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RFC 3810

 
 
 

Multicast Listener Discovery Version 2 (MLDv2) for IPv6

Part 3 of 3, p. 37 to 62
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7.2.  MLD State Maintained by Multicast Routers

   Multicast routers that implement the MLDv2 protocol 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 on which MLD runs, 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, Filter Timer,
       Router 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 the Router 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.

7.2.1.  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
   summarize the total listening state of a multicast address to a
   minimum set such that all nodes' listening states are respected.  The
   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 7.4 describes the changes of the Router
   Filter Mode per Multicast Address Record received.

   A router is in INCLUDE mode for a specific multicast address on a
   given interface if all the listeners on the link interested in that
   address are in INCLUDE mode.  The router state is represented through
   the notation INCLUDE (A), where A is called the "Include List".  The
   Include List is the set of sources that one or more listeners on the
   link have requested to receive.  All the sources from the Include
   List will be forwarded by the router.  Any other source that is not
   in the Include List will be blocked by the router.

   A router is in EXCLUDE mode for a specific multicast address on a
   given interface if there is at least one listener in EXCLUDE mode
   interested in that address on the link.  Conceptually, when a
   Multicast Address Record is received, the Router Filter Mode for that

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   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 set to EXCLUDE. Nevertheless,
   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 Filter Timer expires, and is
   explained in detail in section 7.5.

   When the router is in EXCLUDE mode, the router state is represented
   through the notation EXCLUDE (X,Y), where X is called the "Requested
   List" and Y is called the "Exclude List".  All sources, except those
   from the Exclude List, will be forwarded by the router.  The
   Requested List has no effect on forwarding.  Nevertheless, it has to
   be maintained for several reasons, as explained in section 7.2.3.

   The exact handling of both the INCLUDE and EXCLUDE mode router state,
   according to the received reports, is presented in details in Tables
   7.4.1 and 7.4.2.

7.2.2.  Definition of Filter Timers

   The Filter Timer is only used when the router is in EXCLUDE mode for
   a specific multicast address, and it represents the time for the
   Router Filter Mode of the multicast address to expire and switch to
   INCLUDE mode.  A Filter Timer is a decrementing timer with a lower
   bound of zero.  One Filter Timer exists per multicast address record.
   Filter Timers are updated according to the types of Multicast Address
   Records received.

   If a Filter Timer expires, with the Router Filter Mode for that
   multicast address being EXCLUDE, it means that there are no more
   listeners in EXCLUDE mode on the attached link.  At this point, the
   router transitions to INCLUDE filter mode.  Section 7.5 describes the
   actions taken when a Filter Timer expires while in EXCLUDE mode.

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

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     Router               Filter
   Filter Mode          Timer Value          Actions/Comments
   -----------       -----------------       ----------------

     INCLUDE             Not Used            All listeners in
                                             INCLUDE mode.

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

     EXCLUDE             Timer == 0          No more listeners in
                                             EXCLUDE mode for the
                                             multicast address.
                                             If the Requested List
                                             is empty, delete
                                             Multicast Address
                                             Record.  If not, switch
                                             to INCLUDE filter mode;
                                             the sources in the
                                             Requested List are
                                             moved to the Include
                                             List, and the Exclude
                                             List is deleted.

7.2.3.  Definition of Source Timers

   A Source Timer is a decrementing timer with a lower bound of zero.
   One Source Timer is kept per source record.  Source timers are
   updated according to the type and filter mode of the Multicast
   Address Record received.  Section 7.4 describes the setting of source
   timers per type of Multicast Address Records received.

   In the following, abbreviations are used for several variables (all
   of which are described in detail in section 9).  The variable MALI
   stands for the Multicast Address Listening Interval, which is the
   time in which multicast address listening state will time out.  The
   variable LLQT is the Last Listener Query Time, which is the total
   time the router should wait for a report, after the Querier has sent
   the first query.  During this time, the Querier 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 modification of the information passed
   to the routing protocol.

   If the router is in INCLUDE filter mode, a source can be added to the
   current Include List if a listener in INCLUDE mode sends a Current
   State or a State Change Report which includes that source.  Each
   source from the Include List is associated with a source timer that

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   is updated whenever a listener in INCLUDE mode sends a report that
   confirms its interest in that specific source.  If the timer of a
   source from the Include List expires, the source is deleted from the
   Include List.  If there are no more source records left, the
   multicast address record is deleted from the router.

