Re: Packet network reliability

31 Mar 1987 14:01:02 CST

I'm in the same boat with Dave; VC versus datagram is one of my buttons.

I'm much more familiar with the BBN way of doing things than with PDNs. I can
see where PDNs would want to have very tight control over everything going on
in the network. Tightly binding resources in a single virtual path through the
network does let you know what resources are going where and for how long. I
think it also tends to stabilize network delays around some median which might
be nice. You can also establish stable (fixed) routing. 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 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. 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.
BBN gets around these two disadvantages at a cost. The Milnet for instance
is gettingvery large, very fast. I don't know the formula for computing the
amount of bandwidth required for the internal routing updates, but I would
suspect it would go up exponentially with the number of nodes(and connectivity
of them). If anybody knows the numbers I'd like to have them.

The current limit of 256 nodes on one network has already been reached for
Milnet. They nodes aren't installed and operational yet, but the addresses
have been allocated to future nodes. And even with the size of the backbone,
there are nodes with 12-18 hosts connected or slated for connection. Maybe
we need "butter" switches?

Darrel B.

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