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2.4 Routing Algorithm
The routing algorithm directs each packet to its destination
along a path for which the total estimated transit time is
smallest. This path is not determined in advance. Instead, each
IMP individually decides onto which of its output lines to
transmit a packet addressed to another destination. This selection
is made by a fast and simple table lookup procedure. For each
~osslble
destlnatlon, an entry ln the table designates the appropriate
next le~. These entries reflect line or IMP trouble,
traffic congestion, and current local subnet connectivity. This
routing table is updated about every two-thirds of a second, as
described below.
Each IMP estimates the delay it expects a packet to encounter
in reaching every possible destination over each of its
output lines. It selects the minimum delay estimate for each
destination and periodically passes these estimates to its immediate
neighbors. Each IMP then constructs its own routing table by
combining its neighbors' estimates with its own estimates of the
delay to each neighbor. The estimated delay to each neighbor is
based upon both queue lengths and the recent performance of the
connecting communication circuit. For each destination, the table
is then made to specify that selected output line for which the
sum of the estimated delay to the neighbor plus the neighbor's
delay to the destination is smallest.
The routing table is periodically and dynamically updated
to adjust for changing conditions in the network. The system
is adaptive to the ups and downs of lines, IMPs, and congestion;
it does not require the IMP to know the topology of the network.
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