   Besides this "soft leave" mechanism, there is also a "fast leave"
   scheme in MLDv2; it is also based on the use of source timers.  When
   a node in INCLUDE mode expresses its desire to stop listening to a
   specific source, all the multicast routers on the link lower their
   timer for that source to a small interval of LLQT milliseconds.  The
   Querier then sends then a Multicast Address and Source Specific
   Query, to verify whether there are other listeners for that source on
   the link, or not.  If a corresponding report is received before the
   timer expires, all the multicast routers on the link update their
   source timer.  If not, the source is deleted from the Include List.
   The handling of the Include List, according to the received reports,
   is detailed in Tables 7.4.1 and 7.4.2.

   Source timers are treated differently when the Router Filter Mode for
   a multicast address is EXCLUDE.  For sources from the Requested List
   the source timers have running values; these sources are forwarded by
   the router.  For sources from the Exclude List the source timers are
   set to zero; these sources are blocked by the router.  If the timer
   of a source from the Requested List expires, the source is moved to
   the Exclude List.  The router informs then the routing protocol that
   there is no longer a listener on the link interested in traffic from
   this source.

   The router has to maintain the Requested List for two reasons:

   o  To keep track of sources that listeners in INCLUDE mode listen to.
      This is necessary in order to assure a seamless transition of the
      router to INCLUDE mode, when there will be no listener in EXCLUDE
      mode left.  This transition should not interrupt the flow of
      traffic to the listeners in INCLUDE mode still interested in that
      multicast address.  Therefore, at the moment of the transition,
      the Requested List should represent the set of sources that nodes
      in INCLUDE mode have explicitly requested.

      When the router switches to INCLUDE mode, the sources in the
      Requested List are moved to the Include List, and the Exclude List
      is deleted.  Before the switch, the Requested List can contain an
      inexact guess at the sources that listeners in INCLUDE mode listen
      to - might be too large or too small.  These inexactitudes are due
      to the fact that the Requested List is also used for fast blocking
      purposes, as described below.  If such a fast blocking is
      required, some sources may be deleted from the Requested List (as

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      shown in Tables 7.4.1 and 7.4.2) in order to reduce router state.
      Nevertheless, in each such case the Filter Timer is updated as
      well.  Therefore, listeners in INCLUDE mode will have enough time,
      before an eventual switching, to reconfirm their interest in the
      eliminated source(s), and rebuild the Requested List accordingly.
      The protocol ensures that when a switch to INCLUDE mode occurs,
      the Requested List will be accurate.  Details about the transition
      of the router to INCLUDE mode are presented in Appendix A3.

   o  To allow a fast blocking of previously unblocked sources.  If the
      router receives a report that contains such a request, the
      concerned sources are added to the Requested List.  Their timers
      are set to a small interval of LLQT milliseconds, and a Multicast
      Address and Source Specific Query is sent by the Querier, to check
      whether there are nodes on the link still interested in those
      sources, or not.  If no node confirms its interest in receiving a
      specific source, the timer of that source expires.  Then, the
      source is moved from the Requested List to the Exclude List.  From
      then on, the source will be blocked by the router.

   The handling of the EXCLUDE mode router state, according to the
   received reports, is detailed in Tables 7.4.1 and 7.4.2.

   When the Router Filter Mode for a multicast address is EXCLUDE,
   source records are only deleted when the Filter Timer expires, or
   when newly received Multicast Address Records modify the source
   record list of the router.

7.3.  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.

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     Router
   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

    EXCLUDE            TIMER > 0         Suggest to forward traffic
                                         from source

    EXCLUDE            TIMER == 0        Suggest to not forward
                                         traffic from source.  Move
                                         the source from the
                                         Requested List to the
                                         Exclude List (DO NOT remove
                                         source record)

    EXCLUDE         No Source Element    Suggest to forward traffic
                                         from all sources

7.4.  Action on Reception of Reports

   Upon reception of an MLD message that contains 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
   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.

7.4.1.  Reception of Current State Records

   When receiving Current State Records, a router updates both its
   Filter Timer and its 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.

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   If the router is in INCLUDE filter mode for a multicast address, we
   will use the notation INCLUDE (A), where A denotes the associated
   Include List.  If the router is in EXCLUDE filter mode for a
   multicast address, we will use the notation EXCLUDE (X,Y), where X
   and Y denote the associated Requested List and Exclude List
   respectively.

   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.  'Filter Timer = J' means
   that the Filter Timer for the multicast address should be set to
   value J.

   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)
                                                      Filter 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)
                                                      Filter Timer=MALI

7.4.2.  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.

   The Querier 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.

