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RFC 3810

 
 
 

Multicast Listener Discovery Version 2 (MLDv2) for IPv6

Part 2 of 3, p. 13 to 36
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5.  Message Formats

   MLDv2 is a sub-protocol of ICMPv6, that is, MLDv2 message types are a
   subset 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

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   Address, an IPv6 Hop Limit of 1, and an IPv6 Router Alert option
   [RFC2711] 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.)  MLDv2 Reports can be sent with the source address set to
   the unspecified address [RFC3513], if a valid link-local IPv6 source
   address has not been acquired yet for the sending interface.  (See
   section 5.2.13. for details.)

   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 = decimal 143).  See
      section 11 for IANA considerations.

   To assure the interoperability with nodes that implement MLDv1 (see
   section 8), an implementation of MLDv2 must also support the
   following two message types:

   o  Version 1 Multicast Listener Report (Type = decimal 131) [RFC2710]

   o  Version 1 Multicast Listener Done (Type = decimal 132) [RFC2710]

   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.

5.1.  Multicast Listener Query Message

   Multicast Listener Queries are sent by multicast routers in Querier
   State to query the multicast listening state of neighboring
   interfaces.  Queries have the following format:

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     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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5.1.1.  Code

   Initialized to zero by the sender; ignored by receivers.

5.1.2.  Checksum

   The standard ICMPv6 checksum; it covers the entire MLDv2 message,
   plus a "pseudo-header" of IPv6 header fields [RFC2463].  For
   computing the checksum, the Checksum field is set to zero.  When a
   packet is received, the checksum MUST be verified before processing
   it.

5.1.3.  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 to listen 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 the tuning of the burstiness of MLD traffic on a link.

5.1.4.  Reserved

   Initialized to zero by the sender; ignored by receivers.

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5.1.5.  Multicast Address

   For a General Query, the Multicast Address field is set to zero.  For
   a Multicast Address Specific Query or Multicast Address and Source
   Specific Query, it is set to the multicast address being queried (see
   section 5.1.10, below).

5.1.7.  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.

5.1.8.  QRV (Querier's Robustness Variable)

   If non-zero, the QRV field contains the [Robustness Variable] value
   used by the Querier.  If the Querier's [Robustness Variable] exceeds
   7 (the maximum value of the QRV field), the QRV field 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 they use the default [Robustness
   Variable] value specified in section 9.1, or a statically configured
   value.

5.1.9.  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)

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   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 9.2.

5.1.10.  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 link with an MTU of 1500 octets, the IPv6 header (40 octets)
   together with the Hop-By-Hop Extension Header (8 octets) that
   includes the Router Alert option consume 48 octets; the MLD fields up
   to the Number of Sources (N) field consume 28 octets; thus, there are
   1424 octets left for source addresses, which limits the number of
   source addresses to 89 (1424/16).

5.1.11.  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.

5.1.12.  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 above.

5.1.13.  Query Variants

   There are three variants of the Query message:

   o  A "General Query" is sent by the Querier 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.

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   o  A "Multicast Address Specific Query" is sent by the Querier 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, while the Number of Sources (N) field is set to zero.

   o  A "Multicast Address and Source Specific Query" is sent by the
      Querier 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 of
      interest, while the Source Address [i] field(s) contain(s) the
      source address(es) of interest.

5.1.14.  Source Addresses for Queries

   All MLDv2 Queries MUST be sent with a valid IPv6 link-local source
   address.  If a node (router or host) receives a Query message with
   the IPv6 Source Address set to the unspecified address (::), or any
   other address that is not a valid IPv6 link-local address, it MUST
   silently discard the message and SHOULD log a warning.

5.1.15.  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. This
   might be useful, e.g., for debugging purposes.

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5.2.  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 = 143   |    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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   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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5.2.1.  Reserved

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

5.2.2.  Checksum

   The standard ICMPv6 checksum; it covers the entire MLDv2 message,
   plus a "pseudo-header" of IPv6 header fields [RFC2460, RFC2463].  In
   order to compute the checksum, the Checksum field is set to zero.
   When a packet is received, the checksum MUST be verified before
   processing it.

