CoRE P. van der Stok
Internet-Draft Philips Research
Intended status: Informational K. Lynn
Expires: September 15, 2011 Consultant
March 14, 2011
CoAP Utilization for Building Control
draft-vanderstok-core-bc-03
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. 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 part (fragments are not supported in
CoAP.) Next, it is shown that DNS can be used to locate URIs on the
scale necessary in large commercial BAC deployments. Finally, a
method is proposed for mapping URIs onto legacy BAC resources, e.g.,
to facilitate application-layer gateways.
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,
and (2) service discovery is complimentary to resource discovery and
facilitates control network scaling.
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 September 15, 2011.
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Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
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the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
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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 . . . . . . . . . . . . . . . . . 9
4. Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.1. DNS-Based Service Discovery . . . . . . . . . . . . . . . 11
4.2. Service vs Host Names . . . . . . . . . . . . . . . . . . 12
4.3. Browsing for Services . . . . . . . . . . . . . . . . . . 12
4.4. Resource vs Service Discovery . . . . . . . . . . . . . . 12
4.5. CoRE Link Extensions for DNS-SD . . . . . . . . . . . . . 13
4.5.1. Service Name "sn" attribute . . . . . . . . . . . . . 13
4.5.2. Service Type "st" attribute . . . . . . . . . . . . . 14
4.5.3. Service Subtype "ss" attribute . . . . . . . . . . . . 14
5. Data Representations in CoAP . . . . . . . . . . . . . . . . . 14
5.1. Network architectures . . . . . . . . . . . . . . . . . . 15
5.2. Gateways to legacy networks . . . . . . . . . . . . . . . 17
5.3. Commissioning CoAP devices . . . . . . . . . . . . . . . . 18
5.3.1. DNS-SD server present . . . . . . . . . . . . . . . . 19
5.3.2. DNS-SD server not present . . . . . . . . . . . . . . 20
6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . 20
7. Security considerations . . . . . . . . . . . . . . . . . . . 21
8. IANA considerations . . . . . . . . . . . . . . . . . . . . . 22
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 22
10. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 22
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 22
11.1. Normative References . . . . . . . . . . . . . . . . . . . 22
11.2. Informative References . . . . . . . . . . . . . . . . . . 24
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 25
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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.
The term "service" often means different things to different
communities and even 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 (or server).
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 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 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. 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 nodes in a typical building control application
are resource constrained, making the standardization of a lightweight
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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
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 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 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 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
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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 nodes that is targeted. A group scope may be defined as
link-local, or as a tree maintained by IP-multicast or an 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 protocol oriented assumptions are:
- Nodes may be addressed by one or more groups.
- 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.
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 6.1 of
[I-D.ietf-core-coap] and is compatible with the "http" scheme
specification [RFC2616]. The host part of the authority may be
represented either as a literal IP address or as a fully qualified
domain name. While scheme is irrelevant from the perspective of the
service, it is used in service discovery to identify the protocol
used to access the service.
TBD: we have yet to fully explore the utility of a separate scheme
(e.g., "coapm") to support group communication models as described in
[I-D.rahman-core-groupcomm].
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 node or group of
nodes, 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 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.
The CoAP spec [I-D.ietf-core-coap] states "When a CoAP server is
hosted by a 6LoWPAN node, 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 it is conventional to treat
different ports as representing different authorities and there is no
assurance that a coap server will consistently acquire the same
dynamic port.
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
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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 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. //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 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 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 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
than a standardization concern.
2.3. 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], but its
current impact is limited.
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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 node with name
t-sensor1 exists, is of a given standard type (e.g. ZigBee
temperature sensor), and that this standard type has the readable
attribute: temperature. When commands are sent to a group of nodes
it MUST be the case that the targeted resource has 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 chosen data scheme
(in the case of BACnet, /bacnet/device/object/property).
A better long-term solution is to build on the concepts presented in
[I-D.ietf-core-link-format] and identify resources of a given object
model in terms of a registered "/.well-known" prefix. 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 pathnames that may occur under this prefix.
This allows the XXX standard development organization to assume
responsibility for defining the name space and resources associated
with the prefix. The registered /.well-known/XXX URI effectively
defines a standard object model, or schema. Manufacturers may
optionally define proprietary resources that can be discovered
dynamically using methods described below.
