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 draft-lynn-core-discovery-mapping-02 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 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 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 http://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 April 25, 2013. Copyright Notice Copyright (c) 2012 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 (http://trustee.ietf.org/license-info) 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 Lynn & Shelby Expires April 25, 2013 [Page 1] Internet-Draft CoRE Discovery Mapping October 2012 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 Lynn & Shelby Expires April 25, 2013 [Page 2] Internet-Draft CoRE Discovery Mapping October 2012 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 [RFC6690]. 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 [I-D.vial-core-link-format-wadl]. 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", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in "Key words for use in RFCs to Indicate Requirement Levels" [RFC2119]. Lynn & Shelby Expires April 25, 2013 [Page 3] Internet-Draft CoRE Discovery Mapping October 2012 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. Lynn & Shelby Expires April 25, 2013 [Page 4] Internet-Draft CoRE Discovery Mapping October 2012 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 specification. DNS-SD service names are limited to 255 octets and are of the form: Service Name = .. The 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. may identify a building zone as shown in the examples of [I-D.vanderstok-core-bc]. The 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 is "lamp._sub._dali._udp". Only the rightmost pair of labels is used in SRV and TXT record names. The default part of the service name may be set at the factory or during the commissioning process. It SHOULD uniquely identify an instance of within a . 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= (to support different versions of a service description). An example string is: | 0x08 | t | x | t | v | e | r | = | 1 | Lynn & Shelby Expires April 25, 2013 [Page 5] Internet-Draft CoRE Discovery Mapping October 2012 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 The Resource Instance "ins" attribute maps to the 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 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. Lynn & Shelby Expires April 25, 2013 [Page 6] Internet-Draft CoRE Discovery Mapping October 2012 2.2. Mapping Resource Type "rt" attribute into The resource type "rt" attribute is mapped into the 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 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 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. 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 [I-D.cheshire-dnsext-dns-sd].] 2.4. TXT Record key=value strings The resource is exported to the TXT record key=value string "path=". The Interface Description "if" attribute is exported to the TXT record key=value string "if=". 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]. Lynn & Shelby Expires April 25, 2013 [Page 7] Internet-Draft CoRE Discovery Mapping October 2012 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 , and registered in the correct (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 exported: Agent RD | | | --- GET /rd-lookup/res?exp ------------------------------> | | | | | | <-- 2.05 Content ";exp; ------------ | | rt="dali.light";ins="FrontSpot" | | | Req: GET /rd-lookup/res?exp Res: 2.05 Content ; exp;ct=41;rt="dali.light";ins="FrontSpot" The agent subsequenly registers the following DNS-SD RRs: node1.example.com. 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 node1.example.com. IN TXT txtver=1 IN TXT path=/light/27 Lynn & Shelby Expires April 25, 2013 [Page 8] Internet-Draft CoRE Discovery Mapping October 2012 4. IANA Considerations This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC. 5. Security Considerations [TBD] 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. Lynn & Shelby Expires April 25, 2013 [Page 9] Internet-Draft CoRE Discovery Mapping October 2012 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 [I-D.cheshire-dnsext-dns-sd] Cheshire, S. and M. Krochmal, "DNS-Based Service Discovery", draft-cheshire-dnsext-dns-sd-11 (work in progress), December 2011. [I-D.ietf-core-coap] Shelby, Z., Hartke, K., Bormann, C., and B. Frank, "Constrained Application Protocol (CoAP)", draft-ietf-core-coap-12 (work in progress), October 2012. [I-D.shelby-core-coap-req] Shelby, Z., Stuber, M., Sturek, D., Frank, B., and R. Kelsey, "CoAP Requirements and Features", draft-shelby-core-coap-req-02 (work in progress), October 2010. [I-D.shelby-core-resource-directory] Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource Directory", draft-shelby-core-resource-directory-04 (work in progress), July 2012. [I-D.vanderstok-core-bc] Stok, P. and K. Lynn, "CoAP Utilization for Building Control", draft-vanderstok-core-bc-05 (work in progress), October 2011. [I-D.vial-core-link-format-wadl] 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 http://www.dns-sd.org/ServiceTypes.html, 2012. Lynn & Shelby Expires April 25, 2013 [Page 10] Internet-Draft CoRE Discovery Mapping October 2012 Authors' Addresses Kerry Lynn Consultant Phone: +1 978 460 4253 Email: kerlyn@ieee.org Zach Shelby Sensinode Kidekuja 2 Vuokatti 88600 FINLAND Phone: +358407796297 Email: zach@sensinode.com Lynn & Shelby Expires April 25, 2013 [Page 11]