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draft Row creation with SNMPv1 Oct 19, 1993
Row creation with SNMPv1
October 19, 1993
Steven Waldbusser
Carnegie Mellon University
waldbusser@cmu.edu
<draft-waldbusser-v1rows-00.txt>
Status of this Memo
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas, and
its Working Groups. Note that other groups may also distribute working
documents as Internet Drafts.
Internet Drafts are valid for a maximum of six months and may be
updated, replaced, or obsoleted by other documents at any time. It is
inappropriate to use Internet Drafts as reference material or to cite
them other than as a "work in progress".
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1. Introduction
This memo describes how the RowStatus textual convention, as defined in
RFC 1443 [2], is used with version one of SNMP. The RowStatus textual
convention, and the interactions between a manager and agent that use
it, have been described in terms of SNMPv2 protocol interactions. If
the RowStatus textual convention is used in an SNMPv2 MIB, this memo
shows the interactions that an SNMPv1 manager will have with a SNMPv1
only, or bi-lingual SNMPv1/SNMPv2 agent that implements this MIB.
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2. The SNMPv2 Network Management Framework
The SNMPv2 Network Management Framework consists of four major
components. They are:
o RFC 1442 which defines the SMI, the mechanisms used for describing
and naming objects for the purpose of management.
o RFC 1213 defines MIB-II, the core set of managed objects for the
Internet suite of protocols.
o RFC 1445 which defines the administrative and other architectural
aspects of the framework.
o RFC 1448 which defines the protocol used for network access to
managed objects.
The Framework permits new objects to be defined for the purpose of
experimentation and evaluation.
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3. Overview
The RowStatus textual convention is used to manage the creation and
deletion of conceptual rows, and is used as the value of the SYNTAX
clause for the status column of a conceptual row (as described in
Section 7.7.1 of [1].)
Before the definition of RowStatus in SNMPv2, the only mechanism
commonly used to manage the creation and deletion of conceptual rows
with SNMPv1 was the EntryStatus textual convention in the RMON MIB. It
is recommended that new MIBs use the RowStatus textual convention
because it provides a simple migration path to SNMPv2. This memo
describes how to use SNMPv2's RowStatus with SNMPv1.
4. Use of RowStatus with SNMPv1
A RowStatus column has six defined values:
- 'active', which indicates that the conceptual row is available
for use by the managed device;
- 'notInService', which indicates that the conceptual row exists in
the agent, but is unavailable for use by the managed device (see
NOTE below);
- 'notReady', which indicates that the conceptual row exists in the
agent, but is missing information necessary in order to be
available for use by the managed device;
- 'createAndGo', which is supplied by a management station wishing
to create a new instance of a conceptual row and to have it
available for use by the managed device;
- 'createAndWait', which is supplied by a management station
wishing to create a new instance of a conceptual row but not to
have it available for use by the managed device; and,
- 'destroy', which is supplied by a management station wishing to
delete all of the instances associated with an existing conceptual
row.
Whereas five of the six values (all except 'notReady') may be
specified in a management protocol set operation, only three values
will be returned in response to a management protocol
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retrieval operation: 'notReady', 'notInService' or 'active'. That
is, when queried, an existing conceptual row has only three states:
it is either available for use by the managed device (the status
column has value 'active'); it is not available for use by the
managed device, though the agent has sufficient information to make
it so (the status column has value 'notInService'); or, it is not
available for use by the managed device, because the agent lacks
sufficient information (the status column has value 'notReady').
NOTE WELL
This textual convention may be used for a MIB table, irrespective
of whether the values of that table's conceptual rows are able to
be modified while it is active, or whether its conceptual rows must
be taken out of service in order to be modified. That is, it is
the responsibility of the DESCRIPTION clause of the status column
to specify whether the status column must be 'notInService' in
order for the value of some other column of the same conceptual row
to be modified.
