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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: April 22, 2013                                     Silicon Labs
                                                        October 19, 2012


       Multicast Protocol for Low power and Lossy Networks (MPL)
                    draft-ietf-roll-trickle-mcast-02

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 drive
   packet transmissions for both control and data-plane packets.
   Specific 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 April 22, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   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.  Overview . . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Message Formats  . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  MPL Option . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  ICMPv6 MPL Message . . . . . . . . . . . . . . . . . . . .  8
       4.2.1.  MPL Window . . . . . . . . . . . . . . . . . . . . . .  9
   5.  MPL Forwarder Behavior . . . . . . . . . . . . . . . . . . . . 11
     5.1.  Multicast Packet Dissemination . . . . . . . . . . . . . . 11
       5.1.1.  Trickle Parameters and Variables . . . . . . . . . . . 12
       5.1.2.  Proactive Propagation  . . . . . . . . . . . . . . . . 12
       5.1.3.  Reactive Propagation . . . . . . . . . . . . . . . . . 13
     5.2.  Sliding Windows  . . . . . . . . . . . . . . . . . . . . . 13
     5.3.  Transmission of MPL Multicast Packets  . . . . . . . . . . 15
     5.4.  Reception of MPL Multicast Packets . . . . . . . . . . . . 16
     5.5.  Transmission of ICMPv6 MPL Messages  . . . . . . . . . . . 16
     5.6.  Reception of ICMPv6 MPL Messages . . . . . . . . . . . . . 17
   6.  MPL Parameters . . . . . . . . . . . . . . . . . . . . . . . . 19
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 21
   9.  Security Considerations  . . . . . . . . . . . . . . . . . . . 22
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 23
     10.2. Informative References . . . . . . . . . . . . . . . . . . 23
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 24


















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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
   topology and forwarding mechanisms.  Traditional IP multicast
   forwarding typically relies on topology maintenance mechanisms to
   forward multicast messages 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 packets away from the LBR.  For the same reasons, a LLN
   device may not be able to maintain a multicast forwarding topology
   when operating with limited memory.

   Furthermore, the dynamic properties of wireless networks can make the
   cost of maintaining a multicast forwarding 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 forwarding 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", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [RFC2119].

   The following terms are used throughout this document:

   MPL forwarder       An IPv6 router that subscribes to the MPL
                       multicast group and participates in disseminating
                       MPL multicast packets.

   MPL multicast scope The multicast scope that MPL uses when forwarding
                       MPL multicast packets.  In other words, the
                       multicast scope of the IPv6 Destination Address
                       of an MPL multicast packet.

   MPL domain          A connected set of MPL forwarders that define the
                       extent of the MPL dissemination process.  As a
                       form of flood, all MPL forwarders in an MPL
                       domain will receive MPL multicast packets.  The
                       MPL domain MUST be composed of at least one MPL
                       multicast scope and MAY be composed of multiple
                       MPL multicast scopes.

   MPL seed            A MPL forwarder that begins the dissemination
                       process for an MPL multicast packet.  The MPL
                       seed may be different than the source of the
                       original multicast packet.

   MPL seed identifier An identifier that uniquely identifies an MPL
                       forwarder within its MPL domain.

   original multicast packet  An IPv6 multicast packet that is
                       disseminated using MPL.

   MPL multicast packet  An IPv6 multicast packet that contains an MPL
                       Hop-by-Hop Option.  When either source or
                       destinations are beyond the MPL multicast scope,
                       the MPL multicast packet is an IPv6-in-IPv6
                       packet that contains an MPL Hop-by-Hop Option in
                       the outer IPv6 header and encapsulates an
                       original multicast packet.  When both source and
                       destinations are within the MPL multicast scope,
                       the MPL Hop-by-Hop Option may be included
                       directly within the original multicast packet.





