7. Elements of Procedure This section describes the actions of protocol entities (master agents and subagents) implementing the AgentX protocol. Note, however, that it is not intended to constrain the internal architecture of any conformant implementation. The actions of AgentX protocol entities can be broadly categorized under two headings, each of which is described separately: (1) processing AgentX administrative messages (e.g., registration requests from a subagent to a master agent); and (2) processing SNMP messages (the coordinated actions of a master agent and one or more subagents in processing, for example, a received SNMP GetRequest-PDU). 7.1. Processing AgentX Administrative Messages This subsection describes the actions of AgentX protocol entities in processing AgentX administrative messages. Such messages include those involved in establishing and terminating an AgentX session between a subagent and a master agent, those by which a subagent requests allocation of instance index values, and those by which a subagent communicates to a master agent which MIB regions it supports. Processing is defined specifically for each PDU type in its own section. For the master agent, many of these PDU types require the same initial processing steps. This common processing is defined here, and referenced as needed in the PDU type-specific descriptions. Common Processing: The master agent initially processes a received AgentX PDU as follows: 1) An agentx-Response-PDU is created, res.sysUpTime is set to the value of sysUpTime.0 for the default context, res.error is set to `noAgentXError', and res.index is set to 0. 2) If the received PDU cannot be parsed, res.error is set to ` parseError'. Examples of a parse error are:
- PDU length (h.payload) too short to contain current
construct (Object Identifier header indicates more sub-
identifiers, VarBind v.type indicates data follows, etc)
- An unrecognized value is encountered for h.type, v.type,
etc.
3) Otherwise, if h.sessionID does not correspond to a currently
established session with this subagent, res.error is set to
`notOpen'.
4) Otherwise, if the NON_DEFAULT_CONTEXT bit is set and the master
agent does not support the indicated context, res.error is set
to `unsupportedContext'. If the master agent does support the
indicated context, the value of res.sysUpTime is set to the
value of sysUpTime.0 for that context.
Note: Non-default contexts might be added on the fly by the master
agent, or the master agent might require such non-default
contexts to be pre-configured. The choice is
implementation-specific.
5) If resources cannot be allocated or some other condition
prevents processing, res.error is set to `processingError'.
6) At this point, if res.error is not `noAgentXError', the
received PDU is not processed further. If the received PDU's
header was successfully parsed, the AgentX-Response-PDU is sent
in reply. If the received PDU contained a VarBindList which
was successfully parsed, the AgentX-Response-PDU contains the
identical VarBindList. If the received PDU's header was not
successfully parsed or for some other reason the master agent
cannot send a reply, processing is complete.
7.1.1. Processing the agentx-Open-PDU
When the master agent receives an agentx-Open-PDU, it processes it as
follows:
1) An agentx-Response-PDU is created, res.sysUpTime is set to the
value of sysUpTime.0 for the default context, res.error is set to
`noAgentXError', and res.index is set to 0.
2) If the received PDU cannot be parsed, res.error is set to
`parseError'.
3) Otherwise, if the master agent is unable to open an AgentX session
for any reason, res.error is set to `openFailed'.
4) Otherwise: The master agent assigns a sessionID to the new
session and puts the value in the h.sessionID field of the
agentx-Response-PDU. This value must be unique among all existing
open sessions.
The master agent retains session-specific information from the PDU
for this session:
- The NETWORK_BYTE_ORDER value in h.flags is retained. All
subsequent AgentX protocol operations initiated by the master
agent for this session must use this byte ordering and set this
bit accordingly.
The subagent typically sets this bit to correspond to its native
byte ordering, and typically does not vary byte ordering for an
initiated session. The master agent must be able to decode each
PDU according to the h.flag NETWORK_BYTE_ORDER bit in the PDU, but
does not need to toggle its retained value for the session if the
subagent varies its byte ordering.
- The o.timeout value is used in calculating response timeout
conditions for this session. This field is also referenced in
the AgentX MIB (a work-in-progress) by the agentxSessionTimeout
object.
- The o.id and o.descr fields are used for informational
purposes. These two fields are also referenced in the AgentX
MIB (a work-in-progress) by the agentxSessionObjectID object,
and by the agentxSessionDescr object.
5) The agentx-Response-PDU is sent with the res.error field
indicating the result of the session initiation.
If processing was successful, an AgentX session is considered
established between the master agent and the subagent. An AgentX
session is a distinct channel for the exchange of AgentX protocol
messages between a master agent and one subagent, qualified by the
session-specific attributes listed in 4) above. AgentX session
establishment is initiated by the subagent. An AgentX session can be
terminated by either the master agent or the subagent.
7.1.2. Processing the agentx-IndexAllocate-PDU
When the master agent receives an agentx-IndexAllocate-PDU, it
performs the common processing described in section 7.1, "Processing
AgentX Administrative Messages". If as a result res.error is
`noAgentXError', processing continues as follows:
1) Each VarBind in the VarBindList is processed until either all are
successful, or one fails. If any VarBind fails, the agentx-
Response-PDU is sent in reply containing the original VarBindList,
with res.index set to indicate the failed VarBind, and with
res.error set as described subsequently. All other VarBinds are
ignored; no index values are allocated.
VarBinds are processed as follows:
- v.name is the OID prefix of the MIB OBJECT-TYPE for which a
value is to be allocated.
- v.type is the syntax of the MIB OBJECT-TYPE for which a value is
to be allocated.
- v.data indicates the specific index value requested. If the
NEW_INDEX or the ANY_INDEX bit is set, the actual value in
v.data is ignored and an appropriate index value is generated.
a) If there are no currently allocated index values for v.name in
the indicated context, and v.type does not correspond to a
valid index type value, the VarBind fails and res.error is set
to `indexWrongType'.
b) If there are currently allocated index values for v.name in the
indicated context, but the syntax of those values does not
match v.type, the VarBind fails and res.error is set to
`indexWrongType'.
c) Otherwise, if both the NEW_INDEX and ANY_INDEX bits are clear,
allocation of a specific index value is being requested. If
the requested index is already allocated for v.name in the
indicated context, the VarBind fails and res.error is set to
`indexAlreadyAllocated'.
d) Otherwise, if the NEW_INDEX bit is set, the master agent should
generate the next available index value for v.name in the
indicated context, with the constraint that this value must not
have been allocated (even if subsequently released) to any
subagent since the last re-initialization of the master agent.
