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RFC 2926 - Conversion of LDAP Schemas to and from SLP Templates


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Network Working Group                                           J. Kempf
Request for Comments: 2926                        Sun Microsystems, Inc.
Category: Informational                                         R. Moats
                                                            Coreon, Inc.
                                                           P. St. Pierre
                                                  Sun Microsystems, Inc.
                                                          September 2000

          Conversion of LDAP Schemas to and from SLP Templates

Status of this Memo

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

Copyright Notice

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

Abstract

   This document describes a procedure for mapping between Service
   Location Protocol (SLP) service advertisements and lightweight
   directory access protocol (LDAP) descriptions of services.  The
   document covers two aspects of the mapping.  One aspect is mapping
   between SLP service type templates and LDAP directory schema.
   Because the SLP service type template grammar is relatively simple,
   mapping from service type templates to LDAP types is straightforward.
   Mapping in the other direction is straightforward if the attributes
   are restricted to use just a few of the syntaxes defined in RFC 2252.
   If arbitrary ASN.1 types occur in the schema, then the mapping is
   more complex and may even be impossible.  The second aspect is
   representation of service information in an LDAP directory.  The
   recommended representation simplifies interoperability with SLP by
   allowing SLP directory agents to backend into LDAP directory servers.
   The resulting system allows service advertisements to propagate
   easily between SLP and LDAP.

Table of Contents

   1.0 Introduction ................................................  2
   2.0 Mapping SLP Templates to LDAP Schema ........................  3
     2.1 Mapping from SLP Attribute Types to LDAP Attribute Types ..  8
       2.1.1 Integer ...............................................  8
       2.1.2 String ................................................  8
       2.1.3 Boolean ...............................................  9
       2.1.4 Opaque ................................................  9
     2.2 Keyword Attributes ........................................  9
     2.3 Template Flags ............................................  9
       2.3.1 Multi-valued ..........................................  9
       2.3.2 Optional .............................................. 10
       2.3.3 Literal ............................................... 10
       2.3.4 Explicit Matching ..................................... 10
     2.4 Default and Allowed Value Lists ........................... 10
     2.5 Descriptive Text .......................................... 11
     2.6 Generating LDAP Attribute OIDs ............................ 11
     2.7 Example ................................................... 11
   3.0 Attribute Name Conflicts .................................... 15
   4.0 Mapping from Schema to Templates ............................ 15
     4.1 Mapping LDAP Attribute Types to SLP Attribute Types ....... 16
     4.2 Mapping ASN.1 Types to SLP Types .......................... 17
       4.2.1 Integer ............................................... 18
       4.2.2 Boolean ............................................... 18
       4.2.3 Enumerated ............................................ 18
       4.2.4 Object Identifier ..................................... 19
       4.2.5 Octet String .......................................... 19
       4.2.6 Real .................................................. 19
     4.3 Example ASN.1 Schema ...................................... 19
   5.0 Representing SLP Service Advertisements in an LDAP DIT ...... 22
   6.0 Internationalization Considerations ......................... 24
   7.0 Security Considerations ..................................... 24
   8.0 References .................................................. 25
   9.0 Authors' Addresses .......................................... 26
   10.0 Full Copyright Statement ................................... 27

1.0 Introduction

   SLP templates [1] are intended to create a simple encoding of the
   syntactic and semantic conventions for individual service types,
   their attributes, and conventions.  They can easily be generated,
   transmitted, read by humans and parsed by programs, as it is a string
   based syntax with required comments.  Directory schemas serve to
   formalize directory entry structures for use with LDAP [2] These
   directories serve to store information about many types of entities.
   Network services are an example of one such entity.

   Interoperability between SLP and LDAP is important so clients using
   one protocol derive benefit from services registered through the
   other. In addition, LDAP directory servers can serve as the backend
   for SLP directory agents (DAs) if interoperability is possible In
   order to facilitate interoperability, this document creates mappings
   between the SLP template grammar and LDAP directory schema, and
   establishes some conventions for representing service advertisements
   in LDAP directories. The goal of the translation is to allow SLPv2
   queries (which are syntactically and semantically equivalent to
   LDAPv3 string queries [7]) to be submitted to an LDAP directory
   server by an SLP DA backended into LDAP without extensive processing
   by the DA.

   The simple notation and syntactic/semantic attribute capabilities of
   SLP templates map easily into directory schemas, and are easily
   converted into directory schemas, even by automated means.  The
   reverse may not be true. If the LDAP schema contains attributes with
   unrecognized or complex syntaxes, the translation may be difficult or
   impossible.  If, however, the LDAP schema only uses a few of the
   common syntaxes defined in RFC 2252 [8], then the translation is more
   straightforward. In addition, to foster complete bidirectionality,
   the mapping must follow a very specific representation in its DESC
   attributes.

   This document outlines the correct mappings for SLP templates into
   the syntactic representation specified for LDAP directory schema by
   RFC 2252 [8]. This syntax is a subset of the ASN.1/BER described in
   the X.209 specification [9], and is used by the LDAPv3 [2] directory
   schema.  Likewise, rules and guidelines are proposed to facilitate
   consistent mapping of ASN.1 based schemas to be translated in the SLP
   template grammar. Finally, a proposal for a representation of service
   advertisements in LDAP directory services is made that facilitates
   SLP interoperability.

