Network Working Group N. Kushalnagar
Internet-Draft Intel Corp
Expires: August 28, 2006 G. Montenegro
Microsoft Corporation
February 24, 2006
6LoWPAN: Overview, Assumptions, Problem Statement and Goals
draft-ietf-6lowpan-problem-02.txt
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Copyright (C) The Internet Society (2006).
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. Additional
goals may be found necessary over time and may be added to this
document.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements notation . . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Assumptions . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. IP Connectivity . . . . . . . . . . . . . . . . . . . . . 5
4.2. Topologies . . . . . . . . . . . . . . . . . . . . . . . . 6
4.3. Limited Packet Size . . . . . . . . . . . . . . . . . . . 6
4.4. Limited configuration and management . . . . . . . . . . . 6
4.5. Service discovery . . . . . . . . . . . . . . . . . . . . 7
4.6. Security . . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Goals . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 9
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . . 11
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . . . 14
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1. Introduction
Low-power wireless personal area networks (LoWPANs) comprise devices
that conform to the IEEE 802.15.4-2003 standard by the IEEE
[ieee802.15.4]. The IEEE 802.15.4 devices are characterized by short
range, low bit rate, low power and low cost.
This document gives an overview of LoWPANs and describes how they
benefit from IP and IPv6 networking. It describes the requirements
of LoWPANs with regards to IP layer and above. It spells out the
underlying assumptions of IP for LoWPANs. Finally, it describes
problems associated with enabling IP communication between devices in
LoWPAN, and defines goals to address these in a prioritized manner.
Admittedly, not all items on this list are necessarily appropriate
tasks for the IETF. Nevertheless, they are documented here to give a
general overview of the larger problem. This is useful both to
structure work within the IETF as well as to understand better how to
coordinate with external organizations.
1.1. Requirements notation
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Overview
A LoWPAN is a simple low cost communication network that allows
wireless connectivity in applications with limited power and relaxed
throughput requirements. A LoWPAN typically includes devices that
work together to connect the physical environment to real-world
applications, e.g., wireless sensors. LoWPANs conform to the IEEE
802.15.4-2003 standard. [ieee802.15.4].
Some of the characteristics of LoWPANs are:
1. Small packet size. Given that the maximum physical layer packet
is 127 bytes, the resulting maximum frame size at the media
access control layer is 102 octets. Link-layer security imposes
further overhead, which in the maximum case (21 octets of
overhead in the AES-CCM-128 case, versus 9 and 13 for AES-CCM-32
and AES-CCM-64, respectively) leaves 81 octets for data packets.
2. Support for both 16-bit short or IEEE 64-bit extended media
access control addresses.
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3. Low bandwidth. Data rates of 250 kbps, 40 kbps and 20 kbps for
each of the currently defined physical layers (2.4 GHz, 915 MHz
and 868 MHz, respectively).
4. Topologies include star and mesh operation.
5. Low power, typically some or all devices are battery operated.
6. Low cost, typically associated with sensors, switches, etc.
These drive some of the other characteristics such as low
processing, low memory, etc. Numerical values for "low" have
not been explicitly mentioned here as historically the costs
tend to change over time.
7. Large number of devices expected to be deployed during the life-
time of the technology. This number is expected to dwarf the
number of deployed personal computers, for example.
8. Location of the devices are typically not predefined, thus these
devices are deployed in an adhoc fashion. Furthermore,
sometimes the location of these devices may not be easily
accessible. Additionally these devices may move to new
locations.
9. Devices within LoWPANs have a higher possibility of being
unreliable due to variety of reasons: uncertain radio
connectivity, battery drain, device lockups, physical tampering,
etc.
10. Devices within LoWPANs have a higher possibility of being
unavailable because often these devices are in sleep mode or in
a power down mode to conserve power.
The following sections take into account these characteristics in
describing the assumptions, problems statement and goals for LoWPANs.
3. Assumptions
Given the small packet size of LoWPANs, this document presumes
applications typically send small amounts of data. However, the
protocols themselves do not restrict bulk data transfers.
LoWPANs as described in this document are based on IEEE 802.15.4-
2003. It is possible that the specification may undergo changes in
the future and may change some of the requirements mentioned above.
