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Terminology for Constrained Node Networks

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Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7228.
Authors Carsten Bormann , Mehmet Ersue , Ari Keränen
Last updated 2013-08-15 (Latest revision 2013-07-09)
Replaces draft-bormann-lwig-terms
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Zhen Cao
Shepherd write-up Show Last changed 2013-05-19
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LWIG Working Group                                            C. Bormann
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Informational                                  M. Ersue
Expires: January 10, 2014                         Nokia Siemens Networks
                                                              A. Keranen
                                                           July 09, 2013

               Terminology for Constrained Node Networks


   The Internet Protocol Suite is increasingly used on small devices
   with severe constraints, creating constrained node networks.  This
   document provides a number of basic terms that have turned out to be
   useful in the standardization work for constrained environments.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Constrained Nodes . . . . . . . . . . . . . . . . . . . .   3
     2.2.  Constrained Networks  . . . . . . . . . . . . . . . . . .   4
       2.2.1.  Challenged Networks . . . . . . . . . . . . . . . . .   5
     2.3.  Constrained Node Networks . . . . . . . . . . . . . . . .   5
       2.3.1.  LLN ("low-power lossy network") . . . . . . . . . . .   6
       2.3.2.  LoWPAN, 6LoWPAN . . . . . . . . . . . . . . . . . . .   6
   3.  Classes of Constrained Devices  . . . . . . . . . . . . . . .   8
   4.  Power Terminology . . . . . . . . . . . . . . . . . . . . . .  10
     4.1.  Scaling Properties  . . . . . . . . . . . . . . . . . . .  10
     4.2.  Classes of Energy Limitation  . . . . . . . . . . . . . .  10
     4.3.  Strategies of Using Power for Communication . . . . . . .  11
   5.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   7.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   8.  Informative References  . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   Small devices with limited CPU, memory, and power resources, so
   called constrained devices (also known as sensor, smart object, or
   smart device) can constitute a network, becoming "constrained nodes"
   in that network.  Such a network may itself exhibit constraints, e.g.
   with unreliable or lossy channels, limited and unpredictable
   bandwidth, and a highly dynamic topology.

   Constrained devices might be in charge of gathering information in
   diverse settings including natural ecosystems, buildings, and
   factories and sending the information to one or more server stations.
   Constrained devices may work under severe resource constraints such
   as limited battery and computing power, little memory, as well as
   insufficient wireless bandwidth and ability to communicate.  Other
   entities on the network, e.g., a base station or controlling server,
   might have more computational and communication resources and could
   support the interaction between the constrained devices and
   applications in more traditional networks.

   Today diverse sizes of constrained devices with different resources
   and capabilities are becoming connected.  Mobile personal gadgets,
   building-automation devices, cellular phones, Machine-to-machine
   (M2M) devices, etc.  benefit from interacting with other "things"

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   nearby or somewhere in the Internet.  With this, the Internet of
   Things (IoT) becomes a reality, built up out of uniquely identifiable
   and addressable objects (things).  And over the next decade, this
   could grow to large numbers [fifty-billion] of Internet-connected
   constrained devices, greatly increasing the Internet's size and

   The present document provides a number of basic terms that have
   turned out to be useful in the standardization work for constrained
   environments.  The intention is not to exhaustively cover the field,
   but to make sure a few core terms are used consistently between
   different groups cooperating in this space.

   In this document, the term "byte" is used in its now customary sense
   as a synonym for "octet".  Where sizes of semiconductor memory are
   given, the prefix "kibi" (1024) is combined with "byte" to
   "kibibyte", abbreviated "KiB", for 1024 bytes [ISQ-13].

2.  Terminology

   The main focus of this field of work appears to be _scaling_:

   o  Scaling up Internet technologies to a large number [fifty-billion]
      of inexpensive nodes, while

   o  scaling down the characteristics of each of these nodes and of the
      networks being built out of them, to make this scaling up
      economically and physically viable.

   The need for scaling down the characteristics of nodes leads to
   _constrained nodes_.

2.1.  Constrained Nodes

   The term "constrained node" is best defined by contrasting the
   characteristics of a constrained node with certain widely held
   expectations on more familiar Internet nodes:

   Constrained Node:  A node where some of the characteristics that are
      otherwise pretty much taken for granted for Internet nodes in 2013
      are not attainable, often due to cost constraints and/or physical
      constraints on characteristics such as size, weight, and available
      power and energy.