5.2.3.  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.

5.2.4.  Multicast Address Record

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

5.2.5.  Record Type

   It specifies the type of the Multicast Address Record.  See section
   5.2.12 for a detailed description of the different possible Record
   Types.

5.2.6.  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.

5.2.7.  Number of Sources (N)

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

5.2.8.  Multicast Address

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

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5.2.9.  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.

5.2.10.  Auxiliary Data

   The Auxiliary Data field, if present, contains additional information
   that pertain 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 such data
   present in any received Multicast Address Record.  The semantics and
   the internal encoding of the Auxiliary Data field are to be defined
   by any future version or extension of MLD that uses this field.

5.2.11.  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.

5.2.12.  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.  A MODE_IS_INCLUDE Record is never sent
             with an empty source list.

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        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
             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, whether the source list changes at the same
      time or not.  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

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           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
   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.

   Multicast Address Records with an unrecognized Record Type value MUST
   be silently ignored, with the rest of the report being processed.

   In the rest of this document, we use the following notation to
   describe the contents of a Multicast Address Record that pertains 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:

   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.

5.2.13.  Source Addresses for Reports

   An MLDv2 Report MUST be sent with a valid IPv6 link-local source
   address, or the unspecified address (::), if the sending interface
   has not acquired a valid link-local address yet.  Sending reports
   with the unspecified address is allowed to support the use of IP
   multicast in the Neighbor Discovery Protocol [RFC2461].  For
   stateless autoconfiguration, as defined in [RFC2462], a node is
   required to join several IPv6 multicast groups, in order to perform
   Duplicate Address Detection (DAD).  Prior to DAD, the only address

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   the reporting node has for the sending interface is a tentative one,
   which cannot be used for communication.  Thus, the unspecified
   address must be used.

   On the other hand, routers MUST silently discard a message that is
   not sent with a valid link-local address, without taking any action
   on the contents of the packet.  Thus, a Report is discarded if the
   router cannot identify the source address of the packet as belonging
   to a link connected to the interface on which the packet was
   received.  A Report sent with the unspecified address is also
   discarded by the router.  This enhances security, as unidentified
   reporting nodes cannot influence the state of the MLDv2 router(s).
   Nevertheless, the reporting node has modified its listening state for
   multicast addresses that are contained in the Multicast Address
   Records of the Report message.  From now on, it will treat packets
   sent to those multicast addresses according to this new listening
   state.  Once a valid link-local address is available, a node SHOULD
   generate new MLDv2 Report messages for all multicast addresses joined
   on the interface.

5.2.14.  Destination Addresses for Reports

   Version 2 Multicast Listener Reports are sent with an IP destination
   address of FF02:0:0:0:0:0:0:16, to which all MLDv2-capable multicast
   routers listen (see section 11 for IANA considerations related to
   this special destination address).  A node that operates in version 1
   compatibility mode (see details in section 8) 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.  This might be useful, e.g., for debugging
   purposes.

5.2.15.  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.

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   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, then:

   o  if its Type is not IS_EX or TO_EX, it is split into multiple
      Multicast Address Records; each such record contains a different
      subset of the source addresses, and is sent in a separate Report.

   o  if its Type is IS_EX or TO_EX, a single Multicast Address Record
      is sent, with as many source addresses as can fit; the remaining
      source addresses are not reported.  Although 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.

6.  Protocol Description for Multicast Address Listeners

   MLD is an asymmetric protocol, as it specifies 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; it receives and it
   responds to its own MLD messages, as well as to those of its
   neighbors.)  The multicast router part of MLDv2 is described in
   section 7.

   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 are connected to the same link.

   For interoperability with multicast routers that run the MLDv1
   protocol, nodes maintain a Host Compatibility Mode variable for each
   interface on which multicast reception is supported.  This section
   describes the behavior of multicast address listener nodes on
   interfaces for which Host Compatibility Mode = MLDv2.  The algorithm
   for determining Host Compatibility Mode, and the behavior if its
   value is set to MLDv1, are described in section 8.

