Internet Engineering Task Force (IETF) B. Haberman, Ed.
Request for Comments: 9776 JHU APL
STD: 100 March 2025
Obsoletes: 3376
Updates: 2236
Category: Standards Track
ISSN: 2070-1721
Internet Group Management Protocol, Version 3
Abstract
The Internet Group Management Protocol (IGMP) is the protocol used by
IPv4 systems to report their IP multicast group memberships to
neighboring multicast routers. Version 3 of IGMP (IGMPv3) adds
support for source filtering, that is, the ability for a system to
report interest in receiving packets only from specific source
addresses, or from all but specific source addresses, sent to a
particular multicast address. That information may be used by
multicast routing protocols to avoid delivering multicast packets
from specific sources to networks where there are no interested
receivers.
This document specifies IGMPv3. It is a revised version of RFC 3376
that includes clarifications and fixes for errata, and it is backward
compatible with RFC 3376.
This document updates RFC 2236 and obsoletes RFC 3376.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9776.
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Table of Contents
1. Introduction
1.1. Conventions Used in This Document
2. The Service Interface for Requesting IP Multicast Reception
3. Multicast Reception State Maintained by Systems
3.1. Socket State
3.2. Interface State
4. Message Formats
4.1. Membership Query Message
4.1.1. Max Resp Code
4.1.2. Checksum
4.1.3. Group Address
4.1.4. Flags
4.1.5. S Flag (Suppress Router-Side Processing)
4.1.6. QRV (Querier's Robustness Variable)
4.1.7. QQIC (Querier's Query Interval Code)
4.1.8. Number of Sources (N)
4.1.9. Source Address [i]
4.1.10. Additional Data
4.1.11. Query Variants
4.1.12. IP Destination Addresses for Queries
4.2. Version 3 IGMPv3 Membership Report Message
4.2.1. Reserved
4.2.2. Checksum
4.2.3. Flags
4.2.4. Number of Group Records (M)
4.2.5. Group Record
4.2.6. Record Type
4.2.7. Aux Data Len
4.2.8. Number of Sources (N)
4.2.9. Multicast Address
4.2.10. Source Address [i]
4.2.11. Auxiliary Data
4.2.12. Additional Data
4.2.13. Group Record Types
4.2.14. IP Source Addresses for Reports
4.2.15. IP Destination Addresses for Reports
4.2.16. Notation for Group Records
4.2.17. Membership Report Size
5. Description of the Protocol for Group Members
5.1. Action on Change of Interface State
5.2. Action on Reception of a Query
6. Description of the Protocol for Multicast Routers
6.1. Conditions for IGMP Queries
6.2. IGMP State Maintained by Multicast Routers
6.2.1. Definition of Router Filter-Mode Filter Mode
6.2.2. Definition of Group Timers
6.2.3. Definition of Source Timers
6.3. IGMPv3 Source-Specific Forwarding Rules
6.4. Action on Reception of Reports
6.4.1. Reception of Current-State Records
6.4.2. Reception of Filter-Mode-Change and Source-List-Change
Records
6.5. Switching Router Filter-Modes Filter Modes
6.6. Action on Reception of Queries
6.6.1. Timer Updates
6.6.2. Querier Election
6.6.3. Building and Sending Specific Queries
7. Interoperation With Older Versions of IGMP
7.1. Query Version Distinctions
7.2. Group Member Behavior
7.2.1. In the Presence of Older Version Queriers
7.2.2. In the Presence of Older Version Group Members
7.3. Multicast Router Behavior
7.3.1. In the Presence of Older Version Queriers
7.3.2. In the Presence of Older Version Group Members
8. List of Timers, Counters, and Their Default Values
8.1. Robustness Variable
8.2. Query Interval
8.3. Query Response Interval
8.4. Group Membership Interval
8.5. Other Querier Present Interval
8.6. Startup Query Interval
8.7. Startup Query Count
8.8. Last Member Query Interval
8.9. Last Member Query Count
8.10. Last Member Query Time
8.11. Unsolicited Report Interval
8.12. Older Version Querier Present Interval
8.13. Older Host Present Interval
8.14. Configuring Timers
8.14.1. Robustness Variable
8.14.2. Query Interval
8.14.3. Max Response Time
9. Security Considerations
9.1. Query Message
9.2. Current-State Report Messages
9.3. State-Change Report Messages
9.4. IPsec Usage
10. IANA Considerations
11. References
11.1. Normative References
11.2. Informative References
Appendix A. Design Rationale
A.1. The Need for State-Change Messages
A.2. Host Suppression
A.3. Switching Router Filter Modes from EXCLUDE to INCLUDE
Appendix B. Summary of Changes from IGMPv2
Appendix C. Summary of Changes from RFC 3376
Acknowledgments
Contributors
Author's Address
1. Introduction
The Internet Group Management Protocol (IGMP) is used by IPv4 systems
(hosts and routers) to report their IP multicast group memberships to
any neighboring multicast routers. Note that an IP multicast router
may itself be a member of one or more multicast groups, in which case
it performs both the multicast router part of the protocol (to
collect the membership information needed by its multicast routing
protocol) and the group member part of the protocol (to inform itself
and other, neighboring multicast routers of its memberships).
IGMP is also used for other IP multicast management functions, using
message types other than those used for group membership reporting.
This document specifies only the group membership reporting functions
and messages.
This document specifies Version 3 of IGMP. Version 1, specified in
[RFC1112], was the first widely deployed version and the first
version to become an Internet Standard. Version 2, specified in
[RFC2236], added support for low leave latency, that is, a reduction
in the time it takes for a multicast router to learn that there are
no longer any members of a particular group present on an attached
network. Version 3 adds support for source filtering, that is, the
ability for a system to report interest in receiving packets only
from specific source addresses, as required to support Source-
Specific Multicast (SSM) [RFC3569], or from all but specific source
addresses, sent to a particular multicast address. Version 3 is
designed to be interoperable with Versions 1 and 2.
This document uses "SSM-aware" to refer to systems that support SSM
as defined in [RFC4607].
This document updates [RFC2236] as a proper implementation of Version
3 of IGMP IGMPv3
needs to implement Version 2 IGMPv2 Report and Leave message handling.
This document obsoletes [RFC3376] as it provides clarifications and
fixes for errata in [RFC3376]. Detailed updates for those changes
are described in Appendix C.
1.1. Conventions Used in This Document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2. The Service Interface for Requesting IP Multicast Reception
Within an IP system, there is (at least conceptually) a service
interface used by upper-layer protocols or application programs to
ask the IP layer to enable and disable reception of packets sent to
specific IP multicast addresses. In order to take full advantage of
the capabilities of IGMPv3, a system's IP service interface must
support the following operation:
IPMulticastListen ( socket, interface, multicast-address,
filter-mode, source-list )
where:
* "socket" is an implementation-specific parameter used to
distinguish among different requesting entities (e.g., programs or
processes) within the system; the socket parameter of BSD Unix
system calls is a specific example.
* "interface" is a local identifier of the network interface on
which reception of the specified multicast address is to be
enabled or disabled. Interfaces may be physical (e.g., an
Ethernet interface) or virtual (e.g., the endpoint of a Frame
Relay virtual circuit or the endpoint of an IP-in-IP "tunnel").
An implementation may allow a special "unspecified" value to be
passed as the interface parameter, in which case the request would
apply to the "primary" or "default" interface of the system
(perhaps established by system configuration). If reception of
the same multicast address is desired on more than one interface,
IPMulticastListen is invoked separately for each desired
interface.
* "multicast-address" is the IP multicast address, or group, to
which the request pertains. If reception of more than one
multicast address on a given interface is desired,
IPMulticastListen is invoked separately for each desired multicast
address.
* "filter-mode" may be either INCLUDE or EXCLUDE. In INCLUDE mode,
reception of packets sent to the specified multicast address is
requested only from those IP source addresses listed in the
source-list parameter. In EXCLUDE mode, reception of packets sent
to the given multicast address is requested from all IP source
addresses except those listed in the source-list parameter.
* "source-list" is an unordered list of zero or more IP unicast
addresses from which multicast reception is desired or not
desired, depending on the filter mode. An implementation MAY
impose a limit on the size of source lists, but that limit MUST
NOT be less than 64 addresses per list. When an operation causes
the source list size limit to be exceeded, the service interface
MUST return an error.
For a given combination of socket, interface, and multicast address,
only a single filter mode and source list can be in effect at any one
time. However, either the filter mode or the source list, or both,
may be changed by subsequent IPMulticastListen requests that specify
the same socket, interface, and multicast address. Each subsequent
request completely replaces any earlier request for the given socket,
interface, and multicast address.
Previous versions of IGMP did not support source filters and had a
simpler service interface consisting of Join and Leave operations to
enable and disable reception of a given multicast address (from all
sources) on a given interface. The equivalent operations in the new
service interface follow:
The Join operation is equivalent to:
IPMulticastListen ( socket, interface, multicast-address,
EXCLUDE, {} )
and the Leave operation is equivalent to:
IPMulticastListen ( socket, interface, multicast-address,
INCLUDE, {} )
where {} is an empty source list.
An example of an API providing the capabilities outlined in this
service interface is in [RFC3678].
3. Multicast Reception State Maintained by Systems
3.1. Socket State
For each socket on which IPMulticastListen has been invoked, the
system records the desired multicast reception state for that socket.
That state conceptually consists of a set of records of the form:
(interface, multicast-address, filter-mode, source-list)
The socket state evolves in response to each invocation of
IPMulticastListen on the socket, as follows:
* If the requested filter mode is INCLUDE and the requested source
list is empty, then the entry corresponding to the requested
interface and multicast address is deleted if present. If no such
entry is present, the request is ignored.
* If the requested filter mode is EXCLUDE or the requested source
list is non-empty, then the entry corresponding to the requested
interface and multicast address, if present, is changed to contain
the requested filter mode and source list. If no such entry is
present, a new entry is created, using the parameters specified in
the request.
3.2. Interface State
In addition to the per-socket multicast reception state, a system
must also maintain or compute multicast reception state for each of
its interfaces. That state conceptually consists of a set of records
of the form:
(multicast-address, filter-mode, source-list)
At most, one record per multicast-address exists for a given
interface. This per-interface state is derived from the per-socket
state, but it may differ from the per-socket state when different
sockets have differing filter modes and/or source lists for the same
multicast address and interface. For example, suppose one
application or process invokes the following operation on socket s1:
IPMulticastListen ( s1, i, m, INCLUDE, {a, b, c} )
requesting reception on interface i of packets sent to multicast
address m, only if they come from source a, b, or c. Suppose another
application or process invokes the following operation on socket s2:
IPMulticastListen ( s2, i, m, INCLUDE, {b, c, d} )
requesting reception on the same interface i of packets sent to the
same multicast address m, only if they come from sources b, c, or d.
In order to satisfy the reception requirements of both sockets, it is
necessary for interface i to receive packets sent to m from any one
of the sources a, b, c, or d. Thus, in this example, the reception
state of interface i for multicast address m has filter mode INCLUDE
and source list {a, b, c, d}.
After a multicast packet has been accepted from an interface by the
IP layer, its subsequent delivery to the application or process
listening on a particular socket depends on the multicast reception
state of that socket (and possibly also on other conditions, such as
what transport-layer port the socket is bound to). So, in the above
example, if a packet arrives on interface i, destined to multicast
address m, with source address a, it will be delivered on socket s1
but not on socket s2. Note that IGMP Queries and Reports are not
subject to source filtering and must always be processed by hosts and
routers.
Filtering of packets based upon a socket's multicast reception state
is a new feature of this service interface. The previous service
interface [RFC1112] described no filtering based upon multicast join
state; rather, a join on a socket simply caused the host to join a
group on the given interface, and packets destined for that group
could be delivered to all sockets whether they had joined or not.
