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Versions: (draft-hui-6man-trickle-mcast) 00 01 02 03 04 05 06 07 08 09 10 11 12 RFC 7731

ROLL                                                              J. Hui
Internet-Draft                                                     Cisco
Intended status: Standards Track                               R. Kelsey
Expires: August 29, 2013                                    Silicon Labs
                                                       February 25, 2013


       Multicast Protocol for Low power and Lossy Networks (MPL)
                    draft-ietf-roll-trickle-mcast-04

Abstract

   This document specifies the Multicast Protocol for Low power and
   Lossy Networks (MPL) that 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 uses the Trickle algorithm to
   manage message transmissions for both control and data-plane
   messages.  Different Trickle parameter configurations allow MPL to
   trade between dissemination latency and transmission efficiency.

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 29, 2013.

Copyright Notice

   Copyright (c) 2013 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



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   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
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Applicability Statement  . . . . . . . . . . . . . . . . . . .  6
   4.  Protocol Overview  . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Information Base Overview  . . . . . . . . . . . . . . . .  7
     4.2.  Overview . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Signaling Overview . . . . . . . . . . . . . . . . . . . .  9
   5.  MPL Parameters and Constants . . . . . . . . . . . . . . . . . 10
     5.1.  MPL Multicast Addresses  . . . . . . . . . . . . . . . . . 10
     5.2.  MPL Message Types  . . . . . . . . . . . . . . . . . . . . 10
     5.3.  MPL Seed Identifiers . . . . . . . . . . . . . . . . . . . 10
     5.4.  MPL Forwarder Parameters . . . . . . . . . . . . . . . . . 10
     5.5.  MPL Trickle Parameters . . . . . . . . . . . . . . . . . . 11
   6.  Protocol Message Formats . . . . . . . . . . . . . . . . . . . 13
     6.1.  MPL Option . . . . . . . . . . . . . . . . . . . . . . . . 13
     6.2.  MPL Control Message  . . . . . . . . . . . . . . . . . . . 14
     6.3.  MPL Seed Info  . . . . . . . . . . . . . . . . . . . . . . 15
   7.  Information Base . . . . . . . . . . . . . . . . . . . . . . . 17
     7.1.  Local Interface Set  . . . . . . . . . . . . . . . . . . . 17
     7.2.  Domain Set . . . . . . . . . . . . . . . . . . . . . . . . 17
     7.3.  Seed Set . . . . . . . . . . . . . . . . . . . . . . . . . 17
     7.4.  Buffered Message Set . . . . . . . . . . . . . . . . . . . 17
   8.  MPL Domains  . . . . . . . . . . . . . . . . . . . . . . . . . 19
   9.  MPL Seed Sequence Numbers  . . . . . . . . . . . . . . . . . . 20
   10. MPL Data Messages  . . . . . . . . . . . . . . . . . . . . . . 21
     10.1. MPL Data Message Generation  . . . . . . . . . . . . . . . 21
     10.2. MPL Data Message Transmission  . . . . . . . . . . . . . . 21
     10.3. MPL Data Message Processing  . . . . . . . . . . . . . . . 22
   11. MPL Control Messages . . . . . . . . . . . . . . . . . . . . . 24
     11.1. MPL Control Message Generation . . . . . . . . . . . . . . 24
     11.2. MPL Control Message Transmission . . . . . . . . . . . . . 24
     11.3. MPL Control Message Processing . . . . . . . . . . . . . . 25
   12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 27
   13. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 28
     13.1. MPL Option Type  . . . . . . . . . . . . . . . . . . . . . 28
     13.2. MPL ICMPv6 Type  . . . . . . . . . . . . . . . . . . . . . 28
     13.3. Well-known Multicast Addresses . . . . . . . . . . . . . . 28
   14. Security Considerations  . . . . . . . . . . . . . . . . . . . 29
   15. Normative References . . . . . . . . . . . . . . . . . . . . . 30
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 31



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1.  Introduction

   Low power and Lossy Networks 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.  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 a LLN Border Router (LBR) and use source routing to
   forward messages away from the LBR.  For the same reasons, a 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 this protocol.  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.





