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Versions: 00 01 02 03 04 05 06 07 08 RFC 3810

INTERNET-DRAFT                                           R. Vida, Editor
Obsoletes RFC 2710                                      L. Costa, Editor
                                                                    LIP6

Expires May 2003                                           November 2002


         Multicast Listener Discovery Version 2 (MLDv2) for IPv6
                       <draft-vida-mld-v2-06.txt>


Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 10 of [RFC 2026].

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
   groups may also distribute working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time. It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Copyright Notice

   Copyright (C) The Internet Society (2002). All Rights Reserved.

Abstract

   This document specifies Version 2 of the Multicast Listener Discovery
   Protocol, MLDv2.  MLD is the protocol used by an IPv6 router to
   discover the presence of multicast listeners (that is, nodes wishing
   to receive multicast packets) on its directly attached links, and to
   discover specifically which multicast addresses are of interest to
   those neighboring nodes.

   MLDv2 is derived from version 3 of IPv4's Internet Group Management
   Protocol, IGMPv3.  Compared to the previous version, MLDv2 adds
   support for "source filtering", that is, the ability for a node to
   report interest in listening to packets *only* from specific source
   addresses, or from *all but* specific source addresses, sent to a
   particular multicast address.

   This document obsoletes RFC 2710.





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                            Table of Contents


   1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  The Service Interface for Requesting IP Multicast Reception .   3
   3.  Multicast Listening State Maintained by Nodes . . . . . . . .   4
   4.  Message Formats . . . . . . . . . . . . . . . . . . . . . . .   7
   5.  Description of the Protocol for Multicast Address Listeners .  18
   6.  Description of the Protocol for Multicast Routers . . . . . .  24
   7.  Interoperation with Older Versions of MLD . . . . . . . . . .  35
   8.  List of Timers, Counters and their Default Values . . . . . .  39
   9.  Security Considerations . . . . . . . . . . . . . . . . . . .  43
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  44
   11. References. . . . . . . . . . . . . . . . . . . . . . . . . .  44
   12. Editors' Contact Information. . . . . . . . . . . . . . . . .  45
   13. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . .  45
   14. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . .  46
   Appendix A. Design Rationale. . . . . . . . . . . . . . . . . . .  47
   Appendix B. Summary of Changes from MLDv1 . . . . . . . . . . . .  48
   Full Copyright Statement. . . . . . . . . . . . . . . . . . . . .  50


1.  Introduction

   The Multicast Listener Discovery Protocol (MLD) is used by IPv6
   routers to discover the presence of multicast listeners (that is,
   nodes wishing to receive multicast packets) on their directly
   attached links, and to discover specifically which multicast
   addresses are of interest to those neighboring nodes.  Note that a
   multicast router may itself be a listener of one or more
   multicast addresses, in which case it performs both the "multicast
   router part" of the protocol (to collect the multicast listener
   information needed by its multicast routing protocol) and the
   "multicast address listener part" of the protocol (to inform itself
   and other neighboring multicast routers of its listening state).

   This document specifies Version 2 of MLD.  The previous version of
   MLD became an Internet Standard and is specified in [RFC 2710].  In
   this document we will refer to it as MLDv1.  MLDv2 is a translation
   of the IGMPv3 protocol [RFC 3376] for IPv6 semantics.

   Version 2 of the MLD protocol, when compared to the previous version,
   adds support for "source filtering", that is, the ability for a node
   to report interest in listening to packets *only* from specific
   source addresses, as required to support Source-Specific Multicast
   [SSM], or from *all but* specific source addresses, sent to a
   particular multicast address.  Version 2 is designed to be
   interoperable with the previous version.

   The capitalized 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

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   [RFC 2119].  Due to the lack of italics, emphasis is indicated herein
   by bracketing a word or phrase in "*" characters.


2.  The Service Interface for Requesting IP Multicast Reception

   Within an IP system, there is (at least conceptually) a service
   interface used by upper-layer protocols or application programs to
   ask the IP layer to enable and disable reception of packets sent to
   specific IP multicast addresses.  In order to take full advantage of
   the capabilities of MLDv2, a node's IP service interface must support
   the following operation:

     IPv6MulticastListen ( socket, interface, IPv6 multicast address,
                           filter mode, source list )

   where:

   o "socket" is an implementation-specific parameter used to
     distinguish among different requesting entities (e.g., programs or
     processes) within the node; the socket parameter of BSD Unix system
     calls is a specific example.

   o "interface" is a local identifier of the network interface on which
     reception of the specified multicast address is to be enabled or
     disabled. Interfaces may be physical (e.g., an Ethernet interface)
     or virtual (e.g., the endpoint of a Frame Relay virtual circuit or
     the endpoint of an IP-in-IP "tunnel").  An implementation may allow
     a special "unspecified" value to be passed as the interface
     parameter, in which case the request would apply to the "primary"
     or "default" interface of the node (perhaps established by system
     configuration).  If reception of the same multicast address is
     desired on more than one interface, IPv6MulticastListen is invoked
     separately for each desired interface.

   o "IPv6 multicast address" is the multicast address to which the
     request pertains.  If reception of more than one multicast address
     on a given interface is desired, IPv6MulticastListen is invoked
     separately for each desired address.

   o "filter mode" may be either INCLUDE or EXCLUDE.  In INCLUDE mode,
     reception of packets sent to the specified multicast address is
     requested *only* from those source addresses listed in the source
     list parameter.  In EXCLUDE mode, reception of packets sent to the
     given multicast address is requested from all source addresses
     *except* those listed in the source list parameter.

   o "source list" is an unordered list of zero or more unicast
     addresses from which multicast reception is desired or not desired,
     depending on the filter mode.  An implementation MAY impose a limit
     on the size of source lists, but that limit MUST NOT be less than
     64 addresses per list. When an operation causes the source list

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     size limit to be exceeded, the service interface MUST return an
     error.

   For a given combination of socket, interface, and IPv6 multicast
   address, only a single filter mode and source list can be in effect
   at any one time.  However, either the filter mode or the source list,
   or both, may be changed by subsequent IPv6MulticastListen requests
   that specify the same socket, interface, and IPv6 multicast address.
   Each subsequent request completely replaces any earlier request for
   The given socket, interface and multicast address.

   The previous version of MLD did not support source filters and had a
   simpler service interface consisting of Start Listening and Stop
   Listening operations to enable and disable listening to a given
   multicast address (from *all* sources) on a given interface.  The
   equivalent operations in the new service interface follow:
   The Start Listening operation is equivalent to:

      IPv6MulticastListen ( socket, interface, IPv6 multicast address,
                            EXCLUDE, {} )

   and the Stop Listening operation is equivalent to:

      IPv6MulticastListen ( socket, interface, IPv6 multicast address,
                            INCLUDE, {} )

   where {} is an empty source list.

   An example of an API providing the capabilities outlined in this
   service interface is given in [FILTER-API].


3.  Multicast Listening State Maintained by Nodes


3.1.  Socket State

   For each socket on which IPv6MulticastListen has been invoked, the
   node records the desired multicast listening state for that socket.
   That state conceptually consists of a set of records of the form:

      (interface, IPv6 multicast address, filter mode, source list)

   The socket state evolves in response to each invocation of
   IPv6MulticastListen on the socket, as follows:

   o If the requested filter mode is INCLUDE *and* the requested source
     list is empty, then the entry corresponding to the requested
     interface and multicast address is deleted if present.  If no
     such entry is present, the request is ignored.



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   o If the requested filter mode is EXCLUDE *or* the requested source
     list is non-empty, then the entry corresponding to the requested
     interface and multicast address, if present, is changed to
     contain the requested filter mode and source list.  If no such
     entry is present, a new entry is created, using the parameters
     specified in the request.


3.2.  Interface State

   In addition to the per-socket multicast listening state, a node must
   also maintain or compute multicast listening state for each of its
   interfaces.  That state conceptually consists of a set of records of
   the form:

          (IPv6 multicast address, filter mode, source list)

   At most one record per multicast-address exists for a given
   interface.  This per-interface state is derived from the per-socket
   state, but may differ from the per-socket state when different
   sockets have differing filter modes and/or source lists for the same
   multicast address and interface.  For example, suppose one
   application or process invokes the following operation on socket s1:

         IPv6MulticastListen ( s1, i, m, INCLUDE, {a, b, c} )

   requesting reception on interface i of packets sent to multicast
   address m, *only* if they come from source a, b, or c.  Suppose
   another application or process invokes the following operation on
   socket s2:

          IPv6MulticastListen ( s2, i, m, INCLUDE, {b, c, d} )

   requesting reception on the same interface i of packets sent to the
   same multicast address m, *only* if they come from sources b, c, or
   d. In order to satisfy the reception requirements of both sockets,
   it is necessary for interface i to receive packets sent to m from any
   one of the sources a, b, c, or d.  Thus, in this example, the
   listening state of interface i for multicast address m has filter
   mode INCLUDE and source list {a, b, c, d}.

   After a multicast packet has been accepted from an interface by the
   IP layer, its subsequent delivery to the application or process
   listening on a particular socket depends on the multicast listening
   state of that socket (and possibly also on other conditions, such as
   what transport-layer port the socket is bound to).  So, in the above
   example, if a packet arrives on interface i, destined to multicast
   address m, with source address a, it may be delivered on socket s1
   but not on socket s2. Note that MLDv2 messages are not subject to
   source filtering and must always be processed by hosts and routers.



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   Filtering of packets based upon a socket's multicast reception state
   is a new feature of this service interface.  The previous service
   interface described no filtering based upon multicast listening
   state; rather, a Start Listening operation on a socket simply caused
   the node to start to listen to a multicast address on the given
   interface, and packets sent to that multicast address could be
   delivered to all sockets whether they had started to listen or not.

   The general rules for deriving the per-interface state from the per-
   socket state are as follows:  for each distinct (interface, IPv6
   multicast address) pair that appears in any socket state, a per-
   interface record is created for that multicast address on that
   interface.  Considering all socket records containing the same
   (interface, IPv6 multicast address) pair,

   o if *any* such record has a filter mode of EXCLUDE, then the filter
     mode of the interface record is EXCLUDE, and the source list of the
     interface record is the intersection of the source lists of all
     socket records in EXCLUDE mode, minus those source addresses that
     appear in any socket record in INCLUDE mode.  For example, if the
     socket records for multicast address m on interface i are:

            from socket s1:  ( i, m, EXCLUDE, {a, b, c, d} )
            from socket s2:  ( i, m, EXCLUDE, {b, c, d, e} )
            from socket s3:  ( i, m, INCLUDE, {d, e, f} )

     then the corresponding interface record on interface i is:

                             ( m, EXCLUDE, {b, c} )

     If a fourth socket is added, such as:

            From socket s4:  (i, m, EXCLUDE, {} )

     then the interface record becomes:

                             (m, EXCLUDE, {} )

   o if *all* such records have a filter mode of INCLUDE, then the
     filter mode of the interface record is INCLUDE, and the source list
     of the interface record is the union of the source lists of all the
     socket records.  For example, if the socket records for multicast
     address m on interface i are:

            from socket s1:  ( i, m, INCLUDE, {a, b, c} )
            from socket s2:  ( i, m, INCLUDE, {b, c, d} )
            from socket s3:  ( i, m, INCLUDE, {e, f} )

     then the corresponding interface record on interface i is:

                      ( m, INCLUDE, {a, b, c, d, e, f} )


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   An implementation MUST NOT use an EXCLUDE interface record for a
   multicast address if all sockets for this multicast address are in
   INCLUDE state.  If system resource limits are reached when an
   interface state source list is calculated, an error MUST be returned
   to the application which requested the operation.