   Except when used as elements in the definition of LDAP schemas, 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 [16].

2.0 Mapping SLP Templates to LDAP Schema

   We define the following abstract object class as the parent class for
   all services.  Any specific service type is a subclass of this, with
   its own attributes:

      ( 1.3.6.1.4.1.6252.2.27.6.2.1
        NAME 'slpService'
        DESC 'parent superclass for SLP services'
        ABSTRACT
        SUP top
        MUST  ( template-major-version-number $
                template-minor-version-number $
                description $
                template-url-syntax $
                service-advert-service-type $
                service-advert-scopes )
        MAY   ( service-advert-url-authenticator $
                service-advert-attribute-authenticator ) )

   The attributes correspond to various parts of the SLP service
   template and SLP service advertisement.

   SLP service type templates begin with four definitions that set the
   context of the template:

      template-type - This defines the service type of the template. The
      service type can be a simple service type, like "service:ftp", an
      abstract service type, like "service:printer" or a concrete
      service type, like "service:printer:lpr". The type name can
      additionally include a naming authority, for example
      "service:printer.sun:local".  The name that appears in this field
      omits the "service:" prefix.

      template-version - A string containing a major and minor version
      number, separated by a period.

      template-description - A block of human readable text describing
      what the service type does.

      template-url-syntax - An ABNF [6] grammar describing the service
      type specific part of the service URL.

   The SLP template-type definition is used as the name of the LDAP
   object class for the template, a subclass of the "slpService" class,
   together with the "service" prefix to indicate that the name is for a
   service. In the translating service type name, colons and the period
   separating the naming authority are converted into hyphens. If the
   template defines an SLP concrete type, the concrete type name is
   used; the abstract type name is never used.  For example, the
   template for "service:printer:lpr" is translated into an LDAP object
   class called "service-printer-lpr". Furthermore, if the type name
   contains a naming authority, the naming authority name must be

   included. For example, the service type name
   "service:printer.sun:local" becomes "service-printer-sun-local".  The
   LDAP object class is always "STRUCTURAL".

   The template-version definition is partitioned into two attributes,
   template-major-version-number and template-minor-version-number. The
   LDAP definition for these attributes is:

      ( 1.3.6.1.4.1.6252.2.27.6.1.1
        NAME 'template-major-version-number'
        DESC 'The major version number of the service type template'
        EQUALITY integerMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
        SINGLE-VALUE
      )

      ( 1.3.6.1.4.1.6252.2.27.6.1.2
        NAME 'template-minor-version-number'
        DESC 'The minor version number of the service type template'
        EQUALITY integerMatch
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
        SINGLE-VALUE
      )

   The template-url-syntax definition in the SLP template is described
   by the following attribute:

      ( 1.3.6.1.4.1.6252.2.27.6.1.3
        NAME 'template-url-syntax'
        DESC 'An ABNF grammar describing the service type
              specific part of the service URL'
        EQUALITY caseExactIA5Match
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.26
        SINGLE-VALUE
      )

   The template-description attribute is translated into the X.520
   standard attribute "description" [3].

   We further establish the convention that SLP template characteristics
   that can't be translated into LDAP are inserted into the DESC field
   of the object class definition. The items are separated by empty
   lines (consisting of two "LINE FEED" characters), are preceded by a
   LINE FEED character, and are tagged at the  beginning of the line to
   indicate what they represent.   This allows the template to be
   reconstructed from the schema by properly parsing the comments.

   The bulk of an SLP template consists of attribute definitions.  There
   are four items in an SLP template attribute definition that need to
   be mapped into LDAP:

      Attribute Name - Since SLPv2 attribute names are defined to be
      compatible with LDAPv3, SLP attributes map directly into LDAP
      attributes with no change. Similarly, LDAP attributes map directly
      to SLP attributes.

      Attribute Type - The SLP attribute type is mapped into the LDAP
      attribute type.

      Attribute Flags - The SLP attribute flags are mapped into
      characteristics of the LDAP attribute definition, or into the DESC
      field if no equivalent LDAP attribute definition characteristic
      occurs.

      Default and Allowed Values - These must be handled by the client
      or a DA enabled to handle templates, as in SLP. For reference,
      however, they should be included in the DESC field of the LDAP
      attribute definition.

      Descriptive Text - The SLP template descriptive text should be
      mapped into the DESC field.

   We discuss mapping of types, flags, default and allowed values, and
   descriptive text in the subsections below.

   OIDs for SLP template conversion schema elements are standardized
   under the enterprise number of SrvLoc.Org (6252) [18].

   For purposes of representing an SLP entry, we also define two
   standardized LDAP syntaxes and attributes with standardized OIDs.

      ( 1.3.6.1.4.1.6252.2.27.6.2.2
        DESC 'SLP Service Type'
      )

   Defines the syntax for the service type name. The syntax is defined
   in the BNF for the service URL in RFC 2609 Section 2.1 [1].

      ( 1.3.6.1.4.1.6252.2.27.6.2.3
        DESC 'SLP Scope'
      )

   Defines the syntax for the scope name. The syntax is defined in the
   BNF for scope names in RFC 2608 Section 6.4.1 [5].

