RFC 3810
Multicast Listener Discovery Version 2 (MLDv2) for IPv6
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 [RFC3513], if a valid link-local IPv6 source address has not been acquired yet for the sending interface. (See section 5.2.13. for details.) There are two MLD message types of concern to the MLDv2 protocol described in this document: o Multicast Listener Query (Type = decimal 130) o Version 2 Multicast Listener Report (Type = decimal 143). See section 11 for IANA considerations. To assure the interoperability with nodes that implement MLDv1 (see section 8), an implementation of MLDv2 must also support the following two message types: o Version 1 Multicast Listener Report (Type = decimal 131) [RFC2710] o Version 1 Multicast Listener Done (Type = decimal 132) [RFC2710] Unrecognized message types MUST be silently ignored. Other message types may be used by newer versions or extensions of MLD, by multicast routing protocols, or for other uses. In this document, unless otherwise qualified, the capitalized words "Query" and "Report" refer to MLD Multicast Listener Queries and MLD Version 2 Multicast Listener Reports, respectively.5.1. Multicast Listener Query Message
Multicast Listener Queries are sent by multicast routers in Querier State to query the multicast listening state of neighboring interfaces. Queries have the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 130 | Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Maximum Response Code | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* *
| |
* Multicast Address *
| |
* *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Resv |S| QRV | QQIC | Number of Sources (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
* *
| |
* Source Address [1] *
| |
* *
| |
+- -+
| |
* *
| |
* Source Address [2] *
| |
* *
| |
+- . -+
. . .
. . .
+- -+
| |
* *
| |
* Source Address [N] *
| |
* *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
5.1.1. Code
Initialized to zero by the sender; ignored by receivers.5.1.2. Checksum
The standard ICMPv6 checksum; it covers the entire MLDv2 message, plus a "pseudo-header" of IPv6 header fields [RFC2463]. For computing the checksum, the Checksum field is set to zero. When a packet is received, the checksum MUST be verified before processing it.5.1.3. Maximum Response Code
The Maximum Response Code field specifies the maximum time allowed before sending a responding Report. The actual time allowed, called the Maximum Response Delay, is represented in units of milliseconds, and is derived from the Maximum Response Code as follows: If Maximum Response Code < 32768, Maximum Response Delay = Maximum Response Code If Maximum Response Code >=32768, Maximum Response Code represents a floating-point value as follows: 0 1 2 3 4 5 6 7 8 9 A B C D E F +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ |1| exp | mant | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ Maximum Response Delay = (mant | 0x1000) << (exp+3) Small values of Maximum Response Delay allow MLDv2 routers to tune the "leave latency" (the time between the moment the last node on a link ceases to listen to a specific multicast address and the moment the routing protocol is notified that there are no more listeners for that address). Larger values, especially in the exponential range, allow the tuning of the burstiness of MLD traffic on a link.5.1.4. Reserved
Initialized to zero by the sender; ignored by receivers.
5.1.5. Multicast Address
For a General Query, the Multicast Address field is set to zero. For a Multicast Address Specific Query or Multicast Address and Source Specific Query, it is set to the multicast address being queried (see section 5.1.10, below).5.1.7. S Flag (Suppress Router-Side Processing)
When set to one, the S Flag indicates to any receiving multicast routers that they have to suppress the normal timer updates they perform upon hearing a Query. Nevertheless, it does not suppress the querier election or the normal "host-side" processing of a Query that a router may be required to perform as a consequence of itself being a multicast listener.5.1.8. QRV (Querier's Robustness Variable)
If non-zero, the QRV field contains the [Robustness Variable] value used by the Querier. If the Querier's [Robustness Variable] exceeds 7 (the maximum value of the QRV field), the QRV field is set to zero. Routers adopt the QRV value from the most recently received Query as their own [Robustness Variable] value, unless that most recently received QRV was zero, in which case they use the default [Robustness Variable] value specified in section 9.1, or a statically configured value.5.1.9. QQIC (Querier's Query Interval Code)
The Querier's Query Interval Code field specifies the [Query Interval] used by the Querier. The actual interval, called the Querier's Query Interval (QQI), is represented in units of seconds, and is derived from the Querier's Query Interval Code as follows: If QQIC < 128, QQI = QQIC If QQIC >= 128, QQIC represents a floating-point value as follows: 0 1 2 3 4 5 6 7 +-+-+-+-+-+-+-+-+ |1| exp | mant | +-+-+-+-+-+-+-+-+ QQI = (mant | 0x10) << (exp + 3)
Multicast routers that are not the current Querier adopt the QQI value from the most recently received Query as their own [Query Interval] value, unless that most recently received QQI was zero, in which case the receiving routers use the default [Query Interval] value specified in section 9.2.5.1.10. Number of Sources (N)
