Internet DRAFT - draft-ietf-lwig-guidance
draft-ietf-lwig-guidance
LWIG Working Group C. Bormann, Ed.
Internet-Draft Universitaet Bremen TZI
Intended status: Informational February 25, 2013
Expires: August 29, 2013
Guidance for Light-Weight Implementations of the Internet Protocol Suite
draft-ietf-lwig-guidance-03
Abstract
Implementation of Internet protocols on small devices benefits from
light-weight implementation techniques, which are often not
documented in an accessible way.
This document provides a first outline of and some initial content
for the Light-Weight Implementation Guidance document planned by the
IETF working group LWIG.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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This Internet-Draft will expire on August 29, 2013.
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Copyright (c) 2013 IETF Trust and the persons identified as the
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Objectives . . . . . . . . . . . . . . . . . . . . . . . 4
1.2. Call for contributions . . . . . . . . . . . . . . . . . 5
1.3. Terminology used in this document . . . . . . . . . . . . 5
1.4. Scope of the present document . . . . . . . . . . . . . . 6
1.5. Terminology boilerplate . . . . . . . . . . . . . . . . . 6
2. Drawing the Landscape . . . . . . . . . . . . . . . . . . . . 6
2.1. Design Objectives . . . . . . . . . . . . . . . . . . . . 6
2.2. Implementation Styles . . . . . . . . . . . . . . . . . . 7
2.3. Roles of nodes . . . . . . . . . . . . . . . . . . . . . 8
2.4. Overview over the document . . . . . . . . . . . . . . . 8
3. Data Plane Protocols . . . . . . . . . . . . . . . . . . . . 8
3.1. Link Adaptation Layer . . . . . . . . . . . . . . . . . . 8
3.1.1. Fragmentation in a 6LoWPAN Route-Over Configuration . 8
3.1.1.1. Implementation Considerations for Not-So-
Constrained Nodes . . . . . . . . . . . . . . . . 10
3.2. Network Layer . . . . . . . . . . . . . . . . . . . . . . 10
3.3. Transport Layer . . . . . . . . . . . . . . . . . . . . . 10
3.3.1. TCP . . . . . . . . . . . . . . . . . . . . . . . . . 10
3.3.1.1. Absolutely required TCP behaviors for proper
functioning and interoperability . . . . . . . . 11
3.3.1.2. Strongly encouraged, but non-essential, behaviors
of TCP . . . . . . . . . . . . . . . . . . . . . 12
3.3.1.3. Experimental extensions that are not yet standard
practices . . . . . . . . . . . . . . . . . . . . 13
3.3.1.4. Others . . . . . . . . . . . . . . . . . . . . . 13
3.4. Application Layer . . . . . . . . . . . . . . . . . . . . 14
3.4.1. General considerations about Application Programming
Interfaces (APIs) . . . . . . . . . . . . . . . . . . 14
3.4.2. Constrained Application Protocol (CoAP) . . . . . . . 14
3.4.2.1. Message Layer Processing . . . . . . . . . . . . 15
3.4.2.2. Message Parsing . . . . . . . . . . . . . . . . . 16
3.4.2.3. Storing Used Message IDs . . . . . . . . . . . . 17
3.4.3. (Other Application Protocols...) . . . . . . . . . . 20
4. Control Plane Protocols . . . . . . . . . . . . . . . . . . . 20
4.1. Link Layer Support . . . . . . . . . . . . . . . . . . . 20
4.2. Network Layer . . . . . . . . . . . . . . . . . . . . . . 20
4.3. Routing . . . . . . . . . . . . . . . . . . . . . . . . . 21
4.4. Host Configuration and Lookup Services . . . . . . . . . 21
4.5. Network Management . . . . . . . . . . . . . . . . . . . 21
4.5.1. SNMP . . . . . . . . . . . . . . . . . . . . . . . . 21
4.5.1.1. Background . . . . . . . . . . . . . . . . . . . 21
4.5.1.2. Revisiting SNMP implementation for resource
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constrained devices . . . . . . . . . . . . . . . 22
4.5.1.3. Proposed approach for building an memory
efficient SNMP agent . . . . . . . . . . . . . . 22
4.5.1.4. Example . . . . . . . . . . . . . . . . . . . . . 23
4.5.1.5. Further improvements . . . . . . . . . . . . . . 25
4.5.1.6. Conclusion . . . . . . . . . . . . . . . . . . . 26
5. Security protocols . . . . . . . . . . . . . . . . . . . . . 26
5.1. Cryptography for Constrained Devices . . . . . . . . . . 26
5.2. Transport Layer Security . . . . . . . . . . . . . . . . 26
5.3. Network Layer Security . . . . . . . . . . . . . . . . . 26
5.4. Network Access Control . . . . . . . . . . . . . . . . . 26
5.4.1. PANA . . . . . . . . . . . . . . . . . . . . . . . . 26
5.4.1.1. PANA AVPs . . . . . . . . . . . . . . . . . . . . 27
5.4.1.2. PANA Phases . . . . . . . . . . . . . . . . . . . 27
5.4.1.3. PANA session state parameters . . . . . . . . . . 29
6. Wire-Visible Constraints . . . . . . . . . . . . . . . . . . 31
7. Wire-Invisible Constraints . . . . . . . . . . . . . . . . . 31
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 32
9. Security Considerations . . . . . . . . . . . . . . . . . . . 32
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 32
10.1. Contributors . . . . . . . . . . . . . . . . . . . . . . 32
11. References . . . . . . . . . . . . . . . . . . . . . . . . . 33
11.1. Normative References . . . . . . . . . . . . . . . . . . 33
11.2. Informative References . . . . . . . . . . . . . . . . . 33
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 34
1. Introduction
Today's Internet is experienced by users as a set of applications,
such as email, instant messaging, and social networks. There are
substantial differences in performance between the various end
devices with these applications, but in general end devices
participating in the Internet today are considered to have relatively
high performance.
More and more communications technology is being embedded into our
environment. Different types of devices in our buildings, vehicles,
equipment and other objects have a need to communicate. It is
expected that most of these devices will employ the Internet Protocol
suite. The term "Internet of Things" denotes a trend where a large
number of devices directly benefit from communication services that
use Internet protocols. Many of these devices are not primarily
computing devices operated by humans, but exist as components in
buildings, vehicles, and the environment. There will be a lot of
variation in the computing power, available memory, communications
bandwidth, and other capabilities between different types of these
devices. With many low-cost, low-power and otherwise constrained
devices, it is not always easy to embed all the necessary features.
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Historically, there has been a trend to invent special "light-weight"
_protocols_ to connect the most constrained devices. However, much
of this development can simply run on existing Internet protocols,
provided some attention is given to achieving light-weight
_implementations_. In some cases the new, constrained environments
can indeed benefit from protocol optimizations and additional
protocols that help optimize Internet communications and lower the
computational requirements. Examples of IETF standardization efforts
targeted for these environments include the "IPv6 over Low power WPAN
(6LoWPAN)", "Routing Over Low power and Lossy networks (ROLL)", and
"Constrained RESTful Environments (CoRE)" working groups. More
generally, however, techniques are required to implement both these
optimized protocols as well as the other protocols of the Internet
protocol suite in a way that makes them applicable to a wider range
of devices.
1.1. Objectives
The present document, a product of the IETF Light-Weight
Implementation Guidance (LWIG) Working Group, focuses on helping the
implementers of the smallest devices. The goal is to be able to
build minimal yet interoperable IP-capable devices for the most
constrained environments.
