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From: harald.alvestrand@elab-runit.sintef.no (Harald Tveit Alvestrand)
Newsgroups: comp.protocols.iso
Subject: Re: question about SPDU
Message-ID: <1990Jun8.072624.5598@idt.unit.no>
Date: 8 Jun 90 07:26:24 GMT
References: <9006051437.AA12834@ucbvax.Berkeley.EDU>
Sender: news@idt.unit.no (Usenet news admin)
Reply-To: harald.alvestrand@elab-runit.sintef.no
Organization: ELAB-RUNIT, SINTEF, Norway
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The Session layer is actually a collapse of two protocol layers of the
Teletex(1976?) protocol. So, the GT and PT (Grant Token and Please Token) are
used as "here comes a higher-level PDU" signals, carrying no real meaning.
You actually have to read the specs carefully to detect this; they document it
as if you are allowed to concatenate several SPDUs into one TSDU, and
list a few
"allowed concatenation sequences". It took me quite some time to find out that
most PDUs that were in the "document layer" of Teletex were now in a class that
could NOT be first in a concatenation series, but that GT and PT could
be first,
and carried no real meaning if they did not have any parameters (LI=0).

So much for removing layers by cryptic documentation!

Harald Tveit Alvestrand
X.400 expert, UNINETT MHS manager