Network Working Group B. Cain
Request for Comments: 3376 Cereva Networks
Obsoletes: 2236 S. Deering
Category: Standards Track I. Kouvelas
Cisco Systems
B. Fenner
AT&T Labs - Research
A. Thyagarajan
EriCsson
October 2002
Internet Group Management Protocol, Version 3
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document specifies Version 3 of the Internet Group Management
Protocol, IGMPv3. IGMP is the protocol used by IPv4 systems to
report their IP multicast group memberships to neighboring multicast
routers. Version 3 of IGMP adds support for "source filtering", that
is, the ability for a system to report interest in receiving packets
*only* from specific source addresses, or from *all but* specific
source addresses, sent to a particular multicast address. That
information may be used by multicast routing protocols to avoid
delivering multicast packets from specific sources to networks where
there are no interested receivers.
This document obsoletes RFC2236.
Table of Contents
1. IntrodUCtion. . . . . . . . . . . . . . . . . . . . . . . . . 2
2. The Service Interface for Requesting IP Multicast Reception . 3
3. Multicast Reception State Maintained by Systems . . . . . . . 5
4. Message Formats . . . . . . . . . . . . . . . . . . . . . . . 7
5. Description of the Protocol for Group Members . . . . . . . . 19
6. Description of the Protocol for Multicast Routers . . . . . . 24
7. Interoperation with Older Versions of IGMP. . . . . . . . . . 35
8. List of Timers, Counters, and Their Default Values. . . . . . 40
9. Security Considerations . . . . . . . . . . . . . . . . . . . 43
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 47
11. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 47
12. Normative References. . . . . . . . . . . . . . . . . . . . . 47
13. Informative References. . . . . . . . . . . . . . . . . . . . 47
Appendix A. Design Rationale. . . . . . . . . . . . . . . . . 49
Appendix B. Summary of changes from IGMPv2. . . . . . . . . . 50
Authors" Addresses. . . . . . . . . . . . . . . . . . . . . . 52
Full Copyright Statement. . . . . . . . . . . . . . . . . . . 53
1. Introduction
The Internet Group Management Protocol (IGMP) is used by IPv4 systems
(hosts and routers) to report their IP multicast group memberships to
any neighboring multicast routers. Note that an IP multicast router
may itself be a member of one or more multicast groups, in which case
it performs both the "multicast router part" of the protocol (to
collect the membership information needed by its multicast routing
protocol) and the "group member part" of the protocol (to inform
itself and other, neighboring multicast routers of its memberships).
IGMP is also used for other IP multicast management functions, using
message types other than those used for group membership reporting.
This document specifies only the group membership reporting functions
and messages.
This document specifies Version 3 of IGMP. Version 1, specified in
[RFC-1112], was the first widely-deployed version and the first
version to become an Internet Standard. Version 2, specified in
[RFC-2236], added support for "low leave latency", that is, a
reduction in the time it takes for a multicast router to learn that
there are no longer any members of a particular group present on an
attached network. Version 3 adds support for "source filtering",
that is, the ability for a system to report interest in receiving
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 3 is
designed to be interoperable with Versions 1 and 2.
Multicast Listener Discovery (MLD) is used in a similar way by IPv6
systems. MLD version 1 [MLD] implements the functionality of IGMP
version 2; MLD version 2 [MLDv2] implements the functionality of IGMP
version 3.
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
[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 IGMPv3, a system"s IP service interface must
support the following operation:
IPMulticastListen ( socket, interface, 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 system; 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 system (perhaps established by system
configuration). If reception of the same multicast address is
desired on more than one interface, IPMulticastListen is invoked
separately for each desired interface.
o "multicast-address" is the IP multicast address, or group, to which
the request pertains. If reception of more than one multicast
address on a given interface is desired, IPMulticastListen is
invoked separately for each desired multicast 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 IP source addresses listed in the
source-list parameter. In EXCLUDE mode, reception of packets sent
to the given multicast address is requested from all IP source
addresses *except* those listed in the source-list parameter.
o "source-list" is an unordered list of zero or more IP 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
size limit to be exceeded, the service interface MUST return an
error.
