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Re: FW: [802.1] MSTP MIB - mstpMapTable

Harrie Hazewinkel wrote:

Romascanu, Dan (Dan) wrote:

This issue popped-up on the IEEE 802.1 WG list, around a OCTET STRING
object that would exceed 500 octets, and the authors decided to break it
into 'smaller pieces'. While watching this discussion I checked with the MIB review guidelines,
which do not say anything about a recommended size limiting of an OCTET
STRING, excepting the fact that it is recommended to be limited at some
size, especially when the OCTET STRING object is an index. See
http://www.ietf.org/internet-drafts/draft-ietf-ops-mib-review-guidelines
-04.txt Section 4.6.1.4. Is this OK? If so, how does this live together
with Keith's comment?


I do not see a real problem here. 


I do.  This is one of my "Top 10" problems with SNMP.
The data model layer is dependent on the transport layer,
which is a serious design problem.  MIB authors should
not have to worry about fragmentation and reassembly, but
they do have to worry about it if they want their SNMP SW to work.

Note that there is a difference between admin strings which can be
optionally set to large values vs. large formatted strings that the agent
will generate based on internal parameters (e.g., number of VLANS).
Only the latter type should be "pre-fragmented".


Andy



Immagine, cars should be smaller then 2.5 meters wide, since
otherwise the lanes on the road will not wide enough. If you
live in a small italian town with old roads the car may not
exceed 1.9 meters width, since you would otherwise not be
able to go into the little village.
Even though, the little village adds extra limitations to the
width of the car, it does not forbid it. It only cannot
go into the little village.

Therefore, I do see the limitations of the size of an OCTET
STRING is still as defined by ASN.1. However, if you use
some transport mappings extra limitations may apply.

One might argue that this is confusing to MIB designers
and a hint might have been given about it.



Dan




-----Original Message-----
From: IEEE 802.1 [mailto:hdk-0119.ckxbsg@ATT.NET] On Behalf Of Keith
McCloghrie
Sent: Tuesday, September 06, 2005 3:45 PM
To: STDS-802-1-L@listserv.ieee.org
Subject: Re: [802.1] MSTP MIB - mstpMapTable


1. Divide one long OCTET STRING into 4 shorter
   OCTET STRING. I don't see the reason for it.




The reason is the difference between "must" and "recommended".
Specifically, all the transport mappings in RFC 3417 say the equivalent
of:

   When an SNMP entity uses this transport mapping, it must be capable
   of accepting messages up to and including 484 octets in size.  It is
   recommended that implementations be capable of accepting messages of
   up to 1472 octets in size.  Implementation of larger values is
   encouraged whenever possible.

Keith.

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