The Number of Sources (N) field specifies how many source addresses are present in the Query. This number is zero in a General Query or a Multicast Address Specific Query, and non-zero in a Multicast Address and Source Specific Query. This number is limited by the MTU of the link over which the Query is transmitted. For example, on an Ethernet link with an MTU of 1500 octets, the IPv6 header (40 octets) together with the Hop-By-Hop Extension Header (8 octets) that includes the Router Alert option consume 48 octets; the MLD fields up to the Number of Sources (N) field consume 28 octets; thus, there are 1424 octets left for source addresses, which limits the number of source addresses to 89 (1424/16).5.1.11. Source Address [i]
The Source Address [i] fields are a vector of n unicast addresses, where n is the value in the Number of Sources (N) field.5.1.12. Additional Data
If the Payload Length field in the IPv6 header of a received Query indicates that there are additional octets of data present, beyond the fields described here, MLDv2 implementations MUST include those octets in the computation to verify the received MLD Checksum, but MUST otherwise ignore those additional octets. When sending a Query, an MLDv2 implementation MUST NOT include additional octets beyond the fields described above.5.1.13. Query Variants
There are three variants of the Query message: o A "General Query" is sent by the Querier to learn which multicast addresses have listeners on an attached link. In a General Query, both the Multicast Address field and the Number of Sources (N) field are zero.
o A "Multicast Address Specific Query" is sent by the Querier to
learn if a particular multicast address has any listeners on an
attached link. In a Multicast Address Specific Query, the
Multicast Address field contains the multicast address of
interest, while the Number of Sources (N) field is set to zero.
o A "Multicast Address and Source Specific Query" is sent by the
Querier to learn if any of the sources from the specified list for
the particular multicast address has any listeners on an attached
link or not. In a Multicast Address and Source Specific Query the
Multicast Address field contains the multicast address of
interest, while the Source Address [i] field(s) contain(s) the
source address(es) of interest.
5.1.14. Source Addresses for Queries
All MLDv2 Queries MUST be sent with a valid IPv6 link-local source
address. If a node (router or host) receives a Query message with
the IPv6 Source Address set to the unspecified address (::), or any
other address that is not a valid IPv6 link-local address, it MUST
silently discard the message and SHOULD log a warning.
5.1.15. Destination Addresses for Queries
In MLDv2, General Queries are sent to the link-scope all-nodes
multicast address (FF02::1). Multicast Address Specific and
Multicast Address and Source Specific Queries are sent with an IP
destination address equal to the multicast address of interest.
*However*, a node MUST accept and process any Query whose IP
Destination Address field contains *any* of the addresses (unicast or
multicast) assigned to the interface on which the Query arrives. This
might be useful, e.g., for debugging purposes.
5.2. Version 2 Multicast Listener Report Message
Version 2 Multicast Listener Reports are sent by IP nodes to report (to neighboring routers) the current multicast listening state, or changes in the multicast listening state, of their interfaces. Reports have the following format: 0 1 2 3 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Type = 143 | Reserved | Checksum | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Reserved |Nr of Mcast Address Records (M)| +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Multicast Address Record [1] . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Multicast Address Record [2] . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | . | . . . | . | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | | . . . Multicast Address Record [M] . . . | | +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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 fields are set to zero on transmission, and ignored on reception.5.2.2. Checksum
The standard ICMPv6 checksum; it covers the entire MLDv2 message, plus a "pseudo-header" of IPv6 header fields [RFC2460, RFC2463]. In order to compute the checksum, the Checksum field is set to zero. When a packet is received, the checksum MUST be verified before processing it.5.2.3. Nr of Mcast Address Records (M)
The Nr of Mcast Address Records (M) field specifies how many Multicast Address Records are present in this Report.5.2.4. Multicast Address Record
Each Multicast Address Record is a block of fields that contain information on the sender listening to a single multicast address on the interface from which the Report is sent.5.2.5. Record Type
It specifies the type of the Multicast Address Record. See section 5.2.12 for a detailed description of the different possible Record Types.5.2.6. Aux Data Len
The Aux Data Len field contains the length of the Auxiliary Data Field in this Multicast Address Record, in units of 32-bit words. It may contain zero, to indicate the absence of any auxiliary data.5.2.7. Number of Sources (N)
The Number of Sources (N) field specifies how many source addresses are present in this Multicast Address Record.5.2.8. Multicast Address
The Multicast Address field contains the multicast address to which this Multicast Address Record pertains.
