Network Working Group                                     N. Kushalnagar
Internet-Draft                                                Intel Corp
Expires: August 28, 2006                                   G. Montenegro
                                                   Microsoft Corporation
                                                       February 24, 2006

      6LoWPAN: Overview, Assumptions, Problem Statement and Goals

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Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document describes the assumptions, problem statement and goals
   for transmitting IP over IEEE 802.15.4 networks.  The set of goals
   enumerated in this document form an initial set only.  Additional
   goals may be found necessary over time and may be added to this

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Requirements notation  . . . . . . . . . . . . . . . . . .  3
   2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Assumptions  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   4.  Problems . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
     4.1.  IP Connectivity  . . . . . . . . . . . . . . . . . . . . .  5
     4.2.  Topologies . . . . . . . . . . . . . . . . . . . . . . . .  6
     4.3.  Limited Packet Size  . . . . . . . . . . . . . . . . . . .  6
     4.4.  Limited configuration and management . . . . . . . . . . .  6
     4.5.  Service discovery  . . . . . . . . . . . . . . . . . . . .  7
     4.6.  Security . . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Goals  . . . . . . . . . . . . . . . . . . . . . . . . . . . .  7
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  9
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 14

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1.  Introduction

   Low-power wireless personal area networks (LoWPANs) comprise devices
   that conform to the IEEE 802.15.4-2003 standard by the IEEE
   [ieee802.15.4].  The IEEE 802.15.4 devices are characterized by short
   range, low bit rate, low power and low cost.

   This document gives an overview of LoWPANs and describes how they
   benefit from IP and IPv6 networking.  It describes the requirements
   of LoWPANs with regards to IP layer and above.  It spells out the
   underlying assumptions of IP for LoWPANs.  Finally, it describes
   problems associated with enabling IP communication between devices in
   LoWPAN, and defines goals to address these in a prioritized manner.
   Admittedly, not all items on this list are necessarily appropriate
   tasks for the IETF.  Nevertheless, they are documented here to give a
   general overview of the larger problem.  This is useful both to
   structure work within the IETF as well as to understand better how to
   coordinate with external organizations.

1.1.  Requirements notation

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

2.  Overview

   A LoWPAN is a simple low cost communication network that allows
   wireless connectivity in applications with limited power and relaxed
   throughput requirements.  A LoWPAN typically includes devices that
   work together to connect the physical environment to real-world
   applications, e.g., wireless sensors.  LoWPANs conform to the IEEE
   802.15.4-2003 standard. [ieee802.15.4].

   Some of the characteristics of LoWPANs are:

   1.   Small packet size.  Given that the maximum physical layer packet
        is 127 bytes, the resulting maximum frame size at the media
        access control layer is 102 octets.  Link-layer security imposes
        further overhead, which in the maximum case (21 octets of
        overhead in the AES-CCM-128 case, versus 9 and 13 for AES-CCM-32
        and AES-CCM-64, respectively) leaves 81 octets for data packets.

   2.   Support for both 16-bit short or IEEE 64-bit extended media
        access control addresses.

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   3.   Low bandwidth.  Data rates of 250 kbps, 40 kbps and 20 kbps for
        each of the currently defined physical layers (2.4 GHz, 915 MHz
        and 868 MHz, respectively).

   4.   Topologies include star and mesh operation.

   5.   Low power, typically some or all devices are battery operated.

   6.   Low cost, typically associated with sensors, switches, etc.
        These drive some of the other characteristics such as low
        processing, low memory, etc.  Numerical values for "low" have
        not been explicitly mentioned here as historically the costs
        tend to change over time.

   7.   Large number of devices expected to be deployed during the life-
        time of the technology.  This number is expected to dwarf the
        number of deployed personal computers, for example.

   8.   Location of the devices are typically not predefined, thus these
        devices are deployed in an adhoc fashion.  Furthermore,
        sometimes the location of these devices may not be easily
        accessible.  Additionally these devices may move to new

   9.   Devices within LoWPANs have a higher possibility of being
        unreliable due to variety of reasons: uncertain radio
        connectivity, battery drain, device lockups, physical tampering,
   10.  Devices within LoWPANs have a higher possibility of being
        unavailable because often these devices are in sleep mode or in
        a power down mode to conserve power.

   The following sections take into account these characteristics in
   describing the assumptions, problems statement and goals for LoWPANs.

3.  Assumptions

   Given the small packet size of LoWPANs, this document presumes
   applications typically send small amounts of data.  However, the
   protocols themselves do not restrict bulk data transfers.

