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RFC 7731

Multicast Protocol for Low-Power and Lossy Networks (MPL)

Pages: 29
Proposed Standard

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Internet Engineering Task Force (IETF)                            J. Hui
Request for Comments: 7731                                     Nest Labs
Category: Standards Track                                      R. Kelsey
ISSN: 2070-1721                                             Silicon Labs
                                                           February 2016


       Multicast Protocol for Low-Power and Lossy Networks (MPL)

Abstract

This document specifies the Multicast Protocol for Low-Power and Lossy Networks (MPL), which provides IPv6 multicast forwarding in constrained networks. MPL avoids the need to construct or maintain any multicast forwarding topology, disseminating messages to all MPL Forwarders in an MPL Domain. MPL has two modes of operation. One mode uses the Trickle algorithm to manage control-plane and data-plane message transmissions and is applicable for deployments with few multicast sources. The other mode uses classic flooding. By providing both modes and parameterization of the Trickle algorithm, an MPL implementation can be used in a variety of multicast deployments and can trade between dissemination latency and transmission efficiency. Status of This Memo This is an Internet Standards Track document. This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741. Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc7731.
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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.
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Table of Contents

1. Introduction ....................................................4 2. Terminology .....................................................5 3. Applicability Statement .........................................6 4. MPL Protocol Overview ...........................................7 4.1. MPL Domains ................................................7 4.2. Information Base Overview ..................................8 4.3. Protocol Overview ..........................................8 4.4. Signaling Overview ........................................10 5. MPL Parameters and Constants ...................................11 5.1. MPL Multicast Addresses ...................................11 5.2. MPL Message Types .........................................11 5.3. MPL Seed Identifiers ......................................11 5.4. MPL Parameters ............................................11 6. Protocol Message Formats .......................................14 6.1. MPL Option ................................................14 6.2. MPL Control Message .......................................15 6.3. MPL Seed Info .............................................16 7. Information Base ...............................................17 7.1. Local Interface Set .......................................17 7.2. Domain Set ................................................18 7.3. Seed Set ..................................................18 7.4. Buffered Message Set ......................................18 8. MPL Seed Sequence Numbers ......................................19 9. MPL Data Messages ..............................................19 9.1. MPL Data Message Generation ...............................19 9.2. MPL Data Message Transmission .............................20 9.3. MPL Data Message Processing ...............................21 10. MPL Control Messages ..........................................22 10.1. MPL Control Message Generation ...........................22 10.2. MPL Control Message Transmission .........................22 10.3. MPL Control Message Processing ...........................23 11. IANA Considerations ...........................................24 11.1. MPL Option Type ..........................................24 11.2. MPL ICMPv6 Type ..........................................25 11.3. Well-Known Multicast Addresses ...........................25 12. Security Considerations .......................................25 13. References ....................................................26 13.1. Normative References .....................................26 13.2. Informative References ...................................28 Acknowledgements ..................................................29 Authors' Addresses ................................................29
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1. Introduction

Low-Power and Lossy Networks (LLNs) typically operate with strict resource constraints in communication, computation, memory, and energy. Such resource constraints may preclude the use of existing IPv6 multicast routing and forwarding mechanisms. Traditional IP multicast delivery typically relies on topology maintenance mechanisms to discover and maintain routes to all subscribers of a multicast group (e.g., [RFC3973] [RFC4601]). However, maintaining such topologies in LLNs is costly and may not be feasible given the available resources. Memory constraints may limit devices to maintaining links/routes to one or a few neighbors. For this reason, the Routing Protocol for LLNs (RPL) specifies both storing and non-storing modes [RFC6550]. The latter allows RPL routers to maintain only one or a few default routes towards an LLN Border Router (LBR) and use source routing to forward messages away from the LBR. For the same reasons, an LLN device may not be able to maintain a multicast routing topology when operating with limited memory. Furthermore, the dynamic properties of wireless networks can make the cost of maintaining a multicast routing topology prohibitively expensive. In wireless environments, topology maintenance may involve selecting a connected dominating set used to forward multicast messages to all nodes in an administrative domain. However, existing mechanisms often require two-hop topology information, and the cost of maintaining such information grows polynomially with network density. This document specifies the Multicast Protocol for Low-Power and Lossy Networks (MPL), which provides IPv6 multicast forwarding in constrained networks. MPL avoids the need to construct or maintain any multicast routing topology, disseminating multicast messages to all MPL Forwarders in an MPL Domain. By using the Trickle algorithm [RFC6206], MPL requires only small, constant state for each MPL device that initiates disseminations. The Trickle algorithm also allows MPL to be density aware, allowing the communication rate to scale logarithmically with density.
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2. Terminology

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119]. The following terms are used throughout this document: MPL Forwarder - A router that implements MPL. An MPL Forwarder is equipped with at least one MPL Interface. MPL Interface - An MPL Forwarder's attachment to a communications medium, over which it transmits and receives MPL Data Messages and MPL Control Messages according to this specification. An MPL Interface is assigned one or more unicast addresses and is subscribed to one or more MPL Domain Addresses. MPL Domain Address - A multicast address that identifies the set of MPL Interfaces within an MPL Domain. MPL Data Messages disseminated in an MPL Domain have the associated MPL Domain Address as their destination address. MPL Domain - A scope zone, as defined in [RFC4007], in which MPL Interfaces subscribe to the same MPL Domain Address and participate in disseminating MPL Data Messages. MPL Data Message - A multicast message that is used to communicate a multicast payload between MPL Forwarders within an MPL Domain. An MPL Data Message contains an MPL Option in the IPv6 header and has as its destination address the MPL Domain Address corresponding to the MPL Domain. MPL Control Message - A link-local multicast message that is used to communicate information about recently received MPL Data Messages to neighboring MPL Forwarders. MPL Seed - An MPL Forwarder that generates MPL Data Messages and serves as an entry point into an MPL Domain. MPL Seed Identifier - An unsigned integer that uniquely identifies an MPL Seed within an MPL Domain. Node - Used within this document to refer to an MPL Forwarder.
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3. Applicability Statement

