wdiff draft-vanderstok-roll-mcreq-01.txt draft-vanderstok-roll-mcreq-02.txt

ROLL P. van der Stok, Ed.
Internet-Draft Philips Research vanderstok consultancy
Intended status: Informational March 12, 2012 E. Dijk
Expires: September 13, January 12, 2013 A. Lelkens
Philips Research
July 11, 2012

Multicast requirements for control over LLN
draft-vanderstok-roll-mcreq-01
draft-vanderstok-roll-mcreq-02

Abstract

This is a working document intended to focus discussion on
requirements for multicast in Low-power and Lossy Networks in the
area of M2M communication for control purposes. applications. The Trickle
algorithm, which uses random re-broadcasting to assure that messages
arrive at all destinations, is has been proposed as in the Trickle
Multicast Forwarding ROLL WG draft as the basis for a multicast
routing protocol. In this draft additional requirements on Trickle, multicast
routing are presented, such as timeliness and
ordering, are timeliness, motivated by building
control. Re-broadcasting Random re-broadcasting and timeliness can be mutually exclusive properties. To alleviate that
problem, this difficult to
reconcile. This draft considers minimizing re-broadcast by limiting the
number of re-broadcasting devices presents some simulation results in typical
control settings which show that achieving latencies below 400 ms is
feasible with Trickle. Recommendations are proposed for the wireless network. current
Trickle Multicast Forwarding draft to achieve optimal performance and
meet the stated requirements.

Status of this Memo

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provisions of BCP 78 and BCP 79.

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This Internet-Draft will expire on September 13, 2012. January 12, 2013.

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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4
2. Application characteristics . . . . . . . . . . . . . . . . . 4
3. Multicast requirements . . . . . . . . . . . . . . . . . . . . 5 6
4. Wireless link characteristics Performance of Trickle-based multicast . . . . . . . . . . . . 7
4.1. Reasons for using Trickle . . . . . . . . . . . . . . . . 7
5. Recommendation
4.2. Simulation setup . . . . . . . . . . . . . . . . . . . . . 7
4.3. Simulation results . . . . . . . . . . . . . . . . . . . . 8
4.4. Simulation conclusions . . . . . . . . . . . . . . . . . . 9
5. Performance issues of Trickle Multicast Forwarding . . . . . . 9
5.1. Redundancy of Trickle ICMP message . . . . . . . . . . . . 10
5.2. Ability to configure forwarders as data sinks . . . . . . 11
5.3. Issues in the 'consistency' definition . . . . . . . . . . 11
5.4. Window handling without ICMP . . . . . . . . . . . . . . . 12
6. Summary of Recommendations for Trickle Multicast Forwarding . 12
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. 13
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9
9. 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
9.1. 13
10.1. Normative References . . . . . . . . . . . . . . . . . . . 9
9.2. 13
10.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 14

1. Introduction

The ROLL working group is chartered to design and standardize a
routing protocol for resource constrained devices in Low-power and
Lossy Networks (LLN) [I-D.ietf-roll-rpl]. [RFC6550]. The requirements for ROLL are
documented in [RFC5548] [RFC5673] [RFC5826] [RFC5867]. For building
control it is recognized that most communication is local to the
wireless mesh network, and does not necessarily pass through the edge
router. The point-to-point RPL routing algorithm is developed to
efficiently support unicast routing in such applications
[I-D.ietf-roll-p2p-rpl]. The Trickle algorithm was initially
developed to support the RPL routing algorithm [RFC6206], and later
proposed to support general multicast delivery in LLN LLNs in Trickle
Multicast Forwarding (TMF) [I-D.ietf-roll-trickle-mcast].

This draft discusses the multicast requirements for constrained
devices participating in M2M building control networks. An important
requirement is the delivery of control commands to a subset (group)
of neighbouring devices in the LLN within some latency bound. Also,
analyses are provided of how well Trickle algorithm and TMF can meet
these requirements and suggestions for improvement are made.

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 RFC 2119 [RFC2119].
Addtional privileged words are described below.

"TMF" is used as an abbreviation for Trickle Multicast Forwarding as
described in [I-D.ietf-roll-trickle-mcast].

