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CoRE                                                     P. van der Stok
Internet-Draft                                          Philips Research
Intended status: Informational                                   K. Lynn
Expires: May 2, 2012                                          Consultant
                                                        October 30, 2011


                 CoAP Utilization for Building Control
                      draft-vanderstok-core-bc-05

Abstract

   This draft describes an example use of the RESTful CoAP protocol for
   building automation and control (BAC) applications such as HVAC and
   lighting.  A few basic design assumptions are stated first, then URI
   structure is utilized to define group as well as unicast scope for
   RESTful operations.

   This proposal supports the view that 1) service discovery is
   complementary to resource discovery and facilitates control network
   scaling, and 2) building control is likely to move in steps toward
   all-IP control networks based on the legacy efforts provided by DALI,
   LON, BACnet, ZigBee, and other standards.

   The authority portion of the URI is used to identify a device (group)
   and the resulting DNS name is bound to a unicast (multicast) address.
   Group addressing has consequence for the naming convention of the
   resources of a device.  Naming of URI is building or organization
   dependent, must be flexible, and SHOULD conform to some local
   convention.  Naming of resources MUST be standardised preferrable by
   a building control related organisation.

   It is shown that DNS-based service discovery can be used to locate
   URIs on the scale necessary in large commercial BAC deployments.  The
   relation of DNS-SD and a Resource Directory is discussed.  Finally, a
   method is proposed for mapping URIs onto legacy BAC resources, e.g.,
   to discover application-layer gateways, proxies, and their dependent
   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
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.



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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 May 2, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   described in the Simplified BSD License.






























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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  URI structure  . . . . . . . . . . . . . . . . . . . . . . . .  6
     2.1.  Scheme part  . . . . . . . . . . . . . . . . . . . . . . .  7
     2.2.  Authority part . . . . . . . . . . . . . . . . . . . . . .  7
     2.3.  Path part  . . . . . . . . . . . . . . . . . . . . . . . .  8
   3.  Group Naming and Addressing  . . . . . . . . . . . . . . . . . 10
   4.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.1.  Service discovery goals  . . . . . . . . . . . . . . . . . 12
     4.2.  DNS-Based Service Discovery  . . . . . . . . . . . . . . . 13
     4.3.  Browsing for Services  . . . . . . . . . . . . . . . . . . 14
     4.4.  Resource vs Service Discovery  . . . . . . . . . . . . . . 14
   5.  DNS record structure . . . . . . . . . . . . . . . . . . . . . 15
     5.1.  DNS group example  . . . . . . . . . . . . . . . . . . . . 18
     5.2.  Operational use of DNS-SD  . . . . . . . . . . . . . . . . 19
     5.3.  Commissioning CoAP devices . . . . . . . . . . . . . . . . 20
       5.3.1.  DNS-SD server present  . . . . . . . . . . . . . . . . 21
       5.3.2.  DNS-SD server not present  . . . . . . . . . . . . . . 21
     5.4.  Proxy discovery  . . . . . . . . . . . . . . . . . . . . . 22
   6.  Legacy data Representations in CoAP  . . . . . . . . . . . . . 23
     6.1.  Network architectures  . . . . . . . . . . . . . . . . . . 24
     6.2.  Discovery of legacy gateways . . . . . . . . . . . . . . . 26
   7.  Conclusions  . . . . . . . . . . . . . . . . . . . . . . . . . 26
   8.  Security considerations  . . . . . . . . . . . . . . . . . . . 27
   9.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 28
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
   11. Changelog  . . . . . . . . . . . . . . . . . . . . . . . . . . 28
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 29
     12.2. Informative References . . . . . . . . . . . . . . . . . . 30
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 32

















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1.  Introduction

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].

   In addition, the following conventions are used in this document:

   A CoAP end-point, or server, is identified by a unique {IP address,
   port} tuple and characterised by a protocol.  A server is completely
   specified by the authority part of a URI.

   A device is the physical object that is connected to the network.  A
   device may host one or more CoAP servers.

   A service (in the service discovery sense) is a related set of
   resources on a CoAP server.  A URI completely specifies the syntax of
   a service interface.  Metadata describe the semantics of the service
   interface.  The semantics may include the relation between service
   and the hardware connected to the device.  A CoAP server may expose
   one or more services.

   In examples below involving URIs, the authority is preceded by double
   slashes "//" and path is preceded by a single slash "/".  The
   examples may make use of full or partial host names and the
   difference should be clear from the context.

1.2.  Motivation

   The CoAP protocol [I-D.ietf-core-coap] aims at providing a user
   application protocol architecture for a network of devices with a low
   resource provision such as memory, CPU capacity, and energy.  In
   general, IT application manufacturers strive to provide the highest
   possible functionality and quality for a given price.  In contrast,
   the building automation controls market is highly price sensitive and
   manufacturers tend to compete by delivering a given functionality and
   quality for the lowest price.  In the first market a decreasing
   memory price leads to more software functionality, while in the
   second market it leads to a lower Bill of Material (BOM).

   The vast majority of devices in a typical building control
   application is resource constrained, making the standardization of a
   lightweight application protocol like CoAP a necessary requirement
   for IP to penetrate the device market.  The low energy consumption
   requirement of battery-less devices reinforces this approach.  Low



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   resource budget implies low throughput and small packet size as for
   [IEEE.802.15.4].  Reduction of the packet size is obtained by using
   the header reduction of 6LoWPAN [RFC4944] and encouraging small
   payloads.

   Several legacy building control standards (e.g.  [BACnet], [DALI],
   [KNX], [LON], [ZigBee], etc.) have been developed based on years of
   accumulated knowledge and industry cooperation.  These standards
   generally specify a data model, functional interfaces, packet
   formats, and sometimes a physical medium for data objects and
   function invocation.  Many of these industry standards also specify
   proprietary transport protocols, necessitating expensive stateful
   gateways for these standards to interoperate.  Many more recent
   building control network include IP-based standards for transport (at
   least to interconnect islands of functionality) and other functions
   such as naming and discovery.  CoAP will be successful in the
   building control market to the extent that it can represent a given
   standard's data objects and provide functions, e.g. resource
   discovery, that these standards depend on.

   From the above the wanted basic syntax properties can be summarized
   as:

   - Generate small payloads.
   - Compatible with legacy standards (e.g.  LON, BACnet, DALI, ZigBee
     Device Objects).
   - Service/resource discovery in agreement with legacy standards and
     naming conventions.

   This submission defines an approach in which the payload contains
   messages with a syntax defined by legacy control standards.
   Accordingly, the syntax of the service/resource discovery messages
   encapsulates legacy control standard.  The intention is a progressive
   approach to all-IP in building control.  In a first stage standard
   IETF based protocols (e.g.  CoAP, DNS-SD) are used for transport of
   control messages and discovery messages expressed in a legacy syntax.
   This approach enables the reuse of controllers based on the semantics
   of the chosen control standard.  In a later stage a complete redesign
   of the controllers can be envisaged guided by the accumulated
   experience with all-IP control.

   Two concepts, hierarchy and group, are of prime importance in
   building control, particularly in lighting and HVAC.  Many control
   messages or events are multicast from one device to a group of
   devices (e.g. from a light switch to all lights in an area).  The
   scope of a multicast command or discovery message determines the
   group of devices that is targeted.  A group scope may be defined as
   link-local, as a tree maintained by an IP-multicast protocol, or an



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   overlay that corresponds to the logical structure of a building or
   campus and is independent of the underlying network structure.
   Techniques for group communication are discussed in
   [I-D.rahman-core-groupcomm].

