Gugrajah_Yuvaan_ Ramesh_2003.pdf

Routing Protocols for Ad Hoc Networks Chapter 2
the DAG with respect to a destination a logical direction of the link is defined based
on the relative height metric of neighbouring nodes. The destination is assigned a
height metric of zero. If a source has no route to a destination, it broadcasts a route
query packet (QRY), which is propagated outwards by its neighbours. After
receiving the QRY, a node that has a route to the destination broadcasts a route
update packet (UPD) containing its own height relative to the destination. Each node
receiving the UPD sets its own height metric for the required destination one higher
than the node from which it received the UPD. This results in a series of directed
links from the source to the destination in order of decreasing height. As a
consequence of this multiple routes are often present for a given destination, but
none of them are necessarily the shortest route. The analogy can be made between
the path taken from source to destination and a stream flowing downhill.
When a node discovers that a link is broken, it sets its height metric higher than that
of its neighbours, and issues a UPD to that effect reversing the direction of the link
between them. Links are reversed to reflect the change in adapting to the new
reference level. If a node finds that it has no downstream neighbours, the destination
is presumed lost and a clear (CLR) packet is issued to remove invalid links from the
rest of the network.
Timing is an important factor for TORA because the "height" metric is dependent on
the logical time of a link failure and TORA therefore assumes that all nodes have
synchronized clocks. Figure 2-3 shows the height metrics for the destination after a
QRY has been broadcast by the source and the destination has replied with a UPD.
Figure 2-4 shows the re-establishment of a route after failure of a link. The
assignment (x, y) in Figure 2-3 and Figure 2-4 to each node gives the sequence
number x (based on time) for the height metric and the height metric y for the
required destination.
When multiple sets of coordinating nodes are concurrently detecting partitions,
erasing routes, and building new routes, there is a potential for oscillations to occur
in TORA. This instability is similar to the "count-to-infinity" problem in distance
vector routing protocols except that such oscillations are temporary and route
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