Network Working Group L. Feeney Internet-Draft Uppsala University Intended status: Informational V. Fodor Expires: January 4, 2018 KTH July 03, 2017 Inter-network Coexistence in the Internet of Things draft-feeney-t2trg-inter-network-00 Abstract The breadth of IoT applications implies that future wireless environments will be characterized by the presence of many diverse, administratively independent IoT networks operating in the same physical location. In many cases, these networks will use unlicensed spectrum, due to its low cost and ease of deployment. However, this spectrum is becoming increasingly crowded. IoT networks will therefore be subject to wireless interference, both from similar networks and from networks that use the channel in very different ways. To date, there have been few studies or testbeds that fully reflect this aspect of the future IoT operating environment. This document describes some of the main issues in network co-existence in IoT environments, focusing on protocol-level interactions. It identifies two issues for the IRTF t2trg community. The first is to define best practices for performance evaluation and protocol design in the context of inter-network interference. The second is the potential use of higher layer protocols to actively participate in interference mitigation. 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, 2018. Feeney & Fodor Expires January 4, 2018 [Page 1] Internet-Draft inter-network coexistence July 2017 Copyright Notice Copyright (c) 2017 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. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 2. IoT interference challenges . . . . . . . . . . . . . . . . . 3 2.1. Scale . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.2. Independence . . . . . . . . . . . . . . . . . . . . . . 3 2.3. Battery lifetime . . . . . . . . . . . . . . . . . . . . 4 2.4. Resource constraints . . . . . . . . . . . . . . . . . . 4 2.5. Diversity . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Interaction behaviors . . . . . . . . . . . . . . . . . . . . 5 4. Network co-existence in the IRTF/IETF context . . . . . . . . 6 4.1. Responding to link layer evolution . . . . . . . . . . . 7 4.2. Protocol evaluation . . . . . . . . . . . . . . . . . . . 7 4.3. Active mitigation strategies . . . . . . . . . . . . . . 7 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . 10 7. Informative References . . . . . . . . . . . . . . . . . . . 10 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 11 1. Introduction An enormous range of IoT applications are expected to become pervasive in daily life. Networks will be installed in public spaces, businesses, and residences by a wide range of individual, commercial, and government actors. This means that there will be many diverse, administratively independent networks operating in the same physical location. For example, a future home environment may include IoT applications for security, heating and cooling, elder care, air quality monitoring, personal health and fitness, smart home appliances, structural monitoring, lighting, utilities, and entertainment. Feeney & Fodor Expires January 4, 2018 [Page 2] Internet-Draft inter-network coexistence July 2017 Many of these applications will use unlicensed spectrum due to low cost and simplicity of deployment for both the user and developer. In unlicensed spectrum, there is no authority that has a management relationship with (or even knows about) all of the potentially interfering networks that can be present in some location. This means that there is no entity that can coordinate networks' use of the shared wireless channel. Networks will therefore experience interference caused by transmissions from devices belonging to other networks. Much existing work in inter-network interference is analytic, based on statistical distributions of interfering waveforms under various conditions (see e.g. [NIST]). There have also been a few papers proposing interference mitigation strategies for interference between specific protocols (see e.g. [SURVEY]), particularly between IEEE 802.11 and IEEE 802.15.4. In general, existing standards and products tend to rely on frequency agility and low duty cycles to avoid interference. To date, however, there have been very few studies or testbeds that fully reflect the complex interference scenarios of the future IoT environment, particularly in the context of protocol-level interactions. 2. IoT interference challenges The widespread deployment and diversity of IoT networks will create new challenges in managing network coexistence. 2.1. Scale As IoT becomes pervasive, there may be a large number of networks and devices operating in any given location. Devices from the various networks will be topologically inter-mingled. Interaction scenarios will also be highly dynamic, with mobility leading to frequent changes in the set of interfering devices. 