      ( 1.3.6.1.4.1.6252.2.27.6.1.4
        NAME 'service-advert-service-type'
        DESC 'The service type of the service advertisement, including
              the "service:" prefix.'
        EQUALITY caseExactIA5Match
        SYNTAX 1.3.6.1.4.1.6252.2.27.6.2.2
        SINGLE-VALUE
      )

   Defines an attribute for the service type name.

      ( 1.3.6.1.4.1.6252.2.27.6.1.5
        NAME 'service-advert-scopes'
        DESC 'A list of scopes for a service advertisement.'
        EQUALITY caseExactIA5Match
        SYNTAX 1.3.6.1.4.1.6252.2.27.6.2.3
      )

   Defines a multivalued attribute for the scopes.

   Searches for abstract types can be made with an LDAP query that
   wildcards the concrete type. For example, a search for all service
   advertisements of the printer abstract type can be made with the
   following query:

         (service-advert-service-type=service:printer:*)

   SLP specifies that service URLs and attribute lists can be
   accompanied by a structured authenticator consisting of a digital
   signature and information necessary to verify the signature.  A
   syntax and two standardized SLP attributes are defined for this
   purpose:

      ( 1.3.6.1.4.1.6252.2.27.6.2.3 DESC 'SLP Authenticator')

      The syntax of an SLP authenticator is the bytes of the
      authenticator in network byte order, see RFC 2608, Section 9.2
      [5].

      ( 1.3.6.1.4.1.6252.2.27.6.1.6
        NAME 'service-advert-url-authenticator'
        DESC 'The authenticator for the URL, null if none.'
        SYNTAX 1.3.6.1.4.1.6252.2.27.6.2.3
        SINGLE-VALUE
      )

      This attribute contains the SLP URL authenticator, as defined in
      RFC 2608, Section 9.2 [5].

      ( 1.3.6.1.4.1.6252.2.27.6.1.7
        NAME 'service-advert-attribute-authenticator'
        DESC 'The authenticator for the attribute list, null if none.'
        SYNTAX 1.3.6.1.4.1.6252.2.27.6.2.3
        SINGLE_VALUE
      )

      This attribute contains the SLP attribute authenticator, as
      defined in RFC 2608, Section 9.2 [5].

2.1 Mapping from SLP Attribute Types to LDAP Attribute Types

   We define the mapping from SLP attribute types to LDAP as follows:

      SLP Type    ASN.1 Type               LDAP Type
      ---------------------------------------------------
       Integer     INTEGER              INTEGER
       String      DirectoryString      Directory String
       Boolean     BOOLEAN              Boolean
       Opaque      OCTET STRING         Octet String
       Keyword     (N/A)                IA5 String

   The following subsections discuss further details of the mapping.

2.1.1 Integer

   SLP integers compare as integers when performing a query.  LDAP
   integers behave similarly.  Consequently, the mapping from the SLP
   integer type to LDAP is INTEGER, with the integerMatch matching rule.

2.1.2 String

   SLP strings are encoded as described in the SLP protocol
   specification [5].  All value strings are considered case insensitive
   for matching operations.  SLP strings are not null terminated and are
   encoded in UTF-8.

   SLP strings are mapped to the LDAP Directory String type. The
   Directory String type exactly matches the SLP string type, i.e. it is
   a non-null terminated UTF-8 string. The caseIgnoreMatch equality
   rule, caseIgnoreOrderingMatch ordering rule, and
   caseIgnoreSubstringsMatch substring rule are used for comparing
   string attribute values.

2.1.3 Boolean

   Boolean attributes may have one of two possible values.  In SLP,
   these values are represented as strings, TRUE and FALSE.  In SLP's
   string encoding of a boolean value, case does not matter.

   The SLP Boolean type maps directly into an LDAP BOOLEAN. The
   caseIgnoreMatch rule is used for equality matching.

2.1.4 Opaque

   SLP attribute values of type Opaque are represented as OCTET STRING
   in LDAP, and the octetStringMatch matching rule is used to compare
   them.

2.2 Keyword Attributes

   SLP service type templates allow the definition of keyword
   attributes.  Keyword attributes are attributes whose only
   characteristic is their presence. Keyword attributes have no flag
   information, nor any default or allowed values (since, by definition,
   they have no values).

   ASN.1 has no concept of keyword attributes. Keyword attributes are
   translated into a "May" clause in the ASN.1 class definition for the
   service type. If the keyword attribute is present, then its value is
   of no consequence, but for consistency we make it simply the NUL
   character, "\00".

2.3 Template Flags

   SLP template flags can be handled as described in the following
   subsections.

2.3.1 Multi-valued

   Multi-valued attributes are defined in an SLP template using the one
   value.  All values for a given attribute must be of the same type.

   LDAP attribute definitions require that a single valued attribute
   include the SINGLE-VALUE tag if the attribute is single valued.
   Otherwise, the attribute is assumed to be multivalued by default.

2.3.2 Optional

   SLP uses the 'O' flag to indicate an attribute may or may not be
   present.  These optional attributes are defined using the "May"
   clause in the ASN.1 definition class definition for the service type.
   All other attributes must be defined as a "Must".

2.3.3 Literal

   ASN.1 does not have a mechanism to indicate that the values of an
   attribute may not be translated from one language to another, since
   ASN.1 schema are not typically translated. This flag is dropped when
   translating a template, but presence of the flag should be noted in
   the DESC field. It should be placed on a separate line and tagged
   with "Literal:" so the template can be reconstructed from the schema.

