draft-ietf-core-link-format-02.txt   draft-ietf-core-link-format-03.txt 
CoRE Z. Shelby CoRE Z. Shelby
Internet-Draft Sensinode Internet-Draft Sensinode
Intended status: Standards Track December 10, 2010 Intended status: Standards Track March 14, 2011
Expires: June 13, 2011 Expires: September 15, 2011
CoRE Link Format CoRE Link Format
draft-ietf-core-link-format-02 draft-ietf-core-link-format-03
Abstract Abstract
This document defines Web Linking using a link format for use by This document defines Web Linking using a link format for use by
constrained web servers to describe hosted resources, their constrained web servers to describe hosted resources, their
attributes and other relationships between links. Based on the HTTP attributes and other relationships between links. Based on the HTTP
Link Header format defined in RFC5988, the CoRE Link Format is Link Header format defined in RFC5988, the CoRE Link Format is
carried as a payload and is assigned an Internet media type. A well- 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 known URI is defined as a default entry-point for requesting the
links hosted by a server. links hosted by a server.
skipping to change at page 1, line 36 skipping to change at page 1, line 36
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material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 13, 2011. This Internet-Draft will expire on September 15, 2011.
Copyright Notice Copyright Notice
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Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Link Format . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Web Linking in CoRE . . . . . . . . . . . . . . . . . . . 3
2.1. Web Linking in CoRE . . . . . . . . . . . . . . . . . . . 4 1.2. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1.1. Target and context URIs . . . . . . . . . . . . . . . 4 1.2.1. Discovery . . . . . . . . . . . . . . . . . . . . . . 4
2.1.2. Link relations . . . . . . . . . . . . . . . . . . . . 5 1.2.2. Resource Collections . . . . . . . . . . . . . . . . . 5
2.1.3. Use of anchors . . . . . . . . . . . . . . . . . . . . 5 1.2.3. Resource Directory . . . . . . . . . . . . . . . . . . 5
2.2. CoRE link extensions . . . . . . . . . . . . . . . . . . . 5 1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 5
2.2.1. Resource name 'n' attribute . . . . . . . . . . . . . 6 2. Link Format . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.2.2. Interface description 'd' attribute . . . . . . . . . 6 2.1. Target and context URIs . . . . . . . . . . . . . . . . . 7
2.2.3. Content-type code 'ct' attribute . . . . . . . . . . . 6 2.2. Link relations . . . . . . . . . . . . . . . . . . . . . . 7
2.2.4. Maximum size estimate 'sz' attribute . . . . . . . . . 6 2.3. Use of anchors . . . . . . . . . . . . . . . . . . . . . . 8
2.2.5. Observable 'obs' attribute . . . . . . . . . . . . . . 7 3. CoRE link extensions . . . . . . . . . . . . . . . . . . . . . 8
3. Well-known Interface . . . . . . . . . . . . . . . . . . . . . 7 3.1. Resource type 'rt' attribute . . . . . . . . . . . . . . . 8
3.1. Query Filtering . . . . . . . . . . . . . . . . . . . . . 8 3.2. Interface description 'if' attribute . . . . . . . . . . . 9
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 3.3. Content-type code 'ct' attribute . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10 3.4. Maximum size estimate 'sz' attribute . . . . . . . . . . . 9
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10 4. Well-known Interface . . . . . . . . . . . . . . . . . . . . . 10
6.1. Well-known 'core' URI . . . . . . . . . . . . . . . . . . 10 4.1. Query Filtering . . . . . . . . . . . . . . . . . . . . . 10
6.2. New 'hosts' relation type . . . . . . . . . . . . . . . . 11 5. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
6.3. New link-format Internet media type . . . . . . . . . . . 11 6. Security Considerations . . . . . . . . . . . . . . . . . . . 13
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 12 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 13
8. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7.1. Attribute Registry . . . . . . . . . . . . . . . . . . . . 13
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.2. Well-known 'core' URI . . . . . . . . . . . . . . . . . . 14
9.1. Normative References . . . . . . . . . . . . . . . . . . . 14 7.3. New 'hosts' relation type . . . . . . . . . . . . . . . . 14
9.2. Informative References . . . . . . . . . . . . . . . . . . 14 7.4. New link-format Internet media type . . . . . . . . . . . 15
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 14 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
9. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 16
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
10.2. Informative References . . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19
1. Introduction 1. Introduction
The Constrained RESTful Environments (CoRE) working group aims at The Constrained RESTful Environments (CoRE) working group aims at
realizing the REST architecture in a suitable form for the most realizing the Representational State Transfer (REST) architecture
constrained nodes (e.g. 8-bit microcontrollers with limited RAM and [REST] in a suitable form for the most constrained nodes (e.g. 8-bit
ROM) and networks (e.g. 6LoWPAN). CoRE is aimed at machine-to- microcontrollers with limited memoryt) and networks (e.g. 6LoWPAN
machine (M2M) applications such as smart energy and building [RFC4944]). CoRE is aimed at Machine-to-Machine (M2M) applications
automation. such as smart energy and building automation.