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3.  Overview

   MPL delivers IPv6 multicast packets by disseminating them to all MPL
   forwarders within an MPL domain.  MPL dissemination is a form of
   flood.  An MPL forwarder may broadcast/multicast an MPL multicast
   packet out of the same physical interface on which it was received.
   Using link-layer broadcast/multicast allows MPL to forward multicast
   packets without explicitly identifying next-hop destinations.  An MPL
   forwarder may also broadcast/multicast MPL multicast packets out
   other interfaces to disseminate the message across different links.
   MPL does not build or maintain a multicast forwarding topology to
   forward multicast packets.

   Any MPL forwarder may initiate the dissemination process by serving
   as an MPL seed for an original multicast packet.  The MPL seed may or
   may not be the same device as the source of the original multicast
   packet.  When the original multicast packet's source is outside the
   LLN, the MPL seed may be the ingress router.  Even if an original
   multicast packet source is within the LLN, the source may first
   forward the multicast packet to the MPL seed using IPv6-in-IPv6
   tunneling.  Because MPL state requirements grows with the number of
   active MPL seeds, limiting the number of MPL seeds reduces the amount
   of state that MPL forwarders must maintain.

   Because MPL typically broadcasts/multicasts MPL packets out of the
   same interface on which they were received, MPL forwarders are likely
   to receive an MPL multicast packet more than once.  The MPL seed tags
   each original multicast packet with an MPL seed identifier and a
   sequence number.  The sequence number provides a total ordering of
   MPL multicast packets disseminated by the MPL seed.

   MPL defines a new IPv6 Hop-by-Hop Option, the MPL Option, to include
   MPL-specific information along with the original multicast packet.
   Each IPv6 multicast packet that MPL disseminates includes the MPL
   Option.  Because the original multicast packet's source and the MPL
   seed may not be the same device, the MPL Option may be added to the
   original multicast packet en-route.  To allow Path MTU discovery to
   work properly, MPL encapsulates the original multicast packet in
   another IPv6 header that includes the MPL Option.

   Upon receiving a new MPL multicast packet for forwarding, the MPL
   forwarder may proactively transmit the MPL multicast packet packet a
   limited number of times and then falls back into an optional reactive
   mode.  In maintenance mode, an MPL forwarder buffers recently
   received MPL multicast packets and advertises a summary of recently
   received MPL multicast packets from time to time, allowing
   neighboring MPL forwarders to determine if they have any new
   multicast packets to offer or receive.



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   MPL forwarders schedule their packet (control and data) transmissions
   using the Trickle algorithm [RFC6206].  Trickle's adaptive
   transmission interval allows MPL to quickly disseminate messages when
   there are new MPL multicast packets, but reduces transmission
   overhead as the dissemination process completes.  Trickle's
   suppression mechanism and transmission time selection allow MPL's
   communication rate to scale logarithmically with density.












































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4.  Message Formats

4.1.  MPL Option

   The MPL Option is carried 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|   rsv   |   sequence    |      seed-id (optional)       |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   Option Type         XX (to be confirmed by IANA).

   Opt Data Len        Length of the Option Data field in octets.  MUST
                       be set to either 2 or 4.

   S                   2-bit unsigned integer.  Identifies the length of
                       seed-id. 0 indicates that the seed-id is 0 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. 0 indicates that the value in
                       sequence is not the greatest sequence number that
                       was received from the MPL seed.

   rsv                 5-bit reserved field.  MUST be set to zero and
                       incoming MPL multicast packets in which they are
                       not zero MUST be dropped.

   sequence            8-bit unsigned integer.  Identifies relative
                       ordering of MPL multicast packets from the source
                       identified by seed-id.

   seed-id             Uniquely identifies the MPL seed that initiated
                       dissemination of the MPL multicast packet.  The
                       size of seed-id is indicated by the S field.

   The Option Data of the Trickle Multicast option MUST NOT change as
   the MPL multicast packet is forwarded.  Nodes that do not understand



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   the Trickle Multicast option MUST discard the packet.  Thus,
   according to [RFC2460] the three high order bits of the Option Type
   must be set to '010'.  The Option Data length is variable.

   The seed-id uniquely identifies an MPL seed within the MPL domain.
   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 multicast
   packets from the same MPL seed.  The MPL seed MUST increment the
   sequence field's value on each new MPL multicast packet that it
   disseminates.  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.

