replogle@ncsa.uiuc.edu (Joel Replogle) (12/21/90)
John, We did a project with switched-56 to check into the possibility of doing just what you want - use a line only when you need it. In checking into it we found there were a very limited number of sites which had the sw56 service available, and that there were some incompatibilities based on the type of switches (our campus DMS-100's sw56 didn't converse with the local community's AT&T switch). We pursued more of a host-to-network connection in an attempt to avoid the routing issues (whew!). We had a hard time finding equipment to run at 56k, and didn't find anything which didn't bundle X.25 in with it (we *really* didn't want to run over X.25... sigh). Sw56 service is similar to plain-old telephone service in that you dial a number and get a connection. One question we wreslted with and then decided to punt: What do you do with routing to the remote site when the circuit isn't up? a) no route. Means and establish-circuit-based-on-traffic probably wouldn't be able to detect the packets.. unless the detector was located at the same place where the default route originates. b) static route. Detection would work, but how do you avoid black- holing traffic if the connection doesn't come up in time, or the line is busy? Possibly have the detector send back ICMP network unreachables if the line is busy..? Both of these aren't really much of an issue if the detector/switched router is located on the default-originating net. If there is an alternate path via default then both the routing and the call setup can be a little cleaner. Joel Replogle NCSA Networking Development