





Network Working Group                                     N. Kushalnagar
Request for Comments: 4919                                    Intel Corp
Category: Informational                                    G. Montenegro
                                                   Microsoft Corporation
                                                           C. Schumacher
                                                             Danfoss A/S
                                                             August 2007


    IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):
          Overview, Assumptions, Problem Statement, and Goals

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   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.

Table of Contents

   1. Introduction ....................................................2
   2. Overview ........................................................2
   3. Assumptions .....................................................3
   4. Problems ........................................................4
      4.1. IP Connectivity ............................................4
      4.2. Topologies .................................................5
      4.3. Limited Packet Size ........................................6
      4.4. Limited Configuration and Management .......................6
      4.5. Service Discovery ..........................................6
      4.6. Security ...................................................6
   5. Goals ...........................................................7
   6. Security Considerations .........................................9
   7. Acknowledgements ...............................................10
   8. References .....................................................10
      8.1. Normative References ......................................10
      8.2. Informative References ....................................10





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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].  IEEE 802.15.4 devices are characterized by short
   range, low bit rate, low power, and low cost.  Many of the devices
   employing IEEE 802.15.4 radios will be limited in their computational
   power, memory, and/or energy availability.

   This document gives an overview of LoWPANs and describes how they
   benefit from IP and, in particular, IPv6 networking.  It describes
   LoWPAN requirements with regards to the IP layer and the above, and
   spells out the underlying assumptions of IP for LoWPANs.  Finally, it
   describes problems associated with enabling IP communication with
   devices in a LoWPAN, and defines goals to address these in a
   prioritized manner.  Admittedly, not all items on this list may be
   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
   better understand how to coordinate with external organizations.

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 as follows:

   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.

   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.



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

   6.   Low cost.  These devices are typically associated with sensors,
        switches, etc.  This drives some of the other characteristics
        such as low processing, low memory, etc.  Numerical values for
        "low" elided on purpose since costs tend to change over time.

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

   8.   Location of the devices is typically not predefined, as they
        tend to be deployed in an ad-hoc fashion.  Furthermore,
        sometimes the location of these devices may not be easily
        accessible.  Additionally, these devices may move to new
        locations.

   9.   Devices within LoWPANs tend to be unreliable due to variety of
        reasons: uncertain radio connectivity, battery drain, device
        lockups, physical tampering, etc.

   10.  In many environments, devices connected to a LoWPAN may sleep
        for long periods of time in order to conserve energy, and are
        unable to communicate during these sleep periods.

   The following sections take into account these characteristics in
   describing the assumptions, problems statement, and goals for
   LoWPANs, and, in particular, for 6LoWPANs (IPv6-based LoWPAN
   networks).

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






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   While some LoWPAN devices are expected to be extremely limited (the
   so-called "Reduced Function Devices" or RFDs), more capable "Full
   Function Devices" (FFDs) will also be present, albeit in much smaller
   numbers.  FFDs will typically have more resources and may be mains
   powered.  Accordingly, FFDs will aid RFDs by providing functions such
   as network coordination, packet forwarding, interfacing with other
   types of networks, etc.

   The application of IP technology is assumed to provide the following
   benefits:

   1.  The pervasive nature of IP networks allows use of existing
       infrastructure.

   2.  IP-based technologies already exist, are well-known, and proven
       to be working.

   3.  An admittedly non-technical but important consideration is that
       IP networking technology is specified in open and freely
       available specifications, which is favorable or at least able to
       be better understood by a wider audience than proprietary
       solutions.

   4.  Tools for diagnostics, management, and commissioning of IP
       networks already exist.

   5.  IP-based devices can be connected readily to other IP-based
       networks, without the need for intermediate entities like
       translation gateways or proxies.

4.  Problems

   Based on the characteristics defined in the overview section, the
   following sections elaborate on the main problems with IP for
   LoWPANs.

4.1.  IP Connectivity

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

   1.  The many devices in a LoWPAN make network auto configuration and
       statelessness highly desirable.  And for this, IPv6 has ready
       solutions.

   2.  The large number of devices poses the need for a large address
       space, well met by IPv6.




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   3.  Given the limited packet size of LoWPANs, the IPv6 address format
       allows subsuming of IEEE 802.15.4 addresses if so desired.

   4.  Simple interconnectivity to other IP networks including the
       Internet.

   However, given the limited packet size, headers for IPv6 and layers
   above 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 have more capabilities in terms of
   power, computation, etc.  The requirements on the 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 (low
       chattiness) balanced with topology changes and power
       conservation.

   3.  The computation and memory requirements in the routing protocol
       should be minimal to satisfy the low cost and low power
       objectives.  Thus, storage and maintenance of large routing
       tables is detrimental.

   4.  Support for network topologies in which either FFDs or RFDs may
       be battery or mains-powered.  This implies the appropriate
       considerations for routing in the presence of sleeping nodes.

   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 or IEEE 802.11, the goal is to seamlessly
   integrate the networks built over those different technologies.
   This, of course, is a primary motivation to use IP to begin with.








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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.  Along these lines, IPv6's requirement of sub-IP
   reassembly (see Section 5) may pose challenges for low-end LoWPAN
   devices that do not have enough RAM or storage for a 1280-octet
   packet.

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
   expected to have limited display and input capabilities.
   Furthermore, the location of some of these devices may be hard to
   reach.  Accordingly, protocols used in LoWPANs should have minimal
   configuration, preferably work "out of the box", be easy to
   bootstrap, and enable the network to self heal given the inherent
   unreliable characteristic of these devices.  The size constraints of
   the link layer protocol should also be considered.  Network
   management should have little overhead, yet be powerful enough 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

   IEEE 802.15.4 mandates link-layer security based on AES, but it omits
   any details about topics like bootstrapping, key management, and
   security at higher layers.  Of course, a complete security solution
   for LoWPAN devices must consider application needs very carefully.
   Please refer to the security consideration section below for a more
   detailed discussion and in-depth security requirements.