Some of these assumptions are based on the limited capabilities of
devices within LoWPANs. As devices become more powerful, and consume
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less power, some of the requirements mentioned above may be somewhat
relaxed.
Nevertheless, not all devices in a LoWPAN are expected to be
extremely limited. This is true of so-called "Reduced Function
Devices" (RFDs), but not necessarily of "Full Function Devices"
(FFDs). These will also be present albeit in much smaller numbers,
and will typically have more resources and be mains powered.
Accordingly, FFDs will aid RFDs by providing functions such as
network coordination, packet forwarding, interfacing with other types
of networks, etc.
IP technology is assumed to provide the following benefits:
1. The pervasive nature of IP networks allows use of existing
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 exists.
5. IP based devices can more easily be connected to other IP based
networks, without the need for translation gateways and the like.
4. Problems
Based on the characteristics defined in the overview section, the
following sections elaborate on the main problems with IP for LoWPANs
Note that a common underlying goal is to reduce packet overhead,
bandwidth consumption, and processing requirements.
4.1. IP Connectivity
The requirement for IP connectivity within a LoWPAN is driven by the
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.
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4. Simple interconnectivity to other IP networks including the
Internet.
However, given the limited packet size, headers for IPv6 and above
layers must be compressed whenever possible.
4.2. Topologies
LoWPANs must support various topologies including mesh and star.
Mesh topologies imply multi-hop routing, to a desired destination.
In this case, intermediate devices act as packet forwarders at the
link layer (akin to routers at the network layer). Typically these
are "full function devices" that has more capabilities in terms of
power, computation, etc. The requirements that apply on the chosen
routing protocol are:
1. Given the minimal packet size of LoWPANs, the routing protocol
must impose low (or no) overhead on data packets, hopefully
independently of the number of hops.
2. The routing protocols should have low routing overhead (less
chatty) balanced with topology changes and power conservation.
3. The computation and memory requirements in the routing protocol
should be minimal to satisfy low cost and low power
characteristics. Thus storage and maintaining of large routing
tables may be detrimental.
As with mesh topologies, star topologies include provisioning a
subset of devices with packet forwarding functionality. If, in
addition to IEEE 802.15.4, these devices use other kinds of network
interfaces such as ethernet, IEEE 802.11, etc., the goal is to
seamlessly integrate the networks built over those different
technologies. This, or course, is a primary motivation to use IP to
begin with.
4.3. Limited Packet Size
Applications within LoWPANs are expected to originate small packets.
Adding all layers for IP connectivity should still allow transmission
in one frame without incurring excessive fragmentation and
reassembly. Furthermore, protocols must be designed or chosen so
that the individual "control/protocol packets" fit within a single
802.15.4 frame.
4.4. Limited configuration and management
As alluded to above, devices within LoWPANs are expected to be
deployed in exceedingly large numbers. Additionally, they are
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expected to have limited display and input capabilities.
Furthermore, the location of some of these devices may be hard to
access. As such, protocols designed for LoWPANs should have minimal
configuration, preferably work "out of the box", provide easy
bootstrapping, and the network should be able to self heal given the
inherent unreliable characteristic of these devices. The network
management should have less overhead yet be powerful to control dense
deployment of devices.
4.5. Service discovery
LoWPANs require simple service discovery network protocols to
discover, control and maintain services provided by devices. In some
cases, especially in dense deployments, abstraction of several nodes
to provide a service may be beneficial. In order to enable such
features, new protocols may have to be designed.
4.6. Security
Security for LoWPAN devices must be carefully considered depending
upon the application needs. IEEE 802.15.4 provides AES link layer
security. Due to the nature of 6LoWPAN devices, security solutions
that need excessive computing, or bandwidth may not be suitable for
LoWPAN devices. Please refer to security consideration section below
for an in depth requirements for security.
5. Goals
Goals mentioned here may point at relevant work that can be done
within the IETF (e.g., specification required to transmit IP, profile
of best practices for transmitting IP packets, and associated upper
level protocols, etc). It may also point at work to be done in other
standards bodies that exist or may exist in the future (e.g.,
desirable changes or profiles relevant to IEEE 802.15.4, W3C, etc).
