CoRE Z. Shelby
Internet-Draft Sensinode
Intended status: Standards Track October 25, 2010
Expires: April 28, 2011
CoRE Link Format
draft-ietf-core-link-format-01
Abstract
This document defines a link format for use by constrained CoAP web
servers to describe URIs of resources offered along with other
attributes. Based on the HTTP Link Header format, 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 list of links to resources hosted by a server.
Status of this Memo
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This Internet-Draft will expire on April 28, 2011.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Link Format . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Target and context URIs . . . . . . . . . . . . . . . . . 4
2.2. Link relation 'rel' usage . . . . . . . . . . . . . . . . 5
2.3. Description 'd' usage . . . . . . . . . . . . . . . . . . 5
2.4. Alternative URI 'sh' usage . . . . . . . . . . . . . . . . 5
2.5. Resource name 'n' usage . . . . . . . . . . . . . . . . . 5
2.6. Content-type code 'ct' usage . . . . . . . . . . . . . . . 5
2.7. Resource identifier 'id' usage . . . . . . . . . . . . . . 6
2.8. Examples . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Well-known Interface . . . . . . . . . . . . . . . . . . . . . 6
3.1. Query Filtering . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
5.1. Well-known 'core' URI . . . . . . . . . . . . . . . . . . 9
5.2. New link-format Internet media type . . . . . . . . . . . 9
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 10
7. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 10
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
8.1. Normative References . . . . . . . . . . . . . . . . . . . 11
8.2. Informative References . . . . . . . . . . . . . . . . . . 11
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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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 [I-D.shelby-core-coap-req].
The discovery of resources offered 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. In this document we refer to the discovery of
resources offered by a CoAP server as resource discovery.
The core function of such a discovery mechanism is to provide URIs
("links") for the resources offered, complemented by information
describing the relationship between the resource description and each
resource as well as other attributes. When such a collection of
attributed resource references (links) is offered as a resource of
its own (as opposed to as HTTP headers delivered with a different
resource), we speak of its representation as a link-format.
This document specifies a link-format for use in CoRE resource
discovery by extending the HTTP Link Header Format
[I-D.nottingham-http-link-header] to describe resources hosted by a
constrained server. 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 to resources hosted by a server.
2. Link Format
CoRE resource discovery extends the HTTP Link Header format specified
in [I-D.nottingham-http-link-header] which is specified in Augmented
Backus-Naur Form (ABNF) notation [RFC2616]. The format does not
require special XML or binary parsing, and is extensible.
This link format is used for a similar purpose to that described in
[I-D.nottingham-http-link-header], to describe one or more
relationships between resources. However in this specification the
link format is extended with specific constrained M2M link
parameters, links are carried as a payload rather than in a message
header, and a default interface is defined to discover resources
described by these links.
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[] did not require an Internet media
type for this link format, as it assumes to be carried in an HTTP
header. This specification thus requests a Internet media type for
this format (see Section 5.2).
The CoRE link format uses the ABNF description and associated rules
in Section 5 of [I-D.nottingham-http-link-header]. In addition, the
URI, URI-reference and pchar rules are taken from [RFC3986]. The
"Link:" text is omitted as that is part of the HTTP Link Header.
Multiple link descriptions are separated by commas. The CoRE link
format MUST use the US-ASCII character set (support for RFC2231
encoding of non-ASCII content TBD). The following CoRE specific
link-extension parameters to the format are defined:
link-extension = ( "d" "=" <"> URI-reference <">)
link-extension = ( "sh" "=" <"> URI-reference <">)
link-extension = ( "n" "=" quoted-string )
link-extension = ( "ct" "=" integer )
link-extension = ( "id" "=" quoted-string )
integer = 1*DIGIT
2.1. Target and context URIs
Each link description 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 description is by default the
URI of the resource that returned the link-format representation.
Thus each link can be thought of as describing a target resource
hosted by the server in the absence of further relation information.
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). 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
link entries, each describing a resource hosted by a server. See
Section 5 of [RFC3986] for a description of how URIs are constructed
from URI references.
As per Section 5.2 of [I-D.nottingham-http-link-header] a link
description MAY include an "anchor" attribute, in which case the
context is the URI included in that attribute. This can be used to
describe a relationship between two resources. A consuming
implementation can however choose to ignore such links. It is not
expected that most implementations will be able to derive useful
information from explicitly anchored links.
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2.2. Link relation 'rel' usage
Link descriptions in CoRE are typically used to describe entry points
to services hosted by the server, and thus in the absence of the rel
attribute the registered "service" relation type is assumed. In the
CoRE link format the service relation type indicates that the link is
a service hosted by the server (in the absence of the anchor
attribute). A description can make use of any registered relation
type or extension types in the form of a URI by including the rel
attribute.
2.3. Description 'd' usage
The description "d" attribute can provide a URI to a specific
interface definition used to access the target resource. This could
be for example a URI to the WADL definition of the target resource.
Multiple description attributes MAY appear in a link description.
2.4. Alternative URI 'sh' usage
This attribute can be included to define an alternative short URI
which can also be used to access the target resource. Multiple
alternative short URI attributes MAY appear in a link description.
2.5. Resource name 'n' usage
The resource name "n" attribute is used to assign either a human
readable or a semantically important name to a resource. In the case
of a temperature sensor resource the name could be something like
"Temperature in Centigrade", a URI to an ontology like
"http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature" or an
application-specific semantic name like "TemperatureC". Multiple
name attributes MAY appear in a link description.