   The above rules for deriving the interface state are (re-)evaluated
   whenever an IPv6MulticastListen invocation modifies the socket state
   by adding, deleting, or modifying a per-socket state record.  Note
   that a change of socket state does not necessarily result in a change
   of interface state.


4.  Message Formats

   MLDv2 is a sub-protocol of ICMPv6, that is, MLDv2 message types are a
   subset of the set of ICMPv6 messages, and MLDv2 messages are
   identified in IPv6 packets by a preceding Next Header value of 58.
   All MLDv2 messages described in this document MUST be sent with a
   link-local IPv6 Source Address (or the unspecified address, if the
   node has not yet acquired such an address), an IPv6 Hop Limit of 1,
   and an IPv6 Router Alert option [RFC 2711] in a Hop-by-Hop Options
   header.  (The Router Alert option is necessary to cause routers to
   examine MLDv2 messages sent to IPv6 multicast addresses in which the
   routers themselves have no interest.)

   There are two MLD message types of concern to the MLDv2 protocol
   described in this document:

   o Multicast Listener Query (Type = decimal 130)

   o Version 2 Multicast Listener Report (Type = [Value to be allocated
     by the IANA]).  See section 10 for IANA considerations.

   An implementation of MLDv2 must also support the following two
   message types, for interoperation with the previous version of MLD
   (see section 7):

   o Version 1 Multicast Listener Report (Type = decimal 131) [RFC 2710]

   o Version 1 Multicast Listener Done (Type = decimal 132) [RFC 2710]

   Unrecognized message types MUST be silently ignored.  Other message
   types may be used by newer versions or extensions of MLD, by
   multicast routing protocols, or for other uses.

   In this document, unless otherwise qualified, the capitalized words
   "Query" and "Report" refer to MLD Multicast Listener Queries and MLD
   Version 2 Multicast Listener Reports, respectively.




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4.1.  Multicast Listener Query Message

   Multicast Listener Queries are sent by multicast routers to
   query the multicast listening state of neighboring interfaces.
   Queries have 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 = 130   |      Code     |           Checksum            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Maximum Response Code      |           Reserved            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   *                                                               *
   |                                                               |
   *                       Multicast Address                       *
   |                                                               |
   *                                                               *
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | Resv  |S| QRV |     QQIC      |     Number of Sources (N)     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   *                                                               *
   |                                                               |
   *                       Source Address [1]                      *
   |                                                               |
   *                                                               *
   |                                                               |
   +-                                                             -+
   |                                                               |
   *                                                               *
   |                                                               |
   *                       Source Address [2]                      *
   |                                                               |
   *                                                               *
   |                                                               |
   +-                              .                              -+
   .                               .                               .
   .                               .                               .
   +-                                                             -+
   |                                                               |
   *                                                               *
   |                                                               |
   *                       Source Address [N]                      *
   |                                                               |
   *                                                               *
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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4.1.1.  Code

   Initialized to zero by the sender; ignored by receivers.


4.1.2.  Checksum

   The standard ICMPv6 checksum, covering the entire MLDv2 message plus
   a "pseudo-header" of IPv6 header fields [RFC 2463].  For computing
   the checksum, the Checksum field is set to zero.  When receiving
   packets, the checksum MUST be verified before processing a packet.


4.1.3.  Maximum Response Code

   The Maximum Response Code field specifies the maximum time allowed
   before sending a responding Report. The actual time allowed, called
   the Maximum Response Delay, is represented in units of milliseconds
   and is derived from the Maximum Response Code as follows:

   If Maximum Response Code < 32768,
      Maximum Response Delay = Maximum Response Code

   If Maximum Response Code >=32768, Maximum Response Code represents a
   floating-point value as follows:

     0 1 2 3 4 5 6 7 8 9 A B C D E F
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1| exp |          mant         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Maximum Response Delay = (mant | 0x1000) << (exp+3)

   Small values of Maximum Response Delay allow MLDv2 routers to tune
   the "leave latency" (the time between the moment the last node on a
   link ceases listening to a specific multicast address and the moment
   the routing protocol is notified that there are no more listeners for
   that address).  Larger values, especially in the exponential range,
   allow tuning of the burstiness of MLD traffic on a link.


4.1.4.  Reserved

   Initialized to zero by the sender; ignored by receivers.


4.1.5.  Multicast Address

   The Multicast Address field is set to zero when sending a General
   Query, and set to the multicast address being queried when sending a
   Multicast Address Specific Query or Multicast Address and Source
   Specific Query (see section 4.1.10, below).

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4.1.6.  Resv (Reserved)

   Initialized to zero by the sender; ignored by receivers.


4.1.7.  S Flag (Suppress Router-Side Processing)

   When set to one, the S Flag indicates to any receiving multicast
   routers that they have to suppress the normal timer updates they
   perform upon hearing a Query.  Nevertheless, it does not suppress the
   querier election or the normal "host-side" processing of a Query that
   a router may be required to perform as a consequence of itself being
   a multicast listener.


4.1.8.  QRV (Querier's Robustness Variable)

   If non-zero, the QRV field contains the [Robustness Variable] value
   used by the Querier, i.e., the sender of the Query.  If the Querier's
   [Robustness Variable] exceeds 7, the maximum value of the QRV field,
   the QRV is set to zero.  Routers adopt the QRV value from the most
   recently received Query as their own [Robustness Variable] value,
   unless that most recently received QRV was zero, in which case the
   receivers use the default [Robustness Variable] value specified in
   section 8.1 or a statically configured value.


4.1.9.  QQIC (Querier's Query Interval Code)

   The Querier's Query Interval Code field specifies the [Query
   Interval] used by the querier. The actual interval, called the
   Querier's Query Interval (QQI), is represented in units of seconds
   and is derived from the Querier's Query Interval Code as follows:

   If QQIC < 128, QQI = QQIC

   If QQIC >= 128, QQIC represents a floating-point value as follows:

       0 1 2 3 4 5 6 7
      +-+-+-+-+-+-+-+-+
      |1| exp | mant  |
      +-+-+-+-+-+-+-+-+

   QQI = (mant | 0x10) << (exp + 3)

   Multicast routers that are not the current querier adopt the QQI
   value from the most recently received Query as their own [Query
   Interval] value, unless that most recently received QQI was zero, in
   which case the receiving routers use the default [Query Interval]
   value specified in section 8.2.



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4.1.10.  Number of Sources (N)

   The Number of Sources (N) field specifies how many source addresses
   are present in the Query.  This number is zero in a General Query or
   a Multicast Address Specific Query, and non-zero in a Multicast
   Address and Source Specific Query.  This number is limited by the MTU
   of the link over which the Query is transmitted.  For example, on
   an Ethernet with an MTU of 1500 octets, the IPv6 header (40 octets)
   including the Router Alert option in the Hop-By-Hop Extension Header
   (8 octets) consumes 48 octets, and the MLD fields up to including the
   Number of Sources (N) field consume 28 octets, leaving 1424 octets
   for source addresses, which limits the number of source addresses to
   89 (1424/16).


4.1.11.  Source Address [i]

   The Source Address [i] fields are a vector of n unicast addresses,
   where n is the value in the Number of Sources (N) field.


4.1.12.  Additional Data

   If the Payload Length field in the IPv6 header of a received Query
   indicates that there are additional octets of data present, beyond
   the fields described here, MLDv2 implementations MUST include those
   octets in the computation to verify the received MLD Checksum, but
   MUST otherwise ignore those additional octets.  When sending a Query,
   an MLDv2 implementation MUST NOT include additional octets beyond the
   fields described here.


4.1.13.  Query Variants

   There are three variants of the Query message:

   o A "General Query" is sent by a multicast router to learn which
     multicast addresses have listeners on an attached link.  In a
     General Query, both the Multicast Address field and the Number of
     Sources (N) field are zero.

   o A "Multicast Address Specific Query" is sent by a multicast router
     to learn if a particular multicast address has any listeners on an
     attached link.  In a Multicast Address Specific Query, the
     Multicast Address field contains the multicast address of interest,
     and the Number of Sources (N) field contains zero.

   o A "Multicast Address and Source Specific Query" is sent by a
     multicast router to learn if any of the sources from the specified
     list for the particular multicast address has any listeners on an
     attached link or not.  In a Multicast Address and Source Specific
     Query, the Multicast Address field contains the multicast address

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     of interest, and the Source Address [i] field(s) contain the source
     address(es) of interest.


4.1.14.  Destination Addresses for Queries

   In MLDv2, General Queries are sent to the link-scope all-nodes
   multicast address (FF02::1).  Multicast Address Specific and
   Multicast Address and Source Specific Queries are sent with an IP
   destination address equal to the multicast address of interest.
   *However*, a node MUST accept and process any Query whose IP
   Destination Address field contains *any* of the addresses (unicast or
   multicast) assigned to the interface on which the Query arrives.


4.2.  Version 2 Multicast Listener Report Message

   Version 2 Multicast Listener Reports are sent by IP nodes to report
   (to neighboring routers) the current multicast listening state, or
   changes in the multicast listening state, of their interfaces.
   Reports have 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 = TBA   |    Reserved   |           Checksum            |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |           Reserved            |Nr of Mcast Address Records (M)|
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                  Multicast Address Record [1]                 .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                  Multicast Address Record [2]                 .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                               .                               |
     .                               .                               .
     |                               .                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                  Multicast Address Record [M]                 .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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   where TBA = value To Be Allocated by the IANA, as specified in
   section 4.

   Each Multicast Address Record has the following internal format:

     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |  Record Type  |  Aux Data Len |     Number of Sources (N)     |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     *                                                               *
     |                                                               |
     *                       Multicast Address                       *
     |                                                               |
     *                                                               *
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     *                                                               *
     |                                                               |
     *                       Source Address [1]                      *
     |                                                               |
     *                                                               *
     |                                                               |
     +-                                                             -+
     |                                                               |
     *                                                               *
     |                                                               |
     *                       Source Address [2]                      *
     |                                                               |
     *                                                               *
     |                                                               |
     +-                                                             -+
     .                               .                               .
     .                               .                               .
     .                               .                               .
     +-                                                             -+
     |                                                               |
     *                                                               *
     |                                                               |
     *                       Source Address [N]                      *
     |                                                               |
     *                                                               *
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     |                                                               |
     .                                                               .
     .                         Auxiliary Data                        .
     .                                                               .
     |                                                               |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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4.2.1.  Reserved

   The Reserved fields are set to zero on transmission, and ignored on
   reception.


4.2.2.  Checksum

   The standard ICMPv6 checksum, covering the entire MLDv2 message plus
   a "pseudo-header" of IPv6 header fields [RFC 2460, RFC 2463].  For
   computing the checksum, the Checksum field is set to zero.  When
   receiving packets, the checksum MUST be verified before processing a
   message.