MPL is an IPv6 multicast forwarding protocol designed for the communication characteristics and resource constraints of LLNs. By implementing controlled disseminations of multicast messages using the Trickle algorithm, MPL is designed for networks that communicate using low-power and lossy links with widely varying topologies in both the space and time dimensions. While designed specifically for LLNs, MPL is not limited to use over such networks. MPL may be applicable to any network where no multicast routing state is desired. MPL may also be used in environments where only a subset of links are considered low-power and lossy links. A host need not be aware that their multicast is supported by MPL as long as its attachment router forwards multicast messages between the MPL Domain and the host. However, a host may choose to implement MPL so that it can take advantage of the broadcast medium inherent in many LLNs and receive multicast messages carried by MPL directly. MPL is parameterized to support different dissemination techniques. In one parameterization, MPL may utilize the classic flooding method that involves having each device receiving a message rebroadcast the message. In another parameterization, MPL may utilize Trickle's [RFC6206] "polite gossip" method, which involves transmission suppression and adaptive timing techniques. [Clausen2013] questions the efficiency of Trickle's "polite gossip" mechanism in some multicast scenarios, so by also including a classic flooding mode of operation MPL aims to be able to perform satisfactorily in a variety of situations. To support efficient message delivery in networks that have many poor links, MPL supports a reactive forwarding mode that utilizes MPL Control Messages to summarize the current multicast state. The MPL Control Message size grows linearly with the number of simultaneous MPL Seeds in the MPL Domain -- 4 octets per MPL Seed. When reactive forwarding is not enabled, MPL Control Messages are not transmitted, and the associated overhead is not incurred. This document does not specify a cryptographic security mechanism for MPL to ensure that MPL messages are not spoofed by anyone with access to the LLN. In general, the basic ability to inject messages into an LLN may be used as a denial-of-service attack, regardless of what forwarding protocol is used. For these reasons, LLNs typically employ link-layer security mechanisms to mitigate an attacker's ability to inject messages. For example, the IEEE 802.15.4 [IEEE802.15.4] standard specifies frame security mechanisms using
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   AES-128 to support access control, message integrity, message
   confidentiality, and replay protection.  However, if the attack
   vector includes attackers that have access to the LLN, then MPL
   SHOULD NOT be used.

4. MPL Protocol Overview

The goal of MPL is to deliver multicast messages to all interfaces that subscribe to the multicast messages' destination address within an MPL Domain.

4.1. MPL Domains

An MPL Domain is a scope zone, as defined in [RFC4007], in which MPL Interfaces subscribe to the same MPL Domain Address and participate in disseminating MPL Data Messages. When participating in only one MPL Domain, the MPL Domain Address is the ALL_MPL_FORWARDERS multicast address with Realm-Local scope ("scop" value 3) [RFC7346]. When an MPL Forwarder participates in multiple MPL Domains simultaneously, at most one MPL Domain may be assigned an MPL Domain Address equal to the ALL_MPL_FORWARDERS multicast address. All other MPL Domains MUST be assigned a unique MPL Domain Address that allows the MPL Forwarder to identify each MPL Domain. The MPL Domains SHOULD be configured automatically based on some underlying topology. For example, when using RPL [RFC6550], MPL Domains may be configured based on RPL Instances. When MPL is used in deployments that use administratively defined scopes that cover, for example, multiple subnets based on different underlying network technologies, Admin-Local scope (scop value 4) or Site-Local scope (scop value 5) SHOULD be used. An MPL Forwarder MAY participate in additional MPL Domains identified by other multicast addresses. An MPL Interface MUST subscribe to the MPL Domain Addresses for the MPL Domains that it participates in. The assignment of other multicast addresses is out of scope. For each MPL Domain Address that an MPL Interface subscribes to, the MPL Interface MUST also subscribe to the same MPL Domain Address with Link-Local scope (scop value 2) when reactive forwarding is in use (i.e., when communicating MPL Control Messages).
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4.2. Information Base Overview

A node records necessary protocol state in the following information sets: o The Local Interface Set records the set of local MPL Interfaces and the unicast addresses assigned to those MPL Interfaces. o The Domain Set records the set of MPL Domain Addresses and the local MPL Interfaces that subscribe to those addresses. o A Seed Set records information about received MPL Data Messages received from an MPL Seed within an MPL Domain. Each MPL Domain has an associated Seed Set. A Seed Set maintains the minimum sequence number for MPL Data Messages that the MPL Forwarder is willing to receive or has buffered in its Buffered Message Set from an MPL Seed. MPL uses Seed Sets and Buffered Message Sets to determine when to accept an MPL Data Message, process its payload, and retransmit it. o A Buffered Message Set records recently received MPL Data Messages from an MPL Seed within an MPL Domain. Each MPL Domain has an associated Buffered Message Set. MPL Data Messages resident in a Buffered Message Set have sequence numbers that are greater than or equal to the minimum threshold maintained in the corresponding Seed Set. MPL uses Buffered Message Sets to store MPL Data Messages that may be transmitted by the MPL Forwarder for forwarding.

4.3. Protocol Overview

MPL achieves its goal by implementing a controlled flood that attempts to disseminate the multicast data message to all interfaces within an MPL Domain. MPL performs the following tasks to disseminate a multicast message: o When having a multicast message to forward into an MPL Domain, the MPL Seed generates an MPL Data Message that includes the MPL Domain Address as the IPv6 Destination Address, the MPL Seed Identifier, a newly generated sequence number, and the multicast message. If the multicast destination address is not the MPL Domain Address, IP-in-IP tunneling [RFC2473] is used to encapsulate the multicast message in an MPL Data Message, preserving the original IPv6 Destination Address.
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   o  Upon receiving an MPL Data Message, the MPL Forwarder extracts the
      MPL Seed and sequence number and determines whether or not the MPL
      Data Message was previously received using the MPL Domain's Seed
      Set and Buffered Message Set.

