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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

McCloghrie Experimental [Page 1] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

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
    objects.
 For information on coexistence between SNMPv1 and SNMPv2, consult
 [10].

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.

McCloghrie Experimental [Page 2] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

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
 identifications.

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],

McCloghrie Experimental [Page 3] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

 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-
 trees.
 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.

McCloghrie Experimental [Page 4] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

 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

   identified.

(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.

McCloghrie Experimental [Page 5] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

 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
   requests;

(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:
  1. 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;
  1. 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

McCloghrie Experimental [Page 6] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

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

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
 multi-threading.
 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
 addresses.

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
 contexts.

McCloghrie Experimental [Page 7] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

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
 entities.
 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.)

McCloghrie Experimental [Page 8] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

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

   ASN.1 OBJECT IDENTIFIER value.
 As such, identically named instances of a particular object type must
 be contained within different SNMPv2 contexts.  That is, a particular

McCloghrie Experimental [Page 9] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

 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
 belongs:

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
   family.

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
 context.
 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
 context).

McCloghrie Experimental [Page 10] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

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

McCloghrie Experimental [Page 11] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

 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:
             GetBulkRequest
             GetNextRequest
             GetRequest
             Inform
             Report
             Response
             SNMPv2-Trap
             SetRequest

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
 Response-PDU).
 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

McCloghrie Experimental [Page 12] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

 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.

McCloghrie Experimental [Page 13] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

5. Editor's Address

 Keith McCloghrie
 Cisco Systems, Inc.
 170 West Tasman Drive
 San Jose, CA  95134-1706
 US
 Phone: +1 408 526 5260
 EMail: kzm@cisco.com

6. Acknowledgements

 This document is the result of significant work by three major
 contributors:
   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.

McCloghrie Experimental [Page 14] RFC 1909 An SNMPv2 Administrative Infrastructure February 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
   1996.

[11] McCloghrie, K., and F. Kastenholz, "Evolution of the Interfaces

   Group of MIB-II", RFC 1573, Cisco Systems, FTP Software, January
   1994.

[12] Information processing systems - Open Systems Interconnection -

   Specification of Abstract Syntax Notation One (ASN.1),
   International Organization for Standardization.  International
   Standard 8824, (December, 1987).

McCloghrie Experimental [Page 15] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

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.

McCloghrie Experimental [Page 16] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

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.

McCloghrie Experimental [Page 17] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

 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
 internets.
 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.

McCloghrie Experimental [Page 18] RFC 1909 An SNMPv2 Administrative Infrastructure February 1996

 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
   role.
 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.

McCloghrie Experimental [Page 19]

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