Internet Draft Protocol Operations for SNMPv2 September 1995
Protocol Operations
for Version 2 of the
Simple Network Management Protocol (SNMPv2)
Fri Sep 08 1995
draft-various-snmpv2-proto-syn-01.txt
Tell U. Later
snmpv2@tis.com
Status of this Memo
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running list of open issues
reference list
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acknowledgements
authors
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1. Introduction
A management system contains: several (potentially many) manageable
nodes, each with a processing entity, termed an agent, which has access
to management instrumentation; at least one management station; and, a
management protocol. The management protocol is used to convey
management information between the agents and management stations; and,
for manager-to-manager communications, between management stations.
Operations of the protocol are carried out under an administrative
framework which defines authentication, authorization, access control,
and privacy policies.
Management stations execute management applications which monitor and
control managed elements. Managed elements are devices such as hosts,
routers, terminal servers, etc., which are monitored and controlled via
access to their management information.
Management information is viewed as a collection of managed objects,
residing in a virtual information store, termed the Management
Information Base (MIB). Collections of related objects are defined in
MIB modules. These modules are written using a subset of OSI's Abstract
Syntax Notation One (ASN.1) [@ref asn.1], termed the Structure of
Management Information (SMI) [@ref v2smi].
The management protocol, version 2 of the Simple Network Management
Protocol, provides for the exchange of messages which convey management
information between the agents and the management stations. The form of
these messages is a message "wrapper" which encapsulates a Protocol Data
Unit (PDU). The form and meaning of the "wrapper" is determined by an
administrative framework which defines both authentication and
authorization policies.
It is the purpose of this document, Protocol Operations for SNMPv2, to
define the operations of the protocol with respect to the sending and
receiving of the PDUs.
1.1. A Note on Terminology
For the purpose of exposition, the original Internet-standard Network
Management Framework, as described in RFCs 1155, 1157, and 1212, is
termed the SNMP version 1 framework (SNMPv1). The current framework is
termed the SNMP version 2 framework (SNMPv2).
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2. Overview
2.1. Roles of Protocol Entities
A SNMPv2 entity may operate in a manager role or an agent role.
A SNMPv2 entity acts in an agent role when it performs SNMPv2 management
operations in response to received SNMPv2 protocol messages (other than
an inform notification) or when it sends trap notifications.
A SNMPv2 entity acts in a manager role when it initiates SNMPv2
management operations by the generation of SNMPv2 protocol messages or
when it performs SNMPv2 management operations in response to received
trap or inform notifications.
A SNMPv2 entity may support either or both roles, as dictated by its
implementation and configuration. Further, a SNMPv2 entity can also act
in the role of a proxy agent, in which it appears to be acting in an
agent role, but satisfies management requests by acting in a manager
role with a remote entity. The use of proxy agents and the transparency
principle that defines their behavior is described in [@ref v2admin].
2.2. Management Information
The term, variable, refers to an instance of a non-aggregate object type
defined according to the conventions set forth in the SMI [@ref v2smi]
or the textual conventions based on the SMI [@ref tc]. The term,
variable binding, normally refers to the pairing of the name of a
variable and its associated value. However, if certain kinds of
exceptional conditions occur during processing of a retrieval request, a
variable binding will pair a name and an indication of that exception.
A variable-binding list is a simple list of variable bindings.
The name of a variable is an OBJECT IDENTIFIER which is the
concatenation of the OBJECT IDENTIFIER of the corresponding object-type
together with an OBJECT IDENTIFIER fragment identifying the instance.
The OBJECT IDENTIFIER of the corresponding object-type is called the
OBJECT IDENTIFIER prefix of the variable.
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2.3. Access to Management Information
Three types of access to management information are provided by the
protocol. One type is a request-response interaction, in which a SNMPv2
entity, acting in a manager role, sends a request to a SNMPv2 entity,
acting in an agent role, and the latter SNMPv2 entity then responds to
the request. This type is used to retrieve or modify management
information associated with the managed device.
A second type is also a request-response interaction, in which a SNMPv2
entity, acting in a manager role, sends a request to a SNMPv2 entity,
also acting in a manager role, and the latter SNMPv2 entity then
responds to the request. This type is used to notify a SNMPv2 entity,
acting in a manager role, of management information associated with
another SNMPv2 entity, also acting in a manager role.
The third type of access is an unconfirmed interaction, in which a
SNMPv2 entity, acting in an agent role, sends a unsolicited message,
termed a trap, to a SNMPv2 entity, acting in a manager role, and no
response is returned. This type is used to notify a SNMPv2 entity,
acting in a manager role, of an exceptional situation, which has
resulted in changes to management information associated with the
managed device.
2.4. Retransmission of Requests
For all types of requests in this protocol, the receiver is required
under normal circumstances, to generate and transmit a response to the
originator of the request. Whether or not a request should be
retransmitted if no corresponding response is received in an appropriate
time interval, is at the discretion of the application originating the
request. This will normally depend on the urgency of the request.
However, such an application needs to act responsibly in respect to the
frequency and duration of re-transmissions.
2.5. Message Sizes
The maximum size of a SNMPv2 message is limited by the minimum of:
(1) the maximum message size which the destination SNMPv2 entity can
accept; and,
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(2) the maximum message size which the source SNMPv2 entity can
generate.
The former may be known on a per-recipient basis; and in the absence of
such knowledge, is indicated by transport domain used when sending the
message. The latter is imposed by implementation-specific local
constraints.
