INTERNET-DRAFT R. Vida, Editor
Obsoletes RFC 2710 L. Costa, Editor
LIP6
Expires May 2003 November 2002
Multicast Listener Discovery Version 2 (MLDv2) for IPv6
<draft-vida-mld-v2-06.txt>
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 10 of [RFC 2026].
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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The list of current Internet-Drafts can be accessed at
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Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document specifies Version 2 of the Multicast Listener Discovery
Protocol, MLDv2. MLD is the protocol used by an IPv6 router to
discover the presence of multicast listeners (that is, nodes wishing
to receive multicast packets) on its directly attached links, and to
discover specifically which multicast addresses are of interest to
those neighboring nodes.
MLDv2 is derived from version 3 of IPv4's Internet Group Management
Protocol, IGMPv3. Compared to the previous version, MLDv2 adds
support for "source filtering", that is, the ability for a node to
report interest in listening to packets *only* from specific source
addresses, or from *all but* specific source addresses, sent to a
particular multicast address.
This document obsoletes RFC 2710.
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Table of Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Service Interface for Requesting IP Multicast Reception . 3
3. Multicast Listening State Maintained by Nodes . . . . . . . . 4
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 7
5. Description of the Protocol for Multicast Address Listeners . 18
6. Description of the Protocol for Multicast Routers . . . . . . 24
7. Interoperation with Older Versions of MLD . . . . . . . . . . 35
8. List of Timers, Counters and their Default Values . . . . . . 39
9. Security Considerations . . . . . . . . . . . . . . . . . . . 43
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 44
11. References. . . . . . . . . . . . . . . . . . . . . . . . . . 44
12. Editors' Contact Information. . . . . . . . . . . . . . . . . 45
13. Authors . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
14. Acknowledgements. . . . . . . . . . . . . . . . . . . . . . . 46
Appendix A. Design Rationale. . . . . . . . . . . . . . . . . . . 47
Appendix B. Summary of Changes from MLDv1 . . . . . . . . . . . . 48
Full Copyright Statement. . . . . . . . . . . . . . . . . . . . . 50
1. Introduction
The Multicast Listener Discovery Protocol (MLD) is used by IPv6
routers to discover the presence of multicast listeners (that is,
nodes wishing 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 which case it performs both the "multicast
router part" of the protocol (to collect the multicast listener
information needed by its multicast routing protocol) and the
"multicast address listener part" of the protocol (to inform itself
and other neighboring multicast routers of its listening state).
This document specifies Version 2 of MLD. The previous version of
MLD became an Internet Standard and is specified in [RFC 2710]. In
this document we will refer to it as MLDv1. MLDv2 is a translation
of the IGMPv3 protocol [RFC 3376] for IPv6 semantics.
Version 2 of the MLD protocol, when compared to the previous version,
adds support for "source filtering", that is, the ability for a node
to report interest in listening to packets *only* from specific
source addresses, as required to support Source-Specific Multicast
[SSM], or from *all but* specific source addresses, sent to a
particular multicast address. Version 2 is designed to be
interoperable with the previous version.
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
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[RFC 2119]. Due to the lack of italics, emphasis is indicated herein
by bracketing a word or phrase in "*" characters.
2. 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 and 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 or
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
the endpoint of 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 those 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, but that limit MUST NOT be less than
64 addresses per list. When an operation causes the source list
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size limit to be exceeded, the service interface MUST 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. However, 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 previous version of MLD did not support source filters and had a
simpler service interface consisting 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 follow:
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 providing the capabilities outlined in this
service interface is given in [FILTER-API].
3. Multicast Listening State Maintained by Nodes
3.1. 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 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 corresponding to the requested
interface and multicast address is deleted if present. If no
such entry is present, the request is ignored.
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o If the requested filter mode is EXCLUDE *or* the requested source
list is non-empty, then the entry corresponding 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.
3.2. 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 the per-socket state when different
sockets have differing filter modes and/or source lists for the same
multicast address and interface. For example, suppose one
application or process invokes the following operation on socket s1:
IPv6MulticastListen ( s1, i, m, INCLUDE, {a, b, c} )
requesting reception on interface i of packets sent to multicast
address m, *only* if they come from source 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
listening 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.
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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, and 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 socket state, a per-
interface record is created for that multicast address on that
interface. Considering all socket records containing 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} )
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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 an
interface state source list is calculated, an error MUST be returned
to the application which requested the operation.
