INTERNET-DRAFT April 1991
SNMP Administrative Model
DRAFT
April 8, 1991
1 Introduction
This paper 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.
This paper also 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.
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2 Elements of the Model
2.1 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 2.2). 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 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 param-
eters by which all protocol messages originated by the
party are authenticated as to origin and integrity,
o a single privacy protocol and associated secret by which
all protocol messages received by the party are protected
from disclosure,
o a single MIB view (see section 2.6) 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 can be represented by an
ASN.1 value with the syntax
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SnmpParty ::= SEQUENCE {
partyIdentity
OBJECT IDENTIFIER,
partyTDomain
OBJECT IDENTIFIER,
partyTAddr
OCTET STRING,
partyProxyFor
OBJECT IDENTIFIER,
partyAuthProt
OBJECT IDENTIFIER,
partyAuthClock
TimeTicks,
partyAuthLastMsg
TimeTicks,
partyAuthNonce
INTEGER,
partyAuthPrivate
OCTET STRING,
partyAuthPublic
OCTET STRING,
partyAuthLifetime
INTEGER,
partyPrivProt
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 called the identity of the
party and corresponds to the party identity.
o Its partyTDomain component is called the transport
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domain and indicates the kind of transport service by
which the party receives network management traffic.
An example of a transport domain is rfc1157Domain
(SNMP over UDP).
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 partyAuthProt component is called the authentica-
tion protocol and identifies a protocol by which all mes-
sages generated by the party are authenticated as to origin
and integrity. In this context, the value noAuth signifies
that messages generated by the party are not authenti-
cated as to origin and integrity.
o Its partyAuthClock component is called the authenti-
cation clock and represents a notion of the current time
that is specific to the party. The significance of this com-
ponent is specific to the authentication protocol.
o Its partyAuthLastMsg component is called the ratchet
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 an integer associated with the most
recent, authentic protocol message generated by the party.
The significance of this component is specific to the
authentication protocol.
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o Its partyAuthPrivate component is called the private
authentication key and represents any secret value that
may be 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 partyPrivProt component is called the privacy
protocol and identifies a protocol 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.
o Its partyPrivPrivate component is called the private
privacy key and represents any secret value that may be
needed to support 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.
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.
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2.2 SNMP Protocol Entity
A SNMP protocol entity is an actual process which performs
network management operations by generating and/or respond-
ing to SNMP protocol messages in the manner specified in [1].
When a protocol entity is acting as a particular SNMP party
(see section 2.1), 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 protocol entity maintains a local
database that represents all SNMP parties known to it -- those
whose operation is performed locally, those whose operation
is performed through communication with remote proxied
parties, and those whose operation is performed by remote
parties. In addition, every SNMP protocol entity maintains
a local database that represents an access control policy (see
section 2.11) that defines the access privileges associated with
known SNMP parties.
2.3 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 implementations of the SNMP (in graphics workstations,
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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.
2.4 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 2.3).
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.
2.5 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.
2.6 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:
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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
that are mutually disjoint.
2.7 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,
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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].
2.8 SNMP Authenticated Management Commu-
nication
A SNMP authenticated management communication is a SNMP
management communication (see section 2.7) 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
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}
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.
2.9 SNMP Private Management Communication
A SNMP private management communication is a SNMP
authenticated management communication (see section 2.8)
that is (possibly) protected from disclosure. A private
management communication is represented by an ASN.1 value
with the syntax
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:
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o Its privDst component is called the private destination
and identifies the SNMP party to which the communica-
tion is directed.
o Its privData component is called the private data
and represents the (possibly encrypted) serialization
(according to the conventions of [3] and [1]) of a
SNMP authenticated management communication (see
section 2.8).
2.10 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).
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.
Get 1
GetNext 2
GetResponse 4
Set 8
Trap 16
Table 1: Management Communication Classes
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2.11 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 particular 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 SNMP management commu-
nications classes that a particular subject is authorized to
request of a particular target.
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
}
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For each such value that represents one part of a SNMP access
policy the following statements are true:
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.
