Networking Working Group D. Culler, Ed. Internet-Draft Arch Rock (& UC Berkeley) Intended status: Informational JP. Vasseur, Ed. Expires: December 31, 2007 Cisco Systems, Inc June 29, 2007 Routing Requirements for Low-Power Wireless Networks draft-culler-rl2n-routing-reqs-00.txt Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. 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." The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt. The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html. This Internet-Draft will expire on December 31, 2007. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract The need for high quality routing for Sensor networks comprised of highly constrained devices (CPU, memory, ...) in sometimes unstable wireless environments is critical now and will continue to increase. Interest in this class of applications has grown dramatically in recent years and a routing solution addressing the specific environments of such networks is highly required considering the numerous, incompatible open-source and proprietary routing protocols Culler & Vasseur Expires December 31, 2007 [Page 1] Internet-Draft draft-culler-rl2n-routing-reqs-00.txt June 2007 as well as several industrial forums. The aim of this document is to define the routing requirements for Sensor Networks at the IP layer. Such routing protocol(s) would need to address several unique aspects of this class of embedded devices and would operate in networks comprising links of various nature. Requirements Language 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 [RFC2119]. Table of Contents 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Unique Routing Requirements of Low Power Wireless Networks . . 3 2.1. Spatially-Driven Multihop . . . . . . . . . . . . . . . . 3 2.2. Light Footprint . . . . . . . . . . . . . . . . . . . . . 4 2.3. Small MTU . . . . . . . . . . . . . . . . . . . . . . . . 5 2.4. Deep power management . . . . . . . . . . . . . . . . . . 5 2.5. Heterogeneous Capabilities . . . . . . . . . . . . . . . . 6 2.6. Highly Variable Connectivity . . . . . . . . . . . . . . . 6 2.7. Structured Workload and Traffic Pattern . . . . . . . . . 7 2.8. Partial Information . . . . . . . . . . . . . . . . . . . 8 2.9. Quality of Service Capable Routing . . . . . . . . . . . . 8 2.10. Date Aware routing . . . . . . . . . . . . . . . . . . . . 8 3. Relationship with other Routing Protocols . . . . . . . . . . 8 4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 5. Security Considerations . . . . . . . . . . . . . . . . . . . 9 6. Manageability Considerations . . . . . . . . . . . . . . . . . 9 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10 8.1. Normative References . . . . . . . . . . . . . . . . . . . 10 8.2. Informative References . . . . . . . . . . . . . . . . . . 10 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 10 Intellectual Property and Copyright Statements . . . . . . . . . . 11 Culler & Vasseur Expires December 31, 2007 [Page 2] Internet-Draft draft-culler-rl2n-routing-reqs-00.txt June 2007 1. Introduction The need for high quality routing for wireless networks comprised of highly constrained (memory, power, bandwidth, CPU ...) and typically embedded devices in a potentially variable environment is critical now and will continue to increase. Interest in this class of applications, including sensor networks, device networks, environmental monitoring, home automation, building automation, process control, automated meter readings, condition-based maintenanace, security, and others, has grown dramatically in recent years; a routing solution addressing the specific environments of such networks is highly required considering the numerous, incompatible open-source and proprietary routing protocols that have emerged, as well as several industrial forums that have emerged over the IEEE 802.15.4 link and various proprietary links. The IETF 6LoWPAN working group has defined a format for IPv6 over 802.15.4 with extensive header compression, fragmentation for very small link frames, and support for mesh routing under the IP link. The aim of this document is to define the routing requirements for low power wireless networks at the IP layer. As such, it pertains to collections of IEEE 802.15.4 devices, but is not limited to communication within a single IP link. It pertains to IP level routing among multiple such PANs, routing among IEEE 802.15.4 PANS and other links, and routing in other low power wireless networks. Such routing protocol(s) would need to address several unique aspects of this class of embedded devices and would operate in networks comprising links of various nature. Considering the variety of Sensor and Controller-based applications, there may not be a single routing protocol satisfying the entire list of requirements, in which case it may be decided to define a limited set of routing protocols that could be combined to satify the overall objective. 2. Unique Routing Requirements of Low Power Wireless Networks Sensor networks and related networks of low-power, emebedded devices present a variety of unique routing requirements driven partly by implementation technology constraints, partly by the domain of usage, and partly by application characteristics. These issues are listed roughly in order of criticality. 