CoRE Z. Shelby
Internet-Draft Sensinode
Intended status: Standards Track December 10, 2010
Expires: June 13, 2011
CoRE Link Format
draft-ietf-core-link-format-02
Abstract
This document defines Web Linking using a link format for use by
constrained web servers to describe hosted resources, their
attributes and other relationships between links. Based on the HTTP
Link Header format defined in RFC5988, the CoRE Link Format is
carried as a payload and is assigned an Internet media type. A well-
known URI is defined as a default entry-point for requesting the
links hosted by a server.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on June 13, 2011.
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include Simplified BSD License text as described in Section 4.e of
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Link Format . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Web Linking in CoRE . . . . . . . . . . . . . . . . . . . 4
2.1.1. Target and context URIs . . . . . . . . . . . . . . . 4
2.1.2. Link relations . . . . . . . . . . . . . . . . . . . . 5
2.1.3. Use of anchors . . . . . . . . . . . . . . . . . . . . 5
2.2. CoRE link extensions . . . . . . . . . . . . . . . . . . . 5
2.2.1. Resource name 'n' attribute . . . . . . . . . . . . . 6
2.2.2. Interface description 'd' attribute . . . . . . . . . 6
2.2.3. Content-type code 'ct' attribute . . . . . . . . . . . 6
2.2.4. Maximum size estimate 'sz' attribute . . . . . . . . . 6
2.2.5. Observable 'obs' attribute . . . . . . . . . . . . . . 7
3. Well-known Interface . . . . . . . . . . . . . . . . . . . . . 7
3.1. Query Filtering . . . . . . . . . . . . . . . . . . . . . 8
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6.1. Well-known 'core' URI . . . . . . . . . . . . . . . . . . 10
6.2. New 'hosts' relation type . . . . . . . . . . . . . . . . 11
6.3. New link-format Internet media type . . . . . . . . . . . 11
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12
8. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . . 14
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14
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1. Introduction
The Constrained RESTful Environments (CoRE) working group aims at
realizing the REST architecture in a suitable form for the most
constrained nodes (e.g. 8-bit microcontrollers with limited RAM and
ROM) and networks (e.g. 6LoWPAN). CoRE is aimed at machine-to-
machine (M2M) applications such as smart energy and building
automation.
The discovery of resources hosted by a constrained server is very
important in machine-to-machine applications where there are no
humans in the loop and static interfaces result in fragility. The
discovery of resources provided by an HTTP Web Server is typically
called Web Discovery and the description of relations between
resources is called Web Linking [RFC5988]. In this document we refer
to the discovery of resources hosted by a constrained web server,
their attributes and other resource relations as CoRE Resource
Discovery.
The main function of such a discovery mechanism is to provide URIs
(called links) for the resources hosted by the server, complemented
by attributes about those resources and possible further link
relations. In CoRE this collection of links is carried as a resource
of its own (as opposed to as HTTP headers delivered with a specific
resource). This document specifies a link format for use in CoRE
Resource Discovery by extending the HTTP Link Header Format [RFC5988]
to describe these link descriptions. The CoRE Link Format is carried
as a payload and is assigned an Internet media type. A well-known
URI "/.well-known/core" is defined as a default entry-point for
requesting the list of links about resources hosted by a server, and
thus performing CoRE Resource Discovery.
2. Link Format
The CoRE Link Format extends the HTTP Link Header format specified in
[RFC5988], which is specified in Augmented Backus-Naur Form (ABNF)
notation. The format does not require special XML or binary parsing,
is fairly compact, and is extensible - all important characteristics
for CoRE. It should be noted that this link format is just one
serialization of typed links defined in [RFC5988], others include
Atom or HTTP links. It is expected that resources discovered in the
CoRE Link Format may also be made available in alternative formats on
the greater Internet.
Section 5 of [RFC5988] did not require an Internet media type for the
defined link format, as it was defined to be carried in an HTTP
header. This specification thus defines a Internet media type for
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the CoRE Link Format (see Section 6.3).