      *  If the sequence number is less than the lower-bound sequence
         number maintained in the Seed Set or a message with the same
         sequence number exists within the Buffered Message Set, the MPL
         Forwarder marks the MPL Data Message as old.

      *  Otherwise, the MPL Forwarder marks the MPL Data Message as new.

   o  For each newly received MPL Data Message, an MPL Forwarder updates
      the Seed Set, adds the MPL Data Message into the Buffered Message
      Set, processes its payload, and multicasts the MPL Data Message a
      number of times on all MPL Interfaces participating in the same
      MPL Domain to forward the message.

   o  Each MPL Forwarder may periodically link-local multicast MPL
      Control Messages on MPL Interfaces to communicate information
      contained in an MPL Domain's Seed Set and Buffered Message Set.

   o  Upon receiving an MPL Control Message, an MPL Forwarder determines
      whether or not there are any new MPL Data Messages that have yet
      to be received by the MPL Control Message's source and multicasts
      those MPL Data Messages.

   MPL's configuration parameters allow two forwarding strategies for
   disseminating MPL Data Messages via MPL Interfaces:

   Proactive Forwarding  - With proactive forwarding, an MPL Forwarder
      schedules transmissions of MPL Data Messages using the Trickle
      algorithm, without any prior indication that neighboring nodes
      have yet to receive the message.  After transmitting the MPL Data
      Message a limited number of times, the MPL Forwarder may terminate
      proactive forwarding for the MPL Data Message.

   Reactive Forwarding  - With reactive forwarding, an MPL Forwarder
      link-local multicasts MPL Control Messages using the Trickle
      algorithm [RFC6206].  MPL Forwarders use MPL Control Messages to
      discover new MPL Data Messages that have not yet been received.
      When discovering that a neighboring MPL Forwarder has not yet
      received an MPL Data Message, the MPL Forwarder schedules those
      MPL Data Messages for transmission using the Trickle algorithm.
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   Note that, when used within the same MPL Domain, proactive and
   reactive forwarding strategies are not mutually exclusive and may be
   used simultaneously.  For example, upon receiving a new MPL Data
   Message when both proactive and reactive forwarding techniques are
   enabled, an MPL Forwarder will proactively retransmit the MPL Data
   Message a limited number of times and schedule further transmissions
   upon receiving MPL Control Messages.

4.4. Signaling Overview

MPL generates and processes the following messages: MPL Data Message - Generated by an MPL Seed to deliver a multicast message across an MPL Domain. The MPL Data Message's source is an address in the Local Interface Set of the MPL Seed that generated the message and is valid within the MPL Domain. The MPL Data Message's destination is the MPL Domain Address corresponding to the MPL Domain. An MPL Data Message contains: * The Seed Identifier of the MPL Seed that generated the MPL Data Message. * The sequence number of the MPL Seed that generated the MPL Data Message. * The original multicast message. MPL Control Message - Generated by an MPL Forwarder to communicate information contained in an MPL Domain's Seed Set and Buffered Message Set to neighboring MPL Forwarders. An MPL Control Message contains a list of tuples for each entry in the Seed Set. Each tuple contains: * The minimum sequence number maintained in the Seed Set for the MPL Seed. * A bit-vector indicating the sequence numbers of MPL Data Messages resident in the Buffered Message Set for the MPL Seed, where the first bit represents a sequence number equal to the minimum threshold maintained in the Seed Set. * The length of the bit-vector.
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5. MPL Parameters and Constants

This section describes various program and networking parameters and constants used by MPL.

5.1. MPL Multicast Addresses

MPL makes use of MPL Domain Addresses to identify MPL Interfaces of an MPL Domain. By default, MPL Forwarders subscribe to the ALL_MPL_FORWARDERS multicast address with Realm-Local scope (scop value 3) [RFC7346]. For each MPL Domain Address that an MPL Interface subscribes to, the MPL Interface MUST also subscribe to the MPL Domain Address with Link-Local scope (scop value 2) when reactive forwarding is in use. MPL Forwarders use the link-scoped MPL Domain Address to communicate MPL Control Messages to neighboring (i.e., on-link) MPL Forwarders.

5.2. MPL Message Types

MPL defines an IPv6 Option for carrying an MPL Seed Identifier and a sequence number within an MPL Data Message. The IPv6 Option Type has value 0x6D. MPL defines an ICMPv6 Message (MPL Control Message) for communicating information contained in an MPL Domain's Seed Set and Buffered Message Set to neighboring MPL Forwarders. The MPL Control Message has ICMPv6 Type 159.

5.3. MPL Seed Identifiers

MPL uses MPL Seed Identifiers to uniquely identify MPL Seeds within an MPL Domain. For each MPL Domain that the MPL Forwarder serves as an MPL Seed, the MPL Forwarder MUST have an associated MPL Seed Identifier. An MPL Forwarder MAY use the same MPL Seed Identifier across multiple MPL Domains, but the MPL Seed Identifier MUST be unique within each MPL Domain. The mechanism for assigning and verifying uniqueness of MPL Seed Identifiers is not specified in this document.

5.4. MPL Parameters

PROACTIVE_FORWARDING - A boolean value that indicates whether or not the MPL Forwarder schedules MPL Data Message transmissions after receiving them for the first time. PROACTIVE_FORWARDING has a default value of TRUE. All MPL Interfaces on the same link SHOULD be configured with the same value of PROACTIVE_FORWARDING. An implementation MAY choose to vary the value of
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      PROACTIVE_FORWARDING across interfaces on the same link if
      reactive forwarding is also in use.  The mechanism for setting
      PROACTIVE_FORWARDING is not specified within this document.

   SEED_SET_ENTRY_LIFETIME  - The minimum lifetime for an entry in the
      Seed Set.  SEED_SET_ENTRY_LIFETIME has a default value of
      30 minutes.  It is RECOMMENDED that all MPL Forwarders use the
      same value for SEED_SET_ENTRY_LIFETIME for a given MPL Domain and
      use a default value of 30 minutes.  Using a value of
      SEED_SET_ENTRY_LIFETIME that is too small can cause the duplicate
      detection mechanism to fail, resulting in an MPL Forwarder
      receiving a given MPL Data Message more than once.  The mechanism
      for setting SEED_SET_ENTRY_LIFETIME is not specified within this
      document.

