Applicability Statement: The use of the RPL protocol suite in Home Automation and Building Control
draft-ietf-roll-applicability-home-building-04
The information below is for an old version of the document.
| Document | Type |
This is an older version of an Internet-Draft that was ultimately published as RFC 7733.
|
|
|---|---|---|---|
| Authors | Anders Brandt , Emmanuel Baccelli , Robert Cragie , Peter Van der Stok | ||
| Last updated | 2014-06-30 | ||
| Replaces | draft-brandt-roll-rpl-applicability-home-building | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews | |||
| Additional resources | Mailing list discussion | ||
| Stream | WG state | WG Document | |
| Document shepherd | Yvonne-Anne Pignolet | ||
| IESG | IESG state | Became RFC 7733 (Proposed Standard) | |
| Consensus boilerplate | Unknown | ||
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-ietf-roll-applicability-home-building-04
Roll A. Brandt
Internet-Draft Sigma Designs
Intended status: Informational E. Baccelli
Expires: January 1, 2015 INRIA
R. Cragie
Gridmerge
P. van der Stok
Consultant
June 30, 2014
Applicability Statement: The use of the RPL protocol suite in Home
Automation and Building Control
draft-ietf-roll-applicability-home-building-04
Abstract
The purpose of this document is to provide guidance in the selection
and use of protocols from the RPL protocol suite to implement the
features required for control in building and home environments.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at 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 1, 2015.
Copyright Notice
Copyright (c) 2014 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
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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. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Relationship to other documents . . . . . . . . . . . . . 4
1.2. Requirements language . . . . . . . . . . . . . . . . . . 4
1.3. Terminology . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Required Reading . . . . . . . . . . . . . . . . . . . . 4
1.5. Out of scope requirements . . . . . . . . . . . . . . . . 4
2. Deployment Scenario . . . . . . . . . . . . . . . . . . . . . 5
2.1. Network Topologies . . . . . . . . . . . . . . . . . . . 6
2.2. Traffic Characteristics . . . . . . . . . . . . . . . . . 7
2.2.1. General . . . . . . . . . . . . . . . . . . . . . . . 8
2.2.2. Source-sink (SS) communication paradigm . . . . . . . 8
2.2.3. Publish-subscribe (PS, or pub/sub)) communication
paradigm . . . . . . . . . . . . . . . . . . . . . . 8
2.2.4. Peer-to-peer (P2P) communication paradigm . . . . . . 9
2.2.5. Peer-to-multipeer (P2MP) communication paradigm . . . 9
2.2.6. Additional considerations: Duocast and N-cast . . . . 9
2.2.7. RPL applicability per communication paradigm . . . . 10
2.3. Layer-2 applicability . . . . . . . . . . . . . . . . . . 11
3. Using RPL to meet Functional Requirements . . . . . . . . . . 11
4. RPL Profile . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.1. RPL Features . . . . . . . . . . . . . . . . . . . . . . 12
4.1.1. RPL Instances . . . . . . . . . . . . . . . . . . . . 13
4.1.2. Storing vs. Non-Storing Mode . . . . . . . . . . . . 13
4.1.3. DAO Policy . . . . . . . . . . . . . . . . . . . . . 13
4.1.4. Path Metrics . . . . . . . . . . . . . . . . . . . . 13
4.1.5. Objective Function . . . . . . . . . . . . . . . . . 13
4.1.6. DODAG Repair . . . . . . . . . . . . . . . . . . . . 13
4.1.7. Multicast . . . . . . . . . . . . . . . . . . . . . . 14
4.1.8. Security . . . . . . . . . . . . . . . . . . . . . . 15
4.1.9. P2P communications . . . . . . . . . . . . . . . . . 15
4.1.10. IPv6 address configuration . . . . . . . . . . . . . 15
4.2. Layer 2 features . . . . . . . . . . . . . . . . . . . . 15
4.2.1. Specifics about layer-2 . . . . . . . . . . . . . . . 15
4.2.2. Services provided at layer-2 . . . . . . . . . . . . 15
4.2.3. 6LowPAN options assumed . . . . . . . . . . . . . . . 15
4.2.4. MLE and other things . . . . . . . . . . . . . . . . 15
4.3. Recommended Configuration Defaults and Ranges . . . . . . 16
4.3.1. Trickle parameters . . . . . . . . . . . . . . . . . 16
4.3.2. Other Parameters . . . . . . . . . . . . . . . . . . 16
5. MPL Profile . . . . . . . . . . . . . . . . . . . . . . . . . 16
5.1. Recommended configuration Defaults and Ranges . . . . . . 17
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5.1.1. Trickle parameters . . . . . . . . . . . . . . . . . 17
5.1.2. Other parameters . . . . . . . . . . . . . . . . . . 18
6. Manageability Considerations . . . . . . . . . . . . . . . . 18
7. Security Considerations . . . . . . . . . . . . . . . . . . . 18
7.1. Security considerations during initial deployment . . . . 18
7.2. Security Considerations during incremental deployment . . 19
7.3. Security Considerations for P2P uses . . . . . . . . . . 19
7.4. MPL routing . . . . . . . . . . . . . . . . . . . . . . . 20
7.5. RPL Security features . . . . . . . . . . . . . . . . . . 20
8. Other related protocols . . . . . . . . . . . . . . . . . . . 20
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 21
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 21
11. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 21
12. References . . . . . . . . . . . . . . . . . . . . . . . . . 22
12.1. Normative References . . . . . . . . . . . . . . . . . . 22
12.2. Informative References . . . . . . . . . . . . . . . . . 25
Appendix A. RPL shortcomings in home and building deployments . 26
A.1. Risk of undesired long P2P routes . . . . . . . . . . . . 26
A.1.1. Traffic concentration at the root . . . . . . . . . . 26
A.1.2. Excessive battery consumption in source nodes . . . . 26
A.2. Risk of delayed route repair . . . . . . . . . . . . . . 26
A.2.1. Broken service . . . . . . . . . . . . . . . . . . . 27
Appendix B. Communication failures . . . . . . . . . . . . . . . 27
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 28
1. Introduction
Home automation and building control applications share a substantial
number of properties.
o Both (home and building) can be disconnected from the ISP and they
will (must) continue to provide control to the occupants of the
home c.q. building. This has an impact on routing because most
control communication does (must) not pass via the border routers.
o Both are confronted with unreliable links and want instant and
very reliable reactions. This has impact on routing because of
timeliness and multipath routing.
o The difference between the two mostly appears in the
commissioning, maintenance and user interface which does not
affect the routing.