   While this is less than satisfying as a rigorous definition, it is
   grounded in the state of the art and clearly sets apart constrained
   nodes from server systems, desktop or laptop computers, powerful
   mobile devices such as smartphones etc.  There may be many design

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   considerations that lead to these constraints, including cost, size,
   weight, and other scaling factors.

   (An alternative name, when the properties as a network node are not
   in focus, is "constrained device".)

   There are multiple facets to the constraints on nodes, often applying
   in combination, e.g.:

   o  constraints on the maximum code complexity (ROM/Flash);

   o  constraints on the size of state and buffers (RAM);

   o  constraints on the available power.

   Section 3 defines a small number of interesting classes ("class-N"
   for N=0,1,2) of constrained nodes focusing on relevant combinations
   of the first two constraints.  With respect to available power,
   [RFC6606] distinguishes "power-affluent" nodes (mains-powered or
   regularly recharged) from "power-constrained nodes" that draw their
   power from primary batteries or by using energy harvesting; more
   detailed power terminology is given in Section 4.

   The use of constrained nodes in networks often also leads to
   constraints on the networks themselves.  However, there may also be
   constraints on networks that are largely independent from those of
   the nodes.  We therefore distinguish _constrained networks_ and
   _constrained node networks_.

2.2.  Constrained Networks

   We define "constrained network" in a similar way:

   Constrained Network:  A network where some of the characteristics
      pretty much taken for granted with link layers in common use in
      the Internet by 2013, are not attainable.

   Again, there may be several reasons for this:

   o  cost constraints on the network,

   o  constraints of the nodes (for constrained node networks),

   o  physical constraints (e.g., power constraints, environmental
      constraints, media constraints such as underwater operation,
      limited spectrum for very high density, electromagnetic

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   o  regulatory constraints, such as very limited spectrum availability
      (including limits on effective radiated power and duty cycle), or
      explosion safety,

   o  technology constraints, such as older and lower speed technologies
      that are still operational and may need to stay in use for some
      more time.

   Constraints may include:

   o  low achievable bit rate (including limits on duty cycle),

   o  high packet loss, packet loss (delivery rate) variability,

   o  severe penalties for using larger packets (e.g., high packet loss
      due to link layer fragmentation),

   o  lack of (or severe constraints on) advanced services such as IP

2.2.1.  Challenged Networks

   A constrained network is not necessarily a _challenged_ network

   Challenged Network:  A network that has serious trouble maintaining
      what an application would today expect of the end-to-end IP model,
      e.g., by:

   o  not being able to offer end-to-end IP connectivity at all;

   o  exhibiting serious interruptions in end-to-end IP connectivity;

   o  exhibiting delay well beyond the Maximum Segment Lifetime (MSL)
      defined by TCP [RFC0793].

   All challenged networks are constrained networks in some sense, but
   not all constrained networks are challenged networks.  There is no
   well-defined boundary between the two, though.  Delay-Tolerant
   Networking (DTN) has been designed to cope with challenged networks

2.3.  Constrained Node Networks

   Constrained Node Network:  A network whose characteristics are
      influenced by being composed of a significant portion of
      constrained nodes.

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   A constrained node network always is a constrained network because of
   the network constraints stemming from the node constraints, but may
   also have other constraints that already make it a constrained

2.3.1.  LLN ("low-power lossy network")

   A related term that has been used recently is "low-power lossy
   network" (LLN).  In its terminology document, the ROLL working group
   is saying [I-D.ietf-roll-terminology]:

      LLN: Low power and Lossy networks (LLNs) are typically composed of
      many embedded devices with limited power, memory, and processing
      resources interconnected by a variety of links, such as IEEE
      802.15.4 or Low Power WiFi.  There is a wide scope of application
      areas for LLNs, including industrial monitoring, building
      automation (HVAC, lighting, access control, fire), connected home,
      healthcare, environmental monitoring, urban sensor networks,
      energy management, assets tracking and refrigeration.. [sic]

   In common usage, LLN often stands for "the network characteristics
   that RPL has been designed for".  Beyond what is said in the ROLL
   terminology document, LLNs do appear to have significant loss at the
   physical layer, with significant variability of the delivery rate,
   and some short-term unreliability, coupled with some medium term
   stability that makes it worthwhile to construct medium-term stable
   directed acyclic graphs for routing and do measurements on the edges
   such as ETX [RFC6551].  Actual "low power" does not seem to be
   required for an LLN [I-D.hui-vasseur-roll-rpl-deployment], and the
   positions on scaling of LLNs appear to vary widely

   The ROLL terminology document states that LLNs typically are composed
   of constrained nodes; this is also supported by the design of
   operation modes such as RPL's "non-storing mode".  So, in the
   terminology of the present document, an LLN seems to be a constrained
   node network with certain network characteristics, which include
   constraints on the network as well.

