Example of TORA operations

A

B

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X

• From last time, this was the DAG that was built.

• A was the source and X was the destination.

Link 1

Link 4

Link 3

Link 2

Link 5

Link 6

Link 7

Link 8

Reacting to failures: Route Maintenance• Let Link 4 fail.

• At this time notice that other than the destination all nodes still have an outbound link.

• Thus, none of the nodes generate an UPD message.

• The DAG is still OK !

• This is especially attractive when the network is dense – most nodes have many outbound links.

A

B

C

D

E

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X

Link 1

Link 4

Link 3

Link 2

Link 5

Link 6

Link 7

Link 8

• Let Link 7 fail.

• Now, Node E does not have any outbound links !

• Thus, we resort to full link reversal at E.

• E generates a new reference level which is 1, sets the oid to E and transmits an UPD message.

• It also reverses the direction of all its inbound links.

A

B

C

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Link 1Link 3

Link 2

Link 5

Link 6

Link 7

Link 8

(1,E,0,0,E)

• At this, Node C no longer has outbound links.

• It resorts to partial link reversal – reverses the direction of its links to A and B and transmits an UPD.

• It also sets its own offset to –1 to ensure that it is at a lower level compared to E.

A

B

C

D

E

Y

X

Link 3

Link 2

Link 5

Link 6

Link 8

(1,E,0,-1,C)

Link 1

• Now the situation repeats at B.

• After B reverses its links and transmits an UPDATE.

A

B

C

D

E

Y

X

Link 3

Link 2

Link 5

Link 6

Link 8

(1,E,0,-2,B)

Link 1

•The situation repeats at A.

•This is now a full reversal. A got updates from B and C.

•Thus, A stays at the same level as C, but indicates the full reversal by flipping ri.

• This causes a partial reversal at B.

A

B

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D

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Link 3

Link 2

Link 5

Link 6

Link 8

(1,E,1,-2,A)

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B

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Link 3

Link 2

Link 5

Link 6

Link 8

(1,E,1,-3,B)

Link 1

Link 1

•Finally an update is generated at C.

• DAG is restored !

A

B

C

D

E

Y

X

Link 3

Link 2

Link 5

Link 6

Link 8

(1,E,1,-4,C)

Link 1

• Now let Link 5 fail. This causes a network partition.

•E,D,Y and X are ok.

• C has no outbound links.

• It creates a new reference level which is 2, and sets the oid to C and sends a UPD.

• This causes B to have no outbound links.

A

B

C

D

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Y

X

Link 3

Link 2

Link 5

Link 6

Link 8

(2,C,0,0,C)A

B

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Link 3

Link 2

Link 6

Link 8

Link 1

Link 1

• B resorts to partial reversal.

• It reverses its link to A and broadcasts an update.

• Now A does not have outbound links.

• It resorts to a full reversal. At full reversal ri is flipped.

• This causes a partial reversal at B.

(2,C,0,-1,B)

A

B

CLink 3

Link 2

Link 1

(2,C,1,-1,A)

A

B

CLink 3

Link 2

Link 1

• B’s UPD message after the partial reversal creates the same situation at C.

• This would cause C to realize that there is no path to X. It sets its height to NULL and sends an UPD to A and B.

• Now the nodes realize that there is no path to X.

(2,C,1,-2,B)

A

B

CLink 3

Link 2

Link 1

• Important Point : Height is reduced with respect to node that created the reference level.

Advantages:

• That of an on-demand routing protocol – create a DAG only when necessary.

• Multiple paths created.

• Good in dense networks.

Disadvantages

• Same as on-demand routing protocols.

• Not much used since DSR and AODV outperform TORA.

• Not scalable by any means.

References

• Chapter 8 of book.

• V.D.Park and Scott.M.Corson, “A Highly Adaptive Distributed Routing Algorithm for Mobile Wireless Networks”, Proceedings of INFOCOM 1997.

Associativity Based Routing (ABR)

• Proposed by C-K.Toh currently at Georgia Tech.

• Introduces a new metric for routing which is called “Degree of Association Stability”.

