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RFC 1909 - An Administrative Infrastructure for SNMPv2

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Network Working Group                              K. McCloghrie, Editor
Request for Comments: 1909                           Cisco Systems, Inc.
Category: Experimental                                     February 1996

              An Administrative Infrastructure for SNMPv2

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Table of Contents

   1. Introduction ................................................    2
   2. Overview ....................................................    2
   2.1 Contexts ...................................................    3
   2.2 Authorization: Access Rights and MIB Views .................    3
   2.3 Authentication and Privacy .................................    4
   2.4 Access Control .............................................    5
   2.5 Security Models ............................................    5
   2.6 Proxy ......................................................    5
   3. Elements of the Model .......................................    7
   3.1 SNMPv2 Entity ..............................................    7
   3.2 SNMPv2 Agent ...............................................    7
   3.3 SNMPv2 Manager .............................................    8
   3.4 SNMPv2 Dual-Role Entity ....................................    8
   3.5 View Subtree and Families ..................................    9
   3.6 MIB View ...................................................    9
   3.7 SNMPv2 Context .............................................   10
   3.7.1 Local SNMPv2 Context .....................................   11
   3.7.2 Proxy SNMPv2 Context .....................................   11
   3.8 SNMPv2 PDUs and Operations .................................   12
   3.8.1 The Report-PDU ...........................................   12
   3.9 SNMPv2 Access Control Policy ...............................   13
   4. Security Considerations .....................................   13
   5. Editor's Address ............................................   14
   6. Acknowledgements ............................................   14
   7. References ..................................................   14
   Appendix A Disambiguating the SNMPv2 Protocol Definition .......   16
   Appendix B Who Sends Inform-Requests?  .........................   17
   Appendix B.1 Management Philosophy .............................   17
   Appendix B.2 The Danger of Trap Storms .........................   17
   Appendix B.3 Inform-Requests ...................................   18

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.

   It is the purpose of this document, An Administrative Infrastructure
   for SNMPv2, to define an administrative framework which realizes
   effective management in a variety of configurations and environments.
   The SNMPv2 framework is fully described in [1-6].  This framework is
   derived from the original Internet-standard Network Management
   Framework (SNMPv1), which consists of these three documents:

      STD 16, RFC 1155 [7] which defines the Structure of Management
      Information (SMI), the mechanisms used for describing and naming
      objects for the purpose of management.

      STD 16, RFC 1212 [8] which defines a more concise description
      mechanism, which is wholly consistent with the SMI.

      STD 15, RFC 1157 [9] which defines the Simple Network Management
      Protocol (SNMP), the protocol used for network access to managed

   For information on coexistence between SNMPv1 and SNMPv2, consult

2.  Overview

   A management domain typically contains a large amount of management
   information.  Each individual item of management information is an
   instance of a managed object type.  The definition of a related set
   of managed object types is contained in a Management Information Base
   (MIB) module.  Many such MIB modules are defined.  For each managed
   object type it describes, a MIB module defines not only the semantics
   and syntax of that managed object type, but also the method of
   identifying an individual instance so that multiple instances of the
   same managed object type can be distinguished.

2.1.  Contexts

   Typically, there are many instances of each managed object type
   within a management domain.  For simplicity, the method for
   identifying instances specified by the MIB module does not allow each
   instance to be distinguished amongst the set of all instances within
   the management domain; rather, it allows each instance to be
   identified only within some scope or "context", where there are
   multiple such contexts within the management domain.  Often, a
   context is a physical device, or perhaps, a logical device, although
   a context can also encompass multiple devices, or a subset of a
   single device, or even a subset of multiple devices.  Thus, in order
   to identify an individual item of management information within the
   management domain, its context must be identified in addition to its
   object type and its instance.

   For example, the managed object type, ifDescr [11], is defined as the
   description of a network interface.  To identify the description of
   device-X's first network interface, three pieces of information are
   needed, e.g., device-X (the context), ifDescr (the managed object
   type), and "1" (the instance).