The discovery of resources hosted by a constrained server is very The discovery of resources hosted by a constrained server is very
important in machine-to-machine applications where there are no important in machine-to-machine applications where there are no
humans in the loop and static interfaces result in fragility. The humans in the loop and static interfaces result in fragility. The
discovery of resources provided by an HTTP Web Server is typically discovery of resources provided by an HTTP [RFC2616] Web Server is
called Web Discovery and the description of relations between typically called Web Discovery and the description of relations
resources is called Web Linking [RFC5988]. In this document we refer between resources is called Web Linking [RFC5988]. In this document
to the discovery of resources hosted by a constrained web server, we refer to the discovery of resources hosted by a constrained web
their attributes and other resource relations as CoRE Resource server, their attributes and other resource relations as CoRE
Discovery. Resource Discovery.
The main function of such a discovery mechanism is to provide URIs The main function of such a discovery mechanism is to provide
(called links) for the resources hosted by the server, complemented Universal Resource Indicators (URIs, called links) for the resources
by attributes about those resources and possible further link hosted by the server, complemented by attributes about those
relations. In CoRE this collection of links is carried as a resource resources and possible further link relations. In CoRE this
of its own (as opposed to as HTTP headers delivered with a specific collection of links is carried as a resource of its own (as opposed
resource). This document specifies a link format for use in CoRE to HTTP headers delivered with a specific resource). This document
Resource Discovery by extending the HTTP Link Header Format [RFC5988] specifies a link format for use in CoRE Resource Discovery by
to describe these link descriptions. The CoRE Link Format is carried extending the HTTP Link Header Format [RFC5988] to describe these
as a payload and is assigned an Internet media type. A well-known link descriptions. The CoRE Link Format is carried as a payload and
URI "/.well-known/core" is defined as a default entry-point for is assigned an Internet media type. A well-known URI "/.well-known/
requesting the list of links about resources hosted by a server, and core" is defined as a default entry-point for requesting the list of
thus performing CoRE Resource Discovery. links about resources hosted by a server, and thus performing CoRE
Resource Discovery.
1.1. Web Linking in CoRE
What is the difference between the CoRE Link Format and [RFC5988]?
Technically the CoRE Link Format is a serialization of a typed link
as specified in [RFC5988], used to describe relationships between
resources, so-called "Web Linking". 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 also defines a new relation type "hosts",
which indicates that the resource is hosted by the server from which
the link document was requested.
Why not just use the HTTP Link Header? In HTTP, the Link Header can
be used to carry link information about a resource along with an HTTP
response. This works well for the typical use case for a web server
and browser, where further information about a particular resource is
useful after accessing it. In CoRE the main use case for Web Linking
is the discovery of which resources a server hosts in the first
place. Although some resources may have further links associated
with them, this is expected to be an exception. For that reason the
CoRE Link Format serialization is carried as a resource
representation of a well-known URI. The CoRE Link Format does re-use
the format of the HTTP Link Header serialization defined in
[RFC5988].
1.2. Use Cases
Typical use cases for Web Linking on today's web include e.g.
describing the author of a web page, or describing relations between
web pages (next chapter, previous chapter etc.). Web Linking can
also be applied to M2M applications, where typed links are used to
assist a machine client in finding and understanding how to use
resources on a server. In this section a few use cases are described
for how the CoRE Link Format could be used in M2M applications. For
further technical examples see Section 5. As there are a large range
of M2M applications, these use cases are purposely generic. This
document assumes that different deployments or application domains
will define the appropraite REST interface descriptions along with
Resource Types to make discovery meaniningful.
1.2.1. Discovery
In M2M application, for example home or building automation, there is
a need for local clients and servers to find and interact with each
other without human intervention. The CoRE Link Format can be used
by servers in such environments to enable Resource Discovery of the
resources hosted by the server.
Resource Discovery can be performed either unicast or multicast.
When a server's IP address is already known, either a priori or
resolved via the Domain Name System (DNS), unicast disovery is
performed in order to locate the entry point to the resource of
interest. This is performed using a GET to /.well-known/core on the
server, which returns a payload in the CoRE Link Format. A client
would then match the appropriate Resource Type, Interface Description
and possible Content-Type for its application. These attributes may
also be included in the query string in order to filter the number of
links returned in a response.
Multicast resource discovery is useful when a client needs to locate
a resource within a limited scope, and that scope supports IP
multicast. A GET request to the appropriate multicast address is
made for /.well-known/core. In order to limit the number and size or
responses, a query string is recommended with the known attributes.
Typically a resource would be discovered based on its Resource Type
and/or Interface Description, along with possible application
specific attributes.
1.2.2. Resource Collections
RESTful designs of M2M interfaces often make use of collections of
resources. For example an index of temprature sensors on a data
collection node or a list of alarms on a home security controller.
The CoRE Link Format can be used to make it possible to find the
entry point to a collection and traverse its members. The entry
point of a collection would always be included in /.well-known/core
to enable its discovery. The members of the collection can be
defined either through the interface description of the resource
along with a parameter resource for the size of the collection, or by
using the link format to describe each resource in the collection.
These links could be located under /.well-known/core or hosted for
example in the root resource of the collection.