Each transport mapping for the SNMPv2 indicates the minimum message size
which a SNMPv2 implementation must be able to produce or consume.
Although implementations are encouraged to support larger values
whenever possible, a conformant implementation must never generate
messages larger than allowed by the receiving SNMPv2 entity.
One of the aims of the GetBulkRequest-PDU, specified in this protocol,
is to minimize the number of protocol exchanges required to retrieve a
large amount of management information. As such, this PDU type allows a
SNMPv2 entity acting in a manager role to request that the response be
as large as possible given the constraints on message sizes. These
constraints include the limits on the size of messages which the SNMPv2
entity acting in an agent role can generate, and the SNMPv2 entity
acting in a manager role can receive.
However, it is possible that such maximum sized messages may be larger
than the Path MTU of the path across the network traversed by the
messages. In this situation, such messages are subject to
fragmentation. Fragmentation is generally considered to be harmful
[@ref fragmentation], since among other problems, it leads to a decrease
in the reliability of the transfer of the messages. Thus, a SNMPv2
entity which sends a GetBulkRequest-PDU must take care to set its
parameters accordingly, so as to reduce the risk of fragmentation. In
particular, under conditions of network stress, only small values should
be used for max-repetitions.
2.6. Transport Mappings
It is important to note that the exchange of SNMPv2 messages requires
only an unreliable datagram service, with every message being entirely
and independently contained in a single transport datagram. Specific
transport mappings and encoding rules are specified elsewhere [@ref tm].
However, the preferred mapping is the use of the User Datagram Protocol
[@ref udp].
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3. Definitions
SNMPv2-PDU DEFINITIONS ::= BEGIN
IMPORTS
ObjectName, ObjectSyntax, Integer32
FROM SNMPv2-SMI;
-- protocol data units
-- These are carried in an SnmpV2Message defined in [@ref v2admin] or
-- an SNMPV2CMessage defined in [@ref v2Cadmin] |
PDUs ::=
CHOICE {
get-request
GetRequest-PDU,
get-next-request
GetNextRequest-PDU,
get-bulk-request
GetBulkRequest-PDU,
response
Response-PDU,
set-request
SetRequest-PDU,
inform-request
InformRequest-PDU,
snmpV2-trap
SNMPv2-Trap-PDU
}
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-- PDUs
GetRequest-PDU ::=
[0]
IMPLICIT PDU
GetNextRequest-PDU ::=
[1]
IMPLICIT PDU
Response-PDU ::=
[2]
IMPLICIT PDU
SetRequest-PDU ::=
[3]
IMPLICIT PDU
-- [4] is obsolete
GetBulkRequest-PDU ::=
[5]
IMPLICIT BulkPDU
InformRequest-PDU ::=
[6]
IMPLICIT PDU
SNMPv2-Trap-PDU ::=
[7]
IMPLICIT PDU
Report-PDU ::=
[8]
IMPLICIT PDU
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max-bindings
INTEGER ::= 2147483647
PDU ::=
SEQUENCE {
request-id
Integer32,
error-status -- sometimes ignored
INTEGER {
noError(0),
tooBig(1),
noSuchName(2), -- for proxy compatibility
badValue(3), -- for proxy compatibility
readOnly(4), -- for proxy compatibility
genErr(5),
noAccess(6),
wrongType(7),
wrongLength(8),
wrongEncoding(9),
wrongValue(10),
noCreation(11),
inconsistentValue(12),
resourceUnavailable(13),
commitFailed(14),
undoFailed(15),
authorizationError(16),
notWritable(17),
inconsistentName(18)
},
error-index -- sometimes ignored
INTEGER (0..max-bindings),
variable-bindings -- values are sometimes ignored
VarBindList
}
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BulkPDU ::= -- MUST be identical in
SEQUENCE { -- structure to PDU
request-id
Integer32,
non-repeaters
INTEGER (0..max-bindings),
max-repetitions
INTEGER (0..max-bindings),
variable-bindings -- values are ignored
VarBindList
}
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-- variable binding
VarBind ::=
SEQUENCE {
name
ObjectName,
CHOICE {
value
ObjectSyntax,
unSpecified -- in retrieval requests
NULL,
-- exceptions in responses
noSuchObject[0]
IMPLICIT NULL,
noSuchInstance[1]
IMPLICIT NULL,
endOfMibView[2]
IMPLICIT NULL
}
}
-- variable-binding list
VarBindList ::=
SEQUENCE (SIZE (0..max-bindings)) OF
VarBind
END
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4. Protocol Specification
4.1. Common Constructs
The value of the request-id field in a Response-PDU takes the value of
the request-id field in the request PDU to which it is a response. By
use of the request-id value, a SNMPv2 application can distinguish the
(potentially multiple) outstanding requests, and thereby correlate
incoming responses with outstanding requests. In cases where an
unreliable datagram service is used, the request-id also provides a
simple means of identifying messages duplicated by the network. Use of
the same request-id on a retransmission of a request allows the response
to either the original transmission or the retransmission to satisfy the
request. However, in order to calculate the round trip time for
transmission and processing of a request-response transaction, the
SNMPv2 application needs to use a different request-id value on a
retransmitted request. The latter strategy is recommended for use in
the majority of situations.
A non-zero value of the error-status field in a Response-PDU is used to
indicate that an exception occurred to prevent the processing of the
request. In these cases, a non-zero value of the Response-PDU's error-
index field provides additional information by identifying which
variable binding in the list caused the exception. A variable binding
is identified by its index value. The first variable binding in a
variable-binding list is index one, the second is index two, etc.
