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rfc:rfc3810

Network Working Group R. Vida, Ed. Request for Comments: 3810 L. Costa, Ed. Updates: 2710 LIP6 Category: Standards Track June 2004

      Multicast Listener Discovery Version 2 (MLDv2) for IPv6

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2004).

Abstract

 This document updates RFC 2710, and it specifies Version 2 of the
 Multicast Listener Discovery Protocol (MLDv2).  MLD is used by an
 IPv6 router to discover the presence of multicast listeners on
 directly attached links, and to discover which multicast addresses
 are of interest to those neighboring nodes.  MLDv2 is designed to be
 interoperable with MLDv1.  MLDv2 adds the ability for a node to
 report interest in listening to packets with a particular multicast
 address only from specific source addresses or from all sources
 except for specific source addresses.

Vida & Costa Standards Track [Page 1] RFC 3810 MLDv2 for IPv6 June 2004

Table of Contents

 1.  Introduction. . . . . . . . . . . . . . . . . . . . . . . . .   2
 2.  Protocol Overview . . . . . . . . . . . . . . . . . . . . . .   3
 3.  The Service Interface for Requesting IP Multicast Reception .   9
 4.  Multicast Listening State Maintained by Nodes . . . . . . . .  11
 5.  Message Formats . . . . . . . . . . . . . . . . . . . . . . .  13
 6.  Protocol Description for Multicast Address Listeners. . . . .  27
 7.  Protocol Description for Multicast Routers. . . . . . . . . .  34
 8.  Interoperation with MLDv1 . . . . . . . . . . . . . . . . . .  48
 9.  List of Timers, Counters, and their Default Values. . . . . .  51
 10. Security Considerations . . . . . . . . . . . . . . . . . . .  55
 11. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  56
 12. References. . . . . . . . . . . . . . . . . . . . . . . . . .  56
 13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .  57
 Appendix A. Design Rationale. . . . . . . . . . . . . . . . . . .  58
 Appendix B. Summary of Changes from MLDv1 . . . . . . . . . . . .  59
 Editors' Contact Information. . . . . . . . . . . . . . . . . . .  61
 Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . .  61
 Full Copyright Statement. . . . . . . . . . . . . . . . . . . . .  62

1. Introduction

 The Multicast Listener Discovery Protocol (MLD) is used by IPv6
 routers to discover the presence of multicast listeners (i.e., nodes
 that wish to receive multicast packets) on their directly attached
 links, and to discover specifically which multicast addresses are of
 interest to those neighboring nodes.  Note that a multicast router
 may itself be a listener of one or more multicast addresses; in this
 case it performs both the "multicast router part" and the "multicast
 address listener part" of the protocol, to collect the multicast
 listener information needed by its multicast routing protocol on the
 one hand, and to inform itself and other neighboring multicast
 routers of its listening state on the other hand.
 This document specifies Version 2 of MLD.  The previous version of
 MLD is specified in [RFC2710].  In this document we will refer to it
 as MLDv1.  MLDv2 is a translation of the IGMPv3 protocol [RFC3376]
 for IPv6 semantics.
 The MLDv2 protocol, when compared to MLDv1, adds support for "source
 filtering", i.e., the ability for a node to report interest in
 listening to packets *only* from specific source addresses, as
 required to support Source-Specific Multicast [RFC3569], or from *all
 but* specific source addresses, sent to a particular multicast
 address.  MLDv2 is designed to be interoperable with MLDv1.

Vida & Costa Standards Track [Page 2] RFC 3810 MLDv2 for IPv6 June 2004

 The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
 "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 [RFC2119].  Due to the lack of italics, emphasis is indicated herein
 by bracketing a word or phrase in "*" characters.  Furthermore,
 square brackets are used to denote the value of the enclosed
 variable, as opposed to the variable itself, written without
 brackets.

