RFC1303 - A Convention for Describing SNMP-based Agents

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　　Network Working Group K. McCloghrie
Request For Comments: 1303 Hughes LAN Systems
M. Rose
Dover Beach Consulting
February 1992

A Convention for Describing SNMP-based Agents

Status of This Memo

This memo provides information for the Internet community. It does
not specify an Internet standard. Distribution of this memo is
unlimited.

Abstract

This memo suggests a straight-forward approach towards describing
SNMP-based agents.

Table of Contents

1. The Network Management Framework ............................ 2
2. Objects ..................................................... 2
3. Describing Agents ........................................... 3
3.1 Definitions ................................................ 4
3.2 Mapping of the MODULE-CONFORMANCE macro .................... 5
3.2.1 Mapping of the LAST-UPDATED clause ....................... 6
3.2.2 Mapping of the PRODUCT-RELEASE clause .................... 6
3.2.3 Mapping of the DESCRIPTION clause ........................ 6
3.2.4 Mapping of the SUPPORTS clause ........................... 6
3.2.4.1 Mapping of the INCLUDES clause ......................... 6
3.2.4.2 Mapping of the VARIATION clause ........................ 6
3.2.4.2.1 Mapping of the SYNTAX clause ......................... 6
3.2.4.2.2 Mapping of the WRITE-SYNTAX clause ................... 7
3.2.4.2.3 Mapping of the ACCESS clause ......................... 7
3.2.4.2.4 Mapping of the CREATION-REQUIRES clause .............. 7
3.2.4.2.5 Mapping of the DEFVAL clause ......................... 7
3.2.4.2.6 Mapping of the DESCRIPTION clause .................... 7
3.3 Refined Syntax ............................................. 7
3.4 Usage Example .............................................. 8
4. Acknowledgements ............................................ 11
5. References .................................................. 11
6. Security Considerations...................................... 12
7. Authors' Addresses........................................... 12

1. The Network Management Framework

The Internet-standard Network Management Framework consists of
three components. They are:

RFC1155 [1] which defines the SMI, the mechanisms used for
describing and naming objects for the purpose of management.
RFC1212 [2] defines a more concise description mechanism,
which is wholly consistent with the SMI.

RFC1213 [3] which defines MIB-II, the core set of managed
objects for the Internet suite of protocols.

RFC1157 [4] which defines the SNMP, the protocol used for
network access to managed objects.

The Framework permits new objects to be defined for the
purpose of experimentation and evaluation.

2. Objects

Managed objects are accessed via a virtual information store,
termed the Management Information Base or MIB. Within a given
MIB module, objects are defined using RFC1212's OBJECT-TYPE
macro. At a minimum, each object has a name, a syntax, an
access-level, and an implementation-status.

The name is an object identifier, an administratively assigned
name, which specifies an object type. The object type
together with an object instance serves to uniquely identify a
specific instantiation of the object. For human convenience,
we often use a textual string, termed the OBJECT DESCRIPTOR,
to also refer to the object type.

The syntax of an object type defines the abstract data
structure corresponding to that object type. The ASN.1[5]
language is used for this purpose. However, RFC1155
purposely restricts the ASN.1 constructs which may be used.
These restrictions are explicitly made for simplicity.

The access-level of an object type defines whether it makes
"protocol sense" to read and/or write the value of an instance
of the object type. (This access-level is independent of any
administrative authorization policy.)

The implementation-status of an object type indicates whether
the object is mandatory, optional, obsolete, or deprecated.

3. Describing Agents

When a MIB module is written, it is divided into units of
conformance termed groups. If an agent claims conformance to
a group, then it must implement each and every object within
that group. Of course, for whatever reason, an agent may
implement only a subset of the groups within a MIB module. In
addition, the definition of some MIB objects leave some
aspects of the definition to the discretion of an implementor.

Practical experience has demonstrated a need for concisely
describing the capabilities of an agent with regards to the
MIB groups that it implements. This memo defines a new macro,
MODULE-CONFORMANCE, which allows an agent implementor to
describe the precise level of support which an agent claims in
regards to a MIB group, and to bind that description to the
sysObjectID associated with the agent. In particular, some
objects may have restricted or augmented syntax or access-
levels.

If the MODULE-CONFORMANCE invocation is given to a
management-station implementor, then that implementor can
build management applications which optimize themselves when
communicating with a particular agent. For example, the
management-station can maintain a database of these
invocations. When a management-station interacts with an
agent, it retrieves the agent's sysObjectID. Based on this,
it consults the database. If an entry is found, then the
management application can optimize its behavior accordingly.