2.1. Spatially-Driven Multihop The low transmission power of PAN (Personal Area Network) radios, as typically defined by the collection of IEEE 802.15 links, implies Culler & Vasseur Expires December 31, 2007 [Page 3] Internet-Draft draft-culler-rl2n-routing-reqs-00.txt June 2007 that the range is relatively short; multiple hops are required to achieve communication over greater distances. Variously referred to as mesh or multihop routing, such multihop routing communication is important from a basic energy viewpoint. The energy cost to traverse a given distance with multiple fixed-power hops grows only linearly with distance, whereas the energy of a single RF "hop" grows as a cubic or higher power of the distance, depending on elevation and other factors. It is also essential from a reliability viewpoint. Lower transmission power generally means lower SNR, relatively high per-hop loss rates and greater sensitivity to fading, interference, attenuation, and occlusion. Mutihop communication permits routing around obstacles and provides temporal diversity through retransmission as well as spatial diversity through multiple receivers, i.e., multipath routing. In addition, with multihop routing use to cover distance, route formation and reliability are intimately linked. Taking a longer hop will typically incur a larger loss rate, while a more reliable hop incurs more transmissions to reach the destination. These issues occur potentially both at layer 2, with IP routing over mesh-routed links, and, of course, at layer 3, with IP routing over similar or dissimilar links. Furthermore, with multiple points of egress between low-power wireless networks and conventional powered networks, route selection over on type of link may be influenced by factors in the low-power links. Within the IETF, working groups are attending to aspects of this issue with, for example, 6LoWPAN considering layer 2 "mesh-under" for IEEE 802.15.4 links and MANET considering layer 3 and higher layer routing in mobile environments with relatively high powered nodes and links. Meanwhile, industry forums, including Zigbee, Zwave, Wireless HART, and ISA SP100.11a, and numerous proprietary offerings address the combination of low-power and wireless, but only within the equivalent of a single IP link and only within the context of stacks vertically integrated from phy to application with no provisions for routing to other kinds of links. 2.2. Light Footprint Integrated CMOS radios typically have sophisticated physical layer and MAC support integrated with the transceiver. However, the network layer over this MAC (or sub-MAC) is generally implemented on a microcontroller device with the capabilities and resources historically associated with serial links (e.g., RS-232 and RS-485). In particular, as of today, these devices have only a few kilobytes of RAM and a few to several tens of kilobytes of program ROM. The memory capacity of these device has been growing, but at much slower rate than the SRAM and DRAM storage found in microprocessor-based systems. The marginal cost of memory in embedded devices is much greater than in conventional computers and standby power consumption Culler & Vasseur Expires December 31, 2007 [Page 4] Internet-Draft draft-culler-rl2n-routing-reqs-00.txt June 2007 increases with RAM capacity due to leakage, so memory capacity impacts the lifetime of battery powered, low-duty cycle devices. Thus, the small memory capacity of these units is fundamental and constrains routing table size, buffer capacity, and all routing state, including neighbor tables, link estimators, sequence number and other caches. For example, link state algorithms, distance vector algorithms, and various intermediates and hybrids may have quite different relative merits when footprint is at premium, as compared to convergence rate, information exchange rate, and so on. Existing routing protocols generally attend to constraints imposed by the links more than to constraints imposed by the nodes that connect those links. The prime exception to this is scalability concerns of very large networks given fixed, albeit powerful, routers. Here we are concerned with how routing protocols scale down to less capable nodes, even a fixed network scale. We are also concerned with how routing protocols can allow more capable nodes to relieve less capable ones, even with common link characterstics. Compression techniques, such as that in 6LoWPAN, enable the opportunity to perform routing on low-power devices (and permit the use of small MTUs and modest forwarding buffers), but do not address the resource requirements of the routing protocols that guide the exchange of such compressed packets 2.3. Small MTU Potentially high bit error rates, limited buffer capacity, limited channel capacity shared among numerous devices, and pervasive hidden- terminal occurrences due to the presence of many devices spread over physical regions all lead to the use of relatively small frames. Thus, per packet routing and header information comes at a premium. These issues, as well as limited energy, storage and bandwidth resources, imply that routing needs to be more aware of underlying physical factors than in traditional, even wireless, networks. For example, protocols involving the exchange of lists of all 1-hop or all 2-hop nighbors may be forced to reckon with longs lists (if the physical density is high compared to the communication