   As specified in [RFC6206], a Trickle timer runs for a defined
   interval and has three configuration parameters: the minimum interval
   size Imin, the maximum interval size Imax, and a redundancy
   constant k.

   This specification defines a fourth Trickle configuration parameter,
   TimerExpirations, which indicates the number of Trickle timer
   expiration events that occur before terminating the Trickle algorithm
   for a given MPL Data Message or MPL Control Message.

   Each MPL Interface uses the following Trickle parameters for MPL Data
   Message and MPL Control Message transmissions:

   DATA_MESSAGE_IMIN  - The minimum Trickle timer interval, as defined
      in [RFC6206], for MPL Data Message transmissions.
      DATA_MESSAGE_IMIN has a default value of 10 times the expected
      link-layer latency.

   DATA_MESSAGE_IMAX  - The maximum Trickle timer interval, as defined
      in [RFC6206], for MPL Data Message transmissions.
      DATA_MESSAGE_IMAX has a default value equal to DATA_MESSAGE_IMIN.

   DATA_MESSAGE_K  - The redundancy constant, as defined in [RFC6206],
      for MPL Data Message transmissions.  DATA_MESSAGE_K has a default
      value of 1.

   DATA_MESSAGE_TIMER_EXPIRATIONS  - The number of Trickle timer
      expirations that occur before terminating the Trickle algorithm's
      retransmission of a given MPL Data Message.
      DATA_MESSAGE_TIMER_EXPIRATIONS has a default value of 3.
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   CONTROL_MESSAGE_IMIN  - The minimum Trickle timer interval, as
      defined in [RFC6206], for MPL Control Message transmissions.
      CONTROL_MESSAGE_IMIN has a default value of 10 times the
      worst-case link-layer latency.

   CONTROL_MESSAGE_IMAX  - The maximum Trickle timer interval, as
      defined in [RFC6206], for MPL Control Message transmissions.
      CONTROL_MESSAGE_IMAX has a default value of 5 minutes.

   CONTROL_MESSAGE_K  - The redundancy constant, as defined in
      [RFC6206], for MPL Control Message transmissions.
      CONTROL_MESSAGE_K has a default value of 1.

   CONTROL_MESSAGE_TIMER_EXPIRATIONS  - The number of Trickle timer
      expirations that occur before terminating the Trickle algorithm
      for MPL Control Message transmissions.
      CONTROL_MESSAGE_TIMER_EXPIRATIONS has a default value of 10.

   As described in [RFC6206], if different nodes have different
   configuration parameters, Trickle may have unintended behaviors.
   Therefore, it is RECOMMENDED that all MPL Interfaces attached to the
   same link of a given MPL Domain use the same values for the Trickle
   parameters above for a given MPL Domain.  The mechanism for setting
   the Trickle parameters is not specified within this document.

   The default MPL parameters specify a forwarding strategy that
   utilizes both proactive and reactive techniques.  Using these default
   values, an MPL Forwarder proactively transmits any new MPL Data
   Messages it receives and then uses MPL Control Messages to trigger
   additional MPL Data Message retransmissions where message drops are
   detected.  Setting DATA_MESSAGE_IMAX to the same value as
   DATA_MESSAGE_IMIN in this case is acceptable, since subsequent MPL
   Data Message retransmissions are triggered by MPL Control Messages,
   where CONTROL_MESSAGE_IMAX is greater than CONTROL_MESSAGE_IMIN.
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6. Protocol Message Formats

Messages generated and processed by an MPL Forwarder are described in this section.

6.1. MPL Option

The MPL Option is carried in MPL Data Messages in an IPv6 Hop-by-Hop Options header, immediately following the IPv6 header. The MPL Option has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Option Type | Opt Data Len | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | S |M|V| rsv | sequence | seed-id (optional) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Option Type 0x6D. Opt Data Len Length of the Option Data field [RFC2460] in octets. S 2-bit unsigned integer. Identifies the length of the seed-id. '0' indicates that the seed-id is the IPv6 Source Address and not included in the MPL Option. '1' indicates that the seed-id is a 16-bit unsigned integer. '2' indicates that the seed-id is a 64-bit unsigned integer. '3' indicates that the seed-id is a 128-bit unsigned integer. M 1-bit flag. '1' indicates that the value in the sequence field is known to be the largest sequence number that was received from the MPL Seed. V 1-bit flag. '0' indicates that the MPL Option conforms to this specification. MPL Data Messages with an MPL Option in which this flag is set to 1 MUST be dropped. rsv 4-bit reserved field. MUST be set to 0 on transmission and ignored on reception.
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   sequence       8-bit unsigned integer.  Identifies relative ordering
                  of MPL Data Messages from the MPL Seed identified by
                  the seed-id.

   seed-id        Uniquely identifies the MPL Seed that initiated
                  dissemination of the MPL Data Message.  The size of
                  the seed-id is indicated by the S field.

   The Option Data (specifically, the M flag) of the MPL Option is
   updated by MPL Forwarders as the MPL Data Message is forwarded.
   Nodes that do not understand the MPL Option MUST discard the MPL Data
   Message.  Thus, according to [RFC2460], the three high-order bits of
   the Option Type are set to '011'.  The Option Data length is
   variable.

   The seed-id uniquely identifies an MPL Seed.  When the seed-id is
   128 bits (S=3), the MPL Seed MAY use an IPv6 address assigned to one
   of its interfaces that is unique within the MPL Domain.  Managing MPL
   Seed Identifiers is not within the scope of this document.

