INTERNET-DRAFT Hudson Ayers
Intended Status: Informational Paul Crews
Expires: December 31, 2018 Hubert Teo
Conor McAvity
Amit Levy
Philip Levis
Stanford University
June 29, 2018
Design Considerations For Low Power Internet Protocols
draft-ayers-low-power-interop-00
Abstract
This document discusses guidelines for specifying low-power Internet
protocols in order to improve implementation interoperability. These
guidelines are based around the importance of balancing memory usage
and energy efficiency, and the importance of not relying on Postel's
law when dealing with low resource devices. This document applies
these guidelines to the IPv6 over low-power wireless personal area
networks (6LoWPAN) Internet Standard, suggesting changes that would
make it more likely for implementations to interoperate.
Status of this Memo
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provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on <Expiry Date>
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Copyright and License Notice
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This document is subject to BCP 78 and the IETF Trust's Legal
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Table of Contents
1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . . . 4
2. 6LoWPAN Interoperability Study . . . . . . . . . . . . . . . . 4
2.1 Incomplete Implementations . . . . . . . . . . . . . . . . 5
2.2 Unrealistic Bounds . . . . . . . . . . . . . . . . . . . . 6
2.2.1 Maximum Header Decompression . . . . . . . . . . . . . . 6
2.2.2 Arbitrary Next Header Compression . . . . . . . . . . . 6
2.3 No Pairing Interoperates . . . . . . . . . . . . . . . . . 7
3 Implementation Concerns . . . . . . . . . . . . . . . . . . . . 9
3.1 Processor Resources . . . . . . . . . . . . . . . . . . . . 9
5 Contributing Factors . . . . . . . . . . . . . . . . . . . . . . 11
6 Design Guidelines . . . . . . . . . . . . . . . . . . . . . . . 11
6.1 Guideline 1: Capability Spectrum . . . . . . . . . . . . . . 11
6.1.1 Guideline 1 Application to 6LoWPAN . . . . . . . . . . . 12
6.2 Guideline 2: Capability Discovery . . . . . . . . . . . . . 13
6.2.1 Guideline 2 Application to 6LoWPAN . . . . . . . . . . . 13
6.3 Guideline 3: Provide Reasonable Bounds . . . . . . . . . . . 14
6.3.1 Guideline 3 Application to 6LoWPAN . . . . . . . . . . . 14
6.4 Guideline 4: Don't Break Layering . . . . . . . . . . . . . 15
6.4.1 Guideline 4 Application to 6LoWPAN . . . . . . . . . . . 16
7 Security Considerations . . . . . . . . . . . . . . . . . . . . 18
8 IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 18
9 References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
9.1 Normative References . . . . . . . . . . . . . . . . . . . 18
9.2 Informative References . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 20
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1 Introduction
Interoperability is critical for the Internet. Not only do edge-
devices need to interoperate with the broader Internet, they should
also interoperate with other devices in the same network.
Historically, though, embedded systems and sensor networks have been
vertical silos of proprietary technologies, each using custom network
protocols and homogeneous implementations. Networks typically require
specialized gateways and cannot easily include devices from different
vendors for a variety of applications.
Interoperability is just as important in the Internet of Things, but
end hosts in the Internet of Things are less resourceful, more
diverse in capability, and less well audited than typical Internet
end hosts. Using IP allows devices from different manufacturers,
running completely different software stacks, to interoperate, share
services, and compose into larger, more complex applications. This
interoperability should exist not only between IoT devices
communicating with hosts across the broader Internet, but also
between IoT devices in the same low power wireless network. The
presence of such interoperability precludes the need for multiple
gateways to support different devices, simplifies network management,
and allows for efficient, logical communication between nearby
devices.
To address this problem, the IETF published a series of RFCs
detailing a standard format for transmitting IPv6 packets over low-
power wireless link layers such as IEEE 802.15.4 {RFC 4919][RFC
4944][RFC 6282][RFC 6775]. The 6LoWPAN RFCs define a fragmentation
format, a compression format, and more. These 6LoWPAN standards have
been adopted by a number of popular embedded operating systems,
including Contiki {CONTIKI], RiotOS {RIOT], OpenThread [OPENTHREAD],
mbedOS [ARM], and TinyOS [TINYOS].
