Juergen Schoenwaelder
Internet-Draft TU Braunschweig
Expires May 1999 18 November 1998
DateAndTime Extension for the MIB-2 system Group
<draft-schoenw-snmpv2-mib-ext-00.txt>
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Abstract
This memo defines an extension of the Management Information Base
(MIB) for Version 2 of the Simple Network Management Protocol
(SNMPv2) as published in RFC 1907. In particular, it defines an
object which allows to read and modify the notion of wall clock time
accessible by an SNMP agent. The addition of this object allows to
use the DateAndTime textual convention as defined in RFC 1903 even on
devices that do not have a real-time clock.
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Table of Contents
1 Introduction ................................................. 2
2 Problem ...................................................... 2
3 Solution ..................................................... 3
4 Definitions .................................................. 4
5 Acknowledgments .............................................. 4
6 Authors' Address ............................................. 5
1. Introduction
This memo defines an extension of the Management Information Base
(MIB) for Version 2 of the Simple Network Management Protocol
(SNMPv2) as published in RFC 1907. In particular, it defines an
object which allows to read and modify the notion of wall clock time
accessible by an SNMP agent. The addition of this object allows to
use the DateAndTime textual convention as defined in RFC 1903 even on
devices that do not have a real-time clock.
2. Problem
Many MIBs require to time-stamp events. This is usually accomplished
by using either the TimeStamp or the DateAndTime textual convention
defined in RFC 1903. Both approaches have strength and weaknesses:
TimeStamp:
+ One advantage of a TimeStamp is that sysUpTime is always locally
available, even on devices that have no notion of wall clock
time.
+ Further, sysUpTime is supported by the AgentX sub-agent protocol
defined in RFC 2257 since it is exchanged in master/sub-agent
message exchanges.
- A TimeStamp time-stamp is based on sysUpTime and becomes
ambiguous after some time.
- A TimeStamp has itself no meaning - a manager must also retrieve
the current value of sysUpTime and do calculations against its
local notion of date and time in order to calculate when an
event happened.
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- A TimeStamp can not be used to time-stamp events that have
occurred prior to the initialization of the network management
of the system was last re-initialized.
- TimeStamps reported by sub-agents can be ambiguous if the master
agent has been restarted. Resetting or recalculating time stamps
on such an event can be expensive.
DateAndTime:
+ DateAndTime time-stamps report the time an event occurred in the
local notion of date and time. A DateAndTime time-stamp is
therefore self-contained.
+ DateAndTime time-stamps are globally unambiguous if they contain
the offset from UTC. DateAndTime time-stamps are still locally
unambiguous if they miss the offset from UTC.
+ DateAndTime time-stamps can describe events that have occurred
prior to the initialization of the network management subsystem.
+ DateAndTime time-stamps do not require any special attention in
a sub-agent environment when the master agent restarts.
- A DateAndTime time-stamp is slightly longer than a TimeStamp
time-stamp.
- DateAndTime time-stamps requires that the agent has a notion of
wall clock time. This is generally not supported on devices
without a battery powered clock or devices that do not support
time protocols such as NTP. A MIB which records a time-stamp as
a DateAndTime value can thus not me implemented on such a
device.
- The AgentX sub-agent protocol does not allow to share the notion
of DateAndTime between sub-agents. The notion of DateAndTime
must therefore be exchanged with other local mechanisms. (This
is typically a service of the underlying operating system).
3. Solution
The sysDateAndTime object defined below solves the main problem with
DateAndTime time-stamps by providing a mechanism which allows to
configure the date and time with an SNMP set operations. This enables
devices without a battery powered clock and without time protocols to
have a notion of wall clock time.
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In addition, the sysDateAndTime object allows a manager to read the
remote notion of the current date and time in order to detect clock
screw or mis-configurations. This can be of great help when
interpreting DateAndTime time-stamp values from devices that have
mis-configured clocks.
Objects that record a time-stamp can now be written similar to the
following example:
smRunStartTime OBJECT-TYPE
SYNTAX DateAndTime
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The date and time when the execution started. The value
'0000000000000000'H is returned if the script has not
started yet or if the current date and time is not (yet)
known by the agent."
DEFVAL { '0000000000000000'H }
::= { smRunEntry 3 }
4. Definitions
SNMPv2-MIB-EXT-01 DEFINITIONS ::= BEGIN
IMPORTS
MODULE-IDENTITY, OBJECT-TYPE,
FROM SNMPv2-SMI
system
FROM SNMPv2-MIB;
-- This module is not valid SMIv2. The author left out all the
-- administrative stuff in order to keep it small.
sysDateAndTime OBJECT-TYPE
SYNTAX DateAndTime
MAX-ACCESS read-write
STATUS current
DESCRIPTION
"The locally known date and time. The value '0000000000000000'H
is returned on systems without a local clock until the local
date and time is learned, either from a network time protocol
such as NTP or a management set operations on this object."
::= { system 9 }
END
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5. Acknowledgments
The definition of the sysDateAndTime object was inspired by a remark
Eamonn McManus <emcmanus@gr.opengroup.org> about implementing
DateAndTime objects of the Script MIB in an embedded system without a
hardware clock.
The comparison of the advantages and disadvantages of the frequently
used TimeStamp mechanisms was inspired by discussions about the
TimeStamp textual convention in the SMI design team and on the SNMPv3
mailing list.
6. Authors' Address
Juergen Schoenwaelder Email: schoenw@ibr.cs.tu-bs.de
TU Braunschweig Tel: +49 531 391-3283
Bueltenweg 74/75
38106 Braunschweig
Germany
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