Path: utzoo!utgpu!news-server.csri.toronto.edu!rpi!zaphod.mps.ohio-state.edu!cis.ohio-state.edu!ucbvax!eng.xyplex.com!rlstewart
From: rlstewart@eng.xyplex.com (Bob Stewart)
Newsgroups: comp.protocols.tcp-ip
Subject: Re: What are the rules for Object Instances?
Message-ID: <9106040008.AA24232@xap.xyplex.com>
Date: 4 Jun 91 00:08:55 GMT
References: <23109@shlump.lkg.dec.com>
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I would have responded directly to the questioner, but the address full of !s
and dec.coms scared me off.  Besides, the answer may be of general interest.

SNMP object instancing is designed to be simple and regular, so, in a sense,
every object has an instance identifier, whether it can actually have multiple
instances or not.  To completely identify the object, that instance identifier
goes on the end of the object identifier.  For an object with only one
possible instance, such as sysDescr, the instance ID is 0.  For an object with
multiple instances, such as ifSpeed, the instance identifier suits the object
and is non-zero.  The instance identifier for an interface is ifIndex, a small
integer, which gets tacked onto the end of the ifSpeed object identifer to say
which instance you want.  A zero on the end of ifSpeed is not valid.  A
non-zero on the end of sysDescr is not valid.

So why is the observed behavior confusing?  It may be bugs or it may be agents
that are trying to be extremely tolerant.  It also may be the difference
between get and get-next.  Strictly speaking, for get and set you must supply
the entire object identifier, including instance identifier.  On the other
hand, get-next is so powerful, you can leave off as much of the object
identifier as you want to, and the agent will find what's lexically next.  A
handy result of this power is that you can get-next an object identifier with
the instance completely missing, and get-next will return the first instance
of that object.  Using that full object identifier in your next get-next will
get you the 2nd instance, and so on until there are no more instances and
you'll get whatever lexically follows the last instance.  You can thus
discover all the instance identifiers.

I hope that helps.  Once you're immersed in this stuff it's easy to forget
what it took to get that far under.

	Bob