Service Location Working Group Erik Guttman
INTERNET DRAFT Charles Perkins
James Kempf
31 October 1997 Sun Microsystems
Service Templates and service: Schemes
draft-ietf-svrloc-service-scheme-05.txt
Status of This Memo
This document is a submission by the Service Location Working Group
of the Internet Engineering Task Force (IETF). Comments should be
submitted to the srvloc@corp.home.net mailing list.
Distribution of this memo is unlimited.
This document is an Internet-Draft. Internet-Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
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Abstract
The ''service:'' URL scheme name is used to define URLs (called
''service: URLs'' in this document) that are primarily intended to be
used by the Service Location Protocol in order to distribute service
access information. These schemes provide an extensible framework
for client-based network software to obtain configuration information
required to make use of network services. When registering a
service: URL, the URL SHOULD be accompanied by a set of well-defined
attributes which define the service. These attributes SHOULD
convey configuration information to client software, or service
characteristics meaningful to end users.
This document describes a formal procedure for defining and
standardizing new service types and attributes for use with the
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''service:'' scheme. The formal descriptions of service types and
attributes are templates that are human and machine understandable.
They SHOULD be used by administrative tools to parse service
registration information and by client applications to provide
localized translations of service attribute strings.
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Contents
Status of This Memo i
Abstract i
1. Introduction 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Service Location Protocol . . . . . . . . . . . . . . . . 4
2. Related work 4
3. Service URL Syntax and Semantics 4
3.1. Service URL Syntax . . . . . . . . . . . . . . . . . . . 4
3.2. Service URL Semantics . . . . . . . . . . . . . . . . . . 6
3.3. Use of service: URLs . . . . . . . . . . . . . . . . . . 7
3.4. Specifying the Service Type-Specific URL Syntax . . . . . 8
3.5. Accommodating Abstract Service Types . . . . . . . . . . 8
3.5.1. Advertising Abstract Service Types . . . . . . . 9
4. Syntax and Semantics of Service Type Specifications 10
4.1. Syntax of Service Type Templates . . . . . . . . . . . . 10
4.2. Semantics of Service Type Templates . . . . . . . . . . . 13
4.2.1. Definition of a Service Template . . . . . . . . 13
4.2.2. Service Type . . . . . . . . . . . . . . . . . . 14
4.2.3. Service Type Language . . . . . . . . . . . . . . 14
4.2.4. Version Number . . . . . . . . . . . . . . . . . 14
4.2.5. Description . . . . . . . . . . . . . . . . . . . 14
4.2.6. Syntax of the Service Type-specific URL Part . . 15
4.2.7. Attribute Definition . . . . . . . . . . . . . . 15
5. A Process For Standardizing New Service Types 19
6. IANA Considerations 20
7. Internationalization Considerations 21
7.1. Language Identification and Translation . . . . . . . . . 21
8. Security Considerations 22
A. Service Template Examples 22
A.1. ACAP . . . . . . . . . . . . . . . . . . . . . . . . . . 23
A.2. Abstract Service Template Example: Network-Management . 24
A.3. Concrete Service Template Example: Network-Management:SNMP
24
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A.4. service: URLs and SLP . . . . . . . . . . . . . . . . . . 26
1. Introduction
This document describes a URL scheme, called service: URL, which
defines network access information for network services using a
formal notation. In addition it describes how to define a set of
attributes to associate with a service: URL. These attributes will
allow end users and programs to select between network services of
the same type that have different capabilities. The attributes
are defined in a template document that is readable by people and
machines.
A client uses attributes to select a particular service. Service
selection occurs by obtaining the service: URL that offers the right
configuration for the client. Service type templates define the
syntax of service: URLs for a particular service type, as well as the
attributes which accompany a service: URL in a service registration.
Templates are used for the following distinct purposes:
1. Standardization
The template is reviewed before it is standardized. Once it is
standardized, all versions of the template are archived by IANA.
2. Service Registration
Servers making use of the Service Location Protocol [15] register
themselves and their attributes. They use the templates to
generate the service registrations. In registering, the service
must use the specified values for its attributes.
3. Client presentation of Service Information
Client applications may display service information. The
template provides type information and explanatory text which may
be helpful in producing user interfaces.
4. Internationalization
Entities with access to the template for a given service type in
two different languages may translate between the two languages.
A service may register itself in more than one language using
templates, though it has been configured by an operator who
registered service attributes in a single language.
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All grammar encoding follows the Augmented BNF (ABNF) [9] for syntax
specifications.
1.1. Terminology
This section introduces some terminology for describing service:
URLs.
service scheme
A URL scheme whose name starts with the string "service:" and
is followed by the service type name, constructed according to
the rules in this document. An example is "service:lpr:" for
the lpr print service [14].
service: URL
A URL constructed according to the service scheme definition.
It typically provides at least the following: The name of an
access protocol, and an address locating this service. The
service: URL may include url path information specific to the
type of service, as well as attribute information encoded
according to the URL grammar. The service: URL is used by
the Service Location Protocol to register and discover the
location of services. It may be used by other protocols and in
documents as well.
service type
A name identifying the semantics by which the remainder of
the service: URL is to be understood. It may denote either a
particular network protocol, or an abstract service associated
with a variety of protocols. If the service type denotes a
particular protocol, then the service type name SHOULD either
be assigned the name of a particular well known port [3]
by convention or or be the Assigned Numbers name for the
service [1].
abstract service type
A service type name which is associated with a variety of
different protocols. An example is given in Section A.
