Internet Engineering Task Force (IETF) M. Ellison
Request for Comments: 5935 Ellison Software Consulting
Category: Standards Track B. Natale
ISSN: 2070-1721 MITRE
August 2010
Expressing SNMP SMI Datatypes in XML Schema Definition Language
Abstract
This memo defines the IETF standard expression of Structure of
Management Information (SMI) base datatypes in XML Schema Definition
(XSD) language. The primary objective of this memo is to enable the
production of XML documents that are as faithful to the SMI as
possible, using XSD as the validation mechanism.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5935.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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RFC 5935 Expressing SNMP SMI Datatypes in XSD August 2010
Table of Contents
1. Introduction ....................................................2
2. Conventions .....................................................4
3. Requirements ....................................................4
4. XSD for SMI Base Datatypes ......................................5
5. Rationale .......................................................8
5.1. Numeric Datatypes ..........................................8
5.2. OctetString ................................................9
5.3. Opaque ....................................................10
5.4. IpAddress .................................................10
5.5. ObjectIdentifier ..........................................10
6. Security Considerations ........................................11
7. IANA Considerations ............................................11
7.1. SMI Base Datatypes Namespace Registration .................12
7.2. SMI Base Datatypes Schema Registration ....................12
8. Acknowledgements ...............................................12
9. References .....................................................13
9.1. Normative References ......................................13
9.2. Informative References ....................................13
1. Introduction
Numerous use cases exist for expressing the management information
described by SMI Management Information Base (MIB) modules in XML
[XML]. Potential use cases reside both outside and within the
traditional IETF network management community. For example,
developers of some XML-based management applications may want to
incorporate the rich set of data models provided by MIB modules.
Developers of other XML-based management applications may want to
access MIB module instrumentation via gateways to SNMP agents. Such
applications benefit from the IETF standard mapping of SMI datatypes
to XML datatypes via XSD [XMLSchema], [XSDDatatypes].
MIB modules use SMIv2 [RFC2578] to describe data models. For legacy
MIB modules, SMIv1 [RFC1155] was used. MIB data conveyed in variable
bindings ("varbinds") within protocol data units (PDUs) of SNMP
messages use the primitive, base datatypes defined by the SMI.
The SMI allows for the creation of derivative datatypes, "textual
conventions" ("TCs") [RFC2579]. A TC has a unique name, has a syntax
that either refines or is a base SMI datatype, and has relatively
precise application-level semantics. TCs facilitate correct
application-level handling of MIB data, improve readability of MIB
modules by humans, and support appropriate renderings of MIB data.
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Values in varbinds corresponding to MIB objects defined with TC
syntax are always encoded as the base SMI datatype underlying the TC
syntax. Thus, the XSD mappings defined in this memo provide support
for values of MIB objects defined with TC syntax as well as for
values of MIB objects defined with base SMI syntax. Using the
translation of TC into base SMI datatypes any MIB module that uses
TCs can be mapped into XSD using the mappings defined in this memo.
For example, for IP addresses (both IPv4 and IPv6), MIB objects
defined using the InetAddress TC (as per [RFC4001]) are encoded using
the base SMI datatype underlying the InetAddress TC syntax rather
than the IpAddress base datatype.
Various independent schemes have been devised for expressing SMI
datatypes in XSD. These schemes exhibit a degree of commonality,
especially concerning numeric SMI datatypes, but these schemes also
exhibit sufficient differences, especially concerning the non-numeric
SMI datatypes, precluding uniformity of expression and general
interoperability.
Throughout this memo, the term "fidelity" refers to the quality of an
accurate, consistent representation of SMI data values and the term
"faithful" refers to the quality of reliably reflecting the semantics
of SMI data values. Thus defined, the characteristics of fidelity
and being faithful are essential to uniformity of expression and
general interoperability in the XML representation of SMI data
values.
The primary purpose of this memo is to define the standard expression
of SMI base datatypes in XML documents that is both uniform and
interoperable. This standard expression enables Internet operators,
management application developers, and users to benefit from a wider
range of management tools and to benefit from a greater degree of
unified management. Thus, standard expression enables and
facilitates improvements to the timeliness, accuracy, and utility of
management information.