3. Group Naming and Addressing
Given a network configuration and associated prefixes, 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 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, ..."
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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" 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]. 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 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 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-Host option [I-D.ietf-core-coap] need not be sent
as part of the message. However to identify the resource that is
addressed, a short version of the resource path can be inserted as an
option as explained in [I-D.ietf-core-link-format].
The binding of a group FQDN to multicast address (i.e., creation of
the AAAA record in the DNS zone server) happens during the
commissioning process. Resolution of the group name to a multicast
address happens at restart of a source or receiver node. 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 its name. In the limit, the group can degrade to
a single controller that represents a non-networked subsystem
replacing the original networked group of nodes. Group membership
may be managed by a protocol such as Multicast Listener Discovery
[RFC5790].
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". Assuming that
belonging to a group has as only consequence for the group member
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that 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 server but not necessarily on the low-resource
destinations or sources. Assuming that resolution of addresses only
happens at node start-up, the complexity of the DNS server need not
affect the responsiveness of the nodes.
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
(multicast) address. Naming is building or organization dependent,
must be flexible, and does not require standardization efforts but
SHOULD conform to some uniform convention.
4. Discovery
4.1. 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 such as CoAP nodes 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>, where the service type for CoAP
nodes is "_coap._udp". The identifier "_udp" provides a transport
protocol hint as required by the SRV record definition [RFC2782] and
"_coap" identifies the application protocol. A PTR record with the
label "_coap._udp" is defined for each CoAP end-point in the zone,
and this record's value is set to the service instance name (which in
turn identifies to SRV and TXT records for the CoAP end-point).
DNS-SD also supports one level of subtype, which can be used to
locate coap services based on object model (schema), for example:
_bacnet._sub._coap._udp, _dali._sub._coap._udp, or
_zigbee._sub._coap._udp. The maximum length of the type and subtype
fields is 14 octets, which could allow for "schema-function"
descriptors such as _dali-light, _dali-switch, etc.
The Location part of the service name is identical to the global (DNS
subdomain) part of the authority in URIs that identify the resources
on this node or group and may identify a building zone as in the
examples above.
The Instance part of the service name may be changed during the
commissioning process. It must be unique within the subdomain. The
complete service name uniquely identifies an SRV and a TXT record in
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the DNS zone. The granularity of a service name MAY be at the group
or node level, or it could represent a particular resource within a
coap node. The SRV record contains the host (AAAA record) name and
port of the service. In the case where a service name identifies a
particular functional entry point, the path part of the URI may be
placed in the TXT record.
4.2. Service vs Host Names
In general, the authority "www.example.com" does not refer to a
canonical host name (the label of a AAAA record). Logically, it
refers to the "world wide web service" for the example.com domain.
Literally, the "www" is probably the label of a CNAME record that
names a AAAA record that may in turn specify more than one address
(in the case of round-robin load leveling between identical origin
server instances).
The SRV record functions something like the CNAME in this case,
except that it is capable of resolving to an IP address plus a
listening socket (though, as we said, the use of dynamic sockets is
not recommended in URIs). An optional TXT record may be configured
wih same name as the SRV record and be used to store context-
dependent key=value pairs. For example, a multi-function device
might define a service name for each "base URI" that locates the
start of an object resource (e.g. abs-path=/.well-known/zigbee/
sensor/). Thus, the URI coap://host.example.com/temp might resolve
through DNS-SD lookups to
coap://[fdfd::1234]/.well-known/zigbee/sensor/temp.
4.3. Browsing for Services
CoAP nodes in a given subdomain may be enumerated by sending a DNS
query for PTR records named _coap._udp to the authoritative server
for that zone. A list of names for SRV records matching that
ServiceType.Location 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. Apart from defining the standard resources
identified by schema=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.
4.4. Resource vs Service Discovery
While service discovery is concerned with finding the IP address,
port, protocol, and possibly path of a named service, resource
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discovery is a fine-grained enumeration of resource URIs within a web
service. [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 that identify all
resources present on the node that are exposed as functions.
