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RFC 1351

Network Working Group                                          J. Davin
Request for Comments: 1351          MIT Laboratory for Computer Science
                                                              J. Galvin
                                      Trusted Information Systems, Inc.
                                                          K. McCloghrie
                                               Hughes LAN Systems, Inc.
                                                              July 1992


                       SNMP Administrative Model

Table of Contents

   1.    Abstract  . . . . . . . . . . . . . . . . . . . . . . . . .  2
   2.    Introduction  . . . . . . . . . . . . . . . . . . . . . . .  2
   3.    Elements of the Model . . . . . . . . . . . . . . . . . . .  2
   3.1   SNMP Party  . . . . . . . . . . . . . . . . . . . . . . . .  2
   3.2   SNMP Protocol Entity  . . . . . . . . . . . . . . . . . . .  6
   3.3   SNMP Management Station . . . . . . . . . . . . . . . . . .  6
   3.4   SNMP Agent  . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.5   View Subtree  . . . . . . . . . . . . . . . . . . . . . . .  7
   3.6   MIB View  . . . . . . . . . . . . . . . . . . . . . . . . .  7
   3.7   SNMP Management Communication . . . . . . . . . . . . . . .  8
   3.8   SNMP Authenticated Management Communication . . . . . . . .  9
   3.9   SNMP Private Management Communication   . . . . . . . . . .  9
   3.10  SNMP Management Communication Class . . . . . . . . . . . . 10
   3.11  SNMP Access Control Policy  . . . . . . . . . . . . . . . . 11
   3.12  SNMP Proxy Party  . . . . . . . . . . . . . . . . . . . . . 12
   3.13  Procedures  . . . . . . . . . . . . . . . . . . . . . . . . 13
   3.13.1  Generating a Request  . . . . . . . . . . . . . . . . . . 13
   3.13.2  Processing a Received Communication . . . . . . . . . . . 15
   3.13.3  Generating a Response . . . . . . . . . . . . . . . . . . 17
   4.    Application of the Model  . . . . . . . . . . . . . . . . . 17
   4.1   Non-Secure Minimal Agent Configuration  . . . . . . . . . . 17
   4.2   Secure Minimal Agent Configuration  . . . . . . . . . . . . 20
   4.3   Proxy Configuration   . . . . . . . . . . . . . . . . . . . 21
   4.3.1   Foreign Proxy Configuration . . . . . . . . . . . . . . . 22
   4.3.2   Native Proxy Configuration  . . . . . . . . . . . . . . . 25
   4.4   Public Key Configuration  . . . . . . . . . . . . . . . . . 27
   4.5   MIB View Configurations . . . . . . . . . . . . . . . . . . 29



Davin, Galvin, & McCloghrie                                     [Page 1]

RFC 1351 SNMP Administrative Model July 1992 5. Compatibility . . . . . . . . . . . . . . . . . . . . . . . 33 6. Security Considerations . . . . . . . . . . . . . . . . . . 33 7. References . . . . . . . . . . . . . . . . . . . . . . . . 8. Authors' Addresses . . . . . . . . . . . . . . . . . . . . 34 1. Abstract This memo presents an elaboration of the SNMP administrative model set forth in [1]. This model provides a unified conceptual basis for administering SNMP protocol entities to support o authentication and integrity, o privacy, o access control, and o the cooperation of multiple protocol entities. Please send comments to the SNMP Security Developers mailing list (snmp-sec-dev@tis.com). 2. Introduction This memo presents an elaboration of the SNMP administrative model set forth in [1]. It describes how the elaborated administrative model is applied to realize effective network management in a variety of configurations and environments. The model described here entails the use of distinct identities for peers that exchange SNMP messages. Thus, it represents a departure from the community-based administrative model set forth in [1]. By unambiguously identifying the source and intended recipient of each SNMP message, this new strategy improves upon the historical community scheme both by supporting a more convenient access control model and allowing for effective use of asymmetric (public key) security protocols in the future. 3. Elements of the Model 3.1 SNMP Party A SNMP party is a conceptual, virtual execution context whose operation is restricted (for security or other purposes) to an administratively defined subset of all possible operations of a particular SNMP protocol entity (see Section 3.2). Whenever a SNMP protocol entity processes a SNMP message, it does so by acting as a SNMP party and is thereby restricted to the set of operations defined Davin, Galvin, & McCloghrie [Page 2]
