CoRE P. van der Stok
Internet-Draft Philips Research
Intended status: Informational K. Lynn
Expires: January 10, 2011 Consultant
July 9, 2010
CoAP Utilization for Building Control
draft-vanderstok-core-bc-01
Abstract
This I-D describes an example use of the RESTful CoAP protocol for
building control applications such as HVAC and lighting. A few basic
design assumptions are stated first. The URI structure is exploited
to define multicast as well as unicast scopes. RFC 3986 defines the
URI components as (1) a scheme, (2) an authority, used here to locate
the building, area, or node under control, (3) a path, used here to
locate the resource under control, and (4) a query and fragment part,
where fragments are not supported in CoAP.
This proposal supports the view that (1) 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,
(2) service discovery is complimentary to resource discovery and
facilitates control network scaling, and (3) the provision of a
reliable group communication protocol is essential to support
building control applications.
Status of this Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 10, 2011.
Copyright Notice
van der Stok & Lynn Expires January 10, 2011 [Page 1]
Internet-Draft CoAP Utilization for Building Control July 2010
Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Table of Contents
1. Conventions and Terminology Used in this Document . . . . . . 3
2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. URI structure . . . . . . . . . . . . . . . . . . . . . . . . 5
3.1. Scheme part . . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Authority part . . . . . . . . . . . . . . . . . . . . . . 6
3.3. Path part . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Group Addressing . . . . . . . . . . . . . . . . . . . . . . . 7
5. Reliable multicast . . . . . . . . . . . . . . . . . . . . . . 9
6. Application examples . . . . . . . . . . . . . . . . . . . . . 10
7. Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 12
8. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 15
9. Security considerations . . . . . . . . . . . . . . . . . . . 15
10. IANA considerations . . . . . . . . . . . . . . . . . . . . . 16
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
12.1. Normative References . . . . . . . . . . . . . . . . . . . 16
12.2. Informative References . . . . . . . . . . . . . . . . . . 17
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 18
van der Stok & Lynn Expires January 10, 2011 [Page 2]
Internet-Draft CoAP Utilization for Building Control July 2010
1. Conventions and Terminology Used in this Document
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].
The term "service" may mean different things to different communities
and sometimes different things to the same community. In building
control protocol standards, service is often used to refer to a
function in the RPC sense. In this context, we generally substitute
the term "function". In the IETF community, service may often refer
to an abstract capability such as "datagram delivery". In this
submission we use the term service, in the sense defined by "DNS-
based Service Discovery" [I-D.cheshire-dnsext-dns-sd], as equivalent
to a CoAP end-point.
A CoAP end-point is identified by the authority part of a URI. We
refer to this end-point (which is resolved to an {IP address, port}
tuple) as a "node". By "device" we generally mean the physical
object handled by the installer. While a device may host more than
one service, for simplicity we assume here that a given device may
only host a single CoAP node.
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 fully qualified or partial domain names and
the difference should be clear from the context.
2. Motivation
The CoAP protocol [I-D.ietf-core-coap] aims at providing a user
application protocol architecture that is targeted to a network of
nodes 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 controls market is highly price sensitive and
manufacturers tend to compete by delivering a given functionality and
quality for the lowest price.
The vast majority of nodes in a typical building control application
are resource constrained, making the standardization of a lightweight
application protocol like CoAP a necessary requirement for IP to
penetrate the device market. This approach is further indicated by
the low energy consumption requirement of battery-less nodes. Low
resource budget implies low throughput and small packet size as for
[IEEE.802.15.4]. Reduction of the packet size is obtained by using
van der Stok & Lynn Expires January 10, 2011 [Page 3]
Internet-Draft CoAP Utilization for Building Control July 2010
the header reduction of 6LoWPAN [RFC4944] and encouraging small
payloads.