2.12 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. There are multiple reasons why a SNMP native proxy
party might be used. For example, 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
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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 to a SNMP proxy party. In particular:
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 implementa-
tion. To this dichotomy are attributable many of the most valu-
able benefits in both the information and distribution models
of the management framework, and it is the architectural cor-
nerstone 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 implementa-
tion strategy (or any other!) does not merit special 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 the interaction between a management station and a proxy
agent not supply information to the station about the nature or
progress of the mechanisms by which its requests are realized.
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That is, it should seem -- except for any distinction in an
underlying transport address -- to the management station as
if it were interacting 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.
2.13 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.
2.13.1 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
specifies the desired management operation.
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:
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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 Snmp-
MgmtCom 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.
2.13.2 Processing a Received Communication
This section describes the procedure followed by a SNMP
protocol entity whenever a management communication is
received.
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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 performed 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) from the privData component
of said SnmpPrivMsg value according to the privacy
protocol in use.
In particular, if the privacy protocol recorded for the party
is identified as 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 com-
ponent of the authData component of the SnmpAu-
thMsg value.
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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 accord-
ing to the authentication protocol and other relevant in-
formation 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 SnmpMgmt-
Com value.
14. If the management communication class is not among the
access privileges, then the received message is discarded
after first checking if the class is not 16 (i.e., not a trap),
and if so, generating a response to the originating SNMP
party on behalf of the receiving SNMP party for which the
var-bind-list and request-id components are identical
to those of the received request, and for which the value
of the error-index component is zero, and for which the
value of the error-status component is readOnly.
15. If the proxied party associated with the receiving SNMP
party in the local database is identified as noProxy,
then the SNMP management communication represented
by the SnmpMgmtCom value is performed by the
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receiving SNMP protocol entity with respect to the MIB
view identified with the receiving SNMP party identity
according to the procedures set forth in [1].
16. If the proxied party associated with the receiving
SNMP party in the local database is not identified as
noProxy, then the SNMP management communication
represented by the SnmpMgmtCom value is performed
by appropriate cooperation between the receiving SNMP
party and the identified proxied party.
In particular, if the transport domain associated with
the identified proxied party in the local database is
rfc1157Domain, then the operation requested by the
received message is performed by the generation of a
corresponding request to the proxied party on behalf of
the receiving party. If the received message requires a
response from the local SNMP protocol entity, then that
response is subsequently generated from the response (if
any) received from the proxied party corresponding to the
newly generated request.
2.13.3 Generating a Response
This section describes the procedure followed by a SNMP
protocol entity whenever a response to a management request
is generated.
The procedure for generating a response to a SNMP
management request is identical to the procedure for
transmitting a request (see section 2.13.1), except for the
derivation of the transport domain and address information.
In this case, the response is transmitted using the transport
domain and address (not from the local database but) from
which the corresponding request originated.
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3 Application of the Model
This section describes how the administrative model set forth
above is applied to realize effective network management in a
variety of configurations and environments. Several types of
administrative configurations are identified and examples are
given of how each may be realized.
3.1 Non-Secure Minimal Agent Configuration
This section presents an example configuration for a minimal,
non-secure SNMP agent that interacts with one or more SNMP
management stations. Table 2 presents information about
SNMP parties that is known both to the minimal agent and
to the manager, while Table 3 presents similarly common
information about the local access policy.
As represented in Table 2, the example agent party operates
at UDP port 161 at IP address 1.2.3.4 using the party identity
gracie; the example manager operates at UDP port 2001 at
IP address 1.2.3.5 using the identity george. At minimum,
a non-secure SNMP agent implementation must provide for
administrative configuration (and non-volatile storage) of the
identities and transport addresses of two SNMP parties: itself
and a remote peer. Strictly speaking, other information about
these two parties (including access policy information) need not
be configurable.
