LPWAN GAP Analysis
draft-minaburo-lpwan-gap-analysis-01

Abstract

Low Power Wide Area Networks (LPWAN) are different technologies covering different applications based on long range, low bandwidth and low power operation. The use of IETF protocols in the LPWAN technologies should contribute to the deployment of a wide number of applications in an open and standard environment where actual technologies will be able to communicate. This document makes a survey of the principal characteristics of these technologies and covers a cross layer analysis on how to adapt and use the actual IETF protocols, but also the gaps for the integration of the IETF protocol stack in the LPWAN technologies.

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 Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet-Drafts is at http://datatracker.ietf.org/drafts/current/.

Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress."

This Internet-Draft will expire on January 4, 2017.

Copyright Notice

Copyright (c) 2016 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

1. Introduction

LPWAN (Low-Power Wide Area Network) technologies are a kind of constrained and challenged networks [RFC7228]. They can operate in license or license-exempt bands to provide connectivity to a vast number of battery-powered devices requiring limited communications. If the existing pilot deployments have shown the huge potential and the industrial interest in their capabilities, the loose coupling with the Internet makes the device management and network operation complex. More importantly, LPWAN devices are, as of today, with no IP capabilities. The goal is to adapt IETF defined protocols, addressing schemes and naming spaces to this constrained environment.

2. Problem Statement

The LPWANs are large-scale constrained networks in the sense of [RFC7228] with the following characteristics:

• very small frame payload as low as 12 bytes. Typical traffic patterns are composed of a large majority of frames with payload size around 15 bytes and a small minority of up to 100 byte frames. Some nodes will exchange less than 10 frames per day.
• very low bandwidth, most LPWAN technologies offer a throughput between 50 bit/s to 250 kbit/s, with a duty cycle of 0.1% to 10% on some ISM bands.
• high packet loss, which can be the result of bad transmission conditions or collisions between nodes.
• variable MTU for a link depending on the used L2 modulation.
• highly asymmetric and in some cases unidirectional links.
• ultra dense networks with thousands to tens of thousands of nodes.
• different modulations and radio channels.
• sleepy nodes to preserve energy.

In the terminology of [RFC7228], these characteristics put LP-WANs into the “challenged network” category where the IP connectivity has to be redefined or modified. Therefore, LP-WANs need to be considered as a separate class of networks. The intrinsic characteristics, current usages and architectures will allow the group to make and justify the design choices. Some of the desired properties are:

• keep compatibility with current Internet:
  • preserve the end-to-end communication principle.
  • maintain independence from L2 technology.
  • use or adapt protocols defined by IETF to this new environment that could be less responsive.
  • use existing addressing spaces and naming schemes defined by IETF.
• ensure the correspondence with the stringent LPWAN requirements, such as:
  • limited number of messages per device.
  • small message size, with potentially no L2 fragmentation.
  • RTTs potentially orders of magnitude bigger than existing constrained networks.
• optimize the protocol stack in order to limit the number of duplicated functionalities; for instance acknowledgements should not be done at several layers.

3. Identified gaps in current IETF groups concerning LPWANs

3.1. IPv6 and LPWAN

IPv6 [RFC2460] has been designed to allocate addresses to all the nodes connected to the Internet. Nevertheless the 40 bytes of overhead introduced by the protocol are incompatible with the LPWAN constraints. If IPv6 were used, several LPWAN frames will be needed just to carry the header. Another limitation comes from the MTU limit, which is 1280 bytes required from the layer 2 to carry IPv6 packet [RFC1981]. This is a side effect of the PMTU discovery mechanism, which allows intermediary routers to send to the source an ICMP message (packet too big) to reduce the size. An attacker will be able to forge this message and reduce drastically the transmission performances. This limit allows to mitigate the impact of this attack.

IPv6 needs a configuration protocol (neighbor discovery protocol, NDP [RFC4861]) to learn network parameters, and the node relation with its neighbor. This protocol generates a regular traffic with a large message size that does not fit LPWAN constraints.

3.2. 6LoWPAN, 6lo and LPWAN

6LoWPAN only resolves the IPv6 constraints by drastically reducing IPv6 overhead to about 4 bytes for ND traffic, but the header compression is not better for an end-to-end communications using global addresses (up to 20 bytes). 6LoWPAN has been initially designed for IEEE 802.15.4 networks with a frame size up to 127 bytes and a throughput of up to 250 kb/s with no duty cycle regarding the usage of the network.

IEEE 802.15.4 is a CSMA/CA protocol which means that every unicast frame is acknowledged. Because IEEE 802.15.4 has its own reliability mechanism by retransmission, 6LoWPAN does not have reliable delivery. Some LPWAN technologies do not provide such acknowledgements at L2 and would require other reliability mechanisms.

6lo extends the usage of 6LoWPAN to other technologies (BLE, DECT, …), with similar characteristics to IEEE 802.15.4. The main constraint in these networks comes from the nature of the devices (constrained devices), whereas in LPWANs it is the network itself that imposes the most stringent constraint.