This binding to sysObjectId may not always suffice to define
all MIB objects to which an agent can provide access. In
particular, this situation occurs where the agent dynamically
learns of the objects it supports, for example, an agent
supporting SMUX peers via the SMUX protocol [6]. In these
situations, additional MIB objects beyond sysObjectID must be
used to name other invocations of the MODULE-CONFORMANCE macro
to augment the description of MIB support provided by the
agent. For example, if an agent's sysObjectID indicates that
it supports the SMUX MIB, then each instance of smuxPidentity
will indicate another MODULE-CONFORMANCE invocation which is
dynamically being supported by the agent.

3.1. Definitions

RFC-1303 DEFINITIONS ::= BEGIN

IMPORTS
ObjectName, ObjectSyntax
FROM RFC1155-SMI
DisplayString
FROM RFC1213-MIB;

MODULE-CONFORMANCE MACRO ::=
BEGIN
TYPE NOTATION ::=
"LAST-UPDATED"
value(update UTCTime)
"PRODUCT-RELEASE"
value(release DisplayString)
"DESCRIPTION"
value(description DisplayString)
ModulePart

VALUE NOTATION ::= -- agent's sysObjectID --
value(VALUE ObjectName)

ModulePart ::= Modules
| empty

Modules ::= Module
| Modules Module

Module ::= -- name of module --
"SUPPORTS" ModuleName
"INCLUDES" "{" Groups "}"
VariationPart

ModuleName ::= identifier ModuleIdentifier

ModuleIdentifier ::=
value (moduleID OBJECT IDENTIFIER)
| empty

Groups ::= Group
| Groups "," Group

Group ::= value(group OBJECT IDENTIFIER)

VariationPart ::= Variations
| empty

Variations ::= Variation
| Variations Variation

Variation ::= "VARIATION" value(object ObjectName)
Syntax WriteSyntax Access
Creation DefaultValue
"DESCRIPTION"
value(limitext DisplayString)

-- must be a refinement for object's SYNTAX
Syntax ::= "SYNTAX" type(SYNTAX)
| empty

-- must be a refinement for object's SYNTAX
WriteSyntax ::= "WRITE-SYNTAX" type(WriteSYNTAX)
| empty

Access ::= "ACCESS" AccessValue
| empty

AccessValue ::= "read-only"
| "read-write"
| "write-only"
| "not-accessible"

Creation ::= "CREATION-REQUIRES" "{" Cells "}"

Cells ::= Cell
| Cells "," Cell

Cell ::= value(cell ObjectName)

DefaultValue ::= "DEFVAL"
"{" value (defval ObjectSyntax) "}"
| empty

END

END

3.2. Mapping of the MODULE-CONFORMANCE macro

It should be noted that the expansion of the MODULE-CONFORMANCE macro
is something which conceptually happens during implementation and not
during run-time.

3.2.1. Mapping of the LAST-UPDATED clause

The LAST-UPDATED clause, which must be present, contains the date and
time that this definition was last edited.

3.2.2. Mapping of the PRODUCT-RELEASE clause

The PRODUCT-RELEASE clause, which must be present, contains a textual
description of the product release which includes this agent.

3.2.3. Mapping of the DESCRIPTION clause

The DESCRIPTION clause, which must be present, contains a textual
description of this agent.

3.2.4. Mapping of the SUPPORTS clause

The SUPPORTS clause, which need not be present, is repeatedly used to
name each MIB module for which the agent claims a complete or partial
implementation. Each MIB module is named by its module name, and
optionally, by its associated OBJECT IDENTIFIER as well. (Note that
only a few MIB modules have had OBJECT IDENTIFIERs assigned to them.)

3.2.4.1. Mapping of the INCLUDES clause

The INCLUDES clause, which must be present for each use of the
SUPPORTS clause, is used to name each MIB group associated with the
SUPPORT clause, which the agent claims to implement.

3.2.4.2. Mapping of the VARIATION clause

The VARIATION clause, which need not be present, is repeatedly used
to name each MIB object which the agent implements in some variant or
refined fashion.

