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)