CoRE Working Group                                               K. Lynn
Internet-Draft                                                Consultant
Intended status: Standards Track                               Z. Shelby
Expires: April 25, 2013                                        Sensinode
                                                        October 22, 2012

        CoRE Link-Format to DNS-Based Service Discovery Mapping


   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
   allow either method to be used to locate service interfaces down to
   the granularity of function sets in mixed HTTP/CoAP environments.

Status of this Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on April 25, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   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 . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Mapping CoRE Link Attributes to DNS-SD Records . . . . . . . .  6
   3.  Examples . . . . . . . . . . . . . . . . . . . . . . . . . . .  8
   4.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   6.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  9
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10

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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 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 [I-D.ietf-core-coap].

   Automated discovery of resources hosted by a constrained server is
   critical in machine-to-machine applications where human intervention
   is minimal and static interfaces result in fragility.  CoRE Resource
   Discovery is intended to support fine-grained discovery of hosted
   resources, their attributes, and possibly other resource relations

   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 Web Application
   Description Language (WADL) document

   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 [I-D.cheshire-dnsext-dns-sd] fields that permits
   discovery of CoAP services by either means.  It also addresses the
   following CoRE charter requirement [I-D.shelby-core-coap-req]:

   REQ8:   A definition of how to use CoAP to advertise about or query
           for a Device's description.  This description may include the
           device name and a list of its Resources, each with a URL, an
           interface description URI (pointing e.g. to a Web Application
           Description Language (WADL) document) and an optional name or
           identifier.  The name taxonomy used for this description will
           be consistent with other IETF work, e.g.
           draft-cheshire-dnsext-dns-sd. [charter]

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in "Key words for use in
   RFCs to Indicate Requirement Levels" [RFC2119].

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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 [RFC5988] 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.

   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.shelby-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.

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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 [I-D.cheshire-dnsext-dns-sd] for a detailed

   DNS-SD service names are limited to 255 octets and are of the form:

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

   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. <Domain> may identify a
   building zone as shown in the examples of [I-D.vanderstok-core-bc].

   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 octets in
   length, which is indicated in the resource record header in the DNS
   response message [I-D.cheshire-dnsext-dns-sd].  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 |

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2.  Mapping CoRE Link Attributes to DNS-SD Records

   [I-D.shelby-core-resource-directory] defines two new CoRE Link Format
   attributes that are particularly useful in conjunction with RDs:

      link-extension    = ( "ins" "=" quoted-string ) ; Max 63 octets
      link-extension    = ( "exp" )

   The Resource Instance "ins" attribute is an identifier for a resource
   that distinguishes it from other similar resources.  This attribute
   SHOULD be unique across resources with the same Resource Type
   attribute in the domain in which it is used.

   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.  If the "exp" attribute is defined for a link, then the
   following CoRE specific target attributes (defined in [RFC6690]) are
   mapped directly into DNS-SD labels.  The values are subject to format
   and length constraints as specified in [I-D.cheshire-dnsext-dns-sd].

2.1.  Mapping Resource Instance "ins" attribute into <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
   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-resistent 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 octets in length and the
   entire service name may not exceed 255 octets.

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2.2.  Mapping Resource Type "rt" attribute into <ServiceType>

   The resource type "rt" attribute is mapped into the <ServiceType>
   part of a DNS-SD service name (as defined in Section 1.4) 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
   octets 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 octets.  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.

2.3.  <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

2.4.  TXT Record key=value strings

   The resource <URI> is exported to the TXT record key=value string

   The Interface Description "if" attribute is exported to the TXT
   record key=value string "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 [I-D.cheshire-dnsext-dns-sd].

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3.  Examples

3.1.  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> (see
   section 2.3).  The agent responsible for exporting records to the DNS
   zone file SHOULD be authenticated to the DNS server.

   The following example shows an agent discovering a resource to be

     Agent                                                           RD
       |                                                             |
       | --- GET /rd-lookup/res?exp ------------------------------>  |
       |                                                             |
       |                                                             |
       | <-- 2.05 Content "<coap://node1/light/27>;exp; ------------ |
       |                   rt="dali.light";ins="FrontSpot"           |
       |                                                             |

   Req: GET /rd-lookup/res?exp

   Res: 2.05 Content

   The agent subsequenly registers the following DNS-SD RRs:    IN AAAA FDFD::1234
   _dali._udp            IN PTR  FrontSpot._dali._udp
   light._sub._dali._udp IN PTR  FrontSpot._dali._udp
   FrontSpot._dali._udp  IN SRV  0 0 5678
                         IN TXT  txtver=1
                         IN TXT  path=/light/27

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4.  IANA Considerations

   This document makes no request of IANA.

   Note to RFC Editor: this section may be removed on publication as an

5.  Security Considerations


6.  Acknowledgments

   Valuable contributions and review comments were made by Anders
   Brandt, Angelo Castellani, Esko Dijk, and Peter van der Stok.

7.  References

7.1.  Normative References

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, November 1987.

   [RFC1035]  Mockapetris, P., "Domain names - implementation and
              specification", STD 13, RFC 1035, November 1987.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

   [RFC2045]  Freed, N. and N. Borenstein, "Multipurpose Internet Mail
              Extensions (MIME) Part One: Format of Internet Message
              Bodies", RFC 2045, November 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3629]  Yergeau, F., "UTF-8, a transformation format of ISO
              10646", STD 63, RFC 3629, November 2003.

   [RFC5198]  Klensin, J. and M. Padlipsky, "Unicode Format for Network
              Interchange", RFC 5198, March 2008.

   [RFC5988]  Nottingham, M., "Web Linking", RFC 5988, October 2010.

   [RFC6335]  Cotton, M., Eggert, L., Touch, J., Westerlund, M., and S.

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              Cheshire, "Internet Assigned Numbers Authority (IANA)
              Procedures for the Management of the Service Name and
              Transport Protocol Port Number Registry", BCP 165,
              RFC 6335, August 2011.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, August 2012.

7.2.  Informative References

              Cheshire, S. and M. Krochmal, "DNS-Based Service
              Discovery", draft-cheshire-dnsext-dns-sd-11 (work in
              progress), December 2011.

              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-12 (work in progress), October 2012.

              Shelby, Z., Stuber, M., Sturek, D., Frank, B., and R.
              Kelsey, "CoAP Requirements and Features",
              draft-shelby-core-coap-req-02 (work in progress),
              October 2010.

              Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource
              Directory", draft-shelby-core-resource-directory-04 (work
              in progress), July 2012.

              Stok, P. and K. Lynn, "CoAP Utilization for Building
              Control", draft-vanderstok-core-bc-05 (work in progress),
              October 2011.

              Vial, M. and Z. Shelby, "Interface description with WADL
              in CoRE", draft-vial-core-link-format-wadl-01 (work in
              progress), September 2011.

   [dns-sd]   "dns-sd service type registration",
              Web, 2012.

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Authors' Addresses

   Kerry Lynn

   Phone: +1 978 460 4253

   Zach Shelby
   Kidekuja 2
   Vuokatti  88600

   Phone: +358407796297

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