   Note that each context has (at least) one globally-unique
   identification within the management domain.  Note also that the same
   item of management information can exist in multiple contexts.  So,
   an item of management information can have multiple globally-unique
   identifications, either because it exists in multiple contexts,
   and/or because each such context has multiple globally-unique

2.2.  Authorization: Access Rights and MIB Views

   For security reasons, it is often valuable to be able to restrict the
   access rights of some management applications to only a subset of the
   management information in the management domain.  To provide this
   capability, access to a context is via a "MIB view" which details a
   specific set of managed object types (and optionally, the specific
   instances of object types) within that context.  For example, for a
   given context, there will typically always be one MIB view which
   provides access to all management information in that context, and
   often there will be other MIB views each of which contains some
   subset of the information.  So, by providing access rights to a
   management application in terms of the particular (subset) MIB view
   it can access for that context, then the management application is
   restricted in the desired manner.

   Since managed object types (and their instances) are identified via
   the tree-like naming structure of ISO's OBJECT IDENTIFIERs [12, 1],

   it is convenient to define a MIB view as the combination of a set of
   "view subtrees", where each view subtree is a sub-tree within the
   managed object naming tree.  Thus, a simple MIB view (e.g., all
   managed objects within the Internet Network Management Framework) can
   be defined as a single view sub-tree, while more complicated MIB
   views (e.g., all information relevant to a particular network
   interface) can be represented by the union of multiple view sub-

   While any set of managed objects can be described by the union of
   some number of view subtrees, situations can arise that would require
   a very large number of view subtrees.  This could happen, for
   example, when specifying all columns in one conceptual row of a MIB
   table because they would appear in separate subtrees, one per column,
   each with a very similar format.  Because the formats are similar,
   the required set of subtrees can easily be aggregated into one
   structure.  This structure is named a family of view subtrees after
   the set of subtrees that it conceptually represents.  A family of
   view subtrees can either be included or excluded from a MIB view.

   In addition to restricting access rights by identifying (sub-)sets of
   management information, it is also valuable to restrict the requests
   allowed on the management information within a particular context.
   For example, one management application might be prohibited from
   write-access to a particular context, while another might be allowed
   to perform any type of operation.

2.3.  Authentication and Privacy

   The enforcement of access rights requires the means not only to
   identify the entity on whose behalf a request is generated but also
   to authenticate such identification.  Another security capability
   which is (optionally) provided is the ability to protect the data
   within an SNMPv2 operation from disclosure (i.e., to encrypt the
   data).  This is particularly useful when sensitive data (e.g.,
   passwords, or security keys) are accessed via SNMPv2 requests.

   Recommendations for which algorithms are best for authentication and
   privacy are subject to change.  Such changes may occur as and when
   new research results on the vulnerability of various algorithms are
   published, and/or with the prevailing status of export control and
   patent issues.  Thus, it is valuable to allow these algorithms to be
   specified as parameters, so that new algorithms can be accommodated
   over time.  In particular, one type of algorithm which may become
   useful in the future is the set of algorithms associated with
   asymmetric (public key) cryptography.

   Note that not all accesses via SNMPv2 requests need to be secure.

   Indeed, there are purposes for which insecure access is required.
   One example of this is the ability of a management application to
   learn about devices of which it has no previous knowledge.  Another
   example is to perform any synchronization which the security
   algorithms need before they can be used to communicate securely.
   This need for insecure access is accommodated by defining one of the
   algorithms for authentication as providing no authentication, and
   similarly, one of the algorithms for privacy as providing no
   protection against disclosure.  (The combination of these two
   insecure algorithms is sometimes referred to as "noAuth/noPriv".)

2.4.  Access Control

   An access control policy specifies the types of SNMPv2 requests and
   associated MIB views which are authorized for a particular identity
   (on whose behalf a request is generated) when using a particular
   level of security to access a particular context.

2.5.  Security Models

   A security model defines the mechanisms used to achieve an
   administratively-defined level of security for protocol interactions:

(1)  by defining the security parameters associated with a
     communication, including the authentication and privacy algorithms
     and the security keys (if any) used.

(2)  by defining how entities on whose behalf requests are generated are

(3)  by defining how contexts are identified.

(4)  by defining the mechanisms by which an access control policy is
     derived whenever management information is to be accessed.

