Path: utzoo!attcan!uunet!cs.utexas.edu!csd4.csd.uwm.edu!mailrus!cornell!uw-beaver!ubc-cs!ugly.cs.ubc.ca!vlee From: vlee@ugly.cs.ubc.ca (viola po-ying lee) Newsgroups: comp.protocols.iso Subject: Re: ISO Session Help! Message-ID: <4756@ubc-cs.UUCP> Date: 17 Aug 89 20:13:57 GMT Sender: news@cs.ubc.ca Reply-To: viola@ubc-idacom.cs.ubc.ca (Viola Lee) Organization: UBC-Idacom Joint Research Project Lines: 28 > > Whenever a category 2 SPDU is sent will a category 0 SPDU > ALWAYS be concatenated onto the front? I read the CCITT 1984 X.225 Recommendation. For concatenation, a category 0 SPDU is always in front of category 2 SPDU(s). As for the valid basic concatenation of DT SPDU, the first pdu is GT SPDU ONLY. PT is not allowed with DT SPDU. There is some rule about this GT. Token item parameter is only present in the GT SPDU if this DT SPDU contains a complete SSDU or the last the segment of a segmented SSDU. (Pages 315-318 of the X.225 document give a good description about valid concatenation) >Can you ever have a TSDU with JUST a data SPDU contained in it? A DT SPDU can be mapped to a TSDU, but it would be considered to be an invalid PDU by the receiver. QUESTION for the ISO Session Layer gurus: Why are some SI values of several SPDUs the same (e.g. MAA and AEA)? If the purpose of SI values is to identify the SPDU type, shouldn't they be all unique? It would be a lot easier to decode a SPDU had the SI values been all unqiue. Viola Lee viola@ubc-idacom.ubc.ca (don't use the vlee@ugly address in the header)