Multicast Listener Discovery Version 2 (MLDv2) for IPv6
RFC 9777 also known as STD 101


Internet Engineering Task Force (IETF) B. Haberman, Ed.
Request for Comments: 9777 JHU APL
STD: 101 March 2025
Obsoletes: 3810 
Updates: 2710 
Category: Standards Track 
ISSN: 2070-1721
 Multicast Listener Discovery Version 2 (MLDv2) for IPv6
Abstract
 This document specifies the Multicast Listener Discovery version 2
 (MLDv2) protocol. 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.
 This document updates RFC 2710 and obsoletes RFC 3810.
Status of This Memo
 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF). It represents the consensus of the IETF community. It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG). Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc9777.
Copyright Notice
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 document authors. All rights reserved.
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Table of Contents
 1. Introduction
 1.1. Conventions Used in This Document
 2. Protocol Overview
 2.1. Building Multicast Address Listening State on Multicast
 Address Listeners
 2.2. Exchanging Messages between the Querier and the Listening
 Nodes
 2.3. Building Multicast Address Listening State on Multicast
 Routers
 3. The Service Interface for Requesting IP Multicast Reception
 4. Multicast Address Listening State Maintained by Nodes
 4.1. Per-Socket State
 4.2. Per-Interface State
 5. Message Formats
 5.1. Multicast Listener Query Message
 5.1.1. Code
 5.1.2. Checksum
 5.1.3. Maximum Response Code
 5.1.4. Reserved
 5.1.5. Multicast Address
 5.1.6. Flags
 5.1.7. S Flag (Suppress Router-Side Processing)
 5.1.8. QRV (Querier's Robustness Variable)
 5.1.9. QQIC (Querier's Query Interval Code)
 5.1.10. Number of Sources (N)
 5.1.11. Source Address [i]
 5.1.12. Additional Data
 5.1.13. Query Variants
 5.1.14. Source Addresses for Queries
 5.1.15. Destination Addresses for Queries
 5.2. Version 2 Multicast Listener Report Message
 5.2.1. Reserved
 5.2.2. Checksum
 5.2.3. Flags
 5.2.4. Nr of Mcast Address Records (M)
 5.2.5. Multicast Address Record
 5.2.6. Record Type
 5.2.7. Aux Data Len
 5.2.8. Number of Sources (N)
 5.2.9. Multicast Address
 5.2.10. Source Address [i]
 5.2.11. Auxiliary Data
 5.2.12. Additional Data
 5.2.13. Multicast Address Record Types
 5.2.14. Source Addresses for Reports
 5.2.15. Destination Addresses for Reports
 5.2.16. Multicast Listener Report Size
 6. Protocol Description for Multicast Address Listeners
 6.1. Action on Change of Per-Interface State
 6.2. Action on Reception of a Query
 6.3. Action on Timer Expiration
 7. Description of the Protocol for Multicast Routers
 7.1. Conditions for MLD Queries
 7.2. MLD State Maintained by Multicast Routers
 7.2.1. Definition of Router Filter Mode
 7.2.2. Definition of Filter Timers
 7.2.3. Definition of Source Timers
 7.3. MLDv2 Source-Specific Forwarding Rules
 7.4. Action on Reception of Reports
 7.4.1. Reception of Current-State Records
 7.4.2. Reception of Filter-Mode-Change and Source-List-Change
 Records
 7.5. Switching Router Filter Modes
 7.6. Action on Reception of Queries
 7.6.1. Timer Updates
 7.6.2. Querier Election
 7.6.3. Building and Sending Specific Queries
 8. Interoperation with MLDv1
 8.1. Query Version Distinctions
 8.2. Multicast Address Listener Behavior
 8.2.1. In the Presence of MLDv1 Routers
 8.2.2. In the Presence of MLDv1 Multicast Address Listeners
 8.3. Multicast Router Behavior
 8.3.1. In the Presence of MLDv1 Routers
 8.3.2. In the Presence of MLDv1 Multicast Address Listeners
 9. List of Timers, Counters, and Their Default Values
 9.1. Robustness Variable
 9.2. Query Interval
 9.3. Query Response Interval
 9.4. Multicast Address Listening Interval
 9.5. Other Querier Present Interval
 9.6. Startup Query Interval
 9.7. Startup Query Count
 9.8. Last Listener Query Interval
 9.9. Last Listener Query Count
 9.10. Last Listener Query Time
 9.11. Unsolicited Report Interval
 9.12. Older Version Querier Present Interval
 9.13. Older Version Host Present Interval
 9.14. Configuring Timers
 9.14.1. Robustness Variable
 9.14.2. Query Interval
 9.14.3. Maximum Response Delay
 10. Security Considerations
 10.1. Query Message
 10.2. Current-State Report Messages
 10.3. State-Change Report Messages
 11. IANA Considerations
 12. References
 12.1. Normative References
 12.2. Informative References
 Appendix A. Design Rationale
 A.1. The Need for State-Change Report Messages
 A.2. Host Suppression
 A.3. Switching Router Filter Modes from EXCLUDE to INCLUDE
 Appendix B. Summary of Changes
 B.1. MLDv1
 B.2. Changes since RFC 3810
 Acknowledgments
 Contributors
 Author's Address
1. Introduction
 The Multicast Listener Discovery (MLD) protocol 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 and to
 inform itself and other neighboring multicast routers of its
 listening state, respectively.