2.3.4 Explicit Matching

   The SLP template syntax uses a flag of 'X' to indicate that an
   attribute must be present in order for the query to be properly
   satisfied.  There is no provision for requiring that particular
   attributes be in a query. Consequently, this flag is dropped when
   translating a template, but presence of the flag should be noted in
   the DESC field. It should be placed on a separate line and tagged
   with "Explicit:" so the template can be reconstructed from the
   schema.

2.4 Default and Allowed Value Lists

   The SLP template grammar provides the capability to define default
   and allowed values for an attribute. The SLP protocol does not
   enforce these restrictions on registered attributes, however.  The
   default and allowed values may be used by client side applications,
   or alternatively it may also be used by DAs to initialize
   registrations having no attributes and to limit attribute values to
   the template allowed values.

   LDAP servers also do not support default and allowed values on
   attributes. Therefore, enforcement of default and allowed values in
   SLP templates is left up to the clients or a DA, if the DA is
   backending into LDAP. The default and allowed values should be
   included in the DESC field. The comments should be placed on separate
   lines and labeled with the "Default:" and "Allowed:" tags to allow
   reconstruction of the template.

2.5 Descriptive Text

   The descriptive text associated with an attribute definition should
   be included in the DESC field. It should start on a separate line and
   begin with the "Description:" tag.

2.6 Generating LDAP Attribute OIDs

   LDAP attributes require an OID. In general, there is no a priori way
   that an algorithm can be defined for generating OIDs, because it will
   depend on the conventions used by the organization developing the
   template. In some cases, an organization's procedure for generating
   OIDs may be regular enough that a template developer can
   algorithmically generate OIDs off of an assigned root. Whatever means
   is used, the template developer should assure that unique OIDs are
   assigned to each SLP attribute that is translated into an LDAP
   attribute.

2.7 Example

   The template included below is a hypothetical abstract printer
   service template, similar to that described in [10].

      template-type = printer

      template-version = 0.0

      template-description =
      The printer service template describes the attributes supported by
      network printing devices.  Devices may be either directly
      connected to a network, or connected to a printer spooler that
      understands the a network queuing protocol such as IPP, lpr or the
      Salutation  Architecture.

      template-url-syntax =
      ;The URL syntax is specific to the printing protocol being
      ;employed

      description = STRING
      # This attribute is a free form string that can contain any
      # site-specific descriptive information about this printer.

      printer-security-mechanisms-supported = STRING L M
      none
      # This attribute indicates the security mechanisms supported
      tls, ssl, http-basic, http-digest, none

      printer-operator = STRING O L M
      # A person, or persons responsible for maintaining a
      # printer on a day-to-day basis, including such tasks
      # as filling empty media trays, emptying full output
      # trays, replacing toner cartridges, clearing simple
      # paper jams, etc.

      printer-location-address = STRING O
      # Physical/Postal address for this device.  Useful for
      # nailing down a group of printers in a very large corporate
      # network.  For example: 960 Main Street, San Jose, CA 95130

      printer-priority-queue = BOOLEAN O
      FALSE
      # TRUE indicates this printer or print queue is a priority
      # queuing device.

      printer-number-up = INTEGER O
      1
      # This job attribute specifies the number of source
      # page-images to impose upon a single side of an instance
      # of a selected medium.
      1, 2, 4

      printer-paper-output = STRING M L O
      standard
      # This attribute describes the mode in which pages output
      # are arranged.

      standard, noncollated sort, collated sort, stack, unknown

   We assume that the concrete type "service:printer:lpr" for printers
   that speak the LPR protocol [4] has the following template
   definition:

      template-type = printer:lpr

      template-version = 0.0

      template-description =
      The printer:lpr service template describes the attributes
      supported by network printing devices that speak the
      LPR protocol. No new attributes are included.

      template-url-syntax = queue
      queue = ;The queue name, see RFC 1179.

   The LDAP class definition for the "service:printer:lpr" concrete
   service type is translated as follows:

   ( ---place the assigned OID here---
     NAME  'service-printer-lpr'
     DESC  'Description: The printer:lpr service template
                 describes the attributes supported by network printing
                 devices that speak the LPR protocol. No new attributes
                 are included.

            URL Syntax: queue
                 queue = ;The queue name, see RFC 1179.'
     SUP   slpService
     MUST  ( description $ security-mechanisms-supported $
     labeledURI)
     MAY   ( operator $ location-address $ priority-queue $
             number-up $ paper-output)
   )

   The attribute definitions are translated as follows:

   ( ---place the assigned OID here---
     NAME 'printer-security-mechanisms-supported'
     DESC 'Description: This attribute indicates the security mechanisms
           supported.

           Default: value

           Allowed: tls, ssl, http-basic, http-digest, none

           Literal:'
     EQUALITY caseIgnoreMatch
     ORDERING caseIgnoreOrderingMatch
     SUBSTR caseIgnoreSubstringsMatch
     SYNTAX 1.3.6.1.4.1.1466.115.121.1.15
   )

   ( ---place the assigned OID here---
     NAME 'printer-operator'
     DESC 'Description: A person, or persons responsible for
           maintaining a printer on a day-to-day basis, including
           such tasks as filling empty media trays, emptying full
           output trays, replacing toner cartridges, clearing simple
           paper jams, etc.