The above rules for deriving the interface state are (re-)evaluated
whenever an IPv6MulticastListen invocation modifies the socket state
by adding, deleting, or modifying a per-socket state record. Note
that a change of socket state does not necessarily result in a change
of interface state.
4. Message Formats
MLDv2 is a sub-protocol of ICMPv6, that is, MLDv2 message types are a
subset of the set 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 (or the unspecified address, if the
node has not yet acquired such an address), an IPv6 Hop Limit of 1,
and an IPv6 Router Alert option [RFC 2711] 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.)
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 = [Value to be allocated
by the IANA]). See section 10 for IANA considerations.
An implementation of MLDv2 must also support the following two
message types, for interoperation with the previous version of MLD
(see section 7):
o Version 1 Multicast Listener Report (Type = decimal 131) [RFC 2710]
o Version 1 Multicast Listener Done (Type = decimal 132) [RFC 2710]
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.
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4.1. Multicast Listener Query Message
Multicast Listener Queries are sent by multicast routers 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] *
| |
* *
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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4.1.1. Code
Initialized to zero by the sender; ignored by receivers.
4.1.2. Checksum
The standard ICMPv6 checksum, covering the entire MLDv2 message plus
a "pseudo-header" of IPv6 header fields [RFC 2463]. For computing
the checksum, the Checksum field is set to zero. When receiving
packets, the checksum MUST be verified before processing a packet.
4.1.3. 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 listening 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 tuning of the burstiness of MLD traffic on a link.
4.1.4. Reserved
Initialized to zero by the sender; ignored by receivers.
4.1.5. Multicast Address
The Multicast Address field is set to zero when sending a General
Query, and set to the multicast address being queried when sending a
Multicast Address Specific Query or Multicast Address and Source
Specific Query (see section 4.1.10, below).
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4.1.6. Resv (Reserved)
Initialized to zero by the sender; ignored by receivers.
4.1.7. 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.
4.1.8. QRV (Querier's Robustness Variable)
If non-zero, the QRV field contains the [Robustness Variable] value
used by the Querier, i.e., the sender of the Query. If the Querier's
[Robustness Variable] exceeds 7, the maximum value of the QRV field,
the QRV 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 the
receivers use the default [Robustness Variable] value specified in
section 8.1 or a statically configured value.
4.1.9. 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 8.2.
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4.1.10. 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 with an MTU of 1500 octets, the IPv6 header (40 octets)
including the Router Alert option in the Hop-By-Hop Extension Header
(8 octets) consumes 48 octets, and the MLD fields up to including the
Number of Sources (N) field consume 28 octets, leaving 1424 octets
for source addresses, which limits the number of source addresses to
89 (1424/16).
4.1.11. 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.
4.1.12. 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 here.
4.1.13. Query Variants
There are three variants of the Query message:
o A "General Query" is sent by a multicast router 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 a multicast router
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,
and the Number of Sources (N) field contains zero.
o A "Multicast Address and Source Specific Query" is sent by a
multicast router 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
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of interest, and the Source Address [i] field(s) contain the source
address(es) of interest.
4.1.14. 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.
4.2. 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 = TBA | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Reserved |Nr of Mcast Address Records (M)|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Multicast Address Record [1] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Multicast Address Record [2] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
. . .
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Multicast Address Record [M] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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where TBA = value To Be Allocated by the IANA, as specified in
section 4.
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 .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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4.2.1. Reserved
The Reserved fields are set to zero on transmission, and ignored on
reception.
4.2.2. Checksum
The standard ICMPv6 checksum, covering the entire MLDv2 message plus
a "pseudo-header" of IPv6 header fields [RFC 2460, RFC 2463]. For
computing the checksum, the Checksum field is set to zero. When
receiving packets, the checksum MUST be verified before processing a
message.
4.2.3. 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.
4.2.4. Multicast Address Record
Each Multicast Address Record is a block of fields containing
information on the sender listening to a single multicast address on
the interface from which the Report is sent.
4.2.5. Record Type
Specifies the type of the Multicast Address Record. See section
4.2.12 for a detailed description of the different possible Record
Types.
4.2.6. 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.
4.2.7. Number of Sources (N)
The Number of Sources (N) field specifies how many source addresses
are present in this Multicast Address Record.
4.2.8. Multicast Address
The Multicast Address field contains the multicast address to which
this Multicast Address Record pertains.
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4.2.9. 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.
