A node disseminates a RREQ when it determines that it needs a route
to a destination and does not have one available. This can happen if
the destination is previously unknown to the node, or if a previously
valid route to the destination expires or is marked as invalid. The
Destination Sequence Number field in the RREQ message is the last
known destination sequence number for this destination and is copied
from the Destination Sequence Number field in the routing table. If
no sequence number is known, the unknown sequence number flag MUST be
set. The Originator Sequence Number in the RREQ message is the
node's own sequence number, which is incremented prior to insertion
in a RREQ. The RREQ ID field is incremented by one from the last
RREQ ID used by the current node. Each node maintains only one RREQ
ID. The Hop Count field is set to zero.