Unfortunately, none of these implementations are complete. Each
implementation supports different subsets of 6LoWPAN. As a result,
devices built using different embedded operating systems cannot
interoperate. In fact, for every possible pairing, one implementation
is likely to transmit 6LoWPAN packets which the other cannot process.
This paper explores the reasons behind the lack of interoperability
in practice, and argues that this results from the protocol too
heavily prioritizing radio efficiency over processor resources, and
failing to consider the broad range of devices which embedded
operating systems will attempt to support. This document proposes
four guidelines for designing interoperable protocols for low-power
wireless networks, and explain them through an example application to
two 6LoWPAN standards - RFC 4944 and RFC 6282. These guidelines are
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informed by an empirical analysis of existing 6LoWPAN implementations
as well as experience implementing a full 6LoWPAN stack for the Tock
operating system [TOCK].
1.1 Terminology
Readers are expected to be familiar with all terms and concepts
discussed in "IPv6 over Low-Power Wireless Personal Area Networks
(6LoWPANs): Overview, Assumptions, Problem Statement, and Goals",
"Transmission of IPv6 Packets over IEEE 802.15.4 Networks",
"Compression Format for IPv6 Datagrams over IEEE 802.15.4-Based
Networks".
Readers would benefit from reading 6LoWPAN Neighbor Discovery (ND),
6LoWPAN routing requirements, and 6LoWPAN design/application spaces
for additional details of 6LoWPAN work.
2. 6LoWPAN Interoperability Study
The IETF's 6LoWPAN working group has been concerned with
interoperability between implementation since inception [6LO-CHART].
Indeed, members of the working group have organized ``Plugtests'',
where vendors verified correct implementation of the 6LowPAN
specifications and tested interoperability with other vendors.
Unfortunately, detailed results of these plugtests are not publicly
available.
A node sending IPv6 packets using 6LoWPAN may fragment the packet or
compress headers in a large number of ways permitted by the
specification. These choices depend both on the properties of the
packet (e.g. whether it is a UDP packet or if the origin and
destinations are in the same subnet) as well as on which compression
and fragmentation options the sender chooses to use. For two nodes to
interoperate, the 6LoWPAN implementation on each node must be able to
receive any packet the other node might send.
This document investigates the interoperability of 6LoWPAN
implementations from five common embedded software platforms: Riot
OS, Contiki, OpenThread, TinyOS, and ARM Mbed. This study is
concerned, specifically, with each implementation's ability to
receive and decode 6LoWPAN packets sent from other implementations.
This document does not explore whether devices built using these
implementations could form a network in the first place, since
6LoWPAN leaves much of the network formation process (e.g. discovery
and joining) unspecified.
The compatibility analysis in this document, between five common
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embedded software frameworks with 6LoWPAN implementations, is based
in three discoveries. First, a determination of how completely each
implementation implements the 6LoWPAN specification was obtained by
directly examining their source code. Second, this code analysis was
extended to verify that under some circumstances, each implementation
sends packets using compression or fragmentation options which
another implementation cannot decode. Finally, it was discovered that
even in cases where two implementations use compatible compression
and fragmentation options, different implementation choices, such as
header decompression bounds, limit their interoperability.
As a result, no pairing of these five implementations is fully
interoperable.
2.1 Incomplete Implementations
The 6LoWPAN protocol consists of a large number of complex and mostly
independent features which use the link-layer frame efficiently via
compression and fragmentation optimizations. Examining the code and
documentation for each of the aforementioned 6LoWPAN stacks reveals
that the stacks do not uniformly implement the specification. In
fact, each specification implements a different subset of the
requirements in the 6LoWPAN specification, and none implements the
entire specification to the letter. A visualization of the
mismatched feature support across different 6LoWPAN implementations
can be found in Table 1. Note that OT is an abbreviation used
throughout this document to refer to OpenThread.
