Transport & Network addressing over TLI

[Ole Frank Larsen]

I have a question regarding transport addressing over TLI. The system
concerned is an Interactive Systems Corp. 386/ix 2.0.2 (System V.3.2).


Imagine that you have a transport provider (TP) running above an X.25
network provider (NP). The TP offers a TLI interface to the Transport
User.

The question is: What format should the Transport addresses (TSAP
addresses) for the TP have? TLI does not specify the format of
addresses.

I understand that Retix uses the following format:

     <nlg><NSAP address><tlg><TSAP selector>

     <nlg>                Length of the NSAP address field.
     <Network address>    NSAP address encoded according to
                          ISO 8348/Addendum 2.
     <tlg>                Length of the TSAP selector field.
     <TSAP selector>      The TSAP selecor identifying the TSAP.

Is this *the* format of a TSAP address?

If it is, then what is the format of the NSAP address? I am familiar
with ISO 8348/Addendum 2 which specifies an NSAP address as an IDP
followed by an DSP, the IDP consisting of an AFI and an IDI.

The problem is that the TP runs above an *X.25* network provider.
X.25 only allows called address to be a 14-octet X.121 address, unless
the extended addressing facility is used. The X.121 addresses do NOT
conform to ISO 8348/Addendum 2.

If the TP allows both 14-octet X.121 addresses and full NSAP addresses
to appear in the NSAP address field of the TSAP address, then how does
the TP distinguish between the two? It *must* be able to tell whether
a given address is a full NSAP address or merely an X.121 address,
because the former should be entered in the extended addressing
facility field of the X.25 packet whereas the X.121 address should be
entered in the called address field.

Is the answer that only full NSAP addresses are allowed?


Any help will be greatly appreciated. If you respond through e-mail,
i'll post a summary (Promise!).

Best Regards
-- 
Ole Frank Larsen.                       RC International, Denmark
ofl at rci.dk                              Hackers do it with fewer instructions