Several legacy building control standards (e.g [BACnet], [LON],
[DALI], [KNX], etc.) have been developed based on years of
accumulated knowledge and industry cooperation. These standards
generally specify a data model, functions, packet formats, and
sometimes the physical medium for data objects and function
invocation. Many of these industry standards also specify lower-
level functionality such as proprietary transport protocols,
necessitating expensive stateful gateways for these standards to
interoperate. Many are in the process of transitioning to IP-based
standards for transport and other functions such as naming and
discovery. CoAP will be succesful 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 basic syntax assumptions 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 aims at 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 is
related to the chosen 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 a room). The
scope of a multicast command or discovery message covers the group of
nodes that is targeted. Defining multicast scopes on the basis of
hop count or the existence of edge routers is not always sufficient
in buildings where the network architecture may be independent of the
van der Stok & Lynn Expires January 10, 2011 [Page 4]
Internet-Draft CoAP Utilization for Building Control July 2010
controlled areas (e.g. rooms) in the building.
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 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 protocol oriented assumptions are:
- A syntactic definition of the multicast scope applicable to all
control application multicasts in the building.
- Nodes may be addressed by more than one group.
- Resources addressed by a group must be uniformly named across all
targeted nodes.
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
node in several building areas by nodes inside or outside the
building, and humans may intervene to change control settings.
3. 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). An example based on RFC 3986 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.
3.1. Scheme part
The default scheme in this submission is "coap" although the
intention is that everything stated below about URIs SHOULD apply
equally to "http" and might be exposed, say, through an http-to-coap
van der Stok & Lynn Expires January 10, 2011 [Page 5]
Internet-Draft CoAP Utilization for Building Control July 2010
gateway. That topic is beyond the scope of this document.
3.2. Authority part
The authority part is either a literal IP address or a DNS name
comprised of a global part specifying the domain and a local part
specifying the logical hierarchical structure of the building control
network, down to the group or node 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.
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 node within the building.
Arriving at the node identified by //bldg.5533BA-125a.nl, the
receiving service can parse the path portion of the URI and perform
the requested method 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
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 basic 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. //node-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 sub-
string: //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
van der Stok & Lynn Expires January 10, 2011 [Page 6]
Internet-Draft CoAP Utilization for Building Control July 2010
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 seems to lead to rather verbose URI strings
in the packet, contrary to the small packet assumption. However, the
design of CoAP is such that the authority portion of the URI need not
be transmitted in requests sent to origin servers. The question
arises as to whether the syntax of the authority part needs to be
standardized for building control. Given the examples later in the
text, this appears more to be the concern of the building owner or
the installer.
3.3. Path part
Every network addressable resource is completely identified by a URI
scheme://authority/path. The path part of the URI specifies the
resource within a given node. The representation of object types and
their associated attributes are typically subjects for
standardization. There is no widely accepted standard for uniformly
naming building control device structure in a URI. A vigorous effort
is undertaken by the oBIX working group of OASIS [oBIX].
When a GET method with an URI like: //t-sensor1.25b006.floor1/
temperature is sent, it represents an a priori understanding that the
node with name t-sensor1 exists, is of a given standard type (e.g
BACnet temperature sensor), and that this standard type has the
readable attribute: temperature. However, in the case of multicast
commands to a group of nodes it is necessary that the targeted
resource have the same path on all targeted nodes. 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 natural data scheme (in the case of BACnet,
device/object/property).
4. Group Addressing
As suggested by the examples above, the scope of the messages can be
logically associated with the URI authority. This provides a better
handle to define the multicast scope than the traditional TTL counter
preventing the multicast message to pass one or more routers. This
more sophisticated scoping mechanism is needed to decouple multicast
scopes from the network layout. This is reflected by the capability
provided in [BACnet] to define the scope of its service/resource
discovery messages.
van der Stok & Lynn Expires January 10, 2011 [Page 7]
Internet-Draft CoAP Utilization for Building Control July 2010
Given a network configuration and associated prefixes, the network
operator needs to define an appropriate set of multicast 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 multicast
implementation. Example multicast groups become:
URI authority Targeted group
//all.bldg6 "all nodes in building 6"
//all.west.bldg6 "all nodes in west wing, building 6"
//all.floor1.west.bldg6 "all nodes on floor 1, west wing, ..."