   As described in "Commercial Building Applications Requirements"
   [I-D.martocci-6lowapp-building-applications] it is typical practice
   to aggregate building control at the room, area, and supervisory
   levels.  Furthermore, networks for different subsystems (lights,
   HVAC, etc.) or based on different legacy standards have historically
   been isolated from each other in so-called "silos".  RESTful web
   services [Fielding] represent one possible way to expose
   functionality and normalize data representations between silos in
   order to facilitate higher order applications such as campus-wide
   energy management.

   Consequently, additional group properties are:

   - Devices may be part of one or more groups.
   - Resources addressed by a group must be uniformly and consistently
     named across all targeted devices.

   For clarity, this I-D limits itself to two types of applications: (1)
   M2M control applications running within a building area without any
   human intervention after commissioning of a given network segment and
   (2) maintenance oriented applications where data are collected from
   devices in several building areas by devices inside or outside the
   building, and humans may intervene to change control settings.  This
   I-D compares commercial building solutions with solutions for the
   home.


2.  URI structure

   This I-D considers three elements of the URI: scheme, authority, and
   path, as defined in "Uniform Resource Identifier (URI): Generic
   Syntax" [RFC3986].  The authority is defined within the context of
   standard DNS host naming, while the path is valid in relation to a
   fully qualified domain name (FQDN) plus optional port (and protocol
   is implicit, based on scheme).  An example based on [RFC3986] is:
   foo://host.example.com:8042/over/there?name=ferret#nose, where "foo"
   is the scheme, "host.example.com:8042" is the authority, "/over/
   there" is the path, "name=ferret" is the query, and "nose" is the
   fragment.  Fragments are not supported in CoAP.







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2.1.  Scheme part

   The CoAP URI scheme syntax is specified in section 6 of
   [I-D.ietf-core-coap] and is compatible with the "http" scheme
   specification [RFC2616].  The scheme is implicit from the perspective
   of the service, but it indicates the protocol used to access the
   service to potential clients.

2.2.  Authority part

   The authority part is either a literal IP address or a DNS name
   comprised of a local part, specifying an individual device or group
   of devices, and a global part specifying a (sub)domain that may
   reflect the logical hierarchical structure of the building control
   network.  The result is said to be a fully qualified domain name
   (FQDN) which is globally unique down to the group or device level.
   An optional port number may be included in the authority following a
   single colon ":" if the service port is other than the default CoAP
   value.  The authority resolves to a {IP-address, port} tuple.  The
   IP-address may be either unicast or multicast.  The authority
   therefore identifies an individual server or a named group of
   servers.

   The CoAP spec [I-D.ietf-core-coap] states "When a CoAP server is
   hosted by a 6LoWPAN device, it SHOULD also support a port in the
   61616-61631 compressed UDP port space defined in [RFC4944]."  As
   shown below, DNS-SD [I-D.cheshire-dnsext-dns-sd] is a viable
   technique for discovering dynamic host and port assignments for a
   given service.  However, the use of dynamic ports in URIs is likely
   to lead to brittle (non-durable) identifiers as there is no assurance
   that a CoAP server will consistently acquire the same dynamic port
   and different {IP-address, port} tuples conventionally represent
   different servers.

   A building can be unambiguously addressed by it GPS coordinates or
   more functionally by its zip or postal code.  For example the Dutch
   Internet provider, KPN, assigns to each subscriber a host name based
   on its postcode.  Analogously, an example authority for a building
   may be given by: //bldg.zipcode-localnr.Country/ or more concretely
   an imaginary address in the Netherlands as: //bldg.5533BA-125a.nl/.
   The "bldg" prefix can specify the target device within the building.
   Arriving at the device identified by //bldg.5533BA-125a.nl, the
   receiving service can parse the path portion of the URI and perform
   the requested actions on the specified resource.

   Buildings have a logical internal structure dependent on their size
   and function.  This ranges from a single hall without any structure
   to a complex building with wings, floors, offices and possibly a



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   structure within individual rooms.  The naming of the building
   control equipment and the actual control strategy are intimately
   linked to the building structure.  It is therefore natural to name
   the equipment based on their location within the building.
   Consequently, the local part of the URI identifying a piece of
   equipment is expressed in the building structure.  An example is:
   //light-27.floor-1.west-wing...

   This proposal assumes a minimal level of cooperation between the IT
   and building management infrastructure, namely the ability of the
   former to delegate DNS subdomains to the latter.  This allows the
   building controls installer to implement an appropriate naming scheme
   with the required granularity.  For institutional real estate such as
   a college or corporate campus, the authority might be based on the
   organization's domain, e.g. //device-or-
   group.floor.wing.bldg.campus.example.com/.  In cases where subdomain
   delegation is not an option, structure can still be represented in a
   "flat" namespace, subject to the 63 octet limit for a DNS label:
   //group1-floor2-west-bldg3-campus.example.com.

   Most communication is device to device (M2M) within the building.
   Often a device needs to communicate to all devices of a given type
   within a given area of the building.  For example a thermostat may
   access all radiator actuators in a zone.  A light switch located at
   room 25b006 of floor one, expressed as:
   //switch0.25b006.floor1.5533BA-125a.nl/, might specify a command to
   light1 within the same room with //light1.25b006.floor1.5533BA-
   125a.nl/.  This approach can lead to rather verbose URI strings in
   the packet, contrary to the small packet assumption.  The question
   arises as to whether the syntax of the authority part needs to be
   standardized for building control.  Given the naming flexibility
   provided by DNS, authority names for building control are more the
   concern of the building owner or the installer than a standardization
   concern.

2.3.  Path part

   The path identifies the addressable attributes of the service at the
   highest possible granularity.  A set of paths defines the syntax of
   the service invocation and constitutes the interface description of
   the service.  Every network service attribute is completely
   identified by a URI scheme://authority/path.  In analogy, the path
   part of the URI specifies the resource of a given server.  The naming
   of the services and their associated attributes are typically
   subjects for standardization.  There is no widely accepted standard
   for uniformly naming building control services in a URI.  A vigorous
   effort is undertaken by the oBIX working group of OASIS [oBIX], but
   its current impact is limited.  There is also an open source point



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   naming effort underway called Project Haystack [HAYSTACK].

   The path is constructed like a file system path name.  It consists of
   a sequence of one or more name fields, with each field preceded with
   a slash, like /func1/subf2/final.  The set of paths is structured as
   a tree.  The last name in a name field sequence is called a leave of
   the tree, and the authority is the root of the path tree of a given
   host.  The semantics of a given sub-tree in the path tree is
   specified by the Interface Description (if=) attribute described in
   [I-D.ietf-core-link-format].  As for file systems some tree naming
   with associated semantics can be standardized such as the de facto PC
   standard directory "documents and settings" with the sub-directories
   "My documents", "usradmin", etc.  When a given body, e.g.  XXX, has
   defined a name structure and semantics for the path tree, we say that
   "if = XXX" when the path tree conforms to the name structure defined
   by XXX.