1.2.3. Resource Directory
In many deployment scenarios, for example constrained networks with
sleeping servers, or large M2M deployments with bandwidth limited
access networks, it makes sense to deploy resource directory entities
which store links to resources stored on other servers. Think of
this as a limited search engine for constrained M2M resources.
The CoRE Link Format can be used by a server to register resources
with a resource directory, or to allow a resource directory to poll
for resources. Resource polling uses the same process as unicast or
multicast discovery, however usually without filtering. Resource
registration can be achived by having each server POST their
resources to /.well-known/core on the resource directory. This in
turn adds links to the resource directory under an appropriate
resource. These links can then be discovered by any client by a
performing a GET on the resource directory using a query string
filter.
1.3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This specification requires readers to be familiar with all the terms
and concepts that are discussed in [RFC5988]. This specification
makes use of the following terminology:
Web Linking
A framework for indicating the relationships between web
resources.
Link
Also called "typed links" in RFC5988. A link is a typed
connection between two resources identified by URIs. Made up of a
context URI, a link relation type, a tarfet URI, and optional
target attributes.
Link Format
A particular serialisation of typed links.
CoRE Link Format
A particular serialization of typed links based the HTTP Link
Header serialization defined in Section 5 of RFC5988, but carried
as a resource representation with a MIME type.
Attribute
Properly called "Target Attribute" in RFC5988. A set of key/value
pairs that descibe the link or its target.
CoRE Resource Discovery
When a client discovers the list of resources hosted by a server,
their attributes and other link relations by accessing /.well-
known/core.
2. Link Format 2. Link Format
The CoRE Link Format extends the HTTP Link Header format specified in The CoRE Link Format extends the HTTP Link Header format specified in
[RFC5988], which is specified in Augmented Backus-Naur Form (ABNF) [RFC5988], which is specified in Augmented Backus-Naur Form (ABNF)
notation. The format does not require special XML or binary parsing, notation [RFC5234]. The format does not require special XML or
is fairly compact, and is extensible - all important characteristics binary parsing, is fairly compact, and is extensible - all important
for CoRE. It should be noted that this link format is just one characteristics for CoRE. It should be noted that this link format
serialization of typed links defined in [RFC5988], others include is just one serialization of typed links defined in [RFC5988], others
Atom or HTTP links. It is expected that resources discovered in the include HTML link, Atom feed links [RFC4287] or HTTP Link Headers.
CoRE Link Format may also be made available in alternative formats on It is expected that resources discovered in the CoRE Link Format may
the greater Internet. also be made available in alternative formats on the greater
Internet. The CoRE Link Format is only expected to be supported in
constrained networks and M2M systems.
Section 5 of [RFC5988] did not require an Internet media type for the 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 defined link format, as it was defined to be carried in an HTTP
header. This specification thus defines a Internet media type for header. This specification thus defines a Internet media type for
the CoRE Link Format (see Section 6.3). the CoRE Link Format (see Section 7.4).
The CoRE link format uses the ABNF description and associated rules The CoRE link format uses the ABNF description and associated rules
in Section 5 of [RFC5988]. In addition, the pchar rule is taken from 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 [RFC3986]. The "Link:" text is omitted as that is part of the HTTP
Link Header. As in [RFC5988], multiple link descriptions are Link Header. As in [RFC5988], multiple link descriptions are
separated by commas. Note that commas can also occur in quoted separated by commas. Note that commas can also occur in quoted
strings and URIs but do not end a description. The CoRE link format strings and URIs but do not end a description. The CoRE link format
MUST use UTF-8 encoding, which SHOULD be in NFC (Unicode MUST use UTF-8 encoding, which SHOULD be in NFC (Unicode
Normalization Form C). See Section 3 of [RFC5198], which explains Normalization Form C). See Section 3 of [RFC5198], which explains
why it useful to represent Unicode in a single unique form. why it useful to represent Unicode in a single unique form.
2.1. Web Linking in CoRE 2.1. Target and context URIs
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 Each link conveys one target URI as a URI-reference inside angle
brackets ("<>"). The context URI of a link (also called base URI in brackets ("<>"). The context URI of a link (also called base URI in
[RFC3986]) conveyed in the CoRE Link Format is by default built from [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 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 this information in the target URI, the context URI is built from the
scheme and authority that was used for referencing the resource scheme and authority that was used for referencing the resource
returning the set of links, replacing the path with an empty path. returning the set of links, replacing the path with an empty path.
Thus by default links can be thought of as describing a target Thus by default links can be thought of as describing a target
resource hosted by the server. Other relations can be expressed by resource hosted by the server. Other relations can be expressed by
skipping to change at page 5, line 5 skipping to change at page 7, line 37
with an explicit relation parameter. This is an important difference 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 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 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 context URI). Thus the HTTP Link Header is by default relating
the target URI to the URI that was requested. In comparison, the the target URI to the URI that was requested. In comparison, the
CoRE link format includes one or more links, each describing a CoRE link format includes one or more links, each describing a
resource hosted by a server by default. Other relations can be resource hosted by a server by default. Other relations can be
expressed by using the anchor parameter. See Section 5 of [RFC3986] expressed by using the anchor parameter. See Section 5 of [RFC3986]
for a description of how URIs are constructed from URI references. for a description of how URIs are constructed from URI references.