range). Alternatively, efforts to limit the degree of the network by adjusting transmission range bring additional physical factors into the purvue of routing. Moreover, such measures to optimize route formation may be at odds with optimizing forwarding cost. 2.4. Deep power management Average transmission rates are very low, relative to channel capacity and powering on the radio to be ready receive costs power consumption that is roughly equal to that of actual transmission or reception. Thus, power budgets tend to be dominated by idle listening costs, Culler & Vasseur Expires December 31, 2007 [Page 5] Internet-Draft draft-culler-rl2n-routing-reqs-00.txt June 2007 unless the receivers are heavily duty cycled. Thus, routing protocols must permit deep power management in the underlying link layers. Currently, these link level techniques fall into three general categories: variants of TDMA either local or global, variants of cluster-based beaconing, and variants of preamble sample. While power management is typically viewed as a layer 2 responsibility, few routing protocols anticipate that the devices responsible for forwarding (and for route maintainence) have their network link off most (often over 99%) of the time. Alternatively, certain link-level power management strategies may introduce extreme constraints on routing protocols. 2.5. Heterogeneous Capabilities While the majority of devices are highly constrained, in many settings they operate in conjunction with more capable devices, including microprocessors hosting the same RF link but with greater RAM capacity, devices on mains power with either large or small storage, devices with directional to high-gain antennas, and devices that bridge or route to higher bandwidth links. The existence of such a wide scope of device types within Sensor Networks must be taken into account by the routing protocol to increase the lifetime and robustness of the most constrained devices. In some cases, it may be advantageous to decrease the routing optimality at the benefit of energy saving for the most constrained devices. Thus the routing protocol must not only be capable of supporting such a wide variety a devices but should consider the device capability as a key element of the routing decision, domain scope for the exchange of routing control plane messages. 2.6. Highly Variable Connectivity In many use cases for low power wireless devices, mobility is a central element. However, even where all the devices are stationary, changes in environmental conditions gives rise to substantial changes in the connectivity relationships. Moving objects, opening and closing of doors, background interference due to machinery, electronic equipment, radios, or other wireless networks, even atmospheric changes which increase or decrease absorption all alter the connection topology over which routing takes place. Thus, routing protocols must be adaptive to a changing underlying topology and able to utilize connectivity and related information, such as link quality or signal strength, to maintain viable paths. For many embedded networks with substantial, often the mobility is structured, rather than ad hoc, such as items moving through a manufacturing process, shipping exchanges, mobile devices moving Culler & Vasseur Expires December 31, 2007 [Page 6] Internet-Draft draft-culler-rl2n-routing-reqs-00.txt June 2007 through a stationary network of similar devices, or collections of devices moving together as a network. The most extreme variations in connectivity, including mobility over large distances and enclosure into RF-opaque settings, give rise to intermittent connectivity (DTN: Delay Tolerant Networks). Many use cases involve logging over long periods of disconnected operation and dispersion of logged data upon arrival and detection of a point of connectivity Such topology changing environments are usually challenging for routing protocols and may lead to frequent rerouting decisions: careful consideration must be given to bound the number of rerouting decisions for the most contrained devices so as to save energy. 2.7. Structured Workload and Traffic Pattern The above characteristics suggest that effective general-purpose routing for low-power wireless networks can be very hard - multiple hops are required over spontaneously varying connections where bandwidth is precious, packets are small and little state can be expended at each router. However, the same observations suggests that routing protocols can take advantage of the constrained setting to simplify the general problem. The workload or traffic pattern of use cases for these networks tend to be highly structured (Point-to-Multipoint or Multipoint-to-point due to the specific role of one or more sinks), unlike the any-to-any data transfers and interactive key-strokes that dominate typical client and server workloads. Instrumentation and monitoring typically involve regular, periodic, or alarm-based collection from a large collection of devices. Configuration, tasking, and management typically involve dissemination of commands to an aggregate of devices. Automation, such as lighting control, involve numerous long-lived aggregates of actuation points and control points. Uses in transportation and shipping involve opportunistic communication bursts upon arrival at suitable way points. General-purpose any-to- any connectivity