4.2.10. Auxiliary Data
The Auxiliary Data field, if present, contains additional information
pertaining 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 auxiliary
data present in any received Multicast Address Record. The semantics
and internal encoding of the Auxiliary Data field are to be defined
by any future version or extension of MLD that uses this field.
4.2.11. 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.
4.2.12. 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, if it is non-empty.
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
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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. 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
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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.
Unrecognized Record Type values MUST be silently ignored.
4.2.13. Source Addresses for Reports
An MLDv2 Report MUST be sent with a valid IPv6 link-local source
address, or the unspecified address (::), if the node has not yet
acquired a valid link-local address. Routers MUST ignore a message
that is not sent with a valid link-local address.
Thus, a Report will be ignored, if the router cannot identify the
source address of the packet as belonging to a link assigned to the
interface on which the packet was received. A Report sent with the
unspecified address will also be ignored by a router. The reporting
node has however modified its listening state for multicast addresses
that are contained in the Multicast Address Records of the Report
message.
4.2.14. Destination Addresses for Reports
Version 2 Multicast Listener Reports are sent with an IP
destination address to which all MLDv2-capable multicast routers
listen (see section 10 for IANA considerations related to this
special destination address). A node that is operating in version 1
compatibility mode 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.
4.2.15. Notation for Multicast Address Records
In the rest of this document, we use the following notation to
describe the contents of a Multicast Address Record pertaining 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:
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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.
4.2.16. Multicast Listener Report Size
If the set of Multicast Address Records required in a Report does not
fit within the size limit of a single Report message (as determined
by the MTU of the link on which it will be sent), the Multicast
Address Records are sent in as many Report messages as needed to
report the entire set.
If a single Multicast Address Record contains so many source
addresses that it does not fit within the size limit of a single
Report message, if its Type is not MODE_IS_EXCLUDE or
CHANGE_TO_EXCLUDE_MODE, it is split into multiple Multicast Address
Records, each containing a different subset of the source addresses
and each sent in a separate Report message. If its Type is
MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, a single Multicast Address
Record is sent, containing as many source addresses as can fit, and
the remaining source addresses are not reported; though 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.
5. Description of the Protocol for Multicast Address Listeners
MLD is an asymmetric protocol, specifying 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, receiving and
responding to its own MLD message transmissions as well as to those
of its neighbors.) The multicast router part of MLDv2 is described
in section 6.
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 is connected to the same link.
For interoperability with multicast routers running the older
version of MLD, nodes maintain a MulticastRouterVersion variable
for each interface on which multicast reception is supported. This
section describes the behavior of multicast address listener nodes
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on interfaces for which MulticastRouterVersion = 2. The algorithm
for determining MulticastRouterVersion, and the behavior for version
1, are described in section 7.
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 two types of events that trigger MLDv2 protocol actions on
an interface:
o a change of the interface listening state, caused by a local
invocation of IPv6MulticastListen.
o reception of a Query.
(Received MLD messages of types other than Query are silently
ignored, except as required for interoperation with the earlier
version of MLD.)
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 8.
5.1. Action on Change of Interface State
An invocation of IPv6MulticastListen may cause the multicast
listening state of an interface to change, according to the rules in
section 3.2. Each such change affects the per-interface entry for a
single multicast address.
A change of 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 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 a filter mode of INCLUDE
and an empty source list.
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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
message.
To cover the possibility of the State Change Report being missed by
one or more multicast routers, it is retransmitted [Robustness
Variable] - 1 more times, at intervals chosen at random from the
range (0, [Unsolicited Report Interval]).
If more changes to the same 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 transmitted report are calculated as follows:
o As was done with the first report, the interface state for the
affected multicast address before and after the latest change is
compared.
o The report records expressing the difference are built according to
the table above. However these records are not transmitted in a
message but instead merged with the contents of the pending report,
to create the new State Change Report.
The rules for merging the difference report resulting from the state
change and the pending 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.
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.
If the interface listening state 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
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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.
When [Robustness Variable] State Change Reports with Filter Mode
Change Records have been transmitted after the last filter mode
change, and if source list changes to the interface listening have
scheduled additional reports, then the next State Change Report will
include Source List Change Records.
Each time a State Change Report is transmitted, the contents are
determined as follows. If the report should contain a Filter Mode
Change Record, 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, an ALLOW and a BLOCK record are included. The
contents of these records are built according to the table below.
Record Sources included
------ ----------------
TO_IN All in the current interface state that must be forwarded
TO_EX All in the current interface state that must be blocked
ALLOW All with retransmission state 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).