The overall objective of this memo, and of any related future memos
as may be published, is to define the XSD equivalent [XSDDatatypes]
of SMIv2 (STD 58) and to encourage XML-based protocols to carry, and
XML-based applications to use, the management information defined in
SMIv2-compliant MIB modules. The use of a standard mapping from
SMIv2 to XML via XSD validation enables and promotes the efficient
reuse of existing and future MIB modules and instrumentation by XML-
based protocols and management applications.
Developers of certain XML-based management applications will find
this specification sufficient for their purposes. Developers of
other XML-based management applications may need to make more
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complete reuse of existing MIB modules, requiring standard XSD
documents for TCs [RFC2579] and MIB structure [RFC2578]. Memos
supporting such requirements are planned, but have not been produced
at the time of this writing.
Finally, it is worthwhile to note that the goal of fidelity to the
SMIv2 standard (STD 58), as specified in the "Requirements" section
below, is crucial to this effort. Fidelity leverages the established
"rough consensus" of the precise SMIv2 data models contained in MIB
modules, and leverages existing instrumentation, the "running code"
implementing SMIv2 data models. This effort does not include any
redesign of SMIv2 datatypes, data structures or textual conventions
in order to overcome known limitations. Such work can be pursued by
other efforts.
2. Conventions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
3. Requirements
The following set of requirements is intended to produce XML
documents that can be validated via the XSD defined in this
specification to faithfully represent values carried "on-the-wire" in
SNMP PDUs as defined by the SMI:
R1. All SMI base datatypes MUST have a corresponding XSD datatype.
R2. SMIv2 is the normative SMI for this document. Prior to mapping
datatypes into XSD, legacy SMIv1 modules MUST be converted (at
least logically) in accordance with Section 2.1, inclusive, of
the "Coexistence" RFC [RFC3584].
R3. The XSD datatype specified for a given SMI datatype MUST be able
to represent all valid values for that SMI datatype.
R4. The XSD datatype specified for a given SMI datatype MUST
represent any special encoding rules associated with that SMI
datatype.
R5. The XSD datatype specified for a given SMI datatype MUST include
any restrictions on values associated with the SMI datatype.
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R6. The XSD datatype specified for a given SMI datatype MUST be the
most logical XSD datatype, with the fewest necessary
restrictions on its set of values, consistent with the foregoing
requirements.
R7. The XML output produced as a result of meeting the foregoing
requirements SHOULD be the most coherent and succinct
representation (i.e., avoiding superfluous "decoration") from
the perspective of readability by humans.
4. XSD for SMI Base Datatypes
This document provides XSD datatype mappings for the SMIv2 base
datatypes only -- i.e., the eleven "ObjectSyntax" datatypes defined
in RFC 2578. These datatypes -- via tag values defined in the SMIv2
to identify them in varbinds -- constrain values carried "on-the-
wire" in SNMP PDUs between SNMP management applications and SNMP
agents:
o INTEGER, Integer32
o Unsigned32, Gauge32
o Counter32
o TimeTicks
o Counter64
o OCTET STRING
o Opaque
o IpAddress
o OBJECT IDENTIFIER
The "BITS" pseudo-type (also referred to as a "construct" in RFC
2578) is treated as a Textual Convention, not a base datatype, for
the purpose of this document.
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BEGIN
Mapping of SMIv2 base datatypes from RFC 2578
Contact: Mark Ellison
Organization: Ellison Software Consulting
Address: 38 Salem Road
Atkinson, NH 03811
USA
Telephone: +1 603-362-9270
E-Mail: ietf@EllisonSoftware.com
Contact: Bob Natale
Organization: MITRE
Address: 300 Sentinel Drive
6th Floor
Annapolis Junction, MD 20701
USA
Telephone: +1 301-617-3008
E-Mail: rnatale@mitre.org
Last Updated: 201002260000Z
Copyright (c) 2010 IETF Trust and the persons identified as
authors of the code. All rights reserved.
Redistribution and use in source and binary forms, with or
without modification, is permitted pursuant to, and subject
to the license terms contained in, the Simplified BSD License
set forth in Section 4.c of the IETF Trust's Legal Provisions
Relating to IETF Documents
(http://trustee.ietf.org/license-info).
This version of this XML Schema Definition (XSD)
document is part of RFC 5935; see the RFC itself for
full legal notices.