If no such value can be generated, the VarBind fails and
res.error is set to `indexNoneAvailable'.
e) Otherwise, if the ANY_INDEX bit is set, the master agent should
generate an index value for v.name in the indicated context,
with the constraint that this value is not currently allocated
to any subagent. If no such value can be generated, then the
VarBind fails and res.error is set to `indexNoneAvailable'.
2) If all VarBinds are processed successfully, the agentx-Response-
PDU is sent in reply with res.error set to `noAgentXError'. A
VarBindList is included that is identical to the one sent in the
agentx-IndexAllocate-PDU, except that VarBinds requesting a
NEW_INDEX or ANY_INDEX value are generated with an appropriate
value.
See section 7.1.4.2, "Registering Stuff" for more information on
how subagents should perform index allocations.
7.1.3. Processing the agentx-IndexDeallocate-PDU
When the master agent receives an agentx-IndexDeallocate-PDU, it
performs the common processing described in section 7.1, "Processing
AgentX Administrative Messages". If as a result res.error is
`noAgentXError', processing continues as follows:
1) Each VarBind in the VarBindList is processed until either all are
successful, or one fails. If any VarBind fails, the agentx-
Response-PDU is sent in reply, containing the original
VarBindList, with res.index set to indicate the failed VarBind,
and with res.error set as described subsequently. All other
VarBinds are ignored; no index values are released.
VarBinds are processed as follows:
- v.name is the name of the index for which a value is to be
released
- v.type is the syntax of the index object
- v.data indicates the specific index value to be released. The
NEW_INDEX and ANY_INDEX bits are ignored.
a) If the index value for the named index is not currently
allocated to this session, the VarBind fails and res.error is
set to `indexNotAllocated'.
2) If all VarBinds are processed successfully, res.error is set to
`noAgentXError' and the agentx-Response-PDU is sent. A
VarBindList is included which is identical to the one sent in the
agentx-IndexDeallocate-PDU.
All released index values are now available, and may be used in
response to subsequent allocation requests for ANY_INDEX values and
in response to subsequent allocation requests for the particular
index value.
7.1.4. Processing the agentx-Register-PDU When the master agent receives an agentx-Register-PDU, it performs the common processing described in section 7.1, "Processing AgentX Administrative Messages". If as a result res.error is `noAgentXError', processing continues as follows: If any of the union of subtrees defined by this MIB region is exactly the same as any subtree defined by a MIB region currently registered within the indicated context, those subtrees are termed "duplicate subtrees". If any of the union of subtrees defined by this MIB region overlaps, or is itself overlapped by, any subtree defined by a MIB region currently registered within the indicated context, those subtrees are termed "overlapping subtrees". 1) If this registration would result in duplicate subtrees registered with the same value of r.priority, the request fails and an agentx-Response-PDU is returned with res.error set to `duplicateRegistration'. 2) Otherwise, if the master agent does not wish to permit this registration for implementation-specific reasons, the request fails and an agentx-Response-PDU is returned with res.error set to `requestDenied'. 3) Otherwise, the agentx-Response-PDU is returned with res.error set to `noAgentXError'. The master agent adds this MIB region to its registration data store for the indicated context, to be considered during the dispatching phase for subsequently received SNMP protocol messages. 7.1.4.1. Handling Duplicate and Overlapping Subtrees As a result of this registration algorithm there are likely to be duplicate and/or overlapping subtrees within the registration data store of the master agent. Whenever the master agent's dispatching algorithm (see section 7.2.1, "Dispatching AgentX PDUs") determines that there are multiple subtrees that could potentially contain the same MIB object instances, the master agent selects one to use, termed the 'authoritative region', as follows:
1) Choose the one whose original agentx-Register-PDU r.subtree
contained the most subids, i.e., the most specific r.subtree.
Note: The presence or absence of a range subid has no bearing
on how "specific" one object identifier is compared to another.
2) If still ambiguous, there were duplicate subtrees. Choose the
one whose original agentx-Register-PDU specified the smaller
value of r.priority.
7.1.4.2. Registering Stuff
This section describes more fully how AgentX subagents use the
agentx-IndexAllocate-PDU and agentx-Register-PDU to achieve desired
configurations.
7.1.4.2.1. Registration Priority
The r.priority field in the agentx-Register-PDU is intended to be
manipulated by human administrators to achieve a desired subagent
configuration. Typically this would be needed where a legacy
application registers a specific subtree, and a different
(configurable) application may need to become authoritative for the
identical subtree.
The result of this configuration (the same subtree registered on
different sessions with different priorities) is that the session
using the better priority (see section 7.1.4.1, "Handling Duplicate
and Overlapping Subtrees") will be authoritative. The other session
will simply never be dispatched to.
This is useful in the case described above, but is NOT useful in
other cases, particularly when subagents share tables indexed by
arbitrary values (see below). In general, subagents should register
using the default priority (127).
7.1.4.2.2. Index Allocation
Index allocation is a service provided by an AgentX master agent. It
provides generic support for sharing MIB conceptual tables among
subagents who are assumed to have no knowledge of each other.
The master agent maintains a database of index objects (OIDs), and,
for each index, the values that have been allocated for it. It is
unaware of what MIB variables (if any) the index objects represent.
By convention, subagents use the MIB variable listed in the INDEX
clause as the index object for which values must be allocated. For
tables indexed by multiple variables, values may be allocated for
each index (although this is frequently unnecessary; see example 2
below). The subagent may request allocation of
a) a specific index value
b) an index value that is not currently allocated
c) an index value that has never been allocated
The last two alternatives reflect the uniqueness and constancy
requirements present in many MIB specifications for arbitrary integer
indexes (e.g., ifIndex in the IF-MIB (RFC 2233 [19]),
snmpFddiSMTIndex in the FDDI MIB (RFC 1285 [20]), or
sysApplInstallPkgIndex in the System Application MIB (RFC 2287
[21])). The need for subagents to share tables using such indexes is
the main motivation for index allocation in AgentX.
