rfc9776v1.txt   rfc9776.txt 
skipping to change at line 94 skipping to change at line 94
4.1.3. Group Address 4.1.3. Group Address
4.1.4. Flags 4.1.4. Flags
4.1.5. S Flag (Suppress Router-Side Processing) 4.1.5. S Flag (Suppress Router-Side Processing)
4.1.6. QRV (Querier's Robustness Variable) 4.1.6. QRV (Querier's Robustness Variable)
4.1.7. QQIC (Querier's Query Interval Code) 4.1.7. QQIC (Querier's Query Interval Code)
4.1.8. Number of Sources (N) 4.1.8. Number of Sources (N)
4.1.9. Source Address [i] 4.1.9. Source Address [i]
4.1.10. Additional Data 4.1.10. Additional Data
4.1.11. Query Variants 4.1.11. Query Variants
4.1.12. IP Destination Addresses for Queries 4.1.12. IP Destination Addresses for Queries
4.2. Version 3 Membership Report Message 4.2. IGMPv3 Membership Report Message
4.2.1. Reserved 4.2.1. Reserved
4.2.2. Checksum 4.2.2. Checksum
4.2.3. Flags 4.2.3. Flags
4.2.4. Number of Group Records (M) 4.2.4. Number of Group Records (M)
4.2.5. Group Record 4.2.5. Group Record
4.2.6. Record Type 4.2.6. Record Type
4.2.7. Aux Data Len 4.2.7. Aux Data Len
4.2.8. Number of Sources (N) 4.2.8. Number of Sources (N)
4.2.9. Multicast Address 4.2.9. Multicast Address
4.2.10. Source Address [i] 4.2.10. Source Address [i]
skipping to change at line 118 skipping to change at line 118
4.2.14. IP Source Addresses for Reports 4.2.14. IP Source Addresses for Reports
4.2.15. IP Destination Addresses for Reports 4.2.15. IP Destination Addresses for Reports
4.2.16. Notation for Group Records 4.2.16. Notation for Group Records
4.2.17. Membership Report Size 4.2.17. Membership Report Size
5. Description of the Protocol for Group Members 5. Description of the Protocol for Group Members
5.1. Action on Change of Interface State 5.1. Action on Change of Interface State
5.2. Action on Reception of a Query 5.2. Action on Reception of a Query
6. Description of the Protocol for Multicast Routers 6. Description of the Protocol for Multicast Routers
6.1. Conditions for IGMP Queries 6.1. Conditions for IGMP Queries
6.2. IGMP State Maintained by Multicast Routers 6.2. IGMP State Maintained by Multicast Routers
6.2.1. Definition of Router Filter-Mode 6.2.1. Definition of Router Filter Mode
6.2.2. Definition of Group Timers 6.2.2. Definition of Group Timers
6.2.3. Definition of Source Timers 6.2.3. Definition of Source Timers
6.3. IGMPv3 Source-Specific Forwarding Rules 6.3. IGMPv3 Source-Specific Forwarding Rules
6.4. Action on Reception of Reports 6.4. Action on Reception of Reports
6.4.1. Reception of Current-State Records 6.4.1. Reception of Current-State Records
6.4.2. Reception of Filter-Mode-Change and Source-List-Change 6.4.2. Reception of Filter-Mode-Change and Source-List-Change
Records Records
6.5. Switching Router Filter-Modes 6.5. Switching Router Filter Modes
6.6. Action on Reception of Queries 6.6. Action on Reception of Queries
6.6.1. Timer Updates 6.6.1. Timer Updates
6.6.2. Querier Election 6.6.2. Querier Election
6.6.3. Building and Sending Specific Queries 6.6.3. Building and Sending Specific Queries
7. Interoperation With Older Versions of IGMP 7. Interoperation With Older Versions of IGMP
7.1. Query Version Distinctions 7.1. Query Version Distinctions
7.2. Group Member Behavior 7.2. Group Member Behavior
7.2.1. In the Presence of Older Version Queriers 7.2.1. In the Presence of Older Version Queriers
7.2.2. In the Presence of Older Version Group Members 7.2.2. In the Presence of Older Version Group Members
7.3. Multicast Router Behavior 7.3. Multicast Router Behavior
skipping to change at line 208 skipping to change at line 208
network. Version 3 adds support for source filtering, that is, the network. Version 3 adds support for source filtering, that is, the
ability for a system to report interest in receiving packets only ability for a system to report interest in receiving packets only
from specific source addresses, as required to support Source- from specific source addresses, as required to support Source-
Specific Multicast (SSM) [RFC3569], or from all but specific source Specific Multicast (SSM) [RFC3569], or from all but specific source
addresses, sent to a particular multicast address. Version 3 is addresses, sent to a particular multicast address. Version 3 is
designed to be interoperable with Versions 1 and 2. designed to be interoperable with Versions 1 and 2.
This document uses "SSM-aware" to refer to systems that support SSM This document uses "SSM-aware" to refer to systems that support SSM
as defined in [RFC4607]. as defined in [RFC4607].
This document updates [RFC2236] as a proper implementation of Version This document updates [RFC2236] as a proper implementation of IGMPv3
3 of IGMP needs to implement Version 2 Report and Leave message needs to implement IGMPv2 Report and Leave message handling.
handling.
This document obsoletes [RFC3376] as it provides clarifications and This document obsoletes [RFC3376] as it provides clarifications and
fixes for errata in [RFC3376]. Detailed updates for those changes fixes for errata in [RFC3376]. Detailed updates for those changes
are described in Appendix C. are described in Appendix C.
1.1. Conventions Used in This Document 1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP "OPTIONAL" in this document are to be interpreted as described in BCP
skipping to change at line 375 skipping to change at line 374
listening on a particular socket depends on the multicast reception listening on a particular socket depends on the multicast reception
state of that socket (and possibly also on other conditions, such as 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 what transport-layer port the socket is bound to). So, in the above
example, if a packet arrives on interface i, destined to multicast example, if a packet arrives on interface i, destined to multicast
address m, with source address a, it will be delivered on socket s1 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 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 subject to source filtering and must always be processed by hosts and
routers. routers.
Filtering of packets based upon a socket's multicast reception state Filtering of packets based upon a socket's multicast reception state
is a new feature of this service interface. The previous service is a feature of this service interface. The previous service
interface [RFC1112] described no filtering based upon multicast join interface [RFC1112] described no filtering based upon multicast join
state; rather, a join on a socket simply caused the host to join a 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 group on the given interface, and packets destined for that group
could be delivered to all sockets whether they had joined or not. could be delivered to all sockets whether they had joined or not.
The general rules for deriving the per-interface state from the per- The general rules for deriving the per-interface state from the per-
socket state are as follows: For each distinct (interface, multicast- socket state are as follows: For each distinct (interface, multicast-
address) pair that appears in any socket state, a per-interface address) pair that appears in any socket state, a per-interface
record is created for that multicast address on that interface. record is created for that multicast address on that interface.
Considering all socket records containing the same (interface, Considering all socket records containing the same (interface,
skipping to change at line 448 skipping to change at line 447
a change of socket state does not necessarily result in a change of a change of socket state does not necessarily result in a change of
interface state. interface state.
4. Message Formats 4. Message Formats
IGMP messages are encapsulated in IPv4 datagrams, with an IP protocol IGMP messages are encapsulated in IPv4 datagrams, with an IP protocol
number of 2. Every IGMP message described in this document is sent 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 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 (e.g., Type of Service 0xc0), and carries an IP Router Alert option
[RFC2113] in its IP header. IGMP message types are registered per [RFC2113] in its IP header. IGMP message types are registered per
[RFC9778]. [BCP57].
There are two IGMP message types of concern to the IGMPv3 protocol There are two IGMP message types of concern to the IGMPv3 protocol
described in this document: described in this document:
+===================+=============================+ +===================+==========================+
| Type Number (hex) | Message Name | | Type Number (hex) | Message Name |
+===================+=============================+ +===================+==========================+
| 0x11 | Membership Query | | 0x11 | Membership Query |
+-------------------+-----------------------------+ +-------------------+--------------------------+
| 0x22 | Version 3 Membership Report | | 0x22 | IGMPv3 Membership Report |
+-------------------+-----------------------------+ +-------------------+--------------------------+
Table 1: New Messages Introduced by IGMPv3 Table 1: New Messages Introduced by IGMPv3
An implementation of IGMPv3 MUST also support the following three An implementation of IGMPv3 MUST also support the following three
message types, for interoperation with previous versions of IGMP (see message types, for interoperation with previous versions of IGMP (see
Section 7): Section 7):
+===================+=============================+===========+ +===================+==========================+===========+
| Type Number (hex) | Message Name | Reference | | Type Number (hex) | Message Name | Reference |
+===================+=============================+===========+ +===================+==========================+===========+
| 0x12 | Version 1 Membership Report | [RFC1112] | | 0x12 | IGMPv1 Membership Report | [RFC1112] |
+-------------------+-----------------------------+-----------+ +-------------------+--------------------------+-----------+
| 0x16 | Version 2 Membership Report | [RFC2236] | | 0x16 | IGMPv2 Membership Report | [RFC2236] |
+-------------------+-----------------------------+-----------+ +-------------------+--------------------------+-----------+
| 0x17 | Version 2 Leave Group | [RFC2236] | | 0x17 | IGMPv2 Leave Group | [RFC2236] |
+-------------------+-----------------------------+-----------+ +-------------------+--------------------------+-----------+
Table 2: Legacy IGMP Messages Table 2: Legacy IGMP Messages
Unrecognized message types MUST be silently ignored. Other message Unrecognized message types MUST be silently ignored. Other message
types may be used by newer versions or extensions of IGMP, by types may be used by newer versions or extensions of IGMP, by
multicast routing protocols, or for other uses. multicast routing protocols, or for other uses.
In this document, unless otherwise qualified, the capitalized words In this document, unless otherwise qualified, the capitalized words
"Query" and "Report" refer to IGMP Membership Queries and IGMP "Query" and "Report" refer to IGMP Membership Queries and IGMPv3
Version 3 Membership Reports, respectively. Membership Reports, respectively.
4.1. Membership Query Message 4.1. Membership Query Message
Membership Queries are sent by IP multicast routers to query the Membership Queries are sent by IP multicast routers to query the
multicast reception state of neighboring interfaces. Queries have multicast reception state of neighboring interfaces. Queries have
the following format: the following format:
0 1 2 3 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 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
skipping to change at line 518 skipping to change at line 517
+- -+ +- -+
| Source Address [N] | | Source Address [N] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IGMPv3 Query Message Figure 1: IGMPv3 Query Message
4.1.1. Max Resp Code 4.1.1. Max Resp Code
The Max Resp Code field specifies the maximum time allowed before The Max Resp Code field specifies the maximum time allowed before
sending a responding report. The actual time allowed, called the sending a responding report. The actual time allowed, called the
"Max Resp Time", is represented in units of 1/10 second and is "Max Response Time", is represented in units of 1/10 second and is
derived from the Max Resp Code as follows: 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 Response Time = Max Resp Code
* If Max Resp Code >= 128, Max Resp Code represents a floating-point * If Max Resp Code >= 128, Max Resp Code represents a floating-point
value as follows: value as follows:
0 1 2 3 4 5 6 7 0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
|1| exp | mant | |1| exp | mant |
+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+
Max Resp Time = (mant | 0x10) << (exp + 3) Max Response Time = (mant | 0x10) << (exp + 3)
Figure 2: Max Resp Code Representation Figure 2: Max Resp Code Representation
Small values of Max Resp Time allow IGMPv3 routers to tune the "leave Small values of Max Response Time allow IGMPv3 routers to tune the
latency" (the time between the moment the last host leaves a group "leave latency" (the time between the moment the last host leaves a
and the moment the routing protocol is notified that there are no group and the moment the routing protocol is notified that there are
more members). Larger values, especially in the exponential range, no more members). Larger values, especially in the exponential
allow tuning of the burstiness of IGMP traffic on a network. range, allow tuning of the burstiness of IGMP traffic on a network.
4.1.2. Checksum 4.1.2. Checksum
The Checksum field is the 16-bit one's complement of the one's The Checksum field is the 16-bit one's complement of the one's
complement sum of the whole IGMP message (the entire IP payload). complement sum of the whole IGMP message (the entire IP payload).
For computing the checksum, the Checksum field is set to zero. When For computing the checksum, the Checksum field is set to zero. When
receiving packets, the checksum MUST be verified before processing a receiving packets, the checksum MUST be verified before processing a
packet [RFC1071]. packet [RFC1071].
4.1.3. Group Address 4.1.3. Group Address
The Group Address field is set to zero when sending a General Query 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 and set to the IP multicast address being queried when sending a
Group-Specific Query or Group-and-Source-Specific Query (see Group-Specific Query or Group-and-Source Specific Query (see
Section 4.1.9, below). Section 4.1.11, below).
4.1.4. Flags 4.1.4. Flags
The Flags field is a bitstring managed by the "IGMP Type Numbers" The Flags field is a bitstring managed by the "IGMP Type Numbers"
registry defined in [RFC9778]. registry defined in [BCP57].
4.1.5. S Flag (Suppress Router-Side Processing) 4.1.5. S Flag (Suppress Router-Side Processing)
When set to one, the S flag indicates to any receiving multicast When set to one, the S flag indicates to any receiving multicast
routers that they are to suppress the normal timer updates they routers that they are to suppress the normal timer updates they
perform upon hearing a Query. It does not, however, suppress the perform upon receiving a Query. It does not, however, suppress the
querier election or the normal "host-side" processing of a Query that 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 router may be required to perform as a consequence of itself being
a group member. a group member.
4.1.6. QRV (Querier's Robustness Variable) 4.1.6. QRV (Querier's Robustness Variable)
If non-zero, the QRV field contains the [Robustness Variable] value 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 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, [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 the QRV is set to zero. Routers adopt the QRV value from the most
recently received Query as their own [Robustness Variable] value, recently received Query as their own [Robustness Variable] value,
skipping to change at line 611 skipping to change at line 610
Multicast routers that are not the current querier adopt the QQI Multicast routers that are not the current querier adopt the QQI
value from the most recently received Query as their own [Query value from the most recently received Query as their own [Query
Interval] value, unless that most recently received QQI was zero, in Interval] value, unless that most recently received QQI was zero, in
which case the receiving routers use the default [Query Interval] which case the receiving routers use the default [Query Interval]
value specified in Section 8.2. value specified in Section 8.2.
4.1.8. Number of Sources (N) 4.1.8. Number of Sources (N)
The Number of Sources (N) field specifies how many source addresses 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 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 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 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 the Query is transmitted. For example, on an Ethernet with an MTU of
1500 octets, the IP header including the Router Alert option consumes 1500 octets, the IP header including the Router Alert option consumes
24 octets, and the IGMP fields up to and including the Number of 24 octets, and the IGMP fields up to and including the Number of
Sources (N) field consume 12 octets, leaving 1464 octets for source Sources (N) field consume 12 octets, leaving 1464 octets for source
addresses, which limits the number of source addresses to 366 addresses, which limits the number of source addresses to 366
(1464/4). (1464/4).