Building a small implementation does not have to be hard. Many small
devices use stripped down versions of general purpose operating
systems and their TCP/IP stacks. However, there are implementations
that go even further in minimization and can exist in as few as a
couple of kilobytes of code, as on some devices this level of
optimization is necessary. Technical and cost considerations may
limit the computing power, battery capacity, available memory, or
communications bandwidth that can be provided. To overcome these
limitations the implementers have to employ the right hardware and
software mechanisms. For instance, certain types of memory
management or even fixed memory allocation may be required. It is
also useful to understand what is necessary from the point of view of
the communications protocols and the application employing them. For
instance, a device that only acts as a client or only requires one
connection can simplify its TCP implementation considerably.
The purpose of this document is to collect experiences from
implementers of IP stacks in constrained devices. The focus is on
techniques that have been used in actual implementations and do not
impact interoperability with other devices. The techniques shall
also not affect conformance to the relevant specifications. We
describe implementation techniques for reducing complexity, memory
footprint, or power usage.
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The topics for this working group will be chosen from Internet
protocols that are in wide use today, such as IPv4 and IPv6; UDP and
TCP; ICMPv4/v6, MLD/IGMP and ND; DNS and DHCPv4/v6; TLS, DTLS and
IPsec; as well as from the optimized protocols that result from the
work of the 6LoWPAN, RPL, and CoRE working groups. This document
will be helpful for the implementers of new devices or for the
implementers of new general-purpose small IP stacks. It is also
expected that the document will increase our knowledge of what
existing small implementations do, and will help in the further
optimization of the existing implementations. In areas where the
considerations for small implementations have already been documented
in an accessible way, we will refer to those documents instead of
duplicating the material here.
Generic hardware design advice and software implementation techniques
are outside the scope of this document. Protocol implementation
experience, however, is the focus. There is no intention to describe
any new protocols or protocol behavior modifications beyond what is
already allowed by existing RFCs, because it is important to ensure
that different types of devices can work together. For example,
implementation techniques relating to security mechanisms are within
scope, but mere removal of security functionality from a protocol is
rarely an acceptable approach.
1.2. Call for contributions
The present draft of the document is an outline that will grow with
the contributions received, which are expressly invited. As this
document focuses on experience from existing implementations, this
requires implementer input; in particular, participation is required
from the implementers of existing small IP stacks. "Small" here is
intended to be applicable approximately to what is described in
Section 2 -\u002D where it is more important that the technique
described is grounded in actual experience than that the experience
is actually from a (very) constrained system.
Only a few subsections are fleshed out in this initial draft;
additional subsections will quickly be integrated from additional
contributors.
1.3. Terminology used in this document
The present document has originally also been used to develop
pertinent terminology. This has been factored out into a separate
document, [I-D.ietf-lwig-terminology], which is now a prerequisite to
reading the present document.
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1.4. Scope of the present document
Using this terminology, we can now more precisely define the scope of
the present document:
This document is about implementation techniques that enable
constrained nodes to form constrained node networks.
Delay-Tolerant Networks (DTNs) are out of scope. (See Section 1.1
above for a further list of non-goals.)
1.5. Terminology boilerplate
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 RFC 2119. As this is
an informational document, the [RFC2119] keywords will only be used
to underscore requirements where similar key words apply in the
context of the specifications the light-weight implementation of
which is being discussed.
The term "byte" is used in its now customary sense as a synonym for
"octet".
2. Drawing the Landscape
There is not a single kind of constrained, Internet-connected device.
To the contrary, the trend is towards much more functional variety of
such devices than is customary today in the Internet. The
terminology document [I-D.ietf-lwig-terminology] introduces a number
of terms that will be used here to locate some of the technique
described in the following sections within certain areas of
applications.
2.1. Design Objectives
o Consideration for design or implementation approaches for
implementation of IP stacks for constrained devices will be
impacted by the RAM usage for these designs. Here the
consideration is what is the best approach to minimize overhead.
o In addition, the impact on throughput in terms of IP protocol
implementation must take into consideration the methods that
minimize overhead but balance performance requirements for the
light-weight constrained devices.
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o Protocol implementation must consider its impact on CPU
utilization. Here guidance will be provided on how to minimize
tasks that require additional CPU execution time.
How does the implementation of the IP stack effect the application
both in terms of performance but also of those same attributes and
requirements (RAM, CPU usage, etc.) that we are examining for the IP
protocol stack?
From performing a synthesis of implementation experiences we will be
able to understand and document the benefits and consequences of
varied approaches. Scaling code and selected approaches in terms of
scaling from, say, a 8-bit micro to a 16-bit micro. Such scaling for
the approach will aid in the development of single code base when
possible.
2.2. Implementation Styles
Compared to personal computing devices, constrained devices tend to
make use of quite different classes of operating systems, if that
term is even applicable.
...
o Single-threaded/giant mainloop
o Event-driven vs. threaded/blocking
* The usual multi-threaded model blocks a thread on primitives
such as connect(), accept() or read() until an external event
takes place. This model is often thought to consume too much
RAM and CPU processing.
* The event driven model uses a non-blocking approach: E.g., when
an application interface sends a message, the routine would
return immediately (before the message is sent). A call-back
facility notifies the application or calling code when the
desired processing is completed. Here the benefit is that no
thread context needs to be preserved for long periods of time.
o Single/multiple processing elements
o E.g., separate radio/network processor
Introduce these briefly: Some techniques may be applicable only to
some of these styles!
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2.3. Roles of nodes
Constrained nodes are by necessity more specialized than general
purpose computing devices; they may have a quite specific role. Some
implementation techniques may also
o Constrained nodes
o Nodes talking to constrained nodes
o Gateways/Proxies
In all these cases, constrained nodes that are "sleepy" pose
additional considerations. (Explain sleepy...) E.g., a node talking
to a sleepy node may need to make special arrangements; this is even
more true where a gateway or proxy interfaces the general Internet
o Bandwidth/latency considerations
2.4. Overview over the document
The following sections will first go through a number of specific
protocol layers, starting from layers of the data plane (link
adaptation, network, transport, application), followed by control
plane protocol layers (link layer support, network layer and routing,
host configuration and lookup services). We then look at security
protocols (general cryptography considerations, transport layer
security, network layer security, network access control). Finally,
we discuss some specific, cross-layer concerns, some "wire-visible",
some of concern within a specific implementation. Clearly, many
topics could be discussed in more than one place in this structure.
The objective is not to have something for each of the potential
topics, but to document the most valuable experience that may be
available.
3. Data Plane Protocols
3.1. Link Adaptation Layer
6LoWPAN
3.1.1. Fragmentation in a 6LoWPAN Route-Over Configuration
Author: Carsten Bormann
6LoWPAN [RFC4944] is an adaptation layer that maps IPv6 with its
minimum MTU of 1280 bytes to IEEE 802.15.4, which has a physical
layer MTU of only 127 bytes (some of which are taken by MAC layer and
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adaptation layer headers). Therefore, the adaptation layer provides
a fragmentation and reassembly scheme that can fragment a single IPv6
packet of up to 1280 bytes into multiple adaptation layer fragments
of up to 127 bytes each (including MAC and adaptation layer
overhead).
In a route-over configuration, implementing this adaptation layer
fragmentation scheme straightforwardly means that reassembly and then
fragmentation are performed at each forwarding hop. As fragments
from several packets may be arriving interleaved with each other,
this approach requires buffer space for multiple MTU-size IPv6
packets.
In a mesh-under configuration, adaptation layer fragments can be
forwarded independently of each other. It would be preferable if
something similar were possible for route-over. Complete
independence in forwarding of adaptation layer fragments is not
possible for route-over, however, as the layer-3 addresses needed for
forwarding are in the initial bytes of the IPv6 header, which is
present only in the first fragment of a larger packet.