To summarize the effect of having a conceptual row with a status
column having a SYNTAX clause value of RowStatus, consider the
following state diagram. This state diagram is the same as that
for SNMPv2's RowStatus, except that all 'inconsistentValue' and
'wrongValue' errors have been translated to 'badValue'.
STATE
+--------------+-----------+-------------+-------------
| A | B | C | D
| |status col.|status column|
|status column | is | is |status column
ACTION |does not exist| notReady | notInService| is active
--------------+--------------+-----------+-------------+-------------
set status |noError ->D|badValue |badValue |badValue
column to | or | | |
createAndGo |badValue | | |
| | | |
--------------+--------------+-----------+-------------+-------------
set status |noError see 1|badValue |badValue |badValue
column to | or | | |
createAndWait |badValue | | |
--------------+--------------+-----------+-------------+-------------
set status |badValue |badValue |noError |noError
column to | | | |
active | | | |
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| | or | |
| | | |
| |see 2 ->D| ->D| ->D
--------------+--------------+-----------+-------------+-------------
set status |badValue |badValue |noError |noError ->C
column to | | | |
notInService | | | |
| | or | | or
| | | |
| |see 3 ->C| ->C|badValue
--------------+--------------+-----------+-------------+-------------
set status |noError |noError |noError |noError
column to | | | |
destroy | ->A| ->A| ->A| ->A
--------------+--------------+-----------+-------------+-------------
set any other |see 4 |noError |noError |noError
column to some| | | |
value | ->A| see 1| ->C| ->D
--------------+--------------+-----------+-------------+-------------
(1) goto B or C, depending on information
available to the agent.
(2) if other variable bindings included in the
same PDU, provide values for all columns which are
missing but required, then return noError and goto
D.
(3) if other variable bindings included in the
same PDU, provide values for all columns which are
missing but required, then return noError and goto
C.
(4) at the discretion of the agent, either noError
or badValue may be returned.
NOTE: Other processing of the set request may
result in a response other than noError being
returned.
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4.1. Conceptual Row Creation with SNMPv1
There are four potential interactions when creating a conceptual
row: selecting an instance-identifier which is not in use; creating
the conceptual row; initializing any objects for which the agent
does not supply a default; and, making the conceptual row available
for use by the managed device.
Interaction 1: Selecting an Instance-Identifier
The algorithm used to select an instance- identifier varies for
each conceptual row. In some cases, the instance-identifier is
semantically significant, e.g., the destination address of a route,
and a management station selects the instance-identifier according
to the semantics.
In other cases, the instance-identifier is used solely to
distinguish conceptual rows, and a management station without
specific knowledge of the conceptual row might examine the
instances present in order to determine an unused instance-
identifier. (This approach may be used, but it is often highly
sub-optimal; however, it is also a questionable practice for a
naive management station to attempt conceptual row creation.)
Alternately, the MIB module which defines the conceptual row might
provide one or more objects which provide assistance in determining
an unused instance-identifier. For example, if the conceptual row
is indexed by an integer-value, then an object having an integer-
valued SYNTAX clause might be defined for such a purpose, allowing
a management station to issue a management protocol retrieval
operation. In order to avoid unnecessary collisions between
competing management stations, 'adjacent' retrievals of this object
should be different.
Finally, the management station could select a pseudo-random number
to use as the index. In the event that this index was already in
use and a badValue was returned in response to the management
protocol set operation, the management station should simply select
a new pseudo-random number and retry the operation.
A MIB designer should choose between the two latter algorithms
based on the size of the table (and therefore the efficiency of
each algorithm). For tables in which a large number of entries are
expected, it is recommended that a MIB object be defined that
returns an acceptable index for creation. For tables with small
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numbers of entries, it is recommended that the latter pseudo-random
index mechanism be used.
Note that the above interaction is the same as that for SNMPv2.
Interaction 2: Creating the Conceptual Row
Once an unused instance-identifier has been selected, the
management station determines if it wishes to create and activate
the conceptual row in one transaction or in a negotiated set of
interactions. Both of these mechanisms are different when using
SNMPv1.