4.2.  ICMPv6 MPL Message

   The MPL forwarder uses ICMPv6 MPL messages to advertise information
   about recently received MPL multicast packets.  The ICMPv6 MPL
   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 Window[1..n]                        .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   IP Fields:

   Source Address      A link-local address assigned to the sending
                       interface.

   Destination Address The link-local all-nodes MPL forwarders multicast
                       address (FF02::TBD).







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   Hop Limit           255

   ICMPv6 Fields:

   Type                XX (to be confirmed by IANA).

   Code                0

   Checksum            The ICMP checksum.  See [RFC4443].

   MPL Window[1..n]    List of one or more MPL Windows (defined in
                       Section 4.2.1).

   An MPL forwarder transmits an ICMPv6 MPL message to advertise
   information about buffered MPL multicast packets.  More explicitly,
   the ICMPv6 MPL message encodes the sliding window state (described in
   Section 5.2) that the MPL forwarder maintains for each MPL seed.  The
   advertisement serves to indicate to neighboring MPL forwarders
   regarding newer messages that it may send or the neighboring MPL
   forwarders have yet to receive.

4.2.1.  MPL Window

   An MPL Window encodes the sliding window state (described in
   Section 5.2 that the MPL forwarder maintains for an MPL seed.  Each
   MPL Window 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
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |     w-min     |   w-len   | S |  seed-id (0, 2 or 16 octets)  |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .              buffered-mpl-packets (0 to 8 octets)             .
     .                                                               .
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


   w-min               8-bit unsigned integer.  Indicates the first
                       sequence number associated with the first bit in
                       buffered-mpl-packets.

   w-len               6-bit unsigned integer.  Indicates the size of
                       the sliding window and the number of valid bits
                       in buffered-mpl-packets.  The sliding window's
                       upper bound is the sum of w-min and w-len.





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   S                   2-bit unsigned integer.  Identifies the length of
                       seed-id. 0 indicates that the seed-id value is 0
                       and not included in the MPL Option. 1 indicates
                       that the seed-id value is a 16-bit unsigned
                       integer. 2 indicates that the seed-id value is a
                       128-bit unsigned integer. 3 is reserved.

   seed-id             Indicates the MPL seed associated with this
                       sliding window.

   buffered-mpl-packets  Variable-length bit vector.  Identifies the
                       sequence numbers of MPL multicast packets that
                       the MPL forwarder has buffered.  The sequence
                       number is determined by w-min + i, where i is the
                       offset within buffered-mpl-packets.

   The MPL Window does not have any octet alignment requirement.


































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5.  MPL Forwarder Behavior

   An MPL forwarder implementation needs to manage sliding windows for
   each active MPL seed.  The sliding window allows the MPL forwarder to
   determine what multicast packets to accept and what multicast packets
   are buffered.  An MPL forwarder must also manage MPL packet
   transmissions.

5.1.  Multicast Packet Dissemination

   MPL uses the Trickle algorithm to control packet transmissions when
   disseminating MPL multicast packets [RFC6206].  MPL provides two
   propagation mechanisms for disseminating MPL multicast packets.

   1.  With proactive propagation, an MPL forwarder transmits buffered
       MPL multicast packets using the Trickle algorithm.  This method
       is called proactive propagation since an MPL forwarder actively
       transmits MPL multicast packets without discovering that a
       neighboring MPL forwarder has yet to receive the message.

   2.  With reactive propagation, an MPL forwarder transmits ICMPv6 MPL
       messages using the Trickle algorithm.  An MPL forwarder only
       transmits buffered MPL multicast packets upon discovering that
       neighboring devices have not yet to receive the corresponding MPL
       multicast packets.

   When receiving a new multicast packet, an MPL forwarder first
   utilizes proactive propagation to forward the MPL multicast packet.
   Proactive propagation reduces dissemination latency since it does not
   require discovering that neighboring devices have not yet received
   the MPL multicast packet.  MPL forwarders utilize proactive
   propagation for newly received MPL multicast packets since they can
   assume that some neighboring MPL forwarders have yet to receive the
   MPL multicast packet.  After a limited number of MPL multicast packet
   transmissions, the MPL forwarder may terminate proactive propagation
   for the MPL multicast packet.