2.2. Independence In unlicensed spectrum, there is not necessarily any trust relationship between networks. Networks with overlapping transmission footprints may well have been deployed by different actors (e.g. in adjacent apartments). There is no single authority that has an administrative relationship with all of the potentially interfering networks in some location. Devices within a network will be able to authenticate themselves to each other, but the network itself may not have any meaningful external identity. This means that there is no entity that all networks can trust to coordinate their access to the shared channel. Feeney & Fodor Expires January 4, 2018 [Page 3] Internet-Draft inter-network coexistence July 2017 2.3. Battery lifetime For battery-powered devices, maximizing lifetime is essential. System design is often dominated by the need to minimize device activity and especially by the need to keep the radio turned off as much as possible. This means that there must be some way for senders and receivers to coordinate their radio operations. These mechanisms often depend heavily on careful timing of radio operations, in addition to (or instead of) exchanging control packets. Examples include the synchronized beacon-enabled PAN, asynchronous wakeup based on low power listening (e.g. ContikiMAC), and TSCH-based scheduled mesh (e.g. WirelessHART). This timing dependence means that may be more sensitive to disruption than might be expected from just considering overall channel utilization and collision probabilities [FF15]. Battery constraints also severely limit devices' ability to listen to the channel to observe the behavior of potentially interfering networks. 2.4. Resource constraints Iot networks may be severely resource constrained. Channel capacity and battery limitations have been discussed above. It is also common that IoT devices have very limited on-board CPU and memory. For many IoT applications, devices must be very low-cost and easily deployed and managed by non-expert users. These factors severely constrain the design space and limit the complexity of any protocol for this environment. 2.5. Diversity Although even identical networks experience inter-network interference, diversity of radios, protocols, and applications creates additional challenges. This diversity is fundamentally due to the diversity of the IoT application requirements, and therefore convergence to a single solution is unlikely. Different kinds of radios use different modulation schemes to encode data on the channel, resulting different patterns of radio energy distributed over time and spectrum. They also divide the spectrum into channels differently. This means that network devices may not be able to directly identify the kind of radio that is causing interference and packet loss. In some cases, it may not be possible for channel sensing mechanisms to reliably detect the presence of interfering transmissions. Feeney & Fodor Expires January 4, 2018 [Page 4] Internet-Draft inter-network coexistence July 2017 Different radio technologies are intended to provide different data rates, transmission powers, and coverage areas. In 2.4GHz spectrum, IEEE 802.15.4 is a low power, low data-rate radio; its short (128B) frames, transmitted at 250kbps, occupy the channel for up to a few ms.. IEEE 802.11 networks have much higher data rates (10's of Mbps) and (usually) higher transmit powers. In sub-GHz spectrum, IEEE 802.15.4g/Wi-SUN (smart utility network) radios has data rates 50-200 kbps to communicate over distances of up to ~1km, while LoRa radios use data rates of <5 kbps to provide communication over 10's of km. These differences imply large differences in packet transmission times, which can range from <1 ms to 100's of ms. Timing parameters in a given MAC layer, such as backoff and retransmission intervals, tend to be proportional to packet transmission times for the underlying PHY. This can reduce the effectiveness of backoff strategies in mixed radio environments, though it can also provide opportunities for co-existence strategies (e.g. ). Even where networks use the same radio and PHY they may only share part of their protocol stack. The range of protocols based on the IEEE 802.15.4 PHY layer is a case in point. It includes beacon- enabled star-topologies (with both CSMA and TDMA/GTS modes), scheduled mesh (e.g. IEEE 802.15.4 TSCH, WirelessHART and ISA 100.11a standards) and decentralized, asynchronous low power MACs (e.g. ContikiMAC). Although IP(v6) is widely (though not universally) used in IEEE 802.15.4-based network stacks, there is also considerable diversity in higher layer protocols. Some networks are unrouted star- topologies, while others use multi-hop routing protocols, such as RPL or SmartMesh. There is also diversity in transport protocols (e.g. TCP, CoAP) and applications (many highly specialized). IoT networks therefore have a wide range of channel utilization patterns. These include synchronous and asynchronous wakeup, periodic announcements and data collection, multi-hop forwarding, and bursty responses to detected events. Communication requirements also vary: While few IoT applications are intended as real time control systems, many are based on "reasonably" reliable and timely delivery of small amounts of sensor data and control traffic. 