   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).

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

   o  a change of the per-interface listening state, caused by a local
      invocation of IPv6MulticastListen;

   o  the firing of a specific timer;

   o  the reception of a Query.

   (Received MLD messages of types other than Query are silently
   ignored, except as required for interoperation with nodes that
   implement MLDv1.)

   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 9.

6.1.  Action on Change of Per-Interface State

   An invocation of IPv6MulticastListen may cause the multicast
   listening state of an interface to change, according to the rules in
   section 4.2.  Each such change affects the per-interface entry for a
   single multicast address.

   A change of per-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 per-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 an
   INCLUDE filter mode and an empty source list.

   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)

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

   To cover the possibility of the State Change Report being missed by
   one or more multicast routers, [Robustness Variable] - 1
   retransmissions are scheduled, through a Retransmission Timer, at
   intervals chosen at random from the range (0, [Unsolicited Report
   Interval]).

   If more changes to the same per-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 Report are calculated as follows:

   o  As for the first Report, the per-interface state for the affected
      multicast address before and after the latest change is compared.

   o  The records that express the difference are built according to the
      table above.  Nevertheless, these records are not transmitted in a
      separate message, but they are instead merged with the contents of
      the pending report, to create the new State Change Report.  The
      rules for calculating this merged 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.  These transmissions
   are necessary in order to ensure that each instance of state change
   is transmitted at least [Robustness Variable] times.

   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.
   Sources in retransmission state can be kept in a per multicast
   address Retransmission List, with a Source Retransmission Counter
   associated to each source in the list.  When a source is included in
   the list, its counter is set to [Robustness Variable].  Each time a
   State Change Report is sent the counter is decreased by one unit.
   When the counter reaches zero, the source is deleted from the
   Retransmission List for that multicast address.

   If the per-interface listening 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

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   applies 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. This can be done through a per multicast
   address Filter Mode Retransmission Counter.  When the filter mode
   changes, the counter is set to [Robustness Variable].  Each time a
   State Change Report is sent the counter is decreased by one unit.
   When the counter reaches zero, i.e., [Robustness Variable] State
   Change Reports with Filter Mode Change Records have been transmitted
   after the last filter mode change, and if source list changes have
   resulted in additional reports being scheduled, then the next State
   Change Report will include Source List Change Records.

   Each time a per-interface listening state change triggers the
   Immediate transmission of a new State Change Report, its contents are
   determined as follows.  If the report should contain a Filter Mode
   Change Record, i.e., the Filter Mode Retransmission Counter for that
   multicast address has a value higher than zero, 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, i.e., the Filter
   Mode Retransmission Counter for that multicast address is zero, an
   ALLOW and a BLOCK record is included.  The contents of these records
   are built according to the table below.

   Record   Sources included
   ------   ----------------
   TO_IN    All in the current per-interface state that must be
            forwarded
   TO_EX    All in the current per-interface state that must be
            blocked
   ALLOW    All with retransmission state (i.e., all sources from the
            Retransmission List) 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).

   The building of a scheduled State Change Report, triggered by the
   firing of a Retransmission Timer, instead of a per-interface
   listening state change, is described in section 6.3.

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6.2.  Action on Reception of a Query

   Upon reception of an MLD message that contains 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
   previously scheduled pending responses and, in many cases, schedule a
   combined response.  Therefore, for each of its interfaces on which it
   operates the listener part of the MLDv2 protocol, the node must be
   able to maintain the following state:

   o  an Interface Timer for scheduling responses to General Queries;

   o  a Multicast Address Timer for scheduling responses to Multicast
      Address (and Source) Specific Queries, for each multicast address
      the node has to report on;

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

   When a new valid General Query arrives on an interface, the node
   checks whether it has any per-interface listening state record to
   report on, or not.  Similarly, when a new valid Multicast Address
   (and Source) Specific Query arrives on an interface, the node checks
   whether it has a per-interface listening state record that
   corresponds to the queried multicast address (and source), or not. If
   it does, a delay for a response is randomly selected in the range (0,
   [Maximum Response Delay]), where Maximum Response Delay is derived
   from the Maximum Response Code inserted 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 rule
   is applied.)

