CoRE                                                P. van der Stok, Ed.
Internet-Draft                                          Philips Research
Intended status: Informational                                   K. Lynn
Expires: September 1, 2012                                    Consultant
                                                               A. Brandt
                                                           Sigma Designs
                                                       February 29, 2012

                 CoRE Discovery, Naming, and Addressing


   This is a working document intended to focus discussion and refine
   draft language for the CoAP protocol specification (or other proposed
   standards) in the areas of discovery, naming, and addressing.
   Engineering tradeoffs become more challenging in constrained
   environments; therefore discovery, naming, and addressing are
   considered within the context of adjacent topics that may impact or
   be impacted by design choices in the subject areas.  Special emphasis
   is placed on group definition and discovery.  Examples show how
   groups can be represented in CoAP Resource Directories or DNS-SD.

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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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on September 1, 2012.

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

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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
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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.3.  Applicability  . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Use Cases  . . . . . . . . . . . . . . . . . . . . . . . . . .  6
     3.1.  Discovery scope  . . . . . . . . . . . . . . . . . . . . .  6
     3.2.  Interactive mapping  . . . . . . . . . . . . . . . . . . .  6
     3.3.  M2M mapping  . . . . . . . . . . . . . . . . . . . . . . .  7
     3.4.  Function Set grouping  . . . . . . . . . . . . . . . . . .  7
     3.5.  Discovery queries  . . . . . . . . . . . . . . . . . . . . 10
     3.6.  Sleeping devices . . . . . . . . . . . . . . . . . . . . . 11
   4.  DNS and RD examples  . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  DNS-SD examples  . . . . . . . . . . . . . . . . . . . . . 12
     4.2.  RD examples  . . . . . . . . . . . . . . . . . . . . . . . 17
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 20
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . . 21
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 21
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 21
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 22
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 23

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1.  Introduction

   The CoRE working group is chartered to design and standardize a
   Constrained Application Protocol (CoAP) for resource constrained
   devices and networks [I-D.ietf-core-coap].  The requirements for CoRE
   are documented in [I-D.shelby-core-coap-req].  This draft discusses
   the requirements on service discovery for M2M and interactive
   applications using resource constrained devices.  We propose the use
   of DNS-SD [I-D.cheshire-dnsext-dns-sd] and Resource Directory (RD)
   [I-D.shelby-core-resource-directory] to satisfy the requirements.
   The proposal relies heavily on naming and addressing conventions.
   Special emphasis is placed on the definition, naming, and discovery
   of groups.

1.1.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].
   Addtional privileged words are described below.

   A "device" is a physical processor connected to at least one link
   through a network interface.  Each interface has at least one IP
   unicast address.  The IP address is optionally bound to a host name,
   which may be a Fully Qualified Domain Name (FQDN).

   An "end-point" corresponds to a "service" and is indentified by a
   unique {protocol, host, port} tuple.  The identity of an endpoint may
   be specified by the 'scheme' and 'authority' parts of a URI
   [RFC3986].  'Protocol' is a label that indicates application and
   transport protocol bindings as well as default port (if port is not
   specified) and possibly default semantics such as web-linking
   [RFC5988].  'Host' corresponds to the [RFC3986] ABNF production as
   updated by [I-D.ietf-6man-uri-zoneid].

   A "service type", e.g. _bldg-ctrl._tcp, is equivalent to the
   'protocol' label described above.  It identifies an application
   protocol, typically defined by a Standards Development Organization
   (SDO), and is ultimately registered with IANA
   [I-D.cheshire-dnsext-dns-sd].  The SDO may additionally specify
   service subtypes (e.g. _light, _onoff-control, _air-flow ...) to
   designate units of functionality, the attributes of the subtypes, and
   the primitives acting on the attributes.

   Any attribute of an end-point that can be acted upon by REST methods
   will be represented as a "resource" and must be identified by a URI,
   that is, an end-point plus a 'path' component [RFC3986].

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   A "Function Set" is a service subtype with a standarized set of
   resources and behaviors that may be accessed through a REST
   interface.  A Function Set will typically be decribed by a base URI
   plus an interface definition as described in
   [I-D.shelby-core-interfaces].  The interface definition may specify
   the naming patterns of subordinate resources and the methods that act
   on them as defined by the SDO.

   A "Collection" is a set of two or more homogeneous subordinate
   resources that may be acted upon in the aggregate by sending messages
   to their parent resource, or individually by sending messages to the
   collection member.

   A "Device type" describes a standardized set of function sets that
   satisfy a use case for a hosting device.

   A "group" is a set of devices.