The general rules for deriving the per-interface state from the per-
socket state are as follows: For each distinct (interface, multicast-
address) pair that appears in any socket state, a per-interface
record is created for that multicast address on that interface.
Considering all socket records containing the same (interface,
multicast-address) pair,
* if any such record has a filter mode of EXCLUDE, then the filter
mode of the interface record is EXCLUDE, and the source list of
the interface record is the intersection of the source lists of
all socket records in EXCLUDE mode, minus those source addresses
that appear in any socket record in INCLUDE mode. For example, if
the socket records for multicast address m on interface i are:
- from socket s1: ( i, m, EXCLUDE, {a, b, c, d} )
- from socket s2: ( i, m, EXCLUDE, {b, c, d, e} )
- from socket s3: ( i, m, INCLUDE, {d, e, f} )
then the corresponding interface record on interface i is:
- ( m, EXCLUDE, {b, c} )
If a fourth socket is added, such as:
- from socket s4: ( i, m, EXCLUDE, {} )
then the interface record becomes:
- ( m, EXCLUDE, {} )
* if all such records have a filter mode of INCLUDE, then the filter
mode of the interface record is INCLUDE, and the source list of
the interface record is the union of the source lists of all the
socket records. For example, if the socket records for multicast
address m on interface i are:
- from socket s1: ( i, m, INCLUDE, {a, b, c} )
- from socket s2: ( i, m, INCLUDE, {b, c, d} )
- from socket s3: ( i, m, INCLUDE, {e, f} )
then the corresponding interface record on interface i is:
- ( m, INCLUDE, {a, b, c, d, e, f} )
An implementation MUST NOT use an EXCLUDE interface record to
represent a group when all sockets for this group are in INCLUDE
state. If system resource limits are reached when an interface
state source list is calculated, an error MUST be returned to the
application that requested the operation.
The above rules for deriving the interface state are (re-)evaluated
whenever an IPMulticastListen invocation modifies the socket state by
adding, deleting, or modifying a per-socket state record. Note that
a change of socket state does not necessarily result in a change of
interface state.
4. Message Formats
IGMP messages are encapsulated in IPv4 datagrams, with an IP protocol
number of 2. Every IGMP message described in this document is sent
with an IP Time-to-Live of 1, IP Precedence of Internetwork Control
(e.g., Type of Service 0xc0), and carries an IP Router Alert option
[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
described in this document:
+===================+=============================+
+===================+==========================+
| Type Number (hex) | Message Name |
+===================+=============================+
+===================+==========================+
| 0x11 | Membership Query |
+-------------------+-----------------------------+
+-------------------+--------------------------+
| 0x22 | Version 3 IGMPv3 Membership Report |
+-------------------+-----------------------------+
+-------------------+--------------------------+
Table 1: New Messages Introduced by IGMPv3
An implementation of IGMPv3 MUST also support the following three
message types, for interoperation with previous versions of IGMP (see
Section 7):
+===================+=============================+===========+
+===================+==========================+===========+
| Type Number (hex) | Message Name | Reference |
+===================+=============================+===========+
+===================+==========================+===========+
| 0x12 | Version 1 IGMPv1 Membership Report | [RFC1112] |
+-------------------+-----------------------------+-----------+
+-------------------+--------------------------+-----------+
| 0x16 | Version 2 IGMPv2 Membership Report | [RFC2236] |
+-------------------+-----------------------------+-----------+
+-------------------+--------------------------+-----------+
| 0x17 | Version 2 IGMPv2 Leave Group | [RFC2236] |
+-------------------+-----------------------------+-----------+
+-------------------+--------------------------+-----------+
Table 2: Legacy IGMP Messages
Unrecognized message types MUST be silently ignored. Other message
types may be used by newer versions or extensions of IGMP, by
multicast routing protocols, or for other uses.
In this document, unless otherwise qualified, the capitalized words
"Query" and "Report" refer to IGMP Membership Queries and IGMP
Version 3 IGMPv3
Membership Reports, respectively.
4.1. Membership Query Message
Membership Queries are sent by IP multicast routers to query the
multicast reception state of neighboring interfaces. Queries have
the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x11 | Max Resp Code | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Group Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags |S| QRV | QQIC | Number of Sources (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address [1] |
+- -+
| Source Address [2] |
+- . -+
. . .
. . .
+- -+
| Source Address [N] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1: IGMPv3 Query Message
4.1.1. Max Resp Code
The Max Resp Code field specifies the maximum time allowed before
sending a responding report. The actual time allowed, called the
"Max Resp Response Time", is represented in units of 1/10 second and is
derived from the Max Resp Code as follows:
* If Max Resp Code < 128, Max Resp Response Time = Max Resp Code
* If Max Resp Code >= 128, Max Resp Code represents a floating-point
value as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1| exp | mant |
+-+-+-+-+-+-+-+-+
Max Resp Response Time = (mant | 0x10) << (exp + 3)
Figure 2: Max Resp Code Representation
Small values of Max Resp Response Time allow IGMPv3 routers to tune the
"leave latency" (the time between the moment the last host leaves a
group and the moment the routing protocol is notified that there are
no more members). Larger values, especially in the exponential
range, allow tuning of the burstiness of IGMP traffic on a network.
4.1.2. Checksum
The Checksum field is the 16-bit one's complement of the one's
complement sum of the whole IGMP message (the entire IP payload).
For computing the checksum, the Checksum field is set to zero. When
receiving packets, the checksum MUST be verified before processing a
packet [RFC1071].
4.1.3. Group Address
The Group Address field is set to zero when sending a General Query
and set to the IP multicast address being queried when sending a
Group-Specific Query or Group-and-Source-Specific Group-and-Source Specific Query (see
Section 4.1.9, 4.1.11, below).
4.1.4. Flags
The Flags field is a bitstring managed by the "IGMP Type Numbers"
registry defined in [RFC9778]. [BCP57].
4.1.5. S Flag (Suppress Router-Side Processing)
When set to one, the S flag indicates to any receiving multicast
routers that they are to suppress the normal timer updates they
perform upon hearing receiving a Query. It does not, however, suppress the
querier
Querier election or the normal "host-side" processing of a Query that
a router may be required to perform as a consequence of itself being
a group member.
4.1.6. QRV (Querier's Robustness Variable)
If non-zero, the QRV field contains the [Robustness Variable] value
used by the querier, i.e., the sender of the Query. If the querier's
[Robustness Variable] exceeds 7, the maximum value of the QRV field,
the QRV is set to zero. Routers adopt the QRV value from the most
recently received Query as their own [Robustness Variable] value,
unless that most recently received QRV was zero, in which case the
receivers use the default [Robustness Variable] value specified in
Section 8.1 or a statically configured value.
4.1.7. QQIC (Querier's Query Interval Code)
The QQIC field specifies the [Query Interval] used by the querier.
The actual interval, called the "Querier's Query Interval (QQI)", is
represented in units of seconds and is derived from the QQIC as
follows:
* If QQIC < 128, QQI = QQIC
* If QQIC >= 128, QQIC represents a floating-point value as follows:
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|1| exp | mant |
+-+-+-+-+-+-+-+-+
QQI = (mant | 0x10) << (exp + 3)
Figure 3: QQIC Representation
Multicast routers that are not the current querier adopt the QQI
value from the most recently received Query as their own [Query
Interval] value, unless that most recently received QQI was zero, in
which case the receiving routers use the default [Query Interval]
value specified in Section 8.2.
4.1.8. Number of Sources (N)
The Number of Sources (N) field specifies how many source addresses
are present in the Query. This number is zero in a General Query or
a Group-Specific Group Specific Query and non-zero in a Group-and-Source-Specific Group-and-Source Specific
Query. This number is limited by the MTU of the network over which
the Query is transmitted. For example, on an Ethernet with an MTU of
1500 octets, the IP header including the Router Alert option consumes
24 octets, and the IGMP fields up to and including the Number of
Sources (N) field consume 12 octets, leaving 1464 octets for source
addresses, which limits the number of source addresses to 366
(1464/4).
4.1.9. Source Address [i]
The Source Address [i] fields are a vector of n IP unicast addresses,
where n is the value in the Number of Sources (N) field.
4.1.10. Additional Data
If the Packet Length field in the IP header of a received Query
indicates that there are additional octets of data present, beyond
the fields described here, IGMPv3 implementations MUST include those
octets in the computation to verify the received IGMP Checksum but
MUST otherwise ignore those additional octets. When sending a Query,
an IGMPv3 implementation MUST NOT include additional octets beyond
the fields described here.
4.1.11. Query Variants
There are three variants of the Query message:
1. A General Query is sent by a multicast router to learn the
complete multicast reception state of the neighboring interfaces
(that is, the interfaces attached to the network on which the
Query is transmitted). In a General Query, both the Group
Address field and the Number of Sources (N) field are zero.
2. A Group-Specific Group Specific Query is sent by a multicast router to learn the
reception state, with respect to a single multicast address, of
the neighboring interfaces. In a Group-Specific Group Specific Query, the Group
Address field contains the multicast address of interest, and the
Number of Sources (N) field contains zero.
3. A Group-and-Source-Specific Group-and-Source Specific Query is sent by a multicast router
to learn if any neighboring interface desires reception of
packets sent to a specified multicast address, from any of a
specified list of sources. In a Group-and-Source-Specific Group-and-Source Specific Query,
the Group Address field contains the multicast address of
interest, and the Source Address [i] fields contain the source
address(es) of interest.
4.1.12. IP Destination Addresses for Queries
In IGMPv3, General Queries are sent with an IP destination address of
224.0.0.1, the all-systems multicast address. Group-Specific Group Specific and
Group-and-Source-Specific
Group-and-Source Specific Queries are sent with an IP destination
address equal to the multicast address of interest. However, a
system MUST accept and process any Query whose IP Destination Address
field contains any of the addresses (unicast or multicast) assigned
to the interface on which the Query arrives.
4.2. Version 3 IGMPv3 Membership Report Message
Version 3
IGMPv3 Membership Reports are sent by IP systems to report (to
neighboring routers) the current multicast reception state, or
changes in the multicast reception state, of their interfaces.
Reports have the following format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type = 0x22 | Reserved | Checksum |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Flags | Number of Group Records (M) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Group Record [1] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Group Record [2] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . |
. . .
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Group Record [M] .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4: IGMPv3 Report Message
where each Group Record has the following internal format:
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Record Type | Aux Data Len | Number of Sources (N) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Multicast Address |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Source Address [1] |
+- -+
| Source Address [2] |
+- -+
. . .
. . .
. . .
+- -+
| Source Address [N] |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| |
. .
. Auxiliary Data .
. .
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 5: IGMPv3 Report Group Record
4.2.1. Reserved
The Reserved field is set to zero on transmission and ignored on
reception.
4.2.2. Checksum
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).
For computing the checksum, the Checksum field is set to zero. When
receiving packets, the checksum MUST be verified before processing a
message.
4.2.3. Flags
The Flags field is a bitstring managed by the "IGMP Type Numbers"
registry defined in [RFC9778]. [BCP57].
4.2.4. Number of Group Records (M)
The Number of Group Records (M) field specifies how many Group
Records are present in this Report.
4.2.5. Group Record
Each Group Record is a block of fields containing information
pertaining to the sender's membership in a single multicast group on
the interface from which the Report is sent.
4.2.6. Record Type
See Section 4.2.13, below.
4.2.7. Aux Data Len
The Aux Data Len field contains the length of the Auxiliary Data
field in this Group Record, in units of 32-bit words. It may contain
zero, to indicate the absence of any auxiliary data.
