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





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          5 Acknowledgements ......................................   29
          6 References ............................................   33
          7 Security Considerations ...............................   35
          8 Authors' Addresses ....................................   35














































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          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).





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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 [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





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          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.









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          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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          2.6.  Transport Mappings