Draft Protocol Operations for SNMPv2 Oct 92
Protocol Operations
for version 2 of the
Simple Network Management Protocol (SNMPv2)
Thu Nov 12 08:51:15 1992 |
Jeffrey D. Case
SNMP Research, Inc.
University of Tennessee, Knoxville
case@cs.utk.edu
Keith McCloghrie
Hughes LAN Systems
kzm@hls.com
Marshall T. Rose
Dover Beach Consulting, Inc.
mrose@dbc.mtview.ca.us
Steven L. Waldbusser
Carnegie Mellon University
waldbusser@andrew.cmu.edu
1. Status of this Memo
This document is an Internet Draft. Internet Drafts are
working documents of the Internet Engineering Task Force
(IETF), its Areas, and its Working Groups. Note that other
groups may also distribute working documents as Internet
Drafts.
Internet Drafts are valid for a maximum of six months and may
be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet Drafts as reference
material or to cite them other than as a "work in progress".
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2. Introduction
A network management system contains: several (potentially,
many) network elements, each with an associated processing
entity, termed an agent, which has access to management
instrumentation; at least one management station; and, a
management protocol, used to convey management information
between the agents and the management stations. Operations of
the protocol are carried out under an administrative framework
which defines both authentication and authorization policies.
Network management stations execute management applications
which monitor and control network elements through access to
their management information. Network elements are devices
such as hosts, routers, terminal servers, network monitors,
etc., which are monitored and controlled through 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)
[1], termed the Structure of Management Information (SMI) [2].
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. +
2.1. 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 +
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(SNMPv2). +
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3. Overview
3.1. 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 [3].
3.2. 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 [2] or the textual conventions based on the SMI [4].
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
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prefix of the variable.
3.3. 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.
3.4. Retransmission of Requests
For all types of request 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.
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3.5. Message Sizes
The maximum size of an SNMPv2 message is limited the minimum
of:
(1) the maximum message size which the destination SNMPv2
entity can accept; and,
(2) the maximum message size which the source SNMPv2 entity
can generate.
The former is indicated by partyMaxMessageSize[5] of the
destination party. 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 [6], 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.
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3.6. 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 [7]. However, the preferred
mapping is the use of the User Datagram Protocol [8].
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4. Definitions
SNMPv2-PDU DEFINITIONS ::= BEGIN
IMPORTS
ObjectName, ObjectSyntax, Integer32
FROM SNMPv2-SMI;
-- protocol data units
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,
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
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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
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)
},
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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5. Protocol Specification
5.1. 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.
5.2. PDU Processing
It is mandatory that all SNMPv2 entities acting in an agent
role be able to generate the following PDU types: Response-PDU
and SNMPv2-Trap-PDU; further, all such implementations must be
able to receive the following PDU types: GetRequest-PDU,
GetNextRequest-PDU, GetBulkRequest-PDU, and SetRequest-PDU.
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It is mandatory that all SNMPv2 entities acting in a manager
role be able to generate the following PDU types: GetRequest-
PDU, GetNextRequest-PDU, GetBulkRequest-PDU, SetRequest-PDU,
InformRequest-PDU, and Response-PDU; further, all such
implementations must be able to receive the following PDU
types: Response-PDU, SNMPv2-Trap-PDU, InformRequest-PDU;
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 and transmitted as specified in [3] (as
modified in [9]). The size of a message is limited only by
constraints on the maximum message size, either a local
limitation or the limit associated with the message's
destination party, i.e., it is not limited by the number of
variable bindings.
On receiving a PDU, a SNMPv2 entity parses the encoded PDU.
If a parse error is detected, the PDU is discarded and the
relevant error counter is incremented. Otherwise, the PDU is
processed according to its PDU type, in the authorization
context of the source and destination parties given by the
message "wrapper" which encapsulated the PDU. By definition,
this authorization context indicates whether the PDU type is
authorized, and determines which variables are accessible by
this request. For PDU types which are not authorized, either
due to policy (e.g., a SetRequest-PDU in a read-only
authorization context), or due to implementation constraints
(e.g., a Response-PDU received by a SNMPv2 entity acting in an
agent role), the PDU is discarded and the relevant error
counter is incremented. If the discarded PDU was one of
following PDU types: GetRequest-PDU, GetNextRequest-PDU,
GetBulkRequest-PDU, SetRequest-PDU, or InformRequest-PDU, then
a Response-PDU is generated and transmitted to the originator
of the request. This Response-PDU is formatted with the same
values in its request-id and variable-bindings fields as the
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discarded PDU, with the value of its error-status field set to
`authorizationError', and the value of its error-index field
set to zero.
5.2.1. 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 does not have an OBJECT
IDENTIFIER prefix which exactly matches the OBJECT
IDENTIFIER prefix of any variable accessible by this
request, then its value field is set to `noSuchObject'.
(2) Otherwise, if the variable binding's name does not
exactly match the name of a variable accessible by this
request, then its value field is set to `noSuchInstance'.