5.2.9. Source Address [i]
The Source Address [i] fields are a vector of n unicast addresses, where n is the value in this record's Number of Sources (N) field.5.2.10. Auxiliary Data
The Auxiliary Data field, if present, contains additional information that pertain to this Multicast Address Record. The protocol specified in this document, MLDv2, does not define any auxiliary data. Therefore, implementations of MLDv2 MUST NOT include any auxiliary data (i.e., MUST set the Aux Data Len field to zero) in any transmitted Multicast Address Record, and MUST ignore any such data present in any received Multicast Address Record. The semantics and the internal encoding of the Auxiliary Data field are to be defined by any future version or extension of MLD that uses this field.5.2.11. Additional Data
If the Payload Length field in the IPv6 header of a received Report indicates that there are additional octets of data present, beyond the last Multicast Address Record, MLDv2 implementations MUST include those octets in the computation to verify the received MLD Checksum, but MUST otherwise ignore those additional octets. When sending a Report, an MLDv2 implementation MUST NOT include additional octets beyond the last Multicast Address Record.5.2.12. Multicast Address Record Types
There are a number of different types of Multicast Address Records that may be included in a Report message: o A "Current State Record" is sent by a node in response to a Query received on an interface. It reports the current listening state of that interface, with respect to a single multicast address. The Record Type of a Current State Record may be one of the following two values: Value Name and Meaning ----- ---------------- 1 MODE_IS_INCLUDE - indicates that the interface has a filter mode of INCLUDE for the specified multicast address. The Source Address [i] fields in this Multicast Address Record contain the interface's source list for the specified multicast address. A MODE_IS_INCLUDE Record is never sent with an empty source list.
2 MODE_IS_EXCLUDE - indicates that the interface has a filter
mode of EXCLUDE for the specified multicast address. The
Source Address [i] fields in this Multicast Address Record
contain the interface's source list for the specified
multicast address, if it is non-empty.
o A "Filter Mode Change Record" is sent by a node whenever a local
invocation of IPv6MulticastListen causes a change of the filter
mode (i.e., a change from INCLUDE to EXCLUDE, or from EXCLUDE to
INCLUDE) of the interface-level state entry for a particular
multicast address, whether the source list changes at the same
time or not. The Record is included in a Report sent from the
interface on which the change occurred. The Record Type of a
Filter Mode Change Record may be one of the following two values:
3 CHANGE_TO_INCLUDE_MODE - indicates that the interface has
changed to INCLUDE filter mode for the specified multicast
address. The Source Address [i] fields in this Multicast
Address Record contain the interface's new source list for
the specified multicast address, if it is non-empty.
4 CHANGE_TO_EXCLUDE_MODE - indicates that the interface has
changed to EXCLUDE filter mode for the specified multicast
address. The Source Address [i] fields in this Multicast
Address Record contain the interface's new source list for
the specified multicast address, if it is non-empty.
o A "Source List Change Record" is sent by a node whenever a local
invocation of IPv6MulticastListen causes a change of source list
that is *not* coincident with a change of filter mode, of the
interface-level state entry for a particular multicast address.
The Record is included in a Report sent from the interface on
which the change occurred. The Record Type of a Source List
Change Record may be one of the following two values:
5 ALLOW_NEW_SOURCES - indicates that the Source Address [i]
fields in this Multicast Address Record contain a list of
the additional sources that the node wishes to listen to,
for packets sent to the specified multicast address. If
the change was to an INCLUDE source list, these are the
addresses that were added to the list; if the change was to
an EXCLUDE source list, these are the addresses that were
deleted from the list.
6 BLOCK_OLD_SOURCES - indicates that the Source Address [i]
fields in this Multicast Address Record contain a list of
the sources that the node no longer wishes to listen to,
for packets sent to the specified multicast address. If the
change was to an INCLUDE source list, these are the
addresses that were deleted from the list; if the change
was to an EXCLUDE source list, these are the addresses that
were added to the list.