4.1.9. Source Address [i] 4.1.9. Source Address [i]
skipping to change at line 645 skipping to change at line 644
4.1.11. Query Variants 4.1.11. Query Variants
There are three variants of the Query message: There are three variants of the Query message:
1. A General Query is sent by a multicast router to learn the 1. A General Query is sent by a multicast router to learn the
complete multicast reception state of the neighboring interfaces complete multicast reception state of the neighboring interfaces
(that is, the interfaces attached to the network on which the (that is, the interfaces attached to the network on which the
Query is transmitted). In a General Query, both the Group Query is transmitted). In a General Query, both the Group
Address field and the Number of Sources (N) field are zero. 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 2. A Group Specific Query is sent by a multicast router to learn the
reception state, with respect to a single multicast address, of reception state, with respect to a single multicast address, of
the neighboring interfaces. In a Group-Specific Query, the Group the neighboring interfaces. In a Group Specific Query, the Group
Address field contains the multicast address of interest, and the Address field contains the multicast address of interest, and the
Number of Sources (N) field contains zero. Number of Sources (N) field contains zero.
3. A Group-and-Source-Specific Query is sent by a multicast router 3. A Group-and-Source Specific Query is sent by a multicast router
to learn if any neighboring interface desires reception of to learn if any neighboring interface desires reception of
packets sent to a specified multicast address, from any of a packets sent to a specified multicast address, from any of a
specified list of sources. In a Group-and-Source-Specific Query, specified list of sources. In a Group-and-Source Specific Query,
the Group Address field contains the multicast address of the Group Address field contains the multicast address of
interest, and the Source Address [i] fields contain the source interest, and the Source Address [i] fields contain the source
address(es) of interest. address(es) of interest.
4.1.12. IP Destination Addresses for Queries 4.1.12. IP Destination Addresses for Queries
In IGMPv3, General Queries are sent with an IP destination address of In IGMPv3, General Queries are sent with an IP destination address of
224.0.0.1, the all-systems multicast address. Group-Specific and 224.0.0.1, the all-systems multicast address. Group Specific and
Group-and-Source-Specific Queries are sent with an IP destination Group-and-Source Specific Queries are sent with an IP destination
address equal to the multicast address of interest. However, a address equal to the multicast address of interest. However, a
system MUST accept and process any Query whose IP Destination Address system MUST accept and process any Query whose IP Destination Address
field contains any of the addresses (unicast or multicast) assigned field contains any of the addresses (unicast or multicast) assigned
to the interface on which the Query arrives. to the interface on which the Query arrives.
4.2. Version 3 Membership Report Message 4.2. IGMPv3 Membership Report Message
Version 3 Membership Reports are sent by IP systems to report (to IGMPv3 Membership Reports are sent by IP systems to report (to
neighboring routers) the current multicast reception state, or neighboring routers) the current multicast reception state, or
changes in the multicast reception state, of their interfaces. changes in the multicast reception state, of their interfaces.
Reports have the following format: Reports have the following format:
0 1 2 3 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 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 | | Type = 0x22 | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Number of Group Records (M) | | Flags | Number of Group Records (M) |
skipping to change at line 750 skipping to change at line 749
The Checksum field is the 16-bit one's complement of the one's The Checksum field is the 16-bit one's complement of the one's
complement sum of the whole IGMP message (the entire IP payload). complement sum of the whole IGMP message (the entire IP payload).
For computing the checksum, the Checksum field is set to zero. When For computing the checksum, the Checksum field is set to zero. When
receiving packets, the checksum MUST be verified before processing a receiving packets, the checksum MUST be verified before processing a
message. message.
4.2.3. Flags 4.2.3. Flags
The Flags field is a bitstring managed by the "IGMP Type Numbers" The Flags field is a bitstring managed by the "IGMP Type Numbers"
registry defined in [RFC9778]. registry defined in [BCP57].
4.2.4. Number of Group Records (M) 4.2.4. Number of Group Records (M)
The Number of Group Records (M) field specifies how many Group The Number of Group Records (M) field specifies how many Group
Records are present in this Report. Records are present in this Report.
4.2.5. Group Record 4.2.5. Group Record
Each Group Record is a block of fields containing information Each Group Record is a block of fields containing information
pertaining to the sender's membership in a single multicast group on pertaining to the sender's membership in a single multicast group on
skipping to change at line 868 skipping to change at line 867
* A Source-List-Change Record is sent by a system whenever a local * A Source-List-Change Record is sent by a system whenever a local
invocation of IPMulticastListen causes a change of the source list invocation of IPMulticastListen causes a change of the source list
that is not coincident with a change of the filter mode, of the that is not coincident with a change of the filter mode, of the
interface-level state entry for a particular multicast address. interface-level state entry for a particular multicast address.
The Record is included in a Report sent from the interface on The Record is included in a Report sent from the interface on
which the change occurred. The Record Type of a Source-List- which the change occurred. The Record Type of a Source-List-
Change Record may be one of the following two values: Change Record may be one of the following two values:
5. ALLOW_NEW_SOURCES - indicates that the Source Address [i] 5. ALLOW_NEW_SOURCES - indicates that the Source Address [i]
fields in this Group Record contain a list of the additional fields in this Group Record contain a list of the additional
sources that the system wishes to hear from, for packets sent sources that the system wishes to receive, for packets sent to
to the specified multicast address. If the change was to an the specified multicast address. If the change was to an
INCLUDE source list, these are the addresses that were added INCLUDE source list, these are the addresses that were added
to the list; if the change was to an EXCLUDE source list, to the list; if the change was to an EXCLUDE source list,
these are the addresses that were deleted from the list. these are the addresses that were deleted from the list.
6. BLOCK_OLD_SOURCES - indicates that the Source Address [i] 6. BLOCK_OLD_SOURCES - indicates that the Source Address [i]
fields in this Group Record contain a list of the sources that 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 system no longer wishes to receive, for packets sent to
the specified multicast address. If the change was to an the specified multicast address. If the change was to an
INCLUDE source list, these are the addresses that were deleted INCLUDE source list, these are the addresses that were deleted
from the list; if the change was to an EXCLUDE source list, from the list; if the change was to an EXCLUDE source list,
these are the addresses that were added to the list. these are the addresses that were added to the list.
If a change of source list results in both allowing new sources and 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 blocking old sources, then two Group Records are sent for the same
multicast address, one of type ALLOW_NEW_SOURCES and one of type multicast address, one of type ALLOW_NEW_SOURCES and one of type
BLOCK_OLD_SOURCES. BLOCK_OLD_SOURCES.
skipping to change at line 905 skipping to change at line 904
the destination subnet. The 0.0.0.0 source address may be used by a 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 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 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 on a LAN. Routers MUST accept a report with a source address of
0.0.0.0. 0.0.0.0.
4.2.15. IP Destination Addresses for Reports 4.2.15. IP Destination Addresses for Reports
Version 3 Reports are sent with an IP destination address of Version 3 Reports are sent with an IP destination address of
224.0.0.22, to which all IGMPv3-capable multicast routers listen. A 224.0.0.22, to which all IGMPv3-capable multicast routers listen. A
system that is operating in version 1 or version 2 compatibility system that is operating in v1 or v2 compatibility modes sends v1 or
modes sends version 1 or version 2 Reports to the multicast group v2 Reports to the multicast group specified in the Group Address
specified in the Group Address field of the Report. In addition, a field of the Report. In addition, a system MUST accept and process
system MUST accept and process any version 1 or version 2 Report any v1 or v2 Report whose IP Destination Address field contains any
whose IP Destination Address field contains any of the addresses of the addresses (unicast or multicast) assigned to the interface on
(unicast or multicast) assigned to the interface on which the Report which the Report arrives.
arrives.
4.2.16. Notation for Group Records 4.2.16. Notation for Group Records
In the rest of this document, we use the following notation to In the rest of this document, we use the following notation to
describe the contents of a Group Record pertaining to a particular describe the contents of a Group Record pertaining to a particular
multicast address: multicast address:
IS_IN ( x ) - Type MODE_IS_INCLUDE, source addresses x * IS_IN ( x ) - Type MODE_IS_INCLUDE, source addresses x
IS_EX ( x ) - Type MODE_IS_EXCLUDE, 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_IN ( x ) - Type CHANGE_TO_INCLUDE_MODE, source addresses x
TO_EX ( x ) - Type CHANGE_TO_EXCLUDE_MODE, source addresses x * TO_EX ( x ) - Type CHANGE_TO_EXCLUDE_MODE, source addresses x
ALLOW ( x ) - Type ALLOW_NEW_SOURCES, source addresses x * ALLOW ( x ) - Type ALLOW_NEW_SOURCES, source addresses x
BLOCK ( x ) - Type BLOCK_OLD_SOURCES, source addresses x * BLOCK ( x ) - Type BLOCK_OLD_SOURCES, source addresses x
where x is either: where x is either:
* a capital letter (e.g., "A") to represent the set of source * a capital letter (e.g., "A") to represent the set of source
addresses or addresses or
* a set expression (e.g., "A+B"), where "A+B" means the union of * 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 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. "A-B" means the removal of all elements of set B from set A.
4.2.17. Membership Report Size 4.2.17. Membership Report Size
If the set of Group Records required in a Report does not fit within 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 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 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. in as many Report messages as needed to report the entire set.
If a single Group Record contains so many source addresses that it If a single Group Record contains so many source addresses that it
does not fit within the size limit of a single Report message, and if does not fit within the size limit of a single Report message, and if
its Type is not MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, it is its Type is not MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, it is
split into multiple Group Records, each containing a different subset split into multiple Group Records, each containing a different subset
of the source addresses and each sent in a separate Report message. 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 If its Type is MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, a single
skipping to change at line 1031 skipping to change at line 1029
+=============+=============+==========================+ +=============+=============+==========================+
| INCLUDE (A) | INCLUDE (B) | ALLOW (B-A), BLOCK (A-B) | | INCLUDE (A) | INCLUDE (B) | ALLOW (B-A), BLOCK (A-B) |
+-------------+-------------+--------------------------+ +-------------+-------------+--------------------------+
| EXCLUDE (A) | EXCLUDE (B) | ALLOW (A-B), BLOCK (B-A) | | EXCLUDE (A) | EXCLUDE (B) | ALLOW (A-B), BLOCK (B-A) |
+-------------+-------------+--------------------------+ +-------------+-------------+--------------------------+
| INCLUDE (A) | EXCLUDE (B) | TO_EX (B) | | INCLUDE (A) | EXCLUDE (B) | TO_EX (B) |
+-------------+-------------+--------------------------+ +-------------+-------------+--------------------------+
| EXCLUDE (A) | INCLUDE (B) | TO_IN (B) | | EXCLUDE (A) | INCLUDE (B) | TO_IN (B) |
+-------------+-------------+--------------------------+ +-------------+-------------+--------------------------+
Table 3 Table 3: Transmitted Group Records for State Changes
If the computed source list for either an ALLOW or a BLOCK State- If the computed source list for either an ALLOW or a BLOCK State-
Change Record is empty, that record is omitted from the Report Change Record is empty, that record is omitted from the Report
message. message.
To cover the possibility of the State-Change Report being missed by To cover the possibility of the State-Change Report being missed by
one or more multicast routers, it is retransmitted [Robustness one or more multicast routers, it is retransmitted [Robustness
Variable] - 1 more times, at intervals chosen at random from the Variable] - 1 more times, at intervals chosen at random from the
range (0, [Unsolicited Report Interval]). range (0, [Unsolicited Report Interval]).
If more changes to the same interface state entry occur before all If more changes to the same interface state entry occur before all
the retransmissions of the State-Change Report for the first change the retransmissions of the State-Change Report for the first change
have been completed, each such additional change triggers the have been completed, each such additional change triggers the
immediate transmission of a new State-Change Report. immediate transmission of a State-Change Report.
The contents of the new transmitted report are calculated as follows. The contents of the new transmitted report are calculated as follows.
As was done with the first report, the interface state for the As was done with the first report, the interface state for the
affected group before and after the latest change is compared. The affected group before and after the latest change is compared. The
report records expressing the difference are built according to report records expressing the difference are built according to
Table 3. However, these records are not transmitted in a message but Table 3. However, these records are not transmitted in a message but
instead are merged with the contents of the pending report to create instead are merged with the contents of the pending report to create
the new State-Change report. The rules for merging the difference the new State-Change report. The rules for merging the difference
report resulting from the state change and the pending report are report resulting from the state change and the pending report are
described below. described below.
The transmission of the merged State-Change Report terminates The transmission of the merged State-Change Report terminates
retransmissions of the earlier State-Change Reports for the same retransmissions of the earlier State-Change Reports for the same
multicast address, and becomes the first of [Robustness Variable] multicast address, and becomes the first of [Robustness Variable]
transmissions of State-Change Reports. transmissions of State-Change Reports.
Each time a source is included in the difference report calculated Each time a source is included in the difference report calculated
above, retransmission state for that source needs to be maintained above, retransmission state for that source needs to be maintained
until [Robustness Variable] State-Change reports have been sent by until [Robustness Variable] State-Change Reports have been sent by
the host. This is done in order to ensure that a series of the host. This is done in order to ensure that a series of
successive state changes do not break the protocol robustness. successive state changes do not break the protocol robustness.
If the interface reception-state change that triggers the new report If the interface reception-state change that triggers the new report
is a filter-mode change, then the next [Robustness Variable] State- is a filter-mode change, then the next [Robustness Variable] State-
Change Reports will include a Filter-Mode-Change Record. This Change Reports will include a Filter-Mode-Change Record. This
applies even if any number of source-list changes occur in that applies even if any number of source-list changes occur in that
period. The host has to maintain retransmission state for the group period. The host has to maintain retransmission state for the group
until the [Robustness Variable] State-Change reports have been sent. until the [Robustness Variable] State-Change Reports have been sent.
When [Robustness Variable] State-Change reports with Filter-Mode- When [Robustness Variable] State-Change Reports with Filter-Mode-
Change Records have been transmitted after the last filter-mode Change Records have been transmitted after the last filter-mode
change, and if source-list changes to the interface reception have change, and if source-list changes to the interface reception have
scheduled additional reports, then the next State-Change report will scheduled additional reports, then the next State-Change Report will
include Source-List-Change Records. include Source-List-Change Records.
Each time a State-Change Report is transmitted, the contents are Each time a State-Change Report is transmitted, the contents are
determined as follows. If the report should contain a Filter-Mode- determined as follows. If the report should contain a Filter-Mode-
Change Record, and if the current filter-mode of the interface is Change Record, and if the current filter-mode of the interface is
INCLUDE, a TO_IN record is included in the report; otherwise, a TO_EX INCLUDE, a TO_IN record is included in the report; otherwise, a TO_EX
record is included. If instead the report should contain Source- record is included. If instead the report should contain a Source-
List-Change Records, an ALLOW and a BLOCK record are included. The List-Change Record, an ALLOW and a BLOCK record are included. The
contents of these records are built according to Table 4. contents of these records are built according to Table 4.