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END
5. Rationale
The XSD datatypes, including any specified restrictions, were chosen
based on fit with the requirements specified earlier in this
document, and with attention to simplicity while maintaining fidelity
to the SMI. Also, the "canonical representations" (i.e., refinements
of the "lexical representations") documented in the W3C XSD
specification [XMLSchema], [XSDDatatypes] are assumed.
5.1. Numeric Datatypes
All of the numeric XSD datatypes specified in the previous section --
INTEGER, Integer32, Unsigned32, Gauge32, Counter32, TimeTicks, and
Counter64 -- comply with the relevant requirements
o They cover all valid values for the corresponding SMI datatypes.
o They comply with the standard encoding rules associated with the
corresponding SMI datatypes.
o They inherently match the range restrictions associated with the
corresponding SMI datatypes.
o They are the most direct XSD datatypes that exhibit the foregoing
characteristics relative to the corresponding SMI datatypes (which
is why no "restriction" statements -- other than the "base" XSD
type -- are required in the XSD).
o The XML output produced from the canonical representation of these
XSD datatypes is also the most direct from the perspective of
readability by humans (i.e., no leading "+" sign and no leading
zeros).
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Special note to application developers: compliance with this schema
in an otherwise correct translation from raw ("on-the-wire"
representation) SNMP MIB data produces values that are faithful to
the original. However, the Gauge32, Counter32, Counter64, and
TimeTicks datatypes have special application semantics that must be
considered when using their raw values for anything other than
display, printing, storage, or transmission of the literal value.
RFC 2578 provides the necessary details.
5.2. OctetString
This XSD datatype corresponds to the SMI "OCTET STRING" datatype.
Several independent schemes for mapping SMI datatypes to XSD have
used the XSD "string" type to represent "OCTET STRING", but this
mapping does not conform to the requirements specified in this
document. Most notably, "string" cannot faithfully represent all
valid values (0 thru 255) that each octet in an "OCTET STRING" can
have -- or at least cannot do so in a way that provides for easy
human readability of the resulting XML output.
Consequently, the XSD datatype "hexBinary" is specified as the
standard mapping of the SMI "OCTET STRING" datatype. In hexBinary,
each octet is encoded as two hexadecimal digits; the canonical
representation limits the set of allowed hexadecimal digits to 0-9
and uppercase A-F.
The hexBinary representation of "OCTET STRING" complies with the
relevant requirements:
o It covers all valid values for the corresponding SMI datatype.
o It complies with the standard encoding rules associated with the
corresponding SMI datatype.
o With the "maxLength" restriction to 65535 octets, the XSD datatype
specification matches the restrictions associated with the
corresponding SMI datatype.
o It is the most direct XSD datatype that exhibits the foregoing
characteristics relative to the corresponding SMI datatype (which
must allow for any valid binary octet value).
o The XML output produced from the canonical representation of this
XSD datatype is not optimal with respect to readability by humans;
however, that is a consequence of the SMI datatype itself. Where
human readability is more of a concern, it is likely that the
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actual MIB objects in question will be represented by textual
conventions that limit the set of values that will be included in
the OctetStrings and will, thus, bypass the hexBinary typing.
5.3. Opaque
The "hexBinary" XSD datatype is specified as the representation of
the SMI "Opaque" datatype generally for the same reasons as
"hexBinary" is specified for the "OctetString" datatype:
o It covers all valid values for the corresponding SMI datatype.
o It complies with the standard encoding rules associated with the
corresponding SMI datatype.
o There are no restriction issues associated with using "hexBinary"
for "Opaque".
o It is the most direct XSD datatype that exhibits the foregoing
characteristics relative to the corresponding SMI datatype (which
must allow for any valid binary octet value).
o The XML output produced from the canonical representation of this
XSD datatype is not optimal with respect to readability by humans;
however, that is a consequence of the SMI datatype itself.
Unmediated "Opaque" data is intended for consumption by
applications, not humans.
5.4. IpAddress
The XSD "string" datatype is the natural choice to represent an
IpAddress as XML output. The "pattern" restriction applied in this
case results in a dotted-decimal string of four values between "0"
and "255" separated by a period (".") character. This pattern also
precludes leading zeros.
Note that the SMI relies upon Textual Conventions (TCs) to specify an
IPv6 address. As such, the representation of an IPv6 address as an
XSD datatype is beyond the scope of this document.
