| Rfc | 3810 |
|---|
| Title | Multicast Listener Discovery Version 2 (MLDv2) for IPv6 |
|---|
| Author | R. Vida,
Ed., L. Costa, Ed. |
|---|
| Date | June 2004 |
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| Format: | TXT, HTML |
|---|
| Updates | RFC2710 |
|---|
| Updated by | RFC4604 |
|---|
| Status: | PROPOSED STANDARD |
|---|
|
Network Working Group R. Vida, Ed.
Request for Comments: 3810 L. Costa, Ed.
Updates: 2710 LIP6
Category: Standards Track June 2004
Multicast Listener Discovery Version 2 (MLDv2) for IPv6
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004).
Abstract
This document updates RFC 2710, and it specifies Version 2 of the
Multicast Listener Discovery Protocol (MLDv2). MLD is used by an
IPv6 router to discover the presence of multicast listeners on
directly attached links, and to discover which multicast addresses
are of interest to those neighboring nodes. MLDv2 is designed to be
interoperable with MLDv1. MLDv2 adds the ability for a node to
report interest in listening to packets with a particular multicast
address only from specific source addresses or from all sources
except for specific source addresses.
Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Protocol Overview . . . . . . . . . . . . . . . . . . . . . . 3
3. The Service Interface for Requesting IP Multicast Reception . 9
4. Multicast Listening State Maintained by Nodes . . . . . . . . 11
5. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 13
6. Protocol Description for Multicast Address Listeners. . . . . 27
7. Protocol Description for Multicast Routers. . . . . . . . . . 34
8. Interoperation with MLDv1 . . . . . . . . . . . . . . . . . . 48
9. List of Timers, Counters, and their Default Values. . . . . . 51
10. Security Considerations . . . . . . . . . . . . . . . . . . . 55
11. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 56
12. References. . . . . . . . . . . . . . . . . . . . . . . . . . 56
13. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 57
Appendix A. Design Rationale. . . . . . . . . . . . . . . . . . . 58
Appendix B. Summary of Changes from MLDv1 . . . . . . . . . . . . 59
Editors' Contact Information. . . . . . . . . . . . . . . . . . . 61
Authors' Addresses. . . . . . . . . . . . . . . . . . . . . . . . 61
Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 62
1. Introduction
The Multicast Listener Discovery Protocol (MLD) is used by IPv6
routers to discover the presence of multicast listeners (i.e., nodes
that wish to receive multicast packets) on their directly attached
links, and to discover specifically which multicast addresses are of
interest to those neighboring nodes. Note that a multicast router
may itself be a listener of one or more multicast addresses; in this
case it performs both the "multicast router part" and the "multicast
address listener part" of the protocol, to collect the multicast
listener information needed by its multicast routing protocol on the
one hand, and to inform itself and other neighboring multicast
routers of its listening state on the other hand.
This document specifies Version 2 of MLD. The previous version of
MLD is specified in [RFC2710]. In this document we will refer to it
as MLDv1. MLDv2 is a translation of the IGMPv3 protocol [RFC3376]
for IPv6 semantics.
The MLDv2 protocol, when compared to MLDv1, adds support for "source
filtering", i.e., the ability for a node to report interest in
listening to packets *only* from specific source addresses, as
required to support Source-Specific Multicast [RFC3569], or from *all
but* specific source addresses, sent to a particular multicast
address. MLDv2 is designed to be interoperable with MLDv1.
The capitalized key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
[RFC2119]. Due to the lack of italics, emphasis is indicated herein
by bracketing a word or phrase in "*" characters. Furthermore,
square brackets are used to denote the value of the enclosed
variable, as opposed to the variable itself, written without
brackets.
2. Protocol Overview
This section gives a brief description of the protocol operation. The
following sections present the protocol details.
MLD is an asymmetric protocol; it specifies separate behaviors for
multicast address listeners (i.e., hosts or routers that listen to
multicast packets) and multicast routers. The purpose of MLD is to
enable each multicast router to learn, for each of its directly
attached links, which multicast addresses and which sources have
interested listeners on that link. The information gathered by MLD
is provided to whichever multicast routing protocol is used by the
router, in order to ensure that multicast packets are delivered to
all links where there are listeners interested in such packets.
Multicast routers only need to know that *at least one* node on an
attached link is listening to packets for a particular multicast
address, from a particular source; a multicast router is not required
to *individually* keep track of the interests of each neighboring
node. (Nevertheless, see Appendix A2 item 1 for discussion.)
