Re: Packet network reliability

Andy Malis (
04 Apr 87 15:38:42 EST (Sat)


I'm afraid your experiment wouldn't cause much as much routing
traffic as you think. Your details on the PSN routing algorithm
were a bit wrong. First, there are three things that can cause a
routing update to be generated: a link going up or down, the
delay to a neighbor PSN changing more than a certain threshold,
or the expiration of 50 seconds from the last time the PSN
generated an update. Second, a PSN can generate an update at
most every 10 seconds, no matter how much is changing in the
network. Third, when a link goes down, it takes 60 seconds to
bring it back up (this ensures that if a PSN was isolated
from the network, it cannot bring up its links until it has
received at least one routing update from every other PSN in the
network; otherwise, its routing database would be incomplete).

The estimated time for a routing update to propagate through the
network is simply the network "diameter" (the minimum-hop path
between the two most logically distant PSNs in the network) times
the average propagation delay across the links. Flooding routing
updates gets the highest priority in the PSN, so intra-PSN delays
are usually negligible.

Since routing updates cannot be generated at a greater rate than
every 10 seconds by each PSN and at least every 50 seconds, it is
easy to calculate the minimum, average, and maximum number of
routing updates per minute on a network. And since the line
up/down protocol keeps links down for at least 60 seconds,
manually making links go up or down will probably not produce any
more updates than you would have had anyway.


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