   When a GET method with an URI like
   "//t-sensor1.25b006.floor1.example.com/temperature" is sent, it
   represents an a priori understanding that the server with name
   t-sensor1 exists, provides a service of a given standard type (with
   associated semantics) (e.g.  ZigBee temperature sensor), and that
   this standard type has the readable attribute: temperature.  When
   commands are sent to a group of servers it MUST be the case that the
   targeted resource has the same path on all targeted servers.
   Therefore, it is necessary to establish at least a local uniform path
   naming convention to achieve this.  One approach is to include the
   name of the standard, e.g.  BACnet, as the first element in the path
   and then employ the standard's chosen data scheme (in the case of
   BACnet, /bacnet/device/object/property).

   The organization responsible for defining a given industry standard
   XXX (e.g.  BACnet, ZigBee, etc.) can register the /.well-known/XXX
   prefix and specify the allowable path-names for a server of a given
   type.  The same body also defines the "if=XXX" attribute.  This
   allows the standards development organization responsibile for XXX to
   define the name space and resources associated with the prefix
   together with the associated semantics.  The registered /.well-known/
   XXX URI effectively defines a standard object model, or schema, for
   services of the XXX application protocol.  Manufacturers may
   optionally define proprietary resources that can be discovered
   dynamically using methods described below.

   Although the authority part names need not always be transported, the
   path names MUST be transported in the CoAP packets.  Therefore, path
   names SHOULD be as short as possible, even at the detriment of the
   clarity of the meaning of the path name.




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3.  Group Naming and Addressing

   Within building control it is necessary to send the same command to a
   set of servers.  Grouping allows to invoke the set of services with
   one application command to be executable within a specified time
   interval.  Given a network configuration, the network operator needs
   to define an appropriate set of groups which can be mapped to the
   building areas.  Knowledge about the hierarchical structure of the
   building areas may assist in defining a network architecture which
   encourages an efficient group communication implementation.  IP-
   multicasting over the group is a possible approach for building
   control, although proxy-based methods may prove to be more
   appropriate in some deployments [I-D.rahman-core-groupcomm].

   Example device groups become:

   URI authority                    Targeted group
   //all.bldg6...                   "all devices in building 6"
   //all.west.bldg6...              "all devices in west wing, building
                                    6"
   //all.floor1.west.bldg6...       "all devices on floor 1, west wing,
                                    ..."
   //all.bu036.floor1.west.bldg6... "all devices in office bu036, ..."

   The granularity of this example is for illustration rather than a
   recommendation.  Experience will dictate the appropriate hierarchy
   for a given structure as well as the appropriate number of groups per
   subdomain.  Note that in this example, the group name "all" is used
   to identify the group of all devices in each subdomain.  In practice,
   "all" could name an address record in each of the DNS zones shown
   above and would bind to a different multicast address [RFC3596] in
   each zone.  Highly granular multicast scopes are only practical using
   IPv6.  The multicast address allocation strategy is beyond the scope
   of this I-D, but various alternatives have been proposed
   [RFC3306][RFC3307][RFC3956].

   To illustrate the concept of multiple group names within a building,
   consider the definition, as done with [DALI], of scenes within the
   context of a floor or a single office.  For example, the setting of
   all blue lights in office bu036 of floor 1 can be realized by
   multicasting a message to the group "//blue-lights.bu036.floor1".
   Each group is associated with a multicast IP address.  Consequently,
   when the application specifies the sending of an "on" message to all
   blue lights in the office, the message is multicast to the associated
   IP address.

   The binding of a group FQDN to a multicast address (i.e., creation of
   the AAAA record in the DNS zone server) happens during the



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   commissioning process.  Resolution of the group name to a multicast
   address happens at restart of a device.  A multicast address and
   associated group name in this context are assumed to be long-lived.
   It can happen that during operation the membership of the group
   changes (less or more lights) but its address is not altered and
   neither is its name.  Group membership may be managed by a protocol
   such as Multicast Listener Discovery [RFC5790].

   Similarly, a group can identify a set of resources of one server.
   For examples a device contains four I/O channels.  The device hosts
   one server with four resources to access each of the four individual
   channels separately.  Commonly, it is also required to access all
   four channels as one group.  An additional path identifies the group
   of services.  An example set of services and service-group is:

        URI path        Targeted group
        /IOchannel/1... "channel 1 of the IO channel device "
        /IOchannel/2... "channel 2 of the IO channel device "
        /IOchannel/3... "channel 3 of the IO channel device "
        /IOchannel/4... "channel 4 of the IO channel device "
        /IOchannel/...  "channel 1 to 4 of the IO channel device "

   A group defines a set of servers possibly containing a set of
   resources.  Grouping of the resources is provided by the device
   manufacturer.  Grouping of the servers is supported by DNS and
   multicast protocols.  The multicast address(es) identify the servers
   belonging to the group.  A given server might belong to a number of
   groups.  For example the server belonging to the "blue-lights" group
   in a given corridor might also belong to the groups: "whole
   building", "given wing", "given floor", "given corridor", and "lights
   in given corridor".  From the perspective of a server, the main
   consequence of joining a group is it should accept packets for an
   additional IP address.  The granularity of the domain names may have
   an impact on the complexity of the DNS infrastructure but not
   necessarily on the low-resource destinations or sources.  Assuming
   that resolution of addresses only happens at device start-up, the
   complexity of the DNS server need not affect the responsiveness of
   the devices.

   In summary, the authority portion of the URI resolves to an IP-
   address and port number, and identifies a server or group of servers.
   Authority naming is building or organization dependent, must be
   flexible, and does not require standardization efforts but SHOULD
   conform to some uniform convention.  Path naming SHOULD conform to
   the naming convention of a standardization body.






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4.  Discovery

4.1.  Service discovery goals

   Service discovery in building control should rely on a minimal need
   for intervention by humans (or complete absence of humans) during
   system setup, bootstrapping, restart, configuration and daily
   operation.  The goals for service discovery area:

   Goal                Goal description
   Return_instance     Return all instances of a given service type
                       within a given domain
   Group_instance      Group a set of instances within a group
                       associated with a domain
   Instance_resolution Resolve the instance name to usable invocation
                       information (e.g.  IP address and port)
   Group_resolution    resolve the group name to usable invocation
                       information (e.g.  IP address and port)

   These goals are necessary to support the operation of commercial
   building control.  Returning the instances results in a list of
   names.  For building control these names can be any sequence of
   characters as long as for each service instance these names are
   unique within the domain.  In [I-D.cheshire-dnsext-dns-sd] the office
   equipment in the IT domain is recommended to use understandable and
   human-readable names.  The Home domain may have a need for human
   understandable names.  This is not the case for the commercial
   building automation domain.  However, uniqueness of the name is
   necessary for the application that needs to address the service in a
   consistent manner.  Given the large number of devices in a building
   (several hundreds to thousands) scaling is an important aspect of the
   service discovery.  A set of central DNS servers will provide the
   scalability.  The expectation is that names need to be managed
   consistently by a central authority which can be supported by the DNS
   server.  Tools will assist the installer and operator of the network
   to do the installation, configuration and maintenance of the network
   structure.  Small devices will use the DNS server to learn the
   communication partners providing a given service within their domain
   and to resolve the IP addresses of the communication partners.

   Within the home it is more important that the names convey the
   purpose of the service to the user reading the names and selecting
   his favored service instance.  Non-unique names, although confusing,
   can probably be handled by the user of these names.  Scalability is
   less of an issue because a smaller number of devices is implicated.
   The network in the home is probably more dynamic than its commercial
   counter-part, with many movements of devices and arrival or removal
   of devices.



