| LWIG Working Group | C. Bormann |
| Internet-Draft | Universitaet Bremen TZI |
| Intended status: Informational | M. Ersue |
| Expires: October 25, 2013 | Nokia Siemens Networks |
| A. Keranen | |
| Ericsson | |
| April 23, 2013 |
Terminology for Constrained Node Networks
draft-ietf-lwig-terminology-04
The Internet Protocol Suite is increasingly used on small devices with severe constraints, creating constrained node networks. This document provides a number of basic terms that have turned out to be useful in the standardization work for constrained environments.
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Small devices with limited CPU, memory, and power resources, so called constrained devices (also known as sensor, smart object, or smart device) can constitute a network, becoming “constrained nodes” in that network. Such a network may itself exhibit constraints, e.g. with unreliable or lossy channels, limited and unpredictable bandwidth, and a highly dynamic topology.
Constrained devices might be in charge of gathering information in diverse settings including natural ecosystems, buildings, and factories and sending the information to one or more server stations. Constrained devices may work under severe resource constraints such as limited battery and computing power, little memory and insufficient wireless bandwidth, and communication capabilities. Other entities on the network, e.g., a base station or controlling server, might have more computational and communication resources and could support the interaction between the constrained devices and applications in more traditional networks.
Today diverse sizes of constrained devices with different resources and capabilities are becoming connected. Mobile personal gadgets, building-automation devices, cellular phones, Machine-to-machine (M2M) devices, etc. benefit from interacting with other “things” nearby or somewhere in the Internet. With this, the Internet of Things (IoT) becomes a reality, built up out of uniquely identifiable and addressable objects (things). And over the next decade, this could grow to large numbers [fifty-billion] of Internet-connected constrained devices, greatly increasing the Internet’s size and scope.
The present document provides a number of basic terms that have turned out to be useful in the standardization work for constrained environments. The intention is not to exhaustively cover the field, but to make sure a few core terms are used consistently between different groups cooperating in this space.
The main focus of this field of work appears to be scaling:
The need for scaling down the characteristics of nodes leads to constrained nodes.
The term “constrained node” is best defined by contrasting the characteristics of a constrained node with certain widely held expectations on more familiar Internet nodes:
While this is less than satisfying as a rigorous definition, it is grounded in the state of the art and clearly sets apart constrained nodes from server systems, desktop or laptop computers, powerful mobile devices such as smartphones etc. There may be many design considerations that lead to these constraints, including cost, size, weight, and other scaling factors.
(An alternative name, when the properties as a network node are not in focus, is “constrained device”.)
There are multiple facets to the constraints on nodes, often applying in combination, e.g.:
Section 3 defines a small number of interesting classes (“class-N” for N=0,1,2) of constrained nodes focusing on relevant combinations of the first two constraints. With respect to available power, [RFC6606] distinguishes “power-affluent” nodes (mains-powered or regularly recharged) from “power-constrained nodes” that draw their power from primary batteries or by using energy harvesting; more detailed power terminology is given in Section 4.
The use of constrained nodes in networks often also leads to constraints on the networks themselves. However, there may also be constraints on networks that are largely independent from those of the nodes. We therefore distinguish constrained networks and constrained node networks.
We define “constrained network” in a similar way:
Again, there may be several reasons for this:
Constraints may include:
A constrained network is not necessarily a challenged network [FALL]:
All challenged networks are constrained networks in some sense, but not all constrained networks are challenged networks. There is no well-defined boundary between the two, though. Delay-Tolerant Networking (DTN) has been designed to cope with challenged networks [RFC4838].
A constrained node network always is a constrained network because of the network constraints stemming from the node constraints, but may also have other constraints that already make it a constrained network.