The CoRE link format uses the ABNF description and associated rules
in Section 5 of [RFC5988]. In addition, the pchar rule is taken from
[RFC3986]. The "Link:" text is omitted as that is part of the HTTP
Link Header. As in [RFC5988], multiple link descriptions are
separated by commas. Note that commas can also occur in quoted
strings and URIs but do not end a description. The CoRE link format
MUST use UTF-8 encoding, which SHOULD be in NFC (Unicode
Normalization Form C). See Section 3 of [RFC5198], which explains
why it useful to represent Unicode in a single unique form.
2.1. Web Linking in CoRE
This link format is used for a similar purpose to that described in
[RFC5988], to describe relationships between resources via typed
links, so-called "Web Linking". However in this specification Web
Linking is extended with specific constrained M2M attributes, links
are carried as a message payload rather than in an HTTP Link Header,
and a default interface is defined to discover resources hosted by a
server. This specification defines a new relation type "hosts",
which indicates that the resource is hosted by the server from which
the link document was requested.
2.1.1. Target and context URIs
Each link conveys one target URI as a URI-reference inside angle
brackets ("<>"). The context URI of a link (also called base URI in
[RFC3986]) conveyed in the CoRE Link Format is by default built from
the scheme and authority parts of the target URI. In the absence of
this information in the target URI, the context URI is built from the
scheme and authority that was used for referencing the resource
returning the set of links, replacing the path with an empty path.
Thus by default links can be thought of as describing a target
resource hosted by the server. Other relations can be expressed by
including an anchor parameter (which defines the context URI) along
with an explicit relation parameter. This is an important difference
to the way the HTTP Link Header format is used, as it is included in
the header of an HTTP response for some URI (this URI is by default
the context URI). Thus the HTTP Link Header is by default relating
the target URI to the URI that was requested. In comparison, the
CoRE link format includes one or more links, each describing a
resource hosted by a server by default. Other relations can be
expressed by using the anchor parameter. See Section 5 of [RFC3986]
for a description of how URIs are constructed from URI references.
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2.1.2. Link relations
Since links in the CoRE Link Format are typically used to describe
resources hosted by a server, and thus in the absence of the relation
parameter the new relation type "hosts" is assumed (see Section 6.2).
The "hosts" relation type indicates that the target URI is a resource
hosted by the server given by the base URI, or, if present, the
anchor parameter.
To express other relations a links can make use of any registered
relation parameter or target attributes by including the relation
parameter. The context of a relation can be defined using the anchor
parameter. In this way, relations between resources hosted on a
server, or between hosted resources and external resources can be
expressed.
2.1.3. Use of anchors
As per Section 5.2 of [RFC5988] a link description MAY include an
"anchor" attribute, in which case the context is the URI included in
that attribute. This is used to describe a relationship between two
resources. A consuming implementation can however choose to ignore
such links. It is not expected that all implementations will be able
to derive useful information from explicitly anchored links.
2.2. CoRE link extensions
The following CoRE specific target attributes are defined. These
attributes describe information useful in accessing the target link
of the relation, and in some cases may be URIs. These URIs MUST be
treated as indicators, and are not meant to be actually retrieved
like a URL. When attributes are compared, they MUST be compared as
strings. Relationships to resources that are meant to be retrieved
should be expressed as separate links using the anchor attribute and
the appropriate relation type.
link-extension = ( "n" "=" quoted-string )
link-extension = ( "d" "=" <"> URI-reference <">)
link-extension = ( "ct" "=" integer )
link-extension = ( "sz" "=" integer )
link-extension = ( "obs" )
integer = 1*DIGIT
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2.2.1. Resource name 'n' attribute
The resource name "n" attribute is used to assign a semantically
important name to a resource. In the case of a temperature sensor
resource this could be an application-specific semantic name like
"TemperatureC", a URN like "urn:temperature:centigrade" or a URI
referencing a specific concept in an ontology like
"http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature". Multiple name
attributes MAY appear in a link.