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   Section 5 presents some examples of DNS structures to show how the
   choice of names influences the granularity of the discovery.  In
   sections 5.1 and 5.3 a grouping example and a commissioning example,
   filling the DNS, are presented.

4.2.  DNS-Based Service Discovery

   DNS-Based Service Discovery (DNS-SD) defines a conventional way to
   configure DNS PTR, SRV, and TXT records to facilitate discovery of
   services within a subdomain, re-using the existing DNS
   infrastructure.  This section gives a cursory overview of DNS-SD; see
   [I-D.cheshire-dnsext-dns-sd] for a complete description.

   A DNS-SD service instance name is of the form
   <Instance>.<ServiceType>.<Location>.

   The Location part of the service name is identical to the DNS
   subdomain part of the authority in URIs that identify the resources
   of this server or group and may identify a building zone as in the
   examples above.

   The ServiceType SHOULD have the form [_subtype._sub.]_type._proto
   (e.g. _temp._sub._bc._udp).  The _proto identifier provides a
   transport protocol hint as required by the SRV record definition
   [RFC2782] and, in the case of CoAP, it is always "_udp".  The _type
   identifier is determined by standards development organization (SDO)
   and MUST be registered with dns-sd.org [dns-sd] (e.g. _bc for
   building control).  The SDO is then free to specifiy one or more
   _subtype identifiers, which must be unique for a given _type (e.g.
   _temp).  The _subtype and _type labels are separated by the literal
   "._sub" label.The maximum length of the type and subtype fields is 14
   octets, but shorter names are encouraged to reduce packet sizes.

   A PTR record with the label "_type._proto" is defined for each server
   in a selected domain, and this record's value is set to the service
   instance name (which in turn identifies the SRV and TXT records for
   the CoAP server).

   The Instance part of the service name may be changed during the
   commissioning process.  It must be unique for a given ServiceType
   within the subdomain.  The complete service name uniquely identifies
   an SRV and a TXT record in the DNS zone.  The granularity of a
   service name MAY be at the group or server level, or it could
   represent a particular resource within a CoAP server.  The SRV record
   contains the host (AAAA record) name and port of the service.  The
   path part of the URI MUST be placed in the TXT record (path=) when
   multiple resources belong to the same service.




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4.3.  Browsing for Services

   Devices in a given Location with given ServiceType, _type._proto, may
   be enumerated by sending a DNS query for PTR records named
   _type._proto to the authoritative server for that zone associated
   with the Location.  A list of instance names for SRV records matching
   that <ServiceType>.<Location> is returned.  Each SRV record contains
   the host name and port of a CoAP server.  The IP address of the
   device is obtained by resolving the host name.  DNS-SD also specifies
   an optional TXT record, having the same name as the SRV record, which
   can contain "key=value" attributes.  Apart from defining standardized
   resources identified by if=XXX, the XXX organization may also define
   the standard "key=value" pairs present in the TXT record, e.g.
   type=switch.  By convention, the first pair is txtver=<number> so
   that different versions of the XXX schema may interoperate.  For
   example: A query is sent to DNS-SD to return all DALI lamps within
   the domain office5/mybuilding and with ServiceType:
   _lamp._sub._dali._udp.  DNS-SD returns the list of all SRV records
   and AAAA records of the devices within the domain providing the
   wanted service.

4.4.  Resource vs Service Discovery

   Service discovery is concerned with finding the IP address, port,
   protocol, and possibly path of a named service.  Resource discovery
   is a fine-grained enumeration of resources (path-names) of a server.
   [I-D.ietf-core-link-format] specifies a resource discovery pattern,
   such that sending a confirmable GET message for the /.well-known/core
   resource returns a set of links available from the server.  These
   links describe resources hosted on that server.

   CoAP link format can be used to enumerate attributes and populate the
   DNS-SD database in a semi-automated fashion.  CoAP resource
   descriptions can be imported into DNS-SD for exposure to service
   discovery as described in [I-D.lynn-core-discovery-mapping].  The
   values stored in the DNS-SD directory are extracted from the
   information stored in the resource directory associated with a set of
   CoAP hosts [I-D.shelby-core-resource-directory].  The resources
   describe how the services can be manipulated in detail and in
   concreto.

   It is assumed that a resource directory exists per 6LoWPAN [RFC4944],
   possibly running on the edge router.  The DNS-SD provides a larger
   scope by storing the info of all services over a set of
   interconnected 6LoWPANs.  Where the resource directory is possibly
   completely adequate for home networks, handling of multiple resource
   directories can be quite cumbersome for the many 6LoWPANs envisaged
   for offices.  However, during network configuration, the resource



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   directory can be used as long as the DNS is not yet accessible.

   The DNS-SD approach is complementary to the more fine-grained
   resource discovery, fits better the concept of service by discovering
   servers with given properties.  DNS-SD supports a hierarchical
   approach to the naming of the services as discussed in section 3.
   DNS-SD provides a directory structure that scales well with the
   network size as shown by its present-day operation.


5.  DNS record structure

   An example is presented which explains the Resource Record (RR)
   structure on the DNS server.  This section follows the mapping
   specified in [I-D.lynn-core-discovery-mapping], which defines how to
   fill the DNS-SD records from the link extension values.  Suppose the
   services are delivered by XXX building control devices.  The example
   subtype- and context- names are assumed to be standardized by the XXX
   alliance.  All devices are situated in one office with location
   office4.bldg8.example.com.  The names in the examples are more
   verbose than recommended to make the examples more readable.  The
   table presents the services provided in the office control network:

             service      ServiceType                  Number
             illumination _OnOff_light._sub._bc._udp   4
             presence     _occup_sensor._sub._bc._udp  1
             temperature  _temp_sensor._sub._bc._udp   1
             shading      _shade_control._sub._bc._udp 1

   In DNS PTR records with as label the ServiceType have as value
   service instance names.  The unique Instance names identify the
   service instances.  In the example, the names contain id-x, with x in
   natural numbers.  The names are usually created at the factory floor
   and somehow attached to the product.  The ServiceTypes have been
   suffixed with .04.b8 to represent office4 in building8.  The same
   suffix is used as PTR label to enemerate all instance of a given
   service, or within a given domain.














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         _OnOff_light._sub._bc._udp.04.b8   PTR id-1._OnOff_light
         bc._udp.04.b8                      PTR id-1._OnOff_light
         04.b8                              PTR id-1._OnOff_light
         _OnOff_light._sub._bc._udp.04.b8   PTR id-2._OnOff_light
         bc._udp.04.b8                      PTR id-2._OnOff_light
         04.b8                              PTR id-2._OnOff_light
         _OnOff_light._sub._bc._udp.04.b8   PTR id-3._OnOff_light
         bc._udp.04.b8                      PTR id-3._OnOff_light
         04.b8                              PTR id-3._OnOff_light
         _OnOff_light._sub._bc._udp.04.b8   PTR id-4._OnOff_light
         bc._udp.04.b8                      PTR id-4._OnOff_light
         04.b8                              PTR id-4._OnOff_light
         _occup_sensor._sub._bc._udp.04.b8  PTR id-5._occup_sensor
         bc._udp.04.b8                      PTR id-5._occup_sensor
         04.b8                              PTR id-5._occup_sensor
         _temp_sensor._sub._bc._udp.04.b8   PTR id-6._temp_sensor
         bc._udp.04.b8                      PTR id-6._temp_sensor
         04.b8                              PTR id-6._temp_sensor
         _shade_control._sub._bc._udp.04.b8 PTR id-7._temp_sensor
         bc._udp.04.b8                      PTR id-7._temp_sensor
         04.b8                              PTR id-7._temp_sensor

   In the above example the id-x identifiers without the subtype suffix
   would be discriminating enough.

