ROLL P. van der Stok, Ed. Internet-Draft Philips Research Intended status: Informational February 24, 2012 Expires: August 27, 2012 Multicast requirements for control over LLN draft-vanderstok-roll-mcreq-00 Abstract This is a working document intended to focus discussion on requirements for multicast in Low-power and Lossy Networks in the area of M2M communication for control purposes. The Trickle algorithm, which uses re-broadcasting to assure that messages arrive at all destinations, is proposed as the ROLL multicast protocol. In this draft additional requirements on Trickle, such as timeliness and ordering, are motivated by building control. Re-broadcasting and timeliness can be mutually exclusive properties. To alleviate that problem, this draft considers the possibility of reducing interference by limiting the number of transmission paths between any two devices in the mesh. 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 August 27, 2012. Copyright Notice Copyright (c) 2012 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 van der Stok Expires August 27, 2012 [Page 1] Internet-Draft MCReq February 2012 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 . . . . . . . . . . . . . . . . . . . . . . . . . 3 1.1. Terminology . . . . . . . . . . . . . . . . . . . . . . . 3 1.2. Motivation . . . . . . . . . . . . . . . . . . . . . . . . 4 2. Application characteristics . . . . . . . . . . . . . . . . . 4 3. Multicast requirements . . . . . . . . . . . . . . . . . . . . 5 4. Wireless link characteristics . . . . . . . . . . . . . . . . 6 5. Recommendation . . . . . . . . . . . . . . . . . . . . . . . . 8 6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 9 7. Security Considerations . . . . . . . . . . . . . . . . . . . 9 8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9 9.1. Normative References . . . . . . . . . . . . . . . . . . . 9 9.2. Informative References . . . . . . . . . . . . . . . . . . 10 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 10 van der Stok Expires August 27, 2012 [Page 2] Internet-Draft MCReq February 2012 1. Introduction The ROLL working group is chartered to design and standardize a routing protocol for resource constrained devices in Low-power and Lossy Networks (LLN) [I-D.ietf-roll-rpl]. The requirements for ROLL are documented in [RFC5548] [RFC5673] [RFC5826] [RFC5867]. For building control it is recognized that most communication is local to the wireless mesh network, and does not necessarily pass through the edge router. The point-to-point RPL routing algorithm is developed to efficiently support such applications [I-D.ietf-roll-p2p-rpl]. The Trickle multicast was developed to support the RPL routing algorithm, and later proposed to support general multicast delivery in LLN. [RFC6206]. This draft discusses the multicast requirements for constrained devices participating in M2M building control networks. An important requirement is the delivery of control commands to a subset (group) of neighbouring devices in the LLN within some latency bound. 1.1. Terminology 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]. Addtional privileged words are described below. A "device" is a physical processor connected to at least one link through a network interface. Each interface has at least one IP unicast address. The IP address is optionally bound to a host name, which may be a Fully Qualified Domain Name (FQDN). One device communicates directly with another device by wirelessly transmitting packets to it over a link. The link quality is divided in three regions: 1. good: where a transmitted packet will be correctly received by a destination with a probability higher than 99%. 2. transitional: where the probability of correct reception fluctuates. 3. bad: where almost no transmission is successfully received. It is empirically known that good links can become bad occasionally due to dynamic effects such as multipath interference. A distinction is made between reception and delivery of a message. A message is received when it is stored in the reception buffer of the receiver after transmission and all error checks have been succesfully passed. The message is delivered when the message is van der Stok Expires August 27, 2012 [Page 3] Internet-Draft MCReq February 2012 passed from the reception buffer to the client application. We also say the client application accepts the message. Broadcasting is used for the link-local sending of one packet to all reachable 1-hop neigbours. This is equivalent to the term link-local multicast. 1.2. Motivation In this draft, we focus and develop discussions on requirements pertaining to multicasting, in the context of building control applications. 2. Application characteristics Multicast is important for building control applications. Two types of applications are considered: 1. Discovery messages to all members of the mesh (multicast GET) 2. Control messages to a subset of the mesh (multicast PUT) The first type requires the message to be sent to a subset which may be randomly distributed over the building area. Some of the destinations return unicast messages to the source. The second type requires the message to be sent to a closely spaced subset. No return messages are generated. This second type is the subject of this draft, although most of the requirements equally apply to case 1. An office building typically consist of multiple floors, divided in working areas. The working areas can be open or enclosed by walls. Within a working area sensors measure temperature, presence, humidity and other parameters. On the basis of these measurements, equipment within the working area can receive commands to change settings. A well-known example is presence detection to switch on or dim lights. The equipment configuration is quite stable, because devices are installed in the ceiling, and modifying (or servicing) the installation can be costly. The equipment is interconnected in a wireless network. The RF transmissions pass through the walls