A multicast router performs the *router part* of the MLDv2 protocol
(described in details in section 7) on each of its directly attached
links. If a multicast router has more than one interface connected
to the same link, it only needs to operate the protocol on one of
those interfaces. The router behavior depends on whether there are
several multicast routers on the same subnet, or not. If that is the
case, a querier election mechanism (described in section 7.6.2) is
used to elect a single multicast router to be in Querier state. This
router is called the Querier. All multicast routers on the subnet
listen to the messages sent by multicast address listeners, and
maintain the same multicast listening information state, so that they
can take over the querier role, should the present Querier fail.
Nevertheless, only the Querier sends periodical or triggered query
messages on the subnet, as described in section 7.1.
A multicast address listener performs the *listener part* of the
MLDv2 protocol (described in details in section 6) on all interfaces
on which multicast reception is supported, even if more than one of
those interfaces are connected to the same link.
2.1. Building Multicast Listening State on Multicast Address Listeners
Upper-layer protocols and applications that run on a multicast
address listener node use specific service interface calls (described
in section 3) to ask the IP layer to enable or disable reception of
packets sent to specific multicast addresses. The node keeps
Multicast Address Listening state for each socket on which the
service interface calls have been invoked (section 4.1). In addition
to this per-socket multicast listening state, a node must also
maintain or compute multicast listening state for each of its
interfaces (section 4.2). Conceptually, that state consists of a set
of records, with each record containing an IPv6 multicast address, a
filter mode, and a source list. The filter mode may be either
INCLUDE or EXCLUDE. In INCLUDE mode, reception of packets sent to
the specified multicast address is enabled *only* from the source
addresses listed in the source list. In EXCLUDE mode, reception of
packets sent to the given multicast address is enabled from all
source addresses *except* those listed in the source list.
At most one record per multicast address exists for a given
interface. This per-interface state is derived from the per-socket
state, but may differ from it when different sockets have differing
filter modes and/or source lists for the same multicast address and
interface. After a multicast packet has been accepted from an
interface by the IP layer, its subsequent delivery to the application
connected to a particular socket depends on the multicast listening
state of that socket (and possibly also on other conditions, such as
what transport-layer port the socket is bound to). Note that MLDv2
messages are not subject to source filtering and must always be
processed by hosts and routers.
2.2. Exchanging Messages between the Querier and the Listening Nodes
There are three types of MLDv2 query messages: General Queries,
Multicast Address Specific Queries, and Multicast Address and Source
Specific Queries. The Querier periodically sends General Queries, to
learn multicast address listener information from an attached link.
These queries are used to build and refresh the Multicast Address
Listener state inside all multicast routers on the link.
Nodes respond to these queries by reporting their per-interface
Multicast Address Listening state, through Current State Report
messages sent to a specific multicast address all MLDv2 routers on
the link listen to. On the other hand, if the listening state of a
node changes, the node immediately reports these changes through a
State Change Report message. The State Change Report contains either
Filter Mode Change records, Source List Change records, or records of
both types. A detailed description of the report messages is
presented in section 5.2.12.
Both router and listener state changes are mainly triggered by the
expiration of a specific timer, or the reception of an MLD message
(listener state change can be also triggered by the invocation of a
service interface call). Therefore, to enhance protocol robustness,
in spite of the possible unreliability of message exchanges, messages
are retransmitted several times. Furthermore, timers are set so as
to take into account the possible message losses, and to wait for
retransmissions.
Periodical General Queries and Current State Reports do not apply
this rule, in order not to overload the link; it is assumed that in
general these messages do not generate state changes, their main
purpose being to refresh existing state. Thus, even if one such
message is lost, the corresponding state will be refreshed during the
next reporting period.
As opposed to Current State Reports, State Change Reports are
retransmitted several times, in order to avoid them being missed by
one or more multicast routers. The number of retransmissions depends
on the so-called Robustness Variable. This variable allows tuning
the protocol according to the expected packet loss on a link. If a
link is expected to be lossy (e.g., a wireless connection), the value
of the Robustness Variable may be increased. MLD is robust to
[Robustness Variable]-1 packet losses. This document recommends a
default value of 2 for the Robustness Variable (see section 9.1).