3. Interaction behaviors Interference between WiFi networks is widely observed, especially in dense residential and urban areas, where there are many independently deployed networks. Wifi is an example of a strongly homogeneous interference environment. Most WiFi networks consist of an AP and associated devices that communicate directly with their AP. This Feeney & Fodor Expires January 4, 2018 [Page 5] Internet-Draft inter-network coexistence July 2017 simplifies intra-network coordination and tends to create a somewhat cellular topology. There is also a strong convention for interference-reducing channel assignments (channels 1, 6, and 11 in 2.4GHz spectrum). Nearly all WiFi devices use the same CSMA-based MAC and traffic is dominated by one application - media streaming - which is supported by adaptive mechanisms throughout the protocol stack. Despite these simplifying factors, interference is still considered a problem, even by the general public. 2.4GHz spectrum is also a heterogeneous interference environment. A common scenario will involve high-power, high-traffic WiFi networks impacting networks based on low-power, low-bitrate radios, such as IEEE 802.15.4. This case has been widely studied and several specific protocol variations have been proposed (e.g. [JP14]), but practical solutions mostly involve identifying and using least interfered channels. But in places where there is a lot of WiFi traffic, there may be very few such channels. As a result, the various low-power IoT networks operating in these areas may be crowded into a very small number of "good" channels. This leads to a variety of interactions among IEEE 802.15.4 based protocols. For example, two networks may allocate TDMA transmission slots that conflict with each other. This is because it not possible for independent networks to explicitly coordinate their slot allocations, while battery constraints make it hard to do a lot of channel sensing. Recent results [FF15] show that interference between both TDMA and CSMA-based IEEE 802.15.4 beacon-enabled PANs can lead to synchronization/ desynchronization patterns and episodes of severe packet loss - even when the channel itself is only lightly loaded. These outages can, in turn, affect the operation of higher-layer protocols. More generally, this shows the negative impact of periodic behavior, which is intended to reduce the radio duty cycle and hence energy consumption. 4. Network co-existence in the IRTF/IETF context A recent, very broad survey of spectrum sharing research is found in [SURVEY]. To date, there have been few studies of the interference scenarios outlined above, particularly at the scale and diversity that are expected in future IoT scenarios. In addition, there are very few testbeds or simulation tools that are intended to reflect the future IoT operating environment. A discussion of the challenges of testing wireless co-existence for both licensed and unlicensed spectrum is found in NIST. However, this document only considers only radio PHY layer interactions. Feeney & Fodor Expires January 4, 2018 [Page 6] Internet-Draft inter-network coexistence July 2017 There is a substantial need for better understanding of network co- existence among the most widely used IoT technologies. This is an open research problem for practical deployment of IoT networks, with several areas of relevance for IETF/IRTF activities. In particular, we highlight 1) developing best practices for protocol performance evaluation and 2) research into higher layer protocols for explicit network coordination as issues that are particularly relevant to IRTF t2trg. 4.1. Responding to link layer evolution Radio technologies and link layer standards will continue to evolve to provide increased resilience to interference. Such developments are naturally relevant to IETF activities regarding the definition of IPv6 over various IoT link layers (e.g. 6low, 6tisch) and closely related higher layers protocols such as RPL and CoAP. 4.2. Protocol evaluation Maximizing channel and battery efficiency and minimizing the impact of both intra- and inter-network interference is largely in the domain of spectrum regulation and the PHY/MAC layer and therefore out-of-scope for IETF/IRTF. Nevertheless, higher-layer protocols are also affected by - and can contribute to - adverse interactions between networks sharing the wireless channel. For example, they may have adaptive behaviors or timing dependencies that are sensitive to patterns of loss and delay created by inter-network interference. Or they may have periodic or bursty communication patterns that contribute to adverse interactions. The performance of existing (and forthcoming) IETF protocols such as 6LoWPAN, 6tisch, 6lpwan, RPL, and CoAP under complex IoT interference scenarios is not well understood. We therefore argue that performance evaluation of IoT protocols should consider whether they will perform acceptably in the presence of diverse networks operating in the same spectrum. This is a poorly understood area and there is a lack of simulation or testbed environments that provide scale and diversity characteristic of future IoT environments. Documentation and advocacy of best practices for protocol evaluation scenarios is therefore relevant to IRTF t2trg activities. 