In the operation of GPSR every node maintains the
knowledge of its one hop neighbors. Each node participating
in routing process selects the next-hop closest to the
destination (figure 1). This procedure is called greedy
forwarding. In GPSR, the locations of neighbors are obtained
by exchanging periodic HELLO messages among nodes.
When node receives a HELLO message from its neighbors, it
sets the HELLO timer (Ht) for each of its neighbors for the
next reception of the HELLO message. If it does not receive
HELLO message from a neighbor before the HELLO timer
expires, it assumes that the neighbor has gone out of range.