4.2.3.  Nr of Mcast Address Records (M)

   The Nr of Mcast Address Records (M) field specifies how many
   Multicast Address Records are present in this Report.


4.2.4.  Multicast Address Record

   Each Multicast Address Record is a block of fields containing
   information on the sender listening to a single multicast address on
   the interface from which the Report is sent.


4.2.5.  Record Type

   Specifies the type of the Multicast Address Record. See section
   4.2.12 for a detailed description of the different possible Record
   Types.


4.2.6.  Aux Data Len

   The Aux Data Len field contains the length of the Auxiliary Data
   Field in this Multicast Address Record, in units of 32-bit words.  It
   may contain zero, to indicate the absence of any auxiliary data.


4.2.7.  Number of Sources (N)

   The Number of Sources (N) field specifies how many source addresses
   are present in this Multicast Address Record.


4.2.8.  Multicast Address

   The Multicast Address field contains the multicast address to which
   this Multicast Address Record pertains.

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4.2.9.  Source Address [i]

   The Source Address [i] fields are a vector of n unicast addresses,
   where n is the value in this record's Number of Sources (N) field.


4.2.10.  Auxiliary Data

   The Auxiliary Data field, if present, contains additional information
   pertaining to this Multicast Address Record.  The protocol specified
   in this document, MLDv2, does not define any auxiliary data.
   Therefore, implementations of MLDv2 MUST NOT include any auxiliary
   data (i.e., MUST set the Aux Data Len field to zero) in any
   transmitted Multicast Address Record, and MUST ignore any auxiliary
   data present in any received Multicast Address Record.  The semantics
   and internal encoding of the Auxiliary Data field are to be defined
   by any future version or extension of MLD that uses this field.


4.2.11.  Additional Data

   If the Payload Length field in the IPv6 header of a received Report
   indicates that there are additional octets of data present, beyond
   the last Multicast Address Record, MLDv2 implementations MUST include
   those octets in the computation to verify the received MLD Checksum,
   but MUST otherwise ignore those additional octets.  When sending a
   Report, an MLDv2 implementation MUST NOT include additional octets
   beyond the last Multicast Address Record.


4.2.12.  Multicast Address Record Types

   There are a number of different types of Multicast Address Records
   that may be included in a Report message:

   o A "Current State Record" is sent by a node in response to a Query
     received on an interface.  It reports the current listening state
     of that interface, with respect to a single multicast address.
     The Record Type of a Current State Record may be one of the
     following two values:

     Value  Name and Meaning
     -----  ----------------
       1    MODE_IS_INCLUDE - indicates that the interface has a filter
            mode of INCLUDE for the specified multicast address. The
            Source Address [i] fields in this Multicast Address Record
            contain the interface's source list for the specified
            multicast address, if it is non-empty.

       2    MODE_IS_EXCLUDE - indicates that the interface has a filter
            mode of EXCLUDE for the specified multicast address. The
            Source Address [i] fields in this Multicast Address Record

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            contain the interface's source list for the specified
            multicast address, if it is non-empty.

   o A "Filter Mode Change Record" is sent by a node whenever a local
     invocation of IPv6MulticastListen causes a change of the filter
     mode (i.e., a change from INCLUDE to EXCLUDE, or from EXCLUDE to
     INCLUDE) of the interface-level state entry for a particular
     multicast address. The Record is included in a Report sent from the
     interface on which the change occurred.  The Record Type of a
     Filter Mode Change Record may be one of the following two values:

       3    CHANGE_TO_INCLUDE_MODE - indicates that the interface has
            changed to INCLUDE filter mode for the specified multicast
            address.  The Source Address [i] fields in this Multicast
            Address Record contain the interface's new source list for
            the specified multicast address, if it is non-empty.

       4    CHANGE_TO_EXCLUDE_MODE - indicates that the interface has
            changed to EXCLUDE filter mode for the specified multicast
            address.  The Source Address [i] fields in this Multicast
            Address Record contain the interface's new source list for
            the specified multicast address, if it is non-empty.

   o A "Source List Change Record" is sent by a node whenever a local
     invocation of IPv6MulticastListen causes a change of source list
     that is *not* coincident with a change of filter mode, of the
     interface level state entry for a particular multicast address.
     The Record is included in a Report sent from the interface on which
     the change occurred.  The Record Type of a Source List Change
     Record may be one of the following two values:

       5    ALLOW_NEW_SOURCES - indicates that the Source Address [i]
            fields in this Multicast Address Record contain a list of
            the additional sources that the node wishes to listen to,
            for packets sent to the specified multicast address.
            If the change was to an INCLUDE source list, these are the
            addresses that were added to the list; if the change was to
            an EXCLUDE source list, these are the addresses that were
            deleted from the list.

       6    BLOCK_OLD_SOURCES - indicates that the Source Address [i]
            fields in this Multicast Address Record contain a list of
            the sources that the node no longer wishes to listen to,
            for packets sent to the specified multicast address.  If the
            change was to an INCLUDE source list, these are the
            addresses that were deleted from the list; if the change was
            to an EXCLUDE source list, these are the addresses that were
            added to the list.

   If a change of source list results in both allowing new sources and
   blocking old sources, then two Multicast Address Records are sent for
   the same multicast address, one of type ALLOW_NEW_SOURCES and one of

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   type BLOCK_OLD_SOURCES.

   We use the term "State Change Record" to refer to either a Filter
   Mode Change Record or a Source List Change Record.

   Unrecognized Record Type values MUST be silently ignored.


4.2.13.  Source Addresses for Reports

   An MLDv2 Report MUST be sent with a valid IPv6 link-local source
   address, or the unspecified address (::), if the node has not yet
   acquired a valid link-local address.  Routers MUST ignore a message
   that is not sent with a valid link-local address.

   Thus, a Report will be ignored, if the router cannot identify the
   source address of the packet as belonging to a link assigned to the
   interface on which the packet was received.  A Report sent with the
   unspecified address will also be ignored by a router.  The reporting
   node has however modified its listening state for multicast addresses
   that are contained in the Multicast Address Records of the Report
   message.


4.2.14.  Destination Addresses for Reports

   Version 2 Multicast Listener Reports are sent with an IP
   destination address to which all MLDv2-capable multicast routers
   listen (see section 10 for IANA considerations related to this
   special destination address).  A node that is operating in version 1
   compatibility mode sends version 1 Reports to the multicast address
   specified in the Multicast Address field of the Report.  In addition,
   a node MUST accept and process any version 1 Report whose IP
   Destination Address field contains *any* of the IPv6 addresses
   (unicast or multicast) assigned to the interface on which the Report
   arrives.


4.2.15.  Notation for Multicast Address Records

   In the rest of this document, we use the following notation to
   describe the contents of a Multicast Address Record pertaining to a
   particular multicast address:

     IS_IN ( x )  -  Type MODE_IS_INCLUDE, source addresses x
     IS_EX ( x )  -  Type MODE_IS_EXCLUDE, source addresses x
     TO_IN ( x )  -  Type CHANGE_TO_INCLUDE_MODE, source addresses x
     TO_EX ( x )  -  Type CHANGE_TO_EXCLUDE_MODE, source addresses x
     ALLOW ( x )  -  Type ALLOW_NEW_SOURCES, source addresses x
     BLOCK ( x )  -  Type BLOCK_OLD_SOURCES, source addresses x

   where x is either:

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     o a capital letter (e.g., "A") to represent the set of source
       addresses,

     or

     o a set expression (e.g., "A+B"), where "A+B" means the union of
       sets A and B,  "A*B" means the intersection of sets A and B, and
       "A-B" means the removal of all elements of set B from set A.


4.2.16.  Multicast Listener Report Size

   If the set of Multicast Address Records required in a Report does not
   fit within the size limit of a single Report message (as determined
   by the MTU of the link on which it will be sent), the Multicast
   Address Records are sent in as many Report messages as needed to
   report the entire set.

   If a single Multicast Address Record contains so many source
   addresses that it does not fit within the size limit of a single
   Report message, if its Type is not MODE_IS_EXCLUDE or
   CHANGE_TO_EXCLUDE_MODE, it is split into multiple Multicast Address
   Records, each containing a different subset of the source addresses
   and each sent in a separate Report message.  If its Type is
   MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, a single Multicast Address
   Record is sent, containing as many source addresses as can fit, and
   the remaining source addresses are not reported; though the choice of
   which sources to report is arbitrary, it is preferable to report the
   same set of sources in each subsequent report, rather than reporting
   different sources each time.


5.  Description of the Protocol for Multicast Address Listeners

   MLD is an asymmetric protocol, specifying separate behaviors for
   multicast address listeners -- that is, hosts or routers that listen
   to multicast packets -- and multicast routers.  This section
   describes the part of MLDv2 that applies to all multicast address
   listeners. (Note that a multicast router that is also a multicast
   address listener performs both parts of MLDv2, receiving and
   responding to its own MLD message transmissions as well as to those
   of its neighbors.)  The multicast router part of MLDv2 is described
   in section 6.

   A node performs the protocol described in this section over all
   interfaces on which multicast reception is supported, even if more
   than one of those interfaces is connected to the same link.

   For interoperability with multicast routers running the older
   version of MLD, nodes maintain a MulticastRouterVersion variable
   for each interface on which multicast reception is supported.  This
   section describes the behavior of multicast address listener nodes

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   on interfaces for which MulticastRouterVersion = 2.  The algorithm
   for determining MulticastRouterVersion, and the behavior for version
   1, are described in section 7.

   The link-scope all-nodes multicast address, (FF02::1), is handled as
   a special case.  On all nodes -- that is all hosts and routers,
   including multicast routers -- listening to packets destined to the
   all-nodes multicast address, from all sources, is permanently enabled
   on all interfaces on which multicast listening is supported.  No MLD
   messages are ever sent regarding neither the link-scope all-nodes
   multicast address, nor any multicast address of scope 0 (reserved) or
   1 (node-local).

   There are two types of events that trigger MLDv2 protocol actions on
   an interface:

   o a change of the interface listening state, caused by a local
     invocation of IPv6MulticastListen.

   o reception of a Query.

   (Received MLD messages of types other than Query are silently
   ignored, except as required for interoperation with the earlier
   version of MLD.)

   The following subsections describe the actions to be taken for each
   case.  Timer and counter names appear in square brackets.  Default
   values for those timers and counters are specified in section 8.


5.1.  Action on Change of Interface State

   An invocation of IPv6MulticastListen may cause the multicast
   listening state of an interface to change, according to the rules in
   section 3.2. Each such change affects the per-interface entry for a
   single multicast address.

   A change of interface state causes the node to immediately transmit a
   State Change Report from that interface.  The type and contents of
   the Multicast Address Record(s) in that Report are determined by
   comparing the filter mode and source list for the affected multicast
   address before and after the change, according to the table below.
   If no interface state existed for that multicast address before the
   change (i.e., the change consisted of creating a new per-interface
   record), or if no state exists after the change (i.e., the change
   consisted of deleting a per-interface record), then the
   "non-existent" state is considered to have a filter mode of INCLUDE
   and an empty source list.





