CoRE K. Lynn
Internet-Draft P. van der Stok
Intended status: Standards Track Consultants
Expires: September 6, 2018 M. Koster
SmartThings
C. Amsuess
Energy Harvesting Solutions
March 05, 2018
CoRE Resource Directory: DNS-SD mapping
draft-ietf-core-rd-dns-sd-01
Abstract
Resource and service discovery are complimentary. Resource discovery
provides fine-grained detail about the content of a server, while
service discovery can provide a scalable method to locate servers in
large networks. This document defines a method for mapping between
CoRE Link Format attributes and DNS-Based Service Discovery fields to
facilitate the use of either method to locate RESTful service
interfaces (APIs) in mixed HTTP/CoAP environments.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 6, 2018.
Copyright Notice
Copyright (c) 2018 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
(https://trustee.ietf.org/license-info) in effect on the date of
Lynn, et al. Expires September 6, 2018 [Page 1]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Resource Discovery . . . . . . . . . . . . . . . . . . . 3
1.3. Resource Directories . . . . . . . . . . . . . . . . . . 4
1.4. DNS-Based Service Discovery . . . . . . . . . . . . . . . 4
2. New Link-Format Attributes . . . . . . . . . . . . . . . . . 5
2.1. Resource Instance attribute "ins" . . . . . . . . . . . . 6
2.2. Export attribute "exp" . . . . . . . . . . . . . . . . . 6
3. Mapping CoRE Link Attributes to DNS-SD Record Fields . . . . 6
3.1. Mapping Resource Instance attribute "ins" to <Instance> . 6
3.2. Mapping Resource Type attribute "rt" to <ServiceType> . . 7
3.3. Domain mapping . . . . . . . . . . . . . . . . . . . . . 7
3.4. TXT Record key=value strings . . . . . . . . . . . . . . 7
3.5. Importing resource links into DNS-SD . . . . . . . . . . 8
4. Examples . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. DNS entries . . . . . . . . . . . . . . . . . . . . . . . 9
5. IANA considerations . . . . . . . . . . . . . . . . . . . . . 9
6. Security considerations . . . . . . . . . . . . . . . . . . . 9
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 9
7.1. Normative References . . . . . . . . . . . . . . . . . . 9
7.2. Informative References . . . . . . . . . . . . . . . . . 10
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11
1. Introduction
The Constrained RESTful Environments (CoRE) working group aims at
realizing the REST architecture in a suitable form for the most
constrained devices (e.g. 8-bit microcontrollers with limited RAM and
ROM) and networks (e.g. 6LoWPAN). CoRE is aimed at machine-to-
machine (M2M) applications such as smart energy and building
automation. The main deliverable of CoRE is the Constrained
Application Protocol (CoAP) specification [RFC7252].
Automated discovery of resources hosted by a constrained server is
critical in M2M applications where human intervention is minimal and
static interfaces result in brittleness. CoRE Resource Discovery is
intended to support fine-grained discovery of hosted resources, their
attributes, and possibly other resource relations [RFC6690].
Lynn, et al. Expires September 6, 2018 [Page 2]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
In contrast, service discovery generally refers to a coarse-grained
resolution of an end-point's IP address, port number, and protocol.
This definition may be extended to include multi-function devices,
where the result of the discovery process may include a path to a
resource representing a RESTful service interface and possibly a
reference to a description of the interface such as a JSON Hyper-
Schema document [I-D.handrews-json-schema-hyperschema].
Resource and service discovery are complimentary in the case of large
networks, where the latter can facilitate scaling. This document
defines a mapping between CoRE Link Format attributes and DNS-Based
Service Discovery (DNS-SD) [RFC6763] fields that permits discovery of
CoAP services by either means. It also addresses the CoRE charter
goal to interoperate with DNS-SD.
1.1. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. The term "byte" is used in its now customary sense as a
synonym for "octet".
This specification requires readers to be familiar with all the terms
and concepts that are discussed in {-link} and [RFC6690]. Readers
should also be familiar with the terms and concepts discussed in
[RFC7252]. To describe the REST interfaces defined in this
specification, the URI Template format is used [RFC6570].
This specification also makes use of the terminology of
[I-D.ietf-core-resource-directory].
