UNIX IPC Datagram Reliability under - (nf)

[Beau James]

With respect to the reliability of the X.25 protocol ...

The X.25 protocol set (level 3, packet; level 2, link; level 1, electrical)
was designed for a network whose general structure looks like

	(DTE) ---- (DCE) .............. (DCE) ---- (DTE)

According to the CCITT spec, X.25 specifies the *interface* to the packet
transmission network - the "----" link in the diagram.  What happens to
packets inside the transmission network on their way to the remote end
DTE is not specified.  The specification does not even tell you how
to implement a DCE, although most of the differences are obvious.

The unreliabilities arise from the fact that there is no part of X.25
specification that is assured to be end-to-end between the DTEs that
have the open virtual circuit.  The CCITT spec specifically leaves that
decision up to the implementors of the packet data network (PDN)
(assumed to be the government communication agencies (PTTs) everywhere
but in the U.S.).

Many control messages can cause duplicate packets end-to-end because
they may be generated locally.  (E.g. DTE RESET: only one end of
the connection may be reset; if so, and that DTE resends unacknowledged
packets, the "remote" DTE may see the same data twice.)  Even the
ordinary data packet acknowledgement scheme is not reliable, since the
DCE may acknowledge successful transmission locally, meaning across the
DTE/DCE interface.  If the data does not get to the remote DTE for any
reason, there is no mechanism for determining which packets got lost.
Not very useful, but all according to the standard.

This is not to say that the X.25 protocols cannot be USED in a reliable,
end-to-end network design, IF the implementor ensures that all the X.25
acknowledgement and control messages have end-to-end significance.  The
Data General Xodiac(TM) network uses X.25 virtual circuits as end-to-end
session connections over several different transmission media, for
example.  But when a PDN is the "transmission medium", the virtual circuits
are not necessarily reliable (it depends on the PDN).  In the final
analysis, each top-level service protocol (mail, file transfer, etc.)
has to provide its own end-to-end reliability, if it cares.

						Beau James
						Sun Microsystems, Inc.