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   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
   Filter Timer for that multicast address is lowered to a small
   interval of Last Listener Query Time milliseconds.  If any multicast
   address records that express EXCLUDE mode interest in the multicast
   address are received within this interval, the Filter 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 the query period (i.e., Last Listener Query Time milliseconds)
   the MLD component in the router continues to suggest to the routing
   protocol to forward traffic from the multicast addresses or sources
   that are queried.  It is not until after Last Listener Query Time
   milliseconds without receiving a record that expresses 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 Querier when a particular report is received.

   We use the following notation for describing the queries that 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 7.6.3. describes the process of building and
   maintaining the state of pending queries.

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   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)
                                                       Filter 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) =
                                                            Filter Timer
                                                       Send Q(MA,A-Y)

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

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

7.5.  Switching Router Filter Modes

   The Filter Timer is used as a mechanism for transitioning the Router
   Filter Mode from EXCLUDE to INCLUDE.

   When a Filter 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.  Thus, the router transitions
   to INCLUDE filter mode for the multicast address.

   A router uses the sources from the Requested List as its state for
   the switch to a filter mode of INCLUDE.  Sources from the Requested
   List are moved in the Include List, while sources from the Exclude
   List are deleted.  For example, if a router's state for a multicast
   address is EXCLUDE(X,Y) and the Filter Timer expires for that

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   multicast address, the router switches to filter mode of INCLUDE with
   state INCLUDE(X).  If at the moment of the switch the Requested List
   (X) is empty, the multicast address record is deleted from the
   router.

7.6.  Action on Reception of Queries

   Upon reception of an MLD message that contains 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.

7.6.1.  Timer Updates

   MLDv2 uses the Suppress Router-Side Processing flag to ensure
   robustness, as explained in section 2.1.  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)       Filter 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.

7.6.2.  Querier Election

   MLDv2 elects a single router per subnet to be in Querier state; all
   the other routers on the subnet should be in Non-Querier state. MLDv2
   uses the same querier election mechanism as MLDv1, namely the IPv6
   address.  When a router starts operating on a subnet, by default it
   considers itself as being the Querier.  Thus, it sends several
   General Queries separated by a small time interval (see sections 9.6
   and 9.7 for details).

   When a router receives a query with a lower IPv6 address than its
   own, it sets the Other Querier Present timer to Other Querier Present
   Timeout; if it was previously in Querier state, it switches to Non-

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   Querier state and ceases to send queries on the link.  After the
   Other Querier Present timer expires, it should re-enter the Querier
   state and begin sending General Queries.

   All MLDv2 queries MUST be sent with the FE80::/64 link-local source
   address prefix.  Therefore, for the purpose of MLDv2 querier
   election, an IPv6 address A is considered to be lower than an IPv6
   address B if the interface ID represented by the last 64 bits of
   address A, in big-endian bit order, is lower than the interface ID
   represented by the last 64 bits of address B.

7.6.3.  Building and Sending Specific Queries

7.6.3.1.  Building and Sending Multicast Address Specific Queries

   When a table action "Send Q(MA)" is encountered, the Filter Timer
   must be lowered to LLQT.  The Querier 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 Filter
   Timer is larger than LLQT, the "Suppress Router-Side Processing" bit
   is set in the query message.

7.6.3.2.  Building and Sending Multicast Address and Source Specific
          Queries

   When a table action "Send Q(MA,X)" is encountered by the Querier in
   the table in section 7.4.2, 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  Add the sources to the Retransmission List;

   o  Set the Source Retransmission Counter for each source to [Last
      Listener Query Count].

   The Querier 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.

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   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 (i.e., sources from the Retransmission List of that multicast
   address), 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.

8.  Interoperation with MLDv1

   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.

8.1.  Query Version Distinctions

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

   MLDv1 Query: length = 24 octets

   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.

8.2.  Multicast Address Listener Behavior

8.2.1.  In the Presence of MLDv1 Routers

   In order to be compatible with MLDv1 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.

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   The Host Compatibility Mode of an interface is set to MLDv1 whenever
   an MLDv1 Multicast Address Listener Query is received on that
   interface.  At the same time, the Older Version Querier Present timer
   for the interface is set to Older Version Querier Present Timeout
   seconds.  The timer is re-set whenever a new MLDv1 Query is received
   on that interface.  If the Older Version Querier Present timer
   expires, the host switches back to Host Compatibility Mode of MLDv2.

   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 Querier 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
   section 5.1.3. is not used.

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

8.2.2.  In the Presence of MLDv1 Multicast Address Listeners

   An MLDv2 host may be placed on a link where there are MLDv1 hosts.  A
   host MAY allow its MLDv2 Multicast Listener Report to be suppressed
   by a Version 1 Multicast Listener Report.