SNMPv2 limits OBJECT IDENTIFIER values to a maximum of 128 sub-
identifiers, where each sub-identifier has a maximum value of 2**32-1.
4.2. PDU Processing
SNMPv2 entities acting in a manager role generate the following PDU
types: GetRequest-PDU, GetNextRequest-PDU, GetBulkRequest-PDU,
SetRequest-PDU, InformRequest-PDU, Response-PDU, and Report-PDU;
further, such implementations receive the following PDU types:
Response-PDU, SNMPv2-Trap-PDU, InformRequest-PDU, and Report-PDU.
SNMPv2 entities acting in an agent role generate the following PDU
types: Response-PDU, SNMPv2-Trap-PDU, and Report-PDU; further, such
implementations receive the following PDU types: GetRequest-PDU,
GetNextRequest-PDU, GetBulkRequest-PDU, and SetRequest-PDU.
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In the elements of procedure below, any field of a PDU which is not
referenced by the relevant procedure is ignored by the receiving SNMPv2
entity. However, all components of a PDU, including those whose values
are ignored by the receiving SNMPv2 entity, must have valid ASN.1 syntax
and encoding. For example, some PDUs (e.g., the GetRequest-PDU) are
concerned only with the name of a variable and not its value. In this
case, the value portion of the variable binding is ignored by the
receiving SNMPv2 entity. The unSpecified value is defined for use as
the value portion of such bindings.
For all generated PDUs, the message "wrapper" to encapsulate the PDU is
generated according to the "Elements of Procedure" of the Administrative
Framework as specified in [@ref v2admin], including the invocation of
the specific procedures called for by the authentication and privacy
mechanisms, if any, in effect for the message. While the definition of
"max-bindings" does impose an upper-bound on the number of variable
bindings, in practice, the size of a message is limited only by
constraints on the maximum message size -- it is not limited by the
number of variable bindings.
On receiving a management communication, the message "wrapper" which
encapsulates the PDU is processed according to the "Elements of
Procedure" of the Administrative Framework as specified in [@ref
v2admin], including the invocation of the specific procedures called for
by the authentication and privacy mechanism, if any, in effect for the
message. If those procedures indicate that the operation contained
within the message is to be performed locally, then those procedures
also indicate the MIB view which is visible to the operation, and, if
applicable, the maximum message size associated with the originator of
the message.
4.2.1. The GetRequest-PDU
A GetRequest-PDU is generated and transmitted at the request of a SNMPv2
application.
Upon receipt of a GetRequest-PDU, the receiving SNMPv2 entity processes
each variable binding in the variable-binding list to produce a
Response-PDU. All fields of the Response-PDU have the same values as
the corresponding fields of the received request except as indicated
below. Each variable binding is processed as follows:
(1) If the variable binding's name exactly matches the name of a
variable accessible by this request, then the variable binding's
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value field is set to the value of the named variable.
(2) Otherwise, if the variable binding's name does not have an OBJECT
IDENTIFIER prefix which exactly matches the OBJECT IDENTIFIER
prefix of any (potential) variable accessible by this request, then
its value field is set to `noSuchObject'.
(3) Otherwise, the variable binding's value field is set to to
`noSuchInstance'.
If the processing of any variable binding fails for a reason other than
listed above, then the Response-PDU is re-formatted with the same values
in its request-id and variable-bindings fields as the received
GetRequest-PDU, with the value of its error-status field set to
`genErr', and the value of its error-index field is set to the index of
the failed variable binding.
Otherwise, the value of the Response-PDU's error-status field is set to
`noError', and the value of its error-index field is zero.
The generated Response-PDU is then encapsulated into a message. If the
size of the resultant message is less than or equal to both a local
constraint and the maximum message size of the originator, it is
transmitted to the originator of the GetRequest-PDU.
Otherwise, an alternate Response-PDU is generated. This alternate
Response-PDU is formatted with the same value in its request-id field as
the received GetRequest-PDU, with the value of its error-status field
set to `tooBig', the value of its error-index field set to zero, and an
empty variable-bindings field. This alternate Response-PDU is then
encapsulated into a message. If the size of the resultant message is
less than or equal to both a local constraint and the maximum message
size of the originator, it is transmitted to the originator of the
GetRequest-PDU. Otherwise, the snmpSilentDrops |
[@ref v2mib4v2] counter is incremented and the resultant message is
discarded.
4.2.2. The GetNextRequest-PDU
A GetNextRequest-PDU is generated and transmitted at the request of a
SNMPv2 application.
Upon receipt of a GetNextRequest-PDU, the receiving SNMPv2 entity
processes each variable binding in the variable-binding list to produce
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a Response-PDU. All fields of the Response-PDU have the same values as
the corresponding fields of the received request except as indicated
below. Each variable binding is processed as follows:
(1) The variable is located which is in the lexicographically ordered
list of the names of all variables which are accessible by this
request and whose name is the first lexicographic successor of the
variable binding's name in the incoming GetNextRequest-PDU. The
corresponding variable binding's name and value fields in the
Response-PDU are set to the name and value of the located variable.
(2) If the requested variable binding's name does not lexicographically
precede the name of any variable accessible by this request, i.e.,
there is no lexicographic successor, then the corresponding
variable binding produced in the Response-PDU has its value field
set to `endOfMibView', and its name field set to the variable
binding's name in the request.