3.2.4.2.1. Mapping of the SYNTAX clause

The SYNTAX clause, which need not be present, is used to provide a
refined SYNTAX for the object named in the correspondent VARIATION
clause. Note that if this clause and a WRITE-SYNTAX clause are both
present, then this clause only applies when instances of the object
named in the correspondent VARIATION clause are read.

Consult Section 3.3 for more information on refined syntax.

3.2.4.2.2. Mapping of the WRITE-SYNTAX clause

The WRITE-SYNTAX clause, which need not be present, is used to
provide a refined SYNTAX for the object named in the correspondent
VARIATION clause when instances of that object are written.

Consult Section 3.3 for more information on refined syntax.

3.2.4.2.3. Mapping of the ACCESS clause

The ACCESS clause, which need not be present, is used to provide a
refined ACCESS for the object named in the correspondent VARIATION
clause.

3.2.4.2.4. Mapping of the CREATION-REQUIRES clause

The CREATION-REQUIRES clause, which need not be present, is used to
name the columnar objects of a conceptual row to which values must be
explicitly assigned, by a SNMP Set operation, before the agent will
create new instances of objects in that row. This clause must not be
present unless the object named in the correspondent VARIATION clause
is a conceptual row, i.e., has a syntax which resolves to a SEQUENCE
containing columnar objects. The objects named in the value of this
clause usually will refer to columnar objects in that row. However,
objects unrelated to the conceptual row may also be specified.

The absence of this clause for a particular conceptual row indicates
that the agent does not support the creation, via SNMP operations, of
new object instances in that row.

3.2.4.2.5. Mapping of the DEFVAL clause

The DEFVAL clause, which need not be present, is used to provide a
refined DEFVAL value for the object named in the correspondent
VARIATION clause. The semantics of this value are identical to those
of the OBJECT-TYPE macro's DEFVAL clause [2].

3.2.4.2.6. Mapping of the DESCRIPTION clause

The DESCRIPTION clause, which must be present for each use of the
VARIATION clause, contains a textual description of the variant or
refined implementation.

3.3. Refined Syntax

The SYNTAX and WRITE-SYNTAX clauses allow an object's Syntax to be
refined. However, not all refinements of syntax are appropriate. In
particular,

(1) the object's primitive or application type (as defined in
the SMI [1]) must not be changed;

(2) an object defined with an SMI type of OBJECT IDENTIFIER,
IpAddress, Counter, or TimeTicks cannot be refined; and,

(3) an object defined to have a specific set of values (e.g.,
an INTEGER with named values) cannot have additional
values defined for it.

3.4. Usage Example

Consider how one might document the 4BSD/ISODE "Secure" SNMP
agent:

FourBSD-ISODE DEFINITIONS ::= BEGIN

IMPORTS
MODULE-CONFORMANCE
FROM RFC-1303
-- everything --
FROM RFCxxxx-MIB
-- everything --
FROM RFC1213-MIB
-- everything --
FROM UNIX-MIB
-- everything --
FROM EVAL-MIB;

fourBSD-isode-support MODULE-CONFORMANCE
LAST-UPDATED "9201252354Z"
PRODUCT-RELEASE "ISODE 7.0 + 'Secure' SNMP
upgrade for SunOS 4.1"
DESCRIPTION "4BSD/ISODE 'Secure' SNMP"

SUPPORTS RFCxxxx-MIB -- SNMP Party MIB --
INCLUDES { partyPublic, partyPrivate }

SUPPORTS RFC1213-MIB
INCLUDES { system, interfaces, at, ip, icmp,
tcp, udp, snmp }

VARIATION atEntry
CREATION-REQUIRES { atPhysAddress }
DESCRIPTION "Address mappings on 4BSD require
both protocol and media addresses"

VARIATION ifAdminStatus

SYNTAX INTEGER { up(1), down(2) }
DESCRIPTION "Unable to set test mode on 4BSD"

VARIATION ifOperStatus
SYNTAX INTEGER { up(1), down(2) }
DESCRIPTION "Information limited on 4BSD"

VARIATION ipDefaultTTL
SYNTAX INTEGER { maxttl(255) }
DESCRIPTION "Hard-wired on 4BSD"

VARIATION ipInAddrErrors
ACCESS not-accessible
DESCRIPTION "Information not available on 4BSD"

VARIATION ipInDiscards
ACCESS not-accessible
DESCRIPTION "Information not available on 4BSD"

VARIATION ipRouteEntry
CREATION-REQUIRES { ipRouteNextHop }
DESCRIPTION "Routes on 4BSD require both
destination and next-hop"