If more changes to the same per-interface state entry occur before
all the retransmissions of the State Change Report for the first
change have been completed, each additional change triggers the
immediate transmission of a new State Change Report. Section 6.1
shows how the content of this new report is computed. Retransmissions
of the new State Change Report will be scheduled as well, in order to
ensure that each instance of state change is transmitted at least
[Robustness Variable] times.
If a node on a link expresses, through a State Change Report, its
desire to no longer listen to a particular multicast address (or
source), the Querier must query for other listeners of the multicast
address (or source) before deleting the multicast address (or source)
from its Multicast Address Listener state and stopping the
corresponding traffic. Thus, the Querier sends a Multicast Address
Specific Query to verify whether there are nodes still listening to a
specified multicast address or not. Similarly, the Querier sends a
Multicast Address and Source Specific Query to verify whether, for a
specified multicast address, there are nodes still listening to a
specific set of sources, or not. Section 5.1.13 describes each query
in more detail.
Both Multicast Address Specific Queries and Multicast Address and
Source Specific Queries are only sent in response to State Change
Reports, never in response to Current State Reports. This
distinction between the two types of reports is needed to avoid the
router treating all Multicast Listener Reports as potential changes
in state. By doing so, the fast leave mechanism of MLDv2, described
in more detail in section 2.2, might not be effective if a State
Change Report is lost, and only the following Current State Report is
received by the router. Nevertheless, it avoids an increased
processing at the router and it reduces the MLD traffic on the link.
More details on the necessity of distinguishing between the two
report types can be found in Appendix A1.
Nodes respond to the above queries through Current State Reports,
that contain their per-interface Multicast Address Listening state
only for the multicast addresses (or sources) being queried.
As stated earlier, in order to ensure protocol robustness, all the
queries, except the periodical General Queries, are retransmitted
several times within a given time interval. The number of
retransmissions depends on the Robustness Variable. If, while
scheduling new queries, there are pending queries to be retransmitted
for the same multicast address, the new queries and the pending
queries have to be merged. In addition, host reports received for a
multicast address with pending queries may affect the contents of
those queries. The process of building and maintaining the state of
pending queries is presented in section 7.6.3.
Protocol robustness is also enhanced through the use of the S flag
(Suppress Router-Side Processing). As described above, when a
Multicast Address Specific or a Multicast Address and Source Specific
Query is sent by the Querier, a number of retransmissions of the
query are scheduled. In the original (first) query the S flag is
clear. When the Querier sends this query, it lowers the timers for
the concerned multicast address (or source) to a given value;
similarly, any non-querier multicast router that receives the query
lowers its timers in the same way. Nevertheless, while waiting for
the next scheduled queries to be sent, the Querier may receive a
report that updates the timers. The scheduled queries still have to
be sent, in order to ensure that a non-querier router keeps its state
synchronized with the current Querier (the non-querier router might
have missed the first query). Nevertheless, the timers should not be
lowered again, as a valid answer was already received. Therefore, in
subsequent queries the Querier sets the S flag.
2.3. Building Multicast Address Listener State on Multicast Routers
Multicast routers that implement MLDv2 (whether they are in Querier
state or not) keep state per multicast address per attached link.
This multicast address listener state consists of a Filter Mode, a
Filter Timer, and a Source List, with a timer associated to each
source from the list. The Filter Mode is used to summarize the total
listening state of a multicast address to a minimum set, such that
all nodes' listening states are respected. The Filter Mode may
change in response to the reception of particular types of report
messages, or when certain timer conditions occur.
A router is in INCLUDE mode for a specific multicast address on a
given interface if all the listeners on the link interested in that
address are in INCLUDE mode. The router state is represented through
the notation INCLUDE (A), where A is a list of sources, called the
"Include List". The Include List is the set of sources that one or
more listeners on the link have requested to receive. All the
sources from the Include List will be forwarded by the router. Any
other source that is not in the Include List will be blocked by the
router.
A source can be added to the current Include List if a listener in
INCLUDE mode sends a Current State or a State Change Report that
includes that source. Each source from the Include List is
associated with a source timer that is updated whenever a listener in
INCLUDE mode sends a report that confirms its interest in that
specific source. If the timer of a source from the Include List
expires, the source is deleted from the Include List.
