Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP Posting-Version: version B 2.10.1 6/24/83 SMI; site sun.uucp Path: utzoo!watmath!clyde!burl!ulysses!harpo!decvax!decwrl!sun!beau From: beau@sun.uucp (Beau James) Newsgroups: net.unix-wizards Subject: Re: UNIX IPC Datagram Reliability under - (nf) Message-ID: <444@sun.uucp> Date: Tue, 7-Feb-84 14:57:48 EST Article-I.D.: sun.444 Posted: Tue Feb 7 14:57:48 1984 Date-Received: Fri, 10-Feb-84 01:28:53 EST References: <235@hou3c.UUCP> Organization: Sun Microsystems, Inc. Lines: 42 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.