In addition to the original attributes that the sink is interested
in, the packets contain an extra attribute E, representing the energy cost of
forwarding the packet from source to current nodes. Every time a node receives an
unseen event packet, it adds the cost for forwarding that packet to attribute E. The
energy cost can be the real estimated energy consumption, or just the number of hops
the packet has been forwarded so far. Therefore when the sink receives the packets,
the energy cost of forwarding the interests along different paths can be collected, and
the path with minimum energy cost is reinforced.
Once a reinforced path is constructed, the protocol tries to increase path sharing
by letting other sources use this path to forward packets, i.e., constructs a tree such
that all sources are connected to the reinforced path using the shortest path in terms
of energy cost. The protocol uses incremental cost messages to achieve this goal.
When nodes on the tree (initially the tree contains only nodes on the reinforced
path) receive an unseen event from other sources, it generates an incremental cost
message that contains the message id corresponding to that event and the additional
energy cost C required for transmitting the event to the tree. The incremental cost
message is only sent along the reinforced path. When a node receives the message, it
searches the packets of corresponding events from its message cache and compares
the energy cost E of the cached packet with C.
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