When the goals fall under the IETF's purview, they serve to point out
what those efforts should strive to accomplish. Regardless of
whether they are pursued within one (or more) new (or existing)
working groups. When the goals do not fall under the purview of the
IETF, documenting them here serves as input to those other
organizations [liaison].
The following are the goals according to priority for LoWPANs:
1. As mentioned in the overview, the protocol data units may be as
small 81 bytes. This is obviously far below the minimum IPv6
packet size of 1280 octets, and in keeping with section 5 of the
IPv6 specification [RFC2460], a fragmentation and reassembly
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adaptation layer must be provided at the layer below IP.
2. Given that in the worst case the maximum size available for
transmitting IP packets over IEEE 802.15.4 frame is 81 octets,
and that the IPv6 header is 40 octets long, (without optional
headers), this leaves only 41 octets for upper-layer protocols,
like UDP and TCP. UDP uses 8 octets in the header and TCP uses
20 octets. This leaves 33 octets for data over UDP and 21 octets
for data over TCP. Additionally, as pointed above, there is also
a need for a fragmentation and reassembly layer, which will use
even more octets leaving very few octets for data. Thus if one
were to use the protocols as is, it would lead to excessive
fragmentation and reassembly even when data packets are just 10s
of octets long. This points to the need for header compression
As there is much published and in-progress standardization work
on header compression, this goal needs to investigate using
existing header compression techniques and if necessary specify
new ones.
3. [I-D.ietf-ipv6-rfc2462bis] specify methods for creating IPv6
stateless address auto configuration. Stateless auto
configuration has an advantage over stateful by having less
configuration overhead on the hosts suitable for LoWPANs. The
goal should specify a method to generate an "interface
identifier" from the EUI-64 [EUI64] assigned to the IEEE 802.15.4
device.
4. A routing protocol to support a multi-hop mesh network is
necessary. There is much published work on adhoc multi hop
routing for devices. Some examples include [RFC3561], [RFC3626],
[RFC3684], all experimental. Also, these protocols are designed
to use IP based addresses that have large overheads. For
example, the AODV [RFC3561] routing protocol uses 48 octets for a
route request based on IPv6 addressing. Given the packet size
constraints, transmitting this packet without fragmentation and
reassembly may be difficult. Thus care should be taken when
using existing protocols or designing new protocols for routing
so that the routing packets fit within a single IEEE 802.15.4
frame.
5. One of the points of transmitting IPv6 packets, is to reuse
existing protocols as much as possible. Network management
functionality is critical for LoWPANs. [RFC3411] specifies
SNMPv3 protocol operations. SNMP functionality may be translated
"as is" to LoWPANs. However, further investigation is required.
SNMPv3 may be found to be not suitable, or it may be only
suitable after adapting it appropriately. This adaptation could
include limiting the data types and simplifying the Basic
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Encoding Rules so as to reduce the size and complexity of the
ASN.1 parser, thereby reducing the memory and processing needs to
better fit into the limited memory and power of LoWPAN devices.
6. It may be the case that transmitting IP over IEEE 802.15.4 would
become more beneficial if implemented in a "certain" way.
Accordingly, implementation considerations are to be documented.
7. As header compression becomes more prevalent, overall performance
will depend even more on efficiency of application protocols.
Heavyweight protocols based on XML such as SOAP [SOAP], may not
be suitable for LoWPANs. As such, more compact encodings (and
perhaps protocols) may become necessary. The goal here is to
specify or suggest modifications to existing protocols so that it
is suitable for LoWPANs. Furthermore, application level
interoperability specifications may also become necessary in the
future and may thus be specified.
8. Security threats at different layers must be clearly understood
and documented. Bootstrapping of devices into a secure network
could also be considered given the location, limited display,
high density and ad hoc deployment of devices.
6. IANA Considerations
This document contains no IANA considerations.
7. Security Considerations
6lowpan applications often require confidentiality and integrity
protection. This can be provided at the application or transport
level, at the network layer, and/or at the link layer, i.e. within
the 6lowpan set of specifications. In all these cases, 6LoWPAN
constraints will influence the choice of a particular protocol. Some
of the more relevant constraints are small code size, low power
operation, low complexity, and small bandwidth requirements.