4.2.8. Number of Sources (N)
The Number of Sources (N) field specifies how many source addresses
are present in this Group Record.
4.2.9. Multicast Address
The Multicast Address field contains the IP multicast address to
which this Group Record pertains.
4.2.10. Source Address [i]
The Source Address [i] fields are a vector of n IP unicast addresses,
where n is the value in this record's Number of Sources (N) field.
4.2.11. Auxiliary Data
The Auxiliary Data field, if present, contains additional information
pertaining to this Group Record. The protocol specified in this
document, IGMPv3, does not define any auxiliary data. Therefore,
implementations of IGMPv3 MUST NOT include any auxiliary data (i.e.,
MUST set the Aux Data Len field to zero) in any transmitted Group
Record and MUST ignore any auxiliary data present in any received
Group Record. The semantics and internal encoding of the Auxiliary
Data field are to be defined by any future version or extension of
IGMP that uses this field.
4.2.12. Additional Data
If the Packet Length field in the IP header of a received Report
indicates that there are additional octets of data present, beyond
the last Group Record, IGMPv3 implementations MUST include those
octets in the computation to verify the received IGMP Checksum but
MUST otherwise ignore those additional octets. When sending a
Report, an IGMPv3 implementation MUST NOT include additional octets
beyond the last Group Record.
4.2.13. Group Record Types
There are a number of different types of Group Records that may be
included in a Report message:
* A Current-State Record is sent by a system in response to a Query
received on an interface. It reports the current reception state
of that interface, with respect to a single multicast address.
The Record Type of a Current-State Record may be one of the
following two values:
1. MODE_IS_INCLUDE - indicates that the interface has a filter
mode of INCLUDE for the specified multicast address. The
Source Address [i] fields in this Group Record contain the
interface's source list for the specified multicast address,
if it is non-empty.
2. MODE_IS_EXCLUDE - indicates that the interface has a filter
mode of EXCLUDE for the specified multicast address. The
Source Address [i] fields in this Group Record contain the
interface's source list for the specified multicast address,
if it is non-empty. An SSM-aware host SHOULD NOT send a
MODE_IS_EXCLUDE record type for multicast addresses that fall
within the SSM address range as they will be ignored by SSM-
aware routers [RFC4604].
* A Filter-Mode-Change Record is sent by a system whenever a local
invocation of IPMulticastListen causes a change of the filter mode
(i.e., a change from INCLUDE to EXCLUDE, or from EXCLUDE to
INCLUDE) of the interface-level state entry for a particular
multicast address. The Record is included in a Report sent from
the interface on which the change occurred. The Record Type of a
Filter-Mode-Change Record may be one of the following two values:
3. CHANGE_TO_INCLUDE_MODE - indicates that the interface has
changed to INCLUDE filter mode for the specified multicast
address. The Source Address [i] fields in this Group Record
contain the interface's new source list for the specified
multicast address, if it is non-empty.
4. CHANGE_TO_EXCLUDE_MODE - indicates that the interface has
changed to EXCLUDE filter mode for the specified multicast
address. The Source Address [i] fields in this Group Record
contain the interface's new source list for the specified
multicast address, if it is non-empty. An SSM-aware host
SHOULD NOT send a CHANGE_TO_EXCLUDE_MODE record type for
multicast addresses that fall within the SSM address range.
* A Source-List-Change Record is sent by a system whenever a local
invocation of IPMulticastListen causes a change of the source list
that is not coincident with a change of the filter mode, of the
interface-level state entry for a particular multicast address.
The Record is included in a Report sent from the interface on
which the change occurred. The Record Type of a Source-List-
Change Record may be one of the following two values:
5. ALLOW_NEW_SOURCES - indicates that the Source Address [i]
fields in this Group Record contain a list of the additional
sources that the system wishes to hear from, receive, for packets sent to
the specified multicast address. If the change was to an
INCLUDE source list, these are the addresses that were added
to the list; if the change was to an EXCLUDE source list,
these are the addresses that were deleted from the list.
6. BLOCK_OLD_SOURCES - indicates that the Source Address [i]
fields in this Group Record contain a list of the sources that
the system no longer wishes to hear from, receive, for packets sent to
the specified multicast address. If the change was to an
INCLUDE source list, these are the addresses that were deleted
from the list; if the change was to an EXCLUDE source list,
these are the addresses that were added to the list.
If a change of source list results in both allowing new sources and
blocking old sources, then two Group Records are sent for the same
multicast address, one of type ALLOW_NEW_SOURCES and one of type
BLOCK_OLD_SOURCES.
We use the term "State-Change Record" to refer to either a Filter-
Mode-Change Record or a Source-List-Change Record.
Unrecognized Record Type values MUST be silently ignored.
4.2.14. IP Source Addresses for Reports
An IGMP report is sent with a valid unicast IPv4 source address for
the destination subnet. The 0.0.0.0 source address may be used by a
system that has not yet acquired an IP address. Note that the
0.0.0.0 source address may simultaneously be used by multiple systems
on a LAN. Routers MUST accept a report with a source address of
0.0.0.0.
4.2.15. IP Destination Addresses for Reports
Version 3 Reports are sent with an IP destination address of
224.0.0.22, to which all IGMPv3-capable multicast routers listen. A
system that is operating in version 1 v1 or version 2 v2 compatibility modes sends version 1 v1 or version 2
v2 Reports to the multicast group specified in the Group Address
field of the Report. In addition, a system MUST accept and process
any version 1 v1 or version 2 v2 Report whose IP Destination Address field contains any
of the addresses (unicast or multicast) assigned to the interface on
which the Report arrives.
4.2.16. Notation for Group Records
In the rest of this document, we use the following notation to
describe the contents of a Group Record pertaining to a particular
multicast address:
* IS_IN ( x ) - Type MODE_IS_INCLUDE, source addresses x
* IS_EX ( x ) - Type MODE_IS_EXCLUDE, source addresses x
* TO_IN ( x ) - Type CHANGE_TO_INCLUDE_MODE, source addresses x
* TO_EX ( x ) - Type CHANGE_TO_EXCLUDE_MODE, source addresses x
* ALLOW ( x ) - Type ALLOW_NEW_SOURCES, source addresses x
* BLOCK ( x ) - Type BLOCK_OLD_SOURCES, source addresses x
where x is either:
* a capital letter (e.g., "A") to represent the set of source
addresses or
* a set expression (e.g., "A+B"), where "A+B" means the union of
sets A and B, "A*B" means the intersection of sets A and B, and
"A-B" means the removal of all elements of set B from set A.
4.2.17. Membership Report Size
If the set of Group Records required in a Report report does not fit within
the size limit of a single Report message (as determined by the MTU
of the network on which it will be sent), the Group Records are sent
in as many Report messages as needed to report the entire set.
If a single Group Record contains so many source addresses that it
does not fit within the size limit of a single Report message, and if
its Type is not MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, it is
split into multiple Group Records, each containing a different subset
of the source addresses and each sent in a separate Report message.
If its Type is MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE, a single
Group Record is sent, containing as many source addresses as can fit,
and the remaining source addresses are not reported; though the
choice of which sources to report is arbitrary, it is preferable to
report the same set of sources in each subsequent report, rather than
reporting different sources each time.
5. Description of the Protocol for Group Members
IGMP is an asymmetric protocol, specifying separate behaviors for
group members -- that is, hosts or routers that wish to receive
multicast packets -- and multicast routers. This section describes
the part of IGMPv3 that applies to all group members. (Note that a
multicast router that is also a group member performs both parts of
IGMPv3, receiving and responding to its own IGMP message
transmissions as well as those of its neighbors. The multicast
router part of IGMPv3 is described in Section 6.)
A system performs the protocol described in this section over all
interfaces on which multicast reception is supported, even if more
than one of those interfaces is connected to the same network.
For interoperability with multicast routers running older versions of
IGMP, systems maintain a MulticastRouterVersion variable for each
interface on which multicast reception is supported. This section
describes the behavior of group member systems on interfaces for
which MulticastRouterVersion = 3. The algorithm for determining
MulticastRouterVersion, and the behavior for versions other than 3,
are described in Section 7.
The all-systems multicast address, 224.0.0.1, is handled as a special
case. On all systems -- that is, all hosts and routers including
multicast routers -- reception of packets destined to the all-systems
multicast address, from all sources, is permanently enabled on all
interfaces on which multicast reception is supported. No IGMP
messages are ever sent regarding the all-systems multicast address.
There are two types of events that trigger IGMPv3 protocol actions on
an interface:
* A change of the interface reception state, caused by a local
invocation of IPMulticastListen.
* The reception of a Query.
(Received IGMP messages of types other than Query are silently
ignored, except as required for interoperation with earlier versions
of IGMP.)
The following subsections describe the actions to be taken for each
of these two cases. In those descriptions, timer and counter names
appear in square brackets. The default values for those timers and
counters are specified in Section 8.
5.1. Action on Change of Interface State
An invocation of IPMulticastListen may cause the multicast reception
state of an interface to change, according to the rules in
Section 3.2. Each such change affects the per-interface entry for a
single multicast address.
A change of interface state causes the system to immediately transmit
a State-Change Report from that interface. The type and contents of
the Group Record(s) in that Report are determined by comparing the
filter mode and source list for the affected multicast address before
and after the change, according to Table 3. If no interface state
existed for that multicast address before the change (i.e., the
change consisted of creating a new per-interface record), or if no
state exists after the change (i.e., the change consisted of deleting
a per-interface record), then the "non-existent" state is considered
to have a filter mode of INCLUDE and an empty source list.
+=============+=============+==========================+
| Old State | New State | State-Change Record Sent |
+=============+=============+==========================+
| INCLUDE (A) | INCLUDE (B) | ALLOW (B-A), BLOCK (A-B) |
+-------------+-------------+--------------------------+
| EXCLUDE (A) | EXCLUDE (B) | ALLOW (A-B), BLOCK (B-A) |
+-------------+-------------+--------------------------+
| INCLUDE (A) | EXCLUDE (B) | TO_EX (B) |
+-------------+-------------+--------------------------+
| EXCLUDE (A) | INCLUDE (B) | TO_IN (B) |
+-------------+-------------+--------------------------+
Table 3 3: Transmitted Group Records for State Changes
If the computed source list for either an ALLOW or a BLOCK State-
Change Record is empty, that record is omitted from the Report
message.
To cover the possibility of the State-Change Report being missed by
one or more multicast routers, it is retransmitted [Robustness
Variable] - 1 more times, at intervals chosen at random from the
range (0, [Unsolicited Report Interval]).
If more changes to the same interface state entry occur before all
the retransmissions of the State-Change Report for the first change
have been completed, each such additional change triggers the
immediate transmission of a new State-Change Report.
The contents of the new transmitted report are calculated as follows.
As was done with the first report, the interface state for the
affected group before and after the latest change is compared. The
report records expressing the difference are built according to
Table 3. However, these records are not transmitted in a message but
instead are merged with the contents of the pending report to create
the new State-Change report. The rules for merging the difference
report resulting from the state change and the pending report are
described below.
The transmission of the merged State-Change Report terminates
retransmissions of the earlier State-Change Reports for the same
multicast address, and becomes the first of [Robustness Variable]
transmissions of State-Change Reports.
Each time a source is included in the difference report calculated
above, retransmission state for that source needs to be maintained
until [Robustness Variable] State-Change reports Reports have been sent by
the host. This is done in order to ensure that a series of
successive state changes do not break the protocol robustness.
If the interface reception-state change that triggers the new report
is a filter-mode change, then the next [Robustness Variable] State-
Change Reports will include a Filter-Mode-Change Record. This
applies even if any number of source-list changes occur in that
period. The host has to maintain retransmission state for the group
until the [Robustness Variable] State-Change reports Reports have been sent.