RFC 1351 SNMP Administrative Model July 1992 for that party. The set of possible operations specified for a SNMP party may be overlapping or disjoint with respect to the sets of other SNMP parties; it may also be a proper or improper subset of all possible operations of the SNMP protocol entity. Architecturally, each SNMP party comprises o a single, unique party identity, o a single authentication protocol and associated parameters by which all protocol messages originated by the party are authenticated as to origin and integrity, o a single privacy protocol and associated parameters by which all protocol messages received by the party are protected from disclosure, o a single MIB view (see Section 3.6) to which all management operations performed by the party are applied, and o a logical network location at which the party executes, characterized by a transport protocol domain and transport addressing information. Conceptually, each SNMP party may be represented by an ASN.1 value with the following syntax: SnmpParty ::= SEQUENCE { partyIdentity OBJECT IDENTIFIER, partyTDomain OBJECT IDENTIFIER, partyTAddr OCTET STRING, partyProxyFor OBJECT IDENTIFIER, partyMaxMessageSize INTEGER, partyAuthProtocol OBJECT IDENTIFIER, partyAuthClock INTEGER, partyAuthLastMsg INTEGER, partyAuthNonce INTEGER, Davin, Galvin, & McCloghrie [Page 3]
RFC 1351 SNMP Administrative Model July 1992 partyAuthPrivate OCTET STRING, partyAuthPublic OCTET STRING, partyAuthLifetime INTEGER, partyPrivProtocol OBJECT IDENTIFIER, partyPrivPrivate OCTET STRING, partyPrivPublic OCTET STRING } For each SnmpParty value that represents a SNMP party, the following statements are true: o Its partyIdentity component is the party identity. o Its partyTDomain component is called the transport domain and indicates the kind of transport service by which the party receives network management traffic. An example of a transport domain is rfc1351Domain (SNMP over UDP, using SNMP parties). o Its partyTAddr component is called the transport addressing information and represents a transport service address by which the party receives network management traffic. o Its partyProxyFor component is called the proxied party and represents the identity of a second SNMP party or other management entity with which interaction may be necessary to satisfy received management requests. In this context, the value noProxy signifies that the party responds to received management requests by entirely local mechanisms. o Its partyMaxMessageSize component is called the maximum message size and represents the length in octets of the largest SNMP message this party is prepared to accept. o Its partyAuthProtocol component is called the authentication protocol and identifies a protocol and a mechanism by which all messages generated by the party Davin, Galvin, & McCloghrie [Page 4]
RFC 1351 SNMP Administrative Model July 1992 are authenticated as to integrity and origin. In this context, the value noAuth signifies that messages generated by the party are not authenticated as to integrity and origin. o Its partyAuthClock component is called the authentication clock and represents a notion of the current time that is specific to the party. The significance of this component is specific to the authentication protocol. o Its partyAuthLastMsg component is called the last-timestamp and represents a notion of time associated with the most recent, authentic protocol message generated by the party. The significance of this component is specific to the authentication protocol. o Its partyAuthNonce component is called the nonce and represents a monotonically increasing integer associated with the most recent, authentic protocol message generated by the party. The significance of this component is specific to the authentication protocol. o Its partyAuthPrivate component is called the private authentication key and represents any secret value needed to support the authentication protocol. The significance of this component is specific to the authentication protocol. o Its partyAuthPublic component is called the public authentication key and represents any public value that may be needed to support the authentication protocol. The significance of this component is specific to the authentication protocol. o Its partyAuthLifetime component is called the lifetime and represents an administrative upper bound on acceptable delivery delay for protocol messages generated by the party. The significance of this component is specific to the authentication protocol. o Its partyPrivProtocol component is called the privacy protocol and identifies a protocol and a mechanism by which all protocol messages received by the party are protected from disclosure. In this context, the value noPriv signifies that messages received by the party are not protected from disclosure. Davin, Galvin, & McCloghrie [Page 5]
