CoRE Working Group K. Lynn
Internet-Draft Consultant
Intended status: Standards Track Z. Shelby
Expires: January 12, 2012 Sensinode
July 11, 2011
CoRE Link-Format to DNS-Based Service Discovery Mapping
draft-lynn-core-discovery-mapping-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 so
the two methods may be used interchangeably to locate services.
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
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Internet-Drafts are draft documents valid for a maximum of six months
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This Internet-Draft will expire on January 12, 2012.
Copyright Notice
Copyright (c) 2011 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
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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 . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 8
6. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . 8
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
7.1. Normative References . . . . . . . . . . . . . . . . . . . 8
7.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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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
[I-D.ietf-core-link-format].
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 as well as 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 satisfies 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].
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1.2. Resource Discovery
The main function of Resource Discovery is to provide Universal
Resource Identifiers (URIs, 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
is carried as a resource of its own (as opposed to HTTP headers
delivered with a specific resource).
[I-D.ietf-core-link-format] 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 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.
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 employing an
entity called a Resource Directory (RD), which hosts descriptions of
resources held on other servers, called end-points (EP), 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 records to facilitate discovery of
services (such as CoAP servers in a subdomain) using the existing DNS
infrastructure. This section gives a cursory 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 = {Instance}.{Service}.{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 {Service} part is composed of at least two labels. The first
label of the pair is an underscore character generally followed by
the application protocol name [I-D.ietf-tsvwg-iana-ports]. The
second label 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 label (beginning with an underscore)
followed by the "_sub" label. An example of the {Service} part is
"_lamp._sub._dali._udp". Only the rightmost pair of labels is used
to name SRV and TXT records.
The default {Instance} part of the service name may be set at the
factory or during the commissioning process. It SHOULD uniquely
identify a {Service} within a {Domain}. Taken together, these
elements comprise a unique name for an SRV record (and optionally a
corresponding TXT record) 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 (and 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 present 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 makes it possible to distinguish 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.
If the "exp" attribute is defined for a link, then the following CoRE
specific target attributes (defined in [I-D.ietf-core-link-format])
are intended to be exported directly into DNS-SD. The values are
subject to format and length constraints as specified in
[I-D.cheshire-dnsext-dns-sd].
2.1. Resource Instance "ins" attribute mapped onto {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 the device's MAC address, serial number,
or any similar hexadecimal string in an attempt to make the name
globally unique.
DNS labels are currently limited to 63 octets in length and the
entire service name may not exceed 255 octets.
2.2. Resource Type "rt" attribute mapped onto {Service}
The resource type "rt" attribute is mapped onto the {Service} part of
a DNS-SD instance name (as defined in Section 1.4) and must conform
to the following format. It must be comprised of Net-Unicode text
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stings, each preceded by an underscore '_' and limited to 16 octets
in length. The strings must be separated by periods '.' and end with
the defined CoAP transport label "_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
[I-D.cheshire-dnsext-dns-sd].]
2.4. TXT Record strings
The resource <URI> is exported to the TXT record string "PATH={URI}".
The Interface Description "if" attribute is exported to the TXT
record 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].
3. Examples
Assuming the ability to query a Resource Directory , or multicast a
GET (?exp) over the local link, CoAP resource discovery can 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 {Service}, and registered in the correct {Domain} [selection
method TBD]. The agent responsible for exporting records to the DNS
zone file SHOULD be authenticated with the DNS server.
[TBD: Examples]
4. IANA Considerations
This document makes no request of IANA.
Note to RFC Editor: this section may be removed on publication as an
RFC.
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5. Security Considerations
[TBD]
6. Acknowledgments
Contributions and review comments were made by Anders Brandt, Angelo
Castellani, 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.
7.2. Informative References
[I-D.cheshire-dnsext-dns-sd]
Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", draft-cheshire-dnsext-dns-sd-10 (work in
progress), February 2011.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
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"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-07 (work in progress), July 2011.
[I-D.ietf-core-link-format]
Shelby, Z., "CoRE Link Format",
draft-ietf-core-link-format-06 (work in progress),
June 2011.
[I-D.ietf-tsvwg-iana-ports]
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",
draft-ietf-tsvwg-iana-ports-10 (work in progress),
February 2011.
[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. and S. Krco, "CoRE Resource Directory",
draft-shelby-core-resource-directory-00 (work in
progress), June 2011.
[I-D.vanderstok-core-bc]
Stok, P. and K. Lynn, "CoAP Utilization for Building
Control", draft-vanderstok-core-bc-04 (work in progress),
July 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-00 (work in
progress), March 2011.
[dns-sd] "dns-sd service type registration",
Web http://www.dns-sd.org/ServiceTypes.html, 2011.
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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
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