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    Old State         New State         State Change Record Sent
    ---------         ---------         ------------------------
    INCLUDE (A)       INCLUDE (B)       ALLOW (B-A), BLOCK (A-B)

    EXCLUDE (A)       EXCLUDE (B)       ALLOW (A-B), BLOCK (B-A)

    INCLUDE (A)       EXCLUDE (B)       TO_EX (B)

    EXCLUDE (A)       INCLUDE (B)       TO_IN (B)

   If the computed source list for either an ALLOW or a BLOCK State
   Change Record is empty, that record is omitted from the Report
   message.

   To cover the possibility of the State Change Report being missed by
   one or more multicast routers, it is retransmitted [Robustness
   Variable] - 1 more times, at intervals chosen at random from the
   range (0, [Unsolicited Report Interval]).

   If more changes to the same interface state entry occur before all
   the retransmissions of the State Change Report for the first change
   have been completed, each such additional change triggers the
   immediate transmission of a new State Change Report.

   The contents of the new transmitted report are calculated as follows:

   o As was done with the first report, the interface state for the
     affected multicast address before and after the latest change is
     compared.

   o The report records expressing the difference are built according to
     the table above. However these records are not transmitted in a
     message but instead merged with the contents of the pending report,
     to create the new State Change Report.

   The rules for merging the difference report resulting from the state
   change and the pending report are described below.

   The transmission of the merged State Change Report terminates
   retransmissions of the earlier State Change Reports for the same
   multicast address, and becomes the first of [Robustness Variable]
   transmissions of the new State Change Reports.

   Each time a source is included in the difference report calculated
   above, retransmission state for that source needs to be maintained
   until [Robustness Variable] State Change Reports have been sent by
   the node. This is done in order to ensure that a series of successive
   state changes do not break the protocol robustness.

   If the interface listening state change that triggers the new report
   is a filter mode change, then the next [Robustness Variable] State
   Change Reports will include a Filter Mode Change Record. This applies

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   even if any number of source list changes occur in that period. The
   node has to maintain retransmission state for the multicast address
   until the [Robustness Variable] State Change Reports have been sent.
   When [Robustness Variable] State Change Reports with Filter Mode
   Change Records have been transmitted after the last filter mode
   change, and if source list changes to the interface listening have
   scheduled additional reports, then the next State Change Report will
   include Source List Change Records.

   Each time a State Change Report is transmitted, the contents are
   determined as follows. If the report should contain a Filter Mode
   Change Record, then if the current filter mode of the interface is
   INCLUDE, a TO_IN record is included in the report, otherwise a TO_EX
   record is included. If instead the report should contain Source List
   Change Records, an ALLOW and a BLOCK record are included.  The
   contents of these records are built according to the table below.

     Record   Sources included
     ------   ----------------
     TO_IN    All in the current interface state that must be forwarded
     TO_EX    All in the current interface state that must be blocked
     ALLOW    All with retransmission state that must be forwarded
     BLOCK    All with retransmission state that must be blocked

   If the computed source list for either an ALLOW or a BLOCK record is
   empty, that record is omitted from the State Change Report.

   Note: When the first State Change Report is sent, the non-existent
   pending report to merge with can be treated as a Source Change
   Report with empty ALLOW and BLOCK records (no sources have
   retransmission state).


5.2.  Action on Reception of a Query

   Upon reception of an MLD message containing a Query, the node checks
   if the source address of the message is a valid link-local address,
   if the Hop Limit is set to 1, and if the Router Alert option is
   present in the Hop-By-Hop Options header of the IPv6 packet.  If any
   of these checks fails, the packet is dropped.

   If the validity of the MLD message is verified, the node starts to
   process the Query.  Instead of responding immediately, the node
   delays its response by a random amount of time, bounded by the
   Maximum Response Delay value derived from the Maximum Response Code
   in the received Query message.  A node may receive a variety of
   Queries on different interfaces and of different kinds (e.g., General
   Queries, Multicast Address Specific Queries, and Multicast Address
   and Source Specific Queries), each of which may require its own
   delayed response.

   Before scheduling a response to a Query, the node must first consider

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   previously scheduled pending responses and in many cases schedule a
   combined response. Therefore, the node must be able to maintain the
   following state:

   o A timer per interface for scheduling responses to General Queries.

   o A per-multicast-address-and-interface timer for scheduling
     responses to Multicast Address Specific and Multicast Address and
     Source Specific Queries.

   o A per-multicast-address-and-interface list of sources to be
     reported in the response to a Multicast Address and Source Specific
     Query.

   When a new Query with the Router Alert option arrives on an
   interface, provided the node has state to report, a delay for a
   response is randomly selected in the range (0, [Maximum Response
   Delay]) where Maximum Response Delay is derived from Maximum Response
   Code in the received Query message.  The following rules are then
   used to determine if a Report needs to be scheduled or not and the
   type of Report to schedule.  The rules are considered in order and
   only the first matching rules is applied.

   1. If there is a pending response to a previous General Query
      scheduled sooner than the selected delay, no additional response
      needs to be scheduled.

   2. If the received Query is a General Query, the interface timer is
      used to schedule a response to the General Query after the
      selected delay. Any previously pending response to a General Query
      is canceled.

   3. If the received Query is a Multicast Address Specific Query or a
      Multicast Address and Source Specific Query and there is no
      pending response to a previous Query for this multicast address,
      then the multicast address timer is used to schedule a report. If
      the received Query is a Multicast Address and Source Specific
      Query, the list of queried sources is recorded to be used when
      generating a response.

   4. If there is already a pending response to a previous Query
      scheduled for this multicast address, and either the new Query is
      a Multicast Address Specific Query or the recorded source list
      associated with the multicast address is empty, then the multicast
      address source list is cleared and a single response is scheduled
      using the multicast address timer. The new response is scheduled
      to be sent at the earliest of the remaining time for the pending
      report and the selected delay.

   5. If the received Query is a Multicast Address and Source Specific
      Query and there is a pending response for this multicast address
      with a non-empty source list, then the multicast address source

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      list is augmented to contain the list of sources in the new Query
      and a single response is scheduled using the multicast address
      timer. The new response is scheduled to be sent at the earliest of
      the remaining time for the pending report and the selected delay.

   When the timer in a pending response record expires, the node
   transmits, on the associated interface, one or more Report messages
   carrying one or more Current State Records (see section 4.2.12), as
   follows:

   1. If the expired timer is the interface timer (i.e., it is a pending
      response to a General Query), then one Current State Record is
      sent for each multicast address for which the specified interface
      has listening state, as described in section 3.2.  The Current
      State Record carries the multicast address and its associated
      filter mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list.
      Multiple Current State Records are packed into individual Report
      messages, to the extent possible.

      This naive algorithm may result in bursts of packets when a node
      listens to a large number of multicast addresses.  Instead of
      using a single interface timer, implementations are recommended to
      spread transmission of such Report messages over the interval
      (0, [Maximum Response Delay]).  Note that any such implementation
      MUST avoid the "ack-implosion" problem, i.e. MUST NOT send a
      Report immediately on reception of a General Query.

   2. If the expired timer is a multicast address timer and the list of
      recorded sources for that multicast address is empty (i.e., it
      is a pending response to a Multicast Address Specific Query), then
      if and only if the interface has listening state for that
      multicast address, a single Current State Record is sent for that
      address.  The Current State Record carries the multicast address
      and its associated filter mode (MODE_IS_INCLUDE or
      MODE_IS_EXCLUDE) and source list, if any.

   3. If the expired timer is a multicast address timer and the list of
      recorded sources for that multicast address is non-empty (i.e., it
      is a pending response to a Multicast Address and Source Specific
      Query), then if and only if the interface has listening state for
      that multicast address, the contents of the responding Current
      State Record is determined from the interface state and the
      pending response record, as specified in the following table:

                         set of sources in the
      interface state   pending response record   Current State Record
      ---------------   -----------------------   --------------------
       INCLUDE (A)                B                   IS_IN (A*B)

       EXCLUDE (A)                B                   IS_IN (B-A)

   If the resulting Current State Record has an empty set of source

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   addresses, then no response is sent.

   Finally, after any required Report messages have been generated, the
   source lists associated with any reported multicast addresses are
   cleared.


6.  Description of the Protocol for Multicast Routers

   The purpose of MLD is to enable each multicast router to learn, for
   each of its directly attached links, which multicast addresses have
   listeners on that link.  MLD version 2 adds the capability for a
   multicast router to also learn which *sources* have listeners among
   the neighboring nodes, for packets sent to any particular
   multicast address.  The information gathered by MLD is provided to
   whichever multicast routing protocol is being used by the router, in
   order to ensure that multicast packets are delivered to all links
   where there are interested listeners.

   This section describes the part of MLDv2 that is performed by
   multicast routers.  Multicast routers may themselves become multicast
   address listeners, and therefore also perform the multicast listener
   part of MLDv2, described in section 5.

   A multicast router performs the protocol described in this section
   over each of its directly attached links.  If a multicast router has
   more than one interface to the same link, it only needs to operate
   this protocol over one of those interfaces.

   For each interface over which the router is operating the MLD
   protocol, the router must configure that interface to listen to all
   link-layer multicast addresses that can be generated by IPv6
   multicasts.  For example, an Ethernet-attached router must set its
   Ethernet address reception filter to accept all Ethernet multicast
   addresses that start with the hexadecimal value 3333 [RFC 2464]; in
   the case of an Ethernet interface that does not support the filtering
   of such a range of multicast address, it must be configured to accept
   ALL Ethernet multicast addresses, in order to meet the requirements
   of MLD.

   On each interface over which this protocol is being run, the router
   MUST enable reception of link-scope "all MLDv2-capable routers"
   multicast address from all sources and MUST perform the multicast
   address listener part of MLDv2 for that address on that interface.

   Multicast routers need to know only that *at least one* node on an
   attached link is listening to packets for a particular multicast
   address from a particular source; a multicast router is not required
   to keep track of the interests of each individual neighboring node.
   (Nevertheless, see Appendix A2 point 1 for discussion.)

   MLDv2 is backward compatible with the previous version of the MLD

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   protocol.  In order to remain backward compatible with older MLD
   nodes, MLDv2 multicast routers MUST also implement the previous
   version of the protocol (see section 7).


6.1.  Conditions for MLD Queries

   Multicast routers send General Queries periodically to request
   Multicast Address Listener information from an attached link.  These
   queries are used to build and refresh the Multicast Address Listener
   state of routers on attached links.

   Nodes respond to these queries by reporting their Multicast Address
   Listening state (and set of sources they listen to) with Current
   State Multicast Address Records in MLDv2 Multicast Listener Reports.

   As a listener of a multicast address, a node may express interest in
   listening or not listening to traffic from particular sources.  As
   the desired listening state of a node changes, it reports these
   changes using Filter Mode Change Records or Source List Change
   Records.  These records indicate an explicit state change in a
   multicast address at a node in either the Multicast Address Record's
   source list or its filter mode.  When Multicast Address Listening is
   terminated at a node or traffic from a particular source is no longer
   desired, a multicast router must query for other listeners of the
   multicast address or of the source before deleting the multicast
   address (or source) from its Multicast Address Listener state and
   pruning its traffic.

