Example of TORA operations
A
B
C
D
E
Y
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
Y
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
D
E
Y
X
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
C
D
E
Y
X
Link 3
Link 2
Link 5
Link 6
Link 8
(1,E,1,-2,A)
A
B
C
D
E
Y
X
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
E
Y
X
Link 3
Link 2
Link 5
Link 6
Link 8
(2,C,0,0,C)A
B
C
D
E
Y
X
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.