+========+==============================+ +========+==============================+
| Record | Sources Included | | Record | Sources Included |
+========+==============================+ +========+==============================+
| TO_IN | All in the current interface | | TO_IN | All in the current interface |
| | state that must be forwarded | | | state that must be forwarded |
+--------+------------------------------+ +--------+------------------------------+
| TO_EX | All in the current interface | | TO_EX | All in the current interface |
| | state that must be blocked | | | state that must be blocked |
+--------+------------------------------+ +--------+------------------------------+
| ALLOW | All with retransmission | | ALLOW | All with retransmission |
| | state that must be forwarded | | | state that must be forwarded |
+--------+------------------------------+ +--------+------------------------------+
| BLOCK | All with retransmission | | BLOCK | All with retransmission |
| | state that must be blocked | | | state that must be blocked |
+--------+------------------------------+ +--------+------------------------------+
Table 4 Table 4: Change Record Construction
If the computed source list for either an ALLOW or a BLOCK record is If the computed source list for either an ALLOW or a BLOCK record is
empty, that record is omitted from the State-Change report. empty, that record is omitted from the State-Change Report.
Note: When the first State-Change report is sent, the non-existent | Note: When the first State-Change Report is sent, the non-
pending report to merge with can be treated as a source-change report | existent pending report to merge with can be treated as a
with empty ALLOW and BLOCK records (no sources have retransmission | Source-Change Report with empty ALLOW and BLOCK records (no
state). | sources have retransmission state).
5.2. Action on Reception of a Query 5.2. Action on Reception of a Query
When a system receives a Query, it does not respond immediately. When a system receives a Query, it does not respond immediately.
Instead, it delays its response by a random amount of time, bounded 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 by the Max Response Time value derived from the Max Resp Code in the
received Query message. A system may receive a variety of Queries on received Query message. A system may receive a variety of Queries on
different interfaces and of different kinds (e.g., General Queries, different interfaces and of different kinds (e.g., General Queries,
Group-Specific Queries, and Group-and-Source-Specific Queries), each Group Specific Queries, and Group-and-Source Specific Queries), each
of which may require its own delayed response. of which may require its own delayed response.
Before scheduling a response to a Query, the system must first Before scheduling a response to a Query, the system must first
consider previously scheduled pending responses as, in many cases, it consider previously scheduled pending responses as, in many cases, it
can schedule a combined response. Therefore, the system must be able can schedule a combined response. Therefore, the system must be able
to maintain the following state: to maintain the following state:
* A timer per interface for scheduling responses to General Queries. * A timer per interface for scheduling responses to General Queries.
* A per-group and interface timer for scheduling responses to Group- * A per-group and Interface Timer for scheduling responses to Group
Specific and Group-and-Source-Specific Queries. Specific and Group-and-Source Specific Queries.
* A per-group and interface list of sources to be reported in the * A per-group and interface list of sources to be reported in the
response to a Group-and-Source-Specific Query. response to a Group-and-Source Specific Query.
When a new Query with the Router Alert option arrives on an When a Query with the Router Alert option arrives on an interface,
interface, provided the system has state to report, a delay for a provided the system has state to report, a delay for a response is
response is randomly selected in the range (0, [Max Resp Time]) where randomly selected in the range (0, [Max Response Time]) where Max
Max Resp Time is derived from Max Resp Code in the received Query Response Time is derived from Max Resp Code in the received Query
message. The following rules are then used to determine if a Report 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 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. are considered in order and only the first matching rule is applied.
1. If there is a pending response to a previous General Query 1. If there is a pending response to a previous General Query
scheduled sooner than the selected delay, no additional response scheduled sooner than the selected delay, no additional response
needs to be scheduled. needs to be scheduled.
2. If the received Query is a General Query, the interface timer is 2. If the received Query is a General Query, the Interface Timer is
used to schedule a response to the General Query after the used to schedule a response to the General Query after the
selected delay. Any previously pending response to a General selected delay. Any previously pending response to a General
Query is canceled. Query is canceled.
3. If the received Query is a Group-Specific Query or a Group-and- 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 Source Specific Query and there is no pending response to a
previous Query for this group, then the group timer is used to previous Query for this group, then the Group Timer is used to
schedule a report. If the received Query is a Group-and-Source- schedule a report. If the received Query is a Group-and-Source
Specific Query, the list of queried sources is recorded to be Specific Query, the list of queried sources is recorded to be
used when generating a response. used when generating a response.
4. If there already is a pending response to a previous Query 4. If there already is a pending response to a previous Query
scheduled for this group, and either the new Query is a Group- scheduled for this group, and either the new Query is a Group
Specific Query or the recorded source-list associated with the Specific Query or the recorded source-list associated with the
group is empty, then the group source-list is cleared and a group is empty, then the group source-list is cleared and a
single response is scheduled using the group timer. The new single response is scheduled using the Group Timer. The new
response is scheduled to be sent at the earliest of the remaining response is scheduled to be sent at the earliest of the remaining
time for the pending report and the selected delay. time for the pending report and the selected delay.
5. If the received Query is a Group-and-Source-Specific Query and 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 there is a pending response for this group with a non-empty
source-list, then the group source list is augmented to contain source-list, then the group source list is augmented to contain
the list of sources in the new Query and a single response is the list of sources in the new Query and a single response is
scheduled using the group timer. The new response is scheduled scheduled using the Group Timer. The new response is scheduled
to be sent at the earliest of the remaining time for the pending to be sent at the earliest of the remaining time for the pending
report and the selected delay. report and the selected delay.
When the timer in a pending response record expires, the system When the timer in a pending response record expires, the system
transmits, on the associated interface, one or more Report messages transmits, on the associated interface, one or more Report messages
carrying one or more Current-State Records (see Section 4.2.13), as carrying one or more Current-State Records (see Section 4.2.13), as
follows: follows:
1. If the expired timer is the interface timer (i.e., it is a 1. If the expired timer is the Interface Timer (i.e., it is a
pending response to a General Query), then one Current-State pending response to a General Query), then one Current-State
Record is sent for each multicast address for which the specified Record is sent for each multicast address for which the specified
interface has reception state, as described in Section 3.2. The interface has reception state, as described in Section 3.2. The
Current-State Record carries the multicast address and its Current-State Record carries the multicast address and its
associated filter mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and associated filter mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and
source list. Multiple Current-State Records are packed into source list. Multiple Current-State Records are packed into
individual Report messages, to the extent possible. individual Report messages, to the extent possible.
This naive algorithm may result in bursts of packets when a This naive algorithm may result in bursts of packets when a
system is a member of a large number of groups. Instead of using system is a member of a large number of groups. Instead of using
a single interface timer, implementations are recommended to a single Interface Timer, implementations are recommended to
spread transmission of such Report messages over the interval (0, spread transmission of such Report messages over the interval (0,
[Max Resp Time]). Note that any such implementation MUST avoid [Max Response Time]). Note that any such implementation MUST
the "ack-implosion" problem, i.e., MUST NOT send a Report avoid the "ack-implosion" problem, i.e., MUST NOT send a Report
immediately on reception of a General Query. immediately on reception of a General Query.
2. If the expired timer is a group timer and the list of recorded 2. If the expired timer is a Group Timer and the list of recorded
sources for that group is empty (i.e., it is a pending response sources for that group is empty (i.e., it is a pending response
to a Group-Specific Query), then if and only if the interface has to a Group Specific Query), then if and only if the interface has
reception state for that group address, a single Current-State reception state for that group address, a single Current-State
Record is sent for that address. The Current-State Record Record is sent for that address. The Current-State Record
carries the multicast address and its associated filter mode carries the multicast address and its associated filter mode
(MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list. (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list.
3. If the expired timer is a group timer and the list of recorded 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 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 response to a Group-and-Source Specific Query), then if and only
if the interface has reception state for that group address, the if the interface has reception state for that group address, the
contents of the responding Current-State Record is determined contents of the responding Current-State Record is determined
from the interface state and the pending response record, as from the interface state and the pending response record, as
specified in Table 5. specified in Table 5.
+=====================+=========================+===============+ +=====================+=========================+===============+
| Per-Interface State | Set of Sources in the | Current-State | | Per-Interface State | Set of Sources in the | Current-State |
| | Pending Response Record | Record | | | Pending Response Record | Record |
+=====================+=========================+===============+ +=====================+=========================+===============+
| INCLUDE (A) | B | IS_IN (A*B) | | INCLUDE (A) | B | IS_IN (A*B) |
+---------------------+-------------------------+---------------+ +---------------------+-------------------------+---------------+
| EXCLUDE (A) | B | IS_IN (B-A) | | EXCLUDE (A) | B | IS_IN (B-A) |
+---------------------+-------------------------+---------------+ +---------------------+-------------------------+---------------+
Table 5 Table 5: Current-State Record Construction
If the resulting Current-State Record has an empty set of source If the resulting Current-State Record has an empty set of source
addresses, then no response is sent. addresses, then no response is sent.
Finally, after any required Report messages have been generated, the Finally, after any required Report messages have been generated, the
source lists associated with any reported groups are cleared. source lists associated with any reported groups are cleared.
6. Description of the Protocol for Multicast Routers 6. Description of the Protocol for Multicast Routers
The purpose of IGMP is to enable each multicast router to learn, for The purpose of IGMP is to enable each multicast router to learn, for
each of its directly attached networks, which multicast addresses are each of its directly attached networks, which multicast addresses are
of interest to the systems attached to those networks. IGMP version of interest to the systems attached to those networks. IGMPv3 adds
3 adds the capability for a multicast router to also learn which the capability for a multicast router to also learn which sources are
sources are of interest to neighboring systems, for packets sent to of interest to neighboring systems, for packets sent to any
any particular multicast address. The information gathered by IGMP particular multicast address. The information gathered by IGMP is
is provided to whichever multicast routing protocol is being used by provided to whichever multicast routing protocol is being used by the
the router, in order to ensure that multicast packets are delivered router, in order to ensure that multicast packets are delivered to
to all networks where there are interested receivers. all networks where there are interested receivers.
This section describes the part of IGMPv3 that is performed by This section describes the part of IGMPv3 that is performed by
multicast routers. Multicast routers may also themselves become multicast routers. Multicast routers may also themselves become
members of multicast groups, and therefore also perform the group members of multicast groups, and therefore also perform the group
member part of IGMPv3, as described in Section 5. member part of IGMPv3, as described in Section 5.
A multicast router performs the protocol described in this section A multicast router performs the protocol described in this section
over each of its directly attached networks. If a multicast router over each of its directly attached networks. If a multicast router
has more than one interface to the same network, it only needs to has more than one interface to the same network, it only needs to
operate this protocol over one of those interfaces. On each operate this protocol over one of those interfaces. On each
skipping to change at line 1272 skipping to change at line 1270
(However, see Appendix A.2, item 1 for discussion.) (However, see Appendix A.2, item 1 for discussion.)
IGMPv3 is backward compatible with previous versions of the IGMP IGMPv3 is backward compatible with previous versions of the IGMP
protocol. In order to remain backward compatible with older IGMP protocol. In order to remain backward compatible with older IGMP
systems, IGMPv3 multicast routers MUST also implement versions 1 and systems, IGMPv3 multicast routers MUST also implement versions 1 and
2 of the protocol (see Section 7). 2 of the protocol (see Section 7).
6.1. Conditions for IGMP Queries 6.1. Conditions for IGMP Queries
Multicast routers send General Queries periodically to request group Multicast routers send General Queries periodically to request group
membership information from an attached network. These queries are membership information from an attached network. These Queries are
used to build and refresh the group membership state of systems on used to build and refresh the group membership state of systems on
attached networks. Systems respond to these queries by reporting attached networks. Systems respond to these Queries by reporting
their group membership state (and their desired set of sources) with their group membership state (and their desired set of sources) with
Current-State Group Records in IGMPv3 Membership Reports. Current-State Records in IGMPv3 Membership Reports.
As a member of a multicast group, a system may express interest in As a member of a multicast group, a system may express interest in
receiving or not receiving traffic from particular sources. As the receiving or not receiving traffic from particular sources. As the
desired reception state of a system changes, it reports these changes desired reception state of a system changes, it reports these changes
using Filter-Mode-Change Records or Source-List-Change Records. using Filter-Mode-Change Records or Source-List-Change Records.
These records indicate an explicit state change in a group at a 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. 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 When a group membership is terminated at a system or traffic from a
particular source is no longer desired, a multicast router must query particular source is no longer desired, a multicast router must query
for other members of the group or listeners of the source before for other members of the group or listeners of the source before
deleting the group (or source) and pruning its traffic. deleting the group (or source) and pruning its traffic.
To enable all systems on a network to respond to changes in group To enable all systems on a network to respond to changes in group
membership, multicast routers send specific queries. A Group- membership, multicast routers send specific queries. A Group
Specific Query is sent to verify there are no systems that desire Specific Query is sent to verify there are no systems that desire
reception of the specified group or to "rebuild" the desired reception of the specified group or to "rebuild" the desired
reception state for a particular group. Group-Specific Queries are reception state for a particular group. Group Specific Queries are
sent when a router receives a State-Change record indicating a system sent when a router receives a State-Change Record indicating a system
is leaving a group. is leaving a group.
A Group-and-Source Specific Query is used to verify there are no A Group-and-Source Specific Query is used to verify there are no
systems on a network that desire receiving traffic from a set of systems on a network that desire receiving traffic from a set of
sources. Group-and-Source Specific Queries list sources for a sources. Group-and-Source Specific Queries list sources for a
particular group that have been requested to no longer be forwarded. particular group that have been requested to no longer be forwarded.
This query is sent by a multicast router to learn if any systems This query is sent by a multicast router to learn if any systems
desire reception of packets to the specified group address from the desire reception of packets to the specified group address from the
specified source addresses. Group-and-Source Specific Queries are specified source addresses. Group-and-Source Specific Queries are
only sent in response to State-Change Records and never in response only sent in response to State-Change Records and never in response
skipping to change at line 1317 skipping to change at line 1315
6.2. IGMP State Maintained by Multicast Routers 6.2. IGMP State Maintained by Multicast Routers
Multicast routers implementing IGMPv3 keep state per group per Multicast routers implementing IGMPv3 keep state per group per
attached network. This group state consists of a filter-mode, a list attached network. This group state consists of a filter-mode, a list
of sources, and various timers. For each attached network running of sources, and various timers. For each attached network running
IGMP, a multicast router records the desired reception state for that IGMP, a multicast router records the desired reception state for that
network. That state conceptually consists of a set of records of the network. That state conceptually consists of a set of records of the
form: form:
(multicast address, group timer, filter-mode, (source records)) (multicast address, Group Timer, Router Filter Mode, (source records))
Each source record is of the form: Each source record is of the form:
(source address, source timer) (source address, Source Timer)
If all sources within a given group are desired, an empty source If all sources within a given group are desired, an empty source
record list is kept with filter-mode set to EXCLUDE. This means record list is kept with filter-mode set to EXCLUDE. This means
hosts on this network want all sources for this group to be hosts on this network want all sources for this group to be
forwarded. This is the IGMPv3 equivalent to an IGMPv1 or IGMPv2 forwarded. This is the IGMPv3 equivalent to an IGMPv1 or IGMPv2
group join. group join.
6.2.1. Definition of Router Filter-Mode 6.2.1. Definition of Router Filter Mode
To reduce internal state, IGMPv3 routers keep a filter-mode per group To reduce internal state, IGMPv3 routers keep a filter mode per group
per attached network. This filter-mode is used to condense the total 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 desired reception state of a group to a minimum set such that all
systems' memberships are satisfied. This filter-mode may change in systems' memberships are satisfied. This filter mode may change in
response to the reception of particular types of group records or response to the reception of particular types of Group Records or
when certain timer conditions occur. In the following sections, we when certain timer conditions occur. In the following sections, we
use the term "router filter-mode" to refer to the filter-mode of a 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 particular group within a router. Section 6.4 describes the changes
of a router filter-mode per group record received. of a Router Filter Mode per Group Record received.
