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Network Working Group                                     N. Kushalnagar
Internet-Draft                                                Intel Corp
Intended status: Informational                             G. Montenegro
Expires: August 30, 2007                           Microsoft Corporation
                                                           C. Schumacher
                                                             Danfoss A/S
                                                       February 26, 2007


      6LoWPAN: Overview, Assumptions, Problem Statement and Goals
                   draft-ietf-6lowpan-problem-08.txt

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   This Internet-Draft will expire on August 30, 2007.

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.





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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  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 . . . . . . . . . . .  7
     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 . . . . . . . . . . . . . . . . . . . . . . . . 12
   Intellectual Property and Copyright Statements . . . . . . . . . . 13






























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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 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
   understand better 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:

   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).





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   4.   Topologies include star and mesh operation.

   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 life-
        time 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
       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 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.
   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



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   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.

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.






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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.  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.


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.



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   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 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 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.

   3.  Address Autoconfiguration: [I-D.ietf-ipv6-rfc2462bis] 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 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.





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   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.
       [RFC3411] specifies SNMPv3 protocol operations.  SNMP
       functionality may be translated "as is" to LoWPANs.  However,
       further investigation is required to determine if it is suitable,
       or if an appropriate adaption is in order.  This adaptation could
       include limiting the data types and simplifying the Basic
       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.  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
       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.  IANA Considerations

   This document contains no IANA considerations.


7.  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



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   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
   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.


8.  Acknowledgements

   Thanks to Geoff Mulligan, Soohong Daniel Park, Samita Chakrabarti,



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   Brijesh Kumar and Miguel Garcia for their comments and help in
   shaping this document.


9.  References

9.1.  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.

9.2.  Informative References

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

   [I-D.ietf-ipv6-rfc2462bis]
              Thomson, S., "IPv6 Stateless Address Autoconfiguration",
              draft-ietf-ipv6-rfc2462bis-08 (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.

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

   [liaison]  "LIASONS",
              IETF http://www.ietf.org/liaisonActivities.html.





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Authors' Addresses

   Nandakishore Kushalnagar
   Intel Corp

   Email: nandakishore.kushalnagar@intel.com


   Gabriel Montenegro
   Microsoft Corporation

   Email: g_e_montenegro@yahoo.com


   Christian Peter Pii Schumacher
   Danfoss A/S

   Email: schumacher@danfoss.com

































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Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
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Acknowledgment

   Funding for the RFC Editor function is provided by the IETF
   Administrative Support Activity (IASA).





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