2.6.  Proxy

   It is an SNMPv2 agent which responds to requests for access to
   management information.  Each such request is contained within an
   SNMPv2 message which provides the capability to perform a single
   operation on a list of items of management information.  Rather than
   having to identify the context as well as the managed object type and
   instance for each item of management information, each SNMPv2 message
   is concerned with only a single context.  Thus, an SNMPv2 agent must
   be able to process requests for all items of management information
   within the one or more contexts it supports.

   In responding to a request, an SNMPv2 agent might be acting as a
   proxy for some other agent.  The term "proxy" has historically been
   used very loosely, with multiple different meanings.  These different
   meanings include (among others):

(1)  the forwarding of SNMPv2 requests on to other SNMP agents without
     regard for what managed object types are being accessed; for
     example, in order to forward SNMPv2 request from one transport
     domain to another, or to translate SNMPv2 requests into SNMPv1

(2)  the translation of SNMPv2 requests into operations of some non-SNMP
     management protocol;

(3)  support for aggregated managed objects where the value of one
     managed object instance depends upon the values of multiple other
     (remote) items of management information.

   Each of these scenarios can be advantageous; for example, support for
   aggregation for management information can significantly reduce the
   bandwidth requirements of large-scale management activities.
   However, using a single term to cover multiple different scenarios
   causes confusion.

   To avoid such confusion, this SNMPv2 administrative framework uses
   the term "proxy" with a much more tightly defined meaning, which
   covers only the first of those listed above.  Specifically, the
   distinction between a "regular SNMPv2 agent" and a "proxy SNMPv2
   agent" is simple:

  -  a proxy SNMPv2 agent is an SNMPv2 agent which forwards requests on
     to other agents according to the context, and irrespective of the
     specific managed object types being accessed;

  -  in contrast, an SNMPv2 agent which processes SNMPv2 requests
     according to the (names of the) individual managed object types and
     instances being accessed, is NOT a proxy SNMPv2 agent from the
     perspective of this administrative model.

   Thus, when an SNMPv2 agent acts as a proxy SNMPv2 agent for a
   particular context, although information on how to forward the
   request is specifically associated with that context, the proxy
   SNMPv2 agent has no need of a detailed definition of the MIB view
   (since the proxy SNMPv2 agent forwards the request irrespective of
   the managed object types).

   In contrast, a SNMPv2 agent operating without proxy must have the
   detailed definition of the MIB view, and even if it needs to issue

   requests to other agents, that need is dependent on the individual
   managed object instances being accessed (i.e., not only on the

3.  Elements of the Model

   This section provides a more formal description of the model.

3.1.  SNMPv2 Entity

   An SNMPv2 entity is an actual process which performs management
   operations by generating and/or responding to SNMPv2 protocol
   messages in the manner specified in [4].  An SNMPv2 entity assumes
   the identity of a particular administrative entity when processing an
   SNMPv2 message.

   An SNMPv2 entity is not required to process multiple protocol
   messages concurrently, regardless of whether such messages require it
   to assume the identity of the same or different administrative
   entity.  Thus, an implementation of an SNMPv2 entity which supports
   more than one administrative entity need not be multi-threaded.
   However, there may be situations where implementors may choose to use

   An SNMPv2 entity listens for incoming, unsolicited SNMPv2 messages on
   each transport service address for which it is configured to do so.
   It is a local matter whether an SNMPv2 entity also listens for SNMPv2
   messages on any other transport service addresses.  In the absence of
   any other information on where to listen, an SNMPv2 entity must
   listen on the transport service addresses corresponding to the
   standard transport-layer "ports" [5] on its local network-layer

3.2.  SNMPv2 Agent

   An SNMPv2 agent is the operational role assumed by an SNMPv2 entity
   when it acts in an agent role.  Specifically, an SNMPv2 agent
   performs SNMPv2 management operations in response to received SNMPv2
   protocol messages (except for inform notifications).

   In order to be manageable, all network components need to be
   instrumented.  SNMPv2 access to the instrumented information is via
   the managed objects supported by an SNMPv2 agent in one or more

3.3.  SNMPv2 Manager

   An SNMPv2 manager is the operational role assumed by an SNMPv2 entity
   when it acts in a manager role on behalf of management applications.
   Specifically, an SNMPv2 manager initiates SNMPv2 management
   operations by the generation of appropriate SNMPv2 protocol messages,
   or when it receives and processes trap and inform notifications.