When [Robustness Variable] State-Change reports Reports with Filter-Mode-
Change Records have been transmitted after the last filter-mode
change, and if source-list changes to the interface reception have
scheduled additional reports, then the next State-Change report Report will
include Source-List-Change Records.
Each time a State-Change Report is transmitted, the contents are
determined as follows. If the report should contain a Filter-Mode-
Change Record, and if the current filter-mode of the interface is
INCLUDE, a TO_IN record is included in the report; otherwise, a TO_EX
record is included. If instead the report should contain a Source-
List-Change Records, Record, an ALLOW and a BLOCK record are included. The
contents of these records are built according to Table 4.
+========+==============================+
| Record | Sources Included |
+========+==============================+
| TO_IN | All in the current interface |
| | state that must be forwarded |
+--------+------------------------------+
| TO_EX | All in the current interface |
| | state that must be blocked |
+--------+------------------------------+
| ALLOW | All with retransmission |
| | state that must be forwarded |
+--------+------------------------------+
| BLOCK | All with retransmission |
| | state that must be blocked |
+--------+------------------------------+
Table 4 4: Change Record Construction
If the computed source list for either an ALLOW or a BLOCK record is
empty, that record is omitted from the State-Change report. Report.
| Note: When the first State-Change report Report is sent, the non-existent non-
| existent pending report to merge with can be treated as a source-change report
| Source-Change Report with empty ALLOW and BLOCK records (no
| sources have retransmission state).
5.2. Action on Reception of a Query
When a system receives a Query, it does not respond immediately.
Instead, it delays its response by a random amount of time, bounded
by the Max Resp Response Time value derived from the Max Resp Code in the
received Query message. A system may receive a variety of Queries on
different interfaces and of different kinds (e.g., General Queries,
Group-Specific
Group Specific Queries, and Group-and-Source-Specific Group-and-Source Specific Queries), each
of which may require its own delayed response.
Before scheduling a response to a Query, the system must first
consider previously scheduled pending responses as, in many cases, it
can schedule a combined response. Therefore, the system must be able
to maintain the following state:
* A timer per interface for scheduling responses to General Queries.
* A per-group and interface timer Interface Timer for scheduling responses to Group- Group
Specific and Group-and-Source-Specific Group-and-Source Specific Queries.
* A per-group and interface list of sources to be reported in the
response to a Group-and-Source-Specific Group-and-Source Specific Query.
When a new Query with the Router Alert option arrives on an interface,
provided the system has state to report, a delay for a response is
randomly selected in the range (0, [Max Resp Response Time]) where Max Resp
Response Time is derived from Max Resp Code in the received Query
message. The following rules are then used to determine if a Report
needs to be scheduled and the type of Report to schedule. The rules
are considered in order and only the first matching rule is applied.
1. If there is a pending response to a previous General Query
scheduled sooner than the selected delay, no additional response
needs to be scheduled.
2. If the received Query is a General Query, the interface timer Interface Timer is
used to schedule a response to the General Query after the
selected delay. Any previously pending response to a General
Query is canceled.
3. If the received Query is a Group-Specific Group Specific Query or a Group-and-
Source-Specific
Source Specific Query and there is no pending response to a
previous Query for this group, then the group timer Group Timer is used to
schedule a report. If the received Query is a Group-and-Source- Group-and-Source
Specific Query, the list of queried sources is recorded to be
used when generating a response.
4. If there already is a pending response to a previous Query
scheduled for this group, and either the new Query is a Group- Group
Specific Query or the recorded source-list associated with the
group is empty, then the group source-list is cleared and a
single response is scheduled using the group timer. Group Timer. The new
response is scheduled to be sent at the earliest of the remaining
time for the pending report and the selected delay.
5. If the received Query is a Group-and-Source-Specific Group-and-Source Specific Query and
there is a pending response for this group with a non-empty
source-list, then the group source list is augmented to contain
the list of sources in the new Query and a single response is
scheduled using the group timer. Group Timer. The new response is scheduled
to be sent at the earliest of the remaining time for the pending
report and the selected delay.
When the timer in a pending response record expires, the system
transmits, on the associated interface, one or more Report messages
carrying one or more Current-State Records (see Section 4.2.13), as
follows:
1. If the expired timer is the interface timer Interface Timer (i.e., it is a
pending response to a General Query), then one Current-State
Record is sent for each multicast address for which the specified
interface has reception state, as described in Section 3.2. The
Current-State Record carries the multicast address and its
associated filter mode (MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and
source list. Multiple Current-State Records are packed into
individual Report messages, to the extent possible.
This naive algorithm may result in bursts of packets when a
system is a member of a large number of groups. Instead of using
a single interface timer, Interface Timer, implementations are recommended to
spread transmission of such Report messages over the interval (0,
[Max Resp Response Time]). Note that any such implementation MUST
avoid the "ack-implosion" problem, i.e., MUST NOT send a Report
immediately on reception of a General Query.
2. If the expired timer is a group timer Group Timer and the list of recorded
sources for that group is empty (i.e., it is a pending response
to a Group-Specific Group Specific Query), then if and only if the interface has
reception state for that group address, a single Current-State
Record is sent for that address. The Current-State Record
carries the multicast address and its associated filter mode
(MODE_IS_INCLUDE or MODE_IS_EXCLUDE) and source list.
3. If the expired timer is a group timer Group Timer and the list of recorded
sources for that group is non-empty (i.e., it is a pending
response to a Group-and-Source-Specific Group-and-Source Specific Query), then if and only
if the interface has reception state for that group address, the
contents of the responding Current-State Record is determined
from the interface state and the pending response record, as
specified in Table 5.
+=====================+=========================+===============+
| Per-Interface State | Set of Sources in the | Current-State |
| | Pending Response Record | Record |
+=====================+=========================+===============+
| INCLUDE (A) | B | IS_IN (A*B) |
+---------------------+-------------------------+---------------+
| EXCLUDE (A) | B | IS_IN (B-A) |
+---------------------+-------------------------+---------------+
Table 5 5: Current-State Record Construction
If the resulting Current-State Record has an empty set of source
addresses, then no response is sent.
Finally, after any required Report messages have been generated, the
source lists associated with any reported groups are cleared.
6. Description of the Protocol for Multicast Routers
The purpose of IGMP is to enable each multicast router to learn, for
each of its directly attached networks, which multicast addresses are
of interest to the systems attached to those networks. IGMP version
3 IGMPv3 adds
the capability for a multicast router to also learn which sources are
of interest to neighboring systems, for packets sent to any
particular multicast address. The information gathered by IGMP is
provided to whichever multicast routing protocol is being used by the
router, in order to ensure that multicast packets are delivered to
all networks where there are interested receivers.
This section describes the part of IGMPv3 that is performed by
multicast routers. Multicast routers may also themselves become
members of multicast groups, and therefore also perform the group
member part of IGMPv3, as described in Section 5.
A multicast router performs the protocol described in this section
over each of its directly attached networks. If a multicast router
has more than one interface to the same network, it only needs to
operate this protocol over one of those interfaces. On each
interface over which this protocol is being run, the router MUST
enable reception of multicast address 224.0.0.22 from all sources
(and MUST perform the group member part of IGMPv3 for that address on
that interface).
Multicast routers need to know only that at least one system on an
attached network is interested in packets to a particular multicast
address from a particular source; a multicast router is not required
to keep track of the interests of each individual neighboring system.
(However, see Appendix A.2, item 1 for discussion.)
IGMPv3 is backward compatible with previous versions of the IGMP
protocol. In order to remain backward compatible with older IGMP
systems, IGMPv3 multicast routers MUST also implement versions 1 and
2 of the protocol (see Section 7).
6.1. Conditions for IGMP Queries
Multicast routers send General Queries periodically to request group
membership information from an attached network. These queries Queries are
used to build and refresh the group membership state of systems on
attached networks. Systems respond to these queries Queries by reporting
their group membership state (and their desired set of sources) with
Current-State Group Records in IGMPv3 Membership Reports.
As a member of a multicast group, a system may express interest in
receiving or not receiving traffic from particular sources. As the
desired reception state of a system changes, it reports these changes
using Filter-Mode-Change Records or Source-List-Change Records.
These records indicate an explicit state change in a group at a
system in either the group record's Group Record's source list or its filter-mode.
When a group membership is terminated at a system or traffic from a
particular source is no longer desired, a multicast router must query
for other members of the group or listeners of the source before
deleting the group (or source) and pruning its traffic.
To enable all systems on a network to respond to changes in group
membership, multicast routers send specific queries. A Group- Group
Specific Query is sent to verify there are no systems that desire
reception of the specified group or to "rebuild" the desired
reception state for a particular group. Group-Specific Group Specific Queries are
sent when a router receives a State-Change record Record indicating a system
is leaving a group.
A Group-and-Source Specific Query is used to verify there are no
systems on a network that desire receiving traffic from a set of
sources. Group-and-Source Specific Queries list sources for a
particular group that have been requested to no longer be forwarded.
This query is sent by a multicast router to learn if any systems
desire reception of packets to the specified group address from the
specified source addresses. Group-and-Source Specific Queries are
only sent in response to State-Change Records and never in response
to Current-State Records. Section 4.1.11 describes each query in
more detail.
6.2. IGMP State Maintained by Multicast Routers
Multicast routers implementing IGMPv3 keep state per group per
attached network. This group state consists of a filter-mode, a list
of sources, and various timers. For each attached network running
IGMP, a multicast router records the desired reception state for that
network. That state conceptually consists of a set of records of the
form:
(multicast address, group timer, filter-mode, Group Timer, Router Filter Mode, (source records))
Each source record is of the form:
(source address, source timer) Source Timer)
If all sources within a given group are desired, an empty source
record list is kept with filter-mode set to EXCLUDE. This means
hosts on this network want all sources for this group to be
forwarded. This is the IGMPv3 equivalent to an IGMPv1 or IGMPv2
group join.
6.2.1. Definition of Router Filter-Mode Filter Mode
To reduce internal state, IGMPv3 routers keep a filter-mode filter mode per group
per attached network. This filter-mode filter mode is used to condense the total
desired reception state of a group to a minimum set such that all
systems' memberships are satisfied. This filter-mode filter mode may change in
response to the reception of particular types of group records Group Records or
when certain timer conditions occur. In the following sections, we
use the term "router filter-mode" Router Filter Mode to refer to the filter-mode of a
particular group within a router. Section 6.4 describes the changes
of a router filter-mode Router Filter Mode per group record Group Record received.
Conceptually, when a group record Group Record is received, the router filter-mode Router Filter Mode
for that group is updated to cover all the requested sources using
the least amount of state. As a rule, once a group record Group Record with a
filter-mode of EXCLUDE is received, the router filter-mode Router Filter Mode for that
group will be EXCLUDE.
When a router filter-mode Router Filter Mode for a group is EXCLUDE, the source record
list contains two types of sources. The first type is the set that
represents conflicts in the desired reception state; this set must be
forwarded by some router on the network. The second type is the set
of sources that hosts have requested to not be forwarded. Appendix A
describes the reasons for keeping two different sets when in EXCLUDE
mode.
When a router filter-mode Router Filter Mode for a group is INCLUDE, the source record
list is the list of sources desired for the group. This is the total
desired set of sources for that group. Each source in the source
record list must be forwarded by some router on the network.
Because a reported group record Group Record with a filter-mode of EXCLUDE will
cause a router to transition its filter-mode for that group to
EXCLUDE, a mechanism for transitioning a router's filter-mode back to
INCLUDE must exist. If all systems with a group record Group Record in EXCLUDE
filter-mode cease reporting, it is desirable for the router filter-
mode Router Filter
Mode for that group to transition back to INCLUDE mode. This
transition occurs when the group timer Group Timer expires and is explained in
detail in Section 6.5.