2.1.2. Link relations 2.2. Link relations
Since links in the CoRE Link Format are typically used to describe 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 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). parameter the new relation type "hosts" is assumed (see Section 7.3).
The "hosts" relation type indicates that the target URI is a resource 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 hosted by the server given by the base URI, or, if present, the
anchor parameter. anchor parameter.
To express other relations a links can make use of any registered To express other relations a links can make use of any registered
relation parameter or target attributes by including the relation relation parameter or target attributes by including the relation
parameter. The context of a relation can be defined using the anchor parameter. The context of a relation can be defined using the anchor
parameter. In this way, relations between resources hosted on a parameter. In this way, relations between resources hosted on a
server, or between hosted resources and external resources can be server, or between hosted resources and external resources can be
expressed. expressed.
2.1.3. Use of anchors 2.3. Use of anchors
As per Section 5.2 of [RFC5988] a link description MAY include an 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 "anchor" attribute, in which case the context is the URI included in
that attribute. This is used to describe a relationship between two that attribute. This is used to describe a relationship between two
resources. A consuming implementation can however choose to ignore resources. A consuming implementation can however choose to ignore
such links. It is not expected that all implementations will be able such links. It is not expected that all implementations will be able
to derive useful information from explicitly anchored links. to derive useful information from explicitly anchored links.
2.2. CoRE link extensions 3. CoRE link extensions
The following CoRE specific target attributes are defined. These The following CoRE specific target attributes are defined. These
attributes describe information useful in accessing the target link attributes describe information useful in accessing the target link
of the relation, and in some cases may be URIs. These URIs MUST be 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 treated as indicators, and are not meant to be actually retrieved
like a URL. When attributes are compared, they MUST be compared as like a URL. When attributes are compared, they MUST be compared as
strings. Relationships to resources that are meant to be retrieved strings. Relationships to resources that are meant to be retrieved
should be expressed as separate links using the anchor attribute and should be expressed as separate links using the anchor attribute and
the appropriate relation type. the appropriate relation type.
link-extension = ( "n" "=" quoted-string ) link-extension = ( "rt" "=" quoted-string )
link-extension = ( "d" "=" <"> URI-reference <">) link-extension = ( "if" "=" quoted-string )
link-extension = ( "ct" "=" integer ) link-extension = ( "ct" "=" integer )
link-extension = ( "sz" "=" integer ) link-extension = ( "sz" "=" integer )
link-extension = ( "obs" )
integer = 1*DIGIT integer = 1*DIGIT
2.2.1. Resource name 'n' attribute 3.1. Resource type 'rt' attribute
The resource name "n" attribute is used to assign a semantically The resource type "rt" attribute is used to assign a semantically
important name to a resource. In the case of a temperature sensor important type to a resource. One can think of this as a noun
resource this could be an application-specific semantic name like describing the resource. In the case of a temperature resource this
"TemperatureC", a URN like "urn:temperature:centigrade" or a URI could be an application-specific semantic type like
referencing a specific concept in an ontology like "OutdoorTemperature", a Universal Resource Name (URN) like
"http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature". Multiple name "urn:temperature:outdoor" or a URI referencing a specific concept in
attributes MAY appear in a link. an ontology like
"http://sweet.jpl.nasa.gov/2.0/phys.owl#Temperature". Multiple
resource type attributes MAY appear in a link.
The name attribute is not meant to used to assign a human readable The resource type attribute is not meant to used to assign a human
name to a resource. The "title" attribute defined in [RFC5988] is readable name to a resource. The "title" attribute defined in
meant for that purpose. [RFC5988] is meant for that purpose.
2.2.2. Interface description 'd' attribute 3.2. Interface description 'if' attribute
The interface description "d" attribute is used to provide a URI or The interface description "if" attribute is used to provide a name,
URN indicating a specific interface definition used to interact with URI or URN indicating a specific interface definition used to
the target resource. This could be for example the URI of a WADL interact with the target resource. One can think of this as
definition of the target resource describing verbs usable on a resource. The interface description
"http://www.example.org/sensor.wadl", or a URN indicating the type of attribute is meant to describe the generic REST interface to interact
interface to the resource "urn:sensor". Multiple description with a resource or a set of resources. It is expected that an
interface description will be re-used by different resource types.
For example the resource types "OutdoorTemperature", "DewPoint" and
"RelHumidity" could all be accessible using the interface description
"http://www.example.org/myapp.wadl#sensor".
The interface description could be for example the URI of a Web
Application Description Language (WADL) definition of the target
resource "http://www.example.org/myapp.wadl#sensor", a URN indicating
the type of interface to the resource "urn:myapp:sensor", or an
application-specific name "Sensor". Multiple interface description
attributes MAY appear in a link. attributes MAY appear in a link.