VARIATION ipRouteType
SYNTAX INTEGER { direct(3), indirect(4) }
WRITE-SYNTAX INTEGER { invalid(2), direct(3),
indirect(4) }
DESCRIPTION "Information limited on 4BSD"

VARIATION ipRouteProto
SYNTAX INTEGER { other(1), icmp (4) }
DESCRIPTION "Information limited on 4BSD"

VARIATION ipRouteAge
ACCESS not-accessible
DESCRIPTION "Information not available on 4BSD"

VARIATION ipNetToMediaEntry
CREATION-REQUIRES { ipNetToMediaPhysAddress }
DESCRIPTION "Address mappings on 4BSD require
both protocol and media addresses"

VARIATION ipNetToMediaType
SYNTAX INTEGER { dynamic(3), static(4) }
WRITE-SYNTAX INTEGER { invalid(2), dynamic(3),
static(4) }
DESCRIPTION "Information limited on 4BSD"

VARIATION tcpConnState
ACCESS read-only
DESCRIPTION "Unable to set this on 4BSD"

VARIATION tcpInErrs
ACCESS not-accessible
DESCRIPTION "Information not available on 4BSD"

VARIATION tcpOutRsts
ACCESS not-accessible
DESCRIPTION "Information not available on 4BSD"

SUPPORTS UNIX-MIB
INCLUDES { agents, mbuf, netstat }

SUPPORTS EVAL-MIB
INCLUDES { functions, expressions }

::= { fourBSD-isode 6 6 }

END

According to this invocation, an agent with a sysObjectID of

{ fourBSD-isode 6 6 }

supports four MIB modules.

From the SNMP Party MIB, all the objects contained in the partyPublic
and partyPrivate groups are supported, without variation.

From MIB-II, all groups except the egp group are supported. However,
the objects

ipInAddrErrors
ipInDiscards
ipRouteAge
tcpInErrs
tcpOutRsts

are not available, whilst the objects

ifAdminStatus
ifOperStatus
ipDefaultTTL
ipRouteType
ipRouteProto
ipNetToMediaType

have a restricted syntax, and the object

tcpConnState

is available only for reading. Note that in the case of the objects

ipRouteType
ipNetToMediaType

the set of values which may be read is different than the set of
values which may be written. Finally, when creating a new row in the
atTable, the set-request must create an instance of atPhysAddress.
Similarly, when creating a new row in the ipRouteTable, the set-
request must create an instance of ipRouteNextHop. Similarly, when
creating a new row in the ipNetToMediaTable, the set-request must
create an instance of ipNetToMediaPhysAddress.

From the UNIX-MIB, all the objects contained in the agents, mbuf, and
netstat groups are supported, without variation.

From the EVAL-MIB, all the objects contained in the functions and
expressions groups are supported, without variation.

4. Acknowledgements

The authors gratefully acknowledge the comments of Mark van der Pol
of Hughes LAN Systems, and David T. Perkins of SynOptics
Communications.

5. References

[1] Rose M., and K. McCloghrie, "Structure and Identification of
Management Information for TCP/IP-based internets", RFC1155,
Performance Systems International, Hughes LAN Systems, May 1990.

[2] Rose, M., and K. McCloghrie, Editors, "Concise MIB Definitions",
RFC1212, Performance Systems International, Hughes LAN Systems,
March 1991.

[3] McCloghrie K., and M. Rose, Editors, "Management Information
Base forNetwork Management of TCP/IP-based internets", RFC1213,
Performance Systems International, March 1991.

[4] Case, J., Fedor, M., Schoffstall, M., and J. Davin, "Simple
Network Management Protocol (SNMP), RFC1157, SNMP Research,
Performance Systems International, Performance Systems
International, MIT Laboratory for Computer Science, May 1990.

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

[6] Rose, M., "SNMP MUX Protocol and MIB", RFC1227, Performance
Systems International, May 1991.

6. Security Considerations

Security issues are not discussed in this memo.

7. Authors' Addresses

Keith McCloghrie
Hughes LAN Systems
1225 Charleston Road
Mountain View, CA 94043

Phone: (415) 966-7934
EMail: kzm@hls.com

Marshall T. Rose
Dover Beach Consulting, Inc.
420 Whisman Court
Mountain View, CA 94043-2112

Phone: (415) 968-1052
EMail: mrose@dbc.mtview.ca.us

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