It is important to note that index allocation and MIB region
registration are not coupled in the master agent. The current state
of index allocations is not considered when processing registration
requests, and the current registry is not considered when processing
index allocation requests. (This is mainly to accommodate non-AgentX
subagents.)
AgentX subagents should follow the model of "first request allocation
of an index, then register the corresponding region". Then a
successful index allocation request gives a subagent a good hint (but
no guarantee) of what it should be able to register. The
registration may fail (with `duplicateRegistration') because some
other subagent session has already registered that row of the table.
The recommended mechanism for subagents to register conceptual rows
in a shared table is
1) Successfully allocate an index value.
2) Use that value to fully qualify the MIB region(s), and attempt to
register using the default priority.
3) If the registration fails with `duplicateRegistration' deallocate
the previously allocated index value(s) for this row and go to
step 1).
Note that index allocation is necessary only when the index in question is an arbitrary value, and hence the subagent has no other reasonable way to determine which index values to use. When index values have intrinsic meaning it is not expected that subagents will allocate their index values. For example, RFC 1514's table of running software processes (hrSWRunTable) is indexed by the system's native process identifier (pid). A subagent implementing the row of hrSWRunTable corresponding to its own process would simply register the region defining that row's object instances without allocating index values. 7.1.4.2.3. Examples Example 1: A subagent implements an interface, and wishes to register a single row of the RFC 2233 ifTable. It requests an allocation for the index object "ifIndex", for a value that has never been allocated (since ifIndex values must be unique). The master agent returns the value "7". The subagent now attempts to register row 7 of ifTable, by specifying a MIB region in the agentx-Register-PDU of 1.3.6.1.2.1.2.2.1.[1-22].7. If the registration succeeds, no further processing is required. The master agent will dispatch to this subagent correctly. If the registration failed with `duplicateRegistration', the subagent should deallocate the failed index, request allocation of a new index i, and attempt to register ifTable.[1-22].i, until successful. Example 2: This same subagent wishes to register ipNetToMediaTable rows corresponding to its interface (ifIndex i). Due to the structure of this table, no further index allocation need be done. The subagent can register the MIB region ipNetToMediaTable.[1-4].i, It is claiming responsibility for all rows of the table whose value of ipNetToMediaIfIndex is i.
Example 3:
A network device consists of a set of processors, each of which
accepts network connections for a unique set of IP addresses.
Further, each processor contains a subagent that implements
tcpConnTable. In order to represent tcpConnTable for the entire
managed device, the subagents need to share tcpConnTable.
In this case, no index allocation need be done at all. Each
subagent can register a MIB region of tcpConnTable.[1-5].a.b.c.d,
where a.b.c.d represents an unique IP address of the individual
processor.
Each subagent is claiming responsibility for the region of
tcpConnTable where the value of tcpConnLocalAddress is a.b.c.d.
Example 4:
The Application Management MIB (RFC 2564 [22]) uses two objects to
index several tables. A partial description of them is:
applSrvIndex OBJECT-TYPE
SYNTAX Unsigned32 (1..'ffffffff'h)
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"An applSrvIndex is the system-unique identifier
of an instance of a service. The value is unique
not only across all instances of a given service,
but also across all services in a system."
applSrvName OBJECT-TYPE
SYNTAX SnmpAdminString
MAX-ACCESS read-only
STATUS current
DESCRIPTION
"The human-readable name of a service. Where
appropriate, as in the case where a service can
be identified in terms of a single protocol, the
strings should be established names such as those
assigned by IANA and found in STD 2 [23], or
defined by some other authority. In some cases
private conventions apply and the string should
in these cases be consistent with these
non-standard conventions. An applicability
statement may specify the service name(s) to be
used."
Since applSrvIndex is an arbitrary value, it would be reasonable
for subagents to allocate values for this index. applSrvName
however has intrinsic meaning and any values a subagent would use
should be known a priori, hence it is not reasonable for subagents
to allocate values of this index.
7.1.5. Processing the agentx-Unregister-PDU
When the master agent receives an agentx-Unregister-PDU, it performs
the common processing described in section 7.1, "Processing AgentX
Administrative Messages". If as a result res.error is `
noAgentXError', processing continues as follows:
1) If u.subtree, u.priority, u.range_subid (and if u.range_subid is
not 0, u.upper_bound), and the indicated context do not match an
existing registration made during this session, the agentx-
Response-PDU is returned with res.error set to `
unknownRegistration'.
2) Otherwise, the agentx-Response-PDU is sent in reply with res.error
set to `noAgentXError', and the previous registration is removed
from the registration data store.
7.1.6. Processing the agentx-AddAgentCaps-PDU
When the master agent receives an agentx-AddAgentCaps-PDU, it
performs the common processing described in section 7.1, "Processing
AgentX Administrative Messages". If as a result res.error is `
noAgentXError', processing continues as follows:
1) The master agent adds this agent capabilities information to the
sysORTable for the indicated context. An agentx-Response-PDU is
sent in reply with res.error set to `noAgentXError'.
7.1.7. Processing the agentx-RemoveAgentCaps-PDU
When the master agent receives an agentx-RemoveAgentCaps-PDU, it
performs the common processing described in section 7.1, "Processing
AgentX Administrative Messages". If as a result res.error is
`noAgentXError', processing continues as follows:
1) If the combination of a.id and the optional a.context does not
represent a sysORTable entry that was added by this subagent
during this session, the agentx-Response-PDU is returned with
res.error set to `unknownAgentCaps'.
2) Otherwise the master agent deletes the corresponding sysORTable
entry and sends in reply the agentx-Response-PDU, with res.error
set to `noAgentXError'.
7.1.8. Processing the agentx-Close-PDU
When the master agent receives an agentx-Close-PDU, it performs the
common processing described in section 7.1, "Processing AgentX
Administrative Messages", with the exception that step 4) is not
performed since the agentx-Close-PDU does may not contain a context
field. If as a result res.error is `noAgentXError', processing
continues as follows:
1) The master agent closes the AgentX session as described below, and
sends in reply the agentx-Response-PDU with res.error set to
`noAgentXError':
- All MIB regions that have been registered during this session
are unregistered, as described in section 7.1.5, "Processing
the agentx-Unregister-PDU".