2.3.2.  LoWPAN, 6LoWPAN

   One interesting class of a constrained network often used as a
   constrained node network is the "LoWPAN" [RFC4919], a term inspired
   from the name of the IEEE 802.15.4 working group (low-rate wireless
   personal area networks (LR-WPANs)).  The expansion of that acronym,
   "Low-Power Wireless Personal Area Network" contains a hard to justify
   "Personal" that is due to the history of task group naming in IEEE
   802 more than due to an orientation of LoWPANs around a single

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   person.  Actually, LoWPANs have been suggested for urban monitoring,
   control of large buildings, and industrial control applications, so
   the "Personal" can only be considered a vestige.  Maybe the term is
   best read as "Low-Power Wireless Area Networks" (LoWPANs) [WEI].
   Originally focused on IEEE 802.15.4, "LoWPAN" (or when used for IPv6,
   "6LoWPAN") is now also being used for networks built from similarly
   constrained link layer technologies [I-D.ietf-6lowpan-btle]
   [I-D.mariager-6lowpan-v6over-dect-ule] [I-D.brandt-6man-lowpanz].

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3.  Classes of Constrained Devices

   Despite the overwhelming variety of Internet-connected devices that
   can be envisioned, it may be worthwhile to have some succinct
   terminology for different classes of constrained devices.  In this
   document, the class designations in Table 1 may be used as rough
   indications of device capabilities:

     | Name        | data size (e.g., RAM) | code size (e.g., Flash) |
     | Class 0, C0 | << 10 KiB             | << 100 KiB              |
     |             |                       |                         |
     | Class 1, C1 | ~ 10 KiB              | ~ 100 KiB               |
     |             |                       |                         |
     | Class 2, C2 | ~ 50 KiB              | ~ 250 KiB               |

        Table 1: Classes of Constrained Devices (KiB = 1024 bytes)

   As of the writing of this document, these characteristics correspond
   to distinguishable clusters of commercially available chips and
   design cores for constrained devices.  While it is expected that the
   boundaries of these classes will move over time, Moore's law tends to
   be less effective in the embedded space than in personal computing
   devices: Gains made available by increases in transistor count and
   density are more likely to be invested in reductions of cost and
   power requirements than into continual increases in computing power.

   Class 0 devices are very constrained sensor-like motes.  Most likely
   they will not be able to communicate directly with the Internet in a
   secure manner.  Class 0 devices will participate in Internet
   communications with the help of larger devices acting as proxies,
   gateways or servers.  Class 0 devices generally cannot be secured or
   managed comprehensively in the traditional sense.  They will most
   likely be preconfigured (and will be reconfigured rarely, if at all),
   with a very small data set.  For management purposes, they could
   answer keepalive signals and send on/off or basic health indications.

   Class 1 devices cannot easily talk to other Internet nodes employing
   a full protocol stack such as using HTTP, TLS and related security
   protocols and XML-based data representations.  However, they have
   enough power to use a protocol stack specifically designed for
   constrained nodes (e.g., CoAP over UDP) and participate in meaningful
   conversations without the help of a gateway node.  In particular,
   they can provide support for the security functions required on a
   large network.  Therefore, they can be integrated as fully developed
   peers into an IP network, but they need to be parsimonious with state

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   memory, code space, and often power expenditure for protocol and
   application usage.

   Class 2 can already support mostly the same protocol stacks as used
   on notebooks or servers.  However, even these devices can benefit
   from lightweight and energy-efficient protocols and from consuming
   less bandwidth.  Furthermore, using fewer resources for networking
   leaves more resources available to applications.  Thus, using the
   protocol stacks defined for very constrained devices also on Class 2
   devices might reduce development costs and increase the

   Constrained devices with capabilities significantly beyond Class 2
   devices exist.  They are less demanding from a standards development
   point of view as they can largely use existing protocols unchanged.
   The present document therefore does not make any attempt to define
   classes beyond Class 2.  These devices can still be constrained by a
   limited energy supply.

