RFC 3810
Multicast Listener Discovery Version 2 (MLDv2) for IPv6
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, and is explained in detail in section 7.5. When the router is in EXCLUDE mode, the router state is represented through the notation EXCLUDE (X,Y), where X is called the "Requested List" and Y is called the "Exclude List". All sources, except those from the Exclude List, will be forwarded by the router. The Requested List has no effect on forwarding. Nevertheless, it has to be maintained for several reasons, as explained in section 7.2.3. The exact handling of both the INCLUDE and EXCLUDE mode router state, according to the received reports, is presented in details in Tables 7.4.1 and 7.4.2.7.2.2. Definition of Filter Timers
The Filter Timer is only used when the router is in EXCLUDE mode for a specific multicast address, and it represents the time for the Router Filter Mode of the multicast address to expire and switch to INCLUDE mode. A Filter Timer is a decrementing timer with a lower bound of zero. One Filter Timer exists per multicast address record. Filter Timers are updated according to the types of Multicast Address Records received. If a Filter Timer expires, with the Router Filter Mode for that multicast address being EXCLUDE, it means that there are no more listeners in EXCLUDE mode on the attached link. At this point, the router transitions to INCLUDE filter mode. Section 7.5 describes the actions taken when a Filter Timer expires while in EXCLUDE mode. The following table summarizes the role of the Filter Timer. Section 7.4 describes the details of setting the Filter Timer per type of Multicast Address Record received.
Router Filter
Filter Mode Timer Value Actions/Comments
----------- ----------------- ----------------
INCLUDE Not Used All listeners in
INCLUDE mode.
EXCLUDE Timer > 0 At least one listener
in EXCLUDE mode.
EXCLUDE Timer == 0 No more listeners in
EXCLUDE mode for the
multicast address.
If the Requested List
is empty, delete
Multicast Address
Record. If not, switch
to INCLUDE filter mode;
the sources in the
Requested List are
moved to the Include
List, and the Exclude
List is deleted.
7.2.3. Definition of Source Timers
A Source Timer is a decrementing timer with a lower bound of zero.
One Source Timer is kept per source record. Source timers are
updated according to the type and filter mode of the Multicast
Address Record received. Section 7.4 describes the setting of source
timers per type of Multicast Address Records received.
In the following, abbreviations are used for several variables (all
of which are described in detail in section 9). The variable MALI
stands for the Multicast Address Listening Interval, which is the
time in which multicast address listening state will time out. The
variable LLQT is the Last Listener Query Time, which is the total
time the router should wait for a report, after the Querier has sent
the first query. During this time, the Querier should send [Last
Member Query Count]-1 retransmissions of the query. LLQT represents
the "leave latency", or the difference between the transmission of a
listener state change and the modification of the information passed
to the routing protocol.
If the router is in INCLUDE filter mode, a source can be added to the
current Include List if a listener in INCLUDE mode sends a Current
State or a State Change Report which includes that source. Each
source from the Include List is associated with a source timer that
is updated whenever a listener in INCLUDE mode sends a report that
confirms its interest in that specific source. If the timer of a
source from the Include List expires, the source is deleted from the
Include List. If there are no more source records left, the
multicast address record is deleted from the router.
Besides this "soft leave" mechanism, there is also a "fast leave"
scheme in MLDv2; it is also based on the use of source timers. When
a node in INCLUDE mode expresses its desire to stop listening to a
specific source, all the multicast routers on the link lower their
timer for that source to a small interval of LLQT milliseconds. The
Querier then sends then a Multicast Address and Source Specific
Query, to verify whether there are other listeners for that source on
the link, or not. If a corresponding report is received before the
timer expires, all the multicast routers on the link update their
source timer. If not, the source is deleted from the Include List.
The handling of the Include List, according to the received reports,
is detailed in Tables 7.4.1 and 7.4.2.
Source timers are treated differently when the Router Filter Mode for
a multicast address is EXCLUDE. For sources from the Requested List
the source timers have running values; these sources are forwarded by
the router. For sources from the Exclude List the source timers are
set to zero; these sources are blocked by the router. If the timer
of a source from the Requested List expires, the source is moved to
the Exclude List. The router informs then the routing protocol that
there is no longer a listener on the link interested in traffic from
this source.
The router has to maintain the Requested List for two reasons:
o To keep track of sources that listeners in INCLUDE mode listen to.
This is necessary in order to assure a seamless transition of the
router to INCLUDE mode, when there will be no listener in EXCLUDE
mode left. This transition should not interrupt the flow of
traffic to the listeners in INCLUDE mode still interested in that
multicast address. Therefore, at the moment of the transition,
the Requested List should represent the set of sources that nodes
in INCLUDE mode have explicitly requested.
When the router switches to INCLUDE mode, the sources in the
Requested List are moved to the Include List, and the Exclude List
is deleted. Before the switch, the Requested List can contain an
inexact guess at the sources that listeners in INCLUDE mode listen
to - might be too large or too small. These inexactitudes are due
to the fact that the Requested List is also used for fast blocking
purposes, as described below. If such a fast blocking is
required, some sources may be deleted from the Requested List (as
shown in Tables 7.4.1 and 7.4.2) in order to reduce router state.