For a given combination of socket, interface, and 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 IPMulticastListen requests that specify
the same socket, interface, and multicast address. Each subsequent
request completely replaces any earlier request for the given socket,
interface and multicast address.
Previous versions of IGMP did not support source filters and had a
simpler service interface consisting of Join and Leave operations to
enable and disable reception of a given multicast address (from *all*
sources) on a given interface. The equivalent operations in the new
service interface follow:
The Join operation is equivalent to
IPMulticastListen ( socket, interface, multicast-address,
EXCLUDE, {} )
and the Leave operation is equivalent to:
IPMulticastListen ( socket, interface, multicast-address,
INCLUDE, {} )
where {} is an empty source list.
An example of an API providing the capabilities outlined in this
service interface is in [FILTER-API].
3. Multicast Reception State Maintained by Systems
3.1. Socket State
For each socket on which IPMulticastListen has been invoked, the
system records the desired multicast reception state for that socket.
That state conceptually consists of a set of records of the form:
(interface, multicast-address, filter-mode, source-list)
The socket state evolves in response to each invocation of
IPMulticastListen 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.
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 reception state, a system
must also maintain or compute multicast reception state for each of
its interfaces. That state conceptually consists of a set of
records of the form:
(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:
IPMulticastListen ( 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:
IPMulticastListen ( 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
reception 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 reception
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 will be delivered on
socket s1 but not on socket s2. Note that IGMP Queries and Reports
are not subject to source filtering and must always be processed by
hosts and routers.
Filtering of packets based upon a socket"s multicast reception
state is a new feature of this service interface. The previous
service interface [RFC1112] described no filtering based upon
multicast join state; rather, a join on a socket simply caused the
host to join a group on the given interface, and packets destined
for that group could be delivered to all sockets whether they had
joined or not.
The general rules for deriving the per-interface state from the
per-socket state are as follows: For each distinct (interface,
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, multicast-address) pair,
o if *any* such record has a filter mode of EXCLUDE, then the filter
mode of the interface record is EXCLUDE, and the source list of the
interface record is the intersection of the source lists of all
socket records in EXCLUDE mode, minus those source addresses that
appear in any socket record in INCLUDE mode. For example, if the
socket records for multicast address m on interface i are:
from socket s1: ( i, m, EXCLUDE, {a, b, c, d} )
from socket s2: ( i, m, EXCLUDE, {b, c, d, e} )
from socket s3: ( i, m, INCLUDE, {d, e, f} )
then the corresponding interface record on interface i is:
( m, EXCLUDE, {b, c} )
If a fourth socket is added, such as:
from socket s4: ( i, m, EXCLUDE, {} )
then the interface record becomes:
( m, EXCLUDE, {} )
o if *all* such records have a filter mode of INCLUDE, then the
filter mode of the interface record is INCLUDE, and the source list
of the interface record is the union of the source lists of all the
socket records. For example, if the socket records for multicast
address m on interface i are:
from socket s1: ( i, m, INCLUDE, {a, b, c} )
from socket s2: ( i, m, INCLUDE, {b, c, d} )
from socket s3: ( i, m, INCLUDE, {e, f} )
then the corresponding interface record on interface i is:
( m, INCLUDE, {a, b, c, d, e, f} )
An implementation MUST NOT use an EXCLUDE interface record to
represent a group when all sockets for this group 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 IPMulticastListen 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
IGMP messages are encapsulated in IPv4 datagrams, with an IP protocol
number of 2. Every IGMP message described in this document is sent
with an IP Time-to-Live of 1, IP Precedence of Internetwork Control
(e.g., Type of Service 0xc0), and carries an IP Router Alert option
[RFC-2113] in its IP header. IGMP message types are registered by
the IANA [IANA-REG] as described by [RFC-3228].