So the focus of this applicability document is control in buildings
and home, involving: reliability, timeliness, and local routing.
The purpose of this document is to give guidance in the use of the
RPL protocol suite to provide the features required by the
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requirements documents "Home Automation Routing Requirements in Low-
Power and Lossy Networks" [RFC5826] and "Building Automation Routing
Requirements in Low-Power and Lossy Networks" [RFC5867] [RFC6997].
1.1. Relationship to other documents
The ROLL working group has specified a set of routing protocols for
Lossy and Low- resource Networks (LLN) [RFC6550]. This applicability
text describes a subset of these protocols and the conditions which
make the subset the correct choice. The text recommends and
motivates the accompanying parameter value ranges. Multiple
applicability domains are recognized including: Building and Home,
and Advanced Metering Infrastructure. The applicability domains
distinguish themselves in the way they are operated, their
performance requirements, and the most probable network structures.
Each applicability statement identifies the distinguishing properties
according to a common set of subjects described in as many sections.
A common set of security threats are described in
[I-D.ietf-roll-security-threats]. The applicability statements
complement the security threats document by describing preferred
security settings and solutions within the applicability statement
conditions. This applicability statement may recommend more light
weight security solutions and specify the conditions under which
these solutions are appropriate.
1.2. Requirements language
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 [RFC2119].
1.3. Terminology
This document uses terminology from [RFC6997],
[I-D.ietf-roll-trickle-mcast], [I-D.ietf-roll-terminology],
[IEEE802.15.4], and [RFC6550].
1.4. Required Reading
Applicable requirements are described in [RFC5826] and [RFC5867]. A
survey of the application field is described in [BCsurvey].
1.5. Out of scope requirements
The considered network diameter is limited to a max diameter of 10
hops and a typical diameter of 5 hops, which captures the most common
cases in home automation and building control networks.
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This document does not consider the applicability of RPL-related
specifications for urban and industrial applications [RFC5548],
[RFC5673], which may exhibit significantly larger network diameters.
2. Deployment Scenario
The use of communications networks in buildings is essential to
satisfy the energy saving regulations. Environmental conditions of
buildings can be adapted to suit the comfort of the individuals
present. Consequently when no one is present, energy consumption can
be reduced. Cost is the main driving factor behind utilizing
wireless networking in buildings. Especially for retrofit, wireless
connectivity saves cabling costs.
A typical home automation network is comprised of less than 100
nodes. Large building deployments may span 10,000 nodes but to
ensure uninterrupted service of light and air conditioning systems in
individual zones of the building, nodes are typically organized in
sub-networks. Each sub-network in a building automation deployment
typically contains tens to hundreds of nodes.
The main purpose of the home or building automation network is to
provide control over light and heating/cooling resources. User
intervention may be enabled via wall controllers combined with
movement, light and temperature sensors to enable automatic
adjustment of window blinds, reduction of room temperature, etc. In
general, the sensors and actuators in a home or building typically
have fixed physical locations and will remain in the same home- or
building automation network.
People expect an immediate and reliable response to their presence or
actions. A light not switching on after entry into a room may lead
to confusion and a profound dissatisfaction with the lighting
product.
Monitoring of functional correctness is at least as important.
Devices typically communicate their status regularly and send alarm
messages notifying a malfunction of equipment or network.
In building control, the infrastructure of the building management
network can be shared with the security/access, the IP telephony, and
the fire/alarm networks. This approach has a positive impact on the
operation and cost of the network; however, care should be taken to
ensure that the availability of the building management network does
not become compromised beyond the ability for critical functions to
perform adequately.
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In homes, the network for audio/video streaming and gaming has
different requirements, where the most important one is the high need
in bandwidth for entertainment not needed for control. It is
expected that the entertainment network in the home will mostly be
separate from the control network, which also lessens the impact on
availability of the control network
2.1. Network Topologies
In general, the home automation network or building control network
consists of wired and wireless sub-networks. In large buildings
especially, the wireless sub-networks can be connected to an IP
backbone network where all infrastructure services are located, such
as DNS, automation servers, etc.
The wireless sub-network can be configured according to any of the
following topologies:
o A stand-alone network of 10-100 nodes without border router. This
typically occurs in the home with a stand-alone control network,
in low cost buildings, and during installation of high end control
systems in buildings.
o A connected network with one border router. This configuration
will happen in homes where home appliances are controlled from
outside the home, possibly via a smart phone, and in many building
control scenarios.
o A connected network with multiple border routers. This will
typically happen in installations of large buildings.
Many of the nodes are battery-powered and may be sleeping nodes which
wake up according to clock signals or external events.
In a building control network, for large installation with multiple
border routers, sub-networks often overlap both geographically and
from a wireless coverage perspective. Due to two purposes of the
network, (i) direct control and (ii) monitoring, there may exist two
types of routing topologies in a given sub-network: (i) a tree-shaped
collection of routes spanning from a central building controller via
the border router, on to destination nodes in the sub-network; and/or
(ii) a flat, un-directed collection of intra-network routes between
functionally related nodes in the sub-network.
The majority of nodes in home and building automation networks are
typically devices with very low memory capacity, such as individual
wall switches. Only a few nodes (such as multi-purpose remote
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controls) are more expensive devices, which can afford more memory
capacity.
2.2. Traffic Characteristics
Traffic may enter the network originating from a central controller
or it may originate from an intra-network node. The majority of
traffic is light-weight point-to-point control style; e.g. Put-Ack
or Get-Response. There are however exceptions. Bulk data transfer
is used for firmware update and logging, where firmware updates enter
the network and logs leave the network. Group communication is used
for service discovery or to control groups of nodes, such as light
fixtures.