Suppose that the managing party george wishes to interrogate
the agent named gracie by issuing a SNMP GetNext request
message. The manager consults its local database of party
information. Because the authentication protocol for the party
george is recorded as noAuth, the GetNext request message
generated by the manager is not authenticated as to origin and
integrity. Because, according to the manager's database, the
privacy protocol for the party gracie is noPriv, the GetNext
request message is not protected from disclosure. Rather, it is
simply assembled, serialized, and transmitted to the transport
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Identity gracie george
(agent) (manager)
Domain rfc1157 rfc1157
Address 1.2.3.4, 161 1.2.3.5, 2001
Proxied Party noProxy noProxy
Auth Prot noAuth noAuth
Auth Priv Key "" ""
Auth Pub Key "" ""
Auth Clock 0 0
Auth Last Msg 0 0
Auth Lifetime 0 0
Priv Prot noPriv noPriv
Priv Priv Key "" ""
Priv Pub Key "" ""
Table 2: Party Information for Minimal Agent
Target Subject Privileges
gracie george 3
george gracie 20
Table 3: Access Information for Minimal Agent
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address (IP address 1.2.3.4, UDP port 161) associated in the
manager's database with the party gracie.
When the GetNext request message is received at the agent,
the identity of the party to which it is directed (gracie) is
extracted from the message, and the receiving protocol entity
consults its local database of party information. Because the
privacy protocol for the party gracie is recorded as noPriv, the
received message is assumed to not be protected from disclosure.
Similarly, the identity of the originating party (george) is
extracted, and the local party database is consulted. Because
the authentication protocol for the party george is recorded
as noAuth, the received message is immediately accepted as
authentic.
The received message is fully processed only if the access
policy database local to the agent authorizes GetNext request
communications by the party george with respect to the
agent party gracie. The access policy database presented as
Table 3 authorizes such communications (as well as GetNext
operations).
When the received request is processed, a GetResponse message
is generated by the agent gracie for the originating peer
george. Because the authentication protocol for gracie
is recorded in the local party database as noAuth, the
GetResponse message generated by the manager is not
authenticated as to origin or integrity. Because, according
to the local database, the privacy protocol for the party
george is noPriv, the response message is not protected from
disclosure. Rather, as required by [1], the response message is
directed to the transport address from which the corresponding
request originated -- without regard for the transport address
associated with george in the local database.
When the generated response is received by the manager, the
identity of the party to which it is directed (george) is extracted
from the message, and the manager consults its local database
of party information. Because the privacy protocol for the party
george is recorded as noPriv, the received response is assumed
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not to be protected from disclosure. Similarly, the identity of
the originating party (gracie) is extracted, and the local party
database is consulted. Because the authentication protocol for
the party gracie is recorded as noAuth, the received response
is immediately accepted as authentic.
The received message is fully processed only if the access
policy database local to the agent authorizes GetResponse
communications by the party gracie with respect to the
manager party george. The access policy database presented
as Table 3 authorizes such response messages (as well as Trap
messages).
3.2 Secure Minimal Agent Configuration
This section presents an example configuration for a secure,
minimal SNMP agent that interacts with a single SNMP
management station. Table 4 presents information about
SNMP parties that is known both to the minimal agent and
to the manager, while Table 5 presents similarly common
information about the local access policy.
The interaction of manager and agent in this configuration is
very similar to that sketched above for the non-secure minimal
agent -- except that all protocol messages are authenticated
as to origin and integrity and protected from disclosure. This
example requires encryption in order to support distribution of
secret keys via the SNMP itself. A more elaborate example
comprising an additional pair of SNMP parties could support
the exchange of non-secret information in authenticated
messages without incurring the cost of encryption.
As represented in Table 4, the example agent party operates
at UDP port 161 at IP address 1.2.3.4 using the party identity
ollie; the example manager operates at UDP port 2001 at IP
address 1.2.3.5 using the identity stan. At minimum, a secure
SNMP agent implementation must provide for administrative
configuration (and non-volatile storage) of relevant information
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Identity ollie stan
(agent) (manager)
Domain rfc1157 rfc1157
Address 1.2.3.4, 161 1.2.3.5, 2001
Proxied Party noProxy noProxy
Auth Prot md4AuthProt md4AuthProt
Auth Priv Key "ABCD" "EFGH"
Auth Pub Key "" ""
Auth Clock 0 0
Auth Last Msg 0 0
Auth Lifetime 500 500
Priv Prot desPrivProt desPrivProt
Priv Priv Key "JKLM" "QRST"
Priv Pub Key "" ""
Table 4: Party Information for Secure Minimal Agent
Target Subject Privileges
ollie stan 3
stan ollie 20
Table 5: Access Information for Secure Minimal Agent
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about two SNMP parties: itself and a remote peer. Both
ollie and stan authenticate all messages that they generate
by using the SNMP authentication protocol md4AuthProt
and their distinct, private authentication keys. Although these
private authentication key values ("ABCD" and "EFGH") are
presented here for expository purposes, knowledge of private
authentication keys is not normally afforded to human beings
and is confined to those portions of the protocol implementation
that require it.