Conceptually, when a group record is received, the router filter-mode Conceptually, when a Group Record is received, the Router Filter Mode
for that group is updated to cover all the requested sources using 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 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 filter-mode of EXCLUDE is received, the Router Filter Mode for that
group will be EXCLUDE. group will be EXCLUDE.
When a router filter-mode for a group is EXCLUDE, the source record 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 that list contains two types of sources. The first type is the set that
represents conflicts in the desired reception state; this set must be represents conflicts in the desired reception state; this set must be
forwarded by some router on the network. The second type is the set forwarded by some router on the network. The second type is the set
of sources that hosts have requested to not be forwarded. Appendix A of sources that hosts have requested to not be forwarded. Appendix A
describes the reasons for keeping two different sets when in EXCLUDE describes the reasons for keeping two different sets when in EXCLUDE
mode. mode.
When a router filter-mode for a group is INCLUDE, the source record 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 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 desired set of sources for that group. Each source in the source
record list must be forwarded by some router on the network. record list must be forwarded by some router on the network.
Because a reported group record with a filter-mode of EXCLUDE will Because a reported Group Record with a filter-mode of EXCLUDE will
cause a router to transition its filter-mode for that group to cause a router to transition its filter-mode for that group to
EXCLUDE, a mechanism for transitioning a router's filter-mode back 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 INCLUDE must exist. If all systems with a Group Record in EXCLUDE
filter-mode cease reporting, it is desirable for the router filter- filter-mode cease reporting, it is desirable for the Router Filter
mode for that group to transition back to INCLUDE mode. This Mode for that group to transition back to INCLUDE mode. This
transition occurs when the group timer expires and is explained in transition occurs when the Group Timer expires and is explained in
detail in Section 6.5. detail in Section 6.5.
6.2.2. Definition of Group Timers 6.2.2. Definition of Group Timers
The group timer is only used when a group is in EXCLUDE mode and it 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 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 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. timer with a lower bound of zero kept per group per attached network.
Group timers are updated according to the types of group records Group timers are updated according to the types of Group Records
received. received.
A group timer expiring when a router filter-mode for the group is 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 means there are no listeners on the attached network in
EXCLUDE mode. At this point, a router will transition to INCLUDE EXCLUDE mode. At this point, a router will transition to INCLUDE
filter-mode. Section 6.5 describes the actions taken when a group filter-mode. Section 6.5 describes the actions taken when a Group
timer expires while in EXCLUDE mode. Timer expires while in EXCLUDE mode.
Table 6 summarizes the role of the group timer. Section 6.4 Table 6 summarizes the role of the Group Timer. Section 6.4
describes the details of setting the group timer per type of group describes the details of setting the Group Timer per type of Group
record received. Record received.
+=============+=======+=========================================+ +=============+=======+=========================================+
| Group | Group | Actions/Comments | | Group | Group | Actions/Comments |
| Filter-Mode | Timer | | | Filter-Mode | Timer | |
| | Value | | | | Value | |
+=============+=======+=========================================+ +=============+=======+=========================================+
| INCLUDE | Timer | All members in INCLUDE mode. | | INCLUDE | Timer | All members in INCLUDE mode. |
| | >= 0 | | | | >= 0 | |
+-------------+-------+-----------------------------------------+ +-------------+-------+-----------------------------------------+
| EXCLUDE | Timer | At least one member in EXCLUDE mode. | | EXCLUDE | Timer | At least one member in EXCLUDE mode. |
skipping to change at line 1408 skipping to change at line 1406
+-------------+-------+-----------------------------------------+ +-------------+-------+-----------------------------------------+
| EXCLUDE | Timer | No more listeners to group. If all | | EXCLUDE | Timer | No more listeners to group. If all |
| | == 0 | source timers have expired, then delete | | | == 0 | source timers have expired, then delete |
| | | Group Record. If there are still | | | | Group Record. If there are still |
| | | source record timers running, switch to | | | | source record timers running, switch to |
| | | INCLUDE filter-mode using those source | | | | INCLUDE filter-mode using those source |
| | | records with running timers as the | | | | records with running timers as the |
| | | INCLUDE source record state. | | | | INCLUDE source record state. |
+-------------+-------+-----------------------------------------+ +-------------+-------+-----------------------------------------+
Table 6 Table 6: Group Timer Actions
6.2.3. Definition of Source Timers 6.2.3. Definition of Source Timers
A source timer is kept per source record and is a decrementing timer 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 with a lower bound of zero. Source timers are updated according to
the type and filter-mode of the group record received. Source timers the type and filter-mode of the Group Record received. Source timers
are always updated (for a particular group) whenever the source is are always updated (for a particular group) whenever the source is
present in a received record for that group. Section 6.4 describes present in a received record for that group. Section 6.4 describes
the setting of source timers per type of group records received. the setting of source timers per type of Group Records received.
A source record with a running timer with a router filter-mode for 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 the group of INCLUDE means that there is currently one or more
systems (in INCLUDE filter-mode) that desire to receive that source. systems (in INCLUDE filter-mode) that desire to receive that source.
If a source timer expires with a router filter-mode for the group of If a Source Timer expires with a Router Filter Mode for the group of
INCLUDE, the router concludes that traffic from this particular INCLUDE, the router concludes that traffic from this particular
source is no longer desired on the attached network and deletes the source is no longer desired on the attached network and deletes the
associated source record. associated source record.
Source timers are treated differently when a router filter-mode for a 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 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 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 one system desires the source. It should therefore be forwarded by a
router on the network. Appendix A describes the reasons for keeping router on the network. Appendix A describes the reasons for keeping
state for sources that have been requested to be forwarded while in state for sources that have been requested to be forwarded while in
EXCLUDE state. EXCLUDE state.
If a source timer expires with a router filter-mode for the group of 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 EXCLUDE, the router informs the routing protocol that there is no
longer a receiver on the network interested in traffic from this longer a receiver on the network interested in traffic from this
source. source.
When a router filter-mode for a group is EXCLUDE, source records are 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 only deleted when the Group Timer expires. Section 6.3 describes the
actions that should be taken dependent upon the value of a source actions that should be taken dependent upon the value of a Source
timer. Timer.
6.3. IGMPv3 Source-Specific Forwarding Rules 6.3. IGMPv3 Source-Specific Forwarding Rules
When a multicast router receives a datagram from a source destined to 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 a particular group, a decision has to be made whether to forward the
datagram onto an attached network or not. The multicast routing datagram onto an attached network or not. The multicast routing
protocol in use is in charge of this decision and should use the protocol in use is in charge of this decision and should use the
IGMPv3 information to ensure that all sources/groups desired on a IGMPv3 information to ensure that all sources/groups desired on a
subnetwork are forwarded to that subnetwork. IGMPv3 information does subnetwork are forwarded to that subnetwork. IGMPv3 information does
not override multicast routing information; for example, if the not override multicast routing information; for example, if the
IGMPv3 filter-mode group for G is EXCLUDE, a router may still forward IGMPv3 filter-mode group for G is EXCLUDE, a router may still forward
packets for excluded sources to a transit subnet. packets for excluded sources to a transit subnet.
To summarize, Table 7 describes the forwarding suggestions made by To summarize, Table 7 describes the forwarding suggestions made by
IGMP to the routing protocol for traffic originating from a source IGMP to the routing protocol for traffic originating from a source
destined to a group. It also summarizes the actions taken upon the 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 expiration of a Source Timer based on the Router Filter Mode of the
group. group.
+=============+==========+=======================================+ +=============+==========+=======================================+
| Group | Group | Action | | Group | Group | Action |
| Filter-Mode | Timer | | | Filter-Mode | Timer | |
| | Value | | | | Value | |
+=============+==========+=======================================+ +=============+==========+=======================================+
| INCLUDE | TIMER > | Suggest to forward traffic from | | INCLUDE | TIMER > | Suggest to forward traffic from |
| | 0 | source. | | | 0 | source. |
+-------------+----------+---------------------------------------+ +-------------+----------+---------------------------------------+
| INCLUDE | TIMER == | Suggest to stop forwarding traffic | | INCLUDE | TIMER == | Suggest to stop forwarding traffic |
| | 0 | from source and remove source record. | | | 0 | from source and remove source record. |
| | | If there are no more source records | | | | If there are no more source records |
| | | for the group, delete group record. | | | | for the group, delete Group Record. |
+-------------+----------+---------------------------------------+ +-------------+----------+---------------------------------------+
| INCLUDE | No | Suggest to not forward source. | | INCLUDE | No | Suggest to not forward source. |
| | Source | | | | Source | |
| | Elements | | | | Elements | |
+-------------+----------+---------------------------------------+ +-------------+----------+---------------------------------------+
| EXCLUDE | TIMER > | Suggest to forward traffic from | | EXCLUDE | TIMER > | Suggest to forward traffic from |
| | 0 | source. | | | 0 | source. |
+-------------+----------+---------------------------------------+ +-------------+----------+---------------------------------------+
| EXCLUDE | TIMER == | Suggest to not forward traffic from | | EXCLUDE | TIMER == | Suggest to not forward traffic from |
| | 0 | source (DO NOT remove record). | | | 0 | source (DO NOT remove record). |
+-------------+----------+---------------------------------------+ +-------------+----------+---------------------------------------+
| EXCLUDE | No | Suggest to forward traffic from | | EXCLUDE | No | Suggest to forward traffic from |
| | Source | source. | | | Source | source. |
| | Elements | | | | Elements | |
+-------------+----------+---------------------------------------+ +-------------+----------+---------------------------------------+
Table 7 Table 7: IGMP Forwarding Recommendations
6.4. Action on Reception of Reports 6.4. Action on Reception of Reports
SSM-aware routers SHOULD ignore records that contain multicast SSM-aware routers SHOULD ignore records that contain multicast
addresses in the SSM address range if the record type is addresses in the SSM address range if the record type is
MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE. SSM-aware routers SHOULD MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE. SSM-aware routers SHOULD
ignore IGMPv1/IGMPv2 Report and IGMPv2 DONE messages that contain ignore IGMPv1/IGMPv2 Report and IGMPv2 DONE messages that contain
multicast addresses in the SSM address range, SHOULD NOT use such multicast addresses in the SSM address range, SHOULD NOT use such
Reports to establish IP forwarding state, and MAY log an error if it Reports to establish IP forwarding state, and MAY log an error if it
receives such a message. receives such a message.
6.4.1. Reception of Current-State Records 6.4.1. Reception of Current-State Records
When receiving Current-State Records, a router updates both its group When receiving Current-State Records, a router updates both its group
and source timers. In some circumstances, the reception of a type of and source timers. In some circumstances, the reception of a type of
group record will cause the router filter-mode for that group to Group Record will cause the Router Filter Mode for that group to
change. Table 8 describes the actions, with respect to state and change. Table 8 describes the actions, with respect to state and
timers that occur to a router's state upon reception of Current- timers that occur to a router's state upon reception of Current-
State Records. State Records.
The following notation is used to describe the updating of source The following notation is used to describe the updating of source
timers. The notation ( A, B ) will be used to represent the total timers. The notation ( A, B ) will be used to represent the total
number of sources for a particular group, where number of sources for a particular group, where
A = set of source records whose source timers > 0 (Sources that at * A = set of source records whose source timers > 0 (Sources that at
least one host has requested to be forwarded) 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) * 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 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 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 INCLUDE, a single set is used to describe the set of sources
requested to be forwarded (e.g., simply (A)). requested to be forwarded (e.g., simply (A)).
In Tables 8 and 9, abbreviations are used for several variables (all In Tables 8 and 9, abbreviations are used for several variables (all
of which are described in detail in Section 8). The variable GMI is of which are described in detail in Section 8). The variable GMI is
an abbreviation for the Group Membership Interval, which is the time an abbreviation for the Group Membership Interval, which is the time
in which group memberships will time out. The variable LMQT is an in which group memberships will time out. The variable LMQT is an
abbreviation for the Last Member Query Time, which is the total time abbreviation for the Last Member Query Time, which is the total time
spent after Last Member Query Count retransmissions. LMQT represents spent after [Last Member Query Count] retransmissions. LMQT
the "leave latency" or the difference between the transmission of a represents the leave latency or the difference between the
membership change and the change in the information given to the transmission of a membership change and the change in the information
routing protocol. given to the routing protocol.
Within the "Actions" section of the router state tables, we use the 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 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 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 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. that the Group Timer for the group should be set to value J.
+=========+========+===========+=================+ +=========+========+===========+=================+
| Router | Report | New | Actions | | Router | Report | New | Actions |
| State | Rec'd | Router | | | State | Rec'd | Router | |
skipping to change at line 1563 skipping to change at line 1562
+---------+--------+-----------+-----------------+ +---------+--------+-----------+-----------------+
| EXCLUDE | IS_IN | EXCLUDE | (A)=GMI | | EXCLUDE | IS_IN | EXCLUDE | (A)=GMI |
| (X,Y) | (A) | (X+A,Y-A) | | | (X,Y) | (A) | (X+A,Y-A) | |
+---------+--------+-----------+-----------------+ +---------+--------+-----------+-----------------+
| EXCLUDE | IS_EX | EXCLUDE | (A-X-Y)=GMI | | EXCLUDE | IS_EX | EXCLUDE | (A-X-Y)=GMI |
| (X,Y) | (A) | (A-Y,Y*A) | Delete (X-A) | | (X,Y) | (A) | (A-Y,Y*A) | Delete (X-A) |
| | | | Delete (Y-A) | | | | | Delete (Y-A) |
| | | | Group Timer=GMI | | | | | Group Timer=GMI |
+---------+--------+-----------+-----------------+ +---------+--------+-----------+-----------------+
Table 8 Table 8: Actions on Current-State Report Reception
6.4.2. Reception of Filter-Mode-Change and Source-List-Change Records 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 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- system sends either a Source-List-Change Record or a Filter-Mode-
Change Record for that group. As with Current-State Records, routers Change Record for that group. As with Current-State Records, routers
must act upon these records and possibly change their own state to must act upon these records and possibly change their own state to
reflect the new desired membership state of the network. reflect the new desired membership state of the network.
Routers must query sources that are requested to be no longer Routers must query sources that are requested to be no longer
forwarded to a group. When a router queries or receives a query for forwarded to a group. When a router queries or receives a query for
a specific set of sources, it lowers its source timers for those a specific set of sources, it lowers its source timers for those
sources to a small interval of Last Member Query Time seconds. If sources to a small interval of [Last Member Query Time] seconds. If
group records are received in response to the queries which express Group Records are received in response to the queries which express
interest in receiving traffic from the queried sources, the interest in receiving traffic from the queried sources, the
corresponding timers are updated. corresponding timers are updated.