   It is interesting to consider the case of managing an SNMPv2 manager.
   It is highly desirable that an SNMPv2 manager, just like any other
   networking application, be instrumented for the purposes of being
   managed.  Such instrumentation of an SNMPv2 manager (just like for
   any other networking application) is accessible via the managed
   objects supported by an SNMPv2 agent.  As such, an SNMPv2 manager is
   no different from any other network application in that it has
   instrumentation, but does not itself have managed objects.

   That is, an SNMPv2 manager does not itself have managed objects.
   Rather, it is an associated SNMPv2 agent supporting managed objects
   which provides access to the SNMPv2 manager's instrumentation.

3.4.  SNMPv2 Dual-Role Entity

   An SNMPv2 entity which sometimes acts in an agent role and sometimes
   acts in a manager role, is termed an SNMPv2 dual-role entity.  An
   SNMPv2 dual-role entity initiates requests by acting in a manager
   role, and processes requests regarding management information
   accessible to it (locally or via proxy) through acting in an agent
   role.  In the case of sending inform notifications, an SNMPv2 dual-
   role entity acts in a manager role in initiating an inform
   notification containing management information which is accessible to
   it when acting in an agent role.

   An SNMPv2 entity which can act only in an SNMPv2 manager role is not
   SNMP-manageable, since there is no way to access its management
   instrumentation.  In order to be SNMP-manageable, an SNMPv2 entity
   must be able to act in an SNMPv2 agent role in order to allow its
   instrumentation to be accessed.  Thus, it is highly desirable that
   all SNMPv2 entities be either SNMPv2 agents or SNMPv2 dual-role

   There are two categories of SNMPv2 dual-role entities:  proxy SNMPv2
   agents and (so-called) mid-level managers.  Proxy SNMPv2 agents only
   forward requests/responses; they do not originate requests.  In
   contrast, mid-level managers often originate requests.  (Note that
   the term proxy SNMPv2 agent does not include an SNMPv2 agent which
   translates SNMPv2 requests into the requests of some other management
   protocol; see section 2.6.)

3.5.  View Subtree and Families

   A view subtree is the set of all MIB object instances which have a
   common ASN.1 OBJECT IDENTIFIER prefix to their names.  A view subtree
   is identified by the OBJECT IDENTIFIER value which is the longest
   OBJECT IDENTIFIER prefix common to all (potential) MIB object
   instances in that subtree.

   A family of view subtrees is a pairing of an OBJECT IDENTIFIER value
   (called the family name) together with a bitstring value (called the
   family mask).  The family mask indicates which sub-identifiers of the
   associated family name are significant to the family's definition.

   For each possible managed object instance, that instance belongs to a
   particular view subtree family if both of the following conditions
   are true:

o    the OBJECT IDENTIFIER name of the managed object instance contains
     at least as many sub-identifiers as does the family name, and

o    each sub-identifier in the OBJECT IDENTIFIER name of the managed
     object instance matches the corresponding sub-identifier of the
     family name whenever the corresponding bit of the associated family
     mask is non-zero.

   When the configured value of the family mask is all ones, the view
   subtree family is identical to the single view subtree identified by
   the family name.

   When the configured value of the family mask is shorter than required
   to perform the above test, its value is implicitly extended with
   ones.  Consequently, a view subtree family having a family mask of
   zero length always corresponds to a single view subtree.

3.6.  MIB View

   A MIB view is a subset of the set of all instances of all object
   types defined according to the SMI [1] within an SNMPv2 context,
   subject to the following constraints:

o    It is possible to specify a MIB view which contains the full set of
     all object instances within an SNMPv2 context.

o    Each object instance within a MIB view is uniquely named by an

   As such, identically named instances of a particular object type must
   be contained within different SNMPv2 contexts.  That is, a particular

   object instance name resolves within a particular SNMPv2 context to
   at most one object instance.

   A MIB view is defined as a collection of view subtree families, where
   each view subtree family has a type.  The type determines whether the
   view subtree family is included in, or excluded from, the MIB view.