6.2.2. Definition of Group Timers
The group timer Group Timer is only used when a group is in EXCLUDE mode and it
represents the time for the filter-mode of the group to expire and
switch to INCLUDE mode. We define a group timer Group Timer as a decrementing
timer with a lower bound of zero kept per group per attached network.
Group timers are updated according to the types of group records Group Records
received.
A group timer Group Timer expiring when a router filter-mode Router Filter Mode for the group is
EXCLUDE means there are no listeners on the attached network in
EXCLUDE mode. At this point, a router will transition to INCLUDE
filter-mode. Section 6.5 describes the actions taken when a group
timer Group
Timer expires while in EXCLUDE mode.
Table 6 summarizes the role of the group timer. Group Timer. Section 6.4
describes the details of setting the group timer Group Timer per type of group
record Group
Record received.
+=============+=======+=========================================+
| Group | Group | Actions/Comments |
| Filter-Mode | Timer | |
| | Value | |
+=============+=======+=========================================+
| INCLUDE | Timer | All members in INCLUDE mode. |
| | >= 0 | |
+-------------+-------+-----------------------------------------+
| EXCLUDE | Timer | At least one member in EXCLUDE mode. |
| | > 0 | |
+-------------+-------+-----------------------------------------+
| EXCLUDE | Timer | No more listeners to group. If all |
| | == 0 | source timers have expired, then delete |
| | | Group Record. If there are still |
| | | source record timers running, switch to |
| | | INCLUDE filter-mode using those source |
| | | records with running timers as the |
| | | INCLUDE source record state. |
+-------------+-------+-----------------------------------------+
Table 6 6: Group Timer Actions
6.2.3. Definition of Source Timers
A source timer Source Timer is kept per source record and is a decrementing timer
with a lower bound of zero. Source timers are updated according to
the type and filter-mode of the group record Group Record received. Source timers
are always updated (for a particular group) whenever the source is
present in a received record for that group. Section 6.4 describes
the setting of source timers per type of group records Group Records received.
A source record with a running timer with a router filter-mode Router Filter Mode for
the group of INCLUDE means that there is currently one or more
systems (in INCLUDE filter-mode) that desire to receive that source.
If a source timer Source Timer expires with a router filter-mode Router Filter Mode for the group of
INCLUDE, the router concludes that traffic from this particular
source is no longer desired on the attached network and deletes the
associated source record.
Source timers are treated differently when a router filter-mode Router Filter Mode for a
group is EXCLUDE. If a source record has a running timer with a
router filter-mode
Router Filter Mode for the group of EXCLUDE, it means that at least
one system desires the source. It should therefore be forwarded by a
router on the network. Appendix A describes the reasons for keeping
state for sources that have been requested to be forwarded while in
EXCLUDE state.
If a source timer Source Timer expires with a router filter-mode Router Filter Mode for the group of
EXCLUDE, the router informs the routing protocol that there is no
longer a receiver on the network interested in traffic from this
source.
When a router filter-mode Router Filter Mode for a group is EXCLUDE, source records are
only deleted when the group timer Group Timer expires. Section 6.3 describes the
actions that should be taken dependent upon the value of a source
timer. Source
Timer.
6.3. IGMPv3 Source-Specific Forwarding Rules
When a multicast router receives a datagram from a source destined to
a particular group, a decision has to be made whether to forward the
datagram onto an attached network or not. The multicast routing
protocol in use is in charge of this decision and should use the
IGMPv3 information to ensure that all sources/groups desired on a
subnetwork are forwarded to that subnetwork. IGMPv3 information does
not override multicast routing information; for example, if the
IGMPv3 filter-mode group for G is EXCLUDE, a router may still forward
packets for excluded sources to a transit subnet.
To summarize, Table 7 describes the forwarding suggestions made by
IGMP to the routing protocol for traffic originating from a source
destined to a group. It also summarizes the actions taken upon the
expiration of a source timer Source Timer based on the router filter-mode Router Filter Mode of the
group.
+=============+==========+=======================================+
| Group | Group | Action |
| Filter-Mode | Timer | |
| | Value | |
+=============+==========+=======================================+
| INCLUDE | TIMER > | Suggest to forward traffic from |
| | 0 | source. |
+-------------+----------+---------------------------------------+
| INCLUDE | TIMER == | Suggest to stop forwarding traffic |
| | 0 | from source and remove source record. |
| | | If there are no more source records |
| | | for the group, delete group record. Group Record. |
+-------------+----------+---------------------------------------+
| INCLUDE | No | Suggest to not forward source. |
| | Source | |
| | Elements | |
+-------------+----------+---------------------------------------+
| EXCLUDE | TIMER > | Suggest to forward traffic from |
| | 0 | source. |
+-------------+----------+---------------------------------------+
| EXCLUDE | TIMER == | Suggest to not forward traffic from |
| | 0 | source (DO NOT remove record). |
+-------------+----------+---------------------------------------+
| EXCLUDE | No | Suggest to forward traffic from |
| | Source | source. |
| | Elements | |
+-------------+----------+---------------------------------------+
Table 7 7: IGMP Forwarding Recommendations
6.4. Action on Reception of Reports
SSM-aware routers SHOULD ignore records that contain multicast
addresses in the SSM address range if the record type is
MODE_IS_EXCLUDE or CHANGE_TO_EXCLUDE_MODE. SSM-aware routers SHOULD
ignore IGMPv1/IGMPv2 Report and IGMPv2 DONE messages that contain
multicast addresses in the SSM address range, SHOULD NOT use such
Reports to establish IP forwarding state, and MAY log an error if it
receives such a message.
6.4.1. Reception of Current-State Records
When receiving Current-State Records, a router updates both its group
and source timers. In some circumstances, the reception of a type of
group record
Group Record will cause the router filter-mode Router Filter Mode for that group to
change. Table 8 describes the actions, with respect to state and
timers that occur to a router's state upon reception of Current-
State Records.
The following notation is used to describe the updating of source
timers. The notation ( A, B ) will be used to represent the total
number of sources for a particular group, where
* A = set of source records whose source timers > 0 (Sources that at
least one host has requested to be forwarded)
* B = set of source records whose source timers = 0 (Sources that
IGMP will suggest to the routing protocol not to forward)
Note that there will only be two sets when a router's filter-mode for
a group is EXCLUDE. When a router's filter-mode for a group is
INCLUDE, a single set is used to describe the set of sources
requested to be forwarded (e.g., simply (A)).
In Tables 8 and 9, abbreviations are used for several variables (all
of which are described in detail in Section 8). The variable GMI is
an abbreviation for the Group Membership Interval, which is the time
in which group memberships will time out. The variable LMQT is an
abbreviation for the Last Member Query Time, which is the total time
spent after Last [Last Member Query Count Count] retransmissions. LMQT
represents the "leave latency" leave latency or the difference between the
transmission of a membership change and the change in the information
given to the routing protocol.
Within the "Actions" section of the router state tables, we use the
notation 'A=J', which means that the set A of source records should
have their source timers set to value J. 'Delete A' means that the
set A of source records should be deleted. 'Group Timer=J' means
that the Group Timer for the group should be set to value J.
+=========+========+===========+=================+
| Router | Report | New | Actions |
| State | Rec'd | Router | |
| | | State | |
+=========+========+===========+=================+
| INCLUDE | IS_IN | INCLUDE | (B)=GMI |
| (A) | (B) | (A+B) | |
+---------+--------+-----------+-----------------+
| INCLUDE | IS_EX | EXCLUDE | (B-A)=0 |
| (A) | (B) | (A*B,B-A) | Delete (A-B) |
| | | | Group Timer=GMI |
+---------+--------+-----------+-----------------+
| EXCLUDE | IS_IN | EXCLUDE | (A)=GMI |
| (X,Y) | (A) | (X+A,Y-A) | |
+---------+--------+-----------+-----------------+
| EXCLUDE | IS_EX | EXCLUDE | (A-X-Y)=GMI |
| (X,Y) | (A) | (A-Y,Y*A) | Delete (X-A) |
| | | | Delete (Y-A) |
| | | | Group Timer=GMI |
+---------+--------+-----------+-----------------+
Table 8 8: Actions on Current-State Report Reception
6.4.2. Reception of Filter-Mode-Change and Source-List-Change Records
When a change in the global state of a group occurs in a system, the
system sends either a Source-List-Change Record or a Filter-Mode-
Change Record for that group. As with Current-State Records, routers
must act upon these records and possibly change their own state to
reflect the new desired membership state of the network.
Routers must query sources that are requested to be no longer
forwarded to a group. When a router queries or receives a query for
a specific set of sources, it lowers its source timers for those
sources to a small interval of Last [Last Member Query Time Time] seconds. If
group records
Group Records are received in response to the queries which express
interest in receiving traffic from the queried sources, the
corresponding timers are updated.
Similarly, when a router queries a specific group, it lowers its
group timer
Group Timer for that group to a small interval of Last [Last Member Query
Time
Time] seconds. If any group records Group Records expressing EXCLUDE mode interest
in the group are received within the interval, the group timer Group Timer for
the group is updated and the suggestion to the routing protocol to
forward the group stands without any interruption.
During a query period (i.e., Last [Last Member Query Time Time] seconds), the
IGMP component in the router continues to suggest to the routing
protocol that it forwards traffic from the groups or sources that it
is querying. It is not until after Last [Last Member Query Time Time] seconds
without receiving a record expressing interest in the queried group
or sources that the router may prune the group or sources from the
network.
Table 9 describes the changes in group state and the action(s) taken
when receiving either Filter-Mode-Change or Source-List-Change
Records. This table also describes the queries that are sent by the
querier when a particular report is received.
We use the following notation for describing the queries that are
sent. We use the notation 'Q(G)' to describe a Group-Specific Group Specific Query
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
a result of the action (e.g., A*B), then no query is sent as a result
of the operation.
In order to maintain protocol robustness, queries sent by actions in
Table 9 need to be transmitted [Last Member Query Count] times, once
every [Last Member Query Interval].
If while scheduling new queries there are already pending queries to
be retransmitted for the same group, the new and pending queries have
to be merged. In addition, received host reports for a group with
pending queries may affect the contents of those queries.
Section 6.6.3 describes the process of building and maintaining the
state of pending queries.
+=========+========+=============+=====================+
| Router | Report | New Router | Actions |
| State | Rec'd | State | |
+=========+========+=============+=====================+
| INCLUDE | ALLOW | INCLUDE | (B)=GMI |
| (A) | (B) | (A+B) | |
+---------+--------+-------------+---------------------+
| INCLUDE | BLOCK | INCLUDE (A) | Send Q(G,A*B) |
| (A) | (B) | | |
+---------+--------+-------------+---------------------+
| INCLUDE | TO_EX | EXCLUDE | (B-A)=0 |
| (A) | (B) | (A*B,B-A) | Delete (A-B) |
| | | | Send Q(G,A*B) |
| | | | Group Timer=GMI |
+---------+--------+-------------+---------------------+
| INCLUDE | TO_IN | INCLUDE | (B)=GMI |
| (A) | (B) | (A+B) | Send Q(G,A-B) |
+---------+--------+-------------+---------------------+
| EXCLUDE | ALLOW | EXCLUDE | (A)=GMI |
| (X,Y) | (A) | (X+A,Y-A) | |
+---------+--------+-------------+---------------------+
| EXCLUDE | BLOCK | EXCLUDE | (A-X-Y)=Group Timer |
| (X,Y) | (A) | (X+(A-Y),Y) | Send Q(G,A-Y) |
+---------+--------+-------------+---------------------+
| EXCLUDE | TO_EX | EXCLUDE | (A-X-Y)=Group Timer |
| (X,Y) | (A) | (A-Y,Y*A) | Delete (X-A) |
| | | | Delete (Y-A) |
| | | | Send Q(G,A-Y) |
| | | | Group Timer=GMI |
+---------+--------+-------------+---------------------+
| EXCLUDE | TO_IN | EXCLUDE | (A)=GMI |
| (X,Y) | (A) | (X+A,Y-A) | Send Q(G,X-A) |
| | | | Send Q(G) |
+---------+--------+-------------+---------------------+
Table 9 9: Actions on Change Record Reception
6.5. Switching Router Filter-Modes Filter Modes
The group timer Group Timer is used as a mechanism for transitioning the router
filter-mode Router
Filter Mode from EXCLUDE to INCLUDE.