Instead of performing a full reassembly, implementations may be able
to optimize this process by not keeping a full reassembly buffer, but
just a runt buffer (called "virtual reassembly buffer" in [WEI]) for
each IP packet. This buffer caches only the datagram_tag field (as
usual combined with the sender's link layer address, the
destination's link layer address and the datagram_size field) and the
IPv6 header including the relevant addresses. Initial fragments are
then forwarded independently (after header decompression/compression)
and create a runt reassembly buffer. Non-initial fragments (which
don't require header decompression/compression in 6LoWPAN) are
matched against the runt buffers by datagram_tag etc. and forwarded
if an IPv6 address is available. (This simple scheme may be
complicated a bit if header decompression/compression of the initial
fragment causes an overflow of the physical MTU; in this case some
overflow data may need to be stored in the runt buffers to be
combined with further fragments or may simply be forwarded as a
separate additional fragment.)
If non-initial fragments arrive out of order before the initial
fragment, a route-over router may want to keep the contents of the
non-initial fragments until the initial fragment is available, which
does need some buffer space. If that is not available, a more
constrained route-over router may simply discard out-of order non-
initial fragments, possibly taking note that there is no point in
forwarding any more fragments with the same combination of 6LoWPAN
datagram_tag field, L2 addresses and datagram_size.
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Runt buffers should time out like full reassembly buffers, and may
either keep a map of fragments forwarded or they may simply be
removed upon forwarding the final fragment, assuming that no out-of-
order fragments will follow.
3.1.1.1. Implementation Considerations for Not-So-Constrained Nodes
[RFC4944] makes no explicit mandates about the order in which
fragments should be sent. Because it is heavily favored by the above
implementation techniques, it is highly advisable for all
implementations to always send adaptation layer fragments in natural
order, i.e., starting with the initial fragment, continuing with
increasing datagram_offset.
3.2. Network Layer
IPv4 and IPv6
3.3. Transport Layer
TCP and UDP
Both TCP and UDP employ 16-bit one's-complement checksums to protect
against transmission errors. A number of RFCs discuss efficient
implementation techniques for computing and updating Internet
Checksums [RFC1071] [RFC1141] [RFC1624]. (Updating the Internet
Checksum, as opposed to computing it from scratch, may be of interest
where a pre-computed packet is provided, e.g., in Flash ROM, and a
copy is made in RAM and updated with some current values, or when the
actual transmitted packet is composed from pre-defined parts in ROM
and new parts in RAM.)
3.3.1. TCP
Ed. Note:
The following outline of a section is an attempt to provide
substructure for a future discussion of TCP-related issues based on
the TCP Roadmap, [RFC4614]. The indented text, as well as the RFC
citations, are copied (and redacted) from there; this certainly needs
to be refined in a future version. (Some additional adaptation of
the material may also be required as RFC 2581 was since obsoleted by
RFC 5681, and RFC 3782 was obsoleted by RFC 6582.)
Author: Yuanchen Ma
In [RFC4614], the TCP related RFCs are summarized. Some RFCs
describe absolutely required TCP behaviors for proper functioning and
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interoperability. Further RFCs describe strongly encouraged, but
non-essential, behaviors. There are also experimental extensions
that are not yet standard practices, but that potentially could be in
the future.
In this subsection, the influence of resource constrained nodes on
TCP implementations are summarized according to the lists of
[RFC4614].
3.3.1.1. Absolutely required TCP behaviors for proper functioning and
interoperability
RFC 793 S: "Transmission Control Protocol", STD 7 (September 1981)
In RFC793, the TCP state machine and event processing, and TCP's
semantics for data transmission, reliability, flow control,
multiplexing, and acknowledgment. For this part, the constraint of
memory will limit the multiplexing capability of TCP. /_text needed
for RFC793_/
RFC 1122 S: "Requirements for Internet Hosts - Communication Layers"
(October 1989)
RFC 2460 S: "Internet Protocol, Version 6 (IPv6) Specification
(December 1998)
RFC 2873 S: "TCP Processing of the IPv4 Precedence Field" (June 2000)
This document [RFC2873] removes from the TCP specification all
processing of the precedence bits of the TOS byte of the IP
header.
These three RFCs mandate the support for IPv6 and TOS in IP header,
which are a must for resource constrained node to implement.
RFC 2581 S: "TCP Congestion Control" (April 1999)
Although RFC 793 did not contain any congestion control
mechanisms, today congestion control is a required component of
TCP implementations. This document [RFC2581] defines the current
versions of Van Jacobson's congestion avoidance and control
mechanisms for TCP, based on his 1988 SIGCOMM paper [Jac88]. RFC
2001 was a conceptual precursor that was obsoleted by RFC 2581.
A number of behaviors that together constitute what the community
refers to as "Reno TCP" are described in RFC 2581.
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RFC 1122 mandates the implementation of a congestion control
mechanism, and RFC 2581 details the currently accepted mechanism.
RFC 2581 differs slightly from the other documents listed in this
section, as it does not affect the ability of two TCP endpoints to
communicate; however, congestion control remains a critical
component of any widely deployed TCP implementation and is
required for the avoidance of congestion collapse and to ensure
fairness among competing flows.
RFC 2988 S: "Computing TCP's Retransmission Timer" (November 2000)
Abstract: "This document defines the standard algorithm that
Transmission Control Protocol (TCP) senders are required to use to
compute and manage their retransmission timer.
3.3.1.2. Strongly encouraged, but non-essential, behaviors of TCP
RFC 1323 S: "TCP Extensions for High Performance" (May 1992)
This document [RFC1323] defines TCP extensions for window scaling,
timestamps, and protection against wrapped sequence numbers, for
efficient and safe operation over paths with large bandwidth-delay
products.
RFC 2675 S: "IPv6 Jumbograms" (August 1999)
IPv6 supports longer datagrams than were allowed in IPv4.
RFC 3168 S: "The Addition of Explicit Congestion Notification (ECN)
to IP" (September 2001)
3.3.1.2.1. Congestion Control and Loss Recovery Extensions
RFC 3042 S: "Enhancing TCP's Loss Recovery Using Limited Transmit"
(January 2001)
Abstract: "This document proposes Limited Transmit, a new
Transmission Control Protocol (TCP) mechanism that can be used to
more effectively recover lost segments when a connection's
congestion window is small
RFC 3390 S: "Increasing TCP's Initial Window" (October 2002)
This document [RFC3390] updates RFC 2581 to permit an initial TCP
window of three or four segments during the slow-start phase,
depending on the segment size.
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RFC 3782 S: "The NewReno Modification to TCP's Fast Recovery
Algorithm" (April 2004)
This document [RFC3782] specifies a modification to the standard
Reno fast recovery algorithm, whereby a TCP sender can use partial
acknowledgments to make inferences determining the next segment to
send in situations where SACK would be helpful but isn't
available.
3.3.1.2.2. SACK-Based Loss Recovery and Congestion Control
RFC 2018 S: "TCP Selective Acknowledgment Options" (October 1996)
This document [RFC2018] defines the basic selective acknowledgment
(SACK) mechanism for TCP.
RFC 2883 S: "An Extension to the Selective Acknowledgement (SACK)
Option for TCP" (July 2000)
This document [RFC2883] extends RFC 2018 to cover the case of
acknowledging duplicate segments.
RFC 3517 S: "A Conservative Selective Acknowledgment (SACK)-based
Loss Recovery Algorithm for TCP" (April 2003)
3.3.1.2.3. Dealing with Forged Segments
RFC 1948 I: "Defending Against Sequence Number Attacks" (May 1996)
RFC 2385 S: "Protection of BGP Sessions via the TCP MD5 Signature
Option" (August 1998)
3.3.1.3. Experimental extensions that are not yet standard practices
The experimental extensions are not mature yet. The contents need to
be validated to be safe and logical behavior. It is not recommended
for the resource constrained node to implement.
