TACs and SLIP...


William Westfield (BILLW@MATHOM.CISCO.COM)
Thu 12 Nov 87 13:57:50-PST


Oops. My question was slightly misunderstood (But it did cause a lot
of interesting responses). Perhaps a more detailed explanation is
called for, if I can do it without getting overly commercial...

There are essentially two models possible for doing a "slip server".
At cisco, we refer to these as the "terminal server" model, and the
"gateway" model.

In the gateway model, each slip connection has its own subnet, possibly
with another network on the other end of the link. Information about
how to reach hosts on the other side is all exchanged via normal routing
protocols, and the box is acting as a real gateway. The host can pick
its own IP address by virtue of pretending to be a gateway between the
SLIP link (whose address is fixed) and the host's network.

In the terminal server model, each slip connection is mapped to a
particular IP address, and the box ARPs for all of the resulting IP
addresses - essentially acting as a giant, extra long, very slow,
multi-port ethernet transceiver. Each slip speaking box gets assigned
an IP address on the same subnet as the terminal server itself. Since
no routing information is exchanged, only one IP address at the other
end is possible, and the slip server gets to pick it.

cisco has implemented the terminal server model of a slip server as
part of our terminal server. If enabled, users could issue a SLIP
command, and the exec prints out info telling them their IP address,
and then it goes into a mode where only IP packets are transferred.
(How to exchange this sort of information in a machine oriented
fashion is an unresolved issue - we're interested in any suggestions).
SLIP lines (actually, lines in general) can run up to 38.4Kbps, and
of course lines can get configured permanently in SLIP mode too.
Up to 96 terminal lines can be put on one box.

We think that the gateway model is also implementable, though the idea
of an 97 way gateway is somewhat mind-boggling. What may happen in
this area is a SLIP driver for the serial (normally HDLC) cards we are
currently using. Up to 6 lines per box would be possible.

The original question I asked was meant to mean:

Given that we have implemented SLIP on our ethernet terminal server,
and that we also sell a DDN terminal server (a "TAC" with 1822 or
X.25 interfaces), should be also attempt to have the DDN Terminal
server ALSO support slip, by way of the logical host mechanism that
exists in the DDN network world.

Im still interested in the answer to that question, but the discussion
on SLIP issues in general is interesting too, though it might be more
appropriate for, say, the PCIP mailing list.

Bill Westfield
cisco Systems.
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