   The sequence field establishes a total ordering of MPL Data Messages
   generated by an MPL Seed for an MPL Domain.  The MPL Seed MUST
   increment the sequence field's value on each new MPL Data Message
   that it generates for an MPL Domain.  Implementations MUST follow the
   Serial Number Arithmetic as defined in [RFC1982] when incrementing a
   sequence value or comparing two sequence values.

   Future updates to this specification may define additional fields
   following the seed-id field.

6.2. MPL Control Message

An MPL Forwarder uses ICMPv6 Messages to communicate information contained in an MPL Domain's Seed Set and Buffered Message Set to neighboring MPL Forwarders. The MPL Control Message has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type | Code | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . MPL Seed Info[0..n] . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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   IP Fields:

   Source Address        An IPv6 address in the AddressSet of the
                         corresponding MPL Interface.  MUST be valid
                         within the MPL Domain.

   Destination Address   The link-scoped MPL Domain Address
                         corresponding to the MPL Domain.

   Hop Limit             255


   ICMPv6 Fields:

   Type                  159

   Code                  0

   Checksum              The ICMP checksum.  See [RFC4443].

   MPL Seed Info[0..n]   List of zero or more MPL Seed Info entries.

   The MPL Control Message indicates the sequence numbers of MPL Data
   Messages that are within the MPL Domain's Buffered Message Set.  The
   MPL Control Message also indicates the sequence numbers of MPL Data
   Messages that an MPL Forwarder is willing to receive.  The MPL
   Control Message allows neighboring MPL Forwarders to determine
   whether or not there are any new MPL Data Messages to exchange.

6.3. MPL Seed Info

The MPL Seed Info encodes the minimum sequence number for an MPL Seed maintained in the MPL Domain's Seed Set. The MPL Seed Info also indicates the sequence numbers of MPL Data Messages generated by the MPL Seed that are stored within the MPL Domain's Buffered Message Set. The MPL Seed Info has the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | min-seqno | bm-len | S | seed-id (0/2/8/16 octets) | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . buffered-mpl-messages (variable length) . . . +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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   min-seqno               8-bit unsigned integer.  The lower-bound
                           sequence number for the MPL Seed.

   bm-len                  6-bit unsigned integer.  The size of
                           buffered-mpl-messages in octets.

   S                       2-bit unsigned integer.  Identifies the
                           length of the seed-id.  '0' indicates that
                           the seed-id value is the IPv6 Source Address
                           and not included in the MPL Seed Info.  '1'
                           indicates that the seed-id value is a 16-bit
                           unsigned integer.  '2' indicates that the
                           seed-id value is a 64-bit unsigned integer.
                           '3' indicates that the seed-id is a 128-bit
                           unsigned integer.

   seed-id                 Variable-length unsigned integer.  Indicates
                           the MPL Seed associated with this MPL
                           Seed Info.

   buffered-mpl-messages   Variable-length bit-vector.  Identifies the
                           sequence numbers of MPL Data Messages
                           maintained in the corresponding Buffered
                           Message Set for the MPL Seed.  The i-th bit
                           represents a sequence number of min-seqno
                           + i.  '0' indicates that the corresponding
                           MPL Data Message does not exist in the
                           Buffered Message Set.  '1' indicates that the
                           corresponding MPL Data Message does exist in
                           the Buffered Message Set.

   The MPL Seed Info does not have any octet alignment requirement.

7. Information Base

7.1. Local Interface Set

The Local Interface Set records the local MPL Interfaces of an MPL Forwarder. The Local Interface Set consists of Local Interface Tuples, one per MPL Interface: (AddressSet). AddressSet - a set of unicast addresses assigned to the MPL Interface.
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7.2. Domain Set

The Domain Set records the MPL Interfaces that subscribe to each MPL Domain Address. The Domain Set consists of MPL Domain Tuples, one per MPL Domain: (MPLInterfaceSet). MPLInterfaceSet - a set of MPL Interfaces that subscribe to the MPL Domain Address that identifies the MPL Domain.

7.3. Seed Set

A Seed Set records a sliding window used to determine the sequence numbers of MPL Data Messages (generated by the MPL Seed) that an MPL Forwarder is willing to accept. An MPL Forwarder maintains a Seed Set for each MPL Domain that it participates in. A Seed Set consists of MPL Seed Tuples: (SeedID, MinSequence, Lifetime). SeedID - the identifier for the MPL Seed. MinSequence - a lower-bound sequence number that represents the sequence number of the oldest MPL Data Message the MPL Forwarder is willing to receive or transmit. An MPL Forwarder MUST ignore any MPL Data Message that has a sequence value less than MinSequence. Lifetime - indicates the minimum remaining lifetime of the Seed Set entry. An MPL Forwarder MUST NOT free a Seed Set entry before the remaining lifetime expires.

7.4. Buffered Message Set

A Buffered Message Set records recently received MPL Data Messages from an MPL Seed within an MPL Domain. An MPL Forwarder uses a Buffered Message Set to buffer MPL Data Messages while the MPL Forwarder is forwarding the MPL Data Messages. An MPL Forwarder maintains a Buffered Message Set for each MPL Domain that it participates in. A Buffered Message Set consists of Buffered Message Tuples: (SeedID, SequenceNumber, DataMessage). SeedID - the identifier for the MPL Seed that generated the MPL Data Message. SequenceNumber - the sequence number for the MPL Data Message. DataMessage - the MPL Data Message.
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   All MPL Data Messages within a Buffered Message Set MUST have a
   sequence number greater than or equal to MinSequence for the
   corresponding SeedID.  When increasing MinSequence for an MPL Seed,
   the MPL Forwarder MUST delete any MPL Data Messages from the
   corresponding Buffered Message Set that have sequence numbers less
   than MinSequence.

8. MPL Seed Sequence Numbers

Each MPL Seed maintains a sequence number for each MPL Domain that it serves. The sequence numbers are included in MPL Data Messages generated by the MPL Seed. The MPL Seed MUST increment the sequence number for each MPL Data Message that it generates for an MPL Domain. Implementations MUST follow the Serial Number Arithmetic as defined in [RFC1982] when incrementing a sequence value or comparing two sequence values. This sequence number is used to establish a total ordering of MPL Data Messages generated by an MPL Seed for an MPL Domain.