There are two IGMP message types of concern to the IGMPv3 protocol
described in this document:
Type Number (hex) Message Name
----------------- ------------
0x11 Membership Query
0x22 Version 3 Membership Report
An implementation of IGMPv3 MUST also support the following three
message types, for interoperation with previous versions of IGMP (see
section 7):
0x12 Version 1 Membership Report [RFC-1112]
0x16 Version 2 Membership Report [RFC-2236]
0x17 Version 2 Leave Group [RFC-2236]
Unrecognized message types MUST be silently ignored. Other message
types may be used by newer versions or extensions of IGMP, by
multicast routing protocols, or for other uses.
In this document, unless otherwise qualified, the capitalized words
"Query" and "Report" refer to IGMP Membership Queries and IGMP
Version 3 Membership Reports, respectively.
4.1. Membership Query Message
Membership Queries are sent by IP multicast routers to query the
multicast reception 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 = 0x11 Max Resp Code Checksum
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Group Address
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Resv S QRV QQIC Number of Sources (N)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Source Address [1]
+- -+
Source Address [2]
+- . -+
. . .
. . .
+- -+
Source Address [N]
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.1.1. Max Resp Code
The Max Resp Code field specifies the maximum time allowed before
sending a responding report. The actual time allowed, called the Max
Resp Time, is represented in units of 1/10 second and is derived from
the Max Resp Code as follows:
If Max Resp Code < 128, Max Resp Time = Max Resp Code
If Max Resp Code >= 128, Max Resp Code represents a floating-point
value as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
1 eXP mant
+-+-+-+-+-+-+-+-+
Max Resp Time = (mant 0x10) << (exp + 3)
Small values of Max Resp Time allow IGMPv3 routers to tune the "leave
latency" (the time between the moment the last host leaves a group
and the moment the routing protocol is notified that there are no
more members). Larger values, especially in the exponential range,
allow tuning of the burstiness of IGMP traffic on a network.
4.1.2. Checksum
The Checksum is the 16-bit one"s complement of the one"s complement
sum of the whole IGMP message (the entire IP payload). For computing
the checksum, the Checksum field is set to zero. When receiving
packets, the checksum MUST be verified before processing a packet.
[RFC-1071]
4.1.3. Group Address
The Group Address field is set to zero when sending a General Query,
and set to the IP multicast address being queried when sending a
Group-Specific Query or Group-and-Source-Specific Query (see section
4.1.9, below).
4.1.4. Resv (Reserved)
The Resv field is set to zero on transmission, and ignored on
reception.
4.1.5. S Flag (Suppress Router-Side Processing)
When set to one, the S Flag indicates to any receiving multicast
routers that they are to suppress the normal timer updates they
perform upon hearing a Query. It does not, however, 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 group member.
4.1.6. 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.
4.1.7. 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.
4.1.8. 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 Group-Specific Query, and non-zero in a Group-and-Source-Specific
Query. This number is limited by the MTU of the network over which
the Query is transmitted. For example, on an Ethernet with an MTU of
1500 octets, the IP header including the Router Alert option consumes
24 octets, and the IGMP fields up to including the Number of Sources
(N) field consume 12 octets, leaving 1464 octets for source
addresses, which limits the number of source addresses to 366
(1464/4).
4.1.9. Source Address [i]
The Source Address [i] fields are a vector of n IP unicast addresses,
where n is the value in the Number of Sources (N) field.
4.1.10. Additional Data
If the Packet Length field in the IP header of a received Query
indicates that there are additional octets of data present, beyond
the fields described here, IGMPv3 implementations MUST include those
octets in the computation to verify the received IGMP Checksum, but
MUST otherwise ignore those additional octets. When sending a Query,
an IGMPv3 implementation MUST NOT include additional octets beyond
the fields described here.
4.1.11. Query Variants
There are three variants of the Query message:
1. A "General Query" is sent by a multicast router to learn the
complete multicast reception state of the neighboring interfaces
(that is, the interfaces attached to the network on which the
Query is transmitted). In a General Query, both the Group Address
field and the Number of Sources (N) field are zero.