           Literal:'
     EQUALITY caseIgnoreMatch
     ORDERING caseIgnoreOrderingMatch
     SUBSTR caseIgnoreSubstringsMatch
     SYNTAX 1.3.6.1.4.1.1466.115.121.1.15
   )

   ( --place the assigned OID here---
     NAME 'printer-location-address'
     DESC 'Description Physical/Postal address for this device.
           Useful for nailing down a group of printers in a very
           large corporate network.  For example: 960 Main Street,
           San Jose, CA 95130.'
     EQUALITY caseIgnoreMatch
     ORDERING caseIgnoreOrderingMatch
     SUBSTR caseIgnoreSubstringsMatch
     SYNTAX 1.3.6.1.4.1.1466.115.121.1.15
     SINGLE-VALUE
   )

   ( ---place the assigned OID here---
     NAME 'printer-priority-queue'
     DESC 'Description: TRUE indicates this printer or print
          queue is a priority queuing device.'
     EQUALITY booleanMatch
     SYNTAX 1.3.6.1.4.1.1466.115.121.1.7
     SINGLE-VALUE
   )

   ( ---place the assigned OID here---
     NAME 'printer-number-up'
     DESC 'Description: This job attribute specifies the number
           of source page-images to impose upon a single side of
           an instance of a selected medium. This attribute is
           INTEGER.

           Default: 1

           Allowed: 1, 2, 3, 4'
     EQUALITY integerMatch
     SYNTAX 1.3.6.1.4.1.1466.115.121.1.27
     SINGLE-VALUE
   )

   ( ---place the assigned OID here---
     NAME 'printer-paper-output'
     DESC 'Description: This attribute describes the mode in
           which pages output are arranged. Default value is
           standard.

           Default: standard

           Allowed: standard, noncollated sort, collated sort,
             stack, unknown.
           Literal:'
     EQUALITY caseIgnoreMatch
     ORDERING caseIgnoreOrderingMatch
     SUBSTR caseIgnoreSubstringsMatch
     SYNTAX 1.3.6.1.4.1.1466.115.121.1.15
   )

3.0 Attribute Name Conflicts

   LDAP has a flat name space, and attribute names and OIDs must be
   unique in a directory server. In order to avoid name conflicts in the
   translation of SLP templates to LDAP schemas, template developers may
   want to consider prepending the name of the service type to the
   attribute. Postprocessing attribute names to make them unique when
   translated is not possible, because it would require the DA to
   rewrite queries before submitting them to the directory server. In
   addition, developers should use standard LDAP attributes when such
   attributes are available.

   In the above example template, the abstract type name "printer" is
   prepended to attributes to avoid conflicts. The standard
   "description" attribute defined by X.520 [3] is used to translate the
   template description attribute.

4.0 Mapping from Schema to Templates

   The reverse mapping from LDAP schema to SLP service type templates
   requires dealing with both LDAP and ASN.1 data types.  RFC 2252
   defines 33 attribute syntaxes that should be supported by LDAP
   directory servers.  These syntaxes are defined using BNF for strings
   or using ASN.1 for binary  valued attributes defined by X.520.

   Mapping of the LDAP data types into SLP template types is fairly
   straightforward, but mapping arbitrary ASN.1 data types is somewhat
   more complicated and requires encoding the ASN.1 data type into a
   string. To a certain extent, this masks the ASN.1 data type because
   it becomes impossible to distinguish between a native string having

   content equivalent to an encoded ASN.1 string. However, inclusion of
   the ASN.1 data type in the comment provides additional information
   should a reverse transformation from SLP to ASN.1 be required.

   The following subsections deal with both LDAP and ASN.1 attribute
   data type mappings.

4.1 Mapping LDAP Attribute Syntaxes to SLP Attribute Types

   The following table contains the mappings for LDAP syntaxes to SLP
   data types:

         LDAP Type                              SLP Type
      --------------------------------------------------------
         ACI Item                                 NA
         Access Point                             NA
         Attribute Type Description               NA
         Audio                                    Opaque
         Binary                                   ASN.1 escape
         Bit String                               String
         Boolean                                  Boolean
         Certificate                              Opaque
         Certificate List                         Opaque
         Certificate Pair                         Opaque
         Country String                           String
         DN                                       String
         Data Quality Syntax                      NA
         Delivery Method                          NA
         Directory String                         String
         DIT Content Rule Description             NA
         DIT Structure Rule Description           NA
         DL Submit Permission                     NA
         DSA Quality Syntax                       NA
         Enhanced Guide                           NA
         Facsimile Telephone Number               String
         Fax                                      Opaque
         Generalized Time                         String
         Guide                                    NA
         IA5 String                               String
         INTEGER                                  Integer
         JPEG                                     Opaque
         LDAP Syntax Description                  NA
         LDAP Schema Definition                   NA
         LDAP Schema Description                  NA
         Master and Shadow Access Points          NA
         Matching Rule Description                NA
         Matching Rule Use Description            NA
         Mail Preference                          NA

