Phil R. Karn (
Tue, 24 Mar 87 21:01:57 est

This is misleading. In "unlayered virtual circuit" networks, connection
state information is kept not only in the end-point switches, but also in
every intermediate switch along the path. For example, I understand that
each GTE Telenet node speaks X.75 to its neighbors, making it an unlayered
VC network. Because of this, a node or link failure ANYWHERE along the
connection path causes an X.25 VC reset, not just failures at the end nodes.

Only in "layered VC" networks such as the DDN (where datagrams are used
internally) can intermediate node and link failures occur without resetting
virtual circuits. Since it is my understanding that the DDN is used
almost exclusively for passing Internet datagrams around, I never could
understand the reason for forcing IP gateways to deal with the gratuitous
complexity of X.25. I can only speculate that the reasons were political
and not technical. However, I am thankful that we will be able to come up on
the ARPANET with HDH instead of X.25.


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