Using the md4AuthProt, the public authentication key
for each SNMP party is irrelevant. Also, because the
md4AuthProt is symmetric in character, the private
authentication key for each party must be known to another
SNMP party with which authenticated communication is
desired. In contrast, asymmetric (public key) authentication
protocols would not depend upon sharing of a private key for
their operation. Although the administrative model set forth
here can easily accommodate asymmetric protocols, none are
currently defined for use with the SNMP.
All protocol messages originated by the party stan are
encrypted on transmission using the desPrivProt privacy
protocol and the private key "QRST"; they are decrypted
upon reception according to the same protocol and key.
Similarly, all messages originated by the party ollie are
encrypted on transmission using the desPrivProt protocol and
private authentication key "JKLM"; they are correspondingly
decrypted on reception.
3.3 Proxy Configuration
This section presents example configurations for SNMP
proxy management. On the one hand, proxy management
via a so-called foreign proxy configuration provides the
capability to manage non-SNMP devices; on the other, it
allows an administrator to shift the computational burden
of rich management functionality away from network devices
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whose primary task is not management, via a native proxy
configuration. Among a variety of other benefits, to the extent
that SNMP proxy agents function as points of aggregation for
management information, configurations of this sort may also
reduce the bandwidth requirements of large-scale management
activities.
3.3.1 Foreign Proxy Configuration
This section presents an example configuration by which a
SNMP management station may manage network elements
that do not themselves support the SNMP. This configuration
centers on a SNMP proxy agent that performs SNMP
management operations by communicating with a non-SNMP
device using a proprietary protocol.
Table 6 presents information about SNMP parties that is
recorded in the local database of the SNMP proxy agent.
Table 7 presents information about SNMP parties that is
recorded in the local database of the SNMP management
station. Table 8 presents information about the access policy
specified by the local administration.
As represented in Table 6, the proxy agent party operates at
UDP port 161 at IP address 1.2.3.5 using the party identity
moe; the example manager operates at UDP port 2002 at
IP address 1.2.3.4 using the identity larry. Both larry
and moe authenticate all messages that they generate by
using the protocol md4AuthProt and their distinct, private
authentication keys. Although these private authentication
key values ("ABCD" and "EFGH") are presented here for
expository purposes, knowledge of private keys is not normally
afforded to human beings and is confined to those portions of
the protocol implementation that require it.
Using the md4AuthProt, the public authentication key
for each SNMP party is irrelevant. Also, because the
md4AuthProt is symmetric in character, the private
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Identity larry moe curly
(manager) (proxy) (proxied)
Domain rfc1157 rfc1157 acmeMgmtPrtcl
Address 1.2.3.4, 2002 1.2.3.5, 161 0x98765432
Proxied Party noProxy curly noProxy
Auth Prot md4AuthProt md4AuthProt noAuth
Auth Priv Key "ABCD" "EFGH" ""
Auth Pub Key "" "" ""
Auth Clock 0 0 0
Auth Last Msg 0 0 0
Auth Lifetime 500 500 0
Priv Prot noPriv noPriv noPriv
Priv Priv Key "" "" ""
Priv Pub Key "" "" ""
Table 6: Party Information for Proxy Agent
Identity larry moe
(manager) (proxy)
Domain rfc1157 rfc1157
Address 1.2.3.4, 2002 1.2.3.5, 161
Proxied Party noProxy noProxy
Auth Prot md4AuthProt md4AuthProt
Auth Priv Key "ABCD" "EFGH"
Auth Pub Key "" ""
Auth Clock 0 0
Auth Last Msg 0 0
Auth Lifetime 500 500
Priv Prot noPriv noPriv
Priv Priv Key "" ""
Priv Pub Key "" ""
Table 7: Party Information for Management Station
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authentication key for each party must be known to another
SNMP party with which authenticated communication is
desired. In contrast, asymmetric (public key) authentication
protocols would not depend upon sharing of a private key for
their operation. Although the administrative model set forth
here can easily accommodate asymmetric protocols, none are
currently defined for use with the SNMP.