3.3.1.4. Others
RFC 2923 I: "TCP Problems with Path MTU Discovery" (September 2000)
From abstract: "This memo catalogs several known Transmission
Control Protocol (TCP) implementation problems dealing with Path
Maximum Transmission Unit Discovery (PMTUD), including the long-
standing black hole problem, stretch acknowlegements (ACKs) due to
confusion between Maximum Segment Size (MSS) and segment size, and
MSS advertisement based on PMTU." [RFC2923]
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3.4. Application Layer
3.4.1. General considerations about Application Programming Interfaces
(APIs)
Author: Carl Williams
Constrained devices are not necessarily in a position to use APIs
that would be considered "standard" for less constrained environments
(e.g., Berkeley sockets or those defined by POSIX).
When an API implements a protocol, this can be based on proxy methods
for remote invocations that underneath rely on the communication
protocol. One of the roles of the API can be exactly to hide the
detail of the transport protocol.
Changes to the lower layers will be made to implement light-weight
stacks so this impacts that implementation and inter-workings with
the API. Similar considerations such as RAM, CPU utilization and
performance requirements apply to the API and its use of the lower
layer resources (i.e., buffers).
Considerations for the proper approach for a developer to request
services from an application program need to be explored and
documented. Such considerations will allow the progression of a
common consistent networking paradigm without inventing a new way of
programming these devices.
In addition, such considerations will take into account the inter-
working of the API with the protocols. Protocols are more complex to
use as they are less direct and take a lot of serializing, de-
serializing and dispatching type logic.
So the connection of the API and the protocols on a constrained
device becomes even more important to balance the requirements of
RAM, CPU and performance.
_** Here we will proceed to collect and document ... insert
experiences from existing API on constrained devices (TBD) **_
3.4.2. Constrained Application Protocol (CoAP)
Author: Olaf Bergmann
The Constrained Application Protocol [I-D.ietf-core-coap] has been
designed specifically for machine-to-machine communication in
networks with very constrained nodes. Typical application scenarios
therefore include building automation and the Internet of Things.
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The major design objectives have been set on small protocol overhead,
robustness against packet loss, and high latency induced by small
bandwidth shares or slow request processing in end nodes. To
leverage integration of constrained nodes with the world-wide
Internet, the protocol design was led by the architectural style that
accounts for the scalability and robustness of the Hypertext Transfer
Protocol [RFC2616].
Lightweight implementations benefit from this design in many
respects: First, the use of Uniform Resource Identifiers (URIs) for
naming resources and the transparent forwarding of their
representations in a server-stateless request/response protocol make
protocol-translation to HTTP a straightforward task. Second, the set
of protocol elements that are inevitable for the core protocol and
thus must be implemented on every node has been kept very small to
avoid unnecessary accumulation of optional features. Options that
-\u002D when present -\u002D are critical for message processing are
explicitly marked as such to force immediate rejection of messages
with unknown critical options. Third, the syntax of protocol data
units is easy to parse and is carefully defined to avoid creation of
state in servers where possible.
Although these features enable lightweight implementations of the
Constrained Application Protocol, there is still a trade-off between
robustness and latency of constrained nodes on one hand and resource
demands (such as battery consumption, dynamic memory needs and static
code-size) on the other. This section gives some guidance on
possible strategies to solve this trade-off for very constrained
nodes (Class 1 in [I-D.ietf-lwig-terminology]). The main focus is on
servers as this is deemed the predominant case where CoAP
applications are faced with tight resource constraints.
Additional considerations for the implementation of CoAP on tiny
sensors are given in [I-D.arkko-core-sleepy-sensors].
3.4.2.1. Message Layer Processing
For constrained nodes of Class 1 or even Class 2, limiting factors
for (wireless) network communication usually are RAM size and battery
lifetime. Most applications therefore try to avoid dealing with
fragmented packets on the network layer and minimize internal buffer
space for both transmit and receive operations. One of the most
expensive operations hence is the retransmission of messages as it
implies additional energy consumption for the (radio) network
interface and occupied RAM storage for the send buffer.
Where multi-threading is not an option at all because no full-fledged
operating system is present, all operations are triggered by a big
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main loop in a send-receive-dispatch cycle. To implement the packet
retransmission, CoAP implementations at least need a separate send
buffer and a decent notion of time, e.g. as a strictly monotonic
increasing tick counter. For platforms that disable clock tick
interrupts in sleep states, the application must take into
consideration the clock deviation that occurs during sleep (or ensure
to remain in idle state until the message has been acknowledged or
the maximum number of retransmissions is reached). Since CoAP allows
up to four retransmissions with a binary exponential back-off it
could take up to 45 seconds until the send operation is complete.
Even in idle state, this means substantial energy consumption for
low-power nodes. Implementers therefore might choose a two-step
strategy: First, do one or two retransmissions and then, in the later
phases of back-off, go to sleep until the next retransmission is due.
In the meantime, the node could check for new messages including the
acknowledgement for any confirmable message to send.
A similar strategy holds for confirmable messages with separate
responses. This concept entitles CoAP servers to return an empty
acknowledgement to indicate that a confirmable request has been
understood and is being processed. Once a proper response has been
generate to fulfill the request, it is sent back as a confirmable
message as well. The server implementation in this case must be able
to map retransmissions of the original request to the ongoing
operation and provide the client-selected Token to map between
original request and the separate response.
Depending on the number of requests that can be handled in parallel,
an implementation might create a stub response filled with any option
that has to be copied from the original request to the separate
response, especially the Token option. The drawback of this
technique is that the server must be prepared to receive
retransmissions of the previous (confirmable) request to which a new
acknowledgement must be generated. If memory is an issue, a single
buffer can be used for both tasks: Only the message type and code
must be updated, changing the message id is optional. Once the
resource representation is known, it is added as new payload at the
end of the stub response. Acknowledgements still can be sent as
described before as long as no additional options are required to
describe the payload.
3.4.2.2. Message Parsing
Both CoAP clients and servers must construct outgoing CoAP PDUs and
parse incoming messages. The basic message header consists of only
four octets and thus can be mapped easily to an internal data
structure, considering the actual byte order of the host. Once the
message is accepted for further processing, the set of options
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contained in the received message must be decoded to check for
unknown critical options. To avoid multiple passes through the
option list, the option parser might maintain a bit-vector where each
bit represents an option number that is present in the received
request. The delta-encoded option number indicates the number of
left-shift operations to apply on a bit mask to set the corresponding
bit.
In addition, the byte index of every option is added to a sparse list
(e.g. a one-dimensional array) for fast retrieval. This
particularly enables efficient reduced-function handling of options
that might occur more than once such as Uri-Path. In this
implementation strategy, the delta is zero for any subsequent path
segment, hence the stored byte index for option 9 (Uri-Path) will be
overwritten to hold a pointer to the last occurrence of that option,
i.e., only the last path component actually matters. (Of course,
this requires choosing resource names where the combination of (final
Uri-Path component, final Uri-Query component) is server-wide unique.
Note: Where skipping all but the last path segment is not feasible
for some reason, resource identification could be ensured by some
hash value calculated over the path segments. For each segment
encountered, the stored hash value is updated by the current
option value. This works if a cheap _perfect hashing_ scheme can
be found for the resource names.
Once the option list has been processed at least up to the highest
option number that is supported by the application, any known
critical option and all elective options can be masked out to
determine if any unknown critical option was present. If this is the
case, this information can be used to create a 4.02 response
accordingly. (Note that the remaining options also must be processed
to add further critical options included in the original request.)
