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From: jc@netrix.nac.dec.com
Newsgroups: comp.protocols.tcp-ip
Subject: What are the rules for Object Instances?
Message-ID: <23109@shlump.lkg.dec.com>
Date: 3 Jun 91 14:17:02 GMT
Sender: news@shlump.lkg.dec.com
Organization: Digital Equipment
Lines: 32

Here's my dumb question of the week: We have a bunch of people working
with  SNMP,  and  one  thing  that  has people confused is the "Object
Instance", which apparently is the name for the  funny  zero  that  is
appended to most (but not all) Object Identifiers.  We have determined
by experiment that with various agents lurking about on  the  network,
this  extra  zero octet is sometimes required, sometimes optional, and
sometimes forbidden.  But we haven't yet intuited the pattern, and TFM
(the  RFCs,  Marshall  Rose's  "Simple Book", etc.) seem to think that
this is so obvious that it isn't worth explaining. It isn't obvious to
us, and existing agents seem to follow a variety of rules.

Does  anyone  know  what the rules are for the use of this extra zero?
When is it required?   When  is  it  forbidden?   Or  is  it  just  an
idiosyncracy of the various agents?

Also,  since  it  is  (in  our  experience)  always  zero, is there an
explanation (other than sheer perversity ;-) that it is there at  all?
Can it ever be nonzero?

For instance, many agents require that the sysDescr  be  requested  as
43.6.1.2.1.1.0  or  they  won't  respond,  while others don't need the
zero.  (Some will also accept 1.3.6.1.2.1.1, but  that's  a  different
problem.  :-)  There seems to be no obvious meaning to multiple (i.e.,
different) values for this variable, so why would an instance ever  be
needed?

Is there some rule in the RFCs that covers this, so we  can  point  at
some of the agents and say "You're doing it wrong" and possibly get it
changed so they're all the same?  Or is it just a  weirdness  that  we
have  to  learn  to live with?  It ain't easy writing clients when the
agents out there are inconsistent.  But maybe  if  we  understood  the
rules better...