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   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
      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.

6.3.  Action on Timer Expiration

   There are several timers that, upon expiration, trigger protocol
   actions on an MLDv2 Multicast Address Listener node.  All these
   actions are related to pending reports scheduled by the node.

   1. If the expired timer is the Interface Timer (i.e., there 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 4.2.  The
      Current State Record carries the multicast address and its

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      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 upon 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., there
      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.,
      there 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
      corresponding Current State Record are determined from the per-
      interface state and the pending response record, as specified in
      the following table:

                             set of sources in the
      per-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
   addresses, then no response is sent.  After the required Report
   messages have been generated, the source lists associated with any
   reported multicast addresses are cleared.

   4. If the expired timer is a Retransmission Timer for a multicast
      address (i.e., there is a pending State Change Report for that
      multicast address), the contents of the report are determined as
      follows.  If the report should contain a Filter Mode Change
      Record, i.e., the Filter Mode Retransmission Counter for that
      multicast address has a value higher than zero, then, if the

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      current filter mode of the interface is INCLUDE, a TO_IN record is
      included in the report; otherwise a TO_EX record is included.  In
      both cases, the Filter Mode Retransmission Counter for that
      multicast address is decremented by one unit after the
      transmission of the report.

      If instead the report should contain Source List Change Records,
      i.e., the Filter Mode Retransmission Counter for that multicast
      address is zero, an ALLOW and a BLOCK record is included.  The
      contents of these records are built according to the table below:

      Record   Sources included
      ------   ----------------
      TO_IN    All in the current per-interface state that must be
               forwarded
      TO_EX    All in the current per-interface state that must be
               blocked
      ALLOW    All with retransmission state (i.e., all sources from the
               Retransmission List) that must be forwarded.  For each
               included source, its Source Retransmission Counter is
               decreased with one unit after the transmission of the
               report.  If the counter reaches zero, the source is
               deleted from the Retransmission List for that multicast
               address.
      BLOCK    All with retransmission state (i.e., all sources from the
               Retransmission List) that must be blocked.  For each
               included source, its Source Retransmission Counter is
               decreased with one unit after the transmission of the
               report.  If the counter reaches zero, the source is
               deleted from the Retransmission List for that multicast
               address.

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

7.  Protocol Description 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 used by the router, in order to ensure
   that multicast packets are delivered to all links where there are
   interested listeners.

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   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 6.

   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 operates 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 [RFC2464]; in the case of an
   Ethernet interface that does not support the filtering of such a
   multicast address range, 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 the 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 only need to know that *at least one* node on an
   attached link listens to packets for a particular multicast address
   from a particular source; a multicast router is not required to
   *individually* keep track of the interests of each neighboring node.
   (Nevertheless, see Appendix A2 item 1 for discussion.)

   MLDv2 is backward compatible with the MLDv1 protocol.  For a detailed
   description of compatibility issues see section 8.

7.1.  Conditions for MLD Queries

   The behavior of a router that implements the MLDv2 protocol depends
   on whether there are several multicast routers on the same subnet, or
   not.  If it is the case, a querier election mechanism (described in
   section 7.6.2) is used to elect a single multicast router to be in
   Querier state.  All the multicast routers on the subnet listen to the
   messages sent by multicast address listeners, and maintain the same
   multicast listening information state, so that they can quickly and
   correctly take over the querier functionality, should the present
   Querier fail.  Nevertheless, it is only the Querier that sends
   periodical or triggered query messages on the subnet.

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   The Querier periodically sends General Queries 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, the Querier 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, the Querier sends 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 the Querier 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 the Querier in
   order 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 5.1.13 describes each query in more detail.


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