Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!usc!snorkelwacker.mit.edu!bloom-beacon!eru!hagbard!sunic!ugle.unit.no!nuug!ifi!jon From: jon@ifi.uio.no (Jon lnes) Newsgroups: comp.protocols.iso Subject: Re: X.25 collisions Message-ID: Date: 3 Feb 91 18:41:34 GMT References: <143094@pyramid.pyramid.com> <1991Feb1.095514.7003@informatik.uni-erlangen.de> Sender: jon@ifi.uio.no (Jon \lnes) Organization: Dept. of Informatics, University of Oslo, Norway Lines: 32 Nntp-Posting-Host: gode.ifi.uio.no In-Reply-To: dkhusema@immd4.informatik.uni-erlangen.de (Dirk Husemann)'s message of 1 Feb 91 09:55:14 GMT Originator: jon@gode.ifi.uio.no About RESTART collisions, X.25 operation in LANs, and selection of DTE or DCE mode for channel number allocation: In a LAN, an X.25 DTE implementation must operate separate DTE entities for each communication partner, as opposed to the one-to-one operation of a conventional DTE-DCE pair. Thus, one node may well act as a DTE for communication with another node, and as a DCE towards a third node. This may be an administrative pain, but it is perfectly possible to configure the information statically. To relieve the network administrator of this work, it is better to go for a more dynamic solution, and one option is the RESTART procedure for selecting DTE or DCE characteristics. The main point is: You do not want to restrict your communication partners to a preconfigured set. If you need to talk to a new node via X.25/LAN, just create a new DTE-entity, and issue a RESTART. Our X.25 on Ethernet UNIX kernel implementation creates DTE entities in mbufs when necessary, and deletes inactive entities when a timer goes off. We implement the RESTART procedure, and we believe we can handle the RESTART collision situation properly. However, the easiest solution is to use the Reference Number Facility - another of these ISO afterthoughts/addendums. With this facility, a DTE originating a CALL uses channel number 0, and includes in the facility the actual channel number for the partner to use. The partner sends the CALL CONFIRMATION on the indicated channel, and includes the Reference Number Facility stating its own selected number. Thus, the two partners will use different channel numbers for incoming and outgoing packets on the same channel. With this solution, the CALL COLLISION situation disappears, and the RESTART procedure can be skipped. We implemented the Reference Number Facility as well, and although in many aspects it is an ungly hack, it makes things a lot more easy. The facility should be mandatory for X.25/LAN operation. Jon Oelnes, Norwegian Computing Center, Oslo E-mail: Jon.Olnes@nr.no or jon@ifi.uio.no