CoRE K. Lynn
Internet-Draft Oracle + Dyn
Intended status: Standards Track P. van der Stok
Expires: April 25, 2019 Consultant
M. Koster
SmartThings
C. Amsuess
Energy Harvesting Solutions
October 22, 2018
CoRE Resource Directory: DNS-SD mapping
draft-ietf-core-rd-dns-sd-03
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
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
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://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, 2019.
Copyright Notice
Copyright (c) 2018 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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(https://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
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. CoRE Resource Discovery . . . . . . . . . . . . . . . . . 3
1.3. CoRE Resource Directories . . . . . . . . . . . . . . . . 4
1.4. DNS-Based Service Discovery . . . . . . . . . . . . . . . 5
2. Mapping from web resources DNS services . . . . . . . . . . . 6
2.1. Domain mapping . . . . . . . . . . . . . . . . . . . . . 6
2.2. ServiceType mapping . . . . . . . . . . . . . . . . . . . 6
2.3. Instance mapping . . . . . . . . . . . . . . . . . . . . 7
3. New Link-Format Attributes . . . . . . . . . . . . . . . . . 7
3.1. Export attribute "exp" . . . . . . . . . . . . . . . . . 7
3.2. Resource Instance attribute "ins" . . . . . . . . . . . . 8
4. Mapping CoRE Link Attributes to DNS-SD Record Fields . . . . 8
4.1. Mapping Resource Instance attribute "ins" to <Instance> . 8
4.2. Mapping Resource Type attribute "rt" to <ServiceType> . . 9
4.3. TXT Record key=value strings . . . . . . . . . . . . . . 9
4.4. Exporting resource links into DNS-SD . . . . . . . . . . 10
5. IANA considerations . . . . . . . . . . . . . . . . . . . . . 10
5.1. Mapping Resource Type into ServiceType . . . . . . . . . 10
6. Security considerations . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Acknowledgments . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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 [RFC7252].
Automated discovery of resources hosted by a constrained server is
critical in M2M applications where human intervention is minimal and
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static interfaces result in brittleness. CoRE Resource Discovery is
intended to support fine-grained discovery of hosted resources, their
attributes, and possibly other resource relations [RFC6690].
In contrast to resource discovery, service discovery generally refers
to a coarser-grained resolution of an endpoint'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
JSON Hyper-Schema document [I-D.handrews-json-schema-hyperschema] per
function.
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 (DNS-SD) [RFC6763] fields that permits discovery of
CoAP services by either method. It also addresses the CoRE charter
goal to interoperate with DNS-SD.
The actual publishing of DNS services on the basis of the contents of
the Resource Directory is the subject of
[I-D.sctl-service-registration].
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]. To describe the REST interfaces defined in this
specification, the URI Template format is used [RFC6570].
This specification also incorporates the terminology of
[I-D.ietf-core-resource-directory].
1.2. CoRE Resource Discovery
[RFC8288] defines a Web Link (link) as a typed connection between two
resources, comprised of:
o a link context,
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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) is typically a
Universal Resource Identifier (URI) [RFC3986].
For example, "https://www.example.com/" has a "canonical" resource at
"https://example.com", which has a "type" of "text/html".
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 (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 [RFC8288] to describe these
link descriptions. The CoRE Link Format is carried as a payload and
is serialized according to one of several Internet media types. CoRE
Resource Discovery is accomplished by sending a GET request to the
well-known URI "/.well- known/core", which is defined as a default
entry-point for requesting the collection of links to resources
hosted by a server.
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 the links. 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. 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 held on other servers (referred to as "end-
points") and allows lookups to be performed for those resources. An
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endpoint is a web server associated with a specific IP address and
port; thus a physical device may host one or more endpoints. End-
points may also act as clients.
The Resource Directory implements a set of REST interfaces for end-
points 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 host 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 that identify the resources
on an individual server or group of servers.
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. For example, an organization such as the
Open Connectivity Foundation [OCF] that specifies resources might
register the application protocol name "_oic", which all servers that
advertise OCF resources would use as part of their ServiceType. The
second label indicates the transport and is typically "_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 "light._sub._oic._udp".
