dial-up SLIP

Craig Partridge (craig@NNSC.NSF.NET)
Sun, 15 Nov 87 11:35:35 -0500

At CSNET we're experimenting with an idea which is close to this.

The basic idea is that when an IP packet hits our gateway destined
for a remote machine we make a phone call, establish the link, and
keep it running as long as there is continued traffic. When the
traffic stops, we shut down the line.

There are lots of nasty little problems in this scheme:

    - The TCP SYN takes a huge hit for establishing the phone call.
    So the initial RTT estimate will be much too high.

    - Topology. That SYN probably can't take multiple hops in which
    the phone gets dialed. (We're using a star topology with gateway
    in the center).

    - How to maintain line quality. We know how variable long-distance
    connectivity is.

Interestingly, keeping track of when the line is busy probably isn't a
problem (we already had to handle that problem in X25Net).

Also note that this system is designed to support more than IP. We want to
be able to use more than IP over the interface (line control packets, ISO IP,
XNS, etc). So we had to put leaders in.

Finally, address mapping is fixed. Each address is assigned, and we
do a deterministic IP address to phone # mapping to do the phone call.
We then log in at the remote end, and start up the line protocol.


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