CoRE Z. Shelby
Internet-Draft Sensinode
Intended status: Standards Track S. Krco
Expires: March 18, 2012 Ericsson
September 15, 2011
CoRE Resource Directory
draft-shelby-core-resource-directory-01
Abstract
In many M2M scenarios, direct discovery of resources is not practical
due to sleeping nodes, disperse networks, or networks where multicast
traffic is inefficient. These problems can be solved by employing an
entity called a Resource Directory (RD), which hosts descriptions of
resources held on other servers, allowing lookups to be performed for
those resources. This document specifies the web interfaces that a
Resource Directory supports in order for web servers to discover the
RD and to registrer, maintain, lookup and remove resources
descriptions. Furthermore, new link attributes useful in conjunction
with an RD are defined.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on March 18, 2012.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Architecture and Use Cases . . . . . . . . . . . . . . . . . . 3
2.1. Use Case: Cellular M2M . . . . . . . . . . . . . . . . . . 4
2.2. Use Case: Home and Building Automation . . . . . . . . . . 5
3. Resource Directory Interfaces . . . . . . . . . . . . . . . . 5
3.1. Discovery . . . . . . . . . . . . . . . . . . . . . . . . 6
3.2. Registration . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Update . . . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4. Validation . . . . . . . . . . . . . . . . . . . . . . . . 10
3.5. Removal . . . . . . . . . . . . . . . . . . . . . . . . . 11
3.6. Lookup . . . . . . . . . . . . . . . . . . . . . . . . . . 12
4. New Link-Format Attributes . . . . . . . . . . . . . . . . . . 14
4.1. Resource Instance 'ins' attribute . . . . . . . . . . . . 14
4.2. Export 'exp' attribute . . . . . . . . . . . . . . . . . . 14
5. Security Considerations . . . . . . . . . . . . . . . . . . . 15
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 15
7. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 15
8. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9.1. Normative References . . . . . . . . . . . . . . . . . . . 15
9.2. Informative References . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
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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 Linking [RFC5988]. The use of Web Linking for the
description and discovery of resources hosted by constrained web
servers is specified by the CoRE Link Format
[I-D.ietf-core-link-format]. This specification however only
describes how to discover resources from the web server that hosts
them by requesting /.well-known/core. In many M2M scenarios, direct
discovery of resources is not practical due to sleeping nodes,
disperse networks, or networks where multicast traffic is
inefficient. These problems can be solved by employing an entity
called a Resource Directory (RD), which hosts descriptions of
resources held on other servers, allowing lookups to be performed for
those resources.
This document specifies the web interfaces that a Resource Directory
supports in order for web servers to discover the RD and to
registrer, maintain, lookup and remove resources descriptions.
Furthermore, new link attributes useful in conjunction with a
Resource Directory are defined. Although the examples in this
document show the use of these interfaces with CoAP
[I-D.ietf-core-coap], they may be applied in an equivalent manner to
HTTP [RFC2616].
2. Architecture and Use Cases
The resource directory architecture is shown in Figure 1. A Resource
Directory (RD) is used as a repository for Web Links [RFC5988] about
resources hosted on other web servers, which are called end-points
(EP). An end-point is a web server associated with a port, thus a
physical node may host one or more end-points. The RD implements a
set of REST interfaces for end-points to register and maintain sets
of Web Links (called resource directory entries), for the RD to
validate entries, and for clients to lookup resources from the RD.
End-points themselves can also act as clients.
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End-points are assumed to proactively register and maintain resource
directory entries on the RD, which are soft state and need to be
periodially refreshed. An EP is provided with interfaces to
register, update and remove a resource directory entry. Furthermore,
a mechanism to discover a RD using the CoRE Link Format is defined.
It is also possible for an RD to proactively discover Web Links from
EPs and add them as resource directory entries, or to validate
existing resource directory entries. A lookup interface for
discovering any of the Web Links held in the RD is provided using the
CoRE Link Format.
Registration Lookup
+----+ | |
| EP |---- | |
+----+ ---- | |
--|- +------+ |
+----+ | ----| | | +--------+
| EP | ---------|-----| RD |----|-----| Client |
+----+ | ----| | | +--------+
--|- +------+ |
+----+ ---- | |
| EP |---- | |
+----+
Figure 1: The resource directory architecture.