- All index values allocated during this session are freed, as
described in section 7.1.3, "Processing the agentx-
IndexDeallocate-PDU".
- All sysORID values that were registered during this session are
removed, as described in section 7.1.7, "Processing the
agentx-RemoveAgentCaps-PDU".
The master agent does not maintain state for closed sessions. If a
subagent wishes to re-establish a session after it has been closed,
it needs to re-register MIB regions, agent capabilities, etc.
7.1.9. Detecting Connection Loss
If a master agent is able to detect (from the underlying transport)
that a subagent cannot receive AgentX PDUs, it should close all
affected AgentX sessions as described in section 7.1.8, "Processing
the agentx-Close-PDU", step 1).
7.1.10. Processing the agentx-Notify-PDU
A subagent sending SNMPv1 trap information must map this into
(minimally) a value of snmpTrapOID.0, as described in 3.1.2 of RFC
1908 [24].
When the master agent receives an agentx-Notify-PDU, it performs the common processing described in section 7.1, "Processing AgentX Administrative Messages". If as a result res.error is `noAgentXError', processing continues as follows: 1) If the first VarBind is sysUpTime.0; (a) if the second VarBind is not snmpTrapOID.0, res.error is set to `processingError' and res.index to 2 (b) otherwise these two VarBinds are used as the first two VarBinds within the generated notification. 2) If the first VarBind is not sysUpTime.0; (a) if the first VarBind is not snmpTrapOID.0, res.error is set to `processingError' and res.index to 1 (b) otherwise this VarBind is used for snmpTrapOID.0 within the generated notification, and the master agent uses the current value of sysUpTime.0 for the indicated context as sysUpTime.0 within the notification. 3) An agentx-Response-PDU is sent containing the original VarBindList, and with res.error and res.index set as described above. If res.error is `noAgentXError', notifications are sent according to the implementation-specific configuration of the master agent. If SNMPv1 Trap PDUs are generated, the recommended mapping is as described in RFC 2089 [25]. If res.error indicates an error in processing, no notifications are generated. Note that the master agent's successful response indicates the agentx-Notify-PDU was received and validated. It does not indicate that any particular notifications were actually generated or received by notification targets. 7.1.11. Processing the agentx-Ping-PDU When the master agent receives an agentx-Ping-PDU, it performs the common processing described in section 7.1, "Processing AgentX Administrative Messages". If as a result res.error is ` noAgentXError', processing continues as follows:
1) An agentx-Response-PDU is sent in reply.
If a subagent does not receive a response to its pings, or if it is
able to detect (from the underlying transport) that the master agent
is not able to receive AgentX messages, then it eventually must
initiate a new AgentX session, re-register its MIB regions, etc.
7.2. Processing Received SNMP Protocol Messages
When an SNMP GetRequest, GetNextRequest, GetBulkRequest, or
SetRequest protocol message is received by the master agent, the
master agent applies its access control policy.
In particular, for SNMPv1 or SNMPv2c protocol messages, the master
agent applies the Elements of Procedure defined in section 4.1 of STD
15, RFC 1157 [8] that apply to receiving entities. For SNMPv3, the
master agent applies an Access Control Model, possibly the View-based
Access Control Model (see RFC 2575 [15]), as described in section
3.1.2 and section 4.3 of RFC 2571 [1].
For SNMPv1 and SNMPv2c, the master agent uses the community string as
an index into a local repository of configuration information that
may include community profiles or more complex context information.
For SNMPv3, the master agent uses the SNMP Context (see section 3.3.1
of RFC 2571 [1]) for these purposes.
If application of the access control policy results in a valid SNMP
request PDU, then an SNMP Response-PDU is constructed from
information gathered in the exchange of AgentX PDUs between the
master agent and one or more subagents. Upon receipt and initial
validation of an SNMP request PDU, a master agent uses the procedures
described below to dispatch AgentX PDUs to the proper subagents,
marshal the subagent responses, and construct an SNMP response PDU.
7.2.1. Dispatching AgentX PDUs
Upon receipt and initial validation of an SNMP request PDU, a master
agent uses the procedures described below to dispatch AgentX PDUs to
the proper subagents.
General Rules of Procedure
While processing a particular SNMP request, the master agent may send
one or more AgentX PDUs on one or more subagent sessions. The
following rules of procedure apply in general to the AgentX master
agent. PDU-specific rules are listed in the applicable sections.
1) Honoring the registry
Because AgentX supports registration of duplicate and overlapping
regions, it is possible for the master agent to obtain a value for
a requested varbind from within multiple registered MIB regions.
The master agent must ensure that the value (or exception)
actually returned in the SNMP response PDU is taken from the
authoritative region (as defined in section 7.1.4.1, "Handling
Duplicate and Overlapping Subtrees").
2) GetNext and GetBulk Processing
The master agent may choose to send agentx-Get-PDUs while
servicing an SNMP GetNextRequest-PDU. The master agent may choose
to send agentx-Get-PDUs or agentx-GetNext-PDUs while servicing an
SNMP GetBulkRequest-PDU. One possible reason for this would be if
the current iteration has targeted instance-level registrations.
The master agent may choose to "scope" the possible instances
returned by a subagent by specifying an ending OID in the
SearchRange. If such scoping is used, typically the ending OID
would be the first lexicographical successor to the target region
that was registered on a session other than the target session.
Regardless of this choice, rule (1) must be obeyed.
The master agent may require multiple request-response iterations
on the same subagent session, to determine the final value of all
requested variables.
All AgentX PDUs sent on the session while processing a given SNMP
request must contain identical values of transactionID. Each
different SNMP request processed by the master agent must present
a unique value of transactionID (within the limits of the 32-bit
field) to the session.
3) Number and order of variables sent per AgentX PDU
For Get/GetNext/GetBulk operations, at any stage of the possibly
iterative process, the master agent may need to dispatch several
SearchRanges to a particular subagent session. The master agent
may send one, some, or all of the SearchRanges in a single AgentX
PDU.
The master agent must ensure that the correct contents and
ordering of the VarBindList in the SNMP Response-PDU are
maintained.
