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