4.3. Active mitigation strategies Interference mitigation is likely to rely primarily on improved resilience and local adaptation in PHY and MAC layer protocols and (to a lesser extent) in higher layer protocols. However, there may Feeney & Fodor Expires January 4, 2018 [Page 7] Internet-Draft inter-network coexistence July 2017 be opportunities for high layer protocols to actively participate in interference mitigation by enabling explicit coordination between networks. When two networks use the same radio and channel, it may be possible for frames transmitted by devices in one network to be successfully received by devices in other networks. These external frames cannot be authenticated and are generally discarded at the MAC layer. However, they might provide a way for networks to exchange signaling information at the network layer, despite their diverse channel access or higher-layer protocols. This could be used for devices to announce their expected channel utilization, for example. There are substantial challenges in developing such a mechanism: 1) There is an enormous diversity of radios, channel access methods and utilization patterns that might need to be described. It is not clear what information should be signaled or what actions a receiver should take in response. 2) Battery lifetime, channel capacity, and device CPU / memory resources continue to be significant limitations. In particular, the radio duty cycle is highly constrained, limiting both sensing and communication.. 3) Any such mechanism must operate effectively in the absence of any administrative or trust relationship between networks. Any proposed solution will therefore need to be resilient to the possibility of incompatible, oblivious, selfish, or even hostile networks participating (or not) in some mitigation mechanism. (See Security Considerations.) 4) The privacy implications of networks sharing information about their activity must be carefully considered. (See Security Considerations.) This remains a very open research area and one that we argue is particularly amenable to standards and interoperability oriented approaches enabled by IRTF t2trg. There may be synergy with IRTF t2trg work in IoT semantic interoperability, allowing IoT networks to describe not only the 'things' they connect, but also themselves. There may also be synergy with IETF activities (e.g. spud/plus) in making signaling information available within encrypted flows. It is also possible to consider the possibility of coordination between networks that use different radios and cannot exchange packets Specialized techniques for very simple low bitrate signaling between networks using different radios have been proposed. It is Feeney & Fodor Expires January 4, 2018 [Page 8] Internet-Draft inter-network coexistence July 2017 also possible to consider explicit mitigation strategies that enable information exchange via some service coordinated in the Internet infrastructure. Inspiration here is from cognitive radio solutions where secondary users obtain information about activity of primary users from trusted sources. 5. Security Considerations This document focuses on co-existence between independently administrated networks operating in the same location. The biggest security challenge is therefore that such networks do not necessarily have any basis for a trust relationship. Regulations concerning unlicensed spectrum control radio behaviors such as power spectral density, channelization, or duty cycle. They do not mandate the use of any specific protocol, nor is it possible to ensure that a potentially interfering network is correctly implementing any particular co-existence mechanism. Any proposed solution will therefore need to be resilient to the possibility of incompatible, oblivious, selfish, or even hostile networks participating (or not) in some interference mitigation mechanism. This is especially true for methods in which networks actively coordinate their use of the shared channel. At a minimum, participating in information exchange should not substantially increase vulnerability to disruption in the case of a malicious (or merely incompatible) actor. IoT networks that try to be friendly toward each other may disclose substantial information about their operation. There are privacy issues associated with IoT networks making such information visible, because of their close coupling