Service Location Working Group Pete St. Pierre
INTERNET DRAFT Sun Microsystems
31 July 1997
Conversion of LDAP Schemas to and from SLP Templates
draft-ietf-svrloc-template-conversion-00.txt
Status of This Memo
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Abstract
LDAP and SLP are both useful mechanisms for locating service related
information on a network. While they do perform similar functions,
the way in which the information they provide is formated is very
different. This document describes a set of rules and mappings for
translating between the ASN.1 based LDAP schema and an SLP Template
as described in the ``Service Template and service: Scheme''
draft.[1]
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Contents
Status of This Memo i
Abstract i
1. Motivation 1
2. ASN.1 and BER Encodings 1
3. Mapping from Templates to Schemas 2
3.1. Data Type Mappings . . . . . . . . . . . . . . . . . . . 2
3.2. Integer . . . . . . . . . . . . . . . . . . . . . . . . . 2
3.3. String . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.4. Boolean . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.5. Opaque . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.6. Enumerations . . . . . . . . . . . . . . . . . . . . . . 4
3.7. Multi-valued Attributes . . . . . . . . . . . . . . . . . 4
3.8. Optional Attributes . . . . . . . . . . . . . . . . . . . 4
3.9. Literal Attributes . . . . . . . . . . . . . . . . . . . 5
3.10. Explicit Matching . . . . . . . . . . . . . . . . . . . . 5
3.11. Sample Translation . . . . . . . . . . . . . . . . . . . 5
4. Mapping from Schemas to Templates 5
4.1. Data Conversions . . . . . . . . . . . . . . . . . . . . 6
4.2. Integer . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.3. Strings . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.4. Boolean . . . . . . . . . . . . . . . . . . . . . . . . . 6
4.5. Octet String . . . . . . . . . . . . . . . . . . . . . . 6
4.6. Enumeration . . . . . . . . . . . . . . . . . . . . . . . 6
4.7. Rules for Other ASN.1 Primitive Types . . . . . . . . . . 7
4.8. Set Of . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.9. Real . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.10. bitstring . . . . . . . . . . . . . . . . . . . . . . . . 8
4.11. Object Identifier . . . . . . . . . . . . . . . . . . . . 8
4.12. Sequence Of . . . . . . . . . . . . . . . . . . . . . . . 8
4.13. Sample Translation . . . . . . . . . . . . . . . . . . . 9
5. Notes on Matching Operators 9
6. References 10
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1. Motivation
SLP templates are intended to create a simple encoding of the
syntactic and semantic conventions for individual service types,
their attributes, and conventions. This can easily be generated,
transmitted, read by humans and parsed by programs, as it is a string
based syntax with required comments.
On the other hand, directory schemas serve to formalize directory
entry formulation for use with X.500 and LDAP. These directories
server to store information about many types of entities. Network
services are one such entity.
The ability to register services across both SLP and schema based
directory services is a much needed capability. In order to
facilitate this, mappings must be created between the SLP template
grammar and the directory schemas
The simple notation and syntactic/semantic attribute capabilities
of SLP will map well into directory schemas. This means that
service templates will easily be converted into directory schemas.
The reverse is not true. Only a certain restricted set of types,
matching rules and encoding conventions will be directly mappable
into service type templates. There are rules to cover the cases
where mapping cannot be done directly. It is believed that the cases
which are not supported are the exception rather than the rule.
This document will outline the correct mappings for the four basic
data types supported by SLP to the ASN.1/BER encoding used by the
LDAP directory schema. Likewise, rules and guidelines will be
propsed to facilitate consistent mapping of ASN.1 based schemas to be
translated in the SLP template grammar.
2. ASN.1 and BER Encodings
ASN.1 defined schemas are assumed to be encoded using the Basic
Encoding Rules(BER) defined in CCITT Recommendation x.209. This
document refers heavily to the X.209 specification for on-the-wire
encoding of ASN.1 values. BER supports 4 types of encodings:
Universal, Application, Context Specific and Private. All SLP types
will map to Universal BER encoded values.
Within the scope of Universal types, there are both primitive
encodings and constructed encodings. A primitive encoding a data
value encoding in which the contents octets directly represent the
value. Constructed encodings are a data value encoding in which the
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contents octets are the complete encoding of one or more other data
values. [2]
This document will deal primarily with mapping ASN.1 primitive
encodings to SLP data types. All discussions of bit ordering assume
bit 8 is the most significant bit.
3. Mapping from Templates to Schemas
3.1. Data Type Mappings
SLP supports four data types. Each of these data types can be mapped
to a specific ASN.1 type. In this way, translation of data types can
be described easily. All SLP data types are encoded as strings in
the protocol.
ASN.1 supports a much larger range of values. As such, a subset
will be selected to map SLP values. ASN.1 uses BER encoding as
described in CCITT Recommendation X.209 [2]. BER encodings are based
on tupples containing a Type, Length and Contents.
Complexity is added when the SLP data type is expressed as an
enumeration. This section describes the translation of each data
type to its corresponding ASN.1 type. A discussion of proper
enumeration handling follows these mappings.