2. A "Group-Specific Query" is sent by a multicast router to learn
the reception state, with respect to a *single* multicast address,
of the neighboring interfaces. In a Group-Specific Query, the
Group Address field contains the multicast address of interest,
and the Number of Sources (N) field contains zero.
3. A "Group-and-Source-Specific Query" is sent by a multicast router
to learn if any neighboring interface desires reception of packets
sent to a specified multicast address, from any of a specified
list of sources. In a Group-and-Source-Specific Query, the Group
Address field contains the multicast address of interest, and the
Source Address [i] fields contain the source address(es) of
interest.
4.1.12. IP Destination Addresses for Queries
In IGMPv3, General Queries are sent with an IP destination address of
224.0.0.1, the all-systems multicast address. Group-Specific and
Group-and-Source-Specific Queries are sent with an IP destination
address equal to the multicast address of interest. *However*, a
system 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 3 Membership Report Message
Version 3 Membership Reports are sent by IP systems to report (to
neighboring routers) the current multicast reception state, or
changes in the multicast reception 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 = 0x22 Reserved Checksum
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Reserved Number of Group Records (M)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

. .
. Group Record [1] .
. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

. .
. Group Record [2] .
. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.
. . .
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

. .
. Group Record [M] .
. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where each Group 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 .
. .

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
4.2.1. Reserved
The Reserved fields are set to zero on transmission, and ignored on
reception.
4.2.2. Checksum
The Checksum is the 16-bit one"s complement of the one"s complement
sum of the whole IGMP message (the entire IP payload). 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. Number of Group Records (M)
The Number of Group Records (M) field specifies how many Group
Records are present in this Report.
4.2.4. Group Record
Each Group Record is a block of fields containing information
pertaining to the sender"s membership in a single multicast group on
the interface from which the Report is sent.
4.2.5. Record Type
See section 4.2.12, below.
4.2.6. Aux Data Len
The Aux Data Len field contains the length of the Auxiliary Data
field in this Group 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 Group Record.
4.2.8. Multicast Address
The Multicast Address field contains the IP multicast address to
which this Group Record pertains.
4.2.9. Source Address [i]
The Source Address [i] fields are a vector of n IP 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 Group Record. The protocol specified in this
document, IGMPv3, does not define any auxiliary data. Therefore,
implementations of IGMPv3 MUST NOT include any auxiliary data (i.e.,
MUST set the Aux Data Len field to zero) in any transmitted Group
Record, and MUST ignore any auxiliary data present in any received
Group Record. The semantics and internal encoding of the Auxiliary
Data field are to be defined by any future version or extension of
IGMP that uses this field.
4.2.11. Additional Data
If the Packet Length field in the IP header of a received Report
indicates that there are additional octets of data present, beyond
the last Group Record, IGMPv3 implementations MUST include those
octets in the computation to verify the received IGMP Checksum, but
MUST otherwise ignore those additional octets. When sending a
Report, an IGMPv3 implementation MUST NOT include additional octets
beyond the last Group Record.
4.2.12. Group Record Types
There are a number of different types of Group Records that may be
included in a Report message:
o A "Current-State Record" is sent by a system in response to a Query
received on an interface. It reports the current reception 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 Group
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 Group
Record contain the interface"s source list for the
specified multicast address, if it is non-empty.
o A "Filter-Mode-Change Record" is sent by a system whenever a local
invocation of IPMulticastListen 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 Group 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 Group 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 system whenever a local
invocation of IPMulticastListen 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 Group Record contain a list of the
additional sources that the system wishes to
hear from, 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 Group Record contain a list of the
sources that the system no longer wishes to
hear from, 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 Group Records are sent for the same
multicast address, one of type ALLOW_NEW_SOURCES and one of type
BLOCK_OLD_SOURCES.
We use the term "State-Change Record" to refer to either a Filter-
Mode-Change Record or a Source-List-Change Record.
Unrecognized Record Type values MUST be silently ignored.