If the processing of any variable binding fails for a reason other than
listed above, then the Response-PDU is re-formatted with the same values
in its request-id and variable-bindings fields as the received
GetNextRequest-PDU, with the value of its error-status field set to
`genErr', and the value of its error-index field is set to the index of
the failed variable binding.
Otherwise, the value of the Response-PDU's error-status field is set to
`noError', and the value of its error-index field is zero.
The generated Response-PDU is then encapsulated into a message. If the
size of the resultant message is less than or equal to both a local
constraint and the maximum message size of the originator, it is
transmitted to the originator of the GetNextRequest-PDU.
Otherwise, an alternate Response-PDU is generated. This alternate
Response-PDU is formatted with the same values in its request-id field
as the received GetNextRequest-PDU, with the value of its error-status
field set to `tooBig', the value of its error-index field set to zero,
and an empty variable-bindings field. This alternate Response-PDU is
then encapsulated into a message. If the size of the resultant message
is less than or equal to both a local constraint and the maximum message
size of the originator, it is transmitted to the originator of the
GetNextRequest-PDU. Otherwise, the snmpSilentDrops |
[@ref v2mib4v2] counter is incremented and the resultant message is
discarded.
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4.2.2.1. Example of Table Traversal
An important use of the GetNextRequest-PDU is the traversal of
conceptual tables of information within a MIB. The semantics of this
type of request, together with the method of identifying individual
instances of objects in the MIB, provides access to related objects in
the MIB as if they enjoyed a tabular organization.
In the protocol exchange sketched below, a SNMPv2 application retrieves
the media-dependent physical address and the address-mapping type for
each entry in the IP net-to-media Address Translation Table [@ref
v2ipmib2] of a particular network element. It also retrieves the value
of sysUpTime [@ref v2mib4v2], at which the mappings existed. Suppose
that the agent's IP net-to-media table has three entries:
Interface-Number Network-Address Physical-Address Type
1 10.0.0.51 00:00:10:01:23:45 static
1 9.2.3.4 00:00:10:54:32:10 dynamic
2 10.0.0.15 00:00:10:98:76:54 dynamic
The SNMPv2 entity acting in a manager role begins by sending a
GetNextRequest-PDU containing the indicated OBJECT IDENTIFIER values as
the requested variable names:
GetNextRequest ( sysUpTime,
ipNetToMediaPhysAddress,
ipNetToMediaType )
The SNMPv2 entity acting in an agent role responds with a Response-PDU:
Response (( sysUpTime.0 = "123456" ),
( ipNetToMediaPhysAddress.1.9.2.3.4 =
"000010543210" ),
( ipNetToMediaType.1.9.2.3.4 = "dynamic" ))
The SNMPv2 entity acting in a manager role continues with:
GetNextRequest ( sysUpTime,
ipNetToMediaPhysAddress.1.9.2.3.4,
ipNetToMediaType.1.9.2.3.4 )
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The SNMPv2 entity acting in an agent role responds with:
Response (( sysUpTime.0 = "123461" ),
( ipNetToMediaPhysAddress.1.10.0.0.51 =
"000010012345" ),
( ipNetToMediaType.1.10.0.0.51 = "static" ))
The SNMPv2 entity acting in a manager role continues with:
GetNextRequest ( sysUpTime,
ipNetToMediaPhysAddress.1.10.0.0.51,
ipNetToMediaType.1.10.0.0.51 )
The SNMPv2 entity acting in an agent role responds with:
Response (( sysUpTime.0 = "123466" ),
( ipNetToMediaPhysAddress.2.10.0.0.15 =
"000010987654" ),
( ipNetToMediaType.2.10.0.0.15 = "dynamic" ))
The SNMPv2 entity acting in a manager role continues with:
GetNextRequest ( sysUpTime,
ipNetToMediaPhysAddress.2.10.0.0.15,
ipNetToMediaType.2.10.0.0.15 )
As there are no further entries in the table, the SNMPv2 entity acting
in an agent role responds with the variables that are next in the
lexicographical ordering of the accessible object names, for example:
Response (( sysUpTime.0 = "123471" ),
( ipNetToMediaNetAddress.1.9.2.3.4 =
"9.2.3.4" ),
( ipRoutingDiscards.0 = "2" ))
This response signals the end of the table to the SNMPv2 entity acting
in a manager role.
It is worthwhile to note that the data an agent accesses may be stored
internally in any arbitrary order. It is the agent's responsibility to
provide external access in the prescribed lexicographic order.
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4.2.3. The GetBulkRequest-PDU
A GetBulkRequest-PDU is generated and transmitted at the request of a
SNMPv2 application. The purpose of the GetBulkRequest-PDU is to request
the transfer of a potentially large amount of data, including, but not
limited to, the efficient and rapid retrieval of large tables.
Upon receipt of a GetBulkRequest-PDU, the receiving SNMPv2 entity
processes each variable binding in the variable-binding list to produce
a Response-PDU with its request-id field having the same value as in the
request. Processing begins by examining the values in the non-repeaters
and max-repetitions fields. If the value in the non-repeaters field is
less than zero, then the value of the field is set to zero. Similarly,
if the value in the max-repetitions field is less than zero, then the
value of the field is set to zero.
