rfc1157.txt

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   Also for the sake of simplicity, the SNMP uses only a subset of the
   basic encoding rules of ASN.1 [10].  Namely, all encodings use the
   definite-length form.  Further, whenever permissible, non-constructor
   encodings are used rather than constructor encodings.  This
   restriction applies to all aspects of ASN.1 encoding, both for the
   top-level protocol data units and the data objects they contain.

3.2.3.  Operations Supported on Management Information

   The SNMP models all management agent functions as alterations or
   inspections of variables.  Thus, a protocol entity on a logically
   remote host (possibly the network element itself) interacts with the
   management agent resident on the network element in order to retrieve
   (get) or alter (set) variables.  This strategy has at least two
   positive consequences:

      (1)  It has the effect of limiting the number of essential
           management functions realized by the management agent to
           two:  one operation to assign a value to a specified
           configuration or other parameter and another to retrieve
           such a value.

      (2)  A second effect of this decision is to avoid introducing
           into the protocol definition support for imperative
           management commands:  the number of such commands is in
           practice ever-increasing, and the semantics of such
           commands are in general arbitrarily complex.

   The strategy implicit in the SNMP is that the monitoring of network
   state at any significant level of detail is accomplished primarily by
   polling for appropriate information on the part of the monitoring
   center(s).  A limited number of unsolicited messages (traps) guide
   the timing and focus of the polling.  Limiting the number of
   unsolicited messages is consistent with the goal of simplicity and
   minimizing the amount of traffic generated by the network management
   function.

   The exclusion of imperative commands from the set of explicitly
   supported management functions is unlikely to preclude any desirable
   management agent operation.  Currently, most commands are requests
   either to set the value of some parameter or to retrieve such a
   value, and the function of the few imperative commands currently
   supported is easily accommodated in an asynchronous mode by this
   management model.  In this scheme, an imperative command might be
   realized as the setting of a parameter value that subsequently
   triggers the desired action.  For example, rather than implementing a
   "reboot command," this action might be invoked by simply setting a
   parameter indicating the number of seconds until system reboot.



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3.2.4.  Form and Meaning of Protocol Exchanges

   The communication of management information among management entities
   is realized in the SNMP through the exchange of protocol messages.
   The form and meaning of those messages is defined below in Section 4.

   Consistent with the goal of minimizing complexity of the management
   agent, the exchange of SNMP messages requires only an unreliable
   datagram service, and every message is entirely and independently
   represented by a single transport datagram.  While this document
   specifies the exchange of messages via the UDP protocol [11], the
   mechanisms of the SNMP are generally suitable for use with a wide
   variety of transport services.

3.2.5.  Definition of Administrative Relationships

   The SNMP architecture admits a variety of administrative
   relationships among entities that participate in the protocol.  The
   entities residing at management stations and network elements which
   communicate with one another using the SNMP are termed SNMP
   application entities.  The peer processes which implement the SNMP,
   and thus support the SNMP application entities, are termed protocol
   entities.

   A pairing of an SNMP agent with some arbitrary set of SNMP
   application entities is called an SNMP community.  Each SNMP
   community is named by a string of octets, that is called the
   community name for said community.

   An SNMP message originated by an SNMP application entity that in fact
   belongs to the SNMP community named by the community component of
   said message is called an authentic SNMP message.  The set of rules
   by which an SNMP message is identified as an authentic SNMP message
   for a particular SNMP community is called an authentication scheme.
   An implementation of a function that identifies authentic SNMP
   messages according to one or more authentication schemes is called an
   authentication service.

   Clearly, effective management of administrative relationships among
   SNMP application entities requires authentication services that (by
   the use of encryption or other techniques) are able to identify
   authentic SNMP messages with a high degree of certainty.  Some SNMP
   implementations may wish to support only a trivial authentication
   service that identifies all SNMP messages as authentic SNMP messages.

   For any network element, a subset of objects in the MIB that pertain
   to that element is called a SNMP MIB view.  Note that the names of
   the object types represented in a SNMP MIB view need not belong to a



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   single sub-tree of the object type name space.

   An element of the set { READ-ONLY, READ-WRITE } is called an SNMP
   access mode.

   A pairing of a SNMP access mode with a SNMP MIB view is called an
   SNMP community profile.  A SNMP community profile represents
   specified access privileges to variables in a specified MIB view. For
   every variable in the MIB view in a given SNMP community profile,
   access to that variable is represented by the profile according to
   the following conventions:

      (1)  if said variable is defined in the MIB with "Access:" of
           "none," it is unavailable as an operand for any operator;

      (2)  if said variable is defined in the MIB with "Access:" of
           "read-write" or "write-only" and the access mode of the
           given profile is READ-WRITE, that variable is available
           as an operand for the get, set, and trap operations;

      (3)  otherwise, the variable is available as an operand for
           the get and trap operations.

      (4)  In those cases where a "write-only" variable is an
           operand used for the get or trap operations, the value
           given for the variable is implementation-specific.