1.2. Resource Discovery
The main function of Resource Discovery is to provide Universal
Resource Identifiers (URIs, also called "links") for the resources
hosted by the server, complemented by attributes about those
resources and perhaps additional link relations. In CoRE this
collection of links and attributes is itself a resource (as opposed
to HTTP headers delivered with a specific resource).
[RFC6690] specifies a link format for use in CoRE Resource Discovery
by extending the HTTP Link Header Format [RFC8288] to describe these
link descriptions. The CoRE Link Format is carried as a payload and
is assigned an Internet media type. A well-known URI "/.well-known/
core" is defined as a default entry-point for requesting the list of
links about resources hosted by a server, and thus performing CoRE
Resource Discovery.
Lynn, et al. Expires September 6, 2018 [Page 3]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
Resource Discovery can be performed either via unicast or multicast.
When a server's IP address is already known, either a priori or
resolved via the Domain Name System (DNS) [RFC1034][RFC1035], unicast
discovery is performed in order to locate a URI for 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 [RFC2045] 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.
1.3. Resource Directories
In many M2M scenarios, direct discovery of resources is not practical
due to sleeping nodes, limited bandwidth, or networks where multicast
traffic is inefficient. These problems can be solved by deploying a
network element called a Resource Directory (RD), which hosts
descriptions of resources held on other servers (referred to as "end-
points") and allows lookups to be performed for those resources. An
end-point is a web server associated with specific IP address and
port; thus a physical device may host one or more end-points. End-
points may also act as clients.
The Resource Directory implements a set of REST interfaces for end-
points to register and maintain sets of Web Links, called resource
directory entries. [I-D.ietf-core-resource-directory] specifies the
web interfaces that an RD supports in order for web servers to
discover the RD and to register, maintain, lookup and remove resource
descriptions; for the RD to validate entries; and for clients to
lookup resources from the RD. Furthermore, new link attributes
useful in conjunction with an RD are defined.
1.4. DNS-Based Service Discovery
DNS-Based Service Discovery (DNS-SD) defines a conventional method of
configuring DNS PTR, SRV, and TXT resource records to facilitate
discovery of services (such as CoAP servers in a subdomain) using the
existing DNS infrastructure. This section gives a brief overview of
DNS-SD; see [RFC6763] for a detailed specification.
DNS-SD service names are limited to 255 bytes and are of the form:
Service Name = <Instance>.<ServiceType>.<Domain>
The service name is the label of SRV/TXT resource records. The SRV
RR specifies the host and the port of the endpoint. The TXT RR
provides additional information in the form of key/value pairs.
Lynn, et al. Expires September 6, 2018 [Page 4]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
The <Domain> part of the service name is identical to the global (DNS
subdomain) part of the authority in URIs that identify the resources
on an individual server or group of servers.
The <ServiceType> part is composed of at least two labels. The first
label of the pair is the application protocol name [RFC6335] preceded
by an underscore character. The second label indicates the transport
and is always "_udp" for CoAP services. In cases where narrowing the
scope of the search may be useful, these labels may be optionally
preceded by a subtype name followed by the "_sub" label. An example
of this more specific <ServiceType> is "lamp._sub._dali._udp". Only
the rightmost pair of labels is used in SRV and TXT record names.
The default <Instance> part of the service name may be set at the
factory or during the commissioning process. It SHOULD uniquely
identify an instance of <ServiceType> within a <Domain>. Taken
together, these three elements comprise a unique name for an SRV/ TXT
record pair within the DNS subdomain.
The granularity of a service name MAY be that of a host or group, or
it could represent a particular resource within a CoAP server. The
SRV record contains the host name (AAAA record name) and port of the
service while protocol is part of the service name. In the case
where a service name identifies a particular resource, the path part
of the URI must be carried in a corresponding TXT record.
A DNS TXT record is in practice limited to a few hundred bytes in
length, which is indicated in the resource record header in the DNS
response message [RFC6763]. The data consists of one or more strings
comprising a key=value pair. By convention, the first pair is
txtver=<number> (to support different versions of a service
description). An example string is:
| 0x08 | t | x | t | v | e | r | = | 1 |
2. 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
[RFC6690]:
link-extension = ( "ins" "=" (ptoken | quoted-string) )
; The token or string is max 63 bytes
link-extension = ( "exp" )
Lynn, et al. Expires September 6, 2018 [Page 5]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
2.1. Resource Instance attribute "ins"
The Resource Instance "ins" attribute is an identifier for this
resource, which makes it possible to distinguish it from other
similar resources. This attribute is similar in use to the
<Instance> portion of a DNS-SD record (see Section 1.4, 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 the directory.