For the GetBulkRequest-PDU type, the successful processing of each
variable binding in the request generates zero or more variable bindings
in the Response-PDU. That is, the one-to-one mapping between the
variable bindings of the GetRequest-PDU, GetNextRequest-PDU, and
SetRequest-PDU types and the resultant Response-PDUs does not apply for
the mapping between the variable bindings of a GetBulkRequest-PDU and
the resultant Response-PDU.
The values of the non-repeaters and max-repetitions fields in the
request specify the processing requested. One variable binding in the
Response-PDU is requested for the first N variable bindings in the
request and M variable bindings are requested for each of the R
remaining variable bindings in the request. Consequently, the total
number of requested variable bindings communicated by the request is
given by N + (M * R), where N is the minimum of: a) the value of the
non-repeaters field in the request, and b) the number of variable
bindings in the request; M is the value of the max-repetitions field in
the request; and R is the maximum of: a) number of variable bindings in
the request - N, and b) zero.
The receiving SNMPv2 entity produces a Response-PDU with up to the total
number of requested variable bindings communicated by the request. The
request-id shall have the same value as the received GetBulkRequest-PDU.
If N is greater than zero, the first through the (N)-th variable
bindings of the Response-PDU are each produced as follows:
(1) The variable is located which is in the lexicographically ordered
list of the names of all variables which are accessible by this
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request and whose name is the first lexicographic successor of the
variable binding's name in the incoming GetBulkRequest-PDU. The
corresponding variable binding's name and value fields in the
Response-PDU are set to the name and value of the located variable.
(2) If the requested variable binding's name does not lexicographically
precede the name of any variable accessible by this request, i.e.,
there is no lexicographic successor, then the corresponding
variable binding produced in the Response-PDU has its value field
set to `endOfMibView', and its name field set to the variable
binding's name in the request.
If M and R are non-zero, the (N + 1)-th and subsequent variable bindings
of the Response-PDU are each produced in a similar manner. For each
iteration i, such that i is greater than zero and less than or equal to
M, and for each repeated variable, r, such that r is greater than zero
and less than or equal to R, the (N + ( (i-1) * R ) + r)-th variable
binding of the Response-PDU is produced as follows:
(1) The variable which is in the lexicographically ordered list of the
names of all variables which are accessible by this request and
whose name is the (i)-th lexicographic successor of the (N + r)-th
variable binding's name in the incoming GetBulkRequest-PDU is
located and the variable binding's name and value fields are set to
the name and value of the located variable.
(2) If there is no (i)-th lexicographic successor, then the
corresponding variable binding produced in the Response-PDU has its
value field set to `endOfMibView', and its name field set to either
the last lexicographic successor, or if there are no lexicographic
successors, to the (N + r)-th variable binding's name in the
request.
While the maximum number of variable bindings in the Response-PDU is
bounded by N + (M * R), the response may be generated with a lesser
number of variable bindings (possibly zero) for either of three reasons.
(1) If the size of the message encapsulating the Response-PDU
containing the requested number of variable bindings would be
greater than either a local constraint or the maximum message size
of the originator, then the response is generated with a lesser
number of variable bindings. This lesser number is the ordered set
of variable bindings with some of the variable bindings at the end
of the set removed, such that the size of the message encapsulating
the Response-PDU is approximately equal to but no greater than
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either a local constraint or the maximum message size of the
originator. Note that the number of variable bindings removed has
no relationship to the values of N, M, or R.
(2) The response may also be generated with a lesser number of variable
bindings if for some value of iteration i, such that i is greater
than zero and less than or equal to M, that all of the generated
variable bindings have the value field set to the `endOfMibView'.
In this case, the variable bindings may be truncated after the (N +
(i * R))-th variable binding.
(3) In the event that the processing of a request with many repetitions
requires a significantly greater amount of processing time than a
normal request, then an agent may terminate the request with less
than the full number of repetitions, providing at least one
repetition is completed.
If the processing of any variable binding fails for a reason other than
listed above, then the Response-PDU is re-formatted with the same values
in its request-id and variable-bindings fields as the received
GetBulkRequest-PDU, with the value of its error-status field set to
`genErr', and the value of its error-index field is set to the index of
the variable binding in the original request which corresponds to the
failed variable binding.
Otherwise, the value of the Response-PDU's error-status field is set to
`noError', and the value of its error-index field to zero.
The generated Response-PDU (possibly with an empty variable-bindings
field) is then encapsulated into a message. If the size of the
resultant message is less than or equal to both a local constraint and
the maximum message size of the originator, it is transmitted to the
originator of the GetBulkRequest-PDU. Otherwise, the snmpSilentDrops |
[@ref v2mib4v2] counter is incremented and the resultant message is
discarded.
4.2.3.1. Another Example of Table Traversal
This example demonstrates how the GetBulkRequest-PDU can be used as an
alternative to the GetNextRequest-PDU. The same traversal of the IP
net-to-media table as shown in Section 4.2.2.1 is achieved with fewer
exchanges.