   A "multicast group" is a group of devices that share a multicast
   address.  The multicast address is optionally bound to a FQDN that
   identifies the multicast group.  For the purposes of this document,
   (multicast) groups generally host the same Function Set.

   A "scope" is a possible set of devices or groups.  A scope may be
   realized logically, e.g. as a DNS domain, or a zone which is a unit
   of delegation (partition) of a domain; physically, e.g. the local
   link; or administratively, e.g. as a set of links.

1.2.  Motivation

   In this draft, we focus and expand discussions on requirements
   pertaining to discovery, naming, and addressing, including REQ8 of
   the CoRE charter:

   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]

   The basis of this draft originated in [I-D.vanderstok-core-bc].

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1.3.  Applicability


2.  Architecture

   This section illustrates the aspects of naming schemes and their
   support by DNS-based Service Discovery (DNS-SD)
   [I-D.cheshire-dnsext-dns-sd] , Extended Multicast DNS (xmDNS)
   [I-D.lynn-dnsext-site-mdns], and the Resource Directory (RD)
   [I-D.shelby-core-resource-directory] on a set of network

   The basic network for low-power nodes can be composed of low-resource
   nodes sharing the same IPv6 prefix and connected to low-power links
   like IEEE 802.15.4, ITU-T G.9959, or Powerline.  The "lowpan" is a
   good example of such a network [RFC4944], [RFC6282].  The network can
   be either isolated or connected.  This draft assumes that application
   profiles are defined above coap or http, for example, applications as
   specified by the ZigBee Smart Energy Profile 2.0 (SEP2), Obix, or
   BACnet IT working groups.  The naming and discovery solutions
   presented here are applicable to multiple interconnected subnets.
   Example network architectures are:

   - An isolated lowpan consists of at least two lowpan devices, one of
     which is an edge router that is not connected to a backbone.  A
     Resource Directory may be situated on the edge router.
     Alternatively, xmDNS responders may execute on each device.
   - A connected lowpan consists of at least two lowpan devices, one of
     which is an edge router that is connected to a backbone.  A
     Resource Directory may be situated on an edge router.
     Alternatively, xmDNS responders may execute on each device or the
     DNS-SD service may be available over the backbone.
   - Interconnected lowpans consist of at least two lowpans connected
     via edge routers to the same backbone.  A Resource Directory may be
     situated on each edge router.  Alternatively, xmDNS responders may
     execute on each device or the DNS-SD service may be available over
     the backbone.
   - A site is a set of interconnected subnets that is locally
     administered.  A site may include zero or more lowpans.  Border
     routers may prevent some messages from passing into or out of the

   In certain scenarios, the domain may correspond to the network
   topology.  In the general case, the domain and network subnet
   structure may differ.

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3.  Use Cases

   The use of service discovery is presented in two environments: (1)
   interactive service discovery, and (2) M2M service discovery.  From
   the use cases we derive the types of queries that service discovery
   should support.  In particular, a primary motivation is the discovery
   of groups that support a given Function Set.

3.1.  Discovery scope

   Service discovery has a scope that can be defined and realized with
   domain names.  The authority part of a URI [RFC3986] can express the
   location of the device hosting the service.  A common example is the
   naming associated with the structure of a building.  A device may
   acquire a FQDN that relates directly to its location in the building.
   For example, (or shorter refers to a power-strip with device name
   "power-strip" (or short "ps") in office4 at floor1 in the building of
   the company  Another naming scheme can be functional
   like, possibly refering to TV number 1
   maintained by the media group of the IT department of the example
   organisation.  Domain naming can be used to express that devices are
   situated in the same building area or belong to the same
   organisational units.  Multiple FQDNs can identify a given device.

   The DNS provides the mapping from Fully Qualified Domain Name (FQDN)
   to network address and vice-versa.  The binding of FQDN to a physical
   device (for example, assigning a given FQDN to the TV in the corner)
   depends on the operational conditions as described below.

3.2.  Interactive mapping

   In the residential context, naming of the device is done by the
   occupant of the home.  After connecting the device to the network, an
   IP-address is assigned, possibly based on the EUI-64 value of the
   network interface.  The occupant can use a remote control with a
   graphical user interface to display all devices that provide a given
   service (e.g. a "lighting" service).  The remote control prompts the
   occupant to identify the (default named) devices, possibly
   accompanied by on/off switching, barcode scanning, or other manual
   intervention.  The occupant can provide a meaningful name that will
   be bound to that device.  The installation steps can be as follows:
   Service discovery returns all interfaces on which the specified
   service is available.  The occupant, with or without additional
   physical support, establishes the binding between an IP address
   (based on MAC address or other unique identifier) and the name of the
   device providing the service, plus its relationship with other
   devices or function sets in the system.  In some cases a device has

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   multiple names, and an additional mapping between user specified name
   and an automatically generated DNS name is supported.