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   MPL Seed Identifier - An unsigned integer that uniquely identifies an
                       MPL Seed within an MPL Domain.

















































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3.  Applicability Statement

   This protocol is an IPv6 multicast forwarding protocol for Low-Power
   and Lossy Networks.  By implementing a controlled dissemination using
   the Trickle algorithm, this protocol is designed for networks that
   communicate using low-power and lossy links with widely varying
   topologies in both the space and time dimensions.












































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4.  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.  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.

4.2.  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 [RFC2473] is used to encapsulate the



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      multicast message in an MPL Data Message, preserving the original
      IPv6 Destination Address.

   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 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.

   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 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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4.3.  Signaling Overview

   This protocol 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 a scope value of 3 (subnet-
   local).

   For each MPL Domain Address that an MPL Interface subscribes to, the
   MPL Interface MUST also subscribe to the MPL Domain Address with a
   scope value of 2 (link-local) 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 MPL_OPT_TYPE.

   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 MPL_ICMP_TYPE.

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 Forwarder Parameters

   PROACTIVE_FORWARDING  A boolean value that indicates whether the MPL
      Forwarder should schedule MPL Data Message transmissions after
      receiving them for the first time.  PROACTIVE_FORWARDING has a
      default value of TRUE.





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   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 values for the
   MPL Forwarder Parameters above for a given MPL Domain.  The mechanism
   for setting the MPL Forwarder Parameters is not specified within this
   document.

5.5.  MPL Trickle Parameters

   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.

   Each MPL Forwarder 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 worst-case 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 5.

   DATA_MESSAGE_TIMER_EXPIRATIONS  The number of Trickle timer
      expirations that occur before terminating the Trickle algorithm
      for MPL Data Message transmissions.
      DATA_MESSAGE_TIMER_EXPIRATIONS has a default value of 3.

   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.






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   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.

   Following [RFC6206], it is RECOMMENDED that all MPL Forwarders 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.


































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6.  Protocol Message Formats

   The protocol 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         MPL_OPT_TYPE

   Opt Data Len        Length of the Option Data field in octets.

   S                   2-bit unsigned integer.  Identifies the length of
                       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
                       sequence 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
                       1 MUST be dropped.

   rsv                 4-bit reserved field.  MUST be set to 0 on
                       transmission and ignored on reception.

   sequence            8-bit unsigned integer.  Identifies relative
                       ordering of MPL Data Messages from the MPL Seed
                       identified by seed-id.




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

   The Option Data (in particular 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 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 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[1..n]                      .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   IP Fields:






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   Source Address      An IPv6 address in the AddressSet of the
                       corresponding MPL Interface and 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                MPL_ICMP_TYPE

   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 there are any new MPL Data Messages to exchange.

6.3.  MPL Seed Info

   An 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
                       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.






















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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.

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 that an MPL Forwarder is willing to
   accept generated by the MPL Seed.  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 sequence value less than 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



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   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.

   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.




































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8.  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.

   By default, an MPL Forwarder SHOULD participate in an MPL Domain
   identified by the ALL_MPL_FORWARDERS multicast address with a scope
   value of 3 (subnet-local).

   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
   a scope value of 2 (link-local) when reactive forwarding is in use
   (i.e. when communicating MPL Control Messages).
































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9.  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.








































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10.  MPL Data Messages

10.1.  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 source address is in the AddressList of an MPL Interface
   corresponding to the MPL Domain Address and the destination address
   is the 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 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.

10.2.  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], this document defines the following:

   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



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      value, and the M flag set, but has a sequence value less than 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 the protocol 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
   MinSequence 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.

10.3.  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.



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   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.

   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.


















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11.  MPL Control Messages

11.1.  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 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 MinSequence 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.

11.2.  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], this document defines the following:

   o  This document defines a "consistent" transmission as receiving an
      MPL Control Message that indicates neither the receiving nor
      transmitting node has any new MPL Data Messages to offer.

   o  This document defines an "inconsistent" transmission as receiving
      an MPL Control Message that indicates either the receiving or
      transmitting node has at least one new MPL Data Message to offer.

   o  This document defines an "event" as increasing MinSequence of any
      entry in the corresponding Seed Set or adding a message to the
      corresponding Buffered Message Set.





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   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.

11.3.  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.