• ABR is free from loops.

• What is association stability ?

How stable are nodes with respect to each other ?

Based on an estimate of this, a route is selected.

• Each node would periodically generate and transmit a beacon to signify its existence.

• Neighboring nodes receive this beacon.

• They, maintain what is known as an associativity table.

• For each beacon received, the associativity of the receiving node with respect to the beaconing node is incremented.

• Thus, to reiterate there is an indication of how stable nodes are with respect to each other.

• Association Stability is defined by the connection stability in “time” and “space” of one node with respect to another.

• Associativity entries (or ticks as they are called) are reset when the neighbors of a node, or the node itself moves out of proximity.

• Goal :?

• Longevity – longer lived routes for stability.

• Lower Overhead (?)

• Three phases of ABR:

Route Discovery

Route Re-construction

Route Deletion

• NOW HERE IS A NEW CONCEPT !!!!!!

• Each node broadcasts a query message (BQ message just to sound different ! ) in order to find a destination.

• In addition to the address, the associativity ticks with respect to their neighbors is appended.

• The receiving node, chooses the best one, i.e., retains only the entry corresponding to itself and its upstream node.

• Thus, at the destination, multiple routes are available.

• It chooses the one that is best in terms of associativity ticks.

• If there is a tie, choose shortest path.

ROUTE DISCOVERY

• Destination generates a REPLY message towards the source.

• Intermediate nodes that forward this message mark the corresponding routes as valid.

• Thus, only one route at a given time.

• Partial route recovery is allowed.

• Intermediate nodes will try and rediscover route from point of failure.

• When routes are no longer valid, they would be erased.

• This is similar to DSR – except for associativity.

• In addition partial route discovery.

ROUTE MAINTENANCE

Advantages:

• Tries to find stable routes – lower overhead in some scenarios.

• Partial recovery may be faster in some cases.

Disadvantages

• Unclear if the overhead incurred in maintaining stability info. is higher than the actual gains. Depends on scenario.

• Partial recovery may lead to longer and less stable routes.

References

• Chapter 9 on book.

• Read about effects of beaconing on battery life.

• C-K. Toh, “A Novel Distributed Routing Protocol to support Ad Hoc Mobile Computing”, Proceedings of IEEE 15th Annual International Phoenix Conference on Computers and Communication”, March 1996.

• E.Royer and C-K.Toh, “A Review of Current Routing Protocols for Ad-Hoc Mobile Networks”, IEEE Personal Comm. Mag. April 1999.

Signal Stability based Adaptive Routing (SSA)

• Prof. Tripathi’s work.

• Reference: R.Dube, C.D.Rais, K.Y.Wang and S.K.Tripathi, “Signal Stability based Adaptive Routing for Ad Hoc Mobile Networks”, IEEE Personal Communications Magazine, February 1997.

Principle of SSA

• Select routes based on the signal strength between nodes and on a node’s location stability.

• Choose routes that have stronger connectivity.

• SSA has two component co-operative protocols:

• The Dynamic Routing Protocol

• The Static Routing Protocol.

The Dynamic Routing Protocol (DRP)

• The DRP is responsible for maintaining what is called the Signal Stability Table and also the Routing Table.

• SST – record of signal strengths of neighboring nodes which is obtained by means of periodic beaconing.

• Quantized levels possible weak channel vs. strong channel.

• When a packet is received, DRP processes the packet, updates the tables and passes the received packet to the SRP.

The Static Routing Protocol (DRP)

• Forwards the packet up to the transport layer if it is the receiver.

• If not, it looks up the routing table and forwards the packet to the appropriate next-hop.

• If no entry is found, it initiates a route search.

The Route Search

• Route requests are propagated throughout the network – however ...

• Forwarded onto the next hop, only if they are received over “strong” channels” and have not yet been previously processed. /* Notice that the second condition prevents looping */.

• The destination chooses the first arriving query message because it is most probable that the packet arrived on the “strongest”, shortest and/or least congested path.

• DRP reverses the route and sends a route-reply back to the sender.