3.3.  M2M mapping

   In the commercial context (e.g. office buildings) it is usual to
   employ a Comissioning Tool (CT) to provide the mapping from physical
   device to IP network address or FQDN.  The professional context is
   more rigid than the home because the absence of devices and also the
   unwanted presence of devices needs to be detected.  Devices are named
   by an architect or installation contractor.  Names can be generated
   automatically and need not be human-friendly.  The CT contains the
   names and the physical locations of the devices.  At commissioning
   time the interfaces have acquired a network address, possibly based
   on the EUI-64.  The physical device is identified by reading in a
   unique identifier (e.g.  EUI-64 of interface, UUID of device) with a
   reader (e.g. barcode reader).  Consequently, the device name to
   network address binding is stored into DNS (or elsewhere).
   Alternatives for identifying devices are pushing buttons on the
   device or remotely switching on/off the device.

3.4.  Function Set grouping

   Groups can be used to express that devices are related (e.g.  HVAC
   equipment controlled by the closest temperature sensor).  Grouping is
   also necessary when a set of Function Sets has to react together,
   more or less synchronously, to a sequence of commands sent by one or
   more devices.  A common example is provided by lighting applications
   where a subset of lights in the building are dimmed to the same
   level, set to the same colour, or switched off simultaneously.
   Another example is provided by a power-strip supporting a set of
   power-outlets.  Power-outlets are switched on/off individually or all
   together.  Other examples concern the home, such as a "sleep mode"
   setting of all media devices in the home when the user activates the
   night scene.

   Group naming is done the same way as for device naming.  Related
   devices are grouped and named.  The group name is constructed like a
   FQDN with the group name as prefix.  Adressing the group can be done
   in two ways: (1) by addressing each Function Set of the group
   individually (which requires serial access), or (2) by defining a
   multicast address for a multicast group.  In the latter case, each
   hosting device must enable reception of the messages sent to the
   multicast address.  The Function Sets of the multicast group must
   have identical port number and path, because their values are
   specified in a single multicast message.

   The Function Set can contain a collection of resources of the same

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   subtype.  The Function Set path postfixed with an identifier refers
   to the individual resources of the collection.  For example: the
   /path/light points to the resource collection of the service subtype
   light.  Each member of the collection can be identified with /path/
   light/x, with x in {1,2,3,..}.  Consequently, the /path/light/1/onoff
   specifies the onoff resource of collection member 1 of Function Set
   with /path/light, and /path/light/onoff specifies the resource onoff
   of all collection members contained by the Function Set with /path/
   light.  When /path/light/onoff is used in a multicast message, it is
   interpreted as a message to a single light resource by devices having
   only one, and to all members of the collection for devices having
   several light resources.

   It is expected that SDOs will define group naming conventions,
   extending the service type name conventions for individual devices.

   Figure 1 illustrates some of the concepts described above.  A device
   is identified with the name "Power-strip".  In the example, no domain
   names are associated with the device, and the name "power-strip"
   resolves to a Unique Local Address (ULA)[RFC4193].  The device
   provides two end-points: one delivers a http service and the second a
   CoAP service.  The http server and CoAP server share the same IP-
   address and use different ports 80 and 61616 respectively.  Two
   different service subtypes, one identified by Function Set, ps, and
   one identified by Function Set, pm, are supported by one end-point.
   The Function Set "Power strip" contains a collection of four
   resources, "Outlet 1" to "Outlet 4", each one accessible via the
   accompanying paths /ps/1 to ps/4.  The path /ps interfaces to the
   entire resource collection.  The attribute "output" is defined in the
   service subtype specification.

   TBD......  Relation with [I-D.shelby-core-interfaces]