Section 3 discusses various ways that abstract types can be
accommodated.
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service registration
A service: URL and optionally a set of attributes comprise
a service registration. This registration is made by or on
behalf of a given service. The URL syntax and attributes must
conform to the service template for the registered service.
service template
A formal description of the service attributes and service
scheme associated with a particular service type.
1.2. Service Location Protocol
The Service Location Protocol [15] allows service: URLs to be
registered and discovered, though service: URLs may be also used in
other contexts.
Client applications discover service registrations by issuing queries
for services of a particular type, specifying the attributes of
the service: URLs to return. Clients retrieve the attributes of a
particular service by supplying its service: URL. Attributes for all
service registrations of a particular type can also be retrieved.
Services may register themselves, or registrations may be made on
their behalf. These registrations contain a service: URL, and
possibly attributes and digital signatures.
2. Related work
The "Finding Stuff" work by Ryan Moats, Martin Hamilton, and
Paul Leach uses service: URLs to provide access information about
arbitrary network protocols through DNS [11]. DNS SRV Resource
Records are a mechanism which provides a way to obtain a service by
type for a given domain [10], without specifying which instance of
the service type meet particular requirements.
3. Service URL Syntax and Semantics
This section describes the syntax and semantics of service: URLs.
3.1. Service URL Syntax
The syntax of the service: URL MUST conform to [6]. The only
exception is that the <password> field has been omitted from the
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<site> production, since plain text transmission of passwords is
now discouraged. Note that the syntax for the <sap> field depends
upon the service type definition. The <sap> field is the service
access point, and describes how to access the service. In addition,
although both upper case and lower case characters are recognized in
the <service-type> field for convenience, the name is case-folded
into lower case. Service types are therefore not distinguished on
the basis of case, so, for example, "http" and "HTTP" designate the
same service type. This is consistent with general URL practice, as
outlined in [7].
The ABNF for a service: URL is:
service: URL = "service:" service-type ":" sap
service-type = abstract-type ":" url-scheme / concrete-type
abstract-type = type-name [ "." naming-auth ]
concrete-type = protocol [ "." naming-auth ]
type-name = resname
naming-auth = resname
url-scheme = resname
; A recognized URL scheme name, standardized
; either through common practice or through
; approval of a standards body.
resname = alpha [ 1*(alpha / digit / "+" / "-") ]
sap = "//" site [ url-part ]
site = [ [ user "@" ] hostport ]
hostport = host [ ":" port ]
host = hostname / hostnumber
hostname = *( domainlabel "." ) toplabel
alphanum = alpha / digit
domainlabel = alphanum / alphanum *[alphanum / "-"] alphanum
toplabel = alpha / alpha *[ alphanum / "-" ] alphanum
hostnumber = ipv4-number / ipv6-number
ipv4-number = 1*3digit 3("." 1*3digit)
ipv6-number = 32hex
3digit = digit digit digit
port = 1*digit
; A port number must be included if the
; protocol field does not have an IANA
; assigned port number.
user = *[ uchar / ";" / "+" / "&" / "=" ]
url-part = [ url-path ] [ attr-list ]
url-path = 1 * ( "/" *xchar )
; Each service type must define its
; own syntax consistent
; with [6].
attr-list = 1 * ( ";" attr-asgn )
attr-asgn = attr-id / attr-id "=" attr-value
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safe = "$" / "-" / "_" / "." / "~"
extra = "!" / "*" / "'" / "(" / ")" / "," / "+"
uchar = unreserved / escaped
xchar = unreserved / reserved / escaped
escaped = "%" hex hex
hex "a" / "b" / "c" / "d" / "e" / digit
reserved = ";" / "/" / "?" / ":" / "@" / "&" / "=" / "+"
unreserved = alpha / digit / safe / extra
Certain characters must be escaped before use. To escape any
character, precede the two digits indicating its ASCII value by '%'.
3.2. Service URL Semantics
The service scheme-specific information following the "service:"
URL scheme identifier provides information necessary to access the
service. As described in the previous subsection, the form of a
service: URL is as follows:
service: URL = "service:" service-type ":" sap
where <sap> has the following form:
//addr-spec/url-path;attr-list
The <service-spec> field includes the <service-type> field. As
discussed in Section 1, the <service-type> can be either a concrete
protocol name, or an abstract type name.
The <service-part> field includes a site specification (the
<site> field) in the format specified by [6]. The <site> field
is typically either a domain name (DNS) or an IP network protocol
address for the service, and possibly a port number. Note that use
of DNS hostnames is preferred for ease of renumbering. The <site>
field can be null if other information in the service URL or service
attributes is sufficient to use the service.
The <sap> field allows more information to be provided (by way of
<url-path> and <attr-list>) that can uniquely locate the service or
resource if the <addr-spec> is not sufficient for that purpose.
An <attr-list> field appears at the end of the <url-part> field,
but is never required to exist in any service location registration.
The <attr-list> field is composed of a list of semicolon (";")
separated attribute assignments of the form:
attr-id "=" attr-value
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or for keyword attributes:
attr-id
Attributes are part of service: URLs when the attributes are required
to access a particular service. For instance, an ACAP [13] service
might require that the client authenticate with it through Kerberos.
Including an attribute in the service registration allows the ACAP
client to make use of the correct SASL [12] authentication mechanism.
