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Protocol Operations for Version 2 of the Simple Network Management Protocol (SNMPv2)
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Network Working Group SNMPv2 Working Group
Request for Comments: 1905 J. Case
Obsoletes:
1448 SNMP Research, Inc.
Category: Standards Track K. McCloghrie
Cisco Systems, Inc.
M. Rose
Dover Beach Consulting, Inc.
S. Waldbusser
International Network Services
January 1996
Protocol Operations
for Version 2 of the
Simple Network Management Protocol (SNMPv2)
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
1. Introduction
A management system contains: several (potentially many) nodes, each
with a 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 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) [
1], termed the Structure of
Management Information (SMI) [
2].
SNMPv2 Working Group Standards Track [Page 1]
RFC 1905
Protocol Operations for SNMPv2 January 1996
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 (STD 16), 1157 (STD
15), and 1212 (STD 16), is termed the SNMP version 1 framework
(SNMPv1). The current framework is termed the SNMP version 2
framework (SNMPv2).
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.
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 [
2] or
the textual conventions based on the SMI [
3]. The term, variable
binding, normally refers to the pairing of the name of a variable and
SNMPv2 Working Group Standards Track [Page 2]
RFC 1905
Protocol Operations for SNMPv2 January 1996
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.
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 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.
SNMPv2 Working Group Standards Track [Page 3]
RFC 1905
Protocol Operations for SNMPv2 January 1996
2.5. Message Sizes
The maximum size of a SNMPv2 message is limited to 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 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
[
4], 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 [
5]. However, the preferred mapping is the use of the User
SNMPv2 Working Group Standards Track [Page 4]
RFC 1905
Protocol Operations for SNMPv2 January 1996
Datagram Protocol [
6].
3. 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,
snmpV2-trap
SNMPv2-Trap-PDU,
report
Report-PDU,
}
-- PDUs
GetRequest-PDU ::=
[0]
IMPLICIT PDU
GetNextRequest-PDU ::=
SNMPv2 Working Group Standards Track [Page 5]
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