The PREF field

• Notice so far that a route-search packet is forwarded only if it arrived on a strong link.

• However, it is possible that no route is found with strong links all the way.

• At this time, the source initiates another route-search and uses what is called the “PREF” field to indicate that weak links are acceptable.

Route Maintenance

• Use a route error message to the source to indicate which channel has failed.

• Source then initiates a new route-search to find a new path to the destination.

Advantages:

• More stable routes since signal strength indicates stability.

• No overhead incurred in dissemination of tables.

Disadvantages

• Of on-demand routing protocols.

• The time over which the signal strength is averaged out might be an issue.

Location Aided Routing (LAR)

• By Nitin Vaidya and Young-bae Ko.

• Reference: “Location Aided Routing (LAR) in Mobile Ad Hoc Networks”, Y.Ko and N.Vaidya, Proceedings of Mobicom 1998.

• Idea: Use Location Information for doing Routing – does the name give a hint or what ?

• The location info is obtained using GPS (Global Positioning System).

• Forward the packet towards the destination instead of forwarding it indiscreetly.

Global Positioning System (GPS)

• It is a system that allows a mobile user to know its physical location.

• There is however some error in the estimated position.

• Different systems have different errors – few meters to about 100 meters.

The notion of expected zone

• If a source (S) knows that a destination (D) was at position L at time t0, it has some notion of where the destination might be at a later time t1.

• If the average speed of D is v, then S might expect that D is within a circular radius of v (t1-t0) which is centered at L.

• However, note that this is only an estimate. If the actual speed was higher, the node might be outside this circle.

• This circular region is called the “expected zone”.

Why is the expected zone important ?

• If we have an on-demand routing protocol, now we can disseminate a query within the expected zone !

• Note that if there is no information whatsoever with respect to the destination, query search would degenerate to pure flooding.

• On the other hand, if we further knew that D was moving north, then the expected zone may be further refined.

The Request Zone

• Source node defines what is known as a request zone.

• A node would forward the request only if it belongs to the request zone.

• Typically, the request zone would include the expected zone.

• Additionally nodes in other regions around the expected zone are to be included in the request zone. Why ?

The Request Zone (Continued)

• Source node might not belong to the expected zone. Thus, other nodes which are not in the expected zone would need to forward request towards the expected zone.

• If with an initial request zone, the destination is not found (because no route exists entirely within the request zone), an expanded request zone may be defined.

• In an extreme case, the expanded request zone might be just doing flooding.

• There is a tradeoff between latency and message overhead.

Defining the Request Zone

S

D

v(t1-t0)

Entire Network

Expected Zone

Request Zone

• In the first scheme, the suggestion is that the request zone be the rectangle that includes the source and the expected zone of the destination.

• Source determines the co-ordinates of the rectangle and includes in query.

• A node upon receiving this, will forward the query only if it is within the rectangle.

• The destination node, Node D would respond with its current location, and current time in its reply message.

• It could also include its current speed, or average speed over a recent time interval.

S

D

v(t1-t0)

Entire Network

Expected Zone

Request Zone

• Initially destination’s location, D is unknown.

• The size of the request zone depends upon the average speed of movement of the destination “v”, and the time elapsed since the last known location of D was recorded.

•If v is large, larger expected zone, larger request zone.

• If communication with D is infrequent, it may result in a larger request zone.

• It may be possible to piggyback location info. on packets other than the query.

Size of the request zone

A Second Possible Scheme for determining the Request Zone.

• Source node knows location (X,Y) of dest at some time t0. It calculates its distance from (X,Y) = Ds.

•It includes both, (X,Y) and Ds in the query message.

• A node I, forwards it if for some parameter d, if its own distance from the destination DI <= Ds + d.

• If this message is further received by node J, node J would forward it only if its distance from the destination DJ <= DI + d.

• Obviously the idea is that you try and get closer and closer to the destination.

• More details in the paper.

• Finally, remember we said GPS could have some error.

• Thus, some leeway has to be provided to take this error into account while determining the expected zone and the request zone.

• That is my brief intro to ABR, SSA and LAR.