2.1. Use Case: Cellular M2M
Over the last few years, mobile operators around the world have
focused on development of M2M solutions in order to expand the
business to the new type of users, i.e. machines. The machines are
connected directly to a mobile network using appropriate embedded air
interface (GSM/GPRS, WCDMA, LTE) or via a gateway providing short and
wide range wireless interfaces. From the system design point of
view, the ambition is to design horizontal solutions that can enable
utilization of machines in different applications depending on their
current availability and capabilities as well as application
requirements, thus avoiding silo like solutions. One of the crucial
enablers of such design is the ability to discover resources
(machines - End Points) capable of providing required information at
a given time or acting on instructions from the end users.
In a typical scenario, during a boot-up procedure (and periodically
afterwards), the machines (EPs) register with a Resource Directory
(for example EPs installed on vehicles enabling tracking of their
position for the fleet management purposes and monitoring environment
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parameters) hosted by the mobile operator or somewhere else in the
network, submiting a description of own capabilities. Due to the
usual network configuration of mobile networks, the EPs attached to
the mobile network do not have routable addresses. Therefore, a
remote server is usually used to provide proxy access to the EPs.
The address of each (proxy) EP on this server is included in the
resource description stored in the RD. The users, for example mobile
applications for environment monitoring, contact the RD, look-up the
EPs capable of providing information about the environment using
appropriate set of tags, obtain information on how to contact them
(URLs of the proxy server) and then initate interaction to obtain
information that is finally processed, displayed on the screen and
usually stored in a database. Similarly, fleet management systems
provide a set of credentials along with the appropriate tags to the
RD to look-up for EPs deployed on the vehicles the application is
responsible for.
2.2. Use Case: Home and Building Automation
Home and commercial building automation systems can benefit from the
use of M2M web services. The use of CoRE in home automation across
multiple subnets is described in [I-D.brandt-coap-subnet-discovery]
and in commercial building automation in [I-D.vanderstok-core-bc].
The discovery requirements of these applications are demanding. Home
automation usually relies on run-time discovery to commision the
system, whereas in building automation a combination of professional
commissioning and run-time discovery. Both home and building
automation involve peer-to-peer interactions between end-points, and
involve battery-powered sleeping devices.
The exporting of resource information to other discovery systems is
also important in these automation applications. In home automation
there is a need to interact with other consumer electronics, which
may already support DNS-SD, and in building automation larger
resource directories or DNS-SD covering multiple buildings.
3. Resource Directory Interfaces
This section defines the REST interfaces between an RD and end-
points, and a lookup interface between an RD and clients. Although
the examples throughout this section assume use of CoAP
[I-D.ietf-core-coap], these REST interfaces can also be realized
using HTTP [RFC2616]. An RD implementing this specification MUST
support the discovery, registration, update, removal and lookup
interfaces defined in this section and MAY support the validation
interface. For the purpose of validation, an end-point implementing
this specification SHOULD support Etag validation on /.well-known/
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core.
3.1. Discovery
Before an end-point can make use of an RD, it must first know its
location and optionally the path of the RD root resource. There can
be several mechanisms for discovering the RD including assuming a
default location (e.g. on an Edge Router in a LoWPAN), by assigning
an anycast address to the RD, using DHCP, or by discovering the RD
using the CoRE Link Format. This section defines discovery of the RD
using the well-known interface of the CoRE Link Format
[I-D.ietf-core-link-format] as the required mechanism. It is however
expected that RDs will also be discoverable via other methods
depending on the deployment.
Discovery is performed by sending either a multicast or unicast GET
request to /.well-known/core and including a Resource Type (rt)
parameter [I-D.ietf-core-link-format] with the value "core-rd" in the
query string. Upon success, the response will contain a payload with
a link format entry for each RD discovered, with the URL indicating
the root resource of the RD. When performing multicast discovery,
the multicast IP address used will depend on the scope required and
the multicast capabilities of the network (TBD if a specific
multicast address should be defined for RDs).