The ACAP server's registration might look like:
service:acap://some.where.net;authentication=KERBEROSV4
Note that there can be other attributes of an ACAP server which are
not be appropriate to include in the URL. For instance, the list
of users who have access to the server is useful for selecting an
ACAP server, but is not required for a client to use the registered
service.
Attributes associated with the service: URL are not typically
included in the service: URL. They are stored and retrieved using
other mechanisms. The service: URL is uniquely identified with a
particular service agent or resource, and is used when registering or
requesting the attribute information. The Service Location Protocol
specifies how such information SHOULD be registered by network
services and obtained by client software.
3.3. Use of service: URLs
The service: URL is intended to allow arbitrary client/server and
peer to peer systems to make use of a standardized dynamic service
access point discovery mechanism.
It is intended that service: URLs be selected according to the
suitability of associated attributes. A client application can
obtain the URLs of several services of the same type and distinguish
the most preferable among them by means of their attributes. The
client uses the service: URL to communicate directly to a service.
Attributes are specified with a formal service template syntax
described in Section 4. If a service: URL registration includes
attributes, the registering agent SHOULD also keep track of the
attributes which characterize the service.
Registrations can be checked against the formal attribute
specification defined in the template by the client or agent
representing the client. Service registration are typically done
using the Service Location Protocol [15] (SLP). SLP provides a
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mechanism for service: URLs to be obtained dynamically, according to
the service's attributes.
It is also possible to obtain service: URLs from documents and using
other protocols. In this case, the URL may not be accompanied by
the service attributes. The context in which the URL appears SHOULD
make it clear, if possible, when the service is appropriate to use.
For example, in a mail message, a service might be recommended for
use when the user is in a branch office. Or, an HTML document might
include a service: URL as a pointer to a service, describing in text
what the service does and who is authorized to use it.
3.4. Specifying the Service Type-Specific URL Syntax
When a service type is specified, the specification includes the
definition of the syntax for all URLs that are registered by services
of that particular type. For instance, the "lpr" service type may be
defined with service: URLs in the following form:
service:printer:lpr://<address of printer>/<queue name>
The section of the URL after the address of the printer:
"/" <queue name>
is specific to the lpr service type and corresponds to the
<url-path> field of the general service: URL syntax. This part is
specified when the lpr service type is specified.
3.5. Accommodating Abstract Service Types
An abstract service type is a service type that can be implemented by
a variety of different service agents.
In order to register an service: URL for an abstract service type the
'abstract-type' grammar rule described in section 3.1 is used. This
will result in a URL which includes enough information to use the
service, namely, the protocol, address and path information. Unlike
'concrete' service: URLs, however, the service type is not enough
to determine the service access. Rather, an abstract service type
denotes a class of service types. The following subsection discusses
this point in more detail.
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3.5.1. Advertising Abstract Service Types
Some services may make use of several protocols that are in common
use and are distinct services in their own right. In these cases an
abstract service type is appropriate. What is essential is that all
the required information for the service is clearly defined.
For example, suppose a network service is being developed for
dynamically loading device drivers. The client requires the
following three pieces of information before it can successfully load
and instantiate the driver:
1. The protocol used to load the driver code, for example, "ftp",
"http" or "tftp"
2. A pathname identifying where the driver code is located, for
example "/systemhost/drivers/diskdrivers.drv",
3. The name of the driver, for example, "scsi".
The temptation is to form the first two items into a URL and embed
that into a service: URL. As an example which should be avoided,
service:ftp:/x3.bean.org/drivers/diskdrivers.drv;driver=scsi
is a service: URL which seems to indicate where to obtain the driver.
Rather, an abstract service-type SHOULD be used. The service type is
not "ftp", as the example indicates. Rather, it is "device-drivers".
The service: URL that should be used, consistent with the rules in
section [6], is the following:
service:device-drivers:ftp://x3.bean.org/drivers/diskdrivers.drv;
driver=scsi;platform=sys3.2-rs3000
Other URLs for the same service using other protocols are also
supported, as in:
service:device-drivers:tftp://x2.bean.org/vol3/disk/drivers.drv;
driver=scsi;platform=sys3.2-rs3000
service:device-drivers:http://www.bean.org/drivers/drivpak.drv;
driver=scsi;platform=sys3.2-rs3000
Using SLP, a search for the service type "device-drivers" may return
all of the three URLs listed above. The client selects the most
appropriate access protocol for the desired resource.
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The fundamental requirement is that the abstract service type MUST
be well specified. This requirement is imposed so that program code
or human users have enough information to access the service. In
every case, a well-specified abstract type will include either an
access protocol and a network address where the service is available,
or an embedded URL for a standardized URL scheme that describes
how to access the service. In the example above, there are three
further requirements: A URL path is included for access protocols
indicating the document to download, and two attributes are included
to characterize the driver.
4. Syntax and Semantics of Service Type Specifications
Service type specifications are documents in a formal syntax defining
properties important to service registration. These properties are:
1. General information on the service type specification itself,
2. The syntax of the service type-specific part of the service URL,
3. The definition of attributes associated with a service.
The service type specification document is the service type template.
The following subsections describe the syntax and semantics of
service type templates.
4.1. Syntax of Service Type Templates
Service template documents are encoded in a simple form. They may be
translated into any language or character set, but the template used
for standardization MUST be encoded in UTF8 [16] and be in English.