5.2. Action on Reception of a Query
Upon reception of an MLD message containing 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
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previously scheduled pending responses and in many cases schedule a
combined response. Therefore, the node must be able to maintain the
following state:
o A timer per interface for scheduling responses to General Queries.
o A per-multicast-address-and-interface timer for scheduling
responses to Multicast Address Specific and Multicast Address and
Source Specific Queries.
o A per-multicast-address-and-interface list of sources to be
reported in the response to a Multicast Address and Source Specific
Query.
When a new Query with the Router Alert option arrives on an
interface, provided the node has state to report, a delay for a
response is randomly selected in the range (0, [Maximum Response
Delay]) where Maximum Response Delay is derived from Maximum Response
Code 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 rules 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
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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.
When the timer in a pending response record expires, the node
transmits, on the associated interface, one or more Report messages
carrying one or more Current State Records (see section 4.2.12), as
follows:
1. If the expired timer is the interface timer (i.e., it 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 3.2. 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 on 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., it
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., it
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 responding Current
State Record is determined from the interface state and the
pending response record, as specified in the following table:
set of sources in the
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
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addresses, then no response is sent.
Finally, after any required Report messages have been generated, the
source lists associated with any reported multicast addresses are
cleared.
6. 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 being 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 5.
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 is operating 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 [RFC 2464]; in
the case of an Ethernet interface that does not support the filtering
of such a range of multicast address, 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 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 need to know only 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 keep track of the interests of each individual neighboring node.
(Nevertheless, see Appendix A2 point 1 for discussion.)
MLDv2 is backward compatible with the previous version of the MLD
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protocol. In order to remain backward compatible with older MLD
nodes, MLDv2 multicast routers MUST also implement the previous
version of the protocol (see section 7).
6.1. Conditions for MLD Queries
Multicast routers send General Queries periodically 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, a multicast router 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, multicast routers send 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 a router 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 a multicast router
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 4.1.13 describes each query in more detail.
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6.2. MLD State Maintained by Multicast Routers
Multicast routers implementing MLDv2 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 running MLD, 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, multicast address timer,
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 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.
6.2.1. 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
condense the total listening state of a multicast address to a
minimum set such that all nodes' listening states are covered. This
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 6.4 describes the changes of a router
filter mode per Multicast Address Record received.
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 EXCLUDE.
When a router filter mode for a multicast address is EXCLUDE, the
source record list contains two types of sources. The first type
is the set which represents conflicts in the desired reception state;
this set must be forwarded by some router on the network. It serves
essentially to rebuild the set of sources not forwarded by the router
when certain timer conditions occur. It will be also used as the new
source list when the router switches back to INCLUDE mode, after its
multicast address timer has expired.
The second type of sources is the set of sources which all nodes have
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requested not to be forwarded. The rules for updating the two sets
of the source record list when the router is in EXCLUDE mode will be
described in section 6.4. Appendix A describes the reasons for
keeping the first set of sources when in EXCLUDE mode.
When a router filter mode for a multicast address is INCLUDE, the
source record list is the list of sources that have listeners for the
specific multicast address. Each source in a source record must be
forwarded by some router on the network.
Because a reported Multicast Address Record with a filter mode of
EXCLUDE will cause a router to transition its filter mode for that
multicast address to EXCLUDE, a mechanism for transitioning a
router's filter mode back to INCLUDE must exist. 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 Multicast Address Timer expires and is
explained in detail in section 6.5.
6.2.2. Definition of Multicast Address Timers
The Multicast Address Timer is only used when a multicast address is
in EXCLUDE mode and it represents the time for the *filter mode* of
the multicast address to expire and switch to INCLUDE mode. We define
a multicast address timer as a decrementing timer with a lower bound
of zero kept per multicast address per attached link. Multicast
address timers are updated according to the types of Multicast
Address Records received.
A Multicast Address Timer expiring when a router filter mode for the
multicast address is EXCLUDE means there are no more listeners on the
attached link in EXCLUDE mode. At this point, a router will
transition to INCLUDE filter mode. Section 6.5 describes the actions
taken when a Multicast Address Timer expires while in EXCLUDE mode.
The following table summarizes the role of the Multicast Address
Timer. Section 6.4 describes the details of setting the Multicast
Address Timer per type of Multicast Address Record received.
Multicast Address Multicast Address
Filter Mode Timer Value Actions/Comments
----------- ----------------- ----------------
INCLUDE Timer >= 0 All listeners in
INCLUDE mode.