(3) Otherwise, the variable binding's value field is set to
the value of the named variable.
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 request's source party, it is transmitted to the
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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 request's source party, it is
transmitted to the originator of the GetRequest-PDU.
Otherwise, the resultant message is discarded.
5.2.2. 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 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.
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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 request's source party, 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 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 request's source party, it is
transmitted to the originator of the GetNextRequest-PDU.
Otherwise, the resultant message is discarded.
5.2.2.1. 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 [10] of a particular network
element. It also retrieves the value of sysUpTime [10], at
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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 )
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 )
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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.
5.2.3. 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.
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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 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
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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 two 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 request's source party, 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 the minimum of the local
constraint and the maximum message size of the request's
source party. Note that the number of variable bindings
removed has no relationship to the values of N, M, or R.
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(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.
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 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 request's
source party, it is transmitted to the originator of the
GetBulkRequest-PDU. Otherwise, the resultant message is
discarded.
5.2.3.1. 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
5.2.2.1 is achieved with fewer exchanges.
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 )
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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.
5.2.4. 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.
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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' or `badValue'. (See
Section 4.1.2 of [11].)
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.
5.2.5. 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
with the same values in its request-id, error-status, error-
index and variable-bindings fields as the received
SetRequest-PDU. If the determined message size is greater
than either a local constraint or the maximum message size of
the request's source party, 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 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 request's
source party, it is transmitted to the originator of the
SetRequest-PDU. Otherwise, the resultant message is
discarded. Regardless, processing of the SetRequest-PDU
terminates.
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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.
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 a variable which
is not accessible by this request, 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 the variable binding's name specifies a
variable which does not exist and could not ever 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.
(3) 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.
(4) Otherwise, if the variable binding's value field
specifies, according to the ASN.1 language, a type which
is inconsistent with that required for the variable, 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.
(5) Otherwise, if the variable binding's value field
specifies, according to the ASN.1 language, a length
which is inconsistent with that required for the
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variable, 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.
(6) 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.
(7) 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.
(8) Otherwise, if the variable binding's value field
specifies a value that could under other circumstances be
assigned to the variable, but is presently inconsistent,
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.
(9) Otherwise, if 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.
(10) 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.
(11) Otherwise, the validation of the variable binding
succeeds.
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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.
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 set to the index of the (first) variable
binding for which the assignment couldn't be undone. Note
that implementations are strongly encouraged to take all
possible measures to avoid use of this error-status.
Finally, the generated Response-PDU is encapsulated into a
message, and transmitted to the originator of the SetRequest-
PDU.
5.2.6. 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.
The destination(s) to which an SNMPv2-Trap-PDU is sent is
determined by consulting the aclTable [5] to find all entries
satisfying the following conditions:
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(1) The value of aclSubject refers to the SNMPv2 entity.
(2) The value of aclPrivileges allows for the SNMPv2-Trap-
PDU.
(3) The notification's administratively assigned name is
present in aclSubject's view. (That is, the set of
entries in the viewTable [5] for which the instance of
viewParty has the same value as aclSubject, define a MIB
view which contains the notification's administratively
assigned name.)
(4) If the OBJECTS clause is present in the invocation of the
corresponding NOTIFICATION-TYPE macro, then the
correspondent variables are all present in aclSubject's
view.
Then, for each entry satisfying these conditions, a SNMPv2-
Trap-PDU is sent from aclSubject to aclTarget. The instance
of snmpTrapNumbers [12] corresponding to aclTarget is
incremented, and is used as the request-id field of the
SNMPv2-Trap-PDU. Then, the variable-bindings field are
constructed as:
(1) The first variable is sysUpTime.0 [10].
(2) The second variable is snmpTrapOID.0 [12], which contains
the administratively assigned name of the notification.
(3) If the OBJECTS clause is present in the invocation of the
corresponding NOTIFICATION-TYPE macro, then each
corresponding variable is copied, in order, to the
variable-bindings field.
(4) At the option of the SNMPv2 entity acting in an agent
role, additional variables may follow in the variable-
bindings field.
5.2.7. 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 in the
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MIB View of a party local to the sending application.
The destination(s) to which an InformRequest-PDU is sent is
determined by inspecting the snmpEventNotifyTable [13], or as
specified by the requesting application. The first two
variable bindings in the variable binding list of an
InformRequest-PDU are sysUpTime.0 [10] and snmpEventID.i [13]
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.
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 request's source party, 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 request's source party, it is transmitted to the
originator of the InformRequest-PDU. Otherwise, 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
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(3) transmits the generated Response-PDU to the originator of
the InformRequest-PDU.
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6. Acknowledgements
This document is based, in part, on RFC 1157. The mechanism
for bulk retrieval is influenced by many experiments,
including RFC1187 and also Greg Satz's work on SNMP over TCP.
Finally, the comments of the SNMP Version 2 working group are
gratefully acknowledged:
Steve Alexander, Interactive Systems
Uri Blumenthal, International Business Machines
Jeffrey D. Case, SNMP Research, Inc.