Nevertheless, in each such case the Filter Timer is updated as
well. Therefore, listeners in INCLUDE mode will have enough time,
before an eventual switching, to reconfirm their interest in the
eliminated source(s), and rebuild the Requested List accordingly.
The protocol ensures that when a switch to INCLUDE mode occurs,
the Requested List will be accurate. Details about the transition
of the router to INCLUDE mode are presented in Appendix A3.
o To allow a fast blocking of previously unblocked sources. If the
router receives a report that contains such a request, the
concerned sources are added to the Requested List. Their timers
are set to a small interval of LLQT milliseconds, and a Multicast
Address and Source Specific Query is sent by the Querier, to check
whether there are nodes on the link still interested in those
sources, or not. If no node confirms its interest in receiving a
specific source, the timer of that source expires. Then, the
source is moved from the Requested List to the Exclude List. From
then on, the source will be blocked by the router.
The handling of the EXCLUDE mode router state, according to the
received reports, is detailed in Tables 7.4.1 and 7.4.2.
When the Router Filter Mode for a multicast address is EXCLUDE,
source records are only deleted when the Filter Timer expires, or
when newly received Multicast Address Records modify the source
record list of the router.
7.3. MLDv2 Source Specific Forwarding Rules
When a multicast router receives a datagram from a source destined to
a particular multicast address, a decision has to be made whether to
forward the datagram on an attached link or not. The multicast
routing protocol in use is in charge of this decision, and should use
the MLDv2 information to ensure that all sources/multicast addresses
that have listeners on a link are forwarded to that link. MLDv2
information does not override multicast routing information; for
example, if the MLDv2 filter mode for a multicast address is EXCLUDE,
a router may still forward packets for excluded sources to a transit
link.
To summarize, the following table describes the forwarding
suggestions made by MLDv2 to the routing protocol for traffic
originating from a source destined to a multicast address. It also
summarizes the actions taken upon the expiration of a source timer
based on the Router Filter Mode of the multicast address.
Router
Filter Mode Source Timer Value Action
----------- ------------------ ------
INCLUDE TIMER > 0 Suggest to forward traffic
from source
INCLUDE TIMER == 0 Suggest to stop forwarding
traffic from source and
remove source record. If
there are no more source
records, delete multicast
address record
EXCLUDE TIMER > 0 Suggest to forward traffic
from source
EXCLUDE TIMER == 0 Suggest to not forward
traffic from source. Move
the source from the
Requested List to the
Exclude List (DO NOT remove
source record)
EXCLUDE No Source Element Suggest to forward traffic
from all sources
7.4. Action on Reception of Reports
Upon reception of an MLD message that contains a Report, the router
checks if the source address of the message is a valid link-local
address, if the Hop Limit is set to 1, and if the Router Alert option
is present in the Hop-By-Hop Options header of the IPv6 packet. If
any of these checks fails, the packet is dropped. If the validity of
the MLD message is verified, the router starts to process the Report.
7.4.1. Reception of Current State Records
When receiving Current State Records, a router updates both its
Filter Timer and its source timers. In some circumstances, the
reception of a type of multicast address record will cause the Router
Filter Mode for that multicast address to change. The table below
describes the actions, with respect to state and timers, that occur
to a router's state upon reception of Current State Records.
If the router is in INCLUDE filter mode for a multicast address, we
will use the notation INCLUDE (A), where A denotes the associated
Include List. If the router is in EXCLUDE filter mode for a
multicast address, we will use the notation EXCLUDE (X,Y), where X
and Y denote the associated Requested List and Exclude List
respectively.
Within the "Actions" section of the router state tables, we use the
notation '(A)=J', which means that the set A of source records should
have their source timers set to value J. 'Delete (A)' means that the
set A of source records should be deleted. 'Filter Timer = J' means
that the Filter Timer for the multicast address should be set to
value J.
Router State Report Received New Router State Actions
------------ --------------- ---------------- -------
INCLUDE (A) IS_IN (B) INCLUDE (A+B) (B)=MALI
INCLUDE (A) IS_EX (B) EXCLUDE (A*B, B-A) (B-A)=0
Delete (A-B)
Filter Timer=MALI
EXCLUDE (X,Y) IS_IN (A) EXCLUDE (X+A, Y-A) (A)=MALI
EXCLUDE (X,Y) IS_EX (A) EXCLUDE (A-Y, Y*A) (A-X-Y)=MALI
Delete (X-A)
Delete (Y-A)
Filter Timer=MALI
7.4.2. Reception of Filter Mode Change and Source List Change Records
When a change in the global state of a multicast address occurs in a
node, the node sends either a Source List Change Record or a Filter
Mode Change Record for that multicast address. As with Current State
Records, routers must act upon these records and possibly change
their own state to reflect the new listening state of the link.