Similarly, when a router queries a specific group, it lowers its Similarly, when a router queries a specific group, it lowers its
group timer for that group to a small interval of Last Member Query Group Timer for that group to a small interval of [Last Member Query
Time seconds. If any group records expressing EXCLUDE mode interest Time] seconds. If any Group Records expressing EXCLUDE mode interest
in the group are received within the interval, the group timer for in the group are received within the interval, the Group Timer for
the group is updated and the suggestion to the routing protocol to the group is updated and the suggestion to the routing protocol to
forward the group stands without any interruption. forward the group stands without any interruption.
During a query period (i.e., Last Member Query Time seconds), the During a query period (i.e., [Last Member Query Time] seconds), the
IGMP component in the router continues to suggest to the routing IGMP component in the router continues to suggest to the routing
protocol that it forwards traffic from the groups or sources that it protocol that it forwards traffic from the groups or sources that it
is querying. It is not until after Last Member Query Time seconds is querying. It is not until after [Last Member Query Time] seconds
without receiving a record expressing interest in the queried group without receiving a record expressing interest in the queried group
or sources that the router may prune the group or sources from the or sources that the router may prune the group or sources from the
network. network.
Table 9 describes the changes in group state and the action(s) taken Table 9 describes the changes in group state and the action(s) taken
when receiving either Filter-Mode-Change or Source-List-Change when receiving either Filter-Mode-Change or Source-List-Change
Records. This table also describes the queries that are sent by the Records. This table also describes the queries that are sent by the
querier when a particular report is received. querier when a particular report is received.
We use the following notation for describing the queries that are We use the following notation for describing the queries that are
sent. We use the notation 'Q(G)' to describe a Group-Specific Query sent. We use the notation 'Q(G)' to describe a Group Specific Query
to G. We use the notation 'Q(G,A)' to describe a Group-and-Source to G. We use the notation 'Q(G,A)' to describe a Group-and-Source
Specific Query to G with source-list A. If source-list A is null as Specific Query to G 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 a result of the action (e.g., A*B), then no query is sent as a result
of the operation. of the operation.
In order to maintain protocol robustness, queries sent by actions in In order to maintain protocol robustness, queries sent by actions in
Table 9 need to be transmitted [Last Member Query Count] times, once Table 9 need to be transmitted [Last Member Query Count] times, once
every [Last Member Query Interval]. every [Last Member Query Interval].
If while scheduling new queries there are already pending queries to If while scheduling new queries there are already pending queries to
skipping to change at line 1654 skipping to change at line 1653
| (X,Y) | (A) | (A-Y,Y*A) | Delete (X-A) | | (X,Y) | (A) | (A-Y,Y*A) | Delete (X-A) |
| | | | Delete (Y-A) | | | | | Delete (Y-A) |
| | | | Send Q(G,A-Y) | | | | | Send Q(G,A-Y) |
| | | | Group Timer=GMI | | | | | Group Timer=GMI |
+---------+--------+-------------+---------------------+ +---------+--------+-------------+---------------------+
| EXCLUDE | TO_IN | EXCLUDE | (A)=GMI | | EXCLUDE | TO_IN | EXCLUDE | (A)=GMI |
| (X,Y) | (A) | (X+A,Y-A) | Send Q(G,X-A) | | (X,Y) | (A) | (X+A,Y-A) | Send Q(G,X-A) |
| | | | Send Q(G) | | | | | Send Q(G) |
+---------+--------+-------------+---------------------+ +---------+--------+-------------+---------------------+
Table 9 Table 9: Actions on Change Record Reception
6.5. Switching Router Filter-Modes 6.5. Switching Router Filter Modes
The group timer is used as a mechanism for transitioning the router The Group Timer is used as a mechanism for transitioning the Router
filter-mode from EXCLUDE to INCLUDE. Filter Mode from EXCLUDE to INCLUDE.
When a group timer expires with a router filter-mode of EXCLUDE, a When a Group Timer expires with a Router Filter Mode of EXCLUDE, a
router assumes that there are no systems with a filter-mode of router assumes that there are no systems with a filter-mode of
EXCLUDE present on the attached network. When a router's filter-mode EXCLUDE present on the attached network. When a router's filter-mode
for a group is EXCLUDE and the group timer expires, the router for a group is EXCLUDE and the Group Timer expires, the Router Filter
filter-mode for the group transitions to INCLUDE. Mode for the group transitions to INCLUDE.
A router uses source records with running source timers as its state 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 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 records with source timers greater than zero (i.e., requested to be
forwarded), a router switches to filter-mode of INCLUDE using those forwarded), a router switches to filter-mode of INCLUDE using those
source records. Source records whose timers are zero (from the source records. Source records whose timers are zero (from the
previous EXCLUDE mode) are deleted. previous EXCLUDE mode) are deleted.
For example, if a router's state for a group is EXCLUDE(X,Y) and the For example, if a router's state for a group is EXCLUDE(X,Y) and the
group timer expires for that group, the router switches to filter- Group Timer expires for that group, the router switches to filter-
mode of INCLUDE with state INCLUDE(X). mode of INCLUDE with state INCLUDE(X).
6.6. Action on Reception of Queries 6.6. Action on Reception of Queries
6.6.1. Timer Updates 6.6.1. Timer Updates
When a router sends or receives a query with a clear Suppress Router- When a router sends or receives a query with a clear Suppress Router-
Side Processing flag, it must update its timers to reflect the Side Processing flag, it must update its timers to reflect the
correct timeout values for the group or sources being queried. correct timeout values for the group or sources being queried.
Table 10 describes the timer actions when sending or receiving a Table 10 describes the timer actions when sending or receiving a
Group-Specific or Group-and-Source Specific Query with the S flag not Group Specific or Group-and-Source Specific Query with the S flag not
set. set.
+========+===================================================+ +========+===================================================+
| Query | Action | | Query | Action |
+========+===================================================+ +========+===================================================+
| Q(G,A) | Source Timer for sources in A are lowered to LMQT | | Q(G,A) | Source Timer for sources in A are lowered to LMQT |
+--------+---------------------------------------------------+ +--------+---------------------------------------------------+
| Q(G) | Group Timer is lowered to LMQT | | Q(G) | Group Timer is lowered to LMQT |
+--------+---------------------------------------------------+ +--------+---------------------------------------------------+
Table 10 Table 10: Timer Updates on Query
When a router sends or receives a query with the S flag set, it will When a router sends or receives a query with the S flag set, it will
not update its timers. not update its timers.
6.6.2. Querier Election 6.6.2. Querier Election
IGMPv3 elects a single querier per subnet using the same querier IGMPv3 elects a single querier per subnet using the same Querier
election mechanism as IGMPv2, namely by IP address. When a router election mechanism as IGMPv2, namely by IP address. When a router
receives a general query with a lower IP address, it sets the Other receives a General Query with a lower IP address, it sets the Other-
Querier Present timer to Other Querier Present Interval and ceases to Querier-Present Timer to [Other Querier Present Interval] and ceases
send general queries on the network if it was the previously elected to send general queries on the network if it was the previously
querier. After its Other-Querier Present timer expires, it should elected querier. After its Other-Querier-Present Timer expires, it
begin sending General Queries. should begin sending General Queries.
If a router receives an older version general query, it MUST use the If a router receives an older version General Query, it MUST use the
oldest version of IGMP on the network. For a detailed description of oldest version of IGMP on the network. For a detailed description of
compatibility issues between IGMP versions, see Section 7. compatibility issues between IGMP versions, see Section 7.
6.6.3. Building and Sending Specific Queries 6.6.3. Building and Sending Specific Queries
6.6.3.1. Building and Sending Group-Specific Queries 6.6.3.1. Building and Sending Group Specific Queries
When a table action "Send Q(G)" is encountered, the group timer must When a table action "Send Q(G)" is encountered, the Group Timer must
be lowered to LMQT. The router must then immediately send a group- be lowered to LMQT. The router must then immediately send a Group
specific query as well as schedule [Last Member Query Count - 1] Specific Query as well as schedule [Last Member Query Count] - 1
query retransmissions to be sent every [Last Member Query Interval] query retransmissions to be sent every [Last Member Query Interval]
over [Last Member Query Time]. over [Last Member Query Time].
When transmitting a group-specific query, if the group timer is When transmitting a Group Specific Query, if the Group Timer is
larger than LMQT, the "Suppress Router-Side Processing" bit is set in larger than LMQT, the "Suppress Router-Side Processing" bit is set in
the query message. the query message.
6.6.3.2. Building and Sending Group-and-Source-Specific Queries 6.6.3.2. Building and Sending Group-and-Source Specific Queries
When a table action "Send Q(G,X)" is encountered by a querier in When a table action "Send Q(G,X)" is encountered by a querier in
Table 9 (Section 6.4.2), the following actions must be performed for Table 9 (Section 6.4.2), the following actions must be performed for
each of the sources in X of group G, with the source timer larger each of the sources in X of group G, with the Source Timer larger
than LMQT: than LMQT:
* Set the number of retransmissions for each source to [Last Member * Set the number of retransmissions for each source to [Last Member
Query Count]. Query Count].
* Lower the source timer to LMQT. * Lower the Source Timer to LMQT.
The router must then immediately send a group and source specific The router must then immediately send a Group-and-Source Specific
query as well as schedule [Last Member Query Count - 1] query Query as well as schedule [Last Member Query Count] - 1 query
retransmissions to be sent every [Last Member Query Interval] over retransmissions to be sent every [Last Member Query Interval] over
[Last Member Query Time]. The contents of these queries are [Last Member Query Time]. The contents of these queries are
calculated as follows. calculated as follows.
When building a group and source specific query for group G, two When building a Group-and-source Specific Query for group G, two
separate query messages are sent for the group. The first one has separate query messages are sent for the group. The first one has
the "Suppress Router-Side Processing" bit set and contains all the the "Suppress Router-Side Processing" bit set and contains all the
sources with retransmission state and timers greater than LMQT. The sources with retransmission state and timers greater than LMQT. The
second has the "Suppress Router-Side Processing" bit clear and second has the "Suppress Router-Side Processing" bit clear and
contains all the sources with retransmission state and timers lower contains all the sources with retransmission state and timers lower
or equal to LMQT. If either of the two calculated messages does not or equal to LMQT. If either of the two calculated messages does not
contain any sources, then its transmission is suppressed. contain any sources, then its transmission is suppressed.
Note: If a group-specific query is scheduled to be transmitted at the | Note: If a Group Specific Query is scheduled to be transmitted
same time as a group and source specific query for the same group, | at the same time as a Group-and-Source Specific Query for the
then transmission of the group and source specific message with the | same group, then transmission of the Group-and-Source Specific
"Suppress Router-Side Processing" bit set may be suppressed. | Query message with the "Suppress Router-Side Processing" bit
| set may be suppressed.
7. Interoperation With Older Versions of IGMP 7. Interoperation With Older Versions of IGMP
IGMP version 3 hosts and routers interoperate with hosts and routers IGMPv3 hosts and routers interoperate with hosts and routers that
that have not yet been upgraded to IGMPv3. This compatibility is have not yet been upgraded to IGMPv3. This compatibility is
maintained by hosts and routers taking appropriate actions depending maintained by hosts and routers taking appropriate actions depending
on the versions of IGMP operating on hosts and routers within a on the versions of IGMP operating on hosts and routers within a
network. network.
7.1. Query Version Distinctions 7.1. Query Version Distinctions
The IGMP version of a Membership Query message is determined as The IGMP version of a Membership Query message is determined as
follows: follows:
* IGMPv1 Query: length = 8 octets AND Max Resp Code field is zero * IGMPv1 Query: length = 8 octets AND Max Resp Code field is zero
skipping to change at line 1790 skipping to change at line 1790
* IGMPv3 Query: length >= 12 octets * IGMPv3 Query: length >= 12 octets
Query messages that do not match any of the above conditions (e.g., a Query messages that do not match any of the above conditions (e.g., a
Query of length 10 octets) MUST be silently ignored. Query of length 10 octets) MUST be silently ignored.
7.2. Group Member Behavior 7.2. Group Member Behavior
7.2.1. In the Presence of Older Version Queriers 7.2.1. In the Presence of Older Version Queriers
In order to be compatible with older version routers, IGMPv3 hosts In order to be compatible with older version routers, IGMPv3 hosts
MUST operate in version 1 and version 2 compatibility modes. IGMPv3 MUST operate in v1 and v2 compatibility modes. IGMPv3 hosts MUST
hosts MUST keep state per local interface regarding the compatibility keep state per local interface regarding the compatibility mode of
mode of each attached network. A host's compatibility mode is each attached network. A host's compatibility mode is determined
determined from the Host Compatibility Mode variable, which can be in from the Host Compatibility Mode variable, which can be in one of
one of three states: IGMPv1, IGMPv2, or IGMPv3. This variable is three states: IGMPv1, IGMPv2, or IGMPv3. This variable is kept per
kept per interface and is dependent on the version of General Queries interface and is dependent on the version of General Queries received
heard on that interface as well as the Older Version Querier Present on that interface as well as the Older-Version-Querier-Present Timer
timers for the interface. for the interface.
In order to switch gracefully between versions of IGMP, hosts keep In order to switch gracefully between versions of IGMP, hosts keep
both an IGMPv1 Querier Present timer and an IGMPv2 Querier Present both an IGMPv1-Querier-Present Timer and an IGMPv2-Querier-Present
timer per interface. IGMPv1 Querier Present is set to Older Version Timer per interface. IGMPv1-Querier-Present Timer is set to [Older
Querier Present Timeout seconds whenever an IGMPv1 Membership Query Version Querier Present Interval] seconds whenever an IGMPv1
is received. IGMPv2 Querier Present is set to Older Version Querier Membership Query is received. IGMPv2-Querier-Present Timer is set to
Present Timeout seconds whenever an IGMPv2 General Query is received. [Older Version Querier Present Interval] seconds whenever an IGMPv2
General Query is received.
The Host Compatibility Mode of an interface changes whenever an older The Host Compatibility Mode of an interface changes whenever an older
version query (than the current compatibility mode) is heard or when version query (than the current compatibility mode) is received or
certain timer conditions occur. When the IGMPv1 Querier Present when certain timer conditions occur. When the IGMPv1-Querier-Present
timer expires, a host switches to Host Compatibility Mode of IGMPv2 Timer expires, a host switches to Host Compatibility Mode of IGMPv2
if it has a running IGMPv2 Querier Present timer. If it does not if it has a running IGMPv2 Querier Present timer. If it does not
have a running IGMPv2 Querier Present timer, then it switches to Host have a running IGMPv2 Querier Present timer, then it switches to Host
Compatibility of IGMPv3. When the IGMPv2 Querier Present timer Compatibility of IGMPv3. When the IGMPv2 Querier Present timer
expires, a host switches to Host Compatibility Mode of IGMPv3. expires, a host switches to Host Compatibility Mode of IGMPv3.
The Host Compatibility Mode variable is based on whether an older The Host Compatibility Mode variable is based on whether an older
version General query was heard in the last Older Version Querier version General Query was received in the last [Older Version Querier
Present Timeout seconds. The Host Compatibility Mode variable value Present Interval] seconds. The Host Compatibility Mode variable
MUST NOT be changed by an older version group-specific query. The value MUST NOT be changed by an older version Group Specific Query.