Although all SNMP agents that use cryptographic keys in
their communication with other protocol entities will almost
certainly engage in private SNMP exchanges to distribute those
keys, in order to simplify this example, neither the management
station nor the proxy agent sends or receives private SNMP
communications. Thus, the privacy protocol for each of them
is recorded as noPriv.
The party curly does not send or receive SNMP protocol
messages; rather, all communication with that party proceeds
via a hypothetical proprietary protocol identified by the
value acmeMgmtPrtcl. Because the party curly does not
participate in the SNMP, many of the attributes recorded for
that party in a local database are ignored.
In order to interrogate the proprietary device associated with
the party curly, the management station larry constructs a
SNMP GetNext request and transmits it to the party moe
operating (see Table 7) at UDP port 161, and IP address 1.2.3.5.
This request is authenticated using the private authentication
key "ABCD."
When that request is received by the party moe, the originator
of the message is verified as being the party larry by using
local knowledge (see Table 6) of the private authentication key
"ABCD." Because party larry is authorized to issue GetNext
requests with respect to party moe by the relevant access
control policy (Table 8), the request is accepted. Because
the local database records the proxied party for party moe as
curly, the request is satisfied by its translation into appropriate
operations of the acmeMgmtPrtcl directed at party curly.
These new operations are transmitted to the party curly at
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the address 0x98765432 in the acmeMgmtPrtcl domain.
When and if the proprietary protocol exchange between the
proxy agent and the proprietary device concludes, a SNMP
GetResponse management operation is constructed by the
SNMP party moe to represent the results. This response
communication is authenticated as to origin and integrity
using the authentication protocol md4AuthProt and private
authentication key "EFGH" specified for transmissions from
party moe. It is then transmitted the SNMP party larry
operating at the management station at IP address 1.2.3.4
and UDP port 2002 (the source address for the corresponding
request).
When this response is received by the party larry, the
originator of the message is verified as being the party moe by
using local knowledge (see Table 7) of the private authentication
key "EFGH." Because party moe is authorized to issue Get
responses with respect to party larry by the relevant access
control policy (Table 8), the response is accepted, and the
interrogation of the proprietary device is complete.
It is especially useful to observe that the database of SNMP
parties recorded at the proxy agent (Table 6) need be
neither static nor configured exclusively by the management
station. For instance, suppose that, in this example, the
acmeMgmtPrtcl was a proprietary, MAC-layer mechanism
for managing stations attached to a local area network. In such
an environment, the SNMP party moe would reside at a SNMP
proxy agent attached to such a LAN and could, by participating
in the LAN protocols, detect the attachment and disconnection
of various stations on the LAN. In this scenario, the SNMP
proxy agent could easily adjust its local database of SNMP
parties to support indirect management of the LAN stations
by the SNMP management station. For each new LAN station
detected, the SNMP proxy agent would add to its database
both an entry analogous to that for party curly (representing
the new LAN station itself) and an entry analogous to that for
party moe (representing a proxy for that new station in the
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SNMP domain).
By using the SNMP to interrogate the database of parties held
locally by the SNMP proxy agent, a SNMP management station
can discover and interact with new stations as they are attached
to the LAN.
3.3.2 Native Proxy Configuration
This section presents an example configuration that supports
SNMP native proxy operations -- indirect interaction between
a SNMP agent and a management station that is mediated by
a second SNMP (proxy) agent.
This example configuration is highly similar to that presented
in the discussion of SNMP foreign proxy in the previous
section. In this example, however, the party associated with
the identity curly receives messages via the SNMP, and,
accordingly interacts with the SNMP proxy agent moe using
authenticated SNMP communications.
Table 9 presents information about SNMP parties that is
recorded in the local database of the SNMP proxy agent.
Table 7 presents information about SNMP parties that is
recorded in the local database of the SNMP management
station. Table 10 presents information about the access policy
specified by the local administration.