RFC 1351 SNMP Administrative Model July 1992 o Its partyPrivPrivate component is called the private privacy key and represents any secret value needed to support the privacy protocol. The significance of this component is specific to the privacy protocol. o Its partyPrivPublic component is called the public privacy key and represents any public value that may be needed to support the privacy protocol. The significance of this component is specific to the privacy protocol. If, for all SNMP parties realized by a SNMP protocol entity, the authentication protocol is noAuth and the privacy protocol is noPriv, then that protocol entity is called non-secure. 3.2 SNMP Protocol Entity A SNMP protocol entity is an actual process which performs network management operations by generating and/or responding to SNMP protocol messages in the manner specified in [1]. When a protocol entity is acting as a particular SNMP party (see Section 3.1), the operation of that entity must be restricted to the subset of all possible operations that is administratively defined for that party. By definition, the operation of a SNMP protocol entity requires no concurrency between processing of any single protocol message (by a particular SNMP party) and processing of any other protocol message (by a potentially different SNMP party). Accordingly, implementation of a SNMP protocol entity to support more than one party need not be multi-threaded. However, there may be situations where implementors may choose to use multi-threading. Architecturally, every SNMP entity maintains a local database that represents all SNMP parties known to it -- those whose operation is realized locally, those whose operation is realized by proxy interactions with remote parties or devices, and those whose operation is realized by remote entities. In addition, every SNMP protocol entity maintains a local database that represents an access control policy (see Section 3.11) that defines the access privileges accorded to known SNMP parties. 3.3 SNMP Management Station A SNMP management station is the operational role assumed by a SNMP party when it initiates SNMP management operations by the generation of appropriate SNMP protocol messages or when it receives and processes trap notifications. Sometimes, the term SNMP management station is applied to partial Davin, Galvin, & McCloghrie [Page 6]
RFC 1351 SNMP Administrative Model July 1992 implementations of the SNMP (in graphics workstations, for example) that focus upon this operational role. Such partial implementations may provide for convenient, local invocation of management services, but they may provide little or no support for performing SNMP management operations on behalf of remote protocol users. 3.4 SNMP Agent A SNMP agent is the operational role assumed by a SNMP party when it performs SNMP management operations in response to received SNMP protocol messages such as those generated by a SNMP management station (see Section 3.3). Sometimes, the term SNMP agent is applied to partial implementations of the SNMP (in embedded systems, for example) that focus upon this operational role. Such partial implementations provide for realization of SNMP management operations on behalf of remote users of management services, but they may provide little or no support for local invocation of such services. 3.5 View Subtree 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. 3.6 MIB View A MIB view is a subset of the set of all instances of all object types defined according to the Internet-standard SMI [2] (i.e., of the universal set of all instances of all MIB objects), subject to the following constraints: o Each element of a MIB view is uniquely named by an ASN.1 OBJECT IDENTIFIER value. As such, identically named instances of a particular object type (e.g., in different agents) must be contained within different MIB views. That is, a particular object instance name resolves within a particular MIB view to at most one object instance. o Every MIB view is defined as a collection of view subtrees. Davin, Galvin, & McCloghrie [Page 7]
RFC 1351 SNMP Administrative Model July 1992 3.7 SNMP Management Communication A SNMP management communication is a communication from one specified SNMP party to a second specified SNMP party about management information that is represented in the MIB view of the appropriate party. In particular, a SNMP management communication may be o a query by the originating party about information in the MIB view of the addressed party (e.g., getRequest and getNextRequest), o an indicative assertion to the addressed party about information in the MIB view of the originating party (e.g., getResponse or trapNotification), or o an imperative assertion by the originating party about information in the MIB view of the addressed party (e.g., setRequest). A management communication is represented by an ASN.1 value with the syntax SnmpMgmtCom ::= [1] IMPLICIT SEQUENCE { dstParty OBJECT IDENTIFIER, srcParty OBJECT IDENTIFIER, pdu PDUs } For each SnmpMgmtCom value that represents a SNMP management communication, the following statements are true: o Its dstParty component is called the destination and identifies the SNMP party to which the communication is directed. o Its srcParty component is called the source and identifies the SNMP party from which the communication is originated. o Its pdu component has the form and significance attributed to it in [1]. Davin, Galvin, & McCloghrie [Page 8]