   Discovery can be done with the following results.  A query with the
   following argument returns

           query argument                    result list
           .04.8                             id-1._OnOff_light
                                             .......
                                             id-7._temp_sensor
           _bc._udp.04.b8                    id-1._OnOff_light
                                             .......
                                             id-7._temp_sensor
           _OnOff_light._sub._bc._udp.04.b8  id-1._OnOff_light
                                             .......
                                             id-4._OnOff_light
           _occup_sensor._sub._bc._udp.04.b8 id-5._occup_sensor

   When other offices are included in the database, the query argument
   04.b8 selects those entries which are associated with office4 in
   building8 and rejects any others.  The example shows clearly the
   query granularity that can be obtained and the care that must be
   exercised when defining the names of the ServiceTypes.

   The service instances (value of PTR records) are the labels of the
   SRV, AAAA and TXT records describing the service instance.  The SRV



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   record specifies the location (authority) and the port number.  In
   the authority o4.b8 refers to office4 in building8.  The AAAA record
   specifies the IP-address, while the TXT record specifies the subtype
   and the data representation of the legacy parser (if = ZigBee).

         id-1._OnOff_light   SRV  light1.o4.b8.example.com Port-x
                             AAAA fdfd::1234
                             TXT  if=ZigBee
         id-2._OnOff_light   SRV  light2.o4.b8.example.com Port-x
                             AAAA fdfd::1235
                             TXT  if=ZigBee
         id-3._OnOff_light   SRV  light3.o4.b8.example.com Port-x
                             AAAA fdfd::1236
                             TXT  if=ZigBee
         id-4._OnOff_light   SRV  light4.o4.b8.example.com Port-x
                             AAAA fdfd::1237
                             TXT  if=ZigBee
         id-5._occup_sensor  SRV  occup.o4.b8.example.com  Port-x
                             AAAA fdfd::1238
                             TXT  if=ZigBee
         id-6._temp_sensor   SRV  temp.o4.b8.example.com   Port-x
                             AAAA fdfd::1239
                             TXT  if=ZigBee
         id-7._shade_control SRV  shade.o4.b8.example.com  Port-x
                             AAAA fdfd::1240
                             TXT  if=ZigBee

   It is possible that the temperature sensor and occupancy sensor are
   delivered on one device.  The consequence is that one device hosts
   two services.  In the DNS table the four lights and the shade
   controller are unaffected.  However, the PTR records with the
   occupancy and temperature sensor point to the same unique identifier
   id-8 that is suffixed with the name of the subtype.  This example
   shows that the subtype suffix is needed to discriminate between the
   two service instances.

            _occup_sensor._sub._bc._udp PTR id-8._occup_sensor
            _temp_sensor._sub._bc._udp  PTR id-8._temp_sensor

   Two SRV records with accompanying AAAA and TXT records describe the
   two servers, each providing one service, in more detail.  The servers
   share the same IP address but are connected to different ports, and
   do have a different paths names.  The TXT record is used to specify
   the path part with "path=".







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          id-8._occup_sensor SRV  occup.o4.b8.example.com Port-x
                             AAAA fdfd::1241
                             TXT  path=/os if=ZigBee
          id-8._temp_sensor  SRV  temp.o4.b8.example.com  Port-y
                             AAAA fdfd::1241
                             TXT  path=/ts if=ZigBee

   The path names /ts and /os are short names for temperature_sensor and
   occupancy_sensor respectively.  Not all multi-function devices will
   use different ports for the individual functions.  It is also quite
   common to use different IP interfaces with different IP addresses,
   reflected by the value of the AAAA records.

5.1.  DNS group example

   Another aspect is the grouping of servers.  Where in the former
   section the names of the services are standardized names, this is
   less probable for the group names.  Usually the group names are
   application specific or are standardized at the manufacturer.  For
   example, assume that a group all-light.o4.b8.example.com is created
   which contains all four lights inside office4.  The accompanying
   ServiceType can be defined as _all_light._sub._bc._udp.  The
   ServiceType suffixed with 04.b8 points to a unique identifier defined
   as _all_light.04.b8, assuming that this is the only _all_light group
   within office 4 of building 8.  The PTR record looks like:

            _all_light._sub._bc._udp.04.b8 PTR _all_light.04.b8

   It is assumed that the group all_light.o4.b8.example.com has received
   a multicast address: ff1e::148.  The accompanying SRV, AAAA, and TXT
   RR become:

         _all_light.04.b8 SRV  all_light.o4.b8.example.com Port-z
                          AAAA ff1e::148
                          TXT  if=ZigBee

   When a multicast message is sent to a group, the path of the accessed
   resource must be strictly the same for all servers.  The naming of
   the path is typically a responsibility for the standardisation
   organisations describing the command set for a given application
   area.  However a constraint exits in the case of mult-function
   devices which host multiple resource of the same type.  For example a
   device with three lamps with corresponding onoff attributes can be
   accessed via the three different paths:







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                              /light/1/onoff
                              /light/2/onoff
                              /light/3/onoff

   A unique path to the onoff resource of all instances of light on this
   device can be provided by /light/onoff.  As this is logically the
   path to a single instance on a mono-function device.  The
   corresponding unique paths for onoff to be used in the multicast
   message becomes /light/onoff.  The corresponding resource records for
   a luminaire, named lm1, in DNS become:

    _light._sub._bc._udp.04.b8 PTR  _all_light.04.b8
    _light._sub._bc._udp.04.b8 PTR  _light_1.04.b8
    _light._sub._bc._udp.04.b8 PTR  _light_2.04.b8
    _light._sub._bc._udp.04.b8 PTR  _light_3.04.b8
    _all_light.04.b8           SRV  all_light.o4.b8.example.com Port-x
                               AAAA ff1e::148
                               TXT  if=ZigBee path=/light
    _light_1.04.b8             SRV  lm1.o4.b8.example.com       Port-z
                               AAAA fdfd::1234
                               TXT  if=ZigBee path=/light/1
    _light_2.04.b8             SRV  lm1.o4.b8.example.com       Port-z
                               AAAA fdfd::1234
                               TXT  if=ZigBee path=/light/2
    _light_3.04.b8             SRV  lm1.o4.b8.example.com       Port-z
                               AAAA fdfd::1234
                               TXT  if=ZigBee path=/light/3

   The entries in DNS can be used to form groups with the light weight
   group management protocol and multicast listener discovery [RFC5790].

5.2.  Operational use of DNS-SD

   The populated DNS-SD server provides the necessary support for the
   applications to execute their control loops with minimum operator
   support.  The operation of the office network can be split up in
   phases.  In a first phase the network is commissioned, such that a
   relation is established between the IP address, the servicetype and
   the domain.  The servicetype can be extracted from the link-format as
   described in [I-D.shelby-core-resource-directory].  After
   commissioning this information is stored in the DNS-SD files.  In a
   second phase groups are formed and group names with their IP address
   are stored in the DNS-SD files.  The IP multicast addresses are
   communicated to the members of the groups.  In the third and final
   phase, applications query DNS-SD to find the IP addresses of the
   services within a given domain, and of the groups within a given
   domain.