The following rules govern the number of VarBinds in a given
AgentX PDU:
a) The subagent must support processing of AgentX PDUs with
multiple VarBinds.
b) When processing an SNMP Set request, the master agent must
send all of the VarBinds applicable to a particular subagent
session in a single agentx-TestSet-PDU.
c) When processing an SNMP Get, GetNext, or GetBulk request,
the master agent may send a single AgentX PDU on the session
with all applicable VarBinds, or multiple PDUs with single
VarBinds, or something in between those extremes. The
determination of which method to use in a particular case is
implementation-specific.
4) Timeout Values
The master agent chooses a timeout value for each MIB region being
queried, which is
a) the value specified during registration of the MIB region,
if it was non-zero
b) otherwise, the value specified during establishment of the
session in which this region was subsequently registered, if
that value was non-zero
c) otherwise, or, if the specified value is not practical, the
master agent's implementaton-specific default value
When an AgentX PDU that references multiple MIB regions is
dispatched, the timeout value used for the PDU is the maximum
value of the timeouts so determined for each of the referenced MIB
regions.
5) Context
If the master agent has determined that a specific non-default
context is associated with the SNMP request PDU, that context is
encoded into the AgentX PDU's context field and the
NON_DEFAULT_CONTEXT bit is set in h.flags.
Otherwise, no context Octet String is added to the PDU, and the
NON_DEFAULT_CONTEXT bit is cleared.
7.2.1.1. agentx-Get-PDU Each variable binding in the SNMP request PDU is processed as follows: (1) Identify the target MIB region. Within a lexicographically ordered set of registered MIB regions, valid for the indicated context, locate the authoritative region (according to section 7.1.4.1, "Handling Duplicate and Overlapping Subtrees") that contains the binding's name. (2) If no such region exists, the variable binding is not processed further, and its value is set to `noSuchObject'. (3) Identify the subagent session in which this region was registered, termed the target session. (4) If this is the first variable binding to be dispatched over the target session in a request-response exchange entailed in the processing of this management request: - Create an agentx-Get-PDU for this session, with the header fields initialized as described above (see section 6.1, "AgentX PDU Header"). (5) Add a SearchRange to the end of the target session's PDU for this variable binding. - The variable binding's name is encoded into the starting OID. - The ending OID is encoded as null. 7.2.1.2. agentx-GetNext-PDU Each variable binding in the SNMP request PDU is processed as follows: (1) Identify the target MIB region. Within a lexicographically ordered set of registered MIB regions, valid for the indicated context, locate the authoritative region (according to section 7.1.4.1, "Handling Duplicate and Overlapping Subtrees") that a) contains the variable binding's name and is not a fully qualified instance, or
b) is the first lexicographical successor to the variable
binding's name.
(2) If no such region exists, the variable binding is not processed
further, and its value is set to `endOfMibView'.
(3) Identify the subagent session in which this region was
registered, termed the target session.
(4) If this is the first variable binding to be dispatched over the
target session in a request-response exchange entailed in the
processing of this management request:
- Create an agentx-GetNext-PDU for the session, with the header
fields initialized as described above (see section 6.1,
"AgentX PDU Header").
(5) Add a SearchRange to the end of the target session's agentx-
GetNext-PDU for this variable binding.
- if (1a) applies, the variable binding's name is encoded into
the starting OID, and the OID's "include" field is set to 0.
- if (1b) applies, the target region's r.subtree is encoded
into the starting OID, and its "include" field is set to 1.
(This is the recommended method. An implementation may
choose to use a Starting OID value that precedes r.subtree,
in which case the include bit must be 0. A starting OID
value that succeeds r.subtree is not permitted.)
- the Ending OID for the SearchRange is encoded to be either
NULL, or a value that lexicographically succeeds the Starting
OID. This is an implementation-specific choice depending on
how the master agent wishes to "scope" the possible returned
instances.
7.2.1.3. agentx-GetBulk-PDU
(Note: The outline of the following procedure is based closely on
section 4.2.3, "The GetBulkRequest-PDU" of RFC 1905 [13]. Please
refer to it for details on the format of the SNMP GetBulkRequest-PDU
itself.)
Each variable binding in the request PDU is processed as follows:
(1) Identify the authoritative target region and target session,
exactly as described for the agentx-GetNext-PDU (see section
7.2.1.2, "agentx-GetNext-PDU").
(2) If this is the first variable binding to be dispatched over the
target session in a request-response exchange entailed in the
processing of this management request:
- Create an agentx-GetBulk-PDU for the session, with the header
fields initialized as described above (see section 6.1,
"AgentX PDU Header").
(3) Add a SearchRange to the end of the target session's agentx-
GetBulk-PDU for this variable binding, as described for the
agentx-GetNext-PDU. If the variable binding was a non-repeater
in the original request PDU, it must be a non-repeater in the
agentx-GetBulk-PDU.
The value of g.max_repetitions in the agentx-GetBulk-PDU may be less
than (but not greater than) the value in the original request PDU.
The master agent may make such alterations due to simple sanity
checking, optimizations for the current iteration based on the
registry, the maximum possible size of a potential Response-PDU,
known constraints of the AgentX transport, or any other
implementation-specific constraint.
7.2.1.4. agentx-TestSet-PDU
AgentX employs test-commit-undo-cleanup phases to achieve "as if
simultaneous" semantics of the SNMP SetRequest-PDU within the
extensible agent. The initial phase involves the agentx-TestSet-PDU.
Each variable binding in the SNMP request PDU is processed in order,
as follows:
(1) Identify the target MIB region and target session exactly as
described in section 7.2.1.1, "agentx-Get-PDU", step 1).
Within a lexicographically ordered set of OID ranges, valid for
the indicated context, locate the authoritative range that
contains the variable binding's name.
(2) If no such target region exists, this variable binding fails
with an error of `notWritable'. Processing is complete for this
request.
(3) If this is the first variable binding to be dispatched over the
target session in a request-response exchange entailed in the
processing of this management request:
- create an agentx-TestSet-PDU for the session, with the header
fields initialized as described above (see section 6.1,
"AgentX PDU Header").
(4) Add a VarBind to the end of the target session's PDU for this
variable binding, as described in section 5.4, "Value
Representation".
Note that all VarBinds applicable to a given session must be sent in
a single agentx-TestSet-PDU.