RFC 1351 SNMP Administrative Model July 1992 3.8 SNMP Authenticated Management Communication A SNMP authenticated management communication is a SNMP management communication (see Section 3.7) for which the originating SNMP party is (possibly) reliably identified and for which the integrity of the transmission of the communication is (possibly) protected. An authenticated management communication is represented by an ASN.1 value with the syntax SnmpAuthMsg ::= [1] IMPLICIT SEQUENCE { authInfo ANY, - defined by authentication protocol authData SnmpMgmtCom } For each SnmpAuthMsg value that represents a SNMP authenticated management communication, the following statements are true: o Its authInfo component is called the authentication information and represents information required in support of the authentication protocol used by the SNMP party originating the message. The detailed significance of the authentication information is specific to the authentication protocol in use; it has no effect on the application semantics of the communication other than its use by the authentication protocol in determining whether the communication is authentic or not. o Its authData component is called the authentication data and represents a SNMP management communication. 3.9 SNMP Private Management Communication A SNMP private management communication is a SNMP authenticated management communication (see Section 3.8) that is (possibly) protected from disclosure. A private management communication is represented by an ASN.1 value with the syntax Davin, Galvin, & McCloghrie [Page 9]
RFC 1351 SNMP Administrative Model July 1992 SnmpPrivMsg ::= [1] IMPLICIT SEQUENCE { privDst OBJECT IDENTIFIER, privData [1] IMPLICIT OCTET STRING } For each SnmpPrivMsg value that represents a SNMP private management communication, the following statements are true: o Its privDst component is called the privacy destination and identifies the SNMP party to which the communication is directed. o Its privData component is called the privacy data and represents the (possibly encrypted) serialization (according to the conventions of [3] and [1]) of a SNMP authenticated management communication (see Section 3.8). 3.10 SNMP Management Communication Class A SNMP management communication class corresponds to a specific SNMP PDU type defined in [1]. A management communication class is represented by an ASN.1 INTEGER value according to the type of the identifying PDU (see Table 1). Get 1 GetNext 2 GetResponse 4 Set 8 Trap 16 Table 1: Management Communication Classes The value by which a communication class is represented is computed as 2 raised to the value of the ASN.1 context-specific tag for the appropriate SNMP PDU. A set of management communication classes is represented by the ASN.1 INTEGER value that is the sum of the representations of the communication classes in that set. The null set is represented by the value zero. Davin, Galvin, & McCloghrie [Page 10]
RFC 1351 SNMP Administrative Model July 1992 3.11 SNMP Access Control Policy A SNMP access control policy is a specification of a local access policy in terms of the network management communication classes which are authorized between pairs of SNMP parties. Architecturally, such a specification comprises three parts: o the targets of SNMP access control - the SNMP parties that may perform management operations as requested by management communications received from other parties, o the subjects of SNMP access control - the SNMP parties that may request, by sending management communications to other parties, that management operations be performed, and o the policy that specifies the classes of SNMP management communications that a particular target is authorized to accept from a particular subject. Access to individual MIB object instances is determined implicitly since by definition each (target) SNMP party performs operations on exactly one MIB view. Thus, defining the permitted access of a (reliably) identified subject party to a particular target party effectively defines the access permitted by that subject to that target's MIB view and, accordingly, to particular MIB object instances. Conceptually, a SNMP access policy is represented by a collection of ASN.1 values with the following syntax: AclEntry ::= SEQUENCE { aclTarget OBJECT IDENTIFIER, aclSubject OBJECT IDENTIFIER, aclPrivileges INTEGER } For each such value that represents one part of a SNMP access policy, the following statements are true: Davin, Galvin, & McCloghrie [Page 11]
