a gradient toward the neighbor from which it received the interest. So, the
gradients determine the path back to the originator of the interest. Sensors that
hold data matching the interest send the requested data along the gradients.
In the exploratory phase, the sensed data is reported with a low rate to the sink.
The exploratory phase is followed by a reinforcement phase to get the data with
higher rate for more accurate event detection through specific nodes along a
specific path. Choosing the neighbor that is to be reinforced can be based on
many criteria. The simplest way is to reinforce the neighbor from which the first
response for the interest is received. This way, the reinforced path will be the
path with the lowest delay.
- The Rumor Routing protocol [ 36 ] is a variation of the DD protocol. Its key
idea is to limit flooding to the nodes that have observed events rather than to
the whole network. It uses agent packets and event tables to minimize the
interest flooding overhead of DD.
- The Gradient-Based Routing (GBR) protocol [ 37 ] is another variant of DD.
The key idea of GBR is memorizing the number of hops when the interest is
diffused. Each node calculates its height which is the minimum number of
hops to reach the BS. The difference between the node height and that of its
neighbor is considered the gradient of the link. The link with the largest
gradient is selected.
- The Reliable Energy Aware Routing (REAR) protocol [ 38 ] is based on
flooding interests but instead of discovering only a single path between a
source and the sink, REAR discovers an additional backup path to the same
source. In discovering the backup path, only the nodes that are not involved in
the service path will broadcast the backup path request to maintain two
completely disjoint paths. REAR also utilizes a path reservation mechanism
in which every intermediate node on the path will mark part of its energy as
reserved for this path.
- The COUGAR protocol [ 39 ] separates the query processing task from the
network layer and adds another layer, the query layer, between the network
and the application layers. It represents the network as a huge database system
with the BS being responsible for generating the query plan, defining the
query flow and its in-network computation and forwarding it to the relevant
nodes [ 2 ].
• Location-Aware Based
- An example is the Geographical and Energy Aware Routing (GEAR) protocol
[ 40 ] which is considered an improvement of the DD protocol. It exploits the
fact that the interests are usually formed for a target region and they carry
geographical information about it. It uses this geographical information to
direct the interests to the target region without flooding. It assumes that each
node has information about the location and current energy level of itself and
its own direct neighbors which are exchanged by the use of periodic Hello
Messages. For forwarding a packet, a node computes a cost for each of the
neighbors that are closer to the target region. This cost is calculated based on