Often, there is a direct physical relation between a controlling
sensor and the controlled equipment. For example the temperature
sensor and thermostat are located in the same room sharing the same
climate conditions. Consequently, the bulk of senders and receivers
are separated by a distance that allows one-hop direct path
communication. A graph of the communication will show several fully
connected subsets of nodes. However, due to interference, multipath
fading, reflection and other transmission mechanisms, the one-hop
direct path may be temporally disconnected. For reliability
purposes, it is therefore essential that alternative n-hop
communication routes exist for quick error recovery. (See Appendix B
for motivation.)
Looking over time periods of a day, the networks are very lightly
loaded. However, bursts of traffic can be generated by pushing
permanently the button of a remote control, the occurrence of a
defect, and other unforeseen events. Under those conditions, the
timeliness must nevertheless be maintained. Therefore, measures are
necessary to remove any unnecessary traffic. Short routes are
preferred. Long multi-hop routes via the border router, should be
avoided whenever possible.
Group communication is essential for lighting control. For example,
once the presence of a person is detected in a given room, lighting
control applies to that room only and no other lights should be
dimmed, or switched on/off. In many cases, this means that a
multicast message with a 1-hop and 2-hop radius would suffice to
control the required lights. The same argument holds for HVAC and
other climate control devices. To reduce network load, it is
advisable that messages to the lights in a room are not distributed
any further in the mesh than necessary based on intended receivers.
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An example of an office surface is shown in [office-light], and the
current use of wireless lighting control products is shown in
[occuswitch].
2.2.1. General
Whilst air conditioning and other environmental-control applications
may accept response delays of tens of seconds or longer, alarm and
light control applications may be regarded as soft real-time systems.
A slight delay is acceptable, but the perceived quality of service
degrades significantly if response times exceed 250 msec. If the
light does not turn on at short notice, a user may activate the
controls again, thus causing a sequence of commands such as
Light{on,off,on,off,..} or Volume{up,up,up,up,up,...}. In addition
the repetitive sending of commands creates an unnecessary loading of
the network, which in turn increases the bad responsiveness of the
network.
2.2.2. Source-sink (SS) communication paradigm
This paradigm translates to many sources sending messages to the same
sink, sometimes reachable via the border router. As such, source-
sink (SS) traffic can be present in home and building networks. The
traffic may be generated by environmental sensors (often present in a
wireless sub-network) which push periodic readings to a central
server. The readings may be used for pure logging, or more often,
processed to adjust light, heating and ventilation. Alarm sensors
may also generate SS style traffic. The central server in a home
automation network will be connected mostly to a wired network
segment of the home network, although it is suspected that cloud
services will become available. The central server in a building
automation network may be connected to a backbone or be placed
outside the building.
With regards to message latency, most SS transmissions can tolerate
worst-case delays measured in tens of seconds. Alarm sensors,
however, represent an exception. Special provisions with respect to
the location of the Alarm server(s) need to be put in place to
respect the specified delays.
2.2.3. Publish-subscribe (PS, or pub/sub)) communication paradigm
This paradigm translates to a number of devices expressing their
interest for a service provided by a server device. For example, a
server device can be a sensor delivering temperature readings on the
basis of delivery criteria, like changes in acquisition value or age
of the latest acquisition. In building automation networks, this
paradigm may be closely related to the SS paradigm given that
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servers, which are connected to the backbone or outside the building,
can subscribe to data collectors that are present at strategic places
in the building automation network. The use of PS will probably
differ significantly from installation to installation.
2.2.4. Peer-to-peer (P2P) communication paradigm
This paradigm translates to a device transferring data to another
device often connected to the same sub-network. Peer-to-peer (P2P)
traffic is a common traffic type in home automation networks. Most
building automation networks rely on P2P traffic, described in the
next paragraph. Other building automation networks rely on P2P
control traffic between controls and a local controller box for
advanced scene and group control. The latter controller boxes can be
connected to service control boxes thus generating more SS or PS
traffic.
P2P traffic is typically generated by remote controls and wall
controllers which push control messages directly to light or heat
sources. P2P traffic has a strong requirement for low latency since
P2P traffic often carries application messages that are invoked by
humans. As mentioned in Section 2.2.1 application messages should be
delivered within a few hundred milliseconds - even when connections
fail momentarily.
2.2.5. Peer-to-multipeer (P2MP) communication paradigm
This paradigm translates to a device sending a message as many times
as there are destination devices. Peer-to-multipeer (P2MP) traffic
is common in home and building automation networks. Often, a
thermostat in a living room responds to temperature changes by
sending temperature acquisitions to several fans and valves
consecutively.
2.2.6. Additional considerations: Duocast and N-cast
This paradigm translates to a device sending a message to many
destinations in one network transfer invocation. Multicast is well
suited for lighting where a presence sensor sends a presence message
to a set of lighting devices. Multicast increases the probability
that the message is delivered within the strict time constraints.
The recommended multicast algorithm (e.g.
[I-D.ietf-roll-trickle-mcast]) assures that messages are delivered to
ALL intended destinations.
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2.2.7. RPL applicability per communication paradigm
In the case of SS over a wireless sub-network to a server reachable
via a border router, the use of RPL [RFC6550] is recommended. Given
the low resources of the devices, source routing will be used for
messages from outside the wireless sub-network to the destination in
the wireless sub-network. No specific timing constraints are
associated with the SS type messages so network repair does not
violate the operational constraints. When no SS traffic takes place,
it is recommended to load only RPL code enabling P2P mode of
operation [RFC6997] to satisfy memory requirements by reducing the
code size.
P2P-RPL [RFC6997] is recommended for all P2P and P2MP traffic, taking
place within a wireless sub-network, to assure responsiveness.
Source and destination are typically close together to satisfy the
living conditions of one room. Consequently, most P2P and P2MP
traffic is 1-hop or 2-hop traffic. Appendix A explains why RPL-P2P
is preferable to RPL for this type of communication. Appendix B
explains why reliability measures such as multi-path routing are
necessary even when 1-hop communication dominates.
