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From: viola@idacom.cs.ubc.ca (Viola Lee)
Newsgroups: comp.protocols.iso
Subject: Re: ISO Session Help!
Message-ID: <4791@ubc-cs.UUCP>
Date: 21 Aug 89 23:25:16 GMT
Sender: news@cs.ubc.ca
Reply-To: viola@idacom.cs.ubc.ca (Viola Lee)
Organization: UBC-Idacom Joint Research Project
Lines: 36

In article 473, I said:
>     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.  

To which Rob Hagens replied:
>>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.

Yes!
Actually, I quoted almost directly from X.225 (CCITT Red Book, Fascicle
VIII.5, page 316, footnote 2):

   ... the Token Item parameter is only present in the GIVE TOKENS
   SPDU if this DATA TRANSFER SPDU contains a complete SSDU, or
   the last segment of a segmented SSDU.
   
I believe ISO 8237 is *very* similar to CCITT X.225, but if it says something
different please enlighten me.  I would be really interested in widening my
view if my information is incorrect.

>>It is possible to distiniguish the PDUs by looking at the parameters that
>>are present.

It is not possible for the MAA and AEA SPDUs.  They have exactly the same
parameters.  Although they generate the same responses at the receiver in
almost all states, there are two exceptions:  STA04A (AWAIT MAA) and STA04B
(AWAIT AEA).  Thus, distinguishing by parameters is not always possible.

Viola Lee