If a change of source list results in both allowing new sources and
blocking old sources, then two Multicast Address Records are sent for
the same multicast address, one of type ALLOW_NEW_SOURCES and one of
type BLOCK_OLD_SOURCES.
We use the term "State Change Record" to refer to either a Filter
Mode Change Record or a Source List Change Record.
Multicast Address Records with an unrecognized Record Type value MUST
be silently ignored, with the rest of the report being processed.
In the rest of this document, we use the following notation to
describe the contents of a Multicast Address Record that pertains to
a particular multicast address:
IS_IN ( x ) - Type MODE_IS_INCLUDE, source addresses x
IS_EX ( x ) - Type MODE_IS_EXCLUDE, source addresses x
TO_IN ( x ) - Type CHANGE_TO_INCLUDE_MODE, source addresses x
TO_EX ( x ) - Type CHANGE_TO_EXCLUDE_MODE, source addresses x
ALLOW ( x ) - Type ALLOW_NEW_SOURCES, source addresses x
BLOCK ( x ) - Type BLOCK_OLD_SOURCES, source addresses x
where x is either:
o a capital letter (e.g., "A") to represent the set of source
addresses,
or
o a set expression (e.g., "A+B"), where "A+B" means the union of
sets A and B, "A*B" means the intersection of sets A and B, and
"A-B" means the removal of all elements of set B from set A.
5.2.13. Source Addresses for Reports
An MLDv2 Report MUST be sent with a valid IPv6 link-local source
address, or the unspecified address (::), if the sending interface
has not acquired a valid link-local address yet. Sending reports
with the unspecified address is allowed to support the use of IP
multicast in the Neighbor Discovery Protocol [RFC2461]. For
stateless autoconfiguration, as defined in [RFC2462], a node is
required to join several IPv6 multicast groups, in order to perform
Duplicate Address Detection (DAD). Prior to DAD, the only address
the reporting node has for the sending interface is a tentative one, which cannot be used for communication. Thus, the unspecified address must be used. On the other hand, routers MUST silently discard a message that is not sent with a valid link-local address, without taking any action on the contents of the packet. Thus, a Report is discarded if the router cannot identify the source address of the packet as belonging to a link connected to the interface on which the packet was received. A Report sent with the unspecified address is also discarded by the router. This enhances security, as unidentified reporting nodes cannot influence the state of the MLDv2 router(s). Nevertheless, the reporting node has modified its listening state for multicast addresses that are contained in the Multicast Address Records of the Report message. From now on, it will treat packets sent to those multicast addresses according to this new listening state. Once a valid link-local address is available, a node SHOULD generate new MLDv2 Report messages for all multicast addresses joined on the interface.5.2.14. Destination Addresses for Reports
Version 2 Multicast Listener Reports are sent with an IP destination address of FF02:0:0:0:0:0:0:16, to which all MLDv2-capable multicast routers listen (see section 11 for IANA considerations related to this special destination address). A node that operates in version 1 compatibility mode (see details in section 8) sends version 1 Reports to the multicast address specified in the Multicast Address field of the Report. In addition, a node MUST accept and process any version 1 Report whose IP Destination Address field contains *any* of the IPv6 addresses (unicast or multicast) assigned to the interface on which the Report arrives. This might be useful, e.g., for debugging purposes.5.2.15. Multicast Listener Report Size
If the set of Multicast Address Records required in a Report does not fit within the size limit of a single Report message (as determined by the MTU of the link on which it will be sent), the Multicast Address Records are sent in as many Report messages as needed to report the entire set.
If a single Multicast Address Record contains so many source
addresses that it does not fit within the size limit of a single
Report message, then:
o if its Type is not IS_EX or TO_EX, it is split into multiple
Multicast Address Records; each such record contains a different
subset of the source addresses, and is sent in a separate Report.
o if its Type is IS_EX or TO_EX, a single Multicast Address Record
is sent, with as many source addresses as can fit; the remaining
source addresses are not reported. Although the choice of which
sources to report is arbitrary, it is preferable to report the
same set of sources in each subsequent report, rather than
reporting different sources each time.
