To circumvent this, GPSR computes a planar
subgraph of this graph in a distributed manner, and applies the right-hand
rule to this subgraph. When this traversal reaches a node that is closer to D
than A, greedy forwarding resumes.
D
A
Fig. 5. Get around void space using right-hand rule [16].
From our discussions so far, two facts are obvious. First, GPSR provides
the functionality to deliver a packet to a node at a specified location. Second,
GHT requires the ability to store an event at the node closest to a specified
location (called the home node for an event). (More precisely, GHT maps
an event??™s name to a geographic location, and one way of implementing a
GHT would be to store the event at the node closest to that location). Thus,
there is a subtle di?®erence between what GPSR provides and what GHTs
require. In particular, GPSR would drop a packet if there exists no node at
the destination location specified in the packet header, while GHT will store
the data at the nearest node to the destination. We will discuss how GHT
uses GPRS.
Assume that GHT hashes an event to a destination location d, and, without
loss of generality, that no node exists at that location (Figure 6). When
GPSR routes the packet containing the event, it will enter perimeter mode at
the home node (by definition, the home node will have no neighbor closer to
the destination than itself). Furthermore, the packet will remain in perimeter
mode and return to the home node in that mode (again, by definition, no
other node on the perimeter can be closer to the destination than the home
node).
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