2.2.3. Content-type code 'ct' attribute 3.3. Content-type code 'ct' attribute
The Content-type code "ct" attribute provides a hint about the The Content-type code "ct" attribute provides a hint about the
Internet media type this resource returns. Note that this is only a Internet media type this resource returns. Note that this is only a
hint, and does not override the Content-type Option of a CoAP hint, and does not override the Content-type Option of a CoAP
response obtained by actually following the link. The value is in response obtained by actually following the link. The value is in
the CoAP identifier code format as a decimal ASCII integer the CoAP identifier code format as a decimal ASCII integer
[I-D.ietf-core-coap]. For example application/xml would be indicated [I-D.ietf-core-coap]. For example application/xml would be indicated
as "ct=41". If no Content-type code attribute is present then as "ct=41". If no Content-type code attribute is present then
nothing about the type can be assumed. The Content-type code nothing about the type can be assumed. The Content-type code
attribute MUST NOT appear more than once in a link. attribute MUST NOT appear more than once in a link.
Alternatively, the "type" attribute MAY be used to indicate an Alternatively, the "type" attribute MAY be used to indicate an
Internet media type as a quoted-string [RFC5988]. It is not however Internet media type as a quoted-string [RFC5988]. It is not however
expected that constrained implementations are able to parse quoted- expected that constrained implementations are able to parse quoted-
string Content-type values. A link MAY include either a ct attribute string Content-type values. A link MAY include either a ct attribute
or a type attribute, but MUST NOT include both. or a type attribute, but MUST NOT include both.
2.2.4. Maximum size estimate 'sz' attribute 3.4. Maximum size estimate 'sz' attribute
The maximum size estimate attribute "sz" gives an indication of the The maximum size estimate attribute "sz" gives an indication of the
maximum size of the link indicated by the target URI. This attribute maximum size of the resource indicated by the target URI. This
is not expected to be included for small resources that can attribute is not expected to be included for small resources that can
comfortably by carried in a single MTU, but SHOULD be included for comfortably by carried in a single Maxiumum Transmission Unit (MTU),
resources larger than that. The maximum size estimate attribute MUST but SHOULD be included for resources larger than that. The maximum
NOT appear more than once in a link. 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 4. Well-known Interface
Resource discovery in CoRE is accomplished through the use of a well- Resource discovery in CoRE is accomplished through the use of a well-
known resource URI which returns a list of links about resources known resource URI which returns a list of links about resources
hosted by that server and other link relations. Well-known resources hosted by that server and other link relations. Well-known resources
have a path component that begins with "/.well-known/" as specified have a path component that begins with "/.well-known/" as specified
in [RFC5785]. This document defines a new well-known resource for in [RFC5785]. This document defines a new well-known resource for
CoRE Resource Discovery "/.well-known/core". A server implementing CoRE Resource Discovery "/.well-known/core".
this specification MUST support this resource on the default port
appropriate for the protocol for the purpose of resource discovery. A server implementing this specification MUST support this resource
It is however up to the application which links are included and how on the default port appropriate for the protocol for the purpose of
they are organized. In the absence of any links, a zero-length resource discovery. It is however up to the application which links
payload is returned. The resource representation of this resource are included and how they are organized. The resource /.well-known/
uses the CoRE Link Format described in Section 2. URIs under the core is meant to be used to return links to the entry points of
path /.well-known/core are reserved by this specification for use by resource interfaces on a server. More sophisticated link
servers in organizing links into multiple resources, each with their organization can be achieved by including links to CoRE Link Format
own URI. resources located elsewhere on the server, for example to achieve an
index. In the absence of any links, a zero-length payload is
returned. The resource representation of this resource MUST be the
CoRE Link Format described in Section 2.
The CoRE resource discovery interface supports the following The CoRE resource discovery interface supports the following
interactions: interactions:
o Performing a GET on /.well-known/core to the default port returns 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 a set of links available from the server (if any) in the CoRE Link
Link Format. These links might describe resources hosted on that Format. These links might describe resources hosted on that
server, on other servers, or express other kinds of link relations server, on other servers, or express other kinds of link relations
as described in Section 2. as described in Section 2.
o Filtering may be performed on any of the link format attributes o Filtering may be performed on any of the link format attributes
using a query string as specified in Section 3.1. For example using a query string as specified in Section 4.1. For example
[GET /.well-known/core?n=TemperatureC] would request resources [GET /.well-known/core?n=TemperatureC] would request resources
with the name TemperatureC. A server is not however required to with the name TemperatureC. A server is not however required to
support filtering. support filtering.
o More capable servers such as proxies could support a resource o More capable servers such as proxies could support a resource
directory by requesting the resource descriptions of other end- directory by requesting the resource descriptions of other end-
points or allowing servers to POST requests to /.well-known/core. points or allowing servers to POST requests to /.well-known/core.
The details of such resource directory functionality is however The details of such resource directory functionality is however
out of scope for this document, and is expected to be specified out of scope for this document, and is expected to be specified
separately. separately.
End-points with a large number of resources SHOULD include resource 4.1. Query Filtering
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 A server implementing this document MAY recognize the query part of a
resource-discovery URI as a filter on the resources to be returned. resource discovery URI as a filter on the resources to be returned.