   To enable all nodes on a link to respond to changes in multicast
   address listening, multicast routers send specific queries.  A
   Multicast Address Specific Query is sent to verify that there are no
   nodes that listen to the specified multicast address or to "rebuild"
   the listening state for a particular multicast address.  Multicast
   Address Specific Queries are sent when a router receives a State
   Change Record indicating that a node ceases to listen to a multicast
   address.  They are also sent in order to enable a fast transition of
   a router from EXCLUDE to INCLUDE mode, in case a received
   State Change Record motivates this action.

   A Multicast Address and Source Specific Query is used to verify that
   there are no nodes on a link which listen to traffic from a specific
   set of sources.  Multicast Address and Source Specific Queries list
   sources for a particular multicast address which have been requested
   to no longer be forwarded.  This query is sent by a multicast router
   to learn if any node listens to packets sent to the specified
   multicast address from the specified source addresses.  Multicast
   Address and Source Specific Queries are only sent in response to
   State Change Records and never in response to Current State Records.
   Section 4.1.13 describes each query in more detail.


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6.2.  MLD State Maintained by Multicast Routers

   Multicast routers implementing MLDv2 keep state per multicast address
   per attached link.  This multicast address state consists of a
   filter mode, a list of sources, and various timers.  For each
   attached link running MLD, a multicast router records the listening
   state for that link.  That state conceptually consists of a set of
   records of the form:

          (IPv6 multicast address, multicast address timer,
                             filter mode, (source records) )

   Each source record is of the form:

          (IPv6 source address, source timer)

   If all sources for a multicast address are listened to, an empty
   source record list is kept with filter mode set to EXCLUDE.  This
   means that nodes on this link want all sources for this multicast
   address to be forwarded.  This is the MLDv2 equivalent of an MLDv1
   listening state.


6.2.1.  Definition of Router Filter Mode

   To reduce internal state, MLDv2 routers keep a filter mode per
   multicast address per attached link.  This filter mode is used to
   condense the total listening state of a multicast address to a
   minimum set such that all nodes' listening states are covered.  This
   filter mode may change in response to the reception of particular
   types of Multicast Address Records or when certain timer conditions
   occur.  In the following sections, we use the term "router filter
   mode" to refer to the filter mode of a particular multicast address
   within a router.  Section 6.4 describes the changes of a router
   filter mode per Multicast Address Record received.

   Conceptually, when a Multicast Address Record is received, the router
   filter mode for that multicast address is updated to cover all the
   requested sources using the least amount of state.  As a rule, once a
   Multicast Address Record with a filter mode of EXCLUDE is received,
   the router filter mode for that multicast address will be EXCLUDE.

   When a router filter mode for a multicast address is EXCLUDE, the
   source record list contains two types of sources.  The first type
   is the set which represents conflicts in the desired reception state;
   this set must be forwarded by some router on the network.  It serves
   essentially to rebuild the set of sources not forwarded by the router
   when certain timer conditions occur.  It will be also used as the new
   source list when the router switches back to INCLUDE mode, after its
   multicast address timer has expired.

   The second type of sources is the set of sources which all nodes have

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   requested not to be forwarded.  The rules for updating the two sets
   of the source record list when the router is in EXCLUDE mode will be
   described in section 6.4.  Appendix A describes the reasons for
   keeping the first set of sources when in EXCLUDE mode.

   When a router filter mode for a multicast address is INCLUDE, the
   source record list is the list of sources that have listeners for the
   specific multicast address. Each source in a source record must be
   forwarded by some router on the network.

   Because a reported Multicast Address Record with a filter mode of
   EXCLUDE will cause a router to transition its filter mode for that
   multicast address to EXCLUDE, a mechanism for transitioning a
   router's filter mode back to INCLUDE must exist.  If all nodes with a
   multicast address record having filter mode set to EXCLUDE cease
   reporting, it is desirable for the router filter mode for that
   multicast address to transition back to INCLUDE mode.  This
   transition occurs when the Multicast Address Timer expires and is
   explained in detail in section 6.5.


6.2.2.  Definition of Multicast Address Timers

   The Multicast Address Timer is only used when a multicast address is
   in EXCLUDE mode and it represents the time for the *filter mode* of
   the multicast address to expire and switch to INCLUDE mode. We define
   a multicast address timer as a decrementing timer with a lower bound
   of zero kept per multicast address per attached link.  Multicast
   address timers are updated according to the types of Multicast
   Address Records received.

   A Multicast Address Timer expiring when a router filter mode for the
   multicast address is EXCLUDE means there are no more listeners on the
   attached link in EXCLUDE mode.  At this point, a router will
   transition to INCLUDE filter mode. Section 6.5 describes the actions
   taken when a Multicast Address Timer expires while in EXCLUDE mode.

   The following table summarizes the role of the Multicast Address
   Timer.  Section 6.4 describes the details of setting the Multicast
   Address Timer per type of Multicast Address Record received.

   Multicast Address    Multicast Address
       Filter Mode          Timer Value          Actions/Comments
       -----------       -----------------       ----------------

         INCLUDE             Timer >= 0          All listeners in
                                                 INCLUDE mode.

         EXCLUDE             Timer > 0           At least one listener
                                                 in EXCLUDE mode.



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         EXCLUDE             Timer == 0          No more listeners in
                                                 EXCLUDE mode to the
                                                 multicast address.
                                                 If all source timers
                                                 have expired then
                                                 delete Multicast
                                                 Address Record. If
                                                 there are still source
                                                 record timers running,
                                                 switch to INCLUDE
                                                 filter mode using those
                                                 source records with
                                                 running timers as the
                                                 INCLUDE source record
                                                 state.


6.2.3.  Definition of Source Timers

   A source timer is kept per source record and is a decrementing timer
   with a lower bound of zero.  Source timers are updated according to
   the type and filter mode of the Multicast Address Record received.
   Section 6.4 describes the setting of source timers per type of
   Multicast Address Records received.

   When the router filter mode for a multicast address is INCLUDE, a
   source record with a running timer means that there are currently one
   or more nodes (in INCLUDE filter mode) which listen to that source.
   If a source timer expires, the router concludes that this particular
   source no longer has listeners on the attached link, and deletes the
   associated source record.

   Source timers are treated differently when a router filter mode for a
   multicast address is EXCLUDE.  If a source record has a running timer
   it means that at least one system desires the source.  It should
   therefore be forwarded by some router on the network.  Appendix A
   describes the reasons for keeping state for sources that have been
   requested to be forwarded while in EXCLUDE state.

   If a source timer expires with a router filter mode for the multicast
   address of EXCLUDE, the router informs the routing protocol that
   there is no longer a listener on the link interested in traffic from
   this source.

   When a router filter mode for a multicast address is EXCLUDE, source
   records are only deleted when the Multicast Address Timer expires or
   when newly received Multicast Address Records modify the source
   record list of the router.  Section 6.3 describes the actions that
   should be taken depending on the value of the source timer.




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6.3.  MLDv2 Source Specific Forwarding Rules

   When a multicast router receives a datagram from a source destined to
   a particular multicast address, a decision has to be made whether to
   forward the datagram on an attached link or not.  The multicast
   routing protocol in use is in charge of this decision, and should use
   the MLDv2 information to ensure that all sources/multicast addresses
   that have listeners on a link are forwarded to that link. MLDv2
   information does not override multicast routing information; for
   example, if the MLDv2 filter mode for a multicast address is EXCLUDE,
   a router may still forward packets for excluded sources to a transit
   link.

   To summarize, the following table describes the forwarding
   suggestions made by MLDv2 to the routing protocol for traffic
   originating from a source destined to a multicast address.  It also
   summarizes the actions taken upon the expiration of a source timer
   based on the router filter mode of the multicast address.

    Multicast Address
       Filter Mode      Source Timer Value           Action
       -----------      ------------------           ------

        INCLUDE            TIMER > 0         Suggest to forward traffic
                                             from source

        INCLUDE            TIMER == 0        Suggest to stop forwarding
                                             traffic from source and
                                             remove source record. If
                                             there are no more source
                                             records, delete multicast
                                             address record

        INCLUDE         No source element    Suggest to not forward
                                             traffic from source

        EXCLUDE            TIMER > 0         Suggest to forward traffic
                                             from source

        EXCLUDE            TIMER == 0        Suggest to not forward
                                             traffic from source
                                             (DO NOT remove record)

        EXCLUDE         No Source Element    Suggest to forward traffic
                                             from source


6.4.  Action on Reception of Reports

   Upon reception of an MLD message containing a Report, the router
   checks if the source address of the message is a valid link-local
   address, if the Hop Limit is set to 1, and if the Router Alert option

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   is present in the Hop-By-Hop Options header of the IPv6 packet.  If
   any of these checks fails, the packet is dropped.

   If the validity of the MLD message is verified, the router starts to
   process the Report.


6.4.1.  Reception of Current State Records

   When receiving Current State Records, a router updates both its
   multicast address and source timers.  In some circumstances, the
   reception of a type of multicast address record will cause the router
   filter mode for that multicast address to change.  The table below
   describes the actions, with respect to state and timers, that occur
   to a router's state upon reception of Current State Records.

   The following notation is used to describe the updating of source
   timers.  The notation ( A, B ) will be used to represent the total
   number of sources for a particular multicast address, where

   A = set of source records whose source timers > 0
        (Sources that at least one node has requested to be forwarded)
   B = set of source records whose source timers = 0
        (Sources that MLD will suggest to the routing protocol not to
         forward)

   Note that there will be two sets only when a router's filter mode for
   a multicast address is EXCLUDE.  When the filter mode is INCLUDE, a
   single set is used to describe the set of sources requested to be
   forwarded (e.g. simply ( A ) ).

   In the following tables, abbreviations are used for several variables
   (all of which are described in detail in section 8).  The variable
   MALI is an abbreviation for the Multicast Address Listening Interval
   which is the time in which multicast address listening will time out.
   The variable LLQT is an abbreviation for the Last Listener Query
   Time, which is the total time the router should wait for a report,
   after having sent a first query.  During this time, the router should
   send [Last Member Query Count]-1 retransmissions of the query.  LLQT
   represents the "leave latency", or the difference between the
   transmission of a listener state change and the change in the
   information given to the routing protocol.

   Within the "Actions" section of the router state tables, we use the
   notation '(A)=J', which means that the set A of source records should
   have their source timers set to value J.  'Delete (A)' means that the
   set A of source records should be deleted.  'Multicast Address Timer
   = J' means that the Multicast Address Timer for the multicast address
   should be set to value J.




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    Router State   Report Received  New Router State   Actions
    ------------   ---------------  ----------------   -------

    INCLUDE (A)       IS_IN (B)     INCLUDE (A+B)      (B)=MALI

    INCLUDE (A)       IS_EX (B)     EXCLUDE (A*B, B-A) (B-A)=0
                                                       Delete (A-B)
                                                       Multicast Address
                                                       Timer = MALI

    EXCLUDE (X,Y)     IS_IN (A)     EXCLUDE (X+A, Y-A) (A)=MALI

    EXCLUDE (X,Y)     IS_EX (A)     EXCLUDE (A-Y, Y*A) (A-X-Y)=MALI
                                                       Delete (X-A)
                                                       Delete (Y-A)
                                                       Multicast Address
                                                       Timer = MALI


6.4.2.  Reception of Filter Mode Change and Source List Change Records

   When a change in the global state of a multicast address occurs in a
   node, the node sends either a Source List Change Record or a Filter
   Mode Change Record for that multicast address.  As with Current State
   Records, routers must act upon these records and possibly change
   their own state to reflect the new listening state of the link.