6LoWPAN has optimized Neighbor Discovery by reducing the message size, the periodic exchanges and removing multicast message for point-to-point exchanges with border router.

3.3. 6tisch and LPWAN

6TiSCH is complementary to LPWA technologies.

A key element of 6tisch is the use of synchronization to enable determinism. TSCH and 6TiSCH may provide a standard scheduling function. An LPWA may or may not support synchronization like the one used in 6tisch. The 6tisch solution is dedicated to mesh networks that operate using 802.15.4e MAC with a deterministic slotted channel. The TSCH can help to reduce collisions and to enable a better balance over the channels. It improves the battery life by avoiding the idle listening time for the return channel.

3.4. ROLL and LPWAN

The LPWANs considered by the WG are based on a star topology, which eliminates the need for routing. Future works may address additional use-cases which may require the adaptation of existing routing protocols or the definition of new ones. For the moment, the work done at the ROLL WG and other routing protocols are out of scope of the LPWAN WG.

3.5. CORE and LPWAN

CoRE provides a resource-oriented application intended to run on constrained IP networks. It may be necessary to adapt the protocols to take into account the duty cycling and the potentially extremely limited throughput. For example, some of the timers in CoAP may need to be redefined. Taking into account CoAP acknowledgements may allow the reduction of L2 acknowledgements. The actual work in progress in the CoRE WG where the COMI/CoOL network management interface which uses Structured Identifiers (SID) to reduce payload size over CoAP proves to be a good solution for the LPWA technologies. The overhead is reduced by adding a dictionary which match a URI to a small identifier and a compact mapping of the YANG model into the CBOR binary representation.

3.6. Security and LPWAN

Most of the LPWA integrate some authentication or encryption mechanisms that may not have been defined by the IETF. The working group will work to integrate these mechanisms to unify management. For the technologies which are not integrating natively security protocols, the group will adapt existing mechanisms to the LPWA constraints. The AAA infrastructure brings a scalable solution. It offers a central management for the security processes, draft-garcia-dime-diameter-lorawan-00 and draft-garcia-radext-radius-lorawan-00 explains the possible security process for a LORAWAN network. The mechanisms basically are divided by: key management protocols, encryption and integrity algorithms used. Most of the solutions do not present a key management procedure to derive specific keys for securing network and or data information. In most cases it is assumed a pre-shared key between the smart object and the communication endpoint.

3.7. Mobility and LPWAN

LPWA nodes can be mobile. However, LPWAN mobility is different than the one specified for Mobile IP. LPWAN, implies sporadic traffic and will rarely be used for high-frequency, real-time communications. The applications do not generate a flow, they need to save energy and most of the time the node will be down. The mobility will imply most of the time a group of devices, which represent a network itself, the the mobility concerns more the gateway than the devices.

3.8. DNS and LPWAN

The purpose of the DNS is to enable applications to name things that have a global unique name. Lots of protocols are using DNS to identify the objects, especially REST and applications using CoAP. Therefore, things should be registred in DNS. DNS is probably a good point of research for the LPWA technologies, while the matching of the name and the IP information can be used to configured the LPWA devices.

4. Annex A -- survey of LPWAN technologies

+-------------+---------------+--------------+--------+
|Technology   |range          | Throughput   |MAC MTU |
+-------------+---------------+--------------+--------+
|LoRa         |2-5 km urban   |0.3 to 50 kbps|256 B   |
|             |<15 km suburban|              |        |
+-------------+---------------+--------------+--------+
|SIGFOX       |10 km urban    |up:100/600 bps| 12/    |
|             |50 km rural    |down: 600 bps | 8 B    |
+-------------+---------------+--------------+--------+
|IEEE802.15.4k| < 20 km LoS   |1.5 bps to    |16/24/  |
|LECIM        | < 5 km NoLoS  | 128 kbps     | 32 B   |
+-------------+---------------+--------------+--------+
|IEEE802.15.4g| 2-3 km LoS    | 4.8 kbps to  |2047 B  |
|SUN          |               |800 kbps      |        |
+-------------+---------------+--------------+--------+
|RPMA         | 65 km LoS     |  up: 624kbps |64 B    |
|             | 20 km NoLoS   |down: 156kbps |        |
|             |               | mob: 2kbps   |        |
+-------------+---------------+--------------+--------+
|DASH-7       | 2 km          |    9 kbps    |256 B   |
|             |               |   55.55 kbps |        |
|             |               |  166.66 kbps |        |
+-------------+---------------+--------------+--------+
|Weightless-w | 5 km urban    | 1 kbps to    |min 10 B|
|             |               | 10 Mbps      |        |
+-------------+---------------+--------------+--------+
|Weightless-n |<5 km urban    | 30 kbps to   |max 20 B|
|             |<30 km suburban| 100kbps      |        |
+-------------+---------------+--------------+--------+
|Weightless-p |> 2 km urban   | up to 100kbps|        |
+-------------+---------------+--------------+--------+
| NB-IoT   *  |        <15 km |  ~  200kbps  | >1000B | 
+-------------+---------------+--------------+--------+
* supports segmentation