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

One device communicates directly with another device by wirelessly
transmitting packets to it over a link. The link quality is divided
in three regions: regions [Zhao]:

1. good: where a transmitted packet will be correctly received by a
destination with a probability higher than 99%. of say 95% or more.
2. transitional: where the probability of correct reception
fluctuates.
3. bad: where almost no transmission is successfully received.

It is empirically known that good links can become bad occasionally
(e.g. once a week for a few minutes)due to dynamic effects such as
multipath interference.

A distinction is made between reception and delivery of a message. A
message is received when it is stored in the reception buffer of the
receiver after transmission and all error checks have been
succesfully passed. The message is delivered when the message is
passed from the reception buffer to the destination application. We
also say the application accepts the message.

Broadcasting is used for the link-local sending of one packet to all
reachable 1-hop neigbours. This is equivalent to the term link-local
multicast.

1.2. Motivation

In this draft, we focus and develop discussions on requirements
pertaining to multicasting, IP multicasting requirements and IP multicast routing,
in the context of building control
applications. applications on LLNs. This draft
aims to show potential (latency) improvements for current proposed
multicast routing approaches, that can be easily attained.

2. Application characteristics

Multicast is important for building control applications. Two types
of applications are considered:

1. Discovery messages to (a subset of) the members of the mesh
(multicast GET)
2. Control messages to a subset of the mesh (multicast PUT)

The first type requires the message to be sent to a (sub)set which
may be randomly distributed over the building area. Some of the
destinations return unicast response messages to the source.

The second type requires the a Non-Confirmable message mostly to be sent
to a closely spaced subset. No return messages are generated. This
second type is the subject of this draft, although most of the
requirements equally apply to case 1.

GET and PUT and GET Confirmable/Non-Confirmable are the message types defined
for CoAP [I-D.ietf-core-coap]. They are thought representative for
the two applications types, as one returns the multicast GET SHOULD return a result
unicast response and the other multicast PUT typically does not. not return a
response in control applications.

An office building typically consist of multiple floors, divided in
working areas. The working areas can be open or enclosed by walls.
Within a working area sensors measure temperature, presence, humidity
and other parameters. On the basis of these measurements, equipment
within the working area can receive commands to change settings. A
well-known example is presence detection to switch on or dim lights.
The equipment configuration is quite stable, because devices are
installed in the ceiling, and modifying (or servicing) the
installation can be costly.

The equipment is interconnected in a wireless network. The RF
transmissions pass through the walls and generate interference to the
wireless equipment in other working areas.

The lay-out of a network may be different from installation to
installation. However, it is expected that many wireless networks
extend over one floor and include several working areas. Another
working hypothesis is that most of the time sensors will multicast
their values to a group of devices within the working area.
Consequently, multicast messages are often meant for a subset of
neigbouring (not necessarily 1-hop) devices.

A LoWPAN is a mesh of wireless devices that share the same IPv6
address prefix. A typical LowPAN in a building may cover the area of
an entire floor. A commercial installation may cover 1000 m2 per
floor. A length of 50 m can easily mean a hop count >5 for a message
to pass from end to end. For example, devices may be installed in
the ceiling in a grid with a grid pattern distance of 40 cm 2 m between
devices.

Messages may consist of sensor measurements performed or commands
issued in a given working area, which then must be acted upon by
neigbouring devices in the same working area. Under this control
pattern, source and sink are located in one working area, and
accordingly sink and source of a multicast message are often between
3 - 6 m from each other. Consequently, it is required to send a
multicast to a subset of the devices in the LoWPAN.

In case of commands to luminaries, messages a command message must be
delivered to all LoWPAN-local multicast group members within a clear
deadline of about 200ms. In [RFC5867] a deadline of 120 ms is
suggested for other building applications.

Although control messages are frequently exchanged between closely
spaced (less than 6 m) devices, it is sometimes necessary, say every
hour or less frequently, necessary to send a
message to a subset of devices covering the whole building. In that
case the multicast message will need to pass the edge router of the lowpan
LoWPAN and to propagate to other subnets. This case is discussed in
more detail in [I-D.ietf-core-groupcomm].