2. Protocol Overview

 This section gives a brief description of the protocol operation. The
 following sections present the protocol details.
 MLD is an asymmetric protocol; it specifies separate behaviors for
 multicast address listeners (i.e., hosts or routers that listen to
 multicast packets) and multicast routers.  The purpose of MLD is to
 enable each multicast router to learn, for each of its directly
 attached links, which multicast addresses and which sources have
 interested listeners on that link.  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 listeners interested in such packets.
 Multicast routers only need to know that *at least one* node on an
 attached link is listening to packets for a particular multicast
 address, from a particular source; a multicast router is not required
 to *individually* keep track of the interests of each neighboring
 node.  (Nevertheless, see Appendix A2 item 1 for discussion.)
 A multicast router performs the *router part* of the MLDv2 protocol
 (described in details in section 7) on each of its directly attached
 links.  If a multicast router has more than one interface connected
 to the same link, it only needs to operate the protocol on one of
 those interfaces.  The router behavior depends on whether there are
 several multicast routers on the same subnet, or not.  If that 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.  This
 router is called the Querier.  All 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 take over the querier role, should the present Querier fail.
 Nevertheless, only the Querier sends periodical or triggered query
 messages on the subnet, as described in section 7.1.

Vida & Costa Standards Track [Page 3] RFC 3810 MLDv2 for IPv6 June 2004

 A multicast address listener performs the *listener part* of the
 MLDv2 protocol (described in details in section 6) on all interfaces
 on which multicast reception is supported, even if more than one of
 those interfaces are connected to the same link.

2.1. Building Multicast Listening State on Multicast Address Listeners

 Upper-layer protocols and applications that run on a multicast
 address listener node use specific service interface calls (described
 in section 3) to ask the IP layer to enable or disable reception of
 packets sent to specific multicast addresses.  The node keeps
 Multicast Address Listening state for each socket on which the
 service interface calls have been invoked (section 4.1).  In addition
 to this per-socket multicast listening state, a node must also
 maintain or compute multicast listening state for each of its
 interfaces (section 4.2).  Conceptually, that state consists of a set
 of records, with each record containing an IPv6 multicast address, a
 filter mode, and a source list.  The filter mode may be either
 INCLUDE or EXCLUDE.  In INCLUDE mode, reception of packets sent to
 the specified multicast address is enabled *only* from the source
 addresses listed in the source list.  In EXCLUDE mode, reception of
 packets sent to the given multicast address is enabled from all
 source addresses *except* those listed in the source list.
 At most one record per multicast address exists for a given
 interface.  This per-interface state is derived from the per-socket
 state, but may differ from it when different sockets have differing
 filter modes and/or source lists for the same multicast address and
 interface.  After a multicast packet has been accepted from an
 interface by the IP layer, its subsequent delivery to the application
 connected to a particular socket depends on the multicast listening
 state of that socket (and possibly also on other conditions, such as
 what transport-layer port the socket is bound to).  Note that MLDv2
 messages are not subject to source filtering and must always be
 processed by hosts and routers.

2.2. Exchanging Messages between the Querier and the Listening Nodes

 There are three types of MLDv2 query messages: General Queries,
 Multicast Address Specific Queries, and Multicast Address and Source
 Specific Queries.  The Querier periodically sends General Queries, to
 learn multicast address listener information from an attached link.
 These queries are used to build and refresh the Multicast Address
 Listener state inside all multicast routers on the link.
 Nodes respond to these queries by reporting their per-interface
 Multicast Address Listening state, through Current State Report
 messages sent to a specific multicast address all MLDv2 routers on