Assuming the ability to multicast the GET over the local link, coap
resource discovery 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 by using /.well-known/core attributes as the basis for a
unique "Instance" name, defaulting to "_coap._udp" for the
ServiceType, and using some means to establish in which subdomain the
service should be registered (TBD). The DNS TXT record can be
populated by importing the other resource description attributes as
they share the same key=value format specified in Section 6 of
[I-D.cheshire-dnsext-dns-sd].
4.5. CoRE Link Extensions for DNS-SD
The following CoRE specific target attributes are proposed as
extensions to [I-D.ietf-core-link-format] to support DNS-SD. The
values are intended to be imported directly into a DNS- SD zone file
are are subject to format and length constraints as specified in
[I-D.cheshire-dnsext-dns-sd].
link-extension = ( "sn" "=" quoted-string )
link-extension = ( "st" "=" quoted-string )
link-extension = ( "ss" "=" quoted-string )
4.5.1. Service Name "sn" attribute
The service name "sn" attribute is the <Instance> portion of a DNS-SD
service instance name. It is stored directly in the DNS as a single
DNS label of canonical precomposed UTF-8 [RFC3629] "Net-Unicode"
(Unicode Normalization Form C) [RFC5198] text. However, to the
extent that the "sn" attribute may be chosen to match the DNS host
name of a proxy or gateway, it SHOULD use the syntax defined in
Section 3.5 of [RFC1034] and Section 2.1 of [RFC1123].
The <Instance> portion of the name of a service being offered on the
network SHOULD be configurable by the user setting up the service, so
that he or she may give it an informative name. However, the device
or service SHOULD NOT require the user to configure a name before it
can be used. A sensible choice of default name can allow the device
or service to be accessed in many cases without any manual
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configuration at all. The default name should be short and
descriptive, and MAY include the device's MAC address, serial number,
or any similar hexadecimal string in an attempt to make the name
globally unique.
DNS labels are currently limited to 63 octets in length and the
entire service instance name may not exceed 255 octets.
4.5.2. Service Type "st" attribute
The service type "st" attribute is composed into the <ServiceType>
portion of a DNS-SD instance name as follows. It is limited to 14
octets of Net-Unicode text. If ommitted, it defaults to "coap". An
underscore '_' is prepended to the value of the "st" attribute, which
is then concatenated with a period '.', and finally the "_udp"
identifier. The resulting string is used to form labels for DNS-SD
records and as such is stored directly in the DNS.
4.5.3. Service Subtype "ss" attribute
The service subtype "st" attribute, if present, follows the format
and composition rules defined in the previous section. It is then
concatenated with a period '.' and the <ServiceType> string defined
above to form additonal labels for DNS-SD records as defined in
Section 4.1.
5. Data Representations in CoAP
Before CoAP devices can come to market, manufacturers must agree that
the type and attributes 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 believe 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. As pointed out earlier, this has
consequences for the naming of resources. In some cases a CoAP
device can handle more than one legacy representation. Given that a
CoAP device can handle representation of standard XXX, this I-D
proposes that such a CoAP device can communicate with legacy devices
via a CoAP/legacy gateway (router).
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5.1. Network architectures
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 nodes -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 IP/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 node returns
data to the requesting zzz;yyy coap host via the same gateway.
+------+ +------+ +------+
| yyy | | zzz | | zzz |
| link | | coap | | coap |
+------+ +------+ +------+
| +---------+
|---------| coap |-->
| | gateway |
+------+ +---------+ +-----------+
| yyy | | zzz ; yyy |
| link | | coap |
+------+ +-----------+
Figure 1: network with multiple representation standards
There are at least four ways that the CoAP hosts can address the
legacy nodes behind the gateway.
- All nodes of legacy network YYY share the same Uri-Host also used
for the coap gateway on the coap network. Every legacy node 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 node.
- All nodes 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 node. The routing
protocol on the coap network makes the message arrive at the coap
gateway. The gateway determines the specified legacy node from
the destination IP address.
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- All nodes of legacy network YYY have different authorities. This
means that the possibly lenghy authority names need to be
transmitted. The gateway recognizes the authorities and maps
authority to legacy node.