+----------------------------------------+-------+--+----+---+------+
| Feature | Stack |
+----------------------------------------+-------+--+----+---+------+
| |Contiki|OT|Riot|Arm|TinyOS|
+----------------------------------------+-------+--+----+---+------+
| Uncompressed IPv6 | o | | o | o | o |
| 6LoWPAN Fragmentation | o |o | o | o | o |
| 1280 byte packets | o |o | o | o | o |
| Dispatch_IPHC header prefix | o |o | o | o | o |
| IPv6 Stateless Address Compression | o |o | o | o | o |
| Stateless multicast address compression| o |o | o | o | o |
| 802.15.4 16 bit short address support | |o | o | o | o |
| IPv6 Address Autoconfiguration | o |o | o | o | o |
| IPv6 Stateful Address Compression | o |o | o | o | o |
| Stateful multicast address compression | |o | o | o | |
| IPv6 TC and Flow label compression | o |o | o | o | o |
| IPv6 NH Compression: Tunneled IPv6 | |o | | o | o |
| IPv6 NH Compression: UDP | o |o | o | o | o |
| UDP port compression | o |o | o | o | o |
| UDP checksum elision | | | | | o |
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| Compression + headers past first frag | | | | | |
+----------------------------------------+-------+--+----+---+------
Table 1: 6LoWPAN Interoperability Matrix
2.2 Unrealistic Bounds
Beyond the variation in what portions of the 6LoWPAN specification
each stack implements, there also exists significant variation in how
each stack handles certain implementation-specific details. Some of
these details have little impact on interoperability, such as
decisions regarding how many fragments a stack holds for a given
packet before dropping all of them, whether to allow for
reconstruction of multiple packets simultaneously, and how long to
hold onto fragments for which the rest of the packet has not yet
arrived. Other details, however, differ in ways that significantly
affect interoperability between stacks. A discussion of two such
details follows:
2.2.1 Maximum Header Decompression
Each of the stacks analyzed imposes some limit on the maximum amount
of header decompression possible for a received packet. Such a limit
is necessary to ensure that packet and fragment buffers within a
stack are large enough for received packets. The maximum amount of
header decompression allowed by the 6LoWPAN specification is about
1200 bytes, basically, if an entire MSS IPv6 packet was sent
containing only compressed headers. Some of the stacks analyzed
decompress fragments directly into the MSS buffer which will
eventually contain the entire IPv6 packet, and thus support this
bound. Other stacks impose significantly lower limits - limits low
enough that packets could easily be constructed within the 6LoWPAN
specification that would exceed these limits. For example, Contiki's
limit of 38 bytes of header decompression is exceeded by any packet
for which the IP header is maximally compressed (38 bytes) and the
UDP header is compressed at all. Accordingly, certain stacks would
send packets with a significant amount of header compression, but
other stacks would silently drop these packets due to lacking buffer
space for fragments requiring that much decompression. Furthermore,
these stacks do not given any indication back to the sender that a
packet has been dropped for this reason, making it difficult for the
sending stack to identify how to adjust its transmission to
successfully deliver data.
2.2.2 Arbitrary Next Header Compression
Several of the 6LoWPAN stacks also impose limits on the arbitrary
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compression/decompression of IPv6 extension headers and next headers
required by the specification. The headers which must be handled are
as follows:
- IPv6 Hop-By-Hop Options Header
- IPv6 Routing Header
- IPv6 Fragment Header
- IPv6 Destination Options Header
- IPv6 Mobility Header
- IPv6 Next Header
- UDP Next Header
Further, 6LoWPAN implementations are expected to be able to
decompress at least one of each of these headers, and up to two
Destination Options headers, in almost any order. Handling all of
these possible cases can result in complex state machines, convoluted
code, and increase in code size and RAM use. Therefore, several of
the stacks examined impose a limit on this arbitrary next header
decompression - namely, Contiki and Riot. Both of these stacks only
check for the UDP Next Header. This greatly simplifies the code
required for decompression of next headers in these stacks as
compared to the others, which require recursion to handle this
arbitrary compression. The offshoot of this simplified code, however,
is that these stacks will drop packets with certain compressed
extension header configurations when other stacks send such messages.
2.3 No Pairing Interoperates
These interoperability concerns are more than theoretical: existing
6LoWPAN stacks generate valid packets that other stacks discard. This
proves that missing receive functionality is not simply a case of
limited 6LoWPAN stacks abstaining from handling packets which no
existing stacks ever generate.
What follows is a listing of each of the 10 possible combinations of
6LoWPAN stacks, accompanied by a single example packet which can be
generated by one of the stacks in the pairing which the other stack
would not receive.