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   In the home, a commissioning phase requiring the intervention of an
   installer (a "truck roll") is to be avoided if possible.  Here the
   first phase consists of the booting up devices which insert their
   services resources to a link-format directory.  The information from
   the resource directory can be inserted into DNS-SD or into xmDNS
   [I-D.lynn-dnsext-site-mdns] when appropriate.  In the second phase
   remote controllers or other hand-held devices can be used to discover
   the services of a given type, to group the services, and to store the
   group names into DNS-SD or xmDNS as appropriate.  Pointing out the
   members of a group can be in any kind of manner from typing members
   in, selecting them from a browser list, etc.

5.3.  Commissioning CoAP devices

   For clarity it is assumed in this section that a device hosts one
   server.  A device has received a unique device identifier at the
   production plant.  Given the authority naming presented in section
   2.2 the authority name represents the location of the host within the
   building.

   Commissioning means the following three actions:

             - Defining the URI (location)
             - Assigning an IP address to the URI
             - mapping the unique device identifier to the URI

   Two cases of the office network are considered for commissioning: (1)
   no 6LBR and no DNS server connected, and (2) a 6LBR connects the
   office network to a DNS server.

   When an architect has designed the building and described all light
   points, ventilators, heating- and cooling units, and sensors, it is
   necessary to identify all these devices spatially and functionally.
   Storing the triple <Instance>.<ServiceType>.<Location> into DNS-SD
   represents the commissioning process.  The Instance is the unique
   identifier given to the device in the factory but which has no
   relation to its later location.  The ServiceType together with the
   Location represent the spatial and functional aspects of the device
   as specified by the architect.

   Design decision: A commissioning tool with access to the network is
   used for the commissioning phase.

   For example, dependent on used technology and production process, the
   following situation (state) may exist in a host after physical
   installation of the devices and before commissioning:





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   -  A given host is unaware of its Location.

   -  A given host knows its ServiceType and Instance.  The Instance is
      also readable by bar code reader.

   -  The commissioning tool knows all Locations to which hosts need to
      be assigned.

   -  Each host has a (site-local) IP address.

   Consider the commissioning process (1) with a central DNS-SD server
   and (2) without a central server using xmDNS.  The commissioning
   processes described below are just examples and should not be taken
   as working procedures for commissioning devices in a building.

5.3.1.  DNS-SD server present

   The installer reads with a bar code reader, attached to the
   commissioning tool, the identifier of the device to commission.  It
   is assumed that the tool can learn the IP address of the device with
   the given identifier.  The tool displays on a screen the physical
   lay-out of the devices within the building.  The installer selects,
   on the screen of the tool, the physical location of the chosen
   device.  From the designated physical location the tool creates the
   URI of the selected device.  The tool inserts the URI and the IP
   address into the DNS server.  For example the light with URI
   light1.o4.b8.example.com is represented with an AAAA record:

                 light1.o4.b8.example.com AAAA fdfd::1234

   The tool reads the service name and type from the device using
   resource information stored according to the link-format
   [I-D.ietf-core-link-format].  With this information the tool
   constructs the PTR, SRV and TXT records according to the example
   presented in section 5.

   This is done for all devices within a given part of the building.
   After the commissioning process, all resources of each device have an
   URI and IP address which are stored in the central DNS-SD server.
   When devices are restarted, the DHCP server may allocate new IP
   addresses to the device and update the DNS server.

5.3.2.  DNS-SD server not present

   It is assumed that the building network is composed of independent
   network segments (possibly a single site) such that each device on a
   given segment can communicate directly with any other device on this
   segment.  The segments are not connected to a 6LBR and have no access



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   to DNS or other servers.  The installer knows these segments and has
   a list of devices for a given segment.  In the tool the installer
   selects the names which belong to the given building segment.  The
   selected names are converted to site-local authorities and stored in
   the tool.  All devices are assumed to have selected a site-local IP
   address.  Assume that every device has a unique barcode within the
   building and that the corresponding device knows the bar code number.
   The installer reads with a bar code reader, attached to the tool, the
   Instance name of the device to commission.  The installer selects, on
   the screen of the tool, the physical location of the chosen device.
   The tool knows the authority of the selected device.  The tool
   broadcasts the bar code number and authority to all connected
   devices.  The device with the given barcode number, extends the
   authority with the path name of the resources.  For each resource,
   the device multicasts the site-local IP-address and the site-local
   URI to the xmDNS servers in the connected devices.  This concludes
   the commissioning of a network segment.  All resources of each device
   have a site-local URI and a site-local IP address which are stored in
   the xmDNS servers.

5.4.  Proxy discovery

   Proxies will be used in CoAP networks for at least two major reasons:
   (1) http/coap proxy, and (2) proxy of service on battery-less device.
   The first proxy is probably implemented as forward proxy, while the
   latter is probably implemented as backward proxy.  The battery-less
   device will at rare occasions (when it is not sleeping) and during
   installation answer the GET /.well-known/core request.  The return
   data are used by the installation tool to make the proxy device
   return the same resource names on /.well-known/core as is returned by
   the sleeping device.  An installation tool installs on the proxy all
   the resources of the sleeping device for which the proxy is assumed
   to answer.  Consequently, the proxy is discovered as a multi-server
   host with as many path names as it proxies sleeping servers.  The
   servers on sleeping devices should not be discoverable via DNS-SD.
   However, AAAA records are generated for the sleeping device host
   name.  This host name is used by the proxy to subscribe to the
   "sporadic" services of the sleeping device.  For example assume two
   sleeping devices, an occupancy sensor and a temperature sensor, and
   one proxy.  Two service types are defined with PTR records in DNS-SD.
   The identifier id-1 of the proxy is used by the installation tool to
   define the Instances.

         _occup_sensor._sub._bc._udp.04.b8 PTR id-1._occup_sensor
         _temp_sensor._sub._bc._udp.04.b8  PTR id-1._temp_sensor

   Two SRV records with accompanying AAAA and TXT records describe the
   two services in more detail.  The services share the same IP address,



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   are connected to the same port, but do have different paths names.
   The TXT record is used to specify the path part with "path=".

        id-1._occup_sensor      SRV  proxy.o4.b8.example.com Port-x
                                AAAA fdfd:: 1241
                                TXT  path=/os if=ZigBee
        id-1._temp_sensor       SRV  proxy.o4.b8.example.com Port-x
                                AAAA fdfd:: 1241
                                TXT  path=/ts if=ZigBee
        sl-ts.o4.b8.example.com AAAA fdfd::1242
        sl-os.o4.b8.example.com AAAA fdfd::1243

   The path names /ts and /os are short names for temperature_sensor and
   occupancy_sensor respectively and were taken over from link-format
   information contained in the sleeping devices.  Two AAAA records are
   provided for the two sleeping devices.  The proxy has used the domain
   names of the sleeping devices to subscribe to the publications of the
   two sleeping devices.