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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 [RFC6763]. 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 |
----------------------------------------
2. Mapping from web resources DNS services
These sections describe how each of the three parts of the Service
Name can be mapped to link attributes.
2.1. 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
[RFC6763] and Section 2 in [I-D.sctl-service-registration]
2.2. ServiceType mapping
ServiceTypes are registered by IANA [st]. They identify services
that can be specified by IETF or any other Standards Development
Organization (SDO). The IANA resource type registry [rt] is based on
the resource type (rt= attribute) [RFC6690] which identifies endpoint
functionality specified by IETF or any other SDO.
It is expected that an endpoint providing a given ServiceType
represents a collection of resources each with its own Resource Type.
The Resource Type of the collection MUST be mapped directly to the
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ServiceType. A registry is required to specify the mapping between
Resource Types and ServiceTypes.
2.3. Instance mapping
The Instance name may be freely chosen by the manufacturer and
inserted in the device. During installation the pre-configured
Instance name can be pre- or post-fixed with a string to make the
(Instance, ServiceType) pair unique within the domain. For manual
discovery it is useful when the Instance name is a human readable
string containing the manufacturer name or the device type.
IoT devices are not necessarily equipped with an Instance name for
DNS-SD. To make the (Instance, ServiceType) pair unique, it is
sufficient to use another unique identifier stored in the device such
as the Public key or UUID of the device. When a human readable name
is required, the interface description (if= attribute) [RFC6690] may
provide for example, a URN that can be made unique by pre- or post-
fixing it with a string as is currently done for the Instance name
devices conforming to DNS-SD specification.
When the device selects the Instance name, the device, registering
with the RD, MUST provide an Instance name in its link. When a third
party device, the Commissioning Tool (CT)
[I-D.ietf-core-resource-directory], selects the Instance name, it
specifies the Instance name when registering the device with the
Resource Directory.
3. 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 useful. This specification
defines the following new attributes for use in the CoRE Link Format
[RFC6690]:
link-extension = ( "exp" )
link-extension = ( "ins" "=" (ptoken | quoted-string) )
; The token or string is max 63 bytes
3.1. 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
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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].
3.2. 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. This attribute is equivalent in use to the
<Instance> portion of a DNS-SD record (see Section 1.4), and SHOULD
be unique across resources with the same Resource Type attribute in
the domain in which it is used. A Resource Instance SHOULD be a
descriptive string like "Ceiling Light, Room 3", but MAY be a short
ID like "AF39", a unique UUID, or fingerprint of a public key. This
attribute is used by a Resource Directory to distinguish between
multiple instances of the same resource type within the directory.
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].
4. Mapping CoRE Link Attributes to DNS-SD Record Fields
4.1. Mapping Resource Instance attribute "ins" to <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, 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.
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4.2. Mapping Resource Type attribute "rt" to <ServiceType>
The <ServiceType> part of a DNS-SD Service Name is derived from the
"rt" attribute and SHOULD conform to the reg-rel-type production of
the Link Format defined in Section 2 of [RFC6690].
In practice, the ServiceType should unambiguously identify inter-
operable devices. It is up to individual SDOs to specify how to map
between their registered Resource Type (rt=) values and ServiceType
values. Two approaches are possible; either a hierachical approach
as in Section 1.4 above, or a flat identifier. Both approaches are
shown below for illustration, but in practice only ONE would be
specified.
In either case, the resulting application protocol name MUST be
composed of at least a single Net-Unicode text string, without
underscore '_' or 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 be
mapped into "_oic-d-light._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 bytes. 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="oic.d.light" might be mapped into "light._sub._oic._udp".
The resulting string is used to form labels for DNS-SD records which
are stored directly in the DNS.
4.3. TXT Record key=value strings
A number of [RFC6763] key/value pairs are derived from link-format
information, to be exported in the DNS-SD as key=value strings in a
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].