The query part should conform to the following syntax. Note that The query part should conform to the following syntax. Note that
this only defines querying for a single parameter at a time. this only defines querying for a single parameter at a time.
filter-query = resource-param "=" query-pattern filter-query = resource-param "=" query-pattern
resource-param = "uri" | parmname resource-param = "uri" | parmname
query-pattern = 1*pchar [ "*" ] query-pattern = 1*pchar [ "*" ]
The resource-param "uri" refers to the URI-reference between the "<" The resource-param "uri" refers to the URI-reference between the "<"
and ">" characters of a link. Other resource-param values refer to and ">" characters of a link. Other resource-param values refer to
the link attribute they name. Filtering is performed by comparing the link attribute they name. Filtering is performed by comparing
the query-pattern against the value of the attribute identified by the query-pattern against the value of the attribute identified by
the resource-param for each link-value in the collection of resources the resource-param for each link-value in the collection of resources
identified by the URI path. identified by the URI path.
If the decoded query-pattern does not end with "*", a link value 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 matches the query only if the value of the attribute or URI-reference
denoted by the resource-param is bytewise identical to the query- denoted by the resource-param is bytewise identical to the query-
pattern. If the decoded query-pattern ends with "*", it is pattern. If the decoded query-pattern ends with "*", it is
sufficient that the remainder of the query-pattern be a prefix of the sufficient that the remainder of the query-pattern be a prefix of the
value denoted by the resource-param. value denoted by the resource-param. It is not expected that very
constrained nodes support filtering. Implementations not supporting
It is not expected that very constrained nodes support filtering. filtering MUST simply ignore the query string and return the whole
Implementations not supporting filtering MUST simply ignore the query resource for unicast requests.
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.
When using a transfer protocol like CoAP that supports multicast When using a transfer protocol like the Constrained Application
requests, special care is taken. A multicast request with a query Protocol (CoAP) that supports multicast requests, special care is
string MUST not be responded to if filtering is not supported (to taken. A multicast request with a query string MUST not be responded
avoid a needless response storm). to if filtering is not supported (to avoid a needless response
storm).
4. Examples 5. Examples
A few examples of typical link descriptions in this format follows. A few examples of typical link descriptions in this format follows.
Multiple resource descriptions in a representation are separated by Multiple resource descriptions in a representation are separated by
commas. Linefeeds never occur in the actual format, but are shown in commas. Linefeeds never occur in the actual format, but are shown in
the example for readability. the example for readability.
This example includes links to two different sensors. This example includes links to two different sensors sharing the same
interface description.
REQ: GET /.well-known/core REQ: GET /.well-known/core
RES: 200 OK RES: 200 OK
</sensors/temp>;ct=41;n="TemperatureC", </sensors/temp>;ct=41;rt="TemperatureC";if="sensor",
</sensors/light>;ct=41;n="LightLux" </sensors/light>;ct=41;rt="LightLux";if="sensor"
This example arranges link descriptions hierarchically, with the This example arranges link descriptions hierarchically, with the
entry point including a link to a sub-resource containing links about entry point including a link to a sub-resource containing links about
the sensors. the sensors.
REQ: GET /.well-known/core REQ: GET /.well-known/core
RES: 200 OK RES: 200 OK
</.well-known/core/sensors>;rel="section";ct=40 </sensors>;rt="index";ct=40
REQ: GET /.well-known/core/sensors REQ: GET /sensors
RES: 200 OK RES: 200 OK
</sensors/temp>;n="TemperatureC", </sensors/temp>;ct=41;rt="TemperatureC";if="sensor",
</sensors/light>;ct=41;n="LightLux" </sensors/light>;ct=41;rt="LightLux";if="sensor"
An example query filter may look like: An example query filter may look like:
REQ: GET /.well-known/core?n=LightLux REQ: GET /.well-known/core?rt=LightLux
RES: 200 OK RES: 200 OK
</sensors/light>;ct=41;n="LightLux" </sensors/light>;ct=41;rt="LightLux";if="sensor"
This example shows the use of an anchor attribute to relate the This example shows the use of an anchor attribute to relate the
temperature sensor resource to an external description and to an temperature sensor resource to an external description and to an
alternative URL. alternative URL.
REQ: GET /.well-known/core REQ: GET /.well-known/core
RES: 200 OK RES: 200 OK
</sensors>;rel="index";n="Sensor Index", </sensors>;ct=40;rt="index";rt="Sensor Index",
</sensors/temp>;n="TemperatureC", </sensors/temp>;rt="TemperatureC";if="sensor",
</sensors/light>;ct=41;n="LightLux", </sensors/light>;ct=41;rt="LightLux";if="sensor",
<http://www.example.com/sensors/temp123>;anchor="/sensors/temp" <http://www.example.com/sensors/t123>;anchor="/sensors/temp"
;rel=describedby, ;rel="describedby",
</t>;anchor="/sensors/temp";rel=alternate </t>;anchor="/sensors/temp";rel="alternate"
If a client is interested to find relations about a particular If a client is interested to find relations about a particular
resource, it can perform a query on the anchor parameter: resource, it can perform a query on the anchor parameter:
REQ: GET /.well-known/core?anchor=/sensors/temp REQ: GET /.well-known/core?anchor=/sensors/temp
RES: 200 OK RES: 200 OK
<http://www.example.com/sensors/temp123>;anchor="/sensors/temp" <http://www.example.com/sensors/temp123>;anchor="/sensors/temp"
;rel=describedby, ;rel="describedby",
</t>;anchor="/sensors/temp";rel=alternate </t>;anchor="/sensors/temp";rel="alternate"
5. Security Considerations 6. Security Considerations
This document needs the same security considerations as described in This document needs the same security considerations as described in
Section 7 of [RFC5988]. The /.well-known/core resource may be Section 7 of [RFC5988]. The /.well-known/core resource may be
protected e.g. using DTLS when hosted on a CoAP server as per protected e.g. using DTLS when hosted on a CoAP server as per
[I-D.ietf-core-coap] Section 10.2. [I-D.ietf-core-coap] Section 10.2.
