Radia Perlman (radia@XX.LCS.MIT.EDU)
Tue 17 Nov 87 11:24:09-EST
Source routing as Jacob Rekhter suggests (i.e. having the first gateway
put the entire route in the header, which it computes based on
a complete map of the network) should not be compared to the
"source routing" advocated by 802.5. In the 802.5 scheme the
route is not calculated by the gateways, but instead the burden
of discovering and maintaining routes is placed on the end stations (maybe
you guys use the word "hosts" -- I'm from a different culture).
Discovery of a route in 802.5 is done by having the source end station
emit a sort of virus packet that proliferates
and travels over all the exponential possible
routes throughout the network, each copy recording the route it has taken.
Then the destination end station returns all the copies it receives
to the source end station, which can then enjoy the task of selecting
the "best route" based on any algorithm it likes.
With anything but a very small and very sparsely connected network,
the exponential overhead inherent in the route discovery process will
make the 802.5 proposal impractical. I believe the January issue
of IEEE Network Magazine will be a special issue on bridges, if people
want more information on the subject.
Anyway, that's not a criticism of Jacob Rekhter's scheme, just of his
comment that his proposal was "similar to 802.5 bridges".
There are lots of advantages to having the first gateway specify
the route. The main disadvantages are:
1) the header gets larger of course -- the networking community has
already been accused of offering a "headergram" rather than a "datagram"
2) the scheme gets more complex (though not impossible) when the
network gets large enough to introduce an extra level of hierarchy.
(So that the source gateway does not have a complete map of the network).
Radia Perlman (Radia@XX.LCS.MIT.EDU)
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