RFC 1351 SNMP Administrative Model July 1992 o Its aclTarget component is called the target and identifies the SNMP party to which the partial policy permits access. o Its aclSubject component is called the subject and identifies the SNMP party to which the partial policy grants privileges. o Its aclPrivileges component is called the privileges and represents a set of SNMP management communication classes that are authorized to be processed by the specified target party when received from the specified subject party. 3.12 SNMP Proxy Party A SNMP proxy party is a SNMP party that performs management operations by communicating with another, logically remote party. When communication between a logically remote party and a SNMP proxy party is via the SNMP (over any transport protocol), then the proxy party is called a SNMP native proxy party. Deployment of SNMP native proxy parties is a means whereby the processing or bandwidth costs of management may be amortized or shifted -- thereby facilitating the construction of large management systems. When communication between a logically remote party and a SNMP proxy party is not via the SNMP, then the proxy party is called a SNMP foreign proxy party. Deployment of foreign proxy parties is a means whereby otherwise unmanageable devices or portions of an internet may be managed via the SNMP. The transparency principle that defines the behavior of a SNMP party in general applies in particular to a SNMP proxy party: The manner in which one SNMP party processes SNMP protocol messages received from another SNMP party is entirely transparent to the latter. The transparency principle derives directly from the historical SNMP philosophy of divorcing architecture from implementation. To this dichotomy are attributable many of the most valuable benefits in both the information and distribution models of the management framework, and it is the architectural cornerstone upon which large management systems may be built. Consistent with this philosophy, although the implementation of SNMP proxy agents in certain environments may resemble that of a transport-layer bridge, this particular implementation strategy (or any other!) does not merit special Davin, Galvin, & McCloghrie [Page 12]
RFC 1351 SNMP Administrative Model July 1992 recognition either in the SNMP management architecture or in standard mechanisms for proxy administration. Implicit in the transparency principle is the requirement that the semantics of SNMP management operations are preserved between any two SNMP 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. For this reason, proxy configurations that admit Set operations that apply to information at multiple locations are discouraged, although such operations are not explicitly precluded by the architecture in those rare cases where they might be supported in a conformant way. Also implicit in the transparency principle is the requirement that, throughout its interaction with a proxy agent, a management station is supplied with no information about the nature or progress of the proxy mechanisms by which its requests are realized. That is, it should seem to the management station -- except for any distinction in underlying transport address -- as if it were interacting via SNMP directly with the proxied device. Thus, a timeout in the communication between a proxy agent and its proxied device should be represented as a timeout in the communication between the management station and the proxy 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 agent and management station. 3.13 Procedures This section describes the procedures followed by a SNMP protocol entity in processing SNMP messages. These procedures are independent of the particular authentication and privacy protocols that may be in use. 3.13.1 Generating a Request This section describes the procedure followed by a SNMP protocol entity whenever either a management request or a trap notification is to be transmitted by a SNMP party. 1. An ASN.1 SnmpMgmtCom value is constructed for which the srcParty component identifies the originating party, for which the dstParty component identifies the receiving party, and for which the other component represents the desired management operation. Davin, Galvin, & McCloghrie [Page 13]
