Service Location Working Group Pete St. Pierre
INTERNET DRAFT Sun Microsystems
4 November 1997
Conversion of LDAP Schemas to and from SLP Templates
draft-ietf-svrloc-template-conversion-01.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 2
2. ASN.1 and BER Encodings 2
3. Mapping from Templates to Schemas 3
3.1. Data Type Mappings . . . . . . . . . . . . . . . . . . . 3
3.2. Integer . . . . . . . . . . . . . . . . . . . . . . . . . 3
3.3. String . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.4. Boolean . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.5. Opaque . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.6. Enumerations . . . . . . . . . . . . . . . . . . . . . . 4
3.7. Multi-valued Attributes . . . . . . . . . . . . . . . . . 5
3.8. Optional Attributes . . . . . . . . . . . . . . . . . . . 5
3.9. Literal Attributes . . . . . . . . . . . . . . . . . . . 5
3.10. Explicit Matching . . . . . . . . . . . . . . . . . . . . 5
3.11. Template for Translation . . . . . . . . . . . . . . . . 6
3.12. Translated Schema . . . . . . . . . . . . . . . . . . . . 7
4. Mapping from Schemas to Templates 9
4.1. Data Type Mappings . . . . . . . . . . . . . . . . . . . 9
4.2. Integer . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.3. Strings . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. Boolean . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.5. Octet String . . . . . . . . . . . . . . . . . . . . . . 10
4.6. Enumeration . . . . . . . . . . . . . . . . . . . . . . . 10
4.7. Rules for Other ASN.1 Primitive Types . . . . . . . . . . 10
4.8. Set Of . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.9. Real . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.10. bitstring . . . . . . . . . . . . . . . . . . . . . . . . 11
4.11. Object Identifier . . . . . . . . . . . . . . . . . . . . 12
4.12. Sequence Of . . . . . . . . . . . . . . . . . . . . . . . 12
4.13. Schema to be Translated . . . . . . . . . . . . . . . . . 13
4.14. SLP Translation . . . . . . . . . . . . . . . . . . . . . 15
5. Notes on Matching Operators 15
A. References 16
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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
serve 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. The X.209
specification contains details of the 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 is a data
value encoding in which the contents octets directly represent the
value. Constructed encodings are data values encoding in which the
contents octets are the complete encoding of one or more other data
values. [2]
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This document will deal primarily with mapping ASN.1 primitive
encodings to SLP data types. 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.
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, so 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.
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 is 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.
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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. These strings are mapped to the
ASN.1 DisplayString 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.
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
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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. The ASN.1 syntax for SET OF is commonly used to
define multi-valued ASN.1 objects that must be of the same 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
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".
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3.11. Template for Translation
The template included below is derived from the printer service
scheme described in [4].
type = printer
version = 0.0
language = en
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.
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.
security-mechanisms-supported = STRING L M
none
# This attribute indicates the security mechanisms supported
tls, ssl, http-basic, http-digest, none
operator = STRING 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.
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
priority-queue = BOOLEAN O
FALSE
# TRUE indicates this printer or print queue is a priority
# queuing device.
number-up = INTEGER O
1
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# 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
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
3.12. Translated Schema
This translated schema uses the template attributes primarily as
comments in the beginning of the schema definition. Since all
Objects must support a cannonical name (cn), we use the URL as
the value for an object cn. This maps well, as a cn identifies a
particular object and a URL identifies a partular resource.
-- 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.
printer OBJECT-CLASS
SUBCLASS OF top
MUST CONTAIN {
cn,
description,
security-mechanisms-supported
}
MAY CONTAIN {
operator,
location-address,
priority-queue,
number-up,
paper-output
}
cn OBJECT-TYPE
SYNTAX DisplayString
DESCRIPTION
"The URL of the printer being described"
description OBJECT-TYPE
SYNTAX DisplayString
DESCRIPTION
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"This attribute is a free form string that can contain
Any site-specific descriptive information about this
printer."
security-mechanisms-supported OBJECT-TYPE
SYNTAX INTEGER {
tls(1)
ssl(2)
http-basic(3)
http-digest(4)
none(5)
}
DESCRIPTION
"This attribute indicates the security mechanisms
supported"
operator OBJECT-TYPE
SYNTAX SET OF DisplayString
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."
location-address OBJECT-TYPE
SYNTAX DisplayString
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"
priority-queue OBJECT-TYPE
SYNTAX BOOLEAN
DESCRIPTION
"TRUE indicates this printer or print queue is a priority
queuing device."
number-up OBJECT-TYPE
SYNTAX INTEGER
DESCRIPTION
"This job attribute specifies the number of source
page-images to impose upon a single side of an instance
of a selected medium."
paper-output OBJECT-TYPE
SYNTAX INTEGER {
standard(1)
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noncollated sort(2)
collated sort(3)
stack(4)
unknown(5)
} DESCRIPTION
"This attribute describes the mode in which pages
output are arranged."
