From: mskuhn@immd4.informatik.uni-erlangen.de (Markus Kuhn)
Subject: X.500 (was: second system syndrome)
Organization: CSD., University of Erlangen, Germany
Date: Sun, 13 Sep 1992 14:45:44 GMT

ppan@nobeltech.se (Per Andersson) writes:
>Oh. I haven't seen any actual X.500 implementation, but: 

I believe that 98% of the people that use X.500 today on the Internet
use the Paradise implementation that comes with ISODE 8.0. It has
a DSA and several different interactive user agents. It's quite fine,
but I feel that it's still a research version not 100% suitable for
everyday usage. E.g. installation is a little bit more complicated
then with other popular programs (e.g. elm, GNU things).

>-How will I get access to names of users at other sites

For the user, there are at least 2 different kinds of access to the
directory database that I've seen implemented:

a) browsing

   The X.500 directory is organized like a tree. The whole world is
   represented by the root, the children of the root are all
   countries and international organizations and below the country
   nodes you have organizations (e.g. my university). These
   organizations are perhaps further subdivided in departments
   (organizational units) like "Computer Science" etc.
   The whole structure looks like a file system with directories
   and files and you can browse in it as you can do this in
   a file system (with commands, with a nice GUI, ...)
   Each entry has attributes associated with it. E.g. my entry
   has attributes that describe my phone number, my address (email
   and postal), my favourite drink and even my face (with a small
   JPEG encoded photo). An access control system allows you to
   restrict some users (e.g. from outside your company) to read
   only selected attributes.

b) searching

   Some of the more intelligent user agents ask you a few questions
   or ask you for a guess where in the tree the person you search might
   be located (you know perhaps the country and a part of the name
   of the organization). Then these programms will perform a search in 
   the database and try to find what you searched for (e.g. a person,
   a department, a printer, a fileserver, a contact person, a document, ...)
   The attributes of the entries found will be displayed to you.

All these accesses to the database are done over a protocol called DAP
(directory access protocol). Two kind of programs are involved: The
DUA (directory user agent) implements the user interface and sends the
querry commands via DAP to the DSA (directory service agent, the server)
which contains a part of the database and querries other DSAs using the
DSP (directory system protocol).

(DUA,DSA,DAP,DSP, all right? ;-)

>-Will I do interactive lookups over public X.25 in perhaps 2400bps ?

Thats possible, of course. There are several public DUAs available on
the Internet and via different X.25 networks (e.g. the German WIN).
Over very slow lines, it might be more comfortable to have a local
DUA and using the directory access protocol over the X.500 line.

>-How do I do store&forward mail, still using x.500 ?

Today, if you want to use X.500, you need access to a DSA
in order to resolve querries, and this DSA must have permanent
contact with other DSAs, so that eventually (with several intermediate
DSAs perhaps) you are in contact with every DSA on this world.
This is no problem if you are permanently connected to a large network
with lots of DSAs that are again connected to other DSAs on other
nets (e.g. Internet, many X.25 nets and big LANs). In a dialup
environment (where you need a store and forward system for mail),
things become a little bit more difficult. The database must have
copies of many entries on a lot of different DSAs, because the DSAs
are not connected all the time. In extreme cases, it might be necessary
to have huge copies of the whole tree in each dialup DSA and differential
updates of the copies will be propagated through the network in a store
and forward manner using DISP (directory information shadowing protocol,
the final X.500 abreviation for today :-). DISP is still a draft standard
today and consequently hasn't been implemented widely.

The X.500 directory system may be useful for both finding persons
on the net and doing routing decisions.

I hope this was an answer to your question (correct me if not!).

Markus

(in the tree: Markus Kuhn, Studenten, Informatik, Uni-Erlangen,
Germany. You should be able to find this entry easily with the 
information in my .signature below using a good DUA.)

-- 
Markus Kuhn, Computer Science student -=-=- University of Erlangen, Germany
Internet: mskuhn@immd4.informatik.uni-erlangen.de  |  X.500 entry available
-A distributed system is one in which the failure of a computer you didn't-
-even know existed can render your own computer unusable. (Leslie Lamport)-
QUIT
omputer you didn't-
-even know existed can render your own computer unusable. (Leslie Lamport)-
Newsgroups: comp.protocols.iso