It is understandable that these constraints have associated
tradeoffs. Thus a threat model for 6LoWPAN devices needs to be first
developed in order to weight any risks against the cost of security
and at the same time make meaningful assumptions and simplifications.
Some examples for threats that would be considered are man in the
middle attacks, denial of service attacks.
A separate set of security considerations might apply to
bootstrapping a 6lowpan device into the network, in particular
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initial key establishment processes. This is generally involved with
other application level transactions and may rely on an application-
specific trust model; thus it will not be part of 6LoWPAN. Some
choices may be to use out of band communication techniques such as
USB, infrared or NFC (Near Field Communication) for the initial key
establishment.
After the initial key establishment, subsequent key management
protocols would fall under the purview of 6LoWPAN. In order to be
able to select (or design) this next set of protocols, there needs to
be a common model of the keying material created by the initial key
establishment. There are a few cryptographic protocols to choose
from. It is to be seen if the protocols available as part of IPsec
meet the constraints of 6LoWPAN.
One argument for using link layer security is that most IEEE 802.15.4
chips already have support for AES link layer security. AES is a
block cipher operating on blocks of fixed length, i.e., 128 bits. To
encrypt longer messages, several modes of operation may be used. The
earliest modes described, such as ECB, CBC, OFB and CFB provide only
confidentiality, and this does not ensure message integrity. Other
modes have been designed which ensure both confidentiality and
message integrity, such as CCM* mode. 6LoWPAN could choose to operate
in one of the modes of operation, but it is desirable to utilize as
much of link level security as possible and build upon it.
For network layer security, two models are applicable: end-to-end
security, e.g. using IPsec transport mode, or security that is
limited to the wireless portion of the network, e.g. using a security
gateway and IPsec tunnel mode. The disadvantage of the latter is the
larger header size, which is significant at the 6lowpan frame MTUs.
To simplify 6lowpan implementations, it would be beneficial to
consider security model needed and identify a preferred set of cipher
suites that are appropriate given the 6lowpan constraints.
8. Acknowledgements
Thanks to :
Geoff Mulligan
Soohong Daniel Park
Samita Chakrabarti
Brijesh Kumar
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for their comments and help shaping this document.
9. References
9.1. Normative References
[EUI64] "GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER (EUI-64)
REGISTRATION AUTHORITY", IEEE http://standards.ieee.org/
regauth/oui/tutorials/EUI64.html.
[I-D.ietf-ipv6-2461bis]
Narten, T., "Neighbor Discovery for IP version 6 (IPv6)",
draft-ietf-ipv6-2461bis-05 (work in progress),
October 2005.
[I-D.ietf-ipv6-rfc2462bis]
Thomson, S., "IPv6 Stateless Address Autoconfiguration",
draft-ietf-ipv6-rfc2462bis-08 (work in progress),
May 2005.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", RFC 2460, December 1998.
[ieee802.15.4]
IEEE Computer Society, "IEEE Std. 802.15.4-2003",
October 2003.
9.2. Informative References
[RFC3411] Harrington, D., Presuhn, R., and B. Wijnen, "An
Architecture for Describing Simple Network Management
Protocol (SNMP) Management Frameworks", STD 62, RFC 3411,
December 2002.
[RFC3561] Perkins, C., Belding-Royer, E., and S. Das, "Ad hoc On-
Demand Distance Vector (AODV) Routing", RFC 3561,
July 2003.
[RFC3626] Clausen, T. and P. Jacquet, "Optimized Link State Routing
Protocol (OLSR)", RFC 3626, October 2003.
[RFC3684] Ogier, R., Templin, F., and M. Lewis, "Topology
Dissemination Based on Reverse-Path Forwarding (TBRPF)",
RFC 3684, February 2004.
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[RFC3756] Nikander, P., Kempf, J., and E. Nordmark, "IPv6 Neighbor
Discovery (ND) Trust Models and Threats", RFC 3756,
May 2004.
[SOAP] "SOAP", W3C 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: gabriel_montenegro_2000@yahoo.com
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