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   | "Power-strip"                                                     |
   | [device] has at least one NIC/IP address, may have a name         |
   |                                                                   |
   | +---------------------------------------------------------------+ |
   | | "HTTP server"                                                 | |
   | | [End-point] http://power-strip (at default TCP port 80)       | |
   | +---------------------------------------------------------------+ |
   |                                                                   |
   | +---------------------------------------------------------------+ |
   | | "CoAP server"                                                 | |
   | | [End-point] coap://power-strip (at default UDP port 61616)    | |
   | |                                                               | |
   | | +-----------------------------------------------------------+ | |
   | | | "Power Meter"                                             | | |
   | | | [Function Set] coap://power-strip/pm                      | | |
   | | +-----------------------------------------------------------+ | |
   | |                                                               | |
   | | +-----------------------------------------------------------+ | |
   | | | "Power strip"                                             | | |
   | | | [Function Set] coap://power-strip/ps                      | | |
   | | |                                                           | | |
   | | | +-------------------------------------------------------+ | | |
   | | | | "Outlet 1"                                            | | | |
   | | | | [Collection member] coap://power-strip/ps/1           | | | |
   | | | |   [resource] coap://power-strip/ps/1/output           | | | |
   | | | |   ...                                                 | | | |
   | | | +-------------------------------------------------------+ | | |
   | | |                                                           | | |
   | | | +-------------------------------------------------------+ | | |
   | | | | "Outlet 2"                                            | | | |
   | | | | [Collection member] coap://power-strip/ps/2           | | | |
   | | | +-------------------------------------------------------+ | | |
   | | |                                                           | | |
   | | | +-------------------------------------------------------+ | | |
   | | | | "Outlet 3"                                            | | | |
   | | | | [Collection member] coap://power-strip/ps/3           | | | |
   | | | +-------------------------------------------------------+ | | |
   | | |                                                           | | |
   | | | +-------------------------------------------------------+ | | |
   | | | | "Outlet 4"                                            | | | |
   | | | | [Collection member] coap://power-strip/ps/4           | | | |
   | | | +-------------------------------------------------------+ | | |
   | | +-----------------------------------------------------------+ | |
   | +---------------------------------------------------------------+ |

       Figure 1: device with end-points, Function Sets and resources

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3.5.  Discovery queries

   Service discovery should support that a device can learn its domain
   and all the devices within a domain providing a given service (e.g.
   temperature measurement).  Devices need to learn the groups to which
   they belong and learn all the members of those groups.  This section
   motivates that a discovery service supports the following queries:

   Goal            Description
   Name_resolution Resolve the group or device name to IP address and
                   optional port number
   Return_device   Return all devices supporting a given service
                   (sub-)type within a given domain
   Create_group    Create a group of devices possibly hosting a given
                   service (sub-)type within a given domain
   Enroll_member   Enroll a given device as member of a given group
   Remove_member   Remove a given device as member of a given group
   Return_group    Return all groups of which a given device is a member
   Return_member   Return devices belonging to a given group

   Name_resolution is supported by DNS and CoAP resource discovery.
   Names are required in the context of home control and manual setup of
   installations.  Names are persistent and meaningful as compared to IP
   addresses and are preferably used in applications when IP addresses
   can change.

   Return_device is the most common use of service discovery and was
   originally dsigned for interactive use.  The canonical IT example is
   finding all printers within a zone, which allows a user to select the
   desired printer from the returned list.  Another example is in the
   context of UPnP [UPNP], where all media players are returned on a
   screen and the user can select the desired media player on the screen
   and play the selected content.  In M2M applications, the returned
   names are not displayed on a screen but an application uses the
   returned list to select a (set of) Function Set(s) to control.
   Consequently, names in M2M applications need not be human
   interpretable (for example, they can be unique numbers).

   Create_group is useful in commissioning scenarios, where devices need
   to be grouped to receive the same command in a possibly synchronous
   fashion.  Groups can also be created to express relations between
   devices such as ownership.  The command creates a group name and
   creates a list of the members of the group.  When the group is a
   multicast group, the command defines a unique multicast address and
   port, and specifies the path.

   Enroll_member supports network and device reconfiguration.  When the
   physical lay-out of an installation changes because devices are

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   added, changed or removed, the associated groups also need to be

   Remove_member, see motivation under Enroll_member

   Return_group is needed to learn the groups of which a device is
   member.  The command is necessary for commissioning purposes where a
   Commissioning Tool (CT) is used.  The CT, on the basis of designs
   provided by architects, decorators, sound/light engineers, defines
   groups and group members and stores that information in the service
   discovery database.  In the next phase, the members of a group need
   to learn their membership from the service discovery to enable
   reception of messages.

   Return_member can be used to learn which devices are member of a
   given group.  This command is useful in connection with Return_group.
   The device knowing to which groups it belongs can establish
   communication with the group members.  For example, membership of a
   group instructs new devices, replacing faulty ones, which other
   devices share access rights or need to be consulted regularly.