//all.bu036.floor1.west.bldg6 "all nodes 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 nodes in each subdomain. In practice,
"all" would 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 possible using
IPv6. The multicast address allocation strategy is beyond the scope
of this I-D, but various alternatives have been proposed
[RFC3306][RFC3307][RFC3956]. Some techniques in this proposal, e.g.
service discovery as described below, can be accomplished with a
single coap-specific multicast address as long as the desired scope
is building-wide.
To illustrate the concept of multiple group names within a given
multicast scope, 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 an 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 uri-authority option
[I-D.ietf-core-coap] need not be sent as part of the message.
A group defines a set of nodes. All resources on a given node are
referenced by the multicast address(es) to which the node belongs. A
given node might belong to a number of groups. For example the node
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".
In summary, the authority portion of the URI is used to identify a
node (group) and the resulting DNS name is bound to a unicast
van der Stok & Lynn Expires January 10, 2011 [Page 8]
Internet-Draft CoAP Utilization for Building Control July 2010
(multicast) address, resulting in an associated unicast (multicast)
scope. Naming is building or organization dependent, must be
flexible, and does not require standardization efforts but SHOULD
conform to some uniform convention. In the context of an
administrated professional building, groups can be defined off-line
and stored in DNS server configuration. Automated enumeration, based
on service discovery methods described below, may be used to locate
nodes and add them to groups during the building commissioning phase.
5. Reliable multicast
A reliable group communication (multicast) is essential for an
efficient building control application. Reliable multicast supports
guaranteed delivery of messages to a group of nodes. The
representative example is a group of lights that need to be switched
on simultaneously. Although the delay between sending the command
message (e.g. from a switch) to the effective switching on of the
lights may be up to one second, all lights in the group should appear
to switch on simultaneously (within an interval of 100-200 msec).
Examples of reliable multicast specifications are cited in
[Mullender]. In the case of real-time control of devices, the
following specification applies:
Validity - If sender sends message, m, to a group, g, of
destinations, a path exists between sender and destinations, and
sender and destinations are correct, all destinations in g
eventually receive m.
Integrity - destination receives m at most once from sender and
only if sender sent m to a group including destination.
Agreement - If a correct destination of g receives m, then all
correct destinations of g receive m.
Timeliness - There is a known constant D such that if m is sent at
time t, no correct destination receives m after t+D.
Assuming that every new multicast message contains a unique
transaction identifier, the integrity requirement can be met by
checking this identifier. The agreement and timeliness requirements
can be met by multicast algorithms developed for real time computing.
It is assumed that the clocks of the nodes are synchronized and
multiple redundant paths can be used to reach all destinations either
directly or via other nodes. Especially for battery-less nodes it is
interesting to note that when the message arrives reliably at one
correct destination, it will be passed on to all other correct
destinations and in the contrary case is received by none
van der Stok & Lynn Expires January 10, 2011 [Page 9]
Internet-Draft CoAP Utilization for Building Control July 2010
[Mullender]. The consequence of such a specification is also that
when a light in a group does not switch on, the lamp is faulty
(either the lamp or the associated node). Satisfaction of the
validity requirement does not rely on returning acknowledgement
messages, but on sufficient redundancy in nodes and network links.
The scope of the multicast helps to send the message only to a subset
of interested nodes. However, a minimum set of nodes is needed to
deliver the message reliably. Dividing the building network in
multicast areas helps to confine the multicast while at the same time
assuring the minimum number of participants. The choice of areas is
a design parameter not discussed here. Such areas can be supported
with route-over or mesh-under routing.