3. Multicast requirements

The Multicast multicast requirements are derived from the characteristics of
the aforementioned applications. A device is said to be correct it
it follows the selected multicast routing algorithm. The application
characteristics and the network installation make it possible to add
an additional set of network properties to make the multicast
algorithm more efficient.

The basic traditional multicast requirements (applying (applicable to both PUT
and
GET)are: GET) are [Mullender]:

o Validity: If sender S sends message, m, to a group, g, of
destinations, a path exists between S and a any destination D, and
if S and D are correct, D eventually accepts m.
o Integrity: A destination D accepts m at most once from sender S
and only if sender S sent m to a group including destination. D.
o Agreement: If a correct destination of g accepts m, then all
correct destinations of g accept m.

The set of intended destination devices is identified by the
multicast (group) IP address. Every device in the associated
multicast group is a destination of the multicast. Each destination
accepts messages with as destination the specified IP multicast
address. Additional multicast requirements are:

o Timeliness: There is a known constant C such that if m is sent at
time t, no correct destination accepts m after t+C.

For lighting control applications the value of C is taken as 200 ms.
This requirement considers only holds for the PUT case and without response from a
destination, but not for the return of GET case where a response in the GET case. is returned.

o Ordering: When m1 and m2 sent to the same group g, and a receiver
in g accepts message m1 before m2, every receiver in g accepts m1
before accepting m2

Ordering applies to both the PUT and GET cases. Ordering can be
partial or total. Partial ordering means that for specified message
pairs, one message of the pair precedes the other. In case of total
ordering, every message pair is ordered. Partial ordering is
obtained by adding message counters in the message such that
destinations can order the messages of a given sender. Messages from
different sources are not ordered. Total ordering can be obtained
with vector clocks or using synchronized clocks. Vector clocks
require a large overhead that increases linearly with the number of
devices in the network. As long as no synchronized clocks are
available, partial ordering seems the most realistic. Total Ordering
is interesting for the discovery application. When two devices
announce themselves simultaneously with conflicting properties, all
participants can come to the same decision by favoring the first
arrival. Partial ordering is necessary when a multicast message
needs multiple packets (for example discovery messages) or when
multicast messages are sent with intervals shorter than the maximum
throughput delay.

4. Wireless link characteristics

It Performance of Trickle-based multicast

In this section we investigate the behavior of the Trickle algorithm
[RFC6206] when used for multicast routing. Rebroadcasting as defined
in Trickle makes meeting tight deadlines a challenge. Simulation
results in this section show for particlar configurations and
parameter settings which end-to-end communication delays can be
expected.

4.1. Reasons for using Trickle

The simplest approach to IP multicast is possible to broadcast from a source
to a set of devices reachable over good links in one hop. This is
not sufficient however, because the set of reachable devices is often
a subset of the set of destination devices. Consequently, additional
measures are needed to make sure that the Agreement requirement is
met. A standard technique, to reach all devices instead of a subset,
stipulates that every receiver of a broadcast message rebroadcasts
this message (flooding). When the multicast destination address of
the message corresponds with a specified multicast address in the
receiver device, the message is delivered. Thanks to this technique
it is assured that when a path exists between the source and the
destination device, the destination device will eventually receive
the message from the sender.

Given the network density described in section 2, the multicast can
generate a broadcast storm with lots of interfering senders. The
technique to prevent the storm, also used in Trickle, is to randomly
delay the a message rebroadcast. However, the long delays can seriously
jeopardize the timeliness Timeliness requirement. This draft proposes
three ways suggested by The following sections give
insight under which conditions the application characteristics, to reduce Timeliness requirement can be met.

4.2. Simulation setup

The simulations were done on a general rectangular network topology
and on an approximation of known building installations. The IEEE
802.15.4 protocol is simulated with CSMA and the interference standard back-off
intervals specified by IEEE 802.15.4. Packets between re-broadcasting devices:

1. Restrict A and B arrive
with a probability dependent on the scope distance but independent of the multicast.
2. Restrict number
direction. A distance of rebroadcasting devices.
3. Weaken 70m is at the Timeliness requirement.