8.3.  Multicast Router Behavior

8.3.1.  In the Presence of MLDv1 Routers

   MLDv2 routers may be placed on a network where there is at least one
   MLDv1 router.  The following requirements apply:

   o  If an MLDv1 router is present on the link, 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;
      if an MLDv1 router is present on the link, the system
      administrator must explicitly configure all MLDv2 routers to act
      in MLDv1 mode. When in MLDv1 mode, the Querier MUST send periodic
      General 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).  The Querier MUST also fill
      in the Maximum Response Delay in the Maximum Response Code field,
      i.e., the exponential algorithm described in section 5.1.3. is not
      used.

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

8.3.2.  In the Presence of MLDv1 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
   MLDv1 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.

   The Multicast Address Compatibility Mode of a multicast address
   record is set to MLDv1 whenever an MLDv1 Multicast Listener Report is
   received for that multicast address.  At the same time, the Older
   Version Host Present timer for the multicast address is set to Older
   Version Host Present Timeout seconds.  The timer is re-set whenever a
   new MLDv1 Report is received for that multicast address.  If the
   Older Version Host Present timer expires, the router switches back to
   Multicast Address Compatibility Mode of MLDv2 for that multicast
   address.

   Note that when a router switches back to MLDv2 Multicast Address
   Compatibility 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.

   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 continues to send MLDv2 queries,
   regardless of its Multicast Address Compatibility Mode.

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9.  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.

9.1.  Robustness Variable

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

9.2.  Query Interval

   The Query Interval variable denotes the interval between General
   Queries sent by the Querier.  Default value: 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.

9.3.  Query Response Interval

   The Maximum Response Delay used to calculate the Maximum Response
   Code inserted into the periodic General Queries.  Default value:
   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].

9.4.  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 ([Robustness Variable] times [Query Interval])
   plus [Query Response Interval].

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9.5.  Other Querier Present Timeout

   The Other Querier Present Timeout is the length of time that must
   pass before a multicast router decides that there is no longer
   another multicast router which should be the Querier.  This value
   MUST be ([Robustness Variable] times ([Query Interval]) plus (one
   half of [Query Response Interval]).

9.6.  Startup Query Interval

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

9.7.  Startup Query Count

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

9.8.  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
   to calculate the Maximum Response Code inserted into Multicast
   Address and Source Specific Query messages.  Default value: 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.

9.9.  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

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   Multicast Address and Source Specific Queries sent before the router
   assumes there are no listeners for a particular source.  Default
   value: [Robustness Variable].

9.10.  Last Listener Query Time

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

9.11.  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
   value: 1 second.

9.12.  Older Version Querier Present Timeout

   The Older Version Querier Present Timeout is the time-out for
   transitioning a host back to MLDv2 Host Compatibility Mode.  When an
   MLDv1 query is received, MLDv2 hosts set their Older Version Querier
   Present Timer to [Older Version Querier Present Timeout].

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

9.13.  Older Version Host Present Timeout

   The Older Version Host Present Timeout is the time-out for
   transitioning a router back to MLDv2 Multicast Address Compatibility
   Mode for a specific multicast address.  When an MLDv1 report is
   received for that multicast address, routers set their Older Version
   Host Present Timer to [Older Version Host Present Timeout].

   This value MUST be ([Robustness Variable] times [Query Interval])
   plus ([Query Response Interval]).

9.14.  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.

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9.14.1.  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
   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 value of the Robustness Variable
   should be increased to allow for the expected level of packet loss.
   However, increasing the value of 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).

9.14.2.  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 value of the Query Interval MUST
   be equal to or greater than the Maximum Response Delay used to
   calculate the Maximum Response Code inserted in General Query
   messages.

9.14.3.  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

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

10.  Security Considerations

   We consider the ramifications of a forged message of each type.  Note
   that before processing an MLD message, nodes verify if the source
   address of the message is a valid link-local address (or the
   unspecified 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.  This
   defends the MLDv2 nodes from acting on forged MLD messages originated
   off-link.  Therefore, in the following we discuss only the effects of
   on-link forgery.

10.1.  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 listeners on that
   link in MLDv1 Host 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.

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10.2.  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.
   Nevertheless, 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 MLDv1
   Multicast Address Compatibility Mode 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 it should only be used in situations where source
   filtering is critical.

10.3.  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 cannot cause loss of desired traffic.