Host Compatibility Mode is set depending on the following: The Host Compatibility Mode is set depending on the following:
+=========================+========================================+ +=========================+========================================+
| Host Compatibility Mode | Timer State | | Host Compatibility Mode | Timer State |
+=========================+========================================+ +=========================+========================================+
| IGMPv3 (default) | IGMPv2 Querier Present not running and | | IGMPv3 (default) | IGMPv2 Querier Present not running and |
| | IGMPv1 Querier Present not running | | | IGMPv1 Querier Present not running |
+-------------------------+----------------------------------------+ +-------------------------+----------------------------------------+
| IGMPv2 | IGMPv2 Querier Present running and | | IGMPv2 | IGMPv2 Querier Present running and |
| | IGMPv1 Querier Present not running | | | IGMPv1 Querier Present not running |
+-------------------------+----------------------------------------+ +-------------------------+----------------------------------------+
| IGMPv1 | IGMPv1 Querier Present running | | IGMPv1 | IGMPv1 Querier Present running |
+-------------------------+----------------------------------------+ +-------------------------+----------------------------------------+
Table 11 Table 11: Host Compatibility Mode Settings
If a host receives a query that causes its Querier Present timers to If a host receives a query that causes its Querier Present timers to
be updated and correspondingly its compatibility mode, it should be updated and correspondingly its compatibility mode, it should
switch compatibility modes immediately. switch compatibility modes immediately.
When Host Compatibility Mode is IGMPv3, a host acts using the IGMPv3 When Host Compatibility Mode is IGMPv3, a host acts using the IGMPv3
protocol on that interface. When Host Compatibility Mode is IGMPv2, protocol on that interface. When Host Compatibility Mode is IGMPv2,
a host acts in IGMPv2 compatibility mode, using only the IGMPv2 a host acts in IGMPv2 compatibility mode, using only the IGMPv2
protocol, on that interface. When Host Compatibility Mode is IGMPv1, protocol, on that interface. When Host Compatibility Mode is IGMPv1,
a host acts in IGMPv1 compatibility mode, using only the IGMPv1 a host acts in IGMPv1 compatibility mode, using only the IGMPv1
protocol on that interface. protocol on that interface.
An IGMPv1 router will send General Queries with the Max Resp Code set An IGMPv1 router will send General Queries with the Max Resp Code set
to 0. This MUST be interpreted as a value of 100 (10 seconds). to 0. This MUST be interpreted as a value of 100 (10 seconds).
An IGMPv2 router will send General Queries with the Max Resp Code set An IGMPv2 router will send General Queries with the Max Resp Code set
to the desired Max Resp Time, i.e., the full range of this field is to the desired Max Response Time, i.e., the full range of this field
linear and the exponential algorithm described in Section 4.1.1 is is linear and the exponential algorithm described in Section 4.1.1 is
not used. not used.
Whenever a host changes its compatibility mode, it cancels all its Whenever a host changes its compatibility mode, it cancels all its
pending response and retransmission timers. pending response and retransmission timers.
An SSM-aware host that receives an IGMPv1 Query, an IGMPv2 General An SSM-aware host that receives an IGMPv1 Query, an IGMPv2 General
Query, or an IGMPv2 Group Specific Query for a multicast address in Query, or an IGMPv2 Group Specific Query for a multicast address in
the SSM address range SHOULD log an error. It is RECOMMENDED that the SSM address range SHOULD log an error. It is RECOMMENDED that
implementations provide a configuration option to disable use of the implementations provide a configuration option to disable use of the
Host Compatibility Mode to allow networks to operate only in SSM Host Compatibility Mode to allow networks to operate only in SSM
mode. This configuration option SHOULD be disabled by default. mode. This configuration option SHOULD be disabled by default.
7.2.2. In the Presence of Older Version Group Members 7.2.2. In the Presence of Older Version Group Members
An IGMPv3 host may be placed on a network where there are hosts that An IGMPv3 host may be placed on a network where there are hosts that
have not yet been upgraded to IGMPv3. A host MAY allow its IGMPv3 have not yet been upgraded to IGMPv3. A host MAY allow its IGMPv3
Membership Record to be suppressed by either a Version 1 Membership Membership Record to be suppressed by either an IGMPv1 Membership
Report, or a Version 2 Membership Report. SSM-aware hosts MUST NOT Report or an IGMPv2 Membership Report. SSM-aware hosts MUST NOT
allow its IGMPv3 Membership Record to be suppressed. allow its IGMPv3 Membership Record to be suppressed.
7.3. Multicast Router Behavior 7.3. Multicast Router Behavior
7.3.1. In the Presence of Older Version Queriers 7.3.1. In the Presence of Older Version Queriers
IGMPv3 routers may be placed on a network where at least one router IGMPv3 routers may be placed on a network where at least one router
on the network has not yet been upgraded to IGMPv3. The following on the network has not yet been upgraded to IGMPv3. The following
requirements apply: requirements apply:
skipping to change at line 1893 skipping to change at line 1894
compatible with IGMPv1 and IGMPv2 MUST have a configuration option compatible with IGMPv1 and IGMPv2 MUST have a configuration option
to act in IGMPv1 or IGMPv2 compatibility modes. When in IGMPv1 to act in IGMPv1 or IGMPv2 compatibility modes. When in IGMPv1
mode, routers MUST send Periodic Queries with a Max Resp Code of 0 mode, routers MUST send Periodic Queries with a Max Resp Code of 0
and truncated at the Group Address field (i.e., 8 bytes long) and and truncated at the Group Address field (i.e., 8 bytes long) and
MUST ignore Leave Group messages. They SHOULD also warn about MUST ignore Leave Group messages. They SHOULD also warn about
receiving an IGMPv2 or IGMPv3 query, although such warnings MUST receiving an IGMPv2 or IGMPv3 query, although such warnings MUST
be rate-limited. When in IGMPv2 mode, routers MUST send Periodic be rate-limited. When in IGMPv2 mode, routers MUST send Periodic
Queries truncated at the Group Address field (i.e., 8 bytes long) Queries truncated at the Group Address field (i.e., 8 bytes long)
and SHOULD also warn about receiving an IGMPv3 query (such and SHOULD also warn about receiving an IGMPv3 query (such
warnings MUST be rate-limited). They also MUST fill in the Max warnings MUST be rate-limited). They also MUST fill in the Max
Resp Time in the Max Resp Code field, i.e., the exponential Response Time in the Max Resp Code field, i.e., the exponential
algorithm described in Section 4.1.1 is not used. algorithm described in Section 4.1.1 is not used.
* If a router is not explicitly configured to use IGMPv1 or IGMPv2 * If a router is not explicitly configured to use IGMPv1 or IGMPv2
and hears an IGMPv1 Query or IGMPv2 General Query, it SHOULD log a and receives an IGMPv1 Query or IGMPv2 General Query, it SHOULD
warning. These warnings MUST be rate-limited. log a warning. These warnings MUST be rate-limited.
* It is RECOMMENDED that implementations provide a configuration * It is RECOMMENDED that implementations provide a configuration
option to disable use of compatibility mode to allow networks to option to disable use of compatibility mode to allow networks to
operate only in SSM mode. This configuration option SHOULD be operate only in SSM mode. This configuration option SHOULD be
disabled by default. disabled by default.
7.3.2. In the Presence of Older Version Group Members 7.3.2. In the Presence of Older Version Group Members
IGMPv3 routers may be placed on a network where there are hosts that IGMPv3 routers may be placed on a network where there are hosts that
have not yet been upgraded to IGMPv3. In order to be compatible with have not yet been upgraded to IGMPv3. In order to be compatible with
older version hosts, IGMPv3 routers MUST operate in version 1 and older version hosts, IGMPv3 routers MUST operate in v1 and v2
version 2 compatibility modes. IGMPv3 routers keep a compatibility compatibility modes. IGMPv3 routers keep a compatibility mode per
mode per group record. A group's compatibility mode is determined Group Record. A group's compatibility mode is determined from the
from the Group Compatibility Mode variable, which can be in one of Group Compatibility Mode variable, which can be in one of three
three states: IGMPv1, IGMPv2, or IGMPv3. This variable is kept per states: IGMPv1, IGMPv2, or IGMPv3. This variable is kept per Group
group record and is dependent on the version of Membership Reports Record and is dependent on the version of Membership Reports received
heard for that group as well as the Older Version Host Present timer for that group as well as the Older-Version-Host-Present Timer for
for the group. the group.
In order to switch gracefully between versions of IGMP, routers keep In order to switch gracefully between versions of IGMP, routers keep
an IGMPv1 Host Present timer and an IGMPv2 Host Present timer per an IGMPv1-Host-Present Timer and an IGMPv2-Host-Present Timer per
group record. The IGMPv1 Host Present timer is set to Older Version Group Record. The IGMPv1-Host-Present Timer is set to [Older Version
Host Present Timeout seconds whenever an IGMPv1 Membership Report is Host Present Interval] seconds whenever an IGMPv1 Membership Report
received. The IGMPv2 Host Present timer is set to Older Version Host is received. The IGMPv2-Host-Present Timer is set to [Older Version
Present Timeout seconds whenever an IGMPv2 Membership Report is Host Present Interval] seconds whenever an IGMPv2 Membership Report
received. is received.
The Group Compatibility Mode of a group record changes whenever an The Group Compatibility Mode of a Group Record changes whenever an
older version report (than the current compatibility mode) is heard older version report (than the current compatibility mode) is
or when certain timer conditions occur. When the IGMPv1 Host Present received or when certain timer conditions occur. When the IGMPv1-
timer expires, a router switches to Group Compatibility Mode of Host-Present Timer expires, a router switches to Group Compatibility
IGMPv2 if it has a running IGMPv2 Host Present timer. If it does not Mode of IGMPv2 if it has a running IGMPv2 Host Present timer. If it
have a running IGMPv2 Host Present timer, then it switches to Group does not have a running IGMPv2 Host Present timer, then it switches
Compatibility Mode of IGMPv3. When the IGMPv2 Host Present timer to Group Compatibility Mode of IGMPv3. When the IGMPv2-Host-Present
expires and the IGMPv1 Host Present timer is not running, a router Timer expires and the IGMPv1-Host-Present Timer is not running, a
switches to Group Compatibility Mode of IGMPv3. Note that when a router switches to Group Compatibility Mode of IGMPv3. Note that
group switches back to IGMPv3 mode, it takes some time to regain when a group switches back to IGMPv3 mode, it takes some time to
source- specific state information. Source-specific information will regain source- specific state information. Source-specific
be learned during the next General Query, but sources that should be information will be learned during the next General Query, but
blocked will not be blocked until [Group Membership Interval] after sources that should be blocked will not be blocked until [Group
that. Membership Interval] after that.
The Group Compatibility Mode variable is based on whether an older The Group Compatibility Mode variable is based on whether an older
version report was heard in the last Older Version Host Present version report was received in the last [Older Version Host Present
Timeout seconds. The Group Compatibility Mode is set depending on Interval] seconds. The Group Compatibility Mode is set depending on
the following: the following:
+==========================+=====================================+ +==========================+=====================================+
| Group Compatibility Mode | Timer State | | Group Compatibility Mode | Timer State |
+==========================+=====================================+ +==========================+=====================================+
| IGMPv3 (default) | IGMPv2 Host Present not running and | | IGMPv3 (default) | IGMPv2 Host Present not running and |
| | IGMPv1 Host Present not running | | | IGMPv1 Host Present not running |
+--------------------------+-------------------------------------+ +--------------------------+-------------------------------------+
| IGMPv2 | IGMPv2 Host Present running and | | IGMPv2 | IGMPv2 Host Present running and |
| | IGMPv1 Host Present not running | | | IGMPv1 Host Present not running |
+--------------------------+-------------------------------------+ +--------------------------+-------------------------------------+
| IGMPv1 | IGMPv1 Host Present running | | IGMPv1 | IGMPv1 Host Present running |
+--------------------------+-------------------------------------+ +--------------------------+-------------------------------------+
Table 12 Table 12: Group Compatibility Mode Settings
If a router receives a report that causes its older Host Present If a router receives a report that causes its older Host Present
timers to be updated and correspondingly its compatibility mode, it timers to be updated and correspondingly its compatibility mode, it
SHOULD switch compatibility modes immediately. SHOULD switch compatibility modes immediately.
When Group Compatibility Mode is IGMPv3, a router acts using the When Group Compatibility Mode is IGMPv3, a router acts using the
IGMPv3 protocol for that group. IGMPv3 protocol for that group.
When Group Compatibility Mode is IGMPv2, a router internally When Group Compatibility Mode is IGMPv2, a router internally
translates the following IGMPv2 messages for that group to their translates the following IGMPv2 messages for that group to their
IGMPv3 equivalents: IGMPv3 equivalents:
+================+===================+ +================+===================+
| IGMPv2 Message | IGMPv3 Equivalent | | IGMPv2 Message | IGMPv3 Equivalent |
+================+===================+ +================+===================+
| Report | IS_EX( {} ) | | Report | IS_EX( {} ) |
+----------------+-------------------+ +----------------+-------------------+
| Leave | TO_IN( {} ) | | Leave | TO_IN( {} ) |
+----------------+-------------------+ +----------------+-------------------+
Table 13 Table 13: IGMPv2 Compatibility
Mode Message Translation
IGMPv3 BLOCK messages are ignored, as are source-lists in TO_EX() IGMPv3 BLOCK messages are ignored, as are source-lists in TO_EX()
messages (i.e., any TO_EX() message is treated as TO_EX( {} )). messages (i.e., any TO_EX() message is treated as TO_EX( {} )).
When Group Compatibility Mode is IGMPv1, a router internally When Group Compatibility Mode is IGMPv1, a router internally
translates the following IGMPv1 and IGMPv2 messages for that group to translates the following IGMPv1 and IGMPv2 messages for that group to
their IGMPv3 equivalents: their IGMPv3 equivalents:
+================+===================+ +================+===================+
| IGMPv2 Message | IGMPv3 Equivalent | | IGMPv2 Message | IGMPv3 Equivalent |
+================+===================+ +================+===================+
| v1 Report | IS_EX( {} ) | | v1 Report | IS_EX( {} ) |
+----------------+-------------------+ +----------------+-------------------+
| v2 Report | IS_EX( {} ) | | v2 Report | IS_EX( {} ) |
+----------------+-------------------+ +----------------+-------------------+
Table 14 Table 14: IGMPv1 Compatibility
Mode Message Translation
In addition to ignoring IGMPv3 BLOCK messages and source-lists in In addition to ignoring IGMPv3 BLOCK messages and source-lists in
TO_EX() messages as in IGMPv2 Group Compatibility Mode, IGMPv2 Leave TO_EX() messages as in IGMPv2 Group Compatibility Mode, IGMPv2 Leave
messages and IGMPv3 TO_IN() messages are also ignored. messages and IGMPv3 TO_IN() messages are also ignored.
8. List of Timers, Counters, and Their Default Values 8. List of Timers, Counters, and Their Default Values
Most of these timers are configurable. If non-default settings are Most timers and counters are configurable. If non-default settings
used, they MUST be consistent among all systems on a single link. are used, they MUST be consistent among all systems on a single link.
Note that parentheses are used to group expressions to make the Note that parentheses are used to group expressions to make the
algebra clear. algebra clear.