         MHS OR Address                           String
         Modify Rights                            NA
         Name and Optional UID                    NA
         Name Form Description                    NA
         Numeric String                           String
         Object Class Description                 NA
         Octet String                             Opaque
         OID                                      String
         Other Mailbox                            String
         Postal Address                           String
         Protocol Information                     NA
         Presentation Address                     String
         Printable String                         String
         Substring Assertion                      NA
         Subtree Specification                    NA
         Supplier Information                     NA
         Supplier or Consumer                     NA
         Supplier And Consumer                    NA
         Supported Algorithm                      NA
         DSE Type                                 NA
         Telephone Number                         String
         Teletex Terminal Identifier              String
         Telex Number                             String
         UTC Time                                 String

4.2 Mapping ASN.1 Types to SLP Types

   ASN.1 employs a much richer set of data types than provided by SLP.
   The table below show the mapping of selected ASN.1 data type to their
   nearest SLP equivalent.  Because of the complexity and flexibility of
   ASN.1, a complete list cannot be provided.

   As sample of some ASN.1 encodings and their mappings to SLP:

      ASN.1 type               SLP type
      -----------------------------------------
      INTEGER                  Integer
      BOOLEAN                  Boolean
      ENUMERATED               String
      OBJECT IDENTIFIER        String
      OCTET STRING             Opaque
      REAL                     String

   Data types that do not map directly to SLP data types should be
   defined as either a String, or as Opaque.  ASN.1 types that may only
   contain valid characters for Strings, as defined in X.680 [9] should
   be encoded as strings.  ASN.1 types such as GraphicString that change
   their character set encoding in part way through a value should not

   be encoded as strings, however, If such types are required, the SLP
   Opaque type should be used. In either case, the first line of the
   help text is used to indicate the original ASN.1 data type.

   The following subsections describe how to convert from the ASN.1 BER
   [9] to the SLP template for the different types in the table above.

4.2.1 Integer

   Both SLP templates and ASN.1 support Integers, so there is a one to
   one mapping between an SLP Integer attribute and an ASN.1 Integer
   attribute.  Details on the encoding of integers is summarized in the
   SLP template to ASN.1 section above.

4.2.2 Boolean

   Boolean values are supported by both SLP and ASN.1, though on wire
   encodings differ.  X.680 [9] specifies zero and non-zero encoding for
   booleans, where SLP encodes booleans using the strings TRUE and
   FALSE.  In general, most LDAP servers will use the LDAP Boolean type
   (which is a string), so again the ASN.1 type should be recorded in
   the comment or it will be lost.

4.2.3 Enumerated

   SLP templates support the concept of enumerations through the listing
   of allowed values in the attribute definition.  These enumerations
   are not strictly binding on clients or DAs, but they are similar to
   the ASN.1 definition of enumerations. BER encodes the ASN.1
   enumeration by passing the number of the element's position in the
   enumeration.  This requires both sides to have knowledge of the
   specific enumeration prior to decoding an enumeration's value. SLP
   provides no specific support for transmitting enumerations. They are
   simply String types. Information on the ASN.1 type and ASN.1 encoding
   of the enumeration values is recorded in the comment.

   Example:

   color-supported = STRING   M
   none
   # ASN.1: Enumeration.
   # ASN.1 Mapping: none = 0, highlight = 1, three color = 2,
   #   four color = 4, monochromatic = 5
   #This attribute specifies whether the Printer supports
   # color and, if so, what type.
   none,highlight,three color,four color,monochromatic

4.2.4 Object Identifier

   Object identifiers(OIDs) are commonly used in the ASN.1 world to
   identify object and attributes.  OIDs are a numerical representation
   of an element's place in the naming hierarchy. Each element at a
   particular level of a hierarchy has a unique number assigned within
   that level of the hierarchy. A sample OID would be the naming tree
   for SNMP MIBs:  iso(1) org(3) dod(6) internet(1) mgmt(2) mib(1) would
   be written as the string "1.3.6.1.2.1".

   Because this representation reduces down to a string of dot separated
   numbers, this maps easily to the SLP String type.  The help text for
   this element should indicate it is an ASN.1 OID

      identifier = STRING
      # ASN.1: OID
      # The object identifier for this SNMP agent.

4.2.5 Octet String

   An ASN.1 octet string should be mapped to an Opaque in an SLP
   template.  An octet string is a sequence of bytes, whereas an Opaque
   is a a string that encodes a sequence of bytes. Again, the ASN.1 type
   is lost unless recorded in the comment.

4.2.6 Real

   There is no direct mapping between floating point numbers and any SLP
   data types.  Attributes having the ASN.1 type of Real are mapped to
   SLP type String.  Comments are added to the attribute help text
   indicating the value was originally an ASN.1 real.  For example:

      weight = STRING
      # ASN.1: Real
      # The objects weight in pounds.

