Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!cs.utexas.edu!uunet!cos!howard
From: howard@cos.com (Howard C. Berkowitz)
Newsgroups: comp.dcom.lans
Subject: Re: What services does X.25 provide?
Summary: Transport expects CO or CL network
Keywords: x.25, services, login, e-mail, file transfer, IPC
Message-ID: <23189@cos.com>
Date: 10 Oct 89 16:58:47 GMT
References: <796@maxim.erbe.se> <3279@wasatch.utah.edu> <522@wet.UUCP> <6624@pdn.paradyne.com>
Organization: Corporation for Open Systems, McLean, VA
Lines: 33

In article <6624@pdn.paradyne.com>, larry@pdn.paradyne.com (Larry Swift) writes:
> In article <22877@cos.com> howard@cos.com (Howard C. Berkowitz) writes:
> >The second is one way to provide the Connectionless Network Service
> >(CLNS); it uses X.25 to provide a technology-specific interface
> >to a packet-switching network, but the service interface shown to Transport
> >is connectionless, based on the OSI Internet Protocol (ISO 8473).
> 
> This description seems to be implying a reason for this interface,
> but if so, it escapes me.  Can you provide some background as to why
> network designers might want a CNLS interface over an inherently
> connection-oriented network layer?  If the Layer 3 path traversed
> multiple subnets with an unreliable segment such as X.25-Ethernet-X.25,
> is the CNLS interface to Layer 4 no less viable than the CONS?  Is it
> simply easier to use?  What else?


One of the uglier problems in current OSI is that Transport (at least
connection-oriented transport) does have expectations about the
characteristics of the underlying Network Service.   Transport
over CNLS cannot directly interoperate with transport over CONS.
A number of factors are involved, including whether Transport depends
on the lower layer (i.e., network) to tell it whether a connection
has broken.

There are a variety of solutions to this problem, including transport
relays.  The definitive solution is still evolving.  Note that
X.25 over LANs is used in Europe as a circumvention to this
difficulty.
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