Contiki, OpenThread : Contiki generated message using uncompressed
IPv6
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Contiki, Riot: Riot generated message using stateful multicast
address compression
Contiki, Mbed: Mbed generated message using compresesd, tunneled IPv6
Contiki, TinyOS: TinyOS generated message containing compressed IPv6
extension headers
OpenThread, Riot: OpenThread generated message containing any of the
IPv6 extension headers, which the OpenThread stack automatically
compresses
OpenThread, Mbed: Mbed generated IPv6 packet containing the IPv6
mobility header
OpenThread, TinyOS: OpenThread generated message for which the
destination address is compressed using stateful multicast
compression
Riot, Mbed: Mbed generated IPv6 message containing any compressed
next header other than the UDP header
Riot, TinyOS: Riot generated message for which the destination
address is compressed using stateful multicast compression
Mbed, TinyOS: Mbed generated Neighbor Discovery message using the
6LoWPAN context option as specified in RFC 6775.
This is a non-exhaustive listing, and for most of these pairings
several message formats exist which could be generated by one that
would be dropped by the other. Each instance for which a claim is
made that packets could be easily generated has been verified via
code analysis.
In addition to this code analysis, tests were performed to present
further evidence that several of these packets formats could easily
be generated via typical use of these 6LoWPAN stacks.
These tests involved slightly modifying basic example networking apps
on each stack, such that the existing 6LoWPAN interface could be used
to send certain packets. These modified examples were flashed onto
embedded hardware platforms supported by each. The transmitted
packets were captured using a wireless packet sniffer, and the
sniffed packets analyzed using Wireshark. This exercise verified that
these non-interoperable packets could in fact be sent. Further
description of this hardware generation of select packets can be
found in [DESIGN].
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3 Implementation Concerns
The 6LoWPAN specification was created with a clear goal---to allow
for IPv6 connectivity over a link-layer with an order of magnitude
smaller frame sizes than Ethernet. Unfortunately, fragmented IPv6 on
its own requires header overhead much greater than typical wireless
protocols designed for low power devices. As a result, the
specification places an extreme focus on minimizing protocol overhead
and, thus, radio utilization.
The primary problem with 6LoWPAN is that this focus was taken too
far. This focus has resulted in complex implementations that require
significant processor resources. In order for devices to
interoperate, they must be able to parse any valid received 6LoWPAN
packet that might be sent by others.
In practice, many 6LoWPAN implementations do not implement the entire
specification and, therefore, are not interoperable. This is not a
result of poor software design, but rather intentional choices to
implement different subsets of the specification that favor limited
RAM and code size, security concerns, and minimizing engineering
effort.
In fact, in some cases even these incomplete 6LoWPAN implementations
systems are too resource intensive for some devices. As a result,
several implementations allow the developer to remove portions of the
6LoWPAN stack during compilation. Even when implementations use
overlapping portions of the specification, additional
interoperability conflicts arise from different choices of memory
bounds for decompression.
3.1 Processor Resources
Evidence that developers of these 6LoWPAN stacks were concerned about
6LoWPAN's consumption of processor resources is baked into the design
of each. One of the primary indicators that each implementation was
concerned with code size is the prevalence of options to compile
limited subsets of the 6LoWPAN stack. For example, Contiki defines
the SICSLOWPAN_CONF_COMPRESSION compilation flag, which can be set to
force all Contiki packets (sent and received!) to be processed as
uncompressed IPv6. Riot presents extensive compilation options for
6LoWPAN, allowing for the exclusion of all IPHC compression, the
exclusion of context based compression alone, the exclusion of
fragmentation, the exclusion of ND, and the exclusion of next header
compression. The Mbed stack allows users to exclude elements of the
IPv6 stack such as security features, routing specific features,
link-layer features, and more. Further, Mbed defines macros which can
be used to save RAM at the expense of flash, or vice-versa. TinyOS by
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default removes all code in a stack that is not being used by an
application, and this can easily be observed by compiling different
6LoWPAN application binaries.
Table 2 shows the code size overhead of each of the five
implementations broken into independent overheads for compression,
fragmentation, mesh and broadcast headers, as well as totals for
6LoWPAN and the entire networking stack including physical layer
drivers, IPv6, UDP, ICMP, etc.