Vida & Costa Standards Track [Page 4] RFC 3810 MLDv2 for IPv6 June 2004

 the link listen to.  On the other hand, if the listening state of a
 node changes, the node immediately reports these changes through a
 State Change Report message.  The State Change Report contains either
 Filter Mode Change records, Source List Change records, or records of
 both types.  A detailed description of the report messages is
 presented in section 5.2.12.
 Both router and listener state changes are mainly triggered by the
 expiration of a specific timer, or the reception of an MLD message
 (listener state change can be also triggered by the invocation of a
 service interface call).  Therefore, to enhance protocol robustness,
 in spite of the possible unreliability of message exchanges, messages
 are retransmitted several times.  Furthermore, timers are set so as
 to take into account the possible message losses, and to wait for
 retransmissions.
 Periodical General Queries and Current State Reports do not apply
 this rule, in order not to overload the link; it is assumed that in
 general these messages do not generate state changes, their main
 purpose being to refresh existing state.  Thus, even if one such
 message is lost, the corresponding state will be refreshed during the
 next reporting period.
 As opposed to Current State Reports, State Change Reports are
 retransmitted several times, in order to avoid them being missed by
 one or more multicast routers.  The number of retransmissions depends
 on the so-called Robustness Variable.  This variable allows tuning
 the protocol according to the expected packet loss on a link.  If a
 link is expected to be lossy (e.g., a wireless connection), the value
 of the Robustness Variable may be increased.  MLD is robust to
 [Robustness Variable]-1 packet losses.  This document recommends a
 default value of 2 for the Robustness Variable (see section 9.1).
 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 additional change triggers the
 immediate transmission of a new State Change Report.  Section 6.1
 shows how the content of this new report is computed. Retransmissions
 of the new State Change Report will be scheduled as well, in order to
 ensure that each instance of state change is transmitted at least
 [Robustness Variable] times.
 If a node on a link expresses, through a State Change Report, its
 desire to no longer listen to a particular multicast address (or
 source),  the Querier must query for other listeners of the multicast
 address (or source) before deleting the multicast address (or source)
 from its Multicast Address Listener state and stopping the
 corresponding traffic.  Thus, the Querier sends a Multicast Address

Vida & Costa Standards Track [Page 5] RFC 3810 MLDv2 for IPv6 June 2004

 Specific Query to verify whether there are nodes still listening to a
 specified multicast address or not.  Similarly, the Querier sends a
 Multicast Address and Source Specific Query to verify whether, for a
 specified multicast address, there are nodes still listening to a
 specific set of sources, or not.  Section 5.1.13 describes each query
 in more detail.
 Both Multicast Address Specific Queries and Multicast Address and
 Source Specific Queries are only sent in response to State Change
 Reports, never in response to Current State Reports.  This
 distinction between the two types of reports is needed to avoid the
 router treating all Multicast Listener Reports as potential changes
 in state.  By doing so, the fast leave mechanism of MLDv2, described
 in more detail in section 2.2, might not be effective if a State
 Change Report is lost, and only the following Current State Report is
 received by the router.  Nevertheless, it avoids an increased
 processing at the router and it reduces the MLD traffic on the link.
 More details on the necessity of distinguishing between the two
 report types can be found in Appendix A1.
 Nodes respond to the above queries through Current State Reports,
 that contain their per-interface Multicast Address Listening state
 only for the multicast addresses (or sources) being queried.
 As stated earlier, in order to ensure protocol robustness, all the
 queries, except the periodical General Queries, are retransmitted
 several times within a given time interval.  The number of
 retransmissions depends on the Robustness Variable.  If, while
 scheduling new queries, there are pending queries to be retransmitted
 for the same multicast address, the new queries and the pending
 queries have to be merged.  In addition, host reports received for a
 multicast address with pending queries may affect the contents of
 those queries.  The process of building and maintaining the state of
 pending queries is presented in section 7.6.3.
 Protocol robustness is also enhanced through the use of the S flag
 (Suppress Router-Side Processing).  As described above, when a
 Multicast Address Specific or a Multicast Address and Source Specific
 Query is sent by the Querier, a number of retransmissions of the
 query are scheduled.  In the original (first) query the S flag is
 clear.  When the Querier sends this query, it lowers the timers for
 the concerned multicast address (or source) to a given value;
 similarly, any non-querier multicast router that receives the query
 lowers its timers in the same way.  Nevertheless, while waiting for
 the next scheduled queries to be sent, the Querier may receive a
 report that updates the timers.  The scheduled queries still have to
 be sent, in order to ensure that a non-querier router keeps its state
 synchronized with the current Querier (the non-querier router might

Vida & Costa Standards Track [Page 6] RFC 3810 MLDv2 for IPv6 June 2004

 have missed the first query).  Nevertheless, the timers should not be
 lowered again, as a valid answer was already received.  Therefore, in
 subsequent queries the Querier sets the S flag.