A template document begins with a block of text assigning values to
five document identification items. The five identification items
can appear in any order within the block, but conventionally the
"type" item, which assigns the service type name, occurs at the very
top of the document in order to provide context for the rest of
the the document. The attribute definition item occurs after the
document identification items.
All items end with a blank line. The reserved characters are ";",
"%", "=", ",", "#", LF, and CR. Reserved characters MUST be escaped.
The escape sequence is the same as described in [6].
The service template is encoded in a UTF8 character set, but
submitted as a part of an internet-draft, which is encoded in ASCII
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characters. All characters which are outside of the ASCII range MUST
be escaped using the % HEX HEX syntax. For example, the letter e
accent aigue would be represented as "%c3%a9". Unfortunately, this
will detract from the readability of the service template in the
internet draft. Hopefully some public domain tools will emerge for
translating escaped UTF8 characters into humanly readable ones.
Values in value lists are separated by commas. A value list is
terminated by a newline not preceded by a comma. If the newline is
preceded by a comma, the value list is interpreted to continue onto
the next line.
Attribute identifiers, attribute type names, and flags are all
case insensitive. For ease of presentation, upper and lower case
characters can be used to represent these in the template document.
Newlines are significant in the grammar. They delimit one item from
another, as well as separating parts of items internally.
String values are considered to be a sequence of non-whitespace
tokens potentially with embedded whitespace, separated from each
other by whitespace. Commas delimit lists of strings. String values
are trimmed so as to reduce any sequence of white space interior to a
string to a single white space. Preceding or trailing white space is
removed. For example:
" some value , another example "
is trimmed to
"some value" and "another example".
Note that there can be no ambiguity in string tokenization because
values in value lists are separated by a comma. String tokens are
not delimited by double quotes (") as is usually the case with
programming languages.
Attribute tags and values are useful for directory look-up. In this
case, decoding of character escapes and trimming white space MUST
be performed before string matching. In addition, string matching
SHOULD be case insensitive.
Templates obey the following ABNF [9] grammar:
template = tem-attrs attr-defs
tem-attrs = schemetype schemevers schemelang
schemetext schemeurl
schemetype = "type" "=" scheme termdef
schemevers = "version" "=" version-no termdef
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schemelang = "language" "=" isolang termdef
schemetext = "description" "=" newline desc-text termdef
schemeurl = "url-syntax" "=" newline url-bnf termdef
url-bnf = *[ com-chars ]
; An ABNF describing the <url-path> production
; in the service: URL grammar of Section 3.1.
scheme = service-type [ "." naming-auth ]
service-type = scheme-name
naming-auth = scheme-name
scheme-name = alpha [1*schemechar] [ "." 1*schemechar ]
schemechar = alpha / digit / "-" / "+" /
version-no = 1*digit "." 1*digit
isolang = 2*3lower-alpha ;see [4]
desc-text = *[ com-chars ]
; A block of free-form text for reading by
; people describing the service in a short,
; informative manner.
termdef = newline newline
attr-defs = *( attr-def / keydef )
attr-def = id "=" attrtail
keydef = id "=" "keyword" newline [help-text] newline
attrtail = type flags newline [value-list] [help-text]
[value-list] newline
id = 1*attrchar
type = "string" / "integer" / "boolean" / "opaque"
flags = ["m"/"M"] ["l"/"L"] ["o"/"O"] ["x"/"X"]
value-list = value newline / value "," value-list /
value "," newline value-list
help-text = 1*( "#" help-line )
; A block of free-form text for reading by
; people describing the attribute and
; its values.
help-line = *[ com-chars ] newline
attrchar = schemechar / ":" / "_" / "$" / "~" / "@" / "." /
"|" / "<" / ">" / "*" / "&"
value = string / integer / boolean / opaque
string = safe-char *[safe-char / white-sp] safe-char
integer = [ "+" | "-" ] 1*digit
boolean = "true" / "false"
opaque = 1*digit ":" 4*radix64-char
; The digits define the original length of
; the opaque value. The restricted character
; string is the radix-64 encoding of the
; opaque value( [8], Sect. 6.8.)
; Newlines are ignored in decoding radix-64
; values.
com-chars = safe-char / white-sp / "," / ";"/ "%"
safe-char = attrchar / escaped / " " / "!" / '"' / "'" /
"|" / "(" / ")" / "+" / "-" / "." / ":" /
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"=" / "?" / "[" / "]" / "{" / "/" / "{" /
"$"
; All UTF8 printable characters are
; included except ",", "%", ";", and "#".
escaped = "%" hex hex
hex = digit / "A" / "B" / "C" / "D" / "E" /
"a" / "b" / "c" / "d" / "e"
white-sp = space / tab
newline = CR / ( CR LF )
4.2. Semantics of Service Type Templates
The service type template defines the service attributes and service:
URL syntax for a particular service type. The attribute definition
includes the attribute type, default values, allowed values and other
information.
4.2.1. Definition of a Service Template
There are six items included in the service template. The semantics
of each item is summarized below.
- type
The scheme name of the service scheme. The scheme name consists
of the service type name and an optional naming authority name,
separated from the service type name by a period. See 4.2.2 for
the conventions governing service type names.
- version
The version number of the service type specification.
- language
The language of the service type specification.
- description
A description of the service suitable for inclusion in text read
by people.
- url-syntax
The syntax of the service type-specific URL part of the service:
URL.
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- attribute definitions
A collection of zero or more definitions for attributes
associated with the service in service registrations.