   A pairing of a SNMP community with a SNMP community profile is called
   a SNMP access policy. An access policy represents a specified
   community profile afforded by the SNMP agent of a specified SNMP
   community to other members of that community.  All administrative
   relationships among SNMP application entities are architecturally
   defined in terms of SNMP access policies.

   For every SNMP access policy, if the network element on which the
   SNMP agent for the specified SNMP community resides is not that to
   which the MIB view for the specified profile pertains, then that
   policy is called a SNMP proxy access policy. The SNMP agent
   associated with a proxy access policy is called a SNMP proxy agent.
   While careless definition of proxy access policies can result in
   management loops, prudent definition of proxy policies is useful in
   at least two ways:

      (1)  It permits the monitoring and control of network elements
           which are otherwise not addressable using the management
           protocol and the transport protocol.  That is, a proxy
           agent may provide a protocol conversion function allowing
           a management station to apply a consistent management



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           framework to all network elements, including devices such
           as modems, multiplexors, and other devices which support
           different management frameworks.

      (2)  It potentially shields network elements from elaborate
           access control policies.  For example, a proxy agent may
           implement sophisticated access control whereby diverse
           subsets of variables within the MIB are made accessible
           to different management stations without increasing the
           complexity of the network element.

   By way of example, Figure 1 illustrates the relationship between
   management stations, proxy agents, and management agents.  In this
   example, the proxy agent is envisioned to be a normal Internet
   Network Operations Center (INOC) of some administrative domain which
   has a standard managerial relationship with a set of management
   agents.


































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   +------------------+       +----------------+      +----------------+
   |  Region #1 INOC  |       |Region #2 INOC  |      |PC in Region #3 |
   |                  |       |                |      |                |
   |Domain=Region #1  |       |Domain=Region #2|      |Domain=Region #3|
   |CPU=super-mini-1  |       |CPU=super-mini-1|      |CPU=Clone-1     |
   |PCommunity=pub    |       |PCommunity=pub  |      |PCommunity=slate|
   |                  |       |                |      |                |
   +------------------+       +----------------+      +----------------+
          /|\                      /|\                     /|\
           |                        |                       |
           |                        |                       |
           |                       \|/                      |
           |               +-----------------+              |
           +-------------->| Region #3 INOC  |<-------------+
                           |                 |
                           |Domain=Region #3 |
                           |CPU=super-mini-2 |
                           |PCommunity=pub,  |
                           |         slate   |
                           |DCommunity=secret|
           +-------------->|                 |<-------------+
           |               +-----------------+              |
           |                       /|\                      |
           |                        |                       |
           |                        |                       |
          \|/                      \|/                     \|/
   +-----------------+     +-----------------+       +-----------------+
   |Domain=Region#3  |     |Domain=Region#3  |       |Domain=Region#3  |
   |CPU=router-1     |     |CPU=mainframe-1  |       |CPU=modem-1      |
   |DCommunity=secret|     |DCommunity=secret|       |DCommunity=secret|
   +-----------------+     +-----------------+       +-----------------+


   Domain:  the administrative domain of the element
   PCommunity:  the name of a community utilizing a proxy agent
   DCommunity:  the name of a direct community


                                 Figure 1
                 Example Network Management Configuration











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3.2.6.  Form and Meaning of References to Managed Objects

   The SMI requires that the definition of a conformant management
   protocol address:

      (1)  the resolution of ambiguous MIB references,

      (2)  the resolution of MIB references in the presence multiple
           MIB versions, and

      (3)  the identification of particular instances of object
           types defined in the MIB.

3.2.6.1.  Resolution of Ambiguous MIB References

   Because the scope of any SNMP operation is conceptually confined to
   objects relevant to a single network element, and because all SNMP
   references to MIB objects are (implicitly or explicitly) by unique
   variable names, there is no possibility that any SNMP reference to
   any object type defined in the MIB could resolve to multiple
   instances of that type.

3.2.6.2.  Resolution of References across MIB Versions

   The object instance referred to by any SNMP operation is exactly that
   specified as part of the operation request or (in the case of a get-
   next operation) its immediate successor in the MIB as a whole.  In
   particular, a reference to an object as part of some version of the
   Internet-standard MIB does not resolve to any object that is not part
   of said version of the Internet-standard MIB, except in the case that
   the requested operation is get-next and the specified object name is
   lexicographically last among the names of all objects presented as
   part of said version of the Internet-Standard MIB.

3.2.6.3.  Identification of Object Instances

   The names for all object types in the MIB are defined explicitly
   either in the Internet-standard MIB or in other documents which
   conform to the naming conventions of the SMI.  The SMI requires that
   conformant management protocols define mechanisms for identifying
   individual instances of those object types for a particular network
   element.

   Each instance of any object type defined in the MIB is identified in
   SNMP operations by a unique name called its "variable name." In
   general, the name of an SNMP variable is an OBJECT IDENTIFIER of the
   form x.y, where x is the name of a non-aggregate object type defined
   in the MIB and y is an OBJECT IDENTIFIER fragment that, in a way



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   specific to the named object type, identifies the desired instance.

   This naming strategy admits the fullest exploitation of the semantics
   of the GetNextRequest-PDU (see Section 4), because it assigns names