Interaction 2a: Creating and Activating the Conceptual Row
When using SNMPv2, the management station first determines those
columns for which it must or must not provide values. SNMPv2
performs this function using the exception mechanism that is not
present in SNMPv1. Depending on the complexity of the table and
the management station's knowledge of the agent's capabilities,
this determination can be made locally by the SNMPv1 management
station. It is recommended that the management station make this
determination locally, as this will increase the efficiency of the
resulting transaction.
Once the column requirements have been determined, a management
protocol set operation is accordingly issued. This operation also
sets the new instance of the status column to 'createAndGo'.
When the agent processes the set operation, it verifies that it has
sufficient information to make the conceptual row available for use
by the managed device. The information available to the agent is
provided by two sources: the management protocol set operation
which creates the conceptual row, and, implementation-specific
defaults supplied by the agent (note that an agent must provide
implementation-specific defaults for at least those objects which
it implements as read-only). If there is sufficient information
available, then the conceptual row is created, a 'noError' response
is returned, the status column is set to 'active', and no further
interactions are necessary (i.e., interactions 3 and 4 are
skipped). If there is insufficient information, then the
conceptual row is not created, and the set operation fails with an
error of 'badValue'. On this error, the management station
determines if the failure was due to the status column or to one of
the other columns by inspection of the error-index value in the
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response to the set. If the error was due to the status column, a
subsequent retrieval of the status object can determine if the
selected instance already existed, in which case we return to
interaction 1, otherwise we must assume that the original operation
failed to specify a required column.
If the error was not due to the status column and was a
'noSuchName' error, a column was either not implemented or not
accessible. It is recommended that the management station remove
the column referenced by the error-index from the list of those
columns for which it will provide values and then repeat
interaction 2, informing the user of the modified request.
Interaction 2b: Negotiating the Creation of the Conceptual Row
The management station issues a management protocol set operation
which sets the desired instance of the status column to
'createAndWait'. If the agent is unwilling to process a request of
this sort, the set operation fails with an error of 'badValue'.
(As a consequence, such an agent must be prepared to accept a
single management protocol set operation, i.e., interaction 2a
above, containing all of the columns indicated by its column
requirements.) Of course a 'badValue' might also be returned
because the selected instance already existed. Thus, if a
'badValue' response is received by the management station, a
subsequent retrieval of the status object can determine if the
selected instance already existed, in which case we return to
interaction 1, otherwise it must assume that the agent is only
willing to accept a single management protocol set operation and
must proceed to interaction 2a.
Otherwise, the agent creates the conceptual row, a 'noError'
response is returned, and the status column is immediately set to
either 'notInService' or 'notReady', depending on whether it has
sufficient information to make the conceptual row available for use
by the managed device. If there is sufficient information
available, then the status column is set to 'notInService';
otherwise, if there is insufficient information, then the status
column is set to 'notReady'. Regardless, we proceed to interaction
3.
Interaction 3: Initializing non-defaulted Objects
The management station can now inspect any defaulted values before
deciding on the final values it may wish to set for each column.
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It then issues a getnext request for the values in all columns
(this request will actually contain Object Names that immediately
precede the desired Object Name for each column value).
In the response, for each column, there are three possible
outcomes:
- a value for the correct column is returned, indicating that the
agent implements the object-type associated with this column and
provided a default value. For those columns to which the agent
provides read-create access, a value return tells the management
station that it may issue additional management protocol set
operations, if it desires, in order to change the value associated
with this column.
- a value for an unexpected column is returned, or - an errorStatus
of 'noSuchName' is returned, This indicates that the agent has not
set a default value for the column. If the access for this column
is read-create, the management station must issue additional
management protocol set operations in order to provide a value
associated with this column.