A related term that has been used recently is “low-power lossy network” (LLN). In its terminology document, the ROLL working group is saying [I-D.ietf-roll-terminology]:
In common usage, LLN often stands for “the network characteristics that RPL has been designed for”. Beyond what is said in the ROLL terminology document, LLNs do appear to have significant loss at the physical layer, with significant variability of the delivery rate, and some short-term unreliability, coupled with some medium term stability that makes it worthwhile to construct medium-term stable directed acyclic graphs for routing and do measurements on the edges such as ETX [RFC6551]. Actual “low power” does not seem to be required for an LLN [I-D.hui-vasseur-roll-rpl-deployment], and the positions on scaling of LLNs appear to vary widely [I-D.clausen-lln-rpl-experiences].
The ROLL terminology document states that LLNs typically are composed of constrained nodes; this is also supported by the design of operation modes such as RPL’s “non-storing mode”. So, in the terminology of the present document, an LLN seems to be a constrained node network with certain network characteristics, which include constraints on the network as well.
One interesting class of a constrained network often used as a constrained node network is the “LoWPAN” [RFC4919], a term inspired from the name of the IEEE 802.15.4 working group (low-rate wireless personal area networks (LR-WPANs)). The expansion of that acronym, “Low-Power Wireless Personal Area Network” contains a hard to justify “Personal” that is due to IEEE politics more than due to an orientation of LoWPANs around a single person. Actually, LoWPANs have been suggested for urban monitoring, control of large buildings, and industrial control applications, so the “Personal” can only be considered a vestige. Maybe the term is best read as “Low-Power Wireless Area Networks” (LoWPANs) [WEI]. Originally focused on IEEE 802.15.4, “LoWPAN” (or when used for IPv6, “6LoWPAN”) is now also being used for networks built from similarly constrained link layer technologies [I-D.ietf-6lowpan-btle] [I-D.mariager-6lowpan-v6over-dect-ule] [I-D.brandt-6man-lowpanz].
Despite the overwhelming variety of Internet-connected devices that can be envisioned, it may be worthwhile to have some succinct terminology for different classes of constrained devices. In this document, the class designations in Table 1 may be used as rough indications of device capabilities:
| Name | data size (e.g., RAM) | code size (e.g., Flash) |
|---|---|---|
| Class 0, C0 | « 10 KiB | « 100 KiB |
| Class 1, C1 | ~ 10 KiB | ~ 100 KiB |
| Class 2, C2 | ~ 50 KiB | ~ 250 KiB |
As of the writing of this document, these characteristics correspond to distinguishable clusters of commercially available chips and design cores for constrained devices. While it is expected that the boundaries of these classes will move over time, Moore’s law tends to be less effective in the embedded space than in personal computing devices: Gains made available by increases in transistor count and density are more likely to be invested in reductions of cost and power requirements than into continual increases in computing power.
Class 0 devices are very constrained sensor-like motes. Most likely they will not be able to communicate directly with the Internet in a secure manner. Class 0 devices will participate in Internet communications with the help of larger devices acting as proxies, gateways or servers. Class 0 devices generally cannot be secured or managed comprehensively in the traditional sense. They will most likely be preconfigured (and will be reconfigured rarely, if at all), with a very small data set. For management purposes, they could answer keepalive signals and send on/off or basic health indications.
Class 1 devices cannot easily talk to other Internet nodes employing a full protocol stack such as using HTTP, TLS and related security protocols and XML-based data representations. However, they have enough power to use a protocol stack specifically designed for constrained nodes (e.g., CoAP over UDP) and participate in meaningful conversations without the help of a gateway node. In particular, they can provide support for the security functions required on a large network. Therefore, they can be integrated as fully developed peers into an IP network, but they need to be parsimonious with state memory, code space, and often power expenditure for protocol and application usage.
Class 2 can already support mostly the same protocol stacks as used on notebooks or servers. However, even these devices can benefit from lightweight and energy-efficient protocols and from consuming less bandwidth. Furthermore, using fewer resources for networking leaves more resources available to applications. Thus, using the protocol stacks defined for very constrained devices also on Class 2 devices might reduce development costs and increase the interoperability.