Each of the following subsections deals with one of these items.
4.2.2. Service Type
The service scheme consists of the service type name and an optional
naming authority name separated from the service type name by a
period. The service scheme is a string that is appended to the
'service:' URL scheme identifier, and is the value of the "type"
item in the template document. If the naming authority name is
absent it is assumed to be IANA.
4.2.3. Service Type Language
The service type language is a RFC 1766 Language Tag defining the
language of the template [4] and is the value of the "language" item.
4.2.4. Version Number
The version number of the service type template is the value of the
"version" item. A draft proposal starts at 0.0, and the minor number
increments once per revision. A standardized template starts at 1.0.
Additions of optional attributes add one to the minor number, and
additions of required attributes, changes of definition, or removal
of attributes add one to the major number. The intent is that an
old service template still accurately, if incompletely, defines the
attributes of a service registration if the template only differs
from the registration in its minor version. See Section 5 for more
detail on how to use the version attribute.
4.2.5. Description
The description is a block of text readable by people in the language
of the template and is the value of the "description" item. It
should be sufficient to identify the service to human readers and
provide a short, informative description of what the service does.
If the service type corresponds to a particular protocol, the
protocol specification must be cited here. The protocol need not be
a standardized protocol. The template might refer to a proprietary
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specification, and refer the reader of the template to a contact
person for further information.
4.2.6. Syntax of the Service Type-specific URL Part
The syntax of the service type-specific part of the service:
URL is provided in the template document as the value of the
"url-syntax" item. The <url-path> field of the service: URL is
designed to provide additional information to locate a service when
the <addr-spec> field is not sufficient. The <url-path> field
distinguishes URLs of a particular service type from those of another
service type. For instance, in the case of the lpr service type, the
<url-path> must include the queue name [14], but other service types
may not require this information.
The syntax for the <url-path> field MUST accompany the definition
of a new service type, unless the URL scheme has already been
standardized and is not a service: URL. The syntax is included in the
template document as an ABNF [9] following the rules for URL syntax
described in [6]. There is no requirement for a service scheme to
support a <url-path>. The <url-path> field can be very simple,
or even omitted. If the URL scheme has already been standardized,
the "url-syntax" item SHOULD include a reference to the appropriate
standardization documents. Abstract service types may defer this
field to the template documents describing their concrete instances.
4.2.7. Attribute Definition
The bulk of the template is typically devoted to defining service
type-specific attributes. An attribute definition precisely
specifies the attribute's type, other restrictions on the attribute
(whether it is multi-valued, optional, etc), some text readable by
people describing the attribute, and lists of default and allowed
values. The only required information is the attribute's type, the
rest are optional. Registration, deregistration and the use of
attributes in queries can be accomplished using the Service Location
Protocol [15] or other means, and discussion of this is beyond the
scope of the document.
Attributes are used to convey information about a given service for
purposes of differentiating different services of the same type.
They convey information to be used in the selection of a particular
service to establish communicate with, either through a program
offering a human interface or programmatically. Attributes can be
encoded in different character sets and in different languages. The
procedure for doing this is described in Section 7.
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An attribute definition begins with the specification of the
attribute's identifier and ends with a single empty line. Attributes
definitions have five components (in order of appearance in a
definition):
1. An attribute identifier which acts as the name of the attribute,
2. Attribute descriptors (type and flags),
3. An optional list of values which are assigned to the attribute by
default,
4. An optional block of text readable by people providing a short,
informative description of the attribute,
5. An optional list of allowed values which restrict the value or
values the attribute can take on.
4.2.7.1. The Attribute Identifier
An attribute definition starts with the specification of the
attribute's identifier. The attribute's identifier functions as the
name of the attribute. Note that the characters used to compose an
attribute identifier are restricted to those characters considered
unrestricted for inclusion in a URL according to [6]. The reason
is that services can display prominent attributes in their service:
URL registrations. Each attribute identifier must be unique in the
template. Since identifiers are case folded, upper case and lower
case characters are the same.
4.2.7.2. The Attribute Type
Attributes can have one of five different types: string, integer,
boolean, opaque, or keyword. The attribute's type specification is
separated from the attribute's identifier by an equal sign ("=") and
follows the equal sign on the same line. The string, signed integer,
and boolean types have the standard programming language or database
semantics. Integers are restricted to those signed values that can
be represented in 32 bits. The character set used to represent
strings is not specified at the time the template is defined, but
rather is determined by the service registration. Booleans have the
standard syntax. Opaques are radix64 numbers [8] that can be used
to represent any other kind of data. Keywords are attributes that
have no characteristics other than their existence (and possibly the
descriptive text in their definition).
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Keyword and boolean attributes impose restrictions on the following
parts of the attribute definition. Keyword attribute definitions
MUST have no flag information following the type definition, nor any
default or allowed values list. Boolean attributes are single value
only, i.e., multi-valued boolean attributes are not allowed.
4.2.7.3. Attribute Flags
Flags determine other characteristics of an attribute. With the
exception of keyword attributes, which may not have any flags,
flags follow the attribute type on the same line as the attribute
identifier, and are separated from each other by whitespace. Flags
may appear in any order after the attribute type. Other information
must not follow the flags, and only one flag identifier of a
particular flag type is allowed per attribute definition.
The semantics of the flags are as follows:
- o or O
Indicates that the attribute is optional. If this flag is
missing, the attribute is required in every service registration.