   Routers must query sources or multicast addresses that are requested
   to be no longer forwarded.  When a router queries or receives a query
   for a specific set of sources, it lowers its source timers for those
   sources to a small interval of Last Listener Query Time milliseconds.
   If multicast address records are received in response to the queries
   which express interest in listening the queried sources, the
   corresponding timers are updated.

   Multicast Address Specific queries can also be used in order to
   enable a fast transition of a router from EXCLUDE to INCLUDE mode, in
   case a received Multicast Address Record motivates this action.  The
   Multicast Address Timer for that multicast address will be lowered to
   a small interval of Last Listener Query Time milliseconds.  If any
   multicast address records expressing EXCLUDE mode interest in the
   multicast address are received within this interval, the multicast
   address timer is updated and the suggestion to the routing protocol
   to forward the multicast address stands without any interruption.  If
   not, the router will switch to INCLUDE filter mode for that multicast
   address.

   During a query period (i.e. Last Listener Query Time milliseconds)
   the MLD component in the router continues to suggest to the routing
   protocol that it forwards traffic from the multicast addresses or
   sources that it is querying.  It is not until after Last Listener
   Query Time milliseconds without receiving a record expressing

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   interest in the queried multicast address or sources that the router
   may prune the multicast address or sources from the link.

   The following table describes the changes in multicast address state
   and the action(s) taken when receiving either Filter Mode Change or
   Source List Change Records.  This table also describes the queries
   which are sent by the router in Querier state when a particular
   report is received.

   We use the following notation for describing the queries which are
   sent.  We use the notation 'Q(MA)' to describe a Multicast Address
   Specific Query to the MA multicast address.  We use the notation
   'Q(MA,A)' to describe a Multicast Address and Source Specific Query
   to the MA multicast address with source list A.  If source list A is
   null as a result of the action (e.g. A*B) then no query is sent as a
   result of the operation.

   In order to maintain protocol robustness, queries defined in the
   Actions column of the table below need to be transmitted [Last
   Listener Query Count] times, once every [Last Listener Query
   Interval] period.

   If while scheduling new queries, there are already pending
   queries to be retransmitted for the same multicast address, the new
   and pending queries have to be merged.  In addition, received host
   reports for a multicast address with pending queries may affect the
   contents of those queries.  Section 6.6.3. describes the process of
   building and maintaining the state of pending queries.

   Router State  Report Received  New Router State     Actions
   ------------  ---------------  ----------------     -------
   INCLUDE (A)     ALLOW (B)      INCLUDE (A+B)        (B)=MALI

   INCLUDE (A)     BLOCK (B)      INCLUDE (A)          Send Q(MA,A*B)

   INCLUDE (A)     TO_EX (B)      EXCLUDE (A*B,B-A)    (B-A)=0
                                                       Delete (A-B)
                                                       Send Q(MA,A*B)
                                                       Multicast Address
                                                       Timer = MALI

   INCLUDE (A)     TO_IN (B)      INCLUDE (A+B)        (B)=MALI
                                                       Send Q(MA,A-B)

   EXCLUDE (X,Y)   ALLOW (A)      EXCLUDE (X+A,Y-A)    (A)=MALI

   EXCLUDE (X,Y)   BLOCK (A)      EXCLUDE (X+(A-Y),Y)  (A-X-Y)=Multicast
                                                       Address Timer
                                                       Send Q(MA,A-Y)




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   EXCLUDE (X,Y)   TO_EX (A)      EXCLUDE (A-Y,Y*A)    (A-X-Y)=Multicast
                                                       Address Timer
                                                       Delete (X-A)
                                                       Delete (Y-A)
                                                       Send Q(MA,A-Y)
                                                       Multicast Address
                                                       Timer = MALI

   EXCLUDE (X,Y)   TO_IN (A)      EXCLUDE (X+A,Y-A)    (A)=MALI
                                                       Send Q(MA,X-A)
                                                       Send Q(MA)


6.5.  Switching Router Filter Modes

   The multicast address timer is used as a mechanism for transitioning
   the router filter mode from EXCLUDE to INCLUDE.

   When a multicast address timer expires with a router filter mode of
   EXCLUDE, a router assumes that there are no nodes with a *filter
   mode* of EXCLUDE present on the attached link.  When a router's
   filter mode for a multicast address is EXCLUDE and the multicast
   address timer expires, the router filter mode for the multicast
   address transitions to INCLUDE.

   A router uses source records with running source timers as its state
   for the switch to a filter mode of INCLUDE.  If there are any source
   records with source timers greater than zero (i.e. requested to be
   forwarded), a router switches to filter mode of INCLUDE using those
   source records.  Source records whose timers are zero (from the
   previous EXCLUDE mode) are deleted.

   For example, if a router's state for a multicast address is
   EXCLUDE(X,Y) and the multicast address timer expires for that
   multicast address, the router switches to filter mode of INCLUDE with
   state INCLUDE(X).


6.6.  Action on Reception of Queries

   Upon reception of an MLD message containing a Query, the router
   checks if the source address of the message is a valid link-local
   address, if the Hop Limit is set to 1, and if the Router Alert option
   is present in the Hop-By-Hop Options header of the IPv6 packet.  If
   any of these checks fails, the packet is dropped.

   If the validity of the MLD message is verified, the router starts to
   process the Query.





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6.6.1.  Timer Updates

   When a router sends or receives a query with a clear Suppress
   Router-Side Processing flag, it must update its timers to reflect the
   correct timeout values for the multicast address or sources being
   queried.  The following table describes the timer actions when
   sending or receiving a Multicast Address Specific or Multicast
   Address and Source Specific Query with the Suppress Router-Side
   Processing flag not set.

    Query       Action
    -----       ------
    Q(MA,A)     Source Timers for sources in A are lowered to LLQT
    Q(MA)       Multicast Address Timer is lowered to LLQT

   When a router sends or receives a query with the Suppress Router-Side
   Processing flag set, it will not update its timers.


6.6.2.  Querier Election

   MLDv2 elects a single router to be in Querier state per subnet using
   the same querier election mechanism as MLDv1, namely by IPv6 address.
   When a router receives a query with a lower IPv6 address, it sets the
   Other Querier Present timer to Other Querier Present Timeout and
   ceases to send queries on the link if it was the previously elected
   querier.  After its Other Querier Present timer expires, it should
   begin sending General Queries.

   If a router receives an older version query, it MUST use the older
   version of MLD on the link.  For a detailed description of
   compatibility issues between MLD versions see section 7.


6.6.3  Building and Sending Specific Queries


6.6.3.1.  Building and Sending Multicast Address Specific Queries

   When a table action "Send Q(MA)" is encountered, then the multicast
   address timer must be lowered to LLQT. The router must then
   immediately send a Multicast Address Specific query as well as
   schedule [Last Listener Query Count - 1] query retransmissions to be
   sent every [Last Listener Query Interval] over [Last Listener Query
   Time].

   When transmitting a Multicast Address Specific Query, if the
   multicast address timer is larger than LLQT, the "Suppress
   Router-Side Processing" bit is set in the query message.




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6.6.3.2.  Building and Sending Multicast Address and Source Specific
          Queries

   When a table action "Send Q(MA,X)" is encountered by a querier in the
   table in section 6.4.2, the following actions must be performed for
   each of the sources in X that send to multicast address MA, with
   source timer larger than LLQT:

   o Lower source timer to LLQT.

   o Set number of retransmissions for each source to [Last Listener
     Query Count].

   The router must then immediately send a Multicast Address and Source
   Specific Query as well as schedule [Last Listener Query Count -1]
   query retransmissions to be sent every [Last Listener Query
   Interval], over [Last Listener Query Time].  The contents of these
   queries are calculated as follows.

   When building a Multicast Address and Source Specific Query for a
   multicast address MA, two separate query messages are sent for the
   multicast address.  The first one has the "Suppress Router-Side
   Processing" bit set and contains all the sources with retransmission
   state and timers greater than LLQT. The second has the "Suppress
   Router-Side Processing" bit clear and contains all the sources with
   retransmission state and timers lower or equal to LLQT. If either of
   the two calculated messages does not contain any sources, then its
   transmission is suppressed.

   Note: If a Multicast Address Specific query is scheduled to be
   transmitted at the same time as a Multicast Address and Source
   specific query for the same multicast address, then transmission of
   the Multicast Address and Source specific message with the "Suppress
   Router-Side Processing" bit set may be suppressed.


7.  Interoperation with Older Versions of MLD

   MLD version 2 hosts and routers interoperate with hosts and routers
   that have not yet been upgraded to MLDv2.  This compatibility is
   maintained by hosts and routers taking appropriate actions depending
   on the versions of MLD operating on hosts and routers within a
   network.


7.1.  Query Version Distinctions

   The MLD version of a Multicast Listener Query message is determined
   as follows:

   MLDv1 Query: length = 24 octets


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   MLDv2 Query: length >= 28 octets

   Query messages that do not match any of the above conditions (e.g., a
   Query of length 26 octets) MUST be silently ignored.


7.2.  Multicast Address Listener Behavior


7.2.1.  In the Presence of Older Version Queriers

   In order to be compatible with older version routers, MLDv2 hosts
   MUST operate in version 1 compatibility mode.  MLDv2 hosts MUST keep
   state per local interface regarding the compatibility mode of each
   attached link.  A host's compatibility mode is determined from the
   Host Compatibility Mode variable which can be in one of the two
   states: MLDv1 or MLDv2.  This variable is kept per interface and is
   dependent on the version of General Queries heard on that interface
   as well as on the Older Version Querier Present timer for the
   interface.  This timer is set to Older Version Querier Present
   Timeout seconds whenever an MLDv1 Multicast Address Listener Query is
   received.

   The Host Compatibility Mode of an interface changes whenever an older
   version General Query (than the current compatibility mode) is heard
   or when certain timer conditions occur.  When the Older Version
   Querier Present timer expires, a host switches to Host Compatibility
   mode of MLDv2.

   The Host Compatibility Mode variable is based on whether an older
   version General Query was heard in the last Older Version Querier
   Present Timeout seconds.  The Host Compatibility Mode is set
   depending on the following:

    Host Compatibility Mode                  Timer State
    -----------------------                  -----------

          MLDv2 (default)      Older Version Querier Present not running

          MLDv1                Older Version Querier Present running

   If a host receives a query which causes its Older Version Querier
   Present timer to be updated and correspondingly its compatibility
   mode, it should switch compatibility modes immediately.

   When Host Compatibility Mode is MLDv2, a host acts using the MLDv2
   protocol on that interface.  When Host Compatibility Mode is MLDv1, a
   host acts in MLDv1 compatibility mode, using only the MLDv1 protocol,
   on that interface.

