obligating packet transmissions to be only through the best delivery path,
SADV improves the data delivery performance.
(d) Route Maintenance
Instead of handling failures of already established routes, route maintenance in
VANETs involves handling failures in establishing routes due to intermittent
It may happen that a node does not find a neighbor that is closer to the destination than
the node itself. This case is known as reaching a local maximum. The routing protocol
should include a mechanism for handling such situations by deploying a recovery
mechanism. Many recovery mechanisms are introduced for the greedy protocols:
- The GPSR protocol recovers from a local maximum by entering a perimeter
mode where it follows a mechanism known as the right-hand rule. This rule
states that when a node enters the recovery mode, it forwards the data to the
neighbor that is sequentially counter-clockwise to the virtual line formed
between the node in the recovery mode and the destination [ 7 ].
- The A-STAR technique for recovery from lack of connectivity is to recompute
the path to the destination from the stuck node by temporarily marking the
disconnected part of the path to be ''out of service''.
- The delay-tolerant protocols (e.g., VADD) recover from disconnections by
having the stuck node store the packet until contact with another node. This
approach is known as store-carry-and-forward.
The selection process returns to the greedy mode once the packet reaches a
node that is closer to the destination than the node encountering the local maxi-
Also, because of the nodes' high mobility, the information obtained and utilized
in the beginning of the transmission may change and become invalid leading to
disconnectivity. Therefore, protocols should utilize mechanisms for maintaining
the route and network connectivity:
- The CAR protocol assumes that route disconnection results from the destination
movement. 7 It depends on the use of guard packets generated at the anchor
points. When a destination changes its direction, it announces that to the nearest
anchor point. When the data packet reaches the old destination's location, it will
be rerouted by the guarding nodes to the new estimated position.
In Ref. [ 75 ], Paul et al. presented a comparison among the well-known VANET
routing protocols by showing the pros and cons for each protocol.
It assumes that there is no disconnection problem among the anchor points.