   An MPL forwarder may optionally use reactive propagation to continue
   the dissemination process with lower communication overhead.  With
   reactive propagation, neighboring MPL forwarders use ICMPv6 MPL
   messages to discover new MPL multicast messages that have not yet
   been received.  When discovering that a neighboring MPL forwarder has
   not yet received a new MPL multicast packet, the MPL forwarder
   enables proactive propagation again.







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5.1.1.  Trickle Parameters and Variables

   As specified in RFC 6206 [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.

   MPL defines a fourth configuration parameter, TimerExpirations, which
   indicates the number of Trickle timer expiration events that occur
   before terminating the Trickle algorithm.

   Each MPL forwarder maintains a separate Trickle parameter set for the
   proactive and reactive propagation methods.  TimerExpirations MUST be
   greater than 0 for proactive propagation.  TimerExpirations MAY be
   set to 0 for reactive propagation, which effectively disables
   reactive propagation.

   As specified in RFC 6206 [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.

5.1.2.  Proactive Propagation

   With proactive propagation, the MPL forwarder transmits buffered MPL
   multicast packets using the Trickle algorithm.  Each buffered MPL
   multicast packet that is proactively being disseminated with
   proactive propagation has an associated Trickle timer.  Adhering to
   Section 5 of RFC 6206 [RFC6206], this document defines the following:

   o  This document defines a "consistent" transmission for proactive
      propagation as receiving an MPL multicast packet that has the same
      MPL seed identifier and sequence number as a buffered MPL packet.

   o  This document defines an "inconsistent" transmission for proactive
      propagation as receiving an MPL multicast packet that has the same
      MPL seed identifier, the M flag set, and has a sequence number
      less than the buffered MPL multicast packet's sequence number.

   o  This document does not define any external "events".

   o  This document defines both MPL multicast packets and ICMPv6 MPL
      multicast packets as Trickle messages.  These messages are defined
      in the sections below.





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   o  The actions outside the Trickle algorithm that the protocol takes
      involve managing sliding window state, and is specified in
      Section 5.2.

5.1.3.  Reactive Propagation

   With reactive propagation, the MPL forwarder transmits ICMPv6 MPL
   messages using the Trickle algorithm.  A MPL forwarder maintains a
   single Trickle timer for reactive propagation with each MPL domain.
   When REACTIVE_TIMER_EXPIRATIONS is 0, the MPL forwarder does not
   execute the Trickle algorithm for reactive propagation and reactive
   propagation is disabled.  Adhering to Section 5 of RFC 6206
   [RFC6206], this document defines the following:

   o  This document defines a "consistent" transmission for reactive
      propagation as receiving an ICMPv6 MPL message that indicates
      neither the receiving nor transmitting node has new MPL multicast
      packets to offer.

   o  This document defines an "inconsistent" transmission for reactive
      propagation as receiving an ICMPv6 MPL message that indicates
      either the receiving or transmitting node has at least one new MPL
      multicast packet to offer.

   o  This document defines an "event" for reactive propagation as
      updating any sliding window (i.e. changing the value of WindowMin,
      WindowMax, or the set of buffered MPL multicast packets) in
      response to receiving an MPL multicast packet.

   o  This document defines both MPL multicast packets and ICMPv6 MPL
      multicast packets as Trickle messages.  These messages are defined
      in the sections below.

   o  The actions outside the Trickle algorithm that the protocol takes
      involve managing sliding window state, and is specified in
      Section 5.2.

5.2.  Sliding Windows

   Every MPL forwarder MUST maintain a sliding window of sequence
   numbers for each MPL seed of recently received MPL packets.  The
   sliding window performs two functions:

   1.  Indicate what MPL multicast packets the MPL forwarder should
       accept.

   2.  Indicate what MPL multicast packets are buffered and may be
       transmitted to neighboring MPL forwarders.



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   Each sliding window logically consists of:

   1.  A lower-bound sequence number, WindowMin, that represents the
       sequence number of the oldest MPL multicast packet the MPL
       forwarder is willing to receive or has buffered.  An MPL
       forwarder MUST ignore any MPL multicast packet that has sequence
       value less than than WindowMin.