When a group timer Group Timer expires with a router filter-mode Router Filter Mode of EXCLUDE, a
router assumes that there are no systems with a filter-mode of
EXCLUDE present on the attached network. When a router's filter-mode
for a group is EXCLUDE and the group timer Group Timer expires, the router
filter-mode Router Filter
Mode for the group transitions to INCLUDE.
A router uses source records with running source timers as its state
for the switch to a filter-mode of INCLUDE. If there are any source
records with source timers greater than zero (i.e., requested to be
forwarded), a router switches to filter-mode of INCLUDE using those
source records. Source records whose timers are zero (from the
previous EXCLUDE mode) are deleted.
For example, if a router's state for a group is EXCLUDE(X,Y) and the
group timer
Group Timer expires for that group, the router switches to filter-
mode of INCLUDE with state INCLUDE(X).
6.6. Action on Reception of Queries
6.6.1. Timer Updates
When a router sends or receives a query with a clear Suppress Router-
Side Processing flag, it must update its timers to reflect the
correct timeout values for the group or sources being queried.
Table 10 describes the timer actions when sending or receiving a
Group-Specific
Group Specific or Group-and-Source Specific Query with the S flag not
set.
+========+===================================================+
| Query | Action |
+========+===================================================+
| Q(G,A) | Source Timer for sources in A are lowered to LMQT |
+--------+---------------------------------------------------+
| Q(G) | Group Timer is lowered to LMQT |
+--------+---------------------------------------------------+
Table 10 10: Timer Updates on Query
When a router sends or receives a query with the S flag set, it will
not update its timers.
6.6.2. Querier Election
IGMPv3 elects a single querier per subnet using the same querier Querier
election mechanism as IGMPv2, namely by IP address. When a router
receives a general query General Query with a lower IP address, it sets the Other
Querier Present timer Other-
Querier-Present Timer to Other [Other Querier Present Interval Interval] and ceases
to send general queries on the network if it was the previously
elected querier. After its Other-Querier Present timer Other-Querier-Present Timer expires, it
should begin sending General Queries.
If a router receives an older version general query, General Query, it MUST use the
oldest version of IGMP on the network. For a detailed description of
compatibility issues between IGMP versions, see Section 7.
6.6.3. Building and Sending Specific Queries
6.6.3.1. Building and Sending Group-Specific Group Specific Queries
When a table action "Send Q(G)" is encountered, the group timer Group Timer must
be lowered to LMQT. The router must then immediately send a group-
specific query Group
Specific Query as well as schedule [Last Member Query Count Count] - 1] 1
query retransmissions to be sent every [Last Member Query Interval]
over [Last Member Query Time].
When transmitting a group-specific query, Group Specific Query, if the group timer Group Timer is
larger than LMQT, the "Suppress Router-Side Processing" bit is set in
the query message.
6.6.3.2. Building and Sending Group-and-Source-Specific Group-and-Source Specific Queries
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
each of the sources in X of group G, with the source timer Source Timer larger
than LMQT:
* Set the number of retransmissions for each source to [Last Member
Query Count].
* Lower the source timer Source Timer to LMQT.
The router must then immediately send a group and source specific
query Group-and-Source Specific
Query as well as schedule [Last Member Query Count Count] - 1] 1 query
retransmissions to be sent every [Last Member Query Interval] over
[Last Member Query Time]. The contents of these queries are
calculated as follows.
When building a group and source specific query Group-and-source Specific Query for group G, two
separate query messages are sent for the group. The first one has
the "Suppress Router-Side Processing" bit set and contains all the
sources with retransmission state and timers greater than LMQT. The
second has the "Suppress Router-Side Processing" bit clear and
contains all the sources with retransmission state and timers lower
or equal to LMQT. If either of the two calculated messages does not
contain any sources, then its transmission is suppressed.
| Note: If a group-specific query Group Specific Query is scheduled to be transmitted
| at the same time as a group and source specific query Group-and-Source Specific Query for the
| same group, then transmission of the group and source specific Group-and-Source Specific
| Query message with the "Suppress Router-Side Processing" bit
| set may be suppressed.
7. Interoperation With Older Versions of IGMP
IGMP version 3
IGMPv3 hosts and routers interoperate with hosts and routers that
have not yet been upgraded to IGMPv3. This compatibility is
maintained by hosts and routers taking appropriate actions depending
on the versions of IGMP operating on hosts and routers within a
network.
7.1. Query Version Distinctions
The IGMP version of a Membership Query message is determined as
follows:
* IGMPv1 Query: length = 8 octets AND Max Resp Code field is zero
* IGMPv2 Query: length = 8 octets AND Max Resp Code field is non-
zero
* IGMPv3 Query: length >= 12 octets
Query messages that do not match any of the above conditions (e.g., a
Query of length 10 octets) MUST be silently ignored.
7.2. Group Member Behavior
7.2.1. In the Presence of Older Version Queriers
In order to be compatible with older version routers, IGMPv3 hosts
MUST operate in version 1 v1 and version 2 v2 compatibility modes. IGMPv3 hosts MUST
keep state per local interface regarding the compatibility mode of
each attached network. A host's compatibility mode is determined
from the Host Compatibility Mode variable, which can be in one of
three states: IGMPv1, IGMPv2, or IGMPv3. This variable is kept per
interface and is dependent on the version of General Queries
heard received
on that interface as well as the Older Version Querier Present
timers Older-Version-Querier-Present Timer
for the interface.
In order to switch gracefully between versions of IGMP, hosts keep
both an IGMPv1 Querier Present timer IGMPv1-Querier-Present Timer and an IGMPv2 Querier Present
timer IGMPv2-Querier-Present
Timer per interface. IGMPv1 Querier Present IGMPv1-Querier-Present Timer is set to Older [Older
Version Querier Present Timeout Interval] seconds whenever an IGMPv1
Membership Query is received. IGMPv2 Querier Present IGMPv2-Querier-Present Timer is set to Older
[Older Version Querier Present Timeout Interval] seconds whenever an IGMPv2
General Query is received.
The Host Compatibility Mode of an interface changes whenever an older
version query (than the current compatibility mode) is heard received or
when certain timer conditions occur. When the IGMPv1 Querier Present
timer IGMPv1-Querier-Present
Timer expires, a host switches to Host Compatibility Mode of IGMPv2
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
Compatibility of IGMPv3. When the IGMPv2 Querier Present timer
expires, a host switches to Host Compatibility Mode of IGMPv3.
The Host Compatibility Mode variable is based on whether an older
version General query Query was heard received in the last Older [Older Version Querier
Present Timeout Interval] seconds. The Host Compatibility Mode variable
value MUST NOT be changed by an older version group-specific query. Group Specific Query.
The Host Compatibility Mode is set depending on the following:
+=========================+========================================+
| Host Compatibility Mode | Timer State |
+=========================+========================================+
| IGMPv3 (default) | IGMPv2 Querier Present not running and |
| | IGMPv1 Querier Present not running |
+-------------------------+----------------------------------------+
| IGMPv2 | IGMPv2 Querier Present running and |
| | IGMPv1 Querier Present not running |
+-------------------------+----------------------------------------+
| IGMPv1 | IGMPv1 Querier Present running |
+-------------------------+----------------------------------------+
Table 11 11: Host Compatibility Mode Settings
If a host receives a query that causes its Querier Present timers to
be updated and correspondingly its compatibility mode, it should
switch compatibility modes immediately.
When Host Compatibility Mode is IGMPv3, a host acts using the IGMPv3
protocol on that interface. When Host Compatibility Mode is IGMPv2,
a host acts in IGMPv2 compatibility mode, using only the IGMPv2
protocol, on that interface. When Host Compatibility Mode is IGMPv1,
a host acts in IGMPv1 compatibility mode, using only the IGMPv1
protocol on that interface.
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).
An IGMPv2 router will send General Queries with the Max Resp Code set
to the desired Max Resp Response Time, i.e., the full range of this field
is linear and the exponential algorithm described in Section 4.1.1 is
not used.
Whenever a host changes its compatibility mode, it cancels all its
pending response and retransmission timers.
An SSM-aware host that receives an IGMPv1 Query, an IGMPv2 General
Query, or an IGMPv2 Group Specific Query for a multicast address in
the SSM address range SHOULD log an error. It is RECOMMENDED that
implementations provide a configuration option to disable use of the
Host Compatibility Mode to allow networks to operate only in SSM
mode. This configuration option SHOULD be disabled by default.
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
have not yet been upgraded to IGMPv3. A host MAY allow its IGMPv3
Membership Record to be suppressed by either a Version 1 an IGMPv1 Membership
Report,
Report or a Version 2 an IGMPv2 Membership Report. SSM-aware hosts MUST NOT
allow its IGMPv3 Membership Record to be suppressed.
7.3. Multicast Router Behavior
7.3.1. In the Presence of Older Version Queriers
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
requirements apply:
* If any older versions of IGMP are present on routers, the querier
MUST use the lowest version of IGMP present on the network. This
must be administratively assured; routers that desire to be
compatible with IGMPv1 and IGMPv2 MUST have a configuration option
to act in IGMPv1 or IGMPv2 compatibility modes. When in IGMPv1
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
MUST ignore Leave Group messages. They SHOULD also warn about
receiving an IGMPv2 or IGMPv3 query, although such warnings MUST
be rate-limited. When in IGMPv2 mode, routers MUST send Periodic
Queries truncated at the Group Address field (i.e., 8 bytes long)
and SHOULD also warn about receiving an IGMPv3 query (such
warnings MUST be rate-limited). They also MUST fill in the Max
Resp
Response Time in the Max Resp Code field, i.e., the exponential
algorithm described in Section 4.1.1 is not used.
* If a router is not explicitly configured to use IGMPv1 or IGMPv2
and hears receives an IGMPv1 Query or IGMPv2 General Query, it SHOULD
log a warning. These warnings MUST be rate-limited.
* It is RECOMMENDED that implementations provide a configuration
option to disable use of compatibility mode to allow networks to
operate only in SSM mode. This configuration option SHOULD be
disabled by default.
7.3.2. In the Presence of Older Version Group Members
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
older version hosts, IGMPv3 routers MUST operate in version 1 v1 and
version 2 v2
compatibility modes. IGMPv3 routers keep a compatibility mode per group record.
Group Record. A group's compatibility mode is determined from the
Group Compatibility Mode variable, which can be in one of three
states: IGMPv1, IGMPv2, or IGMPv3. This variable is kept per
group record Group
Record and is dependent on the version of Membership Reports
heard received
for that group as well as the Older Version Host Present timer Older-Version-Host-Present Timer for
the group.
In order to switch gracefully between versions of IGMP, routers keep
an IGMPv1 Host Present timer IGMPv1-Host-Present Timer and an IGMPv2 Host Present timer IGMPv2-Host-Present Timer per
group record.