3.6.  Sleeping devices

   This section suggests that service discovery of sleeping devices is
   mostly a matter of discovering the proxy.  It is expected that a
   proxy will handle communications for the sleeping device.  The
   message sent to the sleeping device is directed to the proxy.  The
   proxy will send the message on with a delay, or send the result of a
   function on the history of messages, when the sleeping device is
   ready.  The communication protocol between proxy and sleeping device
   is currently proprietary, but efforts are under way to standardize.
   The setting up of the proxy is preferably standardized for a large
   set of proxy types.  During the setting-up process, (offline or
   online) the proxy will take over all the entry-points of the sleeping
   device.  The entry-points of the proxy can be entered into the
   discovery respository and consequently discovered like any other

   For groups, two cases need to be considered (1) sleeping device is
   member of a group and receives group messages, and (2) the sleeping
   device sends messages to a group.  Ad (1), when the sleeping device
   needs to receive messages sent to a group, the proxy will receive
   those messages and the Function Set of the proxy is entered as group
   member to receive the group messages.  Ad (2) When the sleeping
   device sends messages to a group, it is preferable that the sleeping
   device sends just one multicast message to the group to minimize
   energy costs.  It is required that when one member of the group
   receives the message, all other group members receive it as well

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   (unanimity), covered by the "reliability" REQ1 in
   [I-D.ietf-core-groupcomm]).  A simple broadcast over a lowpan will
   not always succceed and additional multicast algorithms like Trickle
   [RFC6206] need to be introduced.

4.  DNS and RD examples

   The following device configuration and environment are assumed for
   the examples.  The devices are placed on floor-x (fx) in two rooms
   room-y (ry) and room-z (rz).  Both rooms contain a powerstrip with a
   powermeter and four power outlets.  In each room there are two
   luminaires and one presence sensor (PIR).  Each luminaire contains a
   dimmable light and a light sensor.  Per floor there is a clock to set
   day and night time modes of the devices.  The domains are:, and  The device names of
   the 4 luminaires are lm00203, lm00204, lm00205, and lm00206.  The
   device names of the two powerstrips are ps0057, and ps0078.  The
   paths of the Function Sets of the luminaire are: /lamp with resource
   /lamp/dim for the dimmable light, and /light with resource /light/
   lumen for the light sensor.  The Function Set path for four outlets
   is /ps with resource /ps/output.  The path of each individual outlet
   is /ps/x with x in {1,2,3,4}, and with resource ps/x/output.  The
   name of the two PIRs is pir.  The entry-point path of the PIR is
   /occup, also being the resource.

   Relating location to the domain name is a relevant example of domain
   naming.  Multiple domain names, related to other application aspects,
   can be specified and applied simultaneously.

   Separate subsections provide examples for discovery of devices and of
   groups.  As described in section 3, devices do not announce
   themselves to the discovery repository, as usual for IT applications,
   but they are entered (partially) with the aid of a central tool, for
   example a Remote Control, dedicated device, IPAD or other means.

4.1.  DNS-SD examples

4.1.1.  Basic Concepts

   In conformance with [I-D.cheshire-dnsext-dns-sd], DNS-based discovery
   uses A or AAAA, PTR, SRV, and TXT Resource Records (RR).  The SRV RR
   [RFC2782] specifies an endpoint.  An associated (identically named)
   TXT RR can contain a URI path.  Together the associated SRV and TXT
   RRs can specify a Function Set. An A or AAAA RR [RFC1035] binds a
   device name or multicast group name to an IP address.  The PTR RR
   binds a service type to an end-point, or a service subtype to a
   Function Set.

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   In cases where the end-point port may be dynamic, e.g. in the IPHC
   [RFC6282] compressible range, a new 'coap+srv' scheme is proposed
   (after [I-D.jennings-http-srv]).  The authority part of a coap+srv
   URI specifies the name and location of an SRV record, which in turn
   contains values for host (IP address) and port.

4.1.2.  Commissioning devices

   Commissioning is the process to store the relation between a FQDN and
   a device.  It is assumed that either a Remote Control (RC) in the
   home or a Commissioning Tool (CT) in the professional domain store
   the relation in DNS.

   In the professional domain, the CT is assumed to contain information
   about the devices as prescibed by architect or installation company.
   The information in the CT contains device name, device domain name,
   and location in the building, but the relation with the installed
   processor, identified with an unique identifier (e.g.  EUI-64) is not
   established.  By reading a bar code (or pushing buttons, switching
   on/off equipment, etc.), the CT learns the identifier of the device
   to be commissioned.  All kinds of techniques can be used to establish
   the relation between IP address and unique identifier.  When the
   identifier is the EUI-64 value, the IP address of the device can be
   constructed.  When the identifier is not the EUI-64, a proprietary
   procol can be used to ask a given device its identifier.  Etc. etc.
   The CT can learn the Function Sets (services) available on the device
   by querying /.well-known/core.  In some cases the CT already obtained
   the Function Sets from a configuration file.  Given these data, the
   CT can enter the devices and its services into DNS.  Either
   automatically, or on instructions of an operator, the CT defines the
   groups in the DNS.