Another approach, favored by some implementations, is to send a
packet n times over a wireless link with given intervals dependent on
deployed physical medium. It can be expected that several techniques
will be advocated in the future. Interoperability between wireless
nodes from different manufacturers participating in a muticast
requires that reliable multicast is standardized. The challenges
posed by the wireless, real-time, and battery-less aspects of these
control networks motivate the specification of an appropriate
(possibly new) multicast protocol.
6. Application examples
It is assumed that devices may exchange messages with a content
defined by one of the existing building control standards e.g BACnet,
LON, DALI, ZigBee Device Objects (ZDO), KNX, and others. All of
these standards have defined concepts like type (class) and type
(class) instances.
Within a given type a number of attributes exists that can be
modified or read with a more or less complex invocation syntax. This
draft proposes that the path portion of the URI first identify the
standard and then continue with a standard dependent syntax, to be
defined by the standardization body interested in utilizing CoAP.
For example, a command to a heating unit with a BACnet interface can
be expressed as //authority/BACnet/BACnet-defined-command or a
command to a DALI light can be expressed as //authority/DALI/
DALI-defined-command. The example request: PUT
//light1.bu036.floor1/DALI/Intensity = 30 would translate to CoAP
header [I-D.ietf-core-coap]:
van der Stok & Lynn Expires January 10, 2011 [Page 10]
Internet-Draft CoAP Utilization for Building Control July 2010
- dest IP address determined by resolving: //light1.bu036.floor1
- T bits to 0: Confirmable message
- Code = 2: PUT method
- OC bits set to 1 (for one Mime option)
- Transaction ID set
- Option type= 1, content type: /application/DALI
- DALI command: set Intensity attribute to 30
The new option content type shows that new application mime types
need to be defined to cover the building control standards: e.g.
/application/DALI, /application/BACnet, etc.
Examples of wireless, battery-less nodes are sensors used for
measuring presence, temperature, light intensity, or humidity.
Battery-less means that the nodes are switched off most of the time
and sporadically power up and send out their current measured value.
This value is either sent to a controller node or to a group of
actuator nodes. Examples are presence detection sent to lamps, or
humidity level to a fan.
The destination nodes of the measurements are probably powered by the
mains or a derivative of the mains. It seems unrealistic to have the
controller or the actuator nodes send request messages to the
battery-less nodes, after which the sender has to wait an interval
determined by the duty cycle of the actuator or controller. More
natural is that the battery-less node wakes up and sends its message
to its controller or actuator nodes which are always ready to receive
a message. For example, without a controller node, the presence
detector can send presence regularly to a group of lights.
The group can be defined on-line, by having the lights subscribe to
the presence service, or the group can be defined off-line by the
manager of the control network. On-line definition is more natural
in a dynamic home environment, while off-line is more natural in the
office environment. Off-line has the added advantage of checking on
missing nodes. For subscription the subscribing nodes have to learn
the IP address(es) of the service(s) to which they want to subscribe.
In case of off-line the servers have to learn the IP address of the
multicast group. The latter can be learned from DHCP options, by
inserting the destination IP address inside the configuration file of
the battery-less node.
van der Stok & Lynn Expires January 10, 2011 [Page 11]
Internet-Draft CoAP Utilization for Building Control July 2010
The CoAP protocol foresees the use of a non confirmable message
packet to send these unsolicited responses to the multicast group or
the single controller. Again the syntax of the commands are most
likely defined by legacy standards. Assuming the DALI standard, the
command PUT //blue-lights.bu036.floor1/DALI/OnOff=on leads to the
following packet lay-out:
- dest multicast IP address determined by: //blue-
lights.bu036.floor1
- T bits to 1: Non Confirmable message
- Code = 2, PUT method
- OC bits set to 1 (for one Mime option)
- Transaction ID set, for prevention of double messages
- Option type= 1, content type: /application/DALI
- DALI command: set OnOff attribute to on
7. Discovery
At a high level the the discovery strategy can be introduced with an
example to create a group "DALI/lights" in a given building domain.