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 ASN.1 encodings and their mappings to SLP:
ASN.1 type xxxx 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 Type Mappings
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 tuples
containing a Type, Length and Contents.
4.2. 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
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SLP template to ASN.1 section above, as well as being explained in
detail in RFC2165[3] and the X.209[2] specification.
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 RFC 2165 [3].
4.4. Boolean
Boolean values are supported by both SLP and ASN.1, though on the
wire encodings will vary. X.209[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
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defined as either a String, or as Opaque. ASN.1 types that may only
contain valid characters for Strings, as defined in X.209[2] should
be encoded as strings. If a value may contain illegal string values,
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 OF. ASN.1 sequences
are made of multiple values of different types. For example, an
attribute named 'Engine' may be defined as:
engine OBJECT-TYPE
SYNTAX SEQUENCE OF {
name DisplayString,
status INTEGER {
unknown(1)
running(2)
shutdown(3)
}
}
DESCRIPTION
"Engine description."
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:
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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
To do this, we are relying on an assumption stated in the service:
Scheme Draft [1] 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. Schema to be Translated
In general, ASN.1 provides a much more robust set of data types than
provided for by SLP. For this reason, it is more complex to convert
LDAP schemas to templates for SLP.
The following schema represents an example of a schema for an
exported filesystem. The section presents it as in ASN.1, while the
following section shows the SLP template translation.
-- 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 {
-- the mount host
cn,
-- the mount point
mountDirectory.
-- the mount type
mountType
}
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MAY CONTAIN {
-- mount options
mountOption,
-- dump frequency
mountDumpFrequency,
-- passno
mountPassNo
}
cn OBJECT-TYPE
SYNTAX DisplayString
DESCRIPTION
"The mount host"
mountDirectory
SYNTAX DisplayString
DESCRIPTION
"The filesystem to mount"
mountType OBJECT-TYPE
SYNTAX INTEGER {
ufs(1)
hsfs(2)
nfs(3)
rfs(4)
}
DESCRIPTION
"The type of the filesystem being mounted"
mountOption OBJECT-TYPE
SYNTAX SET OF DisplayString
DESCRIPTION
"mount options for this filesystem"
mountDumpFrequency OBJECT-TYPE
SYNTAX INTEGER (0..9)
DESCRIPTION
"How often to dump this filesystem"
mountPassNo OBJECT-TYPE
SYNTAX Integer
DESCRIPTION
"Boot time mount pass number"
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4.14. SLP Translation
type = mount
version = 1.0
language = en
description = "This would describe a remote filesystem
access protocol"
url-syntax =
filesystem = 1*[ DIGIT / ALPHA ]
urlpath = "/" filesystem
cn = STRING L
# The mount host
mountDirectory = STRING L
# The filesystem to mount
mountType = STRING L
ufs
# The type of the filesystem being mounted
ufs, hsfs, nfs, rfs
mountOption = STRING M O L
# mount options for this filesystem
mountDumpFrequency = INTEGER O
0
# How often to dump this filesystem
0, 1, 2, 3, 4, 5, 6, 7, 8, 9
mountPassNo = INTEGER O
# Boot time mount pass number
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 semantics. In addition to greaterOrEqual
and lessOrEqual, SLP provides for a simple less or greater match.
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LDAP Search Operators SLP Search Operators
and (&) &
or (|) |
not (!) !=
equalityMatch (=) ==
substrings
greaterOrEqual (>=) >=
lessOrEqual (<=) <=
present (=*) <keyword>
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:
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"
A. References
[1]E. Guttman, C. Perkins, J. Kempf "Service Templates and service:
Schemes", Work in Progress, November, 1997
draft-ietf-svrloc-service-scheme-04.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.
[4]P. St. Pierre, "Definition of printer: URLs for use with Service
Location", Work in Progress, October, 1997
draft-ietf-srvloc-printer-scheme-01.txt
Authors' Addresses
Questions about this memo can be directed to:
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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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