3.4.2.3. Storing Used Message IDs
If CoAP is used directly on top of UDP (i.e., in NoSec mode), it
needs to cope with the fact that the UDP datagram transport can
reorder and duplicate messages. (In contrast to UDP, DTLS has its
own duplicate detection.) CoAP has been designed with protocol
functionality such that rejection of duplicate messages is always
possible. It is at the discretion of the receiver if it actually
wants to make use of this functionality. Processing of duplicate
messages comes at a cost, but so does the management of the state
associated with duplicate rejection. Hence, a receiver may have good
reasons to decide not to do the duplicate rejection. If duplicate
rejection is indeed necessary, e.g., for non-idempotent requests, it
is important to control the amount of state that needs to be stored.
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Author: Esko Dijk
CoAP's duplicate rejection functionality can be straightforwardly
implemented in a CoAP end-point by storing, for each remote CoAP end-
point ("peer") that it communicates with, a list of recently received
CoAP Message IDs (MIDs) along with some timing information. A CoAP
message from a peer with a MID that is in the list for that peer can
simply be discarded.
The timing information in the list can then be used to time out
entries that are older than the _expected extent of the re-ordering_,
an upper bound for which can be estimated by adding the _potential
retransmission window_ ([I-D.ietf-core-coap] section "Reliable
Messages") and the time packets can stay alive in the network.
Such a straightforward implementation is suitable in case other CoAP
end-points generate random MIDs. However, this storage method may
consume substantial RAM in specific cases, such as:
o many clients are making periodic, non-idempotent requests to a
single CoAP server;
o one client makes periodic requests to a large number of CoAP
servers and/or requests a large number of resources; where servers
happen to mostly generate separate CoAP responses (not piggy-
backed);
For example, consider the first case where the expected extent of re-
ordering is 50 seconds, and N clients are sending periodic POST
requests to a single CoAP server during a period of high system
activity, each on average sending one client request per second. The
server would need 100 * N bytes of RAM to store the MIDs only. This
amount of RAM may be significant on a RAM-constrained platform. On a
number of platforms, it may be easier to allocate some extra program
memory (e.g. Flash or ROM) to the CoAP protocol handler process than
to allocate extra RAM. Therefore, one may try to reduce RAM usage of
a CoAP implementation at the cost of some additional program memory
usage and implementation complexity.
Some CoAP clients generate MID values by a using a Message ID
variable [I-D.ietf-core-coap] that is incremented by one each time a
new MID needs to be generated. (After the maximum value 65535 it
wraps back to 0.) We call this behavior "sequential" MIDs. One
approach to reduce RAM use exploits the redundancy in sequential MIDs
for a more efficient MID storage in CoAP servers.
Naturally such an approach requires, in order to actually reduce RAM
usage in an implementation, that a large part of the peers follow the
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sequential MID behavior. To realize this optimization, the authors
therefore RECOMMEND that CoAP end-point implementers employ the
"sequential MID" scheme if there are no reasons to prefer another
scheme, such as randomly generated MID values.
Security considerations might call for a choice for
(pseudo)randomized MIDs. Note however that with truly randomly
generated MIDs the probability of MID collision is rather high in use
cases as mentioned before, following from the Birthday Paradox. For
example, in a sequence of 52 randomly drawn 16-bit values the
probability of finding at least two identical values is about 2
percent.
From here on we consider efficient storage implementations for MIDs
in CoAP end-points, that are optimized to store "sequential" MIDs.
Because CoAP messages may be lost or arrive out-of-order, a solution
has to take into account that received MIDs of CoAP messages are not
actually arriving in a sequential fashion, due to lost or reordered
messages. Also a peer might reset and lose its MID counter(s) state.
In addition, a peer may have a single Message ID variable used in
messages to many CoAP end-points it communicates with, which partly
breaks sequentiality from the receiving CoAP end-point's perspective.
Finally, some peers might use a randomly generated MID values
approach. Due to these specific conditions, existing sliding window
bitfield implementations for storing received sequence numbers are
typically not directly suitable for efficiently storing MIDs.
Table 1 shows one example for a per-peer MID storage design: a table
with a bitfield of a defined length _K_ per entry to store received
MIDs (one per bit) that have a value in the range [MID_i + 1 , MID_i
+ K].
+----------+----------------+-----------------+
| MID base | K-bit bitfield | base time value |
+----------+----------------+-----------------+
| MID_0 | 010010101001 | t_0 |
| | | |
| MID_1 | 111101110111 | t_1 |
| | | |
| ... etc. | | |
+----------+----------------+-----------------+
Table 1: A per-peer table for storing MIDs based on MID\_i
The presence of a table row with base MID_i (regardless of the
bitfield values) indicates that a value MID_i has been received at a
time t_i. Subsequently, each bitfield bit k (0...K-1) in a row i
corresponds to a received MID value of MID_i + k + 1. If a bit k is
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0, it means a message with corresponding MID has not yet been
received. A bit 1 indicates such a message has been received already
at approximately time t_i. This storage structure allows e.g. with
k=64 to store in best case up to 130 MID values using 20 bytes, as
opposed to 260 bytes that would be needed for a non-sequential
storage scheme.
The time values t_i are used for removing rows from the table after a
preset timeout period, to keep the MID store small in size and enable
these MIDs to be safely re-used in future communications. (Note that
the table only stores one time value per row, which therefore needs
to be updated on receipt of another MID that is stored as a single
bit in this row. As a consequence of only storing one time value per
row, older MID entries typically time out later than with a simple
per-MID time value storage scheme. The end-point therefore needs to
ensure that this additional delay before MID entries are removed from
the table is much smaller than the time period after which a peer
starts to re-use MID values due to wrap-around of a peer's MID
variable. One solution is to check that a value t_i in a table row
is still recent enough, before using the row and updating the value
t_i to current time. If not recent enough, e.g. older than N
seconds, a new row with an empty bitfield is created.) [Clearly,
these optimizations would benefit if the peer were much more
conservative about re-using MIDs than currently required in the
protocol specification.]
The optimization described is less efficient for storing randomized
MIDs that a CoAP end-point may encounter from certain peers. To
solve this, a storage algorithm may start in a simple MID storage
mode, first assuming that the peer produces non-sequential MIDs.
While storing MIDs, a heuristic is then applied based on monitoring
some "hit rate", for example, the number of MIDs received that have a
Most Significant Byte equal to that of the previous MID divided by
the total number of MIDs received. If the hit rate tends towards 1
over a period of time, the MID store may decide that this particular
CoAP end-point uses sequential MIDs and in response improve
efficiency by switching its mode to the bitfield based storage.
3.4.3. (Other Application Protocols...)
4. Control Plane Protocols
4.1. Link Layer Support
ARP, ND; 6LoWPAN-ND
4.2. Network Layer
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ICMP, ICMPv6, IGMP/MLD
4.3. Routing
RPL, AODV/DYMO, OLSRv2
4.4. Host Configuration and Lookup Services
DNS, DHCPv4, DHCPv6
4.5. Network Management
SNMP, netconf?
4.5.1. SNMP
Author: Brinda M C
This section describes an approach for developing a light-weight SNMP
agent for resource constrained devices running the 6LoWPAN/RPL
protocol stack. The motivation for the work is driven by two major
factors:
o SNMP plays a vital role in monitoring and managing any operational
network; 6LoWPAN based WSN is no exception to this.
o There is a need for building a light-weight SNMP agent which
consumes less memory and less computational resources.
The following subsections are organized as follows:
o Section 4.5.1.1 provides some background.
o In Section 4.5.1.2, we revisit existing SNMP implementation in the
context of memory constrained devices.
o In Section 4.5.1.3, we present our approach for building a memory
efficient SNMP agent.
o Using a realistic example, in Section 4.5.1.4, we illustrate how
the proposed method can be implemented.
o In Section 4.5.1.5, we explore a few ideas which can further help
in improving the memory utilization.