 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 IGMPv3 [RFC9776] 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 (SSM) [RFC3569], or from *all
 but* specific source addresses, sent to a particular multicast
 address. MLDv2 is designed to be interoperable with MLDv1.
 This document uses "SSM-aware" to refer to systems that support SSM
 as defined in [RFC4607].
 This document obsoletes [RFC3810]. Appendix B.2 lists the main
 changes from [RFC3810].
1.1. Conventions Used in This Document
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.
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 A.2, item 1 for discussion.)
 A multicast router performs the router part of the MLDv2 protocol
 (described in detail 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.
 A Multicast Address Listener performs the listener part of the MLDv2
 protocol (described in detail 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 Address 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 Address Listening state, a node must
 also maintain or compute Multicast Address 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 it may differ from the per-socket state when different
 sockets have differing filter-modes and/or source-lists for the same
 multicast address and interface. 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 Address 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
 Listening 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 that all MLDv2 routers
 on 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 5.2.13.
 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 to not overload the link; it is assumed that in
 general, these messages do not generate state changes as their main
 purpose is 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 Listening state and stopping the
 corresponding traffic. Thus, the Querier sends a Multicast Address
 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 2.3, 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 A.1.
 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
 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 Listening 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 Listening 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
 Sections 7.4.1 and 7.4.2.
 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:
 1. 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, which 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 Sections 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 A.3.
 2. 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
 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 sources, 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 Sections 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:
 * "socket" is an implementation-specific parameter used to
 distinguish among different requesting entities (e.g., programs
 and processes) within the node; the socket parameter of BSD Unix
 system calls is a specific example.
 * "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.
 * "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.
 * "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.
 * "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 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].
4. Multicast Address 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 Address 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:
 * 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.
 * 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 Address Listening state, a
 node must also maintain or compute Multicast Address 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 it may differ from the per-socket state when different
 sockets have differing filter-modes and/or source-lists for the same
 multicast address and interface. For example, suppose one
 application or process invokes the following operation on socket s1:
 IPv6MulticastListen ( s1, i, m, INCLUDE, {a, b, c} )
 requesting reception on interface i of packets sent to multicast
 address m, only if they come from 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 Address
 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 feature of this service interface. The previous
 service interface described no filtering based upon Multicast Address
 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,
 * if any such record has a filter-mode of EXCLUDE, then the filter-
 mode of the interface record is EXCLUDE, and the source-list of
 the interface record is the intersection of the source-lists of
 all socket records in EXCLUDE mode, minus those source addresses
 that appear in any socket record in INCLUDE mode. For example, if
 the socket records for multicast address m on interface i are:
 - from socket s1: ( i, m, EXCLUDE, {a, b, c, d} )
 - from socket s2: ( i, m, EXCLUDE, {b, c, d, e} )
 - from socket s3: ( i, m, INCLUDE, {d, e, f} )
 then the corresponding interface record on interface i is:
 - ( m, EXCLUDE, {b, c} )
 If a fourth socket is added, such as:
 - From socket s4: ( i, m, EXCLUDE, {} )
 then the interface record becomes:
 - ( m, EXCLUDE, {} )
 * 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
 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 [RFC4291], if a valid link-local IPv6 source
 address has not been acquired yet for the sending interface. (See
 Section 5.2.14 for details.)
