ROLL P. van der Stok, Ed.
Internet-Draft vanderstok consultancy
Intended status: Informational E. Dijk
Expires: January 12, 2013 A. Lelkens
Philips Research
July 11, 2012
Multicast requirements for control over LLN
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 applications. The Trickle
algorithm, which uses random re-broadcasting to assure that messages
arrive at all destinations, has been proposed in the Trickle
Multicast Forwarding ROLL WG draft as the basis for a multicast
routing protocol. In this draft additional requirements on multicast
routing are presented, such as timeliness, motivated by building
control. Random re-broadcasting and timeliness can be difficult to
reconcile. This draft 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 current
Trickle Multicast Forwarding draft to achieve optimal performance and
meet the stated requirements.
Status of this Memo
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Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
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document authors. All rights reserved.
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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 . . . . . . . . . . . . . . . . . . . . 6
4. Performance of Trickle-based multicast . . . . . . . . . . . . 7
4.1. Reasons for using Trickle . . . . . . . . . . . . . . . . 7
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 . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 13
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
10.1. Normative References . . . . . . . . . . . . . . . . . . . 13
10.2. Informative References . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 14
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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) [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 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 [Zhao]:
1. good: where a transmitted packet will be correctly received by a
destination with a probability 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
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(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 IP multicasting requirements and IP multicast routing,
in the context of building control 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 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 Confirmable/Non-Confirmable are message types defined
for CoAP [I-D.ietf-core-coap]. They are thought representative for
the two applications types, as the multicast GET SHOULD return a
unicast response and the multicast PUT typically does not return a
response in control applications.
An office building typically consist of multiple floors, divided in
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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 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, 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 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 and to propagate to other subnets. This case is discussed in
more detail in [I-D.ietf-core-groupcomm].
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3. Multicast requirements
The 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 (applicable to both PUT
and GET) are [Mullender]:
o Validity: If sender S sends message, m, to a group, g, of
destinations, a path exists between S and 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 S sent m to a group including 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 only holds for the PUT case without response from a
destination, but not for the GET case where a response 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
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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. 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 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 a message rebroadcast. However, long delays can seriously
jeopardize the Timeliness requirement. The following sections give
insight under which conditions the 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 standard back-off
intervals specified by IEEE 802.15.4. Packets between A and B arrive
with a probability dependent on the distance but independent of the
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direction. A distance of 70m is at the limit of the 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 rectangular grid of 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 defined as in [RFC6206].
4.3. Simulation results
The table below presents some of the 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 observed behavior is close to what is observed on the 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
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 Imin was taken to 10 ms and 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 were received by node (4,4) and how many
times a given message was rebroadcast. For k=3 more messages were
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received and sent. Receiving more messages leads to lower maximum
delays because the 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 a new packet
does not always arrive after the first transmission. This is
probably due to the synchronization of nodes when a new message
arrives, resulting in hidden terminal effects at the destination node
by overlapping sending intervals of its neighbors. For d=70 m,
packets are only received by the direct neighbor along the 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 node (x, y)
sends, packets are received in nodes (x+1, y) and (x,y+1). Given a
Imin value of 10ms there is a large probability that the sending by
nodes (x+1, y) and (x, y+1) overlap, leading to collision of 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 larger than k, thus
leading to long delays. The receiving node (x+1, y+1) sends at
regular intervals, determined by the Imax value, its last received
'old' message. Often the reception of the old message by a neighbor
leads to resending the new message. For that reason the maximum
delay is linked to the maximum interval Imax. Increasing the value
of k increases the probability of reciveing rebroadcast messages.
4.4. Simulation conclusions
The results indicate that for 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 Trickle Multicast Forwarding (TMF) draft
[I-D.ietf-roll-trickle-mcast] differs from direct application of
[RFC6206] in the introduction of multi-source, sliding windows, and
use of ICMP messages. For Trickle parameter k finite, a transmission
event consists of sending a Trickle ICMP advertizement (that
summarizes a forwarder's state i.e. buffered IP multicast packets)
and in addition any multicast messages that need rebroadcasting.
This section analyzes some issues of TMF, in particular its ability
to meet the Timeliness requirement for building control scenarios,
and proposes improvements to address the issues.
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5.1. Redundancy of 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 few
multicast packets are active in the network at a 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 same
information (SeedID, sequence number, M bit) plus the useful
application data.
In such cases were low latency is required, the extra overhead of
sending the ICMP message leads to additional delays, for example in
dense network topologies due to increased congestion. In a
simulation of a building control installation the operation with and
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 the entire group of 131 ms was
observed. The extra overhead generated by ICMP messages led to an
average delay of 197 ms, quite close to the Timeliness bound of 200
ms.
The simulation modeled a single IP multicast message active in a
6LoWPAN network, delivery targeted to a group which is 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 latency issue without increasing k (which would lead
to increased traffic), we propose that:
o sending the Trickle ICMP message is made OPTIONAL as part of a
transmission event, if a Trickle forwarder has any Trickle
Multicast Messages to send in that 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 is sent or not.
o as part of a transmission event, sending the Trickle ICMP message
MAY be done after retransmitting Trickle Multicast Messages. Note
that the TMF draft does not clearly express a preferred order for
Trickle ICMP messages.
These proposed changes are still fully compatible with existing
implementations of TMF.
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5.2. Ability to configure forwarders as data sinks
The current TMF makes a separation between (IP) hosts and Trickle
Multicast forwarders. Nodes that only need to receive IP multicast
packets (not wanting to participate in rebroadcasting) therefore can
be configured as hosts unaware of the multicast routing protocol.
However, this bears the risk that such hosts receive a specific
multicast packet very late or never, because they don't have a way to
signal missing packets to Trickle forwarders. Implementing a node as
a host has the clear advantages that the node does not need to buffer
any Trickle Multicast Messages which can considerably reduce memory
usage.
A solution that enables the best of 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 it does keep sliding window state for messages it accepts,
and sends Trickle ICMP messages, but does not buffer any Trickle
Multicast Messages for retransmission.
5.3. Issues in the 'consistency' definition
In the TMF draft the notion of 'consistency' (as we read it) is 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 following scenario:
o Nodes A, B, C are 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 missing m1
for transmission
o A sends ICMP (m1,m2,m3) but not any multicast message m_i
o B sees a consistency and increments c
o When the Trickle timer at B expires assuming k=1, the scheduled
transmission of 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 improve latency in this scenario is to apply the
suppression only to ICMP messages, not to scheduled multicast
messages (such as m1 by B in the example above). A refinement of
this approach is to maintain a counter c for each SeedID/
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Sequence-number combination, in addition to a global Trickle counter
c. Then, retransmission of 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 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 in its window that has no missing messages relative to
the lower bound of the window. So if a forwarder has a window
containing m1,m2,m4,m5 it retransmits m2, triggering others to 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 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 of missing a multicast 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 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.
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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, Jerald Martocci, Matthieu
Vial, and Nicolas Riou.
10. References
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.
[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.
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Internet-Draft MCReq July 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-13
(work in progress), 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]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-10 (work in progress), 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
van der Stok, et al. Expires January 12, 2013 [Page 14]
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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: armand.lelkens@philips.com
van der Stok, et al. Expires January 12, 2013 [Page 15]