The Querier must query sources or multicast addresses that are
requested to be no longer forwarded. When a router queries or
receives a query for a specific set of sources, it lowers its source
timers for those sources to a small interval of Last Listener Query
Time milliseconds. If multicast address records are received in
response to the queries which express interest in listening the
queried sources, the corresponding timers are updated.
Multicast Address Specific queries can also be used in order to enable a fast transition of a router from EXCLUDE to INCLUDE mode, in case a received Multicast Address Record motivates this action. The Filter Timer for that multicast address is lowered to a small interval of Last Listener Query Time milliseconds. If any multicast address records that express EXCLUDE mode interest in the multicast address are received within this interval, the Filter Timer is updated and the suggestion to the routing protocol to forward the multicast address stands without any interruption. If not, the router will switch to INCLUDE filter mode for that multicast address. During the query period (i.e., Last Listener Query Time milliseconds) the MLD component in the router continues to suggest to the routing protocol to forward traffic from the multicast addresses or sources that are queried. It is not until after Last Listener Query Time milliseconds without receiving a record that expresses interest in the queried multicast address or sources that the router may prune the multicast address or sources from the link. The following table describes the changes in multicast address state and the action(s) taken when receiving either Filter Mode Change or Source List Change Records. This table also describes the queries which are sent by the Querier when a particular report is received. We use the following notation for describing the queries that are sent. We use the notation 'Q(MA)' to describe a Multicast Address Specific Query to the MA multicast address. We use the notation 'Q(MA,A)' to describe a Multicast Address and Source Specific Query to the MA multicast address with source list A. If source list A is null as a result of the action (e.g. A*B), then no query is sent as a result of the operation. In order to maintain protocol robustness, queries defined in the Actions column of the table below need to be transmitted [Last Listener Query Count] times, once every [Last Listener Query Interval] period. If while scheduling new queries, there are already pending queries to be retransmitted for the same multicast address, the new and pending queries have to be merged. In addition, received host reports for a multicast address with pending queries may affect the contents of those queries. Section 7.6.3. describes the process of building and maintaining the state of pending queries.
Router State Report Received New Router State Actions
------------ --------------- ---------------- -------
INCLUDE (A) ALLOW (B) INCLUDE (A+B) (B)=MALI
INCLUDE (A) BLOCK (B) INCLUDE (A) Send Q(MA,A*B)
INCLUDE (A) TO_EX (B) EXCLUDE (A*B,B-A) (B-A)=0
Delete (A-B)
Send Q(MA,A*B)
Filter Timer=MALI
INCLUDE (A) TO_IN (B) INCLUDE (A+B) (B)=MALI
Send Q(MA,A-B)
EXCLUDE (X,Y) ALLOW (A) EXCLUDE (X+A,Y-A) (A)=MALI
EXCLUDE (X,Y) BLOCK (A) EXCLUDE (X+(A-Y),Y) (A-X-Y) =
Filter Timer
Send Q(MA,A-Y)
EXCLUDE (X,Y) TO_EX (A) EXCLUDE (A-Y,Y*A) (A-X-Y) =
Filter Timer
Delete (X-A)
Delete (Y-A)
Send Q(MA,A-Y)
Filter Timer=MALI
EXCLUDE (X,Y) TO_IN (A) EXCLUDE (X+A,Y-A) (A)=MALI
Send Q(MA,X-A)
Send Q(MA)
7.5. Switching Router Filter Modes
The Filter Timer is used as a mechanism for transitioning the Router
Filter Mode from EXCLUDE to INCLUDE.
When a Filter Timer expires with a Router Filter Mode of EXCLUDE, a
router assumes that there are no nodes with a *filter mode* of
EXCLUDE present on the attached link. Thus, the router transitions
to INCLUDE filter mode for the multicast address.