Group Record. The IGMPv1 Host Present timer IGMPv1-Host-Present Timer is set to Older [Older Version
Host Present Timeout Interval] seconds whenever an IGMPv1 Membership Report
is received. The IGMPv2 Host Present timer IGMPv2-Host-Present Timer is set to Older [Older Version
Host Present Timeout Interval] seconds whenever an IGMPv2 Membership Report
is received.
The Group Compatibility Mode of a group record Group Record changes whenever an
older version report (than the current compatibility mode) is heard
received or when certain timer conditions occur. When the IGMPv1 Host Present
timer IGMPv1-
Host-Present Timer expires, a router switches to Group Compatibility
Mode of IGMPv2 if it has a running IGMPv2 Host Present timer. If it
does not have a running IGMPv2 Host Present timer, then it switches
to Group Compatibility Mode of IGMPv3. When the IGMPv2 Host Present timer IGMPv2-Host-Present
Timer expires and the IGMPv1 Host Present timer IGMPv1-Host-Present Timer is not running, a
router switches to Group Compatibility Mode of IGMPv3. Note that
when a group switches back to IGMPv3 mode, it takes some time to
regain source- specific state information. Source-specific
information will be learned during the next General Query, but
sources that should be blocked will not be blocked until [Group
Membership Interval] after that.
The Group Compatibility Mode variable is based on whether an older
version report was heard received in the last Older [Older Version Host Present
Timeout
Interval] seconds. The Group Compatibility Mode is set depending on
the following:
+==========================+=====================================+
| Group Compatibility Mode | Timer State |
+==========================+=====================================+
| IGMPv3 (default) | IGMPv2 Host Present not running and |
| | IGMPv1 Host Present not running |
+--------------------------+-------------------------------------+
| IGMPv2 | IGMPv2 Host Present running and |
| | IGMPv1 Host Present not running |
+--------------------------+-------------------------------------+
| IGMPv1 | IGMPv1 Host Present running |
+--------------------------+-------------------------------------+
Table 12 12: Group Compatibility Mode Settings
If a router receives a report that causes its older Host Present
timers to be updated and correspondingly its compatibility mode, it
SHOULD switch compatibility modes immediately.
When Group Compatibility Mode is IGMPv3, a router acts using the
IGMPv3 protocol for that group.
When Group Compatibility Mode is IGMPv2, a router internally
translates the following IGMPv2 messages for that group to their
IGMPv3 equivalents:
+================+===================+
| IGMPv2 Message | IGMPv3 Equivalent |
+================+===================+
| Report | IS_EX( {} ) |
+----------------+-------------------+
| Leave | TO_IN( {} ) |
+----------------+-------------------+
Table 13 13: IGMPv2 Compatibility
Mode Message Translation
IGMPv3 BLOCK messages are ignored, as are source-lists in TO_EX()
messages (i.e., any TO_EX() message is treated as TO_EX( {} )).
When Group Compatibility Mode is IGMPv1, a router internally
translates the following IGMPv1 and IGMPv2 messages for that group to
their IGMPv3 equivalents:
+================+===================+
| IGMPv2 Message | IGMPv3 Equivalent |
+================+===================+
| v1 Report | IS_EX( {} ) |
+----------------+-------------------+
| v2 Report | IS_EX( {} ) |
+----------------+-------------------+
Table 14 14: IGMPv1 Compatibility
Mode Message Translation
In addition to ignoring IGMPv3 BLOCK messages and source-lists in
TO_EX() messages as in IGMPv2 Group Compatibility Mode, IGMPv2 Leave
messages and IGMPv3 TO_IN() messages are also ignored.
8. List of Timers, Counters, and Their Default Values
Most of these timers and counters are configurable. If non-default settings
are used, they MUST be consistent among all systems on a single link.
Note that parentheses are used to group expressions to make the
algebra clear.
8.1. Robustness Variable
The Robustness Variable allows tuning for the expected packet loss on
a network. If a network is expected to be lossy, the Robustness
Variable may be increased. IGMP is robust to (Robustness Variable -
1) packet losses. The Robustness Variable MUST NOT be zero and
SHOULD NOT be one. Default: 2.
8.2. Query Interval
The Query Interval is the interval between General Queries sent by
the Querier. Default: 125 seconds.
By varying the [Query Interval], Query Interval, an administrator may tune the number
of IGMP messages on the network; larger values cause IGMP Queries to
be sent less often.
8.3. Query Response Interval
The Query Response Interval uses the Max Response Time to calculate
the Max Resp Code that is inserted into the periodic General Queries.
Default: 100 (10 seconds).
By varying the [Query Response Interval], an administrator may tune
the burstiness of IGMP messages on the network; larger values make
the traffic less bursty, as host responses are spread out over a
larger interval. The number of seconds represented by the [Query
Response Interval] must be less than the [Query Interval].
8.4. Group Membership Interval
The Group Membership Interval is the amount of time that must pass
before a multicast router decides there are no more members of a
group or a particular source on a network.
This value MUST be ((the Robustness Variable) ([Robustness Variable] times (the Query
Interval)) [Query Interval])
plus (2 * Query [Query Response Interval). Interval]).
8.5. Other Querier Present Interval
The Other Querier Present Interval is the length of time that must
pass before a multicast router decides that there is no longer
another multicast router that should be the querier. This value MUST
be ((the Robustness Variable) ([Robustness Variable] times (the Query Interval)) [Query Interval]) plus (one
half of one Query (0.5 times
[Query Response Interval). Interval]).
8.6. Startup Query Interval
The Startup Query Interval is the interval between General Queries
sent by a Querier on startup. Default: 1/4 the Query Interval. times [Query Interval].
8.7. Startup Query Count
The Startup Query Count is the number of Queries sent out on startup,
separated by the Startup Query Interval. Default: The Robustness
Variable. [Robustness
Variable].
8.8. Last Member Query Interval
The Last Member Query Interval (LMQI) is the Max Response Time used
to calculate the Max Resp Code that is inserted into Group-Specific Group Specific
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-
Source-Specific
Source Specific Query messages. Default: 10 (1 second).
Note that for values of LMQI greater than 12.8 seconds, a limited set
of values can be represented, corresponding to sequential values of
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
next lower value if the requested value is not exactly representable.
This value may be tuned to modify the "leave latency" leave latency of the network.
A reduced value results in reduced time to detect the loss of the
last member of a group or source.
8.9. Last Member Query Count
The Last Member Query Count is the number of Group-Specific Group Specific Queries
sent before the router assumes there are no local members. The Last
Member Query Count is also the number of Group-and-Source-Specific Group-and-Source Specific
Queries sent before the router assumes there are no listeners for a
particular source. Default: The Robustness Variable. [Robustness Variable].
8.10. Last Member Query Time
The Last Member Query Time is the time value represented by the Last [Last
Member Query Interval, multiplied by the Last Interval] times [Last Member Query Count. Count]. It is not a
tunable value, but it may be tuned by changing its components.
8.11. Unsolicited Report Interval
The Unsolicited Report Interval is the time between repetitions of a
host's initial report of membership in a group. Default: 1 second.
8.12. Older Version Querier Present Interval
The Older Version Querier Present Interval is the timeout for
transitioning a host back to IGMPv3 mode once an older version query
is heard. received. When an older version query is received, hosts set
their
Older Version Querier Present Older-Version-Querier-Present Timer to Older [Older Version Querier
Present
Interval. Interval].
It is RECOMMENDED to use the default values for calculating the
interval value as hosts do not know the values configured on the
querying routers. This value SHOULD be [Robustness Variable] times
[Query Interval] plus (10 times the Max Resp Response Time in the last
received query message).
8.13. Older Host Present Interval
The Older Host Present Interval is the timeout for transitioning a
group back to IGMPv3 mode once an older version report is sent for
that group. When an older version report is received, routers set
their Older Host Present Older-Host-Present Timer to Older [Older Host Present Interval. Interval].
This value MUST be ((the Robustness Variable) ([Robustness Variable] times (the Query
Interval)) [Query Interval])
plus (one Query [Query Response Interval). Interval].
8.14. Configuring Timers
This section is meant to provide advice to network administrators on
how to tune these settings to their network. Ambitious router
implementations might tune these settings dynamically based upon
changing characteristics of the network.
8.14.1. Robustness Variable
The Robustness Variable tunes IGMP to expected losses on a link.
IGMPv3 is robust to (Robustness Variable ([Robustness Variable] - 1) packet losses, e.g.,
if the Robustness Variable is set to the default value of 2, IGMPv3
is robust to a single packet loss but may operate imperfectly if more
losses occur. On lossy subnetworks, the Robustness Variable should
be increased to allow for the expected level of packet loss.
However, increasing the Robustness Variable increases the leave
latency of the subnetwork. (The leave latency is the time between
when the last member stops listening to a source or group and when
the traffic stops flowing.)
8.14.2. Query Interval
The overall level of periodic IGMP traffic is inversely proportional
to the Query Interval. A longer Query Interval results in a lower
overall level of IGMP traffic. The Query Interval MUST be equal to
or longer than the Max Response Time inserted in General Query
messages.
8.14.3. Max Response Time
The burstiness of IGMP traffic is inversely proportional to the Max
Response Time. A longer Max Response Time will spread Report
messages over a longer interval. However, a longer Max Response Time
in Group-Specific Group Specific and Source-and-Group-Specific Source-and-Group Specific Queries extends the
leave latency. (The leave latency is the time between when the last
member stops listening to a source or group and when the traffic
stops flowing.) The expected rate of Report messages can be
calculated by dividing the expected number of Reporters by the Max
Response Time. The Max Response Time may be dynamically calculated
per Query by using the expected number of Reporters for that Query as
follows:
+===========================+===============================+
+======================+============================================+
| Query Type | Expected Number of Reporters |
+===========================+===============================+
+======================+============================================+
| General Query | All systems on the subnetwork |
+---------------------------+-------------------------------+
+----------------------+--------------------------------------------+
| Group-Specific Query Group Specific | All systems that had |
| | expressed interest in the |
| Query | the group on the subnetwork |
+---------------------------+-------------------------------+
+----------------------+--------------------------------------------+
| Source-and-Group-Specific Source-and-Group | All systems on the subnetwork that had |
| Specific Query | that had expressed interest |
| | in the source and group |
+---------------------------+-------------------------------+
+----------------------+--------------------------------------------+
Table 15 15: Expected Number of IGMP Reporters
A router is not required to calculate these populations or tune the
Max Response Time dynamically; these are simply guidelines.
9. Security Considerations
IGMP provides any form of confidentiality. This means any device on
a link can passively receive any IGMP message on the link. Such
access can lead to privacy concerns around potentially sensitive
multicast groups or the ability to identify/map the devices on a
link.
We consider the ramifications of a forged message of each type and
describe the usage of an IPsec Authentication Header (AH) to
authenticate messages if desired.
9.1. Query Message
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
forger. If the forger then sends no more Query messages, other
routers' Other Querier Present timer Other-Querier-Present Timer will time out and one will
resume the role of Querier. During this time, if the forger ignores
Leave messages, traffic might flow to groups with no members for up
to [Group Membership Interval].
A Denial-of-Service (DoS) attack on a host could be staged through
forged Group-and- Source-Specific Source Specific Queries. The attacker can find out
about membership of a specific host with a general query. General Query. After
that, it could send a large number of Group-and-Source-Specific
queries, Group-and-Source Specific
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
sources specified in all of those queries Queries for as long as it takes to
send the delayed response. This would consume both memory and CPU
cycles in order to augment the recorded sources with the source lists
included in the successive queries. Queries.
To protect against such a DoS attack, a host stack implementation
could restrict the number of Group-and-Source-Specific Group-and-Source Specific Queries per
group membership within this interval and/or record only a limited
number of sources.
Forged Query messages from the local network can be easily traced.