+----------+--------+---------+-------------+------+----------------+
| Stack | Code Size Measurements (Bytes) |
+----------+--------+---------+-------------+------+----------------+
| | IP-All | 6Lo-All | Compression | Frag | Mesh/Bcast Hdr |
+----------+--------+---------+-------------+------+----------------+
| Contiki | 37538 | 11262 | 5952 | 3319 | N/A |
| OT | 42262 | 26375 | 4146-20000 | 1310 | 4500 |
| Riot | 30942 | 7500 | >4712 | 1514 | N/A |
| Arm Mbed | 46030 | 22061 | 17900 | 3104 | 1331 |
| TinyOS | 37312 | 16174 | ---- | ---- | 600 |
+----------+--------+---------+-------------+------+----------------+
Table 2: 6LoWPAN Stack Code Size
The methodology use to collect these values can be found in [DESIGN].
These results likely overestimate the overhead of fragmentation and
underestimate the overhead of certain kinds of compression since some
of the complexity of compression is born on the fragmentation logic.
Moreover, for OpenThread and Arm Mbed, which required manual
examination of binaries, the results almost certainly underestimate
the overhead of all 6LoWPAN components since we only counted
procedures which unambiguously implemented particular functionality,
though some of the complexity is implemented in other portions of the
stack. In summary:
- 6LoWPAN stack developers were concerned with processor resource
requirements of the protocol.
- Fragmentation, the only portion of 6LoWPAN that's strictly
necessary for sending IPv6 packets, consumes significantly less
ROM than compression.
- Implementations with more complete adherence to compression
specification consume more code for compression
- Mesh and broadcast headers are relatively expensive given that
few real-world applications use them
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5 Contributing Factors
Several fundamental factors contributed to 6LoWPAN's interoperability
problems.
When writing low power networking specifications, an important
"slider'' exists - the tradeoff between code size and protocol
efficiency. This tradeoff is similar to the historically significant
tradeoff between RAM and code size. Techniques such as advanced MAC
and physical layers, and tracking the state of a network can reduce
packet sizes and, thus, radio energy consumption. However, these
techniques typically require larger and more complex
implementations.
Even moving beyond the constraints of code size, added complexity
harms interoperability in the general case, and complex
implementations are undesirable in the space of low power embedded
devices. Finally, 6LoWPAN failed to consider the reality that some
implementations of the protocol may be incomplete, and accordingly
failed to include any affirmative indications of interoperability
failures, with interoperability failures instead only being visible
as silent packet drops. All of these factors contributed to 6LoWPAN's
interoperability problems, and inspire the guidelines that follow.
6 Design Guidelines
This section describes four protocol design guidelines which, if
followed, lead to low-power protocols that are more likely to have
interoperable implementations. In the next section, these are
further explained by showing how each can be applied to 6LoWPAN.
6.1 Guideline 1: Capability Spectrum
A low power protocol should be implementable on devices which are at
the low end of code and RAM resources. Rather than require every
device pay the potential energy costs of fewer optimizations, a
protocol should support a spectrum of device capabilities. This
spectrum defines a clear ordering via which especially resource
constrained devices can reduce code size or RAM use by eliding
features. Such a spectrum makes a protocol usable by extremely low
resource devices without forcing more resourceful devices to
communicate inefficiently.
This capability spectrum should be a linear scale. For a device to
support capability level N, it must also support all lower capability
levels. More complex configuration approaches (e.g., a set of
independent options) would allow for a particular application or
implementation to be more efficient, picking the features that give
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the most benefit at the least complexity cost. However, this sort of
optimization then makes interoperability more difficult, as two
devices must negotiate which features to use.
6.1.1 Guideline 1 Application to 6LoWPAN
Application of this guideline would require replacing the large
collection of "MUST'' requirements - those "features'' in Table 1 -
into 6 levels of functionality. These levels prioritize features that
provide the best packet size savings given the resulting
implementation complexity. For example, the greatest savings results
from compressing 128-bit IPv6 addresses.