   2.  An upper-bound sequence value, WindowMax, that represents the
       sequence number of the next MPL multicast packet that the MPL
       forwarder expects to receive.  An MPL forwarder MUST accept any
       MPL multicast packet that has sequence number greater than or
       equal to WindowMax.

   3.  A list of MPL multicast packets, BufferedPackets, buffered by the
       MPL forwarder.  Each entry in BufferedPackets MUST have a
       sequence number in the range [WindowMin, WindowMax).

   4.  A timer, HoldTimer, that indicates the minimum lifetime of the
       sliding window.  The MPL forwarder MUST NOT free a sliding window
       before HoldTimer expires.

   When receiving an MPL multicast packet, if no existing sliding window
   exists for the MPL seed, the MPL forwarder MUST create a new sliding
   window before accepting the MPL multicast packet.  The MPL forwarder
   may reclaim memory resources by freeing a sliding window for another
   MPL seed if its HoldTimer has expired.  If, for any reason, the MPL
   forwarder cannot create a new sliding window, it MUST discard the
   packet.

   If a sliding window exists for the MPL seed, the MPL forwarder MUST
   ignore the MPL multicast packet if the packet's sequence number is
   less than WindowMin or appears in BufferedPackets.  Otherwise, the
   MPL forwarder MUST accept the packet and determine whether or not to
   forward the packet and/or pass the packet to the next higher layer.

   When accepting an MPL multicast packet, the MPL forwarder MUST update
   the sliding window based on the packet's sequence number.  If the
   sequence number is not less than WindowMax, the MPL forwarder MUST
   set WindowMax to 1 greater than the packet's sequence number.  If
   WindowMax - WindowMin > MPL_MAX_WINDOW_SIZE, the MPL forwarder MUST
   increment WindowMin such that WindowMax - WindowMin <=
   MPL_MAX_WINDOW_SIZE.  At the same time, the MPL forwarder MUST free
   any entries in BufferedPackets that have a sequence number less than
   WindowMin.

   If the MPL forwarder has available memory resources, it MUST buffer
   the MPL multicast packet for proactive propagation.  If not enough



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   memory resources are available to buffer the packet, the MPL
   forwarder MUST increment WindowMin and free entries in
   BufferedPackets that have a sequence number less than WindowMin until
   enough memory resources are available.  Incrementing WindowMin will
   ensure that the MPL forwarder does not accept previously received
   packets.

   An MPL forwarder MAY reclaim memory resources from sliding windows
   for other MPL seeds.  If a sliding window for another MPL seed is
   actively disseminating messages and has more than one entry in its
   BufferedPackets, the MPL forwarder may free entries for that MPL seed
   by incrementing WindowMin as described above.

   If the MPL forwarder cannot free enough memory resources to buffer
   the MPL multicast packet, the MPL forwarder MUST set WindowMin to 1
   greater than the packet's sequence number.

   When memory resources are available, an MPL forwarder SHOULD buffer a
   MPL multicast packet until the proactive propagation completes (i.e.
   the Trickle algorithm stops execution) and MAY buffer for longer.
   After proactive propagation completes, the MPL forwarder may advance
   WindowMin to the packet's sequence number to reclaim memory
   resources.  When the MPL forwarder no longer buffers any packets, it
   MAY set WindowMin equal to WindowMax.  When setting WindowMin equal
   to WindowMax, the MPL forwarder MUST initialize HoldTimer to
   WINDOW_HOLD_TIME and start HoldTimer.  After HoldTimer expires, the
   MPL forwarder MAY free the sliding window to reclaim memory
   resources.

5.3.  Transmission of MPL Multicast Packets

   The MPL forwarder manages buffered MPL multicast packet transmissions
   using the Trickle algorithm.  When adding a packet to
   BufferedPackets, the MPL forwarder MUST create a Trickle timer for
   the packet and start execution of the Trickle algorithm.