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The SNMPv2 entity acting in a manager role begins by sending a
GetBulkRequest-PDU with the modest max-repetitions value of 2, and
containing the indicated OBJECT IDENTIFIER values as the requested
variable names:
GetBulkRequest [ non-repeaters = 1, max-repetitions = 2 ]
( sysUpTime,
ipNetToMediaPhysAddress,
ipNetToMediaType )
The SNMPv2 entity acting in an agent role responds with a Response-PDU:
Response (( sysUpTime.0 = "123456" ),
( ipNetToMediaPhysAddress.1.9.2.3.4 =
"000010543210" ),
( ipNetToMediaType.1.9.2.3.4 = "dynamic" ),
( ipNetToMediaPhysAddress.1.10.0.0.51 =
"000010012345" ),
( ipNetToMediaType.1.10.0.0.51 = "static" ))
The SNMPv2 entity acting in a manager role continues with:
GetBulkRequest [ non-repeaters = 1, max-repetitions = 2 ]
( sysUpTime,
ipNetToMediaPhysAddress.1.10.0.0.51,
ipNetToMediaType.1.10.0.0.51 )
The SNMPv2 entity acting in an agent role responds with:
Response (( sysUpTime.0 = "123466" ),
( ipNetToMediaPhysAddress.2.10.0.0.15 =
"000010987654" ),
( ipNetToMediaType.2.10.0.0.15 =
"dynamic" ),
( ipNetToMediaNetAddress.1.9.2.3.4 =
"9.2.3.4" ),
( ipRoutingDiscards.0 = "2" ))
This response signals the end of the table to the SNMPv2 entity acting
in a manager role.
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4.2.4. The Response-PDU
The Response-PDU is generated by a SNMPv2 entity only upon receipt of a
GetRequest-PDU, GetNextRequest-PDU, GetBulkRequest-PDU, SetRequest-PDU,
or InformRequest-PDU, as described elsewhere in this document.
If the error-status field of the Response-PDU is non-zero, the value
fields of the variable bindings in the variable binding list are
ignored.
If both the error-status field and the error-index field of the
Response-PDU are non-zero, then the value of the error-index field is
the index of the variable binding (in the variable-binding list of the
corresponding request) for which the request failed. The first variable
binding in a request's variable-binding list is index one, the second is
index two, etc.
A compliant SNMPv2 entity acting in a manager role must be able to
properly receive and handle a Response-PDU with an error-status field
equal to `noSuchName', `badValue', or `readOnly', in addition to the
error-status values generated by SNMPv2 entities. (See Section 3.1.2 of
[@ref coex].)
Upon receipt of a Response-PDU, the receiving SNMPv2 entity presents its
contents to the SNMPv2 application which generated the request with the
same request-id value.
4.2.5. The SetRequest-PDU
A SetRequest-PDU is generated and transmitted at the request of a SNMPv2
application.
Upon receipt of a SetRequest-PDU, the receiving SNMPv2 entity determines
the size of a message encapsulating a Response-PDU having the same
values in its request-id and variable-bindings fields as the received
SetRequest-PDU, and the largest possible sizes of the error-status and
error-index fields. If the determined message size is greater than
either a local constraint or the maximum message size of the originator,
then an alternate Response-PDU is generated, transmitted to the
originator of the SetRequest-PDU, and processing of the SetRequest-PDU
terminates immediately thereafter. This alternate Response-PDU is
formatted with the same values in its request-id field as the received
SetRequest-PDU, with the value of its error-status field set to
`tooBig', the value of its error-index field set to zero, and an empty
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variable-bindings field. This alternate Response-PDU is then
encapsulated into a message. If the size of the resultant message is
less than or equal to both a local constraint and the maximum message
size of the originator, it is transmitted to the originator of the
SetRequest-PDU. Otherwise, the snmpSilentDrops |
[@ref v2mib4v2] counter is incremented and the resultant message is
discarded. Regardless, processing of the SetRequest-PDU terminates.
Otherwise, the receiving SNMPv2 entity processes each variable binding
in the variable-binding list to produce a Response-PDU. All fields of
the Response-PDU have the same values as the corresponding fields of the
received request except as indicated below.
The variable bindings are conceptually processed as a two phase
operation. In the first phase, each variable binding is validated; if
all validations are successful, then each variable is altered in the
second phase. Of course, implementors are at liberty to implement
either the first, or second, or both, of these conceptual phases as
multiple implementation phases. Indeed, such multiple implementation
phases may be necessary in some cases to ensure consistency.
The following validations are performed in the first phase on each
variable binding until they are all successful, or until one fails:
(1) If the variable binding's name specifies an existing or non-
existent variable to which this request is/would be denied access
because it is/would not be in the appropriate MIB view, then the
value of the Response-PDU's error-status field is set to
`noAccess', and the value of its error-index field is set to the
index of the failed variable binding.
(2) Otherwise, if there are no variables which share the same OBJECT
IDENTIFIER prefix as the variable binding's name, and which are
able to be created or modified no matter what new value is
specified, then the value of the Response-PDU's error-status field
is set to `notWritable', and the value of its error-index field is
set to the index of the failed variable binding.
(3) Otherwise, if the variable binding's value field specifies,
according to the ASN.1 language, a type which is inconsistent with
that required for all variables which share the same OBJECT
IDENTIFIER prefix as the variable binding's name, then the value of
the Response-PDU's error-status field is set to `wrongType', and
the value of its error-index field is set to the index of the
failed variable binding.
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(4) Otherwise, if the variable binding's value field specifies,
according to the ASN.1 language, a length which is inconsistent
with that required for all variables which share the same OBJECT
IDENTIFIER prefix as the variable binding's name, then the value of
the Response-PDU's error-status field is set to `wrongLength', and
the value of its error-index field is set to the index of the
failed variable binding.
(5) Otherwise, if the variable binding's value field contains an ASN.1
encoding which is inconsistent with that field's ASN.1 tag, then
the value of the Response-PDU's error-status field is set to
`wrongEncoding', and the value of its error-index field is set to
the index of the failed variable binding. (Note that not all
implementation strategies will generate this error.)