4.2.13. IP Source Addresses for Reports
An IGMP report is sent with a valid IP source address for the
destination subnet. The 0.0.0.0 source address may be used by a
system that has not yet acquired an IP address. Note that the
0.0.0.0 source address may simultaneously be used by multiple systems
on a LAN. Routers MUST accept a report with a source address of
0.0.0.0.
4.2.14. IP Destination Addresses for Reports
Version 3 Reports are sent with an IP destination address of
224.0.0.22, to which all IGMPv3-capable multicast routers listen. A
system that is operating in version 1 or version 2 compatibility
modes sends version 1 or version 2 Reports to the multicast group
specified in the Group Address field of the Report. In addition, a
system MUST accept and process any version 1 or version 2 Report
whose IP Destination Address field contains *any* of the addresses
(unicast or multicast) assigned to the interface on which the Report
arrives.
4.2.15. Notation for Group Records
In the rest of this document, we use the following notation to
describe the contents of a Group 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:
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. Membership Report Size
If the set of Group Records required in a Report does not fit within
the size limit of a single Report message (as determined by the MTU
of the network on which it will be sent), the Group Records are sent
in as many Report messages as needed to report the entire set.
If a single Group 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 Group 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 Group
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 Group Members
IGMP is an asymmetric protocol, specifying separate behaviors for
group members -- that is, hosts or routers that wish to receive
multicast packets -- and multicast routers. This section describes
the part of IGMPv3 that applies to all group members. (Note that a
multicast router that is also a group member performs both parts of
IGMPv3, receiving and responding to its own IGMP message
transmissions as well as those of its neighbors. The multicast
router part of IGMPv3 is described in section 6.)
A system 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 network.
For interoperability with multicast routers running older versions of
IGMP, systems maintain a MulticastRouterVersion variable for each
interface on which multicast reception is supported. This section
describes the behavior of group member systems on interfaces for
which MulticastRouterVersion = 3. The algorithm for determining
MulticastRouterVersion, and the behavior for versions other than 3,
are described in section 7.
The all-systems multicast address, 224.0.0.1, is handled as a special
case. On all systems -- that is all hosts and routers, including
multicast routers -- reception of packets destined to the all-systems
multicast address, from all sources, is permanently enabled on all
interfaces on which multicast reception is supported. No IGMP
messages are ever sent regarding the all-systems multicast address.
There are two types of events that trigger IGMPv3 protocol actions on
an interface:
o a change of the interface reception state, caused by a local
invocation of IPMulticastListen.
o reception of a Query.
(Received IGMP messages of types other than Query are silently
ignored, except as required for interoperation with earlier versions
of IGMP.)
The following subsections describe the actions to be taken for each
of these two cases. In those descriptions, timer and counter names
appear in square brackets. The default values for those timers and
counters are specified in section 8.
5.1. Action on Change of Interface State
An invocation of IPMulticastListen may cause the multicast reception
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 system to immediately transmit
a State-Change Report from that interface. The type and contents of
the Group 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.
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.
As was done with the first report, the interface state for the
affected group before and after the latest change is compared. 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 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 host. This is done in order to ensure that a series of
successive state changes do not break the protocol robustness.
If the interface reception-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 even if any number of source-list changes occur in that
period. The host has to maintain retransmission state for the group
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 reception 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
When a system receives a Query, it does not respond immediately.
Instead, it delays its response by a random amount of time, bounded
by the Max Resp Time value derived from the Max Resp Code in the
received Query message. A system may receive a variety of Queries on
different interfaces and of different kinds (e.g., General Queries,
Group-Specific Queries, and Group-and-Source-Specific Queries), each
of which may require its own delayed response.
Before scheduling a response to a Query, the system must first
consider previously scheduled pending responses and in many cases
schedule a combined response. Therefore, the system must be able to
maintain the following state:
o A timer per interface for scheduling responses to General Queries.
o A per-group and interface timer for scheduling responses to Group-
Specific and Group-and-Source-Specific Queries.
o A per-group and interface list of sources to be reported in the
response to a Group-and-Source-Specific Query.