arises in situations such as management, diagnosis, and field access. In many cases, exploiting such structure may simplify difficult problems arising from resource constraints or variation in connectivity. Thus the routing protocols should support Point to Point, Point to Multipoint and Multipoint to Point routing. However, the highly correlated, repetitive use of particular traffic patterns will typically allow routing protocols to optimize for very common simple cases. Culler & Vasseur Expires December 31, 2007 [Page 7] Internet-Draft draft-culler-rl2n-routing-reqs-00.txt June 2007 2.8. Partial Information The density of connectivity varies dramatically from long nearly- linear structures (e.g., over a transect of land, a bridge or a road) to extremely dense collections in a single RF 'cell' (e.g., parcels on a dock or containers in transport). Thus, routing protocols and addressing should avoid placing arbitrary limits on the underlying connection topology. Conversely, routing with partial information is an important property in the sensor network as it facilitates scaling down of the node or scaling up of the the network to points where algorithmic concepts such "all 1-hop neighbors", "all 2-hop neighbors", all nodes, or all pairs may not be representable with the resources available per node. 2.9. Quality of Service Capable Routing QoS (Quality of Service) capable routing is also important to consider both with the goal of improving service where it is is desirable, but in reducing effort where service requirements are lax. Although many WSN uses initially provide fairly latency in-sensitive monitoring, many applications have emerged that require timely delivery of the vast majority of the readings, eventual delivery of the remainder, time-sensitive delivery of alarms, and/or increasing predictability for soft and moderately real-time. These issues impact path selection and path quality optimization, as well as the impact of protocol and route maintenance traffic on data traffic, especially during times of critical physical change. Thus, the mix of applications with a wide range of requirements in term of path quality leads to the potential requirements for QoS-aware routing. 2.10. Date Aware routing Ultimately, scalability may benefit from the ability to perform computations for data reduction or fusion within the network, not just at the data processing sink level. The most common case being aggregation along a dynamically computed path to a sink. Thus the routing protocol should take points of aggregation (another node capability) into account when calculating routes. 3. Relationship with other Routing Protocols This family of unique characteristics pose unique routing challenges. At the same time, these challenges have deep similarities (and substantial points of difference) with several other IETF routing protocols. Like MANET, the interconnection topology over which routing is performed must, in general, be deduced from observed communication events, in addition to physical wiring or explicit Culler & Vasseur Expires December 31, 2007 [Page 8] Internet-Draft draft-culler-rl2n-routing-reqs-00.txt June 2007 configuration. This topology may be static or dynamic, depending on physical conditions. However, the routing state, neighbor table size, and cache state per node will in many cases be highly constrained. Devices themselves have important structure and characteristics, as many are stationary and some are unconstrained. In general, the average bandwidth and power demand per node should stay bounded and not grow unreasonably with the size of a network. Thus, it may be unacceptable to generate unscoped floods, unless the frequency of floods per node diminishes with the size of the network. In these respects, light footprint routing has much in common with IGP. Effective routing must be carried out in the presence of partial (space limited) and somewhat imperfect information. Note that mixed routing protocol may be considered (Distance Vector and Link state). That said, none of the currently available routing protocol fulfills the requirement of Sensor Networks network listed above. The aforementioned requirements may be conflicting and defining a new routing protocol fully satisfying those requirements might be challenging. The objective of this work would be to define a routing protocol that will satisfy those requirements as much as possible and that would potentially adapt itself to the particular deployment context. 4. IANA Considerations This memo includes no request to IANA. 5. Security Considerations TBD. 6. Manageability Considerations TBD. 7. Acknowledgements 8. References Culler & Vasseur Expires December 31, 2007 [Page 9] Internet-Draft draft-culler-rl2n-routing-reqs-00.txt June 2007 8.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. 8.2. Informative References Authors' Addresses David Culler (editor) Arch Rock (& UC Berkeley) 657 Mission St. Suite 600 San Francisco, CA 94105 USA Email: dculler@archrock.com JP Vasseur (editor) Cisco Systems, Inc 1414 Massachusetts Avenue Boxborough, MA 01719 USA Email: jpv@cisco.com Culler & Vasseur Expires December 31, 2007 [Page 10] Internet-Draft draft-culler-rl2n-routing-reqs-00.txt June 2007 Full Copyright Statement Copyright (C) The IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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