7.2.1.5. Dispatch
A timeout value is calculated for each PDU to be sent, which is the
maximum value of the timeouts determined for each of the PDU's
SearchRanges (as described above in section 7.2.1, "Dispatching
AgentX PDUs", item 4). Each pending PDU is mapped (via its
h.sessionID value) to a particular transport domain/endpoint, as
described in section 8 (Transport Mappings).
7.2.2. Subagent Processing
A subagent initially processes a received AgentX PDU as follows:
- If the received PDU is an agentx-Response-PDU:
1) If there are any errors parsing or interpreting the PDU, it is
silently dropped.
2) Otherwise the response is matched to the original request via
h.packetID, and handled in an implementation-specific manner. For
example, if this response indicates an error attempting to
register a MIB region, the subagent may wish to register a
different region, or log an error and halt, etc.
- If the received PDU is any other type:
1) an agentx-Response-PDU is created whose header fields are
identical to the received request PDU except that h.type is set to
Response, res.error to `noError', res.index to 0, and the
VarBindList to null.
2) If the received PDU cannot be parsed, res.error is set to
`parseError'.
3) Otherwise, if h.sessionID does not correspond to a currently
established session, res.error is set to `notOpen'.
4) At this point, if res.error is not `noError', the received PDU is
not processed further. If the received PDU's header was
successfully parsed, the AgentX-Response-PDU is sent in reply. If
the received PDU's header was not successfully parsed or for some
other reason the subagent cannot send a reply, processing is
complete.
7.2.3. Subagent Processing of agentx-Get, GetNext, GetBulk-PDUs
A conformant AgentX subagent must support the agentx-Get, -GetNext,
and -GetBulk PDUs, and must support multiple variables being supplied
in each PDU.
When a subagent receives an agentx-Get-, GetNext-, or GetBulk-PDU, it
performs the indicated management operations and returns an agentx-
Response-PDU.
Each SearchRange in the request PDU's SearchRangeList is processed as
described below, and a VarBind is added in the corresponding location
of the agentx-Response-PDU's VarbindList. If processing should fail
for any reason not described below, res.error is set to `genErr',
res.index to the index of the failed SearchRange, the VarBindList is
reset to null, and this agentx-Response-PDU is returned to the master
agent.
7.2.3.1. Subagent Processing of the agentx-Get-PDU
Upon the subagent's receipt of an agentx-Get-PDU, each SearchRange in
the request is processed as follows:
(1) The starting OID is copied to v.name.
(2) If the starting OID exactly matches the name of a variable
instantiated by this subagent within the indicated context and
session, v.type and v.data are encoded to represent the
variable's syntax and value, as described in section 5.4, "Value
Representation".
(3) Otherwise, if the starting OID does not match the object
identifier prefix of any variable instantiated within the
indicated context and session, the VarBind is set to
`noSuchObject', in the manner described in section 5.4, "Value
Representation".
(4) Otherwise, the VarBind is set to `noSuchInstance' in the manner
described in section 5.4, "Value Representation".
7.2.3.2. Subagent Processing of the agentx-GetNext-PDU
Upon the subagent's receipt of an agentx-GetNext-PDU, each
SearchRange in the request is processed as follows:
(1) The subagent searches for a variable within the
lexicographically ordered list of variable names for all
variables it instantiates (without regard to registration of
regions) within the indicated context and session, as follows:
- if the "include" field of the starting OID is 0, the
variable's name is the closest lexicographical successor to
the starting OID.
- if the "include" field of the starting OID is 1, the
variable's name is either equal to, or the closest
lexicographical successor to, the starting OID.
- If the ending OID is not null, the variable's name
lexicographically precedes the ending OID.
If a variable is successfully located, v.name is set to that
variable's name. v.type and v.data are encoded to represent the
variable's syntax and value, as described in section 5.4, "Value
Representation".
(2) If the subagent cannot locate an appropriate variable, v.name is
set to the starting OID, and the VarBind is set to `
endOfMibView', in the manner described in section 5.4, "Value
Representation".
7.2.3.3. Subagent Processing of the agentx-GetBulk-PDU
A maximum of N + (M * R) VarBinds are returned, where
N equals g.non_repeaters,
M equals g.max_repetitions, and
R is (number of SearchRanges in the GetBulk request) - N.
The first N SearchRanges are processed exactly as for the agentx-
GetNext-PDU.
If M and R are both non-zero, the remaining R SearchRanges are
processed iteratively to produce potentially many VarBinds. For each
iteration i, such that i is greater than zero and less than or equal
to M, and for each repeated SearchRange s, such that s is greater
than zero and less than or equal to R, the (N+((i-1)*R)+s)-th VarBind
is added to the agentx-Response-PDU as follows:
1) The subagent searches for a variable within the
lexicographically ordered list of variable names for all
variables it instantiates (without regard to registration of
regions) within the indicated context and session, for which
the following are all true:
- The variable's name is the (i)-th lexicographical successor
to the (N+s)-th requested OID.
(Note that if i is 0 and the "include" field is 1, the
variable's name may be equivalent to, or the first
lexicographical successor to, the (N+s)-th requested OID.)
- If the ending OID is not null, the variable's name
lexicographically precedes the ending OID.
If all of these conditions are met, v.name is set to the located
variable's name. v.type and v.data are encoded to represent the
variable's syntax and value, as described in section 5.4, "Value
Representation".
2) If no such variable exists, the VarBind is set to `
endOfMibView' as described in section 5.4, "Value
Representation". v.name is set to v.name of the (N+((i-
2)*R)+s)-th VarBind unless i is currently 1, in which case it
is set to the value of the starting OID in the (N+s)-th
SearchRange.
Note that further iterative processing should stop if
- For any iteration i, all s values of v.type are `
endOfMibView'.
- An AgentX transport constraint or other implementation-
specific constraint is reached.
7.2.4. Subagent Processing of agentx-TestSet, -CommitSet, -UndoSet,
-CleanupSet-PDUs
A conformant AgentX subagent must support the agentx-TestSet,
-CommitSet, -UndoSet, and -CleanupSet PDUs, and must support multiple
variables being supplied in the agentx-TestSet-PDU.