Besides this "soft leave" mechanism, there is also a "fast leave"
scheme in MLDv2; it is also based on the use of source timers. When
a node in INCLUDE mode expresses its desire to stop listening to a
specific source, all the multicast routers on the link lower their
timers for that source to a given value. The Querier then sends a
Multicast Address and Source Specific Query, to verify whether there
are other listeners for that source on the link, or not. If a report
that includes this source is received before the timer expiration,
all the multicast routers on the link update the source timer. If
not, the source is deleted from the Include List. The handling of
the Include List, according to the received reports, is detailed in
Tables 7.4.1 and 7.4.2.
A router is in EXCLUDE mode for a specific multicast address on a
given interface if there is at least one listener in EXCLUDE mode for
that address on the link. When the first report is received from
such a listener, the router sets the Filter Timer that corresponds to
that address. This timer is reset each time an EXCLUDE mode listener
confirms its listening state through a Current State Report. The
timer is also updated when a listener, formerly in INCLUDE mode,
announces its filter mode change through a State Change Report
message. If the Filter Timer expires, it means that there are no
more listeners in EXCLUDE mode on the link. In this case, the router
switches back to INCLUDE mode for that multicast address.
When the router is in EXCLUDE mode, the router state is represented
by the notation EXCLUDE (X,Y), where X is called the "Requested List"
and Y is called the "Exclude List". All sources, except those from
the Exclude List, will be forwarded by the router. The Requested
List has no effect on forwarding. Nevertheless, the router has to
maintain the Requested List for two reasons:
o To keep track of sources that listeners in INCLUDE mode listen to.
This is necessary to assure a seamless transition of the router to
INCLUDE mode, when there is no listener in EXCLUDE mode left.
This transition should not interrupt the flow of traffic to
listeners in INCLUDE mode for that multicast address. Therefore,
at the time of the transition, the Requested List should contain
the set of sources that nodes in INCLUDE mode have explicitly
requested.
When the router switches to INCLUDE mode, the sources in the
Requested List are moved to the Include List, and the Exclude List
is deleted. Before switching, the Requested List can contain an
inexact guess of the sources listeners in INCLUDE mode listen to -
might be too large or too small. These inexactitudes are due to
the fact that the Requested List is also used for fast blocking
purposes, as described below. If such a fast blocking is
required, some sources may be deleted from the Requested List (as
shown in Tables 7.4.1 and 7.4.2) in order to reduce router state.
Nevertheless, in each such case the Filter Timer is updated as
well. Therefore, listeners in INCLUDE mode will have enough time,
before an eventual switching, to reconfirm their interest in the
eliminated source(s), and rebuild the Requested List accordingly.
The protocol ensures that when a switch to INCLUDE mode occurs,
the Requested List will be accurate. Details about the transition
of the router to INCLUDE mode are presented in Appendix A3.
o To allow the fast blocking of previously unblocked sources. If
the router receives a report that contains such a request, the
concerned sources are added to the Requested List. Their timers
are set to a given small value, and a Multicast Address and Source
Specific Query is sent by the Querier, to check whether there are
nodes on the link still interested in those sources, or not. If
no node announces its interest in receiving those specific source,
the timers of those sources expire. Then, the sources are moved
from the Requested List to the Exclude List. From then on, the
sources will be blocked by the router.
The handling of the EXCLUDE mode router state, according to the
received reports, is detailed in Tables 7.4.1 and 7.4.2.
Both the MLDv2 router and listener behaviors described in this
document were defined to ensure backward interoperability with MLDv1
hosts and routers. Interoperability issues are detailed in section
8.
3. The Service Interface for Requesting IP Multicast Reception
Within an IP system, there is (at least conceptually) a service
interface used by upper-layer protocols or application programs to
ask the IP layer to enable or disable reception of packets sent to
specific IP multicast addresses. In order to take full advantage of
the capabilities of MLDv2, a node's IP service interface must support
the following operation:
IPv6MulticastListen ( socket, interface, IPv6 multicast address,
filter mode, source list )
where:
o "socket" is an implementation-specific parameter used to
distinguish among different requesting entities (e.g., programs,
processes) within the node; the socket parameter of BSD Unix
system calls is a specific example.
o "interface" is a local identifier of the network interface on
which reception of the specified multicast address is to be
enabled or disabled. Interfaces may be physical (e.g., an
Ethernet interface) or virtual (e.g., the endpoint of a Frame
Relay virtual circuit or an IP-in-IP "tunnel"). An implementation
may allow a special "unspecified" value to be passed as the
interface parameter, in which case the request would apply to the
"primary" or "default" interface of the node (perhaps established
by system configuration). If reception of the same multicast
address is desired on more than one interface, IPv6MulticastListen
is invoked separately for each desired interface.