An RD implementing this specification MUST support query filtering
for the rt parameter as defined in [I-D.ietf-core-link-format].
The discovery interface is specified as follows:
Interaction: EP -> RD
Path: /.well-known/core
Method: GET
Content-Type: application/link-format (if any)
Parameters:
Resource Type (rt): MUST contain the value "core-rd"
Instance (ins): Used to differentiate between multiple RDs.
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Success: 2.05 "Content" with an application/link-format payload
containing a matching entry for the RD resource.
Failure: 2.05 "Content" (should be a "No Content" code?) with an
empty payload is returned in case no matching entry is found for a
unicast request.
Failure: No error response to a multicast request.
Failure: 4.00 "Bad Request"
The following example shows an end-point discovering an RD using this
interface, thus learning that the base RD resource is at /rd. Note
that it is up to the RD to choose its base RD resource.
End-point RD
| |
| ----- GET /.well-known/core?rt=core-rd ------> |
| |
| |
| <---- 2.05 Content "</rd>; rt="core-rd" ------ |
| |
Req: GET coap://[ff02::1]/.well-known/core?rt=core-rd
Res: 2.05 Content
</rd>; rt="core-rd"; ins="Primary"
3.2. Registration
After discovering the location of an RD, an end-point MAY register
its resources to the RD's registration interface. This interface
accepts a POST from an end-point containing the list of resources to
be added to the directory as the message payload in the CoRE Link
Format along with query string parameters indicating the name of the
end-point, an optional node identifier and the lifetime of the
registration. The end-point name is formed by concatenating the Host
and Instance parameters included with the registration. All
parameters of the registration are optional. In the absense of a
Host parameter, the RD will generate a unique one on behalf of the
end-point. The RD then creates a new resource or updates an existing
resource in the RD and returns its location. An end-point MUST use
that location when refreshing registrations using this interface.
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End-point resources in the RD are kept active for the period
indicated by the lifetime parameter. The end-point is reponsible for
refreshing the entry within this period using either the registration
or update interface.
The registration interface is specified as follows:
Interaction: EP -> RD
Path: /.well-known/core or /{rd-base}
Method: POST
Content-Type: application/link-format
Etag: The Etag option MUST be included to allow an RD to perform
validation in the future.
Parameters:
Lifetime (lt): Lifetime of the registration in seconds. Range of
60-4294967295. If no lifetime is included, a default value of
86400 (24 hours) SHOULD be assumed.
Host (h): The host identifier or name of the registering node.
The maximum length of this parameter is 63 octets. This
parameter is combined with the Instance parameter (if any) to
form the end-point name. If not included, the RD MUST generate
a unique Host name on behalf of the node.
Instance (ins): The instance of the end-point on this host, if
there are multiple. The maximum length of this parameter is 63
octets.
Type (rt): The semantic type of end-point. The maximum length of
this parameter is 63 octets.
Success: 2.01 "Created". The Location header of the new resource
entry for the end-point could be e.g. in the form /{rd-base}/
{end-point name}
Failure: 4.00 "Bad Request". Malformed request.
Failure: 5.03 "Service Unavailable". Service could not perform the
operation.
The following example shows an end-point with the name "node1"
registering two resources to an RD using this interface.
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End-point RD
| |
| --- POST /rd "</sensors..." ----------------> |
| |
| |
| <-- 2.01 Created Location: /rd/node1 --------- |
| |
Req: POST coap://rd.example.org/rd?h=node1<=1024
Etag: 0x3f
Payload:
</sensors/temp>;ct=41;rt="TemperatureC";if="sensor",
</sensors/light>;ct=41;rt="LightLux";if="sensor"
Res: 2.01 Created
Location: /rd/node1
3.3. Update
The update interface is used by an end-point to refresh or update its
registration with an RD. To use the interface, the end-point sends a
PUT request to the resource returned in the Location option in the
response to the first registration. An update MAY contain a payload
in CoRE Link Format if there have been changes since the last
registration or update.
The update interface is specified as follows:
Interaction: EP -> RD
Path: Location returned by registration.
Method: PUT
Content-Type: application/link-format (if any)
Etag: The Etag option MUST be included to allow an RD to compare the
existing entry and perform validation in the future.