   With respect to examining the capabilities of constrained nodes,
   particularly for Class 1 devices, it is important to understand what
   type of applications they are able to run and which protocol
   mechanisms would be most suitable.  Because of memory and other
   limitations, each specific Class 1 device might be able to support
   only a few selected functions needed for its intended operation.  In
   other words, the set of functions that can actually be supported is
   not static per device type: devices with similar constraints might
   choose to support different functions.  Even though Class 2 devices
   have some more functionality available and may be able to provide a
   more complete set of functions, they still need to be assessed for
   the type of applications they will be running and the protocol
   functions they would need.  To be able to derive any requirements,
   the use cases and the involvement of the devices in the application
   and the operational scenario need to be analyzed.  Use cases may
   combine constrained devices of multiple classes as well as more
   traditional Internet nodes.

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4.  Power Terminology

   Devices not only differ in their computing capabilities, but also in
   available electrical power and/or energy.  While it is harder to find
   recognizable clusters in this space, it is still useful to introduce
   some common terminology.

4.1.  Scaling Properties

   The power and/or energy available to a device may vastly differ, from
   kilowatts to microwatts, from essentially unlimited to hundreds of

   Instead of defining classes or clusters, we propose simply stating,
   in SI units, an approximate value for one or both of the quantities
   listed in Table 2:

   | Name   | Definition                                  | SI Unit    |
   | Ps     | Sustainable average power available for the | W (Watt)   |
   |        | device over the time it is functioning      |            |
   |        |                                             |            |
   | Et     | Total electrical energy available before    | J (Joule)  |
   |        | the energy source is exhausted              |            |

             Table 2: Quantities Relevant to Power and Energy

   The value of Et may need to be interpreted in conjunction with an
   indication over which period of time the value is given; see the next

4.2.  Classes of Energy Limitation

   As discussed above, some devices are limited in available energy as
   opposed to (or in addition to) being limited in available power.
   Where no relevant limitations exist with respect to energy, the
   device is classified as E3.  The energy limitation may be in total
   energy available in the usable lifetime of the device (e.g.  a device
   with a non-replaceable primary battery, which is discarded when this
   battery is exhausted), classified as E2.  Where the relevant
   limitation is for a specific period, this is classified as E1, e.g.
   a limited amount of energy available for the night with a solar-
   powered device, or for the period between recharges with a device
   that is manually connected to a charger, or by a periodic (primary)
   battery replacement interval.  Finally, there may be a limited amount
   of energy available for a specific event, e.g.  for a button press in

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   an energy harvesting light switch; this is classified as E0.  Note
   that many E1 devices in a sense also are E2, as the rechargeable
   battery has a limited number of useful recharging cycles.

   In summary, we distinguish (Table 3):

   | Name | Type of energy limitation    | Example Power Source        |
   | E0   | Event energy-limited         | Event-based harvesting      |
   |      |                              |                             |
   | E1   | Period energy-limited        | Battery that is             |
   |      |                              | periodically recharged or   |
   |      |                              | replaced                    |
   |      |                              |                             |
   | E2   | Lifetime energy-limited      | Non-replaceable primary     |
   |      |                              | battery                     |
   |      |                              |                             |
   | E3   | No direct quantitative       | Mains powered               |
   |      | limitations to available     |                             |
   |      | energy                       |                             |

                   Table 3: Classes of Energy Limitation

4.3.  Strategies of Using Power for Communication

   Especially when wireless transmission is used, the radio often
   consumes a big portion of the total energy consumed by the device.
   Design parameters such as the available spectrum, the desired range,
   and the bitrate aimed for, influence the power consumed during
   transmission and reception; the duration of transmission and
   reception (including potential reception) influence the total energy

   Based on the type of the energy source (e.g., battery or mains power)
   and how often device needs to communicate, it may use different kinds
   of strategies for power usage and network attachment.

   The general strategies for power usage can be described as follows:

   Always-on:  This strategy is most applicable if there is no reason
      for extreme measures for power saving.  The device can stay on in
      the usual manner all the time.  It may be useful to employ power-
      friendly hardware or limit the number of wireless transmissions,
      CPU speeds, and other aspects for general power saving and cooling
      needs, but the device can be connected to the network all the

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   Always-off:  Under this strategy, the device sleeps such long periods
      at a time that once it wakes up, it makes sense for it to not
      pretend that it has been connected to the network during sleep:
      The device re-attaches to the network as it is woken up.  The main
      optimization goal is to minimize the effort during such re-
      attachment process and any resulting application communications.

      If the device sleeps for long periods of time, and needs to
      communicate infrequently, the relative increase in energy
      expenditure during reattachment may be acceptable.