4.5.1.1. Background
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Our initial SNMP agent implementation was completely based on Net-
SNMP, well-known open-source network monitoring and management
software. After porting the agent on to the TelosB mote, we observed
that it occupies a text program memory of more than 8 KiB on TinyOS
and Contiki OS platforms. (Note that both these platforms already
use compiler optimizations to minimize the memory footprint.) 8 KiB
is already non-negligible given the 48 KiB program memory limit of
TelosB. Added to this, the memory taken up by 6LoWPAN and the
related protocol stacks are ever growing, causing serious memory
crunch in the resource constrained devices. We reached a situation
where we could not build an image on the TinyOS/Contiki OS platforms
with our SNMP agent.
We came across SNMPv1 agent implementations elsewhere in the
literature which also report similar memory consumption. This
motivated us to have a re-look at the existing SNMP agent
implementation, and explore the possibility of an alternate
implementation using altogether a different approach.
4.5.1.2. Revisiting SNMP implementation for resource constrained
devices
If we look at a typical SNMP agent implementation, we can see that
much of the memory consuming code is pertaining to ASN.1 related SNMP
PDU parsing and SNMP PDU build operations. The SNMP parsing mainly
recovers various fields from the incoming PDU, such as the OIDs,
whereas the SNMP PDU build is the reverse operation of building the
response PDU from the OIDs.
The key observation is that, for a given MIB definition, an OID of
interest contained in the incoming SNMP PDU is already available,
albeit in an encoded form. This enables identifying the OID from the
packet in its "raw" form, simplifying parser operation.
We also can make use of this observation while building the response
SNMP PDU. For a given MIB definition, we can think of statically
having a pre-composed ASN.1 encoded version of OIDs, and use them
while constructing the response SNMP PDU.
4.5.1.3. Proposed approach for building an memory efficient SNMP agent
As noted in the previous section, since an SNMP OID is already
_contained_ in the incoming network PDU, we came up with a simple OID
signature identification method performed directly on the network PDU
through simple memory comparisons and table look-ups. Once the OID
has been identified from the packet "in situ", the corresponding per-
OID processing is carried out. Through this scheme we completely
eliminated expensive SNMP parse operations.
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For the SNMP PDU build, we use _pre-encoded_ OID variables which can
simply be plugged into the network SNMP response packet directly
depending on the request OID. Now that the expensive build operation
is taken care, what remains is the construction of the overall SNMP
pdu which can be built through simple logic. Through this scheme we
completely eliminated expensive SNMP build operations.
Based on these ideas, we have re-architected our original SNMP agent
implementation and with our new implementation we were able to bring
down its text memory usage all the way down to 4 KiB from the native
SNMP agent implementation which occupied 8 KiB.
4.5.1.3.1. Discussion on memory usage
With respect to the memory usage, while we have achieved major
reduction in terms of text program memory, which occupies a major
chunk of memory, a question might come to mind with regard to the
static memory allocation for maintaining the tables. We found that
this is not very significant to start with. Through an efficient
table representation, we further optimized the memory consumption.
We could do so because a typical OID description is mainly dominated
by a fixed part of the hierarchy. This enables us to define few
static prefixes, each corresponding to a particular hierarchy level
of the OID. In the context of 6LoWPAN, it can be expected that the
number of hierarchy levels will be small.
4.5.1.4. Example
This section illustrates the simplicity and practicality of our
approach with an example. Let us consider the fragment of a
representative MIB definition depicted in Figure 1
iso
|
org
|
dod
|
internet
|
mgmt.mib-2
|
lowpanMIB
|
+--lowpanPrimaryStatistics(10)
|
+--PrimeStatsEntry(1)
|
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+-- -R-- INTEGER lowpanMoteBatteryVoltageP(1)
+-- -R-- Counter lowpanFramesReceivedP(2)
+-- -R-- Counter lowpanFramesSentP(3)
+-- -R-- Counter ipv6ForwardedMsgP(4)
+-- -R-- Counter OUTSolicitationP(5)
+-- -R-- Counter OUTAdvertisementP(6)
Figure 1: A fragment of a MIB hierarchy
4.5.1.4.1. Optimized SNMP Parsing
Let us consider a GET request for the OIDs lowpanMoteBatteryVoltageP
and lowpanFramesSentP. Corresponding to these OIDs, a C array dump
of the network PDU of SNMP packet with two OIDs in a variable binding
would look as in Figure 2.
char snmp_get_req_pkt[] = {
0x30, 0x81, 0x3d, 0x02, 0x01, 0x00, 0x04, 0x06,
0x70, 0x75, 0x62, 0x6c, 0x69, 0x63, 0xa0, 0x30,
0x02, 0x04, 0x28, 0x29, 0xe4, 0x5d, 0x02, 0x01,
0x00, 0x02, 0x01, 0x00, 0x30, 0x22, 0x30, 0x0f,
0x06, 0x0b, 0x2b, 0x06, 0x01, 0x02, 0x01, 0x83,
0x90, 0x12, 0x0a, 0x01, 0x01, 0x05, 0x00, 0x30,
0x0f, 0x06, 0x0b, 0x2b, 0x06, 0x01, 0x02, 0x01,
0x83, 0x90, 0x12, 0x0a, 0x01, 0x03, 0x05, 0x00 };
Figure 2: An SNMP packet, represented in C
Inspecting the above packet, we see that the main components of the
PDU are:
1. Version (SNMPv1): [0x02, 0x01, 0x00]
2. Community Name ("public"): [0x04, 0x06, 0x70, 0x75, 0x62, 0x6c,
0x69, 0x63]
3. ASN.1 encoded OIDs for lowpanMoteBatteryVoltageP, and
lowpanFramesReceivedP:
* [0x30, 0x0f, 0x06, 0x0b, 0x2b, 0x06, 0x01, 0x02, 0x01, 0x83,
0x90, 0x12, 0x0a, 0x01, 0x01, 0x05, 0x00]
* [0x30, 0x0f, 0x06, 0x0b, 0x2b, 0x06, 0x01, 0x02, 0x01, 0x83,
0x90, 0x12, 0x0a, 0x01, 0x03, 0x05, 0x00]
There is a significant overlap between the two OIDs, which can be
used to simplify the parsing process. We can, for instance, define
one statically initialized array containing elements common between
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these OIDs. Using this notion of common prefix idea, we can come up
with an optimized table and the OID identification then boils down to
simple memory comparisons within this table. The optimized table
construction will also result in scalability.
4.5.1.4.2. Optimized SNMP Build
Extending the same approach as described above, we can build the GET
response by plugging in pre-encoded OIDs into the response packets.
So, corresponding to the GET request for the OIDs as given in section
4.1, we can define C arrays containing pre-encoded OIDs which can go
into the response packet as in Figure 3.
pdu_batt_volt[] = {
0x30, 0x11, 0x06, 0x0b, 0x2b, 0x06, 0x01, 0x02,
0x01, 0x83, 0x90, 0x12, 0x0a, 0x01, 0x01, 0x02,
0x02, 0x00, 0x00 };
pdu_frames_sent[] = {
0x30, 0x11, 0x06, 0x0b, 0x2b, 0x06, 0x01, 0x02,
0x01, 0x83, 0x90, 0x12, 0x0a, 0x01, 0x03, 0x41,
0x02, 0x00, 0x00 };
Figure 3: Pre-encoded OIDs
Since the ASN.1 basic encoding rules are in TLV format, the offset
within the encoded OID where the value needs to be filled-in can be
obtained from the length field.
The table size optimization discussed in the previous section can be
applied here, too.
Note: Though we have taken a simple example to illustrate the
efficacy of the proposed approach, the ideas presented here can
easily be extended to other scenarios as well.
