Path: utzoo!utgpu!jarvis.csri.toronto.edu!mailrus!tut.cis.ohio-state.edu!ucbvax!CS.WISC.EDU!hagens From: hagens@CS.WISC.EDU Newsgroups: comp.protocols.iso Subject: Re: ISO Session Help! Message-ID: <8908181607.AA05269@janeb.cs.wisc.edu> Date: 18 Aug 89 16:07:35 GMT References: <4756@ubc-cs.UUCP> Sender: daemon@ucbvax.BERKELEY.EDU Organization: The Internet Lines: 38 > > > > 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) No! The GT SPDU is always prepended to a DT SPDU. The token item parameter is present in the GT SPDU iff the ss-user wishes to transfer tokens. If the token item parameter is not present, then the GT has no effect (although it still takes up 2 bytes. > >Can you ever have a TSDU with JUST a data SPDU contained in it? If you mean, just a DT SPDU, the answer is no. You always concatenate. > A DT SPDU can be mapped to a TSDU, but it would be considered to > be an invalid PDU by the receiver. Correct. Don't do this. > 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. The session protocol was designed to be bit compatable with a pre-exsistent protocol (T.61). This ambiguity of SPDU SI values is a result of this. It is possible to distiniguish the PDUs by looking at the parameters that are present. Rob Hagens