A router uses the sources from the Requested List as its state for
the switch to a filter mode of INCLUDE. Sources from the Requested
List are moved in the Include List, while sources from the Exclude
List are deleted. For example, if a router's state for a multicast
address is EXCLUDE(X,Y) and the Filter Timer expires for that
multicast address, the router switches to filter mode of INCLUDE with state INCLUDE(X). If at the moment of the switch the Requested List (X) is empty, the multicast address record is deleted from the router.7.6. Action on Reception of Queries
Upon reception of an MLD message that contains a Query, the router checks if the source address of the message is a valid link-local address, if the Hop Limit is set to 1, and if the Router Alert option is present in the Hop-By-Hop Options header of the IPv6 packet. If any of these checks fails, the packet is dropped. If the validity of the MLD message is verified, the router starts to process the Query.7.6.1. Timer Updates
MLDv2 uses the Suppress Router-Side Processing flag to ensure robustness, as explained in section 2.1. When a router sends or receives a query with a clear Suppress Router-Side Processing flag, it must update its timers to reflect the correct timeout values for the multicast address or sources being queried. The following table describes the timer actions when sending or receiving a Multicast Address Specific or Multicast Address and Source Specific Query with the Suppress Router-Side Processing flag not set. Query Action ----- ------ Q(MA,A) Source Timers for sources in A are lowered to LLQT Q(MA) Filter Timer is lowered to LLQT When a router sends or receives a query with the Suppress Router-Side Processing flag set, it will not update its timers.7.6.2. Querier Election
MLDv2 elects a single router per subnet to be in Querier state; all the other routers on the subnet should be in Non-Querier state. MLDv2 uses the same querier election mechanism as MLDv1, namely the IPv6 address. When a router starts operating on a subnet, by default it considers itself as being the Querier. Thus, it sends several General Queries separated by a small time interval (see sections 9.6 and 9.7 for details). When a router receives a query with a lower IPv6 address than its own, it sets the Other Querier Present timer to Other Querier Present Timeout; if it was previously in Querier state, it switches to Non-
Querier state and ceases to send queries on the link. After the Other Querier Present timer expires, it should re-enter the Querier state and begin sending General Queries. All MLDv2 queries MUST be sent with the FE80::/64 link-local source address prefix. Therefore, for the purpose of MLDv2 querier election, an IPv6 address A is considered to be lower than an IPv6 address B if the interface ID represented by the last 64 bits of address A, in big-endian bit order, is lower than the interface ID represented by the last 64 bits of address B.7.6.3. Building and Sending Specific Queries
7.6.3.1. Building and Sending Multicast Address Specific Queries
When a table action "Send Q(MA)" is encountered, the Filter Timer must be lowered to LLQT. The Querier must then immediately send a Multicast Address Specific query as well as schedule [Last Listener Query Count - 1] query retransmissions to be sent every [Last Listener Query Interval], over [Last Listener Query Time]. When transmitting a Multicast Address Specific Query, if the Filter Timer is larger than LLQT, the "Suppress Router-Side Processing" bit is set in the query message.7.6.3.2. Building and Sending Multicast Address and Source Specific Queries
When a table action "Send Q(MA,X)" is encountered by the Querier in the table in section 7.4.2, the following actions must be performed for each of the sources in X that send to multicast address MA, with source timer larger than LLQT: o Lower source timer to LLQT; o Add the sources to the Retransmission List; o Set the Source Retransmission Counter for each source to [Last Listener Query Count]. The Querier must then immediately send a Multicast Address and Source Specific Query as well as schedule [Last Listener Query Count -1] query retransmissions to be sent every [Last Listener Query Interval], over [Last Listener Query Time]. The contents of these queries are calculated as follows.
When building a Multicast Address and Source Specific Query for a multicast address MA, two separate query messages are sent for the multicast address. The first one has the "Suppress Router-Side Processing" bit set and contains all the sources with retransmission state (i.e., sources from the Retransmission List of that multicast address), and timers greater than LLQT. The second has the "Suppress Router-Side Processing" bit clear and contains all the sources with retransmission state and timers lower or equal to LLQT. If either of the two calculated messages does not contain any sources, then its transmission is suppressed. Note: If a Multicast Address Specific query is scheduled to be transmitted at the same time as a Multicast Address and Source specific query for the same multicast address, then transmission of the Multicast Address and Source Specific message with the "Suppress Router-Side Processing" bit set may be suppressed.8. Interoperation with MLDv1
MLD version 2 hosts and routers interoperate with hosts and routers that have not yet been upgraded to MLDv2. This compatibility is maintained by hosts and routers taking appropriate actions depending on the versions of MLD operating on hosts and routers within a network.8.1. Query Version Distinctions
The MLD version of a Multicast Listener Query message is determined as follows: MLDv1 Query: length = 24 octets MLDv2 Query: length >= 28 octets Query messages that do not match any of the above conditions (e.g., a Query of length 26 octets) MUST be silently ignored.8.2. Multicast Address Listener Behavior
8.2.1. In the Presence of MLDv1 Routers
In order to be compatible with MLDv1 routers, MLDv2 hosts MUST operate in version 1 compatibility mode. MLDv2 hosts MUST keep state per local interface regarding the compatibility mode of each attached link. A host's compatibility mode is determined from the Host Compatibility Mode variable which can be in one of the two states: MLDv1 or MLDv2.