0. Uncompressed IPv6
0a. Uncompressed IPv6
0b. 6LoWPAN Fragmentation and the Fragment Header
0c.1280 Byte Packets
1. IPv6 Compression Basics + Stateless Address Compression
1a. Support for the Dispatch_IPHC Header Prefix
1b. Correctly handle elision of IPv6 length and version
1c. Stateless compression of unicast addresses
1d. Stateless compression of multicast addresses
1e. Compression even when 16 bit addresses are used at the link
layer
1f. IPv6 address autoconfiguration
2. Stateful IPv6 Address Compression
2a. Stateful compression of unicast addresses
2b. Stateful compression of multicast addresses
3. IPv6 Traffic Class and Flow Label Compression
3a. Traffic Class compression 3b. Flow Label Compression 3c. Hop
Limit Compression
4. IPv6 and UDP NH Compression + UDP Port Compression
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4a. Handle Tunneled IPv6 correctly
4b. Handle the compression of the UDP Next Header
4c. Correctly handle elision of the UDP length field
4d. Correctly handle the compression of UDP ports
4e. Correctly handle messages for which headers go on
longer than the first fragment, and the headers in the first
fragment are compressed.
5. Entire Specification
5a. Support the broadcast header and the mesh header as
described in RFC 4944
5b Support compression of all IPv6 Extension headers
The classes in this scale do not precisely reflect the current
feature support of the implementations described above. For example,
Contiki supports UDP port compression (level 5) but does not support
802.15.4 short addresses (level 2) or tunneled IPv6 (level 5):
following this formulation, Contiki only provides level 1 support. If
Contiki supported 16-bit addresses, it would provide level 4 support.
The specific spectrum presented here is based off of measurements of
code size, the saved bits that each additional level of compression
allows for, and observations of existing 6LoWPAN implementations.
6.2 Guideline 2: Capability Discovery
The second guideline immediately follows from the first: if two
implementations may have different capability levels, there should be
an explicit mechanism by which two devices can efficiently discover
what level to use when they communicate
If two devices wish to communicate, they default to the lower of
their supported capability levels. For example, suppose a TinyOS
device supports level 2 and a Contiki device supports level 4;
Contiki must operate at level 2 when communicating with the TinyOS
device. This requires keeping only a few bits of state for any device
to communicate with. Also, note that this state is per-hop; for a
layer 3 protocol like IP, it is stored for link-layer neighbors (not
IP endpoints) and so does not require knowledge of the whole network.
6.2.1 Guideline 2 Application to 6LoWPAN
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6lowpan could implement capability discovery using two mechanisms:
neighbor discovery (ND) and ICMP. Neighbor discovery allows devices
to probe and determine capability levels, while ICMP allows devices
to determine when incompatible features are used, or when ND is not
available.
Neighbor discovery: 6LoWPAN ND should add an option that allows a
device to communicate its capability class during association with a
network. The inclusion of a few extra bits in ND messages would allow
all devices that learn neighbor addresses via ND to also know how to
send packets which that neighbor can receive. This option minimizes
the energy cost of communicating capabilities. It is worth noting
that [RFC7400] already employs a similar method for communicating
whether devices implement General Header Compression: adding such an
option is clearly viable.
ICMP: All IPv6 devices are already required to support ICMP. A new
ICMPv6 message type - 6LoWPAN Class Unsupported - should be added,
which could be sent in response to messages received encoded using a
6LoWPAN class higher than the class of the receiving host. This
would allow for communication of capabilities even in networks not
constructed using IPv6 ND. This ICMPv6 message would allow hosts to
indicate exactly what class the receiving host does support,
preventing any need for repeated retransmissions using different
compression or fragmentation formats.
6.3 Guideline 3: Provide Reasonable Bounds
Specifications should impose reasonable bounds on recursive or
variable features so implementations can bound RAM use. These bounds
have two benefits. First, it allows implementations to safely limit
their RAM use without silent interoperability failures. E.g., today,
if an mbed device sends a 6lowpan packet whose compression is greater
than 38 bytes to a Contiki device, Contiki will silently drop the
packet. Second, it ensures that capability discovery is sufficient to
interoperate.
The original designers of a specification may not know exactly what
these values should be. This is not a new problem: TCP congestion
control, for example, had to specify initial congestion window
values. The bounds should initially be very conservative. Over time,
if increasing resources or knowledge suggests they should grow, then
future devices will have the onus of using fewer resources to
interoperate with earlier ones.
