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# SNMP Appendix B

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## Appendix B

### RowStatus (from RFC1903)

    RowStatus ::= TEXTUAL-CONVENTION
        STATUS       current
        DESCRIPTION
                "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 in RFC1902.)

                The status 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 its status automatically set
                     to active, making 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 make 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 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, and an attempt to
                make it so would fail because the agent has insufficient
                information (the state 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
                     not be `active' in order for the value of some other
                     column of the same conceptual row to be modified.  If
                     such a specification is made, affected columns may be
                     changed by an SNMP set PDU if the RowStatus would not
                     be equal to `active' either immediately before or after
                     processing the PDU.  In other words, if the PDU also
                     contained a varbind that would change the RowStatus
                     value, the column in question may be changed if the
                     RowStatus was not equal to `active' as the PDU was
                     received, or if the varbind sets the status to a value
                     other than 'active'.

                Also note that whenever any elements of a row exist, the
                RowStatus column must also exist.

                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:

                                             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|inconsist- |inconsistent-|inconsistent-
    column to     |       or     |   entValue|        Value|        Value
    createAndGo   |inconsistent- |           |             |
                  |         Value|           |             |
    --------------+--------------+-----------+-------------+-------------
    set status    |noError  see 1|inconsist- |inconsistent-|inconsistent-
    column to     |       or     |   entValue|        Value|        Value
    createAndWait |wrongValue    |           |             |
    --------------+--------------+-----------+-------------+-------------
    set status    |inconsistent- |inconsist- |noError      |noError
    column to     |         Value|   entValue|             |
    active        |              |           |             |
                  |              |     or    |             |
                  |              |           |             |
                  |              |see 2   ->D|          ->D|          ->D
    --------------+--------------+-----------+-------------+-------------
    set status    |inconsistent- |inconsist- |noError      |noError   ->C
    column to     |         Value|   entValue|             |
    notInService  |              |           |             |
                  |              |     or    |             |      or
                  |              |           |             |
                  |              |see 3   ->C|          ->C|wrongValue
    --------------+--------------+-----------+-------------+-------------
    set status    |noError       |noError    |noError      |noError
    column to     |              |           |             |
    destroy       |           ->A|        ->A|          ->A|          ->A
    --------------+--------------+-----------+-------------+-------------
    set any other |see 4         |noError    |noError      |see 5
    column to some|              |           |             |
    value         |              |      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, the return value may be
                either:

                     inconsistentName:  because the agent does not choose to
                     create such an instance when the corresponding
                     RowStatus instance does not exist, or

                     inconsistentValue:  if the supplied value is
                     inconsistent with the state of some other MIB object's
                     value, or

                     noError: because the agent chooses to create the
                     instance.

                If noError is returned, then the instance of the status
                column must also be created, and the new state is B or C,
                depending on the information available to the agent.  If
                inconsistentName or inconsistentValue is returned, the row
                remains in state A.

                (5) depending on the MIB definition for the column/table,
                either noError or inconsistentValue may be returned.

                NOTE: Other processing of the set request may result in a
                response other than noError being returned, e.g.,
                wrongValue, noCreation, etc.

                                  Conceptual Row Creation

                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 an inconsistentValue 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 numbers of entries, it is
                recommended that the latter pseudo-random index mechanism be
                used.

                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.

                Interaction 2a: Creating and Activating the Conceptual Row

                The management station must first determine the column
                requirements, i.e., it must determine those columns for
                which it must or must not provide values.  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 management station.  Alternately,
                the management station issues a management protocol get
                operation to examine all columns in the conceptual row that
                it wishes to create.  In response, for each column, there
                are three possible outcomes:

                     - a value is returned, indicating that some other
                     management station has already created this conceptual
                     row.  We return to interaction 1.

                     - the exception `noSuchInstance' is returned,
                     indicating that the agent implements the object-type
                     associated with this column, and that this column in at
                     least one conceptual row would be accessible in the MIB
                     view used by the retrieval were it to exist. For those
                     columns to which the agent provides read-create access,
                     the `noSuchInstance' exception tells the management
                     station that it should supply a value for this column
                     when the conceptual row is to be created.