   The home domain is different from the professional domain in the
   sense that no configuration information exists.  The RC can for
   example use xmDNS to learn the addresses of all the devices present
   in its site.  The RC can query devices for the presence of a given
   service.  The RC can query DNS for its own domain name and use that
   for the other devices in the site.  Once (new) devices are named,
   this information can be stored in DNS for use in the network.

4.1.3.  device examples

   The relation between device name and IP address is expressed for the
   example devices in the following table.

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       IN AAAA fdfd::1234
       IN AAAA fdfd::1235
        IN AAAA fdfd::1236
           IN AAAA fdfd::1237
       IN AAAA fdfd::1238
       IN AAAA fdfd::1239
        IN AAAA fdfd::1240
           IN AAAA fdfd::1241
            IN AAAA fdfd::1242

   The next part defines the Function Sets related to the device names.
   The names of the SRV RRs (Function Sets) need to be unique to the DNS
   server.  The names of the Function Sets are valid within the
   authority zone,, of the name server.  Consequently, lamp1 is
   short for, sensor1 for, etc.  The
   luminaires with name "lm00xxx" host two Function Sets: a lamp and a

            lamp1     IN SRV 0 0 Port
                      IN TXT path=/lamp
            sensor1   IN SRV 0 0 Port
                      IN TXT path=/light
            lamp2     IN SRV 0 0 Port
                      IN TXT path=/lamp
            sensor2   IN SRV 0 0 Port
                      IN TXT path=/light
            powerc1   IN SRV 0 0 Port
                      IN TXT path=/ps
            presence1 IN SRV 0 0 Port
                      IN TXT path=/occup
            lamp3     IN SRV 0 0 Port
                      IN TXT path=/lamp
            sensor3   IN SRV 0 0 Port
                      IN TXT path=/light
            lamp4     IN SRV 0 0 Port
                      IN TXT path=/lamp
            sensor4   IN SRV 0 0 Port
                      IN TXT path=/light
            powerc2   IN SRV 0 0 Port
                      IN TXT path=/ps
            presence2 IN SRV 0 0 Port
                      IN TXT path=/occup
            timer     IN SRV 0 0 Port
                      IN TXT path=/time

   The above list of SRV RRs specifies the attributes of the Function
   Sets: devices, port numbers, IP addresses and path with the
   accompanying AAAA and TXT records.  The names of the SRV records can

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   be created automatically, as long as they identify the SRV records
   uniquely within the set of RR entries in the DNS zone.  The SRV
   record with name powercx (x = 1,2) stands for power collection x,
   accessed via the path /ps which refers to a Function Set that
   contains a collection of two resources.

   Assuming that the service type "_bc" has the service subtype "_lamp",
   the names of the SRV RRs can also be created from the service subtype
   prefixed by the EUI-64 value.  With the service subtype "_lamp" and
   EUI-64 value "1234" the SRV name 1234_lamp can be created
   automatically instead of lamp1.

   PTR records enable the service discovery.  The names of the PTR
   records are the names of the service types, defined by IANA, and they
   refer to the names of the SRV records.  To support the query "all
   lamps within", the following PTR records need to be added
   for service subtype:_lamp._sub._bc._udp.


   Equally to query to all services within a domain, PTR records with as
   name the building control service, "_bc.udp", refer to all SRV
   records describing Function Sets.

       IN PTR
       IN PTR
       IN PTR
       IN PTR
       IN PTR
       IN PTR etc, etc.

   It is shown above how PTR records support the queries filtered on
   service type.  Filtering on domain can be done adding additional PTR
   records which select the devices of a given type within a given
   domain.  The set of PTR records below filters on all lamps within
   domain ry.fx.

4.1.4.  Group examples

   As an example, five multicast-groups are defined to group all lamps
   on floor "fx", all lamps in office "ry", all lamps in office "rz",
   all power-strips on floor "fx", and all devices in the building

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   controlled by a central timer.  The multicast-group names are entered
   into DNS like the device names to enable resolution from multicast-
   group name to multicast address.

           IN AAAA ff15::11
        IN AAAA ff15::12
        IN AAAA ff15::13
          IN AAAA ff15::14
           IN AAAA ff15::15

   It is expected that SDOs will specify naming conventions for group
   names, extending the service (sub)type names for devices.

   The path and port of the multicast-groups is defined with SRV and TXT
   RRs.  (Remark lampgp1 is short for, etc.)

             lampgp1 IN SRV 0 0 Port
                     IN TXT path=/lamp
             lampgp2 IN SRV 0 0 Port
                     IN TXT path=/lamp
             lampgp3 IN SRV 0 0 Port
                     IN TXT path=/lamp
             powergp IN SRV 0 0 Port
                     IN TXT path=/ps
             timergp IN SRV 0 0 Port
                     IN TXT path=/tm

   The groups for the power strips need extra attention because the
   power strips include a collection of resources.  The path to the
   group can be defined as /ps or as /ps/x with x in {1,2,3,4}.  When
   using /ps/x the group contains the outlet x of the power strips.
   Using the path /ps as done in the table above refers to all outlets
   of a powerstrip.  When sending a message to the power-fx group with
   as path /ps/x then the message will be received by Function Sets with
   path ps/x only.