- The building domains can be resolved according to a domain
specification which is consistent over a set of buildings
maintained by a building control provider
- A query over a specified domain returns the IP addresses of all
nodes in this domain with the reource DALI/lights
- The IP address of multicast group "DALI/lights" is defined
- The multicast group can be realized in two alternative ways:
- A list of IP addresses invoked by unicast.
- Each member allocates the IP multicast address, and receives
all messages sent to this IP address.
van der Stok & Lynn Expires January 10, 2011 [Page 12]
Internet-Draft CoAP Utilization for Building Control July 2010
- Messages can be multicast to the group DALI/lights.
This implies a consistent naming scheme within each node. The above
group definition can be done on-line and off-line.
Service or resource discovery is often scoped according to the
building structure. For example, BACnet defines "Who-Is" and "Who-
Has" functions to locate nodes and resources, respectively, that
match specified criteria (filters) in a defined network scope. CoAP
defines a resource discovery capability, but it is limited to link-
local scope; examples may be found in [I-D.ietf-core-coap]. A
service discovery capability is required to extend discovery to other
subnets.
DNS-based Service Discovery [I-D.cheshire-dnsext-dns-sd] defines a
conventional way to configure DNS PTR, SRV, and TXT records to enable
enumeration of services such as CoAP nodes within subdomains. A
service is specified by a name of the form Instance.Type.Domain,
where the type for CoAP nodes is _coap._udp and the domain is a DNS
domain name that identifies a building zone as in the examples above.
For each CoAP end-point in the zone, a PTR record with the name
_coap._udp is defined and it points to an SRV record having the
Instance.Type.Domain name.
All CoAP nodes in a given subdomain may be enumerated by sending a
DNS query to the authoritative server for that zone for PTR records
named _coap._udp. A list of SRV records is returned. Each SRV
record contains the port and host name of a CoAP node. The IP
address of the node 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. This can be
used to store information about the device, e.g. schema=DALI,
type=switch.
Another feature of DNS-SD is the ability to specify service subtypes
using PTR records. For example, a CoAP node that supports BACnet
commands might be represented with a PTR record having the name
_bacnet._sub._coap._udp. In this way, all BACnet nodes in a
subdomain might be enumerated more efficiently. This technique for
node enumeration can be used to emulate BACnet's "Who-Is" function.
With an enumerated list of nodes, a management workstation may then
perform unicast resource discovery as described in
[I-D.ietf-core-coap]. Alternately, the group multicast addressing
described earlier can be used to scope queries for specific resources
to different subdomains. This technique effectively emulates
BACnet's "Who-Has" function.
van der Stok & Lynn Expires January 10, 2011 [Page 13]
Internet-Draft CoAP Utilization for Building Control July 2010
When for example a lamp wants to discover the controller, it is only
interested in the controllers located in the same office (area) as
itself. Consequently, the service discovery is related to the groups
defined according to the building structure. It is advisable to send
a discovery message to a given group. Also the packet does not need
the complete URI in the URI option. In conformance with RFC 5785
[RFC5785], a packet from a controller with the request to return the
device types of all DALI devices within the office bu036 can look
like:
- dest multicast IP address defined by: //bu036.floor1
- T bits to 0: Confirmable messages
- Code 0: GET method
- OC bits set to 2 (for Mime and URI option)
- Transaction ID set
- Option type= 9, uri-path: LEN=13, ".well-known/r"
- Option type= 1, content type: /application/DALI
- DALI command: "return device type"
The responses from the DALI nodes may look like:
- dest IP address of controller
- T bits to 2: Acknowledgement messages
- CODE = 0, OK
- OC bits set to 1 (for Mime option)
- Transaction ID identical
- Option type= 1, content type: /application/DALI
- DALI command: "type = Lamp"
The rest of the protocol is dictated by the legacy standard in use
but encapsulated within the CoAP discovery messages as shown above.
van der Stok & Lynn Expires January 10, 2011 [Page 14]
Internet-Draft CoAP Utilization for Building Control July 2010
8. Conclusions
This I-D explains how building control is based on a hierarchical
structure of the building areas, and that the logical scope of
control and discovery messages is determined by the building
structure. It is shown that DNS subdomain delegation and naming can
be used to express this hierarchy in the authority portion of the
URI, down to the group or node 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). The authority
portion of the URI is resolved by the client into the unicast or
multicast IP address of the targeted node(s). Taking advantage of
the CoAP design [I-D.ietf-core-coap], the uri-authority option need
not be transmitted in requests to origin servers and thus there is no
performance penalty for using descriptive naming schemes.
