CoRE P. van der Stok
Internet-Draft Consultant
Intended status: Standards Track M. Koster
Expires: January 8, 2020 SmartThings
C. Amsüss
Energy Harvesting Solutions
July 07, 2019

CoRE Resource Directory: DNS-SD mapping


Resource and service discovery are complementary. Resource discovery provides fine-grained detail about the content of a web 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 records to facilitate the use of either method to locate RESTful service interfaces (APIs) in heterogeneous HTTP/CoAP environments.

Status of This Memo

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This Internet-Draft will expire on January 8, 2020.

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Table of Contents

1. Introduction and Background

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 [RFC4944]). 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].

CoRE Link Format [RFC6690] is intended to support fine-grained discovery of hosted resources, their attributes, and possibly other related resources. Automated dynamic discovery of resources hosted by a constrained server is critical in M2M applications, where human intervention is minimal and static configurations result in brittleness.

DNS-Based Service Discovery (DNS-SD) [RFC6763] supports wide-area search for instances of a given service type (i.e. servers that support a particular application protocol stack). A service instance consists of a server’s name, IP address, and port number plus additional meta-data about the server. This data may extend to support multi-function devices, where multiple services are available at the same endpoint. The result of the discovery process may include a path to a resource representing the entry point to each function’s RESTful service interface and possibly a link to a formal description of that interface (e.g. a JSON Hyper-Schema document [I-D.handrews-json-schema-hyperschema]).

Resource and service discovery are complementary 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 records that permits discovery of CoAP services by either method. It also addresses the CoRE charter goal to interoperate with DNS-SD.

The primary use case for mapping between resource and service discovery is to support heterogeneous HTTP/CoAP environments where, for example, HTTP clients may discover and communicate with CoAP servers that are behind a “cross proxy” [RFC8075].

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 conventional sense as a synonym for “octet”.

This specification requires readers to be familiar with all the terms and concepts that are discussed in [RFC6690] and [RFC8288]. Readers should also be familiar with the terms and concepts discussed in [RFC7252].

This specification also incorporates the terminology of [I-D.ietf-core-resource-directory].

In particular, the following terms are used frequently:

Endpoint: a web server associated with a specific IP address and port; thus a physical device may host one or more endpoints. Endpoints may also act as clients.

Link: Web Linking [RFC8288] defines a Web Link (link) as a typed connection between two resources, comprised of:

A link can be viewed as a statement of the form “link context has a link relation type resource at link target, which (optionally) has target attributes”, where link target and context are typically Universal Resource Identifiers (URIs) [RFC3986]. For example, “” has a “canonical” resource at “”, which has a “type” of “text/html”.

1.2. CoRE Resource Discovery

The main function of Resource Discovery is to return links to the resources hosted by a server, complemented by attributes about those resources and additional link relations. In CoRE this collection of links and attributes is itself a resource (in contrast to HTTP, where headers delivered with a specific resource describe its attributes).

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) [RFC1034][RFC1035], unicast discovery 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 [RFC6690]. A client would then match the appropriate Resource Type, Interface Description, and possible media 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.

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 of 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.

1.3. CoRE 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 that originate on other endpoints and allows indirect lookups to be performed for those resources.

The Resource Directory implements a set of REST interfaces for endpoints to register and maintain collections of links, called Resource Directory registrations. [I-D.ietf-core-resource-directory] specifies the web interfaces that an RD supports for endpoints 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.

1.4. DNS-Based Service Discovery

DNS-Based Service Discovery (DNS-SD) defines a conventional method of naming and 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; for a detailed specification see [RFC6763].

DNS-SD Service Names are limited to 255 bytes and are of the form:

      Service Name = <Instance>.<ServiceType>.<Domain>

The Service Name identifies a SRV/TXT Resource Record (RR) pair. The SRV RR specifies the hostname and port of an endpoint. The TXT RR provides additional information in the form of key/value pairs. DNS-Based Service Discovery is accomplished by sending a DNS request for PTR records with the name <ServiceType>.<Domain>, which will return a list of zero or more Service Names.

The <Domain> part of the Service Name is identical to the global (DNS subdomain) part of the authority in URIs [RFC3986] that identify the resources on an individual server or group of servers.

The <ServiceType> part is generally composed of two labels. The first label of the pair is the application protocol name [RFC6335] preceded by an underscore character. For example, an organization such as the Open Connectivity Foundation [OCF] that specifies Resource Types [RFC6335] might register application protocol names beginning with “oic”, which all servers that advertise OCF resources would use as part of their ServiceType. The second label indicates the transport protocol binding and is typically “_udp” for CoAP services.

