LIANE%SBU.UFRGS.ANRS.BR@UICVM.UIC.EDU (06/01/90)
Could someone please give a hint about how to distinguish a GT-SPDU with LI-Lenght Indicator=0 from a DT-SPDU also with LI=0, since both have the same SI=1. Liane
jwf@hpctdia.HP.COM (John Freeborg) (06/08/90)
The concatenation rules specify that a GT SPDU (category 0) must ALWAYS be prepended to a DT SPDU (category 2). So often times you will see a session PDU such as: 01-00-01-00 followed by a bunch of data. This is an "empty" GT SPDU concatenated with a DT SPDU (both LIs are 0). You should never see a DT SPDU by itself since category 2 SPDUs must be concatenated with another SPDU. Hope this helps, John ------------------------------------------------------------------------------ John W. Freeborg Hewlett-Packard Company jwf@hpcsos.col.hp.com Colorado Telecommunications Division jwf%hpctdkg@hplabs.hp.com 5070 Centennial Blvd. 'The opinions expressed are not Colorado Springs, CO 80919 necessarily those of my employer.' (719) 531-4771
harald.alvestrand@elab-runit.sintef.no (Harald Tveit Alvestrand) (06/08/90)
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