🚀 go-pugleaf

 Date: Mon, 10 Dec 1990 16:20:52 +0100
 From: Jacob Palme QZ <JPALME@qz.qz.se>
 Subject: Ports in Abstract Protocol specs
 To: ISO/CCITT Group Communication <ISO-GC@qz.qz.se>, mhsnews@uninett.no,
	 eu-mhsnews@uninett.no
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 I am a little confused on the usage of ports in abstract protocol
 specifications.

 Why is the communication between two agents split into different
 ports? What is the advantage? Is this purely an abstract notation,
 or does it mean that if a UA wants to connect to more than one
 port on a Service Agent, then multiple connections have to
 be opened. If the latter, why is this an advantage?

A port is totally an abstract notation which can be a useful mechanism both for
modeling and for conformance. In general,  however,  it has really not been
used for much in X.400 and (particularly) X.500.  At the end of the last Study
Period there was a suggestion to get rid of the ports from X.500 (I can't
remember if we wanted to make it all one port or just get rid of them
entirely).  It came down to not being worth the fight for a purely
documentation issue.  So that is why there is a statement in X.500 about ports
not being related to conformance.

 One advantage with ports might be do make access control easier,
 if only some UA-s can access some ports which provide some kind
 of privileged access. But if this is the reason for organizaing
 the protocol into ports, then why has X.500 different ports
 for search and retrieval?

Yes,  that has been a suggestion.  Group some operations together and put them
as one level of access and others (say administrative operations) as another
level of access.  Another philosophy suggests that this is really an
interface issue and all you need to do is provide a different user interface
where priviledged copmmands/operations are not available and that ports aren't
necessary to accomplish this.

X.500's ports were done before conformance was considered.  They were grouped
by similar function rather than anything else as I remember.  At this point the
ports are purely historical and should not be taken as having any inner
meaning.

 Can the same operation occur via more than one port?

Sure.

Hope this helps explain one of the more obscure parts of OSI.

Best,
Alyson