1

Next hop 1

Hop count 2

Next hop 2

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”

• 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.
Multiple Loop-free Reverse Paths

• Suppose RREQ from S includes highest seqno for itself.

A

E

D

S

0

B

C

Multiple Loop-free Reverse Paths

1

Reverse Path

A

E

D

S

0

B

C

1

Multiple Loop-free Reverse Paths

2

1

Reverse Path

A

E

D

S

0

B

C

1

2

Multiple Loop-free Reverse Paths

2

1

Reverse Path

A

E

D

S

0

B

C

1

2

Multiple Loop-free Forward Paths

1

2

Forward Path

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

A

E

D

S

0

B

C

2

1

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.
• Too few node disjoint paths in dense networks using flooding.

dest seq. next last hop

no hop hop count

D … N1 L1 …

D … N2 L2 …

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

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 @ 2 packets/s.
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.