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In a manner similar to how the number of parent nodes may be controlled
through the p max parameter, the number of children may be controlled by a parameter
c max . This allows for more control over the structure of the tree and has two
advantages.
• A node is less prone to be overloaded by multicast forwards. The more outgoing
edges a forwarder has the more messages are to be sent. In environments
without native multicast support every inbound message causes the same
number of transmissions as there are children of this forwarder.
• Subscription storms are avoided. As a consequence of a parent node failure, all
former child nodes try to resubscribe to other nodes in a short time frame, resulting
in a subscription burst. This is undesirable in terms of network utilization.
The c max value was set to unlimited, 25 and 10. In Fig. 7 the effects on
the path length are shown. The effects on the success ratio in Fig. 6 are a direct
consequence of the increased path length. The impact of a c max = 25 can be seen
as a compromise between the disadvantage of reduced resilience against churn
and the advantage of improved balance of the dissemination structure. A value
of c max = 10 shows a more grave impact on the performance of the system. When
c max is too low, it becomes more likely that nodes are unable to subscribe because
all parent candidates have already reached their maximum child count. The c max
value should be chosen according to the expected churn, the network transfer
rates and the number of subscribers.
Table II. KadScribe parameters
KadScribe
p max 1, 2, 3
c max ∞, 25, 10
parentTimeout
45 s
childTimeout
45 s
4
3.5
3
path length
2.5
2
1.5
1
0.5
0
inf 25 10
c max
Figure 7. Effect of c max on the path length