The 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,
including short uri if used. 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. This necessitates the
definition of additional IANA mime codes, and the mapping of legacy
specific discovery semantics to CoAP resource discovery messages or
DNS-SD lookups.
It is expected that many control messages are sent by battery-less
sensors with their own specific sending intervals to a group of
actuator nodes or controllers. Given the importance of groups and
associated multicast messages to support legacy standard functions,
the specification of a reliable multicast protocol with related
multicast scope is needed in CoAP.
9. Security considerations
TODO: 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
van der Stok & Lynn Expires January 10, 2011 [Page 15]
Internet-Draft CoAP Utilization for Building Control July 2010
privacy applied to such messages must be block oriented and based on
group keys shared by all targeted nodes. The CoRE security analysis
must be broadened to include multicast scenarios.
10. IANA considerations
This I-D proposes the following additions to the Media type
identifiers in conformance with the proposals done in
[I-D.ietf-core-coap].
Internet media type Code
/application/BACnet xx
/application/DALI xx+1
/application/ZDO xx+2
/application/LON xx+3
/application/KNX xx+4
/application/oBIX+exi xx+5
TODO: Investigate CoAP specific well-known multicast address
assignment?
11. 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, and
Nicolas Riou.
12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[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.
van der Stok & Lynn Expires January 10, 2011 [Page 16]
Internet-Draft CoAP Utilization for Building Control July 2010
[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.
[RFC5785] Nottingham, M. and E. Hammer-Lahav, "Defining Well-Known
Uniform Resource Identifiers (URIs)", RFC 5785,
April 2010.
12.2. Informative References
[I-D.ietf-core-coap]
Shelby, Z., Frank, B., and D. Sturek, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-01
(work in progress), July 2010.
[I-D.cheshire-dnsext-dns-sd]
Cheshire, S. and M. Krochmal, "DNS-Based Service
Discovery", draft-cheshire-dnsext-dns-sd-06 (work in
progress), March 2010.
[I-D.cheshire-dnsext-multicastdns]
Cheshire, S. and M. Krochmal, "Multicast DNS",
draft-cheshire-dnsext-multicastdns-11 (work in progress),
March 2010.
[I-D.martocci-6lowapp-building-applications]
Martocci, J., Schoofs, A., and P. Stok, "Commercial
Building Applications Requirements",
draft-martocci-6lowapp-building-applications-01 (work in
progress), July 2010.
[BACnet] Bender, J. and M. Newman, "BACnet/IP",
Web http://www.bacnet.org/Tutorial/BACnetIP/index.html.
[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.
van der Stok & Lynn Expires January 10, 2011 [Page 17]
Internet-Draft CoAP Utilization for Building Control July 2010
[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
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>.
[oBIX] "oBIX working group", Web http://www.obix.org, 2003.
[Mullender]
Mullender, S., "Distributed Systems, Second Edition",
Section 5 , Addison-Wesley Publishing Company, Inc. ,
ISBN 0-201-62427-3, 1995.
Authors' Addresses
Peter van der Stok
Philips Research
High Tech Campus
Eindhoven, 5656 AA
The Netherlands
Email: peter.van.der.stok@philips.com
URI: http://www.research.philips.com/
Kerry Lynn
Consultant
Phone: +1 978 460 4253
Email: kerlyn@ieee.org
van der Stok & Lynn Expires January 10, 2011 [Page 18]