   LoWPANs as described in this document are based on IEEE 802.15.4-
   2003.  It is possible that the specification may undergo changes in
   the future and may change some of the requirements mentioned above.

   Some of these assumptions are based on the limited capabilities of
   devices within LoWPANs.  As devices become more powerful, and consume

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   less power, some of the requirements mentioned above may be somewhat

   Nevertheless, not all devices in a LoWPAN are expected to be
   extremely limited.  This is true of so-called "Reduced Function
   Devices" (RFDs), but not necessarily of "Full Function Devices"
   (FFDs).  These will also be present albeit in much smaller numbers,
   and will typically have more resources and be mains powered.
   Accordingly, FFDs will aid RFDs by providing functions such as
   network coordination, packet forwarding, interfacing with other types
   of networks, etc.

   IP technology is assumed to provide the following benefits:

   1.  The pervasive nature of IP networks allows use of existing
   2.  IP based technologies already exist, are well known and proven to
       be working.
   3.  An admittedly non-technical but important consideration is that
       intellectual property conditions for IP networking technology are
       either more favorable or at least better understood than
       proprietary and newer solutions.
   4.  Tools for diagnostics, management and commissioning of IP
       networks already exists.
   5.  IP based devices can more easily be connected to other IP based
       networks, without the need for translation gateways and the like.

4.  Problems

   Based on the characteristics defined in the overview section, the
   following sections elaborate on the main problems with IP for LoWPANs
   Note that a common underlying goal is to reduce packet overhead,
   bandwidth consumption, and processing requirements.

4.1.  IP Connectivity

   The requirement for IP connectivity within a LoWPAN is driven by the

   1.  The many devices in a LoWPAN make network auto configuration and
       statelessness highly desirable.  And for this, IPv6 has ready
   2.  The large number of devices poses the need for a large address
       space, well met by IPv6.
   3.  Given the limited packet size of LoWPANs, the IPv6 address format
       allows subsuming of IEEE 802.15.4 addresses if so desired.

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   4.  Simple interconnectivity to other IP networks including the

   However, given the limited packet size, headers for IPv6 and above
   layers must be compressed whenever possible.

4.2.  Topologies

   LoWPANs must support various topologies including mesh and star.

   Mesh topologies imply multi-hop routing, to a desired destination.
   In this case, intermediate devices act as packet forwarders at the
   link layer (akin to routers at the network layer).  Typically these
   are "full function devices" that has more capabilities in terms of
   power, computation, etc.  The requirements that apply on the chosen
   routing protocol are:

   1.  Given the minimal packet size of LoWPANs, the routing protocol
       must impose low (or no) overhead on data packets, hopefully
       independently of the number of hops.
   2.  The routing protocols should have low routing overhead (less
       chatty) balanced with topology changes and power conservation.
   3.  The computation and memory requirements in the routing protocol
       should be minimal to satisfy low cost and low power
       characteristics.  Thus storage and maintaining of large routing
       tables may be detrimental.

   As with mesh topologies, star topologies include provisioning a
   subset of devices with packet forwarding functionality.  If, in
   addition to IEEE 802.15.4, these devices use other kinds of network
   interfaces such as ethernet, IEEE 802.11, etc., the goal is to
   seamlessly integrate the networks built over those different
   technologies.  This, or course, is a primary motivation to use IP to
   begin with.

4.3.  Limited Packet Size

   Applications within LoWPANs are expected to originate small packets.
   Adding all layers for IP connectivity should still allow transmission
   in one frame without incurring excessive fragmentation and
   reassembly.  Furthermore, protocols must be designed or chosen so
   that the individual "control/protocol packets" fit within a single
   802.15.4 frame.

4.4.  Limited configuration and management

   As alluded to above, devices within LoWPANs are expected to be
   deployed in exceedingly large numbers.  Additionally, they are

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   expected to have limited display and input capabilities.
   Furthermore, the location of some of these devices may be hard to
   access.  As such, protocols designed for LoWPANs should have minimal
   configuration, preferably work "out of the box", provide easy
   bootstrapping, and the network should be able to self heal given the
   inherent unreliable characteristic of these devices.  The network
   management should have less overhead yet be powerful to control dense
   deployment of devices.

4.5.  Service discovery

   LoWPANs require simple service discovery network protocols to
   discover, control and maintain services provided by devices.  In some
   cases, especially in dense deployments, abstraction of several nodes
   to provide a service may be beneficial.  In order to enable such
   features, new protocols may have to be designed.