In limit of the application characteristics it is mentioned that most control
messages have transmission
range. Two rectangular meshes were tried: (1) 5 x 5 nodes and (2) 10
x 10 nodes. The distance between two adjoining neigbors was varied
between 5 and 70 m. The total surface for the 10 x 10 mesh varied
accordingly between 45 x 45 m^2 and 630 x 630 m^2. The building
installation approximation consist of a set rectangular grid of destinations which 14 x 7
nodes over a surface of 35 x 15 m^2. Parameters Imin, Imax and k and
variables I, t and c are closely spaced to defined as in [RFC6206].

4.3. Simulation results

The table below presents some of the
source. results on the 5 x 5 mesh.

Imax k Parameter Distance
10m 40m 70m
250ms 1 hopcount 1 2-4 5-9
250ms 1 avg delay 5 ms 40 ms 110 ms
250ms 1 max delay 18 ms 90 ms 1050 ms
250ms 1 msgs sent 0-5 0-11 1-12
250ms 1 msgs received 18-36 3-20 0-20
250ms 3 hopcount 1 2-4 5-9
250ms 3 avg delay 5 ms 40 ms 130 ms
250ms 3 max delay 25 90 ms 260 ms
250ms 3 msgs sent 1-7 3-12 7-13
250ms 3 msgs received 40-60 14-32 9-23
500ms 1 hopcount 1 3-5 5-10
500ms 1 avg delay 5 ms 40 ms 110 ms
500ms 1 max delay 19 ms 100 ms 1500 ms
500ms 1 msgs sent 0-4 0-8 0-10
500ms 1 msgs received 12-26 0-16 0-16
500ms 3 hopcount 1 3-5 5-10
500ms 3 avg delay 5 ms 40 ms 120 ms
500ms 3 max delay 22 80 ms 240 ms
500ms 3 msgs sent 1-8 2-9 5-10
500ms 3 msgs received 28-44 8-27 5-18

The interference between multicast sources can be reduced by
limiting observed behavior is close to what is observed on the scope 10 x 10
mesh and on the installation configuration. Behavior on, for
example, a single row of nodes tends to be quite different and
requires quite different parameter settings. The results in the broadcast message.
table concern node (4,4) which had the longest end-to-end delays of
all nodes. Node (0,0) sent a message every 2 seconds. Individual
packets were lost but all messages arrived at all nodes eventually.
The ensuing proximity
condition can be formulated for both PUT Imin was taken to 10 ms and GET as:

o Proximity condition: A multicast Imax was taken to 250 ms and 500 ms
with quite similar results. Changing the Imax has measurable
influence on the maximum end-to-end delay. The table shows how many
copies of a given message is accepted were received by node (4,4) and how many
times a subset
of devices closely spaced given message was rebroadcast. For k=3 more messages were
received and sent. Receiving more messages leads to lower maximum
delays because the sender.

In practice, this condition means probability of receiving the message early
increases with increasing rebroadcast frequency.

The causes for the large maximum delays (>400ms), occurring at d=70m,
have been investigated in more detail. It is shown that most multicast messages can be
constrained to 1-2 hops. Therefore, it a new packet
does not always arrive after the first transmission. This is recommended
probably due to put the
multicast range under control synchronization of nodes when a new message
arrives, resulting in hidden terminal effects at the multicast source.

Given the stability destination node
by overlapping sending intervals of its neighbors. For d=70 m,
packets are only received by the network configuration, direct neighbor along the configuration
of good links is also stable over long periods (say several days). x-axis or
the y-axis. Consequently, when node (x, y) receives a new message,
it originates probably from (x-1, y) or (x, y-1). When all good links node (x, y)
sends, packets are available, the number received in nodes (x+1, y) and (x,y+1). Given a
Imin value of possible paths
between 10ms there is a source large probability that the sending by
nodes (x+1, y) and each (x, y+1) overlap, leading to collision of its destinations the
messages at node (x+1, y+1). In the following intervals, nodes (x+1,
y) and nodes (x, y+1) receive the last message from their neighbors
and do not repeat the message because c is probably larger than
required given k, thus
leading to long delays. The receiving node (x+1, y+1) sends at
regular intervals, determined by the sporadic failure Imax value, its last received
'old' message. Often the reception of the old message by a good link. Under neighbor
leads to resending the
assumption new message. For that reason the qualities of maximum
delay is linked to the good links maximum interval Imax. Increasing the value
of a given device are
unrelated, k increases the failure probability of good link has no consequence reciveing rebroadcast messages.