SLP Type ASN.1 Type
---------------------------
Integer Integer
String String
Boolean Boolean
Opaque Octet String
3.2. Integer
Both SLP templates and ASN.1 support Integers, therefore there
is a one to one mapping between an SLP Integer attribute and an
ASN.1 Integer attribute. On the wire encoding of these two is very
different, though.
In SLP, all integers are encoded as strings. An integer value of
17869 would be represented by a 5 byte string containing the values
of the characters '1', '7', '8', '6', and '9' in the character set
specified in the request or repsonse packet.
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The ASN.1 Integer type is encoded in BER according to the rules in
section 8 of the X.209 specification.
The encoding of an ASN.1 integer value shall be primitive. The
contents octets shall consist of one or more octets. The rules
ensure that an integer value is always encoded in the smallest
possible number of octets.
Also, The contents octets shall be a two's complement binary number
equal to the integer value, and consisting of bits 8 to 1 of the
first octet, followed by bits 8 to 1 of the second octet, followed by
bits 8 to 1 of each octet in turn up to and including the last octet
of the contents octets.
3.3. String
SLP strings are encoded as described in section 20.5 of the SLP
protocol specification [3]. All value strings are considered case
insensitive for matching operations. As such, these strings are
mapped to the ASN.1 caseIgnoreString syntax.
3.4. 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.
ASN.1 supports a Universal, primitive type of boolean. X.209
specifies that the Contents field of a FALSE boolean value be encoded
as a single octet with a value of zero. A boolean whose value is
TRUE shall be encoded as a single octet whose value shall be any
non-zero value, at the sender's option.
3.5. Opaque
SLP values that are encoded as Opaque are really a series of octets.
While SLP uses the construct of <len>:<radix-64-data>, this maps
very nicely to the tag/length1/value BER encoding of the ASN.1 Octet
String.
The <len> field of the SLP encoding will not match the len field of
the BER encoding, as radix-64 encoding results in a 4 to 3 expansion
of the original data. Likewise, data presented in radix-64 notation
must be converted back to the original byte stream to be encoded in
the Contents field of the BER encoding.
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3.6. Enumerations
The SLP template grammar provides for the definition of enumerations.
Enumerations are defined by listing all possible values for the
attribute following any help text provided for that attribute. While
the template syntax allows for creation of enumerations, the SLP
protocol does not strictly enforce enumerations. These enumerations
are still treated as text strings within the protocol, and values
outside the scope of the enumeration defined may be present. The
template enumeration is intended as a guideline to client side
applications as to what values may be expected.
An ASN.1 enumeration commonly maps a text string to a numerical
value. In the BER encoding, the numerical value is passed as an
integer across the wire. The receiving side must then translate
the the value to the associated string as defined in the ASN.1
description. Because of this difference, SLP values that are
encoded as ASN.1 enumerations must be sure the enumeration covers all
possible values.
3.7. Multi-valued Attributes
Multi-valued attributes are defined in an SLP template using the
'M' flag. This flag indicates that an attribute may have more than
one value. All values for a given attribute must be of the same
encoding type. As such, the ASN.1 syntax for SET OF. Use of ``SET
OF'' assures all values will be of the same encoding type.
3.8. Optional Attributes
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 an ASN.1 definition. All other attributes must be defined
as a ``Must''
3.9. Literal Attributes
ASN.1 does not have a mechanism to indicate that the values of an
attribute may not be translated from one language to another.
3.10. Explicit Matching
The SLP template syntax uses a flag of 'X' to indicate that an
attribute must match exactly with a query made by a client. There
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is, however, no mechanism to prevent clients from using the
sub-string operator with explicit matching attributes. Common
practice would be to map this to the ASN.1 matching syntax of
``MATCHES EXACTLY''.
3.11. Sample Translation
TBD: Take SLP Template for a Service and convert to LDAP schema
4. Mapping from Schemas to Templates
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 enco
ASN.1 type SLP type
---------------------------------------
Integer Integer
Strings String
Boolean Boolean
Octet String Opaque
Enumeration String
Set Of 'M' flag
Real String
Bit String String
Object IdentifierString
Sequence Of Multiple Attributes
4.1. Data Conversions
4.2. Integer
Both SLP templates and ASN.1 support Integers, therefore there
is a one to one mapping between an SLP Integer attribute and an
ASN.1 Integer attribute. On the wire encoding of these two is very
different, though.
Details on the encoding is summarized in the SLP template to ASN.1
section above, as well as being explained in detail in [3] and
[X209].
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4.3. Strings
Strings are supported between both SLP and ASN.1. SLP encoding
of the strings must conform to the rules for handling special
characters, as outlined in [3]
4.4. Boolean
Boolean values are supported by both SLP and ASN.1, though on the
wire encodings will vary. [2] specifies zero and non-zero encoding
for booleans, where SLP encodes booleans using the strings TRUE and
FALSE.
4.5. 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, where an Opaque is
a sequence of bytes that has been encoded using radix64.