As represented in Table 9, the proxy agent party operates at
UDP port 161 at IP address 1.2.3.5 using the party identity
moe; the example manager operates at UDP port 2003 at
IP address 1.2.3.4 using the identity larry; the proxied party
operates at UDP port 161 at IP address 1.2.3.6 using the
party identity curly. All three SNMP parties authenticate
all messages that they generate as to origin and integrity by
using the authentication protocol md4AuthProt and their
distinct, private authentication keys. Although these private
key values ("ABCD," "EFGH," and "JKLM") are presented
here for expository purposes, knowledge of private keys is not
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Target Subject Privileges
moe larry 3
larry moe 20
Table 8: Access Information for Foreign Proxy
Identity larry moe curly
(manager) (proxy) (proxied)
Domain rfc1157 rfc1157 rfc1157
Address 1.2.3.4, 2003 1.2.3.5, 161 1.2.3.6, 161
Proxied Party noProxy curly noProxy
Auth Prot md4AuthProt md4AuthProt md4AuthProt
Auth Priv Key "ABCD" "EFGH" "JKLM"
Auth Pub Key "" "" ""
Auth Clock 0 0 0
Auth Last Msg 0 0 0
Auth Lifetime 500 500 500
Priv Prot noPriv noPriv noPriv
Priv Priv Key "" "" ""
Priv Pub Key "" "" ""
Table 9: Party Information for Proxy Agent
Target Subject Privileges
moe larry 3
larry moe 20
curly moe 3
moe curly 20
Table 10: Access Information for Native Proxy
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normally afforded to human beings and is confined to those
portions of the protocol implementation that require it.
In order to interrogate the proxied device associated with the
party curly, the management station larry constructs a SNMP
GetNext request and transmits it to the party moe operating
(see Table 7) at UDP port 161, and IP address 1.2.3.5. This
request is authenticated using the private authentication key
"ABCD."
When that request is received by the party moe, the originator
of the message is verified as being the party larry by using
local knowledge (see Table 9) of the private authentication key
"ABCD." Because party larry is authorized to issue GetNext
(and Get) requests with respect to party moe by the relevant
access control policy (Table 10), the request is accepted.
Because the local database records the proxied party for party
moe as curly, the request is satisfied by its translation into
a corresponding SNMP GetNext request directed from party
moe to party curly. This new communication is authenticated
using the private authentication key "EFGH" and transmitted
to party curly at the IP address 1.2.3.6.
When this new request is received by the party curly, the
originator of the message is verified as being the party moe by
using local knowledge (see Table 9) of the private authentication
key "EFGH." Because party moe is authorized to issue
GetNext (and Get) requests with respect to party curly by
the relevant access control policy (Table 10), the request is
accepted. Because the local database records the proxied party
for party curly as noProxy, the GetNext request is satisfied
by local mechanisms. A SNMP Get response representing
the results of the query is then generated by party curly.
This response communication is authenticated as to origin and
integrity using the private authentication key "JKLM" and
transmitted to party moe at IP address 1.2.3.5 (the source
address for the corresponding request).
When this response is received by party moe, the originator
of the message is verified as being the party curly by using
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local knowledge (see Table 9) of the private authentication
key "JKLM." Because party curly is authorized to issue Get
responses with respect to party moe by the relevant access
control policy (Table 10), the response is not rejected. Instead,
it is translated into a GetNext response that corresponds to
the original GetNext request from party larry. This response
is authenticated as to origin and integrity using the private
authentication key "EFGH" and is transmitted to the party
larry at IP address 1.2.3.5 (the source address for the original
request).
When this response is received by the party larry, the
originator of the message is verified as being the party moe by
using local knowledge (see Table 7) of the private authentication
key "EFGH." Because party moe is authorized to issue Get
responses with respect to party larry by the relevant access
control policy (Table 10), the response is accepted, and the
interrogation is complete.
3.4 Public Key Configuration
This section presents an example configuration predicated upon
a hypothetical security protocol. This hypothetical protocol
would be based on asymmetric (public key) cryptography
as a means for providing data origin authentication (but
not protection against disclosure). This example illustrates
the consistency of the administrative model with public key
technology, and the extension of the example to support
protection against disclosure should be apparent.