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4.4. 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 Resource Type as
the <ServiceType>, and registered in the correct <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
<coap://[FDFD::1234]:5683/light/1>;
exp;rt="oic.d.light";ins="Spot";
d="office";ep="node1"
The agent subsequently registers the following DNS-SD RRs, assuming a
zone name "example.com" prefixed with "office":
_oic._udp.office.example.com IN PTR
Spot._oic._udp.office.example.com
light._sub._oic._udp.example.com IN PTR
Spot._oic._udp.office.example.com
Spot._oic._udp.office.example.com IN TXT
txtver=1;path=/light/1
Spot._oic._udp.office.example.com IN SRV 0 0 5683
node1.office.example.com.
node1.office.example.com. IN AAAA FDFD::1234
In the above figure the Service Name is chosen as
Spot._oic._udp.office.example.com without the light._sub service
prefix. An alternative Service Name would be:
Spot.light._sub._oic._udp.office.example.com.
5. IANA considerations
5.1. Mapping Resource Type into ServiceType
TBD
6. Security considerations
TBD
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7. References
7.1. Normative References
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<https://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <https://www.rfc-editor.org/info/rfc1035>.
[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123,
DOI 10.17487/RFC1123, October 1989,
<https://www.rfc-editor.org/info/rfc1123>.
[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,
<https://www.rfc-editor.org/info/rfc2045>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, DOI 10.17487/RFC3629, November
2003, <https://www.rfc-editor.org/info/rfc3629>.
[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,
<https://www.rfc-editor.org/info/rfc3986>.
[RFC5198] Klensin, J. and M. Padlipsky, "Unicode Format for Network
Interchange", RFC 5198, DOI 10.17487/RFC5198, March 2008,
<https://www.rfc-editor.org/info/rfc5198>.
[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,
<https://www.rfc-editor.org/info/rfc6335>.
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[RFC6570] Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
and D. Orchard, "URI Template", RFC 6570,
DOI 10.17487/RFC6570, March 2012,
<https://www.rfc-editor.org/info/rfc6570>.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
<https://www.rfc-editor.org/info/rfc6690>.
[RFC6763] Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", RFC 6763, DOI 10.17487/RFC6763, February 2013,
<https://www.rfc-editor.org/info/rfc6763>.
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC8288] Nottingham, M., "Web Linking", RFC 8288,
DOI 10.17487/RFC8288, October 2017,
<https://www.rfc-editor.org/info/rfc8288>.
7.2. Informative References
[I-D.handrews-json-schema-hyperschema]
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.
[I-D.ietf-core-resource-directory]
Shelby, Z., Koster, M., Bormann, C., Stok, P., and C.
Amsuess, "CoRE Resource Directory", draft-ietf-core-
resource-directory-15 (work in progress), October 2018.
[I-D.sctl-service-registration]
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, <https://openconnectivity.org/developer/
specifications>.
[rt] IANA, "Resource Type (rt=) Link Target Attribute
Values", 2012, <https://www.iana.org/assignments/core-
parameters/
core-parameters.xhtml#rt-link-target-att-value>.
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[st] IANA, "Service Name and Transport Protocol Port Number
Registry", 2018, <https://www.iana.org/assignments/
service-names-port-numbers/
service-names-port-numbers.xml>.
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.
Authors' Addresses
Kerry Lynn
Oracle + Dyn
150 Dow Street, Tower Two
Manchester, NH 03101
USA
Phone: +1 978-460-4253
Email: kerlyn@ieee.org
Peter van der Stok
Consultant
Phone: +31 492474673 (Netherlands), +33 966015248 (France)
Email: consultancy@vanderstok.org
URI: www.vanderstok.org
Michael Koster
SmartThings
665 Clyde Avenue
Mountain View, CA 94043
USA
Phone: +1 707-502-5136
Email: Michael.Koster@smartthings.com
Christian Amsuess
Energy Harvesting Solutions
Hollandstr. 12/4
1020
Austria
Phone: +43 664-9790639
Email: c.amsuess@energyharvesting.at
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