4.6.  Security

   Security for LoWPAN devices must be carefully considered depending
   upon the application needs.  IEEE 802.15.4 provides AES link layer
   security.  Due to the nature of 6LoWPAN devices, security solutions
   that need excessive computing, or bandwidth may not be suitable for
   LoWPAN devices.  Please refer to security consideration section below
   for an in depth requirements for security.

5.  Goals

   Goals mentioned here may point at relevant work that can be done
   within the IETF (e.g., specification required to transmit IP, profile
   of best practices for transmitting IP packets, and associated upper
   level protocols, etc).  It may also point at work to be done in other
   standards bodies that exist or may exist in the future (e.g.,
   desirable changes or profiles relevant to IEEE 802.15.4, W3C, etc).
   When the goals fall under the IETF's purview, they serve to point out
   what those efforts should strive to accomplish.  Regardless of
   whether they are pursued within one (or more) new (or existing)
   working groups.  When the goals do not fall under the purview of the
   IETF, documenting them here serves as input to those other
   organizations [liaison].

   The following are the goals according to priority for LoWPANs:

   1.  As mentioned in the overview, the protocol data units may be as
       small 81 bytes.  This is obviously far below the minimum IPv6
       packet size of 1280 octets, and in keeping with section 5 of the
       IPv6 specification [RFC2460], a fragmentation and reassembly

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       adaptation layer must be provided at the layer below IP.

   2.  Given that in the worst case the maximum size available for
       transmitting IP packets over IEEE 802.15.4 frame is 81 octets,
       and that the IPv6 header is 40 octets long, (without optional
       headers), this leaves only 41 octets for upper-layer protocols,
       like UDP and TCP.  UDP uses 8 octets in the header and TCP uses
       20 octets.  This leaves 33 octets for data over UDP and 21 octets
       for data over TCP.  Additionally, as pointed above, there is also
       a need for a fragmentation and reassembly layer, which will use
       even more octets leaving very few octets for data.  Thus if one
       were to use the protocols as is, it would lead to excessive
       fragmentation and reassembly even when data packets are just 10s
       of octets long.  This points to the need for header compression
       As there is much published and in-progress standardization work
       on header compression, this goal needs to investigate using
       existing header compression techniques and if necessary specify
       new ones.

   3.  [I-D.ietf-ipv6-rfc2462bis] specify methods for creating IPv6
       stateless address auto configuration.  Stateless auto
       configuration has an advantage over stateful by having less
       configuration overhead on the hosts suitable for LoWPANs.  The
       goal should specify a method to generate an "interface
       identifier" from the EUI-64 [EUI64] assigned to the IEEE 802.15.4

   4.  A routing protocol to support a multi-hop mesh network is
       necessary.  There is much published work on adhoc multi hop
       routing for devices.  Some examples include [RFC3561], [RFC3626],
       [RFC3684], all experimental.  Also, these protocols are designed
       to use IP based addresses that have large overheads.  For
       example, the AODV [RFC3561] routing protocol uses 48 octets for a
       route request based on IPv6 addressing.  Given the packet size
       constraints, transmitting this packet without fragmentation and
       reassembly may be difficult.  Thus care should be taken when
       using existing protocols or designing new protocols for routing
       so that the routing packets fit within a single IEEE 802.15.4

   5.  One of the points of transmitting IPv6 packets, is to reuse
       existing protocols as much as possible.  Network management
       functionality is critical for LoWPANs.  [RFC3411] specifies
       SNMPv3 protocol operations.  SNMP functionality may be translated
       "as is" to LoWPANs.  However, further investigation is required.
       SNMPv3 may be found to be not suitable, or it may be only
       suitable after adapting it appropriately.  This adaptation could
       include limiting the data types and simplifying the Basic

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       Encoding Rules so as to reduce the size and complexity of the
       ASN.1 parser, thereby reducing the memory and processing needs to
       better fit into the limited memory and power of LoWPAN devices.

   6.  It may be the case that transmitting IP over IEEE 802.15.4 would
       become more beneficial if implemented in a "certain" way.
       Accordingly, implementation considerations are to be documented.

   7.  As header compression becomes more prevalent, overall performance
       will depend even more on efficiency of application protocols.
       Heavyweight protocols based on XML such as SOAP [SOAP], may not
       be suitable for LoWPANs.  As such, more compact encodings (and
       perhaps protocols) may become necessary.  The goal here is to
       specify or suggest modifications to existing protocols so that it
       is suitable for LoWPANs.  Furthermore, application level
       interoperability specifications may also become necessary in the
       future and may thus be specified.