The Host Compatibility Mode of an interface is set to MLDv1 whenever an MLDv1 Multicast Address Listener Query is received on that interface. At the same time, the Older Version Querier Present timer for the interface is set to Older Version Querier Present Timeout seconds. The timer is re-set whenever a new MLDv1 Query is received on that interface. If the Older Version Querier Present timer expires, the host switches back to Host Compatibility Mode of MLDv2. When Host Compatibility Mode is MLDv2, a host acts using the MLDv2 protocol on that interface. When Host Compatibility Mode is MLDv1, a host acts in MLDv1 compatibility mode, using only the MLDv1 protocol, on that interface. An MLDv1 Querier will send General Queries with the Maximum Response Code set to the desired Maximum Response Delay, i.e., the full range of this field is linear and the exponential algorithm described in section 5.1.3. is not used. Whenever a host changes its compatibility mode, it cancels all its pending responses and retransmission timers.8.2.2. In the Presence of MLDv1 Multicast Address Listeners
An MLDv2 host may be placed on a link where there are MLDv1 hosts. A host MAY allow its MLDv2 Multicast Listener Report to be suppressed by a Version 1 Multicast Listener Report.8.3. Multicast Router Behavior
8.3.1. In the Presence of MLDv1 Routers
MLDv2 routers may be placed on a network where there is at least one MLDv1 router. The following requirements apply: o If an MLDv1 router is present on the link, the Querier MUST use the lowest version of MLD present on the network. This must be administratively assured. Routers that desire to be compatible with MLDv1 MUST have a configuration option to act in MLDv1 mode; if an MLDv1 router is present on the link, the system administrator must explicitly configure all MLDv2 routers to act in MLDv1 mode. When in MLDv1 mode, the Querier MUST send periodic General Queries truncated at the Multicast Address field (i.e., 24 bytes long), and SHOULD also warn about receiving an MLDv2 Query (such warnings must be rate-limited). The Querier MUST also fill in the Maximum Response Delay in the Maximum Response Code field, i.e., the exponential algorithm described in section 5.1.3. is not used.
o If a router is not explicitly configured to use MLDv1 and receives
an MLDv1 General Query, it SHOULD log a warning. These warnings
MUST be rate-limited.
8.3.2. In the Presence of MLDv1 Multicast Address Listeners
MLDv2 routers may be placed on a network where there are hosts that
have not yet been upgraded to MLDv2. In order to be compatible with
MLDv1 hosts, MLDv2 routers MUST operate in version 1 compatibility
mode. MLDv2 routers keep a compatibility mode per multicast address
record. The compatibility mode of a multicast address is determined
from the Multicast Address Compatibility Mode variable, which can be
in one of the two following states: MLDv1 or MLDv2.
The Multicast Address Compatibility Mode of a multicast address
record is set to MLDv1 whenever an MLDv1 Multicast Listener Report is
received for that multicast address. At the same time, the Older
Version Host Present timer for the multicast address is set to Older
Version Host Present Timeout seconds. The timer is re-set whenever a
new MLDv1 Report is received for that multicast address. If the
Older Version Host Present timer expires, the router switches back to
Multicast Address Compatibility Mode of MLDv2 for that multicast
address.
Note that when a router switches back to MLDv2 Multicast Address
Compatibility Mode for a multicast address, it takes some time to
regain source-specific state information. Source-specific
information will be learned during the next General Query, but
sources that should be blocked will not be blocked until [Multicast
Address Listening Interval] after that.
When Multicast Address Compatibility Mode is MLDv2, a router acts
using the MLDv2 protocol for that multicast address. When Multicast
Address Compatibility Mode is MLDv1, a router internally translates
the following MLDv1 messages for that multicast address to their
MLDv2 equivalents:
MLDv1 Message MLDv2 Equivalent
------------- ----------------
Report IS_EX( {} )
Done TO_IN( {} )
MLDv2 BLOCK messages are ignored, as are source-lists in TO_EX()
messages (i.e., any TO_EX() message is treated as TO_EX( {} )). On
the other hand, the Querier continues to send MLDv2 queries,
regardless of its Multicast Address Compatibility Mode.
9. List of Timers, Counters, and their Default Values
Most of these timers are configurable. If non-default settings are used, they MUST be consistent among all nodes on a single link. Note that parentheses are used to group expressions to make the algebra clear.9.1. Robustness Variable
The Robustness Variable allows tuning for the expected packet loss on a link. If a link is expected to be lossy, the value of the Robustness Variable may be increased. MLD is robust to [Robustness Variable] - 1 packet losses. The value of the Robustness Variable MUST NOT be zero, and SHOULD NOT be one. Default value: 2.9.2. Query Interval
The Query Interval variable denotes the interval between General Queries sent by the Querier. Default value: 125 seconds. By varying the [Query Interval], an administrator may tune the number of MLD messages on the link; larger values cause MLD Queries to be sent less often.9.3. Query Response Interval
The Maximum Response Delay used to calculate the Maximum Response Code inserted into the periodic General Queries. Default value: 10000 (10 seconds) By varying the [Query Response Interval], an administrator may tune the burstiness of MLD messages on the link; larger values make the traffic less bursty, as host responses are spread out over a larger interval. The number of seconds represented by the [Query Response Interval] must be less than the [Query Interval].9.4. Multicast Address Listening Interval
The Multicast Address Listening Interval (MALI) is the amount of time that must pass before a multicast router decides there are no more listeners of a multicast address or a particular source on a link. This value MUST be ([Robustness Variable] times [Query Interval]) plus [Query Response Interval].