   Low-power:  This strategy is most applicable to devices that need to
      operate on a very small amount of power, but still need to be able
      to communicate on a relatively frequent basis.  This implies that
      extremely low power solutions needs to be used for the hardware,
      chosen link layer mechanisms, and so on.  Typically, given the
      small amount of time between transmissions, despite their sleep
      state these devices retain some form of network attachment to the
      network.  Techniques used for minimizing power usage for the
      network communications include minimizing any work from re-
      establishing communications after waking up, tuning the frequency
      of communications, and other parameters appropriately.

   In summary, we distinguish (Table 4):

   | Name | Strategy   | Ability to communicate                       |
   | S0   | Always-off | Re-attach when required                      |
   |      |            |                                              |
   | S1   | Low-power  | Appears connected, perhaps with high latency |
   |      |            |                                              |
   | S2   | Always-on  | Always connected                             |

           Table 4: Strategies of Using Power for Communication

   Note that the discussion above is at the device level; similar
   considerations can apply at the communications interface level.  This
   document does not define terminology for the latter.

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5.  Security Considerations

   This document introduces common terminology that does not raise any
   new security issue.  Security considerations arising from the
   constraints discussed in this document need to be discussed in the
   context of specific protocols.  For instance, [I-D.ietf-core-coap]
   section 11.6, "Constrained node considerations", discusses
   implications of specific constraints on the security mechanisms

6.  IANA Considerations

   This document has no actions for IANA.

7.  Acknowledgements

   Dominique Barthel and Peter van der Stok provided useful comments;
   Charles Palmer provided a full editorial review.

   Peter van der Stok insisted that we should have power terminology,
   hence Section 4.  The text for Section 4.3 is mostly lifted from
   [I-D.arkko-lwig-cellular] and has been adapted for this document.

8.  Informative References

   [FALL]     Fall, K., "A Delay-Tolerant Network Architecture for
              Challenged Internets", SIGCOMM 2003, 2003.

              Arkko, J., Eriksson, A., and A. Keraenen, "Building Power-
              Efficient CoAP Devices for Cellular Networks", draft-
              arkko-lwig-cellular-00 (work in progress), February 2013.

              Brandt, A. and J. Buron, "Transmission of IPv6 packets
              over ITU-T G.9959 Networks", draft-brandt-6man-lowpanz-02
              (work in progress), June 2013.

              Clausen, T., Verdiere, A., Yi, J., Herberg, U., and Y.
              Igarashi, "Observations of RPL: IPv6 Routing Protocol for
              Low power and Lossy Networks", draft-clausen-lln-rpl-
              experiences-06 (work in progress), February 2013.


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              Vasseur, J., Hui, J., Dasgupta, S., and G. Yoon, "RPL
              deployment experience in large scale networks", draft-hui-
              vasseur-roll-rpl-deployment-01 (work in progress), July

              Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
              Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets
              over BLUETOOTH Low Energy", draft-ietf-6lowpan-btle-12
              (work in progress), February 2013.

              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-12 (work in
              progress), March 2013.

              Mariager, P. and J. Petersen, "Transmission of IPv6
              Packets over DECT Ultra Low Energy", draft-mariager-
              6lowpan-v6over-dect-ule-02 (work in progress), May 2012.

   [ISQ-13]   International Electrotechnical Commission, "International
              Standard -- Quantities and units -- Part 13: Information
              science and technology", IEC 80000-13, March 2008.

   [RFC0793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
              793, September 1981.

   [RFC4838]  Cerf, V., Burleigh, S., Hooke, A., Torgerson, L., Durst,
              R., Scott, K., Fall, K., and H. Weiss, "Delay-Tolerant
              Networking Architecture", RFC 4838, April 2007.

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals", RFC
              4919, August 2007.

   [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.

   [RFC6606]  Kim, E., Kaspar, D., Gomez, C., and C. Bormann, "Problem
              Statement and Requirements for IPv6 over Low-Power

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              Wireless Personal Area Network (6LoWPAN) Routing", RFC
              6606, May 2012.

   [WEI]      Shelby, Z. and C. Bormann, "6LoWPAN: the Wireless Embedded
              Internet", ISBN 9780470747995, 2009.

              Ericsson, "More Than 50 Billion Connected Devices",
              Ericsson White Paper 284 23-3149 Uen, February 2011,

Authors' Addresses

   Carsten Bormann
   Universitaet Bremen TZI
   Postfach 330440
   D-28359 Bremen

   Phone: +49-421-218-63921

   Mehmet Ersue
   Nokia Siemens Networks
   St.-Martinstrasse 76
   81541 Munich

   Phone: +49 172 8432301

   Ari Keranen
   Hirsalantie 11
   02420 Jorvas


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