Re: Packet network reliability


Walt Haas (haas%utah-gr@utah-cs.arpa)
Tue, 31 Mar 87 17:08:28 MST


In article <8703312012.AA25019@ucbvax.Berkeley.EDU>,
AFDDN.TCP-IP@GUNTER-ADAM.ARPA writes:

> Tightly binding resources in a single virtual path through the
> network does let you know what resources are going where and for how long...

True, this was a major motivation for Telenet's conversion to VC internals.

> Some nets I believe have a single route source that has global knowledge of
> the network and returns routes to the net nodes for every connection
> request.

The earliest version of Tymnet used something like this. The system did not
scale up, not so much because of the reliability problem but because of the
enormous amount of traffic to and from the Master User Directory. This
approach is no longer used.

> The disadvantage I see is that every VC will use up a given amount of net
> resources whether data is actually being transmitted across the VC or not.

Only a small table entry is used. This is in practice less costly than
the communications resources used to convey a complete address in every
packet.

> In the cases where static routing is downloaded from a central site, the
> survivability of the network as a whole is only as good as that of the
> central site.

True. Tymnet went to four regional sites for routing information, which
helps with robustness and distributes the traffic a little more evenly.
Telenet assigns net addresses geographically, and each TCO routes based
on the implicit geographic information in the Call Request packet.
Thus each TCO can make routing decisions independently of any central
source of routing information. This is very robust.

Cheers -- Walt



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