Parameters:
Lifetime (lt): Lifetime of the registration in seconds. Range of
60-4294967295. If no lifetime is included, a default value of
86400 (24 hours) SHOULD be assumed.
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Success: 2.04 "Changed" in case the resource and/or lifetime was
successfully updated
Failure: 4.00 "Bad Request". Malformed request.
Failure: 5.03 "Service Unavailable". Service could not perform the
operation.
The following example shows an end-point updating a new set of
resources to an RD using this interface.
End-point RD
| |
| --- PUT /rd/node1 "</sensors..." ------------> |
| |
| |
| <-- 2.04 Changed ---------------------------- |
| |
Req: PUT /rd/node1
Etag: 0x40
Payload:
</sensors/temp/1>;ct=41;ins="Indoor";rt="TemperatureC";if="sensor",
</sensors/temp/2>;ct=41;ins="Outdoor";rt="TemperatureC";if="sensor",
</sensors/light>;ct=41;rt="LightLux";if="sensor"
Res: 2.04 Changed
3.4. Validation
In some cases, an RD may want to validate that it has the latest
version of an end-point's resource. This can be performed with a GET
on the well-known interface of the CoRE Link Format including the
latest Etag stored for that end-point. For the purpose of
validation, an end-point implementing this specification SHOULD
support Etag validation on /.well-known/core.
The validation interface is specified as follows:
Interaction: RD -> EP
Path: /.well-known/core
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Method: GET
Content-Type: application/link-format (if any)
Etag: The Etag option MUST be included
Parameters: None
Success: 2.03 "Valid" in case the Etag matches
Success: 2.05 "Content" in case the Etag does not match, the
response MUST include the most recent resource representation and
its corresponding Etag.
Failure: 4.00 "Bad Request". Malformed request.
The following examples shows a successful validation.
End-point RD
| |
| <--- GET /.well-known/core Etag: 0x40 -------- |
| |
| |
| --- 2.03 Valid -----------------------------> |
| |
Req: GET coap://{end-point}/.well-known/core
Etag: 0x40
Res: 2.03 Valid
3.5. Removal
Although RD entries have soft state and will eventually timeout after
their lifetime, an end-point SHOULD explicitly remove its entry from
the RD if it knows it will no longer be available (for example on
shut-down). This is accomplished using a removal interface on the RD
by performing a DELETE on the end-point resource.
The removal interface is specified as follows:
Interaction: EP -> RD
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Path: Location returned by registration.
Method: DELETE
Content-Type: None
Parameters: None
Success: 2.02 "Deleted" upon successful deletion
Failure: 4.00 "Bad Request". Malformed request.
Failure: 5.03 "Service Unavailable". Service could not perform the
operation.
The following examples shows successful removal of the end-point from
the RD.
End-point RD
| |
| --- DELETE /rd/node1 -----------------------> |
| |
| |
| <-- 2.02 Deleted ---------------------------- |
| |
Req: DELETE /rd/node1
Res: 2.02 Deleted
3.6. Lookup
In order for an RD to be used for discovering resources registered
with it, a lookup interface is provided. This lookup interface is
provided as a default, and it is assumed that RDs may also support
lookups to return resource descriptions in alternative formats (e.g.
Atom or HTML Link) or using more advanced interfaces (e.g. supporting
context or semantic based lookup).
The lookup interface is provided using the CoRE Link Format
[I-D.ietf-core-link-format] resource discovery mechanism on the root
RD resource (/rd in the examples). The scope of the discovery is
controlled by the depth of the resource the query is made on. A
lookup on the root RD resource /rd queries all resource descriptions
on the RD, whereas a lookup on /rd/node1 queries all resource
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descriptions held in the "node1" entry. An RD MUST support the query
filtering defined in [I-D.ietf-core-link-format] to allow for
filtered lookups.
The lookup interface is specified as follows:
Interaction: Client -> RD
Path: /{rd-base} or e.g. /{rd-base}/{end-point}
Method: GET
Content-Type: application/link-format (if any)
Parameters:
Filtering: CoRE Link Format attributes may be included to filter
the lookup.