2.3. Building Multicast Address Listener State on Multicast Routers

 Multicast routers that implement MLDv2 (whether they are in Querier
 state or not) keep state per multicast address per attached link.
 This multicast address listener state consists of a Filter Mode, a
 Filter Timer, and a Source List, with a timer associated to each
 source from the list.  The 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 report
 messages, or when certain timer conditions occur.
 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 a list of sources, 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 source can be added to the current Include List if a listener in
 INCLUDE mode sends a Current State or a State Change Report that
 includes that source.  Each source from the Include List is
 associated with a source timer that 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.
 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
 timers for that source to a given value.  The Querier then sends a
 Multicast Address and Source Specific Query, to verify whether there
 are other listeners for that source on the link, or not.  If a report
 that includes this source is received before the timer expiration,
 all the multicast routers on the link update the 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.

Vida & Costa Standards Track [Page 7] RFC 3810 MLDv2 for IPv6 June 2004

 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 for
 that address on the link.  When the first report is received from
 such a listener, the router sets the Filter Timer that corresponds to
 that address.  This timer is reset each time an EXCLUDE mode listener
 confirms its listening state through a Current State Report.  The
 timer is also updated when a listener, formerly in INCLUDE mode,
 announces its filter mode change through a State Change Report
 message.  If the Filter Timer expires, it means that there are no
 more listeners in EXCLUDE mode on the link.  In this case, the router
 switches back to INCLUDE mode for that multicast address.
 When the router is in EXCLUDE mode, the router state is represented
 by 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, 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 to assure a seamless transition of the router to
    INCLUDE mode, when there is no listener in EXCLUDE mode left.
    This transition should not interrupt the flow of traffic to
    listeners in INCLUDE mode for that multicast address.  Therefore,
    at the time of the transition, the Requested List should contain
    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 switching, the Requested List can contain an
    inexact guess of the sources 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
    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 the 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

Vida & Costa Standards Track [Page 8] RFC 3810 MLDv2 for IPv6 June 2004

    are set to a given small value, 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 announces its interest in receiving those specific source,
    the timers of those sources expire.  Then, the sources are moved
    from the Requested List to the Exclude List.  From then on, the
    sources 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.
 Both the MLDv2 router and listener behaviors described in this
 document were defined to ensure backward interoperability with MLDv1
 hosts and routers.  Interoperability issues are detailed in section
 8.

3. The Service Interface for Requesting IP Multicast Reception

 Within an IP system, there is (at least conceptually) a service
 interface used by upper-layer protocols or application programs to
 ask the IP layer to enable or disable reception of packets sent to
 specific IP multicast addresses.  In order to take full advantage of
 the capabilities of MLDv2, a node's IP service interface must support
 the following operation:
    IPv6MulticastListen ( socket, interface, IPv6 multicast address,
    filter mode, source list )
    where:
 o  "socket" is an implementation-specific parameter used to
    distinguish among different requesting entities (e.g., programs,
    processes) within the node; the socket parameter of BSD Unix
    system calls is a specific example.
 o  "interface" is a local identifier of the network interface on
    which reception of the specified multicast address is to be
    enabled or disabled.  Interfaces may be physical (e.g., an
    Ethernet interface) or virtual (e.g., the endpoint of a Frame
    Relay virtual circuit or an IP-in-IP "tunnel").  An implementation
    may allow a special "unspecified" value to be passed as the
    interface parameter, in which case the request would apply to the
    "primary" or "default" interface of the node (perhaps established
    by system configuration).  If reception of the same multicast
    address is desired on more than one interface, IPv6MulticastListen
    is invoked separately for each desired interface.

