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PROPOSED STANDARD
Errata Exist
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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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

   <?xml version="1.0" encoding="utf-8"?>
   <xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema"
   xmlns="urn:ietf:params:xml:ns:smi:base:1.0"
   targetNamespace="urn:ietf:params:xml:ns:smi:base:1.0"
   elementFormDefault="qualified"
   attributeFormDefault="unqualified"
   xml:lang="en">

     <xs:annotation>
       <xs:documentation>
           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.

       </xs:documentation>



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     </xs:annotation>

     <xs:simpleType name="INTEGER">
       <xs:restriction base="xs:int"/>
     </xs:simpleType>

     <xs:simpleType name="Integer32">
       <xs:restriction base="xs:int"/>
     </xs:simpleType>

     <xs:simpleType name="Unsigned32">
       <xs:restriction base="xs:unsignedInt"/>
     </xs:simpleType>

     <xs:simpleType name="Gauge32">
       <xs:restriction base="xs:unsignedInt"/>
     </xs:simpleType>

     <xs:simpleType name="Counter32">
       <xs:restriction base="xs:unsignedInt"/>
     </xs:simpleType>

     <xs:simpleType name="TimeTicks">
       <xs:restriction base="xs:unsignedInt"/>
     </xs:simpleType>

     <xs:simpleType name="Counter64">
       <xs:restriction base="xs:unsignedLong"/>
     </xs:simpleType>

     <xs:simpleType name="OctetString">
       <xs:restriction base="xs:hexBinary">
         <xs:maxLength value="65535"/>
       </xs:restriction>
     </xs:simpleType>

     <xs:simpleType name="Opaque">
       <xs:restriction base="xs:hexBinary"/>
     </xs:simpleType>

     <xs:simpleType name="IpAddress">
         <xs:restriction base="xs:string">
             <xs:pattern value=
             "(([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])\.){3}
                ([0-9]|[1-9][0-9]|1[0-9][0-9]|2[0-4][0-9]|25[0-5])"/>
         </xs:restriction>
     </xs:simpleType>




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     <xs:simpleType name="ObjectIdentifier">
       <xs:restriction base="xs:string">
         <xs:pattern value=
         "(([0-1](\.[1-3]?[0-9]))|
           (2\.(0|([1-9]\d*))))
           (\.(0|([1-9]\d*))){0,126}"/>
       </xs:restriction>
     </xs:simpleType>

   </xs:schema>
   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,
              <http://www.w3.org/TR/1998/REC-xml-19980210>.

   [XMLSchema]
              World Wide Web Consortium, "XML Schema Part 1: Structures
              Second Edition", W3C XML Schema, October 2004,
              <http://www.w3.org/TR/xmlschema-1/>.

   [XSDDatatypes]
              World Wide Web Consortium, "XML Schema Part 2: Datatypes
              Second Edition", W3C XML Schema, October 2004,
              <http://www.w3.org/TR/xmlschema-2/>.

9.2.  Informative References

   [ASN.1]    International Organization for Standardization,
              "Information processing systems - Open Systems
              Interconnection - Specification of Basic Encoding Rules
              for Abstract Syntax Notation One (ASN.1)", International
              Standard 8825, December 1987.

   [NETCONF-DATATYPES]
              Romascanu, D., "Datatypes for Netconf Data Models", Work
              in Progress, May 2007.






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   [RFC2579]  McCloghrie, K., Perkins, D., and J. Schoenwaelder,
              "Textual Conventions for SMIv2", STD 58, RFC 2579,
              April 1999.

   [RFC3688]  Mealling, M., "The IETF XML Registry", BCP 81, RFC 3688,
              January 2004.

   [RFC4001]  Daniele, M., Haberman, B., Routhier, S., and J.
              Schoenwaelder, "Textual Conventions for Internet Network
              Addresses", RFC 4001, February 2005.

Authors' Addresses

   Mark Ellison
   Ellison Software Consulting
   38 Salem Road
   Atkinson, NH  03811
   USA

   Phone: +1 603-362-9270
   EMail: ietf@ellisonsoftware.com


   Bob Natale
   MITRE
   300 Sentinel Drive
   6th Floor
   Annapolis Junction, MD  20701
   USA

   Phone: +1 301-617-3008
   EMail: rnatale@mitre.org



















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