o "IPv6 multicast address" is the multicast address to which the
request pertains. If reception of more than one multicast address
on a given interface is desired, IPv6MulticastListen is invoked
separately for each desired address.
o "filter mode" may be either INCLUDE or EXCLUDE. In INCLUDE mode,
reception of packets sent to the specified multicast address is
requested *only* from the source addresses listed in the source
list parameter. In EXCLUDE mode, reception of packets sent to the
given multicast address is requested from all source addresses
*except* those listed in the source list parameter.
o "source list" is an unordered list of zero or more unicast
addresses from which multicast reception is desired or not
desired, depending on the filter mode. An implementation MAY
impose a limit on the size of source lists. When an operation
causes the source list size limit to be exceeded, the service
interface SHOULD return an error.
For a given combination of socket, interface, and IPv6 multicast
address, only a single filter mode and source list can be in effect
at any one time. Nevertheless, either the filter mode or the source
list, or both, may be changed by subsequent IPv6MulticastListen
requests that specify the same socket, interface, and IPv6 multicast
address. Each subsequent request completely replaces any earlier
request for the given socket, interface, and multicast address.
The MLDv1 protocol did not support source filters, and had a simpler
service interface; it consisted of Start Listening and Stop Listening
operations to enable and disable listening to a given multicast
address (from *all* sources) on a given interface. The equivalent
operations in the new service interface are as follows:
The Start Listening operation is equivalent to:
IPv6MulticastListen ( socket, interface, IPv6 multicast address,
EXCLUDE, {} )
and the Stop Listening operation is equivalent to:
IPv6MulticastListen ( socket, interface, IPv6 multicast address,
INCLUDE, {} )
where {} is an empty source list.
An example of an API that provides the capabilities outlined in this
service interface is given in [RFC3678].
4. Multicast Listening State Maintained by Nodes
4.1. Per-Socket State
For each socket on which IPv6MulticastListen has been invoked, the
node records the desired multicast listening state for that socket.
That state conceptually consists of a set of records of the form:
(interface, IPv6 multicast address, filter mode, source list)
The per-socket state evolves in response to each invocation of
IPv6MulticastListen on the socket, as follows:
o If the requested filter mode is INCLUDE *and* the requested source
list is empty, then the entry that corresponds to the requested
interface and multicast address is deleted, if present. If no
such entry is present, the request has no effect.
o If the requested filter mode is EXCLUDE *or* the requested source
list is non-empty, then the entry that corresponds to the
requested interface and multicast address, if present, is changed
to contain the requested filter mode and source list. If no such
entry is present, a new entry is created, using the parameters
specified in the request.
4.2. Per-Interface State
In addition to the per-socket multicast listening state, a node must
also maintain or compute multicast listening state for each of its
interfaces. That state conceptually consists of a set of records of
the form:
(IPv6 multicast address, filter mode, source list)
At most one record per multicast address exists for a given
interface. This per-interface state is derived from the per-socket
state, but may differ from it when different sockets have differing
filter modes and/or source lists for the same multicast address and
interface. For example, suppose one application or process invokes
the following operation on socket s1:
IPv6MulticastListen ( s1, i, m, INCLUDE, {a, b, c} )
requesting reception on interface i of packets sent to multicast
address m, *only* if they come from the sources a, b, or c. Suppose
another application or process invokes the following operation on
socket s2:
IPv6MulticastListen ( s2, i, m, INCLUDE, {b, c, d} )
requesting reception on the same interface i of packets sent to the
same multicast address m, *only* if they come from sources b, c, or
d. In order to satisfy the reception requirements of both sockets,
it is necessary for interface i to receive packets sent to m from any
one of the sources a, b, c, or d. Thus, in this example, the
listening state of interface i for multicast address m has filter
mode INCLUDE and source list {a, b, c, d}.
After a multicast packet has been accepted from an interface by the
IP layer, its subsequent delivery to the application or process that
listens on a particular socket depends on the multicast listening
state of that socket (and possibly also on other conditions, such as
what transport-layer port the socket is bound to). So, in the above
example, if a packet arrives on interface i, destined to multicast
address m, with source address a, it may be delivered on socket s1
but not on socket s2. Note that MLDv2 messages are not subject to
source filtering and must always be processed by hosts and routers.