4.3 Example ASN.1 Schema

   The following is an example schema for an exported filesystem.  The
   section presents it as in ASN.1 and the following section shows the
   SLP template translation. The template translation does not capture
   the actual attribute format for the Set type, that would be done in
   the LDAP client software making the translation. Note that even
   though the class definition does not conform with the previously
   defined conventions for SLP classes, the schema can still be
   translated into an SLP template.  The syntax used in this example
   follows

         -- Abstraction of a fstab entry (a "mount").
         -- These lookups would likely be performed by an
         -- an automounter type application.
         mount   OBJECT-CLASS ::= {
                 SUBCLASS OF { top }
                 MUST CONTAIN { mountHost |
                                mountDirectory |
                                mountType
                              }
                 MAY CONTAIN { mountOption |
                               mountDumpFrequency |
                               mountPassNo
                             }
                 ID { <oid1> }
         }

         - The mount host.
         mountHost       ATTRIBUTE ::= {
                         WITH SYNTAX caseIgnoreString
                         EQUALITY MATCHING RULE caseIgnoreMatch
                         SINGLE VALUE
                         ID { <oid2> }
         }

         - The file system to mount.
         mountDirectory  ATTRIBUTE ::= {
                         WITH SYNTAX caseIgnoreString
                         EQUALITY MATCHING RULE caseIgnoreMatch
                         SINGLE VALUE
                         ID { <oid3> }
         }

         - The type of file system being mounted.
         mountType       ATTRIBUTE ::= {
                         WITH SYNTAX INTEGER { ufs(1),
                                               hsfs(2),
                                               nfs(3),
                                               rfs(4)
                                             }
                         EQUALITY MATCHING RULE integerMatch
                         SINGLE VALUE
                         ID { <oid4> }
         }

         - Options for the mount operation.
         mountOption     ATTRIBUTE ::= {
                         WITH SYNTAX caseIgnoreString
                         EQUALITY MATCHING RULE caseIgnoreString
                         ID { <oid5> }
         }

         - How often to dump the file system.
         mountDumpFrequency      ATTRIBUTE :: = {
                                 WITH SYNTAX  INTEGER (0..9)
                                 EQUALITY MATCHING RULE integerMatch
                                 SINGLE VALUE
                                 ID { <oid6> }
         }

         - Boot time mount pass number.
         mountPassNo     ATTRIBUTE ::= {
                         WITH SYNTAX INTEGER
                         EQUALITY MATCHING RULE integerMatch
                         SINGLE VALUE
                         ID { <oid7> }
         }

   The translated SLP template is:

      template-type = mount

      template-version = 1.0

      template-description = "Describes a remote filesystem access
      protocol"

      template-url-syntax =
                   filesystem   = 1*[ DIGIT / ALPHA ]
                   urlpath = "/" filesystem

      mountHost = STRING L
      # ASN.1: Case Ignore String, Single Value
      # The mount host

      mountDirectory = STRING L
      # ASN.1: Case Ignore String, Single Value
      # The filesystem to mount

      mountType = STRING L
      ufs
      # ASN.1: Enumeration, Single Value
      # ASN.1 Mapping: ufs = 1, hsfs = 2, nfs = 3, rfs = 4
      # The type of the filesystem being mounted
      ufs, hsfs, nfs, rfs

      mountOption = STRING M O L
      # ASN.1: Case Ignore String
      # mount options for this filesystem

      mountDumpFrequency = INTEGER O
      0
      # ASN.1: Integer Range, Single Value
      # How often to dump this filesystem
      0, 1, 2, 3, 4, 5, 6, 7, 8, 9

      mountPassNo = INTEGER O
      # ASN.1: Integer, Single Value
      # Boot time mount pass number

5.0 Representing SLP Service Advertisements in an LDAP DIT

   In addition to translating between SLP templates and LDAP schema,
   another area requiring compatibility is the representation of SLP
   service advertisements in an LDAP DIT. A standardized representation
   for service information allows SLP DAs to store service
   advertisements in LDAP, and for LDAP clients to query the DIT for
   those services.  Similarly, if LDAP clients represent service
   information in the same form, SLP clients can benefit from
   interoperability.

   A service advertisement contains the service URL in a 'labeledURI'
   attribute [11]. The labeledURI attribute in a service advertisement
   should only contain the service URL for the service, with no
   additional label. It is recommended that the labeledURI be used as
   the RDN for the service object in the DIT.

   Although service advertisements can appear anywhere within the DIT,
   it is recommended that all services be stored under a single common
   point, or root node, to facilitate searching in a domain. This allows
   a  client to search for all of advertisements of a particular service
   type, say, for all printers.  The recommended parent entry is one
   named "ou=service" below the entry which is the representation of the
   domain, as described in RFC 2247.

   For example, a printer service with labeledURI of
   "service:lpr://printsrv/queue1" in the domain foobar.com advertised
   in the LDAP server that holds the entry "dc=foobar,dc=com" tree has
   the following DN:

   "labeledURI=service:lpr://printsrv/queue1, ou=service, dc=foobar,
   dc=com"

   While this leads to a flat space of service storage, since SLP uses
   search filters from LDAP for searches, these filters can be used for
   one-level searches from the root node.

   The following example illustrates how an advertisement having a
   simple service type is represented. The advertisement (in conceptual
   form) for a printer is:

      Service Type: service:lpr://printsrv/queue1
      Scopes: eng,corp
      Attributes:
        description = A general printer for all to use.
        security-mechanisms-supported = none
      Authentication: none

   The RDN of the object is labeledURI=service:lpr://printsrv/queue1,
   and the following LDAP search filter will return this object, along
   with any others of the service type "service:lpr" that match the
   other attributes:

      (&(service-advert-service-type=service:lpr)
        (service-advert-scopes=eng)
        (service-advert-scopes=corp)
        (description=A general printer for all to use.)
        (security-mechanisms-supported=none))

   Service advertisements in SLP also have a lease time associated with
   them. In LDAP servers that support the extensions for dynamic
   directory services [12], the service advertisement entry objectClass
   should be extended with the dynamicObject class. This allows the
   service advertisement to time out within the LDAP directory server.
   If the LDAP directory server does not support the dynamic directory
   services extension, then advertisement lease timeouts must be handled
   by the SLP agent.