9.5. Other Querier Present Timeout
The Other Querier Present Timeout is the length of time that must pass before a multicast router decides that there is no longer another multicast router which should be the Querier. This value MUST be ([Robustness Variable] times ([Query Interval]) plus (one half of [Query Response Interval]).9.6. Startup Query Interval
The Startup Query Interval is the interval between General Queries sent by a Querier on startup. Default value: 1/4 the [Query Interval].9.7. Startup Query Count
The Startup Query Count is the number of Queries sent out on startup, separated by the Startup Query Interval. Default value: [Robustness Variable].9.8. Last Listener Query Interval
The Last Listener Query Interval is the Maximum Response Delay used to calculate the Maximum Response Code inserted into Multicast Address Specific Queries sent in response to Version 1 Multicast Listener Done messages. It is also the Maximum Response Delay used to calculate the Maximum Response Code inserted into Multicast Address and Source Specific Query messages. Default value: 1000 (1 second). Note that for values of LLQI greater than 32.768 seconds, a limited set of values can be represented, corresponding to sequential values of Maximum Response Code. When converting a configured time to a Maximum Response Code value, it is recommended to use the exact value if possible, or the next lower value if the requested value is not exactly representable. This value may be tuned to modify the "leave latency" of the link. A reduced value results in reduced time to detect the departure of the last listener for a multicast address or source.9.9. Last Listener Query Count
The Last Listener Query Count is the number of Multicast Address Specific Queries sent before the router assumes there are no local listeners. The Last Listener Query Count is also the number of
Multicast Address and Source Specific Queries sent before the router assumes there are no listeners for a particular source. Default value: [Robustness Variable].9.10. Last Listener Query Time
The Last Listener Query Time is the time value represented by the Last Listener Query Interval, multiplied by [Last Listener Query Count]. It is not a tunable value, but may be tuned by changing its components.9.11. Unsolicited Report Interval
The Unsolicited Report Interval is the time between repetitions of a node's initial report of interest in a multicast address. Default value: 1 second.9.12. Older Version Querier Present Timeout
The Older Version Querier Present Timeout is the time-out for transitioning a host back to MLDv2 Host Compatibility Mode. When an MLDv1 query is received, MLDv2 hosts set their Older Version Querier Present Timer to [Older Version Querier Present Timeout]. This value MUST be ([Robustness Variable] times (the [Query Interval] in the last Query received)) plus ([Query Response Interval]).9.13. Older Version Host Present Timeout
The Older Version Host Present Timeout is the time-out for transitioning a router back to MLDv2 Multicast Address Compatibility Mode for a specific multicast address. When an MLDv1 report is received for that multicast address, routers set their Older Version Host Present Timer to [Older Version Host Present Timeout]. This value MUST be ([Robustness Variable] times [Query Interval]) plus ([Query Response Interval]).9.14. Configuring timers
This section is meant to provide advice to network administrators on how to tune these settings to their network. Ambitious router implementations might tune these settings dynamically based upon changing characteristics of the network.
9.14.1. Robustness Variable
The Robustness Variable tunes MLD to expected losses on a link. MLDv2 is robust to [Robustness Variable] - 1 packet losses, e.g., if the Robustness Variable is set to the default value of 2, MLDv2 is robust to a single packet loss but may operate imperfectly if more losses occur. On lossy links, the value of the Robustness Variable should be increased to allow for the expected level of packet loss. However, increasing the value of the Robustness Variable increases the leave latency of the link (the time between when the last listener stops listening to a source or multicast address and when the traffic stops flowing).9.14.2. Query Interval
The overall level of periodic MLD traffic is inversely proportional to the Query Interval. A longer Query Interval results in a lower overall level of MLD traffic. The value of the Query Interval MUST be equal to or greater than the Maximum Response Delay used to calculate the Maximum Response Code inserted in General Query messages.9.14.3. Maximum Response Delay
The burstiness of MLD traffic is inversely proportional to the Maximum Response Delay. A longer Maximum Response Delay will spread Report messages over a longer interval. However, a longer Maximum Response Delay in Multicast Address Specific and Multicast Address And Source Specific Queries extends the leave latency (the time between when the last listener stops listening to a source or multicast address and when the traffic stops flowing.) The expected rate of Report messages can be calculated by dividing the expected number of Reporters by the Maximum Response Delay. The Maximum Response Delay may be dynamically calculated per Query by using the expected number of Reporters for that Query as follows: Query Type Expected number of Reporters ---------- ---------------------------- General Query All nodes on link Multicast Address Specific Query All nodes on the link that had expressed interest in the multicast address Multicast Address and Source All nodes on the link that had Specific Query expressed interest in the source and multicast address
A router is not required to calculate these populations or tune the Maximum Response Delay dynamically; these are simply guidelines.10. Security Considerations
We consider the ramifications of a forged message of each type. Note that before processing an MLD message, nodes verify if the source address of the message is a valid link-local address (or the unspecified address), if the Hop Limit is set to 1, and if the Router Alert option is present in the Hop-By-Hop Options header of the IPv6 packet. If any of these checks fails, the packet is dropped. This defends the MLDv2 nodes from acting on forged MLD messages originated off-link. Therefore, in the following we discuss only the effects of on-link forgery.10.1. Query Message
A forged Query message from a machine with a lower IPv6 address than the current Querier will cause Querier duties to be assigned to the forger. If the forger then sends no more Query messages, other routers' Other Querier Present timer will time out and one will resume the role of Querier. During this time, if the forger ignores Multicast Listener Done Messages, traffic might flow to multicast addresses with no listeners for up to [Multicast Address Listener Interval]. A forged Version 1 Query message will put MLDv2 listeners on that link in MLDv1 Host Compatibility Mode. This scenario can be avoided by providing MLDv2 hosts with a configuration option to ignore Version 1 messages completely. A DoS attack on a node could be staged through forged Multicast Address and Source Specific Queries. The attacker can find out about the listening state of a specific node with a general query. After that it could send a large number of Multicast Address and Source Specific Queries, each with a large source list and/or long Maximum Response Delay. The node will have to store and maintain the sources specified in all of those queries for as long as it takes to send the delayed response. This would consume both memory and CPU cycles in order to augment the recorded sources with the source lists included in the successive queries. To protect against such a DoS attack, a node stack implementation could restrict the number of Multicast Address and Source Specific Queries per multicast address within this interval, and/or record only a limited number of sources.
