Re: network horror stories


Walt Haas (haas%utah-gr@utah-cs.arpa)
Fri, 27 Mar 87 14:38:38 MST


One of the more interesting but little-discussed events in the
history of network engineering occured when Telenet converted
from unreliable-datagram internal architecture to VC internally.
For a good discussion of the whys and wherefores of Telenet internal
architecture see a paper entitled "An X.75 Based Network Architecture"
by D. F Weir, J. B. Holmblad and A. C. Rothberg published in the
"Proceedings of the Fifth International Conference on Computer
Communications", 1980. If you don't feel like chasing the Proceedings
around the library Telenet will probably give you a free copy of the
paper.

The justification for ripping out the datagram code and replacing it
with VC code was economic. There is less waste and better management
of the resource in a VC network. I quote from the cited paper:

 "... in the late 70's it became economically attractive to incur
  additional processing and storage costs in order to reduce
  communications costs...

  ... By establishing fixed paths, virtual circuit routing can
  better balance load as compared with routing in a datagram based
  network ... congestion at a transit point in a virtual circuit
  network can be reflected back to the endpoint nodes to restrict
  flow into the network. This capability results from access to
  the virtual circuit at transit nodes using the logical channel
  number. In a datagram network, knowledge of virtual calls does
  not exist at transit nodes and flow control cannot easily be
  applied to the virtual circuits at the endpoints."

Cheers -- Walt



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