4.6. Enumeration
SLP templates support the concept of enumerations through the listing
of values in the attribute definition. This is similar to the ASN.1
definition of enumerations, though encodings vary. In SLP enumerated
values are passed between client and server as strings. BER encodes
the ASN.1 enumeration by passing the number of the elements position
in the enumeration. This requires both sides to have knowledge of
the specific enumeration prior to decoding an enumerations value.
color-supported = STRING M
none
# This attribute specifies whether the Printer supports
# color and, if so, what type.
none, highlight, three color, four color, monochromatic
4.7. Rules for Other ASN.1 Primitive Types
It is reasonable to think that all ASN.1 data types can be accurately
represented using the very basic data types defined in ASN.1. As
such, 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 [3] should
be encoded as strings. If a value may contain illegal string values,
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the SLP Opaque type should be used. In either case, the first line
of the help text should indicate the original ASN.1 data type.
4.8. Set Of
Sets can be accommodated in an SLP template by specifying the
attribute is multivalued. The flag 'M' is used to indicate an
attribute Can have multiple values. All values must be of the same
type. As such, a multivalued attribute of type string could have
values of "one, 2, three", but the value 2 would be returned as
a string, not an integer. Likewise, a multivalued integer could
not have a value of "1, 2, three", as all values would need to be
converted to strings, which are illegal for an attribute of type
integer.
4.9. Real
There is no direct mapping between floating point numbers and any SLP
data types. As such, attributes should be defined as type String.
Comments can be 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.
4.10. bitstring
While the wire encoding of strings and bitstrings is quite different,
it is not unreasonable to represent a bitstring as a series of ones
and zeros. As such, the ASN.1 bitstring is mapped to the SLP String
type, where all characters in the string are either ones or zeros.
mask = STRING
# ASN.1: Bitstring# The mask used to convert this number.
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4.11. Object Identifier
Object identifiers are commonly used in the ASN.1 world to identify
object and attributes. Object ID's are a numerical representation of
an elements place in the naming heirarchy. Each element at the level
of a heirarchy has a unique number assigned within that level of the
heirarch. A sample object ID 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.
4.12. Sequence Of
The ASN.1 construct 'Sequence Of' is probably the least intuitive to
map to an SLP template. SLP attributes can only contain values of
like type. By definition, this is an ASN.1 set. ASN.1 sequences
are made of multiple values of different types. For example, an
attribute named 'Engine' may be defined as:
SEQUENCE name String, status String
In order to map this to an SLP template, we can create multiple
attributes and rely on the ordering for association. The above might
translate as:
engine-name = STRING M
# The name of one of this crafts engines.
engine-status = String M
unknown
# The name of one of this crafts engines.
unknown, running, shutdown
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To do this, we are relying on an assumption stated in the service:
Scheme Draft [SCHEME] that all values of a multivalued attribute
retain their order. When new values are added, they are added to the
end of the list of values.
As such, if we had
engine-name = right, left
engine-status = running, shutdown
We would assume that the engine named right is running and the engine
named left is shutdown.
4.13. Sample Translation
In general, ASN.1 provides a much more robust set of data types than
provided for by SLP. As such, the more difficult task of converting
schemas from LDAP to templates for SLP.
5. Notes on Matching Operators
While the SLP template grammar does not describe the matching
properties of attributes, ASN.1 does. If chosing to add matching
properties to an SLP template when converting it to an ASN.1 based
schema, the following rules should be kept in mind.
LDAP and SLP support the same matching operations, though using
slightly different matching symantics. In addition to greaterOrEqual
and lessOrEqual, SLP provides for a simple less or greater match.
LDAP Search Operators SLP Search Operators and (&) & or (|) | not
(!) != equalityMatch (=) == substrings greaterOrEqual (>=) >=
lessOrEqual (<=) <= present (=*) <keyword> approxMatch ( =) [TBD: Is
there an equivalent?]
ASN.1 provides for three flavors of substring value matching. These
are initial, any, and final. In specifying the match capability of
an attribute, ASN.1 allows specification that a value may match the
leading part, any part, or the final part of a string value. Using
the SLP search sematics, this is accomplished through the substring
(*) operator. Searching for initial, any or final is handled through
specific placement of the operator. The following example, taken
from RFC2165 illustrates this:
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inital: "bob*" matches "bob", "bobcat", and "bob and sue" final:
"*bob" matches "bob", "bigbob", and "sue and bob" any: "*bob*"
matches "bob", "bobcat", "bigbob", and "a bob I know"
6. References
[1]E. Guttman, C. Perkins, J. Kempf ``Service Templates and service:
Schemes'', Work in Progress, July, 1997
draft-ietf-svrloc-service-scheme-02.txt
[2]CCITT Recommendation X.209, ``Specification of Basic Encoding
Rules for Abstract Syntax Notation One (ASN.1), 1988
[3]J. Veizades, E. Guttman, C. Perkins, and S. Kaplan. ``Service
Location Protocol'', RFC 2165. June 1997.
Authors' Addresses
Questions about this memo can be directed to:
Pete St. Pierre
Sun Microsystems
901 San Antonio Avenue
Palo Alto, CA 94043
USA
Phone: +1 415 786-5790
email: Pete.StPierre@Eng.Sun.COM
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