 There are two MLD message types of concern to the MLDv2 protocol
 described in this document:
 * Multicast Listener Query (Type = decimal 130)
 * 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:
 * Multicast Listener Report (Type = decimal 131) [RFC2710]
 * 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 Address Listening state of neighboring
 interfaces. Queries have the following format:
 0 1 2 3
 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Type = 130 | Code | Checksum |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Maximum Response Code | Reserved |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 * *
 | |
 * Multicast Address *
 | |
 * *
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Flags |S| QRV | QQIC | Number of Sources (N) |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 * *
 | |
 * Source Address [1] *
 | |
 * *
 | |
 +- -+
 | |
 * *
 | |
 * Source Address [2] *
 | |
 * *
 | |
 +- . -+
 . . .
 . . .
 +- -+
 | |
 * *
 | |
 * Source Address [N] *
 | |
 * *
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.1.1. Code
 The Code field is initialized to zero by the sender and ignored by
 receivers.
5.1.2. Checksum
 The Checksum field is the standard ICMPv6 checksum; it covers the
 entire MLDv2 message, plus a "pseudo-header" of IPv6 header fields
 [RFC4443] [RFC8200]. 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
 The Reserved field is set to zero on transmission and ignored on
 reception.
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).
5.1.6. Flags
 Allocation of individual bits within the Flags field is described in
 Section 2.2 of [RFC9778]. Future specifications will define the
 associated meaning tied to any such allocation.
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 receiving 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 QQIC 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 QQIC as
 follows:
 * If QQIC < 128, QQI = QQIC
 * If QQIC >= 128, QQIC represents a floating-point value as follows:
 0 1 2 3 4 5 6 7
 +-+-+-+-+-+-+-+-+
 |1| exp | mant |
 +-+-+-+-+-+-+-+-+
 QQI = (mant | 0x10) << (exp + 3)
 Multicast routers that are not the current Querier adopt the QQI
 value from the most recently received Query as their own [Query
 Interval] value, unless that most recently received QQI was zero, in
 which case the receiving routers use the default [Query Interval]
 value specified in Section 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:
 * 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.
 * 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.
 * 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.
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 Address Listening
 state, or changes in the Multicast Address 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 |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | Flags |Nr of Mcast Address Records (M)|
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 . .
 . Multicast Address Record [1] .
 . .
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 . .
 . Multicast Address Record [2] .
 . .
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | . |
 . . .
 | . |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 | |
 . .
 . Multicast Address Record [M] .
 . .
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 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 .
 . .
 | |
 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.2.1. Reserved
 The Reserved field is set to zero on transmission and ignored on
 reception.
5.2.2. Checksum
 The Checksum Field is the standard ICMPv6 checksum; it covers the
 entire MLDv2 message, plus a "pseudo-header" of IPv6 header fields
 [RFC4443] [RFC8200]. 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. Flags
 Allocation of individual bits within the Flags field is described in
 Section 2.3 of [RFC9778]. Future specifications will define the
 associated meaning tied to any such allocation.
5.2.4. Nr of Mcast Address Records (M)
 The Nr of the Mcast Address Records (M) field specifies how many
 Multicast Address Records are present in this Report.
5.2.5. 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.
5.2.6. Record Type
 The Record Type field specifies the type of the Multicast Address
 Record. See Section 5.2.13 for a detailed description of the
 different possible Record Types.
5.2.7. 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.
5.2.8. Number of Sources (N)
 The Number of Sources (N) field specifies how many source addresses
 are present in this Multicast Address Record.
5.2.9. Multicast Address
 The Multicast Address field contains the multicast address to which
 this Multicast Address Record pertains.
5.2.10. 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.
5.2.11. Auxiliary Data
 The Auxiliary Data field, if present, contains additional information
 that pertains 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.
5.2.12. 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.
5.2.13. Multicast Address Record Types
 There are a number of different types of Multicast Address Records
 that may be included in a Report message:
 * 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:
 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.
 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. An SSM-aware host
 SHOULD NOT send a MODE_IS_EXCLUDE record type for multicast
 addresses that fall within the SSM address range as they will
 be ignored by SSM-aware routers [RFC4604].
 * 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. An SSM-aware
 host SHOULD NOT send a CHANGE_TO_EXCLUDE_MODE record type for
 multicast addresses that fall within the SSM address range.
 * A "Source-List-Change Record" is sent by a node whenever a local
 invocation of IPv6MulticastListen causes a change of source-list
 that is not coincident with a change of filter-mode, of the
 interface-level state entry for a particular multicast address.