EXCLUDE Timer > 0 At least one listener
in EXCLUDE mode.
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EXCLUDE Timer == 0 No more listeners in
EXCLUDE mode to the
multicast address.
If all source timers
have expired then
delete Multicast
Address Record. If
there are still source
record timers running,
switch to INCLUDE
filter mode using those
source records with
running timers as the
INCLUDE source record
state.
6.2.3. Definition of Source Timers
A source timer is kept per source record and is a decrementing timer
with a lower bound of zero. Source timers are updated according to
the type and filter mode of the Multicast Address Record received.
Section 6.4 describes the setting of source timers per type of
Multicast Address Records received.
When the router filter mode for a multicast address is INCLUDE, a
source record with a running timer means that there are currently one
or more nodes (in INCLUDE filter mode) which listen to that source.
If a source timer expires, the router concludes that this particular
source no longer has listeners on the attached link, and deletes the
associated source record.
Source timers are treated differently when a router filter mode for a
multicast address is EXCLUDE. If a source record has a running timer
it means that at least one system desires the source. It should
therefore be forwarded by some router on the network. Appendix A
describes the reasons for keeping state for sources that have been
requested to be forwarded while in EXCLUDE state.
If a source timer expires with a router filter mode for the multicast
address of EXCLUDE, the router informs the routing protocol that
there is no longer a listener on the link interested in traffic from
this source.
When a router filter mode for a multicast address is EXCLUDE, source
records are only deleted when the Multicast Address Timer expires or
when newly received Multicast Address Records modify the source
record list of the router. Section 6.3 describes the actions that
should be taken depending on the value of the source timer.
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6.3. 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.
Multicast Address
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
INCLUDE No source element Suggest to not forward
traffic from source
EXCLUDE TIMER > 0 Suggest to forward traffic
from source
EXCLUDE TIMER == 0 Suggest to not forward
traffic from source
(DO NOT remove record)
EXCLUDE No Source Element Suggest to forward traffic
from source
6.4. Action on Reception of Reports
Upon reception of an MLD message containing 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
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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.
6.4.1. Reception of Current State Records
When receiving Current State Records, a router updates both its
multicast address and 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.
The following notation is used to describe the updating of source
timers. The notation ( A, B ) will be used to represent the total
number of sources for a particular multicast address, where
A = set of source records whose source timers > 0
(Sources that at least one node has requested to be forwarded)
B = set of source records whose source timers = 0
(Sources that MLD will suggest to the routing protocol not to
forward)
Note that there will be two sets only when a router's filter mode for
a multicast address is EXCLUDE. When the filter mode is INCLUDE, a
single set is used to describe the set of sources requested to be
forwarded (e.g. simply ( A ) ).
In the following tables, abbreviations are used for several variables
(all of which are described in detail in section 8). The variable
MALI is an abbreviation for the Multicast Address Listening Interval
which is the time in which multicast address listening will time out.
The variable LLQT is an abbreviation for the Last Listener Query
Time, which is the total time the router should wait for a report,
after having sent a first query. During this time, the router 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 change in the
information given to the routing protocol.
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. 'Multicast Address Timer
= J' means that the Multicast Address Timer for the multicast address
should be set to value J.
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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)
Multicast Address
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)
Multicast Address
Timer = MALI
6.4.2. 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.
Routers 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
Multicast Address Timer for that multicast address will be lowered to
a small interval of Last Listener Query Time milliseconds. If any
multicast address records expressing EXCLUDE mode interest in the
multicast address are received within this interval, the multicast
address 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 a query period (i.e. Last Listener Query Time milliseconds)
the MLD component in the router continues to suggest to the routing
protocol that it forwards traffic from the multicast addresses or
sources that it is querying. It is not until after Last Listener
Query Time milliseconds without receiving a record expressing
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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 router in Querier state when a particular
report is received.
We use the following notation for describing the queries which 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 6.6.3. 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)
Multicast Address
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)=Multicast
Address Timer
Send Q(MA,A-Y)
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EXCLUDE (X,Y) TO_EX (A) EXCLUDE (A-Y,Y*A) (A-X-Y)=Multicast
Address Timer
Delete (X-A)
Delete (Y-A)
Send Q(MA,A-Y)
Multicast Address
Timer = MALI
EXCLUDE (X,Y) TO_IN (A) EXCLUDE (X+A,Y-A) (A)=MALI
Send Q(MA,X-A)
Send Q(MA)
6.5. Switching Router Filter Modes
The multicast address timer is used as a mechanism for transitioning
the router filter mode from EXCLUDE to INCLUDE.