When a new Query with the Router-Alert option arrives on an
interface, provided the system has state to report, a delay for a
response is randomly selected in the range (0, [Max Resp Time]) where
Max Resp Time is derived from Max Resp Code in the received Query
message. The following rules are then used to determine if a Report
needs to be scheduled and the type of Report to schedule. The rules
are considered in order and only the first matching rule is applied.
1. If there is a pending response to a previous General Query
scheduled sooner than the selected delay, no additional response
needs to be scheduled.
2. If the received Query is a General Query, the interface timer is
used to schedule a response to the General Query after the
selected delay. Any previously pending response to a General
Query is canceled.
3. If the received Query is a Group-Specific Query or a Group-and-
Source-Specific Query and there is no pending response to a
previous Query for this group, then the group timer is used to
schedule a report. If the received Query is a Group-and-Source-
Specific Query, the list of queried sources is recorded to be used
when generating a response.
4. If there already is a pending response to a previous Query
scheduled for this group, and either the new Query is a Group-
Specific Query or the recorded source-list associated with the
group is empty, then the group source-list is cleared and a single
response is scheduled using the group 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 Group-and-Source-Specific Query and
there is a pending response for this group with a non-empty
source-list, then the group source list is augmented to contain
the list of sources in the new Query and a single response is
scheduled using the group 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 system
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 reception 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 system
is a member of a large number of groups. Instead of using a
single interface timer, implementations are recommended to spread
transmission of such Report messages over the interval (0, [Max
Resp Time]). 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 group timer and the list of recorded
sources for the that group is empty (i.e., it is a pending
response to a Group-Specific Query), then if and only if the
interface has reception state for that group 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.
3. If the expired timer is a group timer and the list of recorded
sources for that group is non-empty (i.e., it is a pending
response to a Group-and-Source-Specific Query), then if and only
if the interface has reception state for that group 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
addresses, then no response is sent.
Finally, after any required Report messages have been generated, the
source lists associated with any reported groups are cleared.
6. Description of the Protocol for Multicast Routers
The purpose of IGMP is to enable each multicast router to learn, for
each of its directly attached networks, which multicast addresses are
of interest to the systems attached to those networks. IGMP version
3 adds the capability for a multicast router to also learn which
*sources* are of interest to neighboring systems, for packets sent to
any particular multicast address. The information gathered by IGMP
is provided to whichever multicast routing protocol is being used by
the router, in order to ensure that multicast packets are delivered
to all networks where there are interested receivers.
This section describes the part of IGMPv3 that is performed by
multicast routers. Multicast routers may also themselves become
members of multicast groups, and therefore also perform the group
member part of IGMPv3, described in section 5.
A multicast router performs the protocol described in this section
over each of its directly-attached networks. If a multicast router
has more than one interface to the same network, it only needs to
operate this protocol over one of those interfaces. On each
interface over which this protocol is being run, the router MUST
enable reception of multicast address 224.0.0.22, from all sources
(and MUST perform the group member part of IGMPv3 for that address on
that interface).
Multicast routers need to know only that *at least one* system on an
attached network is interested in packets to a particular multicast
address from a particular source; a multicast router is not required
to keep track of the interests of each individual neighboring system.
(However, see Appendix A.2 point 1 for discussion.)
IGMPv3 is backward compatible with previous versions of the IGMP
protocol. In order to remain backward compatible with older IGMP
systems, IGMPv3 multicast routers MUST also implement versions 1 and
2 of the protocol (see section 7).
6.1. Conditions for IGMP Queries
Multicast routers send General Queries periodically to request group
membership information from an attached network. These queries are
used to build and refresh the group membership state of systems on
attached networks. Systems respond to these queries by reporting
their group membership state (and their desired set of sources) with
Current-State Group Records in IGMPv3 Membership Reports.
As a member of a multicast group, a system may express interest in
receiving or not receiving traffic from particular sources. As the
desired reception state of a system changes, it reports these changes
using Filter-Mode-Change Records or Source-List-Change Records.