This attribute MUST be no more than 63 bytes in length. The resource
identifier attribute MUST NOT appear more than once in a link
description. This attribute MAY be used as a query parameter in the
RD Lookup Function Set defined in Section 7 of
[I-D.ietf-core-resource-directory].
2.2. Export attribute "exp"
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
endpoints. 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. This
attribute MAY be used as a query parameter in the RD Lookup Function
Set defined in Section 7 of [I-D.ietf-core-resource-directory].
3. Mapping CoRE Link Attributes to DNS-SD Record Fields
3.1. Mapping Resource Instance attribute "ins" to <Instance>
The Resource Instance "ins" attribute maps to the <Instance> part of
a DNS-SD service name. It is stored directly in the DNS as a single
DNS label of canonical precomposed UTF-8 [RFC3629] "Net-Unicode"
(Unicode Normalization Form C) [RFC5198] text. However, to the
extent that the "ins" attribute may be chosen to match the DNS host
name of a service, it SHOULD use the syntax defined in Section 3.5 of
[RFC1034] and Section 2.1 of [RFC1123].
The <Instance> part of the name of a service being offered on the
network SHOULD be configurable by the user setting up the service, so
Lynn, et al. Expires September 6, 2018 [Page 6]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
that he or she may give it an informative name. However, the device
or service SHOULD NOT require the user to configure a name before it
can be used. A sensible choice of default name can allow the device
or service to be accessed in many cases without any manual
configuration at all. The default name should be short and
descriptive, and MAY include a collision-resistant substring such as
the lower bits of the device's MAC address, serial number,
fingerprint, or other identifier in an attempt to make the name
relatively unique.
DNS labels are currently limited to 63 bytes in length and the entire
service name may not exceed 255 bytes.
3.2. Mapping Resource Type attribute "rt" to <ServiceType>
The resource type "rt" attribute is mapped into the <ServiceType>
part of a DNS-SD service name and SHOULD conform to the reg-rel-type
production of the Link Format defined in Section 2 of [RFC6690]. The
"rt" attribute MUST be composed of at least a single Net-Unicode text
string, without underscore '_' or period '.' and limited to 15 bytes
in length, which represents the application protocol name. This
string is mapped to the DNS-SD <ServiceType> by prepending an
underscore and appending a period followed by the "_udp" label. For
example, rt="dali" is mapped into "_dali._udp".
The application protocol name may be optionally followed by a period
and a service subtype name consisting of a Net-Unicode text string,
without underscore or period and limited to 63 bytes. This string is
mapped to the DNS-SD <ServiceType> by appending a period followed by
the "_sub" label and then appending a period followed by the service
type label pair derived as in the previous paragraph. For example,
rt="dali.light" is mapped into "light._sub._dali._udp".
The resulting string is used to form labels for DNS-SD records which
are stored directly in the DNS.
3.3. Domain mapping
TBD: A method must be specified to determine in which DNS zone the
CoAP service should be registered. See, for example, Section 11 in
[RFC6763].
3.4. TXT Record key=value strings
A number of [RFC6763] key/value pairs are derived from link-format
information, to be exported in the DNS-SD as key=value strings in a
TXT record ([RFC6763], Section 6.3).
Lynn, et al. Expires September 6, 2018 [Page 7]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
The resource <URI> is exported as key/value pair "path=<URI>".
The Interface Description "if" attribute is exported as key/value
pair "if=<Interface Description>".
The DNS TXT record can be further populated by importing any other
resource description attributes as they share the same key=value
format specified in Section 6 of [RFC6763].