Additional advantages of RPL-P2P for home and building automation
networks are, for example:
o Individual wall switches are typically inexpensive devices with
extremely low memory capacities. Multi-purpose remote controls
for use in a home environment typically have more memory but such
devices are asleep when there is no user activity. RPL-P2P
reactive discovery allows a node to wake up and find new routes
within a few seconds while memory constrained nodes only have to
keep routes to relevant targets.
o The reactive discovery features of RPL-P2P ensure that commands
are normally delivered within the 250 msec time window and when
connectivity needs to be restored, it is typically completed
within seconds. In most cases an alternative (earlier discovered)
route will work. Thus, route rediscovery is not even necessary.
o Broadcast storms as happening during route discovery for AODV is
less disruptive for P2P-RPL. P2P-RPL has a "STOP" bit which is
set by the target of a route discovery to notify all other nodes
that no more DIOs should be forwarded for this temporary DAG.
Something looking like a broadcast storm may happen when no target
is responding. And in this case, the Trickle suppression
mechanism kicks in; limiting the number of DIO forwards in dense
networks.
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Due to the limited memory of the majority of devices, RPL-P2P SHOULD
be used with source routing in non-storing mode as explained in
Section 4.1.2.
Multicast with MPL [I-D.ietf-roll-trickle-mcast] is recommended for
N-cast over the wireless network. Configuration constraints that are
necessary to meet reliability and timeliness with MPL are discussed
in Section 4.1.7.
2.3. Layer-2 applicability
This document applies to [IEEE802.15.4] and [G.9959] which are
adapted to IPv6 by the adaption layers [RFC4944] and
[I-D.ietf-6lo-lowpanz].
The above mentioned adaptation layers leverage on the compression
capabilities of [RFC6554] and [RFC6282]. Header compression allows
small IP packets to fit into a single layer 2 frame even when source
routing is used. A network diameter limited to 5 hops helps to
achieve this.
Dropped packets are often experienced in the targeted environments.
ICMP, UDP and even TCP flows may benefit from link layer unicast
acknowledgments and retransmissions. Link layer unicast
acknowledgments SHOULD be enabled when [IEEE802.15.4] or [G.9959] is
used with RPL and RPL-P2P.
3. Using RPL to meet Functional Requirements
RPL-P2P SHOULD be present in home automation and building control
networks, as point-to-point style traffic is substantial and route
repair needs to be completed within seconds. RPL-P2P provides a
reactive mechanism for quick, efficient and root-independent route
discovery/repair. The use of RPL-P2P furthermore allows data traffic
to avoid having to go through a central region around the root of the
tree, and drastically reduces path length [SOFT11] [INTEROP12].
These characteristics are desirable in home and building automation
networks because they substantially decrease unnecessary network
congestion around the root of the tree.
When reliability is required, RPL-P2P enables the establishment of
multiple independent paths. For 1-hop destinations this means that
one 1-hop communication and a second 2-hop communication take place
via a neighbouring node. The same two communication paths can be
achieved by using MPL where the source is a MPL forwarder and a
second MPL forwarder is 1 hop removed from the source and the
destination node. The source multicasts the message, which may be
received by both the destination and the 2nd forwarder. The 2nd
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forwarder forwards the message to the destination, thus providing two
routes from sender to destination.
To provide reliability with multiple paths, RPL-P2P is recommended to
keep two independent P2P paths per destination in the source. When
one P2P path is temporarily impossible, as described in Appendix B,
the alternative P2P path can be used without throwing away the
temporarily failing path. The failing P2P path can be safely thrown
away after 15 minutes. A new route discovery is done when the number
of P2P paths is exhausted, or when a P2P path needs to abandoned
because it fails over a too long period.
4. RPL Profile
RPL-P2P SHOULD be used in home automation and building control
networks. Its reactive discovery allows for low application response
times even when on-the-fly route repair is needed. Non-storing mode
SHOULD be used to reduce memory consumption in repeaters with
constrained memory when source routing is used.
4.1. RPL Features
An important constraint on the application of RPL is the presence of
sleeping nodes.
For example in the stand-alone network, the link layer node (master
node, or coordinator) handing out the logical network identifier and
unique node identifiers may be a remote control which returns to
sleep once new nodes have been added. Due to the absence of the
border router there may be no global routable prefixes at all.
Likewise, there may be no authoritative always-on root node since
there is no border router to host this function.
In a network with a border router and many sleeping nodes, there may
be battery powered sensors and wall controllers configured to contact
other nodes in response to events and then return to sleep. Such
nodes may never detect the announcement of new prefixes via
multicast.
In each of the above mentioned constrained deployments, a link layer
node (e.g. coordinator or master) SHOULD assume the role as
authoritative root node, transmitting singlecast RAs with a ULA
prefix information option to nodes during the inclusion process to
prepare the nodes for a later operational phase, where a border
router is added.
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A border router SHOULD be designed to be aware of sleeping nodes in
order to support the distribution of updated global prefixes to such
sleeping nodes.
One MAY implement gateway-centric tree-based routing and global
prefix distribution as defined by [RFC6550]. This would however only
work for always-on nodes.
4.1.1. RPL Instances
When operating P2P-RPL on a stand-alone basis, there is no
authoritative root node maintaining a permanent RPL DODAG. A node
MUST be able to join one RPL instance as an instance is created
during each P2P-RPL route discovery operation. A node MAY be
designed to join multiple RPL instances.
4.1.2. Storing vs. Non-Storing Mode
Non-storing mode MUST be used to cope with the extremely constrained
memory of a majority of nodes in the network (such as individual
light switches).
4.1.3. DAO Policy
A node MAY be designed to join multiple RPL instances; in that case
DAO policies may be needed.
DAO policy is out of scope for this applicability statement.
4.1.4. Path Metrics
OF0 is RECOMMENDED. [RFC6551] provides other options. Using other
objective functions than OF0 may affect inter-operability.
4.1.5. Objective Function
OF0 MUST be supported and is the RECOMMENDED Objective Function to
use. Other Objective Functions MAY be used as well.
4.1.6. DODAG Repair
Since RPL-P2P only creates DODAGs on a temporary basis during route
repair, there is no need to repair DODAGs.
In general for the SS case, handling of time-varying link
characteristics and availability, local repair is sufficient. The
accompanying process is known as poisoning and is described in
Section 8.2.2.5 of [RFC6550]. Given that the plurality of nodes in
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the building does not move around, creating new DODAGs will not
happen frequently.
4.1.7. Multicast
Commercial light deployments may have a need for multicast to
distribute commands to a group of lights in a timely fashion.