 The Record is included in a Report sent from the interface on
 which the change occurred. The Record Type of a Source-List-
 Change Record may be one of the following two values:
 5. ALLOW_NEW_SOURCES - indicates that the Source Address [i]
 fields in this Multicast Address Record contain a list of the
 additional sources that the node wishes to listen to, for
 packets sent to the specified multicast address. If the
 change was to an INCLUDE source-list, these are the addresses
 that were added to the list; if the change was to an EXCLUDE
 source-list, these are the addresses that were deleted from
 the list.
 6. BLOCK_OLD_SOURCES - indicates that the Source Address [i]
 fields in this Multicast Address Record contain a list of the
 sources that the node no longer wishes to listen to, for
 packets sent to the specified multicast address. If the
 change was to an INCLUDE source-list, these are the addresses
 that were deleted from the list; if the change was to an
 EXCLUDE source-list, these are the addresses that were added
 to the list.
 If a change of source-list results in both allowing new sources and
 blocking old sources, then two Multicast Address Records are sent for
 the same multicast address, one of type ALLOW_NEW_SOURCES and one of
 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:
 * a capital letter (e.g., "A") to represent the set of source
 addresses or
 * 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.
5.2.14. 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 [RFC4861]. For
 stateless autoconfiguration, as defined in [RFC4862], a node is
 required to join several IPv6 multicast groups, in order to perform
 Duplicate Address Detection (DAD). Prior to DAD, the only address
 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.
5.2.15. Destination Addresses for Reports
 Version 2 Multicast Listener Reports are sent with an IP destination
 address of ff02::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.
5.2.16. Multicast Listener Report Size
 If the set of Multicast Address Records required in a Report does not
 fit within the size limit of a single Report message (as determined
 by the MTU of the link on which it will be sent), the Multicast
 Address Records are sent in as many Report messages as needed to
 report the entire set.
 If a single Multicast Address Record contains so many source
 addresses that it does not fit within the size limit of a single
 Report message, then:
 * 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.
 * 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). Multicast listeners MUST send MLD messages for all
 multicast addresses except for the link-scope all-nodes multicast
 address and any multicast addresses of scope less than 2.
 There are three types of events that trigger MLDv2 protocol actions
 on an interface:
 * a change of the per-interface listening state, caused by a local
 invocation of IPv6MulticastListen;
 * the firing of a specific timer; and
 * 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 Address
 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
 Table 1. 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) |
 +-------------+-------------+--------------------------+
 Table 1: State-Change Record Transmission Logic
 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:
 * As for the first Report, the per-interface state for the affected
 multicast address before and after the latest change is compared.
 * 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
 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 Table 2.
 +========+=======================================================+
 | 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. |
 +--------+-------------------------------------------------------+
 Table 2: Per-Interface State-Change Report Contents
 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.
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 fail, 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:
 * an Interface Timer for scheduling responses to General Queries;
 * a Multicast Address Timer for scheduling responses to Multicast
 Address (and Source) Specific Queries, for each multicast address
 the node has to report on; and
 * a per-multicast-address list of sources to be reported in response
 to a Multicast Address and Source Specific Query.
 When a 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 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.)
 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 for use 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
 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
 Table 3.
 +=====================+=========================+===============+
 | Per-Interface | Set of Sources in the | Current-State |
 | State | Pending Response Record | Record |
 +=====================+=========================+===============+
 | INCLUDE (A) | B | IS_IN (A*B) |
 +---------------------+-------------------------+---------------+
 | EXCLUDE (A) | B | IS_IN (B-A) |
 +---------------------+-------------------------+---------------+
 Table 3: Determining Contents of Current-State Record
 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
 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 Table 4.
 +========+=====================================================+
 | 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. |
 +--------+-----------------------------------------------------+
 Table 4: Determining Contents of Source-List-Change Records
 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.
 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, as 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 A.2, 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.
 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 Listening 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 MLD Version 2 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 Listening 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 that listen to traffic from a specific
 set of sources. Multicast Address and Source Specific Queries list
 sources for a particular multicast address that 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.
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
 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;
 see Section 7.5 for more details.
 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 detail in Sections
 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.
 Table 5 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.
 +=========+========+================================================+
 | Router | Filter | Actions/Comments |
 | Filter | Timer | |
 | Mode | Value | |
 +=========+========+================================================+
 | INCLUDE | Not | All listeners in INCLUDE mode. |
 | | Used | |
 +---------+--------+------------------------------------------------+
 | EXCLUDE | Timer | At least one listener in EXCLUDE mode. |
 | | > 0 | |
 +---------+--------+------------------------------------------------+
 | EXCLUDE | Timer | No more listeners in EXCLUDE mode for |
 | | == 0 | 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. |
 +---------+--------+------------------------------------------------+
 Table 5: Filter Timer Management
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 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. 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 Sections 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. Then, the router informs 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:
 1. 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, which 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 Sections 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 A.3.