Vida & Costa Standards Track [Page 9] RFC 3810 MLDv2 for IPv6 June 2004

 o  "IPv6 multicast address" is the multicast address to which the
    request pertains.  If reception of more than one multicast address
    on a given interface is desired, IPv6MulticastListen is invoked
    separately for each desired address.
 o  "filter mode" may be either INCLUDE or EXCLUDE.  In INCLUDE mode,
    reception of packets sent to the specified multicast address is
    requested *only* from the source addresses listed in the source
    list parameter.  In EXCLUDE mode, reception of packets sent to the
    given multicast address is requested from all source addresses
    *except* those listed in the source list parameter.
 o  "source list" is an unordered list of zero or more unicast
    addresses from which multicast reception is desired or not
    desired, depending on the filter mode.  An implementation MAY
    impose a limit on the size of source lists.  When an operation
    causes the source list size limit to be exceeded, the service
    interface SHOULD return an error.
 For a given combination of socket, interface, and IPv6 multicast
 address, only a single filter mode and source list can be in effect
 at any one time.  Nevertheless, either the filter mode or the source
 list, or both, may be changed by subsequent IPv6MulticastListen
 requests that specify the same socket, interface, and IPv6 multicast
 address.  Each subsequent request completely replaces any earlier
 request for the given socket, interface, and multicast address.
 The MLDv1 protocol did not support source filters, and had a simpler
 service interface; it consisted of Start Listening and Stop Listening
 operations to enable and disable listening to a given multicast
 address (from *all* sources) on a given interface.  The equivalent
 operations in the new service interface are as follows:
 The Start Listening operation is equivalent to:
    IPv6MulticastListen ( socket, interface, IPv6 multicast address,
                          EXCLUDE, {} )
 and the Stop Listening operation is equivalent to:
    IPv6MulticastListen ( socket, interface, IPv6 multicast address,
                          INCLUDE, {} )
 where {} is an empty source list.
 An example of an API that provides the capabilities outlined in this
 service interface is given in [RFC3678].

Vida & Costa Standards Track [Page 10] RFC 3810 MLDv2 for IPv6 June 2004

4. Multicast Listening State Maintained by Nodes

4.1. Per-Socket State

 For each socket on which IPv6MulticastListen has been invoked, the
 node records the desired multicast listening state for that socket.
 That state conceptually consists of a set of records of the form:
 (interface, IPv6 multicast address, filter mode, source list)
 The per-socket state evolves in response to each invocation of
 IPv6MulticastListen on the socket, as follows:
 o  If the requested filter mode is INCLUDE *and* the requested source
    list is empty, then the entry that corresponds to the requested
    interface and multicast address is deleted, if present.  If no
    such entry is present, the request has no effect.
 o  If the requested filter mode is EXCLUDE *or* the requested source
    list is non-empty, then the entry that corresponds to the
    requested interface and multicast address, if present, is changed
    to contain the requested filter mode and source list.  If no such
    entry is present, a new entry is created, using the parameters
    specified in the request.

4.2. Per-Interface State

 In addition to the per-socket multicast listening state, a node must
 also maintain or compute multicast listening state for each of its
 interfaces.  That state conceptually consists of a set of records of
 the form:
    (IPv6 multicast address, filter mode, source list)
 At most one record per multicast address exists for a given
 interface.  This per-interface state is derived from the per-socket
 state, but may differ from it when different sockets have differing
 filter modes and/or source lists for the same multicast address and
 interface.  For example, suppose one application or process invokes
 the following operation on socket s1:
    IPv6MulticastListen ( s1, i, m, INCLUDE, {a, b, c} )