6. Protocol Description for Multicast Address Listeners
MLD is an asymmetric protocol, as it specifies separate behaviors for
multicast address listeners -- that is, hosts or routers that listen
to multicast packets -- and multicast routers. This section
describes the part of MLDv2 that applies to all multicast address
listeners. (Note that a multicast router that is also a multicast
address listener performs both parts of MLDv2; it receives and it
responds to its own MLD messages, as well as to those of its
neighbors.) The multicast router part of MLDv2 is described in
section 7.
A node performs the protocol described in this section over all
interfaces on which multicast reception is supported, even if more
than one of those interfaces are connected to the same link.
For interoperability with multicast routers that run the MLDv1
protocol, nodes maintain a Host Compatibility Mode variable for each
interface on which multicast reception is supported. This section
describes the behavior of multicast address listener nodes on
interfaces for which Host Compatibility Mode = MLDv2. The algorithm
for determining Host Compatibility Mode, and the behavior if its
value is set to MLDv1, are described in section 8.
The link-scope all-nodes multicast address, (FF02::1), is handled as
a special case. On all nodes -- that is all hosts and routers,
including multicast routers -- listening to packets destined to the
all-nodes multicast address, from all sources, is permanently enabled
on all interfaces on which multicast listening is supported. No MLD
messages are ever sent regarding neither the link-scope all-nodes
multicast address, nor any multicast address of scope 0 (reserved) or
1 (node-local).
There are three types of events that trigger MLDv2 protocol actions
on an interface:
o a change of the per-interface listening state, caused by a local
invocation of IPv6MulticastListen;
o the firing of a specific timer;
o the reception of a Query.
(Received MLD messages of types other than Query are silently
ignored, except as required for interoperation with nodes that
implement MLDv1.)
The following subsections describe the actions to be taken for each
case. Timer and counter names appear in square brackets. Default
values for those timers and counters are specified in section 9.
6.1. Action on Change of Per-Interface State
An invocation of IPv6MulticastListen may cause the multicast
listening state of an interface to change, according to the rules in
section 4.2. Each such change affects the per-interface entry for a
single multicast address.
A change of per-interface state causes the node to immediately
transmit a State Change Report from that interface. The type and
contents of the Multicast Address Record(s) in that Report are
determined by comparing the filter mode and source list for the
affected multicast address before and after the change, according to
the table below. If no per-interface state existed for that
multicast address before the change (i.e., the change consisted of
creating a new per-interface record), or if no state exists after the
change (i.e., the change consisted of deleting a per-interface
record), then the "non-existent" state is considered to have an
INCLUDE filter mode and an empty source list.
Old State New State State Change Record Sent
--------- --------- ------------------------
INCLUDE (A) INCLUDE (B) ALLOW (B-A), BLOCK (A-B)
EXCLUDE (A) EXCLUDE (B) ALLOW (A-B), BLOCK (B-A)
INCLUDE (A) EXCLUDE (B) TO_EX (B)
EXCLUDE (A) INCLUDE (B) TO_IN (B)
If the computed source list for either an ALLOW or a BLOCK State
Change Record is empty, that record is omitted from the Report.
To cover the possibility of the State Change Report being missed by
one or more multicast routers, [Robustness Variable] - 1
retransmissions are scheduled, through a Retransmission Timer, at
intervals chosen at random from the range (0, [Unsolicited Report
Interval]).
If more changes to the same per-interface state entry occur before
all the retransmissions of the State Change Report for the first
change have been completed, each such additional change triggers the
immediate transmission of a new State Change Report.
The contents of the new Report are calculated as follows:
o As for the first Report, the per-interface state for the affected
multicast address before and after the latest change is compared.
o The records that express the difference are built according to the
table above. Nevertheless, these records are not transmitted in a
separate message, but they are instead merged with the contents of
the pending report, to create the new State Change Report. The
rules for calculating this merged report are described below.
The transmission of the merged State Change Report terminates
retransmissions of the earlier State Change Reports for the same
multicast address, and becomes the first of [Robustness Variable]
transmissions of the new State Change Reports. These transmissions
are necessary in order to ensure that each instance of state change
is transmitted at least [Robustness Variable] times.
Each time a source is included in the difference report calculated
above, retransmission state for that source needs to be maintained
until [Robustness Variable] State Change Reports have been sent by
the node. This is done in order to ensure that a series of
successive state changes do not break the protocol robustness.