Requiring the filtering of packets based upon a socket's multicast
reception state is a new feature of this service interface. The
previous service interface described no filtering based upon
multicast listening state; rather, a Start Listening operation on a
socket simply caused the node to start to listen to a multicast
address on the given interface; packets sent to that multicast
address could be delivered to all sockets, whether they had started
to listen or not.
The general rules for deriving the per-interface state from the per-
socket state are as follows: for each distinct (interface, IPv6
multicast address) pair that appears in any per-socket state, a per-
interface record is created for that multicast address on that
interface. Considering all socket records that contain the same
(interface, IPv6 multicast address) pair,
o if *any* such record has a filter mode of EXCLUDE, then the filter
mode of the interface record is EXCLUDE, and the source list of
the interface record is the intersection of the source lists of
all socket records in EXCLUDE mode, minus those source addresses
that appear in any socket record in INCLUDE mode. For example, if
the socket records for multicast address m on interface i are:
from socket s1: ( i, m, EXCLUDE, {a, b, c, d} )
from socket s2: ( i, m, EXCLUDE, {b, c, d, e} )
from socket s3: ( i, m, INCLUDE, {d, e, f} )
then the corresponding interface record on interface i is:
( m, EXCLUDE, {b, c} )
If a fourth socket is added, such as:
From socket s4: ( i, m, EXCLUDE, {} )
then the interface record becomes:
( m, EXCLUDE, {} )
o if *all* such records have a filter mode of INCLUDE, then the
filter mode of the interface record is INCLUDE, and the source
list of the interface record is the union of the source lists of
all the socket records. For example, if the socket records for
multicast address m on interface i are:
from socket s1: ( i, m, INCLUDE, {a, b, c} )
from socket s2: ( i, m, INCLUDE, {b, c, d} )
from socket s3: ( i, m, INCLUDE, {e, f} )
then the corresponding interface record on interface i is:
( m, INCLUDE, {a, b, c, d, e, f} )
An implementation MUST NOT use an EXCLUDE interface record for a
multicast address if all sockets for this multicast address are in
INCLUDE state. If system resource limits are reached when a per-
interface state source list is calculated, an error MUST be returned
to the application which requested the operation.
The above rules for deriving the per-interface state are
(re)evaluated whenever an IPv6MulticastListen invocation modifies the
per-socket state by adding, deleting, or modifying a per-socket state
record. Note that a change of the per-socket state does not
necessarily result in a change of the per-interface state.
5. Message Formats
MLDv2 is a sub-protocol of ICMPv6, that is, MLDv2 message types are a
subset of ICMPv6 messages, and MLDv2 messages are identified in IPv6
packets by a preceding Next Header value of 58. All MLDv2 messages
described in this document MUST be sent with a link-local IPv6 Source
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. Description of the Protocol for Multicast Routers
The purpose of MLD is to enable each multicast router to learn, for
each of its directly attached links, which multicast addresses have
listeners on that link. MLD version 2 adds the capability for a
multicast router to also learn which *sources* have listeners among
the neighboring nodes, for packets sent to any particular multicast
address. The information gathered by MLD is provided to whichever
multicast routing protocol is used by the router, in order to ensure
that multicast packets are delivered to all links where there are
interested listeners.
This section describes the part of MLDv2 that is performed by
multicast routers. Multicast routers may themselves become multicast
address listeners, and therefore also perform the multicast listener
part of MLDv2, 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.
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.fr
Authors' Addresses
This document was written by:
Rolland Vida, LIP6
EMail: Rolland.Vida@lip6.fr
Luis Henrique Maciel Kosmalski Costa, LIP6
EMail: Luis.Costa@lip6.fr
Serge Fdida, LIP6
EMail: Serge.Fdida@lip6.fr
Steve Deering, Cisco Systems, Inc.
EMail: deering@cisco.com
Bill Fenner, AT&T Labs - Research
EMail: fenner@research.att.com
Isidor Kouvelas, Cisco Systems, Inc.
EMail: kouvelas@cisco.com
Brian Haberman, Caspian Networks
EMail: brian@innovationslab.net
This document is the translation of [RFC3376] for IPv6 semantics. It
was elaborated based on the translation of (RFC 2236) into [RFC2710].
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