- All nodes of legacy network YYY have different ports. This can be
expresed in two ways (1) as :port in the URI, or (2) can be
defined 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 node 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 device from a controller that
can producee zzz commands is given. Assume the resource path /.well-
known/zzz identifies the parser of the ZZZ syntax. A 12 octet
message 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.
Client CoAP/ZZZ
| node
| REQUEST |
|-------- CON [0x5577] PUT /.well-known/ZZZ -------->|
| binary 12 octet string |
| |
| RESPONSE |
|<---------- ACK [0x5577] 2.00 OK ----------------- |
| |
Figure 2: Sending a ZZZ command with coap to coap/ZZZ node
In the example the format of the payload is determined by the zzz
standard. The path /.well-known/ZZZ is obtained from the TXT record
for this service, e.g., schema=ZZZ. The 12 octet binary payload
specifies the zzz command. The port number in the UDP header
identifies the zzz parser.
An example of an invocation of a DALI legacy node behind a gateway is
given. Assume the resource path /.well-known/DALI where the port
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number identifies the DALI node. The application sets a value of 200
in the DALI node in the attribute 256 defined by the DALI spec.
Client DALI/CoAP
| gateway
| REQUEST |
|------- CON [0x5577] PUT /.well-known/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
In the example the format of the payload is determined by the legacy
standard. The path /.well-known/DALI is obtained from the TXT record
for this service, e.g., schema=DALI. The 16-bit binary payload
specifies that the attribute dt of the dali compatible resource is
set to 200. The port number in the UDP header identifies the DALI
node.
5.2. Gateways to legacy networks
Two types of gateways are considered; (1) coap gateway to a single
legacy link, called yyy/coap gateway, and (2) coap gateway to legacy
network, called xxx/coap gateway. The source encapsulates the data
with the corresponding representation inside a CoAP message and sends
these messages to the gateway. In the gateway the XXX/YYY data is
removed from the CoAP message and transported to the desired node.
Returning an answer to the invoking host needs to be done in two
different ways dependent on the addressing type of the XXX/YYY
standard. The IP-node-identifier (INI) can be (1) the IP-address,
(2) the IP-address, port number, (3) the URI, or (4) the path .
- The packet contains a YYY link address. In the gateway two tables
are maintained. Table 1 contains a link from INI to YYY address.
Table 2 contains a link of the source IP address to the
destination YYY address for the active request. A yyy/coap host
with IP address, IPs, sends a request to the INI, IPd, of the
specified YYY device with link address Yd. The packet is routed
to the coap gateway. The gateway strips the link, network and
coap information from the packet and sends the message to the Yd
specified in table 1 with the (Yd, IPd) pair. The gateway stores
the source IP address with the destination YYY address in table 2
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as pair (IPs, Yd). When an answer is returned from Yd, the
gateway creates a new coap packet with the destination address,
IPs, as found in table 2 and sends it on to the yyy/coap host,
IPs.
- The packet contains a XXX network address. In the gateway two
tables are maintained. Table 1 contains a mapping from XXX
network addresses to INI for all XXX devices. Table 2 contains a
mapping from IP addresses to XXX network addresses for IP devices.
The xxx/coap host with IP address, IPs, sends a request to the
INI, IPd, of the specified XXX device with network address Xd.
The packet is routed to the coap gateway. The gateway strips the
link, network and coap information from the packet and sends the
message to the Xd specified in table 1 with the (Xd, IPd) pair
with as source Xs as specified in table 2 with the (Xs, IPs) pair.
When an answer is returned from Xd to Xs, the gateway creates a
new coap packet with the destination address, IPs, as found in
table 2 with the (IPs, Xs) pair and with source address IPd as
found in table 1 with (Dd, IPd) pair. The gateway sends the
answer on to the xxx/coap host, IPs.
It is assumed that the gateway conforms to the core standard at the
internet interfaces. Consequently, all resources are visible at
/.well-known/core by invoking a GET. These entries can be entered
into DNS-SD with a commissioning tools as proposed in section 5.3;
according to the rules specified in section 4. Filling in the
address mapping tables is done in a similar way as done for other
application level gateways (ALG).
5.3. Commissioning CoAP devices
Commissioning means mapping a physical device identified by its
location to an URI. Given that mapping of URI to IP address is done
elsewhere (e.g. DNS), the mapping of location to IP address is done
during commissioning with a commissioning tool. The commissiong is
done for a set of coap nodes interconnected by a wireless network.