with human activity. Particularly for health-related applications, even being able to identify the type of network application or its level of activity may reveal sensitive information. Ideally, it should be possible for a network to both obfuscate its communication patterns (if needed) and to cooperate in minimizing adverse interactions. One maxim that may be useful in designing the set of information that a network discloses as a matter of course with the intention of facilitating coexistence is that the information disclosed should not provide more insight than that information an attacker might have gained by simply observing the network for a while. But note that simply disclosing that information in an accessible way still changes the economy of surveillance -- the objective is that it also changes the economy of coexistence, and these effects need to be carefully weighed against each other. Feeney & Fodor Expires January 4, 2018 [Page 9] Internet-Draft inter-network coexistence July 2017 6. Conclusion Understanding and mitigating the impact of inter-network interference on performance and reliability is essential for successful large- scale deployment of IoT solutions. Particularly in residential and urban environments, high density of WiFi networks will limit the number of "good" channels available to low-power IoT networks. As a result, administratively independent IoT networks - possibly with quite different channel access behavior - are likely to operate on shared channels. Potential interactions among these networks are not well understood. For example: What happens if two or more independent networks using CoAP over RPL over 6LowPAN (or 6tisch) over IEEE 1802.15.4 are operating in the same room? What happens if a beacon-enabled PAN (or a Thread or ZigBee or ContikiMAC network, etc.) is thrown into the mix? Especially in a WiFi heavy environment, the value of channel hopping for interference mitigation in IEEE 802.15.4 networks may be limited. Similarly, how will LPWAN networks such as LoRa and SigFox, with coverage areas of 10's of km sq., interact with other sub-GHz networks such as Wi-SUN/IEEE 802.15.4g? Interference mitigation is largely the domain of spectrum regulation and the PHY/MAC layers. There are IETF/IRTF interests as well, most obviously for IoT protocols such as 6LowPAN, 6tisch, RPL, and CoAP. Two open issues are especially relevant to IRTF t2trg: Many interference scenarios are not well understood, particularly with regard to protocol-level interactions. Best practices for performance evaluation should be developed to reflect future IoT environments. There may also be opportunities for active interference mitigation via explicit coordination and information sharing, topics which are particularly amenable to interoperability and standards oriented research. However, there are substantial research challenges. 7. Informative References [FF15] Feeney, L. and V. Fodor, "Reliability in co-located 802.15.4 personal area networks", Proceedings of the 6th ACM International Workshop on Pervasive Wireless Healthcare - MobiHealth '16 , DOI 10.1145/2944921.2944923, 2016. [IEEE802154] "IEEE Standard for Low-Rate Wireless Networks", IEEE standard, DOI 10.1109/ieeestd.2016.7460875, n.d.. Feeney & Fodor Expires January 4, 2018 [Page 10] Internet-Draft inter-network coexistence July 2017 [JP14] Javed, Q. and R. Prakash, "CHAMELEON: A Framework for Coexistence of Wireless Technologies in an Unlicensed Band", Wireless Personal Communications Vol. 77, pp. 777-808, DOI 10.1007/s11277-013-1536-7, November 2013. [NIST] Koepke, G., Young, W., Ladbury, J., and J. Coder, "Interference and Coexistence of Wireless Systems in Critical Infrastructure", National Institute of Standards and Technology report, DOI 10.6028/nist.tn.1885, July 2015. [SEMINT] Feeney, L., "Exploring semantic interference in heterogeneous sensor networks", Proceeding of the 1st ACM international workshop on Heterogeneous sensor and actor networks - HeterSanet '08 , DOI 10.1145/1374699.1374708, 2008. [SURVEY] Han, Y., Ekici, E., Kremo, H., and O. Altintas, "Spectrum sharing methods for the coexistence of multiple RF systems: A survey", Ad Hoc Networks Vol. 53, pp. 53-78, DOI 10.1016/j.adhoc.2016.09.009, December 2016. Acknowledgements The authors would like to thank Michael Frey, Charalampos Orfanidis, Martin Jacobsson, and Per Gunningberg for their valuable collaboration in simulation and measurement studies of inter-network interference. We would also like to thank Carsten Bormann for his support and encouragement in preparing this document, particularly the discussion of security considerations. Authors' Addresses Laura Marie Feeney Uppsala University Box 337 Uppsala SE-751 05 Sweden Email: lmfeeney@it.uu.se Feeney & Fodor Expires January 4, 2018 [Page 11] Internet-Draft inter-network coexistence July 2017 Viktoria Fodor KTH Osquldas vaeg 10 Stockholm SE-100 44 Sweden Email: vjfodor@kth.se Feeney & Fodor Expires January 4, 2018 [Page 12]