If any columns have not had values supplied, the management station
must supply them at this time. The management station may also
update any default values in any read-create columns that it wishes
to change. The management station will issue one or more set
requests for these columns to supply their initial or updated
values. A 'noSuchName' error received in response to any of these
set requests is an indication that a column was either not
implemented or not accessible. It is recommended that the
management station remove the column referenced by the error-index
from the list of those column for which it will provide values and
repeat the operation, informing the user of the modified request.
Interaction 4: Making the Conceptual Row Available
Once the management station is satisfied with the values associated
with the columns of the conceptual row, it issues a management
protocol set operation to set the status column to 'active'. If
the agent has sufficient information to make the conceptual row
available for use by the managed device, the management protocol
set operation succeeds (a 'noError' response is returned).
Otherwise, the management protocol set operation fails with an
error of 'badValue'.
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NOTE WELL
A conceptual row having a status column with value 'notInService'
or 'notReady' is unavailable to the managed device. As such, it is
possible for the managed device to create its own instances during
the time between the management protocol set operation which sets
the status column to 'createAndWait' and the management protocol
set operation which sets the status column to 'active'. In this
case, when the management protocol set operation is issued to set
the status column to 'active', the values held in the agent
supersede those used by the managed device.
If the management station is prevented from setting the status
column to 'active' (e.g., due to management station or network
failure) the conceptual row will be left in the 'notInService' or
'notReady' state, consuming resources indefinitely. The agent must
detect conceptual rows that have been in either state for an
abnormally long period of time and remove them. This period of
time should be long enough to allow for human response time
(including 'think time') between the creation of the conceptual row
and the setting of the status to 'active'. It is suggested that
this period be approximately 5 minutes in length.
Conceptual Row Suspension
When a conceptual row is 'active', the management station may issue
a management protocol set operation which sets the instance of the
status column to 'notInService'. If the agent is unwilling to do
so, the set operation fails with an error of 'badValue'.
Otherwise, the conceptual row is taken out of service, and a
'noError' response is returned. It is the responsibility of the
the DESCRIPTION clause of the status column to indicate under what
circumstances the status column should be taken out of service
(e.g., in order for the value of some other column of the same
conceptual row to be modified).
Conceptual Row Deletion
For deletion of conceptual rows, a management protocol set
operation is issued which sets the instance of the status column to
'destroy'. This request may be made regardless of the current
value of the status column (e.g., it is possible to delete
conceptual rows which are either 'notReady', 'notInService' or
'active'.) If the operation succeeds, then all instances associated
with the conceptual row are immediately removed."
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5. Acknowledgements
The author wishes to thank Keith McCloghrie for his helpful comments on
the ideas in this document.
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6. References
[1] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Structure
of Management Information for version 2 of the Simple Network
Management Protocol (SNMPv2)", RFC 1442, SNMP Research, Inc.,
Hughes LAN Systems, Dover Beach Consulting, Inc., Carnegie Mellon
University, April, 1993
[2] Case, J., McCloghrie, K., Rose, M., and S. Waldbusser, "Textual
Conventions for version 2 of the Simple Network Management Protocol
(SNMPv2)", RFC 1443, SNMP Research, Inc., Hughes LAN Systems, Dover
Beach Consulting, Inc., Carnegie Mellon University, April, 1993
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7. Security Considerations
Security issues are not discussed in this memo.
8. Author's Address
Steven Waldbusser
Carnegie Mellon University
4910 Forbes Ave
Pittsburgh, PA 15213
US
Phone: +1 412 268 6628
Email: waldbusser@cmu.edu
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Table of Contents
1 Introduction .................................................... 2
2 The SNMPv2 Network Management Framework ......................... 3
3 Overview ........................................................ 4
4 Use of RowStatus with SNMPv1 .................................... 4
4.1 Conceptual Row Creation with SNMPv1 ........................... 7
5 Acknowledgements ................................................ 12
6 References ...................................................... 13
7 Security Considerations ......................................... 14
8 Author's Address ................................................ 14
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