The default <Instance> part of the Service Name SHOULD be set to a default value at the factory and MAY be modified during the commissioning process. It MUST 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 might represent a particular resource within a CoAP server. The SRV record contains the host name (AAAA record name) and port of the endpoint, 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 (See section 6 of [RFC6763]). The data consist 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). Each string is formatted as a single length byte followed by 0-255 bytes of text. 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 often useful. This specification defines the following new attributes for use in the CoRE Link Format [RFC6690] to enable the data-driven mappings described in Section 3:

   link-extension    = ( "exp" )
   link-extension    = ( "ins" "=" (ptoken | quoted-string) )
                       ; The token or string is max 63 bytes
   link-extension    = ( "st" "=" (ptoken | quoted-string) )
                       ; The token or string is max 15 bytes

2.1. Export attribute “exp”

The Export “exp” attribute is used as a flag to indicate that a link description MAY be exported from 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 6 of [I-D.ietf-core-resource-directory].

2.2. 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 in a Resource Directory. This attribute specifies the value to be used for the <Instance> portion of an exported DNS-SD Service Name (see Section 1.4), and SHOULD be unique across resources with the same Resource Type “rt=” attribute in the domain in which it is used.

A Resource Instance SHOULD be a descriptive human readable string like “Ceiling Light, Room 3”. This attribute MUST NOT be 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].

2.3. Service Type attribute “st=”

The Service Type instance “st=” attribute specifies the value to be used for the <ServiceType> portion of an exported DNS-SD Service Name (see Section 1.4). This attribute MUST NOT be more than 15 bytes in length (see [RFC6335], Section 5.1) and MUST be present in the IANA Service Name registry [st].

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 directly 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, if the “ins=” attribute is 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 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 (see Appendix D of [RFC6763]).

DNS labels are limited to 63 bytes in length and the entire Service Name may not exceed 255 bytes.

3.2. Mapping Service Type attribute “st=” to <ServiceType>

The Service Type “st=” attribute maps directly to the <ServiceType> part of a DNS-SD Service Name.

In practice, the ServiceType should unambiguously identify interoperable devices. It is up to individual SDOs to specify how to represent their registered Resource Type “rt=” values as registered application protocol names according to [RFC6335]. The application name is then used as the value of the resource “st=” attribute.

The resulting application protocol name MUST be composed of at least a single Net-Unicode text string, without underscore ‘_’ or period ‘.’ and limited to 15 bytes in length (see Section 5.1 of [RFC6335]). 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=”oic.d.light” might correspond to the registered application protocol name st=”oic-d-light” and would be mapped into Service Type “_oic-d-light._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 which DNS zone the CoAP service description should be exported to. See, for example, Section 11 in [RFC6763] and Section 2 in [I-D.sctl-service-registration].

3.4. TXT Record key=value strings

DNS-SD key/value pairs may be derived from CoRE Link Format information and exported as key=value strings in a DNS-SD TXT record (See Section 6.3 of [RFC6763]).

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. Exporting 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 Service Type attribute as the <ServiceType>, and registered in the appropriate <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

The agent subsequently registers the following DNS-SD RRs, assuming a derived DNS zone name “”:      IN PTR  IN TXT
         txtver=1;path=/light/1;rt=oic.d.light;d=sector IN SRV 
         0 0 5683                 IN AAAA FDFD::1234

4. Exporting Resource Directory Service to DNS

In some cases it is required that one (or more) Resource Directories (RD) in a given DNS domain can be discoverable from DNS. The /.well-known/core resource of the RD should reflect this by specifying the “ins”, “exp”, and the “st” attributes in the the link of the RD service. This document specifies in Section 5 two servicetypes: _rd-lookup-res._udp and _rd-lookup-ep._udp for resource types rt = core.rd-lookup-res and rt = core.rd-lookup-ep respectively. The default coap and coaps ports are respectively: 5683 and 5684.

The value of the instance MAY be specified by the manager of the resource directories. In case of an unmanaged RD (for example in a home network) it is recommended that the ins parameter takes a value provided by an Authorization Server during the acceptance of the RD to the network (see for example section 7 of [I-D.ietf-core-resource-directory]).

With the assumption that the “ins” value is attributed by Authorization Server, and [FDFD::1234] is IP address of RD, Example links for RD are:

     Req: GET coap://[FDFD::1234]/.well-known/core?exp

     Res: 2.05 Content

The link atributes can be exported to RR by the mapping process described in Section 3.