Multicast requests using CoAP for the well-known link-format Multicast requests using CoAP for the well-known link-format
resources could be used to perform denial of service on a constrained resources could be used to perform denial of service on a constrained
network. A multicast request SHOULD only be accepted if the request network. A multicast request SHOULD only be accepted if the request
is sufficiently authenticated and secured. is sufficiently authenticated and secured.
CoRE link format parsers should be aware that a link description may CoRE link format parsers should be aware that a link description may
be cyclical, i.e., contain a link to itself. These cyclical links be cyclical, i.e., contain a link to itself. These cyclical links
could be direct or indirect (i.e., through referenced link could be direct or indirect (i.e., through referenced link
resources). Care should be taken when parsing link descriptions and resources). Care should be taken when parsing link descriptions and
accessing cyclical links. accessing cyclical links.
6. IANA Considerations 7. IANA Considerations
6.1. Well-known 'core' URI 7.1. Attribute Registry
This document defines a registry for the link extension attributes
defined for use with the CoRE Link Format. The name of the registry
is "CoRE Link Format Attributes".
Each entry in the registry must include the attribute name, the
attribute, the format of the attribute and a reference to the
attribute's documentation.
Initial entries in this registry are as follows:
+-----------------------+-----------+---------------+-----------+
| Name | Attribute | Type | Reference |
+-----------------------+-----------+---------------+-----------+
| Resource Type | rt | Quoted String | &SELF; |
| Interface Description | if | Quoted String | &SELF; |
| Content-Type | ct | Integer | &SELF; |
| Maximum Size Estimate | sz | Integer | &SELF; |
+-----------------------+-----------+---------------+-----------+
Table 1: CoAP Option Numbers
New attributes defined for the CoRE Link Format MUST NOT collide with
existing attributes defined in [RFC5988].
The IANA policy for future additions to this registry is "IETF
Review" as described in [RFC5226].
The documentation of an attribute should specify the semantics of the
attribute, including the following properties:
o The meaning of the attribute.
o The format of the attribute's value.
o Whether the attribute can occur multiple times.
o The default value, if any.
7.2. Well-known 'core' URI
This memo registers the "core" well-known URI in the Well-Known URI This memo registers the "core" well-known URI in the Well-Known URI
Registry as defined by [RFC5785]. Registry as defined by [RFC5785].
URI suffix: core URI suffix: core
Change controller: IETF Change controller: IETF
Specification document(s): [[ this document ]] Specification document(s): [[ this document ]]
Related information: None Related information: None
6.2. New 'hosts' relation type 7.3. New 'hosts' relation type
This memo registers the new "hosts" Web Linking relation type as per This memo registers the new "hosts" Web Linking relation type as per
[RFC5988]. [RFC5988].
Relation Name: hosts Relation Name: hosts
Description: Refers to a resource hosted by the server indicated by Description: Refers to a resource hosted by the server indicated by
the link context. the link context.
Reference: [[ this document ]] Reference: [[ this document ]]
Notes: This relation is used in CoRE where links are retrieved as a Notes: This relation is used in CoRE where links are retrieved as a
/.well-known/core resource representation, and by default the context /.well-known/core resource representation, and by default the context
of the links is the server at coap://authority from which /.well- of the links is the server at coap://authority from which /.well-
known/core was requested. known/core was requested.
Application Data: None Application Data: None
6.3. New link-format Internet media type 7.4. New link-format Internet media type
This memo registers the a new Internet media type for the CoRE link This memo registers the a new Internet media type for the CoRE link
format, application/link-format. format, application/link-format.
Type name: application Type name: application
Subtype name: link-format Subtype name: link-format
Required parameters: None Required parameters: None
Optional parameters: The query string may contain uri= to match the Optional parameters: The query string may contain uri= to match the
URI, or any other attribute defined for the link format to match that URI, or any other attribute defined for the link format to match that
attribute. attribute as defined in this document.
Encoding considerations: UTF-8 (NFC) Encoding considerations: UTF-8 (NFC)
Security considerations: None Security considerations: None
Interoperability considerations: Interoperability considerations:
Published specification: [[ this document ]] Published specification: [[ this document ]]
Applications that use this media type: CoAP server and client Applications that use this media type: CoAP server and client
skipping to change at page 12, line 27 skipping to change at page 15, line 48
Macintosh file type code(s): Macintosh file type code(s):
Intended usage: COMMON Intended usage: COMMON
Restrictions on usage: None Restrictions on usage: None
Author: CoRE WG Author: CoRE WG
Change controller: IETF Change controller: IETF
7. Acknowledgments 8. Acknowledgments
Special thanks to Peter Bigot, who has made a considerable number Special thanks to Peter Bigot, who has made a considerable number
reviews and text contributions that greatly improved the document. reviews and text contributions that greatly improved the document.