Constrained devices with capabilities significantly beyond Class 2 devices exist. They are less demanding from a standards development point of view as they can largely use existing protocols unchanged. The present document therefore does not make any attempt to define classes beyond Class 2. These devices can still be constrained by a limited energy supply.
With respect to examining the capabilities of constrained nodes, particularly for Class 1 devices, it is important to understand what type of applications they are able to run and which protocol mechanisms would be most suitable. Because of memory and other limitations, each specific Class 1 device might be able to support only a few selected functions needed for its intended operation. In other words, the set of functions that can actually be supported is not static per device type: devices with similar constraints might choose to support different functions. Even though Class 2 devices have some more functionality available and may be able to provide a more complete set of functions, they still need to be assessed for the type of applications they will be running and the protocol functions they would need. To be able to derive any requirements, the use cases and the involvement of the devices in the application and the operational scenario need to be analyzed. Use cases may combine constrained devices of multiple classes as well as more traditional Internet nodes.
Devices not only differ in their computing capabilities, but also in available electrical power and/or energy. While it is harder to find recognizable clusters in this space, it is still useful to introduce some common terminology.
The power and/or energy available to a device may vastly differ, from kilowatts to microwatts, from essentially unlimited to hundreds of microjoules.
Instead of defining classes or clusters, we propose simply stating, in SI units, an approximate value for one or both of the quantities listed in Table 2:
| Name | Definition | SI Unit |
|---|---|---|
| Ps | Sustainable average power available for the device over the time it is functioning | W (Watt) |
| Et | Total electrical energy available before the energy source is exhausted | J (Joule) |
The value of Et may need to be interpreted in conjunction with an indication over which period of time the value is given; see the next subsection.
As discussed above, some devices are limited in available energy as opposed to (or in addition to) being limited in available power. Where no relevant limitations exist with respect to energy, the device is classified as E3. The energy limitation may be in total energy available in the usable lifetime of the device (e.g. a device with a non-replaceable primary battery, which is discarded when this battery is exhausted), classified as E2. Where the relevant limitation is for a specific period, this is classified as E1, e.g. a limited amount of energy available for the night with a solar-powered device, or for the period between recharges with a device that is manually connected to a charger, or by a periodic (primary) battery replacement interval. Finally, there may be a limited amount of energy available for a specific event, e.g. for a button press in an energy harvesting light switch; this is classified as E0. Note that many E1 devices in a sense also are E2, as the rechargeable battery has a limited number of useful recharging cycles.
In summary, we distinguish (Table 3):
| Name | Type of energy limitation | Example Power Source |
|---|---|---|
| E0 | Event energy-limited | Event-based harvesting |
| E1 | Period energy-limited | Battery that is periodically recharged or replaced |
| E2 | Lifetime energy-limited | Non-replaceable primary battery |
| E3 | No direct quantitative limitations to available energy | Mains powered |
Especially when wireless transmission is used, the radio often consumes a big portion of the total energy consumed by the device. Design parameters such as the available spectrum, the desired range, and the bitrate aimed for, influence the power consumed during transmission and reception; the duration of transmission and reception (including potential reception) influence the total energy consumption.
Based on the type of the energy source (e.g., battery or mains power) and how often device needs to communicate, it may use different kinds of strategies for power usage and network attachment.
The general strategies for power usage can be described as follows:
In summary, we distinguish (Table 4):
| Name | Strategy | Ability to communicate |
|---|---|---|
| S0 | Always-off | Re-attach when required |
| S1 | Low-power | Appears connected, perhaps with high latency |
| S2 | Always-on | Always connected |
This draft introduces common terminology that does not raise any new security issue.
This document has no actions for IANA.
Dominique Barthel and Peter van der Stok provided useful comments; Charles Palmer provided a full editorial review.
Peter van der Stok insisted that we should have power terminology, hence Section 4. The text for Section 4.3 is mostly lifted from [I-D.arkko-lwig-cellular] and has been adapted for this document.