   The members of the groups can be stored in DNS by using the reverse
   DNS resolution technique.  It is not unusual that a given IP address
   refers to multiple FQDNs.  Extrapolating to group names extends the
   reverse DNS resolution in a natural manner.  Below the members of
   group with IP address ff15::11 containing all
   four lamps is shown.

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   With the above table queries like all members of lamp-fx can be

   Additional tables are needed to specify the multicast groups to which
   the Function Set of a device belongs.  A PTR RR with the name of the
   Function Set can refer to the name of the group.  In the table below
   the Function Set "lamp1" is part of the groups "lamp-fx", "lamp-ry"
   and "timer-bldg":

      IN PTR
      IN PTR
      IN PTR

   Consequently, a device can query DNS for the groups to which its
   Function Sets belong, and consecutively enable the reception of the
   associated multicast messages.

4.1.5.  Discovery validation

   This section describes how the disovery requirements are met with

4.2.  RD examples

   Resource discovery in CoAP handles resource paths (called links) for
   the resources hosted on the server, augmented with attributes of
   these resources.  A well-known path "/.well-known/core" [RFC5785] is
   a default entry-point for requesting the list of links on a given
   server [I-D.shelby-core-resource-directory].  The Resource Directory
   (RD) stores links to resources hosted by other servers.  The link-
   format [I-D.ietf-core-link-format] defines link extensions to specify
   the service type and service instance as used by DNS-SD.  When
   querying the Resource Directory for links, filters can be applied to
   return only links with specified attribute values.  A node learns the
   IP-address of the RD by for example sending a multicast request to a
   predefined multicast address registered with IANA, or by assuming
   that the RD is located in the edge router.

   Contrary to DNS-SD, the RD has not defined a process which permits
   SDOs to specify service (sub)types.  Consequently, the same service
   type examples are used for DNS-SD as for RD, where service type
   postfixed with subtype (DNS) equals "resource type" (RD), and

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   Function Set (SRV) name (DNS) equals "Instance" (RD).

   This draft adheres to the mapping between DNS and link-format,
   described in [I-D.lynn-core-discovery-mapping].

4.2.1.  Commissioning devices

   It is assumed that either a Remote Control (RC) in the home or a
   Commissioning Tool (CT) in the professional domain is used to fill in
   the RD. devices cannot enter links into the RD contrary to the
   suggestion in [I-D.shelby-core-resource-directory].  Both the RC or
   the CT have to fill in the RD, because at link creation the RD
   generates a location of the link-entry (e.g. /ed/453), that is
   returned to the creator of the link.  Consequently, the CT or the RD
   need to create the link, because they need the location to update the
   link with additional information.

   In the professional domain, the CT learns the identity of the device
   as described earlier, reads the services of the devices with a GET to
   /.well-known/core on the device, and stores them into the RD.

   In the home domain, the RC can read in the EUI-64 of the device to be
   entered.  The user types in domain names and device names on the RC.
   Consecutively, the RC follows the same procedure as the CT.

4.2.2.  Device examples

   For convenience, it is assumed that DNS contains the mapping from
   FQDN to IP address with AAAA RRs and vice-versa with PTR RRs.  In all
   examples the FQDN is used and not the IP-address.  It is assumed that
   the service type "_bc" has the subtypes: "_lamp", "_sensor",
   "_strip", "_pir", and "_clock".  Registration is done with the
   following statements by CT or RC to the RD with authority:
   // and path /rd.

           POST coap://"lm00203";d=""
           Etag: 0x21
           Res: 2.01 created
           Location: /rd/1234

   Leaving out Etag:, Res:, and Location: lines, the other Function Sets
   are defined with:

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           POST coap://"lm00204";d=""

           POST coap://"lm00205";d=""

           POST coap://"lm00206";d=""

           POST coap://"ps0057";d=""

           POST coap://"ps0058";d=""

           POST coap://"pir";d=""

           POST coap://"pir";d=""

           POST coap://"clock";d=""

   The "ins" value identifies the Function Set uniquely within the
   resources attached to the RD, in the same fashion as done for SRV
   RRs.  It is also possible to postfix the EUI-64 value to the service
   subtype name, (e.g. ins="pir1241" instead of "presence2").