   8.  Security threats at different layers must be clearly understood
       and documented.  Bootstrapping of devices into a secure network
       could also be considered given the location, limited display,
       high density and ad hoc deployment of devices.

6.  IANA Considerations

   This document contains no IANA considerations.

7.  Security Considerations

   6lowpan applications often require confidentiality and integrity
   protection.  This can be provided at the application or transport
   level, at the network layer, and/or at the link layer, i.e. within
   the 6lowpan set of specifications.  In all these cases, 6LoWPAN
   constraints will influence the choice of a particular protocol.  Some
   of the more relevant constraints are small code size, low power
   operation, low complexity, and small bandwidth requirements.

   It is understandable that these constraints have associated
   tradeoffs.  Thus a threat model for 6LoWPAN devices needs to be first
   developed in order to weight any risks against the cost of security
   and at the same time make meaningful assumptions and simplifications.
   Some examples for threats that would be considered are man in the
   middle attacks, denial of service attacks.

   A separate set of security considerations might apply to
   bootstrapping a 6lowpan device into the network, in particular

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   initial key establishment processes.  This is generally involved with
   other application level transactions and may rely on an application-
   specific trust model; thus it will not be part of 6LoWPAN.  Some
   choices may be to use out of band communication techniques such as
   USB, infrared or NFC (Near Field Communication) for the initial key

   After the initial key establishment, subsequent key management
   protocols would fall under the purview of 6LoWPAN.  In order to be
   able to select (or design) this next set of protocols, there needs to
   be a common model of the keying material created by the initial key
   establishment.  There are a few cryptographic protocols to choose
   from.  It is to be seen if the protocols available as part of IPsec
   meet the constraints of 6LoWPAN.

   One argument for using link layer security is that most IEEE 802.15.4
   chips already have support for AES link layer security.  AES is a
   block cipher operating on blocks of fixed length, i.e., 128 bits.  To
   encrypt longer messages, several modes of operation may be used.  The
   earliest modes described, such as ECB, CBC, OFB and CFB provide only
   confidentiality, and this does not ensure message integrity.  Other
   modes have been designed which ensure both confidentiality and
   message integrity, such as CCM* mode. 6LoWPAN could choose to operate
   in one of the modes of operation, but it is desirable to utilize as
   much of link level security as possible and build upon it.

   For network layer security, two models are applicable: end-to-end
   security, e.g. using IPsec transport mode, or security that is
   limited to the wireless portion of the network, e.g. using a security
   gateway and IPsec tunnel mode.  The disadvantage of the latter is the
   larger header size, which is significant at the 6lowpan frame MTUs.
   To simplify 6lowpan implementations, it would be beneficial to
   consider security model needed and identify a preferred set of cipher
   suites that are appropriate given the 6lowpan constraints.

8.  Acknowledgements

   Thanks to :

   Geoff Mulligan

   Soohong Daniel Park

   Samita Chakrabarti

   Brijesh Kumar

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   for their comments and help shaping this document.

9.  References

9.1.  Normative References


              Narten, T., "Neighbor Discovery for IP version 6 (IPv6)",
              draft-ietf-ipv6-2461bis-05 (work in progress),
              October 2005.

              Thomson, S., "IPv6 Stateless Address Autoconfiguration",
              draft-ietf-ipv6-rfc2462bis-08 (work in progress),
              May 2005.

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

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

              IEEE Computer Society, "IEEE Std. 802.15.4-2003",
              October 2003.

9.2.  Informative References

   [RFC3411]  Harrington, D., Presuhn, R., and B. Wijnen, "An
              Architecture for Describing Simple Network Management
              Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
              December 2002.

   [RFC3561]  Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
              Demand Distance Vector (AODV) Routing", RFC 3561,
              July 2003.

   [RFC3626]  Clausen, T. and P. Jacquet, "Optimized Link State Routing
              Protocol (OLSR)", RFC 3626, October 2003.

   [RFC3684]  Ogier, R., Templin, F., and M. Lewis, "Topology
              Dissemination Based on Reverse-Path Forwarding (TBRPF)",
              RFC 3684, February 2004.

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   [RFC3756]  Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
              Discovery (ND) Trust Models and Threats", RFC 3756,
              May 2004.

   [SOAP]     "SOAP", W3C

   [liaison]  "LIASONS",

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Authors' Addresses

   Nandakishore Kushalnagar
   Intel Corp


   Gabriel Montenegro
   Microsoft Corporation


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