   A managed object instance is contained/not contained within the MIB
   view according to the view subtree families to which the instance

o    If a managed object instance belongs to none of the relevant
     subtree families, then that instance is not in the MIB view.

o    If a managed object instance belongs to exactly one of the relevant
     subtree families, then that instance is included in, or excluded
     from, the relevant MIB view according to the type of that subtree

o    If a managed object instance belongs to more than one of the
     relevant subtree families, then that instance is included in, or
     excluded from, the relevant MIB view according to the type of a
     particular one of the subtree families to which it belongs.  The
     particular subtree family is the one for which, first, the
     associated family name comprises the greatest number of sub-
     identifiers, and, second, the associated family name is
     lexicographically greatest.

3.7.  SNMPv2 Context

   An SNMPv2 context is a collection of management information
   accessible by an SNMPv2 entity.  The collection of management
   information identified by a context is either local or proxy.

   For a local SNMPv2 context which is realized by an SNMPv2 entity,
   that SNMPv2 entity uses locally-defined mechanisms to access the
   management information identified by the SNMPv2 context.

   For a proxy SNMPv2 context, the SNMPv2 entity acts as a proxy SNMPv2
   agent to access the management information identified by the SNMPv2

   The term remote SNMPv2 context is used at an SNMPv2 manager to
   indicate a SNMPv2 context (either local or proxy) which is not
   realized by the local SNMPv2 entity (i.e.,  the local SNMPv2 entity
   uses neither locally-defined mechanisms, nor acts as a proxy SNMPv2
   agent, to access the management information identified by the SNMPv2

3.7.1.  Local SNMPv2 Context

   A local context refers to a collection of MIB objects which
   (logically) belong to a single entity within a managed device.  When
   an SNMPv2 entity accesses that management information, it does so
   using locally-defined mechanisms.

   Because a device may contain several such local entities, each local
   context has associated with it a "local entity" name.  Further,
   because management information changes over time, each local context
   also has associated with it an associated temporal domain, termed its
   "local time".  This allows, for example, one context to refer to the
   current values of a device's parameters, and a different context to
   refer to the values that the same parameters for the same device will
   have after the device's next restart.

3.7.2.  Proxy SNMPv2 Context

   A proxy relationship exists when a proxy SNMPv2 agent processes a
   received SNMPv2 message (a request or a response) by forwarding it to
   another entity, solely according to the SNMPv2 context of the
   received message.  Such a context is called a proxy SNMPv2 context.
   When an SNMPv2 entity processes management requests/responses for a
   proxy context, it is operating as a proxy SNMPv2 agent.

   The transparency principle that defines the behavior of an SNMPv2
   entity in general, applies in particular to a proxy SNMPv2 context:

     The manner in which a receiving SNMPv2 entity processes SNMPv2
     protocol messages sent by another SNMPv2 entity is entirely
     transparent to the sending SNMPv2 entity.

   Implicit in the transparency principle is the requirement that the
   semantics of SNMPv2 management operations are preserved between any
   two SNMPv2 peers.  In particular, the "as if simultaneous" semantics
   of a

   Set operation are extremely difficult to guarantee if its scope
   extends to management information resident at multiple network
   locations.  Note however, that agents which support the forwarding of
   Set operations concerning information at multiple locations are not
   considered to be proxy SNMPv2 agents (see section 2.6 above).

   Also implicit in the transparency principle is the requirement that,
   throughout its interaction with a proxy SNMPv2 agent, an SNMPv2
   manager is supplied with no information about the nature or progress
   of the proxy mechanisms used to perform its requests.  That is, it
   should seem to the SNMPv2 manager (except for any distinction in an

   underlying transport address) as if it were interacting via SNMPv2
   directly with the proxied device.  Thus, a timeout in the
   communication between a proxy SNMPv2 agent and its proxied device
   should be represented as a timeout in the communication between the
   SNMPv2 manager and the proxy SNMPv2 agent.  Similarly, an error
   response from a proxied device should - as much as possible - be
   represented by the corresponding error response in the interaction
   between the proxy SNMPv2 agent and SNMPv2 manager.