Several mechanisms exist for achieving such functionality;
[I-D.ietf-roll-trickle-mcast] is RECOMMENDED for home and building
deployments. This section relies heavily on the conclusions of
[RT-MPL].
The density of forwarders and the frequency of message generation are
important aspects to obtain timeliness during control operations. A
high frequency of message generation can be expected when a remote
control button is constantly pressed, or when alarm situations arise.
In [RT-MPL] it is shown that short circuiting the buffering and
retries in the IEEE 802.15.4 MAC reduces packet delays. Message loss
is reduced by adding a real-time packet selection procedure before
submitting a packet to the MAC.
Guaranteeing timeliness is intimately related to the density of the
MPL routers. In ideal circumstances the message is propagated as a
single wave through the network, such that the maximum delay is
related to the number of hops times the smallest repetition interval
of MPL. Each forwarder that receives the message, passes the message
on to the next hop by repeating the message. When several copies of
a message reach the forwarder, it is specified that the copy need not
be repeated. Repetition of the message can be inhibited by a small
value of k. To assure timeliness, the value of k should be chosen
high enough to make sure that messages are repeated at the first
arrival of the message in the forwarder. However, a network that is
too dense leads to a saturation of the medium that can only be
prevented by selecting a low value of k. Consequently, timeliness is
assured by choosing a relatively high value of k but assuring at the
same time a low enough density of forwarders to reduce the risk of
medium saturation. Depending on the reliability of the network
channels, it is advisable to choose the network such that at least 2
forwarders per hop repeat messages to the same set of destinations.
There are no rules about selecting forwarders for MPL. In buildings
with central management tools, the forwarders can be selected, but in
the home is not possible to automatically configure the forwarder
topology at this moment.
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4.1.8. Security
In order to support low-cost devices and devices running on battery,
RPL MAY use either unsecured messages or secured messages. If RPL is
used with unsecured messages, link layer security SHOULD be used (see
Section 7.1). If RPL is used with secured messages, the following
RPL security parameter values SHOULD be used:
o T = '0': Do not use timestamp in the Counter Field.
o Algorithm = '0': Use CCM with AES-128
o KIM = '10': Use group key, Key Source present, Key Index present
o LVL = 0: Use MAC-32
4.1.9. P2P communications
[RFC6997] MUST be used to accommodate P2P traffic, which is typically
substantial in home and building automation networks.
4.1.10. IPv6 address configuration
Assigned IP addresses MUST be routable and unique within the routing
domain.
4.2. Layer 2 features
No particular requirements exist for layer 2 but for the ones cited
in the IP over Foo RFCs. (See Section 2.3)
4.2.1. Specifics about layer-2
Not applicable
4.2.2. Services provided at layer-2
Not applicable
4.2.3. 6LowPAN options assumed
Not applicable
4.2.4. MLE and other things
Not applicable
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4.3. Recommended Configuration Defaults and Ranges
The following sections describe the recommended parameter values for
RPL-P2P and Trickle.
4.3.1. Trickle parameters
Trickle is used to distribute network parameter values to all nodes
without stringent time restrictions. Trickle parameter values are:
o DIOIntervalMin 4 = 16 ms
o DIOIntervalDoublings 14
o DIORedundancyConstant 1
4.3.2. Other Parameters
This section discusses the RPL-P2P parameters.
RPL-P2P [RFC6997] provides the features requested by [RFC5826] and
[RFC5867]. RPL-P2P uses a subset of the frame formats and features
defined for RPL [RFC6550] but may be combined with RPL frame flows in
advanced deployments.
Parameter values for RPL-P2P are:
o MinHopRankIncrease 1
o MaxRankIncrease 0
o MaxRank 6
o Objective function: OF0
5. MPL Profile
MPL is used to distribute values to groups of devices. In MPL, based
on Trickle algorithm, also timeliness should be guaranteed. A
deadline of 200 ms needs to be met when human action is followed by
an immediately observable action such as switching on lights. The
deadline needs to be met in a building where the number of hops from
seed to destination varies between 1 and 10.
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5.1. Recommended configuration Defaults and Ranges
In [RT-MPL] the large contribution of MAC delays is explained when
considering MPL intervals between 10 to 100 ms to meet the 200 ms
deadline. It is recommended to set the number of buffers in the MAC
to 1 and not to repeat a failed transmission after a MAC back-off
interval. MPL already repeats the transmission in a controlled
fashion and the MAC should not add to these repetitions.
When the load on the wireless medium is high, [RT-MPL] recommends to
add a real-time layer between MPL and MAC to throw away too late
messages and favour the most recent ones.
5.1.1. Trickle parameters
This section proposes values for the Trickle parameters used by MPL
for the distribution of packets that need to meet a 200 ms deadline.
The probability of meeting the deadline is increased by (1) choosing
a small Imin value,(2) reducing the number of MPL intervals thus
reducing the load, and (3) reducing the number of MPL forwarders to
also reduce the load.
The consequence of this approach is that the value of k can be larger
than 1 because network load reduction is already guaranteed by the
network configuration.
Under the condition that the density of MPL repeaters can be limited,
it is possible to choose low MPL repeat intervals (Imin) connected to
k values such that k>1. The minimum value of k is related to:
o Value of Imin. The length of Imin determines the number of
packets that can be received within the listening period of Imin.
o Number of repeaters receiving the broadcast message from the same
forwarder or seed. These repeaters repeat within the same Imin
interval, thus increasing the c counter.
Within the first MPL interval a limited number, q, of messages can be
transmitted. Assuming a 3 ms transmission interval, q is given by q
= Imin/3. Assuming that at most q message copies can reach a given
forwarder within the first repeat interval of length Imin, the
related MPL parameter values are suggested in the following sections.
5.1.1.1. Imin
Imin = 10 - 50 ms.
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When Imin is chosen much smaller, the interference between the copies
leads to significant losses given that q is much smaller than the
number of repeated packets. With much larger intervals the
probability that the deadline will be met decreases with increasing
hop count.
5.1.1.2. Imax
Imax = 100 - 400 ms.
The value of Imax is less important than the value of max_expiration.