Sources in retransmission state can be kept in a per multicast
address Retransmission List, with a Source Retransmission Counter
associated to each source in the list. When a source is included in
the list, its counter is set to [Robustness Variable]. Each time a
State Change Report is sent the counter is decreased by one unit.
When the counter reaches zero, the source is deleted from the
Retransmission List for that multicast address.
If the per-interface listening change that triggers the new report is
a filter mode change, then the next [Robustness Variable] State
Change Reports will include a Filter Mode Change Record. This
applies even if any number of source list changes occur in that
period. The node has to maintain retransmission state for the
multicast address until the [Robustness Variable] State Change
Reports have been sent. This can be done through a per multicast
address Filter Mode Retransmission Counter. When the filter mode
changes, the counter is set to [Robustness Variable]. Each time a
State Change Report is sent the counter is decreased by one unit.
When the counter reaches zero, i.e., [Robustness Variable] State
Change Reports with Filter Mode Change Records have been transmitted
after the last filter mode change, and if source list changes have
resulted in additional reports being scheduled, then the next State
Change Report will include Source List Change Records.
Each time a per-interface listening state change triggers the
Immediate transmission of a new State Change Report, its contents are
determined as follows. If the report should contain a Filter Mode
Change Record, i.e., the Filter Mode Retransmission Counter for that
multicast address has a value higher than zero, then, if the current
filter mode of the interface is INCLUDE, a TO_IN record is included
in the report; otherwise a TO_EX record is included. If instead the
report should contain Source List Change Records, i.e., the Filter
Mode Retransmission Counter for that multicast address is zero, an
ALLOW and a BLOCK record is included. The contents of these records
are built according to the table below.
Record Sources included
------ ----------------
TO_IN All in the current per-interface state that must be
forwarded
TO_EX All in the current per-interface state that must be
blocked
ALLOW All with retransmission state (i.e., all sources from the
Retransmission List) that must be forwarded
BLOCK All with retransmission state that must be blocked
If the computed source list for either an ALLOW or a BLOCK record is
empty, that record is omitted from the State Change Report.
Note: When the first State Change Report is sent, the non-existent
pending report to merge with can be treated as a Source Change Report
with empty ALLOW and BLOCK records (no sources have retransmission
state).
The building of a scheduled State Change Report, triggered by the
firing of a Retransmission Timer, instead of a per-interface
listening state change, is described in section 6.3.
6.2. Action on Reception of a Query
Upon reception of an MLD message that contains a Query, the node checks if the source address of the message is a valid link-local address, if the Hop Limit is set to 1, and if the Router Alert option is present in the Hop-By-Hop Options header of the IPv6 packet. If any of these checks fails, the packet is dropped. If the validity of the MLD message is verified, the node starts to process the Query. Instead of responding immediately, the node delays its response by a random amount of time, bounded by the Maximum Response Delay value derived from the Maximum Response Code in the received Query message. A node may receive a variety of Queries on different interfaces and of different kinds (e.g., General Queries, Multicast Address Specific Queries, and Multicast Address and Source Specific Queries), each of which may require its own delayed response. Before scheduling a response to a Query, the node must first consider previously scheduled pending responses and, in many cases, schedule a combined response. Therefore, for each of its interfaces on which it operates the listener part of the MLDv2 protocol, the node must be able to maintain the following state: o an Interface Timer for scheduling responses to General Queries; o a Multicast Address Timer for scheduling responses to Multicast Address (and Source) Specific Queries, for each multicast address the node has to report on; o a per-multicast-address list of sources to be reported in response to a Multicast Address and Source Specific Query. When a new valid General Query arrives on an interface, the node checks whether it has any per-interface listening state record to report on, or not. Similarly, when a new valid Multicast Address (and Source) Specific Query arrives on an interface, the node checks whether it has a per-interface listening state record that corresponds to the queried multicast address (and source), or not. If it does, a delay for a response is randomly selected in the range (0, [Maximum Response Delay]), where Maximum Response Delay is derived from the Maximum Response Code inserted in the received Query message. The following rules are then used to determine if a Report needs to be scheduled or not, and the type of Report to schedule. (The rules are considered in order and only the first matching rule is applied.)
1. If there is a pending response to a previous General Query
scheduled sooner than the selected delay, no additional response
needs to be scheduled.
2. If the received Query is a General Query, the Interface Timer is
used to schedule a response to the General Query after the
selected delay. Any previously pending response to a General
Query is canceled.