These four PDUs are used to collectively perform the indicated management operation. An agentx-Response-PDU is sent in reply to each of the PDUs (except -CleanupSet), to inform the master agent of the state of the operation. The master agent must serialize Set transactions for each session. That is, a session need not handle multiple concurrent Set transactions. These Response-PDUs do not contain a VarBindList. 7.2.4.1. Subagent Processing of the agentx-TestSet-PDU Upon the subagent's receipt of an agentx-TestSet-PDU, each VarBind in the PDU is validated until they are all successful, or until one fails, as described in section 4.2.5 of RFC 1905 [13]. The subagent validates variables with respect to the context and session indicated in the testSet-PDU. If each VarBind is successful, the subagent has a further responsibility to ensure the availability of all resources (memory, write access, etc.) required for successfully carrying out a subsequent agentx-CommitSet operation. If this cannot be guaranteed, the subagent should set res.error to `resourceUnavailable'. As a result of this validation step, an agentx-Response-PDU is sent in reply whose res.error field is set to one of the following SNMPv2 PDU error-status values (see section 3, "Definitions", in RFC 1905 [13]): noError (0), genErr (5), noAccess (6), wrongType (7), wrongLength (8), wrongEncoding (9), wrongValue (10), noCreation (11), inconsistentValue (12), resourceUnavailable (13), notWritable (17), inconsistentName (18) If this value is not `noError', the res.index field must be set to the index of the VarBind for which validation failed.
Implementation of rigorous validation code may be one of the most demanding aspects of subagent development. Implementors are strongly encouraged to do this right, so as to avoid if at all possible the extensible agent's having to return `commitFailed' or `undoFailed' during subsequent processing. 7.2.4.2. Subagent Processing of the agentx-CommitSet-PDU The agentx-CommitSet-PDU indicates that the subagent should actually perform (as described in the post-validation sections of 4.2.5 of RFC 1905 [13]) the management operation indicated by the previous TestSet-PDU. After carrying out the management operation, the subagent sends in reply an agentx-Response-PDU whose res.error field is set to one of the following SNMPv2 PDU error-status values (see section 3, "Definitions", in RFC 1905 [13]): noError (0), commitFailed (14) If this value is `commitFailed', the res.index field must be set to the index of the VarBind (as it occurred in the agentx-TestSet-PDU) for which the operation failed. Otherwise res.index is set to 0. 7.2.4.3. Subagent Processing of the agentx-UndoSet-PDU The agentx-UndoSet-PDU indicates that the subagent should undo the management operation requested in a preceding CommitSet-PDU. The undo process is as described in section 4.2.5 of RFC 1905 [13]. After carrying out the undo process, the subagent sends in reply an agentx-Response-PDU whose res.error field is set to one of the following SNMPv2 PDU error-status values (see section 3, "Definitions", in RFC 1905 [13]): noError (0), undoFailed (15) If this value is `undoFailed', the res.index field must be set to the index of the VarBind (as it occurred in the agentx-TestSet-PDU) for which the operation failed. Otherwise res.index is set to 0. This PDU also signals the end of processing of the management operation initiated by the previous TestSet-PDU. The subagent should release resources, etc. as described in section 7.2.4.4, "Subagent Processing of the agentx-CleanupSet-PDU".
7.2.4.4. Subagent Processing of the agentx-CleanupSet-PDU The agentx-CleanupSet-PDU signals the end of processing of the management operation requested in the previous TestSet-PDU. This is an indication to the subagent that it may now release any resources it may have reserved in order to carry out the management request. No response is sent by the subagent. 7.2.5. Master Agent Processing of AgentX Responses The master agent now marshals all subagent AgentX response PDUs and builds an SNMP response PDU. In the next several subsections, the initial processing of all subagent AgentX response PDUs is described, followed by descriptions of subsequent processing for each specific subagent Response. 7.2.5.1. Common Processing of All AgentX Response PDUs 1) If a response is not received on a session within the timeout interval for this dispatch, it is treated as if the subagent had returned `genErr' and processed as described below. A timeout may be due to a variety of reasons, and does not necessarily denote a failed or malfunctioning subagent. As such, the master agent's response to a subagent timeout is implementation-specific, but with the following constraint: A session that times out on three consecutive AgentX requests is considered unable to respond, and the master agent must close the AgentX session as described in section 7.1.8, "Processing the agentx-Close-PDU", step (2). 2) Otherwise, the h.packetID, h.sessionID, and h.transactionID fields of the AgentX response PDU are used to correlate subagent responses. If the response does not pertain to this SNMP operation, it is ignored. 3) Otherwise, the responses are processed jointly to form the SNMP response PDU. 7.2.5.2. Processing of Responses to agentx-Get-PDUs After common processing of the subagent's response to an agentx-Get- PDU (see section 7.2.5.1, "Common Processing of All AgentX Response PDUs", above), processing continues with the following steps:
1) For any received AgentX response PDU, if res.error is not
`noError', the SNMP response PDU's error code is set to this
value. If res.error contains an AgentX specific value (e.g.
`parseError'), the SNMP response PDU's error code is set to a
value of genErr instead. Also, the SNMP response PDU's error
index is set to the index of the variable binding corresponding to
the failed VarBind in the subagent's AgentX response PDU.
All other AgentX response PDUs received due to processing this
SNMP request are ignored. Processing is complete; the SNMP
Response PDU is ready to be sent (see section 7.2.6, "Sending the
SNMP Response-PDU").
2) Otherwise, the content of each VarBind in the AgentX response PDU
is used to update the corresponding variable binding in the SNMP
Response-PDU.
7.2.5.3. Processing of Responses to agentx-GetNext-PDU and
agentx-GetBulk-PDU
After common processing of the subagent's response to an agentx-
GetNext-PDU or agentx-GetBulk-PDU (see section 7.2.5.1, "Common
Processing of All AgentX Response PDUs", above), processing continues
with the following steps:
1) For any received AgentX response PDU, if res.error is not
`noError', the SNMP response PDU's error code is set to this
value. If res.error contains an AgentX specific value (e.g.
`parseError'), the SNMP response PDU's error code is set to a
value of genErr instead. Also, the SNMP response PDU's error
index is set to the index of the variable binding corresponding to
the failed VarBind in the subagent's AgentX response PDU.