The example configuration comprises a single SNMP agent that
interacts with a single SNMP management station. Tables 11
and 12 present information about SNMP parties that is by
the agent and manager, respectively, while Table 5 presents
information about the local access policy that is known to both
manager and agent.
As represented in Table 11, the example agent party
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Identity ollie stan
(agent) (manager)
Domain rfc1157 rfc1157
Address 1.2.3.4, 161 1.2.3.5, 2004
Proxied Party noProxy noProxy
Auth Prot pkAuthProt pkAuthProt
Auth Priv Key "ABCD" ""
Auth Pub Key "" "efgh"
Auth Clock 0 0
Auth Last Msg 0 0
Auth Lifetime 500 500
Priv Prot noPriv noPriv
Priv Priv Key "" ""
Priv Pub Key "" ""
Table 11: Party Information for Public Key Agent
Identity ollie stan
(agent) (manager)
Domain rfc1157 rfc1157
Address 1.2.3.4, 161 1.2.3.5, 2004
Proxied Party noProxy noProxy
Auth Prot pkAuthProt pkAuthProt
Auth Priv Key "" "EFGH"
Auth Pub Key "abcd" ""
Auth Clock 0 0
Auth Last Msg 0 0
Auth Lifetime 500 500
Priv Prot noPriv noPriv
Priv Priv Key "" ""
Priv Pub Key "" ""
Table 12: Party Information for Public Key Management
Station
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operates at UDP port 161 at IP address 1.2.3.4 using
the party identity ollie; the example manager operates at
UDP port 2004 at IP address 1.2.3.5 using the identity
stan. Both ollie and stan authenticate all messages
that they generate as to origin and integrity by using the
hypothetical SNMP authentication protocol pkAuthProt and
their distinct, private authentication keys. Although these
private authentication key values ("ABCD" and "EFGH") are
presented here for expository purposes, knowledge of private
keys is not normally afforded to human beings and is confined
to those portions of the protocol implementation that require
it.
In most respects, the interaction between manager and agent
in this configuration is almost identical to that in the example
of the minimal, secure SNMP agent described above. The
most significant difference is that neither SNMP party in the
public key configuration has knowledge of the private key by
which the other party authenticates its transmissions as to their
origin and integrity. Instead, for each received authenticated
SNMP communication, the identity of the originator is verified
by applying an asymmetric cryptographic algorithm to the
received message together with the public authentication key
for the originating party. Thus, in this configuration, the agent
knows the manager's public key ("efgh") but not its private key
("EFGH"); similarly, the manager knows the agent's public key
("abcd") but not its private key ("ABCD").
For simplicity, privacy protocols are not addressed in this
example configuration, although their use would be necessary
to the secure, automated distribution of secret keys.
4 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
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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.
5 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.
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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.
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References
[1] Jeffrey D. Case, Mark S. Fedor, Martin L. Schoffstall, and
James R. Davin. A Simple Network Management Protocol.
Request for Comments 1157, DDN Network Information
Center, SRI International, May 1990.
[2] Marshall T. Rose and Keith McCloghrie. Structure and
Identification of Management Information for TCP/IP
based internets. Request for Comments 1155, DDN Network
Information Center, SRI International, May 1990.
[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.
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Contents
1 Introduction 1
2 Elements of the Model 2
2.1 SNMP Party 2
2.2 SNMP Protocol Entity 6
2.3 SNMP Management Station 6
2.4 SNMP Agent 7
2.5 View Subtree 7
2.6 MIB View 7
2.7 SNMP Management Communication 8
2.8 SNMP Authenticated Management Communica-
tion 9
2.9 SNMP Private Management Communication 10
2.10 SNMP Management Communication Class 11
2.11 SNMP Access Control Policy 12
2.12 SNMP Proxy Party 13
2.13 Procedures 15
2.13.1 Generating a Request 15
2.13.2 Processing a Received Communication 16
2.13.3 Generating a Response 19
3 Application of the Model 20
3.1 Non-Secure Minimal Agent Configuration 20
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3.2 Secure Minimal Agent Configuration 23
3.3 Proxy Configuration 25
3.3.1 Foreign Proxy Configuration 26
3.3.2 Native Proxy Configuration 30
3.4 Public Key Configuration 33
4 Compatibility 35
5 Security Considerations 36
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