4.4. Simulation conclusions

The results indicate that for
alternative good links. the network configurations we foresee,
with Trickle it is quite possible to reach average message delivery
latency within the 200 ms range, meeting the Timeliness requirement
for most nodes, and to limit the maximum latency by tuning parameter
k.

5. Performance issues of Trickle Multicast Forwarding

The number Trickle Multicast Forwarding (TMF) draft
[I-D.ietf-roll-trickle-mcast] differs from direct application of paths can be reduced by
specifying
[RFC6206] in the introduction of multi-source, sliding windows, and
use of ICMP messages. For Trickle parameter k finite, a subset transmission
event consists of devices, called relay devices (possibly
equivalent with sending a Trickle ICMP advertizement (that
summarizes a forwarder's state i.e. buffered IP multicast forwarders mentioned packets)
and in
[I-D.ietf-roll-trickle-mcast]), addition any multicast messages that need rebroadcasting.
This section analyzes some issues of TMF, in particular its ability
to rebroadcast messages. A path can
pass from a source via relay devices meet the Timeliness requirement for building control scenarios,
and proposes improvements to address the multicast destinations.
A relay device can also be a destination device. In [RFC5867] it is
mentioned that 1 out issues.

5.1. Redundancy of 2 devices Trickle ICMP message

Summarizing state in an ICMP message is clearly useful to reduce
network traffic, if many IP multicast packets are being buffered in
Trickle multicast forwarders. However, if only one or a relay device. Given few
multicast packets are active in the network densities foreseen for lighting, at a much lower relay density
is possible. The reduction of time, a forwarder
sending ICMP messages generates unnecessary overhead. As an example,
consider a forwarder that stores and needs to rebroadcast a single
multicast message m1. According to TMF, it would need to send an
ICMP message containing information about m1 (SeedID, sequence
number, M bit) and additionally send a Trickle Multicast message with
a Trickle Multicast header option which contains exactly the relay devices reduces same
information (SeedID, sequence number, M bit) plus the risk useful
application data.

In such cases were low latency is required, the extra overhead of
interference in
sending the dense networks described ICMP message leads to additional delays, for example in section 2. An
appropriate condition
dense network topologies due to assure the presence increased congestion. In a
simulation of a path between source building control installation the operation with and destination can be formulated as:

o Multiple relay links: any device has good links
without extra ICMP message was compared for the case that a single
multicast message was active. Without ICMP messages an average
latency of message delivery to at least q
relay devices

The value the entire group of q is determined 131 ms was
observed. The extra overhead generated by the quality ICMP messages led to an
average delay of 197 ms, quite close to the links Timeliness bound of 200
ms.

The simulation modeled a single IP multicast message active in a given
installation.

However, the probability that
6LoWPAN network, delivery targeted to a path was temporarily unavailable
cannot be excluded. The timeliness requirement group which is too strong for
wireless sensor networks, where packets get lost for multiple reasons
like hidden terminal, multipath fading, and others. The timeliness
requirement can be reformulated for a subset of 13
nodes out of 95 nodes total, with a 40-byte data payload, each node
acting as a forwarder, with Trickle parameters k=1, Imin=32 ms,
Imax=128 ms.

To addresss the PUT case as: latency issue without increasing k (which would lead
to increased traffic), we propose that:
o Majority Timeliness: with high probability, p, sending the timeliness
requirent Trickle ICMP message is met; with probability (1-p) made OPTIONAL as part of a subset s
transmission event, if a Trickle forwarder has any Trickle
Multicast Messages to send in g accepts m
after t+C.