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5.  Goals

   The goals mentioned below are general and not limited to IETF
   activities.  As such, they may not only refer to 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).  They also point at work more relevant
   to other standards bodies (e.g., desirable changes to 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 other organizations [LIAISON].

   Note that a common underlying goal is to reduce packet overhead,
   bandwidth consumption, processing requirements, and power
   consumption.

   The following are the goals according to priority for LoWPANs:

   1.  Fragmentation and Reassembly layer: As mentioned in the overview,
       the protocol data units may be as small as 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 adaptation layer must
       be provided at the layer below IP.

   2.  Header Compression: Given that in the worst case the maximum size
       available for transmitting IP packets over an 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, the
       6LoWPAN community needs to investigate using existing header
       compression techniques, and, if necessary, specify new ones.







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   3.  Address Autoconfiguration: [6LoWPAN] specifies methods for
       creating IPv6 stateless address auto configuration.  Stateless
       auto configuration (as compared to stateful) is attractive for
       6LoWPANs, because it reduces the configuration overhead on the
       hosts.  There is a need for a method to generate an "interface
       identifier" from the EUI-64 [EUI64] assigned to the IEEE 802.15.4
       device.

   4.  Mesh Routing Protocol: A routing protocol to support a multi-hop
       mesh network is necessary.  There is much published work on ad-
       hoc 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 Ad hoc On-Demand Distance Vector
       (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 routing protocols (or designing new ones) so that
       the routing packets fit within a single IEEE 802.15.4 frame.

   5.  Network Management: One of the points of transmitting IPv6
       packets is to reuse existing protocols as much as possible.
       Network management functionality is critical for LoWPANs.
       However, management solutions need to meet the resource
       constraints as well as the minimal configuration and self-healing
       functionality described in Section 4.4. The Simple Network
       Management Protocol (SNMP) [RFC3410] is widely used for
       monitoring data sources and sensors in conventional networks.
       SNMP functionality may be translated "as is" to LoWPANs with the
       benefit to utilize existing tools.  However, due to the memory,
       processing, and message size constraints, further investigation
       is required to determine if the use of SNMPv3 is suitable, or if
       an appropriate adaptation of SNMPv3 or use of different protocols
       is in order.

   6.  Implementation Considerations: 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.  Application and higher layer Considerations: 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



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       they are suitable for LoWPANs.  Furthermore, application level
       interoperability specifications may also become necessary in the
       future and may thus be specified.

   8.  Security Considerations: 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.  Security Considerations

   IPv6 over LoWPAN (6LoWPAN) applications often require confidentiality
   and integrity protection.  This can be provided at the application,
   transport, network, and/or at the link layer (i.e., within the
   6LoWPAN set of specifications).  In all these cases, prevailing
   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.

   Given these constraints, first, a threat model for 6LoWPAN devices
   needs to be developed in order to weigh any risks against the cost of
   their mitigations while making meaningful assumptions and
   simplifications.  Some examples for threats that should be considered
   are man-in-the-middle attacks and denial of service attacks.

   A separate set of security considerations apply to bootstrapping a
   6LoWPAN device into the network (e.g., for initial key
   establishment).  This generally involves application level exchanges
   or out-of-band techniques for the initial key establishment, and may
   rely on application-specific trust models; thus, it is considered
   extraneous to 6LoWPAN and is not addressed in these specifications.
   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.

   Beyond initial key establishment, protocols for subsequent key
   management as well as to secure the data traffic do fall under the
   purview of 6LoWPAN.  Here, the different alternatives (TLS, IKE/
   IPsec, etc.) must be evaluated in light of the 6LoWPAN constraints.

   One argument for using link layer security is that most IEEE 802.15.4
   devices 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



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   message integrity, such as CCM* mode. 6LoWPAN networks can operate in
   any of the previous modes, but it is desirable to utilize the most
   secure modes available for link-layer security (e.g., CCM*), 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 is beneficial to
   identify the relevant security model, and to identify a preferred set
   of cipher suites that are appropriate given the constraints.

7.  Acknowledgements

   Thanks to Geoff Mulligan, Soohong Daniel Park, Samita Chakrabarti,
   Brijesh Kumar, and Miguel Garcia for their comments and help in
   shaping this document.

8.  References

8.1.  Normative References

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

   [IEEE802.15.4] IEEE Computer Society, "IEEE Std. 802.15.4-2003",
                  October 2003.

8.2.  Informative References

   [EUI64]        "GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER (EUI-64)
                  REGISTRATION AUTHORITY", IEEE,
                  http://standards.ieee.org/
                  regauth/oui/tutorials/EUI64.html.

   [6LoWPAN]      Thomson, S., Narten, T., and T. Jinmei, "IPv6
                  Stateless Address Autoconfiguration", Work in
                  Progress, May 2005.

   [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.






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   [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.

   [SOAP]         "XML Protocol Working Group", W3C,
                  http://www.w3c.org/2000/xp/Group/.

   [LIAISON]      "IETF Liaison Activities", IETF,
                  http://www.ietf.org/liaisonActivities.html.

Authors' Addresses

   Nandakishore Kushalnagar
   Intel Corp

   EMail: nandakishore.kushalnagar@intel.com


   Gabriel Montenegro
   Microsoft Corporation

   EMail: gabriel.montenegro@microsoft.com


   Christian Peter Pii Schumacher
   Danfoss A/S

   EMail: schumacher@danfoss.com
















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