When a multicast address 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. When a router's
filter mode for a multicast address is EXCLUDE and the multicast
address timer expires, the router filter mode for the multicast
address transitions to INCLUDE.
A router uses source records with running source timers as its state
for the switch to a filter mode of INCLUDE. If there are any source
records with source timers greater than zero (i.e. requested to be
forwarded), a router switches to filter mode of INCLUDE using those
source records. Source records whose timers are zero (from the
previous EXCLUDE mode) are deleted.
For example, if a router's state for a multicast address is
EXCLUDE(X,Y) and the multicast address timer expires for that
multicast address, the router switches to filter mode of INCLUDE with
state INCLUDE(X).
6.6. Action on Reception of Queries
Upon reception of an MLD message containing 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.
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6.6.1. Timer Updates
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) Multicast Address 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.
6.6.2. Querier Election
MLDv2 elects a single router to be in Querier state per subnet using
the same querier election mechanism as MLDv1, namely by IPv6 address.
When a router receives a query with a lower IPv6 address, it sets the
Other Querier Present timer to Other Querier Present Timeout and
ceases to send queries on the link if it was the previously elected
querier. After its Other Querier Present timer expires, it should
begin sending General Queries.
If a router receives an older version query, it MUST use the older
version of MLD on the link. For a detailed description of
compatibility issues between MLD versions see section 7.
6.6.3 Building and Sending Specific Queries
6.6.3.1. Building and Sending Multicast Address Specific Queries
When a table action "Send Q(MA)" is encountered, then the multicast
address timer must be lowered to LLQT. The router 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
multicast address timer is larger than LLQT, the "Suppress
Router-Side Processing" bit is set in the query message.
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6.6.3.2. Building and Sending Multicast Address and Source Specific
Queries
When a table action "Send Q(MA,X)" is encountered by a querier in the
table in section 6.4.2, 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 Set number of retransmissions for each source to [Last Listener
Query Count].
The router 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 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.
7. Interoperation with Older Versions of MLD
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.
7.1. Query Version Distinctions
The MLD version of a Multicast Listener Query message is determined
as follows:
MLDv1 Query: length = 24 octets
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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.
7.2. Multicast Address Listener Behavior
7.2.1. In the Presence of Older Version Queriers
In order to be compatible with older version 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. This variable is kept per interface and is
dependent on the version of General Queries heard on that interface
as well as on the Older Version Querier Present timer for the
interface. This timer is set to Older Version Querier Present
Timeout seconds whenever an MLDv1 Multicast Address Listener Query is
received.
The Host Compatibility Mode of an interface changes whenever an older
version General Query (than the current compatibility mode) is heard
or when certain timer conditions occur. When the Older Version
Querier Present timer expires, a host switches to Host Compatibility
mode of MLDv2.
The Host Compatibility Mode variable is based on whether an older
version General Query was heard in the last Older Version Querier
Present Timeout seconds. The Host Compatibility Mode is set
depending on the following:
Host Compatibility Mode Timer State
----------------------- -----------
MLDv2 (default) Older Version Querier Present not running
MLDv1 Older Version Querier Present running
If a host receives a query which causes its Older Version Querier
Present timer to be updated and correspondingly its compatibility
mode, it should switch compatibility modes immediately.
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 router 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
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section 4.1.3. is not used.
Whenever a host changes its compatibility mode, it cancels all its
pending response and retransmission timers.
7.2.2. In the Presence of Older Version Multicast Address Listeners
An MLDv2 host may be placed on a link where there are hosts that
have not yet been upgraded to MLDv2. A host MAY allow its MLDv2
Multicast Listener Report to be suppressed by Version 1 Multicast
Listener Report.
7.3. Multicast Router Behavior
7.3.1. In the Presence of Older Version Queriers
MLDv2 routers may be placed on a network where there is at least one
router that has not yet been upgraded to MLDv2. The following
requirements apply:
o If an older version of MLD is present on routers, 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. When
in MLDv1 mode, routers MUST send periodic 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). They also MUST fill in the Maximum Response Delay in
the Maximum Response Code field, i.e. the exponential algorithm
described in section 4.1.3. is not used.
o If a router is not explicitly configured to use MLDv1 and hears an
MLDv1 General Query, it SHOULD log a warning. These warnings MUST
be rate-limited.
