(a) Highly Dynamic Topology
Since vehicles are moving at high speeds, especially on highways, the topology
in a VANET is subject to frequent changes. Routing protocols should provide
mechanisms for maintaining the followed routes and handling link changes.
(b) Intermittent Connectivity
It can be a common case in VANETs that a vehicle will not find a neighbor in
its vicinity to forward its data to and it will have to keep the data till it comes into
contact with another vehicle. This will be the case in sparse environments. Even in
dense environments, traffic lights and stop signs may lead to some network par-
titions. Routing protocols should provide mechanisms for handling this intermit-
tent connectivity. The most common mechanisms are the Store-Carry-Forward
mechanism for delay-tolerant networks and either the use of Road Side Units
(RSUs) for relaying messages, or depending on finding an alternative path using a
recovery mechanism in delay-sensitive networks.
(c) Restricted Mobility Patterns
Vehicles mobility patterns are restricted by road topology and speed limits. This
may be considered advantageous because these restricted patterns can help in
predicting future conditions (e.g., traffic conditions and vehicles positions). This
feature will help the routing protocols to make more informed decisions [ 1 ].
(d) Sufficient Resources
Vehicles have several advantages over other types of mobile nodes, including
abundant power, processing, and storage resources; these will provide more
flexibility for routing protocol design. VANET routing protocols can relax the
need for energy-efficient routing mechanisms. In addition, having sufficient pro-
cessing and storage resources, VANET routing protocols do not have to be
compact in size and complexity.
(e) Delay Constraints
As the most common applications supported by VANETs are related to safety,
VANET applications often impose hard delay constraints. Routing protocols
should ensure continuous connectivity for such applications to avoid incurring any
delays due to disconnections and expedite the connection setup times to keep the
transmission delay as low as possible.
(f) Availability of Information Providers
The vehicle's sensor readings can be utilized in the routing protocols to enhance
their functionalities. For example, GPS position information and the vehicle's
speed obtained from the speedometer can be used to assist in designing efficient