RFC 1351 SNMP Administrative Model July 1992 2. The local database is consulted to determine the authentication protocol and other relevant information for the originating SNMP party. 3. An ASN.1 SnmpAuthMsg value is constructed with the following properties: o Its authInfo component is constructed according to the authentication protocol specified for the originating party. In particular, if the authentication protocol for the originating SNMP party is identified as noAuth, then this component corresponds to the OCTET STRING value of zero length. o Its authData component is the constructed SnmpMgmtCom value. 4. The local database is consulted to determine the privacy protocol and other relevant information for the receiving SNMP party. 5. An ASN.1 SnmpPrivMsg value is constructed with the following properties: o Its privDst component identifies the receiving SNMP party. o Its privData component is the (possibly encrypted) serialization of the SnmpAuthMsg value according to the conventions of [3] and [1]. In particular, if the privacy protocol for the receiving SNMP party is identified as noPriv, then the privData component is unencrypted. Otherwise, the privData component is processed according to the privacy protocol. 6. The constructed SnmpPrivMsg value is serialized according to the conventions of [3] and [1]. 7. The serialized SnmpPrivMsg value is transmitted using the transport address and transport domain for the receiving SNMP party. Davin, Galvin, & McCloghrie [Page 14]
RFC 1351 SNMP Administrative Model July 1992 3.13.2 Processing a Received Communication This section describes the procedure followed by a SNMP protocol entity whenever a management communication is received. 1. If the received message is not the serialization (according to the conventions of [3] and [1]) of an ASN.1 SnmpPrivMsg value, then that message is discarded without further processing. 2. The local database is consulted for information about the receiving SNMP party identified by the privDst component of the SnmpPrivMsg value. 3. If information about the receiving SNMP party is absent from the local database, or specifies a transport domain and address which indicates that the receiving party's operation is not realized by the local SNMP protocol entity, then the received message is discarded without further processing. 4. An ASN.1 OCTET STRING value is constructed (possibly by decryption, according to the privacy protocol in use) from the privData component of said SnmpPrivMsg value. In particular, if the privacy protocol recorded for the party is noPriv, then the OCTET STRING value corresponds exactly to the privData component of the SnmpPrivMsg value. 5. If the OCTET STRING value is not the serialization (according to the conventions of [3] and [1]) of an ASN.1 SnmpAuthMsg value, then the received message is discarded without further processing. 6. If the dstParty component of the authData component of the obtained SnmpAuthMsg value is not the same as the privDst component of the SnmpPrivMsg value, then the received message is discarded without further processing. 7. The local database is consulted for information about the originating SNMP party identified by the srcParty component of the authData component of the SnmpAuthMsg value. Davin, Galvin, & McCloghrie [Page 15]
RFC 1351 SNMP Administrative Model July 1992 8. If information about the originating SNMP party is absent from the local database, then the received message is discarded without further processing. 9. The obtained SnmpAuthMsg value is evaluated according to the authentication protocol and other relevant information associated with the originating SNMP party in the local database. In particular, if the authentication protocol is identified as noAuth, then the SnmpAuthMsg value is always evaluated as authentic. 10. If the SnmpAuthMsg value is evaluated as unauthentic, then the received message is discarded without further processing, and an authentication failure is noted. 11. The ASN.1 SnmpMgmtCom value is extracted from the authData component of the SnmpAuthMsg value. 12. The local database is consulted for access privileges permitted by the local access policy to the originating SNMP party with respect to the receiving SNMP party. 13. The management communication class is determined from the ASN.1 tag value associated with the SnmpMgmtCom value. 14. If the management communication class of the received message is either 16 or 4 (i.e., Trap or GetResponse) and this class is not among the access privileges, then the received message is discarded without further processing. 15. If the management communication class of the received message is not among the access privileges, then the received message is discarded without further processing after generation and transmission of a response message. This response message is directed to the originating SNMP party on behalf of the receiving SNMP party. Its var-bind-list and request-id components are identical to those of the received request. Its error-index component is zero and its error-status component is readOnly. 16. If the proxied party associated with the receiving SNMP party in the local database is identified as noProxy, Davin, Galvin, & McCloghrie [Page 16]