                     - the exception `noSuchObject' is returned, indicating
                     that the agent does not implement the object-type
                     associated with this column or that there is no
                     conceptual row for which this column would be
                     accessible in the MIB view used by the retrieval.  As
                     such, the management station cannot issue any
                     management protocol set operations to create an
                     instance of this column.

                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 `inconsistentValue'.
                On this error, the management station can issue a management
                protocol retrieval operation to determine if this was
                because it failed to specify a value for a required column,
                or, because the selected instance of the status column
                already existed.  In the latter case, we return to
                interaction 1.  In the former case, the management station
                can re-issue the set operation with the additional
                information, or begin interaction 2 again using
                `createAndWait' in order to negotiate creation of the
                conceptual row.

                                         NOTE WELL

                     Regardless of the method used to determine the column
                     requirements, it is possible that the management
                     station might deem a column necessary when, in fact,
                     the agent will not allow that particular columnar
                     instance to be created or written.  In this case, the
                     management protocol set operation will fail with an
                     error such as `noCreation' or `notWritable'.  In this
                     case, the management station decides whether it needs
                     to be able to set a value for that particular columnar
                     instance.  If not, the management station re-issues the
                     management protocol set operation, but without setting
                     a value for that particular columnar instance;
                     otherwise, the management station aborts the row
                     creation algorithm.

                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 `wrongValue'.  (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.)  Otherwise,
                the conceptual row is created, 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 must now determine the column
                requirements.  It issues a management protocol get operation
                to examine all columns in the created conceptual row.  In
                the response, for each column, there are three possible
                outcomes:

                     - a value is returned, indicating that the agent
                     implements the object-type associated with this column
                     and had sufficient information to provide a value.  For
                     those columns to which the agent provides read-create
                     access (and for which the agent allows their values to
                     be changed after their creation), 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.

                     - the exception `noSuchInstance' is returned,
                     indicating that the agent implements the object-type
                     associated with this column, and that this column in at
                     least one conceptual row would be accessible in the MIB
                     view used by the retrieval were it to exist. However,
                     the agent does not have sufficient information to
                     provide a value, and until a value is provided, the
                     conceptual row may not be made available for use by the
                     managed device.  For those columns to which the agent
                     provides read-create access, the `noSuchInstance'
                     exception tells the management station that it must
                     issue additional management protocol set operations, in
                     order to provide a value associated with this column.

                     - the exception `noSuchObject' is returned, indicating
                     that the agent does not implement the object-type
                     associated with this column or that there is no
                     conceptual row for which this column would be
                     accessible in the MIB view used by the retrieval.  As
                     such, the management station cannot issue any
                     management protocol set operations to create an
                     instance of this column.

                If the value associated with the status column is
                `notReady', then the management station must first deal with
                all `noSuchInstance' columns, if any.  Having done so, the
                value of the status column becomes `notInService', and we
                proceed to interaction 4.

                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
                `inconsistentValue'.

                                         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.  It is the responsibility of the
                DESCRIPTION clause of the status column to indicate what an
                abnormally long period of time would be.  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'.
                In the absence of such information in the DESCRIPTION
                clause, it is suggested that this period be approximately 5
                minutes in length.  This removal action applies not only to
                newly-created rows, but also to previously active rows which
                are set to, and left in, the notInService state for a
                prolonged period exceeding that which is considered normal
                for such a conceptual row.

                                 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 `wrongValue'.  Otherwise, the conceptual row is
                taken out of service, and a `noError' response is returned.
                It is the responsibility of 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."

        SYNTAX       INTEGER {
                         -- the following two values are states:
                         -- these values may be read or written
                         active(1),
                         notInService(2),

                         -- the following value is a state:
                         -- this value may be read, but not written
                         notReady(3),

                         -- the following three values are
                         -- actions: these values may be written,
                         --   but are never read
                         createAndGo(4),
                         createAndWait(5),
                         destroy(6)
                     }

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