and generate interference to the wireless equipment in other working areas. The lay-out of a network may be different from installation to installation. However, it is expected that many wireless networks extend over one floor and include many working areas. Another van der Stok Expires August 27, 2012 [Page 4] Internet-Draft MCReq February 2012 working hypothesis is that most of the time sensors will multicast their values to a group of devices within the working area. Consequently, multicast messages are often meant for a subset of neigbouring devices. A LoWPAN is a mesh of wireless devices that share the same IPv6 address prefix. A typical LowPAN in a building may cover the area of an entire floor. A commercial installation may cover 1000 m2 per floor. A length of 50 m can easily mean a hop count >5 for a message to pass from end to end. For example, devices may be installed in the ceiling in a grid with a grid pattern distance of 40 cm between devices. Messages may consist of sensor measurements performed or commands issued in a given working area, which then must be acted upon by neigbouring devices in the same working area. Given that source and sink are located in one working area, sink and source of a multicast message are often between 3 - 6 m from each other. Consequently, it is required to send a multicast to a subset of the devices in the LoWPAN. In case of commands to luminaries, messages must be delivered within a clear deadline of about 200ms. In [RFC5867] a deadline of 120 ms is suggested for other building applications. Although most control messages are exchanged between closely spaced devices, it is sometimes necessary, say every hour or less frequently, to send a message to a subset of devices covering the whole building. In that case the multicast message will need to pass the edge router of the lowpan and to propagate to other subnets. 3. Multicast requirements The Multicast requirements are derived from the characteristics of the applications. A device is said to be correct it it follows the multicast algorithm. The application characteristics and the network installation make it possible to add an additional set of network properties to make the multicast algorithm more efficient. The basic traditional multicast requirements (PUT and GET)are: o Validity: If sender S sends message, m, to a group, g, of destinations, a path exists between S and a destination D, and S and D are correct, D eventually accepts m. o Integrity: A destination accepts m at most once from sender and only if sender sent m to a group including destination. van der Stok Expires August 27, 2012 [Page 5] Internet-Draft MCReq February 2012 o Agreement: If a correct destination of g accepts m, then all correct destinations of g accept m. The set of intended destination devices is identified by the multicast (group) IP address. Every device in the associated multicast group is a destination of the multicast. Each destination accepts messages with as destination the specified IP multicast address. Additional multicast requirements are: o Timeliness: There is a known constant C such that if m is sent at time t, no correct destination accepts m after t+C. For lighting control applications the value of C is taken as 200 ms. This requirement considers the PUT case and not the return of a response in the GET case. o Ordering: When m1 and m2 sent to the same group g, and a receiver in g accepts message m1 before m2, every receiver in g accepts m1 before accepting m2 Ordering applies to PUT and GET case. Ordering can be partial or total. Partial ordering means that for specified message pairs one of the pair precedes the other. In case of total ordering, every message pair is ordered. Partial ordering is obtained by adding message counters in the message such that destinations can order the messages of a given sender. Messages from different sources are not ordered. Total ordering can be obtained with vector clocks or using synchronized clocks. Vector clocks require a large overhead that increases linearly with the number of devices in the network. As long as no synchronized clocks are available, partial ordering seems the most realistic. Total Ordering is interesting for the discovery application. When two devices announce themselves simultaneously with conflicting properties, all participants can come to the same decision by favoring the first arrival. Partial ordering is necessary when a multicast message needs multiple packets (for example discovery messages) or when multicast messages are sent with intervals shorter than the throughput delay. 4. Wireless link characteristics It is possible to broadcast from a source to a set of devices reachable over good links in one hop. This is not sufficient because the set of reachable devices is often a subset of the set of destination devices. Consequently, additional measures are needed to make sure that the Agreement requirement is met. A standard technique, to reach all devices instead of a subset, stipulates that every receiver of a broadcast message rebroadcasts this message van der Stok Expires August 27, 2012 [Page 6] Internet-Draft MCReq February 2012 (flooding). When the multicast address corresponds with a specified multicast address in the receiver device, the message is delivered. Thanks to this technique it is assured that when a path exists between the source and the destination device, the destination device will eventually receive the message from the sender. Given the network density described above, the multicast can generate a broadcast storm with lots of interfering senders. The technique, also used in Trickle, is to randomly delay the message rebroadcast. However, the long delays can seriously jeopardize the timeliness requirement. This draft proposes three ways suggested by the application characteristics, to reduce the interference between re- broadcasting devices: 1. Restrict the scope of the multicast. 2. Restrict number of rebroadcasting devices. 