4.5.1.5. Further improvements
A few simple methods can reduce the code size as well as generate
computationally inexpensive code. These methods might sound obvious
and trivial but are important for constrained devices.
o If possible, avoid using memory consuming data types such as
floating point while representing a monitored variable when an
equivalent representation of the same that occupies less memory is
adequate. For example, while a battery voltage indication could
take a fractional value between 0 and 3 V, opt for an 8-bit
quantized value.
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o Using meta data in the MIB definition instead of absolute numbers
can bring down the memory and processing significantly and can
improve scalability too especially for a large scale WSN
deployments. Using the same example of battery voltage, one might
think of an OID which represents fewer levels of the battery
voltage signifying high, medium, low, very low.
o While a multi-level hierarchy for MIB definition might improve OID
segregation the flip side is that it increases the overall length
of the OID and results in extra memory and processing overhead.
One may have to make a judicious choice while coming up with the
MIB.
4.5.1.6. Conclusion
This subsection proposes a simple SNMP packet processing based
approach for building a light-weight SNMP agent. While there is
scope for further improvement, we believe that the proposed method
can be a reasonably good starting point for resource constrained
6LoWPAN based networks.
5. Security protocols
5.1. Cryptography for Constrained Devices
5.2. Transport Layer Security
TLS, DTLS, ciphersuites, certificates
5.3. Network Layer Security
IPsec, IKEv2, transforms
Advice for a minimal implementation of IKEv2 can be found in
[I-D.kivinen-ipsecme-ikev2-minimal].
5.4. Network Access Control
(PANA, EAP, EAP methods)
5.4.1. PANA
Author: Mitsuru Kanda
PANA [RFC5191] provides network access authentication between clients
and access networks. The PANA protocol runs between a PANA Client
(PaC) and a PANA Authentication Agent (PAA). PANA carries UDP
encapsulated EAP [RFC3748] and includes various operational options.
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From the point of view of minimal implementation, some of these are
not necessary for constrained devices. This section describes a
minimal PANA implementation for these devices.
The minimization objective for this implementation mainly targets
PaCs because constrained devices often are installed as network
clients, such as sensors, metering devices, etc.
5.4.1.1. PANA AVPs
Each PANA message can carry zero or more AVPs (Attribute-Value Pairs)
within its payload. [RFC5191] specifies nine types of AVPs (AUTH,
EAP-Payload, Integrity-Algorithm, Key-Id, Nonce, PRF-Algorithm,
Result-Code, Session-Lifetime, and Termination-Cause). All of them
are required by all minimal implementations. But there are some
notes.
Integrity-Algorithm AVP and PRF-Algorithm AVP:
All PANA implementations MUST support AUTH_HMAC_SHA1_160 for PANA
message integrity protection and PRF_HMAC_SHA1 for pseudo-random
function (PRF) specified in [RFC5191]. Both of these are based on
SHA-1, which therefore needs to be implemented in a minimal
implementation.
Nonce AVP:
As the basic hash function is SHA-1, including a nonce of 20 bytes in
the Nonce AVP is appropriate ([RFC5191], section 8.5).
5.4.1.2. PANA Phases
A PANA session consists of four phases -\u002D Authentication and
authorization phase, Access phase, Re-Authentication phase, and
Termination phase.
Authentication and authorization phase:
There are two types of PANA session initiation, PaC-initiated session
and PAA-initiated session. The minimal implementation must support
PaC-initiated session and does not need to support PAA-initiated
session. Because a PaC (a constrained device) which may be a
sleeping device, can not receive an unsolicited PANA-Auth-Request
message from a PAA (PAA-initiated session).
EAP messages can be carried in PANA-Auth-Request and PANA-Auth-Answer
messages. In order to reduce the number of messages, "Piggybacking
EAP" is useful. Both the PaC and PAA should include EAP-Payload AVP
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in each of PANA-Auth-Request and PANA-Auth-Answer messages as much as
possible. Figure 4 shows an example "Piggybacking EAP" sequence of
the Authentication and authorization phase.
PaC PAA Message(sequence number)[AVPs]
---------------------------------------------------------------------
-----> PANA-Client-Initiation(0)
<----- PANA-Auth-Request(x)[PRF-Algorithm,Integrity-Algorithm]
// The 'S' (Start) bit set
-----> PANA-Auth-Answer(x)[PRF-Algorithm, Integrity-Algorithm]
// The 'S' (Start) bit set
<----- PANA-Auth-Request(x+1)[Nonce, EAP-Payload]
-----> PANA-Auth-Answer(x+1)[Nonce, EAP-Payload]
<----- PANA-Auth-Request(x+2)[EAP-Payload]
-----> PANA-Auth-Answer(x+2)[EAP-Payload]
<----- PANA-Auth-Request(x+3)[Result-Code, EAP-Payload,
Key-Id, Session-Lifetime, AUTH]
// The 'C' (Complete) bit set
-----> PANA-Auth-Answer(x+3)[Key-Id, AUTH]
// The 'C' (Complete) bit set
Figure 4: Example sequence of the Authentication and authorization
phase for a PaC-initiated session (using "Piggybacking EAP")
Note: It is possible to include an EAP-Payload in both the PANA-Auth-
Request and PANA-Auth-Answer messages with the 'S' bit set. But the
PAA should not include an EAP-Payload in the PANA-Auth-Request
message with the 'S' bit set in order to stay stateless in response
to a PANA-Client-Initiation message.
Access phase:
After Authentication and authorization phase completion, the PaC and
PAA share a PANA Security Association (SA) and move Access phase.
During Access phase, [RFC5191] describes both the PaC and PAA can
send a PANA-Notification-Request message with the 'P' (Ping) bit set
for the peer's PANA session liveness check (a.k.a "PANA ping"). From
the minimal implementation point of view, the PAA should not send a
PANA-Notification-Request message with the 'P' (Ping) bit set to
initiate PANA ping since the PaC may be sleeping. The PaC does not
need to send a PANA-Notification-Request message with the 'P' (Ping)
bit set for PANA ping to the PAA periodically and may omit the PANA
ping feature itself if the PaC can detect the PANA session failure by
other methods, for example, network communication failure. In
conclusion, the PaC does not need to implement the periodic liveness
check feature sending PANA ping but a PaC that is awake should
respond to a incoming PANA-Notification-Request message with the 'P'
(Ping) bit set for PANA ping as possible.
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Re-Authentication phase:
Before PANA session lifetime expiration, the PaC and PAA MUST re-
negotiate to keep the PANA session. This means that the PaC and PAA
enter Re-Authentication phase. Also in the Authentication and
authorization phase, there are two types of re-authentication. The
minimal implementation must support PaC-initiated re-authentication
and does not need to support PAA-initiated re-authentication (again
because the PaC may be a sleeping device). "Piggybacking EAP" is
also useful here and should be used as well. Figure 5 shows an
example "Piggybacking EAP" sequence of the Re-Authentication phase.
PaC PAA Message(sequence number)[AVPs]
---------------------------------------------------------------------
-----> PANA-Notification-Request(q)[AUTH]
// The 'A' (re-Authentication) bit set
<----- PANA-Notification-Answer(q)[AUTH]
// The 'A' (re-Authentication) bit set
<----- PANA-Auth-Request(p)[EAP-Payload, Nonce, AUTH]
-----> PANA-Auth-Answer(p)[EAP-Payload, Nonce, AUTH]
<----- PANA-Auth-Request(p+1)[EAP-Payload, AUTH]
-----> PANA-Auth-Answer(p+1)[EAP-Payload, AUTH]
<----- PANA-Auth-Request(p+2)[Result-Code, EAP-Payload,
Key-Id, Session-Lifetime, AUTH]
// The 'C' (Complete) bit set
-----> PANA-Auth-Answer(p+2)[Key-Id, AUTH]
// The 'C' (Complete) bit set
Figure 5: Example sequence of the Re-Authentication phase for a PaC-
initiated session (using "Piggybacking EAP")
Termination Phase:
The PaC and PAA should not send a PANA-Termination-Request message
except for explicitly terminating a PANA session within the lifetime.
