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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RFC 4919 6LoWPAN Problems and Goals August 2007
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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RFC 4919 6LoWPAN Problems and Goals August 2007
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