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From: karn@jupiter..bellcore.com (Phil R. Karn)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: IP over X.25 (request for info)
Message-ID: <13962@bellcore.bellcore.com>
Date: 10 Feb 89 22:47:40 GMT
References: <8902081711.AA07245@gateway.mitre.org> <11201@ulysses.homer.nj.att.com>
Sender: news@bellcore.bellcore.com
Reply-To: karn@jupiter.bellcore.com (Phil R. Karn)
Organization: Bell Communications Research, Inc
Lines: 18

>The point of doing a X.121->IP address map is to economize on virtual
>circuits.  That is, routes are often symmetric; if a packet from A to B
>goes through gateways G1 and G2, replies from B to A will often go through
>G2 and G1....

On the other hand, CSNET's experience with real commercial X.25 networks
like Telenet has shown that multiple parallel virtual circuits are often
necessary anyway to even approach acceptable performance. The problem is the
small X.25 packet size limit and the tiny packet-layer flow control window.
In the absence of a mechanism for increasing this window you are forced to
open up additional circuits and distribute your traffic among them. Face it,
X.25 was designed for remote slow-speed terminal networking, not serious
computer networking.

I can also say from experience that such a path is an excellent stress test
for your TCP segment reordering code...

Phil