3. If the received Query is a Multicast Address Specific Query or a
Multicast Address and Source Specific Query and there is no
pending response to a previous Query for this multicast address,
then the Multicast Address Timer is used to schedule a report. If
the received Query is a Multicast Address and Source Specific
Query, the list of queried sources is recorded to be used when
generating a response.
4. If there is already a pending response to a previous Query
scheduled for this multicast address, and either the new Query is
a Multicast Address Specific Query or the recorded source list
associated with the multicast address is empty, then the multicast
address source list is cleared and a single response is scheduled,
using the Multicast Address Timer. The new response is scheduled
to be sent at the earliest of the remaining time for the pending
report and the selected delay.
5. If the received Query is a Multicast Address and Source Specific
Query and there is a pending response for this multicast address
with a non-empty source list, then the multicast address source
list is augmented to contain the list of sources in the new Query,
and a single response is scheduled using the Multicast Address
Timer. The new response is scheduled to be sent at the earliest
of the remaining time for the pending report and the selected
delay.
6.3. Action on Timer Expiration
There are several timers that, upon expiration, trigger protocol
actions on an MLDv2 Multicast Address Listener node. All these
actions are related to pending reports scheduled by the node.
1. If the expired timer is the Interface Timer (i.e., there is a
pending response to a General Query), then one Current State
Record is sent for each multicast address for which the specified
interface has listening state, as described in section 4.2. The
Current State Record carries the multicast address and its
associated filter mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and
Source list. Multiple Current State Records are packed into
individual Report messages, to the extent possible.
This naive algorithm may result in bursts of packets when a node
listens to a large number of multicast addresses. Instead of
using a single Interface Timer, implementations are recommended to
spread transmission of such Report messages over the interval (0,
[Maximum Response Delay]). Note that any such implementation MUST
avoid the "ack-implosion" problem, i.e., MUST NOT send a Report
immediately upon reception of a General Query.
2. If the expired timer is a Multicast Address Timer and the list of
recorded sources for that multicast address is empty (i.e., there
is a pending response to a Multicast Address Specific Query), then
if, and only if, the interface has listening state for that
multicast address, a single Current State Record is sent for that
address. The Current State Record carries the multicast address
and its associated filter mode (MODE_IS_INCLUDE or
MODE_IS_EXCLUDE) and source list, if any.
3. If the expired timer is a Multicast Address Timer and the list of
recorded sources for that multicast address is non-empty (i.e.,
there is a pending response to a Multicast Address and Source
Specific Query), then if, and only if, the interface has listening
state for that multicast address, the contents of the
corresponding Current State Record are determined from the per-
interface state and the pending response record, as specified in
the following table:
set of sources in the
per-interface state pending response record Current State Record
------------------- ----------------------- --------------------
INCLUDE (A) B IS_IN (A*B)
EXCLUDE (A) B IS_IN (B-A)
If the resulting Current State Record has an empty set of source
addresses, then no response is sent. After the required Report
messages have been generated, the source lists associated with any
reported multicast addresses are cleared.
4. If the expired timer is a Retransmission Timer for a multicast
address (i.e., there is a pending State Change Report for that
multicast address), the contents of the report are determined as
follows. If the report should contain a Filter Mode Change
Record, i.e., the Filter Mode Retransmission Counter for that
multicast address has a value higher than zero, then, if the
current filter mode of the interface is INCLUDE, a TO_IN record is
included in the report; otherwise a TO_EX record is included. In
both cases, the Filter Mode Retransmission Counter for that
multicast address is decremented by one unit after the
transmission of the report.
If instead the report should contain Source List Change Records,
i.e., the Filter Mode Retransmission Counter for that multicast
address is zero, an ALLOW and a BLOCK record is included. The
contents of these records are built according to the table below:
Record Sources included
------ ----------------
TO_IN All in the current per-interface state that must be
forwarded
TO_EX All in the current per-interface state that must be
blocked
ALLOW All with retransmission state (i.e., all sources from the
Retransmission List) that must be forwarded. For each
included source, its Source Retransmission Counter is
decreased with one unit after the transmission of the
report. If the counter reaches zero, the source is
deleted from the Retransmission List for that multicast
address.
BLOCK All with retransmission state (i.e., all sources from the
Retransmission List) that must be blocked. For each
included source, its Source Retransmission Counter is
decreased with one unit after the transmission of the
report. If the counter reaches zero, the source is
deleted from the Retransmission List for that multicast
address.