There are three measures necessary to defend against externally
forged Queries:
* Routers SHOULD NOT forward Queries. This is easier for a router
to accomplish if the Query carries the Router Alert option.
* Hosts SHOULD ignore v2 or v3 Queries without the Router Alert
option.
* Hosts SHOULD ignore v1, v2, or v3 General Queries sent to a
multicast address other than 224.0.0.1, the all-systems address.
9.2. Current-State Report Messages
A forged Report message may cause multicast routers to think there
are members of a group on a network when there are not. Forged
Report messages from the local network are meaningless, as joining a
group on a host is generally an unprivileged operation, so a local
user may trivially gain the same result without forging any messages.
Forged Report messages from external sources are more troublesome;
there are two defenses against externally forged Reports:
1. Ignore the Report if you cannot identify the source address of
the packet as belonging to a network assigned to the interface on
which the packet was received. This solution means that Reports
sent by mobile hosts without addresses on the local network will
be ignored. Report messages with a source address of 0.0.0.0
SHOULD be accepted on any interface.
2. Ignore Report messages without Router Alert options [RFC2113] and
require routers to not forward Report messages. (The requirement
is not a requirement of generalized filtering in the forwarding
path, as the packets already have Router Alert options in them.)
This solution breaks backwards compatibility with implementations
of IGMPv1 or earlier versions of IGMPv2 that did not require a
Router Alert.
A forged Version 1 v1 Report Message message may put a router into "version 1 "v1 members present"
state for a particular group, meaning that the router will ignore
Leave messages. This can cause traffic to flow to groups with no
members for up to [Group Membership Interval]. This can be solved by
providing routers with a configuration switch to ignore Version 1 v1 messages
completely. This breaks automatic compatibility with Version 1 v1 hosts, so it
should only be used in situations where "fast leave" is critical.
A forged Version 2 v2 Report Message message may put a router into "version 2 "v2 members present"
state for a particular group, meaning that the router will ignore
IGMPv3 source-specific state messages. This can cause traffic to
flow from unwanted sources for up to [Group Membership Interval].
This can be solved by providing routers with a configuration switch
to ignore Version 2 v2 messages completely. This breaks automatic
compatibility with Version 2 v2 hosts, so it should only be used in situations
where source include and exclude is critical.
9.3. State-Change Report Messages
A forged State-Change Report message will cause the Querier to send
out Group-Specific Group Specific or Source-and-Group-Specific Source-and-Group Specific Queries for the group
in question. This causes extra processing on each router and on each
member of the group, but it cannot cause loss of desired traffic.
There are two defenses against externally forged State-Change Report
messages:
1. Ignore the State-Change Report message if you cannot identify the
source address of the packet as belonging to a subnet assigned to
the interface on which the packet was received. This solution
means that State-Change Report messages sent by mobile hosts
without addresses on the local subnet will be ignored. State-
Change Report messages with a source address of 0.0.0.0 SHOULD be
accepted on any interface.
2. Ignore State-Change Report messages without Router Alert options
[RFC2113] and require routers to not forward State-Change Report
messages. (The requirement is not a requirement of generalized
filtering in the forwarding path, as the packets already have
Router Alert options in them.)
9.4. IPsec Usage
In addition to these measures, IPsec in AH mode [RFC4302] may be used
to protect against remote attacks by ensuring that IGMPv3 messages
came from a system on the LAN (or, more specifically, from a system
with the proper key). When using IPsec, the messages sent to
224.0.0.1 and 224.0.0.22 should be authenticated using AH. When
keying, there are two possibilities:
1. Use a symmetric signature algorithm with a single key for the LAN
(or a key for each group). This allows validation that a packet
was sent by a system with the key. This has the limitation that
any system with the key can forge a message; it is not possible
to authenticate the individual sender precisely. It also
requires disabling IPsec's Replay Protection.
2. When appropriate key management standards have been developed,
use an asymmetric signature algorithm. All systems need to know
the public key of all routers, and all routers need to know the
public key of all systems. This requires a large amount of key
management but has the advantage that senders can be
authenticated individually so, e.g., a host cannot forge a
message that only routers should be allowed to send.
This solution only directly applies to Query and Leave messages in
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
communication for arbitrary multicast groups.
10. IANA Considerations
All IGMP types described in this document are managed via [RFC9778]. [BCP57].
IANA has replaced each reference to [RFC3376] with a reference to
this document in both the "IGMP Type Numbers" and "IPFIX Information
Elements" registries.
11. 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,
RFC 1112, DOI 10.17487/RFC1112, August 1989,
<https://www.rfc-editor.org/info/rfc1112>.
[RFC2113] Katz, D., "IP Router Alert Option", RFC 2113,
DOI 10.17487/RFC2113, February 1997,
<https://www.rfc-editor.org/info/rfc2113>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC2236] Fenner, W., "Internet Group Management Protocol, Version
2", RFC 2236, DOI 10.17487/RFC2236, November 1997,
<https://www.rfc-editor.org/info/rfc2236>.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
DOI 10.17487/RFC4302, December 2005,
<https://www.rfc-editor.org/info/rfc4302>.
[RFC4604] Holbrook, H., Cain, B., and B. Haberman, "Using Internet
Group Management Protocol Version 3 (IGMPv3) and Multicast
Listener Discovery Protocol Version 2 (MLDv2) for Source-
Specific Multicast", RFC 4604, DOI 10.17487/RFC4604,
August 2006, <https://www.rfc-editor.org/info/rfc4604>.
[RFC4607] Holbrook, H. and B. Cain, "Source-Specific Multicast for
IP", RFC 4607, DOI 10.17487/RFC4607, August 2006,
<https://www.rfc-editor.org/info/rfc4607>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/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
[RFC1071] Braden, R., Borman, D., and C. Partridge, "Computing the
Internet checksum", RFC 1071, DOI 10.17487/RFC1071,
September 1988, <https://www.rfc-editor.org/info/rfc1071>.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B., and A.
Thyagarajan, "Internet Group Management Protocol, Version
3", RFC 3376, DOI 10.17487/RFC3376, October 2002,
<https://www.rfc-editor.org/info/rfc3376>.
[RFC3569] Bhattacharyya, S., Ed., "An Overview of Source-Specific
Multicast (SSM)", RFC 3569, DOI 10.17487/RFC3569, July
2003, <https://www.rfc-editor.org/info/rfc3569>.
[RFC3678] Thaler, D., Fenner, B., and B. Quinn, "Socket Interface
Extensions for Multicast Source Filters", RFC 3678,
DOI 10.17487/RFC3678, January 2004,
<https://www.rfc-editor.org/info/rfc3678>.
Appendix A. Design Rationale
A.1. The Need for State-Change Messages
IGMPv3 specifies two types of Membership Reports: Current-State and
State Change.
State-Change. This section describes the rationale for needing both
types of Reports.
Routers need to distinguish Membership Reports that were sent in
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
Membership Queries are used mainly to refresh the existing state at
the router; they typically do not cause transitions in state at the
router. Membership Reports that are sent in response to changes in
interface state require the router to take some action in response to
the received report (see Section 6.4).
The inability to distinguish between the two types of reports would
force a router to treat all Membership Reports as potential changes
in state, and it could result in increased processing at the router
as well as an increase in IGMP traffic on the network.
A.2. Host Suppression
In IGMPv1 and IGMPv2, a host would cancel sending pending membership
reports Membership
Reports if a similar report was observed from another member on the
network. In IGMPv3, this suppression of host membership reports Membership Reports has
been removed. The following points explain the reasons behind this
decision.
1. Routers may want to track per-host membership status on an
interface. This allows routers to implement fast leaves (e.g.,
for layered multicast congestion control schemes) as well as
track membership status for possible accounting purposes.
2. Membership Report suppression does not work well on bridged LANs.
Many bridges and Layer 2 / Layer 3 switches that implement IGMP
snooping do not forward IGMP messages across LAN segments in
order to prevent membership report Membership Report suppression. Removing
membership report
Membership Report suppression eases the job of these IGMP
snooping devices.
3. By eliminating membership report Membership Report suppression, hosts have fewer
messages to process; this leads to a simpler state machine
implementation.
4. In IGMPv3, a single membership report Membership Report now bundles multiple
multicast group records Group Records to decrease the number of packets sent.
In comparison, the previous versions of IGMP required that each
multicast group be reported in a separate message.
A.3. Switching Router Filter Modes from EXCLUDE to INCLUDE
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
well (see Section 6.2.1). In EXCLUDE mode, a router forwards traffic
from all sources unless that source exists in the exclusion source
list. If all hosts in EXCLUDE mode cease to exist, it would be
desirable for the router to switch back to INCLUDE mode seamlessly
without interrupting the flow of traffic to existing receivers.
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
router itself is in EXCLUDE mode. If the group timer Group Timer now expires in
EXCLUDE mode, it implies that there are no hosts in EXCLUDE mode on
the network (otherwise, a membership report Membership Report from that host would have
refreshed the group timer). Group Timer). The router can then switch to INCLUDE
mode seamlessly with the list of sources currently being forwarded in
its source list.
Appendix B. Summary of Changes from IGMPv2
While the main additional feature of IGMPv3 is the addition of source
filtering, the following is a summary of other changes from
[RFC2236].
* State is maintained as Group + List-of-Sources, not simply Group
as in IGMPv2.
* Interoperability with IGMPv1 and IGMPv2 systems is defined as
operations on the IGMPv3 state.
* The IP service interface has changed, to allow specification of
source-lists.
* The Querier includes its Robustness Variable and Query Interval in
Query packets to allow synchronization of these variables on non-
Queriers.
* The Max Response Time in Query messages has an exponential range,
changing the maximum from 25.5 seconds to about 53 minutes, for
use on links with a huge number of systems.
* Hosts retransmit state-change messages for increased robustness.
* Additional data sections are defined, to allow later extensions.
* Report packets are sent to 224.0.0.22, to assist Layer 2 switches
in snooping.
* Report packets can contain multiple group records, Group Records, to allow
reporting of full current state using fewer packets.
* Hosts no longer perform suppression, to simplify implementations
and permit explicit membership tracking.
* A new S flag in Query messages fixes robustness issues, which were
also present in IGMPv2.
Appendix C. Summary of Changes from RFC 3376
The following is a list of changes made since [RFC3376] was
published.
* Modified the definition of Older Version Querier Present Interval
to address Erratum 4375.
* Modified the metadata to fix the Obsoletes vs. Updates
relationship with [RFC2236] per Erratum 1501.
* Updated the introductory text to describe the Updates relationship
with [RFC2236] per Erratum 7339.
* Updated the definition of Group Membership Interval to address
Erratum 6725.
* Updated the text relating to the router filter-mode Router Filter Mode to address
Erratum 5562.
* Clarified the use of General Queries in the Querier election
process.
Acknowledgments
We would like to thank Ran Atkinson, Luis Costa, Toerless Eckert,
Dino Farinacci, Serge Fdida, Wilbert de Graaf, Sumit Gupta, Mark
Handley, Bob Quinn, Michael Speer, Dave Thaler, and Rolland Vida for
comments and suggestions on [RFC3376].
Stig Venaas, Hitoshi Asaeda, and Mike McBride have provided valuable
feedback on this specification, and we thank them for their input.
Contributors
Brad Cain, Steve Deering, Isidor Kouvelas, Bill Fenner, and Ajit
Thyagarajan are the authors of [RFC3376], which forms the bulk of the
content contained herein.
Anuj Budhiraja, Toerless Eckert, Olufemi Komolafe, and Tim Winters
have contributed valuable content to this specification.
Author's Address
Brian Haberman (editor)
Johns Hopkins University Applied Physics Lab
Email: brian@innovationslab.net