These records indicate an explicit state change in a group at a
system in either the group record"s source list or its filter-mode.
When a group membership is terminated at a system or traffic from a
particular source is no longer desired, a multicast router must query
for other members of the group or listeners of the source before
deleting the group (or source) and pruning its traffic.
To enable all systems on a network to respond to changes in group
membership, multicast routers send specific queries. A Group-
Specific Query is sent to verify there are no systems that desire
reception of the specified group or to "rebuild" the desired
reception state for a particular group. Group-Specific Queries are
sent when a router receives a State-Change record indicating a system
is leaving a group.
A Group-and-Source Specific Query is used to verify there are no
systems on a network which desire to receive traffic from a set of
sources. Group-and-Source Specific Queries list sources for a
particular group which have been requested to no longer be forwarded.
This query is sent by a multicast router to learn if any systems
desire reception of packets to the specified group address from the
specified source addresses. Group-and-Source Specific Queries are
only sent in response to State-Change Records and never in response
to Current-State Records. Section 4.1.11 describes each query in
more detail.
6.2. IGMP State Maintained by Multicast Routers
Multicast routers implementing IGMPv3 keep state per group per
attached network. This group state consists of a filter-mode, a list
of sources, and various timers. For each attached network running
IGMP, a multicast router records the desired reception state for that
network. That state conceptually consists of a set of records of the
form:
(multicast address, group timer, filter-mode, (source records))
Each source record is of the form:
(source address, source timer)
If all sources within a given group are desired, an empty source
record list is kept with filter-mode set to EXCLUDE. This means
hosts on this network want all sources for this group to be
forwarded. This is the IGMPv3 equivalent to a IGMPv1 or IGMPv2 group
join.
6.2.1. Definition of Router Filter-Mode
To reduce internal state, IGMPv3 routers keep a filter-mode per group
per attached network. This filter-mode is used to condense the total
desired reception state of a group to a minimum set such that all
systems" memberships are satisfied. This filter-mode may change in
response to the reception of particular types of group 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 group within a router. Section 6.4 describes the changes
of a router filter-mode per group record received.
Conceptually, when a group record is received, the router filter-mode
for that group is updated to cover all the requested sources using
the least amount of state. As a rule, once a group record with a
filter-mode of EXCLUDE is received, the router filter-mode for that
group will be EXCLUDE.
When a router filter-mode for a group 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. The second type is the set
of sources which hosts have requested to not be forwarded. Appendix
A describes the reasons for keeping this second set when in EXCLUDE
mode.
When a router filter-mode for a group is INCLUDE, the source record
list is the list of sources desired for the group. This is the total
desired set of sources for that group. Each source in the source
record list must be forwarded by some router on the network.
Because a reported group record with a filter-mode of EXCLUDE will
cause a router to transition its filter-mode for that group to
EXCLUDE, a mechanism for transitioning a router"s filter-mode back to
INCLUDE must exist. If all systems with a group record in EXCLUDE
filter-mode cease reporting, it is desirable for the router filter-
mode for that group to transition back to INCLUDE mode. This
transition occurs when the group timer expires and is explained in
detail in section 6.5.
6.2.2. Definition of Group Timers
The group timer is only used when a group is in EXCLUDE mode and it
represents the time for the *filter-mode* of the group to expire and
switch to INCLUDE mode. We define a group timer as a decrementing
timer with a lower bound of zero kept per group per attached network.
Group timers are updated according to the types of group records
received.
A group timer expiring when a router filter-mode for the group is
EXCLUDE means there are no listeners on the attached network in
EXCLUDE mode. At this point, a router will transition to INCLUDE
filter-mode. Section 6.5 describes the actions taken when a group
timer expires while in EXCLUDE mode.
The following table summarizes the role of the group timer. Section
6.4 describes the details of setting the group timer per type of
group record received.
Group
Filter-Mode Group Timer Value Actions/Comments
----------- ----------------- ----------------
INCLUDE Timer >= 0 All members in INCLUDE
mode.
