CoRE                                                             K. Lynn
Internet-Draft                                                Oracle+Dyn
Intended status: Standards Track                         P. van der Stok
Expires: September 12, 2019                                   Consultant
                                                               M. Koster
                                                              C. Amsuess
                                             Energy Harvesting Solutions
                                                          March 11, 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

   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
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   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
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   This Internet-Draft will expire on September 12, 2019.

Copyright Notice

   Copyright (c) 2019 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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   ( in effect on the date of
   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 and Background . . . . . . . . . . . . . . . . .   2
     1.1.  Terminology . . . . . . . . . . . . . . . . . . . . . . .   3
     1.2.  CoRE Resource Discovery . . . . . . . . . . . . . . . . .   4
     1.3.  CoRE Resource Directories . . . . . . . . . . . . . . . .   5
     1.4.  DNS-Based Service Discovery . . . . . . . . . . . . . . .   5
   2.  New Link-Format Attributes  . . . . . . . . . . . . . . . . .   6
     2.1.  Export attribute "exp"  . . . . . . . . . . . . . . . . .   7
     2.2.  Resource Instance attribute "ins="  . . . . . . . . . . .   7
     2.3.  Service Type attribute "st="  . . . . . . . . . . . . . .   7
   3.  Mapping CoRE Link Attributes to DNS-SD Record Fields  . . . .   7
     3.1.  Mapping Resource Instance attribute "ins=" to <Instance>    7
     3.2.  Mapping Service Type attribute "st=" to <ServiceType> . .   8
     3.3.  <Domain> Mapping  . . . . . . . . . . . . . . . . . . . .   8
     3.4.  TXT Record key=value strings  . . . . . . . . . . . . . .   9
     3.5.  Exporting resource links into DNS-SD  . . . . . . . . . .   9
   4.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  10
   5.  Security considerations . . . . . . . . . . . . . . . . . . .  10
   6.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  10
     6.1.  Normative References  . . . . . . . . . . . . . . . . . .  10
     6.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Appendix A.  Acknowledgments  . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  13

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

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   intervention is minimal and static configurations result in

   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

   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",
   "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

   This specification also incorporates the terminology of

   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.

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

   o  a link context,

   o  a link relation type (see Section 2.1 of [RFC8288],

   o  a link target, and

   o  optionally, target attributes (see Section 2.2 of [RFC8288]).

   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.

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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

   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

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   the transport protocol binding and is typically "_udp" for CoAP

   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

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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

   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].

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]

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   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

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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  IN SRV  0 0 5683  IN AAAA  FDFD::1234

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4.  IANA considerations

   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.  Security considerations


6.  References

6.1.  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., Ed., "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,

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   [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,

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6.2.  Informative References

              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.

              Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
              Amsuess, "CoRE Resource Directory", draft-ietf-core-
              resource-directory-19 (work in progress), January 2019.

              Cheshire, S. and T. Lemon, "Service Registration Protocol
              for DNS-Based Service Discovery", draft-sctl-service-
              registration-02 (work in progress), July 2018.

   [OCF]      Open Connectivity Foundation, "OCF Specification 2.0",
              2018, <

   [REST]     Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures", Ph.D. Dissertation,
              University of California, Irvine, 2000,

   [rt]       IANA, "Resource Type (rt=) Link Target Attribute
              Values", 2012, <

   [st]       IANA, "Service Name and Transport Protocol Port Number
              Registry", 2018, <

Appendix A.  Acknowledgments

   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.

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

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Authors' Addresses

   Kerry Lynn
   150 Dow Street, Tower Two
   Manchester, NH  03101

   Phone: +1 978-460-4253

   Peter van der Stok

   Phone: +31 492474673 (Netherlands), +33 966015248 (France)

   Michael Koster
   665 Clyde Avenue
   Mountain View, CA  94043

   Phone: +1 707-502-5136

   Christian Amsuess
   Energy Harvesting Solutions
   Hollandstr. 12/4

   Phone: +43 664-9790639

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