   While the service advertisement schema outlined in this section is
   primarily for SLP DAs that use LDAP as a backing store, if LDAP
   agents register services using the same format, complete
   interoperability with SLP is achieved.

6.0 Internationalization Considerations

   SLP specifies that an RFC 1766 [13] language code accompanies every
   service advertisement. Language codes for service advertisements in
   LDAP must be represented according to RFC 2596 [14].

   RFC 2596 prohibits language codes in DNs, and specifies that a
   directory server which does not support language codes must treat an
   attribute with a language code as an unrecognized attributes.
   According to RFC 2596, language codes are appended to attribute names
   with a semicolon (";"). For example, the following attribute/value
   pair is in the German locale:

      (address;lang-de=44 Bahnhofstrasse, 2365 Weibstadt, Deutschland)

   An attribute with a language tag in a specific locale is considered a
   separate attribute from attributes in other locales.

   If the service advertisement is in the default SLP locale ("en", no
   dialect), then the language code need not be appended to the
   attribute name.

   SLP queries in locales other than the default need not be rewritten
   to include language tags before being submitted to the directory
   server.  RFC 2596 specifies that all entries that match are returned,
   including those with language tags, without requiring the language
   tags to be explicitly present in the query. The SLP DA can then
   postprocess the result to select the entries from the required
   locale.

7.0 Security Considerations

   SLP authenticators are stored with the service advertisement in the
   DIT, as discussed in Section~7ef{slpdit}. LDAP clients need to use
   LDAP authentication [15] to assure that they are connecting with a
   secure server. In particular, SLP DAs that use LDAP as a back end
   store and that implement SLP authentication MUST use LDAP
   authentication to assure that the LDAP entries for their service
   registrations are secure.

Acknowledgements

   Many thanks are due to Mark Wahl whose detailed and insightful
   comments were instrumental in helping improve the technical accuracy
   of this document with respect to LDAP.

8.0 References

   [1]  Guttman, E., Perkins, C. and J. Kempf, "Service Templates and
        service: Schemes", RFC 2609, April 1999.

   [2]  Wahl, W., Howes, T. and S. Kille, "Lightweight Directory Access
        Protocol (v3)", RFC 2251, December 1997.

   [3]  International Telecommunications Union. The Directory:Selected
        Attribute Types.  ITU Recommendation X.520. August, 1997.

   [4]  McLaughlin, L., "Line Printer Daemon Protocol, RFC 1179, August
        1990.

   [5]  Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
        Location Protocol Version 2", RFC 2608, April 1999.

   [6]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
        Specifications: ABNF", RFC 2234, November 1997.

   [7]  Howes, T., "The String Representation of LDAP Search Filters",
        RFC 2254, December 1997.

   [8]  Wahl, W., Coulbeck, A., Howe, T. and S. Kille, "Lightweight
        Directory Access Protocol (v3): Attribute Syntax Definition",
        RFC 2252, December 1997.

   [9]  ITU-T Rec. X.680. Abstract Syntax Notation One (ASN.1) -
        Specification of Basic Notation. 1994.

   [10] Fleming, P., Jones, K., Lewis, H., and McDonald, I., "Internet
        Printing Protocol (IPP): LDAP Schema for Printer Services", Work
        in Progress.

   [11] Smith, M., "Definition of an X.500 Attribute Type and an Object
        Class to Hold Uniform Resource Identifiers (URIs)", RFC 2079,
        January 1997.

   [12] Yaacovi, Y., Wahl, M. and T. Genovese, "Lightweight Directory
        Access Protocol (v3): Extensions for Dynamic Directory
        Services", RFC 2589, May 1999.

   [13] Alvestrand, H., "Tags for the Identification of Languages", RFC
        1766, December 1997.

   [14] Wahl, M. and T. Howes, "Use of Language Codes in LDAP", RFC
        2596, May 1999.

   [15] Wahl, M., Alvestrand, H., Hodges, J. and R. Morgan,
        "Authentication Methods for LDAP", RFC 2829, May 2000.

   [16] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [17] Dubuisson, O. ASN.1: Communication between Heterogeneous
        Systems. OSS Nokalva, 2000.

   [18] http://www.srvloc.org

9.0 Authors' Addresses

   James Kempf
   Sun Microsystems
   901 San Antonio Avenue
   Palo Alto, CA 94303
   USA

   Phone: +1 650 786-5890
   EMail: james.kempf@sun.com

   Ryan Moats
   Coreon, Inc.
   15621 Drexel Circle
   Omaha, NE, 68135
   USA

   EMail: rmoats@coreon.net

   Pete St. Pierre
   Sun Microsystems
   901 San Antonio Avenue
   Palo Alto, CA 94303
   USA

   Phone: +1 415 786-5790
   EMail: Pete.StPierre@Eng.Sun.COM

10.  Full Copyright Statement

   Copyright (C) The Internet Society (2000).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
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   The limited permissions granted above are perpetual and will not be
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   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

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