If the computed source list for either an ALLOW or a BLOCK record
is empty, that record is omitted from the State Change Report.
7. Protocol Description for Multicast Routers
The purpose of MLD is to enable each multicast router to learn, for
each of its directly attached links, which multicast addresses have
listeners on that link. MLD version 2 adds the capability for a
multicast router to also learn which *sources* have listeners among
the neighboring nodes, for packets sent to any particular multicast
address. The information gathered by MLD is provided to whichever
multicast routing protocol is used by the router, in order to ensure
that multicast packets are delivered to all links where there are
interested listeners.
This section describes the part of MLDv2 that is performed by multicast routers. Multicast routers may themselves become multicast address listeners, and therefore also perform the multicast listener part of MLDv2, described in section 6. A multicast router performs the protocol described in this section over each of its directly attached links. If a multicast router has more than one interface to the same link, it only needs to operate this protocol over one of those interfaces. For each interface over which the router operates the MLD protocol, the router must configure that interface to listen to all link-layer multicast addresses that can be generated by IPv6 multicasts. For example, an Ethernet-attached router must set its Ethernet address reception filter to accept all Ethernet multicast addresses that start with the hexadecimal value 3333 [RFC2464]; in the case of an Ethernet interface that does not support the filtering of such a multicast address range, it must be configured to accept ALL Ethernet multicast addresses, in order to meet the requirements of MLD. On each interface over which this protocol is being run, the router MUST enable reception of the link-scope "all MLDv2-capable routers" multicast address from all sources, and MUST perform the multicast address listener part of MLDv2 for that address on that interface. Multicast routers only need to know that *at least one* node on an attached link listens to packets for a particular multicast address from a particular source; a multicast router is not required to *individually* keep track of the interests of each neighboring node. (Nevertheless, see Appendix A2 item 1 for discussion.) MLDv2 is backward compatible with the MLDv1 protocol. For a detailed description of compatibility issues see section 8.7.1. Conditions for MLD Queries
The behavior of a router that implements the MLDv2 protocol depends on whether there are several multicast routers on the same subnet, or not. If it is the case, a querier election mechanism (described in section 7.6.2) is used to elect a single multicast router to be in Querier state. All the multicast routers on the subnet listen to the messages sent by multicast address listeners, and maintain the same multicast listening information state, so that they can quickly and correctly take over the querier functionality, should the present Querier fail. Nevertheless, it is only the Querier that sends periodical or triggered query messages on the subnet.
The Querier periodically sends General Queries to request Multicast Address Listener information from an attached link. These queries are used to build and refresh the Multicast Address Listener state of routers on attached links. Nodes respond to these queries by reporting their Multicast Address Listening state (and set of sources they listen to) with Current State Multicast Address Records in MLDv2 Multicast Listener Reports. As a listener of a multicast address, a node may express interest in listening or not listening to traffic from particular sources. As the desired listening state of a node changes, it reports these changes using Filter Mode Change Records or Source List Change Records. These records indicate an explicit state change in a multicast address at a node in either the Multicast Address Record's source list or its filter mode. When Multicast Address Listening is terminated at a node or traffic from a particular source is no longer desired, the Querier must query for other listeners of the multicast address or of the source before deleting the multicast address (or source) from its Multicast Address Listener state and pruning its traffic. To enable all nodes on a link to respond to changes in multicast address listening, the Querier sends specific queries. A Multicast Address Specific Query is sent to verify that there are no nodes that listen to the specified multicast address or to "rebuild" the listening state for a particular multicast address. Multicast Address Specific Queries are sent when the Querier receives a State Change Record indicating that a node ceases to listen to a multicast address. They are also sent in order to enable a fast transition of a router from EXCLUDE to INCLUDE mode, in case a received State Change Record motivates this action. A Multicast Address and Source Specific Query is used to verify that there are no nodes on a link which listen to traffic from a specific set of sources. Multicast Address and Source Specific Queries list sources for a particular multicast address which have been requested to no longer be forwarded. This query is sent by the Querier in order to learn if any node listens to packets sent to the specified multicast address, from the specified source addresses. Multicast Address and Source Specific Queries are only sent in response to State Change Records and never in response to Current State Records. Section 5.1.13 describes each query in more detail.