 2. 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 Sections 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, Table 6 below 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.
 +=========+=========+=========================================+
 | Router | Source | Action |
 | Filter | Timer | |
 | Mode | Value | |
 +=========+=========+=========================================+
 | INCLUDE | TIMER > | Suggest to forward traffic from source. |
 | | 0 | |
 +---------+---------+-----------------------------------------+
 | INCLUDE | TIMER | Suggest to stop forwarding traffic from |
 | | == 0 | source and remove the source record. |
 | | | If there are no more source records, |
 | | | delete the Multicast Address Record. |
 +---------+---------+-----------------------------------------+
 | INCLUDE | No | Suggest to not forward traffic from |
 | | Source | source. |
 | | Element | |
 +---------+---------+-----------------------------------------+
 | EXCLUDE | TIMER > | Suggest to forward traffic from source. |
 | | 0 | |
 +---------+---------+-----------------------------------------+
 | EXCLUDE | TIMER | Suggest to not forward traffic from |
 | | == 0 | source. Move the source from the |
 | | | Requested List to the Exclude List (DO |
 | | | NOT remove the source record). |
 +---------+---------+-----------------------------------------+
 | EXCLUDE | No | Suggest to forward traffic from all |
 | | Source | sources. |
 | | Element | |
 +---------+---------+-----------------------------------------+
 Table 6: MLD Forwarding Recommendations
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 fail, the packet is dropped. If the validity of
 the MLD message is verified, the router starts to process the Report.
 SSM-aware routers SHOULD ignore records that contain multicast
 addresses in the SSM address range if the record type is
 MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE. SSM-aware routers SHOULD
 ignore MLDv1 Report and DONE messages that contain multicast
 addresses in the SSM address range, SHOULD NOT use such Reports to
 establish IP forwarding state, and MAY log an error if it receives
 such a message.
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. Table 7 describes
 the actions, with respect to state and timers, that occur to a
 router's state upon reception of Current-State Records.
 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 set A of the source records should
 have their source timers set to value J. 'Delete (A)' means that set
 A of the source records should be deleted. 'Filter Timer = J' means
 that the Filter Timer for the multicast address should be set to
 value J.
 +=========+==========+=========+======================+
 | Router | Report | New | Actions |
 | State | Received | Router | |
 | | | State | |
 +=========+==========+=========+======================+
 | INCLUDE | IS_IN | INCLUDE | (B)=MALI |
 | (A) | (B) | (A+B) | |
 +---------+----------+---------+----------------------+
 | INCLUDE | IS_EX | EXCLUDE | (B-A)=0 |
 | (A) | (B) | (A*B, | |
 | | | B-A) | Delete (A-B) |
 | | | | |
 | | | | Filter Timer=MALI |
 +---------+----------+---------+----------------------+
 | EXCLUDE | IS_IN | EXCLUDE | (A)=MALI |
 | (X,Y) | (A) | (X+A, | |
 | | | Y-A) | |
 +---------+----------+---------+----------------------+
 | EXCLUDE | IS_EX | EXCLUDE | (A-X-Y)=MALI |
 | (X,Y) | (A) | (A-Y, | |
 | | | Y*A) | Delete (X-A) |
 | | | | |
 | | | | Delete (Y-A) |
 | | | | |
 | | | | Filter Timer=MALI |
 +---------+----------+---------+----------------------+
 Table 7: Actions for Received Current-State Records
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 that express interest in listening to the
 queried sources, the corresponding timers are updated.
 Multicast Address Specific queries can also be used in order to
 enable a fast transition of a router from EXCLUDE to INCLUDE mode, in
 case a received Multicast Address Record motivates this action. The
 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, and 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.
 Table 8 describes the changes in multicast address state and the
 action(s) taken when receiving either Filter-Mode-Change or Source-
 List-Change Records. Table 8 also describes the queries that are
 sent by the Querier when a particular report is received.
 We use the following notation to describe 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 Table 8 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.