The agreement requirement specifies that all destinations in g accept transmission event. A Trickle
Multicast forwarder may decide per transmission event (depending
on internal state e.g. number of buffered messages) whether the
ICMP message eventually. Consequently, there is a (low) probability
(1-p) that members sent or not.
o as part of g accept a transmission event, sending the Trickle ICMP message
MAY be done after t+C. Probability p
and subset s are specified as function of retransmitting Trickle Multicast Messages. Note
that the installation and linked TMF draft does not clearly express a preferred order for
Trickle ICMP messages.
These proposed changes are still fully compatible with the value existing
implementations of q. For TMF.

5.2. Ability to configure forwarders as data sinks

The current TMF makes a lighting application this means separation between (IP) hosts and Trickle
Multicast forwarders. Nodes that only need to receive IP multicast
packets (not wanting to participate in
general all lights switch on/off within 200 ms and quite
infrequently, (say once a month) one out rebroadcasting) therefore can
be configured as hosts unaware of all lights swictches on/
off the multicast routing protocol.
However, this bears the risk that such hosts receive a bit later (say specific
multicast packet very late or never, because they don't have a few seconds).

Using rebroadcast with way to
signal missing packets to Trickle forwarders. Implementing a node as
a frequency host has the clear advantages that decreases with increasing
density the node does not need to buffer
any Trickle Multicast Messages which can considerably reduce memory
usage.

A solution that enables the probability best of interference (as in Trickle)
assures both worlds is to allow Trickle
Multicast forwarders to act as 'data sinks' only i.e. not acting as a
repeater. We propose that:
o a Trickle Multicast forwarder MAY act as a data sink, which means
that missed it does keep sliding window state for messages are eventually repeated. It should be
noted that it accepts,
and sends Trickle ICMP messages, but does not buffer any Trickle
Multicast Messages for retransmission.

5.3. Issues in the relay nodes can be consistent while a receiver node 'consistency' definition

In the TMF draft the notion of 'consistency' (as we read it) is
not. Relay nodes cannot detect based
on information received in Trickle ICMP messages only, not on
information received from incoming Trickle Multicast Messages. This
operation can lead to unnecessary delays in certain use cases.
Consider the inconsistency, and following scenario:

o Nodes A, B, C are thus
required to rebroadcast Trickle Multicast forwarders; where A cannot
hear C and C cannot hear A
o A stores messages m1,m2,m3, B stores m1,m2,m3, C stores m2,m3
o C sends ICMP(m2,m3)
o B sees an inconsistency based on this and schedules the latest missing m1
for transmission
o A sends ICMP (m1,m2,m3) but not any multicast message continuously, or receiver
nodes rebroadcast their status with m_i
o B sees a low frequency.

5. Recommendation

From consistency and increments c
o When the text above emerges a number Trickle timer at B expires assuming k=1, the scheduled
transmission of recommendations m1 is cancelled
o C does not get m1 from B, at least not during this round.

Eventually C will get m2, after more rounds (when B transmits before
A does), but later than necessary.

A first approach to make it
possible improve latency in this scenario is to put propagation characteristics of apply the
suppression only to ICMP messages, not to scheduled multicast
algorithm under application control.
1. Take into account timeliness and partial ordering requirements
messages (such as m1 by B in
multicast algorithm.
2. Exploit the small range example above). A refinement of most multicasts used
this approach is to maintain a counter c for control
purposes and put multicast range under application control.
3. Introduce each SeedID/
Sequence-number combination, in addition to a subset global Trickle counter
c. Then, retransmission of devices Trickle Multicast Messages is only
suppressed for those messages that have been received at least k
times. ICMP suppression is still based on the global Trickle counter
c as relay devices to reduce in the current TMF draft.

5.4. Window handling without ICMP

A forwarder that does not support sending ICMP advertizements could
advertize its state by retransmitting the multicasat message with the
largest number of rebroadcasting devices.
4. Introduce meachanisms to remove inconsistencies in receiver nodes
in spite its window that has no missing messages relative to
the lower bound of consistent relay nodes.
5. Use majority timeliness requirement the window. So if a forwarder has a window
containing m1,m2,m4,m5 it retransmits m2, triggering others to choose send
m3 (and maybe higher numbers). If it encounters an inconsistency,
i.e. seeing a multicast with a number lower than its own upperbound,
it itself would send out the messages that have a higher number than
the received multicast message (excluding the ones that it has
received at least k tines during the current Trickle interval).