3.8.  SNMPv2 PDUs and Operations

   An SNMPv2 PDU is defined in [4].  Each SNMPv2 PDU specifies a
   particular operation, one of:


3.8.1.  The Report-PDU

   [4] requires that an administrative framework which makes use of the
   Report-PDU must define its usage and semantics.  With this
   administrative framework, the Report-PDU differs from the other PDU
   types described in [4] in that it is not a protocol operation between
   SNMPv2 managers and agents, but rather is an aspect of error-
   reporting between SNMPv2 entities. Specifically, it is an interaction
   between two protocol engines.

   A communication between SNMPv2 entities is in the form of an SNMPv2
   message.  Such a message is formatted as a "wrapper" encapsulating a
   PDU according to the "Elements of Procedure" for the security model
   used for transmission of the message.

   While processing a received communication, an SNMPv2 entity may
   determine that the received message is unacceptable due to a problem
   associated with the contents of the message "wrapper".  In this case,
   an appropriate counter is incremented and the received message is
   discarded without further processing (and without transmission of a

   However, if an SNMPv2 entity acting in the agent role makes such a
   determination, then after incrementing the appropriate counter, it
   may be required to generate a Report-PDU and to send it to the

   transport address which originated the received message.

   If the agent is able to determine the value of the request-id field
   of the received PDU [4], then it must use that value for the
   request-id field of the Report-PDU.  Otherwise, the value 2147483647
   is used.

   The error-status and error-index fields of the Report-PDU are always
   set to zero.  The variable-bindings field contains a single variable:
   the identity of the counter which was incremented and its new value.

   There is at least one case in which a Report-PDU must not be sent by
   an SNMPv2 entity acting in the agent role: if the received message
   was tagged as a Report-PDU.  Particular security models may identify
   other such cases.

3.9.  SNMPv2 Access Control Policy

   An SNMPv2 access policy specifies the types of SNMPv2 operations
   authorized for a particular identity using a particular level of
   security, and if the operation is to be performed on a local SNMPv2
   context, two accessible MIB views.  The two MIB views are a read-view
   and a write-view.  A read-view is a set of object instances
   authorized for the identity when reading objects.  Reading objects
   occurs when processing a retrieval (get, get-next, get-bulk)
   operation and when sending a notification.  A write-view is the set
   of object instances authorized for the identity when writing objects.
   Writing objects occurs when processing a set operation.  An
   identity's access rights may be different at different agents.

   A security model defines how an SNMPv2 access policy is derived;
   however, the application of an SNMPv2 access control policy is
   performed only:

o    on receipt of GetRequest, GetNextRequest, GetBulkRequest, and
     SetRequest operations; and

o    prior to transmission of SNMPv2-Trap and Inform operations.

   Note that application of an SNMPv2 access control policy is never
   performed for Response or Report operations.

4.  Security Considerations

   Security issues are not directly discussed in this memo.

5.  Editor's Address

   Keith McCloghrie
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA  95134-1706

   Phone: +1 408 526 5260
   EMail: kzm@cisco.com

6.  Acknowledgements

   This document is the result of significant work by three major

     Keith McCloghrie (Cisco Systems, kzm@cisco.com)
     Marshall T. Rose (Dover Beach Consulting, mrose@dbc.mtview.ca.us)
     Glenn W. Waters (Bell-Northern Research Ltd., gwaters@bnr.ca)

   The authors wish to acknowledge James M. Galvin of Trusted
   Information Systems who contributed significantly to earlier work on
   which this memo is based, and the general contributions of members of
   the SNMPv2 Working Group, and, in particular, Aleksey Y. Romanov and
   Steven L. Waldbusser.

   A special thanks is extended for the contributions of:

     Uri Blumenthal (IBM)
     Shawn Routhier (Epilogue)
     Barry Sheehan (IBM)
     Bert Wijnen (IBM)

7.  References

[1]  The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     S. Waldbusser, "Structure of Management Information for Version 2
     of the Simple Network Management Protocol (SNMPv2)", RFC 1902,
     January 1996.

[2]  The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     S. Waldbusser, "Textual Conventions for Version 2 of the Simple
     Network Management Protocol (SNMPv2)", RFC 1903, January 1996.

[3]  The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     S., Waldbusser, "Conformance Statements for Version 2 of the Simple
     Network Management Protocol (SNMPv2)", RFC 1904, January 1996.