 +=========+==========+====================+========================+
 | Router | Report | New Router State | Actions |
 | State | Received | | |
 +=========+==========+====================+========================+
 | INCLUDE | ALLOW | INCLUDE(A+B) | (B)=MALI |
 | (A) | (B) | | |
 +---------+----------+--------------------+------------------------+
 | INCLUDE | BLOCK | INCLUDE(A) | Send Q(MA,A*B) |
 | (A) | (B) | | |
 +---------+----------+--------------------+------------------------+
 | INCLUDE | TO_EX | EXCLUDE(A*B,B-A) | (B-A)=0, Delete (A-B), |
 | (A) | (B) | | Send Q(MA,A*B), Filter |
 | | | | Timer=MALI |
 +---------+----------+--------------------+------------------------+
 | INCLUDE | TO_IN | INCLUDE(A+B) | (B)=MALI, Send |
 | (A) | (B) | | Q(MA,A-B) |
 +---------+----------+--------------------+------------------------+
 | EXCLUDE | ALLOW | EXCLUDE(X+A,Y-A) | (A)=MALI |
 | (X,Y) | (A) | | |
 +---------+----------+--------------------+------------------------+
 | EXCLUDE | BLOCK | EXCLUDE(X+(A-Y),Y) | (A-X-Y)=Filter Timer, |
 | (X,Y) | (A) | | Send Q(MA,A-Y) |
 +---------+----------+--------------------+------------------------+
 | EXCLUDE | TO_EX | EXCLUDE(A-Y,Y*A) | (A-X-Y)=Filter Timer, |
 | (X,Y) | (A) | | Delete (X-A), Delete |
 | | | | (Y-A), Send Q(MA,A-Y), |
 | | | | Filter Timer=MALI |
 +---------+----------+--------------------+------------------------+
 | EXCLUDE | TO_IN | EXCLUDE(X+A,Y-A) | (A)=MALI, Send |
 | (X,Y) | (A) | | Q(MA,X-A), Send Q(MA) |
 +---------+----------+--------------------+------------------------+
 Table 8: Multicast Router State Transitions
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
 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 fail, 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 S flag to ensure robustness, as explained in
 Section 2.1. When a router sends or receives a query with a clear S
 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 S flag not set.
 +=========+=====================================================+
 | Query | Action |
 +=========+=====================================================+
 | Q(MA,A) | Source Timers for sources in A are lowered to LLQT. |
 +---------+-----------------------------------------------------+
 | Q(MA) | The Filter Timer is lowered to LLQT. |
 +---------+-----------------------------------------------------+
 Table 9: Timer Actions for Query Message Transmission
 When a router sends or receives a query with the S 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 Interval]; if it was previously in Querier state, it switches
 to Non- 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
 Table 8 (Section 7.4.2), the following actions must be performed for
 each of the sources in X that send to multicast address MA, with the
 Source Timer larger than LLQT:
 * lower Source Timer to LLQT;
 * add the sources to the Retransmission List; and
 * 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.
 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 that can be in one of the two states:
 MLDv1 or MLDv2.
 The Host Compatibility Mode of an interface is set to MLDv1 whenever
 an MLDv1 Multicast Address Listener General 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 Interval]
 seconds. The timer is reset whenever a new MLDv1 General 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.
 An SSM-aware host that receives an MLDv1 General Query or MLDv1 Group
 Specific Query for a multicast address in the SSM address range
 SHOULD log an error. It is RECOMMENDED that implementations provide
 a configuration option to disable the use of Host Compatibility Mode
 to allow networks to operate only in SSM mode. This configuration
 option SHOULD be disabled by default.
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 MLD Version 2 Multicast Listener Report to be
 suppressed by an MLDv1 Multicast Listener Report (Type = decimal
 131).
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:
 * 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.
 * 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.
 * It is RECOMMENDED that implementations provide a configuration
 option to disable use of compatibility mode to allow networks to
 operate only in SSM mode. This configuration option SHOULD be
 disabled by default.
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
 (Type = decimal 131) 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 Interval] seconds. The
 timer is reset whenever a new MLDv1 Report is received for that
 multicast address. If the Older-Version-Host-Present Timer expires,
 the router switches back to the 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 (Table 10).
 +===============+==================+
 | MLDv1 Message | MLDv2 Equivalent |
 +===============+==================+
 | Report | IS_EX( {} ) |
 +---------------+------------------+
 | Done | TO_IN( {} ) |
 +---------------+------------------+
 Table 10: MLD Message Translation
 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.
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 Query Response Interval is the Maximum Response Delay used to
 calculate the Maximum Response Code that is 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 [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 2 times [Query Response Interval].