   An MLDv1 router will send General Queries with the Maximum Response
   Code set to the desired Maximum Response Delay, i.e. the full range
   of this field is linear and the exponential algorithm described in

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   section 4.1.3. is not used.

   Whenever a host changes its compatibility mode, it cancels all its
   pending response and retransmission timers.


7.2.2.  In the Presence of Older Version Multicast Address Listeners

   An MLDv2 host may be placed on a link where there are hosts that
   have not yet been upgraded to MLDv2.  A host MAY allow its MLDv2
   Multicast Listener Report to be suppressed by Version 1 Multicast
   Listener Report.


7.3.  Multicast Router Behavior


7.3.1.  In the Presence of Older Version Queriers

   MLDv2 routers may be placed on a network where there is at least one
   router that has not yet been upgraded to MLDv2.  The following
   requirements apply:

   o If an older version of MLD is present on routers, the querier MUST
     use the lowest version of MLD present on the network.  This must be
     administratively assured; routers that desire to be compatible with
     MLDv1 MUST have a configuration option to act in MLDv1 mode.  When
     in MLDv1 mode, routers MUST send periodic Queries truncated at the
     Multicast Address field (i.e. 24 bytes long), and SHOULD also warn
     about receiving an MLDv2 query (such warnings must be
     rate-limited). They also MUST fill in the Maximum Response Delay in
     the Maximum Response Code field, i.e. the exponential algorithm
     described in section 4.1.3. is not used.

   o If a router is not explicitly configured to use MLDv1 and hears an
     MLDv1 General Query, it SHOULD log a warning.  These warnings MUST
     be rate-limited.


7.3.2.  In the Presence of Older Version Multicast Address Listeners

   MLDv2 routers may be placed on a network where there are hosts that
   have not yet been upgraded to MLDv2.  In order to be compatible with
   older version hosts, MLDv2 routers MUST operate in version 1
   compatibility mode.  MLDv2 routers keep a compatibility mode per
   multicast address record.  The compatibility mode of a multicast
   address is determined from the Multicast Address Compatibility Mode
   variable which can be in one of the two following states: MLDv1 or
   MLDv2.  This variable is kept per multicast address record and is
   dependent on the version of Multicast Listener Reports heard for that
   multicast address as well as the Older Version Host Present timer for
   the multicast address.  This timer is set to Older Version Host

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   Present Timeout seconds whenever an MLDv1 Multicast Listener Report
   is received.

   The Multicast Address Compatibility Mode of a multicast address
   record changes whenever an older version report (than the current
   compatibility mode) is heard or when certain timer conditions occur.
   When the Older Version Host Present timer expires, a router switches
   to Multicast Address Compatibility mode of MLDv2.  Note that when a
   router switches back to MLDv2 mode for a multicast address, it takes
   some time to regain source-specific state information.
   Source-specific information will be learned during the next General
   Query, but sources that should be blocked will not be blocked until
   [Multicast Address Listening Interval] after that.

   The Multicast Address Compatibility Mode variable is based on whether
   an older version report was heard in the last Older Version Host
   Present Timeout seconds.  The Multicast Address Compatibility Mode is
   set depending on the following:

   Multicast Address Compatibility Mode          Timer State
   ------------------------------------          -----------

               MLDv2 (default)             Older Version Host Present
                                                          not running

               MLDv1                       Older Version Host Present
                                                              running

   If a router receives a report which causes its older Host Present
   timer to be updated and correspondingly its compatibility mode, it
   SHOULD switch compatibility modes immediately.

   When Multicast Address Compatibility Mode is MLDv2, a router acts
   using the MLDv2 protocol for that multicast address.  When Multicast
   Address Compatibility Mode is MLDv1, a router internally translates
   the following MLDv1 messages for that multicast address to their
   MLDv2 equivalents:

      MLDv1 Message                 MLDv2 Equivalent
      -------------                 ----------------

         Report                        IS_EX( {} )

         Done                          TO_IN( {} )

   MLDv2 BLOCK messages are ignored, as are source-lists in TO_EX()
   messages (i.e. any TO_EX() message is treated as TO_EX( {} )).  On
   the other hand, the Querier router continues to send MLDv2 queries,
   regardless of its compatibility mode.




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8.  List of Timers, Counters, and their Default Values

   Most of these timers are configurable.  If non-default settings are
   used, they MUST be consistent among all nodes on a single link. Note
   that parentheses are used to group expressions to make the algebra
   clear.


8.1.  Robustness Variable

   The Robustness Variable allows tuning for the expected packet loss on
   a link.  If a link is expected to be lossy, the Robustness Variable
   may be increased.  MLD is robust to (Robustness Variable - 1) packet
   losses.  The Robustness Variable MUST NOT be zero, and SHOULD NOT be
   one.  Default: 2.


8.2.  Query Interval

   The Query Interval is the interval between General Queries sent by
   the Querier.  Default: 125 seconds.

   By varying the [Query Interval], an administrator may tune the number
   of MLD messages on the link; larger values cause MLD Queries to be
   sent less often.


8.3.  Query Response Interval

   The Maximum Response Delay used to calculate the Maximum Response
   Code inserted into the periodic General Queries. Default: 10000 (10
   seconds)

   By varying the [Query Response Interval], an administrator may tune
   the burstiness of MLD messages on the link; larger values make the
   traffic less bursty, as host responses are spread out over a larger
   interval.  The number of seconds represented by the [Query Response
   Interval] must be less than the [Query Interval].


8.4.  Multicast Address Listening Interval

   The Multicast Address Listening Interval (MALI) is the amount of time
   that must pass before a multicast router decides there are no more
   listeners of a multicast address or a particular source on a link.
   This value MUST be ((the Robustness Variable) times (the Query
   Interval)) plus (one Query Response Interval).


8.5.  Other Querier Present Timeout

   The Other Querier Present Timeout is the length of time that must

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   pass before a multicast router decides that there is no longer
   another multicast router which should be the querier.  This value
   MUST be ((the Robustness Variable) times (the Query Interval)) plus
   (one half of one Query Response Interval).


8.6.  Startup Query Interval

   The Startup Query Interval is the interval between General Queries
   sent by a Querier on startup.  Default: 1/4 the Query Interval.


8.7.  Startup Query Count

   The Startup Query Count is the number of Queries sent out on startup,
   separated by the Startup Query Interval.  Default: the Robustness
   Variable.


8.8.  Last Listener Query Interval

   The Last Listener Query Interval is the Maximum Response Delay used
   to calculate the Maximum Response Code inserted into Multicast
   Address Specific Queries sent in response to Version 1 Multicast
   Listener Done messages.  It is also the Maximum Response Delay used
   in calculating the Maximum Response Code inserted into Multicast
   Address and Source Specific Query messages.  Default: 1000 (1
   second).

   Note that for values of LLQI greater than 32.768 seconds, a limited
   set of values can be represented, corresponding to sequential values
   of Maximum Response Code.  When converting a configured time to a
   Maximum Response Code value, it is recommended to use the exact value
   if possible, or the next lower value if the requested value is not
   exactly representable.

   This value may be tuned to modify the "leave latency" of the link.  A
   reduced value results in reduced time to detect the departure of the
   last listener for a multicast address or source.


8.9.  Last Listener Query Count

   The Last Listener Query Count is the number of Multicast Address
   Specific Queries sent before the router assumes there are no local
   listeners.  The Last Listener Query Count is also the number of
   Multicast Address and Source Specific Queries sent before the router
   assumes there are no listeners for a particular source.  Default: the
   Robustness Variable.




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8.10.  Last Listener Query Time

   The Last Listener Query Time is the time value represented by the
   Last Listener Query Interval, multiplied by the Last Listener Query
   Count.  It is not a tunable value, but may be tuned by changing its
   components.


8.11.  Unsolicited Report Interval

   The Unsolicited Report Interval is the time between repetitions of a
   node's initial report of interest in a multicast address.  Default: 1
   second.


8.12.  Older Version Querier Present Timeout

   The Older Version Querier Present Timeout is the time-out for
   transitioning a host back to MLDv2 mode once an older version query
   is heard.  When an older version query is received, hosts set their
   Older Version Querier Present Timer to Older Version Querier Present
   Timeout.

   This value MUST be ((the Robustness Variable) times (the Query
   Interval in the last Query received)) plus (one Query Response
   Interval).


8.13.  Older Version Host Present Timeout

   The Older Version Host Present Timeout is the time-out for
   transitioning a multicast address back to MLDv2 mode once an older
   version report is sent for that multicast address.  When an older
   version report is received, routers set their Older Version Host
   Present Timer to Older Version Host Present Timeout.

   This value MUST be ((the Robustness Variable) times (the Query
   Interval)) plus (one Query Response Interval).


8.14.  Configuring timers

   This section is meant to provide advice to network administrators on
   how to tune these settings to their network.  Ambitious router
   implementations might tune these settings dynamically based upon
   changing characteristics of the network.


8.14.1.  Robustness Variable

   The Robustness Variable tunes MLD to expected losses on a link.
   MLDv2 is robust to (Robustness Variable - 1) packet losses, e.g. if

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   the Robustness Variable is set to the default value of 2, MLDv2 is
   robust to a single packet loss but may operate imperfectly if more
   losses occur.  On lossy links, the Robustness Variable should be
   increased to allow for the expected level of packet loss.  However,
   increasing the Robustness Variable increases the leave latency of the
   link (the time between when the last listener stops listening to a
   source or multicast address and when the traffic stops flowing).


8.14.2.  Query Interval

   The overall level of periodic MLD traffic is inversely proportional
   to the Query Interval.  A longer Query Interval results in a lower
   overall level of MLD traffic.  The Query Interval MUST be equal to or
   longer than the Maximum Response Delay used to calculate the Maximum
   Response Code inserted in General Query messages.


8.14.3.  Maximum Response Delay

   The burstiness of MLD traffic is inversely proportional to the
   Maximum Response Delay.  A longer Maximum Response Delay will spread
   Report messages over a longer interval.  However, a longer Maximum
   Response Delay in Multicast Address Specific and Multicast Address
   And Source Specific Queries extends the leave latency (the time
   between when the last listener stops listening to a source or
   multicast address and when the traffic stops flowing.)  The expected
   rate of Report messages can be calculated by dividing the expected
   number of Reporters by the Maximum Response Delay.  The Maximum
   Response Delay may be dynamically calculated per Query by using the
   expected number of Reporters for that Query as follows:

   Query Type                         Expected number of Reporters
   ----------                         ----------------------------

   General Query                      All nodes on link

   Multicast Address Specific Query   All nodes on the link that had
                                      expressed interest in the
                                      multicast address

   Multicast Address and Source       All nodes on the link that had
    Specific Query                    expressed interest in the source
                                      and multicast address

   A router is not required to calculate these populations or tune the
   Maximum Response Delay dynamically; these are simply guidelines.






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9.  Security Considerations

   We consider the ramifications of a forged message of each type. Note
   that the requirement that nodes verify that the IPv6 Source Address
   of all received MLD messages is a link-local address (or the
   unspecified address) defends them from acting on forged MLD messages
   originated off-link. We discuss thus only the effects of on-link
   forgery.


