Re: Telephone Access Controllers (TACs) and SLIP...

Ron Broersma (
Wed, 18 Nov 87 12:10:01 PST

We have implemented such a service locally. Our slip "server" (currently on
a vax running 4.3) listens on a bunch of tty ports allocated to this service
for slip requests. As soon as it gets carrier on one of the ports, it accepts
various requests, one of which is "slip <userid> <password>". Then it sends
back the IP address, netmask, and default gateway to be used by the caller.
The caller can then send IP packets out its RS-232 port, packaged with
slip. This is the way many of our PCs talk via our Sytek LAN if they don't
have ethernet access.

The reason we bypass getty/login is for speed reasons. In some cases the userid
and password are optional which allows very quick setup of the SLIP link. It
doesn't have to be done this way, most would want to implement this scheme
by logging in through a login port and then issue a command to enter SLIP mode.

The way we generate IP addresses is to have a specific address per port on the
server. All these ports are on the same subnet. It doesn't really seem to
matter that the PCs accessing the net via SLIP don't have fixed IP addresses.
You only have to allocate one address per server port and you don't have to
worry about setting up a new IP address every time somebody gets a new PC (and
we already have over 3000 of 'em). Routing is easy since we put them all on
a separate subnet.

It would be very nice if we could agree on a "SLIP link establishment protocol"
so we don't come up with multiple schemes for doing the same thing, all
totally incompatible. My scheme works but could be improved. It solves the
general problem for us but should allow the case where the client has a fixed
IP address and/or hostname.


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