(6) Otherwise, if the variable binding's value field specifies a value
which could under no circumstances be assigned to the variable,
then the value of the Response-PDU's error-status field is set to
`wrongValue', and the value of its error-index field is set to the
index of the failed variable binding.
(7) Otherwise, if the variable binding's name specifies a variable
which does not exist and could not ever be created (even though
some variables sharing the same OBJECT IDENTIFIER prefix might
under some circumstances be able to be created), then the value of
the Response-PDU's error-status field is set to `noCreation', and
the value of its error-index field is set to the index of the
failed variable binding.
(8) Otherwise, if the variable binding's name specifies a variable
which does not exist but can not be created under the present
circumstances (even though it could be created under other
circumstances), then the value of the Response-PDU's error-status
field is set to `inconsistentName', and the value of its error-
index field is set to the index of the failed variable binding.
(9) Otherwise, if the variable binding's name specifies a variable
which exists but can not be modified no matter what new value is
specified, then the value of the Response-PDU's error-status field
is set to `notWritable', and the value of its error-index field is
set to the index of the failed variable binding.
(10) Otherwise, if the variable binding's value field specifies a value
that could under other circumstances be held by the variable, but
is presently inconsistent or otherwise unable to be assigned to the
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variable, then the value of the Response-PDU's error-status field
is set to `inconsistentValue', and the value of its error-index
field is set to the index of the failed variable binding.
(11) When, during the above steps, the assignment of the value specified
by the variable binding's value field to the specified variable
requires the allocation of a resource which is presently
unavailable, then the value of the Response-PDU's error-status
field is set to `resourceUnavailable', and the value of its error-
index field is set to the index of the failed variable binding.
(12) If the processing of the variable binding fails for a reason other
than listed above, then the value of the Response-PDU's error-
status field is set to `genErr', and the value of its error-index
field is set to the index of the failed variable binding.
(13) Otherwise, the validation of the variable binding succeeds.
At the end of the first phase, if the validation of all variable
bindings succeeded, then the value of the Response-PDU's error-status
field is set to `noError' and the value of its error-index field is
zero, and processing continues as follows.
For each variable binding in the request, the named variable is created
if necessary, and the specified value is assigned to it. Each of these
variable assignments occurs as if simultaneously with respect to all
other assignments specified in the same request. However, if the same
variable is named more than once in a single request, with different
associated values, then the actual assignment made to that variable is
implementation-specific.
If any of these assignments fail (even after all the previous
validations), then all other assignments are undone, and the Response-
PDU is modified to have the value of its error-status field set to
`commitFailed', and the value of its error-index field set to the index
of the failed variable binding.
If and only if it is not possible to undo all the assignments, then the
Response-PDU is modified to have the value of its error-status field set
to `undoFailed', and the value of its error-index field is set to zero.
Note that implementations are strongly encouraged to take all possible
measures to avoid use of either `commitFailed' or `undoFailed' - these
two error-status codes are not to be taken as license to take the easy
way out in an implementation.
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Finally, the generated Response-PDU is encapsulated into a message, and
transmitted to the originator of the SetRequest-PDU.
4.2.6. The SNMPv2-Trap-PDU
A SNMPv2-Trap-PDU is generated and transmitted by a SNMPv2 entity acting
in an agent role when an exceptional situation occurs.
An SNMPv2 entity wishing to send an SNMPv2-Trap, provides:
- A trap type (a snmpTrapOID.0 value)
- A context local entity value
- A variable binding list
A variable binding list is constructed with the first two variable
bindings in the variable binding list as sysUpTime.0 [@ref v2mib4v2] and
snmpTrapOID.0 [@ref v2mib4v2] respectively, followed by any additional
variables which the generating SNMPv2 entity provides to include within
this SNMPv2-Trap-PDU.
The notifyTable is scanned in conjunction with the v2ContextTable to
select all notifyTable entries such that the context local entity
provided by the application is equal to the context local entity
associated with the context named in the notifyTable entry.
For each such selected entry in which all variable bindings in the
constructed variable binding list are included in the view named by that
selected entry,
- the values for notifySPI, notifyIdentityName, notifyContextName,
and notifyTransportLabel are extracted from the selected entry;
- a request id is generated, and a pdu is built with the generated
value of request id, zero-valued error status and error index
fields, and the constructed variable bindings list;
- a ScopedPDU is constructed using the extracted value of
notifyContextName, the local value of snmpID, and the just-built
PDUs value; and
- According to the SNMPv2 administrative model, [@ref v2admin], a
requesting application must provide the required sPI, authSnmpID,
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identityName, ScopedPDU, and transport information in order to
generate a message. Consequently, the generating entity then
requests that an appropriate SnmpV2Message value be generated and
sent, using
- the extracted value of notifySPI as the sPI;
- the local value of snmpID as the authSnmpID;
- the selected entry's value of notifyIdentityName as the identityName;
- the ScopedPDU value; and
- the [possibly multiple] destinations named by the extracted value of
notifyTransportLabel.
In the interest of clarity, it is worth noting that the request-id of
the generated traps is the same within a given selected notifyTable
entry, but different between notifyTable entries. That is, this
procedure generates potentially multiple trap messages due to
potentially multiple notifyTable entries being selected and potentially
multiple transportTable entries associated with one or more selections.
