📄 rfc1448.txt
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Network Working Group J. Case
Request for Comments: 1448 SNMP Research, Inc.
K. McCloghrie
Hughes LAN Systems
M. Rose
Dover Beach Consulting, Inc.
S. Waldbusser
Carnegie Mellon University
April 1993
Protocol Operations
for version 2 of the
Simple Network Management Protocol (SNMPv2)
Status of this Memo
This RFC specifes an IAB standards track protocol for the
Internet community, and requests discussion and suggestions
for improvements. Please refer to the current edition of the
"IAB Official Protocol Standards" for the standardization
state and status of this protocol. Distribution of this memo
is unlimited.
Table of Contents
1 Introduction .......................................... 2
1.1 A Note on Terminology ............................... 2
2 Overview .............................................. 3
2.1 Roles of Protocol Entities .......................... 3
2.2 Management Information .............................. 3
2.3 Access to Management Information .................... 4
2.4 Retransmission of Requests .......................... 4
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 ............................ 15
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 .............. 21
4.2.4 The Response-PDU .................................. 22
4.2.5 The SetRequest-PDU ................................ 23
4.2.6 The SNMPv2-Trap-PDU ............................... 26
4.2.7 The InformRequest-PDU ............................. 27
Case, McCloghrie, Rose & Waldbusser [Page i]
RFC 1448 Protocol Operations for SNMPv2 April 1993
5 Acknowledgements ...................................... 29
6 References ............................................ 33
7 Security Considerations ............................... 35
8 Authors' Addresses .................................... 35
Case, McCloghrie, Rose & Waldbusser [Page 1]
RFC 1448 Protocol Operations for SNMPv2 April 1993
1. Introduction
A network 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 both authentication and
authorization policies.
Network management stations execute management applications
which monitor and control network elements. Network elements
are devices such as hosts, routers, terminal servers, 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.
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).
Case, McCloghrie, Rose & Waldbusser [Page 2]
RFC 1448 Protocol Operations for SNMPv2 April 1993
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 [3].
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 [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
Case, McCloghrie, Rose & Waldbusser [Page 3]
RFC 1448 Protocol Operations for SNMPv2 April 1993
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.
Case, McCloghrie, Rose & Waldbusser [Page 4]
RFC 1448 Protocol Operations for SNMPv2 April 1993
2.5. Message Sizes
The maximum size of a 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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