Tracy Cox, Bellcore
James R. (Chuck) Davin, Bellcore
Mike Davison, FiberCom
Taso N. Devetzis, Bellcore
Gary W. Haney, Martin Marietta Energy Systems
Matt Hecht, SNMP Research, Inc.
Susan E. Hicks, Martin Marietta Energy Systems
Satish Joshi, SynOptics
Mark Kepke, Hewlett-Packard
Ken Key, SNMP Research, Inc.
Michael Kornegay, Visisoft
Deidre C. Kostick, Bellcore
Cheryl Krupczak, Georgia Tech
Robert C. Lushbaugh, Martin Marietta Energy Systems
Keith McCloghrie, Hughes LAN Systems
Dave Minnich, FiberCom
Dave Perkins, SynOptics
Marshall T. Rose, Dover Beach Consulting, Inc.
Shawn A. Routhier, Epilogue Technology
Jon Saperia, Digital Equipment Corporation
Bob Stewart, Xyplex (chair)
Robert Synder, Cisco Systems
Maurice Turcotte, Racal Datacom
Steven L. Waldbusser, Carnegie Mellon University
Bert Wijnen, International Business Machines
Peter Wilson, 3Com
Steven Wong, Digital Equipment Corporation
Chris Young, Cabletron
Kiho Yum, 3Com
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7. References
[1] Information processing systems - Open Systems
Interconnection - Specification of Abstract Syntax
Notation One (ASN.1), International Organization for
Standardization. International Standard 8824, (December,
1987).
[2] J.D. Case, K. McCloghrie, M.T. Rose, S.L. Waldbusser,
Structure of Management Information for version 2 of the
Simple Network Management Protocol, Internet-Draft,
(October 7, 1992).
[3] J.R. Davin, J.M. Galvin, K. McCloghrie, SNMP
Administrative Model. Request for Comments 1351, (July,
1992).
[4] J.D. Case, K. McCloghrie, M.T. Rose, S.L. Waldbusser,
Textual Conventions for version 2 of the the Simple
Network Management Protocol (SNMPv2), Internet-Draft,
(October 7, 1992).
[5] K. McCloghrie, J.R. Davin, J.M. Galvin, Definitions of
Managed Objects for Administration of SNMP Parties.
Request for Comments 1353, (July, 1992).
[6] C. Kent, J. Mogul, Fragmentation Considered Harmful,
Proceedings, ACM SIGCOMM '87, Stowe, VT, (August 1987).
[7] J.D. Case, K. McCloghrie, M.T. Rose, S.L. Waldbusser,
Transport Mappings for version 2 of the Simple Network
Management Protocol (SNMPv2), Internet-Draft, (October 7,
1992).
[8] J. Postel, User Datagram Protocol. Request for Comments
768, (November 1980).
[9] J.D. Case, K. McCloghrie, M.T. Rose, S.L. Waldbusser,
Introduction to version 2 of the Internet-standard
Network Management Framework, Internet-Draft, (October 7,
1992).
[10] K. McCloghrie and M.T. Rose, Management Information Base
for Network Management of TCP/IP-based internets: MIB-
II. Request for Comments 1213, (March, 1991).
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[11] J.D. Case, K. McCloghrie, M.T. Rose, S.L. Waldbusser,
Coexistence between version 1 and version 2 of the
Internet-standard Network Management Framework,
Internet-Draft, (October 7, 1992).
[12] J.D. Case, K. McCloghrie, M.T. Rose, S.L. Waldbusser,
Management Information Base for version 2 of the Simple
Network Management Protocol, Internet-Draft, (October 7,
1992).
[13] J.D. Case, K. McCloghrie, M.T. Rose, S.L. Waldbusser,
Manager to Manager Management Information Base,
Internet-Draft, (October 7, 1992).
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Table of Contents
1 Status of this Memo ................................... 1
2 Introduction .......................................... 2
2.1 A Note on Terminology ............................... 2
3 Overview .............................................. 4
3.1 Roles of Protocol Entities .......................... 4
3.2 Management Information .............................. 4
3.3 Access to Management Information .................... 5
3.4 Retransmission of Requests .......................... 5
3.5 Message Sizes ....................................... 6
3.6 Transport Mappings .................................. 7
4 Definitions ........................................... 8
5 Protocol Specification ................................ 13
5.1 Common Constructs ................................... 13
5.2 PDU Processing ...................................... 13
5.2.1 The GetRequest-PDU ................................ 15
5.2.2 The GetNextRequest-PDU ............................ 16
5.2.2.1 Example of Table Traversal ...................... 17
5.2.3 The GetBulkRequest-PDU ............................ 19
5.2.3.1 Another Example of Table Traversal .............. 22
5.2.4 The Response-PDU .................................. 23
5.2.5 The SetRequest-PDU ................................ 24
5.2.6 The SNMPv2-Trap-PDU ............................... 27
5.2.7 The InformRequest-PDU ............................. 28
6 Acknowledgements ...................................... 31
7 References ............................................ 32
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