- m or M
Indicates that the attribute can take on multiple values. If
this flag is present, every value in a multi-valued attribute
has the same type as the type specified in the type part of the
attribute definition. Boolean attributes must not include this
flag.
- l or L
Indicates that attribute is literal, i.e. is not meant to be
translated into other languages. If this flag is present, the
attribute is not considered to be readable by people and should
not be translated when the template is translated. See Section 7
for more information about translation.
- x or X
Indicates that clients MUST specify the attribute and a value in
a service query in order to narrowly focus which service: URLs
are returned. The query will be rejected by DA's that utilize
templates if the attribute is not included, regardless of whether
the other attributes match.
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The values for multivalued attributes are an unordered set.
Deletions of individual values from a multivalued attribute are not
supported, and deletion of the attribute causes the entire set of
values to be removed.
4.2.7.4. Default Value or List
If the attribute definition includes a default value or, in the
case of multivalued attributes, a default values list, it begins
on the second line of the attribute definition and continues
over the following lines until a line ends without a comma. As a
consequence, newlines cannot be embedded in values unless escaped.
See Section 3.1.
Particular attribute types and definitions restrict the default
values list. Keyword attributes must not have a list of defaults.
If an optional attribute's definition has an allowed values list,
then a default value or list is not optional but required. The
motivation for this is that defining an attribute with an allowed
values list is meant to restrict the values the attribute can take
on, and requiring a default value or list assures that the default
value is a member of the given set of allowed values.
The default value or list indicates what values the attribute is
given if no values are assigned to the attribute when a service
is registered. If an optional attribute's definition includes no
default value or list, the following defaults are assigned:
1. String values are assigned the empty string,
2. Integer values are assigned zero,
3. Boolean values are assigned false,
4. Opaque values are assigned a byte array containing no values,
5. Multi-valued attributes are initialized with a single value.
For purposes of translating nonliteral attributes, the default values
list is taken to be an ordered set, and translations MUST maintain
that order.
4.2.7.5. Descriptive Text
Immediately after the default values list, if any, a block of
descriptive text SHOULD be included in the attribute definition.
This text is meant to be readable by people, and should include
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a short, informative description of the attribute. It may also
provide additional information, such as a description of the allowed
values. This text is primarily designed for display by interactive
browsing tools. The descriptive text is set off from the surrounding
definition by a crosshatch character ("#") at the beginning of
the line. The text should not, however, be treated as a comment
by parsing and other tools, since it is an integral part of the
attribute definition. Within the block of descriptive text, the text
is transferred verbatim, including indentation and line breaks, so
any formatting is preserved.
4.2.7.6. Allowed Values List
Finally, the attribute definition concludes with an optional
allowed values list. The allowed values list, if any, follows the
descriptive text, or, if the descriptive text is absent, the initial
values list. The syntax of the allowed values list is identical to
that of the initial values list. The allowed values list is also
terminated by a line that does not end in a comma. If the allowed
values list is present, assignment to attributes is restricted to
members of the list.
As with the default values list, the allowed values list is also
considered to be an ordered set for purposes of translation.
4.2.7.7. Conclusion of An Attribute Definition
An attribute definition concludes with a single empty line.
5. A Process For Standardizing New Service Types
New service types can be suggested simply by providing a service type
template and a short description about how to use the service. The
template MUST have its "version" template attribute set to 0.0.
The minor version number increments once with each change until it
achieves 1.0. There is no guarantee any version of the service
template is backward compatible before it reaches 1.0.
Once a service template has reached 1.0, the definition is "frozen"
for that version. New templates must be defined, of course, to
refine that definition, but the following rules must be followed:
- Any new optional attribute defined for the template increases
the minor version number by one. All other attributes for the
version must continue to be supported as before. A client which
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supports 1.x can still use later versions of 1.y (where x<y) as
it ignores attributes it doesn't know about.
- Adding a required attribute, removing support for an attribute
or changing definition of an attribute requires changing the
major version number of a service template. A client application
may be unable to make use of this information, or it may need
to obtain the most recent service template to help the user
interpret the service information.
The template should be submitted as an 'individual contribution'
Internet Draft. The Internet Draft must include a 'template begins
here' and 'template ends here' marking, in text, so that it is
trivial to cut and paste the template from the internet draft.
A notice must be posted to the SVRLOC WG mailing list for review.
Ideally, experts in the implementation and deployment of the
particular protocol are consulted so as to add or delete attributes
or change their definition to make the template as useful as
possible. The mailing list will be maintained even when the SVRLOC
WG goes dormant for the purpose of discussing service templates.
All published versions of the template must be available on-line,
including obsolete ones.
Once consensus is achieved, the template should be reissued with
possible corrections, having its Version number set to 1.0. If there
is no comment on the template after 3 months, it should be considered
to have been accepted. See Section 6 for details on how templates
are submitted to an IANA registry of templates.
6. IANA Considerations
The Applications Area directors appoint a set of reviewers, including
a 'lead reviewer'. Any of these reviewers may ask for clarification
of a service template. If no reviewers dissent, the lead reviewer
will submit the template to the IANA for inclusion in a registry.
Mailing list participants supply input to the process but do not make
the decision whether to accept a service template or request changes
or clarifications.
The service template file has a naming convention:
<service-type> "." <version-no> "." <isolang>
Each of these fields are defined in Section 3. They correspond
to the values of the template fields "type", "version" and
"lang". The files for the example templates in this document
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are called "acap.0.0.en", "network-management.0.0.en" and
"network-management:snmp.0.0.en". See Section A.