7.3.2. In the Presence of Older Version 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
older version 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. This variable is kept per multicast address record and is
dependent on the version of Multicast Listener Reports heard for that
multicast address as well as the Older Version Host Present timer for
the multicast address. This timer is set to Older Version Host
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Present Timeout seconds whenever an MLDv1 Multicast Listener Report
is received.
The Multicast Address Compatibility Mode of a multicast address
record changes whenever an older version report (than the current
compatibility mode) is heard or when certain timer conditions occur.
When the Older Version Host Present timer expires, a router switches
to Multicast Address Compatibility mode of MLDv2. Note that when a
router switches back to MLDv2 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.
The Multicast Address Compatibility Mode variable is based on whether
an older version report was heard in the last Older Version Host
Present Timeout seconds. The Multicast Address Compatibility Mode is
set depending on the following:
Multicast Address Compatibility Mode Timer State
------------------------------------ -----------
MLDv2 (default) Older Version Host Present
not running
MLDv1 Older Version Host Present
running
If a router receives a report which causes its older Host Present
timer to be updated and correspondingly its compatibility mode, it
SHOULD switch compatibility modes immediately.
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 router continues to send MLDv2 queries,
regardless of its compatibility mode.
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8. 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.
8.1. Robustness Variable
The Robustness Variable allows tuning for the expected packet loss on
a link. If a link is expected to be lossy, the Robustness Variable
may be increased. MLD is robust to (Robustness Variable - 1) packet
losses. The Robustness Variable MUST NOT be zero, and SHOULD NOT be
one. Default: 2.
8.2. Query Interval
The Query Interval is the interval between General Queries sent by
the Querier. Default: 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.
8.3. Query Response Interval
The Maximum Response Delay used to calculate the Maximum Response
Code inserted into the periodic General Queries. Default: 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].
8.4. 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 ((the Robustness Variable) times (the Query
Interval)) plus (one Query Response Interval).
8.5. Other Querier Present Timeout
The Other Querier Present Timeout is the length of time that must
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pass before a multicast router decides that there is no longer
another multicast router which should be the querier. This value
MUST be ((the Robustness Variable) times (the Query Interval)) plus
(one half of one Query Response Interval).
8.6. Startup Query Interval
The Startup Query Interval is the interval between General Queries
sent by a Querier on startup. Default: 1/4 the Query Interval.
8.7. Startup Query Count
The Startup Query Count is the number of Queries sent out on startup,
separated by the Startup Query Interval. Default: the Robustness
Variable.
8.8. 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
in calculating the Maximum Response Code inserted into Multicast
Address and Source Specific Query messages. Default: 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.
8.9. 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: the
Robustness Variable.
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8.10. Last Listener Query Time
The Last Listener Query Time is the time value represented by the
Last Listener Query Interval, multiplied by the Last Listener Query
Count. It is not a tunable value, but may be tuned by changing its
components.
8.11. 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: 1
second.
8.12. Older Version Querier Present Timeout
The Older Version Querier Present Timeout is the time-out for
transitioning a host back to MLDv2 mode once an older version query
is heard. When an older version query is received, hosts set their
Older Version Querier Present Timer to Older Version Querier Present
Timeout.
This value MUST be ((the Robustness Variable) times (the Query
Interval in the last Query received)) plus (one Query Response
Interval).
8.13. Older Version Host Present Timeout
The Older Version Host Present Timeout is the time-out for
transitioning a multicast address back to MLDv2 mode once an older
version report is sent for that multicast address. When an older
version report is received, routers set their Older Version Host
Present Timer to Older Version Host Present Timeout.
This value MUST be ((the Robustness Variable) times (the Query
Interval)) plus (one Query Response Interval).
8.14. 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.
8.14.1. 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
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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 Robustness Variable should be
increased to allow for the expected level of packet loss. However,
increasing 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).
8.14.2. 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 Query Interval MUST be equal to or
longer than the Maximum Response Delay used to calculate the Maximum
Response Code inserted in General Query messages.
8.14.3. 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.
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9. Security Considerations
We consider the ramifications of a forged message of each type. Note
that the requirement that nodes verify that the IPv6 Source Address
of all received MLD messages is a link-local address (or the
unspecified address) defends them from acting on forged MLD messages
originated off-link. We discuss thus only the effects of on-link
forgery.
9.1. 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 receivers on that
link in version 1 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.
9.2. 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.
However, 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 "older
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version listener present" state 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 should only be used in situations where source
include and exclude is critical.
9.3. 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 can not cause loss of desired traffic.
