SLIP software available

Russ Hobby (!
25 Nov 87 20:53:12 GMT

Yes, we have a SLIP server implementation now ready. The reason I have
not posted it sooner is because we have been working with the Sun NFS
people on the method of establishing a dialin slip connection and have
been refitting some of their suggestions into our version.

The software allows PCs to make dialup SLIP connections to the campus IP
network. We are also have worked out a method of abbreviated serial line
IP (ASLIP) packeting that makes dialup IP networks more efficient. The
ASLIP software is new to this version and Sun has not seen it yet.

Here is how the system works. The user logs on to the host that is
acting as the gateway, a 4.3 bsd system. He then types in the command
"slip" and he becomes a host on the network. He can then use all the
programs that come with the CMU/MIT PC/IP or Phil Karn's system. To make
connecting to the network a little easier, we have written a program
that will do the complete login automatically. The program has a user
configurable script file that specifies a sequence of strings to send
out the serial line and responses to wait for coming back. It has its
own simple language with branching depending on if the correct response
came back. The net result is that after the script has been set up, the
user types in one command on the PC to connect to the network.

Unlike some PC/IPs, our system assumes that each PC (or actually each
user) has its own, permanent IP address. Security is provided by logon
security on the gateway host. The IP address of the PC is associated
with the usercode on the gateway host. The network connection for that
PC's IP address can only be started from a user logged in with the
correct usercode. The system also makes sure that the IP address is not
already connected before making a new connection.

The way we have set up IP address for the PCs is to have a separate
subnet that contains all the PCs. This way the gateway host needs only
to advertise that it is a route to that subnet and all the PCs are
covered. In essence the gateway host is the network for all SLIP
connections on that subnet.

The abbreviated packets work on the assumption that the connecting
computer is an endnode and will not be doing any routing. In this case
many of the fields in the IP packet are unnecessary. ASLIP uses the
minimum header size based on this assumption. With ASLIP the header size
is either 8 or 4 bytes, much smaller than the standard 40 bytes. The
host that is acting as the ASLIP gateway rebuilds the outgoing packets
to legal IP packets before sending them out the network and abbreviates
the incoming packets from the network. The same server software handles
both SLIP and ASLIP. It only goes into ASLIP mode once it has received
an ASLIP packet from the PC, thus if only SLIP packets are sent, it will
stay in regular SLIP mode. I will post more details on the ASLIP
protocol later.

Also there have been some terminal server vendors interested in this
project. It should not be much work to turn a terminal server into an
ASLIP or SLIP server and that would make it cheaper than using a VAX as
the gateway. Plus there would not be as much maintenance and downtime
with a simple server box.

The software is available via anonymous FTP from and is in
the directory dist/slip/. This includes all software to run the
SLIP/ASLIP server on a 4.3 bsd system, and the modifications to CMU
PC/IP software to implement ASLIP and the auto-login program, plus fix a
couple bugs. See the README file in this directory for a discription of
what the various tar files give you.

The next thing we want to add to the system is BOOTP so that the PC
software does not have to be configured for IP address, but rather get
it from the server.

                                Russell Hobby
                         Data Communications Manager
     U. C. Davis
     Computing Services BITNET: Computing Services BITNET: RDHOBBY@UCDAVIS
     Davis Ca 95616 UUCP: ...!ucbvax!ucdavis!rdhobby
     (916) 752-0236 INTERNET:

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