RFC 1351 SNMP Administrative Model July 1992
RFC 1351 SNMP Administrative Model July 1992 Party Identity Status Family Name Family Mask lucy include internet ""h Table 13: View Definition for Minimal Agent Using this convention for abbreviating MIB view definitions, some of the most common definitions of MIB views may be conveniently expressed. For example, Table 13 illustrates the MIB view definitions required for a minimal SNMP entity that locally realizes a single SNMP party for which the associated MIB view embraces all instances of all MIB objects defined within the internet network management framework. The represented table has a single entry. The SNMP party (lucy) for which that entry defines the MIB view is identified in the first column. The status of that entry (include) signifies that any MIB object instance belonging to the subtree family represented by that entry may appear in the MIB view for party lucy. The family name for that entry is internet, and the zero-length family mask value signifies that the relevant subtree family corresponds to the single view subtree rooted at that node. Another example of MIB view definition (see Table 14) is that of a SNMP protocol entity that locally realizes multiple SNMP parties with distinct MIB views. The MIB view associated with the party lucy comprises all instances of all MIB objects defined within the internet network management framework, except those pertaining to the administration of SNMP parties. In contrast, the MIB view attributed to the party ricky contains only MIB object instances defined in the system group of the internet-standard MIB together with those object instances by which SNMP parties are administered. A more complicated example of MIB view configuration illustrates the abbreviation of related collections of view subtrees by view subtree families (see Table 15). In this Party Identity Status Family Name Family Mask lucy include internet ""h lucy exclude snmpParties ""h ricky include system ""h ricky include snmpParties ""h Table 14: View Definition for Multiple Parties example, the MIB view associated with party lucy includes all object instances in the system group of the internet-standard MIB together with some information related to the second network interface attached to the managed device. However, this interface-related information does not include the speed of the interface. The family Davin, Galvin, & McCloghrie [Page 31]
RFC 1351 SNMP Administrative Model July 1992 mask value "FFA0"h in the second table entry signifies that a MIB object instance belongs to the relevant subtree family if the initial prefix of its name places it within the ifEntry portion of the registration hierarchy and if the eleventh subidentifier of its name is 2. The MIB object instance representing the speed of the second network interface belongs to the subtree families represented by both the second and third entries of the table, but that particular instance is excluded from the MIB view for party lucy because the lexicographically greater of the relevant family names appears in the table entry with status exclude. The MIB view for party ricky is also defined in this example. The MIB view attributed to the party ricky includes all object instances in the icmp group of the internet-standard MIB, together with all information relevant to the fifth network interface attached to the managed device. In addition, the MIB view attributed to party ricky includes the number of octets received on the fourth attached network interface. While, as suggested by the examples above, a wide range of MIB view configurations are efficiently supported by the abbreviated representation of [5], prudent MIB design can sometimes further reduce the size and complexity of the most Party Identity Status Family Name Family Mask lucy include system ""h lucy include { ifEntry 0 2 } "FFA0"h lucy exclude { ifSpeed 2 } ""h ricky include icmp ""h ricky include { ifEntry 0 5 } "FFA0"h ricky include { ifInOctets 4 } ""h Table 15: More Elaborate View Definitions likely MIB view definitions. On one hand, it is critical that mechanisms for MIB view configuration impose no absolute constraints either upon the access policies of local administrations or upon the structure of MIB namespaces; on the other hand, where the most common access policies are known, the configuration costs of realizing those policies may be slightly reduced by assigning to distinct portions of the registration hierarchy those MIB objects for which local policies most frequently require distinct treatment. The relegation in [5] of certain objects to a distinct arc in the MIB namespace is an example of this kind of optimization. Davin, Galvin, & McCloghrie [Page 32]