EXCLUDE Timer > 0 At least one member in
EXCLUDE mode.
EXCLUDE Timer == 0 No more listeners to
group. If all source
timers have expired then
delete Group 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 group record received. Source timers
are always updated (for a particular group) whenever the source is
present in a received record for that group. Section 6.4 describes
the setting of source timers per type of group records received.
A source record with a running timer with a router filter-mode for
the group of INCLUDE means that there is currently one or more
systems (in INCLUDE filter-mode) which desire to receive that source.
If a source timer expires with a router filter-mode for the group of
INCLUDE, the router concludes that traffic from this particular
source is no longer desired on the attached network, and deletes the
associated source record.
Source timers are treated differently when a router filter-mode for a
group is EXCLUDE. If a source record has a running timer with a
router filter-mode for the group of EXCLUDE, it means that at least
one system desires the source. It should therefore be forwarded by a
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 group of
EXCLUDE, the router informs the routing protocol that there is no
longer a receiver on the network interested in traffic from this
source.
When a router filter-mode for a group is EXCLUDE, source records are
only deleted when the group timer expires. Section 6.3 describes the
actions that should be taken dependent upon the value of a source
timer.
6.3. IGMPv3 Source-Specific Forwarding Rules
When a multicast router receives a datagram from a source destined to
a particular group, a decision has to be made whether to forward the
datagram onto an attached network or not. The multicast routing
protocol in use is in charge of this decision, and should use the
IGMPv3 information to ensure that all sources/groups desired on a
subnetwork are forwarded to that subnetwork. IGMPv3 information does
not override multicast routing information; for example, if the
IGMPv3 filter-mode group for G is EXCLUDE, a router may still forward
packets for excluded sources to a transit subnet.
To summarize, the following table describes the forwarding
suggestions made by IGMP to the routing protocol for traffic
originating from a source destined to a group. It also summarizes
the actions taken upon the expiration of a source timer based on the
router filter-mode of the group.
Group
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 for the group, delete
group record.
INCLUDE No Source Elements Suggest to not forward 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 Elements Suggest to forward traffic
from source
6.4. Action on Reception of Reports
6.4.1. Reception of Current-State Records
When receiving Current-State Records, a router updates both its group
and source timers. In some circumstances, the reception of a type of
group record will cause the router filter-mode for that group 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 group, where
A = set of source records whose source timers > 0 (Sources that at
least one host has requested to be forwarded)
B = set of source records whose source timers = 0 (Sources that IGMP
will suggest to the routing protocol not to forward)
Note that there will only be two sets when a router"s filter-mode for
a group is EXCLUDE. When a router"s filter-mode for a group 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
GMI is an abbreviation for the Group Membership Interval, which is
the time in which group memberships will time out. The variable LMQT
is an abbreviation for the Last Member Query Time, which is the total
time spent after Last Member Query Count retransmissions. LMQT
represents the "leave latency", or the difference between the
transmission of a membership 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. "Group Timer=J" means
that the Group Timer for the group should be set to value J.
Router State Report Rec"d New Router State Actions
------------ ------------ ---------------- -------
INCLUDE (A) IS_IN (B) INCLUDE (A+B) (B)=GMI
INCLUDE (A) IS_EX (B) EXCLUDE (A*B,B-A) (B-A)=0
Delete (A-B)
Group Timer=GMI
EXCLUDE (X,Y) IS_IN (A) EXCLUDE (X+A,Y-A) (A)=GMI
EXCLUDE (X,Y) IS_EX (A) EXCLUDE (A-Y,Y*A) (A-X-Y)=GMI
Delete (X-A)
Delete (Y-A)
Group Timer=GMI
6.4.2. Reception of Filter-Mode-Change and Source-List-Change Records
When a change in the global state of a group occurs in a system, the
system sends either a Source-List-Change Record or a Filter-Mode-
Change Record for that group. As with Current-State Records, routers
must act upon these records and possibly change their own state to
reflect the new desired membership state of the network.
Routers must query sources that are requested to be no longer
forwarded to a group.