   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



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   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.












































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12.  Acknowledgements

   The authors would like to acknowledge the helpful comments of Robert
   Cragie, Esko Dijk, Ralph Droms, Paul Duffy, Ulrich Herberg, Owen
   Kirby, Joseph Reddy, Don Sturek, Dario Tedeschi, and Peter van der
   Stok, which greatly improved the document.













































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13.  IANA Considerations

   This document defines one IPv6 Option, a type that must be allocated
   from the IPv6 "Destination Options and Hop-by-Hop Options" registry
   of [RFC2780].

   This document defines one ICMPv6 Message, a type that must be
   allocated from the "ICMPv6 "type" Numbers" registry of [RFC4443].

   This document registers two well-known multicast addresses from the
   IPv6 multicast address space.

13.1.  MPL Option Type

   IANA is requested to allocate an IPv6 Option Type from the IPv6
   "Destination Options and Hop-by-Hop Options" registry of [RFC2780],
   as specified in Table 1 below:

   +--------------+-----+-----+--------------+-------------+-----------+
   |   Mnemonic   | act | chg |     rest     | Description | Reference |
   +--------------+-----+-----+--------------+-------------+-----------+
   | MPL_OPT_TYPE |  01 |  1  |      TBD     |  MPL Option |    This   |
   |              |     |     |  (suggested  |             |  Document |
   |              |     |     | value 01101) |             |           |
   +--------------+-----+-----+--------------+-------------+-----------+

                   Table 1: IPv6 Option Type Allocation

13.2.  MPL ICMPv6 Type

   IANA is requested to allocate an ICMPv6 Type from the "ICMPv6 "type"
   Numbers" registry of [RFC4443], as specified in Table 2 below:

      +---------------+------+---------------------+---------------+
      |    Mnemonic   | Type |         Name        |   Reference   |
      +---------------+------+---------------------+---------------+
      | MPL_ICMP_TYPE |  TBD | MPL Control Message | This Document |
      +---------------+------+---------------------+---------------+

                   Table 2: IPv6 Option Type Allocation

13.3.  Well-known Multicast Addresses

   IANA is requested to allocate an IPv6 multicast address, with Group
   ID in the range [0x01,0xFF] for 6LoWPAN compression [RFC6282],
   "ALL_MPL_FORWARDERS" from the "Variable Scope Multicast Addresses"
   sub-registry of the "INTERNET PROTOCOL VERSION 6 MULTICAST ADDRESSES"
   registry.



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14.  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. The former side effect allows
   an attacker to halt the forwarding process of any MPL Data Messages
   being disseminated.  The latter side effect allows an attacker to
   prevent MPL Forwarders from accepting new MPL Data Messages that an
   MPL Seed generates while the sequence number is less than
   MinSequence.

   More generally, the basic ability to inject messages into a Low-power
   and Lossy Network can be used as a denial-of-service attack
   regardless of what forwarding protocol is used.  For these reasons,
   Low-power and Lossy Networks typically employ link-layer security
   mechanisms to disable an attacker's ability to inject messages.

   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 message's destination address.

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
























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15.  Normative References

   [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
              August 1996.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC2460]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

   [RFC2473]  Conta, A. and S. Deering, "Generic Packet Tunneling in
              IPv6 Specification", RFC 2473, December 1998.

   [RFC2780]  Bradner, S. and V. Paxson, "IANA Allocation Guidelines For
              Values In the Internet Protocol and Related Headers",
              BCP 37, RFC 2780, March 2000.

   [RFC4007]  Deering, S., Haberman, B., Jinmei, T., Nordmark, E., and
              B. Zill, "IPv6 Scoped Address Architecture", RFC 4007,
              March 2005.

   [RFC4443]  Conta, A., Deering, S., and M. Gupta, "Internet Control
              Message Protocol (ICMPv6) for the Internet Protocol
              Version 6 (IPv6) Specification", RFC 4443, March 2006.

   [RFC6206]  Levis, P., Clausen, T., Hui, J., Gnawali, O., and J. Ko,
              "The Trickle Algorithm", RFC 6206, March 2011.

   [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              September 2011.

   [RFC6550]  Winter, T., Thubert, P., 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, March 2012.














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Authors' Addresses

   Jonathan W. Hui
   Cisco
   170 West Tasman Drive
   San Jose, California  95134
   USA

   Phone: +408 424 1547
   Email: jonhui@cisco.com


   Richard Kelsey
   Silicon Labs
   25 Thomson Place
   Boston, Massachusetts  02210
   USA

   Phone: +617 951 1225
   Email: richard.kelsey@silabs.com































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