   After PROACTIVE_TIMER_EXPIRATIONS Trickle timer events, the MPL
   forwarder MUST stop executing the Trickle algorithm.  When a buffered
   MPL multicast packet does not have an active Trickle timer, the MPL
   forwarder MAY free the buffered packet by advancing WindowMin to 1
   greater than the packet's sequence number.

   Each interface that supports MPL is configured with exactly one MPL
   multicast scope.  The MPL multicast scope MUST be site-local or
   smaller and defaults to link-local.  A scope larger than link-local
   MAY be used only when that scope corresponds exactly to the MPL
   domain.




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   An MPL domain may therefore be composed of one or more MPL multicast
   scopes.  For example, the MPL domain may be composed of a single MPL
   multicast scope when using a site-local scope.  Alternatively, the
   MPL domain may be composed of multiple MPL multicast scopes when
   using a link-local scope.

   IPv6-in-IPv6 encapsulation MUST be used when using MPL to forward an
   original multicast packet whose source or destination address is
   outside the MPL multicast scope.  IPv6-in-IPv6 encapsulation is
   necessary to support Path MTU discovery when the MPL forwarder is not
   the source of the original multicast packet.  IPv6-in-IPv6
   encapsulation also allows an MPL forwarder to remove the MPL Option
   when forwarding the original multicast packet over a link that does
   not support MPL.  The destination address scope for the outer IPv6
   header MUST be the MPL multicast scope.

   When an MPL domain is composed of multiple MPL multicast scopes (e.g.
   when the MPL multicast scope is link-local), an MPL forwarder MUST
   decapsulate and encapsulate the original multicast packet when
   crossing between different MPL multicast scopes.  In doing so, the
   MPL forwarder MUST duplicate the MPL Option, unmodified, in the new
   outer IPv6 header.

   The IPv6 destination address of the MPL multicast packet is the all-
   MPL-forwarders multicast address (TBD).  The scope of the IPv6
   destination address is set to the MPL multicast scope.

5.4.  Reception of MPL Multicast Packets

   Upon receiving an MPL multicast packet, the MPL forwarder first
   determines whether or not to accept and buffer the MPL multicast
   packet based on its MPL seed and sequence value, as specified in
   Section 5.2.

   If the MPL forwarder accepts the MPL multicast packet, the MPL
   forwarder determines whether or not to deliver the original multicast
   packet to the next higher layer.  For example, if the MPL multicast
   packet uses IPv6-in-IPv6 encapsulation, the MPL forwarder removes the
   outer IPv6 header, which also removes MPL Option.

5.5.  Transmission of ICMPv6 MPL Messages

   The MPL forwarder generates and transmits a new ICMPv6 MPL message
   whenever Trickle requests a transmission.  The MPL forwarder includes
   an encoding of each sliding window in the ICMPv6 MPL message.

   Each sliding window is encoded using an MPL Window entry, defined in
   Section 5.2.  The MPL forwarder sets the MPL Window fields as



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   follows:

   S  If the MPL seed identifier is 0, set S to 0.  If the MPL seed
      identifier is within the range [1, 65535], set S to 2.  Otherwise,
      set S to 3.

   w-min  Set to the lower bound of the sliding window (i.e.
      WindowMin).

   w-len  Set to the length of the window (i.e.  WindowMax - WindowMin).

   seed-id  If S is non-zero, set to the MPL seed identifier.

   buffered-mpl-packets  Set each bit that represents a sequence number
      of a packet in BufferedPackets to 1.  Set all other bits to 0.
      The i'th bit in buffered-mpl-packets represents a sequence number
      of w-min + i.

5.6.  Reception of ICMPv6 MPL Messages

   An MPL forwarder processes each ICMPv6 MPL message that it receives
   to determine if it has any new MPL multicast packets to receive or
   offer.

   An MPL forwarder determines if a new MPL multicast packet has not
   been received from a neighboring node if any of the following
   conditions hold true:

   1.  The ICMPv6 MPL message includes an MPL Window for an MPL seed
       that does not have a corresponding sliding window entry on the
       MPL forwarder.