All other AgentX response PDUs received due to processing this
SNMP request are ignored. Processing is complete; the SNMP
response PDU is ready to be sent (see section 7.2.6, "Sending the
SNMP Response-PDU").
2) Otherwise, the content of each VarBind in the AgentX response PDU
is used to update the corresponding VarBind in the SNMP response
PDU.
After all expected AgentX response PDUs have been processed, if any
VarBinds still contain the value `endOfMibView' in their v.type
fields, processing must continue:
3) A new iteration of AgentX request dispatching is initiated (as
described in section 7.2.1.2, "agentx-GetNext-PDU"), in which only
those VarBinds whose v.type is `endOfMibView' are processed.
4) For each such VarBind, an authoritative target MIB region is
identified in which the master agent expects to find suitable MIB
variables. The target session is the one on which this new target
region was registered.
The starting OID in each SearchRange is set to the value of v.name
for the corresponding VarBind, and its "include" field is set to
0.
5) The value of transactionID must be identical to the value used
during the previous iteration.
6) The AgentX PDUs are sent on the target session(s), and the
responses are received and processed according to the steps
described in section 7.2.5, "Master Agent Processing of AgentX
Responses".
7) This process continues iteratively until a complete SNMP
Response-PDU has been built, or until there remain no
authoritative MIB regions to query.
Note that r.subtree for the new target region identified in step 4)
may not lexicographically succeed r.subtree for the region that has
returned `endOfMibView'. For example, consider the following
registry:
session A `mib-2' (1.3.6.1.2.1)
session B `ip' (1.3.6.1.2.1.4)
session C `tcp' (1.3.6.1.2.1.6)
If while processing a GetNext-Request-PDU session B returns
`endOfMibView' for a variable name within 1.3.6.1.2.1.4, the target
MIB region identified in step 4) would be 1.3.6.1.2.1 (since it may
contain variables whose names precede 1.3.6.1.2.1.6).
Note also that if session A returned variables from within
1.3.6.1.2.1.6, they must be discarded since session A is NOT
authoritative for that region.
7.2.5.4. Processing of Responses to agentx-TestSet-PDUs
After common processing of the subagent's response to an agentx-
TestSet-PDU (see section 7.2.5.1, "Common Processing of All AgentX
Response PDUs", above), processing continues with the further
exchange of AgentX PDUs. The value of h.transactionID in the agentx-CommitSet, -UndoSet, and -CleanupSet-PDUs must be identical to the value sent in the testSet-PDU. The state transitions and PDU sequences are depicted in section 7.3, "State Transitions". The set of all sessions who have been sent an agentx-TestSet-PDU for this particular transaction are referred to below as "involved sessions". 1) If any target session's response is not `noError', all other agentx-Response-PDUs received due to processing this SNMP request are ignored. An agentx-CleanupSet-PDU is sent to all involved sessions. Processing is complete; the SNMP response PDU is constructed as described below in 7.2.6, "Sending the SNMP Response-PDU". 2) Otherwise an agentx-CommitSet-PDU is sent to all involved sessions. 7.2.5.5. Processing of Responses to agentx-CommitSet-PDUs After common processing of the subagent's response to an agentx- CommitSet-PDU (see section 7.2.5.1, "Common Processing of All AgentX Response PDUs", above), processing continues with the following steps: 1) If any response is not `noError', the SNMP response PDU's error code is set to this value. If res.error contains an AgentX specific value (e.g. `parseError'), the SNMP response PDU's error code is set to a value of genErr instead. Also, the SNMP response PDU's error index is set to the index of the VarBind corresponding to the failed VarBind in the agentx-TestSet-PDU. An agentx-UndoSet-PDU is sent to each target session that has been sent an agentx-CommitSet-PDU. An agentx-CleanupSet-PDU is sent to the remainder of the involved sessions. 2) Otherwise an agentx-CleanupSet-PDU is sent to all involved sessions. Processing is complete; the SNMP response PDU is constructed as described below in section 7.2.6, "Sending the SNMP Response-PDU".
7.2.5.6. Processing of Responses to agentx-UndoSet-PDUs After common processing of the subagent's response to an agentx- UndoSet-PDU (see section 7.2.5.1, "Common Processing of All AgentX Response PDUs", above), processing continues with the following steps: 1) If any response is `undoFailed' the SNMP response PDU's error code is set to this value. Also, the SNMP response PDU's error index is set to 0. 2) Otherwise, if any response is not `noError' the SNMP response PDU's error code is set to this value. Also, the SNMP response PDU's error index is set to the index of the VarBind corresponding to the failed VarBind in the agentx-TestSet-PDU. If res.error is an AgentX specific value (e.g. `parseError'), the SNMP response PDU's error code is set to a value of genErr instead. 3) Otherwise the SNMP response PDU's error code and error index were set in section 7.2.5.5 step 1) 7.2.6. Sending the SNMP Response-PDU Once the processing described in section 7.2.5, "Master Agent Processing of AgentX Responses" is complete, there is an SNMP response PDU available. The master agent now implements the Elements of Procedure for the applicable version of the SNMP protocol in order to encapsulate the PDU into a message, and transmit it to the originator of the SNMP management request. Note that this may involve altering the PDU contents (for instance, to replace the original VarBinds if an error condition is to be returned). The response PDU may also be altered in order to support the SNMPv1 PDU. In such cases the required PDU mapping is that defined in RFC 2089 [25]. (Note in particular that the rules for handling Counter64 syntax may require re-sending AgentX GetBulk or GetNext PDUs until a VarBind of suitable syntax is returned.) 7.2.7. MIB Views AgentX subagents are not aware of MIB views, since view information is not contained in AgentX PDUs. As stated above, the descriptions of procedures in section 7, "Elements of Procedure", of this memo are not intended to constrain the internal architecture of any conformant implementation. In particular, the master agent procedures described in section 7.2.1,
"Dispatching AgentX PDUs" and in section 7.2.5, "Master Agent Processing of AgentX Responses" may be altered so as to optimize AgentX exchanges when implementing MIB views. Such optimizations are beyond the scope of this memo. But note that section 7.2.3, "Subagent Processing of agentx-Get, GetNext, GetBulk- PDUs", defines subagent behavior in such a way that alteration of SearchRanges may be used in such optimizations.
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