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Fig. 4. The Prototype Sensor
emit. The listener responsibility is to decide whether they accept the messages, and
thus become the receiver. To this eect, and to keep the trac at the IP layer for
exibility, all the trac for the opportunistic support is encapsulated in broadcast
UDP packets (by default over port 8181).
Another interesting problem is time-stamping of messages: as sensor and mobile
nodes do not have reliable clocks, and the transit time through the network is
highly variable, it is dicult to know when exactly the notication was generated.
Our solution is to record the in-transit time of each message in a special eld. Each
time a node forwards a message, the time that the message spent stored in the node
is added to the in-transit eld. In this manner, the issuing time of a message is
computed by the receiving node in reference to its local time. For the purposes of
DEMOS project, the precision of this method is satisfactory.
6 Management Footprint
The focus of this paper is on the feasibility of embedding a high-level self-management
system into the constrained devices of a network of sensors. With this goal we have
set up a scenario with one sensor measuring the temperature of the air and pub-
lishing it, and a client producing an increasing number of dierent subscriptions.
6.1 Experiment
The objective of this experiment is to show that the footprint of the system pre-
sented above in Section 4.1 is small enough for the targeted constrained devices.
 
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