8.1. Robustness Variable 8.1. Robustness Variable
The Robustness Variable allows tuning for the expected packet loss on The Robustness Variable allows tuning for the expected packet loss on
a network. If a network is expected to be lossy, the Robustness a network. If a network is expected to be lossy, the Robustness
Variable may be increased. IGMP is robust to (Robustness Variable - Variable may be increased. IGMP is robust to (Robustness Variable -
1) packet losses. The Robustness Variable MUST NOT be zero and 1) packet losses. The Robustness Variable MUST NOT be zero and
SHOULD NOT be one. Default: 2. SHOULD NOT be one. Default: 2.
8.2. Query Interval 8.2. Query Interval
The Query Interval is the interval between General Queries sent by The Query Interval is the interval between General Queries sent by
the Querier. Default: 125 seconds. the Querier. Default: 125 seconds.
By varying the [Query Interval], an administrator may tune the number By varying the Query Interval, an administrator may tune the number
of IGMP messages on the network; larger values cause IGMP Queries to of IGMP messages on the network; larger values cause IGMP Queries to
be sent less often. be sent less often.
8.3. Query Response Interval 8.3. Query Response Interval
The Query Response Interval uses the Max Response Time to calculate The Query Response Interval uses the Max Response Time to calculate
the Max Resp Code that is inserted into the periodic General Queries. the Max Resp Code that is inserted into the periodic General Queries.
Default: 100 (10 seconds). Default: 100 (10 seconds).
By varying the [Query Response Interval], an administrator may tune By varying the [Query Response Interval], an administrator may tune
skipping to change at line 2044 skipping to change at line 2047
the traffic less bursty, as host responses are spread out over a the traffic less bursty, as host responses are spread out over a
larger interval. The number of seconds represented by the [Query larger interval. The number of seconds represented by the [Query
Response Interval] must be less than the [Query Interval]. Response Interval] must be less than the [Query Interval].
8.4. Group Membership Interval 8.4. Group Membership Interval
The Group Membership Interval is the amount of time that must pass The Group Membership Interval is the amount of time that must pass
before a multicast router decides there are no more members of a before a multicast router decides there are no more members of a
group or a particular source on a network. group or a particular source on a network.
This value MUST be ((the Robustness Variable) times (the Query This value MUST be ([Robustness Variable] times [Query Interval])
Interval)) plus (2 * Query Response Interval). plus (2 * [Query Response Interval]).
8.5. Other Querier Present Interval 8.5. Other Querier Present Interval
The Other Querier Present Interval is the length of time that must The Other Querier Present Interval is the length of time that must
pass before a multicast router decides that there is no longer pass before a multicast router decides that there is no longer
another multicast router that should be the querier. This value MUST another multicast router that should be the querier. This value MUST
be ((the Robustness Variable) times (the Query Interval)) plus (one be ([Robustness Variable] times [Query Interval]) plus (0.5 times
half of one Query Response Interval). [Query Response Interval]).
8.6. Startup Query Interval 8.6. Startup Query Interval
The Startup Query Interval is the interval between General Queries The Startup Query Interval is the interval between General Queries
sent by a Querier on startup. Default: 1/4 the Query Interval. sent by a Querier on startup. Default: 1/4 times [Query Interval].
8.7. Startup Query Count 8.7. Startup Query Count
The Startup Query Count is the number of Queries sent out on startup, The Startup Query Count is the number of Queries sent out on startup,
separated by the Startup Query Interval. Default: The Robustness separated by the Startup Query Interval. Default: [Robustness
Variable. Variable].
8.8. Last Member Query Interval 8.8. Last Member Query Interval
The Last Member Query Interval (LMQI) is the Max Response Time used The Last Member Query Interval (LMQI) is the Max Response Time used
to calculate the Max Resp Code that is inserted into Group-Specific to calculate the Max Resp Code that is inserted into Group Specific
Queries sent in response to Leave Group messages. It is also the Max Queries sent in response to Leave Group messages. It is also the Max
Response Time used in calculating the Max Resp Code for Group-and- Response Time used in calculating the Max Resp Code for Group-and-
Source-Specific Query messages. Default: 10 (1 second). Source Specific Query messages. Default: 10 (1 second).
Note that for values of LMQI greater than 12.8 seconds, a limited set Note that for values of LMQI greater than 12.8 seconds, a limited set
of values can be represented, corresponding to sequential values of of values can be represented, corresponding to sequential values of
Max Resp Code. When converting a configured time to a Max Resp Code Max Resp Code. When converting a configured time to a Max Resp Code
value, it is recommended to use the exact value, if possible, or the value, it is recommended to use the exact value, if possible, or the
next lower value if the requested value is not exactly representable. next lower value if the requested value is not exactly representable.
This value may be tuned to modify the "leave latency" of the network. This value may be tuned to modify the leave latency of the network.
A reduced value results in reduced time to detect the loss of the A reduced value results in reduced time to detect the loss of the
last member of a group or source. last member of a group or source.
8.9. Last Member Query Count 8.9. Last Member Query Count
The Last Member Query Count is the number of Group-Specific Queries The Last Member Query Count is the number of Group Specific Queries
sent before the router assumes there are no local members. The Last sent before the router assumes there are no local members. The Last
Member Query Count is also the number of Group-and-Source-Specific Member Query Count is also the number of Group-and-Source Specific
Queries sent before the router assumes there are no listeners for a Queries sent before the router assumes there are no listeners for a
particular source. Default: The Robustness Variable. particular source. Default: [Robustness Variable].
8.10. Last Member Query Time 8.10. Last Member Query Time
The Last Member Query Time is the time value represented by the Last The Last Member Query Time is the time value represented by [Last
Member Query Interval, multiplied by the Last Member Query Count. It Member Query Interval] times [Last Member Query Count]. It is not a
is not a tunable value, but it may be tuned by changing its tunable value, but it may be tuned by changing its components.
components.
8.11. Unsolicited Report Interval 8.11. Unsolicited Report Interval
The Unsolicited Report Interval is the time between repetitions of a The Unsolicited Report Interval is the time between repetitions of a
host's initial report of membership in a group. Default: 1 second. host's initial report of membership in a group. Default: 1 second.
8.12. Older Version Querier Present Interval 8.12. Older Version Querier Present Interval
The Older Version Querier Present Interval is the timeout for The Older Version Querier Present Interval is the timeout for
transitioning a host back to IGMPv3 mode once an older version query transitioning a host back to IGMPv3 mode once an older version query
is heard. When an older version query is received, hosts set their is received. When an older version query is received, hosts set
Older Version Querier Present Timer to Older Version Querier Present their Older-Version-Querier-Present Timer to [Older Version Querier
Interval. Present Interval].
It is RECOMMENDED to use the default values for calculating the It is RECOMMENDED to use the default values for calculating the
interval value as hosts do not know the values configured on the interval value as hosts do not know the values configured on the
querying routers. This value SHOULD be [Robustness Variable] times querying routers. This value SHOULD be [Robustness Variable] times
[Query Interval] plus (10 times the Max Resp Time in the last [Query Interval] plus (10 times the Max Response Time in the last
received query message). received query message).
8.13. Older Host Present Interval 8.13. Older Host Present Interval
The Older Host Present Interval is the timeout for transitioning a The Older Host Present Interval is the timeout for transitioning a
group back to IGMPv3 mode once an older version report is sent for group back to IGMPv3 mode once an older version report is sent for
that group. When an older version report is received, routers set that group. When an older version report is received, routers set
their Older Host Present Timer to Older Host Present Interval. their Older-Host-Present Timer to [Older Host Present Interval].
This value MUST be ((the Robustness Variable) times (the Query This value MUST be ([Robustness Variable] times [Query Interval])
Interval)) plus (one Query Response Interval). plus [Query Response Interval].
8.14. Configuring Timers 8.14. Configuring Timers
This section is meant to provide advice to network administrators on This section is meant to provide advice to network administrators on
how to tune these settings to their network. Ambitious router how to tune these settings to their network. Ambitious router
implementations might tune these settings dynamically based upon implementations might tune these settings dynamically based upon
changing characteristics of the network. changing characteristics of the network.
8.14.1. Robustness Variable 8.14.1. Robustness Variable
The Robustness Variable tunes IGMP to expected losses on a link. The Robustness Variable tunes IGMP to expected losses on a link.
IGMPv3 is robust to (Robustness Variable - 1) packet losses, e.g., if IGMPv3 is robust to ([Robustness Variable] - 1) packet losses, e.g.,
the Robustness Variable is set to the default value of 2, IGMPv3 is if the Robustness Variable is set to the default value of 2, IGMPv3
robust to a single packet loss but may operate imperfectly if more is robust to a single packet loss but may operate imperfectly if more
losses occur. On lossy subnetworks, the Robustness Variable should losses occur. On lossy subnetworks, the Robustness Variable should
be increased to allow for the expected level of packet loss. be increased to allow for the expected level of packet loss.
However, increasing the Robustness Variable increases the leave However, increasing the Robustness Variable increases the leave
latency of the subnetwork. (The leave latency is the time between latency of the subnetwork. (The leave latency is the time between
when the last member stops listening to a source or group and when when the last member stops listening to a source or group and when
the traffic stops flowing.) the traffic stops flowing.)
8.14.2. Query Interval 8.14.2. Query Interval
The overall level of periodic IGMP traffic is inversely proportional The overall level of periodic IGMP traffic is inversely proportional
to the Query Interval. A longer Query Interval results in a lower to the Query Interval. A longer Query Interval results in a lower
overall level of IGMP traffic. The Query Interval MUST be equal to overall level of IGMP traffic. The Query Interval MUST be equal to
or longer than the Max Response Time inserted in General Query or longer than the Max Response Time inserted in General Query
messages. messages.
8.14.3. Max Response Time 8.14.3. Max Response Time
The burstiness of IGMP traffic is inversely proportional to the Max The burstiness of IGMP traffic is inversely proportional to the Max
Response Time. A longer Max Response Time will spread Report Response Time. A longer Max Response Time will spread Report
messages over a longer interval. However, a longer Max Response Time messages over a longer interval. However, a longer Max Response Time
in Group-Specific and Source-and-Group-Specific Queries extends the in Group Specific and Source-and-Group Specific Queries extends the
leave latency. (The leave latency is the time between when the last leave latency. (The leave latency is the time between when the last
member stops listening to a source or group and when the traffic member stops listening to a source or group and when the traffic
stops flowing.) The expected rate of Report messages can be stops flowing.) The expected rate of Report messages can be
calculated by dividing the expected number of Reporters by the Max calculated by dividing the expected number of Reporters by the Max
Response Time. The Max Response Time may be dynamically calculated Response Time. The Max Response Time may be dynamically calculated
per Query by using the expected number of Reporters for that Query as per Query by using the expected number of Reporters for that Query as
follows: follows:
+===========================+===============================+ +======================+============================================+
| Query Type | Expected Number of Reporters | | Query Type | Expected Number of Reporters |
+===========================+===============================+ +======================+============================================+
| General Query | All systems on the subnetwork | | General Query | All systems on the subnetwork |
+---------------------------+-------------------------------+ +----------------------+--------------------------------------------+
| Group-Specific Query | All systems that had | | Group Specific | All systems that had expressed interest in |
| | expressed interest in the | | Query | the group on the subnetwork |
| | group on the subnetwork | +----------------------+--------------------------------------------+
+---------------------------+-------------------------------+ | Source-and-Group | All systems on the subnetwork that had |
| Source-and-Group-Specific | All systems on the subnetwork | | Specific Query | expressed interest in the source and group |
| Query | that had expressed interest | +----------------------+--------------------------------------------+
| | in the source and group |
+---------------------------+-------------------------------+
Table 15 Table 15: Expected Number of IGMP Reporters
A router is not required to calculate these populations or tune the A router is not required to calculate these populations or tune the
Max Response Time dynamically; these are simply guidelines. Max Response Time dynamically; these are simply guidelines.
9. Security Considerations 9. Security Considerations
IGMP provides any form of confidentiality. This means any device on IGMP provides any form of confidentiality. This means any device on
a link can passively receive any IGMP message on the link. Such a link can passively receive any IGMP message on the link. Such
access can lead to privacy concerns around potentially sensitive access can lead to privacy concerns around potentially sensitive
multicast groups or the ability to identify/map the devices on a multicast groups or the ability to identify/map the devices on a
skipping to change at line 2206 skipping to change at line 2206
We consider the ramifications of a forged message of each type and We consider the ramifications of a forged message of each type and
describe the usage of an IPsec Authentication Header (AH) to describe the usage of an IPsec Authentication Header (AH) to
authenticate messages if desired. authenticate messages if desired.
9.1. Query Message 9.1. Query Message
A forged Query message from a machine with a lower IP address than A forged Query message from a machine with a lower IP address than
the current Querier will cause Querier duties to be assigned to the the current Querier will cause Querier duties to be assigned to the
forger. If the forger then sends no more Query messages, other forger. If the forger then sends no more Query messages, other
routers' Other Querier Present timer will time out and one will routers' Other-Querier-Present Timer will time out and one will
resume the role of Querier. During this time, if the forger ignores resume the role of Querier. During this time, if the forger ignores
Leave messages, traffic might flow to groups with no members for up Leave messages, traffic might flow to groups with no members for up
to [Group Membership Interval]. to [Group Membership Interval].
A Denial-of-Service (DoS) attack on a host could be staged through A Denial-of-Service (DoS) attack on a host could be staged through
forged Group-and- Source-Specific Queries. The attacker can find out forged Group-and- Source Specific Queries. The attacker can find out
about membership of a specific host with a general query. After about membership of a specific host with a General Query. After
that, it could send a large number of Group-and-Source-Specific that, it could send a large number of Group-and-Source Specific
queries, each with a large source list and the Maximum Response Time Queries, each with a large source list and the Maximum Response Time
set to a large value. The host will have to store and maintain the set to a large value. The host will have to store and maintain the
sources specified in all of those queries for as long as it takes to 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 send the delayed response. This would consume both memory and CPU
cycles in order to augment the recorded sources with the source lists cycles in order to augment the recorded sources with the source lists
included in the successive queries. included in the successive Queries.
To protect against such a DoS attack, a host stack implementation To protect against such a DoS attack, a host stack implementation
could restrict the number of Group-and-Source-Specific Queries per could restrict the number of Group-and-Source Specific Queries per
group membership within this interval and/or record only a limited group membership within this interval and/or record only a limited
number of sources. number of sources.
Forged Query messages from the local network can be easily traced. Forged Query messages from the local network can be easily traced.
There are three measures necessary to defend against externally There are three measures necessary to defend against externally
forged Queries: forged Queries:
* Routers SHOULD NOT forward Queries. This is easier for a router * Routers SHOULD NOT forward Queries. This is easier for a router
to accomplish if the Query carries the Router Alert option. to accomplish if the Query carries the Router Alert option.
skipping to change at line 2265 skipping to change at line 2265
SHOULD be accepted on any interface. SHOULD be accepted on any interface.
2. Ignore Report messages without Router Alert options [RFC2113] and 2. Ignore Report messages without Router Alert options [RFC2113] and
require routers to not forward Report messages. (The requirement require routers to not forward Report messages. (The requirement
is not a requirement of generalized filtering in the forwarding is not a requirement of generalized filtering in the forwarding
path, as the packets already have Router Alert options in them.) path, as the packets already have Router Alert options in them.)