10. IANA Considerations
A special IP multicast address called *all MLDv2-capable routers*
should be allocated by IANA, as stated in section 4.2.14. Version 2
Multicast Listener Reports will be sent to this special address.
IANA should allocate as well a special ICMP message type number for
Version 2 Multicast Listener Report messages, as specified in section
4.
11. References
11.1. Normative References
[RFC 2026] Bradner, S., "The Internet Standards Process - Revision
3", BCP 9, RFC 2026, November 1996.
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2373] Hinden, R. and S. Deering, "IP Version 6 Addressing
Architecture", RFC 2373, July 1998.
[RFC 2460] Deering, S. and R. Hinden, "Internet Protocol,
Version 6 (IPv6) Specification", RFC 2460, December
1998.
[RFC 2463] Conta, A. and S. Deering, "Internet Control Message
Protocol (ICMPv6) for the Internet Protocol Version 6
(IPv6) Specification", RFC 2463, December 1998.
[RFC 2464] Crawford, M., "Transmission of IPv6 Packets over
Ethernet Networks", RFC 2464, December 1998.
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INTERNET-DRAFT MLDv2 November 2002
[RFC 2710] Deering, S., Fenner, W., Haberman, B., "Multicast
Listener Discovery (MLD) for IPv6", RFC 2710, November
1999.
[RFC 2711] Partridge, C., Jackson, A., "IPv6 Router Alert Option,"
RFC 2711, November 1999.
11.2. Informative References
[FILTER-API] Thaler, D., B. Fenner, and B. Quinn, "Socket Interface
Extensions for Multicast Source Filters", Work in
Progress, June 2002. (draft-ietf-magma-msf-api-03.txt)
[RFC 2236] Fenner, W., "Internet Group Management Protocol,
Version 2", RFC 2236, November 1997.
[RFC 3376] Cain, B., Deering, S., Fenner,B., Kouvelas, I.,
Thyagarajan, A., "Internet Group Management Protocol,
Version 3", RFC 3376, May 2002.
[SSM] Bhattacharyya, S., et al., "An Overview of Source-
Specific Multicast (SSM)", Work in progress, November
2002. (draft-ietf-ssm-overview-04.txt)
12. Editors' Contact Information
Rolland Vida
LIP6, Universite Pierre et Marie Curie
8, rue du Capitaine Scott
75015 Paris, France
phone: +33-1.44.27.71.26
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: +33-1.44.27.87.72
email: Luis.Costa@lip6.fr
13. Authors
This document was written by:
Rolland Vida, LIP6
email: Rolland.Vida@lip6.fr
Luis Henrique Maciel Kosmalski Costa, LIP6
email: Luis.Costa@lip6.fr
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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: bkhabs@nc.rr.com
This document is the translation of [RFC 3376] for IPv6 semantics.
It was elaborated based on the translation of [RFC 2236] into
[RFC 2710].
14. Acknowledgements
We would like to thank Francis Dupont, Konstantin Kabassanov, Remi
Zara, Hitoshi Asaeda, Shinsuke Suzuki, and Erik Nordmark for their
valuable comments and suggestions on this document.
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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 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 interface state require the router
to take some action in response to the received report (see Section
6.4).
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 cancel sending a pending multicast listener
report if a similar report was observed from another listener on the
link. In MLDv2, this suppression of multicast listener reports has
been removed. The following points explain the reasons behind this
decision.
1. Routers may want to track per-host multicast listener status on an
interface. This allows routers to implement fast leaves (e.g. for
layered multicast congestion control schemes) as well as track
listener status for possible accounting purposes.
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. Removing multicast listener report suppression eases
the job of these MLD snooping devices.
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
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multicast address be reported in a separate message.
A.3 Switching router filter modes from EXCLUDE to INCLUDE
If there exist nodes in both EXCLUDE and INCLUDE modes for a single
multicast address on a link, the router must be in EXCLUDE mode as
well (see section 6.2.1). In EXCLUDE mode, a router forwards traffic
from all sources unless that source exists in the exclusion source
list. If all nodes in EXCLUDE mode cease to exist, 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 multicast
address timer now expires in EXCLUDE mode, 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 multicast address
timer). The router can then switch to INCLUDE mode seamlessly with
the list of sources currently being forwarded in its source list.
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 4.1).
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.
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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 65,535 seconds 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. This allows explicit listener tracking.
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.
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Full Copyright Statement
Copyright (C) The Internet Society (2002). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction of any
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included on all such copies and derivative works. However, this
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the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
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