The name attribute is not meant to used to assign a human readable
name to a resource. The "title" attribute defined in [RFC5988] is
meant for that purpose.
2.2.2. Interface description 'd' attribute
The interface description "d" attribute is used to provide a URI or
URN indicating a specific interface definition used to interact with
the target resource. This could be for example the URI of a WADL
definition of the target resource
"http://www.example.org/sensor.wadl", or a URN indicating the type of
interface to the resource "urn:sensor". Multiple description
attributes MAY appear in a link.
2.2.3. Content-type code 'ct' attribute
The Content-type code "ct" attribute provides a hint about the
Internet media type this resource returns. Note that this is only a
hint, and does not override the Content-type Option of a CoAP
response obtained by actually following the link. The value is in
the CoAP identifier code format as a decimal ASCII integer
[I-D.ietf-core-coap]. For example application/xml would be indicated
as "ct=41". If no Content-type code attribute is present then
nothing about the type can be assumed. The Content-type code
attribute MUST NOT appear more than once in a link.
Alternatively, the "type" attribute MAY be used to indicate an
Internet media type as a quoted-string [RFC5988]. It is not however
expected that constrained implementations are able to parse quoted-
string Content-type values. A link MAY include either a ct attribute
or a type attribute, but MUST NOT include both.
2.2.4. Maximum size estimate 'sz' attribute
The maximum size estimate attribute "sz" gives an indication of the
maximum size of the link indicated by the target URI. This attribute
is not expected to be included for small resources that can
comfortably by carried in a single MTU, but SHOULD be included for
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resources larger than that. The maximum size estimate attribute MUST
NOT appear more than once in a link.
2.2.5. Observable 'obs' attribute
The observable attribute "obs" is used to indicate that this resource
supports the observation feature defined in [I-D.ietf-core-observe].
This attribute is used as a flag, and thus it has no value component.
The observable attribute MUST NOT appear more than once in a link.
3. Well-known Interface
Resource discovery in CoRE is accomplished through the use of a well-
known resource URI which returns a list of links about resources
hosted by that server and other link relations. Well-known resources
have a path component that begins with "/.well-known/" as specified
in [RFC5785]. This document defines a new well-known resource for
CoRE Resource Discovery "/.well-known/core". A server implementing
this specification MUST support this resource on the default port
appropriate for the protocol for the purpose of resource discovery.
It is however up to the application which links are included and how
they are organized. In the absence of any links, a zero-length
payload is returned. The resource representation of this resource
uses the CoRE Link Format described in Section 2. URIs under the
path /.well-known/core are reserved by this specification for use by
servers in organizing links into multiple resources, each with their
own URI.
The CoRE resource discovery interface supports the following
interactions:
o Performing a GET on /.well-known/core to the default port returns
a set of links available from the CoAP server (if any) in the CoRE
Link Format. These links might describe resources hosted on that
server, on other servers, or express other kinds of link relations
as described in Section 2.
o Filtering may be performed on any of the link format attributes
using a query string as specified in Section 3.1. For example
[GET /.well-known/core?n=TemperatureC] would request resources
with the name TemperatureC. A server is not however required to
support filtering.
o More capable servers such as proxies could support a resource
directory by requesting the resource descriptions of other end-
points or allowing servers to POST requests to /.well-known/core.
The details of such resource directory functionality is however
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out of scope for this document, and is expected to be specified
separately.
End-points with a large number of resources SHOULD include resource
descriptions only for important links and MAY organize their resource
descriptions into a hierarchy of link resources. This is done by
including links in the /.well-known/core list which point to other
resource lists using the "section" relation type, e.g. </.well-known/
core/sensors>. Such a hierarchy MUST be under the /.well-known/core
path.