Given an Imin value of 10 ms, the 3rd MPL interval has a value of
10*2*2 = 40 ms. When Imin has a value of 40 ms, the 3rd interval has
a value of 160 ms. Given that more than 3 intervals are unnecessary,
the Imax does not contribute much to the performance.
5.1.2. Other parameters
Other parameters are the k parameter and the max_expiration
parameter.
k > q (see condition above). Under this condition and for small
Imin, a value of k=2 or k=3 is usually sufficient to minimize the
losses of packets in the first repeat interval.
max_expiration = 2 - 4. Higher values lead to more network load
while generating copies which will probably not meet their deadline.
6. Manageability Considerations
Manageability is out of scope for home network scenarios. In
building automation scenarios, central control could be applied based
on MIBs.
7. Security Considerations
Refer to the security considerations of [RFC6997], [RFC6550],
[I-D.ietf-roll-trickle-mcast], and the counter measures discussed in
sections 6 and 7 of [I-D.ietf-roll-security-threats].
7.1. Security considerations during initial deployment
At initial deployment the network is incrementally increased and
secured at the link layer. Wireless mesh networks are typically
secured at the link-layer in order to prevent unauthorized parties
from accessing the information exchanged over the links. It is good
practice to create a network of nodes which share the same keys for
link layer encryption and exclude nodes sending non encrypted
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messages. Together with authentication of the sources, it is
possible to prevent unauthorized nodes joining the mesh. This is
ensured with the Protocol for carrying Authentication for Network
Access (PANA) Relay Element [RFC6345] with the use of PANA [RFC5191]
for network access. A new DTLS based protocol is proposed in
[I-D.kumar-dice-dtls-relay].
This recommendation is in line with the couter measures described in
section 6.1.1 of [I-D.ietf-roll-security-threats]
Unauthorized nodes can access the nodes of the mesh via a router.
End-to-end security between applications is recommended by using DTLS
[RFC6347] or TLS [RFC5246].
7.2. Security Considerations during incremental deployment
Communications network security is based on providing integrity
protection and encryption to messages. This can be applied at
various layers in the network protocol stack based on using various
credentials and a network identity.
The credentials which are relevant in the case of RPL are: (i) the
credential used at the link layer in the case where link layer
security is applied (see Section 7.1) or (ii) the credential used for
securing RPL messages. In both cases, the assumption is that the
credential is a shared key. Therefore, there MUST be a mechanism in
place which allows secure distribution of a shared key and
configuration of network identity. Both MAY be done using (i) pre-
installation using an out-of-band method, (ii) delivered securely
when a device is introduced into the network or (iii) delivered
securely by a trusted neighbouring device. The shared key MUST be
stored in a secure fashion which makes it difficult to be read by an
unauthorized party.
Securely delivering a key means that the delivery mechanism MUST have
data origin authentication, confidentiality and integrity protection.
On reception of the delivered key, freshness of the delivered key
MUST be ensured. Securely storing a key means that the storage
mechanism MUST have confidentiality and integrity protection and MUST
only be accessible by an authorized party.
7.3. Security Considerations for P2P uses
Refer to the security considerations of [RFC6997]. Many initiatives
are under way to provide light weight security such as:
[I-D.ietf-dice-profile] and [I-D.keoh-dice-multicast-security].
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7.4. MPL routing
The routing of MPL is determined by the enabling of the interfaces
for specified Multicast addresses. The specification of these
addresses can be done via a CoAP application as specified in
[I-D.ietf-core-groupcomm]. An alternative is the creation of a MPL
MIB and use of SNMPv3 [RFC3411] or CoMI [I-D.vanderstok-core-comi] to
specify the Multicast addresses in the MIB. The application of
security measures for the specification of the multicast addresses
assures that the routing of MPL packets is secured.
7.5. RPL Security features
This section follows the structure of section 7, "RPL security
features" of [I-D.ietf-roll-security-threats], where a thorough
analysis of security threats and proposed counter measures relevant
to RPL and MPL are done.
In accordance with section 7.1 of [I-D.ietf-roll-security-threats],
"Confidentiality features", a secured RPL protocol must implement
payload protection, as explained in Section 7.1 of this document.
The attributes key-length and life-time of the keys depend on
operational conditions, maintenance and installation procedures.
Section 7.2 of this document recommends measures to assure integrity
in accordance with section 7.2 of [I-D.ietf-roll-security-threats],
"Integrity features".
The provision of multiple paths recommended in section 7.3
"Availability features" of [I-D.ietf-roll-security-threats] is also
recommended from a reliability point of view. Randomly choosing
paths MAY be supported.
Key management discussed in section 7.4, "Key Management" of
[I-D.ietf-roll-security-threats], is not standardized and discussions
continue.
Section 7.5, "Considerations on Matching Application Domain Needs" of
[I-D.ietf-roll-security-threats] applies as such.
8. Other related protocols
Application transport protocols may be CoAP over UDP or equivalents.
Typically, UDP is used for IP transport to keep down the application
response time and bandwidth overhead.
Several features required by [RFC5826], [RFC5867] challenge the P2P
paths provided by RPL. Appendix A reviews these challenges. In some
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cases, a node may need to spontaneously initiate the discovery of a
path towards a desired destination that is neither the root of a DAG,
nor a destination originating DAO signalling. Furthermore, P2P paths
provided by RPL are not satisfactory in all cases because they
involve too many intermediate nodes before reaching the destination.
9. IANA Considerations
No considerations for IANA pertain to this document.
10. Acknowledgements
This document reflects discussions and remarks from several
individuals including (in alphabetical order): Mukul Goyal, Sandeep
Kumar, Jerry Martocci, Charles Perkins, Michael Richardson, and Zach
Shelby
11. Changelog
Changes from version 0 to version 1.
o Adapted section structure to template.
o Standardized the reference syntax.
o Section 2.2, moved everything concerning algorithms to section
2.2.7, and adapted text in 2.2.1-2.2.6.
o Added MPL parameter text to section 4.1.7 and section 4.3.1.
o Replaced all TODO sections with text.
o Consistent use of border router, monitoring, home- and building
network.
o Reformulated security aspects with references to other
publications.
o MPL and RPL parameter values introduced.