Both the PaC and PAA know their own PANA session lifetime expiration.
This means the PaC and PAA should not send a PANA-Termination-Request
message when the PANA session lifetime expired because of reducing
message processing cost.
5.4.1.3. PANA session state parameters
All PANA implementations internally keep PANA session state
information for each peer. At least, all minimal implementations
need to keep PANA session state parameters below (in the second
column storage sizes are given in bytes):
+----------------------+------------+-------------------------------+
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| State Parameter | Size | Comment |
+----------------------+------------+-------------------------------+
| PANA Phase | 1 | Used for recording the |
| Information | | current PANA phase. |
| | | |
| PANA Session | 4 | |
| Identifier | | |
| | | |
| PaC's IP address and | 6 or 18 | IP Address length (4 bytes |
| UDP port number | | for IPv4 and 16 bytes for |
| | | IPv6) plus 2 bytes for UDP |
| | | port number. |
| | | |
| PAA's IP address and | 6 or 18 | IP Address length (4 bytes |
| UDP port number | | for IPv4 and 16 bytes for |
| | | IPv6) plus 2 bytes for UDP |
| | | port number. |
| | | |
| Outgoing message | 4 | Next outgoing request message |
| sequence number | | sequence number. |
| | | |
| Incoming message | 4 | Next expected incoming |
| sequence number | | request message sequence |
| | | number. |
| | | |
| A copy of the last | variable | Necessary to be able to |
| sent message payload | | retransmit the message |
| | | (unless it can be |
| | | reconstructed on the fly). |
| | | |
| Retransmission | 4 | |
| interval | | |
| | | |
| PANA Session | 4 | |
| lifetime | | |
| | | |
| PaC nonce | 20 | Generated by PaC and carried |
| | | in the Nonce AVP. |
| | | |
| PAA nonce | 20 | Generated by PAA and carried |
| | | in the Nonce AVP. |
| | | |
| EAP MSK Identifier | 4 | |
| | | |
| EAP MSK value | *) | Generated by EAP method and |
| | | used for generating |
| | | PANA_AUTH_KEY. |
| | | |
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| PANA_AUTH_KEY | 20 | Necessary for PANA message |
| | | protection. |
| | | |
| PANA PRF algorithm | 4 | Used for generating |
| number | | PANA_AUTH_KEY. |
| | | |
| PANA Integrity | 4 | Necessary for PANA message |
| algorithm number | | protection. |
+----------------------+------------+-------------------------------+
*) (Storage size depends on the key derivation algorithm.)
Note: EAP parameters except for MSK have not been listed here. These
EAP parameters are not used by PANA and depend on what EAP method you
choose.
6. Wire-Visible Constraints
o Checksum
o MTU
o Fragmentation and reassembly
o Options -\u002D implications of leaving some out
o Simplified TCP optimized for LLNs
o Out-of-order packets
7. Wire-Invisible Constraints
o Buffering
o Memory management
o Timers
o Energy efficiency
o API
o Data structures
o Table sizes (somewhat wire-visible)
o Improved error handling due to resource overconsumption
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8. IANA Considerations
This document makes no requirements on IANA. (This section to be
removed by RFC editor.)
9. Security Considerations
(TBD.)
10. Acknowledgements
Much of the text of the introduction is taken from the charter of the
LWIG working group and the invitation to the IAB workshop on
Interconnecting Smart Objects with the Internet. Thanks to the
numerous contributors. Angelo Castellani provided comments that led
to improved text.
10.1. Contributors
The RFC guidelines no longer allow RFCs to be published with a large
number of authors. As there are many authors that have contributed
to the sections of this document, their names are listed in the
individual section headings as well as alphabetically listed with
their affiliations below.
+------------+--------------------+---------------------------------+
| Name | Affiliation | Contact |
+------------+--------------------+---------------------------------+
| Brinda M C | Indian Institute | brinda@ece.iisc.ernet.in |
| | of Science | |
| | | |
| Carl | MCSR Labs | carlw@mcsr-labs.org |
| Williams | | |
| | | |
| Carsten | Universitaet | cabo@tzi.org |
| Bormann | Bremen TZI | |
| | | |
| Esko Dijk | Philips Research | esko.dijk@philips.com |
| | | |
| Mitsuru | Toshiba | mitsuru.kanda@toshiba.co.jp |
| Kanda | | |
| | | |
| Olaf | Universitaet | bergmann@tzi.org |
| Bergmann | Bremen TZI | |
| | | |
| Yuanchen | Hitachi (China) | ycma@hitachi.cn |
| Ma | R&D Corporation | |
| | | |
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| ... | ... | |
+------------+--------------------+---------------------------------+
11. References
11.1. Normative References
[I-D.ietf-lwig-terminology]
Bormann, C. and M. Ersue, "Terminology for Constrained
Node Networks", draft-ietf-lwig-terminology-00 (work in
progress), February 2013.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC4944] Montenegro, G., Kushalnagar, N., Hui, J., and D. Culler,
"Transmission of IPv6 Packets over IEEE 802.15.4
Networks", RFC 4944, September 2007.
11.2. Informative References
[I-D.arkko-core-sleepy-sensors]
Arkko, J., Rissanen, H., Loreto, S., Turanyi, Z., and O.
Novo, "Implementing Tiny COAP Sensors", draft-arkko-core-
sleepy-sensors-01 (work in progress), July 2011.
[I-D.ietf-6lowpan-btle]
Nieminen, J., Savolainen, T., Isomaki, M., Patil, B.,
Shelby, Z., and C. Gomez, "Transmission of IPv6 Packets
over BLUETOOTH Low Energy", draft-ietf-6lowpan-btle-12
(work in progress), February 2013.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)", draft-ietf-
core-coap-13 (work in progress), December 2012.
[I-D.kivinen-ipsecme-ikev2-minimal]
Kivinen, T., "Minimal IKEv2", draft-kivinen-ipsecme-
ikev2-minimal-01 (work in progress), October 2012.
[I-D.mariager-6lowpan-v6over-dect-ule]
Mariager, P. and J. Petersen, "Transmission of IPv6
Packets over DECT Ultra Low Energy", draft-mariager-
6lowpan-v6over-dect-ule-02 (work in progress), May 2012.
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[RFC1071] Braden, R., Borman, D., Partridge, C., and W. Plummer,
"Computing the Internet checksum", RFC 1071, September
1988.
[RFC1141] Mallory, T. and A. Kullberg, "Incremental updating of the
Internet checksum", RFC 1141, January 1990.
[RFC1624] Rijsinghani, A., "Computation of the Internet Checksum via
Incremental Update", RFC 1624, May 1994.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC3748] Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
3748, June 2004.
[RFC4614] Duke, M., Braden, R., Eddy, W., and E. Blanton, "A Roadmap
for Transmission Control Protocol (TCP) Specification
Documents", RFC 4614, September 2006.
[RFC5191] Forsberg, D., Ohba, Y., Patil, B., Tschofenig, H., and A.
Yegin, "Protocol for Carrying Authentication for Network
Access (PANA)", RFC 5191, May 2008.
[WEI] Shelby, Z. and C. Bormann, "6LoWPAN: the Wireless Embedded
Internet", ISBN 9780470747995, 2009.
Author's Address
Carsten Bormann (editor)
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63921
Email: cabo@tzi.org
Bormann Expires August 29, 2013 [Page 34]
