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 RFC2252.
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 RFC2252 [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
RFC2252 [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 RFC2119 [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 RFC2609 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 RFC2608 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 RFC2608, 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
RFC2608, 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 RFC2608, 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 RFC1179.
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 RFC1179.'
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. RFC2252
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 RFC2247.
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 RFC1766 [13] language code accompanies every
service advertisement. Language codes for service advertisements in
LDAP must be represented according to RFC2596 [14].
RFC2596 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 RFC2596, 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. RFC2596 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", RFC2609, April 1999.
[2] Wahl, W., Howes, T. and S. Kille, "Lightweight Directory Access
Protocol (v3)", RFC2251, December 1997.
[3] International Telecommunications Union. The Directory:Selected
Attribute Types. ITU Recommendation X.520. August, 1997.
[4] McLaughlin, L., "Line Printer Daemon Protocol, RFC1179, August
1990.
[5] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service
Location Protocol Version 2", RFC2608, April 1999.
[6] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC2234, November 1997.
[7] Howes, T., "The String Representation of LDAP Search Filters",
RFC2254, December 1997.
[8] Wahl, W., Coulbeck, A., Howe, T. and S. Kille, "Lightweight
Directory Access Protocol (v3): Attribute Syntax Definition",
RFC2252, 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)", RFC2079,
January 1997.
[12] Yaacovi, Y., Wahl, M. and T. Genovese, "Lightweight Directory
Access Protocol (v3): Extensions for Dynamic Directory
Services", RFC2589, 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", RFC2829, May 2000.
[16] Bradner, S., "Key Words for Use in RFCs to Indicate Requirement
Levels", BCP 14, RFC2119, 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
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