Success: 2.05 "Content" with an application/link-format payload
containing a matching entries for the lookup.
Failure: 2.05 "Content" (should be a "No Content" code?) with an
empty payload is returned in case no matching entry is found for a
unicast request.
Failure: No error response to a multicast request.
Failure: 4.00 "Bad Request". Malformed request.
Failure: 5.03 "Service Unavailable". Service could not perform the
operation.
The following example shows a client performing a lookup on an RD
using this interface.
Client RD
| |
| ----- GET /rd/node1?rt=Temperature ----------------------> |
| |
| |
| <-- 2.05 Content "<coap://node1/temp>;rt="Temperature" ---- |
| |
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Req: GET /rd/node1?rt=Temperature
Res: 2.05 Content
<coap://node1/temp>;rt="Temperature"
4. New Link-Format Attributes
When using the CoRE Link Format to describe resources being
discovered by or posted to a resource directory service, additional
information about those resources is useful. This specification
defines the following new attributes for use in the CoRE Link Format
[I-D.ietf-core-link-format]:
link-extension = ( "ins" "=" quoted-string ) ; Max 63 octets
link-extension = ( "exp" )
4.1. Resource Instance 'ins' attribute
The Resource Instance "ins" attribute is an identifier for this
resource, which makes it possible to distinguish from other similar
resources. This attribute is similar in use to the "Instance"
portion of a DNS-SD record, and SHOULD be unique across resources
with the same Resource Type attribute in the domain it is used. A
Resource Instance might be a descriptive string like "Ceiling Light,
Room 3", a short ID like "AF39" or a unique UUID or iNumber. This
attribute is used by a Resource Directory to distinguish between
multiple instances of the same resource type within a system.
This attribute MUST be no more than 63 octets in length. The
resource identifier attribute MUST NOT appear more than once in a
link description.
4.2. Export 'exp' attribute
The Export "exp" attribute is used as a flag to indicate that a link
description MAY be exported by a resource directory to external
directories.
The CoRE Link Format is used for many purposes between CoAP end-
points. Some are useful mainly locally, for example checking the
observability of a resource before accessing it, determining the size
of a resource, or traversing dynamic resource structures. However,
other links are very useful to be exported to other directories, for
example the entry point resource to a functional service.
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5. Security Considerations
This document needs the same security considerations as described in
Section 7 of [RFC5988] and Section 6 of [I-D.ietf-core-link-format].
The /.well-known/core resource may be protected e.g. using DTLS when
hosted on a CoAP server as described in [I-D.ietf-core-coap].
6. IANA Considerations
"core-rd" resource type needs to be registered if an appropriate
registry is created.
"ins" and "exp" attributes need to be registered when a future Web
Linking attribute is created.
7. Acknowledgments
Szymon Sasin, Carsten Bormann, Kerry Lynn, Peter van der Stok, Anders
Brandt and Linyi Tian have provided helpful comments, discussions and
ideas to improve and shape this document. The authors would also
like to thank their collagues from the EU FP7 SENSEI project, where
many of the resource directory concepts were originally developed.
8. Changelog
9. References
9.1. Normative References
[I-D.ietf-core-link-format]
Shelby, Z., "CoRE Link Format",
draft-ietf-core-link-format-06 (work in progress),
June 2011.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
9.2. Informative References
[I-D.brandt-coap-subnet-discovery]
Brandt, A., "Discovery of CoAP servers across subnets",
draft-brandt-coap-subnet-discovery-00 (work in progress),
March 2011.
[I-D.ietf-core-coap]
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Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-06 (work in progress), May 2011.
[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-01 (work in progress),
April 2010.
[I-D.vanderstok-core-bc]
Stok, P. and K. Lynn, "CoAP Utilization for Building
Control", draft-vanderstok-core-bc-03 (work in progress),
March 2011.
[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.
Authors' Addresses
Zach Shelby
Sensinode
Kidekuja 2
Vuokatti 88600
FINLAND
Phone: +358407796297
Email: zach@sensinode.com
Srdjan Krco
Ericsson
Phone:
Email: srdjan.krco@ericsson.com
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