10.2. Current State Report messages
A forged Report message may cause multicast routers to think there are listeners of a multicast address on a link when there are not. Nevertheless, since listening to a multicast address on a host is generally an unprivileged operation, a local user may trivially gain the same result without forging any messages. A forged Version 1 Report Message may put a router into MLDv1 Multicast Address Compatibility Mode for a particular multicast address, meaning that the router will ignore MLDv2 source specific state messages. This can cause traffic to flow from unwanted sources for up to [Multicast Address Listener Interval]. This can be solved by providing routers with a configuration switch to ignore Version 1 messages completely. This breaks automatic compatibility with Version 1 hosts, so it should only be used in situations where source filtering is critical.10.3. State Change Report messages
A forged State Change Report message will cause the Querier to send out Multicast Address Specific or Multicast Address and Source Specific Queries for the multicast address in question. This causes extra processing on each router and on each listener of the multicast address, but cannot cause loss of desired traffic.11. IANA Considerations
IANA has assigned the IPv6 link-local multicast address FF02:0:0:0:0:0:0:16, called "all MLDv2-capable routers", as described in section 5.2.14. Version 2 Multicast Listener Reports will be sent to this special address. In addition, IANA has assigned the ICMPv6 message type value of 143 for Version 2 Multicast Listener Report messages, as specified in section 4.12. References
12.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.
[RFC2463] Conta, A. and S. Deering, "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", RFC 2463, December 1998. [RFC2464] Crawford, M., "Transmission of IPv6 Packets over Ethernet Networks", RFC 2464, December 1998. [RFC2710] Deering, S., Fenner, W. and B. Haberman, "Multicast Listener Discovery (MLD) for IPv6", RFC 2710, October 1999. [RFC2711] Partridge, C. and A. Jackson, "IPv6 Router Alert Option," RFC 2711, October 1999. [RFC3513] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6) Addressing Architecture, RFC 3513, April 2003.12.2. Informative References
[RFC2461] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery for IP Version 6 (IPv6)", RFC 2461, December 1998. [RFC2462] Thomson, S. and T. Narten, "IPv6 Stateless Address Autoconfiguration", RFC 2462, December 1998. [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A. Thyagarajan, "Internet Group Management Protocol, Version 3", RFC 3376, October 2002. [RFC3569] Bhattacharyya, S., Ed., "An Overview of Source- Specific Multicast (SSM)", RFC 3569, July 2003. [RFC3678] Thaler, D., Fenner, B. and B. Quinn, "Socket Interface Extensions for Multicast Source Filters", RFC 3678, January 2004.13. Acknowledgments
We would like to thank Hitoshi Asaeda, Randy Bush, Francis Dupont, Ted Hardie, Russ Housley, Konstantin Kabassanov, Erik Nordmark, Shinsuke Suzuki, Margaret Wasserman, Bert Wijnen, and Remi Zara for their valuable comments and suggestions on this document.
APPENDIX A. Design Rationale
A.1. The Need for State Change Messages
MLDv2 specifies two types of Multicast Listener Reports: Current State and State Change. This section describes the rationale for the need for both these types of Reports. Routers need to distinguish Multicast Listener Reports that were sent in response to Queries from those that were sent as a result of a change in the per-interface state. Multicast Listener Reports that are sent in response to Multicast Address Listener Queries are used mainly to refresh the existing state at the router; they typically do not cause transitions in state at the router. Multicast Listener Reports that are sent in response to changes in the per-interface state require the router to take some action in response to the received report (see Section 7.4.). The inability to distinguish between the two types of reports would force a router to treat all Multicast Listener Reports as potential changes in state and could result in increased processing at the router as well as an increase in MLD traffic on the link.A.2. Host Suppression
In MLDv1, a host would not send a pending multicast listener report if a similar report was sent by another listener on the link. In MLDv2, the suppression of multicast listener reports has been removed. The following points explain this decision. 1. Routers may want to track per-host multicast listener status on an interface. This would allow routers to implement fast leaves (e.g., for layered multicast congestion control schemes), as well as track listener status for possible security or accounting purposes. The present specification does not require routers to implement per-host tracking. Nevertheless, the lack of host suppression in MLDv2 makes possible to implement either proprietary or future standard behavior of multicast routers that would support per-host tracking, while being fully interoperable with MLDv2 listeners and routers that implement the exact behavior described in this specification. 2. Multicast Listener Report suppression does not work well on bridged LANs. Many bridges and Layer2/Layer3 switches that implement MLD snooping do not forward MLD messages across LAN segments in order to prevent multicast listener report suppression.