9. MPL Data Messages

9.1. MPL Data Message Generation

MPL Data Messages are generated by MPL Seeds when these messages enter the MPL Domain. All MPL Data Messages have the following properties: o The IPv6 Source Address MUST be an address in the AddressSet of a corresponding MPL Interface and MUST be valid within the MPL Domain. o The IPv6 Destination Address MUST be set to the MPL Domain Address corresponding to the MPL Domain. o An MPL Data Message MUST contain an MPL Option in its IPv6 header to identify the MPL Seed that generated the message and the ordering relative to other MPL Data Messages generated by the MPL Seed. When the destination address is an MPL Domain Address and the source address is in the AddressList of an MPL Interface that belongs to that MPL Domain Address, the application message and the MPL Data Message MAY be identical. In other words, the MPL Data Message may contain a single IPv6 header that includes the MPL Option. Otherwise, IPv6-in-IPv6 encapsulation MUST be used to satisfy the MPL Data Message requirements listed above [RFC2473]. The complete IPv6-in-IPv6 message forms an MPL Data Message. The outer IPv6
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   header conforms to the MPL Data Message requirements listed above.
   The encapsulated IPv6 datagram encodes the multicast data message
   that is communicated beyond the MPL Domain.

9.2. MPL Data Message Transmission

An MPL Forwarder manages transmission of MPL Data Messages in its Buffered Message Sets using the Trickle algorithm [RFC6206]. An MPL Forwarder MUST use a separate Trickle timer for each MPL Data Message that it is actively forwarding. In accordance with Section 5 of RFC 6206 [RFC6206], the following items apply: o This document defines a "consistent" transmission as receiving an MPL Data Message that has the same MPL Domain Address, seed-id, and sequence value as the MPL Data Message managed by the Trickle timer. o This document defines an "inconsistent" transmission as receiving an MPL Data Message that has the same MPL Domain Address, seed-id value, and the M flag set, but has a sequence value less than that of the MPL Data Message managed by the Trickle timer. o This document does not define any external "events". o This document defines MPL Data Messages as Trickle messages. o The actions outside the Trickle algorithm that MPL takes involve managing the MPL Domain's Seed Set and Buffered Message Set. As specified in [RFC6206], a Trickle timer has three variables: the current interval size I, a time within the current interval t, and a counter c. MPL defines a fourth variable, e, which counts the number of Trickle timer expiration events since the Trickle timer was last reset. After DATA_MESSAGE_TIMER_EXPIRATIONS Trickle timer events, the MPL Forwarder MUST disable the Trickle timer. When a buffered MPL Data Message does not have an associated Trickle timer, the MPL Forwarder MAY delete the message from the Buffered Message Set by advancing the MinSequence value of the corresponding MPL Seed in the Seed Set. When the MPL Forwarder no longer buffers any messages for an MPL Seed, the MPL Forwarder MUST NOT increment MinSequence for that MPL Seed. When transmitting an MPL Data Message, the MPL Forwarder MUST either set the M flag to zero or set it to a level that indicates whether or not the message's sequence number is the largest value that has been received from the MPL Seed.
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9.3. MPL Data Message Processing

Upon receiving an MPL Data Message, the MPL Forwarder first processes the MPL Option and updates the Trickle timer associated with the MPL Data Message if one exists. Upon receiving an MPL Data Message, an MPL Forwarder MUST perform one of the following actions: o Accept the message and enter the MPL Data Message in the MPL Domain's Buffered Message Set. o Accept the message and update the corresponding MinSequence in the MPL Domain's Seed Set to 1 greater than the message's sequence number. o Discard the message without any change to the MPL Information Base. If a Seed Set entry exists for the MPL Seed, the MPL Forwarder MUST discard the MPL Data Message if its sequence number is less than MinSequence or exists in the Buffered Message Set. If a Seed Set entry does not exist for the MPL Seed, the MPL Forwarder MUST create a new entry for the MPL Seed before accepting the MPL Data Message. If memory is limited, an MPL Forwarder SHOULD reclaim memory resources by: o Incrementing MinSequence entries in a Seed Set and deleting MPL Data Messages in the corresponding Buffered Message Set that fall below the MinSequence value. o Deleting other Seed Set entries that have expired and the corresponding MPL Data Messages in the Buffered Message Set. If the MPL Forwarder accepts the MPL Data Message, the MPL Forwarder MUST perform the following actions: o Reset the Lifetime of the corresponding Seed Set entry to SEED_SET_ENTRY_LIFETIME. o If PROACTIVE_FORWARDING is TRUE, the MPL Forwarder MUST initialize and start a Trickle timer for the MPL Data Message.
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   o  If the MPL Control Message Trickle timer is not running and
      CONTROL_MESSAGE_TIMER_EXPIRATIONS is non-zero, the MPL Forwarder
      MUST initialize and start the MPL Control Message Trickle timer.

   o  If the MPL Control Message Trickle timer is running, the MPL
      Forwarder MUST reset the MPL Control Message Trickle timer.

10. MPL Control Messages

10.1. MPL Control Message Generation

An MPL Forwarder generates MPL Control Messages to communicate an MPL Domain's Seed Set and Buffered Message Set to neighboring MPL Forwarders. Each MPL Control Message is generated according to Section 6.2, with an MPL Seed Info entry for each entry in the MPL Domain's Seed Set. Each MPL Seed Info entry has the following content: o S set to the size of the seed-id field in the MPL Seed Info entry. o min-seqno set to the MinSequence value of the MPL Seed. o bm-len set to the size of buffered-mpl-messages in octets. o seed-id set to the MPL Seed Identifier. o buffered-mpl-messages with each bit representing whether or not an MPL Data Message with the corresponding sequence number exists in the Buffered Message Set. The i-th bit represents a sequence number of min-seqno + i. '0' indicates that the corresponding MPL Data Message does not exist in the Buffered Message Set. '1' indicates that the corresponding MPL Data Message does exist in the Buffered Message Set.