Changes from version 1 to version 2.
o Clarified common characteristics of control in home and building.
o Clarified failure behaviour of point to point communication in
appendix.
o Changed examples, more hvac and less lighting.
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o Clarified network topologies.
o replaced reference to smart_object paper by reference to I-D.roll-
security-threats
o Added a concise definition of secure delivery and secure storage
o text about securing network with PANA
Changes from version 2 to version 3.
o Changed security section to follow the structure of security
threats draft.
o Added text to DODAG repair sub-section
Changes from version 3 to version 4.
o Renumbered sections and moved text to conform to applicability
template
o Extended MPL parameter value text
o Added references to building control products
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.
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
[RFC5191] Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
Yegin, "Protocol for Carrying Authentication for Network
Access (PANA)", RFC 5191, May 2008.
[RFC5246] Dierks, T. and E. Rescorla, "The Transport Layer Security
(TLS) Protocol Version 1.2", RFC 5246, August 2008.
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[RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel,
"Routing Requirements for Urban Low-Power and Lossy
Networks", RFC 5548, May 2009.
[RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney,
"Industrial Routing Requirements in Low-Power and Lossy
Networks", RFC 5673, October 2009.
[RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation
Routing Requirements in Low-Power and Lossy Networks", RFC
5826, April 2010.
[RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen,
"Building Automation Routing Requirements in Low-Power and
Lossy Networks", RFC 5867, June 2010.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
September 2011.
[RFC6345] Duffy, P., Chakrabarti, S., Cragie, R., Ohba, Y., and A.
Yegin, "Protocol for Carrying Authentication for Network
Access (PANA) Relay Element", RFC 6345, August 2011.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, January 2012.
[RFC6550] Winter, T., Thubert, P., Brandt, A., Hui, J., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Vasseur, JP., and R.
Alexander, "RPL: IPv6 Routing Protocol for Low-Power and
Lossy Networks", RFC 6550, March 2012.
[RFC6551] Vasseur, JP., Kim, M., Pister, K., Dejean, N., and D.
Barthel, "Routing Metrics Used for Path Calculation in
Low-Power and Lossy Networks", RFC 6551, March 2012.
[RFC6554] Hui, J., Vasseur, JP., Culler, D., and V. Manral, "An IPv6
Routing Header for Source Routes with the Routing Protocol
for Low-Power and Lossy Networks (RPL)", RFC 6554, March
2012.
[RFC6997] Goyal, M., Baccelli, E., Philipp, M., Brandt, A., and J.
Martocci, "Reactive Discovery of Point-to-Point Routes in
Low-Power and Lossy Networks", RFC 6997, August 2013.
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[I-D.ietf-6lo-lowpanz]
Brandt, A. and J. Buron, "Transmission of IPv6 packets
over ITU-T G.9959 Networks", draft-ietf-6lo-lowpanz-05
(work in progress), May 2014.
[I-D.ietf-roll-trickle-mcast]
Hui, J. and R. Kelsey, "Multicast Protocol for Low power
and Lossy Networks (MPL)", draft-ietf-roll-trickle-
mcast-09 (work in progress), April 2014.
[I-D.ietf-roll-security-threats]
Tsao, T., Alexander, R., Dohler, M., Daza, V., Lozano, A.,
and M. Richardson, "A Security Threat Analysis for Routing
Protocol for Low-power and lossy networks (RPL)", draft-
ietf-roll-security-threats-07 (work in progress), June
2014.
[I-D.ietf-roll-terminology]
Vasseur, J., "Terms used in Routing for Low power And
Lossy Networks", draft-ietf-roll-terminology-13 (work in
progress), October 2013.
[I-D.ietf-dice-profile]
Hartke, K. and H. Tschofenig, "A DTLS 1.2 Profile for the
Internet of Things", draft-ietf-dice-profile-01 (work in
progress), May 2014.
[I-D.keoh-dice-multicast-security]
Keoh, S., Kumar, S., Garcia-Morchon, O., Dijk, E., and A.
Rahman, "DTLS-based Multicast Security in Constrained
Environments", draft-keoh-dice-multicast-security-07 (work
in progress), May 2014.
[I-D.kumar-dice-dtls-relay]
Kumar, S., Keoh, S., and O. Garcia-Morchon, "DTLS Relay
for Constrained Environments", draft-kumar-dice-dtls-
relay-01 (work in progress), April 2014.
[I-D.ietf-core-groupcomm]
Rahman, A. and E. Dijk, "Group Communication for CoAP",
draft-ietf-core-groupcomm-19 (work in progress), June
2014.
[I-D.vanderstok-core-comi]
Stok, P. and B. Greevenbosch, "CoAp Management
Interfaces", draft-vanderstok-core-comi-04 (work in
progress), May 2014.
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[IEEE802.15.4]
"IEEE 802.15.4 - Standard for Local and metropolitan area
networks -- Part 15.4: Low-Rate Wireless Personal Area
Networks", <IEEE Standard 802.15.4>.
[G.9959] "ITU-T G.9959 Short range narrow-band digital
radiocommunication transceivers - PHY and MAC layer
specifications", <ITU-T G.9959>.
[BCsurvey]
Kastner, W., Neugschwandtner, G., Soucek, S., and H.
Newman, "Communication Systems for Building Automation and
Control", Proceedings of the IEEE Vol 93, No 6, June 2005.
12.2. Informative References
[SOFT11] Baccelli, E., Phillip, M., and M. Goyal, "The P2P-RPL
Routing Protocol for IPv6 Sensor Networks: Testbed
Experiments", Proceedings of the Conference on Software
Telecommunications and Computer Networks, Split, Croatia,,
September 2011.
[INTEROP12]
Baccelli, E., Phillip, M., Brandt, A., Valev , H., and J.
Buron , "Report on P2P-RPL Interoperability Testing",
RR-7864 INRIA Research Report RR-7864, January 2012.
[RT-MPL] van der Stok, P., "Real-Time multicast for wireless mesh
networks using MPL", White paper,
http://www.vanderstok.org/papers/Real-time-MPL.pdf, April
2014.
[occuswitch]
Lighting, Philips., "OccuSwitch wireless", Brochure, http:
//www.philipslightingcontrols.com/assets/cms/uploads/files
/osw/MK_OSWNETBROC_5.pdf, May 2012.