3. By eliminating multicast listener report suppression, hosts have
fewer messages to process; this leads to a simpler state machine
implementation.
4. In MLDv2, a single multicast listener report now bundles multiple
multicast address records to decrease the number of packets sent.
In comparison, the previous version of MLD required that each
multicast address be reported in a separate message.
A.3. Switching router filter modes from EXCLUDE to INCLUDE
If on a link there are nodes in both EXCLUDE and INCLUDE modes for a
single multicast address, the router must be in EXCLUDE mode as well
(see section 7.2.1). In EXCLUDE mode, a router forwards traffic from
all sources except those in the Exclude List. If all nodes in
EXCLUDE mode cease to exist or to listen, it would be desirable for
the router to switch back to INCLUDE mode seamlessly, without
interrupting the flow of traffic to existing listeners.
One of the ways to accomplish this is for routers to keep track of
all sources that nodes that are in INCLUDE mode listen to, even
though the router itself is in EXCLUDE mode. If the Filter Timer for
a multicast address expires, it implies that there are no nodes in
EXCLUDE mode on the link (otherwise a multicast listener report from
that node would have refreshed the Filter Timer). The router can
then switch to INCLUDE mode seamlessly; sources from the Requested
List are moved to the Include List, while sources from the Exclude
List are deleted.
APPENDIX B. Summary of Changes from MLDv1
The following is a summary of changes from MLDv1, specified in RFC
2710.
o MLDv2 introduces source filtering.
o The IP service interface of MLDv2 nodes is modified accordingly.
It enables the specification of a filter mode and a source list.
o An MLDv2 node keeps per-socket and per-interface multicast
listening states that include a filter mode and a source list for
each multicast address. This enables packet filtering based on a
socket's multicast reception state.
o MLDv2 state kept on routers includes a filter mode and a list of
sources and source timers for each multicast address that has
listeners on the link. MLDv1 routers kept only the list of
multicast addresses.
o Queries include additional fields (section 5.1). o The S flag (Suppress Router-Side Processing) is included in queries in order to fix robustness issues. o The Querier's Robustness Variable and Query Interval Code are included in Queries in order to synchronize all MLDv2 routers connected to the same link. o A new Query type (Multicast Address and Source Specific Query) is introduced. o The Maximum Response Delay is not directly included in the Query anymore. Instead, an exponential algorithm is used to calculate its value, based on the Maximum Response Code included in the Query. The maximum value is increased from 65535 milliseconds to about 140 minutes. o Reports include Multicast Address Records. Information on the listening state for several different multicast addresses can be included in the same Report message. o Reports are sent to the "all MLDv2-capable multicast routers" address, instead of the multicast address the host listens to, as in MLDv1. This facilitates the operation of layer-2 snooping switches. o There is no "host suppression", as in MLDv1. All nodes send Report messages. o Unsolicited Reports, announcing changes in receiver listening state, are sent [Robustness Variable] times. RFC 2710 is less explicit. o There are no Done messages. o Interoperability with MLDv1 systems is achieved by MLDv2 state operations. o In order to ensure interoperability, hosts maintain a Host Compatibility Mode variable and an Older Version Querier Present timer per interface. Routers maintain a Multicast Address Compatibility Mode variable and an Older Version Host Present timer per multicast address.
Editors' Contact Information
Rolland Vida LIP6, Universite Pierre et Marie Curie 8, rue du Capitaine Scott 75015 Paris, France Phone: +33-1.44.27.30.58 EMail: Rolland.Vida@lip6.fr Luis Henrique Maciel Kosmalski Costa LIP6, Universite Pierre et Marie Curie 8, rue du Capitaine Scott 75015 Paris, France Phone: +33-1.44.27.30.58 EMail: Luis.Costa@lip6.frAuthors' Addresses
This document was written by: Rolland Vida, LIP6 EMail: Rolland.Vida@lip6.fr Luis Henrique Maciel Kosmalski Costa, LIP6 EMail: Luis.Costa@lip6.fr Serge Fdida, LIP6 EMail: Serge.Fdida@lip6.fr Steve Deering, Cisco Systems, Inc. EMail: deering@cisco.com Bill Fenner, AT&T Labs - Research EMail: fenner@research.att.com Isidor Kouvelas, Cisco Systems, Inc. EMail: kouvelas@cisco.com Brian Haberman, Caspian Networks EMail: brian@innovationslab.net This document is the translation of [RFC3376] for IPv6 semantics. It was elaborated based on the translation of (RFC 2236) into [RFC2710].
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