9.5. Other Querier Present Interval
 The Other Querier Present Interval is the length of time that must
 pass before a multicast router decides that there is no longer
 another multicast router that should be the Querier. This value MUST
 be ([Robustness Variable] times ([Query Interval]) plus (0.5 times
 [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 (LLQI) is the Maximum Response Delay
 used to calculate the Maximum Response Code inserted into Multicast
 Address Specific Queries sent in response to MLDv1 Multicast Listener
 Done (Type = decimal 132) 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
 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] times [Last Listener Query Count]. It
 is not a tunable value, but it 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 Interval
 The Older Version Querier Present Interval is the timeout 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 Interval].
 This value MUST be ([Robustness Variable] times [Query Interval] in
 the last Query received) plus ([Query Response Interval]).
9.13. Older Version Host Present Interval
 The Older Version Host Present Interval is the timeout 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 Interval].
 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.
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 (as shown in Table 11)
 per Query by using the expected number of Reporters for that Query.
 +=======================+==========================================+
 | Query Type | Expected Number of Reporters |
 +=======================+==========================================+
 | General Query | All nodes on the link. |
 +-----------------------+------------------------------------------+
 | Multicast Address | All nodes on the link that had expressed |
 | Specific Query | interest in the multicast address. |
 +-----------------------+------------------------------------------+
 | Multicast Address and | All nodes on the link that had expressed |
 | Source Specific Query | interest in the source and multicast |
 | | address. |
 +-----------------------+------------------------------------------+
 Table 11: Maximum Response Delay Calculation
 A router is not required to calculate these populations or tune the
 Maximum Response Delay dynamically; these are simply guidelines.
10. Security Considerations
 MLD does not contain any cryptographic protection, thus its messages
 are not authenticated, the message contents are not confidential, and
 any message can be replayed. The ability to replay messages does not
 affect the behavior of the protocol itself.
 Replaying messages can lead to multicast forwarding state to remain
 active beyond the needs of group members on a link. Excessive
 retention of multicast state may lead to resource exhaustion in some
 devices.
 The lack of confidentiality allows any device with access to the link
 to determine which multicast groups are being requested. This is a
 privacy issue as some multicast content may be sensitive.
 The lack of authentication allows for the creation of forged
 messages. 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, we discuss only the effects
 of on-link forgery in the following section.
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.
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. If a large number of
 forged Report messages are generated, a multicast router may consume
 significant resources maintaining multicast forwarding state.
 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 it cannot cause loss of desired traffic.
11. IANA Considerations
 IANA has assigned the IPv6 link-local multicast address ff02::16,
 called "all MLDv2-capable routers", as described in Section 5.2.15.
 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,
 DOI 10.17487/RFC2119, March 1997,
 <https://www.rfc-editor.org/info/rfc2119>.
 [RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet
 Networks", RFC 2464, DOI 10.17487/RFC2464, December 1998,
 <https://www.rfc-editor.org/info/rfc2464>.
 [RFC2710] Deering, S., Fenner, W., and B. Haberman, "Multicast
 Listener Discovery (MLD) for IPv6", RFC 2710,
 DOI 10.17487/RFC2710, October 1999,
 <https://www.rfc-editor.org/info/rfc2710>.
 [RFC2711] Partridge, C. and A. Jackson, "IPv6 Router Alert Option",
 RFC 2711, DOI 10.17487/RFC2711, October 1999,
 <https://www.rfc-editor.org/info/rfc2711>.
 [RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
 Architecture", RFC 4291, DOI 10.17487/RFC4291, February
 2006, <https://www.rfc-editor.org/info/rfc4291>.
 [RFC4443] Conta, A., Deering, S., and M. Gupta, Ed., "Internet
 Control Message Protocol (ICMPv6) for the Internet
 Protocol Version 6 (IPv6) Specification", STD 89,
 RFC 4443, DOI 10.17487/RFC4443, March 2006,
 <https://www.rfc-editor.org/info/rfc4443>.
 [RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet
 Group Management Protocol Version 3 (IGMPv3) and Multicast
 Listener Discovery Protocol Version 2 (MLDv2) for Source-
 Specific Multicast", RFC 4604, DOI 10.17487/RFC4604,
 August 2006, <https://www.rfc-editor.org/info/rfc4604>.
 [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
 IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
 <https://www.rfc-editor.org/info/rfc4607>.