11.  IANA Considerations

   IANA has assigned the IPv6 link-local multicast address
   FF02:0:0:0:0:0:0:16, called "all MLDv2-capable routers", as described
   in section 5.2.14.  Version 2 Multicast Listener Reports will be sent
   to this special address.

   In addition, IANA has assigned the ICMPv6 message type value of 143
   for Version 2 Multicast Listener Report messages, as specified in
   section 4.

12.  References

12.1.  Normative References

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

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

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   [RFC2463]    Conta, A. and S. Deering, "Internet Control Message
                Protocol (ICMPv6) for the Internet Protocol Version 6
                (IPv6) Specification", RFC 2463, December 1998.

   [RFC2464]    Crawford, M., "Transmission of IPv6 Packets over
                Ethernet Networks", RFC 2464, December 1998.

   [RFC2710]    Deering, S., Fenner, W. and B. Haberman, "Multicast
                Listener Discovery (MLD) for IPv6", RFC 2710, October
                1999.

   [RFC2711]    Partridge, C. and A. Jackson, "IPv6 Router Alert
                Option," RFC 2711, October 1999.

   [RFC3513]    Hinden, R. and S. Deering, "Internet Protocol Version 6
                (IPv6) Addressing Architecture, RFC 3513, April 2003.

12.2.  Informative References

   [RFC2461]    Narten, T., Nordmark, E. and W. Simpson, "Neighbor
                Discovery for IP Version 6 (IPv6)", RFC 2461, December
                1998.

   [RFC2462]    Thomson, S. and T. Narten, "IPv6 Stateless Address
                Autoconfiguration", RFC 2462, December 1998.

   [RFC3376]    Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A.
                Thyagarajan, "Internet Group Management Protocol,
                Version 3", RFC 3376, October 2002.

   [RFC3569]    Bhattacharyya, S., Ed., "An Overview of Source- Specific
                Multicast (SSM)", RFC 3569, July 2003.

   [RFC3678]    Thaler, D., Fenner, B. and B. Quinn, "Socket Interface
                Extensions for Multicast Source Filters", RFC 3678,
                January 2004.

13.  Acknowledgments

   We would like to thank Hitoshi Asaeda, Randy Bush, Francis Dupont,
   Ted Hardie, Russ Housley, Konstantin Kabassanov, Erik Nordmark,
   Shinsuke Suzuki, Margaret Wasserman, Bert Wijnen, and Remi Zara 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 the per-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 the per-interface
   state require the router to take some action in response to the
   received report (see Section 7.4.).

   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 not send a pending multicast listener report
   if a similar report was sent by another listener on the link.  In
   MLDv2, the suppression of multicast listener reports has been
   removed.  The following points explain this decision.

   1. Routers may want to track per-host multicast listener status on an
      interface.  This would allow routers to implement fast leaves
      (e.g., for layered multicast congestion control schemes), as well
      as track listener status for possible security or accounting
      purposes.  The present specification does not require routers to
      implement per-host tracking.  Nevertheless, the lack of host
      suppression in MLDv2 makes possible to implement either
      proprietary or future standard behavior of multicast routers that
      would support per-host tracking, while being fully interoperable
      with MLDv2 listeners and routers that implement the exact behavior
      described in this specification.

   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.

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   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
      multicast address be reported in a separate message.

A.3.  Switching router filter modes from EXCLUDE to INCLUDE

   If on a link there are nodes in both EXCLUDE and INCLUDE modes for a
   single multicast address, the router must be in EXCLUDE mode as well
   (see section 7.2.1).  In EXCLUDE mode, a router forwards traffic from
   all sources except those in the Exclude List.  If all nodes in
   EXCLUDE mode cease to exist or to listen, 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 Filter Timer for
   a multicast address expires, 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 Filter Timer).  The router can
   then switch to INCLUDE mode seamlessly; sources from the Requested
   List are moved to the Include List, while sources from the Exclude
   List are deleted.

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.

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   o  Queries include additional fields (section 5.1).

   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.

   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 65535 milliseconds 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.

   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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Editors' Contact Information

   Rolland Vida
   LIP6, Universite Pierre et Marie Curie
   8, rue du Capitaine Scott
   75015 Paris, France

   Phone: +33-1.44.27.30.58
   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: +33-1.44.27.30.58
   EMail: Luis.Costa@lip6.fr

Authors' Addresses

   This document was written by:

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

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

   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: brian@innovationslab.net

   This document is the translation of [RFC3376] for IPv6 semantics.  It
   was elaborated based on the translation of (RFC 2236) into [RFC2710].

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Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
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Acknowledgement

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