07 Sep 86 17:32:27 EDT (Sun)
Vint et al,
A minor clarification. There is no general algorithm in use
which spreads traffic across multiple gateways. There is however
a special-purpose table-driven algorithm in the Arpanet/Milnet
mailbridges, which attempts to spread traffic amongst the seven
parallel mailbridges. It is effective only for hosts which are
physically connected to the Arpanet or Milnet, but not gateways,
because hosts listen (should at least) to redirects but gateways
do not. Fortunately this covers a lot of the traffic; without
this mechanism, the mailbridge would be a highly reliable
bottleneck (more so than it is now), since only one path would be
"best", with six !! idle backup gateways at any time.
Unfortunately, with more LANs all the time, this interim approach
is clearly limited.
There is a feature in design stages now for the C/30 family,
which will permit multipath routing to occur -- i.e., between
points A and B several parallel paths might exist, and all might
be in use to carry traffic. With such a mechanism, for example,
a network with only 9.6 kilobit/sec trunks could achieve a
host-host throughput greater than 9.6 by using the multiple
paths. This is a very hard problem, to achieve stability and
efficiency and avoid behavior like the Atlantic two-step.
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