In particular, Peter is responsible for the ABNF descriptions and the In particular, Peter is responsible for the ABNF descriptions and the
idea for a new "hosts" relation type. idea for a new "hosts" relation type.
Thanks to Mark Nottingham and Eran Hammer-Lahav for discussions and Thanks to Mark Nottingham and Eran Hammer-Lahav for the discussions
ideas that led to this draft, and to Carsten Bormann and Martin and ideas that led to this draft, and to Carsten Bormann, Martin
Thomson for extensive comments and contributions that improved the Thomson and Peter Saint-Andre for extensive comments and
text. contributions that improved the text.
Thanks to Michael Stuber, Richard Kelsey, Cullen Jennings, Guido Thanks to Michael Stuber, Richard Kelsey, Cullen Jennings, Guido
Moritz, Peter Van Der Stok, Adriano Pezzuto, Lisa Dussealt, Alexey Moritz, Peter Van Der Stok, Adriano Pezzuto, Lisa Dussealt, Alexey
Melnikov, Gilbert Clark, Salvatore Loreto, Petri Mutka, Szymon Sasin, Melnikov, Gilbert Clark, Salvatore Loreto, Petri Mutka, Szymon Sasin,
Robert Quattlebaum, Robert Cragie, Angelo Castellani, Tom Herbst, Ed Robert Quattlebaum, Robert Cragie, Angelo Castellani, Tom Herbst, Ed
Beroset, Gilman Tolle, Robby Simpson, Colin O'Flynn and David Ryan Beroset, Gilman Tolle, Robby Simpson, Colin O'Flynn and David Ryan
for helpful comments and discussions that have shaped the document. for helpful comments and discussions that have shaped the document.
8. Changelog 9. Changelog
Changes from ietf-02 to ietf-03:
o Removed 'obs' attribute definition, now defined in the CoAP
Observation spec (#99).
o Changed Resource name (n=) to Resource type (rt=) and d= to if=
(#121).
o Hierarchical organization of links under /.well-known/core
removed (#95).
o Bug in Section 3.1 on byte-wise query matching fixed (#91).
o Explanatory text added about alternative Web link formats (#92).
o Fixed a bug in Section 2.2.4 (#93).
o Added use case examples (#89).
o Clarified how the CoRE link format is used and how it differs
from RFC5988 (#90, #98).
o Changed the Interface definition format to quoted-string to
match the resource type.
o Added an IANA registry for CoRE Link Format attributes (#100).
Changes from ietf-01 to ietf-02: Changes from ietf-01 to ietf-02:
o Added references to RFC5988 (#41). o Added references to RFC5988 (#41).
o Removed sh and id link-extensions (#42). o Removed sh and id link-extensions (#42).
o Defined the use of UTF-8 (#84). o Defined the use of UTF-8 (#84).
o Changed query filter definition for any parameter (#70). o Changed query filter definition for any parameter (#70).
skipping to change at page 13, line 47 skipping to change at page 17, line 47
o Clarified that filtering is optional, and the query string is to o Clarified that filtering is optional, and the query string is to
be ignored if not supported (and the URL path processed as be ignored if not supported (and the URL path processed as
normally). normally).
o Required support of wildcard * processing if filtering is o Required support of wildcard * processing if filtering is
supported. supported.
o Removed the aussumption of a default content-type assumption. o Removed the aussumption of a default content-type assumption.
9. References 10. References
9.1. Normative References 10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform [RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66, Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005. RFC 3986, January 2005.
[RFC5234] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010. [RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
9.2. Informative References 10.2. Informative References
[I-D.ietf-core-coap] [I-D.ietf-core-coap]
Shelby, Z., Frank, B., and D. Sturek, "Constrained Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
Application Protocol (CoAP)", draft-ietf-core-coap-03 "Constrained Application Protocol (CoAP)",
(work in progress), October 2010. draft-ietf-core-coap-04 (work in progress), January 2011.
[I-D.ietf-core-observe] [REST] Fielding, "Architectural Styles and the Design of Network-
Hartke, K. and Z. Shelby, "Observing Resources in CoAP", based Software Architectures", , 2000, <http://
draft-ietf-core-observe-00 (work in progress), www.ics.uci.edu/~fielding/pubs/dissertation/top.htm>.
October 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.
[RFC4287] Nottingham, M., Ed. and R. Sayre, Ed., "The Atom
Syndication Format", RFC 4287, December 2005.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network [RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, March 2008. Interchange", RFC 5198, March 2008.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known [RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785, Uniform Resource Identifiers (URIs)", RFC 5785,
April 2010. April 2010.
Author's Address Author's Address
Zach Shelby Zach Shelby
Sensinode Sensinode
Kidekuja 2 Kidekuja 2
Vuokatti 88600 Vuokatti 88600
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