6. Summary of relay
devices with respect Recommendations for Trickle Multicast Forwarding

From the analyses above emerge a number of recommendations that aim
to reduce transmission latency of multicast messages and to reduce
the probability that of missing a device misses its multicast reception deadline.
6. message. In summary, the
following adaptations to TMF [I-D.ietf-roll-trickle-mcast] are
proposed which can be applied independently of each other:
1. Efficient retransmission: sending the Trickle ICMP message is
made OPTIONAL as part of a transmission event if a Trickle
forwarder already has any Trickle Multicast Messages to send.
2. Allow data sinks: a Trickle Multicast forwarder MAY refrain from
buffering any Trickle Multicast Messages for retransmission.
3. Consistency improvement: When a transmission is suppressed, a
forwarder MAY only suppress ICMP but not suppress transmission of
a multicast message that was scheduled due to a detected
inconsistency. This approach could be refined by keeping in
addition to a global Trickle consistency counter c, separate
counters c per SeedID/sequence-number combination suppressing
only messages transit through an edge router.

6. seen at least k times.
4. Window handling without ICMP: forwarders without ICMP sending
capability can ask for retransmissions by rebroadcasting
multicast messages

7. IANA Considerations

This document makes no request of IANA.

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

8. Security Considerations

TBD

9. Acknowledgments

This I-D has benefited from conversations with and comments from
Anders Brandt, Kerry Lynn, Zach Shelby, Emmanuel Frimout, Michael
Verschoor, Jamie Mc Cormack, Dee Denteneer, Esko van Dijk, Jerald Martocci, Matthieu
Vial, and Nicolas Riou.

10. References

9.1.

10.1. Normative References

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

[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.

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

9.2. Informative References

[I-D.ietf-roll-rpl]

[RFC6550] Winter, T., Thubert, P., Brandt, A., Vasseur, J., Hui, J., Pister, K., Thubert, P., Kelsey, R.,
Levis, P., Pister, K., Struik, R., Kelsey, R., Clausen, T., Vasseur, JP., and T.
Winter, R.
Alexander, "RPL: IPv6 Routing Protocol for Low power Low-Power and
Lossy Networks", draft-ietf-roll-rpl-19 (work in
progress), RFC 6550, March 2011. 2012.

10.2. Informative References

[I-D.ietf-roll-p2p-rpl]
Goyal, M., Baccelli, E., Philipp, M., Brandt, A., and J.
Martocci, "Reactive Discovery of Point-to-Point Routes in
Low Power and Lossy Networks", draft-ietf-roll-p2p-rpl-09 draft-ietf-roll-p2p-rpl-13
(work in progress), March June 2012.

[I-D.ietf-roll-trickle-mcast]
Hui, J. and R. Kelsey, "Multicast Forwarding Using
Trickle", draft-ietf-roll-trickle-mcast-00 (work in
progress), April 2011.

[I-D.ietf-core-coap]
Frank, B., Bormann, C.,
Shelby, Z., Hartke, K., Bormann, C., and Z. Shelby, B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-08
draft-ietf-core-coap-10 (work in progress), October 2011.

Author's Address June 2012.

[I-D.ietf-core-groupcomm]
Rahman, A. and E. Dijk, "Group Communication for CoAP",
draft-ietf-core-groupcomm-02 (work in progress),
July 2012.

[Zhao] Zhao, J. and R. Govindan, "Understanding Packet Delivery
Performance in Dense Wireless Sensor Networks", senSys ,
2003.

[Mullender]
Mullender, S., "Distributed Systems, Second Edition",
Section 5 , Addison-Wesley Publishing Company, Inc. ,
ISBN 0-201-62427-3, 1995.

Authors' Addresses

Peter van der Stok (editor)
vanderstok consultancy
Kamperfoelie 8
Helmond, 5708 DM
The Netherlands

Email: consultancy@vanderstok.org
Esko Dijk
Philips Research
High Tech Campus 34-1
Eindhoven, 5656 AA
The Netherlands

Email: esko.dijk@philips.com

Armand Lelkens
Philips Research
High Tech Campus 34-1
Eindhoven, 5656 AA
The Netherlands

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