The netwok can contain zero or more 6LR routers and zero or more
6LBR. The IP infrastructure (DNS, DHCP servers) is connected to the
coap network via the 6LBR. Two cases are considerd for
commissioning: (1) no 6LBR and no DNS server connected, and (2) a
6LBR connects 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 provide identifiers for all these devices. The triple
Instance.ServiceType.Location, as proposed in DNS-SD, is used to
describe the commssioning process. The identifiers of the devices
often reflect their action domain which is linked to their physical
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location. The authority can be equated to the Location identifier.
The Location uniquely identifies the device. The Instance is the
unique identifier given to the device in the factory but which has no
relation to its later function. The ServiceType together with the
Location fully determine the semantics of the data returned by the
device. Commissioning is the process of mapping the Location to
Instance for all installed hosts. The mapping is stored in a
discovery repository such that applications can communicate with the
required devices.
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) exists in a host after physical
installation of the devices:
- 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 Location's to which hosts need to
be assigned.
- A DHCP server (neigbor discovery) provided each host with a (link-
local) IP address.
Consider the commissioning process (1) with a central DNS-SD server
and (2) without a central server using mDNS.
5.3.1. DNS-SD server present
The names with the corresponding authority prefixes can be grouped by
the tool and appropriate group names can be assigned. The names
stored in the tool are inserted into the DNS-SD server, respecting
all DNS rules with respect to domain naming, and authority structure.
Assuming that the authority syntax corresponds with the structure of
the building, the installer can select on a screen the subset of
names belonging to the building part he wants to commission. The
installer reads with a bar code reader, attached to the tool, the
identifier 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
informs the DNS-SD server that the read barcode belongs to the
selected authority. This is done for all devices within a given part
of the building. Once this manual commissioning part is done the
nodes conclude the commissioning process:
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- Each node asks the DNS-SD server the authority belonging to its
barcode.
- Each node updates the authority in the DNS-SD server with its IP
address.
- Each node appends for each resource the path name to the authority
and inserts it into the DNS-SD server with its IP address.
After the commissioning process, all resources of each node have an
URI and IP address which are stored in the central DNS-SD server.
When nodes are restarted, the DHCP server allocates IP addresses to
the node and updates 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 link) such that each node on a
given segment can communicate directly with any other node on this
segment. The segments are not connected to a 6LBR and have no access
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 link-local authorities and stored in
the tool. All nodes are assumed to have selected a link-local IP
address. Assume that every device has a unique barcode within the
building and that the corresponding node knows the bar code number.
The installer reads with a bar code reader, attached to the tool, the
identifier 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 nodes.
The node with the given barcode number, extends the authority with
the path name of the resources. For each resource, the node
multicasts the link-local IP-address and the link-local URI to the
mDNS servers in the connected nodes. This concludes the
commissioning of a network segment. All resources of each node have
a link-local URI and a link-local IP address which are stored in the
mDNS servers.
6. 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
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
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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, using
conventional DNS, 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-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 url to distinguish between resources on a given node,
resulting in very compact identifiers.
DNS-SD [I-D.cheshire-dnsext-dns-sd] can be used to scale up service
discovery beyond the local link. 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 node, locate coap nodes by subtype,
or bind service names for particular coap URIs.
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. This necessitates the definition of an additional IANA mime
code, and the mapping of legacy specific discovery semantics to CoAP
resource discovery messages or DNS-SD lookups.
7. 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 nodes. The CoRE security analysis
must be broadened to include multicast scenarios.
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8. 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.
9. 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.
10. 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.
11. References
11.1. 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.
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[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.
[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.
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11.2. 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-04 (work in progress), January 2011.
[I-D.ietf-core-link-format]
Shelby, Z., "CoRE Link Format",
draft-ietf-core-link-format-02 (work in progress),
December 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.
[I-D.rahman-core-groupcomm]
Rahman, A., "Group Communication for CoAP",
draft-rahman-core-groupcomm-04 (work in progress),
March 2011.
[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.
[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/
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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.
[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.
Authors' Addresses
Peter van der Stok
Philips Research
High Tech Campus
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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