No Internet Draft describing a service type template will be accepted
unless it includes a security considerations section and contact
information for the template document author.
The IANA will maintain a registry containing both the service type
templates, and the template description document containing the
service type template, including all previous versions. The IANA
will receive notice by email from the reviewers, which will contain a
reference to the Internet Draft that contains the service template.
This Internet Draft will be edited to remove the Internet Draft
headers and replace them with a simple header stating "This document
contains a Service Type Template."
Should any trademark or copyright issues arise due to the naming of
the Service Type or attributes in the Service Template, the offending
names may have to be changed. The owner of the trademark may
demand that this be done. The filer of the Template that requires
renaming will decide the new names to use. If such issues arise,
the committee of reviewers in consultation with the IESG directorate
will proceed to satisfy these conditions. The IANA should simply
notify the committee and they will pursue the action: The IANA is
not expected to resolve trademark issues with Service Type templates.
7. Internationalization Considerations
The service: URL must be encoded using the rules set forth in [6].
The character set encoding is limited to specific ranges within the
US-ASCII character set [5].
The template is encoded in UTF8 characters.
7.1. Language Identification and Translation
The language used in attribute strings should be identified using the
"language" template item as defined by [4].
A program can translate a service registration from one language to
another provided it has both the template of the language for the
registration and the template of the desired target language. All
standardized attributes are in the same order in both templates.
All non-arbitrary strings, including the descriptive help text, is
directly translatable from one language to another. Non-literal
attribute definitions, attribute identifiers, attribute type names,
attribute flags, and the boolean constants "true" and "false" are
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never translated. Translation of attribute identifiers is prohibited
because, as with domain names, they can potentially be part of a
service: URL and therefore their character set is restricted. In
addition, as with variable identifiers in programming languages, they
could become embedded into program code.
All strings used in attribute values are assumed translatable unless
explicitly defined as being literal, so that best effort translation
(see below) does not modify strings which are meant to be interpreted
by a program, not a person.
There are two ways to go about translation: standardization and best
effort.
When the service type is standardized, more than one document can
be submitted for review. One service type description is approved
as a master, so that when a service type template is updated in one
language, all the translations (at least eventually) reflect the same
semantics.
If no document exists describing the standard translation of the
service type, a 'best effort' translation for strings should be done.
8. Security Considerations
Service type templates provide information that is used to interpret
information obtained by the Service Location Protocol. If these
templates are modified or false templates are distributed, services
may not correctly register themselves, or clients might not be able
to interpret service information.
The service: URLs themselves specify the service access point and
protocol for a particular service type. These service: URLs could
be distributed and indicate the location of a service other than
that normally want to used. The Service Location Protocol [15]
distributes service: URLs and has an authentication mechanism that
allows service: URLs of registered services to be signed and for the
signatures to be verified by clients.
Each Service Template will include a security considerations section
which will describe security issues with using the service scheme for
the specific Service Type.
A. Service Template Examples
The text in the template example sections is to be taken as being a
single file.
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The ACAP example shows how to use service templates for an
application that has very few attributes. Clients request the ACAP
server where their user data is located by including their user name
as the value of the user attribute.
The Network-Management example shows how abstract service types are
defined and how a corresponding concrete instance is defined. A
system might support any of several Network-Management services.
Here we give only one concrete instance of the abstract type. It is
not necessary to register concrete templates for an abstract service
type if the abstract service type template is completely clear as to
what possible values can be used as a concrete type, and what their
interpretation is.
A.1. ACAP
-------------------------template begins here-----------------------
type=ACAP
version=0.0
lang=en
description=
The ACAP service URL provides the location of an ACAP service.
url-syntax=
url-path= ; There is no URL path defined for an ACAP URL.
users= string M L O
# The list of all users which the ACAP server supports.
groups= string M L O
# The list of all groups which the ACAP server supports.
--------------------------template ends here------------------------
The Internet Draft describing the ACAP scheme template must indicate
contact information and security considerations, e.g.,
contact="Erik Guttman" <Erik.Guttman@sun.com>
security considerations=
If the USER and GROUPS attributes are included a
possibility exists that the list of identities for users or groups
can be discovered. This information would otherwise be difficult
to discover.
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A.2. Abstract Service Template Example: Network-Management
The Internet Draft for the service type template contains the
following text:
-------------------------template begins here-----------------------
type=Network-Management
version=0.0
lang=en
description=
This is an abstract service type. The purpose of the network-
management service type is to organize into a single category
information crucial to properly managing networked hosts. This
will allow all network-management services of a host, as well
as basic host configuration to be obtained by a single query
using SLP.
url-syntax=
url-path= ; Depends on the concrete service type.
; See these templates.
--------------------------template ends here------------------------
In addition, the following format might be used for the needed
contact and security considerations information.
..........
contact="Erik Guttman" <Erik.Guttman@sun.com>
security considerations=
See the security considerations of the concrete service types.
A.3. Concrete Service Template Example: Network-Management:SNMP
-------------------------template begins here-----------------------
type=service:network-management:snmp
version=0.0
lang=en
description=
The 'service:network-management:SNMP:' URL provides information
about the SNMP manageability of a given host. Namely, if this
URL exists for a host (denoted by the <addr-spec> in the URL,)
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the host supports SNMP. The path contains an enumeration of
the MIB groups that are supported by the host. The OID "1.3.6.1."
is assumed as a prefix to each of the OID terms below.
url-syntax=
url-path = ( [port-list] [comm-string] [oid-list])
; None of the attributes listed in the URL path
; are required. They MAY be included.
port-list = ";ports=" port-list
ports = port / port "," ports
; See the Service URL <port> production rule.