6.3.1 Guideline 3 Application to 6LoWPAN
Section 2 discussed two unreasonable bounds which affect 6LoWPAN
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interoperability. The first is the 1280 byte bound on maximum header
decompression (the amount a header will grow when decompressed). A
bound allows implementations to conserve RAM. As a result, some
implementations impose their own lower bounds, but these bounds do
not agree so some stacks cannot decompress some packets sent by other
stacks. The lack of a bound on arbitrary next header compression was
demonstrated as adding significant complexity to implementations to
service packets which should rarely be used.
To address this, maximum header decompression in 6LoWPAN packets
should be bounded to 50 bytes. This bound allows for significant RAM
savings in implementations that decompress first fragments into the
same buffer in which the fragment was originally held prior to any
copying into a 1280 byte buffer.
Second, the requirement for compression of interior headers for
tunneled IPv6 should be removed. Currently, section 4.2 of RFC 6282
states "When the identified next header is an IPv6 Header...The
following bytes MUST be encoded using LOWPAN_IPHC''. This is
problematic because it places no bound on how many tunneled IPv6
headers may need to be compressed or decompressed, creating locations
in code that require unbounded amounts of recursion. Implementations
should adjust their path MTU constraints and responses to support
inserting source routing headers, rather than tunnel IPv6.
This change would limit the complexity of arbitrary next header
compression slightly. In addition, an ordering should be imposed on
the order of IPv6 extension options if they are to be compressed.
This would allow for implementations to avoid recursive functions to
decompress these headers, and instead use simple if/else statements.
If for some reason IPv6 extension headers must be placed in a
different order for a particular packet, those options must be sent
uncompressed.
6.4 Guideline 4: Don't Break Layering
Designers should ensure that interoperability is a central priority
for specifications throughout the design process, and that
interoperability is not simply assumed from the fact that devices
will be communicating via a shared protocol. In particular,
specifications should be careful that considerations introduced to
save energy in certain scenarios should not make assumptions about
the rest of the stack. Layering is a foundational network design
principle. As the difficulty NATs introduced to Internet connectivity
in the early 2000s demonstrated, breaking layering can introduce
unforeseen and extremely difficult to fix interoperability problems.
The appeal of cross-layer optimization in embedded systems is even
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stronger than in traditional computers. Designed for a specific
application, a developer can understand and know exactly how the
entire system works, from hardware to application code. However,
while this whole-system knowledge makes sense for a particular device
or iteration of an application, long-lived systems will evolve and
change. This is especially true if the device will need to
interoperate with new gateways or application devices. Furthermore,
as embedded systems have grown more complex, their software has begun
to resemble more traditional systems. Rather than write software from
scratch every time, systems use and draw on existing operating
systems as well as libraries. By breaking layering, cross-layer
optimizations require that developers own and customize the entire
software stack.
6.4.1 Guideline 4 Application to 6LoWPAN
UDP checksum compression, as defined in section 4.3.2 of RFC 6282,
should be removed from the 6LoWPAN specification. The RFC says that a
higher layer may request the checksum be elided if it has an
integrity mechanism that covers the UDP header. At first glance, this
seems sufficient: if the UDP header is covered by a message integrity
code (MIC) or other checksum, then corrupted packets will be
correctly dropped.
However, it misses an important error case: if the UDP ports are
corrupted, then a packet missing a checksum may be delivered to the
wrong application, and this incorrect application may not impose a
replacement integrity measure or know one exists. It therefore cannot
verify the MIC. Furthermore, protecting the header with a link-layer
MIC is insufficient, as it only protects packets against sub-link
corruption.
The end-to-end principle [E2E], foundational to all modern network
design, says that only endpoints can verify correct communication.
The only place that can safely verify the UDP header is the UDP
stack. It is worth noting that the seminal example that led to
definition of the end-to-end principle was a memory corruption:
packets held in memory to be sent were corrupted before being sent.
The recommended workarounds in RFC 6282 are vulnerable to such an
event. A packet sent by an application that elides the UDP checksum
could be corrupted in memory before the link-layer MIC is computed.
Such a packet would be successfully received by the destination and
dispatched to the wrong application, which would not check the
application-level MIC.
The payoff of UDP checksum compression is not even significant - 2
bytes of checksum is a small portion of a 127 byte frame. The
problematic nature of UDP checksum compression is further
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demonstrated by the fact that only one of the five stacks analyzed in
this document implements the feature.