5. IANA considerations

Two registries are affected by this document: (1) “RD Parameters” registry under “Core Parameters” registry, and (2) Service Name and Transport Protocol Port Number Registry

5.1. RD Parameters Registry

This specification defines new parameters for the registry “RD Parameters” provided under “CoRE Parameters” (TBD).

   | Full name      | Short | Validity      | Use | Description        |
   | ServiceType    | st    |               | RLA | Name of the        |
   |                |       |               |     |Service Type,       |
   |                |       |               |     | max 63 bytes       |
   | Resource       | ins   |               | RLA | Instance identifier|
   | Instance       |       |               |     | of the resource    |
   |                |       |               |     |                    |
   | Export         | exp   |               | RLA | flag to indicate   |
   |                |       |               |     | exportation        |

5.2. Service Name and Transport Protocol Port Number Registry

This specification defines new parameters for the Service Name and Transport Protocol Port Number Registry:

* _rd-lookup-res._udp at ports 5683 and 5684
* _rd-lookup-ep._udp at ports 5683 and 5684

6. Security considerations

Malicious nodes can export fake link attributes to DNS. It is recommended that the RD can be authenticated, and is authorized to both join the network and export its link attributes. Authentication is specified in [I-D.ietf-ace-oauth-authz].

7. Contributors

Keryy lynn was the initiator of, and major contributor to this document. 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.

8. Acknowledgments

The authors wish to thank Stuart Cheshire, Ted Lemon, and David Thaler for their thorough reviews and clarifying suggestions.

9. References

9.1. Normative References

[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application and Support", STD 3, RFC 1123, DOI 10.17487/RFC1123, October 1989.
[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.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO 10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November 2003.
[RFC3986] Berners-Lee, T., Fielding, R. and L. Masinter, "Uniform Resource Identifier (URI): Generic Syntax", STD 66, RFC 3986, DOI 10.17487/RFC3986, January 2005.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J. and D. Culler, "Transmission of IPv6 Packets over IEEE 802.15.4 Networks", RFC 4944, DOI 10.17487/RFC4944, September 2007.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008.
[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.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, DOI 10.17487/RFC6690, August 2012.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013.
[RFC7252] Shelby, Z., Hartke, K. and C. Bormann, "The Constrained Application Protocol (CoAP)", RFC 7252, DOI 10.17487/RFC7252, June 2014.
[RFC8075] Castellani, A., Loreto, S., Rahman, A., Fossati, T. and E. Dijk, "Guidelines for Mapping Implementations: HTTP to the Constrained Application Protocol (CoAP)", RFC 8075, DOI 10.17487/RFC8075, February 2017.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288, DOI 10.17487/RFC8288, October 2017.

9.2. Informative References

[I-D.handrews-json-schema-hyperschema] Andrews, H. and A. Wright, "JSON Hyper-Schema: A Vocabulary for Hypermedia Annotation of JSON", Internet-Draft draft-handrews-json-schema-hyperschema-01, January 2018.
[I-D.ietf-ace-oauth-authz] Seitz, L., Selander, G., Wahlstroem, E., Erdtman, S. and H. Tschofenig, "Authentication and Authorization for Constrained Environments (ACE) using the OAuth 2.0 Framework (ACE-OAuth)", Internet-Draft draft-ietf-ace-oauth-authz-24, March 2019.
[I-D.ietf-core-resource-directory] Shelby, Z., Koster, M., Bormann, C., Stok, P. and C. Amsuess, "CoRE Resource Directory", Internet-Draft draft-ietf-core-resource-directory-23, July 2019.
[I-D.sctl-service-registration] Cheshire, S. and T. Lemon, "Service Registration Protocol for DNS-Based Service Discovery", Internet-Draft draft-sctl-service-registration-02, July 2018.
[OCF] Foundation, O., "OCF Specification 2.0", 2018.
[REST] Fielding, R., "Architectural Styles and the Design of Network-based Software Architectures", 2000.
[rt] IANA, ., "Resource Type (rt=) Link Target Attribute Values", 2012.
[st] IANA, ., "Service Name and Transport Protocol Port Number Registry", 2018.

Authors' Addresses

Peter van der Stok Consultant Phone: +31 492474673 (Netherlands), +33 966015248 (France) EMail: URI:
Michael Koster SmartThings 665 Clyde Avenue Mountain View, CA 94043 USA Phone: +1 707-502-5136 EMail:
Christian Amsüss Energy Harvesting Solutions Hollandstr. 12/4 1020 Austria Phone: +43 664-9790639 EMail:
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