[4]  The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     S. Waldbusser, "Protocol Operations for Version 2 of the Simple
     Network Management Protocol (SNMPv2)", RFC 1905, January 1996.

[5]  The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     Waldbusser, S., "Transport Mappings for Version 2 of the Simple
     Network Management Protocol (SNMPv2)", RFC 1906, January 1996.

[6]  The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     Waldbusser, S., "Management Information Base for Version 2 of the
     Simple Network Management Protocol (SNMPv2)", RFC 1907,
     January 1996.

[7]  Rose, M., and K. McCloghrie, "Structure and Identification of
     Management Information for TCP/IP-based internets", STD 16, RFC
     1155, May 1990.

[8]  Rose, M., and K. McCloghrie, "Concise MIB Definitions", STD 16,
     RFC 1212, March 1991.

[9]  Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple
     Network Management Protocol", STD 15, RFC 1157, SNMP Research,
     Performance Systems International, MIT Laboratory for Computer
     Science, May 1990.

[10] The SNMPv2 Working Group, Case, J., McCloghrie, K., Rose, M., and
     Waldbusser, S., "Coexistence between Version 1 and Version 2 of the
     Internet-standard Network Management Framework", RFC 1908, January

[11] McCloghrie, K., and F. Kastenholz, "Evolution of the Interfaces
     Group of MIB-II", RFC 1573, Cisco Systems, FTP Software, January

[12] Information processing systems - Open Systems Interconnection -
     Specification of Abstract Syntax Notation One (ASN.1),
     International Organization for Standardization.  International
     Standard 8824, (December, 1987).

APPENDIX A - Disambiguating the SNMPv2 Protocol Definition

The descriptions in [4] of the role in which an SNMPv2 entity acts when
sending an Inform-Request PDU are ambiguous.  The following updates
serve to remove those ambiguities.

(1)  Add the following sentence to section 2.1:

          Further, when an SNMPv2 entity sends an inform notification,
          it acts in a manager role in respect to initiating the
          operation, but the management information contained in the
          inform notification is associated with that entity acting in
          an agent role.  By convention, the inform is sent from the
          same transport address as the associated agent role is
          listening on.

(2)  Modify the last sentence of the second paragraph in section 2.3:

          This type is used by one SNMPv2 entity, acting in a manager
          role, to notify another SNMPv2 entity, also acting in a
          manager role, of management information associated with the
          sending SNMPv2 entity acting in an agent role.

(3)  Modify the second paragraph of section 4.2 (concerning the
     generation of Inform-Request PDUs):

          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,
          and Response-PDU; further, all such implementations must be
          able to receive the following PDU types: Response-PDU,
          SNMPv2-Trap-PDU, InformRequest-PDU.  It is mandatory that all
          dual-role SNMPv2 entities must be able to generate an Inform-
          Request PDU.

(4)  Modify the first paragraph of section 4.2.7:

          An InformRequest-PDU is generated and transmitted at the
          request of an application in a SNMPv2 entity acting in a
          manager role, that wishes to notify another application (via
          an SNMPv2 entity also acting in a manager role) of information
          in a MIB view which is accessible to the sending SNMPv2 entity
          when acting in an agent role.

APPENDIX B - Who Sends Inform-Requests?

B.1.   Management Philosophy

   Ever since its beginnings as SGMP, through its definition as SNMPv1,
   and continuing with the definition of SNMPv2, SNMP has embodied more
   than just a management protocol and the definitions of MIB objects.
   Specifically, SNMP has also had a fundamental philosophy of
   management, consisting of a number of design strategies.  These
   strategies have always included the following:

(1)  The impact of incorporating an SNMP agent into a system should be
     minimal, so that both: a) it is feasible to do so even in the
     smallest/cheapest of systems, and b) the operational role and
     performance of a system is not compromised by the inclusion of an
     SNMP agent.  This promotes widespread development, which allows
     ubiquitous deployment of manageable systems.

(2)  Every system (potentially) incorporates an SNMP agent.  In
     contrast, the number of SNMP managers is limited.  Thus, there is a
     significantly larger number of SNMP agents than SNMP managers.
     Therefore, overall system development/complexity/cost is optimized
     if the SNMP agent is allowed to be simple and any required
     complexity is performed by an SNMP manager.