3.1. Query Filtering
A server implementing this document MAY recognize the query part of a
resource-discovery URI as a filter on the resources to be returned.
The query part should conform to the following syntax. Note that
this only defines querying for a single parameter at a time.
filter-query = resource-param "=" query-pattern
resource-param = "uri" | parmname
query-pattern = 1*pchar [ "*" ]
The resource-param "uri" refers to the URI-reference between the "<"
and ">" characters of a link. Other resource-param values refer to
the link attribute they name. Filtering is performed by comparing
the query-pattern against the value of the attribute identified by
the resource-param for each link-value in the collection of resources
identified by the URI path.
If the decoded query-pattern does not end with "*", a link value
matches the query only if the value of the attribute or URI-reference
denoted by the resource-param is bytewise identical to the query-
pattern. If the decoded query-pattern ends with "*", it is
sufficient that the remainder of the query-pattern be a prefix of the
value denoted by the resource-param.
It is not expected that very constrained nodes support filtering.
Implementations not supporting filtering MUST simply ignore the query
string and return the whole resource for unicast requests. An
implementation supporting filtering MUST return only those entries
for which the value of the link attribute identified by the resource-
param contains the query-pattern as a substring. If resource
descriptions are organized hierarchically, a query on the root
resource /.well-known/core MUST return all matching resource
descriptions from the entire hierarchy.
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When using a transfer protocol like CoAP that supports multicast
requests, special care is taken. A multicast request with a query
string MUST not be responded to if filtering is not supported (to
avoid a needless response storm).
4. Examples
A few examples of typical link descriptions in this format follows.
Multiple resource descriptions in a representation are separated by
commas. Linefeeds never occur in the actual format, but are shown in
the example for readability.
This example includes links to two different sensors.
REQ: GET /.well-known/core
RES: 200 OK
</sensors/temp>;ct=41;n="TemperatureC",
</sensors/light>;ct=41;n="LightLux"
This example arranges link descriptions hierarchically, with the
entry point including a link to a sub-resource containing links about
the sensors.
REQ: GET /.well-known/core
RES: 200 OK
</.well-known/core/sensors>;rel="section";ct=40
REQ: GET /.well-known/core/sensors
RES: 200 OK
</sensors/temp>;n="TemperatureC",
</sensors/light>;ct=41;n="LightLux"
An example query filter may look like:
REQ: GET /.well-known/core?n=LightLux
RES: 200 OK
</sensors/light>;ct=41;n="LightLux"
This example shows the use of an anchor attribute to relate the
temperature sensor resource to an external description and to an
alternative URL.
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REQ: GET /.well-known/core
RES: 200 OK
</sensors>;rel="index";n="Sensor Index",
</sensors/temp>;n="TemperatureC",
</sensors/light>;ct=41;n="LightLux",
<http://www.example.com/sensors/temp123>;anchor="/sensors/temp"
;rel=describedby,
</t>;anchor="/sensors/temp";rel=alternate
If a client is interested to find relations about a particular
resource, it can perform a query on the anchor parameter:
REQ: GET /.well-known/core?anchor=/sensors/temp
RES: 200 OK
<http://www.example.com/sensors/temp123>;anchor="/sensors/temp"
;rel=describedby,
</t>;anchor="/sensors/temp";rel=alternate
5. Security Considerations
This document needs the same security considerations as described in
Section 7 of [RFC5988]. The /.well-known/core resource may be
protected e.g. using DTLS when hosted on a CoAP server as per
[I-D.ietf-core-coap] Section 10.2.
Multicast requests using CoAP for the well-known link-format
resources could be used to perform denial of service on a constrained
network. A multicast request SHOULD only be accepted if the request
is sufficiently authenticated and secured.
CoRE link format parsers should be aware that a link description may
be cyclical, i.e., contain a link to itself. These cyclical links
could be direct or indirect (i.e., through referenced link
resources). Care should be taken when parsing link descriptions and
accessing cyclical links.
