Path: utzoo!attcan!uunet!cos!howard
From: howard@cos.com (Howard C. Berkowitz)
Newsgroups: comp.dcom.lans
Subject: Re: What services does X.25 provide?
Summary: Window size negotiable
Keywords: x.25, services, login, e-mail, file transfer, IPC
Message-ID: <22878@cos.com>
Date: 30 Sep 89 13:10:53 GMT
References: <796@maxim.erbe.se> <3279@wasatch.utah.edu> <522@wet.UUCP> <6576@pdn.paradyne.com>
Organization: Corporation for Open Systems, McLean, VA
Lines: 57

In article <6576@pdn.paradyne.com>, larry@pdn.paradyne.com (Larry Swift) writes:
> In article <17683@bellcore.bellcore.com> karn@jupiter.bellcore.com (Phil R. Karn) writes:
>     Telenet's network seems to act as though the D-bit were always set. That
>     ...
>     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.
> 	...
>     Face it, X.25 is a disaster for anything other than remote slow speed
>     terminal multiplexing. It is not suitable for serious computer
>     networking.

I agree that X.25 does not always have optimal performance characteristics,
although it can sometimes be tuned (see below).  Something to bear in
mind, however, is that X.25 is the only interface available on a 
reasonably worldwide basis.  European ministries of posts and telegraphs
have historically been EXTREMELY reluctant to permit (and they can enforce
this) use of non-X.25 networks.  

Currently planned data interfaces to ISDN are oriented to X.25, although
one can set up a 64KBPS connection between pairs of points and run
anything over them, the PTT willing.

So, while X.25 is not ideal, it will probably be only thing usable worldwide
for 10-20 years or so.  There may be faster and more effective protocols
on Son of Internet (T3 or whatever), but vendors will need to look at
these as adjuncts to X.25, simply because these improved protocols
will not be available everywhere.

Just as much as we need new effective protocols, we need to know
how to wrest the best possible performance from X.25.
>     
> Seems like a quantum leap from an obvious hack to a blanket condemnation
> of X.25.  It would be easy (and obvious?) for a private X.25 network to
> open the window size to a more optimal value based on number of hops.


1984 X.25 provides a mechanism for negotiating a non-default (default
normally being 2) window size; whether a given network supports
nonstandard window sizes is a good question when selecting among
network vendors.  

Using modulo 8 sequence numbering, windows up to 7 can be supported,
and up to 127 with modulo 128.


Other performance parameters, such as maximum data packet size
and a throughput objective, can be negotiated.  Implementation of
these negotiation features is network dependent.
-- 
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