Path: utzoo!attcan!utgpu!watmath!att!bellcore!jupiter!karn
From: karn@jupiter (Phil R. Karn)
Newsgroups: comp.dcom.lans
Subject: Re: What services does X.25 provide?
Keywords: x.25, services, login, e-mail, file transfer, IPC
Message-ID: <17683@bellcore.bellcore.com>
Date: 21 Sep 89 23:05:16 GMT
References: <796@maxim.erbe.se> <3279@wasatch.utah.edu> <522@wet.UUCP> <1989Sep18.020822.16329@cit5.cit.oz> <727@idacom.UUCP>
Sender: news@bellcore.bellcore.com
Reply-To: karn@jupiter.bellcore.com (Phil R. Karn)
Organization: Bell Communications Research, Inc
Lines: 32

In article <727@idacom.UUCP> danny@idacom.UUCP (Danny Wilson) writes:
>Although the spec states that if you use the 'D' bit in a data packet,
>I have not run into an actual switch yet that supports this functionality.
>
>This includes some switches that actually implement the '84 standard.
>
>Anyone aware of a switch that implements this function?

Telenet's network seems to act as though the D-bit were always set. That
is, packet-layer RRs are propagated end-to-end. Apparently they do this
as more of an ad-hoc congestion-avoidance scheme than out of concern for
reliable data transfer. But this works strongly against performance even
when there is no danger of congestion, because the default X.25 packet
layer window is only 2 128-byte packets, and the extra delay incurred by
the end-to-end RRs means that the maximum achievable throughput over a
single virtual circuit can be abysmal. This results in hacks like
"downward multiplexing", where a datagram-based DTE (e.g., the CSNET
IP-on-X.25 gateway) opens multiple virtual circuits to the same
destination and spreads its datagrams among them.  It often takes 4 or 5
parallel virtual circuits to keep a single 9.6kb/s access link busy.

The result is unnecessarily complicated code, lots of dropped packets
and thrashing of virtual circuit connections because of table limits,
and a high rate of out-of-order packet delivery. When I wrote my TCP, I
found the best way to *really* test it under fire was to connect across
a CSNET IP-on-X.25 gateway.

Face it, X.25 is a disaster for anything other than remote slow speed
terminal multiplexing. It is not suitable for serious computer
networking.

Phil