This solution breaks backwards compatibility with implementations This solution breaks backwards compatibility with implementations
of IGMPv1 or earlier versions of IGMPv2 that did not require a of IGMPv1 or earlier versions of IGMPv2 that did not require a
Router Alert. Router Alert.
A forged Version 1 Report Message may put a router into "version 1 A forged v1 Report message may put a router into "v1 members present"
members present" state for a particular group, meaning that the state for a particular group, meaning that the router will ignore
router will ignore Leave messages. This can cause traffic to flow to Leave messages. This can cause traffic to flow to groups with no
groups with no members for up to [Group Membership Interval]. This members for up to [Group Membership Interval]. This can be solved by
can be solved by providing routers with a configuration switch to providing routers with a configuration switch to ignore v1 messages
ignore Version 1 messages completely. This breaks automatic completely. This breaks automatic compatibility with v1 hosts, so it
compatibility with Version 1 hosts, so it should only be used in should only be used in situations where "fast leave" is critical.
situations where "fast leave" is critical.
A forged Version 2 Report Message may put a router into "version 2 A forged v2 Report message may put a router into "v2 members present"
members present" state for a particular group, meaning that the state for a particular group, meaning that the router will ignore
router will ignore IGMPv3 source-specific state messages. This can IGMPv3 source-specific state messages. This can cause traffic to
cause traffic to flow from unwanted sources for up to [Group flow from unwanted sources for up to [Group Membership Interval].
Membership Interval]. This can be solved by providing routers with a This can be solved by providing routers with a configuration switch
configuration switch to ignore Version 2 messages completely. This to ignore v2 messages completely. This breaks automatic
breaks automatic compatibility with Version 2 hosts, so it should compatibility with v2 hosts, so it should only be used in situations
only be used in situations where source include and exclude is where source include and exclude is critical.
critical.
9.3. State-Change Report Messages 9.3. State-Change Report Messages
A forged State-Change Report message will cause the Querier to send A forged State-Change Report message will cause the Querier to send
out Group-Specific or Source-and-Group-Specific Queries for the group out Group Specific or Source-and-Group Specific Queries for the group
in question. This causes extra processing on each router and on each in question. This causes extra processing on each router and on each
member of the group, but it cannot cause loss of desired traffic. member of the group, but it cannot cause loss of desired traffic.
There are two defenses against externally forged State-Change Report There are two defenses against externally forged State-Change Report
messages: messages:
1. Ignore the State-Change Report message if you cannot identify the 1. Ignore the State-Change Report message if you cannot identify the
source address of the packet as belonging to a subnet assigned to source address of the packet as belonging to a subnet assigned to
the interface on which the packet was received. This solution the interface on which the packet was received. This solution
means that State-Change Report messages sent by mobile hosts means that State-Change Report messages sent by mobile hosts
without addresses on the local subnet will be ignored. State- without addresses on the local subnet will be ignored. State-
skipping to change at line 2338 skipping to change at line 2336
authenticated individually so, e.g., a host cannot forge a authenticated individually so, e.g., a host cannot forge a
message that only routers should be allowed to send. message that only routers should be allowed to send.
This solution only directly applies to Query and Leave messages in This solution only directly applies to Query and Leave messages in
IGMPv1 and IGMPv2 as Reports are sent to the group being reported, IGMPv1 and IGMPv2 as Reports are sent to the group being reported,
and it is not feasible to agree on a key for host-to-router and it is not feasible to agree on a key for host-to-router
communication for arbitrary multicast groups. communication for arbitrary multicast groups.
10. IANA Considerations 10. IANA Considerations
All IGMP types described in this document are managed via [RFC9778]. All IGMP types described in this document are managed via [BCP57].
IANA has replaced each reference to [RFC3376] with a reference to IANA has replaced each reference to [RFC3376] with a reference to
this document in both the "IGMP Type Numbers" and "IPFIX Information this document in both the "IGMP Type Numbers" and "IPFIX Information
Elements" registries. Elements" registries.
11. References 11. References
11.1. Normative References 11.1. Normative References
[BCP57] Best Current Practice 57,
<https://www.rfc-editor.org/info/bcp57>.
At the time of writing, this BCP comprises the following:
Haberman, B., Ed., "IANA Considerations for Internet Group
Management Protocols", BCP 57, RFC 9778,
DOI 10.17487/RFC9778, March 2025,
<https://www.rfc-editor.org/info/rfc9778>.
[RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5, [RFC1112] Deering, S., "Host extensions for IP multicasting", STD 5,
RFC 1112, DOI 10.17487/RFC1112, August 1989, RFC 1112, DOI 10.17487/RFC1112, August 1989,
<https://www.rfc-editor.org/info/rfc1112>. <https://www.rfc-editor.org/info/rfc1112>.
[RFC2113] Katz, D., "IP Router Alert Option", RFC 2113, [RFC2113] Katz, D., "IP Router Alert Option", RFC 2113,
DOI 10.17487/RFC2113, February 1997, DOI 10.17487/RFC2113, February 1997,
<https://www.rfc-editor.org/info/rfc2113>. <https://www.rfc-editor.org/info/rfc2113>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, Requirement Levels", BCP 14, RFC 2119,
skipping to change at line 2383 skipping to change at line 2390
August 2006, <https://www.rfc-editor.org/info/rfc4604>. August 2006, <https://www.rfc-editor.org/info/rfc4604>.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for [RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, DOI 10.17487/RFC4607, August 2006, IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
<https://www.rfc-editor.org/info/rfc4607>. <https://www.rfc-editor.org/info/rfc4607>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC [RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174, 2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>. May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9778] Haberman, B., Ed., "IANA Considerations for Internet Group
Management Protocols", BCP 57, RFC 9778,
DOI 10.17487/RFC9778, March 2025,
<https://www.rfc-editor.org/info/rfc9778>.
11.2. Informative References 11.2. Informative References
[RFC1071] Braden, R., Borman, D., and C. Partridge, "Computing the [RFC1071] Braden, R., Borman, D., and C. Partridge, "Computing the
Internet checksum", RFC 1071, DOI 10.17487/RFC1071, Internet checksum", RFC 1071, DOI 10.17487/RFC1071,
September 1988, <https://www.rfc-editor.org/info/rfc1071>. September 1988, <https://www.rfc-editor.org/info/rfc1071>.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A. [RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002, 3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<https://www.rfc-editor.org/info/rfc3376>. <https://www.rfc-editor.org/info/rfc3376>.
skipping to change at line 2413 skipping to change at line 2415
[RFC3678] Thaler, D., Fenner, B., and B. Quinn, "Socket Interface [RFC3678] Thaler, D., Fenner, B., and B. Quinn, "Socket Interface
Extensions for Multicast Source Filters", RFC 3678, Extensions for Multicast Source Filters", RFC 3678,
DOI 10.17487/RFC3678, January 2004, DOI 10.17487/RFC3678, January 2004,
<https://www.rfc-editor.org/info/rfc3678>. <https://www.rfc-editor.org/info/rfc3678>.
Appendix A. Design Rationale Appendix A. Design Rationale
A.1. The Need for State-Change Messages A.1. The Need for State-Change Messages
IGMPv3 specifies two types of Membership Reports: Current-State and IGMPv3 specifies two types of Membership Reports: Current-State and
State Change. This section describes the rationale for needing both State-Change. This section describes the rationale for needing both
types of Reports. types of Reports.
Routers need to distinguish Membership Reports that were sent in Routers need to distinguish Membership Reports that were sent in
response to Queries from those that were sent as a result of a change response to Queries from those that were sent as a result of a change
in interface state. Membership reports that are sent in response to in interface state. Membership reports that are sent in response to
Membership Queries are used mainly to refresh the existing state at Membership Queries are used mainly to refresh the existing state at
the router; they typically do not cause transitions in state at the the router; they typically do not cause transitions in state at the
router. Membership Reports that are sent in response to changes in router. Membership Reports that are sent in response to changes in
interface state require the router to take some action in response to interface state require the router to take some action in response to
the received report (see Section 6.4). the received report (see Section 6.4).
The inability to distinguish between the two types of reports would The inability to distinguish between the two types of reports would
force a router to treat all Membership Reports as potential changes force a router to treat all Membership Reports as potential changes
in state, and it could result in increased processing at the router in state, and it could result in increased processing at the router
as well as an increase in IGMP traffic on the network. as well as an increase in IGMP traffic on the network.
A.2. Host Suppression A.2. Host Suppression
In IGMPv1 and IGMPv2, a host would cancel sending pending membership In IGMPv1 and IGMPv2, a host would cancel sending pending Membership
reports if a similar report was observed from another member on the Reports if a similar report was observed from another member on the
network. In IGMPv3, this suppression of host membership reports has network. In IGMPv3, this suppression of host Membership Reports has
been removed. The following points explain the reasons behind this been removed. The following points explain the reasons behind this
decision. decision.
1. Routers may want to track per-host membership status on an 1. Routers may want to track per-host membership status on an
interface. This allows routers to implement fast leaves (e.g., interface. This allows routers to implement fast leaves (e.g.,
for layered multicast congestion control schemes) as well as for layered multicast congestion control schemes) as well as
track membership status for possible accounting purposes. track membership status for possible accounting purposes.
2. Membership Report suppression does not work well on bridged LANs. 2. Membership Report suppression does not work well on bridged LANs.
Many bridges and Layer 2 / Layer 3 switches that implement IGMP Many bridges and Layer 2 / Layer 3 switches that implement IGMP
snooping do not forward IGMP messages across LAN segments in snooping do not forward IGMP messages across LAN segments in
order to prevent membership report suppression. Removing order to prevent Membership Report suppression. Removing
membership report suppression eases the job of these IGMP Membership Report suppression eases the job of these IGMP
snooping devices. snooping devices.
3. By eliminating membership report suppression, hosts have fewer 3. By eliminating Membership Report suppression, hosts have fewer
messages to process; this leads to a simpler state machine messages to process; this leads to a simpler state machine
implementation. implementation.
4. In IGMPv3, a single membership report now bundles multiple 4. In IGMPv3, a single Membership Report now bundles multiple
multicast group records to decrease the number of packets sent. multicast Group Records to decrease the number of packets sent.
In comparison, the previous versions of IGMP required that each In comparison, the previous versions of IGMP required that each
multicast group be reported in a separate message. multicast group be reported in a separate message.
A.3. Switching Router Filter Modes from EXCLUDE to INCLUDE A.3. Switching Router Filter Modes from EXCLUDE to INCLUDE
If hosts exist in both EXCLUDE and INCLUDE modes for a single If hosts exist in both EXCLUDE and INCLUDE modes for a single
multicast group in a network, the router must be in EXCLUDE mode as multicast group in a network, the router must be in EXCLUDE mode as
well (see Section 6.2.1). In EXCLUDE mode, a router forwards traffic well (see Section 6.2.1). In EXCLUDE mode, a router forwards traffic
from all sources unless that source exists in the exclusion source from all sources unless that source exists in the exclusion source
list. If all hosts in EXCLUDE mode cease to exist, it would be list. If all hosts in EXCLUDE mode cease to exist, it would be
desirable for the router to switch back to INCLUDE mode seamlessly desirable for the router to switch back to INCLUDE mode seamlessly
without interrupting the flow of traffic to existing receivers. without interrupting the flow of traffic to existing receivers.
One of the ways to accomplish this is for routers to keep track of One of the ways to accomplish this is for routers to keep track of
all sources desired by hosts that are in INCLUDE mode even though the all sources desired by hosts that are in INCLUDE mode even though the
router itself is in EXCLUDE mode. If the group timer now expires in router itself is in EXCLUDE mode. If the Group Timer now expires in
EXCLUDE mode, it implies that there are no hosts in EXCLUDE mode on EXCLUDE mode, it implies that there are no hosts in EXCLUDE mode on
the network (otherwise, a membership report from that host would have the network (otherwise, a Membership Report from that host would have
refreshed the group timer). The router can then switch to INCLUDE refreshed the Group Timer). The router can then switch to INCLUDE
mode seamlessly with the list of sources currently being forwarded in mode seamlessly with the list of sources currently being forwarded in
its source list. its source list.
Appendix B. Summary of Changes from IGMPv2 Appendix B. Summary of Changes from IGMPv2
While the main additional feature of IGMPv3 is the addition of source While the main additional feature of IGMPv3 is the addition of source
filtering, the following is a summary of other changes from filtering, the following is a summary of other changes from
[RFC2236]. [RFC2236].
* State is maintained as Group + List-of-Sources, not simply Group * State is maintained as Group + List-of-Sources, not simply Group
skipping to change at line 2508 skipping to change at line 2510
changing the maximum from 25.5 seconds to about 53 minutes, for changing the maximum from 25.5 seconds to about 53 minutes, for
use on links with a huge number of systems. use on links with a huge number of systems.
* Hosts retransmit state-change messages for increased robustness. * Hosts retransmit state-change messages for increased robustness.
* Additional data sections are defined, to allow later extensions. * Additional data sections are defined, to allow later extensions.
* Report packets are sent to 224.0.0.22, to assist Layer 2 switches * Report packets are sent to 224.0.0.22, to assist Layer 2 switches
in snooping. in snooping.
* Report packets can contain multiple group records, to allow * Report packets can contain multiple Group Records, to allow
reporting of full current state using fewer packets. reporting of full current state using fewer packets.
* Hosts no longer perform suppression, to simplify implementations * Hosts no longer perform suppression, to simplify implementations
and permit explicit membership tracking. and permit explicit membership tracking.
* A new S flag in Query messages fixes robustness issues, which were * A new S flag in Query messages fixes robustness issues, which were
also present in IGMPv2. also present in IGMPv2.
Appendix C. Summary of Changes from RFC 3376 Appendix C. Summary of Changes from RFC 3376
skipping to change at line 2534 skipping to change at line 2536
* Modified the metadata to fix the Obsoletes vs. Updates * Modified the metadata to fix the Obsoletes vs. Updates
relationship with [RFC2236] per Erratum 1501. relationship with [RFC2236] per Erratum 1501.
* Updated the introductory text to describe the Updates relationship * Updated the introductory text to describe the Updates relationship
with [RFC2236] per Erratum 7339. with [RFC2236] per Erratum 7339.
* Updated the definition of Group Membership Interval to address * Updated the definition of Group Membership Interval to address
Erratum 6725. Erratum 6725.
* Updated the text relating to the router filter-mode to address * Updated the text relating to the Router Filter Mode to address
Erratum 5562. Erratum 5562.
* Clarified the use of General Queries in the Querier election * Clarified the use of General Queries in the Querier election
process. process.
Acknowledgments Acknowledgments
We would like to thank Ran Atkinson, Luis Costa, Toerless Eckert, We would like to thank Ran Atkinson, Luis Costa, Toerless Eckert,
Dino Farinacci, Serge Fdida, Wilbert de Graaf, Sumit Gupta, Mark Dino Farinacci, Serge Fdida, Wilbert de Graaf, Sumit Gupta, Mark
Handley, Bob Quinn, Michael Speer, Dave Thaler, and Rolland Vida for Handley, Bob Quinn, Michael Speer, Dave Thaler, and Rolland Vida for
 End of changes. 186 change blocks. 
371 lines changed or deleted 373 lines changed or added

This html diff was produced by rfcdiff 1.48.