9.1.  Query Message

   A forged Query message from a machine with a lower IPv6 address than
   the current Querier will cause Querier duties to be assigned to the
   forger.  If the forger then sends no more Query messages, other
   routers' Other Querier Present timer will time out and one will
   resume the role of Querier.  During this time, if the forger ignores
   Multicast Listener Done Messages, traffic might flow to multicast
   addresses with no listeners for up to [Multicast Address Listener
   Interval].

   A forged Version 1 Query message will put MLDv2 receivers on that
   link in version 1 compatibility mode.  This scenario can be avoided
   by providing MLDv2 hosts with a configuration option to ignore
   Version 1 messages completely.

   A DoS attack on a node could be staged through forged Multicast
   Address and Source Specific Queries. The attacker can find out about
   the listening state of a specific node with a general query. After
   that it could send a large number of Multicast Address and Source
   Specific queries, each with a large source list and/or long Maximum
   Response Delay.  The node will have to store and maintain the sources
   specified in all of those queries for as long as it takes to send the
   delayed response.  This would consume both memory and CPU cycles in
   order to augment the recorded sources with the source lists included
   in the successive queries.

   To protect against such a DoS attack, a node stack implementation
   could restrict the number of Multicast Address and Source Specific
   Queries per multicast address within this interval, and/or record
   only a limited number of sources.


9.2.  Current State Report messages

   A forged Report message may cause multicast routers to think there
   are listeners of a multicast address on a link when there are not.
   However, since listening to a multicast address on a host is
   generally an unprivileged operation, a local user may trivially gain
   the same result without forging any messages.

   A forged Version 1 Report Message may put a router into "older

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   version listener present" state for a particular multicast address,
   meaning that the router will ignore MLDv2 source specific state
   messages.  This can cause traffic to flow from unwanted sources for
   up to [Multicast Address Listener Interval].  This can be solved by
   providing routers with a configuration switch to ignore Version 1
   messages completely.  This breaks automatic compatibility with
   Version 1 hosts, so should only be used in situations where source
   include and exclude is critical.

9.3.  State Change Report messages

   A forged State Change Report message will cause the Querier to send
   out Multicast Address Specific or Multicast Address and Source
   Specific Queries for the multicast address in question.  This causes
   extra processing on each router and on each listener of the multicast
   address, but can not cause loss of desired traffic.


10.  IANA Considerations

   A special IP multicast address called *all MLDv2-capable routers*
   should be allocated by IANA, as stated in section 4.2.14.  Version 2
   Multicast Listener Reports will be sent to this special address.
   IANA should allocate as well a special ICMP message type number for
   Version 2 Multicast Listener Report messages, as specified in section
   4.


11.  References


11.1.  Normative References

   [RFC 2026]    Bradner, S., "The Internet Standards Process - Revision
                 3", BCP 9, RFC 2026, November 1996.

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

   [RFC 2373]    Hinden, R. and S. Deering, "IP Version 6 Addressing
                 Architecture", RFC 2373, July 1998.

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

   [RFC 2463]    Conta, A. and S. Deering, "Internet Control Message
                 Protocol (ICMPv6) for the Internet Protocol Version 6
                 (IPv6) Specification", RFC 2463, December 1998.

   [RFC 2464]    Crawford, M., "Transmission of IPv6 Packets over
                 Ethernet Networks", RFC 2464, December 1998.

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   [RFC 2710]    Deering, S., Fenner, W., Haberman, B., "Multicast
                 Listener Discovery (MLD) for IPv6", RFC 2710, November
                 1999.

   [RFC 2711]    Partridge, C., Jackson, A., "IPv6 Router Alert Option,"
                 RFC 2711, November 1999.


11.2.  Informative References

   [FILTER-API]  Thaler, D., B. Fenner, and B. Quinn, "Socket Interface
                 Extensions for Multicast Source Filters", Work in
                 Progress, June 2002.  (draft-ietf-magma-msf-api-03.txt)

   [RFC 2236]    Fenner, W., "Internet Group Management Protocol,
                 Version 2", RFC 2236, November 1997.

   [RFC 3376]    Cain, B., Deering, S., Fenner,B., Kouvelas, I.,
                 Thyagarajan, A., "Internet Group Management Protocol,
                 Version 3", RFC 3376, May 2002.

   [SSM]         Bhattacharyya, S., et al., "An Overview of Source-
                 Specific Multicast (SSM)", Work in progress, November
                 2002.  (draft-ietf-ssm-overview-04.txt)


12.  Editors' Contact Information

   Rolland Vida
   LIP6, Universite Pierre et Marie Curie
   8, rue du Capitaine Scott
   75015 Paris, France
   phone: +33-1.44.27.71.26
   email: Rolland.Vida@lip6.fr

   Luis Henrique Maciel Kosmalski Costa
   LIP6, Universite Pierre et Marie Curie
   8, rue du Capitaine Scott
   75015 Paris, France
   phone: +33-1.44.27.87.72
   email: Luis.Costa@lip6.fr


13.  Authors

This document was written by:

   Rolland Vida, LIP6
   email: Rolland.Vida@lip6.fr

   Luis Henrique Maciel Kosmalski Costa, LIP6
   email: Luis.Costa@lip6.fr

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   Serge Fdida, LIP6
   email: Serge.Fdida@lip6.fr

   Steve Deering, Cisco Systems, Inc.
   email: deering@cisco.com

   Bill Fenner, AT&T Labs - Research
   email: fenner@research.att.com

   Isidor Kouvelas, Cisco Systems, Inc.
   email: kouvelas@cisco.com

   Brian Haberman, Caspian Networks
   email: bkhabs@nc.rr.com

   This document is the translation of [RFC 3376] for IPv6 semantics.
   It was elaborated based on the translation of [RFC 2236] into
   [RFC 2710].


14.  Acknowledgements

   We would like to thank Francis Dupont, Konstantin Kabassanov, Remi
   Zara, Hitoshi Asaeda, Shinsuke Suzuki, and Erik Nordmark for their
   valuable comments and suggestions on this document.




























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APPENDIX A.  Design Rationale


A.1  The Need for State Change Messages

   MLDv2 specifies two types of Multicast Listener Reports: Current
   State and State Change.  This section describes the rationale for the
   need for both these types of Reports.

   Routers need to distinguish Multicast Listener Reports that were sent
   in response to Queries from those that were sent as a result of a
   change in interface state.  Multicast Listener Reports that are sent
   in response to Multicast Address Listener Queries are used mainly to
   refresh the existing state at the router; they typically do not cause
   transitions in state at the router.  Multicast Listener Reports that
   are sent in response to changes in interface state require the router
   to take some action in response to the received report (see Section
   6.4).

   The inability to distinguish between the two types of reports would
   force a router to treat all Multicast Listener Reports as potential
   changes in state and could result in increased processing at the
   router as well as an increase in MLD traffic on the link.


A.2  Host Suppression

   In MLDv1, a host would cancel sending a pending multicast listener
   report if a similar report was observed from another listener on the
   link.  In MLDv2, this suppression of multicast listener reports has
   been removed.  The following points explain the reasons behind this
   decision.

   1. Routers may want to track per-host multicast listener status on an
      interface. This allows routers to implement fast leaves (e.g.  for
      layered multicast congestion control schemes) as well as track
      listener status for possible accounting purposes.

   2. Multicast Listener Report suppression does not work well on
      bridged LANs.  Many bridges and Layer2/Layer3 switches that
      implement MLD snooping do not forward MLD messages across LAN
      segments in order to prevent multicast listener report
      suppression.  Removing multicast listener report suppression eases
      the job of these MLD snooping devices.

   3. By eliminating multicast listener report suppression, hosts have
      fewer messages to process; this leads to a simpler state machine
      implementation.

   4. In MLDv2, a single multicast listener report now bundles multiple
      multicast address records to decrease the number of packets sent.
      In comparison, the previous version of MLD required that each

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      multicast address be reported in a separate message.


A.3 Switching router filter modes from EXCLUDE to INCLUDE

   If there exist nodes in both EXCLUDE and INCLUDE modes for a single
   multicast address on a link, the router must be in EXCLUDE mode as
   well (see section 6.2.1).  In EXCLUDE mode, a router forwards traffic
   from all sources unless that source exists in the exclusion source
   list.  If all nodes in EXCLUDE mode cease to exist, it would be
   desirable for the router to switch back to INCLUDE mode seamlessly
   without interrupting the flow of traffic to existing listeners.

   One of the ways to accomplish this is for routers to keep track of
   all sources that nodes that are in INCLUDE mode listen to, even
   though the router itself is in EXCLUDE mode.  If the multicast
   address timer now expires in EXCLUDE mode, it implies that there are
   no nodes in EXCLUDE mode on the link (otherwise a multicast listener
   report from that node would have refreshed the multicast address
   timer).  The router can then switch to INCLUDE mode seamlessly with
   the list of sources currently being forwarded in its source list.


APPENDIX B.  Summary of Changes from MLDv1

   The following is a summary of changes from MLDv1, specified in
   RFC 2710.

   o MLDv2 introduces source filtering.

   o The IP service interface of MLDv2 nodes is modified accordingly.
     It enables the specification of a filter mode and a source list.

   o An MLDv2 node keeps per-socket and per-interface multicast
     listening states that include a filter mode and a source list for
     each multicast address. This enables packet filtering based on a
     socket's multicast reception state.

   o MLDv2 state kept on routers includes a filter mode and a list of
     sources and source timers for each multicast address that has
     listeners on the link. MLDv1 routers kept only the list of
     multicast addresses.

   o Queries include additional fields (section 4.1).

   o The S flag (Suppress Router-Side Processing) is included in queries
     in order to fix robustness issues.

   o The Querier's Robustness Variable and Query Interval Code are
     included in Queries in order to synchronize all MLDv2 routers
     connected to the same link.


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   o A new Query type (Multicast Address and Source Specific Query) is
     introduced.

   o The Maximum Response Delay is not directly included in the Query
     anymore.  Instead, an exponential algorithm is used to calculate
     its value, based on the Maximum Response Code included in the
     Query.  The maximum value is increased from 65,535 seconds to about
     140 minutes.

   o Reports include Multicast Address Records.  Information on the
     listening state for several different multicast addresses can be
     included in the same Report message.

   o Reports are sent to the "all MLDv2-capable multicast routers"
     address, instead of the multicast address the host listens to, as
     in MLDv1.  This facilitates the operation of layer-2 snooping
     switches.

   o There is no "host suppression", as in MLDv1.  All nodes send Report
     messages.  This allows explicit listener tracking.

   o Unsolicited Reports, announcing changes in receiver listening
     state, are sent [Robustness Variable] times. RFC 2710 is less
     explicit.

   o There are no Done messages.

   o Interoperability with MLDv1 systems is achieved by MLDv2 state
     operations.

   o In order to ensure interoperability, hosts maintain a
     Host Compatibility Mode variable and an Older Version Querier
     Present timer per interface.  Routers maintain a Multicast Address
     Compatibility Mode variable and an Older Version Host Present timer
     per multicast address.


















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Full Copyright Statement

   Copyright (C) The Internet Society (2002).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.


























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