6. IANA Considerations
6.1. Well-known 'core' URI
This memo registers the "core" well-known URI in the Well-Known URI
Registry as defined by [RFC5785].
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URI suffix: core
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
6.2. New 'hosts' relation type
This memo registers the new "hosts" Web Linking relation type as per
[RFC5988].
Relation Name: hosts
Description: Refers to a resource hosted by the server indicated by
the link context.
Reference: [[ this document ]]
Notes: This relation is used in CoRE where links are retrieved as a
/.well-known/core resource representation, and by default the context
of the links is the server at coap://authority from which /.well-
known/core was requested.
Application Data: None
6.3. New link-format Internet media type
This memo registers the a new Internet media type for the CoRE link
format, application/link-format.
Type name: application
Subtype name: link-format
Required parameters: None
Optional parameters: The query string may contain uri= to match the
URI, or any other attribute defined for the link format to match that
attribute.
Encoding considerations: UTF-8 (NFC)
Security considerations: None
Interoperability considerations:
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Published specification: [[ this document ]]
Applications that use this media type: CoAP server and client
implementations for resource discovery and HTTP applications that use
the link-format as a payload.
Additional information:
Magic number(s):
File extension(s):
Macintosh file type code(s):
Intended usage: COMMON
Restrictions on usage: None
Author: CoRE WG
Change controller: IETF
7. Acknowledgments
Special thanks to Peter Bigot, who has made a considerable number
reviews and text contributions that greatly improved the document.
In particular, Peter is responsible for the ABNF descriptions and the
idea for a new "hosts" relation type.
Thanks to Mark Nottingham and Eran Hammer-Lahav for discussions and
ideas that led to this draft, and to Carsten Bormann and Martin
Thomson for extensive comments and contributions that improved the
text.
Thanks to Michael Stuber, Richard Kelsey, Cullen Jennings, Guido
Moritz, Peter Van Der Stok, Adriano Pezzuto, Lisa Dussealt, Alexey
Melnikov, Gilbert Clark, Salvatore Loreto, Petri Mutka, Szymon Sasin,
Robert Quattlebaum, Robert Cragie, Angelo Castellani, Tom Herbst, Ed
Beroset, Gilman Tolle, Robby Simpson, Colin O'Flynn and David Ryan
for helpful comments and discussions that have shaped the document.
8. Changelog
Changes from ietf-01 to ietf-02:
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o Added references to RFC5988 (#41).
o Removed sh and id link-extensions (#42).
o Defined the use of UTF-8 (#84).
o Changed query filter definition for any parameter (#70).
o Added more example, now as a separate section (#43).
o Mentioned cyclical links in the security section (#57).
o Removed the sh and id attributes, added obs and sz attributes
(#42).
o Improved the context and relation description wrt RFC5988 and
requested a new "hosts" default relation type (#85).
Changes from ietf-00 to ietf-01:
o Editorial changes to correct references.
o Formal definition for filter query string.
o Removed URI-reference option from "n" and "id".
o Added security text about multicast requests.
Changes from shelby-00 to ietf-00:
o Fixed the ABNF link-extension definitions (quotes around URIs,
integer definition).
o Clarified that filtering is optional, and the query string is to
be ignored if not supported (and the URL path processed as
normally).
o Required support of wildcard * processing if filtering is
supported.
o Removed the aussumption of a default content-type assumption.
9. References
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9.1. Normative References
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
9.2. Informative References
[I-D.ietf-core-coap]
Shelby, Z., Frank, B., and D. Sturek, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-03
(work in progress), October 2010.
[I-D.ietf-core-observe]
Hartke, K. and Z. Shelby, "Observing Resources in CoAP",
draft-ietf-core-observe-00 (work in progress),
October 2010.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
April 2010.
Author's Address
Zach Shelby
Sensinode
Kidekuja 2
Vuokatti 88600
FINLAND
Phone: +358407796297
Email: zach@sensinode.com
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