10.2. MPL Control Message Transmission

An MPL Forwarder transmits MPL Control Messages using the Trickle algorithm. An MPL Forwarder maintains a single Trickle timer for each MPL Domain. When CONTROL_MESSAGE_TIMER_EXPIRATIONS is 0, the MPL Forwarder does not execute the Trickle algorithm and does not transmit MPL Control Messages. In accordance with Section 5 of RFC 6206 [RFC6206], the following items apply: o This document defines a "consistent" transmission as receiving an MPL Control Message that results in a determination that neither the receiving nor transmitting node has any new MPL Data Messages to offer.
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   o  This document defines an "inconsistent" transmission as receiving
      an MPL Control Message that results in a determination that either
      the receiving or transmitting node has at least one new MPL Data
      Message to offer.

   o  The Trickle timer is reset in response to external "events".  This
      document defines an "event" as increasing the MinSequence value of
      any entry in the corresponding Seed Set or adding a message to the
      corresponding Buffered Message Set.

   o  This document defines an MPL Control Message as a Trickle message.

   As specified in [RFC6206], a Trickle timer has three variables: the
   current interval size I, a time within the current interval t, and a
   counter c.  MPL defines a fourth variable, e, which counts the number
   of Trickle timer expiration events since the Trickle timer was last
   reset.  After CONTROL_MESSAGE_TIMER_EXPIRATIONS Trickle timer events,
   the MPL Forwarder MUST disable the Trickle timer.

10.3. MPL Control Message Processing

An MPL Forwarder processes each MPL Control Message that it receives to determine if it has any new MPL Data Messages to receive or offer. An MPL Forwarder determines if a new MPL Data Message has not been received from a neighboring node if any of the following conditions hold true: o The MPL Control Message includes an MPL Seed that does not exist in the MPL Domain's Seed Set. o The MPL Control Message indicates that the neighbor has an MPL Data Message in its Buffered Message Set with sequence number greater than MinSequence (i.e., the i-th bit is set to 1 and min-seqno + i > MinSequence) and is not included in the MPL Domain's Buffered Message Set. When an MPL Forwarder determines that it has not yet received an MPL Data Message buffered by a neighboring device, the MPL Forwarder MUST reset its Trickle timer associated with MPL Control Message transmissions. If an MPL Control Message Trickle timer is not running, the MPL Forwarder MUST initialize and start a new Trickle timer.
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   An MPL Forwarder determines if an MPL Data Message in the Buffered
   Message Set has not yet been received by a neighboring MPL Forwarder
   if any of the following conditions hold true:

   o  The MPL Control Message does not include an MPL Seed for the MPL
      Data Message.

   o  The MPL Data Message's sequence number is greater than or equal to
      min-seqno and not included in the neighbor's corresponding
      Buffered Message Set (i.e., the MPL Data Message's sequence number
      does not have a corresponding bit in buffered-mpl-messages
      set to 1).

   When an MPL Forwarder determines that it has at least one MPL Data
   Message in its corresponding Buffered Message Set that has not yet
   been received by a neighbor, the MPL Forwarder MUST reset the MPL
   Control Message Trickle timer.  Additionally, for each of those
   entries in the Buffered Message Set, the MPL Forwarder MUST reset the
   Trickle timer and reset e to 0.  If a Trickle timer is not associated
   with the MPL Data Message, the MPL Forwarder MUST initialize and
   start a new Trickle timer.

11. IANA Considerations

This document defines one IPv6 Option, a type that has been allocated from the IPv6 "Destination Options and Hop-by-Hop Options" registry of [RFC2780]. This document defines one ICMPv6 Message, a type that has been allocated from the "ICMPv6 'type' Numbers" registry of [RFC4443]. This document registers a well-known multicast address from the "Variable Scope Multicast Addresses" registry of [RFC3307].

11.1. MPL Option Type

IANA has allocated an IPv6 Option Type from the IPv6 "Destination Options and Hop-by-Hop Options" registry of [RFC2780], as specified in Table 1 below: +-----------+-----+-----+-------+-------------+-----------+ | Hex Value | act | chg | rest | Description | Reference | +-----------+-----+-----+-------+-------------+-----------+ | 0x6D | 01 | 1 | 01101 | MPL Option | RFC 7731 | +-----------+-----+-----+-------+-------------+-----------+ Table 1: IPv6 Option Type Allocation
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   Note: IANA has marked the value 0x4D (previously "MPL Option") as
   "Deprecated".

11.2. MPL ICMPv6 Type

IANA has allocated an ICMPv6 Type from the "ICMPv6 'type' Numbers" registry of [RFC4443], as specified in Table 2 below: +------+---------------------+-----------+ | Type | Name | Reference | +------+---------------------+-----------+ | 159 | MPL Control Message | RFC 7731 | +------+---------------------+-----------+ Table 2: ICMPv6 Type Allocation

11.3. Well-Known Multicast Addresses

IANA has allocated an IPv6 multicast address, with Group ID in the range [0x01,0xFF] for IPv6 over Low-Power Wireless Personal Area Network (6LoWPAN) compression [RFC6282], "ALL_MPL_FORWARDERS" from the "Variable Scope Multicast Addresses" sub-registry of the "IPv6 Multicast Address Space Registry" [RFC3307], as specified in Table 3 below: +---------------------+--------------------+-----------+------------+ | Address(es) | Description | Reference | Date | | | | | Registered | +---------------------+--------------------+-----------+------------+ | FF0X:0:0:0:0:0:0:FC | ALL_MPL_FORWARDERS | RFC 7731 | 2013-04-10 | +---------------------+--------------------+-----------+------------+ Table 3: Variable Scope Multicast Address Allocation