3. Weaken the Timeliness requirement. In the application characteristics it is mentioned that most control messages have a set of destinations which are closely spaced to the source. The interference between multicast sources can be reduced by limiting the scope of the broadcast message. The ensuing proximity condition can be formulated for PUT and GET as: o Proximity condition: A multicast message is accepted by a subset of devices closely spaced to the sender. In practice, this condition means that most multicast messages can be constrained to 1-2 hops. It is recommended to put the multicast range under control of the multicast source. Given the stability of the network configuration, the configuration of good links is also stable over long periods (say several days). When all good links are available, the number of possible paths between a source and each of its destinations is probably larger than required given the sporadic failure of a good link. Under the assumption that the qualities of the good links of a given device are unrelated, the failure of good link has no consequence for alternative good links. Given the installation characteristics above, the number of paths between source and destinations is much larger than required to assure that a source device remains connected to all its destinations when a good link fails. The number of paths can be reduced by specifying a subset of devices, called relay devices, to rebroadcast messages. A path can pass from a source via relay devices to the multicast destinations. A relay device can also be a destination device. In [RFC5867] it is mentioned that 1 out of 2 devices is a relay device. Given the network densities foreseen for lighting, a much lower relay density is possible. The reduction van der Stok Expires August 27, 2012 [Page 7] Internet-Draft MCReq February 2012 of the relay devices reduces the risk of interference in the dense networks described above. An appropriate condition to assure the presence of a path between source and destination can be formulated as: o Multiple relay links: any device has good links to at least q relay devices The value of q is determined by the quality of the links in a given installation. However, the probability that a path was temporarily unavailable cannot be excluded. The timeliness requirement is too strong for wireless sensor networks, where packets get lost for multiple reasons like hidden terminal, multipath fading, and others. The timeliness requirement can be reformulated for the PUT case as: o Majority Timeliness: There is a known constant C, and a subset s of correct destinations in group g, such that if m is sent to g at time t, with low probability p, all destinations in s accept m after t+C. The agreement requirement specifies that the destinations in s accept the message eventually. Both probability p and set s are specified as function of the installation and linked with the value of q. Using rebroadcast with a low frequency (as in Trickle) assures that missed messages are eventually repeated. For a lighting application this means that in general all lights switch on/off within 200 ms and quite infrequently, (say once a month) one out of all lights swictches on/off a bit later (say a few seconds). 5. Recommendation From the text above emerges a number of recommendations to make it possible to put propagation characteristics of the multicast algorithm under application control. 1. Take into account timeliness and partial ordering requirements in multicast algorithm. 2. Exploit the small range of most multicasts and put multicast range under application control. 3. Introduce a subset of devices as relay devices to reduce the number of rebroadcasting devices. 4. Use majority timeliness requirement to balance the number of relay devices with respect to the probability that a device misses its multicast reception deadline. van der Stok Expires August 27, 2012 [Page 8] Internet-Draft MCReq February 2012 5. Multicast messages transit through an edge router. 6. IANA Considerations This document makes no request of IANA. Note to RFC Editor: this section may be removed on publication as an RFC. 7. Security Considerations TBD 8. Acknowledgments This I-D has benefited from conversations with and comments from Anders Brandt, Kerry Lynn, Zach Shelby, Emmanuel Frimout, Michael Verschoor, Jamie Mc Cormack, Dee Denteneer, Jerald Martocci, Matthieu Vial, and Nicolas Riou. 9. References 9.1. Normative References [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC5548] Dohler, M., Watteyne, T., Winter, T., and D. Barthel, "Routing Requirements for Urban Low-Power and Lossy Networks", RFC 5548, May 2009. [RFC5673] Pister, K., Thubert, P., Dwars, S., and T. Phinney, "Industrial Routing Requirements in Low-Power and Lossy Networks", RFC 5673, October 2009. [RFC5826] Brandt, A., Buron, J., and G. Porcu, "Home Automation Routing Requirements in Low-Power and Lossy Networks", RFC 5826, April 2010. [RFC5867] Martocci, J., De Mil, P., Riou, N., and W. Vermeylen, "Building Automation Routing Requirements in Low-Power and Lossy Networks", RFC 5867, June 2010. [RFC6206] Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko, van der Stok Expires August 27, 2012 [Page 9] Internet-Draft MCReq February 2012 "The Trickle Algorithm", RFC 6206, March 2011. 9.2. Informative References [I-D.ietf-roll-rpl] Brandt, A., Vasseur, J., Hui, J., Pister, K., Thubert, P., Levis, P., Struik, R., Kelsey, R., Clausen, T., and T. Winter, "RPL: IPv6 Routing Protocol for Low power and Lossy Networks", draft-ietf-roll-rpl-19 (work in progress), March 2011. [I-D.ietf-roll-p2p-rpl] Goyal, M., Baccelli, E., Philipp, M., Brandt, A., and J. Martocci, "Reactive Discovery of Point-to-Point Routes in Low Power and Lossy Networks", draft-ietf-roll-p2p-rpl-07 (work in progress), January 2012. Author's Address Peter van der Stok (editor) Philips Research High Tech Campus 34-1 Eindhoven, 5656 AA The Netherlands Email: peter.van.der.stok@philips.com van der Stok Expires August 27, 2012 [Page 10]