   It is important to remark that there are now two services with the
   same resources present on two different devices: the sleeping device
   and its proxy.  When a host invokes the /.well-known/core resource,
   it should be possible to distinguish between the proxy (to be
   invoked) and the sleeping device (not to be invoked).  The
   distinction is necessary once the sleeping device is discoverable and
   the sleeping device is awake from time to time.  It is suggested that
   the link-format syntax allows to make this distinction.


6.  Legacy data Representations in CoAP

   Before CoAP devices can come to market, manufacturers must agree that
   the type and resources of the device can be interpreted according to
   some generally recognized syntax.  At this moment no such generally
   recognized syntax exists for CoAP devices.  We do not expect an IETF
   working group to standardize such a syntax, and we are convinced that
   syntax standardization is the responsibility of industry standards
   organizations.  Given the long history of building control, many
   groups have defined a data representation for building control
   devices for example BACnet, ZigBee, oBIX, LON, KNX, and many others.
   It is our belief that new representations will be defined and must
   coexist with the named legacy ones.

   The CoAP protocol should transport any data representation, and
   certainly the legacy ones.It is expected that a CoAP client can
   handle one or more legacy representation.  Given that a CoAP client
   can handle representation of standard XXX, this I-D proposes that
   such a CoAP device can communicate with legacy devices via a CoAP/



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   legacy gateway (router).

6.1.  Network architectures

     +------+                 +------+     +------+
     | yyy  |                 | zzz  |     | zzz  |
     | link |                 | CoAP |     | CoAP |
     +------+                 +------+     +------+
       |         +---------+
       |---------| CoAP    |-->
       |         | gateway |
     +------+    +---------+        +-----------+
     | yyy  |                       | zzz ; yyy |
     | link |                       |   CoAP    |
     +------+                       +-----------+

         Figure 1: network with multiple representation standards

   Figure 1 represents the network architecture which is expected for
   the purpose of this I-D.  The CoAP gateway connects one link with two
   legacy devices -containing legacy data representation "yyy"- with the
   wireless CoAP network composed of three CoAP hosts.  Two CoAP hosts
   contain the CoAP stack with a zzz representation and one host
   contains the CoAP stack with a zzz and an yyy representation.  The
   yyy hosts can freely communicate according to the yyy link protocol
   over the yyy link.  The zzz CoAP hosts, including the zzz;yyy host
   can freely exchange zzz data representations according to the CoAP
   protocol over the wireless 6LoWPAN network.  The zzz;yyy host can
   send yyy data representations to the CoAP gateway which passes them
   on to the specified yyy legacy host.  The yyy legacy device returns
   data to the requesting zzz;yyy CoAP host via the same gateway.

   The CoAP hosts can address the legacy devices behind the gateway in
   at least 4 ways.

   -  All devices of legacy network YYY share the URI with the CoAP
      gateway.  Every legacy device is a resource for the gateway as
      seen from the CoAP host.  Consequently, the CoAP host sends the
      message to the IP address of the gateway and the gateway parses
      the URI-Path to determine the specified legacy device.

   -  All devices of legacy network YYY have IP addresses different from
      the IP address of the gateway.  Consequently, a CoAP host sends
      the message to the IP address of the specified device.  The
      routing protocol on the CoAP network makes the message arrive at
      the CoAP gateway.  The gateway determines the specified legacy
      device from the destination IP address.




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   -  All devices of legacy network YYY have different authorities.  The
      authorities of the legacy device resolve to an IP address of the
      gateway.  This means that the possibly lengthy authority names
      need to be transmitted.  The gateway recognizes the authorities
      and maps authority to legacy device.

   -  All devices of legacy network YYY have different ports.  This can
      be expressed in two ways (1) as :port in the URI, or (2) in the
      DNS-SD records.  In the latter case the port is defined in the UDP
      header and is efficient in packet header size.

   The major advantage of all four approaches is that the gateway only
   handles the URI or IP address and port number to select the
   destination legacy device independent of the type of legacy device
   and the contents of the legacy payload of the message.  In Figure 1
   the gateway connects to a single link.  For example, this would be
   the case for DALI standard.  Other legacy standards, like BACnet,
   LON, allow networks composed of multiple links.

   An example of an invocation of a ZZZ service (See figure 2).  The
   resource path /ZZZ identifies the parser of the ZZZ syntax.  A 12
   octet string completely describes the ZZZ command.  The host is
   completely identified by the authority in the URI.  The ZZZ parser on
   the host is identified by the port number in the UDP header (not
   shown).


       Client                                             CoAP/ZZZ
         |                                                  device
         |  REQUEST                                           |
         |-------- CON [0x5577] PUT /ZZZ -------->|
         |             binary 12 octet string                 |
         |                                                    |
         |  RESPONSE                                          |
         |<---------- ACK [0x5577] 2.00 OK  ----------------- |
         |                                                    |


       Figure 2: Sending a ZZZ command with CoAP to CoAP/ZZZ device

   An example of an invocation of a DALI legacy device behind a gateway
   is given in figure 3.  The resource path /DALI identifies the DALI
   parser.  The application sets a value of 200 in the DALI device in
   the resource 256 defined by the DALI spec.







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       Client                                             DALI/CoAP
         |                                                 gateway
         |  REQUEST                                           |
         |------- CON [0x5577] PUT /DALI -------->|
         | binary 16 bit payload dt*256 + 200                 |
         |                                                    |
         |  RESPONSE                                          |
         |<---------- ACK [0x5577] 2.00 OK  ----------------- |
         |                                                    |


      Figure 3: Sending a DALI setting with CoAP to CoAP/DALI gateway

6.2.  Discovery of legacy gateways

   Discovery of legacy gateways is not very different from discovery of
   proxies in section 5.4. the consequences for discovery are listed for
   the four modes of addressing legacy devices via a gateway of section
   6.1.

   -  The gateway presents a list of resources representing the legacy
      devices.  Discovery is done as for other CoAP devices.

   -  Each legacy device has a different IP address.  The gateway must
      create entries in the DNS for as many legacy devices.  The
      authority of the legacy device is the authority of the gateway
      with a ServiceType to be specified by the gateway.

   -  All devices of legacy network YYY have different authorities.  In
      this case each legacy device has the same IP address as the
      gateway.  The gateway must create entries in the DNS for as many
      legacy devices.

   -  All devices of legacy network YYY have different ports.  The
      gateway must create entries in the DNS for as many legacy devices.
      Each entry has the authority of the gateway with a different
      ServiceType and a different port number.


7.  Conclusions

   This I-D explains how naming in building control is based on a
   hierarchical structure of the building areas.  It is shown that DNS
   naming can be used to express this hierarchy in the authority portion
   of the URI, down to the group or device level.  The hierarchical
   naming scheme need not be standardized, but rather can be designed to
   suit the application.  However, it is recommended that the scheme be
   employed consistently throughout the delegated subdomain(s).



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   The authority portion of the URI is resolved by the client, using
   conventional DNS, into the unicast or multicast IP address of the
   targeted device(s).  Taking advantage of the CoAP design
   [I-D.ietf-core-coap], the URI-Host option need not be transmitted in
   requests to origin servers and thus there is no performance penalty
   for using descriptive naming schemes.  The CoAP design allows sending
   a short URI to distinguish between resources on a given device,
   resulting in very compact identifiers.