12. Security Considerations

MPL uses sequence numbers to maintain a total ordering of MPL Data Messages from an MPL Seed. The use of sequence numbers allows a denial-of-service attack where an attacker can spoof a message with a sufficiently large sequence number to (i) flush messages from the Buffered Message List and (ii) increase the MinSequence value for an MPL Seed in the corresponding Seed Set. In both cases, the side effect allows an attacker to halt the forwarding process of any MPL Data Messages being disseminated and prevents MPL Forwarders from accepting new MPL Data Messages that an MPL Seed generates while the sequence number is less than MinSequence or until the corresponding Seed Set Entry expires. The net effect applies to both proactive and reactive forwarding modes.
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   In general, the basic ability to inject messages into an LLN may be
   used as a denial-of-service attack, regardless of what forwarding
   protocol is used.  Because MPL is a dissemination protocol, the
   ability to spoof MPL messages allows an attacker to affect an entire
   MPL Domain.  For these reasons, LLNs typically employ link-layer
   security mechanisms to mitigate an attacker's ability to inject
   messages.  For example, the IEEE 802.15.4 [IEEE802.15.4] standard
   specifies frame security mechanisms using AES-128 to support access
   control, message integrity, message confidentiality, and replay
   protection.  However, if the attack vector includes attackers that
   have access to the LLN, then MPL SHOULD NOT be used.

   To prevent attackers from injecting packets through an MPL Forwarder,
   the MPL Forwarder MUST NOT accept or forward MPL Data Messages from a
   communication interface that does not subscribe to the MPL Domain
   Address identified in the message's destination address.

   MPL uses the Trickle algorithm to manage message transmissions;
   therefore, the security considerations described in [RFC6206] apply.

13. References

13.1. Normative References

[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982, DOI 10.17487/RFC1982, August 1996, <http://www.rfc-editor.org/info/rfc1982>. [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, <http://www.rfc-editor.org/info/rfc2119>. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, DOI 10.17487/RFC2460, December 1998, <http://www.rfc-editor.org/info/rfc2460>. [RFC2473] Conta, A. and S. Deering, "Generic Packet Tunneling in IPv6 Specification", RFC 2473, DOI 10.17487/RFC2473, December 1998, <http://www.rfc-editor.org/info/rfc2473>. [RFC2780] Bradner, S. and V. Paxson, "IANA Allocation Guidelines For Values In the Internet Protocol and Related Headers", BCP 37, RFC 2780, DOI 10.17487/RFC2780, March 2000, <http://www.rfc-editor.org/info/rfc2780>.
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   [RFC3307]  Haberman, B., "Allocation Guidelines for IPv6 Multicast
              Addresses", RFC 3307, DOI 10.17487/RFC3307, August 2002,
              <http://www.rfc-editor.org/info/rfc3307>.

   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              DOI 10.17487/RFC4007, March 2005,
              <http://www.rfc-editor.org/info/rfc4007>.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, Ed., "Internet
              Control Message Protocol (ICMPv6) for the Internet
              Protocol Version 6 (IPv6) Specification", RFC 4443,
              DOI 10.17487/RFC4443, March 2006,
              <http://www.rfc-editor.org/info/rfc4443>.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, DOI 10.17487/RFC6206,
              March 2011, <http://www.rfc-editor.org/info/rfc6206>.

   [RFC6282]  Hui, J., Ed., and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              DOI 10.17487/RFC6282, September 2011,
              <http://www.rfc-editor.org/info/rfc6282>.

   [RFC6550]  Winter, T., Ed., Thubert, P., Ed., Brandt, A., Hui, J.,
              Kelsey, R., Levis, P., Pister, K., Struik, R., Vasseur,
              JP., and R. Alexander, "RPL: IPv6 Routing Protocol for
              Low-Power and Lossy Networks", RFC 6550,
              DOI 10.17487/RFC6550, March 2012,
              <http://www.rfc-editor.org/info/rfc6550>.

   [RFC7346]  Droms, R., "IPv6 Multicast Address Scopes", RFC 7346,
              DOI 10.17487/RFC7346, August 2014,
              <http://www.rfc-editor.org/info/rfc7346>.
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13.2. Informative References

[Clausen2013] Clausen, T., de Verdiere, A., and J. Yi, "Performance Analysis of Trickle as a Flooding Mechanism", The 15th IEEE International Conference on Communication Technology (ICCT2013), DOI 10.1109/ICCT.2013.6820439, November 2013. [IEEE802.15.4] IEEE, "IEEE Standard for Local and metropolitan area networks--Part 15.4: Low-Rate Wireless Personal Area Networks (LR-WPANs)", IEEE 802.15.4, DOI 10.1109/ieeestd.2011.6012487, <http://ieeexplore.ieee.org/servlet/ opac?punumber=6012485>. [RFC3973] Adams, A., Nicholas, J., and W. Siadak, "Protocol Independent Multicast - Dense Mode (PIM-DM): Protocol Specification (Revised)", RFC 3973, DOI 10.17487/RFC3973, January 2005, <http://www.rfc-editor.org/info/rfc3973>. [RFC4601] Fenner, B., Handley, M., Holbrook, H., and I. Kouvelas, "Protocol Independent Multicast - Sparse Mode (PIM-SM): Protocol Specification (Revised)", RFC 4601, DOI 10.17487/RFC4601, August 2006, <http://www.rfc-editor.org/info/rfc4601>.
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Acknowledgements

The authors would like to acknowledge the helpful comments of Robert Cragie, Esko Dijk, Ralph Droms, Paul Duffy, Adrian Farrel, Ulrich Herberg, Owen Kirby, Philip Levis, Kerry Lynn, Joseph Reddy, Michael Richardson, Ines Robles, Don Sturek, Dario Tedeschi, and Peter van der Stok, which greatly improved the document.

Authors' Addresses

Jonathan W. Hui Nest Labs 3400 Hillview Ave. Palo Alto, California 94304 United States Phone: +650 253 2770 Email: jonhui@nestlabs.com Richard Kelsey Silicon Labs 25 Thomson Place Boston, Massachusetts 02210 United States Phone: +617 951 1225 Email: richard.kelsey@silabs.com