3.5. Importing resource links into DNS-SD
Assuming the ability to query a Resource Directory or multicast a GET
(?exp) over the local link, CoAP resource discovery may be used to
populate the DNS-SD database in an automated fashion. CoAP resource
descriptions (links) can be exported to DNS-SD for exposure to
service discovery by using the Resource Instance attribute as the
basis for a unique service name, composed with the Resource Type as
the <ServiceType>, and registered in the correct <Domain>. The agent
responsible for exporting records to the DNS zone file SHOULD be
authenticated to the DNS server. The following example, using the
example lookup location /rd-lookup, shows an agent discovering a
resource to be exported:
Req: GET /rd-lookup/res?exp
Res: 2.05 Content
<coap://[FDFD::1234]:5683/light/1>;
exp;rt="dali.light";ins="Spot";
d="office";ep="node1"
The agent subsequently registers the following DNS-SD RRs, assuming a
zone name "example.com" prefixed with "office":
node1.office.example.com. IN AAAA FDFD::1234
_dali._udp.office.example.com IN PTR
Spot._dali._udp.office.example.com
light._sub._dali._udp.example.com IN PTR
Spot._dali._udp.office.example.com
Spot._dali._udp.office.example.com IN SRV 0 0 5683
node1.office.example.com.
Spot._dali._udp.office.example.com IN TXT
txtver=1;path=/light/1
In the above figure the Service Name is chosen as
Spot._dali._udp.office.example.com without the light._sub service
prefix. An alternative Service Name would be:
Spot.light._sub._dali._udp.office.example.com.
Lynn, et al. Expires September 6, 2018 [Page 8]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
4. Examples
4.1. DNS entries
It may be profitable to discover the light groups for applications,
which are unaware ot the existence of the RD. An agent needs to
query the RD to return all groups which are exported to be inserted
into DNS.
Req: GET /rd-lookup/gp?exp
Res: 2.05 Content
<coap://[FF05::1]/>;exp;gp="grp_R2-4-015;ins="grp1234";
ep="lm_R2-4-015_wndw";
ep="lm_R2-4-015_door
The group with FQDN grp_R2-4-015.bc.example.com can be entered into
the DNS by the agent. The accompanying instance name is grp1234.
The <ServiceType> is chosen to be _group._udp. The agent enters the
following RRs into the DNS.
grp_R2-4-015.bc.example.com. IN AAAA FF05::1
_group._udp.bc.example.com IN PTR
grp1234._group._udp.bc.example.com
grp1234._group._udp.bc.example.com IN SRV 0 0 5683
grp_R2-4-015_door.bc.example.com.
grp1234._group._udp.bc.example.com IN TXT
txtver=1;path=/light/grp1
From then on, applications unaware of the existence of the RD can use
DNS to access the lighting group.
5. IANA considerations
TBD
6. Security considerations
TBD
7. References
7.1. Normative References
Lynn, et al. Expires September 6, 2018 [Page 9]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC6335] Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.
Cheshire, "Internet Assigned Numbers Authority (IANA)
Procedures for the Management of the Service Name and
Transport Protocol Port Number Registry", BCP 165,
RFC 6335, DOI 10.17487/RFC6335, August 2011,
<https://www.rfc-editor.org/info/rfc6335>.
[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/info/rfc6570>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
7.2. Informative References
[I-D.handrews-json-schema-hyperschema]
Andrews, H. and A. Wright, "JSON Hyper-Schema: A
Vocabulary for Hypermedia Annotation of JSON", draft-
handrews-json-schema-hyperschema-01 (work in progress),
January 2018.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
resource-directory-13 (work in progress), March 2018.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
Lynn, et al. Expires September 6, 2018 [Page 10]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>.
[RFC2045] Freed, N. and N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message
Bodies", RFC 2045, DOI 10.17487/RFC2045, November 1996,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
<https://www.rfc-editor.org/info/rfc5198>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
Acknowledgements
This document was split out from [I-D.ietf-core-resource-directory].
Zach Shelby was a co-author of the original version of this draft.
Authors' Addresses
Kerry Lynn
Consultant
Phone: +1 978-460-4253
Email: kerlyn@ieee.org
Peter van der Stok
Consultant
Phone: +31 492474673 (Netherlands), +33 966015248 (France)
Email: consultancy@vanderstok.org
URI: www.vanderstok.org
Lynn, et al. Expires September 6, 2018 [Page 11]
Internet-Draft CoRE Resource Directory: DNS-SD mapping March 2018
Michael Koster
SmartThings
665 Clyde Avenue
Mountain View 94043
USA
Phone: +1 707-502-5136
Email: Michael.Koster@smartthings.com
Christian Amsuess
Energy Harvesting Solutions
Hollandstr. 12/4
1020
Austria
Phone: +43 664-9790639
Email: c.amsuess@energyharvesting.at
Lynn, et al. Expires September 6, 2018 [Page 12]