 [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
 May 2017, <https://www.rfc-editor.org/info/rfc8174>.
 [RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
 (IPv6) Specification", STD 86, RFC 8200,
 DOI 10.17487/RFC8200, July 2017,
 <https://www.rfc-editor.org/info/rfc8200>.
 [RFC9778] Haberman, B., Ed., "IANA Considerations for Internet Group
 Management Protocols", BCP 57, RFC 9778,
 DOI 10.17487/RFC9778, March 2025,
 <https://www.rfc-editor.org/info/rfc9778>.
12.2. Informative References
 [RFC3569] Bhattacharyya, S., Ed., "An Overview of Source-Specific
 Multicast (SSM)", RFC 3569, DOI 10.17487/RFC3569, July
 2003, <https://www.rfc-editor.org/info/rfc3569>.
 [RFC3678] Thaler, D., Fenner, B., and B. Quinn, "Socket Interface
 Extensions for Multicast Source Filters", RFC 3678,
 DOI 10.17487/RFC3678, January 2004,
 <https://www.rfc-editor.org/info/rfc3678>.
 [RFC3810] Vida, R., Ed. and L. Costa, Ed., "Multicast Listener
 Discovery Version 2 (MLDv2) for IPv6", RFC 3810,
 DOI 10.17487/RFC3810, June 2004,
 <https://www.rfc-editor.org/info/rfc3810>.
 [RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
 "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
 DOI 10.17487/RFC4861, September 2007,
 <https://www.rfc-editor.org/info/rfc4861>.
 [RFC4862] Thomson, S., Narten, T., and T. Jinmei, "IPv6 Stateless
 Address Autoconfiguration", RFC 4862,
 DOI 10.17487/RFC4862, September 2007,
 <https://www.rfc-editor.org/info/rfc4862>.
 [RFC9776] Haberman, B., Ed., "Internet Group Management Protocol,
 Version 3", STD 100, RFC 9776, DOI 10.17487/RFC9776, March
 2025, <https://www.rfc-editor.org/info/rfc9776>.
Appendix A. Design Rationale
A.1. The Need for State-Change Report 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 it 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 Layer 2 / Layer 3 switches that
 implement MLD snooping do not forward MLD messages across LAN
 segments in order to prevent Multicast Listener Report
 suppression.
 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
B.1. MLDv1
 The following is a summary of changes from MLDv1, specified in
 [RFC2710].
 * MLDv2 introduces source filtering.
 * The IP service interface of MLDv2 nodes is modified accordingly.
 It enables the specification of a filter-mode and a source-list.
 * An MLDv2 node keeps per-socket and per-interface Multicast Address
 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.
 * 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.
 * Queries include additional fields (Section 5.1).
 * The S flag is included in queries in order to fix robustness
 issues.
 * 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.
 * A new Query type (Multicast Address and Source Specific Query) is
 introduced.
 * 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.
 * Reports include Multicast Address Records. Information on the
 listening state for several different multicast addresses can be
 included in the same Report message.
 * 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.
 * There is no "host suppression", as in MLDv1. All nodes send
 Report messages.
 * Unsolicited Reports, announcing changes in receiver listening
 state, are sent [Robustness Variable] times. [RFC2710] is less
 explicit.
 * There are no Done messages.
 * Interoperability with MLDv1 systems is achieved by MLDv2 state
 operations.
 * 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.
B.2. Changes since RFC 3810
 The following summarizes the changes made since [RFC3810].
 * Added definition of Resv to address Erratum 4773.
 * Added clarifying text on which multicast addresses require the
 sending of MLD messages to address Erratum 5977.
 * Added text to clarify the Group Membership Interval Timer changes
 per Erratum 6725.
 * Changed "Reserved field" to "Flags field" in messages to
 facilitate use of an IANA-managed registry for future bit
 allocations.
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 specification.
 Stig Venaas, Hitoshi Asaeda, and Mike McBride have provided valuable
 feedback on this specification, and we thank them for their input.
Contributors
 Roland Vida, Luis Henrique Maciel Kosmalski Costa, Serge Fdida, Steve
 Deering, Bill Fenner, and Isidor Kouvelas are the authors of RFC
 3810, which makes up the majority of the content in this
 specification.
 Anuj Budhiraja, Toerless Eckert, Olufemi Komolafe, and Tim Winters
 have contributed valuable content to this specification.
Author's Address
 Brian Haberman (editor)
 Johns Hopkins University Applied Physics Lab
 Email: brian@innovationslab.net