Vida & Costa Standards Track [Page 11] RFC 3810 MLDv2 for IPv6 June 2004

 requesting reception on interface i of packets sent to multicast
 address m, *only* if they come from the sources a, b, or c.  Suppose
 another application or process invokes the following operation on
 socket s2:
    IPv6MulticastListen ( s2, i, m, INCLUDE, {b, c, d} )
 requesting reception on the same interface i of packets sent to the
 same multicast address m, *only* if they come from sources b, c, or
 d.  In order to satisfy the reception requirements of both sockets,
 it is necessary for interface i to receive packets sent to m from any
 one of the sources a, b, c, or d.  Thus, in this example, the
 listening state of interface i for multicast address m has filter
 mode INCLUDE and source list {a, b, c, d}.
 After a multicast packet has been accepted from an interface by the
 IP layer, its subsequent delivery to the application or process that
 listens on a particular socket depends on the multicast listening
 state of that socket (and possibly also on other conditions, such as
 what transport-layer port the socket is bound to).  So, in the above
 example, if a packet arrives on interface i, destined to multicast
 address m, with source address a, it may be delivered on socket s1
 but not on socket s2.  Note that MLDv2 messages are not subject to
 source filtering and must always be processed by hosts and routers.
 Requiring the filtering of packets based upon a socket's multicast
 reception state is a new feature of this service interface.  The
 previous service interface described no filtering based upon
 multicast listening state; rather, a Start Listening operation on a
 socket simply caused the node to start to listen to a multicast
 address on the given interface; packets sent to that multicast
 address could be delivered to all sockets, whether they had started
 to listen or not.
 The general rules for deriving the per-interface state from the per-
 socket state are as follows:  for each distinct (interface, IPv6
 multicast address) pair that appears in any per-socket state, a per-
 interface record is created for that multicast address on that
 interface.  Considering all socket records that contain the same
 (interface, IPv6 multicast address) pair,
 o  if *any* such record has a filter mode of EXCLUDE, then the filter
    mode of the interface record is EXCLUDE, and the source list of
    the interface record is the intersection of the source lists of
    all socket records in EXCLUDE mode, minus those source addresses
    that appear in any socket record in INCLUDE mode.  For example, if
    the socket records for multicast address m on interface i are:

Vida & Costa Standards Track [Page 12] RFC 3810 MLDv2 for IPv6 June 2004

       from socket s1:  ( i, m, EXCLUDE, {a, b, c, d} )
       from socket s2:  ( i, m, EXCLUDE, {b, c, d, e} )
       from socket s3:  ( i, m, INCLUDE, {d, e, f} )
    then the corresponding interface record on interface i is:
       ( m, EXCLUDE, {b, c} )
    If a fourth socket is added, such as:
       From socket s4:  ( i, m, EXCLUDE, {} )
    then the interface record becomes:
       ( m, EXCLUDE, {} )
 o  if *all* such records have a filter mode of INCLUDE, then the
    filter mode of the interface record is INCLUDE, and the source
    list of the interface record is the union of the source lists of
    all the socket records.  For example, if the socket records for
    multicast address m on interface i are:
       from socket s1:  ( i, m, INCLUDE, {a, b, c} )
       from socket s2:  ( i, m, INCLUDE, {b, c, d} )
       from socket s3:  ( i, m, INCLUDE, {e, f} )
    then the corresponding interface record on interface i is:
       ( m, INCLUDE, {a, b, c, d, e, f} )
 An implementation MUST NOT use an EXCLUDE interface record for a
 multicast address if all sockets for this multicast address are in
 INCLUDE state.  If system resource limits are reached when a per-
 interface state source list is calculated, an error MUST be returned
 to the application which requested the operation.
 The above rules for deriving the per-interface state are
 (re)evaluated whenever an IPv6MulticastListen invocation modifies the
 per-socket state by adding, deleting, or modifying a per-socket state
 record.  Note that a change of the per-socket state does not
 necessarily result in a change of the per-interface state.

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

Vida & Costa Standards Track [Page 13] RFC 3810 MLDv2 for IPv6 June 2004

 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:

Vida & Costa Standards Track [Page 14] RFC 3810 MLDv2 for IPv6 June 2004

   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]                      *
  |                                                               |
  *                                                               *
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Vida & Costa Standards Track [Page 15] RFC 3810 MLDv2 for IPv6 June 2004

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.