2.6. Content-type code 'ct' usage
The Content-type code "ct" attribute provides a hint about the
Internet media type this resource returns. 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 description.
Alternatively, the "type" attribute MAY be used to indicate an
Internet media type as a quoted-string. It is not however expected
that constrained implementations are able to parse quoted-string
Content-type values.
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2.7. Resource identifier 'id' usage
The resource identifier "id" field is a unique identifier (e.g. UUID
or XRI) for this resource for use in e.g. resource or search
directories. The resource identifier attribute MUST NOT appear more
than once in a link description.
2.8. Examples
A few examples of typical link descriptions in this format follows.
Multiple resource descriptions in a representation are separated by
commas. Commas can also occur in quoted strings and URIs but do not
end a description. Linefeeds never occur in the actual format, but
are shown in the example for readability.
This example includes link descriptions for an index to sensors
hosted by a server, along with links two two different sensors.
GET /.well-known/core
</sensors>;rel="index";n="Sensor Index",
</sensors/temp>;sh="/t";n="TemperatureC",
</sensors/light>;sh="/l";ct=41;n="LightLux"
This example arranges link descriptions hierarchically, with the
entry point including a link description to a sub-resource containing
link descriptions about the sensors.
GET /.well-known/core
</.well-known/core/sensors>;rel="section"
;type="application/link-format"
GET /.well-known/core/sensors
</sensors/temp>;sh="/t";n="TemperatureC",
</sensors/light>;sh="/l";ct=41;n="LightLux"
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 (resource
descriptions) offered by that constrained server. Well-known
resources have a path component that begins with "/.well-known/" as
specified in [RFC5785]. This document defines a new well-known path
prefix for CoRE discovery "/.well-known/core" [Section 5.1]. A
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server implementing this specification MUST support this path prefix
on the default port appropriate for the protocol for the purpose of
resource discovery. It is however up to the application which link
descriptions are included and how they are organized. In the absense
of any links, a zero-length payload is returned. The resource
representation of this resource is described in Section 2.
The CoRE resource discovery interface supports the following
interactions:
o Performing a GET on /.well-known/core to the default port returns
a list of link descriptions available from a CoAP server (if any).
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 accepting [POST /.well-known/core messages] from other
servers. This adds the resources of other end-points as a sub-
resource in which absolute URIs are included for the link-values.
The details of such resource directory functionality is however
out of scope for this document.
End-points with a large number of resources SHOULD include resource
descriptions only for important services or collections and 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, e.g. </.well-known/core/sensors>. Such a
hierarchy SHOULD be under the /.well-known/core path but could be
located elsewhere.
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:
filter-query = resource-param "=" query-pattern
resource-param = "uri" | "d" | "sh" | "n" | "id"
query-pattern = 1*pchar [ "*" ]
The resource-param "uri" refers to the URI-reference between the "<"
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and ">" characters of a link-value. Other resource-param values
refer to the link attribute they name. (TBD: Do we want to add the
resource description attributes that I excluded, or the standard
link-param attributes from I-D.nottingham-http-link-header?)
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 exact
match is performed on the query string, and a 200 OK response is
returned with link descriptions that contains the matching entries
(if any). In contrast, a multicast request with a query string MUST
not be responded to if filtering is not supported (to avoid a
needless response storm). If resource descriptions are organized
hierarchically, a query on the root resource /.well-known/core SHOULD
return all matching resource descriptions from the entire hierarchy.
An example query on the link descriptions from Section 2 may look
like:
GET /.well-known/core?n=LightLux
</sensors/light>;sh="/l";ct=41;n="LightLux"
4. Security Considerations
This document needs the same security considerations as described in
Section 7 of [I-D.nottingham-http-link-header]. 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.
Great care must be taken when processing multicast requests using
CoAP for the well-known link-format resources, as this 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.
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5. IANA Considerations
5.1. Well-known 'core' URI
This memo registers the "core" well-known URI in the Well-Known URI
Registry as defined by [RFC5785].
URI suffix: core
Change controller: IETF
Specification document(s): [[ this document ]]
Related information: None
5.2. 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: US-ASCII
Security considerations: None
Interoperability considerations:
Published specification: [[ this document ]]
Applications that use this media type: CoAP server and client
implementations.
Additional information:
Magic number(s):
File extension(s):
Macintosh file type code(s):
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Intended usage: COMMON
Restrictions on usage: None
Author: CoRE WG
Change controller: IETF
6. Acknowledgments
Special thanks to Mark Nottingham and Eran Hammer-Lahav for
discussions and ideas that led to this draft, and to Carsten Bormann
and Peter Bigot 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, Peter Bigot, Colin O'Flynn and
David Ryan for helpful comments and discussions that have shaped the
document.
7. Changelog
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.
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o Removed the aussumption of a default content-type assumption.
8. References
8.1. Normative References
[I-D.ietf-core-coap]
Shelby, Z., Frank, B., and D. Sturek, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-02
(work in progress), September 2010.
[]
Nottingham, M., "Web Linking",
draft-nottingham-http-link-header-10 (work in progress),
May 2010.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
8.2. Informative References
[I-D.shelby-core-coap-req]
Shelby, Z., Stuber, M., Sturek, D., Frank, B., and R.
Kelsey, "CoAP Requirements and Features",
draft-shelby-core-coap-req-02 (work in progress),
October 2010.
Author's Address
Zach Shelby
Sensinode
Kidekuja 2
Vuokatti 88600
FINLAND
Phone: +358407796297
Email: zach@sensinode.com
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