   2.  The neighbor has a packet in its BufferedPackets that has
       sequence value greater than or equal to WindowMax (i.e. w-min +
       w-len >= WindowMax).

   3.  The neighbor has a packet in its BufferedPackets that has
       sequence number within range of the sliding window but is not
       included in BufferedPackets (i.e. the i'th bit in buffered-mpl-
       packets is set to 1, where the sequence number is w-min + i).

   When an MPL forwarder determines that it has not yet received a new
   MPL multicast packet buffered by a neighboring device, the MPL
   forwarder resets the Trickle timer associated with reactive
   propagation.

   An MPL forwarder determines if an entry in BufferedPackets has not
   been received by a neighboring MPL forwarder if any of the following



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   conditions hold true:

   1.  The ICMPv6 MPL message does not include an MPL Window for the
       packet's MPL seed.

   2.  The packet's sequence number is greater than or equal to the
       neighbor's WindowMax value (i.e. the packet's sequence number is
       greater than or equal to w-min + w-len).

   3.  The packet's sequence number is within the range of the
       neighbor's sliding window [WindowMin, WindowMax), but not
       included in the neighbor's BufferedPacket (i.e. the packet's
       sequence number is greater than or equal to w-min, strictly less
       than w-min + w-len, and the corresponding bit in buffered-mpl-
       packets is set to 0.

   When an MPL forwarder determines that it has at least one buffered
   MPL multicast packet that has not yet been received by a neighbor,
   the MPL forwarder resets the Trickle timer associated with reactive
   propagation.  Additionally, for each buffered MPL multicast packet
   that should be transferred, the MPL forwarder MUST reset the Trickle
   timer and reset e to 0 for proactive propagation.  If the Trickle
   timer for proactive propagation has already stopped execution, the
   MPL forwarder MUST initialize a new Trickle timer and start execution
   of the Trickle algorithm.


























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6.  MPL Parameters

   An MPL forwarder maintains two sets of Trickle parameters for the
   proactive and reactive methods.  The Trickle parameters are listed
   below:

   PROACTIVE_IMIN  The minimum Trickle timer interval, as defined in
      [RFC6206] for proactive propagation.

   PROACTIVE_IMAX  The maximum Trickle timer interval, as defined in
      [RFC6206] for proactive propagation.

   PROACTIVE_K  The redundancy constant, as defined in [RFC6206] for
      proactive propagation.

   PROACTIVE_TIMER_EXPIRATIONS  The number of Trickle timer expirations
      that occur before terminating the Trickle algorithm.  MUST be set
      to a value greater than 0.

   REACTIVE_IMIN  The minimum Trickle timer interval, as defined in
      [RFC6206] for reactive propagation.

   REACTIVE_IMAX  The maximum Trickle timer interval, as defined in
      [RFC6206] for reactive propagation.

   REACTIVE_K  The redundancy constant, as defined in [RFC6206] for
      reactive propagation.

   REACTIVE_TIMER_EXPIRATIONS  The number of Trickle timer expirations
      that occur before terminating the Trickle algorithm.  MAY be set
      to 0, which disables reactive propagation.

   WINDOW_HOLD_TIME  The minimum lifetime for sliding window state.


















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7.  Acknowledgements

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













































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8.  IANA Considerations

   The Trickle Multicast option requires an IPv6 Option Number.



   HEX         act  chg  rest
   ---         ---  ---  -----
     C          01    0  TBD



   The first two bits indicate that the IPv6 node MUST discard the
   packet if it doesn't recognize the option type, and the third bit
   indicates that the Option Data MUST NOT change en-route.




































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9.  Security Considerations

   TODO.
















































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10.  References

10.1.  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.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [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.

   [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.

   [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.

10.2.  Informative References

   [I-D.ietf-roll-terminology]
              Vasseur, J., "Terminology in Low power And Lossy
              Networks", draft-ietf-roll-terminology-06 (work in
              progress), September 2011.















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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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