Vida & Costa Standards Track [Page 16] RFC 3810 MLDv2 for IPv6 June 2004

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)

Vida & Costa Standards Track [Page 17] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 18] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 19] RFC 3810 MLDv2 for IPv6 June 2004

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]                 .
  .                                                               .
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Vida & Costa Standards Track [Page 20] RFC 3810 MLDv2 for IPv6 June 2004

 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                        .
  .                                                               .
  |                                                               |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Vida & Costa Standards Track [Page 21] RFC 3810 MLDv2 for IPv6 June 2004

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.

Vida & Costa Standards Track [Page 22] RFC 3810 MLDv2 for IPv6 June 2004

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.

Vida & Costa Standards Track [Page 23] RFC 3810 MLDv2 for IPv6 June 2004

      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

Vida & Costa Standards Track [Page 24] RFC 3810 MLDv2 for IPv6 June 2004

         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

Vida & Costa Standards Track [Page 25] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 26] RFC 3810 MLDv2 for IPv6 June 2004

 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).

Vida & Costa Standards Track [Page 27] RFC 3810 MLDv2 for IPv6 June 2004

 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)

Vida & Costa Standards Track [Page 28] RFC 3810 MLDv2 for IPv6 June 2004

 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

Vida & Costa Standards Track [Page 29] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 30] RFC 3810 MLDv2 for IPv6 June 2004

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.)

Vida & Costa Standards Track [Page 31] RFC 3810 MLDv2 for IPv6 June 2004

 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

Vida & Costa Standards Track [Page 32] RFC 3810 MLDv2 for IPv6 June 2004

    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

Vida & Costa Standards Track [Page 33] RFC 3810 MLDv2 for IPv6 June 2004

    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. Description of the Protocol for Multicast Routers

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

Vida & Costa Standards Track [Page 34] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 35] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 36] RFC 3810 MLDv2 for IPv6 June 2004

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

Vida & Costa Standards Track [Page 37] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 38] RFC 3810 MLDv2 for IPv6 June 2004

   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

Vida & Costa Standards Track [Page 39] RFC 3810 MLDv2 for IPv6 June 2004

 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

Vida & Costa Standards Track [Page 40] RFC 3810 MLDv2 for IPv6 June 2004

    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.

Vida & Costa Standards Track [Page 41] RFC 3810 MLDv2 for IPv6 June 2004

   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.

Vida & Costa Standards Track [Page 42] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 43] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 44] RFC 3810 MLDv2 for IPv6 June 2004

 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

Vida & Costa Standards Track [Page 45] RFC 3810 MLDv2 for IPv6 June 2004

 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-

Vida & Costa Standards Track [Page 46] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 47] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 48] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 49] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 50] RFC 3810 MLDv2 for IPv6 June 2004

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].

Vida & Costa Standards Track [Page 51] RFC 3810 MLDv2 for IPv6 June 2004

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

Vida & Costa Standards Track [Page 52] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 53] RFC 3810 MLDv2 for IPv6 June 2004

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

Vida & Costa Standards Track [Page 54] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 55] RFC 3810 MLDv2 for IPv6 June 2004

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.

Vida & Costa Standards Track [Page 56] RFC 3810 MLDv2 for IPv6 June 2004

 [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.

Vida & Costa Standards Track [Page 57] RFC 3810 MLDv2 for IPv6 June 2004

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.

Vida & Costa Standards Track [Page 58] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 59] RFC 3810 MLDv2 for IPv6 June 2004

 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.

Vida & Costa Standards Track [Page 60] RFC 3810 MLDv2 for IPv6 June 2004

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].

Vida & Costa Standards Track [Page 61] RFC 3810 MLDv2 for IPv6 June 2004

Full Copyright Statement

 Copyright (C) The Internet Society (2004).  This document is subject
 to the rights, licenses and restrictions contained in BCP 78, and
 except as set forth therein, the authors retain all their rights.
 This document and the information contained herein are provided on an
 "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
 OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
 ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
 INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
 INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
 WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

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 Intellectual Property Rights or other rights that might be claimed to
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 might or might not be available; nor does it represent that it has
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 Copies of IPR disclosures made to the IETF Secretariat and any
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 The IETF invites any interested party to bring to its attention any
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Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Vida & Costa Standards Track [Page 62]

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