(3)  The number of unsolicited messages generated by SNMP agents is
     minimized.  This enables the amount of network management traffic
     to be controlled by the small number of SNMP managers which are
     (more) directly controlled by network operators.  In fact, this
     control is considered of greater importance than any additional
     protocol overhead which might be incurred.  Monitoring of network
     state at any significant level of detail is accomplished primarily
     by SNMP managers polling for the appropriate information, with the
     use of unsolicited messages confined to those situations where it
     is necessary to properly guide an SNMP manager regarding the timing
     and focus of its polling.  This strategy is sometimes referred to
     as "trap-directed polling".

B.2.   The Danger of Trap Storms

   The need for such control over the amount of network management
   traffic is due to the potential that the SNMP manager receiving an
   unsolicited message does not want, no longer wants, or already knows
   of the information contained in the message.  This potential is
   significantly reduced by having the majority of messages be specific
   requests for information by SNMP managers and responses (to those
   requests) from SNMP agents.

   The danger of not having the amount of network management be
   controlled in this manner is the potential for a "storm" of useless
   traps.  As a simple example of "useless", consider that after a
   building power outage, every device in the network sends a coldStart
   trap, even though every SNMP manager and every network operator
   already knows what happened.  For a simple example of "storm",
   consider the result if each transmitted trap caused the sending of
   another.  The greater the number of problems in the state of the
   network, the greater the risk of such a storm occurring, especially
   in the unstructured, heterogeneous environment typical of today's

   While SNMP philosophy considers the above to be more important than
   any lack of reliability in unsolicited messages, some
   users/developers have been wary of using traps because of the use
   (typically) of an unreliable transport protocol and because traps are
   not acknowledged.  However, following this logic would imply that
   having acknowledged-traps would make them reliable; of course, this
   is false since no amount of re- transmission will succeed if the
   receiver and/or the connectivity to the receiver is down.  A SNMP
   manager cannot just sit and wait and assume the network is fine just
   because it is not receiving any unsolicited messages.

B.3.   Inform-Requests

   One of the new features of SNMPv2 is the Inform-request PDU.  The
   Inform-Request contains management information specified in terms of
   MIB objects for a context supported by the sender.  Since by
   definition, an SNMPv2 manager does not itself have managed objects
   (see sections 3.3), the managed objects contained in the Inform-
   request belong to a context of an SNMPv2 agent, just like the managed
   objects contained in an SNMPv2-Trap.

   However, it is not the purpose of an Inform-request to change the
   above described philosophy, i.e., it would be wrong to consider it as
   an "acknowledged trap".  To do so, would make the likelihood and
   effect of trap storms worse.  Recall the building power outage
   example:  with regular traps, the SNMP manager's buffer just
   overflows when it receives messages faster than it can cope with; in
   contrast, if every device in the network were to send a coldStart
   Inform-request, then after a power outage, all will re-transmit their
   Inform-request several times unless the receiving SNMP managers send
   responses.  In the best case when no messages are lost or re-
   transmitted, there are twice as many useless messages; in the worst
   case, the SNMP manager is unable to respond at all and every message
   is re-transmitted its maximum number of times.

   The above serves to explain the rationale behind the definition (see
   Appendix A's update to section 4.2.7 of [4]) that:

     An InformRequest-PDU is generated and transmitted at the request of
     an application in a SNMPv2 entity acting in a manager role, that
     wishes to notify another application (via an SNMPv2 entity also
     acting in a manager role) of information in a MIB view which is
     accessible to the sending SNMPv2 entity when acting in an agent

   This definition says that SNMPv2 agents do not send Inform-Requests,
   which has three implications (ordered in terms of importance):

(1)  the number of devices which send Inform-requests is required to be
     a small subset of all devices in the network;

(2)  while some devices traditionally considered to be SNMP agents are
     perfectly capable of sending Inform-requests, the overall system
     development/complexity/cost is not increased as it would be by
     having to configure/re-configure every SNMPv2 agent as to which
     Inform-requests to send where and how often; and

(3)  the cost of implementing an SNMPv2 agent in the smallest/cheapest
     system is not increased.


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