[ih] How long did it take?

[Craig Partridge]

On Thu, Sep 17, 2026 at 11:28 AM Dave Crocker via Internet-history <
internet-history at elists.isoc.org> wrote:

>
> > I failed to see how X.25 could be used as an internet.
>
> IP/X.25, using X.25 as a link layer.  For CSNet, Purdue wrote a Unix
> driver to do that.
>
> For the original CSNet email service, everything was dial-up, but that
> quickly included dial-up to a commercial x.28/x.29 commercial service.
> This meant that MMDF's packetized phonenet link layer ran on top of the
> commercial X.25 packet service.
>
> That is, one packet layer over another.  Purdue's driver similarly was
> packet over packet.  We each, separately discovered how important it was
> to make sure the top layer packet was no larger than the bottom layer
> one, if possible.  Otherwise, it doubled the commercial service's packet
> count, and charging was by the packet...
>
> d/
>

CSNET used that Purdue code to connect a handful of CSNET sites to the
Internet.

As I recall, and my memory on this is rather hazy, some of those sites were
in Europe so we went from an X.25 provider (AT&T) in the US to an X.25
provider in Europe.  This meant we went through an X.75 gateway that only
allowed, as I recall, *two* X.25 packets in flight in each direction (and
the pages were small - 128 or 256 bytes).  (It is useful to recall that
X.75 terminated the X.25 connections, so it keeping the window small was
important to limit buffering).  So fragmented IP datagrams over X.25 with a
serious delay-bandwidth constraint.  Again, if memory serves, delays were
long (200ms or so).  It worked out to about 8Kbps.

Over time, the reaction was what you'd expect: (a) customers pushed to use
premium dialup modems (the Trailblazer in the late 1980s could hit
19.2Kbps); (b) dialup SLIP; and (c) leased lines.

Craig


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