[office-light]
Clanton and Associates, ., "A Life Cycle Cost Evaluation
of Multiple Lighting Control Strategies", Wireless
Lighting Control, http://www.daintree.net/wp-
content/uploads/2014/02/
clanton_lighting_control_report_0411.pdf, February 2014.
[RTN2011] Holtman, K. and P. van der Stok, "Real-time routing for
low-latency 802.15.4 control networks", International
Workshop on Real-Time Networks; Euromicro Conference on
Real-Time Systems, July 2011.
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[MEAS] Holtman, K., "Connectivity loss in large scale IEEE
802.15.4 network", Private Communication, November 2013.
Appendix A. RPL shortcomings in home and building deployments
A.1. Risk of undesired long P2P routes
The DAG, being a tree structure is formed from a root. If nodes
residing in different branches have a need for communicating
internally, DAG mechanisms provided in RPL [RFC6550] will propagate
traffic towards the root, potentially all the way to the root, and
down along another branch. In a typical example two nodes could
reach each other via just two router nodes but in unfortunate cases,
RPL may send traffic three hops up and three hops down again. This
leads to several undesired phenomena described in the following
sections
A.1.1. Traffic concentration at the root
If many P2P data flows have to move up towards the root to get down
again in another branch there is an increased risk of congestion the
nearer to the root of the DAG the data flows. Due to the broadcast
nature of RF systems any child node of the root is not just directing
RF power downwards its sub-tree but just as much upwards towards the
root; potentially jamming other MP2P traffic leaving the tree or
preventing the root of the DAG from sending P2MP traffic into the DAG
because the listen-before-talk link-layer protection kicks in.
A.1.2. Excessive battery consumption in source nodes
Battery-powered nodes originating P2P traffic depend on the route
length. Long routes cause source nodes to stay awake for longer
periods before returning to sleep. Thus, a longer route translates
proportionally (more or less) into higher battery consumption.
A.2. Risk of delayed route repair
The RPL DAG mechanism uses DIO and DAO messages to monitor the health
of the DAG. In rare occasions, changed radio conditions may render
routes unusable just after a destination node has returned a DAO
indicating that the destination is reachable. Given enough time, the
next Trickle timer-controlled DIO/DAO update will eventually repair
the broken routes, however this may not occur in a timely manner
appropriate to the application. In an apparently stable DAG,
Trickle-timer dynamics may reduce the update rate to a few times
every hour. If a user issues an actuator command, e.g. light on in
the time interval between the last DAO message was issued the
destination module and the time one of the parents sends the next
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DIO, the destination cannot be reached. There is no mechanism in RPL
to initiate restoration of connectivity in a reactive fashion. The
consequence is a broken service in home and building applications.
A.2.1. Broken service
Experience from the telecom industry shows that if the voice delay
exceeds 250ms, users start getting confused, frustrated and/or
annoyed. In the same way, if the light does not turn on within the
same period of time, a home control user will activate the controls
again, causing a sequence of commands such as
Light{on,off,off,on,off,..} or Volume{up,up,up,up,up,...}. Whether
the outcome is nothing or some unintended response this is
unacceptable. A controlling system must be able to restore
connectivity to recover from the error situation. Waiting for an
unknown period of time is not an option. While this issue was
identified during the P2P analysis, it applies just as well to
application scenarios where an IP application outside the LLN
controls actuators, lights, etc.
Appendix B. Communication failures
Measurements on the connectivity between neighbouring nodes are
discussed in [RTN2011] and [MEAS].
The work is motivated by the measurements in literature which affirm
that the range of an antenna is not circle symmetric but that the
signal strength of a given level follows an intricate pattern around
the antenna, and there may be holes within the area delineated by an
iso-strength line. It is reported that communication is not
symmetric: reception of messages from node A by node B does not imply
reception of messages from node B by node A. The quality of the
signal fluctuates over time, and also the height of the antenna
within a room can have consequences for the range. As function of
the distance from the source, three regions are generally recognized:
(1) a clear region with excellent signal quality, (2) a region with
fluctuating signal quality, (3) a region without reception. In the
text below it is shown that installation of meshes with neighbours in
the clear region is not sufficient.
[RTN2011] extends existing work by:
o Observations over periods of at least a week,
o Testing links that are in the clear region,
o Observation in an office building during working hours,
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o Concentrating on one-hop and two-hop routes.
Eight nodes were distributed over a surface of 30m2. All nodes are
at one hop distance from each other and are situated in the clear
region of each other. Each node sends messages to each of its
neighbours, and repeats the message until it arrives. The latency of
the message was measured over periods of at least a week. It is
noticed that latencies longer than a second occurred without apparent
reasons, but only during working days and never in the weekends. Bad
periods could last for minutes. By sending messages via two paths:
(1) one hop path directly, and (2) two hop path via a randomly chosen
neighbour, the probability of delays larger than 100 ms decreased
significantly.
The conclusion is that even for 1-hop communication between not too
distant "Line of Sight" nodes, there are periods of low reception in
which communication deadlines of 200 ms are exceeded. It pays to
send a second message over a 2-hop path to increase the reliability
of timely message transfer.
[MEAS] confirms that temporary bad reception by close neighbours can
occur within other types of areas. Nodes were installed on the
ceiling in a grid with a distance of 30-50 cm between nodes. 200
nodes were distributed over an area of 10m x 5m. It clearly
transpired that with increasing distance the probability of reception
decreases. At the same time a few nodes furthest away from the
sender had a high probability of message reception, while some close
neighbours of the sender did not receive messages. The patterns of
clear reception nodes evolved over time.
The conclusion is that even for direct neighbours reception can
temporarily be bad during periods of several minutes. For a reliable
and timely communication it is imperative to have at least two
communication paths available (e.g. two hop paths next to the 1-hop
path for direct neighbours).
Authors' Addresses
Anders Brandt
Sigma Designs
Email: abr@sdesigns.dk
Emmanuel Baccelli
INRIA
Email: Emmanuel.Baccelli@inria.fr
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Robert Cragie
Gridmerge
Email: robert.cragie@gridmerge.com
Peter van der Stok
Consultant
Email: consultancy@vanderstok.org
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