   DNS-SD [I-D.cheshire-dnsext-dns-sd] can be used to scale up service
   discovery beyond the 6LoWPAN.  DNS-SD can be used to enumerate
   instances of a given service type within a given sub-domain.  This
   affords additional flexibility, such as the ability to discover
   dynamic port assignments for CoAP device, locate CoAP devices by
   subtype, or bind service names for particular CoAP URIs.

   This I-D discusses the addressing, discovery and naming of legacy
   devices behind gateways.  The discovery of backward proxies of
   sleeping devices is handled in a similar fashion.

   A targeted resource is specified by the path portion of the URI.
   Again, this I-D does not mandate a universal naming standard for
   resources but uses examples to show how resources could be named for
   various legacy standards.  An obvious requirement for resources that
   are accessed by multicast is that they MUST all share the same path.
   It is shown that it is possible to transport legacy commands (e.g.
   expressed in BACnet, LON, DALI, ZigBee, etc.) inside a CoAP message
   body.  Entering ServiceTypes particular to a given standard
   necessitates that the standardization body declares the ServiceType
   to dns.org.


8.  Security considerations

   TBD: The detailed CoAP security analysis needs to encompass scenarios
   for building control applications.

   Based on the programming model presented in this I-D, security
   scenarios for building control need to be stated.  Appropriate
   methods to counteract the proposed threats may be based on the work
   done elsewhere, for example in the ZigBee over IP context.

   Multicast messages are, by their nature, transmitted via UDP.  Any
   privacy applied to such messages must be block oriented and based on
   group keys shared by all targeted devices.  The CoRE security
   analysis must be broadened to include multicast scenarios.





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9.  IANA considerations

   This I-D proposes that associations which standardize device
   representations (like BACnet, ZigBee, DALI,...) contact IANA to
   reserve the prefix /.well-known/XXX for the standard XXX.


10.  Acknowledgements

   This I-D has benefited from conversations with and comments from
   Andrew Tokmakoff, Emmanuel Frimout, Jamie Mc Cormack, Oscar Garcia,
   Dee Denteneer, Joop Talstra, Zach Shelby, Jerald Martocci, Anders
   Brandt, Matthieu Vial, Jerome Hamel, George Yianni, and Nicolas Riou.


11.  Changelog

   From bc-01 to bc-02

   - Removed all references to multicast and multicast scope, given
   draft of rahman group communication.

   - Adapted examples to CoAP-2 and core-link drafts.

   - transport short URL for destination recognition.

   - Elaborated legacy discovery under DNS-SD.

   From bc-02 to bc-03

   - Elaboration on gateways, commissioning and legacy networks.

   - Recommendation to extend DNS-SD naming with sn, st, and ss
   attributes.

   From bc-03 to bc-04

   - moved core link extension sub-section to discovery mapping draft

   - extended use of service type

   - gave DNS record examples and worked out multifunction device

   - added proxy discovery and legacy gateway discovery

   - defined path tree and corresponding schema

   - reviewed definition of group, device, server, service (interface),



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   resopurce, and attribute.

   From bc-04 to bc-05

   - extended and corrected examples for multi-function devicesw

   - syntax more compatible with other resource discovery I-Ds

   - abstract adapted

   - more stringent use of the words server, end point, service and
   devices


12.  References

12.1.  Normative References

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

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

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

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

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

   [RFC3306]  Haberman, B. and D. Thaler, "Unicast-Prefix-based IPv6
              Multicast Addresses", RFC 3306, August 2002.

   [RFC3307]  Haberman, B., "Allocation Guidelines for IPv6 Multicast
              Addresses", RFC 3307, August 2002.

   [RFC3596]  Thomson, S., Huitema, C., Ksinant, V., and M. Souissi,
              "DNS Extensions to Support IP Version 6", RFC 3596,
              October 2003.

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




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   [RFC3956]  Savola, P. and B. Haberman, "Embedding the Rendezvous
              Point (RP) Address in an IPv6 Multicast Address",
              RFC 3956, November 2004.

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

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

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

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

   [RFC5790]  Liu, H., Cao, W., and H. Asaeda, "Lightweight Internet
              Group Management Protocol Version 3 (IGMPv3) and Multicast
              Listener Discovery Version 2 (MLDv2) Protocols", RFC 5790,
              February 2010.

12.2.  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.cheshire-dnsext-multicastdns]
              Cheshire, S. and M. Krochmal, "Multicast DNS",
              draft-cheshire-dnsext-multicastdns-14 (work in progress),
              February 2011.

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "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-07 (work in progress),
              July 2011.

   [I-D.martocci-6lowapp-building-applications]
              Martocci, J., Schoofs, A., and P. Stok, "Commercial



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              Building Applications Requirements",
              draft-martocci-6lowapp-building-applications-01 (work in
              progress), July 2010.

   [I-D.rahman-core-groupcomm]
              Rahman, A. and E. Dijk, "Group Communication for CoAP",
              draft-rahman-core-groupcomm-07 (work in progress),
              October 2011.

   [I-D.shelby-core-resource-directory]
              Shelby, Z. and S. Krco, "CoRE Resource Directory",
              draft-shelby-core-resource-directory-01 (work in
              progress), September 2011.

   [I-D.lynn-core-discovery-mapping]
              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.

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

   [BACnet]   Bender, J. and M. Newman, "BACnet/IP",
              Web http://www.bacnet.org/Tutorial/BACnetIP/index.html,
              2000.

   [ZigBee]   Tolle, G., "A UDP/IP Adaptation of the ZigBee Application
              Protocol", draft-tolle-cap-00 (work in progress),
              October 2008.

   [LON]      "LONTalk protocol specification, version 3", 1994.

   [DALI]     "DALI Manual", Web http://www.dali-ag.org/c/manual_gb.pdf,
              2001.

   [KNX]      Kastner, W., Neugschwandtner, G., and M. Koegler, "AN OPEN
              APPROACH TO EIB/KNX SOFTWARE DEVELOPMENT", Web http://
              www.auto.tuwien.ac.at/~gneugsch/
              fet05-openapproach-preprint.pdf, 2005.

   [IEEE.802.15.4]
              "Information technology - Telecommunications and
              information exchange between systems - Local and
              metropolitan area networks - Specific requirements - Part
              15.4: Wireless Medium Access Control (MAC) and Physical



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              Layer (PHY) Specifications for Low Rate Wireless Personal
              Area Networks (LR-WPANs)", IEEE Std 802.15.4-2006,
              June 2006,
              <http://standards.ieee.org/getieee802/802.15.html>.

   [HAYSTACK]
              "Project Haystack", Web http://project-haystack.org/,
              2011.

   [oBIX]     Frank, B., Ed., "oBIX working group",
              Web http://www.obix.org, 2006.

   [Fielding]
              Fielding, R., "Architectural Styles and the Design of
              Network-based Software Architectures, Second Edition",
              Doctoral dissertation , University of California, Irvine ,
              Web http://www.ics.uci.edu/~fielding/pubs/dissertation/
              top.html, 2000.

   [ZigBee-IP]
              "ZigBee Smart Energy Profile 2.0 Application Protocol
              Specification", Draft ZigBee-11167, March 2011.

   [dns-sd]   "dns-sd servicetype registration",
              Web http://www.dns-sd.org/ServiceTypes.html, 2011.


Authors' Addresses

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

   Email: peter.van.der.stok@philips.com


   Kerry Lynn
   Consultant

   Phone: +1 978 460 4253
   Email: kerlyn@ieee.org








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