RFC 1351 SNMP Administrative Model July 1992 5. Compatibility Ideally, all SNMP management stations and agents would communicate exclusively using the secure facilities described in this memo. In reality, many SNMP agents may implement only the insecure SNMP mechanisms described in [1] for some time to come. New SNMP agent implementations should never implement both the insecure mechanisms of [1] and the facilities described here. Rather, consistent with the SNMP philosophy, the burden of supporting both sorts of communication should fall entirely upon managers. Perhaps the best way to realize both old and new modes of communication is by the use of a SNMP proxy agent deployed locally on the same system with a management station implementation. The management station implementation itself operates exclusively by using the newer, secure modes of communication, and the local proxy agent translates the requests of the manager into older, insecure modes as needed. It should be noted that proxy agent implementations may require additional information beyond that described in this memo in order to accomplish the requisite translation tasks implicit in the definition of the proxy function. This information could easily be retrieved from a filestore. 6. Security Considerations It is important to note that, in the example configuration for native proxy operations presented in this memo, the use of symmetric cryptography does not securely prevent direct communication between the SNMP management station and the proxied SNMP agent. While secure isolation of the management station and the proxied agent can, according to the administrative model set forth in this memo, be realized using symmetric cryptography, the required configuration is more complex and is not described in this memo. Rather, it is recommended that native proxy configurations that require secure isolation of management station from proxied agent be implemented using security protocols based on asymmetric (or "public key") cryptography. However, no SNMP security protocols based on asymmetric cryptography are currently defined. In order to participate in the administrative model set forth in this memo, SNMP implementations must support local, non-volatile storage of the local party database. Accordingly, every attempt has been made to minimize the amount of non-volatile storage required. Davin, Galvin, & McCloghrie [Page 33]
RFC 1351 SNMP Administrative Model July 1992 7. References [1] Case, J., M. Fedor, M. Schoffstall, and J. Davin, The Simple Network Management Protocol", RFC 1157, University of Tennessee at Knoxville, Performance Systems International, Performance Systems International, and the MIT Laboratory for Computer Science, May 1990. (Obsoletes RFC 1098.) [2] Rose, M., and K. McCloghrie, "Structure and Identification of Management Information for TCP/IP based internets", RFC 1155, Performance Systems International, Hughes LAN Systems, May 1990. (Obsoletes RFC 1065.) [3] Information Processing -- Open Systems Interconnection -- Specification of Basic Encoding Rules for Abstract Syntax Notation One (ASN.1), International Organization for Standardization/International Electrotechnical Institute, 1987, International Standard 8825. [4] Galvin, J., McCloghrie, K., and J. Davin, "SNMP Security Protocols", RFC 1352, Trusted Information Systems, Inc., Hughes LAN Systems, Inc., MIT Laboratory for Computer Science, July 1992 [5] McCloghrie, K., Davin, J., and J. Galvin, "Definitions of Managed Objects for Administration of SNMP Parties", RFC 1353, Hughes LAN Systems, Inc., MIT Laboratory for Computer Science, Trusted Information Systems, Inc., July 1992. 8. Authors' Addresses James R. Davin MIT Laboratory for Computer Science 545 Technology Square Cambridge, MA 02139 Phone: (617) 253-6020 EMail: jrd@ptt.lcs.mit.edu James M. Galvin Trusted Information Systems, Inc. 3060 Washington Road, Route 97 Glenwood, MD 21738 Phone: (301) 854-6889 EMail: galvin@tis.com Davin, Galvin, & McCloghrie [Page 34]
RFC 1351 SNMP Administrative Model July 1992 Keith McCloghrie Hughes LAN Systems, Inc. 1225 Charleston Road Mountain View, CA 94043 Phone: (415) 966-7934 EMail: kzm@hls.com



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