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4.2.3.  Group examples

   The same five multicast groups of the DNS example are used.  The
   group name to address mapping is specified in DNS with AAAA RRs.  In
   all examples the group name is used and not the IP-address.
   Registration is done with the following statements by CT or RC to the
   RD with authority: / and path /rd, leaving out Etag:,
   Res:, and Location: lines.

           POST coap://"lamp-fx";d=""

           POST coap://"lamp-ry";d=""

           POST coap://"lamp-rz";d=""

           POST coap://"power-fx";d=""

           POST coap://"timer-bldg";d=""

   With the above statements the groups are defined in the RD.  However,
   there is no straight forward mechanism to define the members of a
   multicast group, or to define the inverse: the groups to which a
   Function Set belongs.

4.2.4.  Discovery validation

   This section describes how the disovery requirements are met with the

5.  IANA Considerations

   This document makes no request of IANA.

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

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6.  Security Considerations


7.  Acknowledgments

   Zach Shelby wrote the original Discovery section in
   [I-D.ietf-core-coap] which forms the basis for this draft.  This I-D
   has benefited from conversations with and comments from Emmanuel
   Frimout, Michael Verschoor, Jamie Mc Cormack, Oscar Garcia, Dee
   Denteneer, Joop Talstra, Jerald Martocci, Matthieu Vial, Jerome
   Hamel, George Yianni, and Nicolas Riou.

8.  References

8.1.  Normative References

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

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

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC3986]  Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
              Resource Identifier (URI): Generic Syntax", STD 66,
              RFC 3986, January 2005.

   [RFC4193]  Hinden, R. and B. Haberman, "Unique Local IPv6 Unicast
              Addresses", RFC 4193, October 2005.

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, February 2006.

   [RFC4605]  Fenner, B., He, H., Haberman, B., and H. Sandick,
              "Internet Group Management Protocol (IGMP) / Multicast
              Listener Discovery (MLD)-Based Multicast Forwarding
              ("IGMP/MLD Proxying")", RFC 4605, August 2006.

   [RFC4944]  Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
              "Transmission of IPv6 Packets over IEEE 802.15.4
              Networks", RFC 4944, September 2007.

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   [RFC5234]  Crocker, D. and P. Overell, "Augmented BNF for Syntax
              Specifications: ABNF", STD 68, RFC 5234, January 2008.

   [RFC5785]  Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
              Uniform Resource Identifiers (URIs)", RFC 5785,
              April 2010.

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

   [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              September 2011.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, March 2011.

8.2.  Informative References

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

              Eggert, L., "Congestion Control for the Constrained
              Application Protocol (CoAP)",
              draft-eggert-core-congestion-control-01 (work in
              progress), January 2011.

              Carpenter, B. and R. Hinden, "Representing IPv6 Zone
              Identifiers in Uniform Resource Identifiers",
              draft-ietf-6man-uri-zoneid-00 (work in progress),
              February 2012.

              Frank, B., Bormann, C., Hartke, K., and Z. Shelby,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-08 (work in progress), October 2011.

              Rahman, A. and E. Dijk, "Group Communication for CoAP",
              draft-ietf-core-groupcomm-00 (work in progress),
              January 2012.

              Shelby, Z., "CoRE Link Format",
              draft-ietf-core-link-format-11 (work in progress),

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              January 2012.

              Lynn, K. and Z. Shelby, "CoRE Link-Format to DNS-Based
              Service Discovery Mapping",
              draft-lynn-core-discovery-mapping-01 (work in progress),
              July 2011.

              Lynn, K. and D. Sturek, "Extended Multicast DNS",
              draft-lynn-dnsext-site-mdns-01 (work in progress),
              March 2011.

              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., "CoRE Interfaces",
              draft-shelby-core-interfaces-01 (work in progress),
              January 2012.

              Krco, S. and Z. Shelby, "CoRE Resource Directory",
              draft-shelby-core-resource-directory-02 (work in
              progress), October 2011.

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

              Jennings, C., "DNS SRV Records for HTTP",
              draft-jennings-http-srv-05 (work in progress), March 2009.

              Cheshire, S. and D. Steinberg, "Zero Configuration
              Networking: The Definitive Guide", O'Reilly Media, Inc. ,
              ISBN 0-596-10100-7, 2006.

   [UPNP]     "Universal Plug and Play", Web, 2012.

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Authors' Addresses

   Peter van der Stok (editor)
   Philips Research
   High Tech Campus 34-1
   Eindhoven,   5656 AA
   The Netherlands


   Kerry Lynn

   Phone: +1-978-460-4253

   Anders Brandt
   Sigma Designs
   Emdrupvej 26A, 1.
   Copenhagen O,   2100


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