A node may send a
gratuitous PREP at any time (e.g., as the path to the Root changes or its peer links
change). In the on-demand mode, the PREQ specifies a particular destination node. As
in the pro-active case, the PREQ may specify that intermediate nodes may respond
with a PREP message in case they have a path to the destination of the PREQ. The
Wireless Mesh Networks 346
propagation of the PREQ establishes a set of forward paths from the originator to ??“ in
principle ??“ all other nodes in the mesh. Over time, nodes may receive multiple PREQs
from the same originator; it will retain the most recent forward path information.
Aging of the path data maintained by each node assures that unused forward paths are
deleted.
??? PREP ??“ Path (Set-up) Reply
Nodes that receive a PREQ addressed to themselves, respond with a PREP. Unlike the
PREQ, the PREP is not flooded throughout the mesh but targeted at the originator of
the PREQ. Its propagation establishes the reverse path from destination to originator.
A flag in the PREQ allows intermediate nodes on forward path to respond to the
PREQ if they have a path to the requested destination. In a mesh that is sufficiently
connected, this mechanism considerably reduces the amount of PREQ flooding needed
to set up a path to a given destination.
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