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Network Working Group                                     N. KushalnagarRequest for Comments: 4919                                    Intel CorpCategory: Informational                                    G. Montenegro                                                   Microsoft Corporation                                                           C. Schumacher                                                             Danfoss A/S                                                             August 2007IPv6 over Low-Power Wireless Personal Area Networks (6LoWPANs):Overview, Assumptions, Problem Statement, and GoalsStatus 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 Contents1. Introduction ....................................................22. Overview ........................................................23. Assumptions .....................................................34. Problems ........................................................44.1. IP Connectivity ............................................44.2. Topologies .................................................54.3. Limited Packet Size ........................................64.4. Limited Configuration and Management .......................64.5. Service Discovery ..........................................64.6. Security ...................................................65. Goals ...........................................................76. Security Considerations .........................................97. Acknowledgements ...............................................108. References .....................................................108.1. Normative References ......................................108.2. Informative References ....................................10Kushalnagar, et al.          Informational                      [Page 1]

RFC 4919               6LoWPAN Problems and Goals            August 20071.  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.Kushalnagar, et al.          Informational                      [Page 2]

RFC 4919               6LoWPAN Problems and Goals            August 2007   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.Kushalnagar, et al.          Informational                      [Page 3]

RFC 4919               6LoWPAN Problems and Goals            August 2007   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.Kushalnagar, et al.          Informational                      [Page 4]

RFC 4919               6LoWPAN Problems and Goals            August 2007   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.Kushalnagar, et al.          Informational                      [Page 5]

RFC 4919               6LoWPAN Problems and Goals            August 20074.3.  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 (seeSection 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.Kushalnagar, et al.          Informational                      [Page 6]

RFC 4919               6LoWPAN Problems and Goals            August 20075.  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 withSection 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.Kushalnagar, et al.          Informational                      [Page 7]

RFC 4919               6LoWPAN Problems and Goals            August 2007   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 inSection 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 thatKushalnagar, et al.          Informational                      [Page 8]

RFC 4919               6LoWPAN Problems and Goals            August 2007       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 andKushalnagar, et al.          Informational                      [Page 9]

RFC 4919               6LoWPAN Problems and Goals            August 2007   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.  References8.1.  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,RFC3411, December 2002.Kushalnagar, et al.          Informational                     [Page 10]

RFC 4919               6LoWPAN Problems and Goals            August 2007   [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.comKushalnagar, et al.          Informational                     [Page 11]

RFC 4919               6LoWPAN Problems and Goals            August 2007Full Copyright Statement   Copyright (C) The IETF Trust (2007).   This document is subject to the rights, licenses and restrictions   contained inBCP 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   THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS   OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF   THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.Intellectual Property   The IETF takes no position regarding the validity or scope of any   Intellectual Property Rights or other rights that might be claimed to   pertain to the implementation or use of the technology described in   this document or the extent to which any license under such rights   might or might not be available; nor does it represent that it has   made any independent effort to identify any such rights.  Information   on the procedures with respect to rights in RFC documents can be   found inBCP 78 andBCP 79.   Copies of IPR disclosures made to the IETF Secretariat and any   assurances of licenses to be made available, or the result of an   attempt made to obtain a general license or permission for the use of   such proprietary rights by implementers or users of this   specification can be obtained from the IETF on-line IPR repository athttp://www.ietf.org/ipr.   The IETF invites any interested party to bring to its attention any   copyrights, patents or patent applications, or other proprietary   rights that may cover technology that may be required to implement   this standard.  Please address the information to the IETF at   ietf-ipr@ietf.org.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Kushalnagar, et al.          Informational                     [Page 12]