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7 Security Considerations
This informational document does have some implications for security
if followed.
First, capability advertisements of the type recommended in this
document are liable to leak some information regarding the type of
device sending those advertisements. In any situation for which this
information is priveleged, such advertisements must be suppressed.
Second, implementations should be careful not to take for granted
that the suggestions in this document will be implemented by all
other transmitting devices. Accordingly, though this document
recommends reasonable bounds, receivers still must be careful to
prevent buffer overflows in the event these bounds are not followed.
Finally, it is worth noting that breaking layering has clear security
implications, and that the recommendation in this document to avoid
this practice should be expected to improve security by allowing the
security protocols in place at individual layers to work as intended.
8 IANA Considerations
This is an informational document, and accordingly does not formally
request any IANA changes. However, it is worth noting that the
example application of the guidelines to 6LoWPAN would require some
changes by IANA, if actually implemented.
Namely, IANA would be requested to update some of the "6LoWPAN
Capability Bits" under the "Internet Control Message Protocol Version
6 (ICMPv6) Parameters" registry such that some of the unassigned bits
could be repurposed for capability advertisements as described in
this document.
Additionally, IANA would be requested to update the "IPv6 Neighbor
Discovery Option Formats" registry to include a new ND option format
for capability advertisements [RFC4861].
9 References
9.1 Normative References
[RFC4919] Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
over Low-Power Wireless Personal Area Networks (6LoWPANs):
Overview, Assumptions, Problem Statement, and Goals", RFC
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4919, DOI 10.17487/RFC4919, August 2007, <https://www.rfc-
editor.org/info/rfc4919>.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007, <http://www.rfc-
editor.org/info/rfc4944>.
[RFC6282] Hui, J. and P. Thubert, "Compression Format for IPv6
Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
September 2011, <http://www.rfc-editor.org/info/rfc6282>.
[RFC6775] Shelby, Z., Chakrabarti, S., Nordmark, E., and C. Bormann,
"Neighbor Discovery Optimization for IPv6 over Low-Power
Wireless Personal Area Networks (6LoWPANs)", RFC 6775,
November 2012, <http://www.rfc-editor.org/info/rfc6775>.
9.2 Informative References
[DESIGN] Ayers, H. et al., "Design Considerations for Low Power
Internet Protocols", Arxiv, June 2018.
[TINYOS] TinyOS Alliance, "TinyOS", 2018,
<https://github.com/tinyos/tinyos-main>.
[ARM] ARM Mbed, "ARM Mbed OS", 2018,
<https://github.com/ARMmbed/mbed-os>.
[RIOT] FU Berlin, "Riot OS", 2018, <https://github.com/RIOT-
OS/RIOT>.
[CONTIKI] Dunkels, A., "Contiki OS", 2018,
<https://github.com/contiki-os/contiki>.
[OPENTHREAD] Nest, "OpenThread", 2018,
<https://github.com/openthread/openthread>.
[TOCK] Levy, A., Campbell, B., Pannuto, P., Dutta, P., Levis, P.,
"The Case for Writing a Kernel in Rust", APSys, 2017,
<https://doi.org/10.1145/3124680.3124717>.
[6LO-CHART] Lemon, T., "IPv6 over Low power WPAN WG Charter", IETF,
2005, <https://datatracker.ietf.org/doc/charter-ietf-
6lowpan/>.
[E2E] Saltzer, J. H., Reed, D. P., Clark, D. D., "End-to-end
Arguments in System Design", ACM Trans. Comput. Syst.,
November 1984.
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[RFC7400] Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
IPv6 over Low-Power Wireless Personal Area Networks
(6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
2014, <https://www.rfc-editor.org/info/rfc7400>.
Authors' Addresses
Hudson Ayers
Stanford University
EMail: hayers@stanford.edu
Paul Crews
Stanford University
EMail: ptcrews@stanford.edu
Hubert Hua Kian Teo
Stanford University
EMail: hteo@stanford.edu
Conor McAvity
Stanford University
EMail: cmcavity@stanford.edu
Amit Levy
Stanford University
EMail: levya@cs.stanford.edu
Philip Levis
Stanford University
EMail: pal@stanford.edu
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