; This field is defined as an attribute, below.
comm-string = ";read-community-string=" 1*uchar
; See the Service URL <uchar> production rule.
; This field is defined as an attribute, below.
oid-list = ";oids=" oids
; This field is defined as an attribute, below.
oids = oid / oid "," oids
oid = DIGIT [ "." (DIGIT ".") DIGIT]
ports=integer M L
161
# This attribute must be included. It lists all ports on which
# SNMP Agents are listening.
read-community-string=string L O
# The read community string may be included as an attribute of
# a service:network-managment:snmp: URL. This is useful in
# cases where the community string is PUBLIC and ease of access
# to the SNMP Agent is desired. See the 'security considerations.'
oid=string M L O
# This attribute identifies a list of 'top level' MIB groups.
# This is entirely optional, as such values can be obtained
# directly using SNMP. The value of including this information
# in a service:network-managment:snmp: URL is that it will save
# the Manager time; the MIB information can be obtained along
# with the URL without requiring additional sequential requests
# being sent to the managed system.
--------------------------template ends here------------------------
.........
contact="Erik Guttman" <Erik.Guttman@sun.com>
security considerations=
The read-community-string MUST only be included if the value
of this string is considered to be public. If this attribute
is included, absolutely anyone may access the SNMP Agent and
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get information from it. This may be desirable in some cases.
If this string is considered confidential information, the
read-community-string MUST NOT be included in the URL path
nor in service registrations of the URL made through SLP or
other protocols.
A.4. service: URLs and SLP
A user with an ACAP enabled email client application should not
be bothered with knowing the address of their ACAP server. The
mail client program can use SLP to obtain the ACAP service: URL
automatically, say 'service:acap://server1.nosuch.org', by issuing
a Service Request. In the event that this ACAP server failed, the
Email client can issue the same service request again to find the
backup ACAP server, say 'service:acap://server2.nosuch.org'. In both
cases, the service: URL conforms to the ACAP service template as do
the associated attributes (user and group.)
An SNMP based network Manager can use SLP to obtain
service:network-management:SNMP URLs. This allows the network
Manager to proceed to manage the hosts identified by these URLs
without having to scan networks one address at a time, etc. SLP
provides the capability to send multicast or directed broadcasts to
obtain this information from every managed host.
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References
[1] Protocol and service names, October 1994.
ftp://ftp.isi.edu/in-notes/iana/assignments/service-names.
[2] Address family numbers, October 1995.
ftp://ftp.isi.edu/in-notes/iana/assignments/address-family-numbers.
[3] Port numbers, July 1997.
ftp://ftp.isi.edu/in-notes/iana/assignments/port-numbers.
[4] H. Alvestrand. Tags for the Identification of Languages. RFC
1766, March 1995.
[5] ANSI. Coded Character Set -- 7-bit American Standard code for
Information Interchange. X3.4-1986, 1986.
[6] T. Berners-Lee, R. Fielding, and L. Masinter. Uniform Resource
Locators (URL): Generic Syntax and Semantics. RFC1738 as
amended by RFC1808
[7] T. Berners-Lee, L. Masinter, and M. McCahill. Uniform Resource
Locators (URL). RFC 1738, December 1994.
[8] N. Freed and N. Borenstein. Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies.
RFC 2045, November 1996.
[9] D. Crocker and P Overell. Augmented BNF for Syntax
Specifications: ABNF. draft-ietf-drums-abnf-04.txt, September
1997. (work in progress).
[10] A. Gulbrandsen and P. Vixie. A DNS RR for specifying the
location of services (DNS SRV). RFC 2052, October 1996.
[11] R. Moats and M. Hamilton. Finding Stuff (Providing information
to support service discovery). draft-ietf-svrloc-advertise-00.txt,
February 1997. (work in progress).
[12] J. Myers. Simple Authentication and Security Layer (SASL). RFC
2222, October 1997.
[13] J. G. Myers. ACAP -- Application Configuration Access Prototol.
draft-ietf-acap-spec-04.txt, June 1997. (work in progress).
[14] Pete St. Pierre. Definition of lpr: URLs for use with Service
Location. draft-ietf-svrloc-lpr-scheme-01.txt, November 1997.
(work in progress).
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[15] J. Veizades, E. Guttman, C. Perkins, and S. Kaplan. Service
Location Protocol. RFC 2165, July 1997.
[16] F. Yergeau. UTF-8, a transformation format of unicode and ISO
10646. RFC 2044, October 1996.
Authors' Addresses
Questions about this memo can be directed to:
Erik Guttman Charles E. Perkins James Kempf
Sun Microsystems Sun Microsystems Sun Microsystems
Bahnstr. 2 901 San Antonio Rd. 901 San Antonio Rd.
74915 Waibstadt Palo Alto, CA, 94303 Palo Alto, CA, 94303
Germany USA USA
+49 7263 911484 1 650 786 6464 1 650 786 5890
1 650 786 6445 (fax) 1 650 786 6445 (fax)
erik.guttman@sun.com charles.perkins@sun.com james.kempf@sun.com
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