The request-id should be the same for all traps emitted for a given
event and a given notifyTable entry (where multiple messages are due to
multiple transport entries), but should be different for all traps |
emitted across various notifyTable entries. |
4.2.7. The InformRequest-PDU
An InformRequest-PDU is generated and transmitted at the request an
application in a SNMPv2 entity acting in a manager role, that wishes to
notify another application (in a SNMPv2 entity also acting in a manager
role) of information known by the sending application.
The destination(s) to which an InformRequest-PDU is sent is specified by
the requesting application and/or in consultation with appropriate MIB
objects such as the notificationTable [@ref adminmib].
The first two variable bindings in the variable binding list of an
InformRequest-PDU are sysUpTime.0 [@ref v2mib4v2] and snmpTrapOID.0
[@ref v2mib4v2] respectively. If the OBJECTS clause is present in the
invocation of the corresponding NOTIFICATION-TYPE macro, then each
corresponding variable, as instantiated by this notification, is copied,
in order, to the variable-bindings field. The application may also
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choose to add additional variable bindings to the InformRequest-PDU.
Upon receipt of an InformRequest-PDU, the receiving SNMPv2 entity
determines the size of a message encapsulating a Response-PDU with the
same values in its request-id, error-status, error-index and variable-
bindings fields as the received InformRequest-PDU. If the determined
message size is greater than either a local constraint or the maximum
message size of the originator, then an alternate Response-PDU is
generated, transmitted to the originator of the InformRequest-PDU, and
processing of the InformRequest-PDU terminates immediately thereafter.
This alternate Response-PDU is formatted with the same values in its
request-id field as the received InformRequest-PDU, with the value of
its error-status field set to `tooBig', the value of its error-index
field set to zero, and an empty variable-bindings field. This alternate
Response-PDU is then encapsulated into a message. If the size of the
resultant message is less than or equal to both a local constraint and
the maximum message size of the originator, it is transmitted to the
originator of the InformRequest-PDU. Otherwise, the snmpSilentDrops |
[@ref v2mib4v2] counter is incremented and the resultant message is
discarded. Regardless, processing of the InformRequest-PDU terminates.
Otherwise, the receiving SNMPv2 entity:
(1) presents its contents to the appropriate SNMPv2 application;
(2) generates a Response-PDU with the same values in its request-id and
variable-bindings fields as the received InformRequest-PDU, with
the value of its error-status field is set to `noError' and the
value of its error-index field is zero; and
(3) transmits the generated Response-PDU to the originator of the
InformRequest-PDU.
4.2.8. Report-PDU
A Report-PDU is generated and transmitted at the request of the SNMPv2
protocol entity implementing the administrative model or an
authentication and privacy service within that model. In this regard,
messages containing Report-PDUs are somewhat akin to IP [@ref ip]
messages containing ICMP [@ref icmp] information. The sending
application is the SNMPv2 protocol entity itself or the authentication
and privacy service.
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Messages containing a Report-PDU are almost always generated in response
to a problem or error condition encountered when processing a received
message. While processing a received communication, an SNMPv2 protocol
entity may determine that the message is unacceptable. In this case,
the appropriate counter from the snmp object group [@ref v2mib4v2] or |
from an |
appropriate MIB which accompanies the definition of an authentication
and privacy service, e.g., [@ref newusec]. If, after incrementing the
appropriate counter, the procedures require the generation of a report,
then the PDU is constructed as follows:
the request-id field is set to the value of the request-id of the
received communication which caused the error if it can be determined,
and 2147483647 otherwise;
the error-status and error-index fields of the report PDU are always set
to zero; and
the variable-bindings field.
As of this writing, all uses of the the report PDU require that the
variable-bindings field contain that single variable: the identity of
the statistics counter which was incremented when the error condition
was detected.
Future uses of the Report-PDU may include additional variable-bindings.
Upon receipt of a Report-PDU, the receiving SNMPv2 protocol entity
invokes the proper corrective behavior, (e.g., time synchronization,
proxy error propagation, etc.), if any.
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5. Acknowledgements
To be provided here.
6. References
To be provided here.
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7. Security Considerations
Security issues are not discussed in this memo.
8. Authors' Addresses
Tell U. Later
snmpv2@tis.com
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Table of Contents
1 Introduction .................................................... 3
1.1 A Note on Terminology ......................................... 3
2 Overview ........................................................ 4
2.1 Roles of Protocol Entities .................................... 4
2.2 Management Information ........................................ 4
2.3 Access to Management Information .............................. 5
2.4 Retransmission of Requests .................................... 5
2.5 Message Sizes ................................................. 5
2.6 Transport Mappings ............................................ 6
3 Definitions ..................................................... 7
4 Protocol Specification .......................................... 12
4.1 Common Constructs ............................................. 12
4.2 PDU Processing ................................................ 12
4.2.1 The GetRequest-PDU .......................................... 13
4.2.2 The GetNextRequest-PDU ...................................... 14
4.2.2.1 Example of Table Traversal ................................ 16
4.2.3 The GetBulkRequest-PDU ...................................... 18
4.2.3.1 Another Example of Table Traversal ........................ 20
4.2.4 The Response-PDU ............................................ 22
4.2.5 The SetRequest-PDU .......................................... 22
4.2.6 The SNMPv2-Trap-PDU ......................................... 26
4.2.7 The InformRequest-PDU ....................................... 27
4.2.8 Report-PDU .................................................. 28
5 Acknowledgements ................................................ 30
6 References ...................................................... 30
7 Security Considerations ......................................... 31
8 Authors' Addresses .............................................. 31
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