5.5. ObjectIdentifier
This XSD datatype corresponds to the SMI "OBJECT IDENTIFIER"
datatype.
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The XSD "string" datatype is also the natural choice to represent an
ObjectIdentifier as XML output, for the same reasons as for the
IpAddress choice. The "pattern" restriction applied in this case
results in a dotted-decimal string of up to 128 elements (referred to
as "sub-ids"), each holding an "Unsigned32" integer value.
Note that the first two components of an "OBJECT IDENTIFIER" each
have a limited range of values as indicated in the XSD pattern
restriction and as described in the ASN1.1/BER standard [ASN.1].
There are three values allocated for the root node, and at most 39
values for nodes subordinate to a root node value of 0 or 1.
The minimum length of an "OBJECT IDENTIFIER" is two sub-ids and the
representation of a zero-valued "OBJECT IDENTIFIER" is "0.0".
Note that no explicit "minLength" restriction, which would be "3" to
allow for the minimum of two sub-ids and a single separating dot, is
required since the pattern itself enforces this restriction.
6. Security Considerations
Security considerations for any given SMI MIB module will be relevant
to any XSD/XML mapping of that MIB module; however, the mapping
defined in this document does not itself introduce any new security
considerations.
If and when proxies or gateways are developed to convey SNMP
management information from SNMP agents to XML-based management
applications via XSD/XML mapping of MIB modules based on this
specification and its planned siblings, special care will need to be
taken to ensure that all applicable SNMP security mechanisms are
supported in an appropriate manner yet to be determined.
7. IANA Considerations
In accordance with RFC 3688 [RFC3688], the IANA XML registry has been
updated with the following namespace and schema registrations
associated with this document:
o urn:ietf:params:xml:ns:smi:base:1.0
o urn:ietf:params:xml:schema:base:1.0
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7.1. SMI Base Datatypes Namespace Registration
This document registers a URI for the SMI Base Datatypes XML
namespace in the IETF XML registry. Following the format in RFC
3688, IANA has made the following registration:
URI: urn:ietf:params:xml:ns:smi:base:1.0
Registration Contact: The IESG.
XML: N/A, the requested URI is an XML namespace.
7.2. SMI Base Datatypes Schema Registration
This document registers a URI for the SMI Base Datatypes XML schema
in the IETF XML registry. Following the format in RFC 3688, IANA has
made the following registration:
URI: urn:ietf:params:xml:schema:smi:base:1.0
Registration Contact: The IESG.
XML: Section 4 of this document.
8. Acknowledgements
Dave Harrington provided strategic and technical leadership to the
team that developed this particular specification. Yan Li did much
of the research into existing approaches that was used as a baseline
for the recommendations in this particular specification.
This document owes much to "Datatypes for Netconf Data Models"
[NETCONF-DATATYPES] and to many other sources (including libsmi and
group discussions on the NETCONF mailing lists) developed by those
who have researched and published candidate mappings of SMI datatypes
to XSD.
Individuals who participated in various discussions of this topic at
IETF meetings and on IETF mailing lists include: Ray Atarashi,
Yoshifumi Atarashi, Andy Bierman, Sharon Chisholm, Avri Doria, Rob
Ennes, Mehmet Ersue, David Harrington, Alfred Hines, Eliot Lear,
Chris Lonvick, Faye Ly, Randy Presuhn, Juergen Schoenwaelder, Andrea
Westerinen, and Bert Wijnen.
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9. References
9.1. Normative References
[RFC1155] Rose, M. and K. McCloghrie, "Structure and identification
of management information for TCP/IP-based internets",
STD 16, RFC 1155, May 1990.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2578] McCloghrie, K., Ed., Perkins, D., Ed., and J.
Schoenwaelder, Ed., "Structure of Management Information
Version 2 (SMIv2)", STD 58, RFC 2578, April 1999.
[RFC3584] Frye, R., Levi, D., Routhier, S., and B. Wijnen,
"Coexistence between Version 1, Version 2, and Version 3
of the Internet-standard Network Management Framework",
BCP 74, RFC 3584, August 2003.
[XML] World Wide Web Consortium, "Extensible Markup Language
(XML) 1.0", W3C XML, February 1998,
.
[XMLSchema]
World Wide Web Consortium, "XML Schema Part 1: Structures
Second Edition", W3C XML Schema, October 2004,
RFC, FYI, BCP